Background: Reverse total shoulder arthroplasty (RTSA) is dependent upon the deltoid for overhead function.  Achieving satisfactory range of motion following RTSA can be unpredictable.  Little is known how deltoid muscle volume affects range of motion and outcomes following RTSA.  Initially used to treat rotator cuff deficient conditions the RTSA indications have expanded for use in rotator cuff intact conditions.  We sought to investigate the role of deltoid volume on range of motion and outcomes following RTSA in both rotator cuff intact and deficient conditions. We hypothesized that deltoid volume would directly correlate with range of motion after RTSA with a deficient rotator cuff.

Methods: IRB approval was obtained for the study prior to review of records.  Patients who underwent primary RTSA over a 5-year period from 2010 to 2015 were retrospectively obtained and reviewed. Inclusion criteria included patients with at least 2-year postoperative clinical follow-up including range of motion measurements and outcome scores including American Shoulder and Elbow Surgeons Shoulder (ASES) and Single Assessment Numerical Evaluation (SANE) scores, preoperative computed tomography (CT) and/or magnetic resonance imaging (MRI), and were performed for indications that did not disrupt proximal humeral anatomy.  Final analysis included a total of 107 patients which met inclusion criteria. The rotator cuff integrity was evaluated by 2 musculoskeletal-trained radiologists.  Volumetric deltoid measurements were obtained from preoperative CT or MRI scans using ImageJ digital image processing software.  To keep the location of the volume calculations consistent, the slice 24 mm from the most inferior aspect of the acromion and the 2 slices above and below it were selected for analysis. The cross‐sectional areas of the anterior, middle, and posterior sections of the deltoid were measured for MRI images and the deltoid as a whole was measured for CT images. The deltoid outlines were selected using a freehand selection tool and the pixel count in the cross‐sectional areas was converted to mm2. Deltoid volume was determined as the sum of the 5 slices after the cross‐sectional areas were multiplied by the slice thickness. This process was done separately by 2 researchers and the mean of the two calculations used for the analysis. Satisfactory outcomes were defined as forward elevation (FE) of at least 135o, external rotation (ER) of at least 35o, ASES and SANE scores of at least 70.  Statistical analysis was performed with independent t-tests using mean volumes comparison of satisfactory and unsatisfactory outcomes for FE, ER, ASES and SANE.  The posterior deltoid volume was significantly higher in the group with satisfactory ASES scores 

Results: There were a total of 30 patient who underwent RTSA for rotator cuff intact conditions, of which 18 (60.0%) were able to achieve satisfactory outcomes for ER, 24 (80%) for FE, 17 (65.4%) for ASES scores, and 22 (78.6%) for SANE scores. A total of 77 patients with rotator cuff deficient conditions underwent RTSA with 39 (50.6%) able to achieve satisfactory outcomes for ER, 56 (72.7%) for FE, 42 (60.0%) for ASES scores, and 48 (76.2%) for SANE scores. Total deltoid volume correlated (Pearson correlation coefficient 0.278) with improved FE in rotator cuff deficient patients (p = 0.014).  Mean total deltoid muscle volume was significantly higher in satisfactory (57790.1 mm³ ¬¬¬ 18096.2 mm³) compared with unsatisfactory FE (48579.7 mm³ ¬¬¬ 19506.2 mm³, p = 0.013). Similarly, deltoid muscle volume adjusted for BMI was significantly associated with satisfactory post-operative FE (1862.5  490.8 versus 1567.3  607.3) (p = 0.013). When separated into rotator cuff deficient and intact conditions, total deltoid volume was significantly higher (p=0.030) in patient who achieved satisfactory FE in the deficient group but not the intact group (p=0.533). There was no significant difference with respect to satisfactory versus unsatisfactory outcomes when looking at total deltoid volume as it relates to ER, SANE or ASES scores.  Mean posterior deltoid volume was significantly higher in the ASES satisfactory group in patients with cuff deficient conditions but not cuff intact conditions (p=0.005 vs 0.166).  There was no difference in posterior deltoid volume in patients with satisfactory vs unsatisfactory ER in both groups (p=0.530, 0.782).

Discussion: Preoperative total deltoid muscle volume correlated with postoperative FE.  In patients who achieved at least 135o FE postoperatively the total deltoid volume was significantly larger than those that did not. Preoperative deltoid volume may be one factor in determining the ability to achieve satisfactory FE postoperatively.  This study showed preoperative total deltoid volume plays a more important role in rotator cuff deficient indications for RTSA than those indicated for rotator cuff intact conditions.

Clinical Significance: Our study confirmed our hypothesis that deltoid volume directly correlates to range of motion and with patient satisfaction. The deltoid volume may be a good predictor to determine and discuss expected outcomes after surgery with patients who have a deficient rotator cuff who are undergoing RTSA.

1078 - Deltoid Volume Effects on Range of Motion and Outcomes Following Reverse Total Shoulder Arthroplasty in Rotator Cuff Intact and Deficient Conditions