5.6. Effects of NiTi on fracture healing after intramedullary nailing
5.6.1. General findings
In study IV NiTi was compared to stainless steel after intramedullary nailing. There were no infections in either group. At 1 week, four rats were seen to hobble. Radiographs showed two nails in both groups to have penetrated far to the knee joint in these rats. The rats were killed and excluded from the study. All the other rats used their legs normally after one week. One fast-growing tumor with skin laceration was found in one rat in the stainless steel group 5 weeks after implantation. The rat was killed and also excluded from the study. The histology showed the tumor to be a benign fibroblastoma.
Figure 5-12. The photographs show the bone-implant area (magn. 48x). The black disks on top are metal implants (I) in hard resin embedded samples. The white semicircular areas are holes left by removed implants (IH) in paraffin-embedded samples. The bone wall is located in the middle of each picture and the intramedullary space (IMS) is shown below. The areas of histomorphometric measurement are about the same as those seen in these pictures. The broken lines have been added to clarify the different areas. NWB = new woven bone. EB = eroded bone with resorption pits and rough surface. NLB = new lamellar bone in endosteal surface. PO = bone periosteum. A) NiTi 2 weeks after implantation. There is a new woven bone area left of the outer cortical area. Some eroded bone is also visible, indicating the beginning of the modelation process. B) New woven bone formation was most apparent at 4 weeks in the NiTi group, which is also seen in this photograph. C) NiTi at 8 weeks. A slightly U-shaped cross-sectional bone modelation has developed when periosteal resorption under the implant is compensated by lamellar endosteal callus and lateral new bone bracing. D) Ti-6Al-4V at 8 weeks, when the cortical width value was found to be lower than in the NiTi group. The ragged cortex indicates active resorption, but some new bone can also be seen. E) NiTi 12 weeks after implantation. A close connection with bone is seen. F) NiTi 26 weeks after implantation. There is a thin fibrous layer (FL) between the bone and the hole left by the implant. There is no osteoporosis, continuous resorption or any other sign of harmful irritation.
5.6.2. Histology and morphology
The common histology and morphology of the healing process were similar in the two groups. The osteotomies healed via secondary bone union. At 2 weeks, the callus consisted mainly of fibrous granulation tissue. Chondral tissue was seen around and between the bone fragments and in the endosteal area. Some bony mineralization points were seen. The ends of the osteotomized bones were avascular. At 4 weeks, there was clearly mineralized callus (Figure 5-13A) and woven bone formation. The chondral tissue was mostly seen between the bone heads. New woven bone was found in the peripheral area of the external callus and in the endosteal area. Fibro-cartilage tissue was seen in the middle of the external callus. At 8 weeks, there was clearly more mineralization. Woven bone bridged the fracture site as a mark of bony union. In the metaphyseal area, an intramedullary fibro-cartilaginous layer surrounded the implants (Figure 5-14A and B).
At 12 weeks, there were complete bony unions (Figure 5-13B; Figure 5-15A, B and C), where lamellar bone structures were also evident in polarized light microscopy A three-layer callus structure was sometimes seen, including outer peripheral callus bone, the original bone heads and separate endosteal bony callus. These layers were separated by bone marrow and loose trabecular spaces. At 26 weeks, most samples had a compact, thin, circular bone layer around the nail in the metaphyseal intramedullary area out of the osteotomy site. This formed a close contact with both NiTi and stainless steel. This structure was also clearly visualized by the CT scans. Some cortical ends with normal ossifying and vascularizing processes by osteons were visible (Figure 5-16A and B). Most osteotomies were completely healed. The replacement of woven bone by lamellar bone continued, and remodelation of the whole callus was obvious.
At 60 weeks, the callus contained well-organized lamellar structures. The osteotomy line had disappeared. A thicker peri-implant bone sheet was seen in the metaphyseal area of the intramedullary space (Figure 5-14C). Bone and the NiTi implant formed very close contacts (Figure 5-14D).
5.6.3. Callus size and the consolidation of osteotomy
The maximum width and length of the callus area in the NiTi and StSt groups during the study period are shown in Table 5-2. The results were notably similar between the groups, and no statistical differences were seen. At 60 weeks, the callus was generally thinner and shorter than at the earlier time periods, indicating a normal remodelation and healing response.
Figure 5-13. A) Radiograph of the right femur of a NiTi rat 4 weeks after intramedullary fixation of diaphyseal osteotomy. Cloudy callus has developed and the fixation is stable. B) The right femur 12 weeks after implantation. The osteotomy is well healed.
Figure 5-14. A and B) A mineralized layer around the removed NiTi nail 8 weeks after implantation in the metaphyseal area. I = The hole left by the removed intramedullary nail. The closest layer is composed of a collagen-rich thin fibrous (F) layer. This turns into cartilaginous (C) and bone (B) tissue. HE staining. Magn. 120x and 480x. C) Metaphyseal femur area distal to the osteotomy site. A thick lamellar bone (arrows) sheet around the implant is seen. NiTi 60 weeks after implantation. Magn. 32x. D) Bone envelops very closely the implant head (arrows). Magn. 48x. I= implant. Hard resin embedded section with Goldner-Trichrome staining with the intramedullary nail in place. Slice thickness 30m.
At 2 weeks, there was no overstepping callus formation and a clear osteotomy line was seen in all femurs. At 4 weeks, callus formation was observed in all rats (Figure 5-13A). Most of the osteotomies were classified as showing “good” or “satisfactory” healing in both the NiTi and the stainless steel groups, but there were more “poor” results in the StSt group (27% vs. 20%). There was one rat in the NiTi group and two rats in the StSt group with a 15° antero-posterior malposition. At 8 weeks, more osteotomies were classified as “good” in the NiTi (50%) group than in the stainless steel (36%) group. The total number of rats at this time point was not the same in the two groups, because the rat with the tumor had been excluded from the StSt group. At 12 weeks, the rates of consolidation were equal between the groups. At 26 and 60 weeks, the percentile healing responses were also equal. The consolidation of the osteotomy line as revealed by radiographs is shown in Table 5-3.
Table 5-2. Maximum callus dimensions measured from radiographs. Values are given as mean (mm) ± 1 standard deviation. There is not significant differences between the groups. n=number of the rats.
| Fixation | Time (weeks after implantation) | ||||
|---|---|---|---|---|---|
| 4 | 8 | 12 | 26 | 60 | |
| NiTi | n=15 | n=12 | n=9 | n=6 | n=3 |
| Max width of the callus | 7,5 ± 1,0 | 7,4 ± 1,3 | 7,3 ± 1,0 | 7,8 ± 1,2 | 7,2 ± 0,5 |
| Max length of the callus | 10,5 ± 2,2 | 10,0 ± 2,3 | 11,4 ± 2,4 | 10,4 ± 2,5 | 9,9 ± 1,1 |
| Stainless steel | n=15 | n=11 | n=9 | n=6 | n=3 |
| Max width of the callus | 7,6 ± 1,2 | 7,6 ± 1,6 | 7,5±1,6 | 7,5±2,1 | 6,6 ± 1,5 |
| Max length of the callus | 10,7 ± 2,7 | 10,5 ± 3,4 | 11,4 ± 3,5 | 11,0 ± 5,0 | 8,8 ± 3,4 |
Table 5-3. Union of osteotomy fixed with NiTi or stainless steel intramedullary rod. Values are expressed as percentages.
| Fixation | Time (weeks after implantation) | ||||
|---|---|---|---|---|---|
| 4 | 8 | 12 | 26 | 60 | |
| NiTi | n=15 | n=12 | n=9 | n=6 | n=3 |
| Good | 40% | 50% | 55% | 66% | 100% |
| Satisfactory | 40% | 34% | 33% | 17% | 0 |
| Poor | 20% | 16% | 12% | 17% | 0 |
| Stainless steel | n=15 | n=11 | n=9 | n=6 | n=3 |
| Good | 33% | 36% | 55% | 66% | 100% |
| Satisfactory | 40% | 46% | 33% | 17% | 0 |
| Poor | 27% | 18% | 12% | 17% | 0 |
Figure 5-15. Diaphyseal osteotomy fixed with a NiTi intramedullary nail 12 weeks after implantation. The three photographs are linked together (A, B and C). Good contact between the bone and the nail is evident (black arrows). B) Orderly callus with healed osteotomy (open arrows) is seen. C= callus. Hard resin embedded section with Goldner-Trichrome staining with the intramedullary nail in place. Slice thickness 30m. Magn. 32x. C) In the metaphyseal area, a thin layer of lamellar bone (asterisk) demarcates the nail bed quite closely, indicating good tissue tolerance. AC= articular cartilage of knee joint.
Figure 5-16. A) Remodelation of the original bone ends. Mineralized chondral tissue between the bone ends has been replaced by secondary osteons (white arrows) at the osteotomy line. NiTi 26 weeks after implantation. Magn. 120x. B) Remodelation unit at work. A cutting cone with multinuclear osteoclasts (OC) resorbs the woven bone, and lamellar bone (LB) is formed by osteoblasts (OB). NiTi 26 weeks after implantation. Magn. 480x.