Elevated BMI Is a Risk Factor for Failure to Achieve Minimal Clinically Important Difference After TKA

Among patients who undergo total knee arthroplasty (TKA), obesity is well known to be associated with longer operative time, longer hospital stays and increased likelihood of discharge to rehabilitation facilities. Now, researchers at Massachusetts General Hospital have defined another reason to target preoperative obesity and overweight: the conditions contribute to poorer subjective outcomes of TKA.

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Akhil Katakam, MBA, orthopaedic surgery research fellow, Charles R. Bragdon, PhD, former associate director of clinical studies at the Harris Orthopaedic Laboratory, Christopher M. Melnic, MD, orthopedic surgeon, and Hany S. Bedair, MD, chair of the Center for Hip & Knee Replacement, all of the Department of Orthopaedic Surgery at Massachusetts General Hospital, and a colleague report the details in The Journal of Arthroplasty.

Study Methods

Mass General and six affiliated academic and community hospitals maintain an arthroplasty registry. The researchers used it to identify 1,059 patients who underwent primary TKA between January 2016 and December 2019 and completed the Knee Injury and Osteoarthritis Outcome Score–Physical Function Short Form (KOOS-PS), both before TKA and approximately one year afterward.

The KOOS-PS assesses the difficulty of performing seven activities: rising from bed, putting on socks/stockings, rising from sitting, bending to the floor, twisting/pivoting on the injured knee, kneeling and squatting. The raw score of 0–28 is converted to a true interval score of 0–100.

The primary outcome of interest in this study was the failure to achieve the one-year minimal clinically important difference (MCID) on the KOOS-PS. The MCID proved to be 8.2, calculated as one-half the standard deviation of change from preoperative to postoperative assessments for the entire cohort.

Results

On multivariable logistic regression analysis, failure to achieve the one-year MCID was associated with:

• Increasing BMI as a continuous variable (OR, 1.03; P=0.03); each increase in BMI by 1 kg/m² increased the risk of failure to achieve the MCID by 3%
• Revision within the postoperative year (OR, 6.63; P<0.001)
• Increasing preoperative KOOS-PS score (OR, 1.06; P<0.001)

The risk of failure to achieve the MCID grew stronger when BMI was evaluated as a categorical variable. Relative to normal BMI (18.5–25 kg/m²):

• Overweight (25–30 kg/m²)—OR, 1.77 (P=0.01)
• Obesity class I (30–35 kg/m²)—OR, 1.76 (P=0.01)
• Obesity class II (35–40 kg/m²)—OR, 1.83 (P=0.02)
• Obesity class III (>40 kg/m²)—OR, 2.06 (P=0.03)

Counseling Patients

Most patients in all BMI classes achieved the MCID, including 71% of those in the obesity class III cohort. This study should not dissuade surgeons from offering TKA to patients with elevated BMI. Rather, the findings can guide counseling to help patients manage their expectations.

Multiple studies have shown that reported satisfaction after TKA is more strongly associated with patient expectations before the procedure than with physical function afterward. Therefore, patients who are overweight or obese should be informed that, compared with people of normal BMI, they have less chance of achieving a difference in their knee functioning that's perceptible to them.

This caution is particularly important for patients whose preoperative score on the KOOS-PS (or a similar patient-reported outcome measure) shows relatively high functioning and relatively low pain.

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