Elsevier

Medical Engineering & Physics

Volume 51, January 2018, Pages 31-40
Medical Engineering & Physics

Measurement of physical activity in the pre- and early post-operative period after total knee arthroplasty for Osteoarthritis using a Fitbit Flex device

https://doi.org/10.1016/j.medengphy.2017.10.007Get rights and content

Highlights

  • Pre- and early post-operative daily step count measured with Fitbit Flex devices for patients with OA undergoing TKA.

  • Pre-operative step count, SF-12 PCS score and BMI were strong correlators with post- operative stepcount.

  • Semi-individualised benchmarks based on correlating scores and grouping are presented.

  • A stepwise multivariate regression model is used to develop a patient specific 6 week step count target.

Abstract

Total knee arthroplasty (TKA) is a standard treatment for patients with end stage knee Osteoarthritis (OA) to reduce pain and restore function. The aim of this study was to assess pre- and early post-operative physical activity (PA) with Fitbit Flex devices for patients with OA undergoing TKA and determine any benchmarks for expected post-operative activity. Significant correlations of pre-operative step count, post-operative step count, Body Mass Index (BMI) and Short Form 12 Physical Component Summaries (SF-12 PCS) were found. Mean step counts varied by 3,203 steps per day between obese and healthy weight patients, and 3,786 steps per day between those with higher and lower SF-12 PCS scores, suggesting the need for benchmarks for recovery that vary by patient pre-operative factors. A backwards stepwise regression model developed to provide patient specific step count predictions at 6 weeks had an R2 of 0.754, providing a robust patient specific benchmark for post-operative recovery, while population means from BMI and SF-12 subgroups provide a clinically practical alternative.

Introduction

The aim of total knee arthroplasty (TKA) as a treatment for Osteoarthritis (OA) is the restoration of function, a reduction in pain, satisfaction with surgical outcome and restoration of a healthy lifestyle [1].

The need for physical activity (PA) as part of a healthy lifestyle is undisputed for all of us including for patients with OA. Although it has been found that pain and discomfort experienced during PA limits patients with OA from reaching recommended levels of PA, it is expected that PA levels after TKA would improve [2], [3], [4].

PA can be directly measured or patient self-reported in Patient Reported Outcome Measures (PROMs). Direct measures of PA can include, accelerometry, pedometry, heart rate monitoring, global positioning systems and direct observation. There are also a number of techniques such as doubly labelled water and calorimetry that measure energy expenditure, a related concept often substituted for PA in studies [5]. Historically, many of these techniques have been impractical for use outside of research studies due to their cost or technical requirement. PROMs assessments of PA have lower costs and technical barrier, buthave relatively poor validity and correlation to objective measures [6].

Wearable wireless activity monitors such as the Fitbit Flex are an increasingly low cost and clinically accessible option for monitoring step counts. Pedometers and step count as a measurement holistically has been validated as a means of capturing relative PA between subjects, showing robust construct and convergent validity [7], [8]. In particular, convergent validity was well demonstrated between directly observed and accelerometric measurements of PA and step count as measured by the activity trackers [7]. Fitbit Flex devices have been shown to be valid and reliable assessment tools for ambulation in normal subjects, reporting both high inter- and intra-device reliability [9], [10] and consistency with other objective measures of activity level [11], [12].

PROMs measure health-related quality-of-life. In addition to those used in assessments of physical activity, some are specific to the knee and one popular one is the Western Ontario and McMaster University Osteoarthritis Index (WOMAC). The Knee Injury and Osteoarthritis Outcome Score (KOOS) is self-administrated and was adapted from the WOMAC [13]. Scores can be calculated for pain, symptoms, activities of daily life function (ADL), sport and recreation function and knee-related quality of life (QOL). The Medical Outcomes Short Form-12 (SF-12) is a shorter version of the Medical Outcomes Study Short Form-36 (SF-36) [14] consisting of 12 questions regarding holistic health that produces summary scores of physical and mental functioning: the Physical Component Summary (PCS) and the Mental Component Summary (MCS) [15], [16]. Scores for both the KOOS and SF-12 are transformed into a 0–100 scale where zero represents extreme health or knee problems and 100 represent no problems.

Generally accepted pre-operative indicators for outcome after TKA include age, gender, BMI, diagnosis leading to surgery and function/pain as measured by PROMs, socioeconomic circumstances and anxiety/depression. Clinically important indicators of poor TKA outcomes such as pain were diagnosis of rheumatoid arthritis (RA) versus OA and area deprivation (living in a poorer area) whereas age and gender were specifically associated with function outcomes [17].

Like age and gender, a patient's pre-operative physical functioning and PA levels could be a predictor of restoration of function after TKA. Brandes et al. [18] found moderate correlations between pre-operative and post-operative PA as measured by step count suggesting that a higher pre-operative PA level could serve as a moderate predictor for higher PA levels post-operatively. However, pre-operative baseline PA levels influencing PA for OA patients after TKA are not known [3]. PA levels in the early recovery period is also not known although early mobilisation after TKA surgery can result in reduced length of stay (LOS) in hospital without an increase in negative outcomes [19]. Across populations, even when experiences of pain and stiffness, functional capacity and self-reported physical functioning were improved by TKA, it has been shown that actual directly measured PA increased only slightly six months after surgery [20].

The aim of this study was to assess pre- and early post-operative physical activity (PA) with Fitbit Flex devices for patients with OA undergoing TKA and determine any benchmarks for expected post-operative activity. Meaningful correlations could be used to determine simple, clinically applicable benchmarking rules for expected recovery. More sophisticated modelling was also employed to predict baseline step counts at post-operative time points based on pre-operative step count, PROMs, demographic data and hospital Length of Stay (LOS). This produces a less clinically applicable but more personalised and relevant post-operative benchmark.

Section snippets

Method

A total of 94 patients undergoing TKA were recruited to the study over a 21 month period, from December 2013 to September 2015. Ethics was approved by St Vincent's Human Research Ethics Committee (SVH 13/034).

Exclusion criteria for this study were rheumatoid arthritis and a fixed flexion deformity of >15°, or patients who were wheelchair bound or otherwise completely immobilised in a pre-operative state. The Fitbit Flex, a small, lightweight commercially available wristband containing a

Results

A total of 94 subjects were recruited to the study over a 21 month period, from December 2013 to September 2015. Of the 94 patients recruited, 3 did not return any devices or contribute any data to the study and were not carried forward in any analysis, leaving a total of 91 patients. Step count data was complete for 69 of 94 patients in the pre-operative period, 68 in the day 2–4 post-operative period and 68 at 6 weeks post-operatively. Patient demographics and summary results are shown in

Discussion

From a series of 91 subjects undergoing total knee arthroplasty we assessed the mean daily step count as 6409 + /− 3228 pre-operatively, 1170 +/− 857 2–4 days post-operatively and 6231 +/− 2924 at 6 weeks post-operatively. Step count at all time points (pre-operatively, days 2–4 post-operatively and 6 weeks post-operatively) correlated positively with higher scores on the Short Form-12 Physical Component Score (SF-12 PCS) score and the Knee Osteoarthritis and Outcome Score Activities of Daily

Conclusion

Significant correlations exist between all pre-operative, days 2–4 post-operative and 6 week post-operative step count, BMI and SF-12 PCS score for patients undergoing TKA. Obese patients reach a mean of 4819 steps compared to 7151 for overweight patients and 8022 steps for normal weight patients. Similarly, segmentation by SF-12 PCS score produced expected step counts of 4556, 6758 and 8342. Pre-operative step count of the individual patient or the mean of the BMI or SF-12 group they fall into

Acknowledgements

The authors have no competing interests and would like to acknowledge 360 Knee Systems for funding for this study.

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