Abstract
Introduction
Successful cementless acetabular designs require sufficient initial stability between implant and bone (with interfacial motions <150 μm) and close opposition between the porous coating and the reamed bony surface of the acetabulum to obtaining bone ingrowth and secondary stability. While prior generations of cementless components showed good clinical results for long term fixation, modern designs continue to trend toward increased porosity and improved frictional characteristics to further enhance cup stability.
Objectives
We intend to experimentally assess the differences in initial stability between a hemispherical acetabular component with a highly porous trabecular tantalum fixation surface (Continuum® Acetabular System, Zimmer Inc, Warsaw, IN)(Fig 1) and a hemispherical component with the new highly porous Trabecular Titanium® surface (Delta TT, Lima Corporate, Italy)(Fig 2) manufactured by electron beam melting.
Material and methods
A total of 16 cups were used, 8 for each type. Each cup was used 4 times. Cups were implanted in polyurethane foam blocks with 1mm interference fit and subsequently edge loaded to failure. Two different foam block densities (0.24 g/cm3 and 0.32g/cm3) were used to model low- and high-density bone stock. Each cup was seated into a block under displacement control using a servohydraulic test machine (MTS Bionix 858, Eden Praire, MN) to engage the locking mechanism until axial forces reach 8 to 10 kN. During insertion, force and displacement were recorded to determine the implantation force for each component. After seating, initial acetabular component fixation was assessed using an edge loading test. Descriptive statistics are presented as means and standard deviations for continuous variables. The Kruskal-Wallis test was used to assess the effect of Cup on the outcomes: (1) Insertion force, (2) Insertion energy, (3) Ultimate load, (4) Yield load, and (5) Ultimate Energy. Pairwise comparisons were done using Mann-Whitney U test for significant outcomes and multiple comparisons were adjusted using Bonferroni correction. All analyses were performed with SAS version 9.3 (SAS Institute, Inc., Cary, NC, USA); a p-value less than 0.05 was considered statistically significant.
Results
Delta TT cup required the same seating force (p=0.014) and 18% higher insertion energy (p=0.002) for fully seating compared to Continuum cup, however this difference is not clinically relevant. Delta TT cup exibithed more stability, as exibithed by significantly higher (35%) energy to ultimate load (p=0.014). No statistical differences were found in Ultimate load and Yield load among the 2 cups. Cups in higher density foam required higher force and energy to be seated. In edge load testing higher densities blocks generated higher force and energy accross all cup designs.
Conclusions
The result of this study indicate increased interface stability in Trabecular Titanium cup compared to Porous tantalum cup with a low incresing in the energy required for fully seating.
