Does BMI Matter in Total Joint Surgery?  

Does BMI Matter in Total Joint Surgery? The answer might surprise you.

A Note from CODE: Welcome to Professor’s Corner, a monthly series brought to you by Steve Wood, Ph.D.

Are patient weight and size among the patient-specific factors that impact orthopedic outcomes in regard to implant surgery? Intuitively, operating on a patient with a high body-mass index (BMI) presents additional challenges to a surgical team. Does BMI also affect functional outcomes? To inquire about this relationship, CODE Technology ran an analysis using patient-reported outcome (PRO) data from the CODE Orthopedic Registry. We did this for total knee replacements (TKA) and total hip replacements (THA).

THA Results

For THA, our analysis set was 386 patients. For these patients, we obtained pre-op, 3-month, and 1-year post-op HOOS scores and BMI data. Interestingly, we found that BMI is negatively correlated with orthopedic outcomes during the pre-op timeframe. But, there was no significant correlation between BMI and the 3-month and 1-year post-op HOOS scores (overall and sub-scores). Results varied a bit by age and gender, but a significant relationship was not present in this data.

TKA Results

With TKA data there were 622 patient-reported outcomes analyzed. This includes BMI data, and pre-op and post-op KOOS scores. As we saw with the THA results, there were no significant correlations between BMI scores and post-op KOOS scores. These results echo those from a 2012 published study* that deployed the Oxford Knee Score as a measure of post-op functional outcome.

Speaking with other surgeons, we encountered a guideline that limits or declines surgery on patients with a BMI of 40 or greater. To consider the efficacy of this approach relative to functional outcomes, we segmented both data sets at this level and compared average overall HOOS and KOOS scores, post-op to see if this rule mattered.

Interestingly, there was no significant difference in average HOOS or KOOS scores (Overall and Junior) between the set of patients with BMI scores of 39 or less and those with BMI scores of 40 or more.

Where do we go from here?

As the intuition that a higher BMI would lead to lower functional outcomes is so appealing, other factors should also be considered. Additional analysis with BMI in concert with other patient-specific patient data is warranted. However, these results indicate that patients with higher BMI scores are achieving comparable functional outcome scores to those with lower indexes.

*The Association Between Body Mass Index and the Outcomes of Total Knee Arthroplasty.”
Paul Baker et al, The Journal of Bone and Joint Surgery, 2012;94:1501-8.