Reconstruction of massive rotator cuff rupture using a deltoid muscle flap]

In extensive rotator cuff tears with rupture of the supra- and infraspinatus muscles, we have obtained good and very good results persisting for 3 years or longer only in 40% of cases treated by resection and debridement, and in 25% of our cases the head of the humerus was found to be no longer centrally located in the joint socket on X-ray examination 10 years after treatment. It was therefore proposed that an active muscle flap taken from the anterior part of the deltoid muscle (part III according to Fick) should be used to span the trophic defect in the rotator cuff, being sutured into healthy tissue following freshening up. This seems logical, since the transferred muscle flap remains innervated and vascularized and also works synergistically with the rest of the rotator cuff. The flap interposed in this way between the greater tubercle of the humerus and the acromion functions in the same way as a double-bellied muscle and prevents displacement of the head of the humerus. In this way an active rotator cuff is produced, which also takes over the function of stabilizing the humeral head. After follow-up of at least 1 year (average 19 months), the result is aesthetically satisfactory in all 50 shoulders treated in this way; 47 are free of pain or painful only some of the time; in 32 active elevation of over 120 degrees is possible; 21 have symmetrical force ratios in elevation and 15 for outward rotation in 90 degrees abduction. All reconstructed shoulders were characterized by fatigability in elevation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of experimental suprascapular nerve block on active glenohumeral translations in vivo.

Static superior shoulder instability is associated with long-standing rotator cuff tears. Factors or mechanisms which can prevent superior migration of the humeral head, and therefore allow preservation or restoration of shoulder function despite nonanatomical cuff repair, are poorly understood. The question has therefore arisen, whether centering of the humeral head was the result of active shoulder muscle function. It was the goal of this experimental investigation to (1) determine the pattern of glenohumeral translations during active shoulder abduction measured by open-magnetic resonance imaging (MRI) techniques, and to (2) determine the influence of experimental paralysis of the infra- and supraspinatus muscles on these translations. In contrast to prior experimental investigations, the humeral head remained always centered in the glenoid fossa during active abduction. No superior migration of the humeral head could be provoked with experimental paralysis of the supra- and/or infraspinatus muscles. The hypothesis that static or dynamic superior humeral head displacement is prevented by active-supra- and/or infraspinatus muscle function must therefore be rejected, for the shoulder with a structurally intact muscle-tendon-bone unit.     

Electromyographic analysis of the deltoid and rotator cuff muscles in persons with subacromial impingement

The purpose of this study was to compare subjects with subacromial impingement and subjects with normal shoulders with respect to muscle activity. Fifteen subjects in each group were studied by means of fine-wire electromyography. The middle deltoid and rotator cuff muscles were evaluated during isotonic scaption from 30 to 120 degrees. Overall, the impingement group demonstrated decreased mean muscle activity in comparison with the group of normal subjects. The magnitude of diminished activity was statistically significantly different (P < .05) during the 30- to 60-degrees arc for the infraspinatus, subscapularis, and middle deltoid muscles; in addition, the infraspinatus muscle demonstrated significantly depressed activity during the 60- to 90-degrees arc. In the impingement group, the supraspinatus and teres minor revealed a diminution of muscle function in comparison with shoulders in the normal group; the difference was not significant. This study demonstrates that muscle activity in subjects with impingement is most notably decreased in the first arc of motion. Also of clinical relevance is the fact that the inferior force vector (from the infraspinatus and subscapularis) is less functional in subjects with impingement than is the superior compressive vector (from the supraspinatus). Thus, humeral head depression during the critical first portion of elevation may be insufficient in people with subacromial impingement.     

Joint-preserving treatment options for irreparable rotator cuff tears

An irreparable cuff tear is defined as the inability to achieve direct repair of native tendon to the great tuberosity despite intra- and extra-articular release of the remaining tissue. Three distinct anatomic patterns are identified: posterosuperior cuff tears which involve the supraspinatus, infraspinatus and teres minor; anterosuperior tears which involve the supraspinatus and subscapularis; and global tears which comprise both. Subacromial debridement and tenotomy or tenodesis of the long head of the biceps are proposed for older patients with a functional but very painful shoulder. Partial repair—particularly the infraspinatus and the subscapularis—is indicated for young patients if the muscle is still trophic with a fatty infiltration less than 3. It can be combined with a tendon transfer. In irreparable posterosuperior tears, latissimus dorsi or lower trapezius transfer has been reported to improve active elevation and external rotation. In anterosuperior cuff tears, pectoralis major or latissimus transfer has been used. If the lack of external rotation is isolated with good active forward elevation, the L’Episcopo procedure is the procedure of choice. New techniques with a short follow-up have been proposed recently: implantation of a balloon-shaped, biodegradable spacer in the subacromial space to maintain the position of the humeral head and to facilitate deltoid action; capsular superior reconstruction with a fascia lata or an artificial graft implanted between the superior glenoid rim and the great tuberosity to reproduce the natural capsule of the supra- and infraspinatus and to stabilize the humeral head.     

Electromyographic analysis of the deltoid and rot tor cuff muscles in persons with subacromial impingement

The purpose of this study was to compare subjects with subacromial impingement and subjects with normal shoulders with respect to muscle activity. Fifteen subjects in each group were studied by means of fine-wire electromyography. The middle deltoid and rotator cuff muscles were evaluated during isotonic scaption from 30 to 120 degrees. Overall, the impingement group demonstrated decreased mean muscle activity in comparison with the group of normal subjects. The magnitude of diminished activity was statistically significantly different (P < .05) during the 30- to 60-degrees arc for the infraspinatus, subscapularis, and middle deltoid muscles; in addition, the infraspinatus muscle demonstrated significantly depressed activity during the 60- to 90-degrees arc. In the impingement group, the supraspinatus and teres minor revealed a diminution of muscle function in comparison with shoulders in the normal group; the difference was not significant. This study demonstrates that muscle activity in subjects with impingement is most notably decreased in the first arc of motion. Also of clinical relevance is the fact that the inferior force vector (from the infraspinatus and subscapularis) is less functional in subjects with impingement than is the superior compressive vector (from the supraspinatus). Thus, humeral head depression during the critical first portion of elevation may be insufficient in people with subacromial impingement. (J Shoulder Elbow Surg 2000;9:519-23.)     

Influence of humeral prosthesis height on biomechanics of glenohumeral abduction. An in vitro study

BACKGROUND: During shoulder replacement surgery, the normal height of the proximal part of the humerus relative to the tuberosities frequently is not restored because of differences in prosthetic geometry or problems with surgical technique. The purpose of the present study was to determine the effect of humeral prosthesis height on range of motion and on the moment arms of the rotator cuff muscles during glenohumeral abduction. METHODS: Tendon excursions and abduction angles were recorded simultaneously in six cadaveric specimens during passive glenohumeral abduction in the scapular plane. Moment arms were calculated for each muscle by computing the slope of the tendon excursion-versus-glenohumeral abduction angle relationship. The experiments were carried out with the intact joint and after replacement of the humeral head with a prosthesis that was inserted in an anatomically correct position as well as 5 and 10 mm too high. RESULTS: Insertion of the prosthesis in positions that were 5 and 10 mm too high resulted in significant and marked reductions of the maximum abduction angle of 10 degrees (range, 5 degrees to 18 degrees ) and 16 degrees (range, 12 degrees to 20 degrees ), respectively. In addition, the moment arms of the infraspinatus and subscapularis decreased by 4 to 10 mm. This corresponded to a 20% to 50% decrease of the abduction moment arms of the infraspinatus and an approximately 50% to 100% decrease of the abduction moment arms of the subscapularis, depending on the abduction angle and the part of the muscle being considered. CONCLUSIONS: If a humeral head prosthesis is placed too high relative to the tuberosities, shoulder function is impaired by two potential mechanisms: (1) the inferior capsule becomes tight at lower abduction angles and limits abduction, and (2) the center of rotation is displaced upward in relation to the line of action of the rotator cuff muscles, resulting in smaller moment arms and decreased abduction moments of the respective muscles. Clinical Relevance: In patients managed with shoulder replacement surgery, limitation of range of motion, loss of abduction strength, and overload with long-term failure of the supraspinatus tendon are potential consequences of positioning the humeral head of the prosthesis proximal to the anatomic position.     

Variation in external rotation moment arms among subregions of supraspinatus, infraspinatus, and teres minor muscles.

A rotator cuff tear causes morphologic changes in rotator cuff muscles and tendons and reduced shoulder strength. The mechanisms by which these changes affect joint strength are not understood. This study's purpose was to empirically determine rotation moment arms for subregions of supraspinatus, infraspinatus, and for teres minor, and to test the hypothesis that subregions of the cuff tendons increase their effective moment arms through connections to other subregions. Tendon excursions were measured for full ranges of rotation on 10 independent glenohumeral specimens with the humerus abducted in the scapular plane at 10 and 60 degrees . Supraspinatus and infraspinatus tendons were divided into equal width subregions. Two conditions were tested: tendon divided to the musculotendinous junction, and tendon divided to the insertion on the humerus. Moment arms were determined from tendon excursion via the principle of virtual work. Moment arms for the infraspinatus (p < 0.001) and supraspinatus (p < 0.001) were significantly greater when the tendon was only divided to the musculotendinous junction versus division to the humeral head. Moment arms across subregions of infraspinatus (p < 0.001) and supraspinatus (p < 0.001) were significantly different. A difference in teres minor moment arm was not found for the two cuff tendon conditions. Moment arm differences between muscle subregions and for tendon division conditions have clinical implications. Interaction between cuff regions could explain why some subjects retain strength after a small cuff tear. This finding helps explain why a partial cuff repair may be beneficial when a complete repair is not possible. Data presented here can help differentiate between cuff tear cases that would benefit from cuff repair and cases for which cuff repair might not be as favorable.     

Reverse Shoulder Arthroplasty Combined with a Modified Latissimus Dorsi and Teres Major Tendon Transfer for Shoulder Pseudoparalysis Associated with Dropping Arm

Although a reverse shoulder arthroplasty (RSA) can restore active elevation in the cuff deficient shoulder, it cannot restore active external rotation when both the infraspinatus and teres minor muscles are absent or atrophied. We hypothesized that a latissimus dorsi and teres major (LD/TM) transfer with a concomitant RSA would restore shoulder function and activities of daily living (ADLs). We prospectively followed 11 consecutive patients (mean age, 70 years) with a combined loss of active elevation and external rotation (shoulder pseudoparalysis and dropping arm) who underwent this procedure. All had severe cuff tear arthropathy (Hamada Stage 3, 4, or 5) and severe atrophy or fatty infiltration of infraspinatus and teres minor on preoperative MRI or CT-scan. The combined procedure was performed through a single deltopectoral approach in the same session. Postoperatively, mean active elevation increased from 70 degrees to 148 degrees (+78 degrees ) and external rotation from -18 degrees to 18 degrees (+36 degrees ). The Constant score, subjective assessment and ADLs improved. The combination of a RSA and LD/TM transfer restored both active elevation and external rotation in this selected subgroup of patients with a cuff deficient shoulder and absent or atrophied infraspinatus and teres minor.     

Begutachtung des Rotatorenschadens

The key to analyzing defects and tears in the rotator cuff is a profound knowledge of the anatomical structures and the resulting biomechanics. Defects of the rotator cuff are very frequent shoulder pathologies in the older population. Due to insurance requirements expert opinions have to differentiate between defects as a result of degenerative processes and tears caused through trauma. This article gives some advice for this decision. The rotator cuff is a tendon plate of four muscles inserting on the humeral head that works as a twin force coupling. The tendons of the supraspinatus and the infraspinatus muscles act as a cable system with two anchor points, comparable to a suspension bridge. The function of the delta muscle is particularly supported by the supraspinatus. Therefore, injuries of the central rotator cuff will affect the rotator cable system and the delta muscle. If this cannot be shown, there has to be substantial doubt about any trauma or primary injury. The medical assessment of rotator cuff injuries requires an accurate clinical and radiological diagnosis.     

Latissimus dorsi transfer for the treatment of massive tears of the rotator cuff. A preliminary report

Symptomatic irrepairable rotator cuff tears usually entail complete loss of the substance of the supraspinatus and infraspinatus tendons. Loss of external rotation control and cranial migration of the humeral head on attempted flexion or abduction of the shoulder are the functional hallmarks. Transfer of the latissimus dorsi tendon from the humeral shaft to the superolateral humeral head provides a large, vascularized tendon that can be used to close a massive cuff defect and that exerts an external rotation and head-depressing moment that allow more effective action of the deltoid muscle. This procedure was carried out in 14 patients without any significant complications. Pain relief and functional results in those four cases with a minimum follow-up period of one year (average, 14 months) compared favorably with alternative treatment methods and warrant further anatomic, electromyographic, and clinical investigation.     

Quantitation of in situ contact areas at the glenohumeral joint: a biomechanical study.

Glenohumeral arthritis may result from abnormal articular mechanics, and shoulder reconstructive procedures often rely implicitly on the belief that the restoration of normal articular mechanics is required to obtain satisfactory clinical results. Despite this, limited knowledge of normal or pathologic glenohumeral joint articular mechanics and contact is available. This study uses a stereophotogrammetry technique to determine contact areas in normal cadaver glenohumeral joints with intact ligaments and capsule through a large range of motion using simulated forces of the four rotator cuff muscles and three deltoid heads. All shoulders were first elevated to their maximum elevation in the scapular plane at an external rotation (starting rotation = 40 +/- 8 degrees), which allowed each shoulder to attain its maximal elevation in the scapular plane, and then repeated at 20 degrees internal to this rotation. Contact areas consistently increased with increasing elevation until 120 degrees to an average of 5.07 cm2 before decreasing with further increased elevation to an average of 2.59 cm2 at 180 degrees of total arm elevation. At 20 degrees internal to the starting rotation, contact areas reached high values 60 degrees earlier (averaged 4.56 cm2 at 60 degrees of total arm elevation) and then remained fairly constant through 120 degrees before decreasing with further increased elevation to 2.51 cm2 at 180 degrees total arm elevation. With increasing elevation in the external starting rotation, humeral head contact dramatically migrates from an inferior region to a superocentral-posterior region while glenoid contact shifts posteriorly. When the humeral shaft is positioned 20 degrees internal to the starting rotation, humeral head contact shifts from inferocentral-anterior to superocentral-posterior regions. Simultaneously, a similar posterior shift in glenoid contact is observed. Furthermore, whereas only a small portion of the humeral head surface area is in contact in any given position, contact on the glenoid surface is much more uniformly distributed over its entire articulating surface.     

Optimal normalization tests for shoulder muscle activation: An electromyographic study

To accurately compare electromyographic data from different muscles and different subjects, it is necessary to normalize the integrated data obtained from each muscle. The purpose of this study was to identify the manual muscle testing positions that elicit maximal neural activation (integrated electromyography) of three rotator cuff muscles (supraspinatus, infraspinatus, and subscapularis) and five shoulder synergists (pectoralis major, latissimus dorsi, and anterior, middle, and posterior deltoids). The electromyographic activity of these eight muscles was examined in the nondominant shoulders of nine subjects. Indwelling wire electrodes (supraspinatus, infraspinatus, and subscapularis) and surface adhesive electrodes (pectoralis major, latissimus dorsi, and anterior, middle, and posterior deltoids) were placed. Each subject performed a series of 27 isometric contractions, and optimal tests (maximal neural activation) were identified for each muscle. Four tests were identified that resulted in the maximal neural activation of all eight shoulder muscles: 90° of scapular elevation with ?45° of humeral rotation for the supraspinatus, anterior deltoid, and middle deltoid; external rotation at 90° of scapular elevation and ?45° of humeral rotation for the infraspinatus and posterior deltoid: internal rotation at 90° of scapular elevation and neutral humeral rotation for the subscapularis and latissimus dorsi: and internal rotation at 0° of elevation and neutral rotation for the pectoralis major. These results identify four standard testing positions that will provide reference values for normalization of maximal voluntary contraction for the eight muscles of the shoulder examined in this study. Standardization of these test positions offers normalization guidelines that can be used in future dynamic electromyography studies of the shoulder.     

Dynamic contributions to superior shoulder stability.

It has been suggested that superior decentralization of the humeral head is a mechanical factor in the etiology of degenerative rotator cuff tears. This superior decentralization may be caused by muscular imbalance. The objective of this study was to investigate the contribution of individual shoulder muscles to superior stability of the glenohumeral joint. In 10 fresh frozen cadaver shoulders the tendons of the rotator cuff, teres major, latissimus, pectoralis major, deltoid and biceps were prepared. The shoulders were tested in a shoulder-loading device in 0 degrees, 30degrees, 60 degrees and 90 degrees of glenohumeral abduction. A constant superior force of 20 N was applied to the humerus. Tensile loads were applied sequentially to the tendons in proportion to their cross-sectional areas and translations of the humeral head relative to the glenoid were recorded with a 3Space Fastrak system. Depression of the humeral head was most effectively achieved by the latissimus (5.6 +/- 2.2 mm) and the teres major (5.1 +/- 2.0 mm). Further studies should elucidate their possible in vivo role in the frontal plane force couple to counter balance the deltoid. The infraspinatus (4.6 +/- 2.0 mm) and subscapularis (4.7 +/- 1.9 mm) showed similar effects while the supraspinatus (2.0 +/- 1.4 mm) was less effective in depression. Therefore, the infraspinatus and subscapularis should be surgically repaired whenever possible. The supraspinatus may be of less importance for superior stability than previously assumed.     

The influence of medial reattachment of the torn cuff tendon for retracted rotator cuff tears

We reattached the torn rotator cuff medial to the anatomic cuff insertion site if it was retracted. The purpose was to correlate the amount of medial reattachment of the cuff with shoulder function. We evaluated 63 shoulders in which repaired cuffs were followed with magnetic resonance imaging at a mean follow-up of 8 years. The amount of medial reattachment of the cuff tendon was determined by use of a T2-weighted oblique coronal view, which passed through the center of the humeral head, and was defined as the NCA angle (where N indicates the new cuff insertion point, C indicates the center of the humeral head, and A indicates the anatomic cuff insertion point). Theoretically, the more medially the cuff tendon was reattached, the greater the NCA angle. Neither the Japanese Orthopaedic Association shoulder score nor isometric strength of forward elevation was correlated with the NCA angle. The NCA angle was significantly correlated (P = .001) with the active forward elevation angle, which dramatically decreased at 30 degrees of the NCA angle, approximately 13 mm from the original cuff insertion point, assuming a humeral head radius of 25 mm.     

Shoulder muscle activation during aquatic and dry land exercises in nonimpaired subjects

STUDY DESIGN: Randomized, single blind experimental design using electromyography to measure shoulder muscle activation in nonimpaired subjects. OBJECTIVES: To compare the muscle activation of rotator cuff and shoulder synergists during rehabilitation exercises performed in water or on dry land. BACKGROUND: Early motion is critical to restoration of normal shoulder function. Aquatic therapy has been promoted as a method for increasing range of motion while minimizing stress on the shoulder. METHODS AND MEASURES: The integrated electromyography amplitude of 6 muscles of the shoulder girdle was examined on the nondominant shoulders of 6 subjects (supraspinatus, infraspinatus, and subscapularis, anterior, middle, and posterior deltoids). Each subject performed elevation (0 degree to 90 degrees) in the scapular plane with neutral rotation on land and in water at 3 different speeds of elevation (30 degrees/s, 45 degrees/s, and 90 degrees/s). The mean percentage of the maximal voluntary contraction was determined for each of the 3 test speeds on land and in water. Comparisons between water and dry land were made with a repeated measures analysis of variance. RESULTS: For all 6 muscles tested, muscle activation during the 30 degrees/s test speed and all muscles tested at the 45 degrees/s test speed was significantly less when performed in water versus when performed on land. For example, electromyography activation of the supraspinatus muscle was 16.68% of a maximal voluntary contraction when elevation at was performed at 30 degrees/s on dry land versus 3.93% when performed in water. CONCLUSION: These data suggest that shoulder elevation in the water at slower speeds resulted in a significantly lower activation of the rotator cuff and synergistic muscles. This decreased muscle activation during aquatic physical therapy allows for earlier active motion in the postoperative period without compromising patient safety.     

Biomechanische Evaluation der glenohumeralen Stabilität durch Muskelkraftvektoranalyse

OBJECTIVES: The aim of the study was to quantify the decrease in glenohumeral stability following a global rotator cuff tear and to evaluate the effect of a decreased glenoid inclination angle through analysis of muscle force vectors in a computer model. MATERIAL AND METHODS: The lines of action of eight shoulder muscles were integrated into a standard geometric model. Muscle force magnitudes were estimated based on physiological cross-sectional area and normalized electromyographic activity. The magnitude and elevation angle of the resultant force vector was calculated at 0, 30, 60, and 90 degrees of abduction. A rotator cuff tear was simulated by reduction of the corresponding muscle force vectors. RESULTS: At 0 and 30 degrees of glenohumeral abduction a global rotator cuff tear showed a resultant force vector pointing outside the glenoid. In the computer model, decreasing the inclination angle of the glenoid by 30 degrees increased the stability in rotator cuff-deficient shoulders. CONCLUSIONS: The results of this study provide a biomechanical rationale for clinical complications of global rotator cuff tear such as superior humeral head translation. The decreased glenoid inclination simulated in the computer model may represent a biomechanical basis for the development of new operative techniques to treat global rotator cuff tears.     

Surgical management of complex irreparable rotator cuff deficiency.

The authors surgically treated 23 shoulders in 23 patients with disabling pain associated with irreparable tears of the musculotendinous cuff. In a total of 12 shoulders with preserved passive motion, normal deltoid function, loss of glenohumeral joint surfaces, and sculpturing of the coracoacromial arch, a standard or oversized Neer II humeral prosthesis without glenoid replacement was selected. A total of 11 shoulders that failed to meet these prerequisites or demanded heavy use after operation underwent arthrodesis. Twenty-two patients (12 from the hemiarthroplasty group and 10 from the arthrodesis group) were available for evaluation at an average follow-up period of 37.5 months. Comfort level and overall function were improved in both groups. Active forward elevation improved an average of 44 degrees in the hemiarthroplasty group and an average of 15 degrees in the arthrodesis group. The success of hemiarthroplasty and the problems of glenoid loosening in the presence of cuff deficiency with upward head displacement have led to the conclusion that humeral hemiarthroplasty is the preferred method for managing complex irreparable tears of the rotator cuff in which the articular surface is destroyed, yet the deltoid muscle is functional. Shoulder arthrodesis is reserved for those patients who have both irreparable tears of the rotator cuff and irreparable deficiencies of the deltoid muscle, or the younger patient with demands for substantial strength at low angles of flexion.     

Mechanics of the deltoid muscle. A new approach

The Inman concept of the mechanics of the deltoid describes a vertical upward oriented traction exerted on the upper end of the humerus at the beginning of arm elevation. However, Duchenne de Boulogne showed that the middle deltoid pushes the head downward. When the arm is at rest, the trajectory of the middle deltoid fibers changes by more than 90 degrees so that the humeral head is enveloped by the muscle; this suggests that the deltoid acts on the humeral head like a cable on a pulley. The authors studied the area of contact between the deltoid and the humeral head in three-dimensional reconstructed shoulders. A new model, which includes the pulley effect, was designed to explore the resultant total force applied by the deltoid onto the humerus. In most cases the resultant vertical force was oriented downward. Thus, the conventional model is not complete. The current model indicates that the deltoid prevents upward migration of the humeral head and compresses it against the glenoid. This explains why many shoulders function well despite a massive cuff tear. This also implies that reeducation of the deltoid is a major aspect of the rehabilitation for patients with a rotator cuff tear.     

Shoulder surgery: from cuff repair to joint replacement. An update

REPAIRING FULL THICKNESS CUFF TEARS.--Despite the sound rationale for repairing full-thickness rotator cuff tears, the procedure may fail to restore cuff integrity, which is indispensable to optimal cuff function. The functional role of each cuff muscle and the factors associated with anatomic failure (particularly those related to the muscles and tendons) provide a basis for rational patient selection and for determination of the best surgical strategy on a case-by-case basis. SHOULDER ARTHROPLASTY IN PATIENTS WITH GLENOHUMERAL JOINT DISEASE.--Total shoulder arthroplasty in patients with glenohumeral joint disease provides better outcomes than humeral hemiarthroplasty. The choice between a semi-constrained total prosthesis and a reverse constrained total prosthesis should be based on the nature of the joint disease (either centered humeral head or normal cuff function or migrated humeral head and abnormal cuff function). At present, only the semi-constrained total prosthesis has been proved effective in the long-term when used in a patient with a centered humeral head and active cuff. This provides additional support for repairing cuff tears whenever possible in patients who are still young.     

The effect of infraspinatus disruption on glenohumeral torque and superior migration of the humeral head: a biomechanical study

Rotator cuff ruptures that extend into the infraspinatus tendon may cause dysfunction and superior migration of the humerus. The purpose of this study was to determine whether a threshold size of infraspinatus defect exists beyond which abduction torque generation decreases and superior migration of the humeral head increases. Glenohumeral abduction torque and superior humeral head translations were measured in hanging arms in neutral rotation in cadaver shoulders (n = 10). Loads were applied to the rotator cuff tendons and the middle deltoid. After sequential detachment of the infraspinatus, abduction torque progressively decreased. At three-fifths detachment, abduction torque was significantly lower than after supraspinatus release alone (52% vs 61%, P <.05). Superior translation after complete supraspinatus and infraspinatus detachment increased significantly (P <.05), but no intermediate threshold was detected. Therefore, the entire infraspinatus contributes to abduction torque generation and stabilizes the humeral head against superior subluxation. Even with a tear extending into the superior infraspinatus, the infraspinatus contributes abduction force generation across the glenohumeral joint.