A novel method to assess primary stability of press-fit acetabular cups

Abstract

Initial stability is an essential prerequisite to achieve osseointegration of press-fit acetabular cups in total hip replacements. Most in vitro methods that assess cup stability do not reproduce physiological loading conditions and use simplified acetabular models with a spherical cavity. The aim of this study was to investigate the effect of bone density and acetabular geometry on cup stability using a novel method for measuring acetabular cup micromotion. A press-fit cup was inserted into Sawbones(®) foam blocks having different densities to simulate normal and osteoporotic bone variations and different acetabular geometries. The stability of the cup was assessed in two ways: (a) measurement of micromotion of the cup in 6 degrees of freedom under physiological loading and (b) uniaxial push-out tests. The results indicate that changes in bone substrate density and acetabular geometry affect the stability of press-fit acetabular cups. They also suggest that cups implanted into weaker, for example, osteoporotic, bone are subjected to higher levels of micromotion and are therefore more prone to loosening. The decrease in stability of the cup in the physiological model suggests that using simplified spherical cavities to model the acetabulum over-estimates the initial stability of press-fit cups. This novel testing method should provide the basis for a more representative protocol for future pre-clinical evaluation of new acetabular cup designs.

The effect of dynamic hip motion on the micromotion of press-fit acetabular cups in six degrees of freedom

“…This study used both the experimental system to measure the micromotion of the cup in six DoF under cyclic loading and the acetabular model developed in a previous study [12] ; and combined them with a dynamic hip motion simulator that modelled hip flexion-extension. With this setup, the micromotion of two acetabular cups (a Trident cup with HA coating and a Tritanium cup) were assessed under three different conditions: Static Flexion, Level Walking and Stair Climbing.…”

“…This acetabular model was designed to replicate the structural properties observed around the acetabulum, which comprised compressive forces anteriorly and posteriorly from the acetabular columns and non-supportive areas superiorly and inferiorly caused by the presence of the radiolucent triangle and the acetabular notch, respectively, resulting in a pinching effect. This model was used in a previous study [12] , and others have similarly been used in studies to assess acetabular shell deformation in both THR and hip resurfacing implants [20][21][22] , stress distribution within press-fitted ceramic liners [23] , and cup turn-out due to joint frictional moments [24] .…”

“…Both the Trident and the Tritanium cups were implanted with a 1 mm press-fit into the acetabular cavities using a protocol of five sinusoidal load cycles (between 0.01 kN and 5.0 kN), applied at a frequency of 1 Hz using a single-axis hydraulic machine (Dartec, Series HC10, Zwick Testing Machines Ltd., Leominster, UK) [12] .…”

“…1 ) [12] . The results from this study showed significant levels of micromotion in different directions of motion, highlighting the importance of measuring cup micromotion in all six directions of motion.…”

“…This was carried out by combining both the six DoF measurement system and the acetabular model [12] . developed previously to a dynamic hip motion simulator capable of replicating hip flexion-extension as seen during activities of daily living.…”