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

Background: An osseous defect of the glenoid rim is sometimes caused by mul tiple 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,vith regard to how large a defect must be in order to necessitat e this procedure. Some investigators have proposed that a defect must invol ve at least one-third of the glenoid surface in order to necessitate bone-g rafting. However, it is difficult to determine (1) whether a defect involve s 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 purp oses 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 defe cts on the stability and motion of the shoulder after Bankart repair Methods: The glenoids from sixteen dried scapulae were photographed, and th e 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 desi gn custom templates for the purpose of creating various sizes of osseous; d efects. Ten fresh-frozen cadaveric shoulders then were obtained from indivi duals who had been an average of seventy-nine years old at the time of deat h, and all muscles were removed to expose the joint capsule. With use of a custom multiaxis electromechanical testing machine,vith a six-degrees-of-fr eedom load-cell, the humeral head was translated ten millimeters in the ant eroinferior direction,vith the arm in abduction and external rotation as we ll as in abduction and internal rotation. With a fifty-newton axial force c onstantly 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 hume ral head a normalized distance was determined under eleven sequential condi tions: (1) with the capsule intact, (2) after the creation of a simulated B ankart lesion, (3) after the capsule was repaired, (4) after the creation o f an anteroinferior osseous defect with a width that was 9 percent of the g lenoid length (average,width, 2.8 millimeters), (5) after the capsule was r epaired, (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) afte r the capsule was repaired, (8) after the creation of an osseous defect,vit h a width that was 34 percent of the glenoid length (average width, 10.8 mi llimeters), (9) after the capsule was repaired, (10) after the creation of an osseous defect,vith a,width that was 46 percent of the glenoid length (a verage 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 o f 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 th e 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 gl enoid length (average width, 6.8 millimeters) was significantly smaller tha n the force in shoulders without an osseous defect. The range of external r otation 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 should ers,without a defect (p < 0.0001) because of the pretensioning of the capsu le caused by closing the gap between the detached capsule and the glenoid r im. 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 t he shoulder after Bankart repair. Clinical Relevance: The results of the present study suggest that measures to restore the are of glenoid concavity may be beneficial, in terms of both stability and motion, for patients who have a glenoid defect,vith a width that is at least 21 percent of the glenoid length.