Introduction
Joints are regions where bones are capped and surrounded by connective tissues that firmly hold the bones together and determine the type and degree of movement between them.
Joints are classified both structurally and functionally:
Structural Classification
This classification is based on relative joint mobility.
Diarthroses
Freely movable synovial joints. Examples: elbow, carpal joints, hip, knee, tarsal joints, interphalangeal joints.
Amphiarthroses
Slightly movable joints. Examples: intervertebral discs, joints between articular processes of cervical to lumbar vertebra, costosternal joints (ribs 2-7), pubic symphysis, distal radioulnar joints, tibiofibular joints.
Synarthroses
Joints with little or no movement. Examples: skull sutures, teeth in sockets, epiphyseal plates of long bones, first costosternal joint, mental symphysis.
Functional Classification
This classification is based on the way bones are held together by the joints.
Fibrous Joint
Bones are held together by collagenous fibers extending from the matrix of one bone into the matrix of the next; no joint cavity. Sutures, Gomphoses & Syndesmoses. Examples: skull sutures, teeth in sockets, distal radioulnar joints, tibiofibular joints.
Sutures
Immovable joints that bind the bones of the skull to each other.
- Serrate sutures appear as interlocking wavy lines. Coronal, sagittal & lambdoid sutures.
- Lap or squamous sutures are two bones with overlapping beveled edges. Temporal & parietal bones.
- Plane or butt sutures have straight, non-overlapping edges. Palatine processes of the maxillae.
Gomphoses
Attachment of a tooth to its socket is a joint called a gomphoses. Tooth held in place by fibrous periodontal ligament (collagen fibers that extend from bone of jaw to tooth). Allows tooth to move a little while chewing.
Syndesmoses
Joint in which two bones are bound by a ligament only (interosseous membrane). Most movable of fibrous joints. Interosseous membranes unite radius to ulna and tibia to fibula.
Cartilaginous Joint
Bones are held together by cartilage. Synchondroses & Symphyses. Examples: epiphyseal plate of long bones, costosternal joints, pubic symphysis, intervertebral discs.
Synchondroses
Bones are joined by hyaline cartilage. Rib attachment to sternum. Epiphyseal plate in children binds epiphysis and diaphysis.
Symphyses
Two bones joined by fibrocartilage. Pubic symphysis and intervertebral discs. Only a slight amount of movement is possible.
Bony Joint
Two bones fused by osseous tissue. Synostoses (in early adulthood).
Synostoses
Two bones, once separate, fused by osseous tissue. Ossification occurs with age. Left and right mandible present at birth. Left and right frontal bones present at birth. Epiphyses and diaphysis of the long bones. Bones separated by a joint cavity, lubricated by synovial fluid, enclosed in fibrous joint.
The capsule is mostly freely movable.
General Anatomy Of Synovial Joints
- Articular capsule: fibrous capsule lined by synovial membrane; continuous with periosteum.
- Articular cartilage: hyaline cartilage covering the joint surfaces.
- Synovial (joint) cavity: space holding the synovial fluid.
- Synovial fluid: viscous slippery fluid rich in albumin and hyaluronic acid and similar to raw egg white.
- Meniscus: formed by a pad of fibrocartilage in the jaw, wrist, knee and sternoclavicular joints. Absorbs shock, guides bone movements and distributes forces.
- Tendon: attaches muscle to bone.
- Ligament: attaches bone to bone.
- Bursa: a saclike extension of the joint capsule that extends between nearby structures allowing them to slide more easily past each other.
- Tendon sheaths: elongated cylinders of connective tissue lined with synovial membrane and wraps around a tendon. It is numerous in hand and foot.
Classification Of Synovial Joints
Synovial joints are classified into 6 different types:
Ball-And-Socket Joints
Smooth hemispherical head fits within a cuplike depression. Head of humerus into glenoid cavity of scapula. Head of femur into acetabulum of the hip bone. Multiaxial joint.
Hinge Joints
One bone with a convex surface that fits into a concave depression on the other bone. Ulna and humerus at the elbow joint. Femur and tibia at the knee joint. Finger and toe joints. Monoaxial joint.
Saddle Joints
Each articular surface is shaped like a saddle, concave in one direction and convex in the other. Trapeziometacarpal joint at the base of the thumb. Biaxial joint. More movable than a Condyloid or hinge joint forming the primate opposable thumb.
Pivot Joints
One bone has a projection that fits into a ring-like ligament of another. The first bone rotates on its longitudinal axis relative to the other. Atlantoaxial joint (dens and atlas). Proximal radioulnar joint allows the radius during pronation and supination.
Gliding Joints
Flat articular surfaces in which bones slide over each other. Limited monoaxial joint. Considered amphiarthroses.
Condyloid (Ellipsoid) Joints
Oval convex surface on one bone fits into a similarly shaped depression on the next. Radiocarpal joint of the wrist. Metacarpophalangeal joints at the bases of the fingers. Biaxial joints.
Clinical Correlates
Injuries can occur like:
- Strain: tearing or stretching of a ligament.
- Sprain: tearing or stretching of muscle fibers.
- Torn cartilage
- Joint dislocation
- Bursitis: inflammation of the bursa.
- Tendinitis: inflammation of the tendon.
- Necrosis: death of most joint cells.
Arthritis and artificial joints:
Arthritis is inflammation of one or more joints with pain, swelling, and stiffness.
- Osteoarthritis: results from years of joint wear.
- Rheumatoid arthritis: autoimmune attack on joint.
- Anthroplasty: replacement of a diseased joint with an artificial device called a prosthesis.
- Coxa valga: increase in the angle of inclination between the neck and shaft of the femur.
- Coxa vara: decrease in the angle of inclination between the neck and shaft of the femur.
