Tibial Post Fracture in Posterior-Stabilized Total Knee Arthroplasty: Is Fatigue Alone the Culprit?

Tibial post fracture in posterior-stabilized total knee arthroplasty (PS TKA) is a rare but serious complication resulting from insufficient post strength and polyethylene wear. Although highly cross-linked polyethylene (HXLPE) reduces wear compared to ultra-high-molecular weight polyethylene (UHMWPE), its mechanical strength is not improved. This study reports a series of tibial post fractures and evaluates whether fatigue alone accounts for such failures.

There were six patients (seven knees) who underwent revision for tibial post fractures reviewed retrospectively for demographic, implant, and surgical factors. Complementary cantilever fatigue tests were conducted using PS TKA tibial inserts fabricated from UHMWPE and HXLPE. Reversing antero-posterior (AP) loads were applied via surrogate femoral components at 0° and 75° flexion, without axial compression. There were two post designs tested: a chamfered post and a nonchamfered post with a larger proximal cross-section. Cyclic loading at two Hz began at ± 209 N for 10,000 cycles, with stepwise increase every 10,000 cycles. Additional tests at physiological and supraphysiological loads continued for up to 10 million cycles (Mc), simulating decades of in vivo use.

Fractures occurred a mean of 12.8 years postoperatively (range, 4.5 to 17.8). There were five fractures that had UHMWPE and two that had HXLPE inserts. The overall incidence was 0.22%. All retrieved components were chamfered and exhibited post-base narrowing consistent with wear. In testing, both post designs failed after more than 63,000 cycles at loads exceeding 1,254 N, above physiological AP forces. The nonchamfered design withstood greater than 1,254 N for 10 Mc, while the chamfered design endured 627 N for 10 Mc.

Both UHMWPE and HXLPE posts withstood physiological AP fatigue without failure, indicating that fatigue alone was insufficient to explain clinical fractures. Minimizing wear and optimizing post geometry are critical to preventing late tibial post fracture in PS TKA.