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S3 OBJECTIVE OUTCOME MEASURES IN TOTAL KNEE REPLACEMENT IN RA

Leiden

Outcome measures must be valid, reliable and responsive to change criteria. The most common clinical outcome measures are Knee Society Scores, SF-36 quality of life scores, HAQ and DAS scores. However, performance based measures of functioning may not be dependent on patient report or observer judgment.

Examples of objective pre- and post-operative performance outcome measures are surface Electromyography (EMG) of muscles, kinematics and kinetics (gait analysis). For the evaluation of implant survival after joint arthroplasty, Roentgen Stereophotogrammetric Analysis (RSA) is the golden standard to assess micro-motion of the implants.

Surface EMG can be used to asses the stability of joints before and after intervention. Calibrating of raw EMG data is necessary to compare the data between subjects. It was shown that calibration of EMG data by means of isokinetic contractions on a dynamometer during flexion and extension was more reliable and repeatable than using a Maximum Voluntary Contraction in patients after total knee arthroplasty. After total knee arthroplasty RA patients have a lower net knee joint moment and a higher co-contraction than controls, indicating avoidance of net joint load and an active stabilization of the knee joint.

Fluoroscopy can be used to assess the kinematics of joints. In the pre-operative situation the use of CT models of the involved bones can be matched to the assessed fluoroscopic images. In the post-operative situation CAD models of the implants can be used for this purpose. In this way accurate 3D kinematics of joints can be assessed. During a step-up task of RA patients, the rotating platform of a mobile bearing knee showed no- or far less longitudinal rotation than the femur. Therefore, some of the theoretical advantages of this specific rotating platform knee prosthesis can be questioned. Fluoroscopy has also been used to assess soft tissue artifacts that occur in gait analysis i.e. displacements of skin-mounted markers relative to the underlying bone. The large soft tissue artefacts observed (displacements up to 17 mm and 12 degrees) question the usefulness of parameters found with external movement registration.

In order to assess the micromotion of implants after joint arthroplasty a measurement technique with a much higher accuracy than fluoroscopy is needed. RSA uses tantalum markers as landmarks bony structures and as landmarks on the implant. Recently a new RSA technique has been developed that does not rely on the attachment of artificial markers on the implant but uses CAD models of the implant instead. As an example of RSA as outcome measure, results showed that a calciumphosphate coating improves fixation of tibial components in RA patients, thus preventing mechanical loosening and subsequent long-term revision. In another clinical RSA study, it was found that mobile bearing knees are more predictable and forgiving with respect to micromotion compared to posterior stabilized tibial components in RA patients.

The results obtained by the above described performance outcome measures can be valued since the accuracy and precision of the used outcome measures are all published.

Correspondence should be addressed to ERASS Office, Schulthess Klinik, Lengghalde 2, CH-8008 ZURICH, Switzerland.