BLOG: Improved understanding of patellofemoral dysplasia using 3D prints
I study 3D models of the patellofemoral joint to understand patella tracking, noting the importance of the proximal trochlea and trochlear groove with respect to patella engagement in early knee flexion in cases of patella instability.
The trochlear groove is comparable to a bobsled track, and depth insufficiency of the trochlea can result in displacement of the patella from its intended curvilinear course. This became increasingly easy to grasp using 3D patellofemoral joint (PFJ) models, as I could run my finger along the trochlear groove and simulate patellar tracking through the entire arc of physiologic knee motion. However, I realized that although conventional 2D imaging provided me with a general impression of whether a given set of trochleae and patellae appeared flat or otherwise dysplastic, it had been difficult to mentally visualize the path of the patella and its early engagement with the trochlea using 2D CTs alone.
As I began to create my own 3D-printed PFJ models from patient CTs, I gained a unique appreciation of the limitations of 2D visualizations of 3D relationships and pathologies. As I scrolled through axial and coronal CT slices of the distal femur and patella, I realized how difficult it was to conceptualize changes in the overall morphology of each structure and how these trochlear and patellar contact points may affect each other during the course of development. Differences in surface curvature – concavity and convexity – were difficult to appreciate on conventional radiographs but were easier to see and feel on the 3D models. Corresponding areas of “flatness” along the proximal trochlea and posterior surface of the patella could also be more readily traced along the printed 3D models, conceivably strengthening the notion that trochlear and patellar morphology are intertwined and that biomechanical interactions between the two alter surface shape. Overall, the respective shapes of the trochlea and patella were easier to appreciate in 3D. As I went through the steps of segmenting, creating and playing with my own 3D models of the PFJ from 2D CT images, I found my ability to understand patella tracking on conventional radiographs improved. My conceptualization of joint motion and dysplasia improved, opening a door to new research and clinical questions.
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