The subscapularis muscle is important for normal shoulder function and stability. It constitutes the sole anterior component of the rotator cuff and is the most powerful of the cuff muscles.¹ The subscapularis is a strong internal rotator of the glenohumeral joint, particularly with the shoulder in an adducted and extended position.^2,3 The dynamic force couple created from the coordinated efforts of the subscapularis and the posterior rotator cuff is critical for normal glenohumeral joint kinematics and stability.^4,5 This force couple has been shown experimentally to be an important contributor to humeral head depression throughout multiple positions of glenohumeral abduction.⁶ Loss of subscapularis function commonly results in pain and weakness and occasionally impairment in shoulder function, which may require surgical treatment.^7,8

Fortunately, subscapularis tendon tears are relatively uncommon. Isolated subscapularis tears are even less frequent. Codman⁹ reported involvement of the subscapularis in 3.5% of a series of 200 rotator cuff tears, and Deutsch et al.⁸ noted significant involvement of the subscapularis in 4% of a series of 350 rotator cuff tears. Warner et al.¹⁰ noted involvement of the subscapularis tendon in 4.7% of a series of 407 rotator cuff tears. The majority of subscapularis injuries are associated with tears of the superior rotator cuff (anterosuperior cuff tears) as well.^10,11,12 Isolated subscapularis tears are more commonly associated with trauma in comparison to other types of rotator cuff injuries.^7,13 Traumatic subscapularis tendon tears have been associated with recurrent anterior glenohumeral dislocation in several clinical series.^14,15,16,17 Subscapularis deficiency is also a well-documented complication of open anterior instability and prosthetic humeral replacement.

Repair of acute subscapularis tears has produced excellent clinical results.^7,18,19 Unfortunately, the diagnosis of isolated subscapularis tears is often delayed or missed.⁷ A completely torn subscapularis tendon is prone to retraction and the development of irreversible changes of the muscle. After a delay of several months or longer, repair of the retracted tendon can be very difficult. Inferior clinical results have been reported with delayed repair of subscapularis tears^10,13 and, in many cases, the subscapularis has been found to be irreparable at the time of surgery.^14,20

Muscle transfers have become useful salvage options for patients with irreparable tears of the subscapularis. Options include transfer of the pectoralis major, pectoralis minor, trapezius, latissimus dorsi, teres major, as well as allograft reconstruction.^{11,21,22,23,24} The pectoralis major tendon transfer has produced the most reliable clinical results when compared to other reconstructive options. Several characteristics of the pectoralis major make it favorable for reconstruction of the subscapularis. These include muscle bulk (including elderly patients), a robust tendon, location, similarity of function, and tendon excursion. Pectoralis major tendon transfer has been shown to be beneficial in several clinical situations related to subscapularis insufficiency. Successful results have been reported with associated recurrent anterior shoulder instability secondary to subscapularis deficiency,¹⁴ massive posterior rotator cuff tears,^20,23 and for humeral head containment in the setting of anterosuperior migration.²⁵

The clinical presentation of patients with subscapularis tears is variable. The majority of subscapularis tears are not isolated
but are seen in combination with tears of the superior and posterior cuff. Most patients in this setting are older and present with chronic pain and progressive deterioration of function of the shoulder joint. Acute loss in function in the setting of chronic shoulder symptoms may indicate an acute on chronic rotator cuff injury and should raise the suspicion of subscapularis involvement as well. Isolated injuries of the subscapularis often result from trauma to the shoulder. A forced external rotation moment with or without hyperextension of the adducted shoulder has been reported as a common mechanism of subscapularis injury.^7,8 In these series, the average patient ages were 39 and 50 years, considerably younger than the typical presentation of a massive anterosuperior rotator cuff tear. Anterior shoulder dislocation in the middle-aged patient can result in subscapularis disruption and recurrent instability.^14,17 Subscapularis failure can complicate open instability or prosthetic replacement surgery.

The majority of patients with subscapularis tears will complain of pain that may or may not be localized to the anterior shoulder. Most patients will note increased pain and weakness with both overhead activities and strenuous activities below shoulder level. Isolated tears may produce minimal symptoms and can be easily overlooked. Activities requiring forced internal rotation such as reaching behind the body, placing the hand in a back pocket, or reaching the abdomen are difficult.¹⁸ Sensations of glenohumeral instability are common, especially in more active patients or following prosthetic replacement.

The physical examination of patients with subscapularis tears is significantly influenced by the integrity of the remaining rotator cuff. The majority of patients with isolated tears of the subscapularis can still elevate the arm to the overhead position.^14,18 Tears that also include the posterior cuff often produce significant loss of active elevation due to disruption of the rotator cuff force couple.¹⁰ Isolated subscapularis tears are commonly missed initially and require a high index of suspicion. Subscapularis tears will often result in an increase in external rotation range of motion compared to the opposite shoulder. The abdominal compression and liftoff tests are excellent clinical examination tools that are highly accurate for detecting subscapularis disruption.^2,3,7 However, pain and limited passive range of motion may hinder the accuracy of liftoff test because of the arm position required to perform the maneuver. The strength of the remainder of the rotator cuff should be assessed because of the high prevalence of associated tears of the supraspinatus and infraspinatus muscles. Apprehension in abduction may be seen in those patients with instability. Tears of the subscapularis are often associated with instability of the long head of the biceps tendon.^10,26,27 Biceps provocation tests can clue the clinician to the presence of biceps tendon instability.

Radiographs of the shoulder in patients with isolated subscapularis tears are typically normal. Occasionally subtle anterior translation of the humeral head can be appreciated on the axillary radiograph in patients with subscapularis deficiency. Tears that also include the posterior cuff will often result in superior migration of the humeral head. This is particularly evident on true anteroposterior (AP) radiographs performed with slight abduction of the shoulder. Tears of the subscapularis can accurately be identified with both ultrasound and magnetic resonance imaging (MRI).^28,29,30,31 Associated MRI findings are frequently encountered and fairly specific to subscapularis injuries. These include subluxation or dislocation of the biceps tendon, fluid collections local to the subscapular recess, or subcoracoid bursa and supraspinatus tears.^29,30 MRI evaluation is particularly useful in identifying the degree of tendon retraction and fatty degeneration and atrophy of the subscapularis muscle. Ultrasound examination is more favorable in the postoperative setting because of improved accuracy of rotator cuff imaging over MRI, especially in the setting of implants.³²