Abstract
INTRODUCTION
The cup component of modern resurfacing systems are often coated creating a cementless press-fit fixation in the acetabulum based on surgical under-reaming, also enabling osseoconduction/integration. Due to the higher density of cortical bone along the antero-superior and postero-inferior regions of the acetabulum, the greatest forces occur between the anterior and posterior columns of the pelvis. This produces pinching of the implant that can result in deformation of the cup. Metal shell/modularpress-fit acetabular cups are susceptible to substantial deformation immediately after implantation. This deformation may affect the lubrication, producing point loading and high friction torques between the head and the cup that increase wear and may lead to head clamping and subsequent cup loosening. We sought to test a novel ceramic on ceramic (CoC) hip resurfacing system that should allay any concerns with the Adverse Reaction to Metal Debris associated with metal on metal (MoM) resurfacing devices.
AIM
We sought to quantify the deformation of a novel CoC hip-resurfacing cup after implantation, using a standard surgical technique in a cadaveric model, and compare to the MoM standard. We also assessed if the design clearances proposed for this CoC hip resurfacing implant are compatible with the measured deformations, allowing for an adequate motion of the joint.
METHODS
The pelvis from four fresh frozen cadavers were placed into the lateral position. One surgeon with extensive experience in hip resurfacing surgery (JH) prepared all the pelvises for implantation using a posterior approach to the joint and sequential reaming of the acetabulum to 1mm below the implant outer diameter.
The acetabulum components were then impacted into the prepared pelvis. We used four ceramic and four metal implants of equal and varying size. (2 × (40/46mm, 44/50mm, 50/56mm, 52/58mm)).
The acetabulum cup bearing surface diameter and deformation was measured using a GOM-ATOS optical high precision 3D scanner. 3-Dimensional measurements were taken pre-implantation, immediately after and at 30 minutes following implantation. Two techniques were used to analyse the 3D images: by maximum inscribed diameter and by radial segments. These were compared to the known articulating surface clearance values.
RESULTS
The diameter of the cups in both metal and ceramic systems was reduced after implantation when analysing by maximum inscribed diameter and by radial segments. This deformation was maintained at 30 minutes. We can infer there is no significant bone stress relaxation effect following implantation.
On ceramic cups, the deformation was larger in larger sizes. However, the 44/50 (the second smallest cup) deformed the least. Despite this, the difference in deformation between these two sizes is minimal. The deformation of sizes 50/56 and 52/58 was equivalent. For the metal cups, there was not a clear correlation between the cup size and the deformation. The largest cup size had the same deformation as the smallest size.
CONCLUSIONS
The deformation following implantation of the cup component in a ceramic acetabulum resurfacing behave similarly to a metal implant. Cup deformation measured after implantation is minimal when compared to the minimum design clearance in both systems.
