The Radiographic Prepatellar Fat Thickness Ratio Correlates with Infection Risk Following Total Knee Arthroplasty

Obesity is a known risk factor for surgical site infections (SSI) following total knee arthroplasty (TKA). Current methods use body mass index (BMI) to predict infection risk in patients. However, BMI may not be the most accurate predictor because it does not account for fat distribution and muscle mass is included in the calculation. We sought to assess the impact of subcutaneous fat at the surgical site on risk of infection following a TKA. It has been shown that fat tissue thickness expands without a complementary increase in blood flow, leaving subcutaneous tissue with reduced oxygenation. This wound hypoxia impairs healing by multiple mechanisms; healing wounds have high oxygen demands and leukocytes need oxygen to create reactive oxygen species against infection. We conducted a retrospective study of 530 patients who had TKAs at John Peter Smith Hospital (JPS) from 2006-2010. Pre-operative lateral knee radiographs for each patient were reviewed and measured. Both patellar thickness and prepatellar fat thickness were obtained. Soft tissue thickness was divided by the thickness of the patella to create the prepatellar fat thickness ratio (PFTR). Additionally, diabetes status, smoking status, gender, and BMI were obtained. The infection criteria used was 2 (+) cultures, or 1 (+) culture plus one of the following: gross purulence or [greater than] 10 PMNs/hfp. We expect the PFTR to be a significant predictor of SSI and more accurate than BMI in this regard. It may be beneficial to assess the PFTR in the preoperative evaluation to properly inform the patient of infection risk and allow the physician to take additional precautions to reduce the risk of infection.