ANAT week 3.docx

Week 3 **Module: Bone Tissue (Part 1 of 2)** ===================================== **Learning Outcomes** --------------------- By the end of this module you should be able to: **LO1**: Name the primary germ layer that bone tissue is derived from **LO2**: Revise the funcitons of bone tissue and...

Week 3 **Module: Bone Tissue (Part 1 of 2)** ===================================== **Learning Outcomes** --------------------- By the end of this module you should be able to: **LO1**: Name the primary germ layer that bone tissue is derived from **LO2**: Revise the funcitons of bone tissue and the bones of the skeleton **LO3**: Describe the basic componenets and cell types in bone tissue **LO4**: Compare and contrast the structure of compact and spongy bone **LO5**: Describe the parts of a long bone **LO6**: Describe the external and internal tissue coverings of bone **LO7**: Copmare and contrast the processes of intramembranous and endochondrial ossification and provide examples of bones formed by each **LO8**: Describe the processes of bone growth and bone remodelling **Origin** ---------- ***LO1: Name the primary germ layer that bone tissue is derived from*** Recall from the Connective Tissue Proper module that bone tissue is classified as a specialised connective tissue. Recall from the Embryology module that all tissues in the body are derived from the three primary germ layers of the embryo. The primary germ layer that connective tissue is derived from, and therefore that bone tissue is derived from, is the mesoderm. **Functions** ------------- ***LO2: Revise the functions of bone tissue and the bones of the skeleton*** Bone tissue, also called osseous tissue, is the main component of the bones of the skeleton. Bone tissue and the bones of the skeleton have several functions, which are listed below and were covered in the Skeletal System 1 module last week. It would be a good idea to revisit this module this week to revise these functions. **Components and Cell Types** ----------------------------- ***LO3: Describe the basic components and cell types in bone tissue*** Like all connective tissues, bone tissue has the three basic components of cells, protein fibres and ground substance, with the latter two making up the extracellular matrix. In bone tissue, these components make up the organic element, which comprises around one third of bone mass. The other two thirds of bone mass is comprised of an inorganic element, which is mostly made of calcium phosphate. Within the organic element of bone tissue, the protein fibres are collagen fibres and the ground substance is solid due to the presence of calcium phosphate and other inorganic elements. There are four types of cells in bone tissue, which are osteoprogenitor cells, osteoblasts, osteocytes and osteoclasts ('osteo' = bone). Click on the hotspots on the image below to learn more about these cell types. **Bone Tissue Types** --------------------- ***LO4: Compare and contrast the structure of compact and spongy bone*** Now that we have looked at the basic components and cell types in bone tissue, we are going to look at the types of bone tissue. There are two types of bone tissue, which are compact bone and spongy bone. These two types of bone tissue are made of the same components, but the components are arranged differently in each. ### **Compact Bone** Compact bone is also referred to as dense or cortical bone and forms the external part of the bones of the skeleton. It is solid, hence its name, and has a highly organised structure, with the basic unit of mature compact bone being a cylindrical structure called an osteon or Haversian system. An osteon has several parts including the central canal, concentric lamellae, osteocytes and canaliculi. Click on the hotspots on the image below to learn more about these parts. As well as osteons, compact bone includes other structures including perforating canals, circumferential lamellae and interstitial lamellae. Click on the hotspots on the image below to learn more about these structures. ### **Spongy Bone** Spongy bone is also referred to as cancellous or trabecular bone and forms the internal part of the bones of the skeleton. Unlike compact bone, it appears porous or spongy, hence its name, and does not have the same highly organised structure. Rather than being composed of osteons, spongy bone is composed of a lattice of bony shelves or beams called trabeculae (singular = trabecula; 'trabecula' = beam). When viewed in cross-section, a trabecula looks similar to an osteon in that it is composed of plates of bone tissue called lamellae with osteocytes within lacunae in between adjacent lamellae and canaliculi connecting the osteocytes. However, a trabecula lacks a central canal. Instead, blood vessels and nerves are found in spongy bone within the spaces between trabeculae. These spaces also contain red bone marrow, which is the connective tissue containing the stem cells that give rise to all blood cells. **Long Bones** -------------- ***LO5: Describe the parts of a long bone*** Now that we have looked at the two types of bone tissue, we are going to look at where they are found in the bones of the skeleton, using long bones as an example. Recall from the Skeletal System 1 module that bones can be classified based on their shape. Long bones are the most common bone shape in the body and are classified as such because they are significantly longer than they are wide. A long bone has three main parts, which are the epiphysis, diaphysis and metaphysis. Click on the hotspots on the image below to learn more about these three parts and the type of bone found within each. **Bone Tissue Coverings** ------------------------- ***LO6: Describe the external and internal tissue coverings of bone*** Now that we have looked at where the two types of bone tissue are found in the context of long bones, we are going to look at the tissue coverings of bone, again using long bones as an example. The external and internal surfaces of bone are covered by sheets of tissue called periosteum and endosteum, respectively. Click on the hotspots on the image below to learn more about these tissue coverings. **Ossification** ---------------- ***LO7: Compare and contrast the processes of intramembranous and endochondral ossification and provide examples of bones formed by each*** Now we are going to look at how bone tissue is formed. The process of bone formation is called ossification. There are two types of ossification, which are intramembranous and endochondral ossification. Click on each of the flip cards below to learn more about these two types of ossification. Intramembranous Ossification : This is the less common type of ossification. It involves the formation of bone directly from mesenchyme, which is the embryonic connective tissue derived from the mesoderm primary germ layer of the embryo that ultimately gives rise to all types of connective tissue. Examples of bones which are formed by intramembranous ossification include the flat bones of the skull (e.g. parietal bones), some facial bones (e.g. zygomatic bones, maxillae, mandible) and the clavicles. Endochondral Ossification : This is the more common type of ossification. It involves the formation of bone from a hyaline cartilage model that originates from mesenchyme. The cartilage model grows and is slowly replaced by bone until the bone is fully formed. Most of the bones in the body are formed by endochondral ossification, including the bones of the limbs (e.g. humerus, femur), the vertebrae, the ribs and the pelvis. **Bone Growth and Remodelling** ------------------------------- ***LO8: Describe the process of bone growth and bone remodelling*** Now that we have looked at bone formation, we are going to look at how bones grow and remodel overtime, once again using long bones as an example. Bone growth occurs in both length and width. Growth in length is called interstitial growth and growth in width is called appositional growth. Click on each of the flip cards below to learn more about these processes of bone growth in a long bone. Interstitial Growth : In a long bone, interstitial growth occurs from the epiphysial plates in each metaphysis, where cartilage is organised in different zones showing the transition from cartilage to bone. Appositional Growth : In a long bone, appositional growth occurs from the periosteum. Osteoblasts in the inner cellular layer of the periosteum deposit new bone matrix to increase the width of the bone. At the same time, osteoclasts in the endosteum resorb old bone matrix to increase the size of the medullary cavity. This ensures that the medullary cavity remains in proportion to the width of the bone. Bones do not become dormant once they have reached their full size. Rather, they are continuously remodelling themselves throughout life. The balance of bone formation by osteoblasts and bone resorption by osteoclasts is referred to as bone remodelling. This process is controlled by hormones and mechanical stress placed on bone. When there is an imbalance between bone formation and bone resorption, with bone resorption occurring at a faster rate than bone formation, this can lead to the weakening of bones seen in osteoporosis. This commonly occurs in older adults, particularly older females due to hormonal changes from menopause. **Module: Cartilage (Part 2 of 2)** =================================== **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Name the primary germ layer that cartilage is derived from **LO2**: Describe the functions of cartilage **LO3**: Describe the basic components and cell types in cartilage **LO4**: Describe the structure, function and location of the three types of cartilage **LO5**: Describe the perichondrium and name the types of cartilage covered by perichondrium **Origin** ---------- ***LO1: Name the primary germ layer that cartilage is derived from*** Recall from the Connective Tissue Proper module that cartilage is classified as specialised connective tissue. Recall from the Embryology module that all tissues in the body are derived from the three primary germ layers of the embryo. The primary germ layer that connective tissue is derived from, and therefore that cartilage is derived from, is the mesoderm. **Functions** ------------- ***LO2: Describe the functions of carilage*** Cartilage is a semirigid connective tissue that is weaker but more flexible than bone. It has three main functions, which are support, providing smooth joint surfaces and providing a model for bone formation. Click on the hotspots on the image below to learn more about these three main functions. **Components and Cell Types** ----------------------------- ***LO3: Describe the basic components and cell types in cartilage*** Like all connective tissues, cartilage has the three basic components of cells, protein fibres and ground substance, with the latter two making up the extracellular matrix. The ground substance is semisolid and there is a combination of different protein fibres depending on the type of cartilage. All types of cartilage contain two main types of cells, which are chondroblasts and chondrocytes ('chondro' = cartilage). These are similar to their equivalents in bone tissue. Click on the hotspots on the image below to learn more about these cell types. **Cartilage Types** ------------------- ***LO4: Describe the structure, function and location of the three types of cartilage*** Now that we have looked at the basic components and cell types in cartilage, we are going to look at the types of cartilage. There are three types of cartilage, which are hyaline cartilage, fibrocartilage and elastic cartilage. Click on each of the flip cards below to learn more about these three types of cartilage. **Hyaline Cartilage : Hyaline cartilage is the most common type of cartilage. It has a glass-like appearance on histological slides, which is where it gets its name from. The chondrocytes within their lacunae are scattered throughout the cartilage matrix and the collagen fibres are not readily visible on histological slides. The main function of hyaline cartilage is to support other structures. For this reason, it is found in parts of the respiratory tract including the larynx and trachea, and in the costal cartilages attached to the ribs ('costal' = rib). Another function of hyaline cartilage is to provide a model for bone formation, as it forms the cartilage precursor in endochondral ossification. A specific type of hyaline cartilage called articular cartilage covers the articular surfaces of bones, where it reduces friction between the bones forming the joint.** **Fibrocartilage : Fibrocartilage contains many interwoven collagen fibres, which is where it gets its name from. Unlike in hyaline cartilage, the collagen fibres in fibrocartilage are readily visible on histological slides. The chondrocytes within their lacunae are often arranged in parallel rows between the collagen fibres and there is little ground substance. The main function of fibrocartilage is to provide strength and act as a shock absorber. For this reason, it is found in the intervertebral discs between adjacent vertebrae in the vertebral column, in the pubic symphysis, which is the pad of cartilage between the two pubic bones, and in the wedge-shaped pads of cartilage in the knee joint called menisci.** **Elastic Cartilage : Elastic cartilage contains many interwoven elastic fibres, which is where it gets its name from. The chondrocytes within their lacunae look very similar to those in hyaline cartilage on histological slides, but are often closely packed together and are surrounded by a readily visible meshwork of elastic fibres. The main function of elastic cartilage is to support other structures while allowing for flexibility. For this reason, it is found in the external ear and nose and in the epiglottis of the larynx, which prevents ingested material from entering the airways.** **Perichondrium** ----------------- ***LO5: Describe the perichondrium and name the types of cartilage covered by perichondrium*** **Now that we have looked at the types of cartilage, we are going to look at the perichondrium. This is similar to the periosteum, which is its equivalent in bone tissue. Like the periosteum in bone tissue, the perichondrium is the tough sheet covering the external surface of cartilage ('peri' = around, 'chondrium' = cartilage) and consists of an outer fibrous layer made of dense irregular connective tissue and an inner cellular layer containing cartilage stem cells and chondroblasts. The function of the perichondrium is to protect the cartilage, anchor the cartilage to other structures and provide cells for cartilage growth and repair. Cartilage is avascular, meaning that it lacks blood vessels. Therefore, cartilage must receive its blood supply via diffusion from the blood vessels outside the perichondrium.** **Not all three types of cartilage are covered by perichondrium. Only hyaline cartilage and elastic cartilage are covered by perichondrium, while fibrocartilage lacks perichondrium. The exception to this is articular cartilage, which is the specific type of hyaline cartilage covering the articular surfaces of bones. Unlike all other hyaline cartilage, articular cartilage lacks perichondrium.** **Module: Articular System 1 - Introduction** ============================================= **Learning Outcomes** --------------------- **By the end of this module you should be able to:** **LO1: Define joint or articulation** **LO2: Classify joints structurally and functionally** **LO3: Describe the general structure and provide examples of different types of joints** **LO4: Describe the specific structures associated with synovial joints** **LO5: Explain the movements allowed at different types of joints** **LO6: List the three factors influencing the stability of joints** **Joint or Articulation** ------------------------- **The scientific study of joints is called arthrology. A joint or articulation is the site where two or more bones meet.** **In addition to uniting bones with each other, joints also unite bone with cartilage, cartilage with cartilage and bones with teeth. Joints connect these structures together and provide mobility. Joint structure also resists various forces such as compression, tension and shear stress. Joints have supportive structures that align the participating bones in the joint and protect them from dislocating.** **Joint names are often descriptive of what bones articulate at them. Click on the hotspots next to the numbers on the image below to reveal the joint names.** **Structural Classification of Joints** --------------------------------------- **Joints are classified structurally based on the type of connective tissue that binds the articulating surfaces of the bones and whether a space occurs between the articulating surfaces.** **According to structural classification, there are three types of joints, which are fibrous joints, cartilaginous joints and synovial joints. The structure of a joint determines its function. Click on the cards below to find out more about these three types of joints.** **Fibrous Joint : A fibrous joint occurs where bones are united by dense regular (fibrous) connective tissue** **Cartilaginous Joint: A cartilaginous joint occurs where bones are united by cartilage** **Synovial Joint : A synovial joint has a fluid-filled joint cavity that separates the cartilage-covered articulating surfaces of the bones. The articulating surfaces are enclosed within a capsule and the bones are also united by various ligaments.** **Functional Classification of Joints** --------------------------------------- **In addition to the structural classification of joints, there is also functional classification, which is based on the extent of movement allowed at a joint. The functional classification directly relates to the structural classification of a given joint.** **According to functional classification, there are also three types of joints:** - - - **Typically, fibrous joints are immovable, cartilaginous joints are slightly movable, while synovial joints are freely movable.** **Therefore there is a strong structure-function relationship in joints. This also translates into the stability of joints: more mobility is associated with less stability of a joint.** **Let's have a closer look at each structural joint type and their examples.** **Three Types of Fibrous Joints** --------------------------------- **We will start with fibrous joints, which are the least movable joints.** **In fibrous joints, the bones are united by dense regular (fibrous) connective tissue. No joint cavity is present. Although the majority of these joints are immovable, some of them allow movement depending on the length of the fibers uniting the bones. There are three types of fibrous joints, which are gomphoses, sutures and syndesmoses. Click on the cards below to find out more about each of these types of fibrous joints.** **Gomphoses : Gomphoses ('*gomphosis*' = singular) are the fibrous joints between the roots of individual teeth and the sockets of the mandible or maxilla. The teeth are secured in place by the periodontal ligaments. These are immovable joints, or synarthroses, although they still allow microscopic movements that allow us to determinehow hard we bite down on something or if something is stuck between our teeth.** **Sutures : Sutures ('*suture*' = singular) are the fibrous joints between most bones in the adult skull. For example, the coronal suture unites the frontal and parietal bones. The irregular edges of the skull bones interlock and are united by very short dense connective tissue fibers. Typically, in older adults, the skull sutures ossify gradually.These are immovable joints, or synarthrose, and this immovable nature is a protective adaptation. However,the skull of a newborn has areas called fontanelles which are gaps between the cranial bones filled with membranous connective tissue. This allows movement of the cranial bones so that the skull can mould during passage through the birth canal.** **Syndesmoses : Syndesmoses ('*syndesmosis*' = singular) are fibrous joints where the bones are united by short dense connective tissue fibers but are not interlocked. This arrangement allows for slight movements, and therefore these joints are amphiarthroses. Good examples of this type of joint are the middle radioulnar and middle tibiofibular joints, where the bones are united by an interosseous membrane. The fontanelles in a newborn's skull can be also classified as syndesmoses because they contain membranous connective tissue betweenthe cranial bones. Fontanelles gradually close as the skull enlarges and the cranial bones ossify. While the fontanelles are open, the brain is very vulnerable.** **The Neonatal Skull: Fontanelles** ----------------------------------- The flat cranial bones must grow as the brain expands, so the sutures between them must remain open for some time. In the neonatal skull, there are areas between the growing cranial bones called fontanelles and these evenutally close at different times, e.g. the anterior fontanelle closes by 36 months of age. **Two Types of Cartilaginous Joints** ------------------------------------- Now let's look at cartilaginous joints, which are the second structural joint type. Cartilaginous joints contain a pad of cartilage that is wedged between the ends of bones and like in fibrous joints, there is no joint cavity. Depending on the type of cartilage uniting the bones, cartilaginous joints can be divided into two types, which are synchondroses and symphyses. Click on the cards below to find out more about these two types of cartilaginous joints. **Syncondroses : Synchondroses ('*synchondrosis*' = singular) are cartilaginous joints where the bones are united by hyaline cartilage. These are immovable joints, or synarthroses. A good example of this type of joint is the epiphyseal (growth) plate of long bones in children. These are temporary joints as the growth plates disappear once the ossification process is complete. In adults, good examples include the first sternocostal joint between the manubrium of the sternum (\'*sterno*\') and the costal cartilage of the first rib (\'*costal*\') and the costochondral joints between the ribs ('*costo*') and their costal cartilages ('*chondral*')** **Symphyses : Symphyses ('*symphysis*' = singular) are cartilaginous joints where the bones are lined with hyaline cartilage, which in turn are fused to an intervening pad of fibrocartilage. This arrangement allows for slight movements, and therefore these joints are amphiarthroses. Good examples of this type of joint are the intervertebral discsand the pubic symphysis.** **The Intervertebral Disc** --------------------------- **The intervertebral disc is found in the vertebral column where it units the bodies of adjacent vertebrae to form the intervertebral joint. It is also found between the fifth lumbar vertebra and the sacrum. Similar discs of fibrocartilage arelocated between the two pubic bodies (pubic symphysis) and between the parts of the sternum.** **The intervertebral disc attaches to the vertebral bodies of the adjacent vertebrae it unites via the endplate, which is a layer of hyaline cartilage that covers the superior and inferior aspects of each vertebral body.** **The intervertebral disc consists of two main parts, which are the anulus fibrosus and the nucleus pulposus. Compression of the intervertebral disc results in movements of the vertebral column. Click on the hotspots on the image below to find out more about these two parts.** **Structure of a Synovial Joint** --------------------------------- **Now let\'s look at synovial joints, which are the third structural joint type.** **All synovial joints have several common structures, which are an articular capsule, joint cavity, synovial fluid, articular cartilage and ligaments, as well as nerves and vessels. Click on the hotspots on the image below to find out more about the main structures.** **It is important to note that some synovial joints also contain intra-articular structures that help the joint to function. Good examples of this include menisci (e.g. knee joint), articular discs (e.g. temporomandibular joint), an articular labrum (e.g. shoulder joint) and intra-articular ligaments (e.g. cruciate ligaments of the knee joint).** **The three main factors responsible for the stability of of joints are:** - - - **Bursae and Tendon Sheaths** ----------------------------- **Strictly speaking, bursae and tendon sheaths are not parts of synovial joints, but they are closely associated with them.** **Bursae ('bursa' = singular) are fibrous sacs lined with synovial membrane inside that contain a small amount of synovial fluid. They are located where ligaments, muscles, skin, tendons or bones rub together.** **Tendon sheaths ('sheath' = singular) are elongated bursae that wrap around tendons. They are common where several tendons are crowded together within narrow spaces (e.g. carpal tunnel).** **Bursae : Bursae ('*bursa*' = singular) are fibrous sacs lined with synovial membrane inside that contain a small amount of synovial fluid. They are located where ligaments, muscles, skin, tendons or bones rub together.** **Tendon Sheaths : Tendon sheaths ('*sheath*' = singular) are elongated bursae that wrap around tendons. They are common where several tendons are crowded together within narrow spaces (e.g. carpal tunnel).** **Synovial Joint Types** ------------------------ **Now that we have looked at the general structure of synovial joints, let\'s look at the different types of synovial joints.** **Synovial joints allow movements in the transverse, coronal and sagittal planes. They are all freely movable joints, or diarthroses. However, the type and degree of freedom of movements allowed at a synovial joint depend on the shapes of the articular surfaces of the bones.** **Based on the degree of freedom of movements allowed, synovial joints can be divided into the three categories, which are uniaxial, biaxial and multiaxial. Each category includes different subtypes of joints based on the shapes of the articular surfaces, altogether producing the six types of synovial joints which are plane, hinge, pivot, condyloid, saddle and ball-and-socket. Click on the hotspots on the image below to find out more about the three categories and the six subtypes of synovial joints.** **Next, please look at the pictures of different movement types and read the definitions of these movements. We suggest that you try performing each movement, so that you solidify your understanding.** **Examples of Movements Allowed by Synovial Joints** ---------------------------------------------------- **Flexion - the angle between articulating bones decreases** **Extension - the angle between articulating bones increases** **Lateral Flexion - the spine bends in a lateral direction along the coronal plane** **Abduction - lateral movement of a body part away from the midline** **Adduction - medial movement of a body part toward the midline** **Circumduction - a combination of flexion, abduction, extension and adduction in succession** **Opposition/Reposition - movement of thumb across the palm and back** **Rotation - turning of a bone around its long axis** **Pronation - rotation of the forearm where the palm is turned posterior** **Supination - rotation of the forearm in which the palm is turned anteriorly** **Elevation - movement of a body part inferiorly** **Depression - movement of a body part superiorly** **Protraction - anterior movement of a body part from anatomic position** **Retraction - posterior movement of a body part from anatomic position** **Dorsiflexion - dorsum of foot is brought close to anterior surface of leg (ankle joint movement)** **Plantarflexion - sole of the foot is brought toward posterior surface of leg (ankle joint movement)** **Inversion - twisting motion of the foot that turns the sole medially or inward** **Eversion - twisting motion of the foot that turns the sole laterally or outward** **Factors Influencing the Stability of Joints** ----------------------------------------------- **As we can see, joints, especially synovial joints, allow many different movements and they must be stabilised to avoid constant dislocations or damage. There are three important factors influencing the stability of a joint, which are the articular surfaces, ligaments and muscle tone. Click on the cards below to find out more about each of these factors.** **Articular Surfaces** **The shapes of the articular surfaces determine the movements allowed at a joint. For example, plane synovial joints allow gliding while ball-and-socket joints allow a lot more types of movements. However, the articular surfaces play only a minor role in joint stability.** **Ligaments** **Joint capsules and ligaments can be called 'static' stabilizers because they unite the bones and prevent excessive or undesirable movements. However, these structures are not very effective without muscular support.** **Muscle Tone** **The muscle tendons that cross a joint can be called 'dynamic' stabilizers and they are the most important factor in joint stability. These tendons are kept taut at all times by the tone of their muscles. Thus, muscles stabilize joints during all phases of movement and also when joints are not moving.** **Relationshpi Between Mobility and Stabiliy in Joints** -------------------------------------------------------- **In summary, in this module we have reviewed the three structural joint types in the human body, which are fibrous, cartilaginous and synovial joints, as well as how these relate to the three functional joint types, with fibrous joints typically being immovable, cartilaginous joints typically being slightly movable and synovial joints being freely movable. When it comes to joints, the higher the mobility, the lower the stability, and vice-versa.** **Module: Articular System 2 - Shoulder Joint** =============================================== **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Identify the major joints of the upper limb and define their types according to the structural and functional classification **LO2**: Describe the structure and function of the shoulder joint **Upper Limb Joints** --------------------- **In this module, we will apply what we have learnt in the Articular System 1 module, where we described different types of joints based on their structure and function. We will apply this knowledge in the context of the upper limb joints, focussing particularly on the shoulder joint.** **You may have noticed that most body joints have self-explanatory names that refer to their articulating surfaces and/or bones.** **Now, let's have a closer look at the upper limb joints in more detail.** **Before we proceed, please recall that the upper limb skeleton consists of two groups of bones: bones of the shoulder (pectoral) girdle and bones of the free upper limb. The joints of the shoulder girdle include the sternoclavicular and acromioclavicular joints. The shoulder girdle articulates with the free upper limb at the shoulder joint. The free upper limb includes three main segments - the arm, forearm and hand -- which are interconnected by joints.** **The upper limb joints are designed for high mobility and fine movements required for the positioning and manipulation of the hand. Therefore, most upper limb joints are synovial. The degree and types of movements of these joints depend on their structural subtypes.** **Click on the hotspots next to each number on the image below to learn more about each upper limb joint.** **Shoulder (Glenohumeral) Joint** --------------------------------- **Now let's look at the structure of the shoulder joint, which is a great example of the synovial ball-and-socket joint.** **The anatomical name of the shoulder joint is the glenohumeral joint, which refers to its articulating surfaces: the glenoid fossa (or cavity) of the scapula serves as the 'socket', while the head of the humerus is the 'ball'. The term 'glenoid' comes from Greek word referring to the shallow socket of this joint.** **Note the difference in the size and shape of these articulating surfaces: the humeral head is round and significantly larger than the shallow glenoid fossa. Therefore, the glenoid fossa does not accommodate the entire humeral head at any one time, and the shoulder joint is not a very congruent joint. This feature allows high mobility at the shoulder joint, which is considered the most flexible/movable joint of the body. However, the shoulder joint's stability is sacrificed to allow for its extraordinary mobility.** **The shoulder is a synovial ball-and-socket joint that permits a vast range of movements:** - - - - **Test these movements on yourself. Note that to enable the full range of movements at your shoulder joint, you are required to engage your shoulder girdle joints: the acromioclavicular and sternoclavicular joints.** **Glenoid Labrum** ------------------ **Note that the glenoid fossa of the scapula is shallow and does not accommodate the entire humeral head, making the shoulder joint unstable and not very congruent. The glenoid labrum is a fibrocartilaginous ring that is attached to the circumference of the glenoid fossa. This slightly increases the articular surface area for the head of the humerus and thus, increases joint stability. It also acts as a shock absorber. The labrum ('labrum' means 'lip') is triangular in cross-section: its outer boundary is tall, while the inner free edge is thin and slopes down into the bony glenoid fossa. The superior aspect of the glenoid labrum provides attachment for the tendon of the long head of the biceps brachii muscle.** **Static Stabilisers: Ligaments** --------------------------------- **The shoulder joint has the typical features of a synovial joint: it has a fibrous capsule outside, a synovial cavity inside, and is strengthened by ligaments (i.e. static stabilisers).** **Click on the hotspots next to the highlighted names of the static stabilisers on the image below to learn more about them.** **Dynamic Stabilisers: Rotator Cuff** ------------------------------------- **Shoulder joint ligaments can be called the static stabilisers. However, the most important support for the shoulder joint is provided by the dynamic stabilisers, i.e. muscles surrounding and acting at the joint. Most of the shoulder joint stability is due to the muscle tendons that cross it and reinforce its joint capsule. The shoulder joint is reinforced on three sides by the four muscles that are known collectively as the rotator cuff: supraspinatus, infraspinatus, teres minor and subscapularis.** **These muscles originate at different aspects of the scapula:** - - **All four muscles insert at the proximal humerus, surrounding its head as a cuff. They keep the humeral head in the joint capsule and rotate it at the shoulder joint. Moving the arm vigorously can severely stretch or tear the rotator cuff. More details of the rotator cuff muscles will be studied later.** **Module: Articular System 3 - Knee Joint** =========================================== **Learning Outcomes** --------------------- **By the end of this module, you should be able to:** **LO1: Identify the major joints of the lower limb and define their types according to the structural and functional classification** **LO2: Describe the structure and function of the knee joint** **Lower Limb Joints** --------------------- **In this module, we will apply our understanding of joint types in the context of the lower limb joints. Let's start with identifying the main joints of the lower limb and characterising them based on their structure and function.** **The lower limb joints are designed for high mobility and locomotion. In addition, many of these joints are required to withstand significant weightbearing, much more than the upper limb. Most lower limb joints are synovial. The degrees and types of movements of these joints depend on their structural subtypes.** **Please recall that the lower limb skeleton consists of the pelvic girdle and the free lower limb. The pelvic girdle consists of the two hip bones. Unlike in the upper limb, where there is NO direct articulation between the right and left shoulder girdles, in lower limb, the right and left hip bones articulate directly with one another via the pubic symphysis. This joint is located anteriorly. Posteriorly, each hip bone makes a complex articulation with either side of the sacrum at the sacroiliac joints. The pelvic girdle articulates with the free lower limb at the hip joint. Please recall that the free lower limb includes three main segments (thigh, leg and foot) that are interconnected by synovial joints.** **Click on the hotspots on the image below to learn more about each lower limb joint.** **Knee Joint** -------------- **Now, let's have a closer look at the knee joint. The knee is the largest joint of the human body bringing together three bones: the femur, tibia and patella.** **It is one of the most complicated joints because it consists of three articulations in one:** - - **Note that the fibula is not involved in the knee joint.** **Unless specified otherwise, the term 'knee joint' usually refers to the femorotibial part of the joint. As a typical hinge, it permits flexion and extension. Please note that in flexion, the posterior surfaces of the thigh and leg are brought close together and the angle of the joint decreases. Extension of the knee results in straightening of the leg, i.e., bringing the leg back into the anatomical position. However, the knee joint is a special hinge joint that is referred to as 'modified'. When the knee if flexed, it also permits some medial and lateral rotation of the leg. Rotation of the leg refers to the movement along the longitudinal axis of the leg/tibia, i.e., turning the leg medially or laterally on the flexed knee. These rotations are not possible once the knee is extended. Test these movements on yourself.** **Now, click on the hotspots next to the numbers on the image below to identify bony features associated with the knee joint.** **Static Stabilisers: Ligaments** --------------------------------- **As we have already mentioned, the articulating surfaces (the tibial and femoral condyles) of the knee joint are not congruent. Therefore, this joint's stability depends on the supporting ligaments (static stabilisers) and muscles (dynamic stabilisers). Like other synovial joints, the knee joint is enclosed by a fibrous capsule, which is replaced anteriorly by the patellar bone and the patellar ligament. Internally, the articular surfaces of the tibial and femoral condyles and the patella are lined by hyaline cartilage and the non-articular surfaces are lined by the synovial membrane.** **The ligaments supporting the joint can be divided into two groups: intrinsic (found inside the joint) and extrinsic (located outside the fibrous capsule).** **1. The intrinsic ligaments include the two menisci and the two cruciate ligaments.** **The two C-shaped fibrocartilaginous menisci (lateral and medial) are found inside the joint cavity, bound to the periphery of the tibial condyles. They do not physically attach to the femur. The menisci not only deepen the articular surface on the upper tibia to accommodate for the femoral condyles, but they also provide shock absorption, distribute synovial fluid inside the joint capsule and stabilise the joint. The menisci guide the femoral condyles during knee movements, preventing side-to-side rocking of the femur on the tibia.** **The cruciate ligaments are located at the centre of the joint, crossing each other obliquely like the letter 'X' (hence the name 'cruciate', which means 'cross'). They join and hold the tibia and the femur together. During knee movement, these ligaments wind and unwind around each other. They control the front and back motion of the knee and function to 'lock' the knee when standing. Note that inside the joint cavity, the cruciate ligaments are enclosed by a large synovial fold that excludes these ligaments from the joint's synovial cavity.** **2. The extrinsic ligaments include the two collateral ligaments.** **The articular capsule of the knee joint is reinforced by several important ligaments, including the collateral ligaments. They are located on both (lateral and medial) sides of the joint. The collateral ligaments become taught when the knee is extended, preventing hyperextension of the leg. In addition, these ligaments prevent the leg from moving laterally and medially at the knee.** **Click on the hotspots next to the highlighted labels on the image below to learn more about the static stabilisers of the knee joint.** **On the image below, you can see how the intrinsic ligaments of the knee look from a superior view of the right tibia.** **Note the difference in the shape and size of the lateral and medial menisci and their attachments at the periphery of the tibial condyles, surrounding the articular surfaces on the superior aspect of the tibial condyles that are covered by hyaline cartilage. Also note the tibial attachments of the cruciate ligaments which are located either anterior (for ACL) or posterior (for PCL) to the intercondylar eminence. Proximally, the ACL and PCL attach to the femoral condyles (now shown on this image).** **Now, let's check that you can identify the knee joint features and ligaments on different views. This will reinforce your understanding of the static stabilisers of the knee. As you identify structures, pay attention to the attachments of the knee joint ligaments and link that to the specific functions of ligaments discussed above.** **Dynamic Stabilisers: Muscles** -------------------------------- **Muscles Crossing the Knee Joint** **While the joint capsule and ligaments provide static support to the knee joint, the muscles and their tendons crossing this joint provide dynamic support. Strong muscles reinforce joint stability and help to prevent injuries.** **There are several muscles that cross the knee and reinforce its stability. We will study these muscles in more detail in future modules. For now, let's identify several muscles that cross the knee joint at its anterior and posterior aspects:** - - - **Please note that the abovementioned muscles have their attachments above and below the knee joint (this is why they cross the knee joint). Therefore, when these muscles contract, they produce movements at the knee joint: the anterior muscles produce extension at the knee joint, while the posterior muscles produce flexion at the knee joint.** **Flexion and Extension of the Knee Joint** ------------------------------------------- **Now that we have looked at the structure and function of the knee joint, please watch these three short videos demonstrating flexion and extension at the knee joint.**