Localized vitamin D delivery via 3D-printed nanofiber sheets combined with systemic supplementation enhances tendon-to-bone healing in a rabbit rotator cuff tear model: a preclinical study

Background: Vitamin D is crucial in musculoskeletal health, supporting bone, tendon, and muscle function. However, its localized delivery for tendon healing and muscle regeneration remains underexplored. We hypothesized that combining systemic and localized vitamin D delivery would enhance healing in a rotator cuff repair model. Methods: Sixty-four rabbits were randomly assigned to 2 main groups (n = 32), further divided into 4 subgroups (Group A, A′: Normal diet + sheet without vitamin D; Group B, B′: Normal diet + 3D-printed nanofiber-based vitamin D sheet (VTD sheet); Group C, C′: systemic vitamin D supplementation (VTDS) + sheet without vitamin D; Group D, D′: VTDS + VTD sheet, n = 8 each). The supraspinatus tendons were detached, left detracted for 6 weeks, and repaired using a transosseous method. Groups were assessed at 4- and 12-weeks post-repair. Outcomes included serum 25-OH vitamin D levels, gene expression of COL1, COL3, BMP-2, SCX, SOX9, and ACAN, histologic and biomechanical analysis, muscle fiber cross-sectional area, and vitamin D content in muscle via ELISA. Results: Serum vitamin D levels were highest in Group D and D′ at the time of repair and extraction (P < .001). At 4 weeks post-repair, mRNA expression of COL1 was highest in Group D compared to the other groups (A, B, C, and D; 0.86 ± 0.25, 0.90 ± 0.27, 0.93 ± 0.19, and 1.06 ± 0.25, respectively, P = .046). At 12 weeks post-repair, Group D′ exhibited the highest collagen density (P = .037) and had the greatest load to failure among all groups (A′, B′, C′, D’; 102.3 ± 12.6 N, 99.5 ± 8.3 N, 102.3 ± 18.5 N, and 139.6 ± 25.3 N, respectively, P = .030). Regarding muscle regeneration, the cross-sectional area of muscle fiber was largest in Group D and D′ at 4 and 12 weeks post-repair (P < .05). At 12 weeks after repair, muscle vitamin D levels, as measured by ELISA, were highest in Group D’ (P = .003). Conclusion: In this preclinical rabbit model, dual delivery of vitamin D using localized nanofiber sheets and systemic supplementation enhanced tendon-to-bone healing in a rotator cuff repair model and muscle regeneration compared to single interventions. These findings provide a foundation for future studies aimed at optimizing delivery methods and evaluating safety and efficacy in clinical settings.