Animal experiments were performed to demonstrate the effects of photofunctionalized Ti6Al4V on osseointegration in rats with type 2 diabetes. In this in vivo study, Ti6Al4V implants were inserted into rat femurs, the rats were killed after 2 or 4 weeks and radiological analysis was performed using microcomputed tomography, followed by histological analysis using undecalcified specimens. The study protocol (ethical code number: M19007) was approved by the Animal Research Committee of Hirosaki University, and all experiments were performed according to the Rules for Animal Experimentation of University.
Photofunctionalization of Ti6Al4V implants
The cylinder implants were made from Ti6Al4V (diameter 2 mm, length 8 mm) and provided by B. Braun Aesculap Japan Co., Ltd (Tokyo, Japan). Half of the implants were treated with UV irradiation for 15 min using a photodevice (TheraBeam® affinity; Ushio Inc., Tokyo, Japan) at an intensity of 3 mW/cm2 (Fig. 1A–C). The light source mounted in the TheraBeam affinity was a low-pressure mercury (Hg) lamp, which emitted predominantly UV light of 185-nm and 254-nm. The implants were divided into two groups: Ti6Al4V without UV irradiation (untreated group) and Ti6Al4V with UV irradiation (UV-treated group).
Animals
The Spontaneously Diabetic Torii (SDT) fatty rats (average adult weight of 360.7 ± 18.4 g) reach a high blood glucose level (approximately 200 mg/dL) at 6 weeks of age, and a very high level (approximately 400 mg/dL) at 8 weeks, and thus are a very mature model for type 2 diabetes [24,25,26].
Sprague Dawley (SD) rats (average adult weight of 266.7±13.5 g) were used for the control group and SDT fatty rats for the DM model. All rats were 8-week-old males and purchased from the same company (CLEA Japan, Inc., Tokyo, Japan). The rats were maintained in bracket cages and fed a standard laboratory diet and were able to access water ad libitum under temperature-, humidity-, and lighting-controlled conditions.
A total of 20 rats were divided into four groups, with five in each group as follows: Group I: SD rats implanted for 2 weeks; Group II: DM rats implanted for 2 weeks; Group III: SD rats implanted for 4 weeks; and Group IV: DM rats implanted for 4 weeks.
Surgery
The rats were anesthetized with 1–2% isoflurane. Both hind limbs were shaved, and the incision area (from the distal femur to the knee) was wiped with alcohol before the skin and fascia were opened separately. The flat aspect of each distal femur was exposed and used for implantation. The bilateral distal femurs were drilled using a 2-mm diameter drill. UV-treated implants were inserted into the right femur holes, and untreated implants were inserted into the left femur holes (Fig. 2). After implant placement, the skin and fascia were closed with stitches. At either 2 or 4 weeks after surgery, the rats were euthanized by drawing more than 5 mL of blood directly from the heart, and the femurs were harvested.
Blood glucose analysis
To confirm the establishment of normal and diabetic rats, blood glucose levels were measured just before implantation (0 weeks) and every 2 weeks after the operation until kill. Blood was collected from the tail of the rats, and blood glucose levels were measured using a blood glucometer (Experimental Animal Glucometer SUGL-001, ForaCare Japan, Japan).
Radiological analysis
The specimens were analyzed using microcomputed tomography (Scan Xmate-L090, Comscantecno Co., Ltd., Japan). The imaging conditions were as follows: voltage, 80 kV; current, 100 μA; magnification, 4.942 times; resolution, 20.234 μm/pixel; and slice thickness, 20.234 μm. Three-dimensional bone morphometric analysis was performed using the TRI-3D-BON software (TRI/3D-BON, RATOC System Engineering Co., Ltd., Japan). The bone volume (BV) ratio was defined as the ratio of the mineralized BV within 100 μm from the implant surface. The BV ratio was calculated as the bone occupancy in the area of interest divided by the total area of interest, multiplied by 100.
Histological analysis
The specimens were fixed in 10% buffered formalin and analyzed using microcomputed tomography (Scan Xmate-L090, Comscantecno Co., Ltd., Japan). Specimens were embedded in methyl methacrylate without decalcification [27]. The implants were left in situ for histological analysis. Embedded specimens were cut along the long axis of the implants using a microtome (EXAKT, Norderstedt, Germany).
Each 30–40 µm section was stained green with Villanueva–Goldner and examined by light microscopy (BZ-X700; Keyence Corp., Japan) to evaluate the bone area. For each histological slice, the BIC ratio for each group was calculated using digital image analysis software (ImageJ version 1.48). The BIC ratio was calculated as the length of the bone in direct contact with the surface of the implant divided by the total length of the implant, multiplied by 100 (Fig. 2). The bone in direct contact was defined as the interface at which the bone tissue was located within 20 μm of the implant surface without the intervention of soft tissue.
Statistical analyses
The Mann–Whitney U test was performed to determine differences in blood glucose levels. The Wilcoxon signed-rank test was performed to determine differences in the BIC and BV ratios between the UV-treated and untreated groups at 2 or 4 weeks, respectively. The Mann–Whitney U test was also performed to determine differences in BIC and BV ratios between 2 and 4 weeks in each group. Statistical analyses were performed using SPSS (v 21.0; IBM), and p-values of < 0.05, were considered significant.
The Pearson correlation analysis method was used to analyze the correlation between mean blood glucose level and BIC ratio and between mean blood glucose level and BV ratio. Correlation analysis of all the data was performed, and the data were analyzed after different groupings. Correlation analyses were performed using SPSS (v 21.0; IBM), and p-values of < 0.05 were considered to indicate correlation. When r is > 0, the two variables are positively correlated, and when r < 0, the two variables are negatively correlated. When | r |≥ 0.8, the two variables were highly correlated; when 0.5 ≤| r |< 0.8, the correlation was moderate; when 0.3 ≤| r |< 0.5, there was a low correlation; and when | r |< 0.3, the correlation between the two variables was very weak and was regarded as uncorrelated.