Trapeziectomy (excision of the trapezium, the carpal bone at the base of the thumb, Fig. 1) is the most common surgical treatment for end-stage thumb carpometacarpal (CMC) osteoarthritis (OA)(Weiss and Goodman, 2018), a disease that affects 20–25 % of adults over the age of 50 (Chaisson et al., 1999, Dahaghin et al., 2005, Haugen et al., 2011, Kellgren and Lawrence, 1958, Wilder et al., 2006) and as many as 80 % over the age of 70 (Becker et al., 2013, Haara et al., 2004, Sodha et al., 2005, Wilder et al., 2006). Other treatments include trapeziectomy with suture suspension, implant arthroplasty, and arthrodesis (Burton, 1986). Unlike hip, knee, and shoulder arthroplasty, which are all predictably successful, the complication rate for end-stage CMC OA surgery is high (10 %-39 %), and none of the common surgical treatments have proven superior to the others in terms of post-op pain or function (Brunton and Wilgis, 2010, Martou et al., 2004, Wajon et al., 2015).

The success of hip, knee, and shoulder implants is due in large part to extensive research and standardized pre-clinical mechanical testing using conditions that mimic in vivo loading (Friis, 2017). There is limited comparable research on the thumb or thumb arthroplasty (Crisco and Wolfe, 2017). Our longitudinal study of CMC OA has advanced the understanding of CMC function, but its primary outcome measures were focused on CMC kinematics and the changes in bone morphology associated with OA progression (NIH AR059185). Data on CMC loading is minimal compared to that of other joints. Currently, even the normal pattern of loading in the CMC joint is unknown. The best estimates of the loads at the CMC joint are derived solely from musculoskeletal models (Cooney and Chao, 1977, Gislason et al., 2009, Giurintano et al., 1995, Goislard de Monsabert et al., 2012, Vigouroux et al., 2011, Wu et al., 2015) whose prediction of CMC loading ranges from 5 to 18 times the force at the tip of thumb. In particular, Cooney and Chao estimate thumb CMC compressive forces of ∼100 N and ∼1,200 N for lateral key pinch and grasping, respectively (Cooney and Chao, 1977). However, none of the existing computational models have been validated with in vivo data. The extensive range of surgical treatment options and absence of a clearly superior intervention suggests a paucity of rigorous science on the etiology of load-mediated diseases such as CMC OA (Chaisson et al., 1999, Felson, 2013).

Custom implants instrumented to measure joint contact loads have been developed for the hip (Bergmann et al., 2018, Bergmann et al., 2001, Bergmann et al., 1993, Damm et al., 2010, English and Kilvington, 1979), knee (D’Lima et al., 2011, D’Lima et al., 2008, D’Lima et al., 2007, D’Lima et al., 2006, Kaufman et al., 1996), shoulder (Westerhoff et al., 2009), and spine (Rohlmann et al., 2014, Rohlmann et al., 2007, Rohlmann et al., 1994). The data from these implants has provided critical insight into joint function (D’Lima et al., 2013, Karipott et al., 2018, Ledet et al., 2018) based on direct measurement of joint forces during standardized movements and normal activity. The broad benefit of these studies is the ability to dramatically tighten the design-development-testing loop via the analysis of data collected during normal activities (D’Lima et al., 2013). These findings can provide insight into implant efficacy and performance, potentially identifying early biomarkers of success and failure (D’Lima et al., 2011). Accordingly, an instrumented trapezium implant (iTrapz) could provide critical data that would validate thumb-specific musculoskeletal models, drive the identification of best treatments, enable the development of new treatments, and potentially reduce health care costs (Mahmoudi et al., 2016).

Specifically, understanding the loads at the base of the thumb during activities of daily living would inform approaches to pre-clinical testing for FDA clearance, for example, the development of ASTM standards, which do not currently exist for thumb arthroplasty implants (Crisco and Wolfe, 2017). The ultimate goal for this project is to develop an instrumented replacement trapezium capable of measuring basal thumb loads in vivo. In this study, we examine two strain gauge-based load sensor designs (Fig. 2A & 2B) and report accuracies associated with two different calibration methods, along with models of individual strain gauge failure/dropouts.