Bone Tissue Ablation with sub-µs Pulses of a Q-switch CO₂ Laser: Histological Examination of Thermal Side Effects

Abstract

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The goal of this study is an in vitro evaluation of thermal side-effects by the application of short sub-µs CO₂ laser pulses in combination with an air–water spray on different types of bone tissue. A mechanically Q-switched CO₂ laser delivered 300 ns pulses at 9.6 µm wavelength, which were focused down to a spot size of 440 µm on the tissue (a corresponding energy density of 9 J/cm²). Bone samples (blocks from pig femur, rib, or cartilage) were moved through the beam repeatedly until 1–5 mm deep cuts were produced. An air driven water spray was applied to prevent the tissue dehydration. Subsequent visual and histological examinations revealed no carbonisation, melting traces or fissuring of the tissue. An extremely narrow, 2–6 µm thick thermally altered layer was observed at the cut border in compacta and cartilage. No accumulation of the thermal damage occurred with increasing cut depth. Laser incisions in trabecular tissue were accompanied with a 100–200 µm thick zone of thermal necrosis in bone marrow. The difference from compacta and cartilage can be explained considering the particular character of the spreading of the ablation products in the trabecular meshwork. Minor thermal side effects make the Q-switched and probably other short pulsed CO₂ laser systems interesting for hard tissue surgery.