Anatomic restoration of lateral humeral offset and humeral retroversion optimizes functional outcomes following reverse total shoulder arthroplasty

Cadaveric and computer simulations suggest lateral humeral offset (LHO) and humeral retroversion (HR) are associated with strength and range of motion (ROM) after reverse total shoulder arthroplasty (rTSA), but in vivo data is lacking. This study aimed to evaluate the effects of implant parameters (i.e. LHO and HR) on strength and ROM. LHO and HR were measured using pre-operative and post-operative computed tomography (CT) scans. Postoperative strength was measured across three planes of motion using a Biodex isokinetic dynamometer. Postoperative active and passive ROM during forward elevation, external rotation (ER), and internal rotation (IR) were assessed using a goniometer or spinal level. 30 rTSA patients (14 M, 16F, age: 71.8 ± 6.7yrs) participated with an average postoperative follow-up of 2.4 ± 1.1 years. Regarding strength, higher post-op LHO values were predictive of greater postoperative strength across all movements. However, lateralization of the implant beyond pre-op values (i.e. post-op LHO > pre-op LHO) was associated with poorer strength performance across all ranges of motion. Similar to strength outcomes, greater deviations from pre-op LHO was predictive of poorer IR ROM. Lastly, patients with minimal deviations in HR (post-op HR within 10° of pre-op HR) and minimal deviations in LHO (post-op LHO ≤ pre-op LHO) displayed the greatest postoperative ER ROM. Anatomic restoration of LHO combined with anatomic restoration of HR may be ideal for maximizing strength and ROM following rTSA. Overlateralization beyond anatomic may have negative consequences. Optimal implant lateralization and version may need to be individualized based on preoperative values.