Introduction: With the development of powerful computing tools, it is now possible to quantify variations in skeletal morphology using standardized analytical protocols. In this presentation, we describe the development of computer-based tools to analyze components of femoral deformity in developmental hip dysplasia (DDH) and femoro-acetabular impingement (FAI).

Methods: Typically, three dimensional surface-splined computer models of bones are generated by reconstructing CT scan data. The models generated are then segmented into discrete objects (e.g. the femoral head, neck, shaft, condyles, sulcus, and apophyses) and a coordinate system is attached to each anatomic object to define its relative position and orientation in space. The size of each object can be described by characteristic parameters (eg height, length, width), and its shape with dimensionless ratios (eg width/length). Other methods include principal component analysis which expresses te principal sources of statistical variation in object dimensions, and correspondence analysis, which describes the variation of each point on the surface of a bone compared to the average specimen in the observed population.

Illustrative Applications:

DDH

These methods have been applied to examine systematic variations in the shape and dimensions of the dysplastic femur through reference to data from 171 dysplastic and 84 skeletally normal patients. Of the 171 dysplastic femora, 74 (43%) were graded as Crowe I, 82 (48%) as Crowe stages II or III, and 15 (9%) as Crowe IV. The change in femoral morphology was quantified as a function of the grade of deformity in comparison with normal controls. The principal sources of deformity were also identified.

Results: The results of these analyses will be presented during the lecture.

Conclusions: Mathematical analysis of the shape of bones allows us to describe the type and severity of skeletal deformities in precise quantitative terms. This leads to new, three-dimensional definitions of skeletal phenotypes, and allows automated screening and classification of imaging data sets for the detection of dysmorphic conditions. This approach also has the potential to provide new insights into the true nature of complex deformities presenting for orthopedic treatment.

We studied the morphometry of 35 femora from 31 female patients with developmental dysplasia of the hip (DDH) and another 15 from 15 age- and sex-matched control patients using CT and three-dimensional computer reconstruction models. According to the classification of Crowe et al 15 of the dysplastic hips were graded as class I (less than 50% subluxation), ten as class II/III (50% to 100% subluxation) and ten as class IV (more than 100% subluxation).

The femora with DDH had 10 to 14° more anteversion than the control group independent of the degree of subluxation of the hip. In even the most mildly dysplastic joints, the femur had a smaller and more anteverted canal than the normal control. With increased subluxation, additional abnormalities were observed in the size and position of the femoral head. Femora from dislocated joints had a short, anteverted neck associated with a smaller, narrower, and straighter canal than femora of classes I and II/III or the normal control group.

We suggest that when total hip replacement is performed in the patient with DDH, the femoral prosthesis should be chosen on the basis of the severity of the subluxation and the degree of anteversion of each individual femur.