Abstract
Introduction
Total knee arthroplasty is the standard treatment for advanced knee osteoarthritis. Patient-specific instrument (PSI)has been reported by several authors using different techniques produced by implant companies. The implant manufacturers produce PSI exclusively for their own knee implants and for easy straightforward cases. However, the PSI has become very expensive and unusable as a universal or an open platform. In addition, planning the implant is done by technicians and not by surgeons and needs long waiting time before surgery (6 weeks).
Methods
We proposed a new technique which is a device and method for preparing a knee joint in a patient undergoing TKA surgery of any knee implant (prosthesis). The device is patient specific, based on a method comprised of image-based 3D preoperative planning (CT, MRI or computed X-ray) to design the templates (PSI) that are used to perform the knee surgery by converting them to physical templates using computer-aided manufacturing such as computer numerical control (CNC) or additive-manufacturing technologies. The device and method are used for preparing a knee joint in a universal and open-platform fashion for any currently available knee implant.
Results
All patient-specific implants and any knee implant could be produced. The technique was applied on NExGen implant (Zimmer)on 21 patients, PFC implant (Depuy, J & J) on 5 patients, Scorpio NRG implant (Stryker) on 24 patients and SLK Evo implant (Implant International) on 81 patients. The >15 degrees varus gave a mean of 10.44 degrees in 56.67% of cases and the <15 degrees varus gave a mean of 24.04 degrees in 43.33% of cases. The total varus of 5–30 degrees gave a mean of 16.33 degrees in 90.9% of cases and the total valgus of 20–40 gave a mean of 25 degrees in 9.1% of cases. The fixed flexion deformity of < 20 degrees gave a mean of 9.4 degrees in 75.3% of cases while the fixed flexion deformity of >20 gave a mean of 31.87 degrees in 24.7% of cases.
Discussion
The system is based on CT images, generic data of implant sizes, average bone geometry and standard TKA parameters for bone cutting, mechanical axis and rotation (e.g., zero-degree coronal cut, adjustable posterior slope, femoral flexion, epicondylar axis, no notching or overhang, etc.). The method of planning and completing virtual surgery of TKA includes several steps based on 3D reconstruction and segmentation of computed tomography (CT) or MRI scan data. The universal device and method are suitable to be used for any commercially and currently available knee implant. They are used for all on-shelf implants and all patient-specific instruments.
The device is specifically designed for TKA and the planning is based on the 3D files of a universal TKA prosthesis. There are four standard sizes of the universal TKA prosthesis which were built depending on the average bone geometry. These 4 sizes are 55, 60, 65 and 70 mm. These sizes are consistent with the six most common implants available today: NexGen Zimmer, PFC Depuy, Sigma Knee, Triathlon Stryker, Vanguard Biomet, and Smith & Nephew Proflex. However, for extreme cases, one size above or below the maximum and minimum range can be used. The device has 2 parts: a femoral part and a tibial part, both of which are independent of any commercially available knee implant.
