The instantaneous muscle moment arms of 10 shoulder muscles including the three portions of the deltoid and the rotator cuff and scapulohumeral muscle groups during Four specified glenohumeral motions were calculated. Moment arm values were derived from a plot of tendon excursion versus glenohumeral joint rotation angle during horizontal flexion along the 90 degrees elevation plane and elevation in the sagittal, scapular, and coronal planes. The deltoid had the largest moment arm in elevation. The anterior deltoid has a larger moment arm in the anterior planes, whereas the midportion is greater in the scapular and coronal planes. The muscles with the largest depressor (adductor) moment arms are the pectoralis major, latissimus dorsi, and teres major. Contrary to the Findings of other investigators, the supraspinatus and infraspinatus have a smaller potential elevation torque in the scapular plane than has been previously reported. Furthermore the subscapularis may potentially be a more important elevator in the scapular plane than either the supraspinatus or infraspinatus, especially in the latter phases of motion. The pectoralis major has the largest horizontal Flexion moment arm with the humerus elevated 90 degrees whereas the posterior deltoid and infraspinatus have the largest horizontal extension moment arms in this plane.

The shoulder joint is one of the most complex joints in the body. It consists of three articulations (glenohumeral, acromioclavicular, and sternoclavicular joints) and two gliding mechanisms (subacromial space and scapulothoracic articulation) with 20 surrounding muscles. The biomechanical aspects of the shoulder complex have been researched for more than a century. Starting with the works of Braune and Fischer rl and Fick, 28 a substantial amount of information has been collected, much of which refers to the motion of the humerus relative to the shoulder complex. The muscular system was also studied after the development of electromyographic techniques, most comprehensively by Inman et al. 48 However, recent advances in biomechanical investigation of the shoulder are remarkable. In the last 10 years new technologies were introduced that enabled the investigators to make great progress in shoulder research. This article will review recent progress of biomechanical research on the glenohumeral joint (shoulder joint in a narrow sense): motion, stability, and force.

The morphological characteristics of shoulders with torn rotator cuffs were determined using 41 embalmed specimens. The following parameters were measured in the supraspinatus (SSP), infraspinatus (ISP) and subscapularis (SSC) muscles: the length, thickness and width of the extramuscular tendon; the length of the intramuscular tendon; the length and width of a tear, if present, muscle fibre length; and muscle volume. The cross-sectional area (CSA) of the tendon was measured on the photographic image of slices of the tendon using an image analysis system, and the CSA of the muscle was calculated by dividing the muscle volume by muscle fibre length. The rotator cuff was intact in 11 shoulders. A partial-thickness tear of the cuff was present in 12 shoulders, a full-thickness tear of the SSP in 11 shoulders, and a full-thickness tear of more than 2 tendons in 7. Overall incidence of full-thickness tears of the rotator cuff was 44%, and that of partial-thickness tears 29%. With increase of tear size, the functional tendon length (extramuscular tendon length plus tear length) increased by a statistically significant amount in the SSP, ISP and SSC, whereas muscle fibre length decreased in SSP and ISP. It is concluded that the increased functional tendon length and decreased muscle fibre length are the main morphological changes that make the rotator cuff a physiologically abnormal unit. Surgical repair of the torn cuff would be expected to improve these anatomical changes and restore the kinetics of the glenohumeral joint.(ABSTRACT TRUNCATED AT 250 WORDS)

The relative stabilizing effect of the passive bulk tissues and deltoid to inferior humeral displacement was studied. The humeral head center relative to the glenoid center was determined by a magnetic tracking device in 10 shoulder specimens for the bulk tissue study and in 13 for the deltoid study. Twelve static recordings were obtained per specimen: (1) with and without a 1.5 kg load, (2) with the humerus adducted hanging freely and abducted 90° in neutral rotation, and (3) in three stages of dissection: with all tissues intact, after removal of the tissues superficial to the deltoid, and after removal of the deltoid. In both humeral adduction and abduction, the passive bulk tissues and deltoid did not provide significant stability to the shoulder joint.

The contribution of axial arm rotation to translation of the humeral head with the arm in the hanging position was examined using nine fresh-frozen cadaveric shoulders. Three standard clinical tests were simulated: anterior and posterior translation and sulcus tests. In both the anterior translation and sulcus tests, anterior and inferior displacements were significantly restricted in internal rotation but not in neutral and external rotation. In the posterior translation test, no significant differences in displacement could be observed in internal, neutral, or external rotation. Since anterior and inferior translations of the humeral head were significantly affected by the rotation of the arm, we recommend that the anterior translation and sulcus tests be performed in various rotations.

The motion and laxity of the capitellocondylar unconstrained total elbow prosthesis were assessed, with use of an electromagnetic tracking device and stimulated muscle-loading, after implantation in seventeen cadaveric elbows. The axis of motion of the elbows with the capitellocondylar implants averaged 2.1 +/- 2.3 degrees more varus angulation than that of the intact elbows. This difference may be attributed to the design of the implant, as the 5-degree-valgus humeral component used in this study has a smaller valgus inclination than the articular surface of the distal aspect of the humerus. Although the maximum valgus-varus laxity of the capitellocondylar elbow prostheses was, on the average, 4.3 +/- 2.4 degrees greater than normal (with simulated muscle-loading), the data must be interpreted in light of the fact that this in vitro study did not allow for soft-tissue healing. The prosthetic components tracked well, and there were no dislocations or malarticulations provided that appropriate soft-tissue tensioning and positioning of the components had been achieved at the time of implantation. Sectioning of either the medial or the lateral collateral ligament resulted in gross instability of the joint after capitellocondylar arthroplasty. The ulnar attachment of the medial collateral ligament was found to be vulnerable to injury during the positioning of the ulnar component of this implant.

The stabilising effects on the glenohumeral joint of each of the rotator-cuff muscles and of the biceps were studied with the arm in abduction and external rotation in 13 cadaver shoulders. The muscles were loaded one at a time with forces proportional to their cross-sectional areas. We recorded the positions of the humeral head before and after the application to the humerus of an anterior force of 1.5 kg. When the capsule was intact, the anterior displacement with the subscapularis loaded was significantly larger than with the other muscles loaded (p = 0.0009). With the capsule vented, the displacement with the biceps loaded was significantly smaller than that with the subscapularis loaded (p = 0.0052). After creating an imitation Bankart lesion, the displacement with the biceps loaded was significantly less than with any of the rotator-cuff muscles loaded (p = 0.0132). We conclude that in the intact shoulder, the subscapularis is the least important anterior stabiliser, and that the biceps becomes more important than the rotator-cuff muscles as stability from the capsuloligamentous structure decreases. Strengthening of the biceps as well as the rotator-cuff muscles should be part of the rehabilitation programme for anterior shoulder instability.

Malrotation of the humeral component of the capitellocondylar total elbow replacement is thought but not proved to be one of the major causes of postoperative dislocation. The purpose of this study was to quantitate the effect of malrotation of the humeral component on the kinematics and laxity of the capitellocondylar total elbow prosthesis. Eleven fresh previously frozen elbows were used. With the humeral component in optimal position, external rotation, or internal rotation, movements of the elbow with neutral, valgus, and varus loading were monitored with an electromagnetic tracking device. When the humeral component was positioned in external rotation, the ulna was more valgus and supinated than when the component was in optimal position, and when the component was in internal rotation the ulna was more valgus in extension and more supinated in flexion. Malrotation in external rotation decreased valgus-varus laxity, and malrotation in internal rotation increased rotational laxity. Only one elbow became dislocated, despite constant severe maltracking between the components in all of the specimens. We concluded that although malrotation of the humeral component influences the laxity and causes maltracking, it is not the primary cause of postoperative dislocation. The contribution of other factors should be investigated.

We studied the contributions of the long and short heads of the biceps (LHB, SHB) to anterior stability in 13 cadaver shoulders. The LHB and SHB were replaced by spring devices and translation tests at 90 degrees abduction of the arm were performed by applying a 1.5 kg anterior force. The position of the humeral head was monitored by an electromagnetic tracking device with or without an anterior translational force; with 0 kg, 1.5 kg or 3 kg loads applied on either LHB or SHB tendons in 60 degrees, 90 degrees or 120 degrees of external rotation; and with the capsule intact, vented, or damaged by a Bankart lesion. The anterior displacement of the humeral head under 1.5 kg force was significantly decreased by both the LHB and SHB loading in all capsular conditions when the arm was in 60 degrees or 90 degrees of external rotation. At 120 degrees of external rotation, anterior displacement was significantly decreased by LHB and SHB loading only when there was a Bankart lesion. We conclude that LHB and SHB have similar functions as anterior stabilizers of the glenohumeral joint with the arm in abduction and external rotation, and that their role increases as shoulder stability decreases. Both heads of the biceps have been shown to have a stabilising function in resisting anterior head displacement, and consideration should therefore be given to strengthening the biceps during rehabilitation programmes for chronic anterior instability of the shoulder.

Nine fresh frozen cadaveric shoulders were used to examine the changes in intraarticular pressure (IAP) of the shoulder joint and the position of the humeral head under various loads to determine the relationship between these parameters. All the soft tissues superficial to the rotator cuff were removed. The position of the humerus relative to the scapula was monitored using an electromagnetic tracking system before and after venting the capsule with 0, 0.5, and 1.0 kg of load applied to the humerus inferiorly in addition to the weight of the arm. Simultaneously, the IAP was monitored using a pressure transducer before venting the capsule. Venting had a significant effect on the position of the humeral head: the positions of the head after venting were significantly lowered in all loading conditions. The average IAP was -76 cm H2O without load, and the value decreased in a linear fashion with increased load; the correlations were significant in four of six shoulders without dislocation. The correlations were less significant between the pressure and the position, and there were no significant correlations between the load and the position. IAP, which is determined primarily by the load applied, is important to stabilize the shoulder inferiorly. Because IAP is intimately related to the external load and the response is specific to individuals, examination of IAP response to external load may be helpful in characterizing various instabilities of the shoulder.

Eleven fresh cadaver shoulders were studied to determine the static contribution (bulk effect) of the rotator cuff on inferior glenohumeral stability provided by scapular inclination. All musculature, including the rotator cuff, was removed. The position of the humerus relative to the scapula was recorded using an electromagnetic tracking device under conditions of no force and 1.5 kg of inferior translation force applied to the humerus, with the arm in the hanging position (sulcus test) and then in 90 degrees abduction (Abduction-Inferior Stability test = ABIS test), with the scapula inclined referable to the vertical line at -15 degrees, 0 degrees, 15 degrees and 30 degrees in the sulcus test and at 15 degrees, 30 degrees, 45 degrees and 60 degrees in the ABIS test. In the sulcus test without load, all shoulders dislocated at scapular inclination angles of -15 degrees and 0 degrees, whereas no shoulders dislocated at 30 degrees. The angle of scapular inclination had a significant effect on humeral head positions (p < 0.0001), with the head position at -15 degrees and 0 degree being lower than at 15 degrees, which was lower than at 30 degrees. In the ABIS test, none of the shoulders dislocated, although the effect of the angle of scapular inclination was significant (p < 0.0001), with the position of the humeral head being higher at 15 degrees than at other angles of inclination. Comparison of these data and previously reported data with the cuff intact showed no significant effect of rotator cuff removal on humeral head position and displacement in both tests. Therefore, we conclude that the static condition of the rotator cuff has no significant effect on the stabilizing function of scapular inclination. The stabilizing mechanism of scapular inclination seems to be associated with the bony configuration and/or anatomy and biomechanical properties of the superior capsuloligamentous structures.

The purpose of this study was to determine the structural properties of the capsule of the glenohumeral joint. Twelve fresh frozen cadaveric shoulders were studied. Capsular strips were prepared from four different sites (anterior, posterior, superior, and inferior) of the capsule. One end of the capsular sections was left attached to the humerus, and the other excised was fixed in a clamp of an Instron universal testing machine. Maximum load, strength (maximum stress), and modulus of elasticity of these four capsular portions were measured. The most common mode of failure was tear at the midsubstance (68%), followed by tear at the clamp-capsule junction (23%), and detachment from the humerus (9%). The posterior capsule (1.0 +/- 0.4 mm) was thinner than the anterior (1.8 +/- 0.3 mm), superior (1.6 +/- 0.4 mm), and inferior capsule (1.5 +/- 0.3 mm). Among the four portions of the capsule, the posterior capsule showed the greatest strength (216.6 +/- 58.2 kg/cm2) and modulus of elasticity (683.1 +/- 228.8 kg/cm2), whereas the superior capsule showed the least strength (82.4 +/- 33.5 kg/cm2). There were no significant differences in maximum load. The greater strength of the posterior capsule may be one explanation for the low incidence of posterior shoulder dislocation.

Eleven fresh-frozen cadaver shoulders were studied to examine the influence of scapular inclination on inferior stability of the glenohumeral joint. All muscles except the rotator cuff were removed, and the capsule was vented. Inferior stability tests in the hanging position (sulcus test) and in 90° abduction (abduction inferior stability [ABIS] test) were simulated by the application of a 1.5 kg load with the scapula inclined at - 15°, 0°, 15°, and 30° in the sulcus test and at 15°, 30°, 45°, and 60° in the ABIS test. An electromagnetic tracking device was used to record the position of the humerus in relation to the glenoid. In the sulcus test all of the shoulders dislocated when the scapula was inclined at - 15°. However, when the scapula was inclined at 30°, no shoulder dislocated before loading, and one shoulder dislocated after loading. As a result both the loaded and unloaded positions of the humeral head shifted significantly to the superior direction as the scapular inclination increased (p < 0.0001). In the ABIS test, however, the positions of the humeral head shifted interiorly with an increase in scapular inclination (p < 0.0001), although none of the shoulders dislocated in any of the inclination angles. We conclude that scapular inclination contributes significantly to inferior stability of the glenohumeral joint. Increased scapular inclination prevents inferior displacement of the humeral head, probably because of a bony cam effect that causes tightening of the superior capsule.