Effects of low-intensity pulsed ultrasound on healing of mandibular bone defects: an experimental study in rabbits


Research evidence suggests that low-intensity pulsed ultrasound (LIPU) produces significant osteoinductive effects, accelerating the healing of bone defects. The authors investigated the effects of LIPU on mandibular bone defects in a rabbit model. Fifty-six adult Dutch rabbits were divided randomly into control, LIPU-1 (1 MHz), and LIPU-3 (3 MHz) groups. A mandibular defect was created in all rabbits. The effect of LIPU on mandibular defects was assessed by frequency (1 or 3 MHz) and timing (2 and 4 weeks). Bone mineral density (BMD) was measured and stereology and histology performed; results were compared at the end of 2 and 4 weeks. LIPU-3 resulted in significantly higher bone formation compared to the control group at the end of week 4 on histological assessment ( P = 0.008). BMD was significantly higher at 4 weeks than at 2 weeks ( P = 0.03). LIPU-3 increased the numerical density of osteoblasts and osteocytes at the end of week 4 ( P = 0.05 and P = 0.001, respectively). The results of this study are in favour of using LIPU 3 MHz to accelerate mandibular bone healing. However, this study suggests that a frequency of 3 MHz and the longer application of LIPU 3 MHz for 4 weeks can only promote 8% mandibular bone healing in rabbits. Therefore, the use of LIPU has no really convincing, consistent clinical effects on maxillofacial bone healing.

The healing of bone defects in the maxillofacial region is always a challenge in cases of congenital or traumatic defects or tumour resection. Rapid soft tissue growth inside the defect may block bone formation at the defect periphery, resulting in impaired defect healing. A relative lack of certain tissue factors in the centre of the defect, which originate from the edge of the defect, is believed to limit the bone healing process.

Several methods for managing bone defects have been introduced following research in the field of maxillofacial bone regeneration, such as the use of bone grafts and/or barrier membranes. Recently, the effect of ultrasound on bone defect healing has been assessed. Well-designed prospective studies have been reported, indicating that low-intensity pulsed ultrasound (LIPU) can accelerate fracture healing of the long bones in animal models and the fracture repair process in the tibia and radius.

Reports on the application of LIPU in the maxillofacial area are conflicting ( Table 1 ). Ustun et al. reported that LIPU may have positive effects on osseointegration and the stability of dental implants. However, Schortinghuis et al., in two separate studies, reported that there was no statistically significant difference in the percentage of defect closure between groups of rats exposed to LIPU and control rats. El-Bialy et al., in two different studies, showed that LIPU 1.5 MHz accelerated bone formation in mandibular bone distraction and suggested that the later stages of healing were enhanced more by pulsed ultrasound. Erdogan et al. found considerable contributions of LIPU (1.5 MHz) to bone healing in mandibular fractures.

Table 1
Overview of previously reported studies on the application of low-intensity pulsed ultrasound (LIPU) on mandibular defects.
Reference Species ( n ) Frequency, MHz Follow-up, weeks Evaluation methods Results
El-Bialy et al., 2002 Rabbit (21) 1.5 4 Bone photodensity, vibratory coherence, mechanical stiffness, histological studies Accelerated bone formation evaluated by photodensitometric, vibratory, elastic, and histological techniques
Schortinghuis et al., 2004 Rat (72) 1.5 2, 4 Microradiographs No statistically significant difference in the percentage of defect closure between the groups
Schortinghuis et al., 2005 Rat (64) 1.5 2, 4 Microradiographs No statistically significant difference in the percentage of defect closure between the groups
Erdogan et al., 2006 Rabbit (30) 1.5 3 Three-point bending test, digital radiodensitometric analysis, histological and histomorphometric examinations Considerable contributions to bone healing in mandibular fractures
El-Bialy et al., 2008 Rabbit (36) 1.5 1, 2, 3, 4 Quantitative bone density (QBD), mechanical testing, and histological examination Earlier stages of bone healing were enhanced more by continuous ultrasound, whereas late stages were enhanced more by pulsed ultrasound

These previously reported studies used 1.5 MHz ultrasound in the maxillofacial area. Although some compared the bone healing process based on weeks post application, the effect of different frequencies has not been evaluated in this region. Taking into account the reported positive influences of ultrasound on the bone healing processes with the disturbed function of impaired bone healing in large maxillofacial defects, it was decided to investigate the potential of LIPU to stimulate mandibular bone defect healing at two different frequencies, over 2 and 4 weeks, in a rabbit model.

Materials and methods

The study was approved by the university ethics committee. Fifty-six mature male Dutch rabbits weighing a mean 2.4 ± 0.2 kg and aged a mean 18 ± 2 months were included ( Table 2 ). Unilateral mandibular defects were created in all animals and they were subsequently divided into three treatment groups, which were subdivided further into six subgroups. The first two subgroups of animals (G1 and G2, n = 11 in each) received 1 MHz ultrasound treatment daily, 10 min/day, for 2 weeks (G1) or 4 weeks (G2). The second two subgroups (G3 and G4, n = 11 in each) received 3 MHz ultrasound with the same protocol for 2 weeks (G3) or 4 weeks (G4). Animals in the two control groups (G5 and G6, n = 6 in each) received sham application for 2 weeks (G5) or 4 weeks (G6).

Table 2
Study groups for the application of low-intensity pulsed ultrasound (LIPU) on mandibular defects.
Group LIPU Time of sampling after LIPU, weeks Number of rabbits
G1 1 MHz 2 11
G2 1 MHz 4 11
G3 3 MHz 2 11
G4 3 MHz 4 11
G5 (control 1) None 2 6
G6 (control 2) None 4 6

Surgical procedure

All rabbits underwent general anaesthesia by intramuscular injection of 44 mg/kg ketamine 10% (Alfasan International BV, Woerden, the Netherlands) and 8 mg/kg xylazine 2% (Alfasan International BV). After placing the animal in a supine position, the mandible was exposed with a 3-cm incision in the diastema area. A unilateral extraoral submandibular approach was used and a 5 mm × 2 mm × 1 mm bone defect was created 5 mm medial to the mental foramen on one side using a round bur (ELA Carbide; Emil Lange – Zahnbohrerfabrik e.K., Engelskirchen, Germany); all layers were then sutured ( Fig. 1 ).

Fig. 1
Approach to the rabbit mandible for creation of the bone defect (arrow).

Postoperative care

An analgesic (flunixin 5%, 0.15 mg/kg; Erfan Daru Pharmaceutical, Tehran, Iran) and an antibiotic (penicillin 22,000 IU/kg; Erfan Daru Pharmaceutical) were administered intramuscularly preoperatively and twice per day for four postoperative days. The rabbits were housed in separate cages and fed soft food for 1 week. After the first week, a normal diet was resumed. Food and water intake and the weights of the animals were monitored and recorded daily. Animals that had a weight loss of more than 20% of their initial body weight were excluded from the study.

Ultrasound application

On the second postoperative day, the application of LIPU was started for the animals in the four experimental groups. A commercially available therapeutic ultrasound device (new version 215× class 1 type BF; Nvin Medical Engineering Co., Isfahan, Iran) was used for the ultrasound treatment. The device transmits pulsed ultrasound signals with an operating frequency of 1–3 MHz, which produces 0.5 W/cm 2 average temporal and spatial intensity. Ultrasound applications were performed after placing the animal in a box in order to restrict excessive movements. No sedative agent was given during the ultrasound treatments. An investigator observed the whole ultrasound treatment session to ensure continuity of the application.

The 10-min sessions were repeated on a daily basis for 2 or 4 weeks according to the study group (G1–G4). The same procedure was applied to the animals in the two control groups, without activating the ultrasound device (G5 and G6). On the day after the last LIPU application, all animals were killed with an intravenous injection of 100 mg/kg sodium thiopental (Rotexmedica, Trittau, Germany). The whole mandibles were harvested and soft tissues were stripped off. The mandibles were split at the midline using a scalpel. Animals in groups G1, G3, and G5 were killed on postoperative day 15, while animals in G2, G4, and G6 were killed on postoperative day 30. Bone mineral density (BMD) measurements and stereology and histological examinations were performed on the harvested hemimandibles.

BMD assessment

The BMD of the prepared regions of the mandible was measured using dual-energy X-ray absorptiometry (DXA) (Hologic, Lunar Corp., Madison, WI, USA); the DXA device was calibrated daily, in accordance with the manufacturer’s recommendations. The regional high-resolution mode of the small animal scan protocol was used.

To measure BMD using DXA, the specimens were positioned centrally at the bottom of a square, thin-walled plastic container. The level in the water bath was set at 8 cm. One at a time, a hemimandible was placed into the container and the device was set to start the test. Each test took about 3 min. A specific area of 0.47 cm 2 was selected and focused on. This area contained the defect created in the mandible. The mandibular bone density and bone content of the rabbits in each group was measured. All DXA measurements and analyses were performed by the same investigator.

Histopathology assessment

The specimens were fixed in 4% neutralized formalin for 2 weeks, decalcified in 10% formic acid, dehydrated, and finally embedded in paraffin. Coronal sections approximately 5 μm in thickness were prepared through the centre of the defects, stained with haematoxylin–eosin (H&E), and observed under an optical microscope (Nikon, Tokyo, Japan). A single blinded pathologist scored the stained sections according to the amount of mineralization in the fracture gap, using the grading system described by Perry et al. With this system, grade 1 indicates fibrous union, grade 2 indicates predominantly fibrous tissue with some cartilage, grade 3 indicates equal amounts of fibrous tissue and cartilage, grade 4 indicates all cartilage, grade 5 indicates predominantly cartilage with some woven bone, grade 6 indicates equal amounts of cartilage and woven bone, grade 7 indicates predominantly woven bone with some cartilage, grade 8 indicates woven bone, grade 9 indicates woven bone and some lamellar bone, and grade 10 indicates lamellar bone. The mean score was calculated for each group.

Stereological study

Volume density

The volume density, V v (bone or cavity), of the trabecular bone and the bone marrow cavity was estimated using the point-counting method and the following formula: V v (bone or cavity) = P (bone or cavity)/ P (ref), where P (bone or cavity) and P (ref) are the total numbers of points hitting the bone, cavity, and the reference space ( Fig. 2 ).

Fig. 2
Point-counting method. The total number of points hitting the bone (or cavity) is divided by the number of points overlaying the reference space. H&E staining; scale bar = 100 μm.

Numerical density

The optical disector was used to estimate the total number of osteoblast, osteocyte, and osteoclast cells in the defect region of the mandible. For this purpose, the authors used software designed at the Histomorphometry and Stereology Research Centre, Shiraz University of Medical Sciences. The area of the defect was demarcated and counts were done at random locations. The area of the counting frame (a/f) was 21 μm × 21 μm. Section thickness was used as the height of the disector ( h ), excluding the 4-μm thick guard zones at the top and bottom of each section. To calculate the suitable guard zone, z -axis distribution of the nuclei was plotted.

The counted cells were scored and grouped in 10 histograms from percentiles 0–100 through the mandible tissue section, from the upper surface (0%) to the lower surface (100%).

Fig. 3 shows the z -axis distribution of the nuclei. The upper and lower 20% of the histogram were considered as the guard zones and the counting box was placed over the remaining 60% ( h ). According to the histogram, under-sampling was balanced out and corrected. Any nucleus coming into maximal focus within the next focal sampling plane was selected if it was located completely or partly inside the counting frame and did not touch the exclusion line ( Fig. 4 ).

Fig. 3
Distribution of the nuclei of the osteoblasts in the section of the mandible ( z -axis). Each bar represents the percentage of nuclei in 10% of the section thickness. The vertical dashed line indicates how the cell nuclei would present if all cells were equally distributed throughout the z -axis, i.e. 10% of all nuclei in each 10% percentile bar.

Fig. 4
Optical disector. (A) An unbiased counting frame is overlaid on the histological image of the bone. (B) Any cell whose nucleus comes into focus in the height of the disector is counted (arrows). H&E staining; scale bar = 20 μm.

Post-shrinkage thickness was measured during cell counting and used to determine an average thickness of 20 μm ( t ). All cells were distinguished based on differences in morphological characteristics and were counted separately within the frame. In order to determine cell densities, osteoblast, osteocyte, and osteoclast cells counted in the defect area were divided by the total volume of the counting frames as follows:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Nv(Cells/area of defect)=∑Q−∑p×a/f×h×tBA’>Nv(Cells/area of defect)=Qp×a/f×h×tBANv(Cells/area of defect)=∑Q−∑p×a/f×h×tBA
N v ( Cells/area of defect ) = ∑ Q − ∑ p × a / f × h × t B A
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Jan 17, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Effects of low-intensity pulsed ultrasound on healing of mandibular bone defects: an experimental study in rabbits
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