Elsevier

Clinical Biomechanics

Volume 18, Issue 3, March 2003, Pages 231-236
Clinical Biomechanics

The influence of surgical malalignment on the contact pressures of fixed and mobile bearing knee prostheses––a biomechanical study

https://doi.org/10.1016/S0268-0033(02)00189-4Get rights and content

Abstract

Objective. To investigate the effect of surgical malalignment on contact pressures of fixed and mobile bearing knee prostheses.

Design. An experimental set-up was used to measure contact pressure on the tibial component of fixed and mobile bearing knee prostheses subjected to a compression load and surgical malalignment situations were simulated.

Background. Mobile bearing knee prostheses were designed to decrease tibiofemoral contact pressure by providing both high congruity and mobility. It was also assumed to accommodate surgical malalignment. However, few studies have reported the effect of malalignment of the tibiofemoral joint on contact pressure of fixed and mobile bearing knee prostheses.

Methods. Surgical malalignment situations were simulated to evaluate contact characteristics of tibial component of fixed and mobile bearing knee prostheses. The simulated malalignment conditions include the medial–lateral translation (0.5 and 1 mm), anterior–posterior translation (2 and 4 mm) and internal–external rotation (1°, 3°, 5° and 10°) of the femoral component relative to the tibial component. Fuji pressure sensitive film was used to measure the contact pressure.

Results. The greatest increase of maximum contact pressure in the anterior–posterior maltranslation was 7.63% and 7.62% relative to the neutral contact situation in the fixed and mobile bearing designs respectively. In the medial–lateral maltranslation, there was 23.3% in the fixed bearing design and was 22.0% in the mobile bearing design. In the internal/external malrotation, the greatest increase of maximum contact pressure in the fixed bearing design was 27.1%, which was much higher than the mobile bearing design (22.4%).

Conclusions. The mobile bearing design can reduce maximum contact pressure more significantly than the fixed bearing design when malalignment conditions of the tibiofemoral joint occurs, especially in the internal/external malrotation. The mobile bearing design offers the advantage of self-adjusting over the fixed bearing design to accommodate surgical malalignment.

Relevance This study revealed that the mobile bearing design has smaller maximum contact pressures than the fixed bearing design in knee prosthesis under malalignment biomechanical tests. This result indicates that there is an advantage for a mobile bearing design over a fixed bearing design to accommodate malalignment conditions caused by surgical technique or soft tissues imbalance in total knee arthroplasty.

Introduction

Polyethylene wear of articular surface is a well-recognized complication of total knee arthroplasty. Factors related to failure of bearing surface of knee prosthesis include material properties of polyethylene, sterilization method, soft tissues balance, the patient’s daily activity and the contact pressures on the articular surfaces (Bartel et al., 1986; Collier et al., 1991; McNamara et al., 1994). An asymmetrical wear of the polyethylene component was the common pattern in many retrieval analyses (Wasielewski et al., 1994). Therefore, implant alignment and soft tissues balance and the wear of tibial polyethylene component has produced a lot of publications related to these areas (Liau et al., 1999; Liau et al., 2002).

The tibial component in total knee prostheses can be either a fixed or a mobile bearing. The long-term result between fixed bearings and mobile bearings is still a controversial issue (Lewis et al., 1982; Bert et al., 1998; Sanchez-Sotelo et al., 1999). From the biomechanical point of view, mobile bearing design can reduce polyethylene wear and minimize loosening by providing both congruity and mobility in the tibiofemoral joint (Goodfellow et al., 1986). In addition, another advantage of mobile bearing design is to accommodate any surgical malalignment of tibial baseplate and to allow intraoperative adjustment of joint space (Buechel and Pappas, 1989; Buechel and Pappas, 1987).

Many biomechanical studies have demonstrated that contact pressure on the tibial polyethylene component is closely related to polyethylene wear (Rose et al., 1980; Wright and Bartel, 1986). However, most studies in biomechanical tests, finite element analyses and analytical approaches assumed the femoral and tibial components were under an ideal contact alignment. The malalignment of knee prostheses was thought to accelerate polyethylene wear (Rose et al., 1980). Only a few researchers focused on the contact pressures on the tibial polyethylene components under malaligned conditions. Liau et al. (1999) used Fuji pressure sensitive film to study the influence of contact alignment of the tibiofemoral joint of knee prostheses in in vitro biomechanical test. They also used finite element analysis to investigate the effects of malalignment on the stresses in three different conformity designs of the tibiofemoral joint (Liau et al., 2002). Matsuda et al. (1999) utilized the digital electronic pressure sensor to investigate the effect of varus tilt on contact pressures in total knee prostheses. These studies focused on fixed bearing design with different articular curvatures. Matsuda et al. (1998) also measured the contact pressures on the upper- and under-surface of the tibial polyethylene components with a neutral and malrotated tibial tray of three mobile bearing designs and one fixed bearing design. They demonstrated that mobile bearing designs appear to offer advantages over the fixed bearing design when moderate rotational malalignment of the tibial component occurs. However, only the effect of rotational malalignment was considered in their study. To investigate the advantages of mobile bearing designs for accommodating surgical malalignments in the medial–lateral (ML) and anterior–posterior (AP) translations and internal–external (IE) rotation more clearly, this study was undertaken to compare the contact pressures on the tibial component in fixed bearing and mobile bearing knee prostheses under malaligned conditions.

Section snippets

Methods

A commercial knee system (U-knee system, United Orthopedic Co., Taipei, Taiwan) with a flat-on-flat design in the coronal plane and a semi-conformity design in the sagittal plane and 5 degrees of posterior slope in the tibial insert was used. The current design in the tibial component of U-knee system is a fixed bearing. The locking mechanism of the tibial component was modified to become a mobile bearing system. The mobile bearing knee prosthesis allows 1 mm translation in both ML and AP

Results

The typical results of the contact pressure distribution of the neutral and malalignment positions measured by Fuji prescale sensitive film were shown in Fig. 3. The maximum contact pressure (MCP) of fixed bearing design was 23.6 MPa, which was significantly higher than the mobile bearing design (22.3 MPa) (P<0.05) under the normal contact alignment. The MCP of fixed bearing design was also significantly higher than the mobile bearing design when the femoral component was translated posteriorly

Discussion

Mobile bearing knee prosthesis was designed to decrease the prevalence of prosthesis loosening and polyethylene wear (Goodfellow et al., 1986). It can also accommodate to surgical malalignment because of high mobility of the bearing element. When malalignment of prosthesis occurred, the loading distribution in the tibiofemoral joint would change, which may further alter stress distribution on the contact surface and soft tissues tension at knee joint. Therefore, to approach the actual

Conclusion

The mobile bearing design has lower MCPs than the fixed bearing design when malalignment of the tibiofemoral joint of knee prosthesis occurs. The mobile bearing design offers the advantage of self-adjusting over the fixed bearing design to accommodate surgical malalignment.

References (19)

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