Development of a Comprehensive Osteochondral Allograft MRI Scoring System (OCAMRISS) with Histopathologic, Micro-Computed Tomography, and Biomechanical Validation

Abstract

Objective: To describe and apply a semi-quantitative MRI scoring system for multi-feature analysis of cartilage defect repair in the knee by osteochondral allografts, and to correlate this scoring system with histopathologic, micro-computed tomography (μCT), and biomechanical reference standards using a goat repair model.

Design: Fourteen adult goats had two osteochondral allografts implanted into each knee: one in the medial femoral condyle (MFC) and one in the lateral trochlea (LT). At 12 months, goats were euthanized and MRI was performed. Two blinded radiologists independently rated nine primary features for each graft, including cartilage signal, fill, edge integration, surface congruity, calcified cartilage integrity, subchondral bone plate congruity, subchondral bone marrow signal, osseous integration, and presence of cystic changes. Four ancillary features of the joint were also evaluated, including opposing cartilage, meniscal tears, synovitis, and fat-pad scarring. Comparison was made with histological and μCT reference standards as well as biomechanical measures. Interobserver agreement and agreement with reference standards was assessed. Cohen's kappa, Spearman's correlation, and Kruskal-Wallis tests were used as appropriate.

Results: There was substantial agreement (κ>0.6, p<0.001) for each MRI feature and with comparison against reference standards, except for cartilage edge integration (κ=0.6). There was a strong positive correlation between MRI and reference standard scores (ρ=0.86, p<0.01). OCAMRISS was sensitive to differences in outcomes between the types of allografts.

Conclusions: We have described a comprehensive MRI scoring system for osteochondral allografts and have validated this scoring system with histopathologic and μCT reference standards as well as biomechanical indentation testing.

Keywords: MRI scoring system; cartilage repair; osteochondral allografts.

Figure 1.

Medial femoral condyle allograft with fresh storage and good performance (OCAMRISS TS9-MRI 3 points and TS9-REF 4 points; cartilage stiffness = 4.2 MPa). Sagittal proton density (PD)–weighted image (A), sagittal 3D ultrashort echo time (UTE) subtraction image (B), hematoxylin and eosin stain (C), and micro–computed tomography (D) demonstrate features as listed in the accompanying table (E).

Figure 2.

Medial femoral condyle allograft stored at 4 °C × 14 days with outstanding performance (OCAMRISS TS9-MRI 1 point and TS9-REF 7 points; cartilage stiffness = 5.1 MPa). Sagittal proton density (PD)–weighted image (A), sagittal 3D ultrashort echo time (UTE) subtraction image (B), hematoxylin and eosin stain (C), and micro–computed tomography (D) demonstrate features as listed in the accompanying table (E).

Figure 3.

Medial femoral condyle allograft with frozen storage and poor performance (OCAMRISS TS9-MRI 12 points and TS9-REF 25 points; cartilage stiffness = 0.2 MPa). Sagittal short TI inversion recovery (STIR) image (A), sagittal 3D ultrashort echo time (UTE) subtraction image (B), hematoxylin and eosin stain (C), and micro–computed tomography (D) demonstrate features as listed in the accompanying table (E).

Figure 4.

Lateral trochlea allograft with frozen storage and poor performance (OCAMRISS TS9-MRI 12 points and TS9-REF 23 points; cartilage stiffness, 0.1 MPa). Axial short TI inversion recovery (STIR) image (A), axial proton density (PD)–weighted image (B), Safranin-O stain (C), and micro–computed tomography (D) demonstrate features as listed in the accompanying table (E).

Figure 5.

Graph of relationship between 9-feature MRI score (TS9-MRI) versus 9-feature reference standard score (TS9-REF). Spearman’s ρ = 0.855, confidence interval [CI] = [0.708, 0.928].

Figure 6.

Graphs of 13-feature MRI score (TS13-MRI) versus biomechanical indentation stiffness for both medial femoral condyle (MFC) and lateral trochlea (LT) grafts (A) and only for MFC grafts (B). Spearman’s ρ for combined MFC and LT grafts was significantly negative (ρ = −0.528, confidence interval [CI] = [−0.746, −0.149]) and the relationship strengthened when evaluating for only MFC grafts (ρ = −0.788, CI = [−0.948, −0.374]) as there was a wider range of stiffness for LT grafts. Red dots represent MFC grafts and blue dots represent LT grafts.

Figure 7.

Boxplots of mean 9-feature MRI score (TS9-MRI) (A) and 9-feature reference standard score (TS9-REF) (B). The Kruskal-Wallis test detected significant differences for TS9-MRI and TS9-REF with the frozen group performing worse than the other three groups (p = 0.007 and p = 0.001, respectively).