Humeral position after reverse shoulder arthroplasty as measured by lateralization and distalization angles and association with acromial stress fracture; a case-control study

Abstract Background Reverse Shoulder Arthroplasty (RSA) has become increasingly utilized in the surgical management for a growing number of indications in increasingly younger and active populations. The complication of Acromial Stress Fracture (ASF) has been described in various patient groups after RSA. Patients who experience ASF after RSA report worse clinical outcomes. Biomechanical studies and some clinical studies have implicated the degree of lateralization and distalization of the humerus as factors associated with development of ASF. Most prior studies utilize scalar or linear methods to measure humeral position following RSA. These scalar measurements do not account for the relative effects of variances in patient stature. Lateralization Shoulder Angle (LSA) and Distalization Shoulder Angle (DSA) are angular methods that have been described as a reproducible assessment of humeral position while controlling for variance in patient stature. Methods A case-control study was performed where cases of ASF after RSA were identified after a retrospective review of all reverse shoulder arthroplasty cases at a single institution using a single implant type. A 1:3 case-control matching scheme was implemented based on age and gender. All surgeries were performed by one of 4 fellowship-trained Shoulder and Elbow surgeons. Preoperative and Postoperative radiographs were assessed and humeral position was measured using LSA and DSA. A Mixed Linear model was utilized to test the difference in means between cases and controls. Results Between January 2012 and December 2015, 689 patients underwent primary RSA. 29 patients (4.2%) sustained acromial stress fracture. Acromial stress fracture occurred at a mean of 7.6 months after surgery (range 1.0-36.8 months). According to the Levy classification, 19 were type 1 fractures, 9 were type 2 and 1 was type 3. An additional 24 patients (3.5%) patients were noted to have experience an acromial stress reaction without evidence for fracture. Mean change in LSA (from preoperative to postoperative) was -19.78 o (SE 2.82) in cases (net medialization) and -12.18 o (SE 1.63) in controls (P= .02). The mean change in DSA was 36.40 o (SE 2.09) in cases and 31.82° (SE 1.21) in controls (P= .06) Conclusion Higher preoperative LSA (humeral lateralization) and greater decrease of LSA (greater net medialization) after surgery were noted to have an association with ASF. Humeral position after RSA as measured by DSA was not associated with radiographically evident ASF in this case-control study controlling for age and gender. Level of Evidence Level III