The range of motion also was affected by the size of the osseous defect. The range of external rotation was restricted when the Bankart lesion was repaired in the presence of an osseous defect that was as large as or larger than that associated with osteotomy 2. This was because repairing the capsule back to the glenoid rim in the presence of an osseous defect was equivalent to tensioning the capsule by closing the gap created by the defect. Lusardi et al.13, in a clinical study of twenty shoulders (nineteen patients), noted that anterior capsulorrhaphy may result in severe limitation of external rotation. The relationship between the amount of imbrication of the capsule and the range of motion also has been investigated in cadaveric studies. Black et al.3, in a study of six cadaveric specimens, demonstrated that two millimeters of imbrication of the anterior part of the capsule caused a 40 percent decrease in external rotation and that seven millimeters of imbrication caused a 67 percent decrease. On the basis of those results, we calculated that the average limitation in external rotation was 25 degrees per centimeter of imbrication. Matsen et al.17 reported that shortening the anterior part of the capsule and the subscapularis tendon by one centimeter limited external rotation of the humerus by approximately 20 degrees. We hypothesized, on the basis of theoretical calculations, that one centimeter of shortening would limit external rotation by 360/(π x humeral head diameter) degrees. As the average diameter of the humeral head was 44.1 ± 4.4 millimeters, we calculated that one centimeter of shortening would limit external rotation by 26 degrees. In fact, the measured limitation was 25 degrees per centimeter of defect. Thus, our hypothesis was proved.
There have been many reports on the clinical results of the Bankart procedure. The presence of a glenoid fracture is known to cause chronic anterior instability2,8. Aston and Gregory2 reported the case of a patient who had recurrent anterior dislocations after sustaining a fracture that resulted in an anterior glenoid fragment involving slightly less than one-fourth of the articular surface. The shoulder did not redislocate after the fragment was fixed. Rowe et al.27 apparently were the first to describe the relationship between the size of the glenoid defect and the outcome of Bankart repair. Those authors did not find any significant difference among shoulders with various sizes of osseous defects involving one-sixth to one-third of the glenoid surface. Similarly, Protzman26 observed no relationship between the presence of a glenoid lesion and the outcome of a modified Bankart procedure. Thomas and Matsen30 and Wirth et al.35 performed a Bankart procedure without bone-grafting for all shoulders that had traumatic recurrent anterior dislocations with or without an osseous defect. The rates of recurrence were 2.6 percent (one of thirty-nine) and 0.9 percent (one of 108), respectively. On the basis of those reports, it appears that bone-grafting may not be necessary when an osseous defect involves less than one-third of the glenoid surface16,17,27.
In interpreting those reports, the greatest problem is that it is not clear how the investigators determined that the defect involved one-third of the glenoid surface. To the best of our knowledge, the only study to have included a quantitative assessment of the osseous defect of the glenoid was reported by Ungersböck et al.32, who evaluated the relationship between the width of the glenoid defect and the outcome of Bankart repair. In that study, none of twenty-six shoulders in which the osseous defect was less than three millimeters wide had a recurrence after the operation compared with one of three shoulders in which the osseous defect was at least three millimeters wide. Although the number of shoulders was small, the results of that study suggest the importance of the anteroinferior part of the glenoid rim to the stability of the shoulder after Bankart repair. The three-millimeter width cited by Ungersböck et al.32 is almost equivalent to the 2.8-millimeter width of the defect produced by osteotomy 1 in the present study. Although those authors reported one redislocation in a shoulder in which the defect was at least three millimeters wide, they did not specify the exact size of the defect in that particular case. In the present study, we expressed the size of the osseous defect as a percentage of the length of the glenoid. For the purpose of comparing our results with those in the literature, we also calculated the size of the defect as a percentage of the area of the entire glenoid surface (Table I). According to this conventional method, osteotomy 2, on which the stability of the repaired shoulder hinged, involved approximately one-fifth of the glenoid surface. To our surprise, however, it looked as if the defect produced by osteotomy 2 involved almost one-third of the glenoid surface. What Rowe et al.27 and others16,17 have described as one-third of the glenoid surface actually may be one-fifth. Thus, an area ratio, which has been commonly used, is not an accurate way of expressing the size of the defect because the determination of area is far more complex than the measurement of length. We believe that it is more accurate to express the size of a defect in terms of width (for example, 6.8 millimeters) or as a percentage of glenoid length (for example, 21 percent) rather than as an area ratio (for example, one-fifth).
The present study had several limitations. First, a simulated Bankart lesion is different from a Bankart lesion in vivo. Previous investigators have observed that a simulated Bankart lesion alone does not cause anterior dislocation1,28. They speculated that other factors, such as tearing of the posterior part of the capsule and functional deficiency of the rotator cuff muscles, also were responsible for anterior dislocation. In the present study, a simulated Bankart lesion was created not only by elevating the labrum from the glenoid but also by disrupting the continuity of the labrum. In our pilot study, we noticed that a shoulder did not become unstable even after the anteroinferior aspect of the labrum was totally elevated from the glenoid if the continuity of the labrum was preserved. As the humeral head passes through the space between the glenoid and the detached labrum during in vivo dislocation, the labrum must become either extremely elongated or disrupted. A review of the methods used by previous investigators1,28 to create a Bankart lesion suggests that the continuity of the labrum is one of the factors that may have affected the results. Elongation of the capsule in shoulders with recurrent anterior dislocation5,31,33 could be another factor that explains the discrepancy between the experimental and clinical settings.
Second, the orientation of the osseous defect in the present study may have been more inferiorly based than is the case in the clinical setting. The shoulder typically dislocates in the anteroinferior direction. However, as the glenoid is tilted anteriorly at approximately 20 degrees9, dislocation actually occurs in a slightly more anterior, rather than anteroinferior, direction in relation to the glenoid. We believed that this discrepancy would have little effect on the results as long as the osseous defect and the translation force were oriented in the same direction.
Third, we measured the range of rotation by applying the torque manually. This may have led to some error in the measurement of the range of motion. We tried to minimize the error by having the same individual (E. I.) apply the torque. The similarity between the range of motion that was measured during the experiment and the range that was predicted on the basis of our theoretical calculations indirectly proves the validity of the experiment.
Fourth, we limited the range of displacement to ten millimeters in order to avoid overloading the specimens. This range of displacement did not cause dislocation or subluxation. However, as the abducted and externally rotated shoulder was very stable in the anteroinferior direction when the capsulolabral structures were intact, the amount of increase in anteroinferior displacement observed in shoulders that had a defect with a width that was 21 percent of the glenoid length (average width, 6.8 millimeters) would be likely, in the clinical situation, to cause apprehension that the shoulder will subluxate anteriorly.
Finally, two test positions were chosen: (1) abduction and external rotation and (2) abduction and internal rotation. Some may argue that abduction and internal rotation is not the position of anterior instability. Others may argue that abduction and neutral rotation should have been chosen to test the effect of glenoid concavity. We had planned to perform the displacement test with the arm in neutral rotation as well as in external and internal rotation. However, in neutral rotation, the head was too unstable to be tested. For example, after osteotomy 4, the head was on the verge of the articular surface of the glenoid and it dislocated as soon as it was moved anteroinferiorly because there was no effect of ligamentous tension. Because of the fifty-newton axial load, the dislocated head could not be reduced during backward movement and, thus, the test had to be discontinued. As our major interest was the effect of ligamentous tension in shoulders with and without an osseous defect, we decided not to perform the displacement test with the arm in neutral rotation. Instead, gross stability with the arm in neutral rotation was checked in a qualitative fashion. Again, the stability of the glenohumeral joint depends on osseous concavity and ligamentous tension. We acknowledge that testing one mechanism in a quantitative way but the other in a qualitative way is a drawback of this study.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding source was the National Institutes of Health.
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