Background

The full leg x-ray is a widely used imaging modality for post-operative assessment of total knee replacement (TKR). However, these assessments require controlled conditions and precise measuring in order to be accurate. inter-observer reliability remains a matter of concern as well. This study examines whether intersurgeon differences are significant.

The use of 3D imaging methodologies in orthopaedics has allowed the introduction of new technologies, such as the design of patient-specific implants or surgical instrumentation. This has introduced the need for high accuracy, in addition to a correct diagnosis. Until recently, little was known about the accuracy of MR imaging to reconstruct 3D models of the skeletal anatomy. This study was conducted to quantify the accuracy of MRI-based segmentation of the knee joint.

Nine knees of unfixed human cadavers were used to compare the accuracy of MR imaging to an optical scan. MR images of the specimens were obtained with a 1.5T clinical MRI scanner (GE Signa HDxt), using a slice thickness of 2 mm and a pixel size of 0.39 mm × 0.39 mm. Manual segmentation of the images was done using Mimics® (Materialise NV, Leuven, Belgium). The specimens were cleaned using an acetone treatment to remove soft-tissue but to keep the cartilage intact. The cleaned bones were optically scanned using a white-light optical scanner (ATOS II by GOM mbH, Braunschweig, Germany) having a resolution of 1.2 million pixels per measuring volume, yielding an accuracy of 0.02 mm. The optical scan of each bone reflects the actual dimensions of the bone and is considered as a ground truth measurement. First, a registration of the optical scan and the MRI-based 3D reconstruction was performed. Then, the optical scan was compared to the 3D model of the bone by calculating the distance of the vertices of the optical scan to the reconstructed 3D object.

Comparison of the 3D reconstruction using MRI images and the optical scans resulted in an average absolute error of 0.67 mm (± 0.52 mm standard deviation) for segmentation of the cartilage surface, with an RMS value of circa twice the pixel size. Segmenting the bone surface resulted in an average absolute error of 0.42 mm (± 0.38 mm standard deviation) and an RMS error of 1.5 times the pixel size. This accuracy is higher than reported previously by White, who compared MRI and CT imaging by looking at the positioning of landmarks on 3D printed models of the segmented images using a calliper [White, 2008]. They reported an average accuracy of 2.15 mm (± 2.44 mm) on bone using MRI images. In comparison, Rathnayaka compared both CT- and MRI-based 3D models to measurements of the real bone using a mechanical contact scanner [Rathnayaka, 2012]. They listed an accuracy of 0.23 mm for MRI segmentation using five ovine limbs.

This study is one of the first to report on the segmentation accuracy of MRI technology on knee cartilage, using human specimens and a clinical scanning protocol. The results found for both bone and cartilage segmentation demonstrate the feasibility of accurate 3D reconstructions of the knee using MRI technology.

In recent years 3D preoperative planning has become increasingly popular with orthopaedic surgeons. One technique that has shown to be successful in transferring this preoperative plan to the operating room is based on surgical templates that guide various surgical instruments. Such a patient-specific template is designed using both the 3D reconstructed anatomy and the preoperative plan and is then typically produced via additive manufacturing technology. The combination of a preoperative plan and a surgical template has the potential to result in a more accurate procedure than an unguided one, when the following three criteria are met: the template needs to achieve a stable fit on the surgical field, it needs to fit in a unique position, and the surgeon needs to be able to determine the correct, planned position during the surgery. When the template fails one of these conditions, it can be used incorrectly. Consequently the process could result in an inaccurate outcome.

This research focuses on modelling the stability of a surgical template on bone. The relationship between the contact surface of the template and the resulting stability is investigated with a focus on methods to quantify the template stability. The model calculates a quality score on the designed contact surface, which reflects the likelihood of positioning the template on the bone in a stable position. The model used in this study has been experimentally validated to verify its ability to provide a reliable indication of the template stability. This was analysed using finite element analysis where multiple templates and support models with different contact surface shapes were created. The application of forces and moments in varying directions was simulated. Stability is then defined as the ability of a template to resist an applied force or moment. The displacements of the templates were computed and analysed. The results show a minimal displacement of less than 0.01 mm and a maximal displacement larger than 10 mm. The former is considered to be a very stable template design; the latter to be very unstable and hence, would result in an insecure contact.

The geometry of the contact surface had a clear influence on the template stability. Overall, the coverage of curvature variations improved the stability of the template. The displacements of the different finite element simulations were used as criterion for ranking the tested template designs according to their stability on their corresponding model surface. This ranking is then compared to that resulting from the quality score of the stability model. Both rankings showed a similar trend. This evaluation phase thus indicates that the developed stability model can be used to predict the stability of a surgical template during the preoperative design process.