More passive internal tibial rotation with posterior cruciate ligament retention than with excision in a medial pivot TKA implanted with unrestricted caliper verified kinematic alignment

Abstract

Purpose: Excision of the posterior cruciate ligament (PCL) is recommended when implanting a medial pivot (MP) total knee arthroplasty (TKA) to reduce the risk of limiting flexion by over-tensioning the flexion space. The present study determined whether PCL retention (1) limits internal tibial rotation and (2) causes anterior lift-off of the insert in 90° flexion after implantation of an MP design with unrestricted caliper verified kinematic alignment (KA).

Methods: Four surgeons implanted an MP TKA design with medial ball-in-socket and lateral flat tibial insert in ten fresh-frozen cadaveric knees. Before and after PCL excision, trial inserts with medial goniometric markings measured the angular I-E tibial orientation relative to the trial femoral component's medial condyle in extension and at 90° flexion, and the surgeon recorded the occurrence of anterior lift-off of the insert at 90° flexion.

Results: PCL retention resulted in greater internal tibial rotation than PCL excision, with mean values of 15° vs 7° degrees from maximum extension to 90° flexion, respectively (p < 0.0007). At 90° flexion, no TKAs with PCL retention and one TKA with PCL excision had anterior lift-off of the insert (N.S.).

Conclusions: This preliminary study of ten cadaveric knees showed that PCL retention restored more passive internal tibial rotation than PCL excision with a negligible risk of anterior lift-off. However, in vivo analysis from multiple authors with a larger sample size is required to recommend PCL retention with an MP TKA design implanted with unrestricted caliper verified KA.

Keywords: Calipered; Insert thickness; Posterior cruciate ligament; Tibial rotation; Total knee arthroplasty; Total knee replacement.

Fig. 1

Schematics show a left trial insert goniometer with an anteromedial scale in 5° intervals. The I–E rotation of the tibia relative to a longitudinal line on the medial condyle of the trial femoral component (circles) was 3° external in extension and 12° internal at 90° of flexion resulting in 15° of internal rotation like the native knee. The medial pivot insert is composed of a medial ball-in-socket and a lateral flat articular surface

Fig. 2

Intraoperative photographs of a left TKA in 90° flexion show examples of no anterior lift-off and lift-off of the trial insert (not goniometric) from the trial baseplate, which indicates an over-tensioned flexion space and PCL

Fig. 3

Worksheet for intraoperatively recording serial verification checks for a right knee with a varus deformity showing the caliper measurements of bone resections for a femoral component with a 9 mm-thick distal femoral condyles and 8 mm-thick posterior femoral condyles. The thicknesses of the distal and posterior femoral resections are adjusted so that they equal the thickness of the component within 0 ± 0.5 mm after compensating for 2 mm of cartilage wear when present and a ~ 1 mm kerf from the saw cut

Fig. 4

Composite shows the decision-tree followed by the surgeons when they performed unrestricted caliper verified KA TKA. The technique sets the components to restore the pre-arthritic distal and posterior femoral and proximal tibial joint lines within 0 ± 0.5 mm, which restores native tibial compartment forces without the release of healthy ligaments, including the PCL [29, 33, 34]

Fig. 5

Composite shows the method for positioning the TKA and measuring the angular tibial orientation of the goniometer insert relative to the femoral component with the knee passively extended and with the TKA in 90° flexion with the heel resting on the bump supporting the leg without the surgeon applying rotation to the tibia

Fig. 6

Box plots of ten cadaveric knees show the mean internal tibial rotation from maximum extension to 90° of flexion (transverse line in the middle of the green diamond) of 15° with PCL retention was significantly greater than the 7° after PCL excision after implantation of a medial ball-in-socket and lateral flat tibial insert TKA with unrestricted calipered KA (p < 0.0007). The top and bottom edges of the green diamond indicate the 95% confidence interval limits