Intraoperative Measurements of Reverse Total Shoulder Arthroplasty Joint Reaction Forces Using an Instrumented Trial Prosthesis

Introduction Persistent problems and relatively high complication rates with reverse total shoulder arthroplasty (RTSA) are reported (1, 2). It is assumed that some of these complications are affected by improper intraoperative soft tissue tension. Achieving proper intraoperative soft tissue tension is an obvious surgical goal. However, intraoperative soft tissue tension measurements and methods for RTSA have not been reported. One way to quantify soft tissue tension is to measure intraoperative joint forces using an instrumented prosthesis. Hence, we have developed an instrumented RTSA to measure shoulder joint forces intraoperatively. The goal of this study was to measure intraoperative shoulder joint forces during RTSA. Materials and Methods The instrumented shoulder prosthesis measures the contact force vector between the glenosphere and humeral tray. This force sensor is a custom instrumented trial implant that can be used with an existing RTSA system (EQUINOXE, Exactech Inc, Gainesville, FL) just as a standard trial implant is used. Four uniaxial foil strain gauges (QFLG-02-11-3LJB, Tokyo Sokki Kenkyujo Co., Ltd., JP) are instrumented inside the sensor. Using a calibration matrix, the three force components were calculated from four strain gauge outputs (3). Sixteen patients who underwent RTSA took part in this IRB approved study. All patients were greater than 50 years of age and willing to review and sign the study informed consent form. After obtaining informed consent for surgery, a standard deltopectoral approach to the shoulder was performed. The instrumented trial prostheses were assembled on the glenoid baseplate instead of a standard glenosphere. After the joint was reduced, joint forces were recorded during cyclic rotation, flexion, scapular plane movement (scaption), and adduction of the shoulder. Strain gauge outputs were recorded during these movements as well as the neutral position just before movements. Mean values of forces with each motion were compared by one-way analysis of variance (ANOVA). A multiple comparisons test was subsequently performed to examine differences between motions. Results Three sensors failed due to intraoperative breakage of strain gauge wires, leaving 13 subjects with measured joint reaction forces. During abduction, for example, the force vector varied from superior to antero-medial, and the resultant joint force in abduction was 83 N in a representative subject (Figure 1). Average joint reaction forces decreased with shoulder motion from a neutral position to external rotation or scaption. Conversely, they increased with flexion or abduction (Figure 2). Mean force values were not the same for each movement (p = 0.018). Forces recorded during flexion and scaption movements differed significantly (p = 0.012). Discussion The intraoperative forces acting in the RTSA have been measured for the first time, and these measurements are ongoing. We expect more measurements will permit surgeons objectively to place and align implant components to achieve predictable and durable RTSA results.