The Physics and Engineering of Laser Ablation of Hard Tissue for Osteotomy Purposes

Till today bone surgeries (osteotomies) are performed with conventional mechanical tools like drills and saws. Because of massive contact pressure, vibration and friction these conventional methods carry great risks of damaging the surrounding tissue as well as the bone itself in form of mechanical trauma and thermal damage even if performed with great care. Moreover, a broadening of cuts and a subsequent deposition of metal shavings and bacterial contamination are negative side effects. The more complicated the anatomical location and the closer the surgery is to nerves and vascular structures the higher the risk of collateral damage. A beneficial alternative presents contact-free laser osteotomy which offers blood-loss reduction, vibration-reduction, a free choice of the cutting geometry, a small operation field and the prevention of massive bone flour and metal abrasion. The successful development of a laser osteotome was so far limited by technological and engineering drawbacks. Now, this next step for osteotomies is feasible with the introduction of computers in surgical navigation, pre-operative planning, medical imaging, medical robotics, software development and suitable laser sources. This thesis describes the engineering of a suitable optical system, i.e. a cutting laser head, and examines the resulting laser light and hard tissue. First and foremost, the laser head must ensure a most effective tissue ablation without inducing any mechanical or thermal damage to the treated or surrounding tissue. This is achieved by beam shaping and guiding elements and the development of a tissue cooling system, i.e. a nozzle array. Furthermore, it must have a large working distance to the operating field, be light enough not to overload the medical robot it is attached to. Both are achieved through a compact and lightweight laser head design. Since the work for this thesis was part of the development of a commercial computer assisted laser osteotome, aspects like safety and ergonomics were taken into account, e.g. Visual cameras for intervention observation and recording. During the development of the laser head, the interaction of laser light and hard tissue was investigated. After developing the laser head to a state that it was used successfully for three animal studies, in depth analyses for the resulting minimized heat induction during the laser osteotomies were investigated.