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Dynamic Contributions to Superior Shoulder Stability A.M Journal of Ort hopaed ic Research ELSEVIER Journal of Orthopaedic Research 19 (1001) 206-212 www.elsevier.nl/locate/orthres Dynamic contributions to superior shoulder stability A.M. Halder, K.D. Zhao, S.W. O'Driscoll, B.F. Morrey, K.N. An * hfajo Clinic, Orthopedic Biamechanic~Lahouatory, ,700 First Street S U', Rochrster, hfN 5590.5, LISA Received 1 November 1999; accepted 24 April 2000 Abstract 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 O", 30°, 60" and 90" 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 f 2.2 mm) and the teres major (5.1 k 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 i2.0 mm) and sub- scapularis (4.7 i 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. Published by Elsevier Science Ltd on behalf of Orthopaedic Research Society. Introduction ruption on the bursa1 and articular side [21], as well as sclerosis of the acromion and thickening of the coraco- Rotator cuff lesions are among the most common acromial ligament [4]. This would further narrow the shoulder diseases. However, the origin of rotator cuff acromio-humeral interval initiating a vicious cycle degeneration is still unclear. Possible explanations in- eventually leading to rotator cuff rupture. However, clude intrinsic factors such as decreased circulation in superior decentralization of the humeral head may be the critical area with limited vascularization [29] or the cause as well as the result of rotator cuff thinning. changes in collagen synthesis and turnover [30]. Ex- Several studies identified static structures contribut- trinsic factors include increased shear and compressive ing to superior stability of the glenohumeral joint. In forces as a result of a narrowed acromio-humeral in- conjunction with the insertion of the long head of biceps terval [24,31]. It is probable that a combination of in- tendon. the glenoid labrum has a stabilizing effect [26]. trinsic and extrinsic factors is present prior to rotator The size of the rotator cuff interval also seems to be cuff failure [32]. important [ 131. Negative intraarticular pressure has a A decrease in acromio-humeral distance may be due stabilizing effect in all directions but only at low loads to osteophytes on the undersurface of the acromion or ~141. hypertrophy of the coracoacromial ligament [ 1,9,22], as Few studies have focused on dynamic stabilizers of well as superior migration of the humeral head the glenohumeral joint in the superior-inferior direction [7,34,31,391. Superior decentralization of the humeral [6,11,24,31.34,37]. Their results are controversial and the head may increase shear and compressive forces on the relative contributions to superior stability of different rotator cuff tendons. This potentially causes fiber dis- shoulder muscles are not yet known. However, this would be of tremendous importance in order to address the most effective muscles in conservative treatment of the impingement syndrome as well as postoperative re- habiliation. Furthermore, it would help to direct rotator *Corresponding author. Tel.: + 1-507-281-2262; fax: + 1-507-284- 5392. cuff surgery to the most relevant structures. The objec- E-ma2 address: aii.kainan(a~mayo.edu(K.N. An). tive of this study was to investigate the contribution of 0736-0266/01/$ - see front matter Published by Elsevier Science Ltd on behalf of Orthopaedic Research Society. PII: S0736-0266(00)00028-0 A. hf. Hulder rt al. I Journal of'Orthopaedic Rrsrurch 19 (2001) 206-212 207 different muscles to superior gelnohumeral stability. Chester, Vermont, USA) measured the three-dimensional positions and orientations of sensors attached to the specimen in relation to the These contributions may be relevant to our under- source of electromagnetic waves. standing of the etiology and prevention of rotator cuff The glenohumeral joint was positioned according to laser-pointers degeneration. which were located superiorly and laterally and whose beam inter- sected at the center of rotation of the hinged arc. The scapulae were aligned and rigidly mounted in the shoulder-testing device so that the medial margin of the scapula was in line with the vertical axis of the Materials and methods device, and the humeral head was in the center of the pivoting arc. Nylon loops were sutured to the tendons and were connected to the Ten fresh frozen cadaver shoulders without rotator cuff tears or pneumatic actuators by strings that were guided by pulleys. The po- radiological evidence of glenohumeral osteoarthritis were used in the sitions of the pulleys were carefully adjusted so that the strings imitated study. During dissection, preparation and testing the specimens were muscle lines of action by running through the centroids of each muscle moistened using physiologic saline solution to prevent dehydration. All [16]. Finally, 3Space'" sensors were attached to the proximal humerus soft tissues superficial to the muscles were removed except rotator cuff close to the head and to the spine of the scapula. The source of the muscles, teres major, latissimus, deltoid and pectoralis major muscles electromagnetic waves was rigidly mounted in line with the vertical and their respective tendons. All muscles, except the deltoid, were el- axis of the shoulder-testing device. evated from the bone and resected at the musculotendinous junction. To selectively investigate the stabilizing function of the tested The deltoid was preserved. Their tendons were carefully separated muscles only translations of the humeral head relative to the glenoid from surrounding soft tissue to ensure unrestricted movement. Nylon were allowed while the humerus was kept in a fixed degree of abduc- strings were sutured to each margin of the flat tendons to allow even tion in the scapular plane. As tightness of the ligaments in the extremes loading. Fiberglass rods were cemented into the medullary canals of of motion kept in the muscles from generating vertical translations the proximal humeri to control position in the frontal plane. Thinner [13,38] the humerus was locked in neutral rotation. The experiment rods were fixed in drill holes perpendicular to the humeral shaft in the started in the hanging arm position and was performed in 30", 60". and neutral position employing the bicipital groove as a landmark to 90" of glenohumeral abduction (Fig. 2). The joints were confirmed to control rotation. The scapulae were mounted onto a Plexiglas plate. be vented prior to testing. Of the muscles tested, the pectoralis major The shoulder-testing device (Fig. 1) was made of non-metal mate- and the latissimus did not originate from the scapula. Therefore, ad- rials to avoid interference with the electromagnetic tracking device. A justing the pulleys according to the scapular tilt [28] changed their lines hinged Plexiglas are attached to the humerus allowed control of the of action. degree of abduction. Through variable-position pulleys, strings from The potentials of different muscles to reverse superior translation of the tendons were connected to pneumatic actuators (Airpot Corpo- the humeral head were measured. Superior translation of the humeral ration, Norwalk, Connecticut, USA). A commercially-available com- head was accomplished by application of a constant superior force of puter controlled by Labview'" software (National Instruments 20 N. The depressor effects of the supraspinatus, infraspinatus, teres Corporation 1994, Austin, Texas, USA) drove electro-pneumatic minor, teres major, latissimus, superior and inferior part of the sub- valves (Proportion-Air 1997, McCordsville, Indiana, USA) to load the scapularis, pectoralis major, and long head of biceps were tested. The pneumatic actuators. Custom-made load cells on each cylinder verified tendons were loaded sequentially in random order in line of muscle the applied loads. A 3Space'" Fastrak system (Polhemus 1993, Col- action and proportional to their respectivc physiologic cross-sectional Pulley Actuator Weight Actuator Control System Fig. 1. Shoulder testing device. A hinged Plexiglas arc attached to the humerus allowed control of abduction and rotation. Strings from the tendons were connected to pneumatic actuators through pulleys that were variable in position. A commerically-available computer drove electropneumatic valves to load the pneumatic actuators. Custom-made load cells on each cylinder verified the applied loads. A 3Space'" Fastrak system attached to the specimen measured the three-dimensional positions and orientation of sensors in relation to the source of electromagnetic waves. A.M. Hulder et ul. I Journul of Orthopurdic Rescurch 19 (2001)206-212 3-Space Sensor Humerus Scapula 3-Space Sensor Pulley String 3-Space Sensor Ninged Arc Pulley Weight Actuator Fig. 2. Testing of the infraspinatus muscle in the hanging arm position. The humertls was locked in neutral rotation. A constant superior force of 20 N was applied to the humerus, and the tendon was loaded in line of muscle action proportional to its cross-sectional area by computer-controlled pneumatic actuators.
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