L. Borgstede, L. Kirkpatrick, T. Johnson, J. Mason, Zimmer, Inc., Warsaw, IN
Modeling and testing of the patellofemoral joint (PFJ) presents simulation challenges. Chair rising, stair and squatting activities create demands on the PFJ approaching 3BW at 110o of tibiofemoral flexion. Retrievals of well functioning patellae may show deformation, creep and or polishing in contact areas while other components may also exhibit pitting, delamination and/or fracture. A method was developed to evaluate PFJ implant durability by in-vitro cyclic loading using an AMTI Knee simulator. A loading cycle of one squat followed by two walking motions was created to apply a distribution of high and low demand loads to the implant. Articulation was affected by mounting the femoral component on the arbor and the patellar component on the vertical actuator. Input waveforms were generated to control load, flexion and contact location. The patella fixture support was altered to minimize medial-lateral shear during waking while constraining the fixture so that a medially directed force, about 8-10% of the joint force, was applied to the patellar component during squat. Studies show that as deep flexion occurs, the patella transitions off the femur at approximately 100-120o of tibiofemoral flexion. Thus, simulation was limited to less than 120o of flexion. Tibiofemoral flexion from 0 to 120o was simulated by applying patellofemoral flexion from 0 to 84o. Joint force during the squat ranged from 0.5-3.0BW (bodyweight, BW=103 kg). Joint force during walk ranged from 0.2-0.5BW. The test duration consists of 219,000 cycles (219,000 squat and 438,000 walk motions). This is equivalent to 30 squats per day for 20 years. Contact and damage patterns created by this method are consistent with retrieved implants. Specimen fracture occurred near the 219,000-cycle end point, while pitting and delamination has occurred at cycle counts ranging from 13,000-30,000 cycles
Summary: Patellofemoral joint simulation presents challenges. Here, we develop a method to evaluate patellofemoral joint implant durability by in-vitro cyclic loading using an AMTI knee simulator. Contact and damage patterns created by this method are consistent with retrieved implants.
