Model-based Roentgen Stereophotogrammetry (MBRSA) of Radiopaque Polyethylene prosthesis: a Pilot Study

Background Ultra-high molecular weight polyethylene (PE) is the articulating surface of the majority of orthopaedic replacements. PE has a low X-ray attenuation, hence the follow-up of the PE prosthesis by direct imaging (X-ray, RSA) is difficult. The radiopacity can be enhanced by diffusing an iodised oil (Lipiodol), for early diagnosis of failure. Aim To assess the precision of MBRSA to detect radiopaque PE components. Method and Materials This study used 4 Oxford Partial Knee (OPK) bearings (Zimmer-Biomet, UK) with different levels of radiopacity. First, a phantom was used consisting of an OPK tibial tray and a femoral component both rigidly clamped to polycarbonate rods with 6 beads attached to each rod and the OPK bearing positioned between the other OPK components. Second , the tibial tray was removed and the bearing was fixed onto a Sawbone tibia with 6 beads. The phantoms were positioned in a standing RSA setup in front of a carbon uniplanar calibration box (Leiden, NL). Stereo-radiographs of 7 successive poses were acquired from the phantoms. Translational and rotational movements were applied to mimic clinical positioning variation. Zero motion between the markers and the implants was assumed, hence the measured migrations were the experimental errors including the radiopacity level, model accuracy, parts symmetry and the RSA algorithm. Result and finding For the highest level of radiopacity, the largest standard deviation for any translation and rotation was 0.05 mm and 0.46˚ respectively. The precision level decreased with decreasing radiopacity level. One-way ANOVA showed no difference when comparing this precision with published precision levels of metallic prosthesis and the femoral component of this study (P>0.05). Conclusion This novel bearing can be analysed with MBRSA with a precision comparable to metal implants.