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.

A good understanding of musculoskeletal pathologies not only requires a good knowledge of normal human anatomy but also an insight in human evolution and development. Biomechanical studies of the musculoskeletal system have greatly improved our understanding of the human musculoskeletal system via medical imaging, modeling and simulation techniques. The same techniques are, however, also used in the study of nonhuman species and a comparison of human and nonhuman data can yield interesting insight in form-function relationships and mechanical constraints on motion.

Anatomical and biomechanical studies on dogs and rabbits have already yielded valuable insight in disease mechanisms and development of musculoskeletal pathologies such as osteoarthritis (OA). Nonhuman primates have, however, rarely been studied in this context, though they may prove particularly valuable as they can provide us with an evolutionary context of modern human anatomy and pathology. The high prevalence of osteoarthritis in modern humans and its rare occurrence in wild primates has previously been explained as due to human joints being ‘underutilized’ or ‘underdesigned’. Modern humans are highly specialized for bipedalism, while nonhuman primates typically use a wide range of locomotor modes and joint postures to travel through the three-dimensionally complex forest canopy. These hypotheses can, however, be challenged, as it seems more likely that the low occurrence of OA in wild primates is due to a combination of underreporting of the disease and absence of the ageing effect in these species. Our understanding of musculoskeletal function and disease in modern humans would clearly benefit from more studies investigating the occurrence and characteristics of OA in nonhuman primates.