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Objectives: Bone loss in anterior glenohumeral instability has significant clinical implications and is responsible for surgical failure. Previous work has focused on glenoid and humeral head defects separately. There is no prior biomechanical work evaluating the combined effect of these lesions. The purpose of this study is to determine the effect of the combination of humeral head and glenoid bone loss on glenohumeral joint translation in a bipolar bone loss model with a Bankart lesion and after Bankart repair. Our hypothesis is that when both humeral head and glenoid defects occur together, the amount of bone loss required to compromise soft tissue Bankart repair is less than compared to when glenoid and humeral head lesions occur in isolation. Methods: Eight cadaveric shoulders were dissected to expose the intact capsule and glenohumeral joint. The set-up and testing was as described by Itoi ,Sekiya and Yamamoto1,2,3.The humeral shaft and scapula were potted and the shoulder mounted on a custom shoulder testing device permitting 6 degrees of freedom. The test positions were 60 degrees of glenohumeral abduction and 60 degrees of external rotation. A 50N compressive load was applied to the glenohumeral joint. A MTS 858 Servohydraulic test system (MTS Systems, Eden Prairie, MN) was used to translate the humeral head anteriorly 10mm at a rate of 2.0mm/sec. The peak force required to translate the humeral head 10mm was recorded. Three trials were performed in each condition, and the mean value was used for data analysis. All Bankart lesions were made sharply from the 2 o’clock to 6 o’clock position for a right shoulder. Bankart repair was made with transosseous tunnels using high strength suture. A digital micrometer was used to measure and create glenoid defects with a saw parallel to the anterior glenoid. Hill-Sachs lesions were made from 3D models created from a clinical database of computerized tomographic images of 142 patients with shoulder instability (Figure 1). A bell-shape curve was created and the 50th percentile lesion for size and location was selected, 1.47 cm3. Testing was conducted in the following sequence for each specimen:(1) intact (2)posterior capsulotomy, (3)bankart lesion, (4)bankart repair, (5) 2mm glenoid defect with Bankart lesion, (6)bankart repair, (3) Hill Sachs (1.47cm3) lesion, bankart lesion and 2mm defect, (7)bankart repair, (8) 4mm defect with Hill Sachs defect and Bankart lesion,(9)bankart repair, (10)6mm glenoid defect with Hill Sachs, defect and Bankart lesion (11)bankart repair. Results: A linear mixed-effects approach was used to examine force needed to translate for each condition. A statistically significant reduction in force needed to translate was observed with Bankart repair performed with a 1.47 cm3 Hill-Sachs defect and a 2mm (p=0.01) and 4mm (p=0.04) glenoid bone loss compared to Bankart repair with glenoid bone loss alone (Figure 2). A 2mm glenoid defect with the Hill-Sachs defect resulted in a 25% reduction in stability. A 6mm glenoid defect with the Hill-Sachs lesion resulted in a 50% reduction in load to translation compared to a Bankart repair with no bone loss. Conclusion: As little as a 2mm glenoid defect together with a Hill-Sachs lesion of the dimensions observed in 50% of an actual clinical practice will compromise a soft tissue Bankart repair alone. Combined lesions , even when small, may require surgical strategies that address bone defects to optimize outcomes.

The Effect of Combined Glenoid and Humeral Head Defects on Glenohumeral Translation