Jordy van Sambeeck

Summary, general discussion and future perspectives 9 147 of flexion, in which situation the patella normally already has entered the trochlear groove. A second disadvantage of the PTI is that patellar height can be influenced by trochlear dysplasia. Responsiveness is the ability of an instrument to detect change over time in the construct to be measured 1 . This is problematic for pre- and postoperative measurement after tibial tubercle distalization using the IS or MIS. These measurement methods measure distance from the patella to the tibial tuberosity, this distance is not changed with tibial tubercle distalization and is therefore not responsive. All of the currently available measurement methods have inherent shortcomings in measurement characteristics such as reliability, validity and responsiveness. As briefly described above: there is no perfect measurement conceivable. A combination of two measurements might provide the best of both worlds. We would propose to measure patellar height on CR using CD in combination with the PTI on MRI. The CD can be used to screen for patella alta on CR, has a reliability very close to IS and has the advantage of responsiveness on tibial tubercle distalization. When surgery is considered, additional MRI could be performed to be able to measure the PTI, which also has a high reliability and uses the femur as reference point, which might be a more valid and clinically relevant reference point and could be used to guide the decision on whether to perform distalization or not. For determining the amount of distalization, the PTI could also be taken into account, aiming for a PTI between 12.5 and 50%. The surgeon can further specify the goal for an individual patient based on patient specific anatomy. Next to the measurement characteristics, the accuracy of imaging is influenced by various variables which affect the quality of imaging and the reliability of measurements. Awareness of these limitations is essential in interpreting the results of conventional radiograph based measurements, and translating these measurements into decision making tools for preoperative planning in daily practice. A possible limitation of our study is that most conventional radiographs were not perfect lateral views, which decreases uniformity in position of the knee. Koëter et al. demonstrated the importance of position of the X-ray tube, they found that a radiograph in 5 degrees rotation can result in false measurements 2 . Historically, CT and MRI are performed with the patient supine, on CT with the knees in neutral position and on MRI with the knees in approximately 20 degrees of flexion in a coil. There is often heterogeneity between imaging protocols between hospitals. With the patient supine and the knee in neutral position, there is no dynamic stabilization of the patella and the position is dependent on leg axis (or position) and static anatomic stabilizers. Kaiser et al. demonstrated that in this position patellar tilt and patellofemoral axial engagement are determined by type of trochlear dysplasia, knee torsion and tibial tubercle-trochlear groove distance (TT-TG), all static anatomical factors 3 . However, dynamic stabilization can play an