Formation of a new enthesis after attachment of the supraspinatus tendon: A quantitative histologic study in rabbits

Abstract

Purpose. To quantify in a longitudinal study non-chondrocytic cells and chondrocytes, tissular architecture as well as extracellular matrix restoration during the formation of an enthesis following supraspinatus tendon attachment to the humerus. Methods. In 89 rabbits, one supraspinatus fibrocartilaginous enthesis was resected and the tendon either attached to the greater tuberosity (n = 75) or not attached ( n = 14). The animals were sacrificed after 2, 6, 8, 12 or 24 weeks. The operated and contralateral shoulders were processed for histologic sections. Number of non-chondrocytes, chondrocytes and alignment of chondrocytes in rows were assessed histologically. Extracellular matrix restoration was measured based on ( 1 ) area of toluidine blue metachromasia indicating proteoglycan content and (2) on area of diffracted polarized light indicating spatial collagen fiber alignment. Results. In the attached tendon, the number of non-chondrocytic cells sharply increased at 2 weeks, progressively decreased thereafter but remained higher than controls at all time points. Chondrocytes appeared at 2 weeks and their number reached control levels by 6 weeks (136 f 14 vs 144 5 15 controls, p .05). The percentage of chondrocytes aligned in rows increased from 19 k 4% at 2 weeks to reach near normal values at 24 weeks (71 f 3% vs 78 5 2% controls, p > .05). Area of metachromasia increased from 0.1 f 0.1 mm2 at 2 weeks to 3.8 k 0.3 mm2 at 24 weeks, still below contralateral enthesis levels (4.6 f 0.1 mm2, p < .05). Area of diffracted polarized light enlarged from 12 5 2 x lo3 pm2 at 2 weeks to 151 k 19 x lo3 pm2 at 24 weeks, still significantly smaller than contralateral levels (177 k 13 x lo3 pm', p < .05). Neither chondrocytes nor metachromasia were observed in the non-attached tendons. Conclusion. A new enthesis was formed after attachment of the supraspinatus tendon into bony trough. Histomorphometry allowed to document extensive non-chondrocytic proliferation that was followed by appearance of chondrocytes and their spatial organization, a process was complete by 24 weeks. Extracellular matrix formation as well as spatial alignment of collagen fibers were delayed and not complete by 24 weeks. This first longitudinal investigation on the formation of the supraspinatus enthesis using quantitative outcome measures cautions against too early and too aggressive a rehabilitation program.

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Imaging of the rabbit supraspinatus enthesis at 7 Tesla: a 4‐week time course after repair surgery and effect of channeling

“…The enthesis contains four distinct zones visible by microscopy: bone (zone 1), mineralized fibrocartilage (zone 2), nonmineralized fibrocartilage (zone 3), and tendon proper (zone 4) . Quantitative histologic measures have provided insights into the reformation of a SSP enthesis after surgical repair in animal models . Changes included an increase in chondrocyte number 2 weeks after SSP tendon attachment with progressive return to normal cellularity by 6 weeks.…”