PATIENT SPECIFIC TEMPLATES FOR TOTAL KNEE REPLACEMENT

Abstract

Aim: To develop a 3-D pre-surgical planner that facilitates selection and placement of correct prosthetic components in the joint, and the design of patient specific templates to use intra-operatively to reproduce the pre-planned implantation procedure, in total knee replacement (TKR) surgery.

Design/Methods: The process begun with loading of pre-operative CT scan data of cadaver knee, onto medical software, followed by reconstructions of 3D models of the joint. Then measurements of anterior-posterior diameter of the femoral condyles of the 3D models of the joint were used to select and import a correct CAD drawing of prostheses from a database of electronic files available in a range of sizes. The selected prosthetic components were positioned and aligned on the 3-d model of the joint, making sure that the anterior flange of the femoral prosthesis component did not violate superior cortical bone of trochlea. Whilst the tibial stem was placed central within the medullar space of the bone, and the plane of the tibial cut was perpendicular to the long axis of the tibia. The planned data were next exported to a CAD environment where template to prepare the bone to receive the prostheses, was designed. A template was designed to press fit on a bone (e.g. femur), via minimum number of cylindrical protrusions with their ends made to conform to the geometry of that bone at the regions of contact. The integrated surgical tools were secured to the bones with pins through each of the protrusions, and were equipped with saw guide slits for cutting the bone, and with drill guides for drilling the fixation holes. Thereafter the files describing templates and prosthetic components selected for cadaveric joint concerned were sent to rapid prototyping machine for manufacturing.

Results: Fourteen procedures were performed on cadaveric knees to date. Visual examination of the joint has revealed the 3-D planning system enabled correct selection of appropriate prosthetic components and alignment, as evidenced by absence of protrusions or overhanging beyond the edges of the bones. The resected bone surfaces were visually smooth and flat. Gaps between the bones and the internal surfaces of the prosthetic components were measured using steel shim gauges, and largest recorded was 0.9mm. Laxity between the femur and tibia was absent and the joint attained full range of flexion. Dimensional deviations of post-operative scans of the prepared bones from the pre-planned ones were between 0.5 and 0.9mm. The templates after their use were shown capable to withstand the rigors of theatre environment.

Conclusion: With the planning software, it has been shown that it is possible to design a simple to use implantation guidance system according to the final position of the restorative prosthesis and the bone pathological condition. Pre-operative planner system relieves the clinician from multiple intra-operative decisions. The system is ideal for critical anatomical situations and eliminates possible manual placement errors such as those from extra and intra-medullary alignment tool. Less inventory required of both implants and instrumentation means reduced complexity of procedure, surgical time and cost.