In this large consecutive series of 22,289 knees (16,136 patients) treated with total knee arthroplasty, increasing BMI was associated with increasing risk of reoperations, implant revision or removal, and infection. The threshold for BMI at which the risk of complications began to increase was between 30 and 35 kg/m2 (depending on the variable), and the increase was progressively more pronounced for higher BMIs and was especially notable in morbidly obese patients (those with a BMI of ≥40 kg/m2). BMI was associated with a higher risk of revision for mechanical failure, including aseptic loosening and polyethylene wear, with a peak at a BMI of about 40 kg/m2; however, this risk declined in patients with the highest BMIs. BMI did not have a significant association with risk of revision for tibiofemoral instability, knee manipulation under anesthesia, and venous thromboembolism.
Periprosthetic joint infection was the complication most strongly associated with high BMI. The correlation between infection and obesity has been shown by other authors15-17,22-30, who have reported widely variable levels of risk at different BMI thresholds. There are likely multiple factors that contribute to this finding, including the large adipose tissue layer (large potential dead space that poses an infection risk), comorbidities such as diabetes mellitus (which poses an independent infection risk)16,17,26, and longer operative times (also an independent risk factor for infection)31 in patients with higher BMIs. Furthermore, some studies have suggested a proinflammatory state in obese patients, with potentially an impaired immune response to infection32-34.
Revision for mechanical implant failure etiologies, including aseptic loosening, bearing surface wear, and implant fracture, is one of the most common causes of implant revision in total knee arthroplasty35. There remains a paucity of information linking obesity with mechanical failure after total knee arthroplasty. In fact, although Kerkhoffs et al. demonstrated an increased overall implant revision rate associated with obesity, there were not enough studies in their meta-analysis to support an association between obesity and aseptic implant failure13. Our results demonstrate increased rates of mechanical failure for patients with a BMI of 35 to 39.99 kg/m2. This increased failure rate was seen for both revision for aseptic loosening and revision for polyethylene wear, with a peak at about a BMI of 40 kg/m2 and a decline with a BMI of >40 to 45 kg/m2. We hypothesize that the increased stress on implant interfaces and bearing surfaces may account for the higher aseptic loosening and wear rates seen in the patients with a BMI of 35 to 39.99 kg/m2. The lack of strongly increased risk of revision for aseptic implant loosening and wear above a BMI of 45 kg/m2 may possibly represent lower activity levels in this group of patients19,20.
The results of this study may be compared and contrasted with those of a recent paper using the same methodology on total hip arthroplasty36. As with knee arthroplasty, the risks of reoperation, implant removal or revision, and infection were found to correlate strongly with BMI. However, the risk of infection was even more extreme for patients with a high BMI after total hip arthroplasty than for patients with a high BMI after total knee arthroplasty. For total hip arthroplasty, no correlation was found between BMI and aseptic implant loosening, but such a correlation was demonstrated for patients who had undergone knee arthroplasty, with a peak at a BMI of about 40 kg/m2.
The results of this study should be interpreted in the light of certain limitations. This series consisted of patients treated at a single institution. Although this limited generalizability, it also represented a strength by minimizing potential institutionally based confounders. Also, although the duration of follow-up was limited for some patients, the Kaplan-Meier survivorship analysis accounted for the differences in patient follow-up. We accounted for age, sex, diagnoses, and surgical indications, but we did not account for other patient and surgically related factors, such as the type of implants used. For the complication of infection, high BMI is correlated with risk of diabetes mellitus, which is an independent risk factor for infection, and high BMI also may correlate with other nutritional factors such as serum albumin level that also affect infection risk. Separating the effects of these factors from BMI in individual patients was beyond the scope of this study and represents an area for further study. The frequency of patients with high BMI progressively increased during the time period covered by the study and the practice of knee surgery also changed during the time period of the study. For this reason, the analysis also was performed with a multivariate model including the date of the surgical procedure, which did not dramatically change the results. Finally, significant findings of a small magnitude in large study cohorts may not represent clinically important differences for individual patients.
The major strengths of this study included a large patient population, prospective data collection, and the long period (28 years) of analysis. Furthermore, there was completeness of follow-up for specific end points. Finally, the uniqueness of this study involves the examination of the shape of the curve correlating the effect of BMI over the observed range to each outcome of interest.
In conclusion, this series contributes to our understanding regarding the association between BMI and the outcomes after total knee arthroplasty, including reoperation, revision, and common complications. These data may be used by patients, surgeons, and physicians to make informed decisions concerning the relative risks and benefits of elective total knee arthroplasty in patients with different BMIs. Both the provider and the patient may consider the relative risk of a complication as well as the absolute risk of a complication when considering the risks and benefits of an elective procedure. Although the efficacy of preoperative interventions to reduce BMI on preventing complications remains unknown, these findings highlight the potential for collaborative efforts between care providers to reduce complications by modifying preoperative risk factors, such as BMI, prior to elective procedures. In health-care policy and the evolution of different delivery models, these findings may be considered as part of preoperative risk stratification.
Tables showing multivariate analyses among total knee arthroplasties stratified by BMI for reoperation, revision or implant removal, and complication risk and for revision or implant removal risk for mechanical failure are available with the online version of this article as a data supplement at jbjs.org.
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