MEASURING DISPLACEMENTS OF THE TOTAL KNEE ARTHROPLASTY MOBILE PLATEAU USING TWO RADIO-OPAQUE MARKERS

Abstract

Purpose: Kinematics of the total knee arthroplasty plateau has been widely studied. Many methods can be used: opto-electronic captors, electromagnetic knee device, radiography, repositioning with the CAO model. We report a simple method based on calculating the position of two radio-opaque markers and the contours of the prosthetic components.

Material and methods: Twenty patients with a unilateral mobile plateau total knee arthroplasty were studied. An image amplifier linked to a digital recorder (frequency 25 Hz), CAO models of the implant, and adapted software (Matlab) were used. Each patient performed a series of movements, standing up from the sitting position, within the fluoroscopic field. Each sequence was digitalized. Kinematic images were sampled at 6 Hz. Different parameters were measured on each sampled image: position of two radio-opaque markers included in the polyethylene insert and characteristic dimensions of the prosthesis components. These 2D recordings were used to deduct the relative 3D position of each of the prosthetic components. Laws of analytic geometry and functional analysis were used to resolve the triangular matrices needed to transform the 2D measures into 3D values. Angular and linear positions of the prosthetic components were established for different times t. Reconcatenation by time produced an kinematic analysis of the pros-thesis behaviour.

Results: This method allowed us to establish the kinematics of the total knee arthroplasty mobile plateau with a precision of 0.2 mm and 0.4°. The proposed analysis method is reliable and precise. It is less costly in development time than methods based on automatic repositioning of 3D models of the implant on fluoroscopic images.

Conclusion: The measurement method proposed requires radio-opaque markers positioned in orthogonal directions so they are visible during movements. Although we used a semi-automatic calculation protocol, totally automatic systems can be applied to process fluoroscopic images.