Supplementary Material for: Acoustic Emission Foreshocks and Aftershocks from Skull Fracture Caused by Surgical Clamping

Abstract
Background and Objectives: The clamping force for surgical head immobilization must be sufficiently large enough to prevent slippage but not to induce fracturing. Motivated by an unmet need to detect skull fractures during surgery to trigger remedial action, we sought to demonstrate a method of viable measurement and interpretation of acoustic emissions (AE) generated by a fracturing skull prior, during, and after a fracture.
Methods: Escalating clamping force via skull pins affixed in an Integra Life Sciences Mayfield® skull clamp was applied to a formalin-fixed cadaveric head. Variable and pathologic skull thickness was simulated by drilling the skull to reduce thickness at pin locations on the frontal or temporal bone. AE was monitored via an in-house developed device.
Results: This method produced a mixture of “punch-out type” skull fractures with some occurrences of larger area “cave-in” type fractures. The recorded AE waveforms, amplitudes, and temporal patterns showed the skull can fail not only immediately upon application of clamping, but also in delayed manners seconds or minutes after force application. A stereotypical, escalating rate of event occurrence and amplitude was detectable prior to delayed fractures (foreshocks) up to a peak event correlating with macroscopic fracture. A stereotypical hyperbolic decay of event rates was detectable immediately post-fracture (aftershocks). Other acoustic sources like drilling were distinguishable.
Conclusion: Skull fractures produce stereotypical AE, possibly identifiable intra-operatively. Detecting patterns indicative of impending or recent fracture may allow for immediate intervention, avoiding severe complications. This represents the first reported evidence of detectable foreshock and aftershock AE sequences from bone fracture and points to striking parallels with seismicity generated by earthquakes, thus enabling tools from geophysics to be applied to detect imminent and recent bone failure.