Joints (anatomy)
Joints are crucial structures in the human body that connect individual bones, allowing for movement and providing mechanical support within the musculoskeletal system. They are composed of strong and flexible soft tissues, enabling various types of motion facilitated by muscles. Joints can be categorized based on their structure, function, and biomechanics, with classifications including cartilaginous, fibrous, bony, and synovial joints. Depending on their mobility, they are further divided into immovable (synarthrosis), slightly movable (amphiarthrosis), and freely movable (diarthrosis) joints, with examples such as the skull, vertebrae, and major limb joints respectively.
Functionally, joints permit movements like gliding, angular, circumduction, and rotation, determined by the shapes of the bones and surrounding ligaments and muscles. However, they are susceptible to a range of disorders, from minor sprains to chronic conditions like osteoarthritis and rheumatoid arthritis, which can lead to pain, inflammation, and loss of mobility. Treatment options vary from lifestyle changes and physical therapy to medications and surgical interventions, emphasizing the importance of early diagnosis and effective management to maintain joint health and quality of life. Understanding joint anatomy and function is essential for appreciating their role in overall physical well-being and addressing the challenges posed by joint diseases.
Joints (anatomy)
Anatomy or system affected: Bones, knees, ligaments, musculoskeletal system, tendons
Definition: The connection of two or more bones which may or not allow a range of movement
Structure and Functions
In the musculoskeletal system, 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, 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 and other bones, bones and cartilage, bones and teeth, and 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 include 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 the number and configuration of articulating elements regarding the movement that they allow. Therefore, joints are subdivided as simple or compound based on 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 or 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. As of the early 2020s, one of every four adults in the US had some type of joint disease. It was also the primary cause of work disability in the US. Although genetics and traumatic injuries impact 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 cartilage, which eventually causes joint pain, stiffness, or disability. Microtraumatic injuries lead to low levels of inflammation which, over time, destroy the protective cartilage of 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. Weight loss can lower stress on joints, and stretching improves flexibility, and relieves minor pains. 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 has grown beyond the controversy about its effectiveness. Still, research is needed to determine the therapeutic mechanisms of glucosamine. Although established treatments or surgical procedures may slow 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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