Study on TiO2 Nanofilm That Reduces the Heat Production of Titanium Alloy Implant in Microwave Irradiation and Does Not Affect Fracture Healing

Abstract

Background. Metal implants can produce heat and damage adjacent tissues under microwave irradiation, which makes local metal implants in the body a contraindication for microwave therapy. However, with the wide application of titanium alloy implants which have low permeability and low conductivity, this concept has been challenged. Our team members have confirmed through previous research that continuous low-power microwave irradiation does not cause thermal damage to the surrounding tissues of the titanium alloy. Is there any other way to further increase the dose of microwave irradiation while reducing the heat production of titanium alloy implants? In this study, the effect of TiO2 nanofilm on reducing the heat production of titanium alloy implants in microwave field was verified by animal experiments, and the effect of TiO2 nanofilm on fracture healing was observed. Methods. 30 rabbits were selected. In the experiment of temperature measurement, 10 rabbits were randomly divided into experimental group ( $n=5$ ) and control group ( $n=5$ ), and the contralateral lower limb of the rabbits in experimental group was set as the sham operation group. The right femurs in the experimental group were implanted with Ti6Al4V plates coated with TiO2 nanofilm, and the right femurs in the control group were implanted with common titanium alloy plates without TiO2 nanofilm. The same surgical procedure was used in the sham operation group, but no plate was implanted. The temperature of the deep tissue above the metal implant was measured with an anti-interference thermocouple thermometer during 20 minutes of microwave irradiation. The other 20 rabbits were randomly divided into two groups, experimental group ( $n=10$ ) and control group ( $n=10$ ). The femoral shaft fracture models were established again. Ti6Al4V plates coated with TiO2 nanofilm and common titanium alloy plates were implanted in the two groups, respectively, and both groups were exposed to continuous microwave irradiation with a power of 40 W or 60 W for 30 days after operation. The fracture healing was evaluated by X-ray at 0 day, 14 days, and 30 days after microwave irradiation, respectively. The animals were sacrificed at 30 days after operation for histopathological assessment. Results. The temperature in the experimental group, control group, and sham operation group increased significantly after 40 W and 60 W microwave irradiation ( $2.18\pm 0.15$ °C~ $6.02\pm 0.38$ °C). When exposed to 40 W microwave, the temperature rise of the control group was $4.0\pm 0.34$ °C, which was significantly higher than that of the experimental group $2.82\pm 0.15$ °C ( $P<0.01$ ) and the sham operation group $2.18\pm 0.33$ °C ( $P<0.01$ ). There was no significant difference in temperature rise between the experimental group and the sham operation group ( $P=0.21$ ). When exposed to 60 W microwave, the temperature rise of the control group was $6.02\pm 0.38$ °C, which was significantly higher than that of the experimental group $3.66\pm 0.14$ °C ( $P<0.01$ ) and sham operation group $2.96\pm 0.22$ °C ( $P<0.01$ ), and there was no significant difference between the experimental group and the sham operation group ( $P=0.32$ ). X-ray evaluation showed that there was no significant difference in callus maturity between the experimental group and the control group at 14 days ( $P=0.554$ ), but there was significant difference in callus maturity between the two groups at 30 days ( $P=0.041$ ). The analysis of bone histologic and histomorphometric data at 30 days was also consistent with this. Conclusion. Under the animal experimental condition, compared with the common titanium alloy implant, the TiO2 nanofilm can reduce the heat production of the titanium alloy implant in the 2450 MHz microwave field and has no adverse effect on fracture healing. This study opens up a promising new idea for the application of microwave therapy to metal implants in human body.