Published 10/1/2012
Maureen Leahy

TKA Improves Function Regardless of Patient’s BMI

Obese, nonobese patients experience similar improvement

Patients with a higher body mass index (BMI) experience similar levels of improvement after total knee arthroplasty (TKA) as patients with a lower BMI, according to a study that was published recently in The Journal of Bone and Joint Surgery.

“Obesity rates are reaching epidemic proportions in both the United States and Europe, resulting in more obese and morbidly obese patients needing knee arthroplasty,” said Paul Baker, MBBS, MSc, FRCS, the study’s lead author. “Previous studies, however, have been unable to gauge the impact of BMI upon knee function and general health following TKA. This is the first study to quantify this effect.”

Study design
Using Patient Reported Outcome Measures (PROMs) data linked to the corresponding National Joint Registry of England and Wales records, the researchers analyzed the outcomes of primary TKAs performed between May 2008 and September 2010. Their goal was to establish the nature of the relationship between BMI and pre- and postoperative PROMs and to determine if improvement in these scores was dependent on BMI.

PROMs included pre- and postoperative validated assessments of knee-specific health using the Oxford Knee Score (OKS); general health was assessed using the EuroQol-5D (EQ-5D) and the EQ-5D visual analog score (VAS). Patients were also asked whether they had been readmitted to the hospital or had a reoperation and whether postoperative complications such as wound or bleeding problems had developed.

The researchers analyzed the out- comes of 13,673 patients (6,117 males; 7,556 females; mean age: 69.7 years) who had undergone primary TKA for the treatment of osteoarthritis and whose BMI was between 15 and 60. Patients were stratified into the following groups:

  • Group 1 (BMI: 15 to 24.9; n = 1,292)
  • Group II (BMI: 25 to 39.9; n = 11,363)
  • Group III (BMI: 40 to 60; n = 1,018)

Outcomes were comparable
The researchers found that pre- and postoperative scores in all PROMs decreased as BMI increased; yet, linear regression analysis indicated a trend for slight improvement in scores in patients with higher BMIs. After adjusting for age, sex, ASA grade, number of comorbidities, and general health rating, the mean improvement in the OKS was similar in the three groups (Group I = 15.4; Group II = 15.1; Group III = 15.9; P = 0.02). Improvements in the EQ-5D and EQ-5DVAS were also comparable (
Table 1).

Wound complication rates were significantly higher (P < 0.001) in Group III (17 percent), compared to Group I (9 percent) and Group II (12 percent). The rates of bleeding problems, readmission, and reoperation were similar among the three groups.

“Significantly, despite lower postoperative functional scores, patients with morbid obesity (BMI > 40) gained as much improvement in knee-specific function as patients with a ‘normal’ BMI (18.5 to 25),” said Dr. Baker. “Importantly, we found that this effect was consistent across the BMI range, meaning that improvement in scores did not ‘tail off’ above a specific BMI threshold. Thus, all patients derived equivalent functional benefit following TKA irrespective of their preoperative BMI. In addition, although the rates of self-reported wound complications were greater in the morbidly obese group, the rates of readmission and reoperation at a minimum of 6 months following surgery were not.”

Dr. Baker concluded, “The previous historical view that increasing BMI is associated with poor surgical outcomes is becoming outdated. Our work complements existing contemporary reports of equivalent rates of complications and mid-term implant survival in patients irrespective of their preoperative BMI. Taken together, these reports imply that it is unfair to deny patients the benefits of surgery based solely upon their BMI. This study also helps to quantify the level of function and rates of complications to be expected after TKA in the morbidly obese patient population. This will promote a more informed consent process and more realistic expectations prior to surgery.”

Maureen Leahy is assistant managing editor of AAOS Now. She can be reached at leahy@aaos.org

Dr. Baker’s coauthors of “The Association Between Body Mass Index and the Outcomes of Total Knee Arthroplasty” are Tim Petheram, MBBS, MSc, MRCS; Simon Jameson, MBBS, MRCS; Mike Reed, MD, FRCS; Paul Gregg, MD, FRCS; and David Deehan, MD, BSc, FRCS.

Disclosures: no information available.

Bottom Line

  • Historically, higher BMI has been associated with poor surgical outcomes in total knee arthroplasty (TKA).
  • Previous studies have been unable to quantify the effect of BMI on knee function and general health following TKA.
  • In this study based on patient-reported outcome measures, the researchers found that patients derived equivalent functional benefit following TKA, irrespective of their preoperative BMI.
  • The researchers believe that potential candidates for TKA should not be denied surgery based solely on their BMI.


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  2. Yeung E, Jackson M, Sexton S, Walter W, Zicat B, Walter W. The effect of obesity on the outcome of hip and knee arthroplasty. Int Orthop. 2011 Jun;35(6):929-34.