Adequate canal-filling ratio and extension stem length reduce radiographic loosening in revision total knee arthroplasty using hybrid fixation with grit-blasted, fluted stems.

Abstract

The extension stem plays a critical role in ensuring mechanical stability during revision total knee arthroplasty (RTKA). We hypothesized that higher canal-filling ratios (CFR) and sufficient extension stem lengths are associated with reduced radiographic loosening. This study aimed to test this hypothesis in revision total knee arthroplasty (RTKA) using hybrid fixation and to determine the optimal cutoff values. We retrospectively analyzed patients undergoing RTKA from 2002 to 2020. Inclusion criteria were minor bone defects, revision for aseptic reasons or reimplantation after infection eradication, hybrid fixation, implantation with the U2 PSA™ Knee System (United Orthopedics ®, Taiwan), and ≥24-month follow-up. Prosthesis loosening was radiographically defined using the modified Knee Society scoring system. Receiver operating characteristic curve analysis was used to determine cutoff values for the CFR and stem length related to prosthesis loosening. Univariate logistic regression analysis was used to assess the relationship between CFR-related parameters and prosthesis loosening. Radiographic loosening occurred in 21 out of 79 cases. Demographics did not differ between loosening and non-loosening groups. Univariate logistic regression analysis identified protective factors against loosening, including a femoral CFR ≥ 0.83 with a stem length ≥20.7 mm, and a tibial CFR ≥ 0.77 with a stem length ≥18.7 mm. To mitigate the risk of post-RTKA aseptic loosening using hybrid fixation, we recommend a target femoral CFR ≥ 0.83 with a stem length exceeding 20.7 mm and a tibial CFR ≥ 0.77 with a stem length exceeding 18.7 mm.