Jordy van Sambeeck

General introduction 17 1 patellar height can also be determined on MRI. CT scanning provides information on anatomic risk factors such as bony anatomy of the trochlea, lateralization of the tibial tubercle and patellar height but does not provide information on the cartilaginous and tendinous structures. There are some specific issues that should be considered when interpreting measurements of radiographic imaging of the patellofemoral joint. These include patient position, leg position, load of the leg, difference in measurement for different imagingmethods and variability of measurements. For example, it should be taken into account that MRI and CT are performed with the patient supine without any load on the leg, this gives different results than CR concerning patellar height. Importance of leg position was previously illustrated by false outcomes of trochlear anatomy on CR with minimal rotation aberrations of 5° 9 . Different leg position between CT- and MRI-scans could result in different outcomes of patellar height measurements 10 . A CT-scan is made with the knee in full extension, an MRI is typically made with the knee in 20° flexion. Hochreiter et al. demonstrated high variability in values of measurements that describe the anatomy of the ‘normal’ trochlea such as sulcus angle, trochlear depth and lateral femoral trochlear inclination 11 . This might be a result of the complex three dimensional anatomy of the trochlea which include differences in the transverse, coronal and sagittal plane, as well as of differences due to heterogeneity in methodology. Trochlear anatomy was originally assessed on conventional radiographs conform the description of Dejour, whose classification is still used in practice 4 (figure 6 and 7). Despite that the Dejour classification is a simplification of the three dimensional anatomy of the trochlea, it is useful to discriminate between mild (type A) and high grade (type B-D) trochlear dysplasia. Therefore, it can be helpful to direct the type of surgical treatment. The increased availability of CT and MR imaging after the initial description of Dejour has led to more possible and often more reproducible measurements of the trochlear anatomy. New additional assessment methods to classify trochlear morphology might lead to new classification methods and treatment strategies.