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CHAPTER The Skeletal System 5 5.1 The skeletal system consists...

CHAPTER The Skeletal System 5 5.1 The skeletal system consists of connective tissue 131 5.2 Bones develop from cartilage 133 5.3 Mature bone undergoes remodeling and repair 134 5.4 Bones fit together to form the skeleton 136 5.5 Joints form connections between bones 142 5.6 Diseases and disorders of the skeletal system 145 Colored X-ray of the cervical spine of a healthy 29-year-old male. Key Concepts The skeletal system is composed of bones, ligaments, and cartilage. The skeletal system supports and protects the other organ systems of the body and provides a structure that enables movement. Bones consist of living cells surrounded by extracellular deposits of calcium minerals. Bone tissue undergoes constant replacement, remodeling, and repair. Bones store minerals and produce the cellular components of blood (red blood cells, white blood cells, and platelets). Ligaments comprised of connective tissue hold bones together. When damaged, ligaments are slow to heal because they have very few living cells and a poor blood supply. Joints are the points of contact between bones. In a movable joint, bone surfaces are covered by a layer of smooth cartilage and lubricated with fluid to reduce friction and wear. 129 CURRENT ISSUE Questions to Consider 1 Do you approve of harvesting bone A Black Market in Human Bones? from cadavers for the purpose of process- ing the tissue into bone-based products for patients, provided the tissue is legally Alistair Cooke, famed host of the PBS series Masterpiece Theatre, died in 2004 at the obtained? age of 95. His body lay in a New York 2 Do you think it should be legal to sell City funeral parlor for a few days awaiting a cadaver or its parts? cremation. But before Cooke’s body was cremated, it was secretly carved up in a are used in more than 600,000 surgical back room and his bones were removed. procedures every year. The patient pays all Authorities allege that his bones were fees incurred in the handling, processing, then sold for a substantial profit, to be testing, and shipping of the products, but the transplanted into patients in desperate tissue banks themselves do not make a profit. need of tissue grafts. Cooke’s family, who had not given Illegal Body Parts Enter permission for his body parts to be donated, knew nothing of this until police contacted Human bones and bone products ready for the Supply Chain shipment to hospitals. them after the funeral. Understandably, The imbalance between supply and they were appalled. But there is an even demand for human body parts leaves the more horrifying side to this story: Cooke injuries, transplanted vertebrae relieve back entire system vulnerable to exploitation. died of lung cancer that had spread to his pain, and veins and heart valves are used in In the Alistair Cooke case, prosecutors bones. Could his deadly cancer have been heart surgeries. The bones, tendons, veins, alleged that Michael Mastromarino, an oral transmitted to the people who received and heart valves from just one corpse can be surgeon who had lost his license, arranged his bone tissue? It’s unlikely since bone worth over $200,000 to surgeons, hospitals, for a Brooklyn funeral parlor to deliver products generally are sterilized, but the and recipients. bodies to a secret operating room. There, answer to this question may not be known Under federal law, it is illegal to sell Mastromarino and his accomplices removed for decades. human body parts for a profit—they can only body parts before the bodies were buried be donated, either by the patient while he or or cremated. Authorities say that the men she is still alive or by the family after death. paid the funeral parlor up to $1,000 per Recycling Body Parts: Several hundred licensed nonprofit tissue body and then sold the harvested tissues for A Legitimate Industry banks in the United States receive donated up to $7,000 per body to a legitimate but The process of harvesting tissues from a tissues and test them for infectious diseases unsuspecting tissue-processing company. human corpse for transplant into a patient such as HIV, syphilis, and the viruses that In some cases, Mastromarino and his is a legitimate industry that serves urgent cause hepatitis (inflammation of the liver). accomplices falsified records indicating medical needs. Harvested bones are used To reduce the chances of tissue products the deceased’s age and cause of death. to repair fractures and replace cancerous transmitting disease, authorities impose Mr. Cooke died of cancer at the age of 95, bone. Bone pins and powdered bone created strict guidelines that specify what types of but his records were falsified to indicate that from donated bone is used in dental surgery; tissues may be harvested, from whom they he died at age 85 of a heart attack. They also bone paste plugs holes. Tendons and may be harvested, and how they must be allegedly looted body parts from a 43-year- ligaments taken from donors are used to processed. For instance, to prevent any risk old woman who had died of ovarian cancer; repair joints and tissues damaged by sports of transmitting cancer, federal guidelines they then forged a signature on a consent prohibit the use of bones form and listed the cause of death as a head from cancer patients for injury. Prosecutors eventually identified over tissue implants. a thousand corpses from which body parts After donation, bone were taken without permission between 2001 tissue is shaped into and 2005. usable forms, such In 2008, Mr. Mastromarino pled guilty as pins, plates, and in a plea bargain in exchange for providing powders. The final information about others who were involved. products are sterilized He was expected to spend at least 18 years and shipped to hospitals in prison, but he died of liver and bone and surgeons all over cancer in 2013. An accomplice and seven the country, where they funeral home directors received lesser sentences. Donated bones are cleaned, The Alistair Cooke case is not the first sterilized, and shaped into such incident, and it is not likely to be the bone products. last. In 1999, the University of California ➔ 130  CH APTER 5 The Skeletal System 131 at Irvine discovered that the director of its should only have to pay the legitimate Safeguards need to be put in place to Willed Body Program was selling human costs associated with the body parts prevent abuses so that we can be assured spines to a Phoenix hospital for $5,000 processing industry—not the added fees of the legitimacy and the safety of the apiece. And in 2008, the director of UCLA’s paid to traffickers in illegal body parts. supply of human body parts. Willed Body Program was sentenced to four years in prison for selling more than a million dollars worth of body parts. SUMMARY Regulators say that abuses such as these are most likely to occur when relatively poorly It is illegal to buy or sell human body parts for a profit. Patients or their families can paid directors (including funeral home donate body parts only to nonprofit tissue banks. directors) have access to valuable body parts Legitimately donated tissues are tested, sterilized, processed, and transplanted into and when oversight is lax. UC Irvine and patients who urgently need them. UCLA have both tightened their oversight Only about 20,000 cadavers are donated annually for body parts—not enough to meet procedures as a result of the scandals. the growing legitimate demand for human tissues and tissue products. The donation of a single cadaver to a nonprofit tissue-processing company can The supply/demand imbalance may be contributing to a black market in body parts benefit several dozen patients. Patients harvested illegally from cadavers. The human body is capable of an awesome array of Ligaments consist of dense fibrous connective tissue—they physical activities. With training, some individuals can bind the bones to each other. Cartilage is a specialized run a mile in less than four minutes or lift more than connective tissue consisting primarily of fibers of collagen their own weight. Exquisitely sensitive motor skills allow and elastin in a gel-like fluid called ground substance. us to thread a needle, turn our head to focus on a single Cartilage has several functions, including reducing friction star, and throw a baseball into the strike zone. Considered in joints. individually, any one of these activities may not seem amazing, but for a single structure (the human body) to be capable of all of them is remarkable indeed. From Bones are the hard elements of the skeleton an engineering standpoint it would be like designing a Most of the mass of bones consists of nonliving extracellular bulldozer that is strong enough to flatten a building, yet crystals of calcium minerals that give bones their hard, rigid delicate enough to pick up a dime. appearance and feel. But bone is actually a living tissue The one organ system that makes all this possible is that contains several types of living cells involved in bone the skeletal system. Without the skeletal system, we’d be formation and remodeling, plus nerves and blood vessels. about as capable of movement as a jellyfish or a slug. In Indeed, bones bleed when cut during orthopedic surgery or partnership with our muscles, the bones of our skeletal when they break. system permit us to stand upright and to move about. The Bones perform five important functions. The first skeletal system supports and encloses our internal organs three—support, protection, and movement—are the same and provides a structure for our external covering, the skin. as the functions of the skeleton overall, which is, after And last but not least, bones of the skeletal system surround all, primarily bone. The rigid support structure of bones and protect the brain, our body’s central control system. allows us to sit and to stand upright. The bones of the In this chapter, we describe the structure and development skeleton also support, surround, and protect many of our of the bones that comprise the skeletal system, how bones soft internal organs, such as the lungs, liver, and spleen. The develop, and how they remodel and repair themselves. We’ll attachment of bones to muscles makes it possible for our review how bones fit together to form the skeleton, look at bodies to move. how joints enable bones and muscles to work in unison, and The fourth and fifth functions of bones—blood cell consider what can go wrong with the skeletal system. formation and mineral storage—are harder to remember, but they are just as important. Cells in certain bones are the only source of new red and white blood cells and 5.1 The skeletal system consists platelets for blood. Without this ability to produce new blood cells, we would die within months. Bones also serve of connective tissue as an important long-term storage depot for two minerals, The skeletal system comprises three types of connective calcium and phosphate, that can be drawn from bone when tissue—bones, ligaments, and cartilage. Bones are the hard necessary (for example, during pregnancy to support the elements of the skeleton with which we are most familiar. growth of a fetus). 132 C HA P TE R 5 The Skeletal System Bone contains living cells hard, relatively strong trabeculae (from Latin, meaning “little beams”). Although spongy bone is as hard as compact bone A typical long bone, so called because it is longer than it (it is spongy only in appearance), it is less dense, allowing is wide, consists of a cylindrical shaft (called the diaphysis) bones to be light but strong. with an enlarged knob called an epiphysis at each end Taking an even closer look at compact bone (Figure 5.1a). Dense compact bone forms the shaft and (Figure 5.1c), we see that it is made up largely of covers each end, and less dense spongy bone fills the inner extracellular deposits of calcium phosphate enclosing and regions of the epiphyses. surrounding living cells called osteocytes (from the Greek A central cavity in the diaphysis is filled with yellow bone words for “bone” and “cells”). Osteocytes are arranged in marrow. Yellow bone marrow is primarily fat that can be rings in cylindrical structures called osteons (sometimes utilized for energy. In certain long bones, most notably the called Haversian systems). Osteocytes nearest the center of an long bones of the upper arms and legs (humerus and femur, osteon receive nutrients by diffusion from blood vessels that respectively), the spaces within spongy bone are filled with pass through a central canal (Haversian canal). red bone marrow. Special cells called stem cells in the red bone As bone develops and becomes hard, the osteocytes marrow produce red and white blood cells and platelets. become trapped in hollow chambers called lacunae The outer surface of the bone is covered by a tough (Figure 5.1d). However, the osteocytes remain in direct contact layer of connective tissue, the periosteum, which contains with each other via thin canals called canaliculi. Within specialized bone-forming cells. If an epiphysis of a long the canaliculi, extensions of the cell cytoplasm in adjacent bone forms a movable joint with another bone, the joint osteocytes are joined together by gap junctions (channels that surface is covered by a smooth layer of cartilage that reduces permit the movement of ions, water, and other molecules friction. between two adjacent cells). By exchanging nutrients across A closer look at a section of bone taken from an gap junctions, osteocytes can be supplied with nutrients even epiphysis (Figure 5.1b) reveals that compact bone is a nearly though most osteocytes are not located near a blood vessel. solid structure, with central canals containing nerves and Waste products produced by the osteocytes are exchanged in the blood vessels. In contrast, spongy bone is a latticework of Central canal Osteon Spongy bone (spaces contain Osteon Osteocytes Epiphysis red bone marrow) c) A photograph of an osteon of compact bone showing osteocytes embedded within the Compact solid structure. bone Yellow bone Lacuna marrow Osteocyte Blood Diaphysis vessel Canalicula Periosteum d) A single osteocyte in a lacuna. Osteocytes remain Central cavity in contact with each other by (contains yellow cytoplasmic extensions into bone marrow) the canaliculi between cells. Spongy bone Compact bone Epiphysis Blood vessels and b) A closer view of a section of bone. nerve in central canal Compact bone is a nearly solid structure with central canals for the blood vessels and nerves. a) A partial cut through a long bone. Spongy bone is spongy in appearance only. Figure 5.1 Structure of bone.  CH APTER 5 The Skeletal System 133 opposite direction and are removed from the bone by blood There are three types of cartilage in the human skeleton. vessels. In spongy bone, osteocytes do not need to rely on Fibrocartilage consists primarily of collagen fibers arranged central canals for nutrients and waste removal. The slender in thick bundles. It withstands both pressure and tension trabecular structure of spongy bone gives each osteocyte well. The intervertebral disks between the vertebrae, and access to nearby blood vessels in red bone marrow. also certain disklike supportive structures in the knee joint called menisci, are made of fibrocartilage. Hyaline cartilage If osteocytes did not have gap junctions in their cell is a smooth, almost glassy cartilage of thin collagen fibers. ­membranes, would they be able to survive? Explain. Hyaline cartilage forms the embryonic structures that later become the bones. It also covers the ends of mature bones Ligaments hold bones together in joints, creating a smooth, low-friction surface. Elastic cartilage is mostly elastin fibers, so it is highly flexible. It Ligaments attach bone to bone. Ligaments consist of dense lends structure to the outer ear and to the epiglottis, a flap fibrous connective tissue, meaning that they are a regular of tissue that covers the larynx during swallowing. array of closely packed collagen fibers all oriented in the same direction, with just a few fibroblasts in between. Recap Bones contribute to support, movement, and protection. (Recall that fibroblasts are cells that produce and secrete Bones also produce the blood cells and store minerals. Ligaments the proteins that compose collagen, elastic, and reticular hold bones together, and cartilage provides support. fibers.) Ligaments confer strength to certain joints while still permitting movement of the bones in relation to each other. 5.2 Bones develop from cartilage Cartilage lends support In the earliest stages of fetal development, the rudimentary Cartilage contains fibers of collagen and/or elastin in a models of future bones are created out of hyaline cartilage ground substance of water and other materials. Cartilage, (Figure 5.2a) by cartilage-forming cells called chondroblasts. smoother and more flexible than bone, is found where Most chondroblasts are short-lived, however; within support under pressure is needed and movement is necessary. 2–3 months they begin to die, and as they do the cartilage Adolescence Cartilage Childhood growth Fetus: At plate 2–3 months Fetus: First 2 months Developing periosteum Compact bone Blood containing vessel osteocytes a) Chondroblasts form b) The cartilage starts to c) Osteoblasts secrete hyaline cartilage, creating dissolve and the periosteum osteoid and enzymes, a rudimentary model of begins to develop. Newly facilitating the deposition Cartilage future bone. developing blood vessels of hard hydroxyapatite growth transport osteoblasts into the crystals. plate area from the periosteum. d) The growth plates in long bones move farther apart. The bone lengthens and widens. Figure 5.2 How bone develops. The first two phases of bone development occur in the fetus. Bones continue to grow longer throughout childhood and adolescence because of growth at the growth plates. 134 C HA P TE R 5 The Skeletal System dissolves, making room for blood vessels (Figure 5.2b). At the replaced by bone tissue. At this point, the bones can no longer same time, the periosteum begins to form on the outer surface lengthen, though they can continue to grow in width. of the developing bone. The blood vessels carry bone-forming cells called osteoblasts (from the Greek words for “bone” Recap Bone-forming cells called osteoblasts produce a and “to build”) into the area from the developing periosteum. protein mixture (including collagen) that becomes bone’s Osteoblasts secrete a mixture of proteins (including collagen) structural framework. Osteoblasts also secrete an enzyme that called osteoid, which forms a matrix that provides internal facilitates mineral deposition. structure and strength to bone (Figure 5.2c). Osteoblasts also secrete enzymes that facilitate the crystallization of hard mineral salts of calcium phosphate around and between the osteoid 5.3 Mature bone undergoes matrix. As more and more calcium phosphate is deposited, the remodeling and repair osteoblasts become embedded in the hardening bone tissue. In Even though bones stop growing longer, they do not remain mature compact bone, approximately one-third of the structure the same throughout life. Bone is a dynamic tissue that is osteoid and two-thirds is crystals of calcium phosphate. undergoes constant replacement, remodeling, and repair. Eventually, the rate at which osteoblasts produce the Remodeling may be so extensive that there is a noticeable osteoid matrix and stimulate the mineral deposits declines, change in bone shape over time, even in adults. and osteoblasts become mature osteocytes embedded in Bone remodeling and repair are in part due to a third their individual lacunae. Mature osteocytes continue to type of bone cell called an osteoclast (from the Greek maintain the bone matrix, however. Without them, the words for “bone” and “to break”). Osteoclasts cut through matrix would slowly disintegrate. mature bone tissue, dissolving the hard calcium phosphate Some bones, called long bones, continue to lengthen crystals and digesting the osteoid matrix in their path. throughout childhood and adolescence. This is because The released calcium and phosphate ions enter the blood. chondroblasts and osteoblasts remain active in a narrow The areas from which bone has been removed attract new strip of cartilage called the growth plate (or epiphyseal osteoblasts, which lay down new osteoid matrixes and plate) (Figure 5.2d). Chondroblast activity (and hence stimulate the deposition of new calcium phosphate. the deposition of new cartilage) is concentrated on the Table 5.1 summarizes the four types of cells that outside of the plate, whereas the conversion of the cartilage contribute to bone development and maintenance. model to bone by osteoblasts is concentrated on the inside of the plate (Figure 5.3). In effect, the bone lengthens Bones can change in shape, size, and strength as the two growth plates migrate farther and farther apart. Bones also grow in width as osteoblasts lay down more bone Over time, constant remodeling can actually change the on the outer surface just below the periosteum. shape of a bone. The key is that compression stress on a Bone development is controlled by hormones, chemicals bone, such as the force of repeated jogging on the legs, causes secreted by the endocrine glands. The most important tiny electrical currents within the bone. These electrical hormone in preadolescents is growth hormone, which currents stimulate the bone-forming activity of osteoblasts. stimulates the bone-lengthening activity of the growth plate. The compressive forces and the electric currents are greatest During puberty, the sex hormones (testosterone and estrogen) at the inside curvature of the long bone undergoing stress also stimulate the growth plate, at least initially. But at about (Figure 5.4a). Thus, in the normal course of bone turnover, age 18 in females and 21 in males, these same sex hormones new bone is laid down in regions under high compressive signal the growth plates to stop growing, and the cartilage is stress and bone is resorbed in areas of low compressive stress (Figure 5.4b). The final shape of a bone, then, tends to match the compressive forces to which it is exposed (Figure 5.4c). Chondroblasts Weight-bearing exercise increases overall bone mass and deposit new strength. The effect is pronounced enough that the bones of cartilage at the outer surface trained athletes may be visibly thicker and heavier than those Osteoblasts Table 5.1 Cells involved in bone development convert cartilage and maintenance to bone at the inner surface Type of cell Function Chondroblasts Cartilage-forming cells that build a model of the future bone Osteoblasts Young bone-forming cells that cause the hard extracellular matrix of bone to develop Figure 5.3 How long bones increase in length. Osteocytes Mature bone cells that maintain the structure of bone What two substances do osteoblasts produce that lead to the Osteoclasts Bone-dissolving cells formation of hard bone?  CH APTER 5 The Skeletal System 135 Compressive force Bone removed New bone here added here a) The application of force b) Over time, bone is c) The final result is to a slightly bent bone deposited on the inside a bone matched to produces a greater curvature and removed the compressive compressive force on the from the outside force to which it is inside curvature. curvature. exposed. Compressive force produces weak electrical currents, which stimulate osteoblasts. Figure 5.4 Bone remodeling. of nonathletes. You don’t have to be a professional athlete of an imbalance over many years in the rates of activities of to get this benefit, however. If you begin a regular program these two types of bone cells. of any weight-bearing exercise, such as jogging or weight lifting, your bones will become denser and stronger as your Swimming is considered good exercise for muscle mass and osteoblasts produce more bone tissue. overall cardiorespiratory fitness. How would you expect swimming to affect bone mass and density? MJ’s BlogInFocus   Should you have a bone density scan to determine whether you’re losing bone mass? Is it Bone cells are regulated by hormones currently recommended? To find out, visit MJ’s blog Like bone growth, the rates of activities of osteoblasts and in the Study Area in MasteringBiology and look under osteoclasts in adulthood are regulated by hormones that “Bone Density Scans.” function to maintain calcium homeostasis. When blood http://goo.gl/RRyMxp levels of calcium fall below a given point, parathyroid hormone (PTH) stimulates osteoclasts to secrete more bone-dissolving enzymes. The increased activity of osteoclasts causes more Homeostasis of bone structure depends on the precise bone to be dissolved, releasing calcium and phosphate into the balance of the activities of osteoclasts and osteoblasts. bloodstream. If calcium levels rise, then another hormone called Osteoporosis is a common condition in which bones lose calcitonin stimulates osteoblast activity, causing calcium and a great deal of mass (seemingly becoming “porous”) because phosphate to be removed from blood and deposited in bone. 136 C HA P TE R 5 The Skeletal System Although the total bone mass of young adults doesn’t change much, it’s estimated that almost 10% of their bones may be Axial skeleton Appendicular skeleton remodeled and replaced each year. We’ll review the regulation of blood calcium concentration in more detail when we discuss the endocrine system. Cranium Suppose a man is not getting sufficient calcium in his diet, such (skull) that his blood calcium level is chronically low. Would his PTH levels and calcitonin levels be low, normal, or high? Explain. Maxilla Mandible Bones undergo repair A broken (fractured) bone undergoes a repair process that can take weeks to months, depending on your age and the Clavicle bone involved. Immediately after a fracture, blood vessels Scapula supplying the bone bleed into the area, producing a mass of Sternum clotted blood called a hematoma. Inflammation, swelling, and Ribs pain are likely to occur during this stage. The repair process begins within days as fibroblasts migrate to the area. Some Humerus of the fibroblasts become chondroblasts, and together they produce a tough fibrocartilage bond called a callus between Vertebrae the two broken ends of the bone. A callus can be felt as a hard, raised ring at the point of the break. Then osteoclasts arrive and begin to remove dead fragments of the original bone and Ulna the blood cells of the hematoma. Finally, osteoblasts arrive Radius to deposit osteoid matrix and encourage the crystallization of calcium phosphate minerals, converting the callus into bone. Carpals Eventually, the temporary union becomes dense and hard again. Bones rarely break in the same place twice because the Metacarpals repaired union remains slightly thicker than the original bone. Recently, it has been discovered that the application of weak electrical currents to the area of a broken bone can increase the rate of healing. It is thought that electrical Sacrum Phalanges current works by attracting osteoclasts and osteoblasts to the area under repair. Recap Healthy bone replacement and remodeling depend on Coxal bone the balance of activities of bone-resorbing osteoclasts and bone- forming osteoblasts. When a bone breaks, a fibrocartilage callus Femur forms between the broken ends and is later replaced with bone. Patella 5.4 Bones fit together to form the skeleton Tibia Now that we have reviewed the dynamic nature of bone Fibula tissue, we turn to how bones are classified and organized in the human skeleton (Figure 5.5). Bones can be classified into four types based on shape: long, short, flat, and irregular. So Tarsals far we have discussed long bones, which include the bones of Metatarsals the limbs and fingers. Short bones (the bones of the wrists) are approximately as wide as they are long. Flat bones (including Phalanges the cranial bones, the sternum, and the ribs) are thin, flattened, and sometimes curved, with only a small amount Figure 5.5 The human skeleton. of spongy bone sandwiched between two layers of compact bone. Irregular bones such as the coxal (hip) bones and the On this and subsequent figures, find the anatomical terms vertebrae include a variety of shapes that don’t fit into the corresponding to the following common names: breastbone, other categories. A few flat and irregular bones, including the collarbone, shoulder blade, hip bone, thighbone, shinbone.  CH APTER 5 The Skeletal System 137 sternum and the hip bones, contain red bone marrow that are joined tightly together except for the mandible, which produces blood cells. attaches to the temporal bone by a joint that, because it The human skeleton comprises 206 bones and the permits a substantial range of motion, allows us to speak various connective tissues that hold them together. The and chew. skeleton has three important functions. First, it serves as a Curving underneath to form the back and base of the structural framework for support of the soft organs. Second, skull is the occipital bone. Near the base of the occipital it protects certain organs from physical injury. The brain, for bone is a large opening called the foramen magnum (Latin example, is enclosed within the bones of the skull, and the for “great opening”). This is where the vertebral column heart and lungs are protected by a bony cage consisting of connects to the skull and the spinal cord enters the skull to ribs, the sternum, and vertebrae. Third, because of the way communicate with the brain. that the bony elements of the skeleton are joined together Several of the cranial and facial bones contain air spaces at joints, the presence of the skeleton permits flexible called sinuses, which make the skull lighter and give the movement of most parts of the body. This is particularly true human voice its characteristic tone and resonance. Each of the hands, feet, legs, and arms. sinus is lined with tissue that secretes mucus, a thick, sticky The skeleton is organized into the axial skeleton and the fluid that helps trap foreign particles in incoming air. The appendicular skeleton. sinuses connect to the nasal cavity via small passageways through which the mucus normally drains. However, if you The axial skeleton forms the midline of the body develop a cold or respiratory infection, the tissue lining your sinuses can become inflamed and block these passages. Sinus The axial skeleton consists of the skull (including the inflammation is called sinusitis. If fluid accumulates inside maxilla and mandible), sternum, ribs, and vertebral column the sinuses, the resulting sensation of pressure may give you (including the sacrum; see Figure 5.5). a “sinus headache.” The skull: cranial and facial bones The human skull (cranium) comprises over two dozen bones that protect the brain and form the structure of the face. Figure 5.6 illustrates Temporal bone Parietal bone some of the more important bones of the skull. Frontal bone The cranial bones are flat bones in the skull that enclose Sphenoid bone and protect the brain. Starting at the front of the skull, the frontal bone comprises the forehead and the upper ridges of Ethmoid bone the eye sockets. At the upper left and right sides of the skull are the two parietal bones, and forming the lower left and Lacrimal bone right sides are the two temporal bones. Each temporal bone is pierced by an opening into the ear canal that allows sounds Nasal bone to travel to the eardrum. Between the frontal bone and the Zygomatic bone temporal bones is the sphenoid bone, which forms the back Maxilla of both eye sockets. The ethmoid bone contributes to the eye Occipital sockets and also helps support the nose. bone The facial bones compose the front of the skull. On either Mandible side of the nose are the two maxilla (maxillary) bones, which External auditory form part of the eye sockets and contain the sockets that meatus anchor the upper row of teeth. The hard palate (the “roof” Maxilla of the mouth) is formed by the maxilla bones and the two palatine bones. Behind the palatine bones is the vomer bone, Zygomatic bone which is part of the nasal septum that divides the nose Palatine bone into left and right halves. The two zygomatic bones form the Sphenoid bone cheekbones and the outer portion of the eye sockets. Vomer bone The two small, narrow nasal bones underlie only the upper bridge of the nose; the rest of the fleshy protuberance Foramen magnum called the nose is made up of cartilage and other connective tissue. Part of the space formed by the maxillary and nasal bones is the nasal cavity. The small lacrimal bones, at the Occipital bone inner eye sockets, are pierced by a tiny opening through which the tear ducts drain tears from the eye sockets into the nasal cavity. Figure 5.6 The human skull. Except for the mandible, which has The mandible, or lower jaw, contains the sockets that a hinged joint with the temporal bone, the bones of the skull are house the lower row of teeth. All the bones of the skull joined tightly together. Their function is protection, not movement. 138 C HA P TE R 5 The Skeletal System 1 2 3 Cervical 4 vertebrae 5 (7) 6 7 1 2 3 4 5 Larynx 6 Thoracic 7 vertebrae (12) 8 9 10 11 12 1 2 Hyoid bone Lumbar 3 Figure 5.7 The hyoid bone. vertebrae (5) 4 5 The hyoid bone The hyoid bone (Figure 5.7) does not make direct contact with the other bones of the axial Sacrum skeleton; it is attached to the temporal bone only by (5 fused) ligaments. It serves as a point of attachment for muscles of the Coccyx tongue, the larynx, and the pharynx. Because of its position, (4 fused) it is rarely broken by accidental injury. In cases of suspected homicide, however, a broken hyoid bone is considered to be a strong indication of deliberate strangulation. Figure 5.8 The vertebral column. Vertebrae are named and numbered according to their location. The vertebral column The vertebral column: the body’s main axis The is moderately flexible because of the presence of joints and vertebral column (the backbone or spine) is the main intervertebral disks. axis of the body (Figure 5.8). It supports the head, protects the spinal cord, and serves as the site of attachment for the A closer look at vertebrae (Figure 5.9a) shows four limbs and various muscles. It consists of a column of how they are stacked on each other and how they are 33 irregular bones called vertebrae (singular: vertebra) that joined. Vertebrae share two points of contact, called extend from the skull to the pelvis. When viewed from the articulations, located behind their main body. There are side, the vertebral column is somewhat curved, reflecting also articulations with the ribs. The spinal cord passes slight differences in structure and size of vertebrae in the through a hollow cavity between the articulations and various regions. the main body. Neighboring vertebrae are separated We classify the vertebral column into five anatomical from each other by a flat, elastic, compressible regions: intervertebral disk composed of a soft gelatinous Cervical (neck)—7 vertebrae. center and a tough outer layer of fibrocartilage. Thoracic (the chest or thorax)—12 vertebrae. Intervertebral disks serve as shock absorbers, protecting Lumbar (the lower portion or “small” of the back, which the delicate vertebrae from the impact of walking, forms the lumbar curve of the spine)—5 vertebrae. jumping, and other movements. In conjunction with Sacral (in the sacrum or upper pelvic region)—In the course the vertebral joints, vertebral disks also permit a limited of evolution, the 5 sacral vertebrae have become fused. degree of movement. This lends the vertebral column Coccygeal (the coccyx or tailbone)—4 fused vertebrae. greater flexibility, allowing us to bend forward, lean The coccyx is all that remains of the tails of our ancient backward, and rotate the upper body. ancestors. It is an example of a vestigial structure, An especially strong impact or sudden movement meaning one that no longer has any function. can compress an intervertebral disk, forcing the softer  CH APTER 5 The Skeletal System 139 Spinal cord Articulations with another vertebra Intervertebral disk Spinal nerve Articulation with ribs Main bodies of vertebrae a) Healthy disks. Herniated area pressing against a nerve b) A herniated disk. Figure 5.9 Vertebrae. ‫هنا بتجي الصورة وسهم‬ ‫واملطلوب منك االسم‬ center to balloon outward, press against spinal nerves, and ( Ribs & Cartilage) ‫اذا كان السهم بني ال‬ cause intense back pain. This condition is referred to as a ‫مارح يجونك االثنني في الخيارات واملوجود هو الصح‬ “herniated” or “slipped disk” (Figure 5.9b), and it occurs most often in the lumbar vertebrae. Occasionally the disk may rupture, releasing its soft, pulpy contents. The pain that C7 accompanies a herniated disk can be alleviated by surgery T1 1 to remove the damaged disk, relieving the pressure against the nerve. However, surgical correction of a herniated disk reduces spinal flexibility somewhat because the two adjacent 2 vertebrae must be fused together with bone grafts. Sternum 3 (breastbone) Generally, the bony vertebral column does an effective 4 job of shielding the softer spinal cord, which consists of Ribs 5 nervous tissue that connects the brain to the rest of the body. 6 However, injury to the vertebral column can damage the Cartilage T11 7 spinal cord or even sever it, resulting in partial or complete paralysis of the body below that point. Persons with suspected T12 8 9 vertebral injuries should not be moved until a physician can Vertebral column L1 10 12 assess the situation, because any twisting or bending could L2 cause additional, perhaps permanent, damage to the spinal 11 Floating ribs cord. You may have noticed that when athletes are injured on the field, they are instructed to lie absolutely still until a Figure 5.10 Ribs. The 12 pairs of ribs are numbered according trainer and physician have examined them thoroughly. to their attachment to the thoracic vertebrae. Only the first seven pairs attach directly to the sternum. The ribs and sternum: protecting the chest cavity What function do the ribs and sternum have that other parts of the Humans have 12 pairs of ribs (Figure 5.10). One end of skeleton do not have? How might this explain the fact that the ribs each rib branches from the thoracic region of the vertebral and sternum are connected by flexible cartilage rather than by bone? 140 C HA P TE R 5 The Skeletal System column. The other ends of the upper seven pairs attach via joint in the body. The upper arm can rotate in roughly a cartilage to the sternum, or breastbone, a flat blade-shaped circle, the arm can bend in one dimension and rotate, and bone composed of three separate bones that fuse during the wrist and fingers can all bend and rotate to varying development. Rib pairs 8–10 are joined to the seventh rib degrees. We also have “opposable thumbs,” meaning we can by cartilage, and thus attach indirectly to the sternum. The place them opposite our other fingers. The opposable bottom two pairs of ribs are called floating ribs because they thumb has played an important role in our evolutionary do not attach to the sternum at all. history, as it makes it easier to grasp and manipulate tools The ribs, sternum, and vertebral column form a and other objects. protective rib cage that surrounds and shields the heart, lungs, We pay a price for this flexibility, because freedom of and other organs of the chest (thoracic) cavity. The rib cage movement also means relative instability. If you fall on your also helps us breathe, because muscles between the ribs lift arm, for example, you might dislocate your shoulder joint them slightly during breathing, expanding the chest cavity or crack a clavicle. In fact, the clavicle is one of the most and inflating the lungs. The base of the sternum is connected frequently broken bones in the body. to the diaphragm, a muscle that is important to breathing. Although our upper limbs are well adapted to a wide range of movements, too much of one kind of motion can Humans have more sacral vertebrae than most mammals do, be harmful. Repetitive motions—performing the same task and these sacral bones are fused into an unusually strong struc- over and over—can lead to health problems called repetitive ture. Given what you know about the functions of the vertebral stress syndromes. Depending on the part of the body that column, why do you think that is? is overused, these injuries can take many forms. A well- known repetitive stress syndrome is carpal tunnel syndrome, The appendicular skeleton: pectoral a condition often due to repetitive typing at a computer keyboard. The carpal bones of the wrist are held together by girdle, pelvic girdle, and limbs a sheath of connective tissue. The blood vessels, nerves, and Those parts of the body that attach to the axial skeleton tendons to the hand and fingers pass through the sheath are called appendages, from the Latin word meaning “to via the “carpal tunnel.” Overuse of the fingers and hands hang upon.” The second division of the human skeleton, the appendicular skeleton, includes the arms, legs, and their attachments to the trunk, which are the pectoral and pelvic girdles. Clavicle (collarbone) The pectoral girdle lends flexibility to the upper limbs The pectoral girdle, a supportive frame for Pectoral girdle the upper limbs, consists of the right and left clavicles Scapula (collarbones) and right and left scapulas (shoulder (shoulder blade) blades) (Figure 5.11). The clavicles extend across the top of the chest and attach to the scapulas, the triangular bones in the upper back. Humerus (upper arm) The arm and hand consist of 30 different bones. The upper end of the humerus, the long bone of the upper arm, fits into a socket in the scapula. The other end of the humerus meets with the ulna and radius, the two bones of the forearm, at the elbow. If you’ve ever hit your elbow and experienced a painful tingling, you know why this area is Ulna nicknamed the “funny bone,” you’ve just struck the ulnar nerve that travels along the elbow. Forearm The lower ends of the ulna and radius meet the carpal bones, a group of eight small bones that make up the wrist. Radius The five metacarpal bones form the palm of the hand, and they join with the 14 phalanges, which form the fingers 8 Carpals (wrist) and thumb. The pectoral girdle and arms are particularly well 5 Metacarpals (hand) adapted to permit a wide range of motion. They connect to the rest of the body via muscles and tendons—a relatively 14 Phalanges (finger bones) loose method of attachment. This structure gives the upper body of humans a degree of dexterity unsurpassed among large animals. We can rotate our upper arms almost 360 Figure 5.11 Bones of the right side of the pectoral girdle degrees—a greater range of movement than with any other and the right arm and hand.  CH APTER 5 The Skeletal System 141 produces swelling and inflammation of the tendons, which attach to the sacral region of the vertebral column in back, causes them to press against the nerve supplying the hand. then curve forward to meet in front at the pubic symphysis, The result may be pain, tingling, or numbness in the wrist where they are joined by cartilage. You can feel the upper and hand. Mild episodes of carpal tunnel syndrome respond curves of the coxal bones (the iliac region) as your hip bones. to rest and pain relievers. Severe cases can be treated with Together, these structures form the pelvis. surgery to relieve the pressure. The primary function of the pelvic girdle is to support the weight of the upper body against the force of gravity. It The pelvic girdle supports the body The pelvic girdle also protects the organs inside the pelvic cavity and serves as consists of the two coxal bones and the sacrum and coccyx a site of attachment for the legs. The structure of the pelvic of the vertebral column (Figure 5.12). The coxal bones girdle reflects a trade-off between dexterity and stability. Partly because the pelvic girdle and lower limbs are larger and more firmly connected to the rest of the body than the pectoral girdle and upper limbs, the lower limbs are less dexterous than the upper limbs. The femur (thighbone) is the longest and strongest bone in the body. When you jog or jump, your femurs are exposed to forces of impact of several tons per square inch. Coxal bones and The rounded upper end of each femur fits securely into a sacrum (pelvis) socket in a coxal bone, creating a stable joint that effectively supports the body while permitting movement. The lower end of the femur intersects at the knee joint with the larger of the two bones of the lower leg, the tibia, which, in turn, makes contact with the thinner fibula. The patella, or kneecap, is a triangle-shaped bone that protects and Pubic stabilizes the knee joint. symphysis At the ankle, the tibia and fibula join with the seven tarsal bones that make up the ankle and heel. Five long bones, the metatarsals, form the foot. The 14 bones of the toes, like those of the fingers, are called phalanges. In adult women, the pelvic girdle is broader and shallower than it is in men, and the pelvic opening is Femur (upper leg) wider (Figure 5.13). The wider pelvic opening allows for safe passage of a baby’s head during labor and delivery. Differences in pelvic structures between men and women Patella (knee cap) also account for the different degrees of hip sway between men and women as they walk. These characteristic differences appear during puberty when a woman’s body begins to produce sex hormones. The sex hormones trigger a process of bone remodeling that shapes the female pelvic girdle to adapt for pregnancy and birth. Tibia Lower leg Fibula 7 Tarsals (ankle) 5 Metatarsals (foot) 14 Phalanges (toe bones) Male Female Figure 5.13 The pelvis. Note the wider pelvic opening (arrows) Figure 5.12 Bones of the pelvic girdle and the left leg and foot. in the female. 142 C HA P TE R 5 The Skeletal System Recap The skull and vertebral column protect the brain Joints vary from immovable to freely movable and spinal cord, the rib cage protects the organs of the chest Joints vary considerably from basically immovable to freely cavity, and the pelvic girdle supports the body’s weight and movable. Types of joints include fibrous, cartilaginous, and protects the pelvic organs. The upper limbs are capable of a synovial joints. wide range of motions (dexterous movement). The lower limbs Fibrous joints are immovable. At birth, the flat bones in are stronger but less dexterous than the upper limbs. a baby’s skull are separated by relatively large spaces filled with fibrous connective tissue. These “soft spots,” called fontanels, enable the baby’s head to change shape slightly 5.5 Joints form connections so that it can squeeze safely through the mother’s pelvic between bones opening during childbirth (Figure 5.14). The presence of joints also allows for brain growth and development after We now turn to the structures and tissues that hold the birth. During childhood, these fibrous joints gradually skeleton together while still permitting us to move about harden. By the time we reach adulthood, the joints have freely: joints, ligaments, and tendons. Joints, also called become thin lines, or sutures, between skull bones (review articulations, are the points of contact between bones. Figure 5.6). These immovable joints firmly connect the Ligaments and tendons are connective tissues that stabilize bones that protect and stabilize the skull and brain. many joints. Cartilaginous joints, in which the bones are connected by hyaline cartilage, are slightly movable, allowing for some degree of flexibility. Examples include the cartilaginous joints that connect the vertebrae in the backbone and those that attach the lower ribs to the sternum. The most freely movable joints are synovial joints, in which the bones are separated by a thin fluid-filled cavity. The two bones of a synovial joint are fastened together and stabilized by ligaments. The interior of the cavity is lined with a synovial membrane, which secretes synovial fluid to lubricate and cushion the joint. To reduce friction even further, the articulating surfaces of the two bones are covered with a tough but smooth layer of hyaline cartilage. Together, the synovial membrane and the surrounding hyaline cartilage constitute the joint capsule. Different types of synovial joints permit different kinds of movements. A hinge joint, such as the knee and elbow, gets its name because it allows movement in one plane Anterior Posterior like the hinges on a door. The knee joint is strong enough fontanel fontanel to withstand hundreds of pounds of force, yet it is flexible enough to swing freely in one direction (Figure 5.15). To reduce friction, there are small disks of cartilage on either side of the knee called menisci (singular: meniscus). The knee joint also includes 13 small sacs of fluid, called bursae (singular: bursa), for additional cushioning. The entire joint is wrapped in strong ligaments that attach bone to bone and tendons that attach bone to muscle. Note the two cruciate ligaments (posterior and anterior) that join the tibia to the femur bone. The anterior cruciate ligament is sometimes injured when the knee is hit with great force from the side. A second type of synovial joint, a ball-and-socket joint, permits an even wider range of movement. Examples include MJ’s BlogInFocus   Why are knee and hip replacement surgeries on the rise? Visit MJ’s blog in the Study Area in MasteringBiology and look under “Knee and Sphenoid Mastoid Hip Surgeries.” fontanel fontanel Figure 5.14 Fontanels. The four fontanels allow a baby’s head to http://goo.gl/AarPvE change shape slightly during childbirth.  CH APTER 5 The Skeletal System 143 HEALTH & WELLNESS Treating a Sprained Ankle For a severe sprain, many physicians every hour or 45 minutes every hour and advise the frequent application of cold a half. In other words, keep the sprain to the sprained area during the first 24 cold for about half the time, for as long hours, followed by a switch to heat. Why as you can stand it. The in-between the switch, and what is the logic behind periods ensure adequate blood flow for Treat sprains first with cold, then later with heat. the timing of cold versus heat? The big- tissue metabolism. It’s also a good idea gest immediate problem associated with to keep the ankle wrapped in an elastic injured generally pay the price in a longer a sprain is damage to small blood vessels bandage and elevated between cooling recovery time. and subsequent bleeding into the tissues. treatments, to prevent swelling. If you’re After 24 hours there shouldn’t be Most of the pain associated with a sprain having trouble remembering all this, any more bleeding from small vessels. is due to the bleeding and swelling, not remember the acronym “RICE”—Rest, The damage has been minimized, so now damage to ligaments themselves. The Ice, Compression, Elevation. the goal is to speed the healing process. immediate application of cold constricts The key to a quick recovery from a Heat dilates the blood vessels, improves blood vessels in the area and prevents sprain is rapid application of the RICE the supply of nutrients to the area, and most of the bleeding. The prescription is method. Athletes who try to “work through attracts blood cells that begin the process generally to cool the sprain for 30 minutes the pain” by continuing to compete while of tissue repair. Femur Hyaline Thigh cartilage muscles Posterior Tendon cruciate ligament Anterior Patella cruciate ligament Joint capsule Menisci Tibia Ligaments Fibula Ligament Patella Tendon a) A cutaway anterior view of the right b) A view of the knee with muscles, tendons, knee with muscles, tendons, and the and ligaments in their normal position joint capsule removed and the bones surrounding the intact joint capsule. The pulled slightly apart so that the two combination of ligaments, tendons, and muscles menisci are visible. holds the knee tightly together. Figure 5.15 The knee joint is a hinged synovial joint. What is the difference between a ligament and a tendon? Hint: Find all the ligaments and tendons in this figure and notice what they are attached to. 144 C HA P TE R 5 The Skeletal System Circumduction: Movement of a limb so Abduction: that it describes a cone Movement of a limb away from a body’s midline Abduction Imaginary cone of movement Adduction: Movement of a limb toward the body’s midline Adduction Rotation: Movement of a body part around its own a) Abduction and adduction. axis Extension Flexion Extension: Increases the angle of a joint Flexion: Decreases the angle of a joint c) Flexion and extension. b) Rotation and circumduction. Figure 5.16 Types of movements made possible by synovial joints. Which of these types of movement can be produced by a hinge joint? Which by a ball-and-socket joint? Which by a fibrous joint? the joint between the femur and the coxal bone (see seen, the bones of a synovial joint are held tightly together Figure 5.12) and between the humerus and the pectoral by ligaments. They are stabilized even more by tendons, girdle (see Figure 5.11). In both cases, the rounded head of another type of tough connective tissue, which join the the bone fits into a socket, allowing movement in all planes. bones to muscles. Ligaments and tendons contain collagen Figure 5.16 illustrates the different types of movements arranged in parallel fibers, making ligaments and tendons as made possible by hinge and ball-and-socket joints. Note that strong and as flexible as a twisted nylon rope. In addition, you can rotate your arm and your leg because the shoulder muscle contraction strengthens and stabilizes certain joints and hip are ball-and-socket joints, but you cannot rotate the at the very moment they need it the most. hinge joint in your knee. To appreciate the role of muscle contraction in stabilizing a joint, try this simple experiment. Sit in a low chair, stretch Ligaments, tendons, and muscles one of your legs straight out in front of you with your heel resting on the floor, and relax your muscles. Move your kneecap strengthen and stabilize joints (patella) from side to side gently with your hand. Notice how Thanks to its design, a synovial joint can withstand easily you can shift it out of position. Now, without changing tremendous pounding day after day, year after year without position, tense the muscle of your thigh and again try to move wearing out. But where does it get its strength? For that, your kneecap with your hand. See the difference? The patella we turn to ligaments, tendons, and muscles. As we have is attached to the tibia by a ligament and to the muscles of the  CH APTER 5 The Skeletal System 145 thigh by a tendon (review Figure 5.14b). Contraction of the thigh muscle (as when you take a step while walking) puts tension on the tendon and the ligament. The increased tension holds the patella and the rest of the joint firmly in place. If you move your hand to just below the kneecap, you can feel the tightening of the patellar ligament as you alternately contract and relax your thigh muscle. Recap Joints are the points of contact between bones. Fibrous joints are immovable in adults, cartilaginous joints permit some movement, and synovial joints are highly movable. Synovial joints are held together by ligaments and lubricated by synovial fluid. a) A scanning electron micrograph (SEM) of normal bone. 5.6 Diseases and disorders of the skeletal system In this chapter, we have already discussed several health conditions related to the skeletal system, including fractures and carpal tunnel syndrome. Now we look at several more. Osteoporosis is caused by excessive bone loss Osteoporosis is a condition caused by excessive bone loss over time (Figure 5.17), leading to brittle, easily broken bones. Symptoms include hunched posture (Figure 5.18), difficulty walking, and an increased likelihood of bone fractures, especially of the spine and hip. Osteoporosis is a major health problem in the United States. Over 10 million b) SEM of a bone showing osteoporosis. Americans have the condition, and it accounts for more than 1.5 million debilitating fractures every year. Figure 5.17 Bone Loss in osteoporosis. A very slow progressive bone loss occurs in both men and women after age 35 because of a slight imbalance between the rates of bone breakdown by osteoclasts and new bone formation by osteoblasts. Overall, the rate of bone loss in men (and in women before menopause) is only about 0.4% per year. That means that on average, a man will lose only about 20% of his bone mass by age 85—not enough to cause disability in most cases. For women, it’s a different story, because a decline in estrogen after menopause leads to a more rapid rate of bone loss in the decade immediately after menopause—as high as 2 to 3% per year. After that, the rate of loss begins to decline slowly toward 0.4% again. Nevertheless, women tend to lose considerably more bone mass over a lifetime than men, which is why women are more prone to osteoporosis. Other risk factors include smoking, a sedentary lifestyle, low calcium intake, and being underweight. Figure 5.18 Osteoporosis. Osteoporosis can lead to repeated compression fractures of the spine and a permanent change in spine curvature. MJ’s BlogInFocus What is the effect of smoking on bone deposition in young women? To find out, visit MJ’s The good news is that osteoporosis can be prevented. blog in the Study Area in MasteringBiology and look Two important strategies: get enough calcium and vitamin D, under “Smoking and Bone Deposition.” and maintain a consistent exercise program throughout your life. Calcium is crucial for the formation of new bone tissue. http://goo.gl/LvvPLV Current recommendations call for a daily intake of about 1,000 to 1,500 mg per day for adults, but women who have 146 C HA P TE R 5 The Skeletal System gone through menopause may benefit from even higher Causes of bursitis and tendinitis may include tearing injuries intakes. Both men and women can benefit from weight- to tendons, physical damage caused by blows to the joint, and bearing exercise (such as walking) and strength training some bacterial infections. Like ligaments, tendons and the tissues (such as lifting weights), because these activities increase lining the bursae are not well supplied with blood vessels, so they bone mass. For women especially, estrogen replacement do not heal quickly. Treatment usually involves applying cold therapy after menopause can slow the rate of bone loss. during the first 24 hours and heat after that, resting the injured Several medications are available to treat osteoporosis. area, and taking pain-relieving medications. “Tennis elbow” is a A class of drugs called biphosphonates (alendronate and painful condition caused by either bursitis or tendinitis. Other risedronate) act by inhibiting the bone-resorbing function of common locations for pain include the knee, shoulder, and the osteoclasts. The FDA recently approved a new biphosphonate Achilles tendon that pulls up the back of the heel. medication, Boniva Injection, which can be administered intravenously every three months. Teriparatide, a medication Arthritis is inflammation of joints that is a fragment of the normal parathyroid hormone By their nature, joints are exposed to high compressive forces molecule, is the first osteoporosis medication that can actually and are prone to excessive wear caused by friction. Arthritis stimulate the activity of the bone-forming osteoblasts. is a general term for joint inflammation. The most common type of arthritis is osteoarthritis, a degenerative (“wear-and- Sprains mean damage to ligaments tear”) condition that affects about 20 million Americans, A sprain is due to stretched or torn ligaments. Often, it is most over age 45. In osteoarthritis, the cartilage covering the accompanied by internal bleeding with subsequent bruising, ends of the bones wears out. With time the bone thickens swelling, and pain. The most common example is a sprained and may form bony spurs, which further restrict joint ankle. Sprains take a long time to heal because the ligaments movement. The result is increased friction between the bony have few cells and a poor blood supply. Minor sprains, in which surfaces, and the joint becomes inflamed and painful. Over- the ligaments are only stretched, usually mend themselves with the-counter medications can reduce the inflammation and time. If a large ligament is torn completely, it generally does pain, and surgical joint replacements for severe osteoarthritis not heal by itself, and surgery may be necessary to remove it. are fairly routine today. Injections of hyaluronic acid, a Sometimes the joint can be stabilized with a piece of tendon or component of hyaline cartilage, can also reduce arthritic by repositioning other ligaments. Torn ligaments in the knee knee pain. Many physicians advise people with osteoarthritis are particularly troublesome because they often leave the knee to exercise regularly, which helps preserve the joints’ healthy joint permanently unstable and prone to future injuries. range of motion. Several promising new treatments to reduce joint inflammation are still in the experimental stage. Which is likely to heal faster—a broken bone or a sprained Osteoarthritis should not be confused with rheumatoid ligament? Why? arthritis. Rheumatoid arthritis also involves joint inflammation, but it is caused by the body’s own immune system, which Bursitis and tendin