ITRM20110659A1 - INTERVERTEBRAL DISC PROSTHESES FOR THE STABILIZATION OF THE LUMBAR COLUMN AND ITS KIT. - Google Patents

Description

INTERVERTEBRAL DISC PROSTHESIS FOR STABILIZATION

OF THE LUMBAR COLUMN AND RELATIVE KIT.

The present invention relates to an intervertebral disc prosthesis for the stabilization of the lumbar spine and relative kit.

More specifically, the invention relates to a prosthesis of the so-called type for correcting the spinal column, capable of restoring the physiological range of motion of two vertebral bodies of the spinal column.

In the following the description will be mainly addressed to the correction of the lumbar area of the spinal column, but it is quite clear that the same should not be considered limited to this specific use.

As is well known, the spine is often subject to various pathologies. In many cases, it is also necessary to replace the intervertebral disc, and in particular both the external annulus and the nucleus pulposus.

Low back pain, or lumbar pain, is one of the pathologies for which the intervertebral disc is most frequently replaced with the implantation of a prosthesis. This pathology is very frequent and can also cause severe pain to those affected. Prosthetic replacement surgery is generally performed especially for chronic low back pain refractory to conservative or pharmacological treatments.

Currently, there are a large number of commercially available intervertebral disc prostheses, which however have several problems.

The first limitation of known prostheses is that they tend to be very rigid from a mechanical point of view. In particular, these prostheses, precisely because of the position for which they are intended, are subjected to considerable mechanical stresses, due to the numerous flexion, rotation and torsion movements to which the human spine is subjected. The intervertebral disc prostheses currently available on the market are very rigid and often do not allow the person to whom they are implanted to recover all possible movements.

In fact, from a strictly functional point of view, it has been observed that the prostheses according to the known technique have a reduced level of reactivation of the lordotic alignment, not being able to maintain the physiological ranges of movement due to flexion / extension, rotation and lateral inclination, with the consequence for the patient of considerable limitations in movements.

A second problem that the prostheses according to the known art have, much more serious than the first, is that they generally have large dimensions. This implies that it is not possible to install them between the vertebrae by acting with a well-known and minimally invasive surgical operation of discectomy, accessing the site where said prosthesis must be implanted posterior to the patient, as it would be more natural and much less invasive.

Among other things, regarding the kinematics aspects, currently the prosthetic disc replacement technology has two basic types of approaches available based on the degree of freedom allowed with respect to uniaxial rotation:

to. unconstrained devices with variable center of rotation that allow segmental rotation and translation through three axes X, Y, Z; b. constrained devices with fixed center of rotation, so that the anteroposterior and lateral translation movements are prevented, while the rotational movements are allowed.

Unconstrained devices require the presence of an intact ligament structure. On the other hand, the restricted ones do not guarantee the restoration of the totality of the movements allowed by the natural disk.

The prostheses available today must be implanted through a surgical operation by accessing from the abdomen. This obviously implies a series of operative and post-operative complications, as well as greater risks in general, including:

- the solicitation of vital parts;

- greater difficulty for the surgeon to operate, which also translates into a decrease in the probability of success of the operation;

- fewer patients who can afford to undergo the operation, for example for reasons of age, health and the like.

Last but not least, lastly, the prostheses of the known art are also very expensive.

In light of the above, therefore, the purpose of the present invention is to propose an intervertebral disc prosthesis for the stabilization of the spinal column, in particular of the lumbar spine, which can be installed posteriorly between two vertebral bodies by means of a simple discectomy (so-called minimally invasive access), following established surgical techniques.

It is also an aim of the present invention to propose a prosthesis which has mechanical and kinematic properties comparable to those of a human intervertebral disc, allowing the patient to perform all possible movements with the vertebral column.

Finally, the aim of the present invention is to propose a kit for the use of said prosthesis.

Therefore, the specific object of the present invention is an intervertebral disc prosthesis interposable between two vertebral bodies, characterized by the fact of comprising at least one upper arm with mainly longitudinal extension, at least one lower arm having a mainly longitudinal extension and an intermediate junction element, interposed between said at least one upper arm and said at least one lower arm, to which it is coupled in rotation by coupling means, said prosthesis assuming a closed operating position, in which said arms are arranged with said longitudinal axes substantially aligned, and an operating position of opening, in which said arms have misaligned axes.

Always according to the invention, said prosthesis can comprise a first upper arm and a second upper arm, and a first lower arm and a second lower arm, said first upper arm and second upper arm and said first lower arm and second lower arm being reciprocally coupled in such a way as to be able to assume at least two reciprocal positions, respectively with parallel longitudinal axes and orthogonal longitudinal axes.

Still according to the invention, in said operating open position the longitudinal axes of said first upper arm and said second upper arm can form a substantially right angle, the longitudinal axes of said first lower arm and said second lower arm can form a substantially angle straight, and the longitudinal axes of said first upper arm and said second lower arm can be substantially parallel.

Further according to the invention, the external surface of said arms can have a semi-threading which in said closed operative position such as to constitute a complete thread, and such that in said open operative position said semi-threads can be anchored to the vertebral bodies between which it is interposable .

Advantageously according to the invention, said intermediate joining element can comprise a first body and a second body coupled by means of a ball joint.

Always according to the invention, said first body has an upper surface intended to be housed in a circular seat of the arm to which it approaches, and a lower surface provided with a cavity, and said second body has a convex upper surface, suitable to be inserted in said cavity, with joint or compression coupling, so as to form a spherical joint, and a lower surface intended to be housed in a circular seat of the arm to which it approaches.

Still according to the invention, said circular seat can comprise steps arranged circularly and having a progressive increasing height, and said upper surface of said first body and said lower surface of said second body each comprise a projection adapted to interact with said steps of the respective circular seat .

Furthermore, according to the invention, said first and second bodies of said intermediate joining element comprise a series of perimeter teeth respectively arranged laterally.

Advantageously according to the invention, said at least one upper arm and said at least one lower arm have a central hole, and said coupling means comprise two tightening nuts each inserted in said holes, respectively of said at least one upper arm and of at least one arm lower, and coupled to said intermediate joining element.

Still according to the invention, each of said first and second tightening nut has a substantially cross-shaped opening and a helical groove on the lateral surface, and by the fact that it comprises an anchoring pin including a pair of fins, substantially at 180 °, a with respect to the other, which are inserted in the slots of said cross opening and adapted to act on said helical groove, so that following the rotation of said at least one upper and lower arm, there is an exit of said anchoring pin from said tightening nut.

A further object of the present invention is an intervertebral disc prosthesis kit, comprising at least one intervertebral disc prosthesis as defined above, and at least one insertion-extraction device comprising: a support element comprising a stem having a first end, on which a handle and a first and a second knob, and a second extremity; a gripper comprising two jaws pivoted centrally in correspondence with a fulcrum having a hole, each jaw comprising a handle, suitable to be arranged on said second end of said support element, and a gripping element and being able to act on one of the teeth of said intermediate element, said pliers further comprising a threaded rod rotatable by means of said first knob, passing through said hole and having an end of stroke, said rod being engaged to a slide via a threaded hole; and a pair of articulated arms coupled by means of said central threaded hole of said slide to said threaded rod, one end of said articulated arms being pivoted in correspondence with said threaded hole, while the other end is pivoted in proximity to said element of gripping said jaws.

Always according to the invention, each of said gripping elements of said jaws comprises a screw, operable with said second knob, adapted to interact with a respective series of perimeter teeth of said intermediate joining element.

Still according to the invention, said kit can comprise an element for completing the thread able to complete the threading of said prosthesis made by means of the half-threads in said closed operative position.

The present invention will now be described for illustrative but not limitative purposes, according to its preferred embodiments, with particular reference to the figures of the attached drawings, in which:

Figure 1 shows a perspective view of an embodiment of the intervertebral disc prosthesis according to the invention in the open position;

figure 2 shows a top view of the prosthesis of figure 1;

figure 3 is an exploded view of the prosthesis of figures 1;

Figure 4 shows a perspective view of the prosthesis according to Figure 1 in the closed position; Figure 5 shows an intermediate element of the intervertebral disc prosthesis according to the invention;

figure 6 shows the coupling with an arm of the prosthesis of the intermediate element according to figure 5;

figure 7 shows an exploded view of a fixing nut with the respective anchoring pin of the prosthesis according to the invention;

figure 8 shows a side view of an intervertebral disc prosthesis according to the invention placed between two vertebral bodies;

figure 9 shows a further lateral view of an intervertebral disc prosthesis according to the invention placed between two vertebral bodies;

Figure 10 shows a perspective view of a prosthesis insertion-extraction device according to the present invention;

figure 11 shows a detail of the insertion-extraction device according to figure 10;

figure 12 shows a pliers of the insertion-extraction device according to figure 10;

Figure 13 shows a detail of the gripper of the insertion-extraction device according to Figure 10;

Figure 14 shows the jaws of the pliers of the insertion-extraction device according to the figure

10;

figure 15a shows a first step of coupling a prosthesis according to the invention to a pliers of the insertion-extraction device;

Figure 15b shows a second step of coupling a prosthesis according to the invention to a pliers of the insertion-extraction device;

figure 15c shows a third step of coupling a prosthesis according to the invention to a pliers of the insertion-extraction device;

figure 15d shows a fourth step of coupling a prosthesis according to the invention to a pliers of the insertion-extraction device;

figure 16 shows a plan view of the prosthesis in the open operative position by means of the insertion-extraction device;

figure 17 shows a perspective view of the prosthesis according to figure 16; And

figure 18 shows a cross section of the prosthesis according to the invention.

In the various figures similar parts will be indicated with the same numerical references.

With reference to figures 1 - 4, it is possible to observe the parts of an embodiment of the intervertebral prosthesis 1 according to the invention.

As can be seen, said prosthesis 1 comprises an intermediate joining element 50, to which a first pair of upper arms 2 and a second pair of lower arms 3 are coupled. Each pair of upper arms 2 and lower 3 comprises a coupling arm 20 ' , 30 'with said intermediate joining element 50 and a second arm 20, 30. The arms 20, 20' and 30, 30 'of each pair are mutually superimposed and mutually rotating with respect to a central axis and made in such a way as to couple reciprocally by grafting, when parallel, with respect to each other.

Each of said arms 20, 20 ', 30 and 30' has a central hole, respectively 21, 21 ', 31 and 31'.

Circular seats 40 are made around the holes 21 'and 31' (only one is visible in the figure) in which steps 41 are provided, arranged circularly and having a progressive increasing height, the function of which will be better specified below.

Finally, each of said arms 20, 20 ', 30 and 30' has internal channels, respectively 22, 22 ', 32 and 32', the function of which will be better defined below.

Said intermediate joining element 50 (see also Figure 5) comprises a first body 51, having an upper surface 52 which is housed in the circular seat 40 of the arm 20 ', and a lower surface provided with a cavity (not visible in the figures), and a second body 51 ', having a convex upper surface 52', able to fit into said cavity, with joint or compression coupling, so as to form a spherical joint, and a lower surface 53 'which is housed in the circular seat 40 of the arm 30 '.

With reference to figures 5 and 6, analyzing in greater detail the intermediate joining element 50, it can be observed that said first body 51 and said second body 51 'have each side of the perimeter teeth 54 and 54' respectively, made in such a way as to allow the coupling of a device for inserting / extracting the prosthesis 1, which will be better described below.

Furthermore, said first and second bodies 51 and 51 'comprise, on the respective upper surfaces 52 and lower 53', protrusions 55 and 55 ', adapted to interact with said steps 41 of said circular seats 40. The structure of the lower surface (convex ) of the body 51 and of the upper (concave) surface 52 'of the body 51' allows their reciprocal rotation (the purpose of which will be clearer in the remainder of this description). The provision of the steps 41 of increasing height and of the protrusions 55, 55 'on the surfaces of the bodies 51, 51' which are housed in the housings 40 allows, following the relative rotation of the two bodies 51, 51 ', to modify the height of the prosthesis 1.

Two clamping nuts 60 couple, respectively, in the holes 21, 21 'and 31, 31' of the arms 20, 20 'and 30, 30', respectively. Said nuts 60 each have (see also Figure 7) tongues 61 for coupling with the arm, 20 or 30 respectively, and, on the upper surface, a substantially cross opening 62 for the insertion of a respective pin anchor 70, having a pair of fins 71, substantially at 180 ° with respect to each other, which are inserted into the slots of said cross-shaped opening 62.

A helical groove 63 is formed on the side of said nut 60, in which, after the insertion of the pin 70, said tabs 71 act, so that, following the rotation of the arms 20, 20 'and 30, 30', there is an exit of the pin 70 from the nut 60, and therefore from the prosthesis 1, obtaining an anchorage to the vertebral bodies A and B by means of the tip 73 of the pin 70.

Said prosthesis 1 can therefore assume a closed operative position, in which said arms 20, 20 ', 30 and 30' are arranged with their own longitudinal axes aligned and mutually coupled by grafting, as shown in figure 4, and an operative position open (Figures 1 - 3), in which said arms 20, 20 ', 30 and 30' are arranged with their longitudinal axes misaligned, and in particular in the present embodiment the arms of each pair 2 or 3 are arranged with the axes forming a substantially right angle, as a limit position.

The external surface of the arms 20, 20 'and 30, 30' has a configuration having a half thread 80 such as to constitute, in the closed operative position of said prosthesis 1, a thread.

Figures 8 and 9 show the prosthesis 1 arranged between two vertebral bodies A and B, in the open position. In particular, figure 8 shows how the prosthesis 1 according to the invention can be stressed by bending and compression (observe the arrows in the figure).

Furthermore, the fixing of the prosthesis 1 to the vertebral bodies is obtained through the action of the half-threads made on the pairs of arms 2 and 3, and through the lifting of the anchoring pins 70 (described in detail previously).

In particular, said prosthesis 1 is inserted between the vertebral bodies in the closed position. Once said prosthesis 1 is inserted by screwing between two vertebral bodies A and B and is opened, the threading made by means of the half-threads 80 of the arms 20, 20 'and 30, 30' is inserted into the bone tissue of said vertebral bodies A and B including the prosthesis 1 can be placed, ensuring primary fixation and obtaining a high resistance to retropulsion, thus allowing, in particular, an optimal restoration of the correct segmented lumbar lordosis.

Consider that the intermediate joining element 50 is made of a material capable of guaranteeing certain mechanical characteristics and of withstanding the load conditions typical of the lumbar spine, which, as known, is subject to different mechanical stresses due to rotation and torsion, as well as bending. Said material is also inert from a biological point of view, thus presenting a low probability of rejection.

The intermediate joint element 50, therefore, allows the maintenance of the physiological range of motion by flexion / extension, rotation and lateral inclination, as well as the restoration of the height of the relative segment, of the anatomical balance and of the stability of the spinal column.

With reference now to figures 10 - 14, the structure of the device for inserting and extracting said prosthesis 1 into / from a pair of vertebral bodies can be seen.

Said insertion-extraction device 90 comprises a support element 100 consisting of a stem 101, having a first and a second end 102 and 103 respectively. On said first end 102 there are provided a handle 104 and a first and a second knob, respectively 105 and 106, arranged superimposed.

A gripper 110 is coupled to the end 103 of the stem 101. Said gripper 110 consists of two jaws 111, centrally pivoted at the fulcrum 112, each having a handle 111 'and a gripping element 111 ". Said fulcrum 112 also has a hole 113 through which a rod 114 passes with an end of stroke 115 which acts as a guide for a slide 116 '. A pair of articulated arms 116 is coupled, by means of said slide 116'. , moreover, it passes through a central threaded hole 117 obtained in said slide 116 '. One end of the articulated arms 116 is pivoted in correspondence with said threaded hole 117 (i.e. the slide 116'), while the other end is hinged in proximity of the gripping element 111 "of the jaws 111.

Two screws 118 are provided internally on the gripping elements 111 ", substantially arch-shaped, each acting on one of the teeth 54 and 54 'of the intermediate element 50.

When said insertion-extraction device 90 is coupled to said prosthesis 1, the front parts of the articulated arms 116 are inserted into the internal channels 22, 22 ', 32 and 32' of said arms 20, 20 ', 30 and 30'. In this way, said mechanism is able to allow the passage from said closed operative position to said open operative position of said prosthesis 1.

Observing figures 15a - 15d, it is possible to observe the coupling of the prosthesis 1 to the forceps 100, which occurs initially by coupling said ends 11 'and 111' 'between the perimeter teeth 54 and 54'.

Preferably, the prosthesis 1 also comprises an element for completing the thread 119, which can be previously inserted in the insertion and extraction device 90, which compensates for the shaped parts of the arms 20, 20 ', 30 and 30'.

Once the pliers 110 have been coupled with the prosthesis 1, it is inserted between the two vertebral discs A and B. Then, the knob 105 is operated, which transmits the rotary movement by means of a hollow shaft (not visible in the figure) passing inside the support element 100. This hollow shaft is coupled with the rod 114, which being threaded allows the sliding of the slide 116 'and therefore the opening movement of the arms of the prosthesis 1. it acts on the push rod 114 which determines, by means of the action of the articulated arms 116, the rotation of the pairs of arms 2 and 3 with respect to the intermediate element 50, as can be seen in figures 16 and 17.

At the same time, the tip 73 of the anchoring pin 70 rises, anchoring itself to the vertebral bodies A and B.

The screws 108, while the gripper 100 is still closed and tightened with the gripping elements 101 "the intermediate element 50, are activated by means of said knob 96 The knob 106 transmits the rotary movement by means of a further shaft (not visible in the figure) concentric with said hollow shaft passing inside the body of the caliper 110. This further shaft is provided in its other end with a toothed wheel (not visible in the figure) which drives two secondary shafts (also not visible in the figure) passing inside the gripping elements 111 'and 111' '(see figure 13). This pair of secondary shafts which are located inside the gripping elements 111' and 111 '' are coupled to the other end, with screws 118 (see figure 14).

By rotating, said screws 108 each act on the perimeter teeth 54 and 54 'respectively, increasing the height of the prosthesis 1.

Subsequently, it is sufficient to remove the support element 90 from the grips 94 of the gripper 100, open the gripper 100 by widening the jaws 101 and uncouple said gripper 100 from said open prosthesis 1.

The surgical technique that can be used with the present prosthesis 1 is performed under general anesthesia. When the prosthesis 1 according to the invention is in the closed position, it assumes a longitudinal cylindrical shape with a lateral thread, simulating a screw. This allows a posterior insertion of the same, through a discectomy.

To implant said prosthesis 1, in particular, the following steps are followed:

- placing the prosthesis 1 in the closed operative position;

- coupling the prosthesis 1 to said insertion-extraction device 90;

perform a discectomy and a posterior laminotomy, i.e. the posterior vertebral arch is opened, making a minimally invasive opening, to contain the diameter of the intervertebral prosthesis 1;

- screwing of the prosthesis 1 limitedly to the part of the sub-chondral bone, but not cancellous, by means of the insertion-extraction device 90; and - opening or expansion of the prosthesis 1 and removal of the insertion-extraction device 90.

The extraction of the prosthesis 1 is very easy thanks to the cross section that it presents, which is visible in figure 18. When it has to be removed, it is sufficient to couple the prosthesis 1 with the forceps 100, lower the completion of the thread 119 and rotate the forceps around its longitudinal axis and extract it from the vertebral bodies.

Different embodiments of the intervertebral prosthesis 1 are possible, which can also provide a different number of upper arms, in the limit only one and a different number of lower arms, also in this case in the limit only one.

An advantage of the present invention is that of allowing its implantation by acting posteriorly by performing a discectomy and a posterior laminotomy on the patient, a procedure consolidated in surgical practice which therefore does not introduce additional surgical risks.

Another advantage of the present invention is that the intermediate element guarantees a range of movements close to that which characterizes each level of the lumbar spine (Yamamoto).

A further advantage of the present invention is that it allows to respect the anatomy of the neuronal arch in order to maintain greater stability of the treated segment, thanks to the fact that the interbody prosthesis under examination allows, thanks to its shape and characteristic, to be accessible posterior and minimally invasive, as well as thanks to the preservation of the functional part of the joint facets. Its design allows a great ease of insertion, while the cylindrical body, the thread and the fixing pins ensure excellent stability, guaranteeing primary bone fixation and avoiding migration.

Another advantage of the invention is that the prosthesis is equipped with a mechanism that allows it to make its height variable.

Furthermore, the artificial disc must allow the normal movements of the spinal column in such a way that it itself (artificial disc) molds itself on the movement of the vertebrae. Furthermore, the prosthesis is suitable to withstand both normal and maximum load conditions, it works as a stand-alone device if the posterior elements are intact, it is resistant to wear and allows osseointegration at the bone-prosthesis interface.

A further advantage of the present invention is that the prosthesis allows to restore a center of rotation closer to the physiological one than those with a fixed instantaneous rotation center (CIR) and to transfer less stress on the facet joints. In unconstrained prostheses such as the one under examination, the role played by the surrounding soft tissues is very important to ensure stability, to preserve the posterior longitudinal ligament as much as possible (PLL Posterior Longitudinal Ligament) so as to increase the stability of the implant, limiting the forces in hyperflexion. Furthermore, uncostrained prostheses are more tolerant of positions that differ slightly from the optimal positioning, forgiving the surgeon, in the first experiences in the prosthetic field, limited positioning errors.

The present invention has been described for illustrative but not limitative purposes, according to its preferred embodiments, but it is understood that variations and / or modifications may be made by those skilled in the art without thereby departing from the relative scope of protection. , as defined by the appended claims.

Claims (7)

CLAIMS 1. Intervertebral disc prosthesis (1) interposable between two vertebral bodies (A, B), characterized by the fact of understanding at least one upper arm (20, 20 ') with mainly longitudinal extension, at least one lower arm (30, 30 ') having a mainly longitudinal extension e an intermediate joint element (50), interposed between said at least one upper arm (20, 20 ') and said at least one lower arm (30, 30'), to which it is coupled in rotation by means of coupling means (60), said prosthesis (1) assuming a closed operative position, in which said arms (20, 20 '; 30, 30') are arranged with said longitudinal axes substantially aligned, and an open operative position, in which said arms (20, 20 '; 30, 30') have misaligned axes, said prosthesis (1) being characterized in that said at least one upper arm (20, 20 ') and said at least one lower arm (30, 30') have a central hole (21, 21 ', 31, 31'), and said coupling means comprise two tightening nuts (60) each inserted in said holes (21, 21 ', 31, 31'), respectively of said at least one upper arm (20, 20 ') and of at least one lower arm (30 , 30 '), and coupled to said intermediate joining element (50). 2. Prosthesis (1) according to claim 1, characterized in that it comprises a first upper arm (20) and a second upper arm (20 '), and a first lower arm (30) and a second lower arm (30') , said first upper arm and second upper arm and said first lower arm and second lower arm being mutually coupled so as to be able to assume at least two reciprocal positions, respectively with parallel longitudinal axes and orthogonal longitudinal axes. 3. Prosthesis (1) according to claim 2, characterized in that, in said open operating position, the longitudinal axes of said first upper arm (20) and said second upper arm (20 ') form a substantially right angle, the longitudinal axes of said first lower arm (30) and said second lower arm (30 ') form a substantially right angle, and the longitudinal axes of said first upper arm (20) and said second lower arm (30 ') are substantially parallel. 4 Prosthesis (1) according to any one of the preceding claims, characterized in that each of said clamping nuts (60) has a substantially cross-shaped opening (62) and a helical groove (63) on the lateral surface, and by the fact that comprise an anchoring pin (70) including a pair of fins (71), substantially at 180 ° with respect to each other, which are inserted into the slots of said cross opening (62) and adapted to act on said helical groove (63 ), so that following the rotation of said at least one upper and lower arm (20, 20 ', 30, 30'), said anchoring pin (70) exits from said nut clamping (60). 5 Intervertebral Disc Prosthesis Kit, Including - at least one intervertebral disc prosthesis (1) as defined in claims 1-4, and at least one insertion-extraction device (90) comprising: - a support element (100) comprising a stem (101) having a first end (102), on which a handle (104) and a first and a second knob (105, 106) are provided, and a second end (103 ); - a pliers (110) comprising two jaws (111) pivoted centrally at a fulcrum (112) having a hole (113), each jaw (111) comprising a handle (111 '), suitable to be arranged on said second end ( 103) of said support element (100), and a gripping element (111 ") and being able to act on one of the teeth (54, 54 ') of said intermediate element (50), said gripper (110) further comprising a threaded rod (114) rotatable by means of said first knob (105), passing through said hole (113) and having an end of stroke (115), said rod (114) being engaged to a slide (116 ') mediated a threaded hole (117); e - a pair of articulated arms (116) coupled by means of said central threaded hole (117) of said slide (116 ') to said threaded rod (114), one end of said articulated arms (116) being pivoted at said threaded hole (117), while the other end is pivoted near said gripping element (111 ") of said jaws (111). 6 Kit according to claim 5, characterized in that each of said gripping elements (111 ") of said jaws (111) comprises a screw (118), which can be operated with said second knob (106), adapted to interact with a respective series of perimeter teeth (54, 54 ') of said intermediate joining element (50). 7. Kit according to any one of claims 5 or 6, characterized in that it comprises an element for completing the thread (119) adapted to complete the threading of said prosthesis (1) made by means of the half-threads (80) in said operative position closed .