ITBO20100597A1 - INTERVERTEBRAL DISC THERDESIS - Google Patents

Description

INTERVERTEBRAL DISC PROSTHESIS

DESCRIPTION OF THE INVENTION

The invention fits into the field of disc prostheses for intervertebral implants.

In the intervertebral disc, the annulus fibrosus, or fibrous ring, encloses the nucleus pulposus. In a healthy intervertebral disc, the nucleus pulposus supports the biomechanical movement of the vertebral column and the disc bears the load and compressive forces to which the vertebral column is constantly subjected. Unfortunately, the intervertebral discs are subject to degenerative phenomena, particularly at the lumbar level, due to the greater load they have to bear.

Until a few years ago, the only treatment for disc pathologies was vertebral arthrodesis, a surgical operation through which the degenerated intervertebral disc unable to support said load is removed and is obtained, with various techniques, a bone fusion between the two vertebrae affected by the degenerated disc. This surgical treatment relieves the patient of pain but causes the loss of movement between the two vertebrae, which are thus rigidly fixed to each other. This can lead, over time, to a functional overload of the two discs close to the removed one which can develop a pathology detectable biomechanically and by radiography. In a non negligible percentage of cases, this pathology is also associated with symptoms such as to require further arthrodesis interventions also on the neighboring vertebrae with consequent loss of mobility of a further sector of the vertebral column.

Intervertebral disc prostheses have been proposed relatively recently as an alternative to arthrodesis in the treatment of degenerative disc disease when the pathology found does not require blocking of the two vertebrae adjacent to the degenerated disc.

The implantation of these disc prostheses to replace the intervertebral disc or nucleus pulposus is preferred to arthrodesis as it allows the biomechanical movement of the two vertebrae involved in the implant. These disc prostheses have been designed to relieve back pain and support the movements of the spine and typically include a ball joint that allows the aforementioned biomechanical movement. Therefore, the prostheses include two components: one provided with a seat and the other provided with a part shaped to engage with the seat to identify the ball joint together with the latter. Typically this part consists of a portion of a sphere (hereinafter referred to briefly as `` spherical portion '')

One type of disc prosthesis is the so-called â € œtotal disc prosthesisâ € which completely replaces the degenerated intervertebral disc and requires the removal of both the nucleus pulposus and the fibrous ring. These prostheses must therefore be able to withstand the compressions to which the replaced intervertebral disc is subjected as well as to support the biomechanical movement of the vertebrae involved in the implant. At the lumbar or sacral level, where the vertebrae have to support a greater weight, the dimensions of the total disc prostheses are such as to allow their implantation only through an anterior surgical access route which can lead to significant complications including male infertility.

A first category of total disc prostheses consists of prostheses composed of two metal components in which the upper one is provided with a first side, intended to counter the lower vertebral plate of a first vertebra, and a second side, opposite on the first side, where there is a seat. The lower component has a first side intended to counter face the upper vertebral plate of a second vertebra, adjacent and lower to the first vertebra, and a second side, opposite the first side, provided with a spherical portion to engage with the seat of the upper component, identifying together with the latter a spherical joint.

To implant the prosthesis, the two components must be pre-assembled so that the spherical portion engages in the seat creating the aforementioned ball joint. Due to the type of material used, that is to say a metal compatible with the implants, this disc prosthesis has a low friction of the seat and spherical portion that define the spherical joint. On the other hand, this prosthesis does not have elasticity characteristics and therefore is not able to cushion the axial compression shocks to which the spinal column is subjected, acting on the vertebrae.

In a second category of total disc prostheses, the latter are made up of three components, one lower, one intermediate and one upper. The lower component is metallic and is provided with a first side, intended to counter the upper vertebral plate of a first vertebra, and a second side, opposite the first side, comprising a housing. The upper component is metallic, it is substantially identical to and performs the same function as the upper component of two-component prostheses: it therefore has a seat. The intermediate component is made of plastic material, typically HDPE. It is intended to be interposed between the upper and lower components, to be supported by the housing of the lower component and has a spherical portion at the top that is shaped to engage with the seat of the upper component to identify a joint jointly with the latter. spherical.

To implant the disc prosthesis, it is pre-assembled so that the intermediate component is received and supported by the housing of the lower component and the spherical portion engages in the seat creating the aforementioned ball joint.

Thanks to the intermediate component in HDPE, the three-component disc prostheses have an elasticity comparable to that of the healthy intervertebral disc and therefore are theoretically able to elastically absorb axial compression shocks acting on the vertebrae.

Unfortunately, especially at the lumbar or sacral level, where the load is greater, there may be compression forces acting on the vertebrae affected by the implant such as to deform, at least temporarily, the spherical portion. This deformation results in a decrease in the contact surface of the spherical portion with the seat in the joint and asymmetrical wear of the spherical portion as a result of the friction of the spherical joint. There is therefore the risk that in the long term this wear could lead to the blockage of the ball joint and therefore the loss of movement of the vertebrae affected by the implant with the same negative consequences relating to the arthrodesis described above.

Replacing the intervertebral disc with a total disc prosthesis is a fairly recent technique and there is not yet enough feedback on any long-term complications. Therefore it is the individual surgeon who chooses whether to use a two or three-component disc prosthesis depending on whether he gives greater importance to the reliability of the ball joint over time or to the ability to elastically absorb compression forces and axial shocks. which the vertebrae are subjected to. Since the implantation of a total disc prosthesis involves considerable risks of complications related to the anterior surgical approach, disc prostheses have been proposed with reduced dimensions and such as to allow their implantation through a posterior surgical approach. These disc prostheses, called nucleus, replace only the pulpy nucleus of the intervertebral disc and therefore have to withstand lower compressions than total disc prostheses. The core disc prostheses currently used are typically made up of three components shaped in a manner substantially similar to the components of the three-component total disc prosthesis with the difference that they are made entirely of PEEK. Despite its small size, PEEK gives core disc prostheses sufficient compressive strength. This resistance, with the same size of the prosthesis, cannot be obtained with other plastic materials, such as HDPE. On the other hand, the elasticity of these core prostheses due to PEEK is lower than that of the healthy nucleus pulposus. Furthermore, core disc prostheses are still subject to deformation and wear of the spherical portion, although to a lesser extent than total three-component disc prostheses in which the intermediate component is made of HDPE. Therefore, even core disc prostheses present the risk of locking the ball joint.

The object of the present invention is to reduce and / or eliminate the aforementioned disadvantages relating to known disc prostheses.

The aforementioned purposes are achieved with an intervertebral disc prosthesis, comprising: ∠’a lower component provided with a first side intended to counter face the upper vertebral plate of a first vertebra;

∠’an upper component equipped with: a first side intended to counter the lower vertebral plate of a second vertebra adjacent and superior to said first vertebra; and a second side, opposite said first side, comprising a seat;

∠’an intermediate component intended to be interposed between the lower component and the upper component and provided at the top with a shaped part to engage with the seat to identify a ball joint jointly with the seat;

characterized by further understanding:

- elastic means intended to be functionally interposed between the lower component and the intermediate component to react elastically to compression forces acting along the spine once the aforementioned prosthesis has been implanted between the aforementioned first and second vertebrae.

Said elastic means allow to obtain a disc prosthesis with an elasticity comparable to that of the healthy nucleus pulposus and allow to use for the production of the intermediate and superior components of materials having characteristics of greater rigidity, non-deformability and resistance to wear compared to those currently used in known types of prostheses.

Preferably these materials are metallic materials. Therefore the deformation and wear phenomena due to the compression of the vertebrae and the friction of the joint are minimized, in particular the aforementioned asymmetrical wear. Consequently, the ball joint of the proposed prosthesis is reliable even in the long term.

Prostheses according to the invention are therefore preferred in which the part of the intermediate component shaped to engage with the seat and the seat itself are provided with a layer of metal in correspondence with the relative surfaces intended to mutually contact each other. Preferably, said part of the intermediate component is entirely made of metal. When the upper component and the intermediate component are made of metal, the risk of wear of the contact surfaces of the ball joint is further reduced.

Prostheses according to the invention are also preferred in which the lower component is also made of metal since they have a greater resistance to compression.

Obviously the metal used for the production of these prostheses is compatible with the implants. Titanium is particularly preferable as it provides fewer artifacts on radiographs.

Similarly to the known art, said part of the intermediate component that can be engaged in the seat can be shaped as a portion of a sphere and the prosthesis according to the invention can be pre-assembled by positioning the elastic means on the lower component in such a way as to be supported from the latter. The intermediate component must then be supported by the elastic means and the relative part must be engaged with the seat to identify the spherical joint together with the latter.

The characteristics of the invention will be made clear in the following description of a preferred embodiment of the disc prosthesis, in accordance with what is reported in the claims and with the aid of the attached drawing tables, in which:

Figs.1 and 2 are exploded perspective views respectively from above and from below of an embodiment of the proposed disc prosthesis;

Fig.3 is a front view of the assembled disc prosthesis of Fig.1 and 2;

Fig.4 is a section of Fig.3 according to the IV-IV plane of Fig.3;

Fig.5 is a side view of the assembled disc prosthesis of Fig.1;

Fig.6 is a section of Fig.5 according to the VI-VI plane of Fig.5; And

Fig.7 is a further front view of the assembled disc prosthesis of Fig.1.

For the purposes of the present description, the same numerical references in the figures will imply similar characteristics unless otherwise indicated.

With reference to the attached figures, the intervertebral disc prosthesis 1 according to the invention comprises a lower, an upper, an intermediate component and elastic means. The lower component 20 is provided with a first side 21 intended to counter face the upper vertebral plate of a first vertebra. The upper component 10 is provided with a first side 11, intended to face the lower vertebral plate of a second vertebra adjacent and superior to said first vertebra, and a second side 12, opposite to said first side 11, comprising a seat 15. The intermediate component 30 is intended to be interposed between the lower component 10 and the upper component 20 and is provided at the top with a part 35 shaped to engage with the seat 15 to identify a spherical joint together with the seat 15. The elastic means 40 are intended to be functionally interposed between the lower component 20 and the intermediate component 30 to react elastically to compression forces acting along the vertebral column once the aforementioned prosthesis has been implanted between the aforementioned first and second vertebrae.

In certain positions of the spinal column and depending on the type of elastic means used to assemble the prosthesis, the shocks due to the compressive force acting on the vertebrae affected by the implant could also lead to permanent deformation of the elastic means. Therefore, disc prostheses are preferred in which the lower component 20 is provided with a limit switch 28, 29 arranged to limit the excursion of the elastic means 40 to avoid permanent deformation of the elastic means 40.

The limit switch, according to an embodiment not shown, may have a suitable protuberance present on the second side 12 of the upper component.

Advantageously, as in the embodiment shown, the limit switch 28, 29 is arranged to abut the intermediate component 30 when the elastic means reach the maximum allowed excursion.

For this purpose, the intermediate component 30 can comprise: a cap 36 provided with a cap conforming the aforementioned part 35 shaped to engage with the seat 15; and a stem 33 that originates from the lower wall of the cap 36, while the lower component 20 comprises on a second side 22, opposite the relative first side 11, a housing 24 for jointly receiving the elastic means 40 and the stem 33 (Fig .1).

When the bottom of the housing 24 defines the aforementioned limit switch 28 to abut the shank 33, the lower component 20 can abut the shank 33 by finding its relative lower contact surface 38.

When the cap 36 is provided with a peripheral annular contact surface 39 provided in correspondence with the lower wall of the cap 36, the lower component 20 comprises on the second side 22 and the aforementioned limit switch 29, which is annular, concentric and external to the € ™ housing 24 to abut the peripheral annular contact surface 39 of the cap 36.

In the prosthesis shown, the intermediate component 30 is shaped like a mushroom and has both a shank 33 provided with a lower contact surface 38 and a cap 36 provided at the bottom with the aforementioned peripheral annular contact surface 39. Therefore it can simultaneously meet the lower component 20 in two different limit switches; that is to say both the limit switch 28 provided in the bottom wall of the housing 24 and the aforementioned annular limit switch 29. In this way the intermediate component 30 encounters the lower component 20 on a larger surface than a single limit switch 28, 29 decreasing the pressure acting on the single limit switches 28 and 29.

As shown in Figs. 4 and 6, once the prosthesis 1 has been pre-assembled, the elastic means 40 support the intermediate component 30. When the assembled prosthesis 1 is implanted, the compressive forces acting on the vertebrae deform the means rubber bands, compressing them. Typically, the elastic means 40 will be designed to be partially compressed when the patient is in an upright position and is not subjected to strain and the load acting on the vertebrae affected by the implant is substantially given by the weight of the part of the body overlying the vertebrae. .

Following an axial shock, for a short period a greater load weighs on said vertebrae further compresses the elastic means 40 and the upper end 42 of the elastic means approaches the lower component making an excursion F. The intermediate component 30 supported by the elastic means, it approaches the lower component 20 until it encounters the limit switch 28, 29. In this way the elastic means 40 cannot be further compressed and are protected from permanent deformation. The resistance of the intervertebral disc prosthesis 1 is given by the nature of the materials of which its lower, intermediate and upper components 10, 30, 20 are made. Therefore, prostheses in which said components 10, 30, 20 are made of metal are advantageous.

Once this greater load has been released, at least partially, the reaction of the elastic means 40 will be directed to raise the relative upper end 42 and consequently the intermediate component e 30 will be moved away from the lower component 20. These elastic means 40 therefore act as â € œshock absorbâ € as they are able to cushion the compressive forces acting on the vertebrae allowing the intermediate component to be able to oscillate vertically.

Preferably said elastic means 40 are made of medical grade PEEK. They can comprise at least one spring, which is preferably shaped like a cup spring 45 provided with an axial through hole 43.

Advantageously, the intermediate component 30 is shaped like a mushroom whose stem 33 can be engaged in the through hole 43 of the cup spring 45 and whose cap 36, once the stem 33 is engaged in the through hole 43 of the cup spring 45, is It is intended to meet, with the relative lower wall from which stem 33 originates, the upper end 42 of the cup spring 45. The lower component 10, the cup spring 45 and the stem 33 are arranged in such a way that once the prosthesis has been assembled with the stem 33 engaged in the through hole 43, the cup spring 45 supports the intermediate component 30.

The prostheses 1 according to the invention are preferred, in which the bottom of the housing 24 is provided with a central recessed seat, so that a first annular wall 25 remains identified, raised and external to the recessed central seat, which I support the cup spring 45, and a second wall, inside the recessed central seat, which defines the aforementioned limit switch 28 to abut the stem 33.

Preferably said elastic means 40 are made of medical grade PEEK. They may comprise at least one spring, which is preferably shaped like a cup spring 45.

When the cup spring 45 is arranged with the concavity facing the lower component 20 it can support the intermediate component 30 more easily.

In the event that a single cup spring 45 is present, as in the embodiment illustrated in Figs. 1-7, this spring 45 may have a rather squat shape with a high thickness value S. In this way it is possible obtain a sufficient elasticity of the prosthesis 1 even with a high compressive force acting on the vertebrae.

The proposed disc prosthesis 1 may be a total disc prosthesis intended to be implanted to replace an intervertebral disc for an anterior surgical approach. In this case, the elastic means 40 may also not comprise a cup spring 45 and / or may not be made of PEEK. In fact, even with elastic means 40 shaped in another way and / or in other materials, it is possible to obtain total disc prostheses having an elasticity comparable to that of the nucleus pulposus, in particular if it is intended to be implanted in a position of the vertebral column in where the weight to be supported is less.

Advantageously, the disc prosthesis 1 according to the invention is sized to replace the nucleus pulposus of an intervertebral disc and therefore allows implantation via a posterior surgical access route. The reduced dimensions of this core disc prosthesis, linked to the compression that the core prosthesis must withstand, can be advantageously obtained with the embodiment of Figs. 1-7, in which the disc prosthesis 1 comprises lower and upper components 20, 10 metal; elastic means 40 of PEEK, shaped like a cup spring 45; and an intermediate component 30, entirely in metal, shaped in the shape of a mushroom as previously described with the stem 33 engageable in the through hole of the cup spring 45. In particular, in this embodiment of the core disc prosthesis 1 shown in Figure 1 -7, the PEEK cup spring 45 guarantees a flexion F of 0.3 mm when subjected to a compression force of 120 kg Fig. 6. Furthermore, the metal structure of the disc core prosthesis 1, once assembled, guarantees a load of break similar to that of the vertebrae and equal to 300 kg. Preferably, the components of the disc prosthesis are shaped in such a way as to obtain, in plan view, a spherical joint arranged centrally with a center of rotation which, once the prosthesis is implanted, must be closer to the lower vertebra than to the upper one. As can be seen in Figs. 3-7 the first sides 11, 21 and the second sides 12, 22, of the lower and upper components have an inclination such as to allow the upper component 20 to incline in all directions by at least 4 °, preferably 4.5 °, with respect to the lower component 10. In Fig. 7 the disc prosthesis 1 has one of the possible inclinations of the upper component 10 with respect to the lower one 20.

Advantageously, the disc prosthesis 1 comprises a plurality of anchoring teeth 14 which, once penetrated into the vertebral plates, avoid displacements of said prosthesis 1. These teeth can be arranged on the first side 21 of the lower component 10 and on the first side of the upper component. 20.

The surfaces in contact with the vertebral plates, on the first sides 11 and 21 of the lower 20 and upper 10 components, may comprise compositions or substances intended to facilitate the fusion of these components with the vertebrae. For example, the prosthesis 1 can comprise a layer of hydroxyapatite on said first sides 11 and 21.

It is understood that what above has been described by way of non-limiting example, therefore any variants of a practical-applicative nature are intended to fall within the protective scope of the invention as described above and claimed hereinafter.

Claims (13)

CLAIMS 1. Intervertebral disc prosthesis, comprising: a lower component (20) provided with a first side (21) intended to counter face the upper vertebral plate of a first vertebra; an upper component (10) provided with: a first side (11) intended to face the lower vertebral plate of a second vertebra adjacent and superior to said first vertebra; and a second side (12), opposite to said first side (11), comprising a seat (15); an intermediate component (30) intended to be interposed between the lower component (10) and the upper component (20) and provided at the top with a part (35) shaped to engage with the seat (15) to identify jointly with the seat (15) a ball joint; characterized by further understanding: elastic means (40) intended to be functionally interposed between the lower component (20) and the intermediate component (30) to react elastically to compressive forces acting along the spinal column once the aforementioned prosthesis has been implanted between the aforementioned ones and second vertebra. 2. Prosthesis according to claim 1, in which the part (35) of the intermediate component (30) shaped to engage with the seat (15) and the seat itself (15) are provided with a layer of metal in correspondence with the relative intended surfaces to contact each other. Prosthesis according to the preceding claim, in which the upper component (20) and the intermediate component (30) are made of metal. 4. Prosthesis according to any one of the preceding claims, wherein the lower component (20) is made of metal. 5. Prosthesis according to any one of the preceding claims, in which the lower component (20) is provided with a limit switch (28, 29) arranged to limit the excursion of the elastic means (40). 6. Prosthesis according to the preceding claim, in which the limit switch (28, 29) is arranged to abut the intermediate component (30) when the elastic means reach the maximum allowed excursion. 7. Prosthesis according to claim 6, wherein: the intermediate component (30) comprises: a cap (36) provided with a cap conforming the aforementioned part (35) shaped to engage with the seat (15); a stem (33) originating from the lower wall of the cap (36); a peripheral annular contact surface (39) provided at the lower wall of the cap (36); and the lower component (20) comprises: on a second side (22), opposite the relative aforementioned first side (11), a housing (24) for jointly receiving the elastic means (40) and the stem (33); and the aforementioned limit switch (29), which is annular, concentric and external to the housing (24) to meet the peripheral annular contact surface (39) of the cap (36). 8. Prosthesis according to claim 6, wherein: the intermediate component (30) comprises: a cap (36) provided with a cap conforming the aforementioned part (35) shaped to engage with the seat (15); and a stem (33) originating from the lower wall of the cap (36); and the lower component (20) comprises on a second side (22), opposite to the relative aforementioned first side (11), a housing (24) for jointly receiving the elastic means (40) and the stem (33), the bottom of the € ™ housing (24) defining the aforementioned limit switch (28) to abut the stem (33). Prosthesis according to any one of the preceding claims, wherein the elastic means (40) comprise at least one cup spring (45) provided with an axial through hole (43). 10. Prosthesis according to the preceding claim, in which the intermediate component (30) is shaped like a mushroom whose stem (33) can be engaged in the through hole (43) of the cup spring (45) and whose cap (36 ), once the stem (33) is engaged in the through hole (43) of the cup spring (45), it is destined to meet, with the relative lower wall from which the stem (33) originates, the upper extremity (42) of the Belleville washer (45). 11. Prosthesis according to any one of claims 9 to 10, in which the bottom of the housing (24) is provided with a central recessed seat, so that a first annular wall (25) is identified, raised and external to the seat central recessed seat, which supports the cup spring (45), and a second wall, inside the recessed central seat, which defines the aforementioned limit switch (28) to abut the stem (33). Prosthesis according to any one of the preceding claims, in which the elastic means (40) are made of PEEK. 13. Prosthesis according to any preceding claim, wherein the prosthesis (1) is sized to replace the nucleus pulposus of an intervertebral disc.