Inferior Alveolar Nerve Transposition—An In Vitro Comparison Between Piezosurgery and Conventional Bur Use

Dental implant placement in the severely atrophic posterior mandible can be compromised by an inadequate vertical volume of bone above the neurovascular bundle. Although lateralization of the inferior alveolar nerve (IAN) is often performed as an alternative to augmentation techniques, there is a risk of nerve injury through primary surgical trauma or secondary complications such as edema or hematoma. In spite of possible surgical reconstruction of nerves, efforts have been made to minimize these disadvantages.1–3 Initially, a corticotomy that includes the mental foramen is performed, then the remaining cancellous lamella covering the IAN is removed.4,5 To protect the nerve, a thin bent spatula inserted through the mental foramen is placed between the neurovascular bundle and the cancellous lamella. The IAN is retracted from the mandibular canal, the implants are inserted, and the nerve is positioned lateral to the implants. The anterior incisive branch is not affected. Transposition of the IAN requires a corticotomy including the mental foramen. After transection of the anterior incisive branch the nerve is transposed posteriorly.4,6,7 Despite extreme caution and measures, injuries caused by compression or direct trauma by the rotating instrument cannot be excluded. A reduction in IAN lesions has been reported with the use of ultrasonic devices.8 New cutting devices using piezoelectricity possess a high oscillating frequency of over 20 000 cycles per second and an oscillating motion of 20 to 200 μm; when touching mineralized hard tissue, they work like a pneumatic hammer, and small particles of bone are shattered to form microcracks and debris.9,10 Such an effect does not occur in soft tissue because of its elasticity. In contrast to conventional ultrasonic cutting devices, the piezoelectric device is 3 times more powerful and has a modified oscillating movement. This results in an improved bone cut and decreased heat development.8,11,12 

The aim of this study is to compare the effects of piezoelectric and conventional drilling instruments on the IAN and the adjacent bone in cadaver sheep mandibular specimens after transposition of the IAN.

Ten sheep mandibles were dissected from the surrounding soft tissue and the periostium without harming the mental and mandibular nerves. On the left side, the canal was opened with a diamond-coated spherical drill under constant irrigation with saline solution. The IAN was protected by a thin bent spatula inserted in the mental foramen.13 Preparation of the right side was carried out with the piezoelectric device (Mectron Piezosurgery Device, Mectron, Genova, Italy) mounted with a diamond-coated spherical tip. The device consisted of a piezoelectric hand piece and a regulation panel. Functional frequencies of 25 to 29 kHz and digital modulation in “boosted” mode with 30 kHz were selected. A jet of physiological saline solution provided constant cooling. The highest power mode boosted was selected.14 

The bone areas of interest were cross sectioned with a circular saw (Figure 1). The first cut was placed at the level of the mental foramen. The anterior section served as a control for the surface measurement. The genuine inferior alveolar canal showed a similar cross section to the manipulated area. The samples were fixed in 3.5% formaldehyde (Figure 1).

Surface measurements were carried out with a laser surface measurement (LSM) system (UBM, Messtechnik GmbH, Ettlingen, Germany).15 To compare the surface structure produced by both preparation methods, the 10 bone sections of each group were cut longitudinally at the anterior side of the mandibular canal, providing a horizontal surface necessary for the laser measurements in an area of 2.0 × 0.5 mm. The values shown in Table 1 were measured on all 40 bone pieces.

The samples were dehydrated with ascending alcohol concentrations. For the detection of microcracks, the samples were stained en bloc with 1% basic fuchsin (basic fuchsin, Merck 15937, Darmstadt, Germany).16–18 The samples were then embedded in Technovit (Technovit 7200 VLC, Heraeus Kulzer, Wehrtheim, Germany). After 12 hours curing under ultraviolet light, 250-μm thick sections were cut (Microslice 2, Metals Research, Cambridge, UK). These were mounted (Cyanacrylat, Loctite, München, Germany) onto acrylicglass (50 × 50 × 3 mm³, 05370, Mertin, Freiburg, Germany) and ground to a thickness of 100 μm.

The initial investigation was performed at ×100 and ×200 magnification under a fluorescent light microscope (Axio Cam 2 Plus, Zeiss, Germany). Microcracks appeared orange under green light (λ = 546). To obtain quantitative data from the defect zone, the broadest part was observed at the lower magnification. Measurements were evaluated with ImageJ V1.3 (Java Applet, freeware; Sun Microsystems, Inc, Santa Clara, Calif) in a blinded format. The thicknesses of the defects were determined with the plumb bob.

The samples were submerged in 50% formic acid for 1 week and dehydrated with ascending alcohol concentrations for 12 hours. Finally, the samples were embedded in paraffin, sectioned (8 μm), and stained (hematoxylin and eosin [H&E] and Elastica van Giesson [EVG]).

After exposure, the IAN was carefully removed from the canal starting at the mental foramen. A circular stitch (Vicryl, 5-0, Ethilon, Lenneke Marelaan, Belgium) was inserted for further identification of the prepared lateral side. The nerve was then mobilized between the mental foramen and the first molar. A second stitch was placed at the level of the first molar. The IAN was sectioned and removed posteriorly from these marks. The pieces of nerve were submerged in 3.5% formaldehyde for 1 week. India ink staining was carried out to mark the anterior side of the nerve, allowing spatial orientation after cutting.

The nerves were cut perpendicular to the fascicles within the anterior section and lengthwise within the posterior section. All samples were then dehydrated with ascending alcohol concentrations, embedded in paraffin, sectioned (8 μm), and stained (H&E, EVG, Goldner 1). Evaluation of the nerve cross sections was performed with a microscope (Axio Cam 2 Plus) at ×25 and ×100 magnification.

The results of the topographic measurements of original bone, bone dissected with the rotary bur, and bone dissected with the piezoelectric device are shown in Table 2. Figure 2A through C shows the 3-dimensional reconstructions of measurement points (LSM). The resolution of measurements was 1000 points/1 mm to 500 points/0.5 mm. Figure 3A through C demonstrates the histological photomicrographs of bone fragments (decalcification).

One-way analysis of variance (SPSS version 9.0) showed significant differences among the values for all groups (P_RA = .046; P_Rmax = .020; P_Rt = .014).

The rotary bur produced a smooth cut bone edge. No separation of the bone lamella occurred (Figure 4A and B). The defect zone of the samples prepared with the piezoelectric device had an average depth of approximately 170 μm (Table 3). The defect zone created with the rotary instrument had an average depth of approximately 54 μm.

The piezoelectric device produced an irregular surface. The structure of the bone worked loose, comparable with a brittle mineral (Figure 4C and D). Application of the Student t test for independent samples (SPSS version 9.0) revealed a significant difference between the 2 groups (F = 7.252, thus no equal variances; P = .000).

The selected localization allowed mobilization and removal of the nerve section. The India ink marks at the anterior epineurium could be easily identified in all samples.

The vessels were always visible on the anterior half of the crosswise-cut nerves (Figure 5B and E). All sections of nerve exhibited a polyfascicular structure. Because the section was anterior, the nerve demonstrated fewer fascicles and more fat cells in the internal epineurium.19 

The external epineurium showed longitudinal collagen fibers.20 The perineurium consisted of perineural epithelium with predominantly longitudinal collagen fibrils and fibrocytes between them.

The histological pictures of both groups displayed typical characteristics of the different methods of preparation. No lesions could be observed, and isolated bone particles were found at the epineurium in the group prepared with the rotary bur (Figure 5A through C). With the piezoelectric device, the epineurium had a rough appearance. Bone particles were found in and on the epineurium (Figure 5D through F).

Other parts of the nerves lying within the epineurium were not affected either by the piezoelectric device or the rotary bur. The extension of the irritation zone, noticed in lengthwise sections (Figure 5D), was visible in all areas touched by the piezoelectric device.

Difficulties with implant insertion arise in cases of atrophic jaws. The limiting factor in such cases is the anatomical course of the IAN. For lateralization or transposition of the nerve, the 2- to 4-mm cortical bone, the thin cancellous bone, and a fibrous sheath (paraneurium) have to be removed. In addition, 64% of patients demonstrate a cortical lamella within the cancellous lamella.21 

A rotary bur is reported to cut faster and produce a deeper cut than a piezoelectric device.22,23 The cutting efficiency of a piezoelectric device depends on the adjustment of the instrument tip, with the highest efficiency when the tip is kept flat.24 The best angle resulting in a higher cutting rate is reported to be between 0° and 10° of the orthogonal preparation axis.22 In comparison, the efficiency of the rotary bur was independent of the adjustments.

In the presented study, it was observed macroscopically that the irrigation liquid rinsed the bone particles into the nerve canal during preparation and exposure of the nerve in both techniques. This was also observed microscopically with a higher particle density on the epineurium after the use of the piezoelectric device. In contrast, the rotating motion of the rotary bur carried splinters away from the area of surgery.

The rotary bur produced regular bone edges, whereas the piezoelectric device produced loosened bone edges. The average surface roughness (Rmax) was significantly higher for the samples prepared with the piezoelectric device (27.26 μm) than for those prepared with the rotary bur (18.80 μm). Similar results have been shown previously.23,25 

Faster initial wound healing and a consequent reduction in infection risk after preparation with ultrasonic instruments have been observed in vivo in animals.25,26 The improved healing tendency could be because of the pronounced bone edge roughness, which would allow a greater number of osteoclasts and osteoblasts to act simultaneously. This effect was observed with defects reaching into the cancellous bone.25 Remaining within the cortex, a rough surface reduced the regeneration process. Beginning at the periostium, a smooth surface produced with a rotary bur seemed to be more conductive to regeneration.23 However, there was no significant difference in wound healing after 42 to 90 days in both studies.25,27 

The formation of microcracks in dentin by the swinging ultrasonic tip has already been observed in root-end preparations.9 According to Burr and Stafford,28 criteria for the identification of microcracks in bone are (1) an intermediate size ranging between the smaller canaliculi and the larger vascular channels (average size of the microcracks produced by stress fractures; eg, in the human rib, it is approximately 90 μm¹⁸), (2) sharp borders with a basic fuchsin area around them, (3) staining throughout the entire depth, and (4) corners of the microcracks that appear more deeply stained than the intervening space. The computation of form and volume is problematic because the 3-dimensional view is lost in transversal cuts. To avoid this problem, Taylor and Lee29 simulated calculations in which the microcracks displayed themselves as ellipses possessing specific axial ratios (4.5:1) with a standardized ratio of 0:1.

In our investigation, some differences must be taken into consideration. The mechanism of the cracking is different. Microcracks formed from stress fractures lie within the bone, but microcracks caused by preparation were at the edge of the bone. The size and number of microcracks were subject to a much larger variance. The course of the microcracks in the 3-dimensional structure presented itself differently. No distinct alignment resulted. Whole chunks of mineralized tissue were loosened. The third dimension could be neglected because it corresponded to the length of the prepared bone. For these reasons, we limit our description to a defect zone. The zone created by the piezoelectric device (approximately 170 μm) was more than 3 times as deep as that of the rotary bur (approximately 54 μm) and showed a more invasive character, which could be an advantage for the healing process. The increased surface area might induce an activation of more osteoclasts and osteoblasts.22,24 An exact differentiation between cancellous bone and cortex was not possible.

The usual definition in the literature of nerve lesions consists of morphological changes and the accompanying loss of function.30,31 Lesions in our study refer only to histomorphological changes, which appear during preparation. Reaction of the dead nerve tissue accompanied with functional loss could not be examined. Histological lesions were limited to the epineurium in the group treated with the piezoelectric device.

The advantage of the ultrasonic instruments is a claimed inactive effect on soft tissue.10 The explanation for this phenomenon is that soft tissue gives way to the instrument's oscillation and thus does not represent a target. The lesions in the epineurium shown in this study can be explained by the roughness of the insert in addition to the relative rigidity of the epineurium. If the tissue has a high rigidity, the moment of inertia is too large to follow the oscillations, and lower-rigidity tissue injuries at the surface will result. Because injuries could be observed only at the surface of the epineurium, a decreased oscillation seems to be linked with increased depth (Figure 6).

The function of the epineurium is to protect the nerve fibers. It contains important cellular components and vessels required for repair mechanisms. However, these fibers are more vulnerable than those of the endoneurium.31,32 A lesion of the epineurium is accompanied by a morphological change. Extraneural scar formation as a consequence of the healing process can compress or rotate the peripheral nerves, and fibrous ankylosis can lead to restriction of the slip motion.

The operational interference after conventional transposition or transposition with a bur frequently causes a well-known sensitivity disturbance of the area served by the IAN. This is probably because of the mobilization and the associated stretching of the nerve, which can be stretched about 10% to 20% before structural changes develop.33 With increasing tension, the nerve moves within its canal and uses the range of the loose flexible fibers. Tearing of the axons occurs only when they are overstretched.30 The prognosis of these lesions after conventional lateralization or transposition of the IAN is reported to be very good (healing between 80% and 100%1,2). Initial healing is observed after 3.8 to 5.7 weeks and complete recovery after 6 months to 1 year.1,5–7 These statements refer to a lateralization without complications.

Further investigation should aim to reveal the damage to the nerve during preparation with the piezoelectric device. This could be done on the basis of in vivo studies with the help of somatosensory-evoked potentials for the objective measurement of nerve damage. If the marginal injuries to the epineurium prove to be tolerable, lateralization with the piezoelectric device might be regarded as a safe and easy method.

In vitro preparation with the piezoelectric device was more invasive to the bone than with the conventional diamond bur. In fact, touching the IAN resulted in roughening of the epineurium, but deeper structures remained unaffected. Therefore, within the parameters of this in vitro study, the degree and risk of injury with the piezoelectric device was lower than with the conventional rotary bur.