Multiparametric MRI of Epiphyseal Cartilage Necrosis (Osteochondrosis) with Histological Validation in a Goat Model

Abstract

Purpose: To evaluate multiple MRI parameters in a surgical model of osteochondrosis (OC) in goats.

Methods: Focal ischemic lesions of two different sizes were induced in the epiphyseal cartilage of the medial femoral condyles of goats at 4 days of age by surgical transection of cartilage canal blood vessels. Goats were euthanized and specimens harvested 3, 4, 5, 6, 9 and 10 weeks post-op. Ex vivo MRI scans were conducted at 9.4 Tesla for mapping the T1, T2, T1ρ, adiabatic T1ρ and TRAFF relaxation times of articular cartilage, unaffected epiphyseal cartilage, and epiphyseal cartilage within the area of the induced lesion. After MRI scans, safranin O staining was conducted to validate areas of ischemic necrosis induced in the medial femoral condyles of six goats, and to allow comparison of MRI findings with the semi-quantitative proteoglycan assessment in corresponding safranin O-stained histological sections.

Results: All relaxation time constants differentiated normal epiphyseal cartilage from lesions of ischemic cartilage necrosis, and the histological staining results confirmed the proteoglycan (PG) loss in the areas of ischemia. In the scanned specimens, all of the measured relaxation time constants were higher in the articular than in the normal epiphyseal cartilage, consistently allowing differentiation between these two tissues.

Conclusions: Multiparametric MRI provided a sensitive approach to discriminate between necrotic and viable epiphyseal cartilage and between articular and epiphyseal cartilage, which may be useful for diagnosing and monitoring OC lesions and, potentially, for assessing effectiveness of treatment interventions.

Fig 1. Safranin O-stained sections of femoral condyle.

Top row: Surgically induced large lesions (3, 5, and 9 weeks post induction). Bottom row: Surgically induced small lesions (4, 6 and 10 weeks post surgical induction). Decreased staining in the chondronecrosis shows a variable degree of pallor. The optical density experiment of the safranin O-stained sections of the femoral condyle was conducted and shown in Fig 2 to estimate the PG loss in the chondronecrosis.

Fig 2. Light absorption (arbitrary units [A.U.]) in safranin O stained sections of femoral condyle.

Top row: Surgically induced large lesions (3, 5, and 9 weeks post induction). Bottom row: Surgically induced small lesions (4, 6 and 10 weeks post surgical induction). The areas of chondronecrosis are outlined in black (the area in (E) is selected based on the PG loss, in which the color shows yellow or light blue). Intralesional color spectrum ranges from yellow to dark blue, as proteoglycans are progressively lost from the cartilage matrix. The late-stage lesions in (C) and (F) similarly demonstrated very low proteoglycan content and either resulted in a marked delay in endochondral ossification (C) or became completely surrounded by bone (F).

Fig 3. Linear regression analysis of the optical density experiment.

(A) Light absorptions (A.U.) for viable (x, R² = 0.129) and necrotic (o, R² = 0.989) epiphyseal cartilage were fitted linearly to different time points, indicating that the PG content remained stable in the viable epiphyseal cartilage, but decreased linearly with time in the areas of chondronecrosis. (B) The % difference between light absorption by viable and necrotic epiphyseal cartilage was fitted linearly to different time points (R² = 0.993). Although slight PG loss was observed in the 6-week sample, this point was excluded from the analysis because no histologically evident chondronecrosis was observed.

Fig 4. Parametric MRI images of femoral condyle.

Relaxation time maps for (A, E) T₂, (B, F) T_1ρ, (C, G) adiabatic T_1ρ, and (D, H) T_RAFF in the medial condyle of the distal femur at 5 weeks (top row, large lesion) and 6 weeks (bottom row, small lesion) post-surgically. Lesion locations are indicated by the arrows in the T₂ maps.

Fig 5. Regression analysis of the percent differences of all parametric MRI relaxation constants and of the light absorption between the viable and necrotic epiphyseal cartilage.

The solid line shows the linear fitting (R² = 0.39) between the percent differences of all MR relaxation constants (Table 4) and the percent difference of the light absorption (Table 2, column 4), resulting in the slope equal to 0.21.