Comparison of flanged and unflanged acetabular cup design. An experimental study using ceramic and cadaveric acetabuli

Computational nodal displacement analysis of acetabulum fossa for injection molded cemented polyethylene acetabular liner

The acetabular liner (AL) is one of the key components that determine the functionality and durability of the total hip joint replacement (THR) device. The performance of Ultra high molecular weight polyethylene (UHMWPE)-based AL depends critically on the manufacturing route and its properties, which are evaluated pre-clinically using a host of experimental and computational analyses. The conventional manufacturing of an AL involves multiple stages, including extrusion/compression molding followed by machining, which is time/cost intensive and leads to material loss. In such a scenario, injection molding is a promising alternative, yet its feasbility remains unexplored for the manufacturing of AL for THA applications. Against this backdrop, the two-fold objectives of this work are to report our recent efforts to establish the efficacy of the injection molding of new generation UHMWPE biomaterial; HU (60 wt% HDPE- 40 wt% UHMWPE blend) for manufacturing AL prototype and to present the key biomechanical response analysis of this prototype, in silico. A range of manufacturing relevant material properties, as well as customized mold design to manufacture HU-based AL with external design features, are discussed. Such guidelines are particularly relevant to mold polymeric parts with a higher thickness (>8 mm). As part of the pre-clinical validation of AL with new design features, a less explored in silico approach to assess biomechanical micro-strain in the acetabulum fossa is presented, and the results are analysed in accordance with the mechanostat theory. The outcomes revealed that for a 100 kg subject weight, average micro-strain in the remodelling region was 1132, while it was determined as 723 for a 55 kg subject weight. Such results highlight the influence of subject weight on micro-strain generation and distribution in the acetabulum fossa. The von Mises stress in AL also increased with subject weight from 17 MPa in a subject weight of 55 kg to 28 MPa in a subject weight of 100 kg. Taken together, this work demonstrates the feasibility and competence of this new generation biomaterial in terms of implant manufacturing via injection molding with a clinically desired biomechanical response.

The addition of a flange does not improve the pressure generated during cemented acetabular cup implantation

Flanged acetabular cups were developed with the rationale that, at insertion, they would increase the pressure of the cement and improve penetration of cement into the acetabular bone. Various studies have been inconclusive regarding their effectiveness. In this work, we aimed to eliminate all confounding factors and measure the pressures generated during acetabular pressurization and cup implantation using a simplified steel acetabulum, high precision pressure transducers, proper surgical techniques and two acetabular cups, identical apart from the addition of a flange to one. It was found that the flanged acetabular component did not significantly increase the pressure in the acetabulum and in some cases reduced the pressures generated when compared to an unflanged cup. The addition of a flange did not reduce the pressure differential between the pole and the rim of the acetabulum, nor did it have a significant effect on pressure lost over the cup implantation period. It was concluded that flanged acetabular cups provide no significant improvement in the pressures generated in the acetabulum during acetabular cup implantation. It is hypothesized that the flange may be seen as a design feature intended to slow the insertion of the cup into the cement, thus requiring the surgeon to apply a larger load in order to correctly position the acetabular cup; in this way larger pressure will be generated.

What is the Optimum Pattern of Pressurisation to Gain Maximum Penetration of Methylmethacrylate Cement into the Reamed Acetabulum?

Introduction

The cemented polyethylene cup has remained the standard acetabular implant for 50 years although there has been little research into cementing techniques. In the past, cement was previously inserted by sequential manual pressurisation (thumbing) but this technique was prone to contamination of the cement leading to weakening of fixation. In recent times, third-generation techniques using sealed pressurisation with rim preparation have been espoused with similar results. We were interested in establishing whether repeated cycles of compression of cement allowing adequate time for relaxation increases its depth of penetration, and the optimum period of relaxation required to achieve this goal.

Method

A single mix of polymethylmethacrylate cement at dough stage was inserted into a model of the reamed acetabulum. Cyclical pressurisation of the cement with 50 N followed nine different patterns to simulate thumbing, constant pressure, and the application of a sealed and unsealed acetabular cup implant.

Results

A constant load was as effective as all variations of repeated cycles of load and relaxation except for 50 N pressure applied for four seconds with four second intervals. A four second interval of relaxation achieved significantly more penetration than five or three seconds.

Following two minutes of constant pressure, the application of a sealed or unsealed thrust of the plunger had no effect on cement penetration.

Conclusion

This study suggests that optimal polymethylmethacrylate cement penetration into the acetabulum occurs with cycled application of load for four seconds followed by four seconds of relaxation. The subsequent pressurisation with either a flanged or unflanged acetabular implant does not appear to improve cement penetration.

The use of a "rim cutter" device and a flanged cup for improving the mantle of the acetabular component of a cemented Exeter total hip arthroplasty

A retrospective study was performed to evaluate the cement mantle in two groups of patients treated with the acetabular components of cemented Exeter total hip arthroplasties (THAs). Two groups of 20 patients were compared: Group 1 received non-flanged acetabular cemented cups (Contemporary, Stryker) and Group 2 received flanged acetabular cemented cups (X3 Rim Fit, Stryker). Cups in Group 2 were implanted after using a rim cutter device. Group 2 showed better penetration of cement in zone 1 (10.76 mm compared with 2.93 mm; p = 0.008) and a thicker cement mantle in zone 1 (3.57 mm compared with 2.89 mm; p = 0.04). More cups in Group 2 had a cement mantle thickness less than 3 mm (30 % in Group 1 compared with 70 % in Group 2; p = 0.0039). No other radiological differences were observed. These results favor the use of a rim cutter device and flanged cup to improve the cement mantle for the acetabular components of cemented Exeter THAs. However, the improvements were less than expected. In view of the results of previous studies, further research is therefore needed to assess the value of this approach in improving the acetabular cement mantle.

Progressive radiolucent lines following the implantation of the cemented Rimfit acetabular component in total hip arthroplasty using the rimcutter technique: cause for concern?

AIMS

The highly cross-linked polyethylene Exeter RimFit flanged cemented acetabular component was introduced in the United Kingdom in 2010. This study aimed to examine the rates of emergence of radiolucent lines observed when the Rimfit acetabular component was implanted at total hip arthroplasty (THA) using two different techniques: firstly, the 'rimcutter' technique in which the flange sits on a pre-prepared acetabular rim; and secondly, the 'trimmed flange' technique in which the flange is trimmed and the acetabular component is seated inside the rim of the acetabulum.

PATIENTS AND METHODS

The radiographs of 150 THAs (75 'rimcutter', 75 'trimmed flange') involving this component were evaluated to assess for radiolucencies at the cement/bone interface by three observers.

RESULTS

Rimfit acetabular components implanted using the rimcutter technique had significantly higher rates of radiolucency than those introduced using the 'trimmed flange' technique one year post-operatively (one zone: 63/75 (84%) vs 17/75 (23%); two zones 42/75 (56%) vs 0/75 (0%); all three zones 17/75 (23%) vs 0/75 (0%):(all p < 0.001).

CONCLUSION

On the basis of these findings, we have stopped using the 'rimcutter' technique when implanting the Rimfit acetabular component and have reverted to the 'trimmed flange' technique.

TAKE HOME MESSAGE

Surgeons should be vigilant of the performance of the Rimfit acetabular component when used alongside the rim cutter device due to an observed higher rate of progressive radiolucencies with this combination of component / technique.