Abstract
Introduction
The trochlea of a typical patellofemoral replacement or anterior flange of a total knee replacement usually extends past the natural trochlea and continues onto the femoral anterior cortex. One reason for this is that it allows a simple patella button to be permanently engaged in the trochlea groove in an attempt to ensure stability. On the natural patella, the apex helps to guide it into the trochlea groove as the knee moves from full extension into flexion.
The aim is to study whether a generalised patella can be created that is close in form to a healthy patella.
Method
MRI scans were taken of 30 patellae. Characteristics of these patellae (height, width, thickness, apex angle) were measured. The apex angle was found to be similar between patellae (mean=126 degrees, sd = 8.8), as were the ratios between height and width (mean width/height = 1.05, sd = 0.07) and between thickness and width (mean width/thickness = 1.8, sd = 0.19).
These patellae were then segmented to create a surface including cartilage, resulting in 30 STL (stereolithography) files in which the surfaces are represented by triangle meshes.
To design the average patella the individual patellae were aligned to a standard frame of reference by placing a set of landmarks on the proximal/distal, medial/lateral and anterior/posterior extents of each (fig.1). The vertical axis was defined as passing parallel to the proximal/distal points and the horizontal as passing parallel to the medial/lateral points when looking along the computed vertical axis. The origin centre of the frame of reference was chosen to be mid-way between these points. The mean width was then computed and each patella scaled linearly around the origin to give them all equal width.
All the aligned patellae were then averaged together to provide a composite cartilaginous patella. The averaging process was achieved by taking one patella as a seed. The patella chosen for seed was that whose parameters were closest to the average width, height and thickness. An approximately normal vector was passed a point ‘P’ on the seeds, and the points at which these intersected the other models were then determined. The closest intersection point to ‘P’ on each model was chosen and these averaged together. ‘P’ is then replaced in the model with this average point. The averaging process then continues with all the remaining points on the seed model in the same manner to build the average models.
Results and Discussion
The mean patella was compared with individual patellae. This comparison was performed by taking each point on the mean patella and finding the closest point on individual patellae - a colour coded map of differences was obtained (fig.2) along with a mean of the absolute difference for each patella. The absolute mean difference ranged from 0.56mm to 1.33mm, averaging at 0.85mm. This shows a reasonable fit between the average patella and each individual example, raising the possibility of using the average shape in future research to develop anatomical patellofemoral replacements and for planning patella resurfacing.
