Quantified Deltoid Muscle Stiffness Can Predict Improved Muscle Strength for Elevation Following Reverse Shoulder Arthroplasty.

: Although the indications for reverse shoulder arthroplasty (RSA) are expanding, an improvement in muscle strength in each patient following RSA remains unclear. The objective was to investigate whether or not improvement in muscle strength for shoulder elevation in patients who underwent RSA was influenced by pre- or postoperative deltoid muscle stiffness measured using shear wave elastography (SWE). : Sixty-five patients who underwent RSA over a 12-month follow-up period were included. Patient characteristics and clinical and radiologic measurements were recorded. Preoperatively and at 3, 6, 9, and 12 months after surgery, deltoid muscle stiffness and muscle strength for scapular-plane abduction were sequentially measured using SWE and a portable dynamometer. In each quarterly period (3-6, 6-9, and 9-12 months), patients were assessed for an improvement in muscle strength and separated into two groups: improved and non-improved. To assess the risk of lack of improvement in each quarterly period, the variables were compared between the groups. : Improvement in muscle strength was observed in 52 patients (80%) at 3-6 months, 46 patients (71%) at 6-9 months, and 39 patients (60%) at 9-12 months. Notably, SWE measurements at the beginning of each period showed significantly greater values in the non-improved group than in the improved group during the subsequent quarterly period. A receiver operating characteristic (ROC) curve analysis suggested that SWE values >45.1-50.0 kPa might be associated with a lack of muscle strength improvement over 3 months with 73-87% specificity and 73-85% sensitivity. : Our study demonstrated that increased deltoid muscle stiffness negatively correlated with an improvement in muscle strength following RSA. According to our results, a postoperative assessment with SWE may be useful for not only improving muscle strength after RSA but also facilitating postoperative improvement by preventing excessive stiffness in the deltoid muscle.