INTRODUCTION/OBJECTIVES: To determine the differences between static and dynamic patella alta measurements for assessing patellar height. Patella alta is an abnormally high patella in relation to the femoral trochlea. Previously, magnetic resonance imaging (MRI) has been used for a more precise patella alta analysis of the overlap between the patellar and trochlear cartilage (Patella-Trochlea Index, (PTI)). METHODS: We included consecutive patients with anterior knee pain who had both MRI and dynamic weight-bearing CBCT available. Patients with previous knee surgery, inadequate imaging quality, or incomplete datasets were excluded. Twenty-nine patients underwent standard static (supine, 3.0T MRI standard knee coil) and dynamic PTI assessment (single-leg, weight-bearing high-resolution cone-beam CT with quadriceps contraction). Conical beam computerized tomography (CBCT) allows the analysis of patellar and trochlear cartilage overlap similar to MRI. Patellar height relative to the trochlea was quantified by PTI (< 15% overlap defines patella alta). The Caton-Deschamps (C-D) and Insall-Salvati (I-S) indices were measured as originally described. The presence and effect of trochlea dysplasia on patella alta was assessed by measuring the lateral trochlear inclination (LTI) angle. A crude comparison between static and dynamic assessment was performed with a paired sample t-test and adjusted comparison in relation to lateral trochlear inclination (LTI) angle with linear mixed model analysis. RESULTS: In standard static MRI, patellar height measurements had lower values than in dynamic assessment with PTI or C-D. More importantly, PTI was 28% lower (95% CI, 21% to 35%, p<0.01) in dynamic assessment than in static assessment, indicating significantly more frequent patella alta diagnosis in dynamic assessment than in static MRI. Mean C-D was 10% less (95% CI, 7% to 14%, p<0.01) in static than in dynamic assessment. However, I-S was similar in both imaging modalities, indicating an invaluable method of patellar alta assessment concerning patellar engagement to the trochlea. The mean distal cartilage margin of the patella in sagittal view was 3.3 mm (range, 0 mm to 6.7 mm) more proximal in dynamic than in static imaging when quadriceps is contracted. CONCLUSION: Diagnosis of patella alta is challenging because dynamic and static imaging modalities result in significant differences when measuring patella-trochlea overlap. A dynamic imaging modality may be considered a more precise method of assessing patellar engagement within the trochlea. The results of this study suggest the diagnostic value of standard static MRI is limited. LEVEL OF EVIDENCE: III.
Rahnel et al. (Fri,) studied this question.