Is BMI Outdated in TKA Patient Selection?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 (BEYOND-BMI)

Is BMI Outdated in TKA Patient Selection?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.

While Body Mass Index (BMI) is currently used as a primary tool for risk stratification in patients undergoing Total Knee Athroplasty(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.

Study Overview

• Status: Not yet recruiting
• Conditions: BMI; Knee Replacement Arthroplasty; Regional Body Composition; Periprosthetic Infection; Knee Arthritis Osteoarthritis

Detailed Description

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.

• Study Type: Observational
• Enrollment (Estimated): 400

Contacts and Locations

• Study Contact: Name: Michelle Stephens; Phone Number: +447976152397; Email: research.sponsorship@elht.nhs.uk

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: No

• Sampling Method: Non-Probability Sample

Study Population

Adults aged over 18 meeting the study criteria who have undergone a primary TKR at East Lancashire Hospital NHS Trust within the defined study period will be included in the study. Patients will be identified using a combination of OPCS codes for primary TKA, theatre logs and electronic operation records. All eligible adults withint the study period will be included for analysis.

Description

Inclusion Criteria:

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

Exclusion Criteria:

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

Study Plan

How is the study designed?

Number of groups / cohorts

1

Cohorts and Interventions

Group / Cohort
Adults over 18 undergoing Primary Total Knee Replacement; This cohort includes adults undergoing Primary Total Knee Replacement within the defined study period meeting the exclusion and exclusion criteria. The end point is 12 months following index procedure with events of interest being periprosthetic joint infection.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Postoperative periprosthetic joint infection	The primary objective is to determine whether an increase in the thickness of the soft tissue (fat) over the front of the shin bone (known as pre-tubercular subcutaneous fat thickness) as measured on a lateral pre-operative x-ray is associated with an increased risk of postoperative infection within 12 months following primary total knee arthroplasty.	12 months from index procedure

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
BMI and risk of post-operative infection	To assess the association between Body Mass Index (BMI) and postoperative infection following TKA	12 months
BMI vs PTFT relationship	To evaluate the relationship between BMI and pre-tubercular subcutaneous fat thickness (PTFT)	12 months
PTFT influence on return to theatre	To assess the association between PTFT and return to theatre (within 30 days and 1 year)	12 months

Collaborators and Investigators

• Sponsor: East Lancashire Hospitals NHS Trust
• Investigators: Study Chair: Lee Hoggett, MBChB, FRCS, East Lancashire NHS Trust; Principal Investigator: Charles Hallett, MBBS, MRCS, East Lancashire NHS Trust

Study record dates

Study Major Dates

• Study Start (Estimated): August 17, 2026
• Primary Completion (Estimated): November 1, 2026
• Study Completion (Estimated): February 1, 2027

Study Registration Dates

• First Submitted: August 10, 2026
• First Submitted That Met QC Criteria: August 10, 2026
• First Posted (Actual): August 13, 2026

Study Record Updates

• Last Update Posted (Actual): August 13, 2026
• Last Update Submitted That Met QC Criteria: August 10, 2026
• Last Verified: August 1, 2026

More Information

Terms related to this study

• Keywords: BMI; Total Knee Replacement; Knee Arthritis; Regional anatomy; Periprosthetic infection
• Other Study ID Numbers: DEV144; 372419 (Other Identifier: IRAS)

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO
• IPD Plan Description: Individual participant data will not be shared because the dataset contains potentially identifiable patient information collected under NHS approvals for the purposes of this study only. Participant consent for public data sharing was not obtained, and sharing individual-level data would be subject to UK data protection legislation, institutional information governance requirements, and sponsor approval.

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No