Assessment of polyethylene wear in THA using Model-Based Roentgen Stereophotogrammetric analysis - an experimental phantom-model setup

Objectives Common reasons for long-term failure of total hip arthroplasty (THA) are polyethylene (PE) wear and aseptic loosening. It has been shown that a 3D wear analysis has higher accuracy in comparison to a 2D method. Currently, Marker-Based Roentgen Stereophotogrammetric Analysis (RSA) is considered as a precise tool to monitor PE wear in vivo. However, RSA requires additional implantation of tantalum markers, which obstructs its routine clinical application. Model-Based RSA (MBRSA) is a feasible approach, using elementary geometrical shape models instead of tantalum markers to measure PE wear.

Interchangeability of MBRSA has been proofed for monitoring implant to bone fixation, but validation for PE wear detection is rare. It is hypothesized thus accuracy and precision of MBRSA approach are superior to conventional PE wear detection of THA, using single plain radiographs (2D wear). Furthermore, the effect of pelvic malpositioning on the PE wear measurements for the both methods should be investigated within an experimental phantom model.

Methods A phantom model was constructed to simulate linear PE wear. It consisted out of a micrometer screw and a femoral bone-implant-model (FBIM), which could be moved precisely around three degrees of freedom (DOF) relative to an acetabular bone-implant-model (ABIM). Relative to the standard “neutral” position, the ABIM could be malpositioned for three DOF (a.p. tilt, obliquity, and rotation). This phantom was located within a radiologic measurement setup, which was able to capture RSA and conventional x-ray images in parallel without moving the phantom model. 3 different wear protocols were performed by FBIM for 7 different orientations of the ABIM. Each protocol was repeated 5 times per ABIM orientation. PE wear for each image series was measured using the MBRSA (Model-Based RSA, v4.1, RSAcore) as well as 2D wear method (mediCAD Classic, v5.1, mediCAD Hectec GmbH).

Results and Conclusion Precision and accuracy were for 2D wear with 0.206 mm and 0.159 mm worse than for MBRSA with 0.043 mm and 0.017 mm respectively. Four pelvic malposition in 2D wear approach and one pelvic malposition in MBRSA indicated significantly different results (p <  0.05) compared to the pelvic neutral position ([Abb. 1]).

Abb. 1 Detected wear by 2D wear and MBRSA in relation to pelvic alignment. X-axis showed grouped positions of the ABIM. Y-axis calculated differences between measurement value and true value (2D wear: blue, MBRSA: green, Stars (*): p <  0.05).

Conclusion Precision and accuracy using MBRSA were by factors of 4 and 9 better than the 2D wear approach. Attention must be paid to potential malposition of the pelvis during the image acquisition because this affects the 2D wear measurement outcome significantly. If this is taken into account, an in vivo wear measurement will be possible using 2D wear technologies.

Stichwörter polyethylene wear, RSA, THA, precision, accuracy, pelvic malposition

Publikationsverlauf

Artikel online veröffentlicht:
15. Oktober 2020