Purpose: In vivo analysis of the kinematics of total knee arthroplasty (TKA) is of particular interest because it i) enables measurements of mobile plateau displacements by degree of freedom (rotation, anteroposterior translation, mediolateral motion); ii) provides crucial information concerning polyethylene wear and loosening.

Material and methods: Twenty patients with a mobile plateau TKA with posterior stabilisation were selected on the basis of three inclusion criteria: follow-up greater than six months, unilateral prosthesis, satisfactory clinical result (HKS = 80). The lateral view on the image amplifier was recorded using a digital recorder while the patient produced flexion-extension movements of the loaded knee in the sitting position. We determined the position of the mobile plateau relative to the tibial base and to the femoral piece throughout the flexion cycle using two radio-opaque markers included in the mobile plateau.

Results: Rotation of the mobile plateau-tibial base interface was observed for all 20 TKA. There was an external rotation from flexion to extension of 3° to 16° in all cases. there was also an anteroposterior translation for 18/20 TKA. For ten TKA this was an anterior translation from flexion to extension. For eight TKA, it was a posterior translation of the mobile plateau. The movement was minimal (mean 2 mm) in all cases.

Discussion: We compared these findings with data in the literature obtained with the same fluoroscopic method. The behaviour observed was intermediary to that for two versions of a reference TKA with a mobile plateau (two mobile menisci, rotating platform).

Conclusion: We demonstrated rotation and anteroposterior translation movements of the mobile plateau total knee arthroplasty studied. The mobile plateau was in a “floating” situation in 18/20 cases. This is theoretically favourable to the transmission of stress forces. An in vitro study using stress gauges to measure stress at interfaces should provide complementary information to this fluoroscopic study.

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.