TYPES OF JOINTS

Skeletal joints, the sites of articulation between one bone or cartilage and another, are generally of three types: fibrous joints (skull-type sutures), cartilaginous and fibrocartilaginous joints (discovertebral joints), and synovial joints (most limb joints).

Fibrous joints, or skull-type articulations, are called synarthroses: one bone is joined to another by an unossified fibrous membrane or residual plate of cartilage. Fibrous joints, such as skull sutures, allow little or no movement. Bones united by ligamentous attachments, such as the proximal and distal tibiofibular joints and the superior sacroiliac joints, are called syndesmoses.

Cartilaginous joints are amphiarthroses and are of two types: fibrocartilaginous and cartilaginous. Bony surfaces coated with hyaline cartilage and united by fibrocartilaginous disks are symphyses (fibrocartilaginous joints). Examples include discovertebral, manubriosternal, xiphisternal, and costosternal joints and symphysis pubis. United epiphyseal hyaline cartilage is an example of a cartilaginous amphiarthroidal joint. Amphiarthroses allow a limited range of movement but provide considerable stability.

Synovial joints or diarthroses are freely movable joints. The bony surfaces are coated with hyaline cartilage and united by a fibrous articular capsule. The synovial membrane lines the inner surface of the capsule but does not cover the articular cartilage (Figure 1-1). The capsule is strengthened by collateral ligaments. Synovial joints comprise most of the joints of the extremities and are the most accessible joints to direct inspection and palpation. Synovial joints share important structural components: subchondral bone, hyaline cartilage, a joint cavity, synovial lining, articular capsule, and supporting ligaments.

FIGURE 1-1 A, Structure of the synovial joint. B, An example of a synovial joint in sagittal section.

Synovial joints serve a variety of functions and differ in configuration, permitting specific movements while restricting others. Synovial joints can be subdivided into seven major types:

STRUCTURE OF SYNOVIAL JOINTS AND SUPPORTING TISSUES

The articular cartilage is an avascular, aneural, resilient, low-friction, load-bearing tissue covering the articulating bony surfaces. It is capable of absorbing impact by virtue of its compressibility, elasticity, low hydraulic permeability, and self-lubrication (squeeze-film hydrodynamic lubrication). It derives its nourishment through diffusion from the synovial fluid that bathes its surface.

The synovial membrane provides an unobtrusive, flexible, low-friction, well-lubricated lining for diarthrodial joints, tendon sheaths, and bursae. Histologically the synovium consists of two layers: an intimal or synovial lining cell layer—made up of one to three layers of cells or synoviocytes—and a subintimal or subsynovial layer of loose, vascular, fibro-fatty connective tissue. Synovium has important phagocytic functions and produces synovial fluid. Table 1-1 shows different types of synovial effusions.

Joint capsules, ligaments, and tendons are dense fibrous tissues with a major role in musculoskeletal function. Collectively they allow and guide joint motion while resisting high tensile loads without deformation. Laxity or adaptive shortening of these structures affects joint motion and may lead to injury (see Figure 1-1).

Joint capsules attach around articular surfaces to form a continuous envelope for the joint. Capsular cells are predominantly fibroblasts with a dense fibrous matrix surrounding them. The capsule is lined with synovium that provides lubrication for the joint. Ligaments attach bone to bone and function to stabilize adjacent bones by restricting abnormal movements. Ligaments may be intracapsular, capsular, or extracapsular, and they share similar cell and matrix characteristics with the joint capsule and tendon tissues.

The anterior cruciate ligament (ACL) is an intracapsular ligament that connects anterior aspect of intercondylar eminence of tibia with medial surface of lateral femoral condyle.

The medial (tibial) collateral ligament (MCL) is a capsular ligament connecting the medial epicondyle of the femur with the medial surface of the medial tibial condyle; it is continuous with the capsule and is essentially a thickening of the capsule.

The lateral (tibial) collateral ligament (LCL) is an extracapsular ligament, not part of the fibrous capsule of the knee; it connects the lateral epicondyle of the femur with the fibular head.

Tendons vary in size and shape, but all attach muscles to bone to transmit the force of muscle contraction to bone. Maintaining tendon tension plays a role in homeostasis of the tendon. Overuse and repetitive trauma results in degenerative changes that outweigh the regenerative process and weaken the tendon.

CATEGORIES OF MUSCULOSKELETAL PROBLEMS

Musculoskeletal problems and rheumatic diseases can be practically classified into five major categories, defined by the tissues predominantly affected: periarticular, articular, bone, nerve, and extraarticular.