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Introduction:The zone of calcified cartilage (ZCC) anchors articular cartilage (AC) to subchondral bone through a layer of intermediate stiffness. The regulation and functional consequences of cartilage calcification may vary with depth from the articular surface. The hypothesis of this study was that the in vitro calcification of immature AC occurs selectively in the deep region and is associated with a local increase in stiffness.Methods:AC and growth plate cartilage (GPC) from calves were incubated in DMEM, 1% fetal bovine serum, 100 µg/mL ascorbate, and ±10 mM β-glycerophosphate (βGP) for up to 3 weeks. To assess the time course and effects of cell viability and βGP, full-depth strips of AC and GPC were analyzed by histology, indentation, and45Ca++uptake. To assess the effect of tissue zone, disks harvested from surface and deep zone AC and from reserve and hypertrophic zone of GPC were incubated independently and analyzed by compression and for45Ca++uptake and biochemical components.Results:The deep ~20% of immature AC calcified within 3 weeks, with calcification dependent on cell viability and βGP. Mineral was deposited continuously around cells in AC but only between cell columns in GPC. The deep zone of AC exhibited a compressive modulus of 0.53 MPa after βGP-induced calcification, ~4-fold stiffer than AC incubated without βGP.Conclusions:Cartilage explants exhibit inherent zone-specific calcification processes, resulting in an increase in stiffness associated with cartilage calcification. Such properties may be useful for engineering a biomimetic ZCC tissue to integrate cartilaginous tissue to bone, thereby forming a mechanically functional osteochondral unit.

Objective: To describe and apply a semiquantitative MRI scoring system for multifeature 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 2 osteochondral allografts implanted into each knee: one in the medial femoral condyle and one in the lateral trochlea. At 12 months, goats were euthanized and MRI was performed. Two blinded radiologists independently rated 9 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 histologic and µCT reference standards as well as biomechanical measures. Interobserver agreement and agreement with reference standards was assessed. Cohen’s κ, 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). Osteochondral allograft MRI scoring system 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.

Objective: Treatments for articular cartilage lesions could benefit from characterization of lesion patterns and their progression to end-stage osteoarthritis. The objective of this study was to identify, quantitatively, topographic patterns of cartilage lesions in the human knee. Design: Photographs were taken of 127 unilateral distal femora (from 109 cadavers and 18 arthroplasty remnants) with full-thickness cartilage lesions. Using digital image analysis, the lesions were localized, and normalized lesion size was determined for the patellofemoral groove (PFG) and the lateral and medial femoral condyles (LFC and MFC, respectively). Samples were classified into patterns using cluster analysis of the lesion size at each compartment. For each pattern, maps showing the extent and frequency of lesions were created. Results: Four main patterns (a-d) were identified (each P < 0.001), with the lesion size varying from small (a) to large in distinct regions (b-d). Pattern b had a predominant lesion (23% area) in the MFC and smaller (Conclusion: The present methods facilitated quantitative identification of distinct topographic patterns of full-thickness cartilage lesions, based on lesion size and location. These results have implications for stratifying osteoarthritis patients using precise quantitative methods and, with additional longitudinal data, targeting cartilage treatments.