The enhancement of periosteal chondrogenesis in organ culture by dynamic fluid pressure

Cartilage repair by autologous periosteal arthroplasty is enhanced by continuous passive motion (CPM) of the joint after transplantation of the periosteal graft. However, the mechanisms by which CPM stimulate chondrogenesis are unknown. Based on the observation that an oscillating intra-synovial pressure fluctuation has been reported to occur during CPM (0.6-10 kPa), it was hypothesized that the oscillating pressure experienced by the periosteal graft as a result of CPM has a beneficial effect on the chondrogenic response of the graft. We have developed an in vitro model with which dynamic fluid pressures (DFP) that mimic those during CPM can be applied to periosteal explants while they are cultured in agarose gel suspension. In this study periosteal explants were treated with or without DFP during suspension culture in agarose, which is conducive to chondrogenesis. Different DFP application times (30 min, 4 h, 24 h/day) and pressure magnitudes (13, 103 kPa or stepwise 13 to 54 to 103 kPa) were compared for their effects on periosteal chondrogenesis. Low levels of DFP (13 kPa at 0.3 Hz) significantly enhanced chondrogenesis over controls (34 ± 7% vs 14 ± 5%; P < 0.05), while higher pressures (103 kPa at 0.3 Hz) completely inhibited chondrogenesis, as determined from the percentage of tissue that was determined to be cartilage by histomorphometry. Application of low levels of DFP to periosteal explants also resulted in significantly increased concentrations of Collagen Type II protein (43 ± 8% vs 10 ± 5%; P < 0.05). New proteoglycan synthesis, as measured by ³⁵S-sulphate uptake was increased by 30% in periosteal explants stimulated with DFP (350 ± 50 DPM vs 250 ± 75 DPM of ³⁵S-sulphate uptake/μg total protein), when compared to controls though this difference was not statistically significant. The DFP effect at low levels was dose-dependant for time of application as well, with 4 h/day stimulation causing significantly higher chondrogenesis than just 30 min/day (34 ± 7 vs 12 ± 4% cartilage; P < 0.05) and not significantly less than that obtained with 24 h/day of DFP (48 ± 9% cartilage, P > 0.05). These observations may partially explain the beneficial effect on cartilage repair by CPM. They also validate an in vitro model permitting studies aimed at elucidating the mechanisms of action of mechanical factors regulating chondrogenesis. The fact that these tissues were successfully cultured in a mechanical environment for six weeks makes it possible to study the actions of mechanical factors on the entire chondrogenic pathway, from induction to maturation. Finally, these data support the theoretical predictions regarding the role of hydrostatic compression in fracture healing.