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Calipered Kinematically Aligned Total Knee Arthroplasty Closely Restores the Tibial Compartment Forces of the Native Knee
Overview
Tibial forces in the medial and lateral compartments following calipered kinematically aligned (KA) total knee arthroplasty (TKA) are of high interest for two reasons. One reason is that knowledge of the tibial compartment forces provides a quantitative indication of how well the goal of restoring biomechanical variables characterizing knee function to native has been achieved. A second is that differences in tibial forces between the medial and lateral compartments serve as an indicator of soft tissue “balancing.” This chapter summarizes the methods and results from two studies, one that measured in vitro tibial compartment forces of a calipered KA TKA during passive motion and another that measured intraoperative tibial compartment forces during passive motion. These results are compared with those of a third study, which measured in vitro tibial compartment forces in the native knee during passive motion. Based on these comparisons, the chapter demonstrates that patient-specific alignment of the limb, knee, and joint lines inherent to KA TKA, in conjunction with the verification checks used in calipered KA TKA, closely restore tibial compartment forces and the balance (i.e., difference) between tibial compartment forces to native, without ligament release.
In Vitro Tibial Compartment Forces During Passive Motion in the Calipered Kinematically Aligned Total Knee Arthroplasty and Native (i.e., healthy) Knee
describes and demonstrates methods used to measure tibial compartment forces during passive motion in vitro.
Description of custom tibial force sensor and methods
To measure tibial compartment forces in vitro, a unique custom tibial force sensor was developed. , The sensor replaced the tibial component (i.e., baseplate and insert) and measured the tibial forces and their locations (i.e., centers of pressure) independently in the medial and lateral compartments over the full area of the articular surfaces. The root mean squared error (RMSE) of the custom tibial force sensor is 6.1 N or 1.3% of 450 N, which is full scale. The articular surfaces of the tibial force sensor were interchangeable to match those of a particular size tibial insert. Hence the substitution of the tibial force sensor in place of the tibial component did not alter the biomechanics of the tibiofemoral joint.
After calipered KA TKA, tibial compartment forces were measured over the range of passive motion (extension to 90 degrees of flexion) in 13 cadaveric knees. Small loads were applied to the tendons of the biceps femoris (15 N), the semimembranosus/semitendinosus (26 N), and the quadriceps (80 N) to maintain the stability of the joint. The mean total tibial force caused by tension in the soft tissues was computed as the difference between the mean measured total tibial force (i.e., sum of medial+lateral) and the average contribution of the applied muscle forces to the total tibial force. The difference in tibial compartment forces was computed as the difference between the medial and lateral tibial forces. Thus, a positive difference indicated that the medial tibial force was greater than the lateral tibial force.
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