Do medial-stabilized TKA designs confer better balance than non-medial-stabilized designs?
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No better balance provided
Total knee arthroplasty ligament balancing is a complex interplay between patients’ inherent laxity/stiffness, bony alignment and soft tissue factors.
As we all know, the knee itself is not purely a hinge joint. There are rotational movements of the femur and tibia in flexion and extension. The tibia internally rotates with respect to the femur from extension to flexion. There are also factors such as femoral rollback and femoral external rotation. From 0° to 120° of flexion, the medial collateral ligament does not tend to move much while the lateral collateral ligament moves posteriorly by 17 mm.
The concept of the medial-stabilized TKA design was to provide a more natural type of knee with a ball-in-socket type of congruency on the medial compartment and more movement laterally. Yet, this may not be necessary. Today, there are an array of options in terms of restoring supposedly normal knee kinematics with asymmetric tibial components with the medial side being larger in surface area than the lateral, more congruency on the medial side, as well as anterior and posterior cam and post mechanisms to allow for stability while reproducing better rollback than a medial-stabilized design.
However, the implant alone cannot compensate for malalignment, so it would behoove the surgeon to be meticulous about femoral and tibial positioning as well as rotation. Furthermore, long-term studies have not shown objective advantages to a medial-stabilized design compared to conventional knee replacements. Therefore, a medial-stabilized design of a knee replacement has not been shown to be superior in terms of longevity or patient-reported outcomes. The decision for a specific implant should be based on the patient’s anatomy, deformity, surgeon experience and implant cost.
- References:
- Kaneko T, et al. J Orthop. 2017;doi:10.1016/j.jor.2016.12.005.
- Murakamia K, et al. J Orthop. 2018;doi:10.1016/j.jor.2018.05.003.
- van der Merwe JM, et al. J Am Acad Orthop Surg Glob Res Rev. 2020;doi:10.5435/JAAOSGlobal-D-19-00047.
- Rina Jain, MD, is an attending orthopedic surgeon with Synergy Orthopedic Specialists Medical Group and a voluntary clinical lecturer at University of California San Diego.
Literature shows no kinematic differences
TKA is a procedure that has yielded great success in the treatment of end-stage knee arthritis. However, many studies have quoted up to 20% of individuals who are not satisfied with their outcomes due to persistent pain and stiffness. Newer techniques in addressing kinematic alignment and balance have been developed in an attempt to improve these outcomes. Medial pivot knees were designed to address the medial pivot pattern seen during gait and, in the majority of patients, during a deep bend. The medial pivot design in most modern TKA implants allows this medial pivot by creating greater conformity on the medial compartment between the femoral condyle and tibial insert. In addition to this increase in conformity medially, the lateral side is typically less congruent to allow more free movement along the lateral arc. Theoretically, this design should confer a sense of motion closer to that of a physiologically “normal” knee. Several studies have conflicting results as to whether this design confers better clinical outcomes. R. Michael Meneghini, MD, and colleagues assessed 152 TKAs using sensor-guided technology to detect this medial pivot phenomenon. Despite controlling for several clinical factors, the outcomes did not differ in medial pivot knee designs. In addition, this sensor technology showed intraoperative medial pivot kinematic patterns in only 40% of TKA patients. When specifically targeting kinematics, the 0° to terminal flexion cohort showed no difference in patient-reported outcomes based on the pivot type. However, many studies have cited differences in medial pivot design vs. PS implant design with respect to Knee Society Score. In my opinion, the literature has not demonstrated superior clinical outcomes or true kinematic differences in medial pivot knees.
- Reference:
- Meneghini M, et al. Orthopaedic Proceedings. 2018;99-B:82.
- Linda I. Suleiman, MD, is assistant professor of orthopedic surgery and medical education, adult hip and knee reconstruction and replacement at Northwestern University Feinberg School of Medicine in Chicago.
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