# Effect of segmental rim defect morphology on press-fit stability of the cementless acetabular cup: a finite element analysis

## TL;DR

This study uses computer modeling to show that large bone defects in weight-bearing areas of the hip can significantly reduce the stability of hip implants after surgery.

## Contribution

The study introduces a finite element analysis to quantify how segmental rim defect morphology affects cementless acetabular cup stability.

## Key findings

- Defects in the weight-bearing zone showed the lowest push-out and lever-out stability.
- Increasing defect width or depth significantly reduced cup stability.
- Weight-bearing zone defects caused the highest micromotion during the gait cycle.

## Abstract

Segmental rim defect (SRD) of the acetabulum is a common type of osteolysis following primary total hip arthroplasty. Accurate quantitative evaluation of severity and morphology of the SRD can provide critical information to surgeons for decision-making on surgical reconstruction. Therefore, we aim to investigate the effect of morphologic features of SRDs on the press-fit stability of cementless cups.

A 3D finite element model of Sawbones hemipelvic bone was developed using CT scan. Submodels with varying SRD geometries and locations were created, followed by Boolean reaming and virtual 1-mm press-fit implantation of a cementless cup. The press-fit stability was evaluated by using the push-out and lever-out tests based on quasi-static non-linear finite element analysis. The peak micromotion (PM) levels of all submodels at the bone-cup interface during the gait cycle were also compared.

Despite the depth and width, the defect in the weight-bearing zone exhibited the lowest average push-out load of 2429 ± 294.19 N and lever-out moment of 100 ± 8.40 Nm comparing to those in other zones. The maximal push-out and lever-out loads of the cup decreased as either the width or depth of defect increased. The percentages of strength reduction for push-out and lever-out tests, respectively up to 47.8 and 34.1%, were significantly higher in the weight-bearing zone comparing to those in the other zones. During the gait cycle, all worst-scenario defects resulted in varying levels of increase in interfacial PM comparing to the intact acetabular rim. In the second half of the gait cycle, the defect in the weight-bearing zone produced the highest level of PM.

In revision total hip arthroplasty (r-THA), a large SRD in the weight-bearing zone can result in the utmost loss of press-fit stability and a detrimental interfacial micromotion, which may lead to early loosening of the cup. Thus, an accurate quantitative evaluation of defect morphology is valuable for surgeons to develop a refined plan for reconstruction of initial stability of a cementless cup.