Multiparameter quantitative computer-assisted tomography assessment of unicompartmental knee arthroplasties

Background: Unicompartmental knee arthroplasty is a popular alternative to total knee replacement in selected patients. Component alignment has not yet been described by computer-assisted tomography (CAT) imaging techniques; these have been developed for total knee arthroplasty analysis. The aims of this study were to report two new technologies; a new unicompartmental knee arthroplasty system was radiographically assessed with a new CAT scan protocol. Methods: In a consecutive cohort study, 60 knees were analysed by the 'UniCAT Protocol'. Patients were implanted with a unicompartmental knee arthroplasty system that uses a unique ligament tensor for femoral component alignment. The uniCAT protocol requires a long anteroposterior and lateral scout scan to measure limb alignment and component orientation. A spiral computer-assisted tomography at the knee is used to measure component rotation. The total scan time was 20 s with a calculated unshielded radiation dose of 1 mSv or less. Results: The mechanical axis had a mean of 2.7 degrees varus. Femoral components were implanted with a mean of 0.37 degrees valgus and 1.3 degrees flexion. Tibial components were implanted with a mean 3.47 degrees varus and 5.1 degrees posterior slope. Femoral components were externally rotated a mean of 3.36 degrees, tibial components were externally rotated 6.59 degrees from the posterior tibia and 5.68 degrees from the transepicondylar axis. Conclusion: The UniCAT protocol uses less radiation than whole-limb spiral scans and is a method that can be used with all modern computer-assisted tomography machines. The coronal and sagital alignment results compare favourably with previous published reports without computer-assisted tomography. Component rotation has not previously been reported and its implications are yet to be defined.