Cross-sectional fat fraction analysis of the gluteus medius and minimus muscle in asymptomatic vs. symptomatic hips using 2-point Dixon MRI

Abstract

Background: To evaluate cross-sectional fat fractions (FF) of the hip abductors in asymptomatic compared to symptomatic hips in elderly patients using 2-point Dixon MRI, and compare them to the Goutallier classification.

Methods: In this retrospective (clinicial trial number: not applicable) single-center study, two radiologists assessed cross-sectional fatty infiltration of the gluteus minimus (Gmin) and medius (Gmed) muscles in both hips of patients with unilateral greater trochanteric pain syndrome (GTPS) using 2-point Dixon MRI-derived fat fractions (FF). Additionally, fatty infiltration was assessed on T1w sequences using the Goutallier classification. Differences in fatty infiltration for both methods between the symptomatic and asymptomatic hips were assessed using the Wilcoxon signed-rank test.

Results: 42 patients (mean age 65.1 ± 13.7 years, 28 females) were analyzed. Median FF in asymptomatic hips was 13.5 ± 15.2% for the Gmin and 14.2 ± 11.6% for the Gmed muscle (vs. 13.9.±14.1% and 16.2 ± 18.4% on the symptomatic side). FF of Gmed were significantly lower on the asymptomatic side than on the symptomatic side (p ≤ .043). No significant difference was observed neither for FF of the Gmin muscle (p ≥.30) nor for the Goutallier gradings of the Gmin (p ≥ .06) and Gmed (p ≥ .13) muscle between the asymptomatic vs. symptomatic sides.

Conclusion: Median FF were close to 14% for Gmin and Gmed in elderly asymptomatic hips, with a tendency for lower fat fractions in males. Significantly higher fat content of the Gmed was present in symptomatic hips, which was solely detectable by FF analysis and not by Goutallier grading. Reproducibility of FF analysis surpassed the Goutallier assessment.

Keywords: 2-point-Dixon; Fat fraction; Hip; Magnetic resonance imaging.

Fig. 1

Flowchart illustrating the patient selection process. The final study sample consisted of 84 hips in 42 patients: Group 1 consisted of the asymptomatic 42 hips, whereas the contralateral symptomatic hips were put in Group 2. MRI of both hips was acquired for a prior prospective study on greater trochanteric pain syndrome, as described in the manuscript text

Fig. 2

Example illustrating the assessment of fat fractions (FF) of the abductor muscles on bilateral hip MRI in a 57-year-old female patient. Symptomatic left (a-c) and asymptomatic right side (d - f); on the Dixon derived axial FF images (a, d), Gmin (orange outline) and Gmed (green outline) muscles were divided into an anterior, middle, and posterior third, respectively. Matching axial T1w turbo-spin-echo (TSE) images (b, e) were used as anatomical references for the outline and the Goutallier grading. Coronal T2w TSE images (c, f) were used to assure consistent measurement at the level of the acetabular roof (red dashed line, f)

Fig. 3

Fat fraction (FF) boxplots of the Gmin and Gmed muscle on the asymptomatic (white) and symptomatic side (grey). The minimum (lower whisker), first quartile (lower box margin), median (thick black line), third quartile (upper box margin), and maximum FF (upper whisker) of the Gmin and Gmed muscles are shown. Each dot above the respective box plot represents a single outlier. The asterisk denotes the statistically significant difference in the mean FF of the Gmed muscle between the asymptomatic and symptomatic sides

Fig. 4

51-year-old female patient with differences in fat fractions (FF) between abductor muscles of the asymptomatic and symptomatic side that are not apparent with the Goutallier grading. Asymptomatic left (a, b) and symptomatic right (c, d) side; on the T1w MR images (b, d), both the Gmin (orange outline) and Gmed (green outline) muscle were assigned the same overall Goutallier grading of 2 on the asymptomatic (b) and symptomatic (d) side. On the Dixon-derived FF-sequences (a, c), the mean FF in this patient was lower on the asymptomatic side compared to the symptomatic side for the Gmin muscle (14.1% vs. 15.0%) and even more so for the Gmed muscle (22.5% vs. 25.8%)

Fig. 5

Bilateral hip MRI in an 80-year-old female patient with right-sided greater trochanteric pain syndrome. Asymptomatic left (a, b) and symptomatic right side (c, d); on the T1w images (b, d), Gmin (green outline) and Gmed (orange outline) muscles were assigned an overall Goutallier grading of 2 on both sides. On the Dixon-derived FF-sequences (a, c), the mean FF for the Gmin muscle on the left and right side (15.0% and 13.2%, respectively) and the Gmed muscle on the asymptomatic side were comparable (15.3%). The FF of the Gmed muscle on the symptomatic side was higher in this patient (20.5%), which was a common finding in this study cohort. In the middle third of the Gmin muscle, there is an increased amount of fat compared to the other muscle parts in this patient, which was also a common finding in the study cohort