Regeneration of large bone defects is a major problem in trauma surgery and orthopedics. At the moment, the gold standard for their treatment consists of autologous bone chips, taken from the iliac crest. However, this procedure has to deal with donor site complications [
1]. In recent times, more and more tissue engineering approaches using cells with osteoinductive potential and different types of scaffolds might circumvent those limitations. The combination of endothelial progenitor cells (EPC) and marrow stromal cells (MSC) with a β-tricalcium phosphate (β-TCP) scaffold was effective in experimental bone healing models [
2,
3]. However, MSC/EPC must be culture-expanded prior use which might increase the risk for genetic alterations [
4] or contamination with pathogens, as well as it takes time and will be prohibitively expensive. Also, the needed growth factors used for EPC differentiation in vitro such as IGF-1 might be able to support transformation of hematopoietic progenitors [
5] from which EPC develop [
6]. Due to these risks and limitations, we started investigating the use of bone marrow mononuclear cells (BMC) for bone healing [
7‐
9]. BMC can be harvested and reintroduced to the patient within hours which is more compatible with the clinical requirement for rapid fracture repositioning. There is no need for a long expansion time minimizing the negative effects. Under the heterogeneous mixture of BMC diverse cell types were found i.e. (immature) lymphocytes, (immature) monocytes, and progenitor cell populations. Several types of cells with regenerative potential like precursors of MSC, hematopoietic stem cells (HSC) as a putative source of EPC, and (immature) monocytes were also found [
8,
10‐
13] Our own previous work demonstrated that BMCs seeded on uncoated β-TCP scaffolds [
8] (1.3 × 10
6 BMC/mL) and transplanted into an experimental femur defect, exerted highly beneficial effects on the bone healing response [
14,
15] qualitatively comparable to those mediated by cultured MSC and EPC [
2,
3]. In our own recently completed phase-I clinical study, we demonstrated that BMC seeded onto pre-implanted β-TCP scaffold in a bone defect was well tolerated and complete bone healing was achieved in all 10 patients after 3 months [
9].
However, the optimal concentration of BMC has not yet been determined for bone healing. With this study, we want to determine the optimal dosage of the BMC in the bone defect to support bone healing.