Overview of Joints

Synovial Joint

freely moveable, separated by synovium, surrounded by joint capsule; however, high mobility causes low stability

Examples: knee, elbow, glenohumoral joint, hip, wrist, ankle, interphalangeal joint, sterno-clavicular joint (flattened ball and socket), trochoid or pivotal joint (allows axial rotation of vertebrae), connections between radius/ulna and tibia/fibula

Mobility, integrity, range of motion is determined by shape of articular surface, ligament structure, and muscle mass

Joint Capsule (Joint Cavity) – surrounds joint – stiffness correlates with mobility of joint

external surface is fibrous (continuation of fibrous periosteum)–thickening of fibrous sacs become supporting ligaments

internal surface is continuous with synovial membrane – produces synovial fluid (resembles egg white)

rich vascular, lymphatic, and nerve supply

minor capsule tear can allow blood to leak into contact with hyaline cartilage Þ mineralization, joint degeneration

bursae – lubricating bag inside or outside some sinovial capsules – bursa fluid, potential space (ziplock with a drop of oil)

Cartilage – two types in every synovial joint:

(1) hyaline cartilage – low coefficient of friction, no nerve/blood supply (receives nutrients from synovial fluid by diffusion)

(2) fibrocartilage – forms meniscus (within joint capsule; rubbery soft cushion – surrounds, restrains, and stabilizes joint)

serves to increase contact area of bones, decrease peak area loads, and enhance active diffusion in cartilage
can cover part of joint, so hyaline cartilage is still opposed (e.g., knee); can also cover entire joint to produce two separate synovial cavities (e.g., sterno-clavicular joint)

Classification of synovial joints – by shape of articular surfaces, which determines motion

(1) Planar (arthroidial) Joint – also called "gliding" or "sliding" joints – motion parallel to plane of joint

examples: articular processes between vertebrae in cervical and thoracic areas, tarso-metatarsal joints II-V

(2) Ball and Socket (enarthrosis) Joint – great mobility (flexion, extension, abduction, adduction, circumduction, axial rotation)

examples: glenohumeral joints, hip joints; also sternoclavicular joint, although its range of motion is not as great

(3) Condylar ('Knuckles') Joints – like double hinges, ovate surfaces in which motion is limited to major and minor axes

examples: metacarpophalangeal joints, knee, and radio-carpal joints

(4) Saddle-Shaped (Sellar) Joints – motion along two orthogonal axes oriented in different directions

allows flexion-extension and abduction/adduction, also a combination of the two resulting in circumduction

example: pollical carpo-metacarpal joint

(5) Hinge (Ginglymus) Joint – single axis joints (flexion-extension)

example: humero-ulnar joint at elbow

(6) Trochoid Joint – axial rotation limited by a series of ligaments

examples: atlanto-axial joint (between vertebrae C1 and C2), proximal radio-ulnar joint (allows pronation/supination)

Pathology of Synovial Joints

bursitis – inflammation of external and internal bursae – look similar, but external is much easier to treat (oral antibiotics)

plica synovialis mediopatellaris – rare thickening of synovium – can encroach into bone and impinge upon medial condyle

meniscal fraying – very common problem, especially on medial side of knee

articular chondrosis - wearing of articular surface, an early sign of osteoarthritis; associated with meniscal fraying

meniscus tears – can be repaired if in outer third (which has its own blood supply); otherwise, must be replaced

removal of meniscus: short-term improvement; greater load, compression, and shearÞ significant osteoarthritis within 10 yrs

articular carilage lesions – graded from I to IV (IV means bone is exposed)

mosaicplasty – harvest osteochondral plugs from non-weightbearing site and transfer to defect

synovitis – hypertrophy of synovial fluid – inflamed synovium (e.g., adhesive capsulitis)– can shave off to cure symptoms

Other Histological Joint Classes

Symphyseal – slightly moveable, connection is sandwich of hyaline and fibrocartilage

hyaline cartilage attached to each bone, fibrocartilage disc between – mostly provides shock absorption

also called amphiarthroidial joints

Examples: annulus fibrosus of intervertebral disk, pubic symphysis – symphyseal are located only on midline of body

Syndesmoses – slightly moveable, connection is fibrous connective tissue

Examples: interosseous membranes, distal tibio-fibular joint, connection of tooth to alveous (Gomphosis), sutures in skull

Synchondroses – immovable, connection is hyaline cartilage

any movement is deformation of cartilage itself – used for structural integrity and growth

Examples: growth plates, rib attachment to sternum, meeting of ileum, ischium, and pubis in acetabulum

Synostoses – immovable, connection is bone (i.e., bones are fused)

Examples: fused cranial sutures, fused diaphysis/epiphysis, bone fusion after joint destruction (e.g., metacarpal fusion)

Note: The intervertebral junction has all five joint types