One in Four Children Experience BMI Increase After Lower Extremity Surgery Without Return to Preoperative Levels: A Retrospective Cohort Study.

INTRODUCTION: Weight and body mass index (BMI) are influenced by many intrinsic and extrinsic factors, including activity level. After undergoing orthopaedic surgery, a period of immobilization and activity restrictions typically follows, which may place patients at risk for an acute increase in weight and BMI. This study aimed to quantify the change in BMI preoperatively to 3 months postoperatively after treatment of lower extremity conditions and assess whether BMI returned to baseline levels by 1 year among patients with a clinically meaningful BMI increase at 3 months (BMI z-score ≥0.20). METHODS: A single-center, retrospective review was performed of patients younger than 18 years treated operatively for lower extremity fractures or undergoing an elective lower extremity surgery over a 3-year period. Multivariable logistic regression was used to evaluate the association between a clinically meaningful BMI increase at 3 months and baseline characteristics to determine potential "at risk" patients. RESULTS: In total 411 patients were included (average age 13.0 ± 3.1 y, 57.9% male). The average change in BMI from baseline to 3 months was 0.33 kg/m² (95% CI: 0.08-0.59), a mean change in BMI z-score of -0.040 (95% CI: -0.093 to 0.013). A clinically meaningful increase in BMI z-score (≥0.20) was found in 109 patients (26.5%). Of these patients, 49 had documented 12 to 18-month follow-up with only 28.6% (95% CI: 15.9-41.2%) returning to their preoperative BMI. The only baseline characteristic significantly associated with an increase in BMI z-score ≥0.20 within 3 months was diagnosis category (obesity-associated conditions vs. fracture and other orthopaedic conditions) [odds ratio (OR): 1.84, 95% CI: 1.04-3.25; P=0.037]. CONCLUSIONS: Postoperatively, when patients are immobilized with weight-bearing and activity restrictions, there is risk of clinically meaningful increases in BMI. This represents an opportunity for orthopaedic providers to incorporate means for weight management during the recovery period, which includes several in-person follow-up appointments. This may include simple education or appropriate referrals and evaluations by primary care providers and weight management specialists. LEVEL OF EVIDENCE: Level IV.