Thermal injury from mastoid bone burrs.

Marked bone temperature elevations and surface osteocyte necrosis result from cutting and diamond burrs used without continuous irrigation. Temperatures generated may injure the facial nerve even if drilling is carried on a millimeter or more distant from the nerve. Irrigation of the operative field maintains bone and nerve temperatures at safe levels. Cutting and diamond burrs obey different thermodynamic principles. Higher rotation speeds, greater applied pressures and greater surface contact area increase cutting burr thermogenesis. Burrs with more flutes generate greater heat than those with fewer flutes. Diamond burr heat generation varies in proportion to applied pressure, but is independent of rotation speed or contact area. Haversian canal blood vessels do not contribute to postoperative revascularization. Rather, vessels immigrate from adjacent intact periosteum, penetrating arterioles and exposed marrow. Surface thermal injury does not impede revascularization and should not impair the healing of tissue grafts applied to the bone surface.