Stem Cell–Conditioned Medium Promotes Graft Remodeling of Midsubstance and Intratunnel Incorporation After Anterior Cruciate Ligament Reconstruction in a Rat Model

Background: Stem cell–conditioned medium (CM) has been increasingly used in regenerative medicine. However, its effect on graft-host integration after anterior cruciate ligament (ACL) reconstruction (ACLR) remains unclear. Purpose: To examine the effect of human bone marrow stem cell (hBMSC)–CM on graft-bone integration and graft midsubstance ligamentization in a rat model of ACLR. Study Design: Controlled laboratory study. Methods: CM was obtained from the supernatant of commercially available hBMSCs in serum-free Dulbecco’s modified Eagle medium (DMEM). In a rat model of an ACL injury, isometric ACLR was performed. Three groups were established: CM injection group (CM; n = 40), control injection group (CI; n = 40) with serum-free DMEM injections, and no injection group (NI; n = 40). An intra-articular injection was performed weekly. Micro–computed tomography was conducted at 2, 4, and 8 weeks postoperatively. Histological and biomechanical analyses were conducted at 4 and 8 weeks postoperatively. The NIH3T3 fibroblast was utilized as a model in vitro to examine the effect of CM using the cell counting kit–8 (CCK-8) assay and immunofluorescence staining of Ki-67, α–smooth muscle actin (α-SMA), and collagen 1 (Col 1). Results: At 4 and 8 weeks, the femoral and tibial bone tunnel areas as well as the interface between the graft and host bone were smaller, while the bone volume/total volume ratio was higher, in the CM group. Sharpey-like fibers formed at 8 weeks in the CM group. At 4 and 8 weeks, more Col 1 was noticed in the CM group than in the NI group (both P < .001) or CI group (both P < .001). Immunohistochemically, the α-SMA–positive area was up-regulated at the graft-bone interface at 4 weeks ( P < .001) and declined at 8 weeks ( P < .001) in the CM group compared with the other 2 groups. At the midsubstance, α-SMA expression decreased from 4 to 8 weeks in all groups and was significantly lower in the CM group than in the NI group ( P < .01) or CI group ( P < .05) at 8 weeks. The CCK-8 assay showed that CM increased NIH3T3 viability ( P < .001) and the level of Ki-67 ( P < .05), α-SMA ( P < .001), and Col 1 ( P < .001) in CM-educated NIH3T3 cells. Conclusion: hBMSC-CM accelerates graft-bone incorporation and midsubstance ligamentization and enhances the proliferation, differentiation, and collagen synthesis of fibroblasts. Clinical Relevance: Graft-host integration is essential after ACLR. The current study identified a novel agent, that is, hBMSC-CM, as a candidate for promoting integration.