Passive glenohumeral joint stabilization: A biomechanical study

Passive glenohumeral joint stability was tested in 10 cadaveric shoulder specimens, before and after venting of the intraarticulor space. Force-displacement diagrams were measured with anterior, posterior, and inferior excursion, in neutral position, in 90° of abduction, and in a combination of 90° of abduction and 90° of external rotation. Displacement at 50 Ns before venting averaged 11.17 mm posteriorly (SD = 6.48 mm), 7.15 mm anteriorly (SD = 5.51 mm), and 3.41 mm inferiorly (SD = 3.37 mm). Venting of the joint increased displacement by 47% anteriorly, 49% posteriorly, and 61 % inferiorly. In addition, mathematic model calculation was used to estimate the destabilizing forces at the glenohumeral joint during physiologic arm motion. This force approximated 970 N, cldarly more than passive stabilizers alone seem to be able to provide. It is therefore assumed that, in addition to passive joint stability, active stabilization plays a major role at the glenohumeral joint.     

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.     

Postimpingement instability following reverse shoulder arthroplasty: a parametric finite element analysis

BackgroundInstability following reverse shoulder arthroplasty is influenced by various factors such as component design, component positioning, and soft tissue tensioning. Patients may achieve glenohumeral motion beyond initial scapular impingement during activities of daily living which could further compound instability. However, instability/subluxation risk postscapular impingement is not well documented. Conventional range of motion analysis tools cannot account for the restraining effect of soft tissues or subluxation risk after impingement. Using a previously validated finite element analysis approach, the purpose of this study was to investigate the effects of glenoid component lateralization and humeral component angle of inclination (AOI), with or without simulated subscapularis repair, on postimpingement subluxation. We hypothesized that lack of subscapularis repair, a valgus humeral component AOI, and glenoid medialization would all result in greater postimpingement instability.MethodsA FE model of the shoulder including the subscapularis tendon and middle deltoid was created, incorporating a general representation of a commercial reverse shoulder arthroplasty implant placed under the direction of a fellowship-trained shoulder surgeon. The deltoid and subscapularis were tensioned and wrapped around the reconstructed glenohumeral joint prior to simulating motion. Humeral rotations were then prescribed to simulate external rotation (neutral to 50°), extension (neutral to 50°), adduction (neutral to 30°), and abduction (neutral to 90°). The effects of three glenosphere lateralization offsets (2, 4, and 10 mm) and 2 humeral liner angles of inclination (varus-150° and valgus-155°) on subluxation propensities were investigated with and without the subscapularis tendon present.ResultsSimulated subscapularis repair resulted in 21%-34% less postimpingement subluxation. Presence of the subscapularis provided stability over a greater range of abduction. Impingement-free range of motion was similar regardless of the presence or absence of the subscapularis. The valgus AOI resulted in 23% less subluxation during abduction. During other motions however, the valgus AOI resulted in 67%-110% greater postimpingement subluxation (subscapularis present), which further worsened without the subscapularis.ConclusionImplant design modifications to improve stability may not be beneficial for all motions, highlighting the importance of directionality when investigating instability. Liner-bone impingement appears to compound instability/subluxation and the subscapularis appears to restrain postimpingement instability.Level of evidenceBasic Science Study; Computer Modeling     

Anterior shoulder stability: Contributions of rotator cuff forces and the capsular ligaments in a cadaver model

The purpose of this study was to quantify in a biomechanical model the contributions to shoulder joint stability that are made by tensions in the four tendons of the rotator cuff and by static resistance of defined portions of the capsular ligaments. A materials testing machine was used to directly determine anterior joint laxity by measurement of the force required to produce a standard anterior subluxation. Shoulders were tested in external or neutral humeral rotation. Data were analyzed by multiway analysis of variance with regression analysis. This model simulated tensions in the rotator cuff musculature by applying static loads at the tendon insertion sites acting along the anatomic lines of action. A load in any of the cuff tendons resulted in a measurable and statistically significant contribution to anterior joint stability. The contributions between different tendons were not significantly different and did not depend on the humeral rotation (neutral or external). In neutral humeral rotation the superior and middle glenohumeral ligaments together function equally with the inferior glenohumeral ligament as primary stabilizers against anterior humeral translation. The posterior capsule is a secondary stabilizer. The external rotation of the abducted humerus increases anterior stability by more than doubling the stability contribution from the inferior glenohumeral ligament. The stability contribution from the posterior capsule is larger in external rotation than in neutral rotation but is still of secondary magnitude. In external rotation the stability contribution of the anterior capsule, including the superior glenohumeral ligament and the middle glenohumeral ligament, becomes insignificant. The model presented here simulates the combined effect of two major sources of shoulder stability. This versatile model permits the direct measurement of the contributions to anterior shoulder stability that are made by tensions in the rotator cuff tendons and by static resistance of defined capsular zones. The use of multiple regression analysis-a standard statistical technique but one relatively new to the orthopaedic literature-permits quantitative determination of the contribution of each independent variable to the dependent variable, shoulder stability.     

Effect of rotator interval closure on glenohumeral stability and motion: a cadaveric study

The effect of rotator interval closure, which is performed as an adjunct to arthroscopic stabilization of the shoulder, has not been clarified. Fourteen fresh-frozen cadaveric shoulders were used. The position of the humeral head was measured using an electromagnetic tracking device with the capsule intact, sectioned, and imbricated between the superior glenohumeral ligament and the subscapularis tendon (SGHL/SSC closure) or between the superior and middle glenohumeral ligaments (SGHL/MGHL closure). The direction of translational loads (10, 20, and 30 N) and arm positions were (1) anterior, posterior, and inferior loads in adduction; (2) anterior load in abduction/external rotation in the scapular plane; and (3) anterior load in abduction/external rotation in the coronal plane. The range of motion was measured using a goniometer under a constant force. Both methods reduced anterior translation in adduction. Only SGHL/MGHL closure reduced anterior translation in abduction/external rotation in the scapular plane and posterior translation in adduction. Both methods reduced the range of external rotation and horizontal abduction. Rotator interval closure is expected to reduce remnant anterior/posterior instability and thereby improve the clinical outcomes of arthroscopic stabilization procedures.     

Dynamic capsuloligamentous anatomy of the glenohumeral joint

Though many anatomic and biomechanical studies have been performed to elucidate capsuloligamentous anatomy of the glenohumeral joint, no previous studies have evaluated capsuloligamentous anatomy during rotator cuff contraction. The purpose of this study was to define and document the orientation and interrelationship between the glenohumeral ligaments during simulated rotator cuff contraction. Six fresh cadaveric shoulders were arthroscoped to document and grade ligamentous anatomy. The superior and middle glenohumeral ligaments and the anterior and posterior bands of the inferior glenohumeral ligament complex were labeled by an arthroscopicassisted technique with a linked metallic bead system. Shoulders were then placed onto an experimental apparatus that simulated rotator cuff function through computer-controlled servo-hydrolic actuators attached to the rotator cuff and biceps by a clamp and cable-and-pulley system. Simulated rotator cuff action and manual placement allowed shoulders to be placed into three positions of rotation (neutral, internal, and external) in three positions of scapular plane abduction (0°, 45°, 90°). Anteroposterior and axillary lateral plane radiographs were taken in each position to document orientation of all four ligaments. Both the superior and middle glenohumeral ligaments were maximally lengthened in 0° and 45° abduction and external rotation and appeared to shorten in all positions of abduction. The anterior and posterior bands of the inferior glenohumeral ligament complex maintained a cruciate orientation in all positions of abduction in the anteroposterior plane, except at 90° abduction and external rotation, where they are parallel. This cruciate orientation is due to the different location of the glenoid origin and humeral insertion of each band and may allow reciprocal tightening of each during rotation. The glenohumeral capsule is composed of discreet ligaments that undergo large charges in orientation during rotation. The superior and middle glenohumeral ligaments appear to complement the inferior glenohumeral ligaments, with the former tightening in adduction and the latter tightening in abduction. This relationship permits the large range of motion normally seen in the glenohumeral joint.     

Functional study of glenohumeral ligaments

BackgroundThe glenohumeral ligaments are passive stabilising anatomical structures of the shoulder which, in synergy with the other active and passive stabilising structures, enable joint movement and cohesion. The purpose of this study is to analyse the isolated and synergic function of the glenohumeral ligaments by using a tetrapolar detection system with computer analysis.MethodsIn a study performed on cadavers after anatomical dissection, detector electrodes were positioned on the individual ligaments and recordings were made of bioelectric impedance and, consequently, the resistance, which is an indicator of the state of tension or relaxation of the ligamentous complex. Predefined positions of the upper limb were adopted—neutral adduction, adduction with external rotation, abduction at 45° with neutral and external rotation, and abduction at 90° with neutral and external rotation.ResultsThe superior glenohumeral ligament is important in stabilisation of the glenohumeral joint in adduction and external rotation. The middle glenohumeral ligament is an important stabilising structure in the positions of adduction and external rotation and abduction up to 45° in external rotation. The resistance, and therefore tension, of the inferior glenohumeral ligament, which is negligible in positions of neutral adduction and adduction in external rotation, increases in value for angles between 45° and 90°, indicating the important stabilising function of this ligament in those positions.ConclusionOur experimental study on cadavers, which involved evaluating the resistance of the glenohumeral ligaments by means of tetrapolar detection and computer analysis of the results, contributes to our knowledge of the functional activity of the anterior portion of the joint capsule.     

Impingement of the deep surface of the subscapularis tendon and the reflection pulley on the anterosuperior glenoid rim: a preliminary report

Sixteen patients underwent detailed arthroscopic evaluation; all had moderate to severe, primarily unexplained shoulder pain provoked by anterior elevation and internal rotation, and all were nonresponsive to subacromial injection of local anesthetic. None of the patients had any symptoms or signs of instability. Partial subscapularis lesions were documented in 10 of 13 patients who had undergone preoperative arthro-magnetic resonance imaging. At arthroscopy, an isolated lesion of the common humeral insertion of the superior glenohumeral and coracohumeral ligaments (a so-called pulley lesion) was found in 3 cases, a lesion associating a pulley and an articular side partial subscapularis lesion in 10 cases, and an isolated articular side partial subscapularis tear in 3 cases. The most painful movement, which consisted of flexion and internal rotation, caused impingement of the involved ligamentous and/or capsular insertions in all patients. If the arm was elevated above 90 degrees, the zone of mechanical contact was between the long head of the biceps and the pulley region and superiormost aspect of the labrum; if elevation was decreased, impingement occurred between the tendinous insertion of the subscapularis and the anterior glenoid labrum and rim. This study suggests that in addition to the posterosuperior impingement of the supraspinatus tendon originally described by Walch, anterosuperior impingement of the deep surface of the subscapularis is a form of intraarticular impingement responsible for painful structural disease 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.     

Repair of tears of the subscapularis

BACKGROUND: Rotator cuff tears involving the subscapularis are less common than those involving the superior aspect of the rotator cuff. The purpose of the present study was to report the results of repair of isolated tears of the subscapularis. METHODS: The records on eighty-four shoulders that had undergone open repair of the subscapularis tendon were reviewed. The mean age of the patients at the time of surgery was 53.2 years. The mean interval from the onset of symptoms to the time of surgery was 12.5 months. Fifty-seven tears were traumatic, and twenty-seven were degenerative. Twenty-three tears involved the superior one-third of the subscapularis tendon, forty-one tears involved the superior two-thirds, and twenty tears were complete. Fifty-four shoulders had a dislocation or subluxation of the long head of the biceps tendon, and ten shoulders had a rupture of the long head of the biceps tendon. Forty-eight shoulders underwent concomitant biceps tenodesis, thirteen shoulders underwent concomitant biceps tenotomy, and four shoulders underwent concomitant recentering of the biceps. Patients were evaluated clinically and radiographically at a mean of forty-five months (range, twenty-four to 132 months) postoperatively. RESULTS: The mean Constant score increased from 55.0 points preoperatively to 79.5 points postoperatively. Seventy-five patients were satisfied or very satisfied with the result. Preoperatively, four shoulders had mild glenohumeral arthritis. Postoperatively, twenty-five shoulders had mild glenohumeral arthritis and two shoulders had moderate glenohumeral arthritis. Tenodesis or tenotomy of the biceps tendon at the time of subscapularis repair was associated with improved subjective and objective results, independent of the preoperative condition of the biceps tendon. CONCLUSIONS: Repair of isolated subscapularis tears yields acceptable improvement in shoulder function in selected patients. Additionally, the results of the present study support routine tenodesis or tenotomy of the long head of the biceps tendon at the time of subscapularis repair.     

Effect of joint compression on inferior stability of the glenohumeral joint

To determine the relative importance of negative intraarticular pressure, capsular tension, and joint compression on inferior stability of the glenohumeral joint we studied 17 fresh, normal adult cadaver shoulders using a "3 degrees of freedom" shoulder test apparatus. Translations were measured in intact and vented shoulders while a 50-N superior and inferior directed force was applied to the shoulder. Three different joint compressive loads (22 N, 111 N, 222 N) were applied externally. Tests were performed in 3 positions of humeral abduction in the scapular plane (0 degree, 45 degrees, 90 degrees) and in 3 positions of rotation (neutral, maximal internal, and maximal external). After tests of the intact and vented shoulder, the glenohumeral ligaments were sectioned and tests were repeated. With minimal joint compression of 22 N, negative intraarticular pressure and capsular tension limited translation of the humeral head on the glenoid. Increasing the joint compressive load to 111 N resulted in a reduction of mean inferior translation from 11.0 mm to 2.0 mm at 0 degree abduction, from 21.5 mm to 1.4 mm at 45 degrees abduction, and from 4.5 mm to 1.2 mm at 90 degrees abduction. With a compressive load of 111 N, venting the capsule or sectioning of glenohumeral ligaments had no effect on inferior stability. Clinical relevance: Glenohumeral joint compression through muscle contraction provides stability against inferior translation of the humeral head, and this effect is more important than negative intraarticular pressure or ligament tension.     

In situ force distribution in the glenohumeral joint capsule during anterior-posterior loading.

Our objective was to examine the function of the glenohumeral capsule and ligaments during application of an anterior-posterior load by directly measuring the in situ force distribution in these structures as well as the compliance of the joint. We hypothesized that interaction between different regions of the capsule due to its continuous nature results in a complex force distribution throughout the glenohumeral joint capsule. A robotic/universal force-moment sensor testing system was utilized to determine the force distribution in the glenohumeral capsule and ligaments of intact shoulder specimens and the joint kinematics resulting from the application of external loads at four abduction angles. Our results suggest that the glenohumeral capsule carries no force when the humeral head is centered in the glenoid with the humerus in anatomic rotation. However, once an anterior-posterior load is applied to the joint, the glenohumeral ligaments carry force (during anterior loading, the superior glenohumeral-coracohumeral ligaments carried 26+/-16 N at 0 degrees and the anterior band of the inferior glenohumeral ligament carried 30+/-21 N at 90 degrees). Therefore, the patient's ability to use the arm with the humerus in anatomic rotation should not be limited following repair procedures for shoulder instability because the repaired capsuloligamentous structures should not carry force during this motion. Separation of the capsule into its components revealed that forces are being transmitted between each region and that the glenohumeral ligaments do not act as traditional ligaments that carry a pure tensile force along their length. The interrelationship of the glenohumeral ligaments forms the biomechanical basis for the capsular shift procedure. The compliance of the joint under our loading conditions indicates that the passive properties of the capsule provide little resistance to motion of the humerus during 10 mm of anterior or posterior translation with anatomic humeral rotation. Finally, this knowledge also enhances the understanding of arm positioning relative to the portion of the glenohumeral capsule that limits translation during examination under anesthesia.     

Muscle activity during shoulder dislocation

EMG activity from eight shoulder muscles in parallel was recorded from 4 patients with generalized joint laxity. During external rotation of the humerus in 45° abduction, humeroscapular dislocation occurred in 2 patients and subluxation in the 2 others. The activity level in the subscapularis was low, and the activation speed was slow. The low muscle activity and delay in activation of the subscapularis muscle may contribute to the instability.     

Muscle activity during shoulder dislocation

EMG activity from eight shoulder muscles in parallel was recorded from 4 patients with generalized joint laxity. During external rotation of the humerus in 45 degrees abduction, humeroscapular dislocation occurred in 2 patients and subluxation in the 2 others. The activity level in the subscapularis was low, and the activation speed was slow. The low muscle activity and delay in activation of the subscapularis muscle may contribute to the instability.     

Tendons, ligaments, and capsule of the rotator cuff. Gross and microscopic anatomy.

We investigated the structure of the myotendinous rotator cuff in thirty-two grossly intact cuffs from thirty fresh cadavera of subjects who had been seventeen to seventy-two years old at the time of death. We studied the gross anatomy of the capsule and ligaments of the cuff, as well as histological sections of the tendons of the subscapularis, supraspinatus, and infraspinatus muscles. The tendons were found to splay out and interdigitate to form a common, continuous insertion on the humerus. The biceps tendon was ensheathed by interwoven fibers derived from the subscapularis and supraspinatus tendons. The anterior margin and bursal surface of the supraspinatus tendon were enveloped by a thick sheet of fibrous tissue derived from the coracohumeral ligament. Fibers from the coracohumeral and glenohumeral ligaments were found concentrated in a plane between the capsule and the tendons of the cuff. Microscopically, in the region of the supraspinatus and infraspinatus tendons, the cuff was composed of five layers defined by the attachments and orientations of the fibrous elements in each of these layers.     

A modified Bristow-Helfet-May procedure for recurrent dislocation and subluxation of the shoulder. Report of two hundred and twelve cases

A modified Bristow-Helfet-May procedure was performed for recurrent dislocation or subluxation of the glenohumeral joint in 207 patients (212 shoulders), whose average age at the time of surgery was 20.3 years (range, fourteen to forty-seven years). The procedure was modified by directing the coracoid segment and conjoined tendon over the superior border rather than through the substance of the subscapularis tendon and muscle. The indications were either documented recurrent anterior dislocation of the glenohumeral joint or subluxation with instability as demonstrated on examination with the patient under anesthesia. The procedure was evaluated on the basis of the rate of recurrence of dislocation and subluxation, postoperative complications, the patients' subjective evaluation, and the effect of the procedure on the motion of the glenohumeral joint and the strength of the muscles of the shoulder as related to overhead throwing. Eight (3.8 per cent) of the shoulders redislocated and ten (4.7 per cent) had one or more subjective episodes of subluxation after the procedure. Complications included postoperative infection in two patients and problems with the screw that required its removal in ten. One hundred and thirty-one (62 per cent) of the patients responded to a questionnaire regarding their subjective evaluation of the results of surgery. Eleven (8 per cent) were unable to perform daily activities that involved overhead work, and forty-five (34 per cent) stated that they still had some degree of discomfort or pain in the shoulder. One hundred and twenty-six patients (96.2 per cent) stated that they were happy with the results of the surgery and would have the procedure again. Thirty patients had Cybex testing of the muscles of the shoulder. Only three (16 per cent) of the nineteen athletes whose dominant arm had been operated on returned to their pre-injury level of throwing. Data obtained with regard to changes in the range of motion and strength of the glenohumeral joint indicate that this loss of throwing ability was not due solely to a loss of glenohumeral motion. It appeared to be also related to a concomitant loss of strength at the extreme of external rotation of the humerus and the initiation of internal rotation of the humerus.     

Humeral avulsion of glenohumeral ligaments

Humeral avulsion of glenohumeral ligaments (HAGL) is an increasingly recognized cause of recurrent shoulder instability. HAGL lesions are the result of acute traumatic glenohumeral subluxation or dislocation. Anterior avulsion of the inferior glenohumeral ligament from the humeral neck is the more common lesion; however, posterior lesions are seen as well. Careful history and physical examination are critical in the diagnosis of HAGL lesions. MRI is the best imaging study for diagnosing these lesions. Injection of intra-articular contrast dye aids in visualization. Most HAGL lesions cause recurrent instability and require surgical repair. Arthroscopic repair with the use of accessory portals has yielded promising results. Excellent results have been achieved with open surgical management using a subscapularis incision. Mini-open techniques involve limited incision in the lower one half of the subscapularis.     

The immediate effects of soft tissue mobilization with proprioceptive neuromuscular facilitation on glenohumeral external rotation and overhead reach

STUDY DESIGN: Randomized controlled 2-group. pretest-posttest, multivariate study of patients with shoulder musculoskeletal disorders. OBJECTIVES: The purpose of this study was to evaluate the immediate effect of soft tissue mobilization (STM) with proprioceptive neuromuscular facilitation (PNF) to increase glenohumeral external rotation at 45 degrees of shoulder abduction and overhead reach. BACKGROUND: It is postulated that limitation in glenohumeral external rotation, when measured at 45 degrees of shoulder abduction, represents subscapularis muscle flexibility deficits and is associated with the inability to fully reach overhead. No research, however, is available to demonstrate whether intervention strategies intended to improve subscapularis flexibility and glenohumeral external rotation range of motion at 45 degrees of shoulder abduction will improve a patient's ability to reach overhead. METHODS AND MEASURES: Twenty patients (10 males, 10 females; age range, 21-83 years) with limited glenohumeral external rotation and overhead reach of 1 year duration or less served as subjects. The subjects were randomly assigned to a treatment group, which consisted of soft tissue mobilization to the subscapularis and proprioceptive neuromuscular facilitation to the shoulder rotators, or a control group. Goniometric measurements of glenohumeral external rotation at 45 degrees abduction and overhead reach were taken preintervention and immediately postintervention for the treatment group or at prerest and postrest periods for the control group. RESULTS: The treatment group improved by a mean of 16.4 degrees (95% confidence interval [CI, 12.5 degrees-20.3 degrees) of glenohumeral external rotation, as compared to less than a 1 degree gain (95% CI, -0.2 degrees-2.0 degrees) in the control group (P < .0005). Overhead reach in the treatment group improved by a mean of 9.6 cm (95% CI, 5.2-14.0 cm) in comparison to a mean gain of 2.4 cm (95% CI, -0.8-5.6 cm) for the control group (P = .009). CONCLUSION: These findings suggest that a single intervention session of STM and PNF was effective for producing immediate improvements in glenohumeral external rotation and overhead reach in patients with shoulder disorders.     

Muscle activity and coordination in the normal shoulder. An electromyographic study

Muscle activity and coordination in ten shoulders were studied in five healthy subjects using electromyography (EMG) recorded during standardized loaded movements, i.e., flexion, extension, abduction, external rotation, and internal rotation at 0 degrees, 45 degrees, and 90 degrees of abduction. Bipolar surface and intramuscular fine-wire electrodes were used, and the EMG signal was low-pass filtered, full-wave rectified, and time-averaged. Activity from the subscapularis, supraspinatus, infraspinatus, pectoralis major (sternoclavicular part), the anterior, middle, and posterior parts of the deltoid, and the latissimus dorsi was recorded in parallel. In order to allow a comparison of the activity in a subject's different muscles and the activity in specific muscles between different individuals, the EMG was normalized. Muscle activity occurred simultaneously in muscles producing the movement and in antagonistic muscles. Coordination due to muscle contractions plays a significant role in stabilizing the shoulder joint. The infraspinatus, subscapularis, and latissimus dorsi acted as stabilizers during flexion; the subscapularis acted as a stabilizer during external rotation and with the supraspinatus during extension.     

Anatomy and functional aspects of the rotator interval

The anatomy of the region between the supraspinatus and subscapularis tendons, called the rotator interval, was studied in 22 shoulders of 12 cadavers. Its function was then examined by sequential cutting of tendon or rotator interval structures. The rotator interval was found to be composed of parts of the supraspinatus, subscapularis, coracohumeral ligament, superior glenohumeral ligament, and glenohumeral joint capsule. A medial part composed of 2 layers was defined and distinguished from a more lateral part composed of 4 layers. The most superior 3 layers of the lateral part formed a fibrous plate. The medial part was found to primarily limit inferior translation and, to a lesser extent, external rotation. The fibrous plate of the lateral part of the rotator interval mainly limited external rotation of the adducted arm. The coracohumeral ligament played a key role in external rotation as well as in inferior translation.