Instrument for minimal invasive surgical operations, comprising a rod or catheter and a tip mounted on the rod or catheter with an elastically deformable element The invention relates to an instrument for minimal invasive surgical operations, comprising a rod or catheter and a tip mounted on the rod or catheter with an elastically deformable element, wherein the elastically deformable element is arranged to apply a contact force with the tip on a tissue that is subjected to the surgical operations.

Although the invention is not restricted thereto, in the following the invention will be particularly elucidated with reference to an application in an instrument for tissue ablation in the heart of a patient.

People suffering from cardiac arrhythmia, irregular heartbeat, can be helped by undergoing a heart ablation procedure. In this kind of procedure, the abnormal electrical pathways that cause the irregular contractions of the heart are interrupted by forming scar tissue. Usually, either radiofre- quency ablation or cryoablation is used to scar affected areas. A heart ablation procedure is a relatively long procedure {taking more than 4h) wherein a flexible catheter is fed through a vein inside the heart. The catheter tip approaches the inner side of the heart wall. Ideally, a path of scarred tissue is

created such that a continuous barrier interrupts the faulty electrical pathways. In practice, this is done by applying multiple adjacent scar dots. In this process it is of paramount importance that the contact force between the catheter tip and the heart is kept below a safety value. Exceeding this force could cause puncturing of the heart wall causing serious complications. On the other hand, if during the time that the ablation instrument is activated (a few seconds) the contact force is too low or the contact separates, the procedure can become ineffective. Considering the cardiac and respiratory motions during the procedure, keeping the force between the limits is far from trivial. The electrophysiologist that per ^¬ forms the procedure can monitor the contact force which is measured at the catheter tip and displayed on a screen. Howev- er, the control that he has over the position and force of the tip is limited because of the long flexible rod or catheter between the tip and the handle. Furthermore in the prior art following the heart movements at the speed of the heartbeat is practically impossible. This combination of challenges and risks makes this procedure a lengthy and technically very challenging intervention.

US2005/0267332 discloses a spring tip flexible electrode, and a method for using that electrode for tissue abla- tion in the heart. The spring tip flexible electrode comprises an enshrouded flexible electrode for applying ablation energy to target the tissue for the formation of spots or continuous linear lesions. The spring of the spring tip may comprise compressible coils, compressible mesh, or compressible bellows. The spring provides axial suspension and is capable of axial compression and extension, and is flexible enough for deflection and bending. The axial suspension of the spring tip facilitates the desired contact between the electrode and the tissue surface and facilitates enhanced tissue contact in dif- ficult environments. A disadvantage is however is that due to the linear characteristics of the spring the contact force may exceed a maximum value, and that the contact force varies together with the movement of the heart so that contact between the electrodes and the heart tissue is not at all times guar- anteed.

The invention has as an object to address these concerns and proposes to that end an instrument for minimal invasive surgical operations, comprising a rod or catheter and a tip mounted on the rod or catheter with an elastically deform- able element, wherein the elastically deformable element is arranged to apply a contact force with the tip on tissue that is subjected to the surgical operations, wherein the elastically deformable element is designed to provide a substantially constant force to the tip when a distance between the tip and the rod or catheter that supports the tip is varied within a predefined range. Preferable embodiments of the instrument of the invention are proposed according to the features of one or more of the appended dependent claims. According to the feature that the elastically deform- able element is designed to provide a substantially constant force to the tip when a distance between the tip and the rod or catheter that supports the tip is varied, the deformable element provides the tip with zero-stiffness when moved in a range in a longitudinal direction of the rod or catheter.

It is further preferred that the deformable element autonomously resists bending in a transverse direction. This avoids the necessity to apply auxiliary features for maintain- ing directional stability of the tip mounted on the deformable element .

One further desirable feature is that the deformable element has a monolithic structure.

In accordance with the invention it is preferable that the elastically deformable element is an endless tape loop spring with predefined parameters relating to at least its material, combination of materials, or positions in the tape loop spring where such material is missing, the tape loop spring width, the tape loop spring material thickness, and the tape loop spring's traverse curvature, wherein the tape loop spring is embodied with two elongated parts in or parallel to a longitudinal direction of the rod or catheter, which elongated parts convert into each other by bends at opposite sides of the elongated parts, and wherein each bend connects the elongated parts to each other, and wherein at least one of the said parameters of the endless tape loop spring is varied along the circumference of the tape loop spring, so that variation of the distance between the tip and the rod or catheter causes a circumferential round going motion of the endless tape loop spring which causes that the bends travel along the circumference of the endless loop while maintaining their position in or parallel to the longitudinal direction of the rod or catheter. In this construction of the instrument of the invention the said variation of at least one of the parameters of the endless tape loop spring along the circumference of the tape loop spring is essential to provide a substantially constant force to the tip when (within a predefined range) a distance between the tip and the rod or catheter that supports the tip is varied. The motion of the endless loop is prefera- bly self-constrained to avoid the need of any additional guid ^¬ ing mechanism. This is preferably achieved by providing that the material of the endless tape loop spring has a transverse curvature in the direction of its width.

As already mentioned it is possible to vary at least one of the parameters in the circumference of the endless tape loop spring, notably one or more of the tape loop spring' s material, combination of materials, or positions in the tape loop spring where such material is missing, the tape loop spring width, the tape loop spring material thickness, and/or the tape loop spring's transverse curvature.

Preferably however the width of the tape loop spring is varied in the circumferential direction of the tape loop spring, which from a constructional point of view is most easy to realize.

The construction of the instrument is then preferably such that the tape loop spring has an unloaded position wherein respective parts of the tape loop spring occupy a preferential position with respect to each other, and wherein then at or near the location of the bends the width of the tape loop spring is smaller than at the two elongated parts between the bends. One possible way of realizing this is that the endless tape loop spring has a width with tapered portions. This is relatively easy to implement and causes that the tapered por- tions have their smallest width at or near the bends when the tape loop spring is in the unloaded position, which corresponds with the lowest amount of potential energy in the tape loop spring. Correspondingly a broader part of the tapered portions are at the bends of the endless tape loop spring when the tape loop spring is in a predefined loaded position. Then the potential energy in the tape loop spring is at a slightly higher value than in the unloaded position.

When it comes to preferable embodiments of the tapered portions, it is found that best results are achieved when the tapered portions are provided with an imaginary top angle or subtended angle in the range of 95 - 135°.

As already mentioned a further preferable feature is that in a transverse direction of the endless tape loop spring, the spring has a curvature with a predefined radius of curvature. This is a simple measure to provide the spring with a high stiffness in the transverse direction so as to resist bending in said direction and to provide directional stability to the tip.

Advantageously the predefined radius of curvature is preferably selected at a value corresponding to the rod or catheter diameter. The elongated parts of the endless tape loop spring are then always at an approximate distance from each other corresponding to the diameter of the rod or cathe- ter that supports the tip and the endless tape loop spring.

The invention will hereinafter be further elucidated with reference to the drawing of an exemplary embodiment of an apparatus according to the invention that is not limiting as to the appended claims .

In the drawing:

-figure 1 shows schematically a heart in which an instrument according to the invention is applied to provide an ablation procedure to certain tissue of the heart;

-figure 2 shows a top part of the instrument of the invention providing a view at its rod or catheter, at an elas- tically deformable element supported by the rod or catheter, and at a tip supported by the elastically deformable element; and

-figure 3 shows a force - distance diagram of the tip that is moved with reference to the rod or catheter.

Whenever in the figures the same reference numerals are applied, these numerals refer to the same parts.

In figure 1 a heart is schematically shown, as well as its inferior vena cava through which an instrument 1 ac- cording to the invention is guided, in particular in this case an ablation catheter.

With further reference to figure 2, the instrument 1 of figure 1 comprises a rod or catheter 2 and a tip 3 mounted on the rod or catheter 2 with an intermediate elastically de- formable element 4, wherein the elastically deformable element 4 is arranged to control a contact force applied with the tip 3 on the tissue of the heart shown in figure 1 that will be subjected to an ablation procedure. The elastically deformable element 4 as shown in figure 2 is designed to provide a substantially constant force to the tip 3 when a distance is varied between the tip 3 and the rod or catheter 2 that indirectly (via the intermediate elas- tically deformable element 4) supports the tip 3. This variation of distance can of course only be done within a predefined range. In figure 2 the tip 3 and the rod or catheter 2 are approximately at their closest position wherein the elastically deformable element 4 practically completely fills in the space between the rod or catheter 2 and the tip 3.

As figure 2 further shows the elastically deformable element 4 is embodied as an endless tape loop spring 5. This endless tape loop spring 5 has predefined parameters relating to at least its material (steel or other suitable material), combination of materials (steel and plastic or other combina ^¬ tions) , or positions in the tape loop spring where such material is missing (for instance in the middle part - not shown in figure 2 ) , the tape loop spring width 6, the tape loop spring material thickness (usually less than a millimetre) , and the tape loop spring's transverse curvature 7. It is essential that at least one of said parameters of the endless tape loop spring 5 is varied along the circumference of the tape loop spring 5.

The tape loop spring 5 is embodied with two elongated parts 8, 9 in or parallel to a longitudinal direction 10 of the rod or catheter 2, which elongated parts 8, 9 convert into each other by bends 11, 12 at opposite sides of the elongated parts 8, 9 so that each bend 11, 12 connects the elongated parts 8, 9 to each other.

Variation of the distance between the tip 3 and the rod or catheter 2 causes a circumferential round going motion of the endless tape loop spring 5 which causes that the bends 11, 12 travel along the circumference of the endless tape loop spring 5 while maintaining their position in or parallel to the longitudinal direction 10 of the rod or catheter 2. Since the tip 3 is mounted on a first part 15 of the tape loop spring 5, and the rod or catheter 2 connects to the tape loop spring 5 at a second part 16 of the tape . loop spring 5, the circumferential round going motion of the endless tape loop spring 5 results in a corresponding change of distance between the tip 3 and the rod or catheter 2 of the instrument 1. Due to the construction of the endless tape loop spring 5 with the at least one parameter that varies along the circumference of the tape loop spring 5, the force F applied by the tip 3 on a tissue that is in contact with the tip 3, will during this varying distance between the tip 3 and the rod or catheter 2 be maintained at a relatively constant value which hardly varies with the changing distance between the tip 3 and the rod or catheter 2.

In figure 2 a preferred embodiment is shown in which the width 6 of the tape loop spring is varied in the circumferential direction of the tape loop spring 5. Figure 2 particularly shows an embodiment wherein the endless tape loop spring 5 has a width 6 with tapered portions that extend along the length of the elongated parts 8, 9 and also in the bends 11, 12. It is however also possible to restrict the tapered portions to extend over a limited circumferential range of the tape loop spring 5.

Because figure 2 shows the situation wherein the tip

3 and the rod or catheter 2 are approximately at their closest position, which corresponds with a loaded position of the endless tape loop spring 5, a broader part of the tapered portions (i.e. broader than the part with the smallest width) is then at the bends 11, 12 of the endless tape loop spring 5. It is remarked however that the tape loop spring 5 also has an unloaded position wherein the respective parts of the tape loop spring 5 occupy a preferential position with respect to each other, and wherein then at or near the location of the bends 11, 12 the width 6 of the tape loop spring 5 will be at its smallest possible value which is in any case smaller than at the two elongated parts 8, 9 between the bends 11, 12.

It is remarked that preferably the tapered portions are provided with an imaginary top angle, or also called sub- tended angle, which lies in the range of 95 - 135°.

Figure 2 further shows that in a transverse direction of the endless tape loop spring 5 that defines the width 6, the spring 5 has a curvature 7 with a predefined radius of curvature. The traverse curvature 7 in trig dirgction of thQ width of the spring 5 supports that the motion of the endless loop is self-constrained and does not need any additional guiding mechanism. The predefined radius of curvature 7 is preferably selected at a value corresponding to the diameter of the rod or catheter 2, which causes that the distance between the elongated portions 8, 9 of the endless tape loop spring 5 will approximately equate with the diameter of the rod or catheter 2.

Figure 3 shows the simulated results of an instrument design as shown in figure 2, wherein the relation is shown between the force F applied with the tip 3 on a tissue treated with the tip, which force is indicated along the y-axis, and the distance D shown along the x-axis which is the distance travelled by the tip 3 with reference to the rod or catheter 2. The graph of figure 3 shows that the force at the tip 3 of the instrument is substantially constant and independent from the distance between the tip 3 and the rod or catheter 2. Correspondingly the tip (3} exhibits zero-stiffness or substantially zero-stiff ess when moved in a range in a longitudinal direction of the rod or catheter 2.

Although the invention has been discussed in the foregoing with reference to an exemplary embodiment of the instrument of the invention, the invention is not restricted to this particular embodiment which can be varied in many ways without departing from the invention. The discussed exemplary embodiment shall therefore not be used to construe the appended claims strictly in accordance therewith. On the contrary the embodiment is merely intended to explain the wording of the appended claims without intent to limit the claims to this exemplary embodiment. The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using this exemplary embodiment.