Tibial Baseplate Microstructure Governs High Cycle Fatigue Fracture In Vivo

Previous studies report rare occurrences of tibial baseplate fractures following primary total knee arthroplasty (TKA). However, at a microstructural scale, it remains unclear how fatigue models developed in vitro apply to fractures in vivo. In this study, we asked: (1) do any clinical factors differentiate fracture patients from a broader revision sample; and (2) in vivo, how does microstructure influence fatigue crack propagation? We identified three fractured tibial baseplates from an institutional review board exempt implant retrieval program. Then, for comparison, we collated clinical data from the same database for n = 2120 revision TKA patients with tibial trays. To identify mechanisms, we characterized fracture features using scanning electron and digital optical microscopy. Additionally, we performed cross sectional analysis using focused ion beam milling. The fracture cohort consisted of moderately to very active patients with increased implantation time (15.6 years) compared to the larger revision patient sample (5.1 years, p = 0.009). We did not find a significant difference in patient weight between the two groups (p = 0.98). Macroscopic fracture features aligned well with both previous retrieval analysis and in vitro baseplate fatigue tests. On a micron scale, we identified striations on each baseplate, demonstrating fatigue as the fracture mechanism. In vivo fatigue fracture processes depended on both the alloy (Ti-6Al-4V vs. CoCrMo) and the microstructure of the alloy formed during manufacturing. For Ti-6Al-4V, the presence of equiaxed or acicular microstructure influenced the fatigue crack propagation, the latter arising from large prior β grains and a Widmanstatten microstructure, degrading fatigue strength. CoCrMo alloy fatigue cracks propagated linearly, crystallographically influenced by planar slip. However, we did not document any features of overload or fast fracture, suggesting a high cycle, low stress fatigue regime. Ultimately, the crack profiles we present here may provide insight into fatigue fractures of modern tibial baseplates.