Rotator Cuff Deficiency Research Articles

An assessment of the inter examiner reliability of clinical tests for subacromial impingement and rotator cuff integrity

Over 30 separate clinical signs for the shoulder have been described, most with little evidence to support their accuracy and reliability. The aim of our study was to evaluate the accuracy and reliability of some of the commonly used tests for rotator cuff disease. Two clinicians, a consultant with an established shoulder practice and a registrar with an interest in shoulder surgery, examined 63 patients with history suggestive of rotator cuff disease. A set of pre-determined clinical tests for impingement syndrome and rotator cuff tear were standardised and agreed upon before the study was commenced. The examination included eliciting a painful arc, the drop arm test, the Neer’s and the Hawkin’s sign for impingement syndrome. Integrity of the individual components of the rotator cuff was assessed by the strength of abduction initiation and at 90° abduction for supraspinatus tear (Jobe’s test), Speed’s and Yergason’s test for biceps pathology, strength of shoulder external rotation for infraspinatus tears and the Gerber lift-off test for subscapularis tears. Accuracy assessments were determined by comparing clinical findings against findings at arthroscopy in 50 of the 63 patients. The two observers did not differ significantly in their assessments. Un-weighted kappa values defining agreement between the two observers and the positive predictive values were evaluated. Our findings indicate that clinical signs can be relied upon for diagnosis of impingement but not for rotator cuff deficiency. Compared with other previously published studies, our study demonstrates that the inter observer agreement is better when the assessors have a special interest and understanding of shoulder disorders.

Reverse Shoulder Arthroplasty for the Treatment of Rotator Cuff Deficiency

Early designs of reverse shoulder arthroplasty components for the treatment of glenohumeral arthritis associated with severe rotator cuff deficiency in some cases have been associated with mechanical failure. The purpose of this study was to perform a prospective outcomes study of reverse shoulder arthroplasty performed with use of 5.0-mm peripheral locking screws for baseplate fixation and a lateralized center of rotation for the treatment of a rotator cuff deficiency. From February 2004 to March 2005, 112 patients (114 shoulders) were treated with a reverse shoulder arthroplasty as part of a United States Food and Drug Administration Investigational Device Exemption study. Ninety-four patients (ninety-six shoulders) were available for a minimum follow-up of two years. Of the ninety-six shoulders, thirty-seven had a primary rotator cuff deficiency, thirty-three had a previous rotator cuff operation, twenty-three had a previous arthroplasty, and three had a proximal humeral nonunion. The patients were prospectively followed clinically (the American Shoulder and Elbow Surgeons [ASES] score, the Simple Shoulder Test [SST], and self-reported satisfaction) and radiographically (mechanical failure, loosening, and notching). Patients were videotaped while performing a standard active range-of-motion protocol before and after treatment. These videos were then analyzed in a blinded fashion by three independent observers using a digital goniometer. At two years, the average total ASES scores had improved from 30 preoperatively to 77.6; the average ASES pain scores, from 15 to 41.6; and the average SST scores, from 1.8 to 6.8 (p < 0.0001 for all). Blinded analysis of range of motion showed that average abduction improved from 61 degrees preoperatively to 109.5 degrees (p < 0.0001); average flexion, from 63.5 degrees to 118 degrees (p < 0.0001); and average external rotation, from 13.4 degrees to 28.2 degrees (p < 0.0001). The patients rated the outcome as excellent in fifty-three shoulders (55%), good in twenty-six (27%), satisfactory in eleven (12%), and unsatisfactory in six (6%). There was no evidence of mechanical failure of the baseplate or scapular notching in any of the patients. Six of the ninety-four patients in this study had a complication. Recent advances in reverse shoulder arthroplasty have allowed for improvement in patient outcomes while minimizing early mechanical failure and scapular notching and decreasing the overall complication rate at short-term follow-up.

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.

Contribution of the Reverse Endoprosthesis to Glenohumeral Kinematics

After placement of a reverse shoulder endoprosthesis, range of motion is usually still compromised. To what extent this occurs from limitation in motion of the reverse endoprosthesis is, however, unclear. We measured the motion pattern of 16 patients (18 shoulders) during three active and passive range of motion tasks using a six degree-of-freedom electromagnetic tracking device. Despite rotator cuff deficiencies, glenohumeral elevation contributed roughly two-thirds of the total thoracohumeral elevation, which is comparable to healthy subjects. However, patients could not actively use the full range of motion provided by the prosthesis. Although we found considerable interindividual differences in shoulder kinematics, the limitation in glenohumeral range of motion appears related to a lack of generated muscle force and not the design of the prosthesis.

Use of the reverse shoulder prosthesis for the treatment of failed hemiarthroplasty in patients with glenohumeral arthritis and rotator cuff deficiency

Use of the reverse shoulder prosthesis for the treatment of failed hemiarthroplasty in patients with glenohumeral arthritis and rotator cuff deficiency

Glenohumeral Arthrodesis After Failed Prosthetic Shoulder Arthroplasty

While there have been numerous reports concerning glenohumeral arthrodesis for many indications, there is little available information specific to glenohumeral arthrodesis performed after failed prosthetic shoulder arthroplasty. The purpose of this study was to report the outcomes of glenohumeral arthrodesis in the setting of severe glenohumeral bone loss and deltoid muscle and rotator cuff insufficiency following failed prosthetic shoulder arthroplasty. We retrospectively reviewed clinical and radiographic data on seven consecutive patients treated with glenohumeral arthrodesis following a failed prosthetic shoulder arthroplasty between 1997 and 2004. The average duration of clinical follow-up was four years (range, 1.5 to eight years). Five of the seven patients demonstrated an intact fusion at the time of the latest follow-up. Four of the seven patients had undergone additional bone-grafting procedures in an effort to obtain union. Two of these patients ultimately had a persistent nonunion despite the additional procedures for bone-grafting and revision of the fixation hardware. Overall, the average subjective clinical outcome score (Penn Shoulder Score) improved significantly from 17 points (range, 8 to 33 points) to 58 points (range, 31 to 77 points) (p = 0.008). The most common complication was delayed union requiring additional procedures for bone-grafting and revision of the fixation hardware. Treatment of a failed prosthetic shoulder arthroplasty with concomitant extensive glenohumeral bone loss and soft-tissue deficiencies is extremely challenging. The results of this study suggest that glenohumeral arthrodesis can yield satisfactory clinical outcomes. However, both the patient and the surgeon should be aware of the complex nature of this surgery and the frequent need for additional surgical procedures to obtain fusion.

A Shoulder Hemiarthroplasty with a Tendon Muscle Transfer for Glenohumeral Arthritis associated with Severe Rotator Cuff Deficiency

A Shoulder Hemiarthroplasty with a Tendon Muscle Transfer for Glenohumeral Arthritis associated with Severe Rotator Cuff Deficiency

In vitro kinematics of the shoulder following rotator cuff injury

Tears in the rotator cuff may result in altered mechanics of the glenohumeral joint. It is not unusual for some patients with cuff tears to exhibit near normal motion with the injured shoulder, while other patients with the same magnitude of injury are unable to achieve full elevation on the injured side. The effect of simulated tears of the rotator cuff on active glenohumeral joint kinematics was investigated by testing eight cadaveric specimens using an in vitro shoulder simulator. Active abduction of the humerus was produced by applying forces to simulate loading of the supraspinatus, subscapularis, infraspinatus/teres minor, and the anterior, middle, and posterior thirds of the deltoid. Three sequential 1cm lesions were created in the rotator cuff, the first two in the supraspinatus tendon and the third in the subscapularis tendon. The plane of abduction moved posteriorly and became more abnormal throughout abduction with increased tear size. No difference was observed in the internal/external rotation of the humerus or the position of the humeral head on the glenoid during elevation. In order to generate the motions achieved by the intact joint, patients with rotator cuff insufficiency likely employ other muscle groups. Retraining muscle groups surrounding the glenohumeral joint may decrease the need for surgical interventions.

Die luxierte Schulterprothese – eine vermeidbare Katastrophe?

Instability after shoulder arthroplasty is a common problem, even though complete dislocations are rare. A distinction can be made between vertical and horizontal instabilities. The most common type of vertical instability is superior migration of the humeral head caused by rotator cuff insufficiency; the shoulder prosthesis should be changed for an inverse prosthesis if the patient is symptomatic. Horizontal instabilities can certainly lead to acute dislocation, but it is far more common for them to result in eccentric loading of the glenoid and in turn to increased wear and loosening. When a prosthesis is first implanted it is essential to reproduce the original bony situation before the deformity caused by arthrosis, arthritis or fracture, as this is the only way to prevent instability. This requires careful preoperative planning including evaluation of CT or MRI scans so that during the operation it will be possible, for example, to reorientate an eccentrically torn glenoid using a bone graft or by eccentric reaming and restore the original torsion. At least as much importance attaches to the treatment of the soft tissue, meaning careful release and later closure of the rotator cuff and capsule complex, as to the bony situation. In the authors' own institution 190 prostheses were implanted between 2000 and 2006 and there were three dislocations (1.6%).

Rotator Cuff Tear Arthropathy

Rotator cuff tear arthropathy represents a spectrum of shoulder pathology characterized by rotator cuff insufficiency, diminished acromiohumeral distance with impingement syndromes, and arthritic changes of the glenohumeral joint. Additional features may include subdeltoid effusion, humeral head erosion, and acetabularization of the acromion. Although the progression of rotator cuff tears seems to play a role in the development of cuff tear arthropathy, information is lacking regarding the natural progression of rotator cuff tears to cuff tear arthropathy. Controversy remains about the role of basic calcium phosphate crystals in the development of cuff tear arthropathy. Nonsurgical management is the first line of treatment in most patients. Traditionally, surgical management of rotator cuff tear arthropathy has been disappointing because of the development of complications long-term and poor patient satisfaction with functional outcomes. Recent studies, however, report promising experience with reverse ball-and-socket arthroplasty.

Impact of Fatty Infiltration of the Teres Minor Muscle on the Outcome of Reverse Total Shoulder Arthroplasty

Reports have demonstrated that reverse shoulder arthroplasty restores overhead elevation but fails to restore active external rotation. The teres minor muscle-tendon unit contributes to active external rotation, and its deficiency may impair the clinical outcome. It was therefore the purpose of this study to evaluate the influence of fatty infiltration of the teres minor muscle on the clinical outcome after reverse total shoulder replacement. Forty-two shoulders in forty-two patients (average age, seventy-one years) with painful cuff tear arthropathy or an irreparable rotator cuff deficiency with pseudoparesis were treated with a reverse Delta-III shoulder arthroplasty and followed clinically for a minimum of twenty-four months. Preoperatively, fatty infiltration of the teres minor was assessed, according to the grading system of Goutallier et al., with use of magnetic resonance imaging. The effect of teres minor fatty infiltration on the subjective and objective outcomes of the reverse shoulder arthroplasty was evaluated. The thirty shoulders with stage-0, 1, or 2 fatty infiltration of the teres minor muscle (group 1) had a significantly better ultimate Constant score, a significantly better subjective shoulder value, and significantly greater preoperative-to-postoperative improvement than the twelve shoulders with stage-3 or 4 fatty infiltration (group 2). In group 1 the relative Constant score increased by an average of 41% and the subjective shoulder value increased by an average of 44%, whereas in group 2 the respective increases were 32% (p = 0.033) and 25% (p = 0.018). Group 1 had an average increase of 6.2 points in the score for extremity positioning, whereas group 2 gained only 5.3 points (p = 0.033). Group 1 had a net gain of 9 degrees of external rotation with the arm at the side compared with an average net loss of 7 degrees in group 2 (p < 0.001). Stage-3 or 4 fatty infiltration of the teres minor compromises the clinical outcome of reverse shoulder arthroplasty for the treatment of irreparable rotator cuff tears.

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Introduction: Arthrodesis of the glenohumeral joint for severe pain and disability after failed glenohumeral arthroplasty is extremely difficult. This is a retrospective review of the use of bulk allograft bone to treat this challenging patient population. Methods: Twelve patients with failed shoulder arthroplasty ages 54-78 with deltoid, rotator cuff deficiency, or pain underwent arthrodesis of the shoulder using a proximal humeral or proximal femoral allograft. The allografts were augmented with autogenous iliac crest bone graft in all cases.

Instability After Shoulder Replacement: Rockin’ &amp; Rollin’

Instability after shoulder replacement is a rare but serious complication. Instability can be due to component malpositioning, bone deficiency, improper soft tissue balancing or soft tissue defects. Posterior glenoid wear, abnormal humeral version, humeral shortening or soft tissue deficiency such as subscapularis insufficiency can result in instability. Patients with rotator cuff deficiency may experience anterosuperior escape as well. Soft tissue balancing should be addressed at the time of surgery. It is important to recognize all of these factors so that each can be addressed at the time of revision and also to help prevent instability during primary surgery.

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Objective: To assess the short-term clinical and radiographic outcomes of the reverse total shoulder arthroplasty. Methods: After FDA approval of the reverse total shoulder arthroplasty (TSA) in 2004, a consecutive series of the first sixty-two patients treated with this prosthesis by a single surgeon was prospectively initiated through an IRB-approved protocol. All patients presented with signs and symptoms of painful glenohumeral arthritis combined with severe rotator cuff deficiency, anterosuperior shoulder instability, and/or a painful, dysfunctional failed shoulder arthroplasty.

Predictors of Scapular Notching in Patients Managed with the Delta III Reverse Total Shoulder Replacement

The reverse Delta III shoulder prosthesis can relieve pain and restore function in patients with cuff tear arthropathy. The most frequently reported radiographic complication is inferior scapular notching. The purpose of the present study was to evaluate the clinical relevance of notching and to determine the anatomic and radiographic parameters that predispose to its occurrence. Seventy-seven consecutive shoulders in seventy-six patients with an irreparable rotator cuff deficiency were managed with a reverse Delta III shoulder arthroplasty and were followed clinically and radiographically for a minimum of twenty-four months. The effects of cranial-caudal glenoid component positioning and the prosthesis-scapular neck angle on the development of inferior scapular notching and clinical outcome were assessed. All shoulders that had development of notching did so in the first fourteen months. Of the seventy-seven shoulders that were studied, thirty-four (44%) had inferior scapular notching, twenty-three (30%) had posterior notching, and six (8%) had anterior notching. Osteophytes along the inferior part of the scapula occurred in twenty-one (27%) of the seventy-seven shoulders. The angle between the glenosphere and the scapular neck (r = 0.667) as well as the craniocaudal position of the glenosphere (r = 0.654) were highly correlated with inferior notching (p < 0.001). A notching index was calculated with use of the height of implantation of the glenosphere and the postoperative prosthesis-scapular neck angle. This allowed prediction of the occurrence of notching with a sensitivity of 91% and specificity of 88%. The height of implantation of the glenosphere had approximately an eight times greater influence on inferior notching than the prosthesis-scapular neck angle did. Inferior scapular notching was associated with a significantly poorer clinical outcome. Inferior scapular notching after reverse total shoulder arthroplasty adversely affects the intermediate-term clinical outcome. It can be prevented by optimal positioning of the glenoid component.

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Introduction: The purpose of this study was to determine if glenosphere tilt relative to the scapular spine in the coronal plane would predispose fixation to mechanical failure. We reviewed the radiographs of 262 patients treated with a Reverse Shoulder Prosthesis (RSP) for rotator cuff insufficiency in the setting of glenohumeral arthritis. The spinospheric angle, the arc subtended by the baseplate and scapular spine in the coronal plane, was measured for each patient. ASES scores, range of motion and catastrophic failures of baseplate loosening were also recorded. Methods: Post-operative radiographs of 262 patients treated with an RSP were retrospectively identified and their spinospheric angles measured and recorded. Measurements were made on digital, Grashey view radiographs using a computer guided goniometer (Medview 5.1; StorComm Inc, Jacksonville, FL). Average time to follow up was 27 months (range 6-84 months). Three blinded observers, none of whom were the primary surgeon, were utilized. Inter-observer scores of reliability and variation coefficients were also calculated. Results: Coronal plane tilt measured 71.8° (range 50°-96°, SD 8.2°) for the 262 patients. In the ten failures mean tilt was 77° (range 71°-84°, SD 5°). This difference was significant (p=0.0099). The time from implantation to failure ranged from 15 to 55 months (mean 32 months). There were 66 patients whose spinospheric angle was 78° or greater. The mean ASES score of 64.6 for this group did not differ significantly from the mean of 64.5 (p=0.95) calculated for the 196 patients with a tilt angle of 77° or less. Range of motion values were also not significantly different between the two groups. Interobserver reliability between the three observers was strong with an average coefficient of variation of 3.1%. Conclusion: Patients who have their glenosphere tilted superiorly have an increased risk of catastrophic mechanical failure. Patients at risk for early failure appear clinically no different from patients not predisposed to catastrophic loosening. If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use). If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use). If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use).

Use of the reverse shoulder prosthesis for the treatment of failed hemiarthroplasty in patients with glenohumeral arthritis and rotator cuff deficiency

We report the use of the reverse shoulder prosthesis in the revision of a failed shoulder hemiarthroplasty in 19 shoulders in 18 patients (7 men, 11 women) with severe pain and loss of function. The primary procedure had been undertaken for glenohumeral arthritis associated with severe rotator cuff deficiency. Statistically significant improvements were seen in pain and functional outcome. After a mean follow-up of 44 months (24 to 89), mean forward flexion improved by 26.4 degrees and mean abduction improved by 35 degrees . There were six prosthesis-related complications in six shoulders (32%), five of which had severe bone loss of the glenoid, proximal humerus or both. Three shoulders (16%) had non-prosthesis related complications. The use of the reverse shoulder prosthesis provides improvement in pain and function for patients with failure of a hemiarthroplasty for glenohumeral arthritis and rotator cuff deficiency. However, high rates of complications were associated with glenoid and proximal humeral bone loss.

The Use of the Reverse Shoulder Prosthesis for the Treatment of Failed Hemiarthroplasty for Proximal Humeral Fracture

Humeral hemiarthroplasty is an established treatment for patients with selected fractures of the proximal part of the humerus. However, a subset of patients have development of glenoid arthritis and rotator cuff deficiency due to tuberosity failure. To date, there has been no reliable salvage procedure for this problem. Over a period of five years, twenty-nine patients (twenty-five women and four men) with a mean age of sixty-nine years (range, forty-two to eighty years) were managed with removal of a hemiarthroplasty prosthesis and revision with a Reverse Shoulder Prosthesis alone or in combination with a proximal humeral allograft. Patients were followed clinically and radiographically for an average of thirty-five months. All patients were evaluated with use of the American Shoulder and Elbow Surgeons score; the Simple Shoulder Test; range-of-motion measurements, including abduction, forward flexion, and external rotation; and a rating scale for overall satisfaction with the outcome of the surgery. Patients were assessed preoperatively and at all follow-up points beginning at three months postoperatively. The average total American Shoulder and Elbow Surgeons score improved from 22.3 preoperatively to 52.1 at the time of the last follow-up (p < 0.001). The average American Shoulder and Elbow Surgeons pain score improved from 12.2 to 34.4 (p < 0.001), and the average American Shoulder and Elbow Surgeons function score improved from 10.1 to 17.7 (p = 0.058). The average Simple Shoulder Test score improved from 0.9 to 2.6 (p = 0.004). Forward flexion improved from 38.1 degrees to 72.7 degrees (p < 0.001), and abduction improved from 34.1 degrees to 70.4 degrees (p < 0.001). The overall complication rate was 28% (eight of twenty-nine). At the time of the latest follow-up, sixteen patients rated the outcome as good or excellent, seven rated it as satisfactory, and six were dissatisfied. Four of the six patients who were dissatisfied had been managed with a Reverse Shoulder Prosthesis alone. The Reverse Shoulder Prosthesis offers a salvage-type solution to the problem of failed hemiarthroplasty due to glenoid arthritis and rotator cuff deficiency following tuberosity failure. The early results reported here are promising. In cases of severe proximal humeral bone deficiency, augmentation of the Reverse Shoulder Prosthesis with a proximal humeral allograft may improve patient satisfaction.

The Use of the Reverse Shoulder Prosthesis for the Treatment of Failed Hemiarthroplasty for Proximal Humeral Fracture

Background: Humeral hemiarthroplasty is an established treatment for patients with selected fractures of the proximal part of the humerus. However, a subset of patients have development of glenoid arthritis and rotator cuff deficiency due to tuberosity failure. To date, there has been no reliable salvage procedure for this problem. Methods: Over a period of five years, twenty-nine patients (twenty-five women and four men) with a mean age of sixty-nine years (range, forty-two to eighty years) were managed with removal of a hemiarthroplasty prosthesis and revision with a Reverse Shoulder Prosthesis alone or in combination with a proximal humeral allograft. Patients were followed clinically and radiographically for an average of thirty-five months. All patients were evaluated with use of the American Shoulder and Elbow Surgeons score; the Simple Shoulder Test; range-of-motion measurements, including abduction, forward flexion, and external rotation; and a rating scale for overall satisfaction with the outcome of the surgery. Patients were assessed preoperatively and at all follow-up points beginning at three months postoperatively. Results: The average total American Shoulder and Elbow Surgeons score improved from 22.3 preoperatively to 52.1 at the time of the last follow-up (p < 0.001). The average American Shoulder and Elbow Surgeons pain score improved from 12.2 to 34.4 (p < 0.001), and the average American Shoulder and Elbow Surgeons function score improved from 10.1 to 17.7 (p = 0.058). The average Simple Shoulder Test score improved from 0.9 to 2.6 (p = 0.004). Forward flexion improved from 38.1° to 72.7° (p < 0.001), and abduction improved from 34.1° to 70.4° (p < 0.001). The overall complication rate was 28% (eight of twenty-nine). At the time of the latest follow-up, sixteen patients rated the outcome as good or excellent, seven rated it as satisfactory, and six were dissatisfied. Four of the six patients who were dissatisfied had been managed with a Reverse Shoulder Prosthesis alone. Conclusions: The Reverse Shoulder Prosthesis offers a salvage-type solution to the problem of failed hemiarthroplasty due to glenoid arthritis and rotator cuff deficiency following tuberosity failure. The early results reported here are promising. In cases of severe proximal humeral bone deficiency, augmentation of the Reverse Shoulder Prosthesis with a proximal humeral allograft may improve patient satisfaction. Level of Evidence: Therapeutic Level IV. See Instructions to Authors for a complete description of levels of evidence.

Biomechanical considerations of the normal and rotator cuff deficient shoulders and the reverse shoulder prosthesis

Summary The treatment of combined glenohumeral arthritis and massive rotator cuff deficiency remains a unique challenge. Prosthetic reconstruction is aimed at restoring the normal kinematics of the glenohumeral joint. The reverse shoulder prosthesis medialises the centre of rotation of the glenohumeral joint, thereby lengthening the deltoid lever arm. The centre of rotation is also fixed at the glenoid bone prosthesis interface and this has the effect of reducing the torque on the glenoid component and lessens the risk of glenoid loosening. To further enhance the function of deltoid, the humerus is lowered relative to the glenoid, thereby increasing the deltoid tension. Although the preliminary results have been encouraging, there are still some unresolved issues such as prosthetic joint instability, scapular notching, wear of the polyethylene cup, fixation of the humeral stem and the lack of external rotation following surgery.