Background/Purpose: Almost 90% of referring physicians think obesity increases the likelihood of poor outcomes after total knee replacement (TKR).  However, current data are conflicting.  The purpose of this study is to assess of the association of body mass index (BMI) with pain, function and satisfaction 2 years after primary TKR.

Methods:  Institutional TKR Registry patients who had a primary TKR between July 2007 and June 2009 and BMI>18.5 were enrolled. Poor pain and function were defined as WOMAC score <= 60. Data were collected prior to surgery and 2 years post-op. Multivariate regressions were performed to evaluate the association between BMI at baseline and poor pain and function at 2 years, controlling for gender, age, race, Deyo Comorbidity score and educational attainment. Expectations were measured with a validated TKR Expectations Survey.

 Results: 2524 patients were included in the analysis. BMI > 40 were more likely to be non-Caucasian, female, have less education and more co-morbidities.  Pre-operatively, both pain and function were least severe in <25 BMI category, increasing as BMI increased. At 2 years, change in WOMAC pain and function showed a step wise, dose dependant improvement across BMI categories, with BMI > 40 showing the most improvement.  At 2 years, there was a statistically significant trend towards lower BMI categories having the least pain (p-value=0.0003) and best function, (p-value<0.0001), but the differences between groups were not clinically significant.  In the multivariate regressions, there were no statistically significant associations between any BMI category or number of co-morbidities and poor pain or function at 2 years. Being female significantly increased the risk of having poor pain (OR 1.6; 95% CI 1.2-2.2) or poor function (OR 1.5; 95% CI 1.1-2.1) at 2 years.  Being Caucasian decreased the risk of poor pain (OR 0.6; 95% CI 0.4-0.9) or poor function (OR 0.5; 95% CI 0.3-0.7).  Having only high school education also increased the risk of poor pain (OR 1.5; 95% CI   1.1- 2.1) and poor function (OR 1.9; 95% CI 1.4-2.6) at 2 years. Age group 61-70 showed a decreased risk of poor pain compared to age <=60, (OR 0.5; 95% CI 0.4- 0.8). At 2 years, 20.4% of patients lost weight, (mean weight loss 0.6 lbs +/- 2.7), with the greatest loss in BMI >40 (2.7 lbs, +/- 5).  There were no significant differences in expectations or satisfaction between BMI categories. 

Conclusion: Although obese patients have worse pain and function at the time they elect TKR, their outcomes at 2 years are not clinically significantly different than other patients.  However, race and educational attainment were significantly associated with poor outcomes.  Obese patients have similar expectations and are as satisfied as patients with lower BMI.  More research should be done on the effect of race and education on TKR outcomes.   Obesity should not be regarded as a risk factor for poor outcomes after primary TKR.

Patient Characteristics	Average Weight	Overweight	Obese class I	Obese class II	Obese class III	P-value
	(18.5 ≤ BMI <25)	(25 ≤ BMI <30)	(30 ≤ BMI <35)	(35 ≤ BMI <40)	(40 ≤ BMI)
	N=523	N=902	N=633	N=289	N=177
Age	71.0 ± 10.0	69.1 ± 9.7	66.7 ± 9.2	64.8 ± 8.9	64.3 ± 7.9	<0.0001
Female	365 (70.3%)	476 (53.0%)	359 (57.2%)	200 (69.4%)	132 (75.4%)	<0.0001
Caucasian	491 (93.9%)	824 (91.4%)	560 (88.5%)	251 (86.9%)	156 (88.1%)	0.0027
High school or less	56 (17.9%)	153 (24.1%)	140 (29.4%)	72 (29.4%)	51 (30.3%)	<0.0001*
Some college or college graduate	254 (81.2%)	416 (65.5%)	286 (60.0%)	130 (53.2%)	82 (48.8%)
Masters professional or doctorate degrees	207 (66.1%)	323 (50.9%)	203 (42.6%)	86 (35.2%)	42 (25.0%)
0 Deyo comorbidities	409 (79.4%)	664 (75.5%)	435 (70.7%)	195 (68.8%)	100 (59.0%)	<0.0001*
1-2 Deyo comorbidities	103 (20.0%)	214 (24.3%)	178 (28.9%)	86 (30.3%)	67 (39.5%)
>= 3 Deyo comorbidities	7 (1.4%)	20 (2.3%)	15 (2.4%)	7 (2.5%)	8 (4.7%)
Pre-Operative WOMAC Pain	59.4 ± 17.1	57.0 ± 17.5	53.6 ± 17.1	51.0 ± 18.3	46.5 ± 16.3	<0.0001
2-Year Post-Operative WOMAC Pain	89.2 ± 15.1	88.9 ± 14.5	86.9 ± 16.2	84.8 ± 17.2	86.0 ± 16.5	0.0001
Change in WOMAC Pain	29.9 ± 20.0	31.9 ± 19.6	33.3 ± 20.2	34.0 ± 21.6	39.3 ± 21.9	<0.0001
Pre-Operative WOMAC Function	58.7 ± 17.5	57.6 ± 17.2	52.6 ± 17.0	49.7 ± 17.7	43.5 ± 15.9	<0.0001
2-Year Post-Operative WOMAC Function	87.9 ± 15.5	87.3 ± 15.0	84.7 ± 17.0	82.8 ± 18.2	82.2 ± 16.6	<0.0001
Change in WOMAC Function	29.0 ± 20.1	30.5 ± 18.8	32.4 ± 19.1	34.1 ± 18.6	38.0 ± 20.5	<0.0001
Change in Weight (lbs)	0.2 ± 1.7	-0.3 ± 1.9	-0.8 ± 2.7	-1.3 ± 3.2	-2.7 ± 5.5	<0.0001
Somewhat/Very Satisfied with Relieving Pain	453 (90.2%)	797 (93.1%)	547 (91.0%)	253 (90.7%)	152 (92.7%)	0.333
Somewhat/Very Satisfied with Improving	413 (83.4%)	695 (82.1%)	495 (83.2%)	230 (82.4%)	123 (76.9%)	0.401
ability to do recreational activities
Overall Somewhat/Very Satisfied with TKR	445 (88.3%)	786 (91.5%)	537 (89.2%)	252 (89.7%)	148 (91.4%)	0.333
*= test for trend

 

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/obesity-is-not-a-risk-factor-for-poor-pain-and-function-two-years-after-total-knee-replacement/