The trochlear isometric point is different in patients with recurrent patellar instability compared to controls: a radiographical study

Abstract

PurposeThe purpose of the study was to investigate the theoretical isometric point based of the curve of the femoral groove and relating it to the origin of the MPFL femoral tunnel on lateral radiograph by comparing a patellar instability cohort with a control cohort.MethodsFrom a Patellar Instability database the radiographs of 40 consecutive patients were analysed to define Schöttle’s point, and the arc of the circle of the trochlear groove. A comparison population of 20 radiographs from comparable patients with tibiofemoral joint disorders was used as a control. The distance from Schöttle’s point to the most anterior part of the groove (extension) was also compared to the distance to the distal end of the roof of the notch (flexion).ResultsThe trochlea was circular in the controls but not the Patellofemoral Instability cohort where trochlear dysplasia is usually present. The difference between the extension and flexion length was a mean of − 2.0 ± 0.5 mm in the controls and + 6.0 ± 0.5 mm in the patellofemoral cohort. In neither cohort did the centre of the circle correspond to Schöttle’s point. The extension distance correlated with the boss height.ConclusionsThe dysplastic trochlea is not circular and the centre of the best matched circle was different to the control trochleae which were circular. The circle centres did not correlate with Schöttle’s point for either cohort, and was more proximal in the Patellofemoral Instability cohort.Clinical relevanceFor the MPFL to have equal tension throughout flexion within the groove, the length should not change. In normal knees the MPFL does not behave isometrically. The change in length, as measured from Schöttle’s point to the trochlea, was greater for patellofemoral instability patients explaining why an isolated MPFL reconstruction in the presence of severe trochlear dysplasia risks poor outcomes. Level of evidence III.

Radiologic Measurements in the Assessment of Patellar Instability: A Systematic Review and Meta-analysis

“… ∥ References 2 –8, 10 –13, 16, 17, 19, 21 –23, 25 –32, 34 –37, 39 –58, 61, 62, 66 –70, 72 –75, 77 –85, 88 –…”

Computed Tomography Imaging Analysis of the MPFL Femoral Footprint Morphology and the Saddle Sulcus: Evaluation of 1094 Knees

“…First, patients with ACLR rather than a healthy population with no knee injuries were included as the control group. Although this population was previously reported as comparable with patellofemoral instability, 8,9 and ACLR did not damage the femoral medial condyle, the underlying differences might prevent the generalization of our results. Second, the determination of the geometric centers of the osseous landmarks could be influenced by the image quality and understanding of the saddle sulcus.…”

“…Concerns have been raised that patients at risk of RPD have anatomically different knees, particularly for the femoral trochlear shapes. 9 , 20 , 39 , 50 Thus, previous laboratory studies 9 , 38 using healthy cadaveric knees would not be fully relied on in routine clinical practice. Patellofemoral kinematics, contact area and pressure, and stability were further reported to be significantly affected by trochlear dysplasia in a simulated in vitro test.…”

“…33 Radiographic reference points have often been used to determine the femoral tunnel position. 4,17,37,41,47,55 However, the accuracy of this method has been debated, 9,22,38,39,60 and the difficulty in obtaining true-lateral radiographs during surgery adds to the confusion. Recently, direct assessment of the femoral anatomy has been proposed for MPFL femoral tunnel placement.…”

The Results of Recurrent Patellar Dislocation Treatment With MPFL Reconstruction in Adolescents With a Minimum Three-year Follow-up: A Prospective Study

“…Interestingly, in a study of 64 patients with a median age of 24 years old, Koenen et al found that uoroscopy enabled more accurate femoral tunnel positioning; however, this particular study did not include any comparison with the results obtained by other positioning methods [18]. Campos et al emphasise that as far as trochlear dysplasia is concerned, the Schöttle's point is not suitable for a femoral tunnel; their study based on 40 cases indicates that it is better to use a more proximal point in this group of patients, and that taking this approach should improve accuracy in pre-op planning [34].…”