Joints: Classification and Types

• Joints are articulations between two or more bones. Articulation or Arthrosis is the Latin term for "joint." Arthrology is the study of joints, while arthritis refers to their inflammation.

Joint Classification Based on Degree of Mobility

• Immovable joints (Synarthroses) exhibit no mobility, for example, cranial sutures in adults and primary cartilaginous joints in children.
• Slightly movable joints (Amphiarthroses) allow some degree of mobility, such as secondary cartilaginous joints and syndesmoses.
• Freely movable joints (Diarthroses) provide maximum degree of mobility, e.g., synovial joints.

Joint Classification Based on Cavity and Connective Tissue

• Fibrous Joints: bones are articulated by fibrous connective tissue with slight or no movement and lack a joint cavity. They include sutures, syndesmoses, and gomphoses.
• Sutures connect articular surfaces with a sutural ligament, primarily connecting skull bones without allowing movement, except for a small degree in growing children. They're further divided based on surface shape:
• Plane Surface Sutures: feature plane and smooth articular surfaces, e.g., the median palatine suture.
• Serrate Sutures: serrated articular surfaces interlock like a jigsaw, e.g., the sagittal suture.
• Denticulate Sutures: margins interlock like teeth of a saw, e.g., the lambdoid suture.
• Squamous Sutures: relatively flat articular surfaces overlap, e.g., the suture between temporal and parietal bones.
• Schindylesis Sutures: a specialized type where a ridge fits into a groove, e.g., the joint between the rostrum of the sphenoid bone and the ala of the vomer bone.
• Syndesmosis Joints: are where adjacent bones are linked by more connective tissue than in sutures, in the form of interosseous ligaments and membranes, like the inferior tibiofibular, interosseous radioulnar, interosseous tibiofibular, and tympano-stapedial joints. They allow slight movement such as pronation and supination.
• Gomphosis Joints: are specialized fibrous joints that fix teeth into the alveolar sockets of the mandible and maxillae, secured by the periodontal ligament.

Cartilaginous Joints

• These link bones via hyaline or fibrous cartilage, lacking a joint cavity. There are two types:
• Primary Cartilaginous Joints (Synchondroses): Bones connected by hyaline cartilage, which are temporary, becoming bone through ossification - they are immovable. Example: joints between the epiphysis and diaphysis of growing bones and the first costosternal joint.
• Secondary Cartilaginous Joints: Have hyaline cartilage plates on articular surfaces connected by fibrocartilage, allowing limited movement, e.g., the symphysis pubis and intervertebral discs.

Synovial Joints

• They are the most common and clinically significant, featuring a cavity filled with synovial fluid between articular surfaces, enclosed by a fibrous capsule and are also know as diarthrodial joints
• Articular surfaces are smooth, covered with hyaline cartilage that is avascular, non-nervous, and nourished by synovial fluid, providing a slippery surface for free movement with low friction.
• The joint cavity is the space between articulating surfaces, covered by an outer fibrous capsule and inner synovial membrane, and contains synovial fluid.
• The fibrous capsule consists of white fibrous connective tissue, permitting movements but resisting dislocations, with nerve endings for preventing sprains and a rich vascular plexus.
• The synovial membrane lines the inner aspect of the fibrous capsule, secreting synovial fluid containing hyaluronic acid for viscosity and covers up to the articular cartilage but not the articular cartilages.
• Synovial fluid is a slippery, egg-white-like fluid in the synovial cavity, containing hyaluronic acid, monocytes, lymphocytes, macrophages, and traces of protein, providing nutrition and lubrication.
• Ligaments are thickened bands of collagen fibers that stabilize synovial joints inside and outside the fibrous capsule, maintaining stability in one plane and preventing unwanted movements.
• Bursae are connective tissue sacs filled with synovial fluid near synovial joints, commonly between tendon and bone, muscle and bone, skin and bone, and tendon and skin, reducing friction and facilitating movement.

Types of Synovial Joints

• Plane Joints: simple joints with flat articular surfaces, permitting sliding movement, e.g., intercarpal and intertarsal joints, and joints between articular processes of adjacent vertebrae.
• Hinge Joints: pulley-shaped articular surfaces resembling door hinges, allowing only flexion and extension, e.g., the humero-ulnar, interphalangeal, knee, and ankle joints.
• Pivot Joints: have a central pivot or axis surrounding a bony or cartilaginous ring; either the pivot or ring rotates, allowing only rotation, e.g., the superior radioulnar and median atlantoaxial joints.
• Condylar Joints: oval convex surface fits into an elliptical concave depression, allowing angular movement in two directions (biaxial), e.g., up and down and side to side. The end of the bone bearing the round articular surface is called the condyle, e.g., right and left temporomandibular and knee joints.
• Ellipsoid Joints: elliptical convex surface articulates with an elliptical concave surface allowing movement in flexion and extension around the transverse axis, abduction and adduction around the anteroposterior axis, and circumduction (combination of these movements), e.g., the radiocarpal, metacarpophalangeal, and metatarsophalangeal joints.
• Saddle Joints: saddle-shaped surfaces are reciprocally concavo-convex like a modified condyloid joint, allowing wide-range movement, including flexion, extension, abduction, and rotation, e.g., the first carpometacarpal joint of the thumb and the sterno-clavicular and incudo-malleolar joints.
• Ball and Socket Joints: ball-shaped head of one bone fits into a cup-like socket allowing free movement including flexion, extension, adduction, abduction, medial rotation, lateral rotation, and circumduction, such as hip (largest) and shoulder joints and the incudo-stapedial joint (smallest).

Applied Anatomy - Arthritis

• Arthritis is inflammation of joints, with three types:
• Osteoarthritis: where damaged cartilages degenerate due to friction or reduced synovial fluid.
• Rheumatoid: Cartilages are eroded by an antibody-antigen reaction, causing chronic, progressive inflammation and distortion.
• Gout: Deposition of uric acid crystals in synovial joints causes redness and inflammation due to hyperuricemia.

Muscle Classifications

• Muscle, a contractile tissue from the mesodermal layer, makes up 40% of body weight and aids in all bodily movements
• Skeletal Muscles (Voluntary/Striated): These are striated with cross striations and supplied by somatic nerves, thus under voluntary control; they are cylindrical, unbranched cells with multinucleated nuclei present peripherally. They aid in motion and include muscles of the limbs like the biceps brachii.
• Smooth Muscles (Involuntary/Non-striated): These are non-striated and they respond slowly to stimuli without getting fatigue easily. It exists circularly inside and longitudinally outside of GIT, They aggregate to form bundles and fascicles, supplied by autonomic nerves, e.g., in the gastrointestinal, respiratory, and urinogenital tracts, blood vessels, and around the eyes.
• Multi-unit muscles contract as one unit under nervous stimuli.
• Unitary muscles contract independently under stimulation.
• Cardiac Muscles: Found in the myocardium, they're striated, branched, and cylindrical, and characterized by presence of presence of intercalated disc and supplied by autonomic nerves.

Key Differences

• Skeletal Muscles are voluntary, unbranched and multinucleated whereas Cardiac Muscles are involuntary, branched with intercalated discs and single nucleus
• Skeletal muscles are attached to the skeletal system to help movement of body parts but Cardiac muscles exist in walls of the heart to pump blood
• Skeletal has more myofibrils and sarcoplasmic reticulum without any rhythmic contraction but Cardiac has less myofibrils and a rhythmic contraction

Facial Muscles

• Facilitating expressions, these muscles regulate openings on face and each opening has dilators and a sphinctor.
• Muscles include orbicularis oculi, levator palpebrae superioris, and occipitofrontalis.
• Oral Fissure consists of the orbicularis oris , levator labii superioris alaeque nasi, Mentalis, Risorius and buccinator [Blowing muscle
• Nostrils Compressor naris and Dilator naris.

Explore joint classifications based on mobility and connective tissue. Learn about immovable (synarthroses), slightly movable (amphiarthroses), and freely movable (diarthroses) joints. Understand fibrous joints like sutures and syndesmoses.