Measurements

Two musculoskeletal fellowship-trained radiologists (see Acknowledgments) selected participants based on the inclusion criteria and deidentified the MRI studies for analysis. They then noted the location of the MPFL tear: femoral-side 1/3, patellar-side 1/3, or midsubstance 1/3.^7–9

Four orthopaedic surgeons (N.L.G., B.M.W., D.M.T., B.J.K.) independently measured patellar morphology, patellar height, trochlear morphology, and patellar alignment. All measurements were obtained using OsiriX imaging software (version 10.0.1; Pixmeo SARL, Bernex, Switzerland).¹⁰ Patellar morphology was based on the classification of Wiberg¹¹ (Figure 1). Patellar height was based on 3 measurements, which had been previously validated using MRI.¹² These measurements were the Caton-Deschamps ratio,¹³ Insall-Salvati index,¹⁴ and patellotrochlear index¹⁵ (Figures 2–4).

Wiberg classification of patella. A. Type 1: medial and lateral facets are gently concave and nearly equal in size. B. Type 2: medial facet is flat or convex and smaller than the concave lateral facet. C. Type 3: medial facet is very small and is usually prominent and convex, while the lateral facet is broad and concave.

Caton-Deschamps ratio: line B/line A. Line B measures from the inferior margin of the patellar articular cartilage to the anterior margin of the tibial plateau. Line A measures the length of the patellar articular cartilage.

Patellotrochlear index: line B/line A. Line B measures the length of the trochlear articular cartilage surface from the superior-most aspect to the inferior-most aspect of where the adjacent patellar cartilage stops. Line A measures the length of the patellar articular cartilage.

Insall-Salvati index: line B/line A. Line B measures from the inferior pole of the patella to the intersection of the superior portion of the patellar tendon insertion. Line A measures the greatest length of the patella from superior pole to inferior pole.

The MRI reference slices for obtaining measurements were standardized and adapted from Barnett et al.¹² Trochlear morphology and patellar alignment were based on the sulcus angle^16,17 and congruence angle,¹⁷ respectively (Figure 5). These measures have been used in previous analyses of MPFL tears in the context of patellar dislocation.^8,9,18

Sulcus angle (∠ABC) and congruence angle (∠DBE) as measured on magnetic resonance imaging.