Abstract
Background
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. The present study determined the differences in the I-E range of rotation and I-E positions at maximum extension and at 90° of flexion that result from the use of insert thicknesses that deviate ± 1mm in thickness from the implanted insert.
Methods
20 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).
Results
For all three inserts, the tibial component progressively internally rotated on the femoral component from maximum extension to maximum 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 position 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 2). At 90° the mean −14.7° internal insert position 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 3).
Discussion and Conclusions
The insert goniometer is an inexpensive, simple, and sensitive instrument that measured the insert position of a medial ball-in-socket PCL retaining implant with a flat lateral insert implanted with calipered KA and showed the I-E rotation matched the pattern of the native knee's screw-home mechanism. 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 position of the insert at maximum extension and a less internal position at 90° when the healthy ligaments were stretched or loosened by 1mm.
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