Page 28 - ISAKOS Newsletter 2016 Volume 1

CURRENT CONCEPTS
Soft Tissue Balancing in Total Knee Arthroplasty
Once appropriately distracted, attention is focused on two scales that correspond to the tensor: the angle (°, positive value in varus ligament balance) between the seesaw and platform plates, and the distance (mm) between the center midpoints of upper surface of the seesaw plate and the proximal tibial cut (mm, joint center / joint component gap). By measuring these angular deviations and distances under a constant joint distraction force, the ligament balance and joint center / joint component gaps can be measured, respectively.
Soft Tissue Balance with Reduced PF Joint and Femoral Component Placement
In a series of intraoperative soft tissue balance assessments, the importance of maintaining a reduced and anatomically oriented PF joint was emphasized, in order to obtain accurate and more physiologically relevant soft tissue balancing. First, the joint component gap kinematics in PS TKA showed a different pattern with the PF joint everted and reduced; the component gap increased throughout knee flexion with the PF joint eversion, while the component gap with the PF joint reduction increased with knee flexion but decreased after 60o of flexion. Second, intra-operative assessment of the joint component gap with reduced PF joint has been reported to predict post-operative flexion. Both an increased value during the extension to flexion gap and a decreased value during the flexion to deep flexion gap with the PF joint reduced, not everted, showed an inverse correlation with the post-operative knee flexion angle, not pre-operative flexion angle. Third, the correlations between the soft tissue balance assessed by the tensor and the navigation system were higher with the reduced PF joint than everted PF joint, suggesting that surgeons should assess soft tissue balance during PS TKA with the PF joint reduced when using a navigation system. In addition to these reports, some recent studies have emphasized the importance of the physiological post-operative knee condition in assessing soft tissue balance with the PF joint reduction.
The main concepts of measurement using the new tensor are different from the conventional tensioning device, with the femoral trial component in place as well as a reduced PF joint. As the next step, accordingly, the difference in soft tissue balancing between the femoral trial component in place and the conventional osteotomized condition was explored. In the intraoperative assessment of soft tissue balance, the joint gap showed significant decrease at extension, not flexion, after the femoral trial prosthesis placement, and varus ligament balances were significantly reduced at extension and increased at flexion after the femoral trial placement.
These changes at extension might have been caused by the tensed posterior structures of the knee with the posterior condyle of the externally rotated aligned femoral trial. At knee flexion, a medial tension in the extensor mechanisms might be increased after the femoral trial placement with the PF joint repaired, and increased ligament balance in varus. We measured the “joint component gap”, which is remarkably different from conventional gap measurement.
The joint component gap is measured with the femoral component in place, whereas the conventional gap measurement is done between the cut surfaces of the femur and tibia. By keeping the femoral component in place, the knee is afforded a greater degree of extension due to its curving arc. In this arrangement, the posterior condyles of the component tighten the posterior capsule, resulting in a smaller joint gap at full extension. In addition, due to the 7-degree posterior slope of the tibia and a slight femoral anterior bowing, we can consider the “conventional extension gap” to be at about 10 degrees of the knee flexion angle. Mitsuyama et al similarly reported on 80 varus type osteoarthritic knees with the offset-type tensor, that selecting larger size of femoral component as well as the femoral component placement reduced the extension gap. They reported that the placement of femoral component reduced the medial and lateral extension gaps by a mean of 1.0 mm and 0.9 mm, respectively, and medial and lateral gaps further decreased by a mean of 2.1 mm and 2.8 mm, respectively, when a specially made femoral component with a posterior condyle enlarged by 4 mm was tested. Mihalko et al stated that the release of more posterior structures had a greater effect on the extension gap than on the flexion gap in explaining the importance of the relationship between posterior structures and the extension gap in a cadaver study. Sugama et al reported in their operative study that a bone cut from the posterior femoral condyles could change the tension of the posterior soft tissue structures and thus alter the width and shape of the extension gap. These previous reports support the above mechanism.
26 ISAKOS NEWSLETTER 2016: Volume I