Background/Purpose: In 2009 over 620,000 total knee replacement (TKR) surgeries were performed. That number is expected to increase to 3.5 million annually by 2030. On average, physical activity and functional ability remain diminished one year post TKR when compared to age matched controls. Many patients who undergo TKR are obese.  The purpose of this study was to examine the relationship of preoperative body mass index (BMI) to objectively measured physical activity levels at 6 months post TKR.

Methods: Subjects were 179 participants of an NIH funded RCT of telephone support intervention following unilateral primary total knee replacement (TKR).  Participants were stratified by BMI (< 30; 30-35; > 35). Participants were asked to wear an accelerometer (Step Activity Monitor™) at the ankle for four consecutive days (2 weekday; 2 weekend) before surgery and at 8 weeks and 6 months post operatively.  The accelerometer monitored the number of steps taken daily. Valid wear days required a minimum of 10 hours of wear time. 

Results: Participants were 68% female with a mean (SD) age of 65.1 (8.61). Mean (SD) BMI at baseline was 32.5 (5.24).  All 179 participants wore the accelerometer preoperatively. 174, 163 and 168 participants had at least one valid wear day preoperatively, at 8 weeks and 6 months after surgery respectively. Mean days worn was 3.3 days.  Participants with BMI > 35 (n=53) took fewer steps than those with BMI < 35 (n=121) preoperatively, at 8 weeks post surgery (BMI > 35, n = 46 and BMI < 35, n = 117) and at 6 months post surgery (BMI > 35, n = 50 and BMI < 35, n = 118) (p < .01) (Figure). At 6 months post surgery, mean steps (SD) were greatest in those with BMI < 30, 8845 (3731), fewer in those with BMI between 30 and 35, 8688 (3386), and fewest in those with BMI > 35, 7196 (3299).  Change in daily steps from baseline to 6 months was similar across all BMI groups.

Conclusion: Subjects with grade 2 obesity or greater (BMI >35) are less physically active throughout the perioperative period of TKR than those with lower BMI.  However high BMI does not preclude improvements in physical activity levels following TKR. An understanding of the association between obesity and physical activity can help inform patients’ and clinicians’ expectations for post-TKR gain and may focus interventions not only on the knee impairments following TKR but also on the modifiable risks to optimal gains in physical activity following TKR. Risk factors such as BMI may be addressed pre- and post-operatively.