Shoulder injuries | throwing athletes

Shoulder injuries in the throwing athlete: primary instability and secondary impingement

The shoulder joint is often injured in the throwing athlete because it has a greater range of movement than any other joint in the body, and because its stability depends upon intact muscles and ligaments rather than supporting bony structures.

Phases of throwing

The five phases of throwing are wind-up, cocking, acceleration, deceleration and follow-through. The forces generated during these phases are considerable and the resulting stresses generated around the shoulder joint make it prone to acute and chronic inflammatory conditions and injuries. A poor throwing technique will exacerbate the potential for chronic inflammatory shoulder conditions.

A good throwing technique requires the athlete to use his body weight and the large muscle groups of the legs, back and trunk to generate kinetic energy across the shoulder in the direction of the thrown object. After the object is thrown, the retained energy in the throwing arm needs to be dissipated back to the large muscles which then absorb it. Poor mechanics during the wind-up and cocking phases require the shoulder muscles themselves to generate the extra required energy to propel the object being thrown. This leads to fatigue of the shoulder muscles, and will ultimately result in injuries.

Once the object is thrown, a poor follow-through will result in excess energy being retained in the soft tissues of the shoulder, rather than returning to be absorbed by the large muscles described above, causing local tissue damage. Dynamic electromyographic analysis has substantiated much of this theory (2,3,4).

Simple anatomy and biomechanics

The shoulder (glenohumeral) joint is a ball (the humeral head) and socket (the glenoid fossa of the scapula) joint which is supported by the glenohumeral ligaments and labrum. The glenohumeral ligaments (inferior, middle and superior) are discrete capsular thickenings that limit excessive rotation and translation of the humeral head. In the overhead throwing athlete, the inferior glenohumeral ligament is the primary anterior stabiliser when the arm is abducted beyond 90° and externally rotated. The labrum is a thickening surrounding the glenoid which acts to deepen the glenoid cavity (the socket).

The shoulder is stabilised by both static and dynamic restraints. Static restraints include the articular anatomy, the labrum, the glenohumeral ligaments and the negative pressure within the joint. Dynamic restraints include joint compression and the steering effect of the rotator cuff muscles (important small muscles around the shoulder).

The rotator cuff muscles consist of the supraspinatus, infraspinatus, teres minor and subscapularis. The subscapularis is an internal rotator of the glenohumeral joint, whereas the infraspinatus and teres minor muscles are external rotators. The rotator cuff as a whole functions to centre the humeral head in the glenoid for stability and to allow maximal leverage during shoulder movements.

Shoulder injuries in the throwing athlete

Any of the dynamic or static restraint mechanisms may be damaged by the throwing actions of the athlete, and there is a considerable overlap of injuries. Additionally, an untreated or unrecognised injury may progress to further injuries within the shoulder.

Common acute overuse injuries include rotator cuff tendinitis and biceps tendinitis. Common chronic injuries include impingement syndrome, rotator cuff tears, glenoid labrum tears and shoulder instability. The athlete will usually complain of anterior shoulder pain that is worst when attempting to increase the speed or power of his / her throw.

Primary instability and secondary impingement

Most athletes with anterior shoulder pain have positive impingement signs and until a few years ago it was believed that they all had primary impingement. They subsequently underwent anterior acromioplasty (removal of the anterior part of the acromion process – the acromion is a bony shelf that juts up from the shoulder blade to provide a kind of protective roof over the shoulder joint) with rotator cuff repair as needed and the results of surgery proved to be inconsistent(5). It is now known that symptomatic throwing athletes often have a primary instability of the shoulder with secondary impingement(6,7). Anterior acromioplasty with excision of the coracoacromial ligament in such individuals may actually increase shoulder instability and magnify symptoms.

Anterior instability may develop after a high-energy trauma but in the throwing athlete it starts as an overuse injury. Chronic overuse can stretch the static stabilisers of the shoulder, causing instability. The scapular and rotator cuff muscles act out of synchrony with each other putting an increased stress on the rotator cuff to keep the head of the humerus in the centre of the glenoid.

As the rotator cuff muscles weaken, the head subluxes anteriorly (moves forward) when the arm is abducted and externally rotated. This anterior subluxation causes a secondary impingement (compressing against) of the rotator cuff on the acromion and the coracoacromial ligaments, bringing on pain.

Clinical examination

Active and passive range of motion, shoulder strength and areas of tenderness should be elicited. Most athletes with shoulder pain have positive impingement signs. Pain during forward flexion while the examiner stabilises the scapula is the primary impingement sign. Pain during active abduction of the internally rotated arm is the secondary impingement sign.

Examination of shoulder stability is important and the signs may be subtle. The apprehension test can be used to detect anterior instability and involves abduction of the shoulder to about 90° followed by external rotation. As the external rotation is increased, the athlete with anterior instability will feel as if the shoulder is going to “pop out” or sublux forward. He/she will try to guard against further external rotation and become very apprehensive. The relocation test is performed in a similar way with the patient lying supine (on his/her back) and applying anterior pressure to the posterior aspect of his humeral head while abducting and externally rotating the arm. If there is anterior instability, this will be painful, but by applying a posteriorly directed force to the humeral head, the pain will ease as the humeral head is placed in the anatomic position. The presence of posterior capsular tightness may be elicited by the presence of decreased internal rotation of the shoulder.

Imaging

Recent studies indicate that MRI is superior to ultrasound and CT scanning in evaluating shoulder pain caused by rotator cuff tears, subacromial impingement and osteoarthritis of the glenohumeral and acromioclavicular joints (8,9,10). Ultrasound examination in the hands of a good musculoskeletal radiologist is considerably cheaper, however, and allows dynamic evaluation. With a good history and examination, however, such imaging may not be required in the vast majority of cases. Plain radiographs should be taken to exclude bony pathology such as fractures, calcific tendinitis, metastatic disease and osteoarthritis. Axillary views may demonstrate signs of instability, namely spurring or erosion of the anterior glenoid or a Hill-Sachs lesion (osteochondral depression on the posterior humeral head caused by impaction of the dislocated humeral head on the glenoid rim).

Other diagnostic tools

Selective local anaesthetic injections can help pinpoint the painful area in the shoulder.

Diagnostic arthroscopy allows excellent visualisation of the glenohumeral joint and the subacromial space with little soft tissue destruction and short rehabilitation period. Whilst the patient is anaesthetised, the presence, degree and direction of the shoulder instability may be evaluated (11). Of course, it is possible to proceed to repair or correct many of the pathological conditions in the shoulder arthroscopically.

Non-operative treatment

The mainstay of initial treatment for primary instability and secondary impingement is non-operative (12). A large study of non-operative management for subacromial impingement syndrome demonstrated that non steroidal anti-inflammatory drugs with specific rehabilitation programmes gave satisfactory results in 67% out of 616 patients and that only 28% needed a subacromial decompression (13). There should be a period of “relative rest” where overhead activity is avoided (14). An individualised physiotherapy programme should then be commenced. Stretching of tight muscle groups whilst avoiding stretching the anterior muscles and capsule in a patient with anterior instability, should be followed by strengthening exercises for the scapular rotators and rotator cuff muscles. This should continue for 6 to 12 months under supervision. If at this time throwing is still not possible because of pain, a surgical procedure to address the problem with the anterior capsule and labrum should be sought. Athletes with documented rotator cuff tears, labral lesions or loose bodies should have these lesions repaired or debrided.

Operative treatment

The athlete with chronic shoulder instability whose ligaments are incompetent, resulting in capsular laxity, needs to have a surgical adjustment to the ligament tension in order to restore ligament balance if non-operative measures have failed. Such procedures are termed capsulorraphies or capsular shifts (they effectively involve a tightening of the capsule to stop unwanted movement). The adjustment is made medially, inferiorly or laterally in the capsule(15,16). Other procedures have been described but are controversial as they work by limiting the range of movement so that the end-range laxity is not challenged. This is obviously not ideal for the athlete. Recent work has been published on laser-assisted capsulorrhaphy(17) and thermal-assisted capsular shrinkage(18) – the jury is still out on these techniques. Primary or secondary impingement can be surgically treated by open or arthroscopic acromioplasty. Care must be taken to avoid removal of the lateral acromion, to prevent deltoid detachment and to remove just enough bone. The aim is that by removing the source of mechanical abrasion of the supraspinatus tendon of the rotator cuff, progression of impingement to partial and full thickness tears will be stopped. However, poor vasculature, tendon nutrition, established fibrosis and composition changes in the tendon mean that the process of degenerative disease and cuff tearing continues despite relief of painful symptoms (19).

The expected outcome after acromioplasty for impingement syndrome, whether performed as an open or arthroscopic procedure, is comparable (20). Approximately 80% of patients will experience satisfactory pain relief (21,22). There are, however, a lack of any standardised assessments, so an accurate comparison between studies is not really possible.Post-operative rehabilitation initially involves the restoration of a pain-free passive range of movement and then the development of active strength. The results of surgery often appear poor for the first three months but tend to improve over the first year. The main advantages of arthroscopic surgery include the shorter hospital stay, less anaesthetic morbidity and reaching rehabilitation landmarks faster(23). Sadly, some studies suggest poorer results where patients are involved in compensation claims (24).

Referred pain from neck pathology should always be excluded. Repetitive stress may also injure the acromioclavicular and sternoclavicular joints. Finally, less common causes of shoulder pain in the throwing athlete should be borne in mind. These include quadrilateral space syndrome, suprascapular nerve entrapment, axillary artery occlusion, axillary vein thrombosis, posterior capsule laxity and glenoid spurs. These diagnoses lie in the domain of the specialist shoulder surgeon.

Alex Watson