A bursa is a fluid-filled sac that facilitates motion of structures that move against each other. It can be found between skin and bone, muscle and bone, tendons and bone, ligaments and bone, and between muscles. The bursae function as padding between structures to reduce friction caused by moving parts [4].

The joint is the musculoskeletal structure most frequently influenced by degenerative disease. Changes are most often associated with aging, excess weight, trauma, and inflammatory conditions. In the presence of these factors, articular cartilage softens, thins, and ulcerates, and the joint surfaces become rough. There may be a narrowing of the joint space and swelling of adjacent soft tissue. The normal smooth-gliding joint action is diminished, and the periosteum becomes irritated by friction, stimulating the growth of bone spurs at the joint margins. The effects of this destruction include joint pain, stiffness, and joint deformity, which can result in slight to moderate limitation of movement. Crunching or grating sounds, called crepitus, may be heard upon movement [8,9].

Joints may be assessed for changes by observation and palpation. Heberden nodes may be noted on the distal interphalangeal joint of patients with osteoarthritis. Likewise, rheumatoid nodules may be noted near the joints of patients with rheumatoid arthritis, even in the absence of other signs. Joints may be compared bilaterally to assess symmetry, position, and changes in alignment [17,19].

Because it is readily available and avoids the use of radiation, ultrasonography is often a good option in the assessment of musculoskeletal disorders and injuries. Ultrasound allows for the visualization of joints, tendons, muscles, bursae, ligaments, cartilage, nerves, fascia, and related soft tissue and can have a role in diagnosis and/or evaluation of disease progression for a variety of conditions. The American Academy of Physical Medicine and Rehabilitation indicates that ultrasound is an essential component in the diagnosis of tendinopathies/tendon tears, nerve entrapments (e.g., carpal tunnel syndrome), and acute or chronic muscle injury [32]. It may also be involved in the evaluation of ligamentous injury and joint instability syndromes, subluxations/dislocations, and fascia injury or inflammation. When joint aspiration is necessary, it may be guided by ultrasound, as may therapeutic injections.

There is wide variability in clinical symptoms in Marfan syndrome, with the most notable occurring in eye, skeleton, connective tissue, and cardiovascular systems. The most common symptom is myopia. Ocular problems are a result of defective supporting tissue of the lens, which can cause bilateral subluxation or total dislocation of the lens. The dislocation is usually upward, but slit-lamp examination is done to detect more subtle variations. Complications such as reduced visual acuity, uveitis, glaucoma, cataracts, and retinal detachment may also occur [33].

Immune dysregulation, in the form of autoimmunity, is thought to be the prime cause of lupus. In patients with lupus, the body produces an accelerated inflammatory response, resulting in the production of autoantibodies, causing immune complexes (antigens combined with antibodies) [49,56]. These autoantibodies and complexes assault the body's own healthy cells and tissues [47,49,50,51]. Symptoms of SLE are the result of the damage to the body's tissues secondary to the immunologic response. One of the hallmark indicators of lupus is the formation of autoantibodies, and the presence of autoantibodies in the blood is a key factor to the diagnosis of lupus [47,49,51].

The most common sprains affect the ankle and occur when inversion of the foot tears a ligament, usually the anterior talofibular ligament. Knee sprains cause swelling, hemarthrosis, significant decrease in range of motion, and joint laxity. Often the person hears a "pop" when the injury occurs and later describes the knee as feeling as it is going to "give way." The medial collateral ligament is most commonly involved [70]. Following the acute injury, patients are usually able to bear weight.

Sacroiliac Joint Syndrome. SI joint syndrome typically manifests as localized pain in the lower back or upper buttock area that overlies the SI joint. Pain is intensified by attempts to walk up stairs, and while pain may be referred to the posterior thigh, extension below the knee is unusual [100]. Tenderness over the SI joint is often found in physical examination, and pain is aggravated by the Patrick test or single-leg standing [97]. The onset of SI joint pain is usually gradual (over months to years), and although etiology is often elusive, trauma, infection, and tumor represent infrequent yet known causes of SI joint pain [100].

Rheumatoid arthritis has no known prevention or cure. Lifelong treatment is usually required, including medication, physical therapy, exercise, and possibly surgery. In order to provide the best outcomes, patients should be educated regarding the most appropriate treatment regimens for their disease manifestations, as earlier rheumatoid arthritis diagnosis can assist in aggressive early treatment for rheumatoid arthritis (when indicated), thereby delaying joint destruction. The 2010 ACR/EULAR Classification Criteria for Rheumatoid Arthritis is now a well-established diagnostic and prognostic tool; as such, guidelines (e.g., the 2016 update of the EULAR Recommendations for the Management of Rheumatoid Arthritis with Synthetic and Biological Disease-Modifying Antirheumatic Drugs) recommend that patients start treatment with a disease-modifying antirheumatic drug (DMARD) immediately following a rheumatoid arthritis diagnosis [189]. Therapeutic goals include preservation of function and quality of life, minimization of pain and inflammation, joint protection, and control of systemic complications, with the ultimate aim being low disease activity or remission [23,24,27,189,190].

Hemangiomas are rare vascular tumors often associated with arteriovenous malformations of skin vascular disease. They tend to affect younger individuals, often teenage girls who have been symptomatic since childhood. The knee is the most commonly involved joint [37].