Roman Preuss

VALIDATION OF AN OPTICAL SYSTEM TO MEASURE ACETABULAR SHELL DEFORMATION IN CADAVERS

Deformation of the acetabular shell at the time of surgery can result in poor performance and early failure of the hip replacement. The study aim was to validate an ATOS III Triple Scan optical measurement system against a co-ordinate measuring machine using in vitro testing and to check repeatability under cadaver laboratory conditions. Two sizes of custom-made acetabular shells were deformed using a uniaxial/two-point loading frame and measured at different loads. Roundness measurements were performed using both the ATOS III Triple Scan optical system and a co-ordinate measuring machine and then compared. The repeatability was also tested by measuring shells pre- and post-insertion in a cadaver laboratory multiple times. The in vitro comparison with the co-ordinate measuring machine demonstrated a maximum difference of 5u2009µm at the rim and 9u2009µm at the measurement closest to the pole of the shell. Maximum repeatability was below 1u2009µm for the co-ordinate measuring machine and 3u2009µm for the ATOS III Triple Scan optical system. Repeatability was comparable between the pre-insertion (below 2u2009µm) and post-insertion (below 3u2009µm) measurements in the cadaver laboratory. This study supports the view that the ATOS III Triple Scan optical system fulfils the necessary requirements to accurately measure shell deformation in cadavers.

A novel method for measuring acetabular cup deformation in cadavers

Press-fit acetabular shells are subject to compressive loading conditions upon insertion during surgery. These conditions may result in deformation of the shell, potentially adversely affecting the performance of any liner fitted within it. This technical note describes the method used to measure acetabular shell deformation in a cadaver study using an optical GOM ATOS Triple Scan system. Six custom-made titanium acetabular shells were scanned prior to and immediately following insertion into three cadavers (one shell per acetabulum). A three-dimensional colour map was created for each shell, and cross sections were taken at the rim to determine the maximum diametrical deformation. Values of between 30 and 150 µm were recorded, with the results suggesting a size-dependent response.

The influence of the strength of bone on the deformation of acetabular shells: a laboratory experiment in cadavers

Concerns have been raised that deformation of acetabular shells may disrupt the assembly process of modular prostheses. In this study we aimed to examine the effect that the strength of bone has on the amount of deformation of the acetabular shell. The hypothesis was that stronger bone would result in greater deformation. A total of 17 acetabular shells were inserted into the acetabula of eight cadavers, and deformation was measured using an optical measuring system. Cores of bone from the femoral head were taken from each cadaver and compressed using a materials testing machine. The highest peak modulus and yield stress for each cadaver were used to represent the strength of the bone and compared with the values for the deformation and the surgeons subjective assessment of the hardness of the bone. The mean deformation of the shell was 129 µm (3 to 340). No correlation was found between deformation and either the maximum peak modulus (r² = 0.011, t = 0.426, p = 0.676) or the yield stress (r² = 0.024, t = 0.614, p = 0.549) of the bone. Although no correlation was found between the strength of the bone and deformation, the values for the deformation observed could be sufficient to disrupt the assembly process of modular acetabular components.

Acetabular shell deformation as a function of shell stiffness and bone strength.

Press-fit acetabular shells used for hip replacement rely upon an interference fit with the bone to provide initial stability. This process may result in deformation of the shell. This study aimed to model shell deformation as a process of shell stiffness and bone strength. A cohort of 32 shells with two different wall thicknesses (3 and 4u2009mm) and 10 different shell sizes (44- to 62-mm outer diameter) were implanted into eight cadavers. Shell deformation was then measured in the cadavers using a previously validated ATOS Triple Scan III optical system. The shell–bone interface was then considered as a spring system according to Hooke’s law and from this the force exerted on the shell by the bone was calculated using a combined stiffness consisting of the measured shell stiffness and a calculated bone stiffness. The median radial stiffness for the 3-mm wall thickness was 4192u2009N/mm (range, 2920–6257u2009N/mm), while for the 4-mm wall thickness the median was 9633u2009N/mm (range, 6875–14,341u2009N/mm). The median deformation was 48u2009µm (range, 3–187u2009µm), while the median force was 256u2009N (range, 26–916u2009N). No statistically significant correlation was found between shell stiffness and deformation. Deformation was also found to be not fully symmetric (centres 180° apart), with a median angle discrepancy of 11.5° between the two maximum positive points of deformation. Further work is still required to understand how the bone influences acetabular shell deformation.