Summary

Patient factors (age, sex, and body mass index) appear to drive survivability outcomes after primary total knee arthroplasty more than the implant factors of the size and/or stem choice.

Abstract

Introduction

Elevated body mass index (BMI), prosthesis design, and implant alignment are contributing variables in total knee arthroplasty (TKA) that have been shown to directly affect functional outcomes and early revision rates. To date, most of the literature has examined the individual impact of BMI, various TKA prosthesis designs, and implant positioning on the associated rate of complications in TKA survivability. However, there has been little research on the influence of component sizing with survivorship. The objective of this study was to determine the relationship between increased BMI and tibial prosthesis design characteristics. Specifically, the study evaluated whether 1) tibia contact surface area, 2) tibial base plate surface area, or 3) tibia stem length was related to higher revision rates or longer implant survivability in TKA patients that had an increased BMI.

Methods

A retrospective review and analysis of primary TKA surgeries performed in the past 20 years using an institutional database. Five implant designs were selected from the institutional database for inclusion. Endpoints were selected as either 1) TKA requiring a revision arthroplasty or 2) a currently surviving implant. Reason for revision included trauma, aseptic loosening, pain, instability, arthrofibrosis/stiffness, or infection. Patient-specific information recorded in the arthroplasty database was retrieved for data analysis including sex, age, derived BMI, implant specific models and sizes, current survival time, and reason for revision if performed. Three implants from the same manufacturer were grouped, as they have the same tibial base plate size. Statistical analysis was conducted using SPSS. A multivariate cox regression model was performed, and significance was accepted at p≤0.05.

Results

A retrospective review of n=8548 primary TKA surgeries were included in the study. Male sex was significantly higher in the Company 1 group (51%) relative to the Company 2 group (40%) and the Company 3 groups (39%) (p<0.01). Age and BMI were not significantly different between groups (p=0.19 and p=0.40, respectively). Cumulative survivability of all implants was 97.7% at 5 years and 97.2% at 10 years. There was no significant correlation between BMI and tibia contact surface area (p=0.10) or tibia stem length (p=0.29). There was no statistically significant difference in the overall revision (p=0.23-0.28) or revision due to infection (p=0.48-0.82) between all implant choices. Male sex (p=0.02) and BMI (p<0.01) were significantly correlated with revision. In all implants, BMI (p<0.01), contact surface area (p=0.04), and male gender (p<0.01) was significantly correlated a required revision due to an infection. When stratified based on BMI, there was a significant decrease in survivability based on BMI ≥ to 40kg/m2 compared to BMI < 30 kg/m2 (p<0.01) and BMI 30-40 kg/m2 (p<0.01).

Conclusion

There was no correlation between BMI and implant size (tibia contact surface area or stem length). All three implant categories demonstrated comparable cumulative and infection-specific revision rates. A BMI ≥ 40 kg/m2 was associated with decreased implant survivability. This suggests that patients with high BMI do not necessarily require larger implants, and surface area and stem length do not seem to affect survivability of the implant.