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The Overhead Shoulder

Shoulder pain affects 18-26% of adults in their lifetime, making it one of the most common regional pain syndromes (Walker-Bone et al, 2004). Risk of shoulder injury in the workplace increases especially with repetitive overhead work, heavy liGing, vibration and working in awkward positions (Linaker & Bone, 2015). Overall, the majority of shoulder injuries are related to overhead activities, excessive time in overhead positions or overload in overhead positions. Symptoms from the shoulder can be persistent and disabling and may enforce significant functional & lifestyle restrictions as well as job loss or early retirement, which can create a financial burden on the family (Kuijpers et al, 2006).

Although we are focusing on injuries aggravated by overhead activities, we need to recognise the complex & multi-factorial nature of shoulder pain. It needs to be considered within a biopsychosocial framework. In each patient a different cluster of potential physical, psychosocial, lifestyle, psychological, pathoanatomical and neurophysiological factors are present and are associated with their disorder. Stress increases cortisol levels and depression and anxiety commonly occur when patients are fearful. Co-morbidities such as obesity, diabetes and underlying systemic diseases impede healing and recovery. Many of these factors are modifiable and if not managed adequately, amplify the risk of injury as well as intensity and duration of symptoms (Karels at al, 2007). Education of the patient around their modifiable risk factors is consequently a vital part of management.

As a brief review of the functional anatomy of the shoulder complex; four articulations (Glenohumeral (GH), Acromioclavicular (AC), Sternoclavicular (SC) & Scapulothoracic (ST)) provide the bony struts around which an intricate ligamentous and muscular system provide both stability and mobility. The maintenance of space between the undersurface of the acromion and the greater tuberosity is essential to minimise compressive forces onto the rotator cuff tendons (supraspinatus & infraspinatus), damaging the nociceptive structures. The superior wall of the subacromial space is formed by the acromion and coracoacromial ligament. The inferior subacromial wall is defined by the humeral head, superior glenohumeral joint and coracohumeral ligament. The acromion, coracoacromial ligament and corocoid collectively form the coracoacromial arch (Seitz et al, 2011). During shoulder elevation the anterior acromion needs to move superiorly to protect the rotator cuff tendons from compressive forces on their bursal surface (Lee et al, 2001). Swelling of bursae and tendons, thickening of ligaments (e.g. coracoacromial or coracohumeral) and other space occupying conditions (e.g. calcification) can all further reduce the volume within the acromiohumeral space.

The rotator cuff tendons, along with biceps, are important to maintain the neutral position of the head of the humerus in the glenoid during overhead activities, integral in maintaining the acromiohumeral and coracohumeral spaces. During shoulder elevation, the coronal plane force couple comes into play. The upward shear of deltoid is counter-acted by the inferior glide from infraspinatus, teres minor and subscapularis to maintain centring of the humeral in the glenoid as the arm is raised above the head. During shoulder rotation, centring of the humeral head in the glenoid is controlled by the transverse force couple of subscapularis anteriorly and infraspinatus/ teres minor posteriorly. Failure of the functioning of either or both force couples (e.g tears of one or more rotator cuff muscles or tear of long head of biceps), will result in superior and / or anterior translation of the humeral head, reducing the acromiohumeral space, particularly with overhead activities. The result is compressive and / or tensile forces on the pain generating structures within the subacromial space. Normal acromiohumeral distance (AHD) is approximately 12mm and can reduce to 6mm in the presence of a full thickness rotator cuff tear (Goutallier et al. 2011), most reliably measured with ultrasound (McCreesh et al, 2015).

Let’s discuss various other factors that can influence the size of the subacromial space (AHD) and predispose to shoulder injury, particularly during overhead activities. These factors are multifactorial, both intrinsic and extrinsic, and may co-exist to differing degrees in each individual patient experiencing pain in overhead positions.

Humeral Retroversion vs Anteversion

Children and primates have a posteriorly rotated head of humerus (Greenberg et al, 2015; Carretero et al, 2023). This retroversion creates a larger acromio-humeral space and is therefore tendon protective. As we get older, and we spend more time at our desks and in a protracted sustained position, our humeral heads become more anteverted, reducing this acromiohumeral space. The exception is young throwers (e.g. baseball pitchers or tennis players) who maintain this retroverted position by repetitively pushing into end range shoulder external rotation, allowing them to throw from a full externally rotated position, increasing throwing power. This retroversion is why children find it comfortable to hang on overhead gym bars whereas as adults we find it uncomfortable or even painful. It also helps us understand why repetitive overhead shoulder activities in adults increases shoulder injury risk.

Glenohumeral Internal Rotation Deficit (GIRD)

Restricted posterior structures (posterior shoulder capsule, tendinous insertions of infraspinatus and teres minor) create a loss of glenohumeral internal rotation (GIRD) and a shiG in GH forces during rotation and overhead activities. Consequent to the GIRD, we see antero-superior translation of the humeral head during GH elevation, and postero-superior translation during GH abduction / external rotation. Both result in mechanical impingement, pain and loss of range of motion (ROM) in overhead positions (Hackney, 1996; Tyler et al, 2000; Kibler et al, 2012).

Scapular Dyskinesis

Optimal scapular resting position with the shoulder in neutral, arm by the side, is in slight upward rotation, the inferior angle more lateral than the root of the spine of the scapula. This sesamoid bone should be flat against the chest wall and not protruding on the vertebral border (vertebral winging) or from the inferior angle (pseudo winging or anterior scapular tilt). This upward rotation of the scapula enhances the passive stability of the GH joint (Mottram 1997). Clinically, if the scapula is in a dysfunctional position at rest, it will behave dysfunctionally during motion.

If you are working overhead, the scapula needs to upwardly rotate (60°), posteriorly tilt (30°), and externally rotate (20°) to maintain the AHD and minimise risk of impingement (Ludewig, 2009).

Abnormal scapular kinematics, due to muscle imbalances, proprioceptive losses, or as a compensation for glenohumeral movement impairments, may reduce the AHD and increase impingement risk during overhead activities (Mackenzie et al, 2015). Changes in scapular kinematics

is variable and inconsistently reported in the literature. Some changes are causative of pain (e.g. downward rotation) and others are considered compensatory and protective (e.g. upward rotation in) as seen in a patients with frozen shoulder, the scapula theoretically trying to increase the size of the acromiohumeral space and minimise pain (Mohamed and Alawna, 2022).

Sternoclavicular (SC) and Acromioclavicular (AC) overhead restriction

During shoulder elevation and other overhead activities, substantial motion is required at the SC and AC joints for optimum mobility. Abnormal scapulothoracic motion is associated with dysfunction motion at the SC and AC joints, with a loss of posterior rotation of these joints during shoulder abduction and external rotation. A loss of clavicular backward rotation and first rib restriction can limit GH lateral rotation ROM with overhead activities, which negatively affects shoulder overhead function, prevents optimal centring of the head of the humerus in the glenoid and predisposes to shoulder posterior impingement (Lawrence et al, 2014).

Postural dysfunction

A thoracic kyphosis changes the rest position of the scapula, usually to a protracted and downwardly rotated position, tilting the glenoid inferiorly. It is also associated with anterior tilting of the scapula. A reduction of the AHD in patients with a slouched posture has been found on ultrasound resulting in various clinical impingement conditions (Maczenzie et al, 2015). A thoracic kyphosis is associated with a loss of GH abduction and elevation (Bullock et al, 2009). Other postural influences which have a negative effect on scapular position and therefore on how the glenoid articulates with the humeral head (maltracking) includes scoliosis, a forward head posture as well as a flattened (inverted) thoracic spine. All these postural dysfunctions result in muscles imbalances, joint incongruity and ligament laxity with a potential for altered neurodynamics. It seems logical that to improve patients pain and dysfunction during overhead activities, one must not ignore more distal factors such as postural dysfunctions, that may be causing scapular dyskinesis, and increasing compressive or tensile forces on the tendons of the cuff.

Space occupying structures reducing the available volume within the subacromial space: Subacromial Bursitis

This bursa is one of up to 12 bursae in the shoulder and is the largest in the body. Reducing friction between rotator cuff tendons during motion and contributing to proprioception at the shoulder, it is highly nociceptive, generating a large amount of pain when swollen and compressed during overhead activities. All shoulder special tests compress and stress not only the rotator cuff tendons, but also the bursae, making diagnosis difficult and differentiation non-specific (Lewis et al, 2015).

Swelling of the RC tendons

Increased thickness of supraspinatus is a feature of rotator cuff tendinopathy (McCreesh et al, 2017; Ishigaki et al, 2022). This means that the tendon occupies more of the sub-acromial space, potentially further increasing tendon compression (Michener et al, 2015). The affected tendon also swells in response to fatigue loading, further enhancing tendinopathy symptoms.

Thickening of the coracoacromial ligament (CAL)

The CAL plays an important role in shoulder biomechanics, joint stability, and proprioception. Thickening of this ligament at the acromial undersurface (coracoacromial arch) reduces the available space within the sub-acromial space. It may be age related and is linked to capsular tightness and rotator cuff tear arthropathy (Rothenberg, 2017).

Management of pain and dysfunction in the shoulder exposed to overhead use

It seems clear that managing the patient with shoulder pain and dysfunction, aggravated by repetitive overhead activities, will necessitate both finding ways to increase the size of the subacromial space as well as to modify time spent in overhead positions. This may include postural correction, stretching the posterior capsule, dynamic scapular motion retraining during overhead loading, retraining motor control of the muscles of force couples that centre the humeral head in the glenoid during shoulder elevation and rotation and reducing bursal and tendon swelling. Education of the overhead worker or athlete is paramount. Modification of job tasks may be necessary in the short to medium term. Avoiding time overhead is tendon protective and manual workers may, for example, be able to stand on a step to reduce shoulder elevation when hammering into a wall.

Loading needs to be reduced and working with shorter levers (flexed elbows) may assist.

Sleep is our most powerful anti-oxidant, so spend time educating the patient on sleep hygiene. Use ICE and not heat to reduce the volume of swollen tissue in the subacromial space (Parle et al, 2016). NSAID’s may assist. An ultrasound guided corticosteroid injection may be necessary to reduce pain in the acute phase if conservative measure are unsuccessful.

Clinically, all rehabilitation exercises should be no more than a 2/10 on the VAS pain scale and no worse up to 24 hours post exercise. Initial exercises should be performed below 90° shoulder elevation where subacromial pressures are least (Wright et al, 2018). Closed chain exercises are safer, so preferentially use these in the early phases. Open chain exercises are more functional, so add these into your program as your rehabilitation program becomes more task specific.

Conclusion

The majority of patients presenting with pain, weakness or dysfunction of the shoulder can relate their injury to spending time overloading the shoulder in an overhead position. There are multiple contributing factors that can reduce the coracoacromial space with resultant compressive or tensile forces on the nociceptive structure within that space, especially overhead. An understanding of the complex biomechanical & multi-factorial pathogenesis will help the healthcare practitioner to better educate and manage this patient group.