THE DESIGN OF STEM INTRODUCERS AND THE SURGICAL TECHNIQUE ARE IMPORTANT IN ACHIEVING AN OPTIMAL IMPLANT-CEMENT INTERFACE

A significant variation exists between polished taper slip (PTS) stem designs, associated fractured rates amongst them, and in the design of stem introducers used. Achieving a conforming implant-cement interface (ICI) is crucial to optimal function of PTS implants. Movement of the stem within the setting cement during surgery should be uniplanar and not be associated with any unplanned deviations. This is, in-part, controlled by the stem introducer can introduce excessive movement potentially leading to incongruent regions between implant and cement mantle creating cement defects and/or unequal cement mantles compromising the ICI. The aim of this study was to assess movement when using different introducer designs. We compared four stem introducer designs used with two commonly used PTS stems. The stems were mounted using a silicone rubber compound to simulate setting cement at different time points (early=soft, late=hard). The stem tips were left clear, and an inertial measurement unit attached to measure acceleration, angular velocity, and rotation. Participating surgeons (n=16) were asked to maintain stem position for ten seconds before releasing the introducer. Simulation of soft cement conditions showed a mean root mean square (RMS) value ranging from 0.10g to 0.30g for acceleration, 12.75°/s to 67.94°/s for angular velocity, and 2.02° to 6.03° for rotation with significant differences (P < 0.05) noted between different stem introducers. Introducers that included an additional mechanism to secure the stem to the introducer showed increased stem movement on detachment compared to simpler designs that did not. Simulation of later insertion during the curing process (hard) showed a similar pattern, with a lower overall range of movement. Our results showed that introducer design had a significant impact on stem movement within the setting cement. Furthermore, its removal earlier in the setting reaction resulted in increased movement. These findings highlight the importance of instrument design and correct technique in achieving the optimal ICI. Surgeons should be mindful of their choice of stem introducer and optimise their technique to account for their choice.