Modeling Ultrafast Laser Ablation on the Glenoid Bone for the Fitting of a Prosthetic Screw

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In order to fit prosthetic screws, mechanical drilling of the bone has been the norm since the development of modern surgery. However, bone reabsorption, hyperthermia and thermo necrosis could occur depending on the exposure time to the drill and elevated temperature in the surrounding bone due to friction and drill pressure. Non-contact ablation using a CO2 laser can potentially increase the accuracy of the bone drilling and reduce the amount of friction applied to the bone, thus reducing the thermal effects on the surrounding tissue. A model of laser ablation of the human glenoid bone was done on COMSOL with a governing equation of transient state heat transfer from laser to bone. This heat transfer was then correlated to bone loss. According to previous studies, bone disintegration occurs at approximately 613 Kelvin. The bone’s geometry was simplified to a 2-D axisymmetric cylinder. The two domains of the 5mm deep screw region were also 2D-axisymmetric cylinders with varying radius and depth. The phase field model was used to take into account the ablation process of bone. Because the bone essentially disintegrates into “gas-like” particles after reaching this temperature, the phase field model was used to determine the downward velocity of the “air-bone” interface. An adaptive mesh was also developed to move in conjunction with the moving interface. The laser pattern consisted of consecutive concentric cylindrical shells, with the first pulse at the center of the targeted ablated site and the following pulses were cylindrical shells of increasing area. However, because the radial scanning speed was extremely small compared to the pulse duration, concentric cylindrical shells were assumed to occur simultaneously, creating a constant area of laser ablation for each of the two screw domains. Because the CO2 laser did not have a significant penetration depth as the heat generated by the laser was absorbed mainly at the bone surface, input laser heating was modeled as constant flux. Finally, the modeling results for laser ablation were compared to factors in mechanical bone drilling. By varying the input flux of the laser within a range of 300 W/cm2 to 1200 W/cm2 and measuring the total ablation time and the total damage in the surrounding tissue, an optimal flux range between 1050 W/cm2 and 1100 W/cm2 was found to minimize the end time (approximately 0.55 seconds) and thermal damage to the surrounding bone (3.5 mm3). Compared to mechanical drilling, laser ablation with the optimized flux value was much faster than mechanical drilling which can drill at approximately 0.33 mm per second. Generally, less surgery time decreases a patient’s risk when under anesthesia. An increased amount of thermal damage may also lead to refractures, loosening of the prosthetic and permanent loss of tissue function. As laser ablation minimized both these parameters, this model demonstrates that laser ablation of bone is a viable method to consider in future surgical orthopedic work.

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Modeling Ultrafast Laser Ablation; Glenoid Bone; Prosthetic Screw


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