Femorotibial kinematics and load patterns after total knee arthroplasty: An in vitro comparison of posterior-stabilized versus medial-stabilized design
Introduction
Femorotibial kinematics and load patterns after total knee arthroplasty (TKA) vary greatly. Non-physiological motion after TKA, for example, paradoxical anterior translation of the femur on the tibia with flexion or posterior subluxation of the femur are described (Dennis et al., 1998, Dennis et al., 2003). Besides the necessary stability of a TKA, physiological femoral posterior translation provides a more natural motion of the knee, resulting in a better functional outcome and satisfaction for the patient after TKA (Pritchett, 2011). Implant design directly influences the kinematics after TKA (Banks and Hodge, 2004). Therefore, “guided motion” knee systems like the PS system were developed in the late 1970s by Insall et al. (Insall et al., 1982) to restore physiological kinematics and make kinematics more predictable. A post on the inlay interacts with an integrated cam in the femoral component, providing a medial and lateral femoral posterior translation with flexion of the knee. With specific investigations of normal knee kinematics (Blaha et al., 2003, Iwaki et al., 2000), other guided motion systems were further developed in the 2000s. The MS design is supposed to provide an antero-posterior (AP) translation of the femur in the lateral compartment, while the medial condyle interacts like a “ball and socket” joint. This medial convexity of the inlay might reduce contact stress and might replicate physiological kinematics more than the PS design. On the other hand, studies have described an antero-lateral pain with guided motion TKA systems and it is assumed that a higher forced lateral AP translation might cause pain in this area (Halewood et al., 2014, Luyckx et al., 2010).
Especially for newly designed or modified implants, an in vitro analysis should be extensively performed for implementation of the implants in vivo. In vitro studies with human specimens are an established method of investigating femorotibial knee kinematics and load patterns after TKA (Arnout et al., 2014, Blaha et al., 2003, Heyse et al., 2010, Steinbruck et al., 2015, Varadarajan et al., 2009, Walker et al., 2011). A specific comparison in terms of kinematics and joint pressure of a PS and a MS knee system was—to the authors knowledge—not performed before. Therefore, the goal of this study was to compare a newly developed MS TKA system with a conventional PS design of the same manufacturer. We firstly hypothesized that the MS design had a higher AP translation and secondly a higher maximum pressure in the lateral femorotibial compartment during flexion of the knee. By having the same bone cuts, fixation points, and tibial baseplate for both the PS and MS knee systems, a direct comparison and interpretation of the results within every single knee specimen could be performed.
Section snippets
Human knee specimens
Twelve human knee specimens (Table 1) were used to investigate knee kinematics after TKA. The local ethics committee of the university approved the acquisition and usage of the human specimens. None of the chosen specimens had any severe varus or valgus deformity ≥ 10°. Muscles were dissected and fat tissue was removed carefully. Special care was taken to preserve the joint capsule and the tendons of the vastus medialis muscle, vastus lateralis muscle, intermedius and rectus femoris muscle, as
Femorotibial translation
According to the mixed effect model, the MS design showed a significant decrease of femorotibial translation in the medial compartment compared to PS design (Table 2, Fig. 4). At 120° of flexion, the PS design displayed a mean posterior translation of the femur related to the tibia of 15.7 mm (SD 7.6 mm), while the MS design translated in the medial compartment of an average of 3.5 mm (SD 4.8 mm) (Fig. 5).
In the lateral compartment, both designs showed a posterior translation of the femur in
Discussion
Our study shows that the PS design enforces a ML posterior translation, while the MS TKA system enables a combination of a posterior translation in the lateral compartment with a medial pivot. According to in vivo and in vitro studies of the physiological movement of the knee during flexion the medial femoral condyle also hardly translates in AP direction while the lateral condyle significantly moves backwards (Iwaki et al., 2000, Johal et al., 2005, Karrholm et al., 2000). Johal et al.
Conclusions
The MS TKA system enables a combination of medial pivot with a lateral translation, which is slightly lower compared to PS TKA. This provides AP stability of the knee and the appearance of antero-lateral pain after implantation is unlikely. Regarding femorotibial pressure distribution, future in vivo analysis and long-term studies are needed in order to support the good outcome and survivorship of medial stabilized knee systems.
Conflict of interest
The authors declare no conflict of interest for this study.
Acknowledgments
We especially thank Professor Michael A Freeman for his personal support in critically reviewing the manuscript. We also thank Tatjana Müller for the preparation of the specimens and her help during the tests. We like to thank the Medacta Company to support this study by donating the used knee prosthesis and their technical and logistical support. The Medacta Company had no involvement in the collection, analysis and interpretation of data and no involvement in the writing of the report; and in
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