Technical Field

The present invention relates to a flexible joint.

Background Art

As is well known, flexible joints are usually employed for making a connection between two movable bodies, such as e.g. two rigid bodies, two mechanical members, two rotating shafts and so on.

One of the main fields of application of the flexible joints is in the connection between rotating shafts which are not perfectly aligned in order to effectively transfer the rotary motion from one shaft to another, thus offsetting their misalignment.

The flexible joint according to the present invention finds a preferred, but not exclusive, application in the context of making orthopedic prostheses in order to join two elements that need to move elastically between each other while keeping the continuity of the connection, by virtue of an appropriate degree of strength.

Flexible joints of known type are generally provided with two interfaces at the end which are adapted to be associated with two bodies to be connected, and with an elastic element placed between the two ends in order to give flexibility to the joint.

Depending on the desired degrees of freedom, the flexible joints may comprise elastic elements of different conformations.

Flexible joints are particularly known to have an elastic element that has a serpentine conformation winding in an open cylinder around a longitudinal axis. Such a conformation is commonly referred to by the terms “twin loop”, “wire binding”, “wire-o” and “double loop wire”.

Flexible joints of this type are particularly advantageous because they have an elastic element that is susceptible to undergo elastic deformation as a result of the application of external tensile, compressive, bending and torsion forces, thus giving the joint great flexibility. In addition, joints provided with such a geometric shape are easily fabricated by means of additive manufacturing, if compared with the more traditional helical springs.

This type of joint, however, has problems particularly related to the conformation with which the elastic element is made. This conformation, in fact, means that, as a result of elastic deformation of the elastic element, very high stresses are generated in the latter which show themselves mainly at the inversion points of the serpentine, thus greatly reducing the strength of the joint. It is also noteworthy that in the area of flexible joints, there is an increasing demand for the development of elastic elements which ensure ever-increasing strength and, at the same time, have small overall dimensions.

Description of the Invention

Therefore, the main aim of the present invention is to devise a flexible joint which is able to greatly reduce stresses in the areas of the elastic element that are most stressed.

A further object of the present invention is to make a very strong flexible joint. Another object of the present invention is to devise a flexible joint which can overcome the aforementioned drawbacks of the prior art within the framework of a simple, rational, easy and effective to use as well as cost-effective solution. Another object of the present invention is to devise a flexible joint which can be effectively made by additive manufacturing.

A further object is to make a flexible joint which can be used to make bone and ligament replacements.

The aforementioned objects are achieved by this flexible joint having the characteristics of claim 1.

Brief Description of the Drawings

Other characteristics and advantages of the present invention will become more apparent from the description of a preferred, but not exclusive, embodiment of a flexible joint, illustrated by way of an indicative, yet non-limiting example, in the attached tables of drawings in which:

Figure 1 is a perspective view of the flexible joint in accordance with the present invention,

Figure 2 is a view of an in-plane development of the elastic element of the joint in Figure 1,

Figure 3 is a vertical longitudinal sectional view of a connecting body of the elastic element in Figure 2,

Figure 4 is a front view of the joint in Figure 1.

Embodiments of the Invention

With particular reference to these figures, reference numeral 1 globally denotes a flexible joint.

The flexible joint 1 is configured to elastically connect two bodies together in order to allow their relative displacements. Such bodies can be, e.g., two misaligned rotating shafts, two rigid bodies, such as e.g. two bones for an orthopedic prosthesis, two mechanical members, and so on.

As visible from Figure 1, the flexible joint 1 extends along its own longitudinal axis Al between one pair of ending portions 2, 3, each intended to be associated with one of the two bodies to be connected.

Appropriately, the flexible joint 1 comprises at least one elastic element 4 winding around a longitudinal axis Al. Specifically, the elastic element 4 has a serpentine conformation winding around the longitudinal axis Al. Preferably, the elastic element 4 winds in an open cylinder pattern around the longitudinal axis Al.

As visible from Figures 1 and 2, the elastic element 4 comprises a plurality of connecting bodies 5 arranged adjacent and alternated to each other. Specifically, each of the connecting bodies 5 defines at least one loop 6 of the elastic element 4 and comprises at least one extreme portion 7 and at least one groove 8. The extreme portions 7 are arranged facing each other, and the grooves 8 are positioned between them. The grooves 8 can also be of the type of a shear, a hole or, in general, simple discontinuities. In actual facts, the elastic element 4 runs along a substantially sinusoidal trajectory which defines a plurality of extreme portions 7 wherein the elastic element 4 has a substantially “U-shaped” conformation, as specified in detail later in the description.

Such a conformation of the elastic element 4 allows making a flexible joint 1 susceptible to undergo elastic deformation due to an external force applied thereon. Such an external force results in a stress on the elastic element 4 which is capable of generating a relevant displacement between the two ends joining elements 18 and 19. Specifically, the elastic element 4 is movable due to an external force from an unloading or home configuration, wherein the elastic element shows no deformation, to a loading configuration wherein the elastic element shows a deformed condition.

The external force applied can generate at least one of: a bending stress, a torsion stress and a tensile/compressive stress.

The term “bending stress” is used to denote a stress which is adapted to produce a curvature of the elastic element 4 and to generate a bending moment as internal reaction.

The term “torsion stress” refers to a stress which generates a relevant rotation of the elements 18 and 19 around the direction of longitudinal development A2 of the elastic element 4 and generates a torque moment as internal reaction.

The terms “tensile stress” and “compressive stress” are intended to denote a stress which generates a force directed substantially parallel to the direction of longitudinal development A2 of the elastic element 4 as internal reaction.

In the unloading configuration, the elastic element 4 is in a home condition wherein it is substantially free of elastic deformation. In the loading configuration, on the other hand, the elastic element 4 has an elastic deformation which generates stresses along the elastic element 4 which occur mainly at the extreme portions 7 of the connecting bodies 5.

Advantageously, each connecting body 5 comprises a recess 9 made at the extreme portion 7 to define a widening of the groove 8. Specifically, the recess 9 is made by material removal which allows widening the groove 8. This recess 9 gives a specific geometry to the elastic element 4, described in detail later in the disclosure, which allows the local stresses generated in the loading configuration where the extreme portion 7 is located to be reduced, redistributing them appropriately and consequently giving high strength to the flexible joint 1.

An in-plane development of the elastic element 4 is shown in Figure 2. This Figure 2 substantially represents the two longitudinal sections of the elastic element 4 projected onto the same plane.

As can be seen from Figure 2, in its in-plane development, the elastic element 4 shows a substantially sinusoidal pattern. In actual facts, the elastic element 4 develops along its own direction of longitudinal development A2 with a substantially sinusoidal pattern. In other words, the elastic element 4 runs along a serpentine trajectory.

Such a pattern defines the plurality of connecting bodies 5 which are arranged alternated or adjacent to each other. In actual facts, the connecting bodies 5 extend along a transverse direction A3 of the elastic element 4.

In detail, in its in-plane development, the elastic element 4 has a centerline L which divides the elastic element 4 into two mutually opposite parts. The centerline L is substantially parallel to the direction of longitudinal development A2 of the elastic element 4.

The connecting bodies 5 extend along the transverse direction A3 starting from the centerline L. In actual facts, the centerline L divides the elastic element 4 into the plurality of connecting bodies 5. Thus, the connecting bodies 5 are arranged alternately with respect to the centerline L. In other words, in its longitudinal development, the elastic element 4 has a midline (identifiable between the outer profile and the inner profile) which oscillates substantially around the centerline L and defining the geometry of the plurality of connecting bodies 5. By moving forward along the centerline L, this oscillation can have constant amplitude (such as in Figure 2) or modulated amplitude and constant frequency (such as in Figure 4) or variable frequency. In this way, the connecting bodies 5 are arranged substantially aligned with each other along the direction of longitudinal development A2 of the elastic element. In addition, the connecting bodies 5 of one pair of consecutive connecting bodies 5 are arranged opposite each other with respect to the centerline L.

As visible from Figures 2 and 3, each connecting body 5 has a substantially “U” shape.

Specifically, each connecting body 5 has a pair of union portions 10 and an extreme portion 7 positioned between the pair of union portions 10. The extreme portion 7 has a curvilinear pattern. On the other hand, each union portion 10 has a substantially linear pattern. The union portions 10 are arranged substantially parallel to each other. Each union portion 10 has a thickness S and a respective rectilinear stretch 15. The rectilinear stretches 15 are spaced apart by a distance D.

At least one union portion 10 of each connecting body 5 is connected to a union portion 10 of a consecutive connecting body 5. In actual facts, the union portions 10 of two consecutive connecting bodies 5 are continuous with each other.

Figure 1, on the other hand, shows the elastic element 4 in a perspective view. In such a view, the connecting bodies 5 are moved closer to each other to wind around the longitudinal axis in an open cylinder pattern, which in this case substantially coincides with the centerline L, so that each connecting body 5 defines a half-loop 6.

Such a conformation is called “open cylinder” because the elastic element does not wind completely around the longitudinal axis, but has an opening where the extreme portions 7 of the connecting bodies 5 are moved closer. The angle of winding is therefore variable and in any case less than 360°.

In actual facts, the elastic element 4 is of the type of a spring and has an overall shape which is commonly referred to by the terms “twin loop”, “wire binding”, “wire-o” or “double loop wire”.

Specifically, the direction of longitudinal development A2 of the elastic element 4 is arranged substantially parallel to the longitudinal axis Al of the flexible joint 1 and each connecting body 5 develops around the longitudinal axis Al of the flexible joint 1. In detail, each connecting body 5 develops along a circumferential arc.

It cannot however be ruled out that the elastic element 4 may have a different conformation in which, e.g., the extreme portions 7 are not moved close to each other. For example, the elastic element 4 may be arranged with its direction of longitudinal development A2 wound around the longitudinal axis Al of the flexible joint 1. In other words, the direction of longitudinal development A2 of the elastic element 4 is arranged substantially along a circumference with its center on the longitudinal axis Al of the flexible joint 1.

Preferably, the elastic element 4 has an elongated main body made at least partly of harmonic steel, stainless steel, titanium or biocompatible metal alloy. Figure 3 shows a view of a vertical longitudinal section of a connecting body 5 of the elastic element 4.

In the remainder of the description and in subsequent claims in order to facilitate and better detail the description of the connecting body 5, reference will be made to such vertical longitudinal section, visible in Figure 3.

In detail, with reference to the aforementioned vertical longitudinal section, each connecting body 5 comprises an inner edge 11 which marginally bounds the groove 8.

In addition, each connecting body 5 has an outer edge 12 opposite the inner edge 11. In actual facts, the inner edge 11 faces the groove 8 of the connecting body 5.

Since the connecting bodies 5 are continuous with each other, as visible from Figure 2, the outer edge 12 of a connecting body 5 is connected to the inner edge 11 of a consecutive connecting body 5.

Appropriately, as will be described in detail later in the description, in the vertical longitudinal section, the inner edge 11 has at least one point of flexure 13 arranged where the recess 9 is located. Such a conformation of the inner edge 11 allows at least one change to be made in the way of the curvature of the inner edge 11 where the recess 9 is located, which allows the stresses generated as a result of the application of an external force on the flexible joint 1 to be redistributed around the point of flexure.

Preferably, in the vertical longitudinal section, the inner edge 11 has at least one pair of points of flexure 13 arranged where the recess 9 is located and substantially opposite each other. It cannot, however, be ruled out that the inner edge 11 may have a different number of points of flexure 13.

As visible from Figure 3, in the vertical longitudinal section, the inner edge 11 has at least one curvilinear stretch 14 marginally bounding the recess 9. In particular, the curvilinear stretch 14 is arranged where the extreme portion 7 of the connecting body 5 is located. In actual facts, the curvilinear stretch 14 marginally bounds the extreme portion 7.

This curvilinear stretch 14 allows making a recess along the inner edge 11 which allows for the redistribution of the stresses generated where the extreme portion 7 is located, thereby decreasing local stresses.

In actual facts, the recess 9 is made by material removal from the connecting body 5 at the extreme portion 7 starting from the inner edge 11, going towards the outer edge 12 by a predefined stretch.

Preferably, the curvilinear stretch 14 has a substantially “C” shape. In particular, the curvilinear stretch 14 has a substantially circumferential arc shape. In actual facts, the curvilinear stretch 14 has a predefined radius of curvature Rl.

Furthermore, in the vertical longitudinal section, the inner edge 11 comprises at least one rectilinear stretch 15 marginally bounding the groove 8 and a connecting stretch 16 positioned between the curvilinear stretch 14 and the rectilinear stretch 15. In detail, the rectilinear stretch 15 bounds the part of the groove 8 which is unrelated to the recess 9. In detail, the rectilinear stretch 15 is arranged where the union portion 10 of the connecting body 5 is located. In actual facts, the rectilinear stretch 15 marginally bounds the union portion 10.

The rectilinear stretch 15 extends between a first end 15a connected to the connecting stretch 16 and a second end 15b. The second end 15b is, in some cases, connected to the outer edge 12 of the adjacent connecting body 5. In the event, on the other hand, of the connecting body 5 being arranged where an ending portion 2, 3 of the joint 1 is located, the second end 15b may be connected to a joining element 18, 19 described in detail later in this description.

The rectilinear stretch 15 is arranged transversely to the direction of longitudinal development of the elastic element 4. In particular, in the unloading configuration, shown in Figures 1 and 2, the rectilinear stretch 15 is substantially perpendicular to the direction of longitudinal development of the elastic element 4. On the other hand, in the loading configuration, shown in Figure 4, the rectilinear stretch 15 is arranged diagonally with respect to the direction of longitudinal development of the elastic element 4.

Appropriately, the connecting stretch 16 is substantially curvilinear. This configuration allows making a recess along a direction transverse to the rectilinear stretch 15 which defines a widening of the groove 8.

In particular, the connecting stretch 16 has a substantially “C” shape. Preferably, the connecting stretch 16 has a substantially circumferential arc shape. In actual facts, the connecting stretch 16 has a predefined radius of curvature R2.

As anticipated above, the inner edge 11 has at least one point of flexure 13. The connecting stretch 16 and the curvilinear stretch 14 join each other where the point of flexure 13 is located. In the present case, in vertical longitudinal section, the connecting stretch 16 has a first direction of curvature, which defines a first convexity, and the curvilinear stretch 14 has a second direction of curvature, which defines a second convexity. The first direction of curvature and the second direction of curvature are opposite each other.

This change in curvature allows stresses to be distributed along the connecting stretch 16 and the curvilinear stretch 14.

Overall, the connecting stretch 16 and the curvilinear stretch 14 define a substantially “S” shape.

The connecting stretch 16 is tangent at one end to the rectilinear stretch 15 and at the other end to the curvilinear stretch 14, having opposite convexity.

The curvilinear stretch 14 is tangent at one end to the rectilinear stretch 15 and at the other end to the linear stretch 17.

The linear stretch 17 has been represented here as rectilinear, but it could also be curved, as long as it is tangent at one end to the stretch 14.

The linear stretch 17 could also be absent, in case the center of curvature of the stretch 14 lies on the axis of symmetry of the recess 9. The radii of curvature Rl, R2 of the curvilinear stretch 14 and of the connecting stretch 16 substantially define the geometry of the inner edge 11 where the recess 9 is located, consequently resulting in the stress distribution.

As anticipated above, the distance between two rectilinear stretches 15 is called D, while the thickness of each union portion 10 is called S.

Appropriately, the connecting stretch 16 has a radius of curvature R2 comprised between S/4 and 2S, preferably S.

In addition, the curvilinear stretch 14 has a radius of curvature Rl comprised between D/2 and (D+S)/2, preferably D/2+S/4.

Conveniently, the inner edge 11 has no comers at least where the recess 9 is located. In other words, the connecting points of the connecting stretch 16 with the curvilinear stretch 14 and the rectilinear stretch 15 are rounded.

As can be seen from Figure 3, the inner edge 11 of each connecting body 5 has a substantially symmetrical conformation with respect to its own line having a section substantially parallel to the rectilinear stretch 15. Such a section line is substantially transverse to the direction of longitudinal development of the elastic element 4.

This section line, again referring to the vertical longitudinal section visible in Figure 3, divides the connecting body into two mirroring parts in which each part comprises a rectilinear stretch 15, a connecting stretch 16 and a curvilinear stretch 14.

The part of description concerning the rectilinear stretch 15, the connecting stretch 16 and the curvilinear stretch 14, therefore, will be intended to refer to both parts of the connecting body 5.

In the present case, the inner edge 11 has a pair of curvilinear stretches 14 opposite each other which marginally bound the recess 9. In particular, the curvilinear stretches 14 are opposite each other with respect to the section line.

Preferably, the radii of curvature R1 of the curvilinear stretches 14 are substantially similar to each other. It cannot, however, be ruled out that the curvilinear stretches 14 may have different radii of curvature R1 from each other.

Preferably, the curvilinear stretches 14 are connected to each other by means of a linear stretch 17. It cannot however be ruled out that the inner edge 11 may be without such a linear stretch 17 and that the curvilinear stretches 14 may be continuous with each other to make a single curvature or join through two suitably connected generic curves, provided the concavity is maintained.

The linear stretch 17, if any, is arranged transversely to the rectilinear stretch 15.

Taken together, the curvilinear stretches 14 and the linear stretch 17 have a substantially “C” shape which marginally bounds the recess 9; if the stretch 17 were not present, a portion of a circumference would be made instead.

As can be seen from Figure 3, in a vertical longitudinal section, the inner edge 11 comprises a pair of rectilinear stretches 15 opposite each other to bound the groove 8. Each of the rectilinear stretches 15 is connected to one of the connecting stretches 16 at their respective ends 15a. Specifically, the inner edge 11 comprises a pair of connecting stretches 16, each positioned between one of either the curvilinear stretches 14 or one of the rectilinear stretches 15.

Preferably, the radii of curvature R2 of the connecting stretches 16 are substantially similar to each other. It cannot, however, be ruled out that the connecting stretches 16 may have radii of curvature R2 different from each other.

As visible in Figure 2, in an unloading configuration of the elastic element 4, the rectilinear stretches 15 are substantially parallel to each other. On the other hand, as visible from Figure 4, in a loading configuration, the rectilinear stretches 15 diverge from each other.

Preferably, the recess 9 makes a decrease in thickness of the elastic element 4 where the extreme portion 7 is located. Since in the preferred embodiment shown in the figures, the outer edge 12 is substantially parallel to the rectilinear stretch 15 of the inner edge 11 even where the recess 9 is located, the thickness is meant as the distance between the inner edge 11 and the outer edge 12 along a direction substantially perpendicular to the rectilinear stretch 15 of the inner edge 11.

It cannot, however, be ruled out that the outer edge 12 may have a different pattern where the recess 9 is located, e.g., it may make a widening that allows the thickness of the elastic element 4 to be maintained substantially constant. In that case, the thickness is meant as the distance between the inner edge 11 and a reference line substantially parallel to the rectilinear stretch 15 of the inner edge 11.

As anticipated above, the elastic element 4 extends between one pair of ending portions 2, 3. Appropriately, the flexible joint 1 comprises one pair of joining elements 18, 19. Each joining element 18, 19 is arranged where one of the ending portions 2, 3 is located and is configured to be connected to one body of a pair of bodies to be connected.

Preferably, each joining element 18, 19 is locked together with one end of the elastic element 4.

Preferably, each joining element 18, 19 is a flange or a sleeve element which can be associated with one of the two bodies to be connected.

Such joining elements 18, 19 are particularly familiar to an expert in the field and therefore will not be described in detail in the context of this description.

The applicants performed several tests aimed at measuring the difference between the stresses located in the extreme portions 7 of the connecting bodies 5 in the presence or absence of the recess 9.

In a first series of tests, the value of the maximum internal stress that is reached by an elastic element 4 without the recess 9 described above was measured following the application of a transverse tensile force equal to 100 N to one ending portion 2, 3 of the body 5, while the other ending portion is fully restrained.

From that first series of tests, a maximum stress value of 4243 MPa was measured for a given geometry made of titanium (elastic modulus of 120 GPa).

In a second series of tests, the value of the maximum internal stress which is reached by an elastic element 4 substantially similar to the elastic element 4 used in the first series of tests was measured in which, however, the aforementioned recess 9 was derived.

From that second set of tests, a maximum stress value of 566 MPa was measured.

It has in practice been ascertained that the described invention achieves the intended objects, and in particular, the fact is emphasized that by means of the flexible joint according to the present invention it is possible to greatly reduce stresses in the areas of the elastic element that are most stressed, thus making a flexible joint that is very strong and, consequently, has small overall dimensions if compared to joints of equal strength.