Abstract
Introduction
Accurate placement of total knee arthroplasty (TKA) components is critical for obtaining good long-term clinical outcome. Several contemporary CT- or MRI-based technologies allow surgeons to pre-plan TKA and translate that planning into the operating room. To evaluate TKA component placement, post-operative CT or MRI scans allow comprehensive 3D measurements. However, these are expensive and difficult to obtain in large numbers, and yield an additional radiation dose to the patient (in case of CT). A potential solution to overcome these hurdles exists in using 2D/3D registration techniques. In this technique, a new tool (the X-ray Module, Mimics®, Materialise NV) is used to align one or more post-operative X-rays with the preoperative CT- or MRI-based 3D planning (Figure 1). The aim of this study was to determine the accuracy of this 2D/3D registration technique for determining 3D position of TKA implant components postoperatively.
Materials and Methods
A TKA was performed in six human cadaver legs. A CT scan was acquired preoperatively and the bones were segmented using Mimics® to obtain 3D bone models. Post-operatively, a high-resolution CT scan with minimization of metal scatters was acquired and bones and implant components were segmented in Mimics® to obtain the ground truth for their relative position. To apply the novel X-ray based post-op analysis, conventional anteroposterior and lateral radiographs were obtained. The accuracy of the X-ray tool was determined by calculating the angles (varus/valgus, flexion/extension, external/internal rotations) and the distances (anterior/posterior, proximal/distal, medial/lateral) between the centers of gravity of the implants from the X-ray based method and the CT-based ground truth in the anatomical coordinate system of the bone. X-ray based alignment was assessed by an orthopedic surgeon (3 repetitions) and Bland-Altman plots were created to visualize the differences between the ground truth and the X-ray based assessment of the implant position.
Results
The differences in rotation between the X-ray and CT analyses are shown in Figure 2 (femur) and Figure 3 (tibia). The average differences between the methods were in the order of 1° or less, except for external/internal rotation, which showed the largest differences (0.23±0.85° for the femur, 0.51±1.91° for the tibia). For the position differences between the X-ray and CT analyses, average differences were smaller than 0.3mm and 0.9mm for the femur and tibia, respectively. Proximal-distal alignment showed larger differences (0.24±0.22 for the femur, −0.87±0.42 mm for the tibia) than the anterior-posterior alignment (−0.07±0.43 for the femur and 0.01±0.21 for the tibia) and the mediolateral alignment (0.17±0.11 for the femur and 0.17±0.15 for the tibia).
Discussion
Sub-degree/millimeter accuracy was achieved in all measurements except external/internal rotation. The accuracy of this technology depends on various factors including image quality, geometry of the 3D models and the experience of the observer. Adding additional diagonal radiographs to the X-ray based analysis may help to improve the 2D/3D registration, which may increase the accuracy of the external/internal rotation measurements. This will be subject to further study.
