Subscapularis Tendon

Bullet Points

Pseudoparalysis is defined as an inability to actively elevate the arm beyond 90 degrees with full passive forward flexion. Prevalence of subscapularis lesion is high (around 50% of arthroscopic rotator cuff procedures, 10% are isolated). The superior two-thirds of the subscapularis ends as a tendon that inserts on the lesser tuberosity of the humerus. The inferior part has no tendon; the muscle attaches directly to the bone. The subscapularis should be considered as two different muscles. It is the largest and most powerful of the rotator cuff muscles. The subscapularis is an internal rotator and is crucial for shoulder elevation. Several special tests have been described to aid in diagnosis, including the lift-off, belly-press, and bear-hug tests. Mulitple magnetic resonance imaging (MRI) views should be analyzed to increas the chance of subscapularis lesion detection. FUSSI and SAM are similar conditions causing abrasion of the upper and medial part of the subscapularis tendon. 90% of patients with a subscapularis tear have associated long head of the biceps lesions or instability. Preservation of the intra-articular biceps will frequently jeopardize the repair. Results are good or excellent in around 80% of cases. Around 90% of patients returned to normal activities, and 90% are satisfied.

Key words

Subscapularis tendon; Rotator cuff lesion; Biomechanics; Classification; FUSSI; SAM; Lafosse; Toussain; Results; Complications; Irreparable; Anatomy; Pectoralis major transfer; Imaging; Pseudoparalysis.

Definition of Pseudoparalytic Shoulder

Pseudoparalysis is defined as an inability to actively elevate the arm beyond 90 degrees with full passive forward flexion. It is also important to note that this is functional limitation and not pain inhibition; this can be distinguished by the inability to hold the arm at 90 degrees and/or an evaluation of motion after a subacromial injection. Anatomically, pseudoparalysis requires the disruption of at least one rotator cable attachment. Recently it has been demonstrated that dysfunction of the entire subscapularis and supraspinatus or three rotator cuff muscles are risk factors for pseudoparalysis.

1. Collin P, Matsumura N, Lädermann A, Denard PJ, Walch G. Relationship between massive chronic rotator cuff tear pattern and loss of active shoulder range of motion. J Shoulder Elbow Surg 2014;23:1195-202. Primary arthroscopic repair can lead to reversal of pre-operative pseudoparalysis in 90% of patients, but in only 43% of revision surgeries.

2. Denard PJ, Lädermann A, Jiwani AZ, Burkhart SS. Functional outcome after arthroscopic repair of massive rotator cuff tears in individuals with pseudoparalysis. Arthroscopy 2012;28:1214-9.

Anecdote

Like teres minor, the subscapularis has been a “forgotten tendon” of the rotator cuff.

3. Walch G, Nové-Josserand L, Liotard JP, Noel E, Tavernier TH, Barthélémy R. Le petit rond (teres minor): l'oublié de la coiffe. In: Blum A TT, Brasseur J-L, Noël E, Walch G, Cotten A, Bard HG,, ed. L'Epaule: une approche pluridisciplinaire. Montpellier:: Sauramps Medical; 2005:237–44. In the 1980s, at a meeting of the American Academy of Orthopaedic Surgeons in Orlando, Christian Gerber presented a new, unknown clinical entity, the “Isolated tear of the subscapularis tendon“. The presentation was based on 16 cases with computed tomography (CT)-arthrography or magnetic resonance imaging (IRM), intraoperative photographs of all cases and included a detailed description of the clinical features including lift- off test, belly-press test and increased passive external rotation. The discusser of the paper felt that this was a nice presentation which unfortunately lacked a bit of relevance as this entity was not observed in the US and was probably a "Swiss disease“. Certainly, he and his experienced expert colleagues had never seen it. Dr Richard Hawkins, who had visited Christian Gerber a few weeks earlier and participated in two isolated subscapularis tendon repairs got up in the audience and told the discusser. "I don’t doubt that you, as I, have never seen an isolated subscapularis tear, but after my visit I can tell you that many have seen us".

Christian Gerber submitted the paper to the American Journal of Bone and Joint Surgery which rejected it, because the reviewers could not see the relevance for the English speaking readership. It was published in the British Journal of Bone and Joint Surgery in 1991.

Isolated subscapularis tears are currently known to represent about 8-10% of all the rotator cuff tendon tears as well in Europe as in America.

Prevalence

Because articular sided tears are not commonly visualized during open approaches, subscapularis tears were previously believed to be rare. They are now recognized to have a prevalence of nearly 30% in all arthroscopic shoulder surgery, and up to 59% in arthroscopic rotator cuff procedures. Isolated subscapularis tears are currently known to represent about 8-10% of all the rotator cuff tendon tears.

4. Arai R, Sugaya H, Mochizuki T, Nimura A, Moriishi J, Akita K. Subscapularis tendon tear: an anatomic and clinical investigation. Arthroscopy 2008;24:997-1004. 5. Barth JR, Burkhart SS, De Beer JF. The bear-hug test: a new and sensitive test for diagnosing a subscapularis tear. Arthroscopy 2006;22:1076-84. 6. Bennett WF. Subscapularis, medial, and lateral head coracohumeral ligament insertion anatomy. Arthroscopic appearance and incidence of "hidden" rotator interval lesions. Arthroscopy 2001;17:173-80. 7. Lafosse L, Jost B, Reiland Y, Audebert S, Toussaint B, Gobezie R. Structural integrity and clinical outcomes after arthroscopic repair of isolated subscapularis tears. J Bone Joint Surg Am 2007;89:1184-93. Anatomy of the Subscapularis The subscapularis muscle arises from the anterior surface of the scapula, courses laterally beneath the coracoid, and becomes tendinous near the level of the glenoid rim. The tendinous footprint is roughly trapezoidal in shape, with its widest portion superior, and has a mean superior to inferior length of 2.5 cm.

8. Richards DP, Burkhart SS, Tehrany AM, Wirth MA. The subscapularis footprint: an anatomic description of its insertion site. Arthroscopy 2007;23:251-4. Macroscopically, the superior two-thirds of the subscapularis ends as a tendon that inserts on the lesser tuberosity of the humerus. The inferior part has no tendon; the muscle attaches directly to the bone, as with the teres minor on the posterior aspect of the humerus. Due to anatomical, biomechanical but also electrophysiological considerations, the subscapularis muscle, usually perceived as a single muscle, should be considered as two different muscles (inferior and superior).

9. Collin P, Lädermann A, Le Bourg M, Walch G. Subscapularis minor--an analogue of the Teres minor? Orthopaedics & traumatology, surgery & research : OTSR 2013;99:S255-8. The superior and inferior subscapularis are innervated by two different nerves.11,12 The superior part is innervated by the upper subscapularis nerve that comes directly from the posterior cord. The inferior part is innervated by the lower subscapularis nerve arising from either the axillary nerve or the thoracodorsal nerve.

10. Kadaba MP, Cole A, Wootten ME, et al. Intramuscular wire electromyography of the subscapularis. J Orthop Res 1992;10:394-7. 11. Kasper JC, Itamura JM, Tibone JE, Levin SL, Stevanovic MV. Human cadaveric study of subscapularis muscle innervation and guidelines to prevent denervation. J Shoulder Elbow Surg 2008;17:659-62. Electrophysiology studies confirm this division and have shown different activity in the superior and inferior parts of the subscapularis.

12. Kadaba MP, Cole A, Wootten ME, et al. Intramuscular wire electromyography of the subscapularis. J Orthop Res 1992;10:394-7. Different activation patterns in the two parts of the subscapularis are also found during a positron emission tomography (PET) study.

13. Omi R, Sano H, Ohnuma M, et al. Function of the shoulder muscles during arm elevation: an assessment using positron emission tomography. Journal of anatomy 2010;216:643-9. The superior margin of the subscapularis marks the inferior border of the rotator interval, which contains the superior glenohumeral ligament and the coracohumeral ligament. The medial sling, composed of a robust deep layer (medial head of the coracohumeral ligament) and a thin superficial layer (superior glenohumeral ligament), has an insertional footprint at the top of the lesser tuberosity that is directly adjacent to the footprint of the superolateral subscapularis on the lesser tuberosity. As such, when the upper subscapularis tears away from its bone attachment, the medial sling (which is directly adjacent to it), also tears away from the bone. As this sling is part of the bicipital pulley, biceps pathology is frequently associated with subscapularis tears.

Biomechanics

The subscapularis is the largest and most powerful of the rotator cuff muscles, providing more than 50% of the rotator cuff force alone.

14. Keating JF, Waterworth P, Shaw-Dunn J, Crossan J. The relative strengths of the rotator cuff muscles. A cadaver study. J Bone Joint Surg Br 1993;75:137-40. Functionally, the subscapularis internally rotates the humerus, contributes to anterior stability of the shoulder, and plays an important role in balancing force couples of the glenohumeral joint. In the coronal plane, the superior deltoid moment is balanced by the inferomedial moment of the inferior rotator cuff (Figure).