Ultra-high molecular weight polyethylene (UHMWPE) is a commonly used as bearing material in joint replacement devices. UHMWPE implants can be hard to see on a standard X-ray because UHMWPE does not readily attenuate X-rays. Radiopaque UHMWPE would enable direct imaging of the bearing both during and after surgery, providing in vivo assessment of bearing position, dislocation or fracture, and potentially a direct measure of wear. The X-ray attenuation of UHMWPE was increased by diffusing an FDA approved contrast agent (Lipiodol) into UHMWPE parts (Zaribaf et al, 2018). The aim of this study was to evaluate the optimal level of radiopacity for a UHMWPE bearing.

Samples of un-irradiated medical grade UHMWPE (GUR 1050) were machined into 4mm standard medium Oxford Unicompartmental bearings. Samples were immersed in Lipiodol Ultra Fluid (Guerbert, France) at elevated temperatures (85 °C, 95 °C and 105 °C) for 24 h to achieve three different levels of radiopacity.

A phantom set-up was used for X-ray imaging; the phantom contained two perspex rods to represent bone, with the metallic tibial tray and polyethylene bearing fixed to the end of one rod and the metallic femoral component fixed to the other rod. Radiographs of the samples were taken (n=5) with the components positioned in full extension. To ensure consistency, the images of all the samples were taken simultaneously alongside an untreated part.

The results of our ongoing study demonstrate that the radiopacity of UHMWPE can be enhanced using Lipiodol and the parts are visible in a clinical radiographs. The identification of the optimal treatment from a clinical perspective is ongoing; we are currently running a survey with clinicians to find the consensus on the optimal radiopacity taking into account the metallic components and alignment. Future work will involve a RSA study to assess the feasibility of measuring wear directly from the bearing.

Study of failed Oxford medial unicompartmental knee replacements at the Royal Cornwall Hospital.

Details of orthopaedic implants in Scotland are recorded on a national database. The results are used by the Scottish Arthroplasty Project to record survival and complication rates for both knee and hip replacements. The aim of our study was to assess the accuracy of recorded data for unicompartmental knee replacements in the West of Scotland. The national database was searched for all unicompartmental knee replacements carried out in the West of Scotland between March 2000 to October 2004. All patient data was then crosschecked with hospital theatre records and case notes for confirmation of accuracy. A total of 88 cases were coded as unicompartment joint replacements in the study period. 63 cases were confirmed as being accurate (71.6%) and 6 as being inaccurate (6.8%). 19 patient details were not available for review either from notes or theatre records (21.6%). Of those coded inaccurately, five were total knee replacements, one cemented hip hemiarthroplasty and one shoulder replacement. One case of miscoding could be accounted for as an error in documentation while in six cases no cause could be identified. Of the 63 knees confirmed as unicompartmental, seven knees had been revised within 5 years, giving a 5 year survival rate of 87.7%. The current system used by the Scottish Arthroplasty Project in Scotland has at least a 6.8% inaccuracy rate when recording unicompartmental knee replacements

Objectives

This study reports on a secondary exploratory analysis of the early clinical outcomes of a randomised clinical trial comparing robotic arm-assisted unicompartmental knee arthroplasty (UKA) for medial compartment osteoarthritis of the knee with manual UKA performed using traditional surgical jigs. This follows reporting of the primary outcomes of implant accuracy and gait analysis that showed significant advantages in the robotic arm-assisted group.