The effect of a glenoid defect on anteroinferior stability of the shoulder after Bankart repair: A cadaveric study

Background: An osseous defect of the glenoid rim is sometimes caused by multiple recurrent dislocations of the shoulder. It is generally thought that a large defect should be treated with bone-grafting, but there is a lack of consensus with regard to how large a defect must be in order to necessitate this procedure. Some investigators have proposed that a defect must involve at least one-third of the glenoid surface in order to necessitate bone- grafting. However, it is difficult to determine (1) whether a defect involves one-third of the glenoid surface and (2) whether a defect of this size is critical to the stability of the shoulder after a Bankart repair. The purposes of the present study were (1) to create and quantify various sizes of osseous defects of the glenoid and (2) to determine the effect of such defects on the stability and motion of the shoulder after Bankart repair. Methods: The glenoids from sixteen dried scapulae were photographed, and the images were scanned into a computer. The average shape of the glenoid was determined on the basis of the scans, and this information was used to design custom templates for the purpose of creating various sizes of osseous defects. Ten fresh-frozen cadaveric shoulders then were obtained from individuals who had been an average of seventy-nine years old at the time of death, and all muscles were removed to expose the joint capsule. With use of a custom multiaxis electromechanical testing machine with a six-degrees-of- freedom load-cell, the humeral head was translated ten millimeters in the anteroinferior direction with the arm in abduction and external rotation as well as in abduction and internal rotation. With a fifty-newton axial force constantly applied to the humerus in order to keep the humeral head centered in the glenoid fossa, the peak force that was needed to translate the humeral head a normalized distance was determined under eleven sequential conditions: (1) with the capsule intact, (2) after the creation of a simulated Bankart lesion, (3) after the capsule was repaired, (4) after the creation of an anteroinferior osseous defect with a width that was 9 percent of the glenoid length (average width, 2.8 millimeters), (5) after the capsule was repaired, (6) after the creation of an osseous defect with a width that was 21 percent of the glenoid length (average width, 6.8 millimeters), (7) after the capsule was repaired, (8) after the creation of an osseous defect with a width that was 34 percent of the glenoid length (average width, 10.8 millimeters), (9) after the capsule was repaired, (10) after the creation of an osseous defect with a width that was 46 percent of the glenoid length (average width, 14.8 millimeters), and (11) after the capsule was repaired. Results: With the arm in abduction and external rotation, the stability of the shoulder after Bankart repair did not change significantly regardless of the size of the osseous defect (p = 0.106). With the arm in abduction and internal rotation, the stability decreased significantly as the size of the osseous defect increased (p < 0.0001): the translation force in shoulders in which the width of the osseous defect was at least 21 percent of the glenoid length (average width, 6.8 millimeters) was significantly smaller than the force in shoulders without an osseous defect. The range of external rotation in shoulders in which the width of the osseous defect was at least 21 percent of the glenoid length was significantly less than that in shoulders without a defect (p < 0.0001) because of the pretensioning of the capsule caused by closing the gap between the detached capsule and the glenoid rim. The average loss of external rotation was 25 degrees per centimeter of defect. Conclusions: An osseous defect with a width that is at least 21 percent of the glenoid length may cause instability and limit the range of motion of the shoulder after Bankart repair. Clinical Relevance: The results of the present study suggest that measures to restore the arc of glenoid concavity may be beneficial, in terms of both stability and motion, for patients who have a glenoid defect with a width that is at least 21 percent of the glenoid length.