A rotator cuff tear can be caused by the weakening of the rotator cuff tendons. This weakening can be caused by age or how often the rotator cuff is used. Adults over the age of 60 are more susceptible to a rotator cuff tear. According to a study in the Journal of Orthopaedic Surgery and Traumatology the frequency of rotator cuff tears can increase with age. The study shows the participants that were the ages of 70–90 years old had a rate of rotator cuff tears that were 1 to 5. The participants who were 90+ years old the frequency of a rotator cuff tear jumped to 1 to 3. This study shows that with an increase in age there is also an increase in the probability of a rotator cuff tear.

After an anterior shoulder dislocation, the risk of a future dislocation is about 20%. This risk is greater in males than females.

According to a study in the Journal of Orthopaedics the prevalence of a rotator cuff tear was considerably greater in males than in females within the ages of 50–60 years old, within the ages of 70–80 years old there wasn’t much difference in prevalence. The data of this study showed that the prevalence of a rotator cuff tear in the general population is 22.1% Yamamoto et al. performed a medical examination on 683 people whom live in a mountain village. The purpose of this study was to determine the prevalence of a rotator cuff tear among a population. Yamamoto found that among the mountain village population, rotator cuff tears were present in 20.7% of the village population. In both of these studies we see that the percentages of the prevalence of a rotator cuff tear are very close in number so these numbers show us the prevalence of rotator cuff tears in the general population.

In an autopsy study of rotator cuff tears, the incidence of partial tears was 28%, and of complete rupture 30%. Frequently, tears occurred on both sides and the frequency increased with age. The frequency was also higher in females. Other cadaver studies have noted intratendinous tears to be more frequent (7.2%) than bursal-sided (2.4%) or articular-sided tears (3.6%). However, clinically, articular-sided tears are found to be 2 to 3 times more common than bursal-sided tears and among a population of young athletes, articular-sided tears constituted 91% of all partial-thickness tears.

In tennis players, about 39.7% have reported current or previous problems with their elbow. Less than one quarter (24%) of these athletes under the age of 50 reported that the tennis elbow symptoms were "severe" and "disabling," while 42% were over the age of 50. More women (36%) than men (24%) considered their symptoms severe and disabling. Tennis elbow is more prevalent in individuals over 40, where there is about a four-fold increase among men and two-fold increase among women. Tennis elbow equally affects both sexes and, although men have a marginally higher overall prevalence rate as compared to women, this is not consistent within each age group, nor is it a statistically significant difference.

Playing time is a significant factor in tennis elbow occurrence, with increased incidence with increased playing time being greatery for respondents under 40. Individuals over 40 who played over two hours doubled their chance of injury. Those under 40 increased it 3.5 fold compared to those who played less than two hours per day.

The incidence of adhesive capsulitis is approximately 3 percent in the general population. Occurrence is rare in children and people under 40 but peaks between 40 and 70 years of age. At least in its idiopathic form, the condition is much more common in women than in men (70% of patients are women aged 40–60). Frozen shoulder is more frequent in diabetic patients and is more severe and more protracted than in the non-diabetic population.

People with diabetes, stroke, lung disease, rheumatoid arthritis, or heart disease are at a higher risk for frozen shoulder. Injury or surgery to the shoulder or arm may cause blood flow damage or the capsule to tighten from reduced use during recovery. Adhesive capsulitis has been indicated as a possible adverse effect of some forms of highly active antiretroviral therapy (HAART). Cases have also been reported after breast and lung surgery.

Another factor of tennis elbow injury is experience and ability. The proportion of players who reported a history of tennis elbow had an increased number of playing years. As for ability, poor technique increases the chance for injury much like any sport. Therefore, an individual must learn proper technique for all aspects of their sport. The competitive level of the athlete also affects the incidence of tennis elbow. Class A and B players had a significantly higher rate of tennis elbow occurrence compared to class C and novice players. However, an opposite, but not statistically significant, trend is observed for the recurrence of previous cases, with an increasingly higher rate as ability level decreases.

Other ways to prevent tennis elbow:

- Decrease the amount of playing time if already injured or feeling pain in outside part of the elbow.

- Stay in overall good physical shape.

- Strengthen the muscles of the forearm: (pronator quadratus, pronator teres, and supinator muscle)—the upper arm: (biceps, triceps)—and the shoulder (deltoid muscle) and upper back (trapezius). Increased muscular strength increases stability of joints such as the elbow.

- Like other sports, use equipment appropriate to your ability, body size, and muscular strength.

- Avoid any repetitive lifting or pulling of heavy objects (especially over your head)

Vibration dampeners (otherwise known as "gummies") are not believed to be a reliable preventative measure. Rather, proper weight distribution in the racket is thought to be a more viable option in negating shock.

Tendon injury and resulting tendinopathy are responsible for up to 30% of consultations to sports doctors and other musculoskeletal health providers. Tendinopathy is most often seen in tendons of athletes either before or after an injury but is becoming more common in non-athletes and sedentary populations. For example, the majority of patients with Achilles tendinopathy in a general population-based study did not associate their condition with a sporting activity. In another study the population incidence of Achilles tendinopathy increased sixfold from 1979-1986 to 1987-1994. The incidence of rotator cuff tendinopathy ranges from 0.3% to 5.5% and annual prevalence from 0.5% to 7.4%.

The literature on the pathophysiology of bursitis describes inflammation as the primary cause of symptoms. Inflammatory bursitis is usually the result of repetitive injury to the bursa.

In the subacromial bursa, this generally occurs due to microtrauma to adjacent structures, particularly the supraspinatus tendon. The inflammatory process causes synovial cells to multiply, increasing collagen formation and fluid production within the bursa and reduction in the outside layer of lubrication (Ishii et al., 1997).

Less frequently observed causes of subacromial bursitis include hemorrhagic conditions, crystal deposition and infection.

Many causes have been proposed in the medical literature for subacromial impingement syndrome. The bursa facilitates the motion of the rotator cuff beneath the arch, any disturbance of the relationship of the subacromial structures can lead to impingement. These factors can be broadly classified as intrinsic such as tendon degeneration, rotator cuff muscle weakness and overuse. Extrinsic factors include bone spurs from the acromion or AC joint, shoulder instability and neurologic problems arising outside of the shoulder.

In 1997 Morrison et al.

published a study that reviewed the cases of 616 patients (636 shoulders) with impingement syndrome (painful arc of motion) to assess the outcome of non-surgical care. An attempt was made to exclude patients who were suspected of having additional shoulder conditions such as, full-thickness tears of the rotator cuff, degenerative arthritis of the acromioclavicular joint, instability of the glenohumeral joint, or adhesive capsulitis. All patients were managed with anti-inflammatory medication and a specific, supervised physical-therapy regimen. The patients were followed up from six months to over six years. They found that 67% (413 patients) of the patients improved, while 28% did not improve and went to surgical treatment. 5% did not improve and declined further treatment.

Of the 413 patients who improved, 74 had a recurrence of symptoms during the observation period and their symptoms responded to rest or after resumption of the exercise program.

The Morrison study shows that the outcome of impingement symptoms varies with patient characteristics. Younger patients ( 20 years or less) and patients between 41 and 60 years of age, fared better than those who were in the 21 to 40 years age group. This may be related to the peak incidence of work, job requirements, sports and hobby related activities, that may place greater demands on the shoulder. However, patients who were older than sixty years of age had the "poorest results". It is known that the rotator cuff and adjacent structures undergo degenerative changes with ageing.

The authors were unable to posit an explanation for the observation of the bimodal distribution of satisfactory results with regard to age. They concluded that it was "unclear why (those) who were twenty-one to forty years old had less satisfactory results". The poorer outcome for patients over 60 years old was thought to be potentially related to "undiagnosed full-thickness tears of the rotator cuff".

Inferior dislocation is the least likely, occurring in less than 1%. This condition is also called luxatio erecta because the arm appears to be permanently held upward or behind the head. It is caused by a hyper abduction of the arm that forces the humeral head against the acromion. Such injuries have a high complication rate as many vascular, neurological, tendon, and ligament injuries are likely to occur from this mechanism of injury.

Most commonly due to anterior shoulder dislocation caused by hyperabduction and external rotation of the arm. Usually in young men who play contact sports (E.g. rugby, football, volleyball, basketball, etc.). Frequent anterior (frontward) subluxation also poses a great risk factor.

Dead arm syndrome starts with repetitive motion and forces on the posterior capsule of the shoulder. The posterior capsule is a band of fibrous tissue that interconnects with tendons of the rotator cuff of the shoulder. Four muscles and their tendons make up the rotator cuff. They cover the outside of the shoulder to hold, protect and move the joint.

Overuse can lead to a buildup of tissue around the posterior capsule called hypertrophy. The next step is tightness of the posterior capsule called posterior capsular contracture. This type of problem reduces the amount the shoulder can rotate inwardly.

Over time, with enough force, a tear may develop in the labrum. The labrum is a rim of cartilage around the shoulder socket to help hold the head of the humerus (upper arm) in the joint. This condition is called a superior labrum anterior posterior (SLAP) lesion. The final outcome in all these steps is the dead arm phenomenon.

The shoulder is unstable and dislocation may come next. Dead arm syndrome will not go away on its own with rest—it must be treated. If there is a SLAP lesion, then surgery is needed to repair the problem. If the injury is caught before a SLAP tear, then physical therapy with stretching and exercise can restore it.

It is common among baseball pitchers as they age, and it can also occur with quarterbacks in football and handball players also as they age.

Shoulder impingement syndrome, also called subacromial impingement, painful arc syndrome, supraspinatus syndrome, swimmer's shoulder, and thrower's shoulder, is a clinical syndrome which occurs when the tendons of the rotator cuff muscles become irritated and inflamed as they pass through the subacromial space, the passage beneath the acromion. This can result in pain, weakness and loss of movement at the shoulder.

Enthesopathies may take the form of spondyloarthropathies (joint diseases of the spine) such as ankylosing spondylitis, plantar fasciitis, and Achilles tendinitis. Enthesopathy can occur at the elbow, wrist, carpus, hip, knee, ankle, tarsus, or heel bone, among other regions. Further examples include:

- Adhesive capsulitis of shoulder

- Rotator cuff syndrome of shoulder and allied disorders

- Periarthritis of shoulder

- Scapulohumeral fibrositis

- Synovitis of hand or wrist

- Periarthritis of wrist

- Gluteal tendinitis

- Iliac crest spur

- Psoas tendinitis

- Trochanteric tendinitis

In horses tendinitis is called a bowed tendon from the appearance of the affected tendon after it heals without treatment. Mesenchymal stem cells, derived from a horse's bone marrow or fat, are currently being used for tendon repair in horses.

The exact etiology of tendinopathy has not been fully elucidated and different stresses may induce varying responses in different tendons. There are multifactorial theories that could include: tensile overload, tenocyte related collagen synthesis disruption, load-induced ischemia, neural sprouting, thermal damage, and adaptive compressive responses. The intratendinous sliding motion of fascicles and shear force at interfaces of fascicles could be an important mechanical factor for the development of tendinopathy and predispose tendons to rupture. Obesity, or more specifically, adiposity or fatness, has also been linked to an increasing incidence of tendinopathy.

The most commonly accepted cause for this condition however is seen to be an overuse syndrome in combination with intrinsic and extrinsic factors leading to what may be seen as a progressive interference or the failing of the innate healing response. Tendinopathy involves cellular apoptosis, matrix disorganization and neovascularization.

Tendinopathy can be induced in animal models by a surgical injury to the tendon. In both sheep shoulder (infraspinatus) and horse forelimb (superficial digitor flexor) tendons, a mid-tendon transection caused pathology in the entire tendon after four and six weeks respectively.

Quinolone antibiotics are associated with increased risk of tendinitis and tendon rupture. A 2013 review found the incidence of tendon injury among those taking fluoroquinolones to be between 0.08 and 0.2%. Fluoroquinolones most frequently affect large load-bearing tendons in the lower limb, especially the Achilles tendon which ruptures in approximately 30 to 40% of cases.

To prevent the problem, a common recommendation is to keep the shoulder joint fully moving to prevent a frozen shoulder. Often a shoulder will hurt when it begins to freeze. Because pain discourages movement, further development of adhesions that restrict movement will occur unless the joint continues to move full range in all directions (adduction, abduction, flexion, rotation, and extension). Physical therapy and occupational therapy can help with continued movement.

Tendinosis of the common extensor tendon of the elbow (“tennis elbow”), as of the Rotator Cuff, is a common cause of pain in the elbow or shoulder.

The general opinion is that tendinosis is due to tendon overuse, and failed healing of the tendon. In addition, the extensor carpi radialis brevis muscle plays a key role.

The most common symptoms in impingement syndrome are pain, weakness and a loss of movement at the affected shoulder. The pain is often worsened by shoulder overhead movement and may occur at night, especially if the patient is lying on the affected shoulder. The onset of the pain may be acute if it is due to an injury or may be insidious if it is due to a gradual process such as an osteoarthritic spur. The pain has been described as dull rather than sharp, and lingers for long periods of a time, making it hard to fall asleep at night. Other symptoms can include a grinding or popping sensation during movement of the shoulder.

The range of motion at the shoulder may be limited by pain. A painful arc of movement may be present during forward elevation of the arm from 60° to 120°. Passive movement at the shoulder will appear painful when a downwards force is applied at the acromion but the pain will ease once the downwards force is removed.

Humeral avulsion of the glenohumeral ligament (HAGL) is defined as an avulsion (tearing away) of the inferior glenohumeral ligament from the anatomic neck of the humerus. In other words, it occurs when we have disruption of the ligaments that join the humerus to the glenoid.

HAGL tends to occur in 7.5-9.3% of cases of anterior shoulder instability. Making it an uncommon cause of anterior shoulder instability.

Avulsion of this ligamentous complex may occur in three sites: glenoid insertion (40%), the midsubstance (35%) and the humeral insertion (25%).

Tendinosis, sometimes called chronic tendinitis, chronic tendinopathy, or chronic tendon injury, is damage to a tendon at a cellular level (the suffix "osis" implies a pathology of chronic degeneration without inflammation). It is thought to be caused by microtears in the connective tissue in and around the tendon, leading to an increase in tendon repair cells. This may lead to reduced tensile strength, thus increasing the chance of tendon rupture. Tendinosis is often misdiagnosed as tendinitis because of the limited understanding of tendinopathies by the medical community. Classic characteristics of "tendinosis" include degenerative changes in the collagenous matrix, hypercellularity, hypervascularity, and a lack of inflammatory cells which has challenged the original misnomer "tendinitis".

As the name implies, movement of the shoulder is severely restricted in people with a "frozen shoulder". This condition, which doctors call adhesive capsulitis, is frequently caused by injury that leads to lack of use due to pain. Rheumatic disease progression and recent shoulder surgery can also cause frozen shoulder. Intermittent periods of use may cause inflammation. Adhesions (abnormal bands of tissue) grow between the joint surfaces, restricting motion. There is also a lack of synovial fluid, which normally lubricates the gap between the arm bone and socket to help the shoulder joint move. It is this restricted space between the capsule and ball of the humerus that distinguishes adhesive capsulitis from a less complicated painful, stiff shoulder. People with diabetes, stroke, lung disease, rheumatoid arthritis, and heart disease, or who have been in an accident, are at a higher risk for frozen shoulder. The condition rarely appears in people under the age of 40.

Tendinitis injuries are common in the upper shoulder and lower section of the elbow (including the rotator cuff attachments), and are less common in the hips and torso. Individual variation in frequency and severity of tendinitis will vary depending on the type, frequency and severity of exercise or use; for example, rock climbers tend to develop tendinitis in their fingers or elbows, swimmers in their shoulders. Achilles tendinitis is a common injury, particularly in sports that involve lunging and jumping, while Patellar tendinitis is a common among basketball and volleyball players owing to the amount of jumping and landing.

A veterinary equivalent to Achilles tendinitis is bowed tendon, tendinitis of the superficial digital tendon of the horse.

In medicine, an enthesopathy refers to a disorder involving the attachment of a tendon or ligament to a bone. This site of attachment is known as the entheses.

If the condition is known to be inflammatory, it can more precisely be called an enthesitis.

Pain is perceived on shoulder motion, especially on certain movements. Often a crossover arm test is utilized in diagnosis because this compresses the AC joint, exacerbating the symptoms. X-rays of the shoulder joint may show either arthritic changes of the ac joint or osteolysis.