A Step in the Right Direction: Body Location Determines Activity Tracking Device Accuracy in Total Knee and Hip Arthroplasty Patients

R. Goel
J. R. Danoff, Zucker School of Medicine at Hofstra/Northwell
M. Petrera
K. Blevins
J. J. Purtill
A. F. Chen


INTRODUCTION: Step counts measured by activity monitoring devices (AMDs) and smartphones (SPs) can objectively measure a patient's activity levels after total hip and knee arthroplasty (total joint arthroplasty [TJA]). This study investigated the use and optimal body location of AMDs and SPs to measure step counts in the postoperative period. METHODS: This was a two-armed, prospective, observational study of TJA inpatients (n = 24) and 2-week status after TJA (n = 25) completing a 100-ft walk. Observer-counted steps were compared with those measured by AMDs (wrist and ankle) and SPs (hip and neck). Acceptable error was defined as <30%. Error rates were treated as both dichotomous and continuous variables. RESULTS: AMD and SP step counts had overall unacceptable error in TJA inpatients. AMDs on the contralateral ankle and SPs on the contralateral hip had error rates less than 30% at 2 weeks postoperatively. Two-week postoperative patients required lower levels of assist (11/25 walker; 4/25 no assist), and significant improvements in stride length (total hip arthroplasty 1.27 versus 1.83 ft/step; total knee arthroplasty 1.42 versus 1.83 ft/step) and cadence (total hip arthroplasty 74.6 versus 166.0 steps/min; total knee arthroplasty 73.5 versus 144.4 steps/min) were seen between inpatient and postoperative patients. Regression analysis found that increases in postoperative day and cadence led to a decrease in device error. CONCLUSION: In inpatients with TJA, AMDs and SPs have unacceptable variability and limited utility for step counting when using a walker. As gait normalizes and the level of ambulatory assist decreases, AMDs on the contralateral ankle and SPs on the contralateral hip demonstrated low error rates. These devices offer a novel method for measurement of objective outcomes and potential for remote monitoring of patient progress after TJA. LEVEL OF EVIDENCE: Level II, prospective, three-armed study, prognostic study.