Structure and Functions

In the musculoskeletal system, the joints are structures that connect individual bones while allowing some type of movement and mechanical support. This skeletal articulation holds together distinct bones with strong but flexible soft tissue that enable movement on components of the skeleton by muscles on opposite sides of the joint that contract or relax. Joints occur between bones and teeth, and between cartilages. Based on their anatomical location, they are grouped between the joints of the trunk and the upper or lower extremity. They can be classified structurally, functionally, or biomechanically.

Structurally, joints may be classified as cartilaginous, fibrous, boney, or synovial, based on the composition of how these bones connect to each other. A cartilaginous joint is connected by hyaline cartilage or fibrocartilage. A fibrous joint is connected by a collagen- and elastin-rich connective tissue. In a bony joint, there is a fusion between bones. Synovial joints are not directly connected but are found within a synovial cavity full of synovial fluid that lubricates and cushions the joint.

Strength and flexibility are important functional features of joints but also contradictory concepts in which greater joint strength translates into less flexibility and otherwise. Functionally, joints are ranked based on the degree of mobility rendered: immobility (synarthrosis), slight mobility (amphiarthrosis), and free mobility (diarthrosis). Synarthroses are immovable joints such as those located between the plates of the skull. Amphiarthroses are joints that allow slight movement such as in the vertebrae. Diarthroses are joints that move freely and are also known as synovial joints. These include the joints in the shoulder, hip, knee, and elbow.

The most common mobile joints present in the body are the hinge joint, pivot joint, ball-and-socket joint, saddle joint, and ellipsoidal joint. Joints allow four types of movement: gliding, angular, circumduction, and rotation. The shape of bones and their articular surfaces, in addition to the ligaments and muscles intersecting the joint, determines the degree of movement permitted at a specific joint.

Biomechanically, joints can be characterized according to number and configuration of articulating elements with regard to the movement that they allow. Therefore, joints are subdivided as simple and compound based upon the number of partaking bones and into combinational and complex joints.

Disorders and Diseases

Problems with joints range from minor injuries (sprains) to serious or chronic joint disease. Age, use, and overuse can diminish joint function or deteriorate this structure further to become diseased. Genetics, direct trauma, misalignment, dislocation, and mechanical loads may also play a role in damaging joints.

Several inflammatory conditions can affect the joints. Synovitis is the inflammation of the lining of the synovial joint, the synovial membrane. The swelling of this membrane causes pain especially when the joint moves. Bursitis (such as tennis elbow) is the inflammation of the bursa that rests between a tendon and skin or between a tendon and bone. The symptoms include joint pain, tenderness, swelling, stiffness, or warmth around the joint.

“Arthritis” is a generic term for a group of chronic medical conditions affecting the joints. The most common one is osteoarthritis (OA), which is characterized by the progressive wearing down of the cartilage in the joints. The symptoms of this degenerative joint disease include swelling, deep aching pain that gets worse after exercise or rainy weather, limited movement, loss of flexibility, stiffness, and grinding of the joint during movement.

Another type of medical condition that afflicts the joints is rheumatoid arthritis (RA), an autoimmune disease. In RA, the body produces an immunocellular reaction that targets the joints and causes inflammation. RA is a chronic autoimmune inflammatory joint disease afflicting the lining of the joints, resulting in painful swelling that will cause bone erosion and joint deformities. Since it is a systemic disease, it may involve other internal organs (lungs, kidneys, heart, or eyes) as well. Some of the associated symptoms are low-grade fever, weight loss, fatigue, morning stiffness, muscle aches, weakness, loss of appetite, skin redness or inflammation, had and foot deformities, and numbness or tingling. Joint loss may appear within the first couple of years after diagnosis. RA frequently involves a lifetime of treatments, medications, exercise, physical therapy, education, and sometimes surgery.

Perspective and Prospects

Joint disorders and diseases were well-known ailments to physicians of ancient Greece and Rome. Evidence of joint problems has been discovered in Egyptian mummies and Roman gladiators. Today, one of every five adults in the United States has some type of joint disease. It is the primary cause of work disability in the United States. Although genetics and traumatic injuries may have a negative impact on joint health, this is a multifactorial disease and, as such, early diagnosis and proper management is essential to improve quality of life. The prevalence of joint disease, however, does not translate into total inevitability. It is possible to conserve joint function and mobility when knowledge, preventive measures, and correct treatments are implemented.

Aging thins the cartilage, which will eventually cause joint pain, stiffness, or disability. Microtraumatic injuries lead to low levels of inflammation, which over time will destroy the protective cartilage at the joints. Excess weight damages the joints in the long run as well, especially the knees, which support body weight. The loss of muscle mass during aging causes the joints to overcompensate by absorbing more of the beating form daily activities and aggravating the affected site.

- Ana Maria Rodriguez-Rojas, M.S.