Abstract
This study concerns intraosseous temperature changes during the use of piezosurgical inserts. On six fresh pig jaws heated to body temperature (36 °C), osteotomies and osteoplasties were performed in vitro with the Piezosurgery ® 3 device (Mectron, Carasco, Italy) and various inserts. The intraosseous temperature increases were measured at a depth of 3 mm and at a distance of 1 mm from the working site using nickel–chromium/nickel temperature sensors. 20 °C Ringer’s solution was used for cooling in an initial test series and 10 °C Ringer’s in a second series. The processed bone was examined using digital volume tomography images to determine the ratio of cortical to cancellous bone thickness. Mean temperature increases of 4.4–10.9 °C were found; maximum temperature peaks were over 47 °C for an average of only 8.5 s. The type of piezosurgical insert had a marked influence on intraosseous temperature generation ( p = 0.026); the thickness of the cortical bone and the temperature of the coolant did not. Coolant temperature had an influence on the bone cooling time ( p = 0.013). The results show that correct use of the piezosurgery device does not give rise to prolonged temperature increases over 47 °C and hence does not cause any irreversible thermal damage in the bone.
Piezosurgery has been used in dentistry for about 20 years and its range of indications has constantly been expanding. As well as osteotomies (bone block harvesting, sinus lifts, and bone splitting), osteoplasties (resective periodontitis and peri-implantitis treatment), tooth extractions and apicectomies, it is increasingly being used for implant insertion and orthodontic microsurgery.
Ultrasonic inserts for bone surgery cut hard, mineralized tissue as well as teeth or bone. The cutting is precise and the width varies between 60 and 200 μm depending on the insert. Soft tissues, such as nerves, blood vessels or the Schneiderian membrane, are not injured because they are able to move with the same frequency of 25–29 kHz or the resulting micro-oscillations of 60–210 μm. Soft tissues can only be cut at frequencies above 50 kHz.
Piezosurgery can be described as a tried and tested, atraumatic surgical method. The operating field is bloodless because of the ultrasound-induced cavitation effect and affords a clear view. Patients may find the low working pressure and the barely perceptible vibrations more acceptable than rotary instruments.
Heat generation, when working with oscillating or rotary inserts on bone, is a frequently discussed problem. The intraosseous heating can be so high that thermal damage occurs in the bony tissue. Irreversible bone damage was observed at temperatures of 47 °C for 1 min, but not at 44 °C for 1 min.
Overall, intraosseous hyperaemia ensues at 40–41 °C and necrosis of lipocytes and initial resorptive processes above 47 °C. Blood flow in the bone stops at 47 °C and above, while irreversible necrosis with possible sequestration occurs at temperatures of 55 °C and higher. As well as the pure temperature increase, the duration of exposure to the effects of heat also plays a role: irreversible destruction is directly proportional to the temperature effect and the time factor.
The exact threshold for irreversible damage in bony tissue has not been conclusively researched. It is mainly given as 47 °C over 1 min, but in some cases this is corrected upwards or downwards.
Several studies have investigated intraosseous heat generation during the use of rotary instruments. Rotational speed contact, pressure cooling, drill design and drill diameter have been described as key factors influencing heat generation.
There have been few studies on the intraosseous temperature change that occurs when oscillating cutting tips or piezosurgical inserts are used. One study investigated heat production when working with an oscillating saw in vitro and in vivo . It found that intermittent working is essential because the coolant will otherwise fail to penetrate the depths of the prepared bone split and hence will not reach the tip of the instrument. Piezosurgical inserts have been tested for intraosseous temperature changes in two current studies. Both were conducted under standardized conditions on non-tooth-bearing bone that was not at body temperature.
The purpose of the present study was to investigate in vitro the influence of the size of an insert, the temperature of the coolant and the thickness of cortical bone on intraosseous temperature generation under conditions as close as possible to clinical practice.
Materials and methods
In this study, six fresh pig mandibles were used in vitro . The rationale for choosing pig jaws was the histological and chemical similarity of pig bone with human bones. The jaws were collected on the day of slaughter directly from the abattoir (Bell, Basel, Switzerland) and heated to body temperature (36 °C) in a warming bath ( Fig. 1 ).
To avoid differences between devices and inserts from different manufacturers, all the osteotomies and osteoplasties were performed with the Piezosurgery ® 3 device (Mectron, Carasco, Italy). The straight bone saws OT7, OT7S-4 and OT7S-3 (Mectron) were used for the osteotomies. Insert OT7 has five teeth and a saw blade thickness of 0.55 mm. The OT7S-4 and OT7S-3 saws have four and three teeth, respectively, and are both 0.35 mm thick at their tip ( Fig. 2 ). The Mectron inserts OP1 and OP3 were used for the osteoplasties ( Fig. 3 ).
The Piezosurgery ® 3 device was operated on its highest setting for all the measurements. As recommended by the manufacturer, this setting was ‘Bone – Cortical’ for inserts OP1, OP3 and OT7 and ‘Bone – Special’ for inserts OT7S-4 and -3.
Water cooling was set to maximum, which is equivalent to a flow rate of 90 ml/min. Ringer’s solution (Braun, Sempach, Switzerland) was used for cooling, which was 20 °C in an initial test series and 10 °C in a second series.
Flexible nickel–chromium/nickel type K thermosensors with a measuring range between −50 °C and +260 °C and a diameter of 0.2 mm at the tip were used for the temperature measurements (B+B Thermo-Technik, Donaueschingen, Germany). They were connected to a thermo measuring device (B+B Thermo-Technik) which was linked to a laptop. The data were recorded and saved using the ‘HandHeld’ program (B+B Thermo-Technik).
The thermosensors for the osteotomies were positioned at 3 mm depth in the bone. They were 1 mm from the eventual piezosurgical cut ( Fig. 4 ). For the osteoplasties the sensor was placed 5 mm apical to the alveolar crest on the buccal side. The channel for the sensor was prepared with a 0.9 mm wide rose-head bur (Dentsply, Maillefer, Ballaigues, Switzerland). To improve heat conductivity, the tips of the thermosensors were wet with a mixture of Vaseline and saline solution in a 1:1 ratio (Vaseline, Hoga Pharm AG, Schlieren, Switzerland, and Ringer’s solution, Braun).
Before every piezosurgical cut, the accurate temperature measurement of the sensors was controlled by measuring the warming bath temperature and comparing it with data from the thermometer integrated in the heating installation of the bath.
The muscles, mucosa and periosteum were removed buccally in the region of the angle of the jaw for the osteotomies and in the region of the molars for the osteoplasties. In the osteotomies, four repetitive measurements per jaw (24 in total) and per insert were performed with 20 °C warm liquid coolant and a corresponding 24 measurements with 10 °C warm coolant. The piezosurgical cuts were prepared to a length of 10 mm and a depth of 5 mm ( Fig. 4 ). The piezo handpiece was guided by hand, as recommended by the manufacturer, with as little pressure as possible and pulling, intermittent movements.
For the osteoplasties, 20 measurements per insert were performed (four repetitive measurements on five jaws). The temperature of the coolant was always 20 °C. To reflect resective periodontal or peri-implantitis therapy, bone was removed cervically in an apical direction with intermittent, pulling movements. The tip was advanced up to 1 mm from the sensor ( Fig. 5 ).
In order to determine sensor depth, digital volume tomography (DVT; Scanora ® 3D, Soredex, Tuusula, Finland) was performed in the area of the angle of the jaw of the pig mandibles and gauged in relation to the average cortical bone thickness. Following the tests, DVTs of all the processed jaws were taken and the exact cortical bone thickness in the area of the piezosurgical cuts was determined.
Statistics
The difference between the highest measured temperature for one measurement and bone temperature was used for the statistical analysis. To analyse correlations between the temperature increases and the different influential variables, such as temperature of irrigating solution, cortical bone thickness and working instruments, mixed-effects models were used whereby the logarithmized values of temperature increase were analysed. Geometric mean ratios and 95% confidence limits were calculated from the corresponding back transformed contrasts of the mixed-effects models. The Wilcoxon rank sum test was used to analyse the significance of the length of time the temperatures were above the threshold. The level of significance was set at 0.05. All the calculations were made using the statistics program R (Version 2.12.2, The R Foundation for Statistical Computing, Vienna, Austria).
Results
Osteotomies
Intraosseous temperature increases occurred with all three-bone saws. Particularly with the largest insert OT7, the median values for maximum measured temperature increases were close to the threshold of 47 °C when body temperature of 36 °C was added: 10.9 °C for 20 °C cooling and 10.45 °C for 10 °C cooling ( Fig. 6 ). Analysis with mixed-effects models revealed a marked difference between the largest and smallest insert (OT7 vs. OT7-S3) in terms of temperature increase (geometric mean ratio 0.48 (95% CI: 0.26, 0.92); p = 0.026).
Lowering coolant temperature did not cause any difference with regard to maximum temperature generation (geometric mean ratio 0.9 (95% CI: 0.68, 1.2); p = 0.47). The maximum measured temperatures appeared only briefly. There were sharp temperature increases when the piezosurgery tip was level with the sensor and hence at a distance of 1 mm from the sensor. Accordingly, the temperature fell rapidly as that distance increased. The average times during which the threshold of 47 °C was exceeded were all less than 10 s ( Table 1 Table 1 ). The longest maximum time measured when the threshold was exceeded was 29 s, well below 1 min, and it was measured with the largest insert ( Table 1 ). When the time factor was incorporated, all the temperature increases were on average below the threshold of 47 °C for 1 min.
