Abstract
Purpose
The tibia first technique in unicompartmental knee arthroplasty (UKA) may have the advantage that surgeons can obtain a balanced flexion-extension gap. However, changes of the soft tissue tension during UKA has not been elucidated yet. The purpose of this study was to examine the correlation between the soft tissue tension before the femoral osteotomy and after the femoral component in place using the tensor in UKA.
Methods
Thirty UKAs for isolated medial compartmental osteoarthritis or idiopathic osteonecrosis were assessed. The mean age was 71.8±8.5 years old (range: 58–85), and the average coronal plane femorotibial angle (FTA) was 181.2±3.2 degree preoperatively. All the patients received a conventional medial Zimmer Unicompartmental High Flex Knee System (Zimmer Inc, Warsaw, Ind). The actual values of the proximal and posterior femoral osteotomy were calculated by adding the thickness of the bone saw blades to the thickness of the bony cut. Using a UKA tensor which designed to facilitate intra-operative soft tissue tension throughout the range of motion (ROM), the original gap before the femoral osteotomy, the component gap after the femoral osteotomy, and component placement were assessed under 20 lb distraction forces. (Figure 1)
Results
The mean actual thickness of the distal femoral osteotomy 6.5 ± 1.3 mm and the posterior femoral osteotomy was 7.4 ± 1.3 mm. The distal thickness of the Zimmer UKA was set to 6.5 mm and the mean posterior thickness of the prosthesis used in this study was 5.8 ± 0.3 mm.
There is a positive correlation between the original and component gap throughout the ROM (R > 0.5). The original and component gap showed the same kinematic pattern from full extension to 90 degrees of knee flexion. However, the component gap showed significantly higher compared to the original gap after 120 degrees of knee flexion (p < 0.001). (Figure 2)
Conclusions
Despite the fact that the component gap showed significantly higher compared to the original gap in deep flexion, there is a positive correlation between the original and component gap throughout the ROM. The discrepancy during deep flexion was due to the posterior design of the prosthesis that is designed to be thinner than the actual thickness of the posterior osteotomy in order to prevent flexion gap tightness.
These results suggest that the tibia first technique with the tensor have the advantage that surgeons can predict final soft tissue tension before femoral osteotomies with the comprehension of the prosthetic design and help restore natural knee kinematics, potentially improving implant survival and functional outcomes.
