The effect of mesenchymal stromal cells on the osseoinduction of impaction grafts.

Revision total hip replacement has a failure rate of up to 25%. Part of the reason for this high failure is the lack of bone stock. In this study, we investigated whether mesenchymal stromal cells (MSCs) or osteoprogenitors (OPs) contribute to bone formation in impacted allograft or an allograft and hydroxyapatite (HA) combination. Eight samples were inserted in the paraspinal muscles of six sheep, which were allograft or an HA/allograft mix, seeded with autologous MSCs, autologous OPs, or no cells (control), and impacted with a clinically relevant force at 3 kN preimplantation. Other groups were HA blocks seeded with or without MSCs. Samples were retrieved at 12 weeks. MSCs incorporated into allograft, HA/allograft, and HA blocks displayed higher mean values of new bone formation compared with the controls, but there was no statistical difference between allograft groups. However, in HA/allograft groups, significantly more new bone was found with MSCs than without cells (p = 0.046) or with OPs (p = 0.028). In the HA blocks, more new bone was found in the MSC group than in the HA block without cells (p = 0.028). In conclusion, MSCs used with combinations of allograft and HA enhance new bone formation in an ovine ectopic site after being subjected to realistic forces that are used during impaction grafting.