5.5. Bone response to NiTi in the regional acceleratory phenomenon (RAP) model

Some changes in new bone formation and cortical bone width were found after periosteal implantation. The results of histomorphometric measurements are shown in Table 5-1 and in the Figure 5-9 and Figure 5-10. Figure 5-11 is a scheme illustrating the area of measurement and the different parameters. At 2 weeks, the new woven bone area (N.Wo.B) was larger with Ti-6Al-4V compared to NiTi (p < 0.01) or StSt, but the erosion area (E.Ar) and the erosion surface perimeter as a fraction of the bone surface perimeter (E.Pm/B.Pm) were also greater with Ti-6Al-4V compared to NiTi, indicating active early modelation. NiTi and StSt had their maximum new bone formation activity at 4 weeks, while the new bone formation of Ti-6Al-4V began to diminish at that point. There were no statistically significant differences between the materials at this point. At 8 weeks, Ti-6Al-4V had its highest erosion activity. Its Ct.Wi was lower than that of StSt (p < 0.005) or NiTi (p < 0.05). NiTi had higher N.Wo.B than StSt. The differences between the tested materials were smallest at 12 weeks. E.Ar was hardly measurable for all materials. At 26 weeks, there was still some new bone formation and bone resorption activity left, indicating that a steady state had not been reached yet. E.Pm/B.Pm remained higher with Ti-6Al-4V. Remodelation occurred with all materials, as the perforated surfaces were partly filled with new bone. There were no statistically significant differences between the materials at 12 and 26 weeks.

Certain common features in bone structure were observed over time in all samples. There was a thin fibrous (< 20-200 µm) layer present between the bone and the implant. The closest contacts were generally found in the 12- and 26-week samples (Figure 5-12E and Figure 5-12F).

The 2-week groups (Figure 5-12A) showed formation of new woven bone, mostly at the verge of the closest implant contact area. Some chondral cells were incidentally seen. In the 4-week groups, the new bone lined the nearest implant contact area even more clearly. The outer lamellar bone began to be destroyed under the implant by osteoclasts, and there was distinct bone porosis. In the 8-week groups, erosion already reached the deeper bone structures (Figure 5-12D). Slightly U-shaped cross-sectional bone modelation was observed. The most radical modifications in bone structure and cortical width were seen at 8 weeks (Figure 5-12C and Figure 5-12D). The formation of endosteal callus with lamellar structure was also clearly discernible. In the 12-week samples, the erosion of bone was less abundant and the endosteal callus was thicker (Figure 5-12E). In the 26-week specimen, this deformation was already reduced (Figure 5-12F). Multiple layers of lamellar and woven bone under the implant were observed, and the lamellar structures under the implant were repaired, indicating remodelation.

In conclusion, an adequate regional acceleratory phenomenon with normal new bone formation was evident with NiTi. The new woven bone formation started earlier in the Ti-6Al-4V than the NiTi group, but at 8 weeks the NiTi and stainless steel groups had greater cortical width compared to the Ti-6Al-4V group. Later than that, no statistical differences were seen.