Abstract
Introduction:
Extensive bone defects of the proximal femur e.g. due to aseptic loosening might require the implantation of megaprostheses. In the literature high loosening rates of such megaprostheses have been reported. However, different fixation methods have been developed to achieve adequate implant stability, which is reflected by differing design characteristics of the commonly used implants. Yet, a biomechanical comparison of these designs has not been reported.
The aim of our study was to analyse potential differences in the biomechanical behaviour of three megaprostheses with different designs by measuring the primary rotational stability in vitro.
Methods:
Four different stem designs [Group A: Megasystem-C® (Link), Group B: MUTARS®(Implantcast), Group C: GMRS™ (Stryker) and Group D: Segmental System (Zimmer); see Fig. 1] were implanted into 16 Sawbones® after generating a segmental AAOS Typ 2 defect.
Using an established method to analyse the rotational stability, a cyclic axial torque of ± 7.0 Nm along the longitudinal stem axis was applied. Micromotions were measured at defined levels of the bone and the implant [Fig. 2]. The calculation of relative micromotions at the bone-implant interface allowed classifying the rotational implant stability.
Results:
All four different implants exhibited low micromotions, indicating adequate primary stability. Lowest micromotions for all designs were located near the femoral isthmus [Fig. 3]. The extent of primary stability and the global implant fixation pattern differed considerably and could be related to the different design concepts.
Discussion:
Compared to other implant designs, all stems resulted in low relative motions regardless their design. The conical Megasystem-C® stem seems to lock in the proximal isthmus of the femur, whereas the MUTARS® stem seems to have a total fixation. Its hexagonal cross-section might have a good interlocking effect against rotational force application. Similarly, the GMRS™ stem shows a total fixation with little tendency to the distal part. The very rough porous-coated surface seems to generate a comparable fixation method to the hexagonal MUTARS® stem. However, the four longitudinal expansions in the proximal part of the GMRS™ stem might not have such a high rotational stability effect as expected. Compared to the other stems, the Segmental System stem showed very low relative micromotions in the proximal part. This sharp fluted stem seems to engrave itself into the bone.
Within this study all stems seemed to achieve an adequate primary rotational stability. We could show that stem design could qualitatively and quantitatively influence the initial fixation behavior of megaprostheses regarding biomechanical tests, like primary stability measurements in synthetic femurs. These experiences should be considered regarding the choice of stem fixation design in specific defect situations.
