Addressing the Inadequacy of BMI: Superior Prediction of Aseptic Loosening Risk in TKA Using Weight-to-Implant Surface Area Ratio

Commentary Aseptic loosening remains the primary cause of revision total knee arthroplasty (TKA), accounting for >37% of single-stage revision surgeries1. While a strong correlation between high body mass index (BMI) and increased risk of aseptic loosening has been well established, recent evidence suggests that BMI may not be an ideal measure for this purpose2. From a biomechanical perspective, aseptic loosening arises from the loss of mechanical fixation at the bone-implant interface, caused by the remodeling process and resorption of trabecular bone3. Consequently, an effective predictor should focus on the mechanical stresses and strains experienced at the bone-implant interface rather than a global index such as BMI. Huebschmann et al. propose the weight-to-tibial baseplate surface-area ratio (weight/SA) as a predictor of aseptic loosening in TKA. Their study analyzed data from 16,368 patients operated on by 56 surgeons, with revisions due to aseptic loosening of the tibial baseplate identified. The authors showed that patients at or above the cutoff weight/SA value of 0.0162 kg/mm2 exhibited a 3-fold increased risk of revision due to aseptic tibial baseplate loosening. Furthermore, factors such as BMI, age, sex, and American Society of Anesthesiologists (ASA) score were not found to be significant predictors in their analysis. None of the patients who had received a tibial stem extension developed tibial baseplate loosening requiring revision; however, their analysis indicated that, among patients without a stem extension, a higher rate of revision for aseptic loosening was observed in patients at or above, compared with below, the cutoff weight/SA value. Given the findings of this study, an important question arises regarding the influence of the baseplate keel design on the risk of aseptic loosening. The reported surface area was that of the baseplate in the axial plane at the bone-implant interface, ignoring the surface area of the keel. The absence of aseptic loosening in patients who had received a stem extension suggests that the bone-implant surface area used should include the area of the keel, as addition of the stem extension does not increase the axial surface area, but notably decreases the risk of aseptic loosening. An inherent limitation of studies related to revision surgeries is the relatively small number of patients with revision in the study cohort. An alternative solution would be to conduct numerical simulations whose models are verified and validated using information obtained from clinical studies, which can then be used to answer questions that are difficult, if not impossible, to address through prospective randomized studies. In summary, Huebschmann et al. have shown that weight/SA as a predictor of the risk of aseptic tibial baseplate loosening in TKA is superior to BMI. Although this is without a doubt a step forward, there are questions that remain unanswered, and further research is necessary to address the gaps in our knowledge of the underlying cause of aseptic loosening in TKA and how to assess the risk associated with it. Using the total implant surface area that is in contact with bone could decrease the effect of keel shape on the analysis and further improve the accuracy of the calculated risk. Additionally, effects of soft-tissue-related conditions, such as sarcopenia and ligamentous laxity, could be studied to determine their contribution to the risk of aseptic loosening in TKA.