Abstract
Introduction
The Flexible Nichidai Knee (FNK) System (Nakashima Medical, Japan) was designed to fit Asian knees. Especially, the posterior stabilized(PS) prosthesis was designed as semi-constrained posterior stabilized system that had a large tibial post and femoral articulation. We hypothesized that the semi-constrained PS implant design would have a positive influence on vivo kinematics after total knee arthroplasty (TKA).
Materials and Methods
A total of 16 patients (21 knees) who had undergone TKA using PS implant were randomly recruited from our database. Of the 16 patients, all patients were women. Fourteen patients had osteoarthritis and 2 patients had rheumatoid arthritis. The average age was 72.3± 9.5 years, and the average postoperative duration was 23.4 ± 19.3 months. The subject performed sequential deep knee bends under WB from 0° to maximum flexion under fluoroscopic monitoring in the sagittal plane. Conversely, under NWB, the patient sat on a chair and was asked to perform active assisted knee flexion. To estimate spatial position and orientation of the artificial knee prosthesis, a 2D to 3D registration technique was used. We evaluated knee range of motion, femoral axial rotation relative to the tibial component, and anteroposterior translation of the femorotibial contact point for both medial and lateral sides. Closest distances between femoral cam and tibial post engagement were measured,
Results
Range of Motion
The mean full extension angle between femoral and tibial components, was −8.1±8.8°and −7.5±5.5°in WB and NWB, respectively. The mean maximum-flexion angle was 110.0±18.1°and 119.3±8.9°in WB and NWB, respectively.
Femoral Axial Rotation
Fig.1 shows the mean degree of femoral axial rotation relative to the tibial components in WB and NWB. The femur was externally rotated 0.7±3.9°and 0.3±4.7°at 0° degree in WB and NWB, respectively. The external rotation increased to 4.8±5.2°and 6.2±5.9°at 120°flexion in WB and NWB, respectively.
Anteroposterior Translation
The mean femorotibial contact point under WB and NWB was shown in Fig.2 for medial contact and Fig. 3 for lateral contact. Under WB, the mean medial contact point moved posteriorly from −1.6±2.0mm at 0° flexion. The point then moved gradually anteriorly with flexion to −9.3±1.5mm at 120°flexion. The mean lateral contact point moved posteriorly from −1.9±1.7mm at 0° flexion, and then moved anteriorly to at −8.9±2.7mm 120° flexion. Under NWB conditions, the mean medial contact point moved posteriorly from −1.1±1.8mm at 0° flexion. The point then moved gradually posteriorly with flexion to −6.6±2.8mm at 120° flexion. The mean lateral contact point in PS TKA moved posteriorly from −4.4±3.3mm at 0° flexion, and then moved posteriorly to −12.3±3.6mm at 120°flexion.
Post-Cam Engagement
The mean knee flexion angle at initial post-cam engagement was 61.9 ± 15.9° under WB and 57.5 ± 16.0° under NWB.
Discussion
Our study showed external rotation and bycondylar posterior rollback pattern in the entire range of knee flexion. The reason for this might be that the post cam design was high(20mm), which does not allow for high external rotation. The in vivo kinematics of the semi-constrained PS FNK prosthesis showed similar kinematic patterns due to the development concept of the implant design.
