WO2015056204A1

WO2015056204A1 - Apparatus for performing pectus excavatum repair

Info

Publication number: WO2015056204A1
Application number: PCT/IB2014/065360
Authority: WO
Inventors: Gastone Ciuti; Leonardo Ricotti; Arianna Menciassi; Marco GHIONZOLI; Antonio MESSINEO
Original assignee: Scuola Superiore Di Studi Universitari E Di Perfezionamento Sant'anna; Universita' Degli Studi Di Firenze
Priority date: 2013-10-16
Filing date: 2014-10-16
Publication date: 2015-04-23
Also published as: ITPI20130090A1

Abstract

An apparatus for performing correction of the pathology of pectus excavatum comprises at least one elongated bar and means of suture suitable to constrain the elongated bar in the vicinity of its first and second ends, in body tissues, either directly or by means of suitable stabilizers. The elongated bar, which must have certain mechanical characteristics of breaking load, yield strength and elasticity to withstand the stresses imposed by the chest, is made of a composite material made from a bioresorbable polymer matrix in which are disposed elongated metal. A method of removing the apparatus provides for performing an incision on one side of the chest, and then pull out one by one the elongated metal, taking advantage of the high flexibility of the foregoing.

Description

DESCRIPTION OF THE INVENTION

Title of Invention

APPARATUS FOR PERFORMING PECTUS EXCAVATUM REPAIR TECHNICAL FIELD

The present invention relates to an elongated bar with optimized structure of the type included in an apparatus for the repair of pectus excavatum disease.

BACKGROUND ART

The pectus excavatum disease is a most common malformation of the chest wall and consists in a more or less evident retroversion of the sternum that causes a depression of the central front portion of the chest.

The technique most commonly used and effective for the treatment of that disease is described in US 6,024,759 A. In this document it is disclosed an apparatus consisting of an elongated bar, preferably with a rectangular section, and that falls within a certain range of dimensions for length, height and thickness, and having appropriate mechanical properties of resistance to deformation or stiffness or yield strength, which is suitable to assume a conformation such as to maintain the chest in the desired shape. The said bar is bent to make it assume a convex shape, it is made to pass below the sternum with its convexity backward directed, it is then rotated 180° so that the convexity is forward directed and so that the sternum is raised by the bar in the desired position, and finally, the bar is constrained in the latter position by means of stitches, which block the ends of the bar to the muscles of the side chest wall, directly or through the interposition of appropriate stabilizer elements. The bar material is preferably stainless steel 316LVM, which is particularly used in medical applications and has physical properties such as to ensure the above mechanical characteristics, with the necessary maximum size in height and thickness of the bar provided for this specific application. The document also provides the possibility to realize the bar in different materials and precisely other biocompatible metal materials such as titanium or cobalt-chromium.

A limitation inherent in the use of these metallic materials is the need to provide in each case, at the end of the treatment period, an operation of removing the metal bar from the body of the patient. This surgical procedure, although less complex than the intervention briefly described above needed for inserting the bar, is still invasive and complex and because of the size of the bar itself and the possible adhesions formed in the time between the bar and the surrounding tissues.

Still in the document US 6,024,759 A, there is also the possibility to realize the bar in a bio- absorbable material which can for example be selected from polymers and co-polymers of PLLA, PGA, PDLA, or other material may be absorbable yet.

The use of a resorbable material could make superfluous the procedure for removal of the bar. However, in the cited document itself, the use of a bar entirely consisting of a resorbable material is proposed only theoretically as currently the physical properties of absorbable materials are not able to ensure the necessary mechanical characteristics when at the same time it remains within the maximum size of the bar provided for this application. Moreover, since the physical properties of absorbable materials also depend on the time, it is even more difficult to study or detect a resorbable material that has the functional life suitable for this type of application in which the bar is left in situ for an average period of about three years.

In many medical applications, a resorbable material is used exclusively as a surface coating of a non-absorbable component, a metallic one or other material, able to ensure the required mechanical and structural characteristics. Similarly, a coating of absorbable material may be applied to a metallic elongated bar, however this could solve the problem of the formation of adhesions between the bar and the surrounding tissues, but it does not eliminate the need to perform an invasive surgery to remove the same .

Still in US 6,024,759 A, it is described that the elongated bar may be a single bar or even a plurality of aligned bars and arranged adjacent to one another, bound or not to each other by removable means or by welding, anyway so as to obtain the required mechanical properties. As shown in Figures 1 A, 1 B and 1 C of the same document the bar can be formed by more bars of cylindrical or rectangular cross-section arranged close to each other, adjacent or overlapping. The use of multiple distinct bars can be advantageous in the procedure of removal of the bar as if they are physically separated from each other, they may be removed one at a time separately with a less invasive procedure as it is sufficient to prepare a channel extraction of a size that is much smaller the lower is the section of the single element of the bar. However, this solution involves additional complications in the already complex procedure for inserting the apparatus, as it is necessary to provide suitable means to maintain mutually bound the individual elements of the bar during the application of the same and further means able to stabilize the position of each single element during the entire treatment period, up to the removal.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to propose an apparatus for performing correction of pectus excavatum disease that can overcome the limitations of the prior art above described.

Further object of the present invention is to propose an apparatus for the correction of pectus excavatum disease that does not cause problems of compatibility with the body tissue and which allows to adjust appropriately, by chemical functionalization, the inflammatory and fibrotic response of the body to the implant.

Another object of the present invention is to propose an apparatus for pectus excavatum repair that is easily removable at the end of the treatment period.

Another object of the present invention is to propose a method for removing from the chest of a patient an apparatus for pectus excavatum repair that it is minimally invasive and simple to implement.

According to one aspect of the present invention, the above objects and others are achieved by means of a device as set forth and characterized in the independent claim 1 .

The claims dependent on claim 1 describe other characteristics of the present invention or variants of the main inventive idea.

Conventionally, an apparatus for performing pectus excavatum repair comprises at least one elongated bar provided with a first and a second end, and suture means suitable to secure said elongated bar in the vicinity of its first and second ends, to body tissues, either directly or by proper stabilizers. The said elongated bar has a size in height such as to be inserted into one intercostal space, and it has yield strength and rigidity such that said bar is adapted to be given a convex shape and it is also adapted, in said convex shape, to be inserted under the sternum of the patient to maintain the latter in the desired shape.

According to the present invention the said elongated bar is made of a matrix of bioresorbable material inside which there is a plurality of elongated elements in metallic material. The elongated elements are arranged aligned with each other according to the direction of the length of the bar and they are spaced with the resorbable material interposed between them. Furthermore, each of the elongated elements has a length substantially equal to the length of the elongated bar.

The matrix of bioresorbable material guarantees the perfect compatibility of the prosthetic implant and holds in place the metal elements which have a structural function and yet, given the very small dimensions in cross-section, they can be removed in an extremely easy procedure, by simply "pulling them out" from the polymer matrix.

Advantageously, the resorbable material is selected from poly (ortho esters), poly-L-lactic acid, homopolymers and copolymers of poly hydroxy butyrate, poly-E- caprolactone, poly (SA-HDA anhydride), poly (BPA iminocarbonate), while the material of the elongated elements is selected from stainless steel, titanium alloy, tungsten alloy.

Still advantageously, each of the elongated elements has a substantially constant section along the whole length with an area not exceeding 20 mm ²· while the ratio between the total area of the plurality of elongated elements and the total area of the section of the elongated bar is not higher than 0.85.

Preferably, the elongated elements have a slab-based geometry, while alternately they have a rod-based geometry or a still different one.

BRIEF DESCRIPTION OF DRAWINGS

These and other features of the invention will be more readily understood from the following description of preferred embodiments thereof, provided as non-limiting examples, with reference to the accompanying figures in which:

Figure 1 shows a perspective view of a prosthetic implant according to the present invention, in particular an implant for pectus excavatum repair; Figure 2 schematically shows a front view of the implant of FIG. 1 implanted in the chest of a patient;

Figure 3 shows a perspective view of an end of an elongated bar according to the present invention: in Fig. 3a a first embodiment is shown, in Fig. 3b a second embodiment is shown;

Figure 4 shows a summary table of the stresses which it is subjected an elongated bar (with metal elements having slab-based geometry, as in Figure 3a) for different combinations of materials;

Figure 5 shows a summary table of the values of elastic deformation percentage which the elongated bar is subjected for the same combinations of materials and still considering a slab-based geometry of the metallic elements;

Figure 6 shows a summary table of the stresses which it is subjected an elongated bar (with metal elements having rod-based geometry, as in figure 3b) for different combinations of materials;

Figure 7 shows a summary table of the values of elastic deformation percentage which the elongated bar is subjected for the same combinations of materials and still considering a rod-based geometry of the metallic elements;

Figure 8 shows a chart of the maximum deflection as a function of the percentage of metal alloy with respect to the polymeric material, for a given combination of materials, and considering both geometries (slab-based Figure 3a and rod-based Figure 3b) for the metal elements.

DESCRIPTION OF PREFERRED EMBODIMENTS

In Figures 1 and 2 it is shown an apparatus 1 , for the correction of pectus excavatum malformation consisting of an elongated bar, 2, provided with a first end 21 , and a second end, 22, in correspondence of which are present constraint means, which may include stabilizers, 3.

Just like according to the prior art, the bar 2, suitably curved, is made to pass below the sternum with the convexity directed backward by inserting it through a cut at one side of the chest. Once the bar has passed through the chest cavity, it is rotated so that the convexity is facing the front side so as to maintain the sternum raised to the desired position. The bar is secured in position thanks to the constraint means present at the first and second ends 21 and 22 comprising the stabilizers 3 provided with holes, 31 , through which they are fixed to the pectoral muscles by suture. To perform the operation of insertion of the apparatus it is therefore necessary to perform at least two incisions, in positions substantially symmetrical, in correspondence of the points in which the first and second ends 21 and 22 of the elongated bar 2 will be located.

As shown in Fig. 3, the elongated bar 2 is made of a composite material consisting of a polymeric matrix bio-resorbable, 23, and a plurality of elongated metal elements, 24, 24', inside the matrix. The elongated elements 24, 24 'are arranged aligned according to the direction of the length of the elongated bar 2, equally spaced with the matrix of bioresorbable material interposed between them. Furthermore, each of the elongated elements 24, 24' extends substantially along the entire length of the elongated bar 2.

In the embodiment of FIG. 3a the elongated elements 24 are shaped in slabs and their section has a width having substantially the same width of the elongated bar 2, while their thickness is very low so that the thickness of the elongated bar 2 is made up of three overlapping elongated elements 24 suitably spaced by interposition of a bioresorbable polymeric material 23.

In the embodiment of FIG. 3b, the elongated elements 24' are metal wires of circular cross section arranged neatly in the section of the elongated bar 2 to form two rows and seven columns suitably spaced by the interposition of a bioresorbable polymeric material.

The elongated metal elements 24, 24', which have the main function of ensuring the load resistance of the elongated bar 2, can of course also have different section and it can be arranged within the bioresorbable polymeric matrix 23 according to different layouts.

The metallic material of the elongated metal elements 24, 24 'is preferably selected from stainless steel AISI 316 L, the titanium alloy Ti-6AI-4V, or tungsten. The tensile strength of these materials ranges from about 400 MPa to about 1800 MPa; the yield strength ranges from about 150 MPa to about 1200 MPa; the elongation at break ranges from about 10% to about 50%, the elastic modulus ranges between about 100 GPa and about 500 GPa, while the Poisson ratio ranges from 0.25 to 0.35.

With regard to the polymeric matrices, these can be formed by a variety of polymers whose profile of degradability is compatible with the estimated times of implantation for the elongated bar 2. The preferred polymer materials for this application include: poly (ortho-esters) such poly (ortho ester) t-CDM: 1 .6-HD = 35:65 and poly (ortho ester) t-CDM: 1 .6-HD = 90: 10; poly-L-lactic acid, homopolymer and copolymers of poly hydroxy butyrate which the 22% HV copolymer, poly- ε caprolactone, poly (SA-HDA anhydride), poly (BPA iminocarbonate).

The tensile strength of these materials ranges from about 4 MPa to about 50 MPa, while the elastic modulus is between about 40 MPa and about 3000 MPa (Engelberg and Kohn 1991 ).

Appropriate finite element simulation showed the mechanical behaviour of an elongated composite bar, made of the materials listed above.

For example, simulations have been performed considering a bar of length 300 mm, width 12.7 mm and thickness 3.5 mm constrained at its ends, and considering the composite structure with three metal thin slabs of thickness 0.9 mm (volume of each thin slab 3321 mm ³ and the ratio between the total volume of metal slabs and the total volume of the elongated bar of 0.75 approximately) that run throughout the matrix, for its length (embodiment of Fig. 3a) and considering a force of 250 N applied to the center of the bar (a value greater than the maximum pressure found in the literature for the correction of pectus excavatum, according to Weber et al. 2006), and were obtained the values of maximum stress, and maximum deformation percentage of the bar, which are reported in the tables of Figure 4 and Figure 5, respectively.

As shown by the values in the tables, all combinations of materials determine a stress on the bar that is lower than the breaking load of all three metals or metal alloys considered. The yield strength of the steel alloy AISI 316 L is exceeded for all combinations of materials, except in association with poly-L-lactic acid that remains slightly below. All combinations of polymeric materials with titanium alloy Ti-6AI-4V and Tungsten, however, provide maximum stress values that are much lower than the yield strength of these two metals. The combination of materials that allows having the lower deformation percentage (and, consequently, the lower total deformation of the bar) is constituted by poly-L-lactic acid and Tungsten.

All materials and combinations of materials listed above are therefore essentially suitable for the execution of an elongated bar for the correction of pectus excavatum according to the present invention.

Further simulations have been performed considering even a bar of length 300 mm, width 12.7 mm and thickness 3.5 mm constrained at its ends, and considering the composite structure with fourteen metal rods with diameter of 1 .45 mm (volume of each rod 1980 mm ³ and the ratio between the total volume of metal rods and total volume of the elongated bar of 0.52 approximately) that run throughout the matrix, for its length (embodiment of Fig. 3b) and whereas still a force of 250 N is applied to the center of the bar. Also in this case were obtained the values of maximum stress, and maximum deformation percentage of the bar, which are reported in the tables of Figure 6 and Figure 7, respectively.

As shown by the values in the tables, the breaking load of the steel alloy AISI 316 L is exceeded in all combinations with different polymers. The yield strength of the titanium alloy Ti-6AI-4V instead is exceeded only in the case where you use poly (SA-HDA anhydride). In the case of using tungsten, the yield strength of the metal is never reached. However, in the case of use of poly (SA-HDA anhydride) in combination with this metal, the maximum deformation percentage of the bar reaches values higher than 19%, which are beyond the limit of elongation of the tungsten (that is about 10%). The combination of materials that allows to have the lower deformation percentage (and, consequently, the lower total deformation of the bar) is still the one formed by poly-L-lactic acid and tungsten, which can therefore be considered, in the present state of the technique, the combination of materials preferred from the structural point of view, but it is not limiting as it can be replaced with other combinations of materials as a result, for example, of evaluations of an economic nature.

As regards the geometry and dimensions of the elongated metal elements, these are defined by identifying a balance between a desired flexibility of the metal elements 24, 24', which makes the removal procedure easier, and their ability to resist high-pressure loads. For example, in the case of the slab-based geometry 24, a decrease in the thickness of the slabs themselves may be desirable, to increase flexibility and thus facilitate their removal (simply "taking them off" from the matrix), once completed the corrective action of the elongated bar 2 .In Figure 8 is shown, for the combination of materials, poly-L-lactic acid + Tungsten, the trend of the maximum stress acting on the elongated bar 2 when the volumetric percentage of metal inside the bar is varied (directly proportional to the thickness of the metal slabs 24 or metal rods).The graph shows with dashed lines the yield strength and the structural limit of the Tungsten. The points identified by diamonds, 71 , in the case of the slabs and by triangles, 72, in the case of the rods refers to the values of maximum stress acting on the bar in correspondence of different sizes of the metallic elements (and thus of different percentages of metal in the structure). It is apparent that the minimum thickness for the metal plates is 0.4 mm and the minimum diameter of the rods is 0.75 mm below these values the yield strength (and also breaking load) of the Tungsten is exceeded.

Obviously, the results of these simulations show the possibility of realizing an elongated bar according to the present invention with numerous combinations of materials, but the above list of the latter should be considered only illustrative and not exhaustive. Furthermore, also as regards the geometry of the elongated metal 24, 24' and of the composite material as a whole, many variants may be possible, without thereby departing from the concept of the present invention.

The use of an elongated bar 2 according to the present invention, made of composite material, makes it possible the implementation of a particular method of removal of the same at the end of the treatment period.

In fact, according to the method of the present invention, to remove the apparatus 1 for the correction of pectus excavatum is sufficient to perform an incision of a maximum length of about 1 .5 cm, and then to withdraw in succession, one at a time, all the elongated metal elements 24, 24', and finally closing the incision by sutures. The high flexibility of each elongated metallic element 24, 24' allows a particularly easy removal and the fact that they are incorporated in the polymeric matrix prevents that they can damage the body tissues. The removal procedure is extremely simple and non-invasive because the need for removal of the elongated bar 2 is limited to the need to remove the elongated metal elements 24, 24'. In fact, the bioresorbable polymer matrix remains in situ and the body gradually resorbs it. Likewise, the stabilizers 3, when present, are made of a bioresorbable polymeric material and therefore they do not need to be removed. Alternatively, in the case that it is necessary a greater structural strength of the stabilizers, these are made of metallic material and coated with bioresorbable polymeric material and in this case the removal method include stages dedicated to the removal of the stabilizers. A further alternative is to use composite stabilizers that, similarly to the bar, are constituted by a bioresorbable matrix within which are positioned metallic structural elements. The latter solution would allow a minimally invasive removal also of the metallic elements of the stabilizers, in addition to those of the bar. The metallic structural elements may have, similarly to what happens in the bar, a slab or rod geometry and are in number and section sufficient to provide the required mechanical properties to the stabilizer element.

Obviously, in addition to the phases of the removal method of the invention the removal procedure could include further steps and surgical procedures deemed necessary or appropriate.

The advantages of an apparatus for the correction of pectus excavatum and a method for the removal of the same according to the present invention remain unchanged even in the presence of further variations or modifications introduced while always remaining within the scope of protection defined by the following claims.

Claims

1 . Apparatus for performing pectus excavatum repair comprising at least one elongated bar provided with a first and a second end, and suture means suitable to secure said elongated bar, in the vicinity of its first and second end, to body tissues, either directly or by proper stabilizers, and wherein said elongated bar has a size in height such as to be inserted in an intercostal space, and it has yield strength and rigidity such that said bar is suitable to be given a convex shape and also suitable, in said convex shape, to be inserted under the sternum of the patient to maintain the latter in the desired shape, characterized in that said elongated bar is made of a matrix of bioresorbable material inside which there is a plurality of elongated elements in metallic material, said elongated elements being arranged aligned according to the direction of the length of said bar, said elongated elements being spaced with interposed said resorbable material, each of said elongated elements having a length substantially equal to the length of said elongated bar.

2. Apparatus according to claim 1 characterized in that said resorbable material is selected from poly (ortho esters), poly-L-lactic acid, homopolymers and copolymers of poly hydroxi butyrate, poly-£-caprolactone, poly (SA-HDA anhydride), poly (BPA iminocarbonate).

3. Apparatus according to any preceding claim characterized in that said elongated elements are made of a material selected from stainless steel, titanium alloy, tungsten.

4. Apparatus according to any preceding claim characterized in that each of said elongated elements and has substantially the same section along the whole length with an area not exceeding 20 mm², the ratio between the total volume of said plurality of elongated elements and the total volume of said elongated bar being not more than 0.85.

5. Apparatus according to any preceding claim characterized in that said elongated elements are evenly distributed in said matrix made of resorbable material.

6. Apparatus according to the previous claim characterized in that said elongated elements have a sheet-based geometry.

7. Apparatus according to claim 5 characterized in that said elongated elements have a rod- based geometry.

8. Apparatus according to any preceding claim characterized in that it comprises stabilizer elements made of a composite material consisting of a matrix in a resorbable material in which are contained metallic structural elements.