Biomechanical research on contour cage with transacetabular screws fixation in revision total hip arthroplasty

A method to assess primary stability of acetabular components in association with bone defects

The objectives of this study were to develop a simplified acetabular bone defect model based on a representative clinical case, derive four bone defect increments from the simplified defect to establish a step-wise testing procedure, and analyze the impact of bone defect and bone defect filling on primary stability of a press-fit cup in the smallest defined bone defect increment. The original bone defect was approximated with nine reaming procedures and by exclusion of specific procedures, four defect increments were derived. The smallest increment was used in an artificial acetabular test model to test primary stability of a press-fit cup in combination with bone graft substitute (BGS). A primary acetabular test model and a defect model without filling were used as reference. Load was applied in direction of level walking in sinusoidal waveform with an incrementally increasing maximum load (300 N/1000 cycles from 600 to 3000 N). Relative motions (inducible displacement, migration, and total motion) between cup and test model were assessed with an optical measurement system. Original and simplified bone defect volume showed a conformity of 99%. Maximum total motion in the primary setup at 600 N (45.7 ± 5.6 µm) was in a range comparable to tests in human donor specimens (36.0 ± 16.8 µm). Primary stability was reduced by the bone defect, but could mostly be reestablished by BGS-filling. The presented method could be used as platform to test and compare different treatment strategies for increasing bone defect severity in a standardized way.

Effect of different motor tasks on hip cup primary stability and on the strains in the periacetabular bone: An in vitro study

BACKGROUND

Excessive prosthesis/bone motions and the bone strains around the acetabulum may prevent osteointegration and lead to cup loosening. These two factors depend on post-operative joint loading. We investigated how Walking (which is often simulated) and Standing-Up from seated (possibly more critical) influence the cup primary stability and periacetabular strains.

METHODS

Twelve composite hemipelvises were used in two test campaigns. Simplified loading conditions were adopted to simulate Walking and Standing-Up. For each motor task, a single-direction force was applied in load packages of increasing amplitude. Stable and unstable uncemented cups were implanted. Digital image correlation was used to measure implant/bone motions (three-dimensional translations and rotations, both permanent and inducible), and the strain distribution around the acetabulum.

FINDINGS

When stable implants were tested, higher permanent cranial translations were found during Walking (however the resultant migrations were comparable with Standing-Up); higher rotations were found for Standing-Up. When unstable implants were tested, motions were 1-2 order of magnitude higher. Strains increased significantly from stable to unstable implants. The peak strains were in the superior aspect of the acetabulum during Walking and in the superior-posterior aspect of the acetabulum and at the bottom of the posterior column during Standing-Up.

INTERPRETATION

Different cup migration trends were caused by simulated Walking and Standing-Up, both similar to those observed clinically. The cup mobilization pattern depended on the different simulated motor tasks. Pre-clinical testing of new uncemented cups could include simulation of both motor tasks. Our study could also translate to indication of what tasks should be avoided.