Restoring the Patient's Pre-Arthritic Posterior Slope Is the Correct Target for Maximizing Internal Tibial Rotation When Implanting a PCL Retaining TKA with Calipered Kinematic Alignment

Abstract

Introduction: The calipered kinematically-aligned (KA) total knee arthroplasty (TKA) strives to restore the patient's individual pre-arthritic (i.e., native) posterior tibial slope when retaining the posterior cruciate ligament (PCL). Deviations from the patient's individual pre-arthritic posterior slope tighten and slacken the PCL in flexion that drives tibial rotation, and such a change might compromise passive internal tibial rotation and coupled patellofemoral kinematics.

Methods: Twenty-one patients were treated with a calipered KA TKA and a PCL retaining implant with a medial ball-in-socket and a lateral flat articular insert conformity that mimics the native (i.e., healthy) knee. The slope of the tibial resection was set parallel to the medial joint line by adjusting the plane of an angel wing inserted in the tibial guide. Three trial inserts that matched and deviated 2°> and 2°< from the patient's pre-arthritic slope were 3D printed with goniometric markings. The goniometer measured the orientation of the tibia (i.e., trial insert) relative to the femoral component.

Results: There was no difference between the radiographic preoperative and postoperative tibial slope (0.7 ± 3.2°, NS). From extension to 90° flexion, the mean passive internal tibial rotation with the pre-arthritic slope insert of 19° was greater than the 15° for the 2°> slope (p < 0.000), and 15° for the 2°< slope (p < 0.000).

Discussion: When performing a calipered KA TKA with PCL retention, the correct target for setting the tibial component is the patient's individual pre-arthritic slope within a tolerance of ±2°, as this target resulted in a 15-19° range of internal tibial rotation that is comparable to the 15-18° range reported for the native knee from extension to 90° flexion.

Keywords: kinematic alignment; rotation; slope; total knee arthroplasty; total knee replacement.

Figure 1

Intraoperative photographs of the medial side of a right knee show the method of setting the tibial resection (blue dotted line) parallel to the medial joint line (green dotted line) by adjusting the plane of an angel wing inserted in the tibial guide (left) and the visual verification check showing the tibial resection matches the patient’s native slope (right).

Figure 2

Schematics of a left TKA show the −12° of internal tibial orientation of the trial insert goniometer relative to the medial femoral condyle (left), and the method of creating the tibial inserts with a 2°> (=2° increased) and 2°< (=2° decreased) slope by pivoting the articular surface about the center (blue circle) of the insert that matched the patient’s native slope.

Figure 3

The figure shows the radiographic method for measuring the posterior slope of the preoperative tibia and postoperative tibial component (left) and the distribution of the difference in slope for the 21 patients as a measure of the reproducibility in setting the tibial component to the patient’s native slope using the angel wing (right).

Figure 4

The figure shows an intraoperative photograph of the insert goniometer of a left TKA in extension reading 8° of external tibial orientation (left) and box plots of 21 patients that show the mean external tibial orientation was not significantly different between the three inserts (right). The top and bottom edges of the green diamond indicate the 95% confidence interval limits.

Figure 5

The figure shows an intraoperative photograph of the insert goniometer of a left TKA in 90° flexion reading −15° of internal tibial orientation (left) and the box plots show the internal tibial orientation for 21 patients and the insert slopes with different letters are significantly different (right).

Figure 6

The box plots show the internal tibial rotation from extension to 90° flexion for 21 patients, and the insert slopes with different letters are significantly different.