Jordy van Sambeeck

General introduction 19 1 PatellarheightmeasurementwasoriginallydescribedonCR.Multiplemeasurement methods are described, several of them are widely used including the Insall-Salvati ratio (IS) 12 , Caton-Deschamps ratio 13 , modified Insall-Salvati ratio (MIS) 14 and Blackburne-Peel ratio (BP) 15 , these will be explained by an illustration in chapter 2. Most of the measurement methods can also be performed on CT or MR images, in which case different normal values might be used 16 . The patellotrochlear index (PTI) is a measurement method that is specifically designed for measurement on MRI images 17 . Lateralization of the tibial tubercle can be measured on CT and MR images. The original measurement method measures the mediolateral distance in millimeters between tibial tuberosity and trochlear groove on axial slices of CT and is called the tibial tubercle-trochlear groove distance (TT-TG) 4 . This measurement can also be performed on MRI images, for which different normal values should be used. Measurement of the TT-TG is still the most often used measurement method of lateralization of the tibial tubercle, despite its known difficulties in patients with trochlear dysplasia 18 . There are alternative measurement methods such as the tibial tubercle – posterior cruciate ligament distance (TT-PCL) and the recently described tibial tubercle – mid inter-epicondylar line trochlea intersection (TT- MIELTI) 19 . However, it is not well known if there is a clinical relevant advantage of these two different measurement methods, further investigation on this is needed. Combining CR, CT and MRI as imaging methods with the possible multiple measurements methods of anatomic risk factors could result in an overload of information, without being able to create a uniform treatment plan which is consistent with all measurements. It is advisable to develop a standard diagnostic imaging workup with a selected amount of measurements on CR and CT or MRI which complement each other. Difficulties that exist in choosing the combination of best imaging modality with the best measurement method include differences in normal values on different imaging modalities, differences in normal values during growth, differences between 2-dimensional and 3-dimensional imaging, differences in the display of bony and soft tissue structures and ease of the measurement method. This is only a limited enumeration of issues without further explanation or discussion that can influence outcome of the measurements.