Influence of stem design on the primary stability of megaprostheses of the proximal femur

Abstract

Purpose Extended bone defects of the proximal femur can be reconstructed by megaprostheses for which aseptic loosening constitutes one of the major failure modes. The basic requirement for long-term success of endoprostheses is primary stability. We therefore assessed whether sufficient primary stability can be achieved by four different megaprostheses in a standardised bone defect of the proximal femur and whether their different design leads to different fixation patterns. Methods Four different designs of proximal femoral replacements were implanted into 16 Sawbones® after preparing segmental bone defects (AAOS type II). Primary rotational stability was analysed by application of a cyclic torque of ±7 Nm and measuring the relative micromotions between bone and implant at different levels. The main fixation zones and differences of fixation patterns of the stem designs were determined by an analysis of variance. Results All four implants exhibited micromotions below 150 μm, indicating adequate primary stability. Lowest micromotions for all designs were located near the femoral isthmus. The extent of primary stability and the global implant fixation pattern differed considerably and could be related to the different design concepts. Conclusions All megaprostheses studied provided sufficient primary stability if the fixation conditions of the femoral isthmus were intact. The design characteristics of the different stems largely determined the extent of primary stability and fixation pattern.Understanding these different fixation types could help the surgeon to choose the most suitable implant if the fixation conditions in the isthmus are compromised.

Influence of cerclages on primary stability of tumor megaprostheses subjected to distal femur defects

“…Therefore, primary stability of stems of the intact bones seems to be adequate. These results are fairly similar to another study [15].…”

“…The more distally the femur is resected, the more conical is the medullary canal and the less distinctive the press-fit situation of the implant-bone interface [3]. Despite of multiplying the micromovements by a physiological load of $25 Nm [13,15], we did not reach the maximal interface slipping movement of 150 mm [21]. Therefore, primary stability of stems of the intact bones seems to be adequate.…”

“…Our small number of cases consequently is a further limitation especially since two out of four stems broke out and initially did not provide any useable data. However, high reproducibility was generated in prior biomechanical studies applying the same experimental setup [13,15], assuming reliability of our data. Nevertheless, one further limitation of our results is the difficulty of direct clinical transfer.…”

“…The distal tip of the stem is cylindrical and polished. After statistical power analysis based on data according to similar studies [13][14][15] with alpha (5%) and beta (20%) errors (power = 80%) and relative movements of 15.2 and 15.8 (SD = 0.2) mdeg/Nm between two groups, the sample size was limited to n = 4.…”

“…Preparing of the medullary cavity of the femora was performed using an appropriate 17 mm hexagonal rasp by one experienced surgeon (O.B). The stem was implanted using the universal material testing machine (Inspekt table blue 20, Hegewald & Peschke, Nossen, Germany) according to a standardized protocol (25 axial loading cycles of 2 kN and 25 axial loading cycles of 4 kN) [13][14][15]. Due to the press-fit situation during the implantation, most femora models showed a provoked crack formation when the implant was set, which confirmed literature results, that fractures may occur during or after implantation when using this prosthesis [5].…”