The invention generally relates to a device for fragmenting substances.
Such a device is used in medicine in the context of electro-hydraulic lithotripsy for fragmentation of hard tissue and of concretions, for example intracorporeal stones, for example kidney stones, urethral stones, bladder stones, gall stones, salivary stones, etc., bone, cartilage, lens material in the eye, bone cement, thrombi, deposits and calcifications.
Moreover, such a device can also be used for stimulation of tissue.
Although the invention is described below with reference to its medical applications, a device of the type mentioned at the outset can also be employed in technical and industrial sectors, for example for fragmentation of lime deposits in pipeline systems.
The electro-hydraulic lithotripsy (EHL) probe known from document U.S. Pat. No. 5,425,735 comprises an electro-hydraulic converter arranged in the probe head. The electro-hydraulic converter has two electrodes which are immersed in a liquid in a liquid chamber within the housing of the electro-hydraulic converter. Arranged distal by and at a slight distance from the electrodes, there is a shock-transmitting element designed as a piston which can move axially in the housing from a proximal end position (rest position) to a distal end position. By applying a current impulse to the two electrodes, the electrical short circuit brings about an explosion-like formation of cavitation bubbles in the conductive liquid in the liquid chamber, and these cavitation bubbles lead to an shock-like pressure wave which moves the shock-transmitting element in shock-like manner in the distal direction away from its proximal rest position, such that a distal end face of the shock-transmitting element can impact the substance that is to be fragmented, in order to crush this substance.
The device known from the aforementioned document to this extent represents an improvement on conventional EHL probes in which no shock-transmitting element is provided and in which the fragmentation is brought about with the aid of the pressure wave that arises as a result of the explosion-like spark discharge on the electrodes. Such a device is known from EP-A-0 640 316, for example. However, the pressure waves acting not just axially but in all directions have the disadvantage of causing soft-tissue trauma, for example as a result of burns caused by sparks during short-circuiting of the electrodes and mechanical defects such as perforations.
The shock-transmitting element of the EHL probe known from U.S. Pat. No. 5,425,735 avoids this problem, since the pressure waves propagating in all directions are channeled axially in the liquid chamber. The shock-transmitting element is moved from the proximal end position to the distal end position by the explosion-like propagation of the pressure waves (shock waves), the distal end position being defined by an abutment which is arranged on the housing of the converter and which, in the known device, is formed by a radial annular surface in the area of the distal end of the housing. Provided on the shock-transmitting element there is a counter-abutment which is designed in the form of a radial annular surface in the area of the proximal end of the shock-transmitting element.
Arranged between the counter-abutment and the abutment there is a compression spring which bears at its distal end on the annular surface on the housing and bears at its proximal end on the annular surface on the shock-transmitting element. The compression spring is thus coupled into the running path of the counter-abutment. The compression spring has the purpose of moving the counter-abutment back again from the distal end position to the proximal end position.
Such a design of a restoring mechanism for the shock-transmitting element has disadvantages, however. Since the compression spring is arranged within the entire possible stroke range of the shock-transmitting element, the following situations may arise.
If the spring is hard, a situation may arise in which the shock-transmitting element executes only a partial stroke of its maximum path of movement between the proximal end position and the distal end position, such that the shock-transmitting element strikes against a “soft” abutment. It is not then possible, however, to optimally accelerate the shock-transmitting element and thereby transmit the kinetic energy of the shock-transmitting element to the fragmentable substance as shock-like as possible and starting from the maximum speed.
If the spring is soft, on the other hand, so that the shock-transmitting element can run as far as the distal end abutment, the spring nevertheless has to be completely compressed, as a result of which part of the kinetic energy imparted to the shock-transmitting element by the pressure wave is dissipated by the spring, which leads to deceleration of the shock-transmitting element. In this case too, therefore, there is no optimum shock-like transfer of energy to the substance that is to be fragmented.
Moreover, DE-A-195 10 920 discloses a device for crushing concretions in the medical sector, said device having a probe whose distal end points in the direction of a concretion that is to be crushed, and whose proximal end is accommodated in a housing in a guide. This housing is thus arranged at the proximal end of the probe shaft, such that the shock wave is generated outside the body. Moreover, the shock wave is not generated directly by means of an electro-hydraulic converter but instead by means of an electromagnetic linear motor.
At the proximal probe end, the probe shaft has an impact surface which is impacted by a mass accelerated in the electromagnetic field in order to introduce impulses into the probe, which impulses are conveyed through the probe in the direction of the concretion that is to be fragmented. At an inlet of a probe guide lying in an abutment surface, the impact surface of the probe forms a defined starting state to which the probe can be returned after each shock impulse. The return of the probe takes place via a damping element in the distal area of the probe, which element, although not being completely compressed, nevertheless represents a soft abutment for the probe, which represents the shock-transmitting element. This is because the action of the damping element is a deceleration of the probe.
A further disadvantage of this known device lies in the fact that the shocks are generated outside the body. Because of the probe shaft length, which has to transmit the shock, and the guiding thereof in an endoscope, for example, or in a hollow organ, friction occurs which greatly attenuates the mechanical shock and thus reduces the fragmentation effect. In certain applications, the probe shaft moreover adopts a curved course inside the body, as a result of which the energy dissipation of the shock inside the probe is further intensified. In addition, the probe shaft cannot be made sufficiently flexible to maintain its suitability for shock transmission.