Abstract
Introduction
In hip arthroplasty, it has been shown that assembly of the femoral head onto the stem remains a non-standardized practice and differs between surgeons [1]. Pennock et al. determined by altering mechanical conditions during seating there was a direct effect on the taper strength [2]. Furthermore, Mali et al. demonstrated that components assembled with a lower assembly load had increased fretting currents and micromotion at the taper junction during cyclic testing [3]. This suggests overall performance may be affected by head assembly method. The purpose of this test was to perform controlled bench top studies to determine the influence of impaction force and compliance of support structure (or damping) on the initial stability of the taper junction.
Materials and Methods
Test Specimens
Testing was performed on 36mm +5mm CoCr heads combined with prototype Ti6Al4V locking taper analogs both machined with approximately a 5.67º taper angle. To minimize sample variation, the locking taper analogs were dimensionally matched with CoCr femoral heads to maintain a uniform angle difference. Prior to testing, samples were cleaned with isopropanol and allowed to dry.
Effect of Peak Force Magnitude
Testing was performed on a rigid setup where a 10N preload was applied to the femoral head axially. Heads were assembled with loads ranging from 2kN–10kN using an impaction tower and seating loads were recorded at a collection rate of 273kHz. After assembly, tensile loads were applied until the taper junction was fully disassembled and distraction loads were recorded at a collection rate of 500Hz.
Effect of Damping
40 durometer rubber pads were placed underneath the trunnions as well as to the striking surface of the impaction tower to simulate compliance in the supporting structure and the surgical instruments. Aside from the added damping, testing was performed identical to the rigid setup tests.
Results
Taper stability (as assessed by disassembly forces) increased linearly with peak assembly force with an R2 value of 0.95 for both rigid and compliant groups (Figure 1). On average a 46% larger input energy was required in the compliant group to achieve a comparable impaction force to the rigid group (Figure 2). However, the correlation between the assembly load and distraction force was not affected.
Discussion
As shown in previous studies, impact force has a large effect on initial taper stability. An interesting finding in this study was that system compliance has a large effect on the applied assembly force. The addition of a compliant setup was intended to simulate a surgical scenario where factors such as the patient's leg positioning, patient mass, surgical instruments, and surgical approach may influence the resulting compliance due to the dissipation of impaction energy and reducing the applied impaction force. Based on test results, surgical procedure as well as patient variables may have a significant effect of initial taper stability.
For any figures or tables, please contact authors directly (see Info & Metrics tab above).
