## 5 Thermal Necrosis: Definition and Mechanisms  
Thermal necrosis occurs in bone tissue around 47 \({}^{\circ}\)C if said temperature is sustained for a minimum of 60 s [24]. This is a critical threshold during orthopedic procedures because several key parts of the interventions generate heat in the surrounding bone tissue, such as drilling or utilizing PMMA bone cement [24].  
During polymerization, the peak temperatures reached can exceed 56 \({}^{\circ}\)C, and as such, this temperature poses a risk of thermal necrosis to the adjacent bone tissue [35]. The duration and extent of the exposure are important factors in determining the degree of necrosis. Short-term exposure to high temperatures can cause immediate cell death (5 s at 50 \({}^{\circ}\)C); longer exposures or higher temperatures exacerbate the damage [25; 35].  
The relationship between temperature and the duration of exposure is logarithmic, meaning that increasing the temperature, even slightly, can significantly lower the time necessary for necrosis to occur. For example, exposing the bone tissue to a temperature of 50 \({}^{\circ}\)C for over a minute has the same effect as exposing the same bone tissue to 60 \({}^{\circ}\)C for a few seconds, each resulting in significant tissue damage and thermal necrosis [35].  
Figure 4: The uses of PMMA bone cement in orthopedics.  
The main culprit of thermal necrosis occurring during the polymerization of PMMA bone cement is the exothermic nature of said reaction [21]. This generates a significant amount of heat that causes local temperatures at the cement-bone interface to increase, this process being detrimental to the bone tissue. The polymerization process of PMMA bone cement can lead to local temperatures exceeding 70 \({}^{\circ}\)C at the interface, this leading to local tissue damage. The extent of the damage is influenced by various factors, such as the thickness of the cement mantle and the volume of the cement used during fixation [36; 37; 38; 39; 40].  
Another factor that influences the extent of thermal necrosis is the exposure time of the bone tissue to elevated temperatures. Even if the peak temperatures of the polymerization process are lower, prolonged exposure to moderately high temperatures can still lead to the necrosis of the bone cells [26]. Aside from the thermal effect of the polymerization process, certain residual unreacted monomers can lead to a form of chemical necrosis [40]. These monomers are toxic to the bone cells, aggravating the damage caused by the high temperatures [36; 41]. The last main factor that can influence the extent of thermal damage occurring at the bone-cement interface is the type of cement used and the preparation method. High-viscosity cements generate more heat during polymerization, as do thicker cement mantles [36; 42].  
In addition to reducing the exothermic peak temperature during PMMA polymerization, it is important to evaluate how these methods impact the degree of polymer conversion. Insufficient polymer conversion results in a higher residual monomer content, which is associated with compromised mechanical properties and increased cytotoxicity. Current evidence suggests that methods such as pre-cooling and the inclusion of phase-change materials (PCMs) are effective at reducing thermal risks but may influence polymerization dynamics. For instance, studies indicate that pre-cooling can prolong the polymerization time, which might delay complete conversion. Conversely, nanoparticle additives like magnesium oxide have shown promise in supporting uniform polymer conversion by promoting better heat distribution. These findings underscore the need for a balanced approach that prioritizes both thermal safety and polymer integrity (Table 1).