Moby Parsons, MD
Instability and scapular stress fractures remain two of the most common complications after reverse Total Shoulder Arthroplasty (rTSA),1 both of which may compromise outcomes and require further surgery. Each is conversely related to the soft tissue tension in the deltoid and remaining rotator cuff which are affected by implant configuration, including glenosphere diameter, glenosphere offset, humeral tray, and liner thickness. These parameters may also affect the resting tension in the remaining cuff which can thereby impact postoperative active shoulder rotation.2
To date, surgeon determination of the optimal construct tension after reduction of rTSA remains largely qualitative and may be based on individual experience derived from ease of reduction, passive range of motion on the table, presence or absence of boney impingement, and “shuck tests” for dislocation. The integrity and repairability of the subscapularis may also play a role. Currently, there remain no quantitative methods to measure soft tissue tension intraoperatively and how it may change during different arm positions that correlate with desired patient function after rTSA.
With more than 15 years of increasing utilization in the United States and even longer abroad, rTSA has now surpassed anatomic shoulder arthroplasty in prevalence giving its widening indications.3 Its use in multiple clinical settings including those with failed prior non-arthroplasty surgery and complex revision cases may challenge surgeons to determine the best configuration and placement in any given case to restore stability, motion and comfort. While preoperative planning platforms and patient-specific instrumentation or navigation can provide surgeons a more quantitative approach to optimize the reconstruction, these platforms cannot reconcile soft tissue tension, which is an important consideration for postoperative function.
To address this gap in knowledge, Verstraete and colleagues recently reported on the development of a humeral trial insert with an intraoperative load sensor (Equinoxe® with VERASENSE™, OrthoSensor, Inc., Dania Beach, FL) that can be used to measure load magnitude and displaying center of load location of the humeral component on the glenosphere component. This sensor has been cleared in the United States for use with the Exactech Equinoxe Shoulder System and contains three load transducers across the articulating surface of the trial insert. The sensor can measure the load magnitude as the shoulder is taken through various positions that replicate normal function and wirelessly transmit this to a tablet for real-time, intraoperative assessment. As with a conventional rTSA, the insert and tray thickness can be altered to change the tension in the construct. These magnitudes provide the first surrogate measure of soft-tissue tension to help guide surgeons on implant choice to reduce potential complications related to being too loose (instability) or too tight (stiffness, stress fractures, pain).
A preliminary cadaveric study of VERASENSE has demonstrated that increasing the tightness of the rTSA construct does result in increased joint load magnitudes, particularly toward the end ranges of motion elevation and rotation.4 Loads were also noted to be highly variable during different motions with a steep increase toward the end range.4 This suggests that clinical decisions on optimal implant configuration should be evaluated throughout the full range of motion and not just on the ease of reduction in a single, “neutral” position. Understanding characteristic load changes, based on the shape of the load curve, can help surgeons determine if shoulders have insufficient end-range tension as may occur in revision cases with bone loss. Loads that exceed a defined magnitude during range of motion trialing may also alert the surgeon to conditions where a patient’s postoperative motion recovery may be impacted by excessive tension in the rTSA which they will have to overcome to achieve higher degrees of elevation and rotation.
Clinical implementation of this technology may ultimately lead to a greater understanding not only of how reverse implants affect shoulder biomechanics, but how real-time, intraoperative assessment of loads can inform surgeons on the best combination of implants to balance mobility and stability. When coupled with preoperative planning and surgical navigation offered by the ExactechGPS® system, VERASENSE can provide data-driven decision-making that is customized to each case. As we gain increasing experience with these technologies and couple this information with clinical outcomes, future analytics capabilities will eventually be able to inform surgeons both preoperatively and intraoperatively on measures that can be taken to maximize patient-specific outcomes.
- Chae J, Siljander M, Wiater JM. Instability in Reverse Total Shoulder Arthroplasty. J Am Acad Orthop Surg. 2018 Sep 1;26(17):587-596.
- Giles JW, Langohr GD, Johnson JA, Athwal GS. The rotator cuff muscles are antagonists after reverse total shoulder arthroplasty. J Shoulder Elbow Surg. 2016 Oct;25(10):1592-600. Epub 2016 Apr 20.
- Wagner ER, Farley KX, Higgins I, Wilson JM, Daly CA, Gottschalk MB. The incidence of shoulder arthroplasty: rise and future projections compared with hip and knee arthroplasty. J Shoulder Elbow Surg. 2020 Dec;29(12):2601-2609. Epub 2020 Jun 9.
- Verstraete MA, Conditt MA, Parsons IM, Greene AT, Roche CP, Decerce J, Jones RB, Youderian AR, Wright TW, Zuckerman JD. Assessment of intraoperative joint loads and mobility in reverse total shoulder arthroplasty through a humeral trial sensor. J Shoulder Elbow Surg. 2020;30(1):2-12.