Abstract
Introduction
A search of the literature indicates several constrained total knee arthroplasty (TKA) systems are at risk for articular surface lockdown bolts backing out. The backing out of a lockdown bolt may lead to an unstable and/or painful knee and may necessitate revision. Upon backing out, the bolt may damage implant components and surrounding tissues. To date, studies in the literature have not simulated or replicated loosening of bolts in TKA. Therefore, the objectives of this study were to 1) develop a set of physiological loading parameters that challenge bolted articular surfaces; 2) evaluate whether significant bolt torque is lost during application of this loading to a CCK device with a bolt as a secondary locking mechanism.
Materials and Methods
Physical test parameters to loosen lockdown bolts were developed based on loading experienced during activities of daily living. Sinusoidal waveforms and timing were used to simulate worst case walking gait conditions. Compared to data from everyday activities in instrumented TKR patients, anterior posterior loads and internal/external torques exceeding the absolute maximums observed were selected. To transfer more shear and torsion to the joint interface, compressive load lower than typically reported for walking gait was used. Frequency was representative of walking gait motion.
The offset in torsional waveform enables a ratcheting motion to drive a loose bolt out of the joint: during external femoral rotation of a left knee, reduced compressive load and posterior directed femoral loading on a CCK spine creates a potential articular surface lift-off. The lift-off may grab the underside of the front bolt shoulder while external (CCW) rotation loosens the bolt. These loading conditions exist during toe-off of walking gait. Two CCK devices were evaluated to capture potential difference in performance: a medium articular surface combination and a smaller articular surface combination. Testing was performed on a load frame capable of rotation and vertical / horizontal translation.
Results
No bolts completely loosened when fully tightened. However, average loss in bolt torque of 39.3% on the medium and 21.5% on the smaller articular surface was observed. Loading led to reorientation of the articular surfaces verified by markings on the components. Additional constructs that were under-tightened intentionally to one-quarter of target torque value lost all bolt torque and completely backed out.
Discussion and Significance
The backing out of lockdown bolts in TKA has been reported in the literature but not replicated in-vitro. A challenging, physiologically relevant set of loading parameters was developed and applied to a CCK device with an articular surface lockdown bolt. Upon loading, the bolts experienced statistically significant loss of bolt torque which may be attributed to articular surface reorientation. Selected loading parameters led to complete bolt back-out in under-tightened constructs.
