Fibula-Tibia Distance and Revision for Instability After Primary Total Knee Arthroplasty: An Exploratory Case-Control...

Background/Objectives: Instability without component loosening is a leading indication for revision total knee arthroplasty (TKA), yet no preoperative radiographic marker of this outcome has been validated. We examined whether preoperative fibula-tibia geometry on standing anteroposterior (AP) radiographs differs between patients subsequently revised for instability and non-revised controls, and whether any difference could be explained by radiographic rotation. Methods: In this exploratory retrospective case-control study, preoperative standing AP radiographs of 22 patients revised for instability without loosening (2014-2024) were compared with 29 primary-TKA controls frequency-matched on age, sex and comorbidity, analyzing the index (operated) knee in both groups. Three blinded orthopedic residents annotated 17 tibiofibular landmarks; 135 pairwise inter-landmark distances normalized to fibular head width were screened, and the distance with the best combined reliability-discrimination rank was carried forward with bootstrap optimism correction and nested repeated cross-validation. Four complementary rotation sensitivity analyses compared two rotation proxies between groups, adjusted the distance for them, decomposed it into vertical and horizontal components with an in silico rotation model, and assessed bilateral agreement. Results: The fibula lateral shaft-tibia lateral meta-diaphysis distance was shorter in revised patients (0.76 ± 0.16 vs. 0.93 ± 0.17 fibular-head-width units; p = 0.0006; area under the curve (AUC) 0.78 [95% CI 0.65-0.90], optimism-corrected 0.72, cross-validated 0.66; inter-rater ICC(A,1) 0.80). No statistically significant difference in either rotation proxy was detected, and the association persisted after adjustment for both proxies and side (odds ratio 4.00 per standard deviation, 95% CI 1.59-10.03). The difference resided mainly in the vertical component, which is less sensitive to simulated axial rotation under the model assumptions, and simulated rotation of 0-30° did not reproduce it. The bilateral agreement was moderate (r = 0.55), but the subsequently revised knee measured shorter than the contralateral knee. Conclusions: In this exploratory cohort a shorter fibula-tibia distance was associated with subsequent revision for instability, and the association persisted across the rotation sensitivity analyses performed. The distance is a hypothesis-generating candidate marker; it does not establish the risk of instability in unselected patients, and prospective, three-dimensional validation is required before any clinical use.