gms | German Medical Science

Objectives: Calipered kinematically aligned (KA) total knee arthroplasty (TKA) restores the in vitro internal-external (I-E) rotation laxities at 0° and 90° of the native knee. Although increasing and decreasing the thickness of the insert in 1 mm increments loosens and tightens the flexion space, there are little data on how this might adversely affect the screw-home mechanism and I-E rotational laxity from maximum extension to 90° flexion. The present study determined the differences in passive internal rotation and I-E orientations at maximum extension and at 90° of flexion that result from the use of insert thicknesses that deviate ± 1mm from the implanted insert.

Methods: Twenty-two patients were treated with a calipered KA and a PCL retaining implant with a 1:1 medial ball-in-socket constraint and a non-constrained lateral flat articular insert surface. Verification checks, that are validated to restore native tibial compartment forces without release of healthy ligaments including the PCL, were used to select the optimal insert thickness. Trial inserts with thicknesses ranging from 10 to 13 mm were 3-D printed with medial goniometric markings that record rotation from 20° external to -20° internal with respect to a sagittal line laser marked on center of the medial condyle of the trial femoral component at maximum extension and 90° of flexion (Figure 1 [Fig. 1]).

Results: For all three inserts, the tibia progressively internally rotated on the femur during flexion. From maximum extension to 90° flexion the -21.7° range of internal rotation for the optimal insert thickness was greater than the -16° for the 1mm thinner insert (p < 0.000), and the -13.1° for the 1mm thicker insert (p < 0.000). At maximum extension, the mean insert orientation of 7° external for the optimal insert thickness was more external than the 4.5° for the 1mm thinner insert (p < 0.000), and the 3.5° for the 1mm thicker insert (p < 0.000) (Figure 1 [Fig. 1]).

At 90° the mean -14.7° internal insert orientation for the optimal insert thickness was more internal than the -11.5° for the 1mm thinner insert (p < 0.000), and the -9.5° for the 1mm thicker insert (p < 0.000) (Figure 1 [Fig. 1]).

Discussion and Conclusions: Restoring the pre-arthritic native ligament laxities is the target, as the insert goniometer detected a 6° loss of internal rotation and a less external orientation at maximum extension and a less internal orientation at 90° when the healthy ligaments were stretched or loosened by 1mm. Using the insert goniometer can serve the arthroplasty surgeon as a verification check that the optimal insert thickness is selected, and the native pre-arthritic laxities are restored.