Subscapularis Injury

Anatomy

The subscapularis muscle is the largest and most powerful of the four rotator cuff muscles. In isolation, it provides approximately 50% of the rotator cuff force. The muscle originates from the anterior surface of the scapula and is typically split up into the upper two-thirds and the lower third. The upper two-thirds has a tendinous insertion on the lesser tuberosity, with the lower third demonstrating a muscular insertion onto the humeral metaphysis as described by Hinton et al. It has also been found that the musculotendinous junction was fully formed approximately 2 cm from the lesser tuberosity. The upper tendinous portion is what can be visualized arthroscopically. Wright et al., in their cadaveric study of six shoulders, demonstrated that the superior 44% of the subscapularis could be visualized arthroscopically. This study also found the course of the axillary nerve passed immediately caudal to the lower border of the subscapularis—approximately 32.8 ± 6.0 mm caudal to its arthroscopically visualized superior tendinous border. More recent cadaveric studies have demonstrated the width of the subscapularis insertion to range from 16 to 20 mm with its length ranging from 25 to 40. The upper portion of the subscapularis tendon interdigitates with the anterior fibers of the supraspinatus tendon to contribute to the rotator interval structure, as well as the transverse humeral ligaments. This interval is important to the athletic shoulder, providing stability in the overhead athlete. Articular blending of fibers is significant in biceps pulley stability. Extra-articular structures of the interval include the coracoid and coracohumeral ligament.

The upper and lower subscapular nerves provide the innervation to the subscapularis muscle. The majority of the muscle is innervated by the upper subscapularis nerve originating from the C5-C6 nerve roots off of the posterior cord of the brachial plexus. The lower subscapularis nerve innervates the axillary portion of the subscapularis as well as the teres major. In a common variation of the nervous supply, the axillary nerve can give off one or two small branches and supply innervation to the axillary portion of the subscapularis. The majority of the blood supply to the subscapularis muscle is derived from the subscapular artery. The subscapular artery, the largest branch of the axillary artery, arises at the lower border of the subscapularis, which it follows to the inferior angle of the scapula.

Several cadaveric studies have shown that the superior portion of the footprint on the lesser tuberosity represents the widest portion of the insertion. The insertion is trapezoidal in geometry, with the widest portion being superior, tapering in width at the distal attachment. Based on these studies, it has been thought that the most superior attachment of the subscapularis is the strongest, and is the location where tears initiate and progress.

Function

The primary function of the subscapularis muscle, along with the pectoralis major, the latissimus dorsi, and teres major muscles, is to internally rotate the shoulder. Favre et al. described the cranial and middle segments of the subscapularis muscle as internal rotators with the arm in an adducted position. The muscle also acts as a force couple along with the posterior rotator cuff (infraspinatus and teres minor). In concert, these muscles provide a stable fulcrum for motion—contributing to the dynamic stability required of the glenohumeral joint. The subscapularis also opposes the action of the deltoid to allow humeral elevation and abduction by the supraspinatus. Given its anterior location with relation to the glenohumeral joint, the subscapularis contributes to humeral head stability opposing anterior subluxation/dislocation.

Mechanism of Injury

There seems to be a bimodal distribution when it comes to subscapularis injuries. In the traumatic setting, typically injuries are sustained to the subscapularis with the arm in hyperextension and external rotation. Gerber and Krushnell described 16 patients who sustained isolated subscapularis tendon tears with a forced external rotation in an adducted arm. Deutsch et al. evaluated a cohort of 14 shoulders with subscapularis injuries. All but three patients sustained a traumatic injury of hyperextension or external rotation of the abducted arm. The average age of their patient population was 39 years, with all of the patients being male.

There has also been an association with anterior glenohumeral dislocation in the older patient population and subscapularis failures. Neviaser and Neviaser described a series of 11 patients with traumatic, recurrent anterior instability of the glenohumeral joint leading to subscapularis insufficiency. The average patient age in their series was 62.7, and in all cases, stability was restored with repair of the anterior capsule and subscapularis. In skeletally immature patients, lesser tuberosity avulsion has been described. A missed diagnosis and subsequent missed repair can lead to significant morbidity (see Fig. 48.1 ).

The majority of acute subscapularis tears occur in conjunction with supraspinatus tears termed “anterosuperior lesions of the rotator cuff.” With more degenerative tears of the subscapularis, coracoid impingement has been implicated. Lo and Burkhart have described the “roller-wringer effect,” which can occur when the coracohumeral interval is less than 6 mm. In this phenomenon, the tip of the coracoid impinges upon the subscapularis tendon, creating tensile forces on the articular surface leading to tensile undersurface fiber failure (TUFF) lesions. Forward elevation, internal rotation, and cross-body adduction may cause impingement of the subscapularis tendon between the tip of the coracoid and lesser tuberosity.

A common associated pathology found concomitantly with subscapularis injuries is biceps injuries. Several studies have demonstrated medial biceps subluxation or frank tearing resulting from subscapularis insufficiency. Concomitant injuries have been studied by Burkhart et al., and described arthroscopically as the “comma sign”—signifying scarring of the disrupted medial biceps sling to the torn subscapularis tendon. Arai et al. reviewed 435 consecutive arthroscopies and found that in all cases where there was biceps instability, there was associated subscapularis tearing. In fact, biceps symptoms and pathology often lead to the diagnosis of subscapularis insufficiency. A likely phenomenon for this association is that the superior border of the subscapularis contributes to the pulley mechanism that retains the biceps tendon groove.

It is less common, but the biceps can dislocate with an intact subscapularis. Here, the lateral pulley is disrupted and the biceps dislocates superficially to the intact medial pulley and subscapularis. In these patients, lateral pulley and supraspinatus tears can lead to this form of biceps instability.

Classification

There have been several proposed classification systems for subscapularis tendon tears. In general, tears are classified as partial thickness, full thickness with no retraction, and full thickness with retraction. Pfirrman et al. classified tears according to three grades. Grade I: involvement of less than 25% superior to inferior length of the tendon. Grade II: greater than 25% of the tendon. Grade III: complete, full-thickness tears. LaFosse et al. classified the subscapularis tendon tears based on intraoperative evaluation and preoperative computed tomography (CT) scan into five types: type I, partial lesion of superior one-third; type II, complete lesion of superior one-third; type III, complete lesion of superior two-thirds; type IV, complete tendon lesion with centered head and fatty degeneration less than or equal to stage 3; and type V, complete lesion with eccentric head and coracoid impingement as well as fatty degeneration greater than stage 3.

More recently, Yoo et al. studied the three-dimensional anatomic footprint of the subscapularis tendon in order to describe a new classification system based on the four facets of the tendon's insertional anatomy. They then confirmed their cadaveric findings arthroscopically and devised a classification system based on five different categories: type I, fraying or longitudinal split of the subscapularis tendon; type IIA, less than 50% subscapularis detachment of the first facet; type IIB, greater than 50% detachment without complete disruption of the lateral hood (approximately one-third of footprint); type III, entire first facet with complete-thickness tear (lateral hood tear); type IV, first and second facets are exposed with much medial retraction of the tendon (approximately two-third of footprint); and type V, complete subscapularis tendon tear involving the muscular portion.