What are joints in anatomy?

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 tissues that enable movement on components of the skeleton by muscles on opposite sides of the joint that contract or relax. Joints occur between bones but also appear between bones and cartilage, 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, bony, 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 the 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. Treatment consists of anti-inflammatory drugs; in addition, cortisone injections directly to the joint may be used. Elevated levels of uric acid that deposit as crystals in joints cause a painful inflammation known as gout. Lifestyle changes, such as reducing the intake of protein and fats, can help gout symptoms. 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. When bursitis is not infected, treatment
involves rest, elevation, ice, massage therapy, pain medication, or anti-inflammatory drugs; infected bursitis demands antibiotic therapy.

“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. OA treatment might include anti-inflammatory medications, pain medications, creams or ointments to rub on the joint, cortisone or hyaluronic acid injections, glucosamine and chondroitin, physical therapy, exercise, weight loss, and braces or orthotics to stabilize the affected joint. For those patients who do not respond well to those treatments, surgical interventions are available.

Soft tissue problems can be corrected surgically using soft tissue techniques. Replacing damaged or missing cartilage is known as cartilage restoration and is performed in younger arthritis patients. In arthroscopy, a small fiber-optic instrument is used to evaluate the inner joint surface, to clean out debris around a degenerative joint, to biopsy synovial membrane, or to reconstruct anatomic joint anomalies such as a torn meniscus, damaged cartilage, or a cruciate ligament. Arthroplasty, or joint replacement surgery, is performed when damage is too severe and realigning or reconstruction is needed in the defective joint. Sometimes a joint is damaged to the point that the bone cannot support a prosthetic device. In this case, arthrodesis, the surgical fusion of the bones, is required; although the joint will lose its flexibility, it will be stable and painless.

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, hand 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. Medications that alleviate RA include nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, immunosuppressants, and disease-modifying antirheumatic drugs
(DMARDs).

An anterior cruciate ligament (ACL) injury occurs when there is a stretch or tear of the knee. These tears might be complete or partial. Football, soccer, skiing, basketball, and martial arts are risky sports that may result in ACL tears. Surgery is required to stabilize and repair the knee. A meniscus repair is needed when this shock-absorbing material is harmed. A graft is done to replace a torn ACL. In an autograft, the surgeon takes material from the patient’s own tendon to restore the injured ACL. In an allograft, the original tendon material is derived from a donor cadaver. Proper rehabilitation follows ACL replacement.

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 from daily activities and aggravating the affected site.

While damaged cartilage is hard to repair or regain, joint movement and agility can be kept by adopting some simple steps for strong joints today and in the future. Preventing joint injuries is the best solution. Weight loss can lower stress on joints. Good stretching improves flexibility, and minor pains are relieved by muscle movement. Exercise reduces stiffness of the joints, and varying fitness routines avoids overtaxing any one area. Muscle and ligament strength training protects joints from damage. Omega-3 fatty acids not only relieve joint pain symptoms but also reduce the inflammation levels that cause more pain. Vitamin D is a protective agent for joints because of its anti-inflammatory properties. The popularity of glucosamine to alleviate joint pain and disability has grown beyond the controversy about its effectiveness. Still, future research is needed to determine the therapeutic mechanisms of glucosamine. Although established treatments or surgical procedures might help slow down or halt joint disease progression following early detection, it is the collaborative efforts of patients and health care providers that can ultimately restore joint health, achieve optimal quality of life, and reduce the financial burden on
society.

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