Differences in the Development of Fibrocartilage Layers in the Quadriceps Tendon and Patellar Tendon Insertions in Rabbits: A Quantitative Study

BACKGROUND: Differences in the development of fibrocartilage layers in quadriceps tendon (QT) and patellar tendon (PT) insertion sites are unclear. Because the mechanical environments for the QT and PT are different, the development of the QT and PT insertions may differ. PURPOSE: To investigate differences in the development of fibrocartilage layers in the QT and PT insertion sites in rabbits through use of quantitative morphometric evaluations. STUDY DESIGN: Descriptive laboratory study. METHODS: This study included 54 male Japanese White rabbits. Animals were euthanized at ages 1 day and 1, 2, 3, 4, 6, 8, 12, and 24 weeks (n = 6 for each age). Chondrocyte number, proliferation, apoptosis, sex-determining region Y box 9 (Sox9)–positive rates, safranin O–stained glycosaminoglycan (GAG) areas, tidemark length, insertion width, and patellar length were evaluated and compared with the same parameters at age 24 weeks and between QT and PT insertion sites. RESULTS: Chondrocyte proliferation was low up to age 2 weeks for QT insertion and low up to 1 week for PT insertion. Chondrocyte apoptosis was high at 1 day and Sox9 expression was low up to 1 week for PT insertion. Sox9 expression was higher in QT than in PT insertion at age 12 weeks. The high chondrocyte count continued to age 1 day in PT insertion and up to 6 weeks in QT insertion. The chondrocyte number was higher in QT than in PT insertion at age 2 weeks. The period of thicker GAG lasted from 2 to 8 weeks in PT insertion and from 1 to 12 weeks in QT insertion. GAG thickness in QT insertion was higher than in PT insertion at age 4 and 12 weeks. CONCLUSION: Development of fibrocartilage layers in QT and PT insertion sites was completed at age 24 weeks in rabbits. However, the period of high chondrocyte count and period of thicker GAG were longer in QT than in PT insertion up to 12 weeks. CLINICAL RELEVANCE: Development of fibrocartilage layers in QT and PT insertions differed in rabbits. Our results may contribute to the development of appropriate treatments based on age and the development of methods for regeneration of the insertion.