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.


CONCLUSIONS
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.
Campbell
David G
DG
Wakefield Orthopaedic Clinic, Adelaide, SA, Australia. dcampbell@woc.com.au
Johnson
Luke J
LJ
West
Simon C
SC
D019645:Arthroplasty, Replacement, Knee; D015331:Cohort Studies; D006801:Humans; D007719:Knee Joint / Q000000981:diagnostic imaging* / Q000601:surgery*; D020370:Osteoarthritis, Knee / Q000601:surgery; D014057:Tomography, X-Ray Computed / Q000379:methods*