Ultrashort laser ablation of load bearing and skull cortical bone tissue: A comparative study

Full text: Ultrashort laser pulses have proved to be a successful tool in many medical applications, however they have yet to experience widespread use for hard tissue ablation. Several studies have been performed to study ultrashort pulsed laser ablation properties of bone tissue and have yielded promising results especially for applications in microsurgical procedures such as in the ear, spine, sinus and skull. Lasers can overcome several problems associated with mechanical tools such as such as drill bit wander and excessive temperature rise (and tissue death) as a result of friction at the tool-tissue interface. Laser pulses in the micro- and nanosecond regime have been found to produce thermally generated defects in bone tissue that result in carbonization around the ablation site and lead to increased healing times paired with a higher risk of infection and bone necrosis. Femtosecond lasers are an excellent candidate for bone cutting and machining due to their 'cold cutting' mechanism; ablation of hard dielectric materials can occur with minimal thermal or mechanical side effects. One of the most important parameters for characterizing laser ablation is the ablation threshold, the minimum amount of energy per unit area required to remove material. Investigations have been conducted in order to determine this parameter in cortical bone and report range of values from ~3.3 Jcm^-2 to 0.6 Jcm^-2. However, few studies have been performed comparing the ablation parameters between cortical bone in load bearing and skull structures. Our previous studies in ex vivo bovine and ovine cortical bone samples in load bearing structures identified the ablation threshold, incubation parameters and optimal factors that affect the laser cutting rate. These results have shown promise with increased accuracy, prevision and no heat damaged to the surrounding structures whilst achieving comparable material removal to conventional microsurgical tools. These results on cortical bone are not necessarily representative for femtosecond laser surgery of all types of bone; the structural characteristics of bones depend upon their location and function within the animal. This study identifies and compares the ultrashort pulsed laser ablation properties of load bearing bone and skull bone using a Ti:Sapphire (Coherent Legend Elite) femtosecond laser (τ = 110 fs, Repetition Rate = 1 kHz, λ = 800 nm). Using the diameter regression technique, the ablation threshold of cortical load bearing and skull bone is determined and incubation effects characterized. 1 mm ablation cut lines have been utilized to characterize the ablation rates and achievable depths as a function of incident laser power. Ablation features dimensions, volumes and quality of cut and collateral tissue damage were measured using several methods, optical profilometry, scanning electron microscopy and optical microscopy. (author)