Optimization of Laser Ablation Parameters for Lumbar Discectomy
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Ashraf, Shaumik; Chan, William; Delgado, Robert; Hassan, Mohamed
Lower back pain, or lumbar pain, is a potentially debilitating chronic condition that will affect an estimated 80% of individuals in their lifetime. Lumbar pain can be caused by herniated lumbar discs, which is a painful ailment that occurs when the nucleolus pulposus or annulus fibrosus of an intervertebral lumbar disc is displaced beyond the intervertebral space and pinches the spinal nerve. The resulting nervous stimulation produces tremendous localized pain in the lower back and, in some cases, referredpain in the legs or arms. Current methods that can treat herniated discs include physical therapy, epidural steroid injections, and surgical removal. The former two solutions only treat mild cases of lumbar disc herniation, and the latter solution has along recovery time with postoperative complications. The application of laser discectomy to the treatment of lumbar pain associated with herniated discs is not novel; lasers have been used as a minimally-invasive treatment of moderately severe cases of herniated discs since the late 20thcentury. This study aims to analyze and improve minimally invasive laser ablation to remove lumbar disc tissue that has been displaced from its proper position. Concentrating an infrared laser beam on the herniated part of the intervertebral disc causes vaporization of the tissue with unparalleled precision. This results in relieved pressure on the spinal nerve and alleviation of pain. Two parameters in laser surgery are power density and separation between pulses, which must be manipulated to minimize surgery time and reduce collateral thermal affliction.Using COMSOL MultiphysicsⓇa herniated intervertebral disc and adjacent vertebrae was designed for laser ablation and transient heating analysis. A 3-dimensional Cartesian geometry was adopted and modeled with conductive heat transfer coupled with volumetric laser-heat generation determined by optical diffusion. The laser ablation physics was modeled by implementing a velocity at nodes that have reached the ablation temperature.The method of evaluating the effectiveness of the simulation is through determination of the mass loss via ablation due to the total energy transferred. This would ensure safe ablation of tissue during a laser discectomy surgery and lower the risk of protein denaturation, excessive water loss (reducing intervertebral disc shock absorption), and irreversible changes to tissue functionality as a result of changes in material properties. As a result, post-operative complications can be evaded and overall treatment of lumbar disc herniation (LDH) will become more efficient. Furthermore, this computational modeling approach was chosen in order to study the effect and impact of parameter variation on the laser ablation procedure, and to computationally optimize the length of the procedure to allowfor mass loss with minimal thermal damage to healthy tissue.Results of this study indicate that mass loss begins at approximately 6.2 seconds, after the temperature of the tissue has been raised to the ablation temperature. Sensitivity analysis of the model revealed that the mass loss is most dependent on the density of the intervertebral tissue and the power of the laser applied. Optimization of the laser ablation surgery occurs at 10.9 seconds, when the difference between the amount of herniated intervertebral damaged and the amount of healthy intervertebral disc and vertebral bone tissue damaged is maximized. More anatomically accurate computational models can be generated by using meshes of CT scans of the intervertebral disc, manually building various types of hernias and then running the same analysis of finding optimal laser parameters. Non-negligible vapor flow resulting from laser ablation can also be incorporated into future models.
Laser Ablation; herniated disc; lumbar pain; intervertebral disc; ablation