Continuous Low-Intensity Ultrasound Promotes Native-to-Native Cartilage Integration

Failure of the host/graft interface to integrate impedes the success of cartilage repair protocols. Continuous low-intensity ultrasound (cLIUS) at the resonant frequency of 5 MHz is proposed as a treatment modality for promoting native-to-native cartilage integration in vitro. Cylindrical incisions (4 mm) simulating chondral discontinuity were made in bovine cartilage and osteochondral explants, and maintained under cLIUS stimulation (14 kPa [5 MHz, 2.5 Vpp], 20 min, four times/day) for 28 days. Incised cartilage and osteochondral explants were categorized into three study groups; Group I: cLIUS was applied immediately upon incision; Group II: cLIUS was applied after 14 days following incision; Group-III: after 14 days following incision, explants were treated with 0.1% hyaluronidase and 30 U/mL collagenase VII. As a separate study group, incised osteochondral explants were treated immediately with cLIUS at a nonresonant frequency of 2 MHz (14 kPa [2 MHz, 6 Vpp], 20 min, four times/day). Cellular migration was analyzed by scratch assays, and by visualizing migrating cells into the hydrogel core of cartilage/hydrogel constructs. Explants under cLIUS (5 MHz) displayed higher percent apposition along with gap closures when compared with untreated controls and explants treated with cLIUS at 2 MHz. cLIUS (5 MHz)-treated explants were immunopositive for type II collagen. The strength of native-to-native cartilage integration was higher (p = 0.005) in cLIUS-treated cartilage explants at 0.19 ± 0.08 MPa as compared with 0.05 ± 0.03 MPa in untreated controls. Enhanced cartilage phenotype coupled with increased cellular migration were noted under cLIUS (5 MHz), alluding to the observed integration between cartilage interfaces. Collectively, cLIUS at cell resonant frequency promoted integrative cartilage repair, therefore, has the potential to improve cartilage repair outcomes. Impact Statement Lack of integration between the host and graft cartilage interfaces impedes the success of cartilage repair techniques. Continuous low-intensity ultrasound (cLIUS) is documented to induce chondrogenesis and chondrocyte phenotype. However, integrative cartilage repair under cLIUS has not been evaluated. Our results demonstrated integration between cartilage interfaces, increased percent apposition, increased strength of integration, and maintenance of cartilage phenotype under cLIUS (5 MHz). Integrative repair under cLIUS (5 MHz) stemmed from enhanced migration of cells and increased expression of cartilage-specific genes, namely SOX9 and COL2A1. Thus, cLIUS has the potential to improve the outcomes of grafting protocols for cartilage repair.