Eiji Itoi

Stabilising function of the biceps in stable and unstable shoulders

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.

Shoulder muscle moment arms during horizontal flexion and elevation.

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.

Effect of Back-Strengthening Exercise on Posture in Healthy Women 49 to 65 Years of Age

OBJECTIVEnTo evaluate the effect of back-strengthening exercise on posture in 60 healthy estrogen-deficient women.nnnDESIGNnThe 60 study subjects were randomly assigned to either an exercise or a control group, and various factors were assessed at time of enrollment in the study and at 2-year follow-up.nnnMATERIAL AND METHODSnThe 32 women in the exercise group were instructed in progressive back-strengthening exercises, whereas the 28 women in the control group had no exercise prescription and were asked to continue their usual physical and dietary activities. At baseline and 2-year follow-up examinations, back extensor strength was measured with a strain-gauge dynamometer, and lateral roentgenograms of the thoracic and lumbar areas of the spine were obtained to measure the angles of thoracic kyphosis, lumbar lordosis, and sacral inclination. The changes in radiographic measurements and back extensor strength were analyzed statistically.nnnRESULTSnBack extensor strength increased significantly in both the exercise and the control groups, but no radiographic measurements were significantly different between these groups. The significant increase in back extensor strength in both groups of healthy women suggested that the original grouping did not accurately reflect the amount of exercise. Thus, the 60 subjects were reclassified for comparison on the basis of increase in back extensor strength--27 with more than or equal to the mean increase of 21.1 kg and 33 with less than 21.1 kg. Furthermore, each of these groups of subjects was subdivided on the basis of degree of thoracic kyphosis. Among the subjects with substantial thoracic kyphosis, those with a significant increase in back extensor strength had a significant decrease in thoracic kyphosis (-2.8 +/- 4.2 degrees; P = 0.041), whereas those with a small increase in strength had a nonsignificant increase in thoracic kyphosis (1.8 +/- 5.3 degrees). The increase in back extensor strength did not seem to affect mild degrees of kyphosis.nnnCONCLUSIONnIncreasing the back extensor strength in healthy estrogen-deficient women helps decrease thoracic kyphosis.

The relevance of the moment arm of shoulder muscles with respect to axial rotation of the glenohumeral joint in four positions

OBJECTIVEnThis study was undertaken to determine the efficiency of the shoulder girdle muscles during axial humeral rotation based on measurements of the moment arms.nnnDESIGNnThe instantaneous muscle moment arms of 10 shoulder muscles, including the three portions of the deltoid, the rotator cuff muscles, teres major, and the thoracohumeral muscle group, were measured during four specified glenohumeral rotations.nnnBACKGROUNDnAxial humeral rotation is a commonly performed movement during activities of daily living and is a targeted motion of shoulder rehabilitation, particularly in those protocols emphasizing rotator cuff strengthening. An understanding of the function of the movers and stabilizers of the shoulder requires such basic information of muscle moment arms.nnnMETHODSnThe instantaneous moment arm values of the muscles were derived from the slope of the plot of tendon excursion versus glenohumeral joint rotation angle. Motion studied included axial rotation with the humerus elevated 90 degrees in the coronal, scapular, and sagittal planes, as well as in the neutral position with the arm at the side.nnnRESULTSnBased on the findings, with the humerus in both neutral and elevated positions, the infraspinatus is potentially the most powerful external rotator, followed by teres minor and posterior deltoid. Subscapularis and possibly pectoralis major are the most effective internal rotators in this position.nnnCONCLUSIONSnThe moment arm in providing axial humeral rotation of 10 shoulder muscles in four planes were obtained. In general, the teres minor and infraspinatus had the largest moment arms in external rotation, and the subscapularis had the largest moment arm in internal rotation. The muscle function for axial humeral rotation was found to be modified by the plane of arm elevation.nnnRELEVANCEnThe data could be used for developing exercise programs in physical therapy.

Scapular inclination and inferior stability of the shoulder

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.

Dynamic anterior stabilisers of the shoulder with the arm in abduction

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.

Superior-Inferior Stability of the Shoulder: Role of the Coracohumeral Ligament and the Rotator Interval Capsule

OBJECTIVEnTo study the superior-inferior stabilizing functions of the coracohumeral ligament (CHL) and the rotator interval capsule (RIC) with use of a material testing machine.nnnMATERIAL AND METHODSnThe axial translations of the humerus with the superior-inferior translation force of 30 N applied were recorded under the following joint capsule conditions: (1) intact, (2) vented, (3) the CHL sectioned, and (4) the RIC incised in six cadaver shoulders. The order of sectioning was changed for conditions 3 and 4 in six other cadaver shoulders.nnnRESULTSnWith the arm in internal and neutral rotations, venting the capsule significantly increased the superior-inferior translation, which was unaffected by further sectioning of the CHL and the RIC. With the arm in external rotation, only the CHL contributed significantly to inferior stability, whereas both this ligament and the RIC contributed to superior stability to a lesser degree.nnnCONCLUSIONnThe CHL is a stabilizer in superior inferior directions with the arm in external rotation, and the intra-articular pressure that is maintained by the intact RIC is a stabilizer in superior-inferior directions with the arm in internal and neutral rotations. These findings may provide a scientific background to support closure of the interval space to stabilize the shoulder and may explain part of the superior instability observed in shoulders with rotator cuff tears.

Motion and laxity of the capitellocondylar total elbow prosthesis

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.

Biomechanical investigation of the glenohumeral joint

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.

Dynamic Stabilizing Function of the Deltoid Muscle in Shoulders with Anterior Instability

Background The stabilizing role of the deltoid muscle has not been extensively studied. Purpose To study the contribution of the deltoid muscle to anterior stability of the shoulder. Study Design Controlled laboratory study. Methods We used nine fresh cadaveric shoulders with the arm at 90° of abduction and 90° of external rotation. The position of the humeral head was monitored by an electromagnetic tracking device with 0 and 1.5 kg of anterior translation force; with 0, 1, 3, and 5 kg of force applied to each of the anterior, middle, and posterior portions of the deltoid muscle; and with the capsule intact, vented, and with a simulated Bankart lesion. Results With the capsule intact, anterior displacement was significantly reduced by application of load to the middle deltoid muscle. After the capsule was vented, load application to the anterior, middle, or posterior deltoid muscle significantly reduced anterior displacement. With a simulated Bankart lesion, effects of muscle loading were most apparent: anterior displacement was significantly reduced with loading of each muscle portion. Conclusion The deltoid muscle is an anterior stabilizer of the glenohumeral joint with the arm in abduction and external rotation. Clinical Relevance The stabilizing function of the deltoid muscle takes on more importance as the shoulder becomes unstable.