Shoulder:Biomechanics

Bullet Points

• The stability of the glenohumeral joint depends on soft tissue stabilizers, bone morphology and dynamic stabilizers such as the rotator cuff and long head of the biceps tendon.

• Shoulder stabilization techniques include anatomic procedures such as repair of the labrum or restoration of bone loss, but also non-anatomic options such as remplissage or tendon transfers.

• Rotator cuff repair should restore the cuff anatomy, reattach the rotator cable and respect the coracoacromial arch whenever possible. Tendon transfer, superior capsular reconstruction or balloon implantation have been proposed for irreparable lesions.

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• Shoulder rehabilitation should focus on restoring balanced glenohumeral and scapular force couples in order to avoid an upward migration of the humeral head and secondary cuff impingement. The primary goal of cuff repair is to be as anatomic as possible and to create a biomechanically favourable environment for tendon healing.

Keywords

Anatomy; glenohumeral instability; humerus; ligaments; rehabilitation; rotator cuff; scapula; therapeutic implications.

Introduction

The biomechanics of the shoulder are highly complex. First, it is composed of four joints (glenohumeral, acromioclavicular, scapulothoracic, and sternoclavicular). The glenohumeral joint has six degrees of freedom and is the most mobile joint in the human body, allowing the hand to reach a wide range of positions. This mobility can be further enhanced by translation of the humeral head on the glenoid, but the consequence of this tremendous mobility is perhaps a predisposition to instability and impingements. Second, mobility is assumed by 18 muscles that act in synergy. Consequently, decoupling/isolating them is impossible, making precise kinematic analysis and clinical examination difficult. Third, the glenohumeral joint has the characteristics of an active non-weight-bearing joint, leading to major bony and muscular modifications and frequent tendon overuse.

When looking at the shoulder as a functional unit, it appears that several factors need consideration. To function normally, the shoulder needs all the anatomic structures to work in a chain. Form will allow function.1 First, the central nervous system provides a signal to the muscletendon unit. By contracting, the muscle transmits its tension to the tendon, which then acts as a lever arm on the joint. To be efficient, such a system requires a stable fulcrum. The necessary stability is provided by static and dynamic factors such as bony contours, ligaments, labrum, capsule, etc.

The specificity of biomechanically relevant parameters, such as, for example, joint reaction forces, is that they cannot be measured in vivo without invasive procedures.2 Our knowledge therefore mainly relies on experimental cadaveric studies3 or computational modelling.4 These simulations have become more sophisticated in recent years, allowing the inclusion of an increasing number of variables with the ability to adjust both pathology and patient-specific characteristics.5 This ongoing process will without doubt call into question prior assumptions and allow further insights into shoulder biomechanics. It is crucial to understand the basic principles of shoulder biomechanics and their modifications in the most common pathologies encountered in daily practice.

Instability

Static stabilizers

Static stability of the glenohumeral joint is provided by the capsulolabral structures as well as the bony anatomy of the glenoid. Historically, significant effort was placed on understanding the importance of the anterior capsulolabral structures, due to the fact that these structures are classically torn in the case of anterior shoulder instability.6 The glenohumeral ligaments are a thickening of the joint capsule and represent the primary static stabilizers. To allow a high degree of shoulder mobility they only become tight at the end-ranges of motion. The superior glenohumeral ligament is tight in adduction, the middle at 45 degrees of abduction and the inferior glenohumeral when the shoulder is brought to 90 degrees of abduction in external rotation.7 The inferior glenohumeral ligament is therefore considered the strongest and most important soft tissue stabilizer. Structurally it can be avulsed from the glenoid side resulting in an antero-inferior labral lesion, as well as from the humeral side resulting in the less-frequent humeral avulsion of the glenohumeral ligament (HAGL) lesion.8,9 The postero-inferior capsule and posterior inferior glenohumeral ligament are not as robust as their anterior counterparts,10 but it is often felt to be necessary to ‘balance’ both inferior ligaments during a soft tissue repair for instability. Laxity is a normal, physiologic and asymptomatic finding, that corresponds to translation of the humeral head in any direction to the glenoid. 11 Hyperlaxity is constitutional, multidirectional, bilateral and asymptomatic. Hyperlaxity of the shoulder is probably best defined as external rotation with the elbow at the side equal to or greater than 85 degrees.12 This nonpathological finding is a risk factor for instability but does not by itself demand treatment unless there is clear pathological laxity. Pathological laxity of the inferior glenohumeral ligament is observed when passive abduction in neutral rotation in the glenohumeral joint is above 105 degrees, there is apprehension above 90 degrees of abduction, or if a difference of more than 20 degrees between the two shoulders is noted.13,14 Pathological laxity is often multidirectional and associated with a redundant capsule leading to an increased glenohumeral volume.15 Biomechanical studies have focused on evaluating the effectiveness of soft tissue procedures to reduce capsular volume. Cadaveric models created by stretching the capsule 10–30% beyond the maximal range of motion, revealed that 1 cm capsular shifts were effective to reduce capsular volume by an average 33.7% (range, 25.3% to