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 have 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 is in place using the tibia first technique in UKA.
Thirty UKAs for isolated medial compartmental osteoarthritis or idiopathic osteonecrosis were assessed. 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 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.
The mean actual thickness of the distal femoral osteotomy was 6.5 ± 1.3 mm and the posterior femoral osteotomy was 7.4 ± 1.3 mm. The distal thickness of the prosthesis 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° of knee flexion. However, the component gap was significantly higher compared to the original gap after 120° of knee flexion (P < 0.001).
Despite the fact that the component gap values were significantly higher compared to the original gap value 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. 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 prosthetic design and help restore natural knee kinematics, potentially improving implant survival and functional outcomes.