Wiberg classification is a system used to describe the shape of the patella based mainly on the asymmetry between the patellar medial and lateral facets on axial views of the patella. Increasing number type indicates a larger degree of asymmetry.
Classification
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type I (or type A)
roughly symmetrical, equal-sized facets
concave facets
although presumably the ideal shape of the patella, it is in fact rather uncommon, occurring in only 10% of the general population
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type II (or type B)
slightly smaller size of the medial facet
flattening of the medial facet
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type III (or type C)
markedly smaller size of the medial facet
more vertical orientation of the medial facet
convex aspect of the medial facet