Abstract
BACKGROUND
Trochlear geometry of modern femoral implants is designed for the mechanical alignment (MA) technique for Total Knee Arthroplasty (TKA). The biomechanical goal is to create a proximalised and more valgus trochlea to better capture the patella and optimize tracking. In contrast, Kinematic alignment (KA) technique for TKA respects the integrity of the soft tissue envelope and therefore aims to restore native articular surfaces, either femoro-tibial or femoro-patellar. Consequently, it is possible that current implant designs are not suitable for restoring patient specific trochlea anatomy when they are implanted using the kinematic technique. This could cause patellar complications, either anterior knee pain, instability or accelerated wear or loosening. The aim of our study is therefore to explore the extent to which native trochlear geometry is restored when the Persona® implant (Zimmer, Warsaw, USA) is kinematically aligned.
METHODS
A retrospective study of a cohort of 15 patients with KA-TKA was performed with the Persona® prosthesis (Zimmer, Warsaw, USA). Preoperative knee MRIs and postoperative knee CTs were segmented to create 3D femoral models. MRI and CT segmentation used Materialise Mimics® and Acrobot Modeller® software, respectively. Persona® implants were laser-scanned to generate 3D implant models. Those implant models have been overlaid on the 3D femoral implant model (generated via segmentation of postoperative CTs) to replicate, in silico, the alignment of the implant on the post-operative bone and to reproduce in the computer models the features of the implant lost due to CT metal artefacts. 3D models generated from post-operative CT and pre-operative MRI were registered to the same coordinate geometry. A custom written planner was used to align the implant, as located on the CT, onto the pre-operative MRI based model (figure 1). In house software enabled a comparison of trochlea parameters between the native trochlea and the performed prosthetic trochlea (figure 2). Parameters assessed included 3D trochlear axis and anteroposterior offset from medial facet, central groove, and lateral facet. Sulcus angle at 30% and 40% flexion was also measured. Inter and intra observer measurement variabilities have been assessed.
RESULTS
Varus-valgus rotation between the native and prosthetic trochleae was significantly different (p<0.001), with the prosthetic trochlear groove being on average 7.9 degrees more valgus. Medial and lateral facets and trochlear groove were significantly understuffed (3 to 6mm) postoperatively in the proximal two thirds of the trochlear, with greatest understuffing for the lateral facet (p<0.05). The mean medio-lateral translation and internal-external rotation of the groove and the sulcus angle showed no statistical differences, pre and postoperatively (figure 3).
CONCLUSION
Kinematic alignment of Persona® implants poorly restores native trochlear geometry. The clinical impact of this finding remains to be defined.
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