Chondroinduction Is the Main Cartilage Repair Response to Microfracture and Microfracture With BST-CarGel

Abstract

Background: Current cartilage repair histological scoring systems are unable to explain the relationship between collagen type II deposition and overall repair quality. Purpose/Hypothesis: The purpose of this study was to develop a novel zonal collagen type (ZCT) 5-point scoring system to measure chondroinduction in human clinical biopsy specimens collected after marrow stimulation. The hypothesis was that the ZCT scores would correlate with the International Cartilage Repair Society–II (ICRS-II) overall histological repair assessment score and glycosaminoglycan (GAG) content. Study Design: Descriptive laboratory study. Methods: After optimizing safranin O staining for GAG and immunostaining for human collagen type II and type I (Col2 and Col1, respectively), serial sections from clinical osteochondral repair biopsy specimens (13 months after microfracture or microfracture with BST-CarGel; n = 39 patients) were stained and 3 blinded readers performed histomorphometry for percentage of staining, ICRS-II histological scoring, polarized light microscopy (PLM) scoring, and 5-point ZCT scoring based on tidemark morphology, zonal distribution of Col2 and Col1, and Col1 percentage stain. Because 1 biopsy specimen was missing bone, 38 biopsy specimens were evaluated for ICRS-II, PLM, and ZCT scores. Results: Chondroinduction was identified in 21 biopsy specimens as a Col2 matrix fused to bone that spanned the deep-middle-superficial zones (“full-thickness hyaline repair”), deep-middle zones, or deep zone (“stalled hyaline”) that was covered with a variable-thickness Col1-positive matrix, and was scored, respectively, as ZCT = 1 (n = 4 biopsy specimens), ZCT = 2 (n = 6) and ZCT = 3 (n = 11). Other biopsy specimens (n = 17) were fibrocartilage (n = 9; ZCT = 4), fibrous tissue (n = 4, ZCT = 5), or non-marrow derived (n = 4; ZCT = 0). Non-marrow derived tissue had a mean mature tidemark score of 84 out of 100 versus a regenerating tidemark score of 24 for all other biopsy specimens ( P = .005). Both "stalled hyaline" repair and fibrocartilage had the same mean Col2 percentage stain; however, fibrocartilage was distinguished by heavy Col1 deposits in the deep zone, a 2-fold higher mean Col1 percentage stain ( P = .001), and lower surface integrity ( P = .03). ZCT scores correlated with GAG content and the ICRS-II overall assessment score, especially when combined with the PLM score for collagen organization ( R = 0.82). Histological scores of the deep zone strongly predicted the ICRS-II overall assessment score ( R = 0.99). Conclusion: The ICRS-II overall repair assessment score and GAG content correlated with the extent of Col2 deposition free of fibrosis in the deep/middle zone rather than bulk accumulation of Col2. Clinical Relevance: Biopsy tissue from the BST-CarGel randomized clinical trial (microfracture without and with BST-CarGel, as treatment groups were not unblinded) showed regenerated tissue consistent with a chondroinduction mechanism in at least half of the treated lesions.