The interaction of shock waves with biological tissue - momentum transfer, the key for tissue stimulation and fragmentation.

Background: Shock waves in medicine have gained enormous importance and have spread since 1980, and the first kidney stone was successfully fragmented in a patient in Munich. Meanwhile, the spectrum of medical applications of shock waves ranges from powerful fragmentation of kidney stones to diverse indications such as wound healing, chronic pelvic pain, spasticity, erectile dysfunction, and others, to neuro-stimulation in the context of Alzheimer's disease. A comprehensive working mechanism for this diverse field of medical indications is still missing.

Objective: Investigation of the physical basis of the working mechanism of shock waves in medical applications.

Methods: We developed a model based on the mechanical forces generated by the momentum transfer at the acoustic interfaces of different layers of biological tissue. The generated forces are strong enough to crash brittle material and provide an adequate mechanical stimulus to activate mechano-transduction and mechano-sensory-transduction with nerve stimulation, thereby affecting the neural memory function of the central nervous system.

Results: The key to generating appropriate forces in the millisecond range is the mechanism of momentum transfer at the interfaces between tissue layers with different acoustic impedances. According to Newton's laws of motion, a change in momentum (momentum transfer) generates force F = d P /d t . The inherent shear forces can stretch biological membranes to release biomolecules such as vascular endothelial growth factor and nitric oxide. A most favorable feature of this mechanism is the selective effect on soft tissue interfaces and small tissue inhomogeneities to generate small forces in the range of few (≤10) Newton to stimulate tissue and nerve cells, while the same shock wave can generate forces ≥200 Newton and more on hard tissue interfaces such as bones or stones.

Conclusion: The mechanism of momentum transfer is the basis for mechano-transduction and mechano-sensory transduction. It offers the opportunity to stimulate peripheral nerves and modify the motor reflex patterns of "pathologic" reflexes by hyper stimulation. The new technique of transcranial pulse stimulation may be based on direct stimulation and reactivation of neurons in the brain. Momentum transfer is the basic physical mechanism and the initiator for successive biological processes in medical shock wave therapy.