Abstract
ABSTRACT Coordinate system definition is a critical element of biomechanical modeling of the knee, and cases of skeletal trauma present major technical challenges. This paper presents a method to define a tibial coordinate system by fitting geometric primitives to surface anatomy requiring minimal user input. The method presented here utilizes a conical fit to both the tibial shaft and femoral condyles to generate independent axes forming the basis of a tibial coordinate system. Definition of the tibial axis showed high accuracy when shape fitting to ≥50 mm of shaft with <3° of angular variation from the axis obtained using the full tibia. Repeatability and reproducibility of the axis was compared using intraclass correlation coefficients which showed excellent intra- and inter-observer agreement across cases. Additionally, shape fitting to the distal femoral condyles showed high accuracy compared to the reference axis established automatically through identifying the medial and lateral epicondyles (<4°). Utilizing geometric primitives to estimate functional axes for the tibia and femur removes reliance on anatomical landmarks that can be displaced by fracture or inaccurately identified by observers. Furthermore, fitting of such primitives provides a more complete understanding of the true bony anatomy, which cannot be done through simple landmark identification.
Highlights
Computed tomography (CT) is commonly utilized to visualize 3D geometry of anatomical structures in ortho pedic research
This study presents a method by which a subjectspecific tibial coordinate system can be established through the fitting of geometric primitives to surface anatomy of the tibia and femur
As indicated by the deviations measured for the tibial axis angles and variability in the origin, the coordinate system pro posed here is both highly repeatable and reproduci ble when the shape fitting technique is applied to the tibial shaft at lengths greater than 50 mm
Summary
Computed tomography (CT) is commonly utilized to visualize 3D geometry of anatomical structures in ortho pedic research In biomechanics, these data can form part of computational models of the musculoskeletal system where knowledge of subject anatomy is required (Arnold et al 2010; Delp and Loan 1995; Gerus et al 2013). Coordinate systems defined by anatomical landmarks are well established in the field of biomechanics (Beardsley et al 2007; Grood and Suntay 1983; Kai et al 2014; Roos et al 2005); these systems rely on the manual identification of anatomical landmarks While these coordinate systems can be applied to the knee with good accuracy, their reliance on anatomical landmarks for both the tibia and femur restricts their application and renders them unusable for cases invol ving skeletal trauma disrupting the normal bony anatomy. This is important when consider ing fractures involving the proximal aspect of the tibia which can involve significant disruption to the articular surface making it near impossible to identify anatomical landmarks on the tibia (Millar et al 2018; Schatzker et al 1979; Thewlis et al 2015)
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