Patellar malalignment: a new method on knee MRI

Abstract

Purpose: The medial patellofemoral ligament (MPFLL)/lateral patellar retinaculum (LPR) ratio were assessed in knees as a means to detect patellar malalignment. We also aimed to evaluate the prevalence of the various types of trochlear dysplasia in patients with patellar malalignment.

Materials and methods: After approval of our institutional ethics committee, we conducted a retrospective study that included 450 consecutive patients to evaluate them for the presence of patellar malalignment. Parameters investigated were the trochlear type, sulcus angle, presence of a supratrochlear spur, MPFLL, LPR, patella alta, and patella baja by means of 1.5T magnetic resonance imaging (MRI). Overall, 133 patients were excluded because of the presence of major trauma, multiple ligament injuries, bipartite patella, and/or previous knee surgery. The Dejour classification was used to assess trochlear dysplasia. Two experienced radiologists (HKY, EEE) evaluated the images. Their concordance was assessed using the kappa (κ) test.

Results: The frequencies of patellar malalignment and trochlear dysplasia were 34.7 and 63.7 %, respectively. The frequency of trochlear dysplasia associated with patellar malalignment was 97.2 %. An MPFLL/LPR ratio of 1.033-1.041 had high sensitivity and specificity for malalignment. The researchers' concordance was good (κ = 0.89, SE = 0.034, P < 0.001).

Conclusion: Trochlear dysplasia is frequently associated with patellar malalignment. An increased MPFLL/LPR ratio is useful for detecting patellar malalignment on knee MRI, which is a novel quantitative method based on ligament length.

Keywords: Knee MRI; Medial patellofemoral ligament; Patella alta- MPFLL/LPR; Trochlear dysplasia.

Fig. 1

Axial proton density fat-saturated magnetic resonance imaging (PD-fatsat MRI) (a Sect. 3 cm above the knee joint). Note the normal trochlear groove and sulcus angle

Fig. 2

Axial PD-fatsat MRI of a Sect. 3 cm above the knee joint. Although there is type A trochlear dysplasia and the sulcus angle is increased to 150°, the trochlea is symmetric

Fig. 3

Axial PD-fatsat MRI (a section 3 cm above the knee joint). a Type B trochlear dysplasia is present. Note the flat trochlear groove and patellar subluxation. b Another patient was diagnosed with type B trochlear dysplasia, patellar subluxation, and patellar chondromalacia

Fig. 4

Axial PD-fatsat MRI (a Sect. 3 cm above the knee joint). Type C trochlear dysplasia is present. Note the trochlear fascial asymmetry, increased lateral convection, and medial facet hypoplasia

Fig. 5

a Axial PD-fatsat MRI shows type D trochlear dysplasia. Note the trochlear surface asymmetry and hump. b Mid-sagittal T1-weighted fast spin echo MRI reveals a supratrochlear spur

Fig. 6

Medial patellofemoral ligament (MPFLL) and lateral patellar retinaculum length (LRR) measurements on axial PD-fatsat MRI crossing through the patellar center. a MPFL/LPR ratio of 42.77/50.20 = 0.85 was within normal limits. b This patient has type B dysplasia and patellar subluxation. MPFL/LPR ratio was 1.51, which was higher than the cutoff value. c This patient had type B dysplasia and patellar subluxation. MPRL/LPR ratio was 1.19