Semi-automated landmark-based 3D analysis reveals new morphometric characteristics in the trochlear dysplastic femur.

[trochlear dysplasia]

The authors hypothesise that the trochlear dysplastic distal femur is not only characterised by morphological changes to the trochlea . The purpose of this study is to describe the morphological characteristics of the trochlear dysplastic femur in and outside the trochlear region with a landmark-based 3 D analysis .Arthro-CT scans of 20 trochlear dysplastic and 20 normal knees were used to generate 3 D models including the cartilage . To rule out size differences , a set of landmarks were defined on the distal femur to isotropically scale the 3 D models to a standard size . A predefined series of landmark-based reference planes were applied on the distal femur . With these landmarks and reference planes , a series of previously described characteristics associated with trochlear dysplasia as well as a series of morphometric characteristics were measured .For the previously described characteristics , the analysis replicated highly significant differences between trochlear dysplastic and normal knees . Furthermore , the analysis showed that , when knee size is taken into account , the cut-off values of the trochlear bump and depth would be 1 mm larger in the largest knees compared to the smallest knees . For the morphometric characteristics , the analysis revealed that the trochlear dysplastic femur is also characterised by a 10 % smaller intercondylar notch , 6 -8 % larger posterior condyles (lateral -medial ) in the anteroposterior direction and a 6 % larger medial condyle in the proximodistal direction compared to a normal femur .This study shows that knee size is important in the application of absolute metric cut-off values and that the posterior femur also shows a significantly different morphology .