[Elastic modulus measured by spherical indentation used toward the clinical application of the brain stiffness]

Palpation of brain stiffness is one of techniques that leads to successful neurosurgical procedures. In order to evaluate brain stiffness quantitatively, we studied the potential clinical applicability of a spherical indenting tactile sensor.

The sensor had a spherical rigid indenter (diameter=5.0 mm; contact pressure=1.0 gf/mm2), and the indenter was rapidly pushed and pulled at a constant speed by a computer-controlled motor. The pressure-depth hysteresis curve was obtained using the sensor, and the shear elastic modulus (G) was calculated on the basis of the Hertz contact theory. We adopted the G-value at the maximum depth (G_max) as an indicator of brain stiffness.

First, to calibrate the sensor, we investigated the elasticity of silicone plates. The optimal settings for clinical application was an indenting speed of 1.5 mm/s and an indenting maximum depth of 2-3 mm. Next, we measured the elasticity of a decompressive site in 7 patients who had been stable for more than 21 days after undergoing decompressive craniectomy. The G_max of the decompressive site was 1.71 ± 0.75 kPa. Finally, we measured the intraoperative brain elasticity in a case of brain tumor with severe brain edema. The transdural elasticity of the edematous brain was G=4.87 kPa, and the direct elasticity of the brain surface decreased to G=4.34 kPa after dura incision.

The spherical indentation method for measuring brain elasticity seems applicable to neurosurgical procedures.