## 4 Discussion  
Thermal osteonecrosis is a complex and multistage process, in which enzymatic and membrane proteins are destroyed, the osteoclastic and osteoblastic activity is significantly lowered, and dehydration as well as desiccation of the cell occurs. All these processes may lead to cell death, impaired healing, prolonged remodeling, and early implant failure. The first symptom of osteonecrosis is usually blood flow reduction, followed by osteoclastic resorption, which can no longer play their role in keeping a healthy structure cortical and spongy bone. Accurate temperature, which causes thermal damage, remains unclear [6; 15]. The aim of the study was to measure the damage done during drilling to the tissues surrounding the implantation site in comparison to the drilling system, its diameter, drilling temperature, and cooling methods. The material was examined under a fluorescent microscope in order to receive the results, which accurately illustrated the thermal damage of the bone. Many factors have been described as the reasons for temperature rise during the dental implant site preparation including bone quality and its cortical thickness, drill type (material, design, diameter and wearing), drilling speed, feed rate and depth, pressure put while drilling, cooling approach and pre-drilling [6]. Previous studies showed that when the temperature during dental implant site preparation rises to 47 \({}^{\circ}\)C and remains so for 1 min, the bone is resorbed and replaced with fat tissue. We may therefore hypothesize, that a temperature under 47 \({}^{\circ}\)C combined with drilling time not exceeding 1 min is safe and protects against osteonecrosis. On the other hand, they revealed that heating the bone during the implant site preparation to the temperature of 50 \({}^{\circ}\)C caused resorption of the bone in the amount of 30% [16]. Many studies have been conducted in order to determine which cooling system and cooling conditions are the most efficient to avoid thermal necrosis. In general, cooling systems are divided into internal and external coolant approaches. Internal cooling systems deliver coolant (fluid or air) through the channel in the drill, whereas an external cooling system delivers it on the outer side of the drill or directly to the drilling point using fully automatic systems or a simple manual syringe [17]. As the depths are unknown for their infiltration sites, which are done by the external cooling systems, there is a limited possibility of cooling in the area where the drill has contact with the bone. In 1975, [PERSON] and [PERSON] introduced the first internally cooled drills, so the coolant could also achieve the top of the rotating drill [18]. What is interesting to note is that another study performed many years after introducing the invention above-mentioned showed that more heat was generated in the superficial part of the bone than at the bottom of the drilling cavity. This means that the external delivery of the coolant should remain a sufficient method of lowering the temperature while drilling [19]. Therefore in our study, an external cooling system was used and evaluated. It was clearly visible that  
Figure 9: The thickness of the compressed tissue area at the top drill [\(\mu\)m]. the lack of any coolant significantly increased the bone tissue destruction significantly in all three drill rotation speeds and both drilling sites (Figure 1 and Table 2). Another extremely significant cooling practice, done while drilling, is irrigation and the amount of coolant. Several studies have compared the efficiency of lowering the temperature depending on the presence of coolant and its amount. [PERSON] compared the drilling with and without coolant, which showed a significant increase in temperature in cases without [20]. [PERSON] et al. compared irrigation volumes of 12 mL/min and 30 mL/min and revealed no significant difference between them [21].