Osteogenesis is key to fracture healing and osteointegration of implanted material. Modification of surfaces on a nanoscale has been shown to affect cell interaction with the material and can lead to preferential osteogenesis. We hypothesised that osteogenesis could be induced in a heterogeneous population of osteoprogenitor cells by circular nanopits on a material surface. Furthermore, we intended to assess any correlation between nanopit depth and osteoinductive potential.

The desired topographies were embossed onto polycaprolactone (PCL) discs using pre-fabricated nickel shims. All pits had a diameter of 30μm and investigated pit depths were 80nm, 220nm and 333nm. Scanning electron microscopy confirmed successful embossing and planar controls were shown to be flat. A bone marrow aspirate was obtained from the femoral neck of a healthy adult undergoing a hip replacement. After establishing a culture, cells were seeded onto the PCL discs, suspended in basal media and incubated. Samples were fixed and stained after three and 28 days.

Cells were stained for the adhesion molecule vinculin after three days. Lowest concentrations of vinculin were seen in the planar control group. Osteoprogenitor cells on the shallowest pits, 80nm, had larger and brighter adhesion complexes. After 28 days, osteocalcin and osteopontin expression were used as markers of cell differentiation into an osteoblastic phenotype. 220nm deep pits consistently produced cells with the highest concentrations of osteopontin (p = 0.017) with a similar trend of osteocalcin expression. Cells on all topographies had higher expression levels than the planar controls.

We demonstrated stimulation of osteogenesis in a heterogeneous population of osteoprogenitor cells. This cell mix is similar to that present in fracture healing and after reaming for intramedullary devices or uncemented implants. All nanopit depths gave promising results with an optimum depth of 220nm after 28 days.