Purpose: Cinematic studies after total knee arthroplasty without an anterior cruciate ligament demonstrate abnormal behaviour compared with the normal knee. The purpose of this cinematic analysis was to examine the knee behaviour after implantation of single-compartment prostheses with an intact anterior cruciate ligament.

Material and methods: The femorotibial contact points were analysed by videofluoroscopy in 20 patients executing a complete weight-bearing extension to flexion movement. These patients had medial (n=16) or lateral (n=4) single-compartment implants. The clinical result in all patients was considered to be very good with a mean HSS score of 97.9 points at a mean 56 months postoperatively. The femorotibial contact points were determined using an automatic computerised adaptation-modelling system. An anterior contact on the medial tibial line in the sagittal plane was positive and a posterior contact was negative. The rotation axis in the craniopodal direction was measured between the anteroposterior longitudinal axis of the femoral component and the fixed axis of the tibial component.

Results: The mean position of the contact point for medial single-compartment prostheses was −90.8 mm in complete extension, −1.4 mm at 30° flexion, −2.4 mm at 60°, and −1.7 mm at 90°. Mean position of the contact point for lateral single-compartment prostheses was −4.0 mm at complete extension, −7.9 mm at 30° flexion, −5.7 mm at 60° and −5/7 mm at 90°. Seven patients with a medial implant and two patients with a lateral implant exhibited paradoxical anterior translation of the femur during flexion. On the average, patients with a medial implant had normal 3.3° axial rotation at 90°; axial rotation was 11.2° for patients with a lateral implant.

Discussion and conclusion: Cinematic analysis of the normal knee has demonstrated anterior femorotibial contact in extension and 14.2 mm posterior rolling of the femoral component during flexion. After total knee arthroplasty without preservation of the anterior cruciate ligament, the rolling movement is limited or absent and a paradoxical anterior translation can be observed. In the present study, the first reported on single-compartment implants, demonstrates that movement is similar to that in the normal knee but with major interindividual variability. A posterior contact at extension and a paradoxical anterior translation can also be observed. This suggests progressive development of anterior cruciate ligament laxity over time, which can at least in part explain the premature polyethylene wear observed after implantation of single-compartment knee implants.

We used fluoroscopy to study the kinematics of the knee in 47 patients with total knee arthroplasty (TKA) and four control subjects with normal knees while performing a single-leg deep-knee bend. The videos were analysed using still photographs taken at 5 degrees increments of flexion. Femorotibial contact points, patellar ligament rotation, and patellar rotation were calculated from each image. Maximum weight-bearing flexion was determined for each knee. Compared with the control group, posterior-cruciate-retaining TKA did not reproduce normal knee kinematics in any case, but showed a starting point posterior to the tibial midline which translated anteriorly with flexion. The curves from successive knee bends could not be consistently reproduced. Under weight-bearing conditions, the maximum flexion for any PCR TKA was 98 degrees and several patients could not flex beyond 70 degrees.