Most revision total knee arthroplasty patients experience excellent clinical outcomes. However, case studies document rare occurrences of metal component fractures promoted by the high cycles and low stresses that occur during daily use. In vivo, the mechanism that initiates metal fatigue fracture remains unclear. To address this gap, we collected implants from two retrieval programs. We asked: (1) what is the role of implant fixation in promoting fatigue fracture? (2) within modular junctions, what damage modes are associated with crack initiation? (3) how do equiaxed Ti-6Al-4V and wrought CoCrMo microstructures influence crack propagation? First, we compared clinical data for n = 8 fracture and n = 489 stemmed revision patients. Then, we performed retrieval analysis, combining radiographs with scanning electron microscopy and X-ray photoelectron spectroscopy. We found no differences in weight (p = 0.07) or implantation time (p = 0.10) when comparing revision and fracture cohorts. Radiographs of fractured components showed insufficient metaphyseal fixation at or near the stem's modular junction. At the taper interface, we identified Ti-6Al-4V thread wear and CoCrMo pitting. Sectioning near a Ti-6Al-4V crack initiation site revealed an oxidized interface and an intergranular corrosion attack. On the free surface, TiO increased (p 2 decreased (p < 0.0001) after 20 min of sputter time. Fatigue cracks propagated through equiaxed Ti-6Al-4V and wrought CoCrMo grains. These data support a hypothesized mechanism where insufficient implant fixation with bone or cement promotes localized damage accumulation within the modular junction, fatigue crack initiation and subsequent fracture.
Kurtz et al. (Wed,) studied this question.