Biomechanical analysis of the subscapularis,infraspinatus and teres minor length and moment arm after reverse shoulder arthroplasty: a cadaveric study

BackgroundReverse shoulder arthroplasty (RSA) affects the length and moment arm of the deltoid and rotator cuff. Currently, RSA is commonly considered for cuff-intact conditions, such as primary glenohumeral osteoarthritis. As such, understanding the effect of contemporary lateralized designs on the rotator cuff is paramount. The purpose of this study was to determine changes in length and moment arm of the subscapularis, infraspinatus and teres minor with implantation of one of 3 RSA designs.MethodsA previously validated model was used in 6 hemi-toraces with the shoulder attached. Suture lines were run through pneumatic cylinders from the insertion to the origin of 10 muscles to apply a constant, stabilizing load. Electromagnetic tracking sensors were fixed to the thorax, scapula, and humerus to record 3-dimensional kinematics. Coordinate systems were established according to ISB recommendations. The origin and insertion of the subscapularis, infraspinatus and teres minor were digitized and tracked. Testing consisted of manually rotating the humerus through 5 cycles of its internal-external rotation arc. Kinematic data was collected at 120 Hz. Testing was performed in 3 positions of abduction: 0°, 30°, and 60°. After testing the intact shoulder, RSA was performed using 3 different configurations: an onlay 135-degree humeral component matched with a 2-mm lateralized glenosphere, the same humeral component with a 6-mm lateralized glenosphere, and an inlay 135-degree humeral component matched with a 10 mm lateralized glenosphere. Minimal muscle operative lengths, maximal muscle operative lengths, and muscle moment arms were computed.ResultsWhen compared with the native shoulder, all 3 configurations of RSA resulted in statistically significant increases in both the minimal and maximal operative lengths of the subscapularis in all abduction positions. The teres minor only showed a statistically significant increase in minimal and maximal length at 60° of abduction. The infraspinatus showed a statistically significant increase in tendon excursion at 0° and 30° of abduction. In 40° of abduction and 40° of internal rotation, all RSA configurations translated in a decreased subscapularis internal rotation moment arm. On the contrary, RSA increased the external rotation moment arm of the infraspinatus in neutral rotation and 0° of abduction.ConclusionImplantation of contemporary lateralized RSA implants led to increased length of the subscapularis to a greater extent than the increased length experienced by the infraspinatus and teres minor. The moment arm of the subscapularis decreased, whereas the moment arm of the teres minor in neutral rotation with the arm in abduction increased.Level of EvidenceLevel III; Basic Science, Biomechanics Study     

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.     

Neer Award 2006: Biomechanical assessment of inferior tuberosity placement during hemiarthroplasty for four-part proximal humeral fractures

Tuberosity malpositioning commonly occurs and is associated with a decline in clinical function after prosthetic shoulder reconstruction for proximal humeral fractures. This study assesses the biomechanical effects of inferior tuberosity position on glenohumeral joint forces and humeral head position at multiple positions. Eight fresh-frozen cadaveric shoulders were tested. Hemiarthroplasty was performed with preservation of anatomic tuberosity height and with 10 mm and 20 mm of inferior tuberosity displacement. The rotator cuff, deltoid, pectoralis major, and latissimus dorsi muscles were statically loaded. Contact forces and humeral head position were recorded within a functional range of motion. Glenohumeral joint forces shifted significantly superiorly (P < .05) at 30 degrees of abduction after both 10 mm and 20 mm of tuberosity displacement. At 60 degrees of glenohumeral abduction, glenohumeral joint forces remained significantly altered after tuberosity displacement of 10 mm and 20 mm compared with the intact height (P < .005). This study demonstrates that, during hemiarthroplasty performed for proximal humeral fractures, malpositioning the tuberosities inferiorly results in significant superior glenohumeral joint force displacement. These findings suggest that the mechanical advantage of the shoulder abductor muscles is compromised with inferior tuberosity malpositioning and may help to explain inferior functional results seen in these patients.     

The entire rotator cuff contributes to elevation of the arm

The function of the infraspinatus, teres minor, and subscapularis during elevation of the arm remains poorly defined. These muscles may generate moments that contribute to abduction of the arm, although they frequently are classified as humeral depressors. The purposes of this study were to measure the contributions to abduction made by the more inferiorly positioned rotator cuff muscles relative to the contributions of the supraspinatus and to determine the range of motion at which the muscles are most effective. Five fresh cadaveric shoulder girdles were mounted in an apparatus designed to simulate contraction of the deltoid and rotator cuff while maintaining the normal relationship between glenohumeral and scapulothoracic motions. The deltoid force required for elevation was measured without simulated contraction of the rotator cuff and with simulated contraction of the entire rotator cuff, of the supraspinatus only, and of the infraspinatus-teres minor and subscapularis only. A significant reduction in deltoid force when other muscle activity was added indicated that the additions contributed significantly to abduction. The deltoid force required with concurrent contraction of the entire rotator cuff averaged 41% less than with the deltoid alone but was not significantly different than with the deltoid and supraspinatus or with the deltoid, infraspinatus-teres minor, and subscapularis. Concurrent application of forces to the supraspinatus or the infraspinatusteres minro and subscapularis significantly reduced the required deltoid force over the range of motion studied by an average of 28 and 36%, respectively. The contributions of the rotator cuff muscles to abduction of the arm were greatest at low abduction angles (30 and 60°) and were insignificant by 120°. The infraspinatus-teres minor and subscapularis contribute significantly to abduction: their contibution was equal to that of the supraspinatus and, like the supraspinatus, they are most effective during the first 90° of abduction.     

Humeral head translation decreases with muscle loading

This study was conducted to determine the effect of in vitro passive and active loading on humeral head translation during glenohumeral abduction. A shoulder simulator produced unconstrained active abduction of the humerus in 8 specimens. Loading of the supraspinatus, subscapularis, infraspinatus/teres minor, and anterior, middle, and posterior deltoid muscles was simulated by use of 4 different sets of loading ratios. Significantly greater translations of the humeral head occurred both in 3 dimensions (P < .001) and in the sagittal plane (P < .005) during passive motion when compared with active motion from 30 degrees to 70 degrees of abduction. In the sagittal plane, passive abduction experienced a resultant translation of 3.8 +/- 1.0 mm whereas the active loading ratios averaged 2.3 +/- 1.0 mm. There were no significant differences in the translations that were produced by the 4 sets of muscle-loading ratios used to achieve active motions. This study emphasizes the importance of the musculature in maintaining normal ball-and-socket kinematics of the shoulder.     

Moment arms of the shoulder muscles during axial rotation

The objective of the present study was to determine the instantaneous moment arms of 18 major muscle sub‐regions crossing the glenohumeral joint in axial rotation of the humerus during coronal‐plane abduction and sagittal‐plane flexion. The tendon‐excursion method was used to measure instantaneous muscle moment arms in eight entire upper‐extremity cadaver specimens. The results showed that the inferior subscapularis was the largest internal rotator; its rotation moment arm peaks were 24.4 and 27.0 mm during abduction and flexion, respectively. The inferior infraspinatus and teres minor were the greatest external rotators; their respective rotation moment arms peaked at 28.3 and 26.5 mm during abduction, and 23.3 and 22.1 mm during flexion. The two supraspinatus sub‐regions were external rotators during abduction and internal rotators during flexion. The latissimus dorsi and pectoralis major behaved as internal rotators throughout both abduction and flexion, with the three pectoralis major sub‐regions and middle and inferior latissimus dorsi displaying significantly larger internal rotation moment arms with the humerus adducted or flexed than when abducted or extended (p < 0.001). The deltoid behaved either as an internal rotator or an external rotator, depending on the degree of humeral abduction and axial rotation. Knowledge of moment arm differences between muscle sub‐regions may assist in identifying the functional effects of muscle sub‐region tears, assist surgeons in planning tendon transfer surgery, and aid in the development and validation of biomechanical computer models. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:658–667, 2011     

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.     

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.     

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.     

Experimental investigation of reaction forces at the glenohumeral joint during active abduction

Reaction forces at the glenohumeral joint counterbalance the mass moment of the upper extremity during shoulder motion and are directly related to the activity of muscles across the joint. Because stability of the glenohumeral joint depends on compression of the humeral head into the glenoid, reaction forces constitute an important aspect of shoulder biomechanics. The objective of this study was to measure reaction forces at the glenohumeral joint during active scapula plane abduction. Furthermore, to clarify the relationship between the deltoid and supraspinatus muscles throughout abduction, this study investigated the effect of 4 variations of applied muscle forces on the magnitude and direction of glenohumeral reaction forces. We used a dynamic shoulder testing apparatus equipped with a force-moment sensor to directly measure reaction forces. Joint reaction forces increased throughout abduction and peaked at approximately 90 degrees for all testing conditions. The largest reaction forces occurred when the ratio of applied forces favored the supraspinatus tendon, whereas simulated paralysis of the supraspinatus resulted in a significant decrease in joint compression. There were no differences in direction of the reaction force between testing conditions. The results of this study indicate that the magnitude of glenohumeral joint reaction forces varies according to the ratio of forces between the supraspinatus and deltoid muscles. Thus, conditions characterized by either deltoid or supraspinatus dysfunction may result in abnormal loading mechanics at the glenohumeral joint. Understanding the relationship between rotator cuff function and glenohumeral reaction forces will aid in clarifying the importance of muscular activity to shoulder stability and strength as it relates to compression of the humeral head.     

Shoulder electromyography in multidirectional instability

We studied shoulder muscle activity in multidirectional instability (MDI) and multidirectional laxity (MDL) of the shoulder, our hypothesis being that altered muscle activity plays a role in their pathogenesis. Six muscles (supraspinatus, infraspinatus, subscapularis, anterior deltoid, middle deltoid, and posterior deltoid) were investigated by use of intramuscular dual fine-wire electrodes in 7 normal shoulders, 5 MDL shoulders, and 6 MDI shoulders. Each subject performed 5 types of exercise (rotation in neutral, 45 degrees of abduction, 90 degrees of abduction, flexion/extension, and abduction/adduction) on an isokinetic muscle dynamometer at two rates, 90 degrees /s and 180 degrees /s. After filtering, rectification, and smoothing, the electromyography signal was normalized by using the peak voltage of the movement cycle. In subjects with MDI, compared with normal subjects, activity patterns of the anterior deltoid were different during rotation in neutral and 90 degrees of abduction, whereas those of the middle and posterior deltoid were different during rotation in 90 degrees of abduction. In subjects with MDL, the posterior deltoid showed increased activity compared with normal subjects during adduction. Activity patterns of the supraspinatus, infraspinatus, and subscapularis appeared similar in both groups. Dual fine-wire electromyography offers insight into the complex role of shoulder girdle muscle function in normal movement and in instability. Altered patterns of shoulder girdle muscle activity and imbalances in muscle forces support the theory that impaired coordination of shoulder girdle muscle activity and inefficiency of the dynamic stabilizers of the glenohumeral joint are involved in the etiology of MDI. Interestingly, the abnormalities are in the deltoid rather than the muscles of the rotator cuff.     

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.     

Analysis of rotator cuff muscles in adult human cadaveric specimens

Management of irreparable massive rotator cuff tears remains a challenging and controversial problem. Defining glenohumeral force relations may allow for the development of treatment strategies based on biomechanical principles. Five fresh-frozen adult human cadaveric shoulder specimens were dissected to determine fiber length, mass, and lever arm of (a) the 3 bellies of the deltoid and (b) the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis). From these data, physiologic cross-sectional areas and moment relations were calculated. These relations provide evidence for a balanced axial force couple between the anterior and posterior rotator cuff. Demonstration of an axial force couple across the glenohumeral joint may have clinical significance for treatment of irreparable massive rotator cuff tears and may explain why many patients with full-thickness rotator cuff tears can regain acceptable shoulder function.     

A new prosthetic design for proximal humeral fractures: reconstructing the glenohumeral unit

A new shoulder prosthesis design for proximal humeral fractures has been developed. The rim of the articular component of this prosthesis has several holes to which the bone-tendon junction of the rotator cuff is fixed, to allow an anatomic reconstruction of the glenohumeral unit. The strength of the tuberosity fixation to this prosthesis is investigated in a cadaveric study. Artificial 4-part fractures were created in 18 human, fresh-frozen, paired shoulder joints with intact rotator cuffs. Two methods of tuberosity fixation were used in a matched-pair fashion. In group I the tuberosities were sutured to the rim of the prosthetic head, and in group II the tuberosities were circumferentially tension band-wired. Strength testing was performed on a material-testing machine, and displacement was recorded with an opto-electronic device. Both fixation methods proved to be equally reliable in the forces exerted during activities of daily living without significant displacement of the fracture fragments.     

Radiographic evaluation of glenohumeral kinematics: a muscle fatigue model

The purpose of this study was to document the effect of muscle fatigue on glenohumeral kinematics. Twelve male volunteers without shoulder disease and with an average age of 27 years were studied. Glenohumeral anteroposterior radiographs were taken at 45 degrees intervals as the arm was abducted in the plane of the scapula from 0 degree to 135 degrees. This series of radiographs was performed both before and immediately after the subject performed a series of deltoid and rotator cuff fatiguing exercises. The average humeral head position or translation before and after muscle fatigue for each arm angle was compared. For all subjects, before fatigue, the position of the humeral head was below the center of the glenoid for all angles of abduction. There was essentially no change in position of the humeral head in the prefatigue state, as the arm was abducted from 0 degree to 135 degrees with no more than an average 0.3 mm of total humeral head excursion. After fatigue, excursion of the humeral head increased to an average of 2.5 mm between the tested positions. The position of the humeral head with the arm at 0 degree of abduction was lower or had migrated inferiorly compared with the rested state, with an average 1.2 mm significant increase in inferior translation. With the initiation of abduction, the humeral head demonstrated significant superior migration or translation in all positions tested. This result has important implications for conservative treatment of shoulder impingement and underscores the importance of rehabilitation to maximize the endurance and strength of the rotator cuff musculature.     

Simulated joint and muscle forces in reversed and anatomic shoulder prostheses

Reversed shoulder prostheses are increasingly being used for the treatment of glenohumeral arthropathy associated with a deficient rotator cuff. These non-anatomical implants attempt to balance the joint forces by means of a semi-constrained articular surface and a medialised centre of rotation. A finite element model was used to compare a reversed prosthesis with an anatomical implant. Active abduction was simulated from 0 degrees to 150 degrees of elevation. With the anatomical prosthesis, the joint force almost reached the equivalence of body weight. The joint force was half this for the reversed prosthesis. The direction of force was much more vertically aligned for the reverse prosthesis, in the first 90 degrees of abduction. With the reversed prosthesis, abduction was possible without rotator cuff muscles and required 20% less deltoid force to achieve it. This force analysis confirms the potential mechanical advantage of reversed prostheses when rotator cuff muscles are deficient.     

Rehabilitation of shoulder joint instabilities

The overall goal of the rehabilitation period is a return to full range of motion and a strengthening of the muscles that have a role in protecting the shoulder from injury. Rehabilitation is accomplished gradually and is performed initially in a limited range that excludes the terminal 30 degrees at either end. Two key muscle groups are to be strengthened. One group is for the glenohumeral joint: the deltoid, latissimus dorsi, pectoralis major, and the rotator cuff muscles--especially the subscapularis muscle for internal rotation and the infraspinatus and the teres minor muscles for external rotation. The second group, the trapezius, rhomboids, and serratus anterior muscles, is strengthened for scapular control. Finally, general conditioning and trunk flexibility exercises are critical for a return to former competitive levels.     

The static rotator cuff does not affect inferior translation of the humerus at the glenohumeral joint.

BACKGROUND: The static contribution of the rotator cuff to the inferior stability of the shoulder is poorly understood. The purpose of this study was to determine the effect of static rotator cuff muscles on the inferior stability of the glenohumeral joint. METHODS: The humeral head positions relative to the glenoid were obtained in 12 shoulder specimens under the following conditions: with and without a 1.5-kg load; with the humerus adducted and abducted 90 degrees; and in three stages of dissection: (1) before release of any of the rotator cuff muscles, (2) after release of the supraspinatus or the cuff muscles other than the supraspinatus, and (3) after release of all of the cuff muscles. The order of release was changed in two ways: release of the supraspinatus followed by the release of other muscles in one group, and the opposite order in the other group. RESULTS: In both adduction and abduction, there were no significant differences in the positions of the humeral head either among the three stages of release or between the two different orders of release. CONCLUSION: The static contribution of the cuff muscles to the inferior stability of the shoulder is insignificant.     

Shoulder prostheses treating cuff tear arthropathy: a comparative biomechanical study.

Painful cuff tear arthropathy (CTA) affects the independence of the elderly. Surgical treatment often consists of joint replacement, the functional outcome of which remains variable. Knowledge of the biomechanical properties of the different prosthetic designs can guide the orthopaedic surgeon in the choice of implant to predict its clinical result. A 3-D computer model of the glenohumeral joint is used to analyse the moment of the deltoid muscle in the scapular plane. A geometrical 3-D ball-and-socket model of the shoulder joint was used to calculate (1) the angle-force relationships, (2) the moment arm of the deltoid muscle and (3) the moment of the deltoid muscle components, for increasing degrees of arm elevation in the scapular plane. In this 3-D model, a clinical thoraco-scapular rhythm analysis was implemented, based on measurements in normal subjects, patients treated with an anatomical prosthesis and patients treated with an inversed delta III prosthesis. These data were compared for 10 different prosthetic treatment options. RESULTS: Muscle angle-force curves show a favourable slope in non-anatomical prosthetic designs, where the centre of rotation of the glenohumeral joint is medialized, the deltoid muscle is elongated and the humeral shaft is lateralized. On the contrary, anatomical prosthetic designs do not perform well in this computer analysis. CONCLUSIONS: From a biomechanical point of view, a shoulder prosthesis which medializes the centre of rotation, lengthens the deltoid muscle and increases the deltoid lever arm, results in a significantly more powerful abduction of the shoulder, despite complete loss of rotator cuff function. RELEVANCE: This study explains why a successful functional outcome can be expected in CTA with a reversed prosthesis.     

Dynamic glenohumeral stability provided by the rotator cuff muscles in the mid-range and end-range of motion. A study in cadavera

BACKGROUND: Both static and dynamic factors are responsible for glenohumeral joint stability. We hypothesized that dynamic factors could potentially operate throughout the entire range of glenohumeral motion, although capsuloligamentous restraints (a static factor) have been thought to be primarily responsible for stability in the end-range of motion. The purpose of this study was to quantitatively compare the dynamic glenohumeral joint stability in the end-range of motion (the position of anterior instability) with that in the mid-range by investigating the force components generated by the rotator cuff muscles. METHODS: Ten fresh-frozen shoulders from human cadavera were obtained, and all soft tissues except the rotator cuff were removed. The glenohumeral capsule was resected after the rotator cuff muscles had been released from the scapula. A specially designed frame positioned the humerus in 60 degrees of abduction and 45 degrees of extension with respect to the scapula. The compressive and shear components on the glenoid were measured before and after a constant force was applied individually to each muscle with the humerus in five different positions (from neutral to 90 degrees of external rotation). The dynamic stability index, a new biomechanical parameter reflecting these force components and the concavity-compression mechanism, was calculated. The higher the dynamic stability index, the greater the dynamic glenohumeral stability. RESULTS: In the mid-range of motion, the supraspinatus and subscapularis provided higher dynamic stability indices than did the other muscles (p < 0.05). On the other hand, when the position of anterior instability was simulated in the end-range of motion, the subscapularis, infraspinatus, and teres minor provided significantly higher dynamic stability indices than did the supraspinatus (p < 0.005). CONCLUSIONS: The rotator cuff provided substantial anterior dynamic stability to the glenohumeral joint in the end-range of motion as well as in the mid-range. CLINICAL RELEVANCE: A glenohumeral joint with a lax capsule and ligaments might be stabilized dynamically in the end-range of motion if the glenoid concavity is maintained and the function of the external and internal rotators, which are efficient stabilizers in this position, is enhanced.