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.
Results and comparison with native knee
Although the mean medial tibial force was greater than the mean lateral tibial force from 0 degrees to 90 degrees flexion, the tibial force in each compartment ( Figs. 12.1A and 12.1B ) showed similar patterns of decrease with flexion. Tibial compartment forces were greatest at extension, decreased rapidly to zero at about 5-degrees flexion, and remained near zero over the remainder of the flexion arc to 90-degrees flexion. The mean total tibial force ( Fig. 12.1C ) was greatest at 0-degrees flexion (mean total = 103 N), decreased to a minimum at 10-degrees flexion (mean total = 0 N), and then increased somewhat during flexion from 15-degrees to 90-degrees flexion (mean total = 36 N at 90-degrees flexion). The mean difference in tibial compartment forces ( Fig. 12.1D ) remained below 31 N (<7 lbs) from 0-degrees to 90-degrees flexion.
The mean tibial compartment forces and the mean total tibial forces after KA TKA in vitro are somewhat lower than those measured in the native knee in vitro ( Fig. 12.1C ). This likely occurred because the contribution of the soft tissue restraints was isolated in the present study and passive muscle forces were absent, whereas both effects were present in the study that measured tibial compartment forces in the native knee. In any case, the low mean tibial compartment forces and the low mean total tibial force indicate that calipered KA TKA reduces the risk of overly tight soft tissue restraints that might cause persistent pain, stiffness, and limited range of motion. ,
The mean differences in tibial forces between compartments closely match the differences of the native knee throughout flexion ( Fig. 12.1D ). This close match indicates that calipered KA TKA restores the tensions in the medial and lateral soft tissue restraints to native and the resulting balance in tibial compartment forces to native.
Intraoperative Tibial Compartment Forces During Passive Motion in the Calipered Kinematically Aligned Total Knee Arthroplasty
describes and demonstrates methods used to measure tibial compartment forces during passive motion intraoperatively.
Description of commercially available tibial force sensor and methods
A commercially available tibial force sensor (VERASENSE, Orthosensor, Dania Beach, FL, USA) was used to intraoperatively measure tibial compartment forces and their locations (i.e., centers of pressure) independently in the medial and lateral compartments ( Fig. 12.2 ). Again, only the tibial forces are of interest in this chapter. Unlike the custom tibial force sensor used in the cadaveric knees, the area of measurement in each compartment was limited to the triangle formed by three imbedded load cells. The RMSE for distributed loading within the triangular area is 17 N. Although this absolute error is almost three times greater than that of the custom tibial force sensor described above, the VERASENSE is still useful as long as mean values are computed over a reasonably large sample (i.e., >30 patients), because computing the mean negates the random error. By replacing the tibial insert with an exact match in size and shape for a particular size insert, the VERASENSE enabled intraoperative measurement of tibial compartment forces without otherwise affecting the biomechanics of the tibiofemoral joint.