Background

Total knee arthroplasty (TKA) is the current standard of care for patients with severe osteoarthritis in their knee. In TKA, the damaged articular surfaces of the femur and tibia are replaced with orthopedic implants (i.e. the femoral and tibial components respectively). However, 20-25% of these patients report dissatisfaction after TKA using the traditional method of TKA, mechanically aligned TKA. By 2030, the prevalence of dissatisfaction could be 700,000 patients/year based on projected estimate of 3.5 million TKAs/year in the United States alone.

The primary determinant of the clinical outcome of TKA is the alignment of the femoral and tibial components relative to their respective bones. The mechanically aligned TKA references anatomic landmarks that are unrelated to the function of the knee. A relatively new method for aligning the components, kinematically aligned TKA (KA TKA), references the articular surfaces with the goal of restoring the alignments of the limb, knee, and joint lines, and hence native knee function.

Although patients report higher satisfaction and function at 6 months to 6 years after KA TKA, two unavoidable changes may prevent KA TKA from restoring passive knee function. The first is the replacement of the articular surfaces with implants of discrete sizes and average shapes. The second is resection of the anterior cruciate ligament (ACL).

Accordingly, the overreaching aim of this research was to provide objective evidence using controlled in vitro experiments to evaluate passive joint function before and after KA TKA. Four metrics were determined to evaluate passive joint function:​ (1) the laxities (relative displacement of the tibia to the femur in a degree of freedom under an applied load), (2) the neutral positions (relative position/orientation of the tibia to the femur in a degree of freedom with no loads applied), (3) the contact force balance (difference between the contact forces in the medial and lateral compartments), and (4) contact kinematics (i.e. motion of the contact locations of the femoral component on the tibial component during flexion).

Aim 1:​ Laxities and Neutral Positions

To determine whether either or both of these unavoidable changes prevent KA TKA from restoring to native the laxities and neutral positions, the primary objectives were to determine how well KA TKA prevents clinically important (1) changes in the laxities and (2) shifts in the neutral positions in varus-valgus rotation, internal-external rotation, anterior-posterior translation, and compression-distraction relative to those of the native knee. The eight laxities and four neutral positions were measured from 0° to 120° of flexion in 30° increments in 13 native human cadaveric knees before and after KA TKA using a six degree-of-freedom load application system. The prevalence of clinically important changes in the 65 measured values (13 knees × 5 flexion angles) of each laxity after KA TKA was relatively low for all eight laxities. The highest prevalence occurred in external rotation laxity (21% exceeded 3.6°) and the lowest occurred in compression laxity (0% exceeded 1 mm). The prevalence of clinically important shifts in the 65 measured values of each neutral position after KA TKA was relatively low for internal-external rotation (20% exceeded 5°) and varus-valgus rotation (5% exceeded 1.5°), but was relatively high for the anterior-posterior (69% exceeded 1.1 mm) and compression-distraction (38% exceeded 1 mm).

Aim 2:​ Laxities and Neutral Positions

Imbalanced contact forces (> 67 N) and abnormal contact kinematics (i.e. anterior translation of the contact locations) of the femur on the tibia during passive flexion are associated with reduced satisfaction and limited flexion respectively after total knee arthroplasty (TKA). Although high satisfaction and function have been reported after kinematically aligned TKA (KA TKA) at 6 months to 6 years, no study has determined how well KA TKA prevents clinically important imbalances and abnormal contact kinematics during passive flexion. Accordingly, the objectives were to determine in vitro how well KA TKA prevents (1) imbalances between the contact forces in the medial and lateral compartments greater than 67 N, and (2) anterior translation of the contact locations in the medial and lateral compartments over an arc of flexion from 0° to 120°. The contact forces and contact locations were determined in both the medial and lateral compartments from 0° to 120° of flexion in 30° increments in six human cadaveric knees following KA TKA using a tibial contact force sensor implanted in place of the standard cruciate-retaining tibial component. The average contact forces remained balanced within 18 N, and the prevalence of clinically important imbalances in the 30 measured values (6 knees × 5 flexion angles) was 7%. The contact locations in both compartments translated posteriorly in all six KA TKA (i.e. prevalence of anterior translation = 0%) by an average of 16 mm and 18 mm in the medial and lateral compartments respectively from 0° to 120° of flexion.

Conclusions

KA TKA prevented clinically important changes/shifts in ≥ 79% of the measured values in ten of the twelve laxities/neutral positions, prevented clinically important contact force imbalances in 93% of the measured values, and prevented abnormal contact kinematics 100% of the knees despite the two unavoidable changes. These findings also help to explain the previously published clinical results that show KA TKA has a low risk of failure at 6 years and leads to high patient satisfaction and function at 6 months to 6 years which adds to the ever-growing compilation of evidence that justifies KA TKA as an acceptable alternative to the traditional alignment goals for TKA.