New Studies on BMI and Hip Replacement

Obesity is one of the most serious public health problems in the 21^st century, and body weight is becoming an important consideration in orthopaedic procedures, especially joint arthroplasty. Two new studies in the February 3, 2016 Journal of Bone & Joint Surgery illuminate the relationship between body mass index (BMI) and hip-arthroplasty outcomes.

In a prognostic study based on registry data (21,361 consecutive hip replacements), Wagner et al. analyzed postsurgical complications and reoperations using BMI as a continuous variable. They found strong associations between increasing BMI and increasing rates of reoperation, implant revision or removal, early hip dislocation, and both superficial and deep infections. Although researchers are just starting to examine the efficacy of preoperative interventions to reduce BMI (see related OrthoBuzz post), Wagner et al. suggest that “collaborative interventions between care providers and patients may be undertaken to modify risk factors, such as BMI, before elective procedures.” A commentary on this study lauds the authors for analyzing BMI with a “dose-response” perspective, but the commentators note that “BMI neither remains constant nor follows a predictable trend over time.”

In a separate therapeutic study by Issa et al., clinical and patient-reported outcomes of primary THA were lower in super-obese patients (BMI ≥ 50 kg/m²) than in matched patients with normal BMI (<30 kg/m²). Specifically, after a mean follow-up of six years, compared with the normal-BMI group, the super-obese group had:

• A 4.5 times higher odds ratio (OR) of undergoing a revision
• A 7.7 times higher OR of surgical complications, including superficial and deep infections
• Significantly lower mean values on the Harris hip score, the physical and mental components of the SF-36, and the UCLA activity score.

Despite these between-group findings, super-obese patients still experienced significant clinical improvements compared with their preoperative status. However, they saw an average of 2.5 previous surgeons who refused to perform the procedure prior to being referred to the authors.