Abstract
Introduction
The marriage of rapid prototyping technologies with Arthroplasty has resulted in the fabrication and use of cutting jigs and guides which are tailored to a patients' individual anatomy. These disposable cutting blocks are designed based on input parameters obtained from pre-operative CT and MRI scans and manufactured using 3-D printers. Indirect benefits include a reduction in inventory and a decrease in the burden for central sterilising units. This approach is advantageous for the surgeon in the attainment of ideal mechanical alignment, which is known to be associated with an improved clinical outcome and implant longevity. This study evaluated the postoperative alignment parameters from a single surgeon series of patients following TKA with the Signature (Biomet) system.
Methods and Materials
The postoperative alignment of a single surgeon series of 60 consecutive patients receiving a Vanguard cruciate retaining TKR (Biomet) using the Signature patient-specific surgical positioning guides was performed. Postoperative CT and preoperative templating MRI scans were imported into Mimics 14.0 (Materialise, Belgium) where specific bony landmarks were identified in both data sets. A subset of these points was used to transform the MRI data into the CT coordinate frame to enable the computation of femoral mechanical alignment in the absence of a full-length lower limb CT scan. CT and transformed MRI landmarks were then imported into ProEngineer (PTC, MA) where angular measurements were made by projecting axes onto anotomical planes. Flexion, rotation, valgus/varus of the femoral component and posterior slope, rotation and valgus/varus of the tibial component were computed. Femoral rotation was referenced to the trans-epicondylar axis as opposed to Whiteside's line. Overall limb alignment was determined based on individual component position.
Results
Results are presented as mean ± standard deviation. Overall, the mean position of the femoral component was found to be 0.8° ± 2.1° in valgus, 4.2° ± 3.4° in flexion and 0.3° ± 2.0° in external rotation. Likewise, the mean position of the tibial component was found to be in 0.4° ± 1.4° in varus, 4.6° ± 2.9° in posterior slope and 1.9° ± 6.6° in external rotation. Overall mean limb alignment was found to be 0.4° ± 2.5° in varus.
Discussion
With respect to varus/valgus alignment of the components, the number of outliers outside the ± 3° accepted range was small (8/60 for the femoral component and 1/60 for the tibial component). This data demonstrates that the early results for knee replacements performed using the Signature patient specific jigs delivers an improved level of prosthetic alignment when compared to published data for standard instrumented knees.
