Is BMI Outdated in TKA Patient Selection? Assessment of the Impact of Pre-tubercular Subcutaneous Tissue Thickness and the Risk of Postoperative Infection Following Total Knee Arthroplasty

Total knee arthroplasty (TKA) is a highly effective intervention for end-stage osteoarthritis, providing significant improvements in pain and function. However, postoperative complications-particularly surgical site infection and periprosthetic joint infection (PJI)-remain among the most serious adverse outcomes. These complications are associated with substantial morbidity, the need for revision surgery, prolonged hospitalisation, and increased healthcare costs.

Obesity is a well-recognised risk factor for complications following TKA, and Body Mass Index (BMI) is widely used as a surrogate measure to guide patient selection. Across the NHS, BMI thresholds are commonly applied, often between 40 and 45, with the aim of reducing postoperative risk.

Despite its widespread use, BMI has well-documented limitations. It does not differentiate between fat and lean mass, nor does it account for fat distribution or regional anatomical variation. As such, BMI may not accurately reflect the local soft tissue characteristics at the surgical site, which are likely to influence wound healing and infection risk.

Recent studies have explored the role of regional soft tissue measurements, such as pre-tubercular subcutaneous fat thickness (PTFT), in predicting postoperative complications. Evidence suggests that increased subcutaneous tissue thickness over the proximal tibia may be associated with higher rates of wound complications and infection following TKA. However, existing studies are limited in number, and there is a lack of data within UK-based populations and NHS clinical practice.

While BMI is currently used as a primary tool for risk stratification in patients undergoing TKA, its limitations raise concerns regarding its accuracy and fairness in guiding surgical decision-making. In particular, reliance on BMI alone may lead to the exclusion of patients who could safely benefit from surgery, while failing to identify others at increased risk due to unfavourable local anatomical factors.

Pre-tubercular subcutaneous fat thickness (PTFT) offers a simple, reproducible, and anatomically relevant measurement that directly reflects the soft tissue envelope at the operative site. As such may provide a more clinically meaningful assessment of infection risk than BMI.

This study aims to evaluate the association between PTFT and postoperative infection following TKA. By doing so, it seeks to determine whether PTFT could serve as an adjunct or alternative risk stratification tool in clinical practice.

The findings of this study have the potential to inform a more individualized approach to patient selection and preoperative counselling, improving shared decision-making and resource utilisation.

Postoperative infection following TKA is influenced by a combination of systemic and local patient factors. Body Mass Index is widely used as a surrogate marker of obesity and surgical risk; however, it is a crude measure that does not account for body composition, fat distribution, or regional anatomical variation.

PTFT represents a localized anatomical measurement of subcutaneous tissue overlying the proximal tibia, directly adjacent to the surgical field in TKA. Increased soft tissue thickness in this region may contribute to impaired wound healing through several mechanisms, including reduced tissue perfusion, increased dead space, prolonged operative time, and greater tension on wound closure.

Previous studies have demonstrated an association between increased subcutaneous tissue thickness and postoperative wound complications and infection following TKA. Unlike BMI, PTFT provides a patient-specific, anatomically relevant measurement that may better reflect the local biological environment influencing surgical outcomes.

This study aim to address this existing knowlege gap through adopting a retrospective observational approach to evaluate whether PTFT is a more accurate predictor of postoperative infection than BMI. By correlating radiographic measurements of PTFT with clinical outcomes, this study aims to address the limitations of BMI-based risk stratification and explore a more individualized approach to patient selection and risk assessment.

Inclusion criteria

• Adults aged 18 years or older at the time of surgery
• Underwent primary total knee replacement surgery
• Procedure performed at East Lancashire Hospitals NHS Trust (Burnley General Hospital or Royal Blackburn Hospital)
• Surgery performed between 1 November 2023 and 30 November 2024
• Availability of a preoperative lateral knee X-ray to allow measurement of soft tissue thickness

Exclusion criteria

• Patients who underwent revision knee replacement surgery
• Patients who underwent partial (unicompartmental) knee replacement surgery
• Patients aged under 18 years at the time of surgery
• Primary procedure performed outside East Lancashire Hospitals NHS Trust
• Absence of a suitable preoperative lateral knee X-ray for measurement of soft tissue thickness
• Patients who have registered a National Data Opt-Out