Background/Purpose: Obesity is an important predictor of functional status and pain after total knee (TKR) and total hip (THR) replacement. However, variations in pre-post TKR and THR changes in function and pain by obesity status remain to be examined. 

Methods: Pre- and 6 month post TKR data on physical function and pain were collected on 1,533 primary TKR and 1,118 primary THR patients between 5/2011 and 10/2012. Data included demographics, comorbidities, operative joint pain severity based on the Knee Injury/Hip Disability and Osteoarthritis Outcome Score (KOOS/HOOS), physical function (SF-36 PCS), mental health (SF-36 MCS), and musculoskeletal burden of illness.  Pre-post changes in PCS and pain were analyzed using descriptive statistics and linear mixed models that accounted for the clustering of patients within clinic. 

Results: Approximately 14% of TKR patients were under or normal weight (BMI<25), 33% overweight (BMI: 25.1-30), 29% obese (BMI: 30.1-35), 15% severely obese (BMI: 35.1-40) and 9% morbidly obese (BMI>40). Patients had average age of 67 years, included 61% women and 93% whites. Patients with greater level of obesity had lower PCS at baseline (p<0.001) and 6 month (p<0.001). However, pre-TKR to 6 month post-TKR change in  PCS  did not differ by level of obesity (p=0.554), and had an overall mean (SE) of 9.7 (0.23). Patients with greater levels of obesity had worse pain scores at baseline (p<0.001) but greater change in pain between pre-TKR and  6 months post-TKR (p=0.001). As a result, average pain scores at 6 months were similar across  levels of obesity (p=0.150), and had an overall mean (SE) of 84.9 (0.40).

Approximately 27% THR patients were under/normal weight (BMI<25), 38% overweight (BMI: 25.1-30), 23% obese (BMI: 30.1-35), 9% severely obese (BMI: 35.1-40) and 4% morbidly obese (BMI>40). Patients had average age of 66 years, included 62% women and 95% whites. Greater level of obesity was associated with lower PCS at baseline (p<0.001) and 6 month (p<0.001). Mean change in pre-to-six month PCS was greater in patients with BMI <35 when compared to BMI>35 ( 14 vs. 11.5)   Greater level of obesity was associated with a poorer baseline pain score (p<0.001) but larger change in post-op pain at (p=0.001). At 6 months, pain scores did not differ by level of obesity (p=0.068) with a mean score greater than 90.

Conclusion: At 6 months after TKR, severely obese patients (BMI>35) reported improvements in pain and function equal to or greater than patients with BMI<35.  At 6 months after THR, all patients reported significant functional gains although patients with BMI>35 had lower mean functional gain than those with BMI<35. All patients reported excellent pain relief.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/does-functional-gain-and-pain-relief-after-tkr-and-thr-differ-by-patient-obese-status/