Bone screw for an osteosynthesis device, assembly formed by a screw, a connector and a nut, and kit comprising at least one such assembly
This invention relates to prosthetic devices implantable on the bones of a patient, and more particularly to bone screws for fixing said devices.
It is particularly, but not exclusively, applicable to bone screws which are to be implanted in vertebrae and on the head of which elements are arranged to connect the vertebra to a rod-like device.
It is common practice in spinal surgery to use osteosynthesis devices, the essential element of which is one or more rods extending over a more or less long part of the spine to correct the relative positioning of two or more vertebrae.
These rods are fixed to the vertebrae by means of bone screws, with a rod-receiving element at the end emerging from the vertebra, i.e. at the part we will call “upper”. This element is often “tulip-shaped”, i.e. with two opposite walls defining an opening into which the stem is inserted.
These walls are internally threaded, and after insertion of the rod into the space defined by the walls, a threaded plug is screwed between the walls to close the space and securely wedge the rod in.
In other variants, which are the ones covered by the present invention, as well as US2016183983, the rod is received not in such a socket, but in a connector that allows the rod to be laterally offset from the longitudinal axis of the screw and to choose an orientation of the rod with respect to the screw that is well suited to the treatment of the patient's pathology.
This connector is then attached to the screw, by insertion on a threaded rod-like element, which will be called a “pin” in the following, and which constitutes the upper part of the screw.
The connector is attached to this pin by means of a nut that engages the threaded rod of the pin and presses the connector against the base of the pin.
Typically, the pin and the lower part of the screw, which has a thread for penetrating and then securing it in the vertebra, are placed in the extension of each other along the same longitudinal axis and form a monobloc unit.
The screw is inserted into the vertebra by the surgeon using an instrument such as a hollow-tipped screwdriver, which engages a surface with flats (e.g. forming a hexagon) which is provided towards the lower end of the pin, at least above the upper end of the threaded portion of the screw.
The rotation of the instrument ensures that the threaded part of the screw penetrates the vertebra.
Once the screw is secured, the pin and its lower end emerge from the vertebra.
The connector for the shaft of the osteosynthesis device can then be inserted onto the pin, the shaft placed in the connector, and the locking nut inserted onto the threaded part of the pin so as to lock the screw-connector-stem assembly in the desired position.
Advantageously, the pin is breakable, i.e. it initially has a zone of lesser resistance at a certain level of its length which can be easily broken by the surgeon.
In this way, the pin can be designed to have a relatively long initial length which facilitates the insertion of the connector and then the insertion of the nut.
When the nut is tightened, the area of least resistance is slightly above the top surface of the nut.
The surgeon then exerts a lateral force on the pin which leads to the pin breaking at its weakest point, thus reducing the length of the pin to what is strictly necessary for the screw-connector-stem-nut assembly to function.
In this way, the pin is no longer excessively long and uncomfortable for the patient.
However, the known devices have a feature which, in use, can be disadvantageous.
After insertion of the connector and/or locking of the nut, the connector and nut assembly is found around the part of the pin with flats, by means of which the screw could be fixed in the vertebra.
In this way, manipulation to adjust the longitudinal position of the pin in relation to the vertebral surface by further tightening or partial loosening of the screw is not possible without complete disassembly of the connector-stem-nut assembly.
The aim of the invention
— is to provide surgeons with a new configuration of a screw with a pin, which is free from the disadvantages that have been reported, i.e. which remains fully accessible and adjustable in height even after the connector, shaft and nut have been fitted.
For this purpose, the invention has as its object a bone screw comprising a lower bone anchoring part provided with a thread, an upper part constituted by a pin with a threaded external surface for receiving a nut, and a seat for receiving a connector intended to connect the screw to a rod of an osteosynthesis device and to be held against the seat by the nut characterized in that said pin is hollow and has an inner space opening at its upper end, the lower part of said inner space ending in a recess provided with flats for receiving a screwdriver.
The pin has a line of least resistance making it breakable.
The invention also relates to an assembly formed by a bone screw, a connector for connecting the screw to a stem of an osteosynthesis device, and a nut for holding the connector against the seat of the screw, characterised in that the bone screw is of the above type.
The nut may have a flange at the bottom to rest on the upper face of the connector to wedge it against the seat after the nut has been screwed on.
The connector may have an upper surface in the shape of a portion of a sphere and the lower surface of the nut is then also in the shape of a portion of a sphere.
The nut may have a tubular top.
Said tubular part of the nut may have a line of least resistance making it breakable.
The inner space of the nut may have flats with which a screwdriver can be engaged when tightening and loosening the nut.
The said flats may then remain on the inner space of the nut after the section of its tubular upper part along the line of least resistance.
The invention also relates to a kit for an osteosynthesis device, of the type comprising atleast one screw-connector-nut assembly, characterised in that at least one screw is of the above type, and in that it also comprises a screwdriver intended to engage with the said impression terminating the lower part of the inner space of the pin.
Said nut may be of the type in which its interior space has flats, and said kit may also include a screwdriver for engaging the flats of the interior space of the nut.
As will be appreciated, the invention is based on the replacement of the usual square outer surface of the pin, allowing the screwdriver to grip the screw and insert it into the bone, with a recess for the engagement of a screwdriver, having squares or a star configuration (of the type commonly known as "Torx*”), this recess being located inside the pin itself, and remaining permanently accessible even after the screw-connector-shaft-nut assembly has been fully assembled and secured.
Furthermore, the bottom of this indentation is most preferably located towards the base of the pin, and the indentation thus remains functional even after the upper part of the pin has been separated along the line of least resistance, if any.
It is therefore possible to readjust the screw's penetration into the patient's bone at any time, simply by reinserting the screwdriver into the pin and rotating it in either direction.
An alternative, however, which would be in accordance with the invention, would be to retain a conventional external surface on the pin, which would be usable when implanting the screw.
The square indentation inside the pin would be added to this, and would be used for later adjustments of the screw drive.
As the outer fluted surface is larger in diameter than the impression, it would allow a relatively high torque to be applied and a larger diameter screwdriver to be used, thus facilitating the initial insertion of the screw into the bone more easily than if only the inner impression to the pin was used, especially at a stage when the screw is not yet firmly anchored in the bone.
Once the screw is anchored in a quasi-definite position, it can no longer tilt, and it becomes easier to turn it with a small diameter screwdriver of the type that will be inserted into the internal recess of the pin.
The invention will be better understood on reading the following description, given with reference to the following attached figures:
- Figure 1 shows a bone screw of the assembly according to the invention in front view;
- Figure 2 shows a side view of the connector intended to be fitted onto the bone screw to connect a rod of a spinal osteosynthesis device;
- Figure 3 shows the same connector in top view; - Figure 4 shows the nut that secures the connector and the rod to the screw in a front view; - Figure 5 shows the same nut in perspective from below; - Figure 6 shows the same nut in perspective from above; - Figure 7, which shows, seen in perspective, the assembly according to the invention formed by the screw, the connector and the nut in the assembled state, with the stem of the osteosynthesis device into which the assembly according to the invention is integrated; - Figure 8 shows a side view of the same assembly with the stem, as well as the bone into which the screw is inserted; - Figure 9 shows the same device, but with a conventional nut, different from the one shown in figures 4-8, seen from the front in longitudinal section. The bone screw 1 of the assembly according to the invention comprises in its lower 16 — part, as is conventional, a thread 2 intended to ensure the anchoring of the screw 1 in the bone for which it is intended, for example a vertebra, as will be given as a non-limiting example. In its upper part, the screw 1 has a cylindrical threaded rod 3 which will be called a “pin”, preferably presenting, as shown, a line of least resistance in the form of a local reduction 4 in its thickness. A connector 5, visible in figures 2 and 3, is fitted to this pin 3. This connector 5 has a body 6 with a perforation 7 which passes right through it and allows the connector 5 to be fitted onto the pin 3. A hook-shaped appendage 8 is integrally formed on the body 6, the opening 9 of which faces the lower part of the connector 5. The screw 1 has a seat 10 on which the underside 11 of the connector 5 rests after it has been pushed in. A nut 12 completes the screw 1-connector 5 assembly to make it operational. In the example shown, it is in the form of a cylindrical tube 13 comprising, on at least the lower part of its internal wall, a thread 14 corresponding to the thread on the pin 3 of the screw 1, and, on its lower part, a collar 15 which is intended to rest on the upper face 16 of the connector 5 so as to wedge, after the nut 12 has been screwed on, the connector 5 against the seat
10. It can be seen from figures 8 and 9 that in their respective final positions, the longitudinal axes of the screw 1 and of the perforation 7 of the connector 5 are not strictly aligned, due to the deviation of the connector 5 imposed by the presence of the rod 20 of the osteosynthesis device and its tightening against the seat 10 and the spherical geometry of the 5-nut connector contact 12, 22. Typically, the angulation between these two axes can be upto 15° in all directions of space. This spherical geometry of the 5-nut connector contact 12, 22 also limits the deformation of the connector hook 8 when the rod is tightened on the seat 10.
In the non-limiting example shown in Figures 4 to 8, the nut 12 has, at the junction between the flange 15 and the cylindrical tube 13, a line of least resistance 17 in the form of a localised decrease in the thickness of the outer wall of the cylindrical tube 13. At the end
5 ofthe installation of the osteosynthesis device, it allows the tube 13 to be separated from the flange 15 by a torsional effort so as to reduce the vertical bulk of the assembly, as will be explained later.
A flat 18 on the flange 15 provides a support for a tool with which this separation can be made.
The separation of the upper breakaway part 19 from the pin 3 is generally carried out in the same movement, by means of a bending force.
In a conventional manner, the assembly whose separate parts have just been described is designed so that, as shown in Figures 7 and 8, when the nut 12 is tightened, the hook 8 of the connector which receives the rod 20 of the osteosynthesis device presses this rod 20 against the seat 10 of the screw 1, the latter being inserted into a bone 21 shown in Figure 8. Figure 9 shows more explicitly, in longitudinal section, this same assembly,
equipped here with a nut 22 of a more usual shape, which differs from the nut 12 shown in the other figures in that it has no tubular portion 13 and has an external surface 23 provided with flats allowing it to be tightened by a screwdriver which is placed on top of it.
This nut 22 has an internal thread 14 corresponding to that of the pin 3, just like the previous nut 12. The height of the nut 22 is designed so that, when it is in the position shown in Figure 9 and secures the screw 1-connector 5-rod assembly 20, the line of least resistance 4 of the pin 3 emerges slightly above the nut 22, so that the pin 3 can be shortened by breaking along the line of least resistance 4.
Figure 9 shows what is an essential feature of the invention.
This feature is that the pin 3 is hollow, in that it has an inner space 24 over its entire height, which opens out into the upper end 25 of the pin 3. This inner space 24 has in its lower part a recess 26 provided with flats or, as shown, in the shape of a star (of the type commonly known as "Torx*”), into which the end of a screwdriver of corresponding shape and dimensions, which can be handled by the surgeon, can be inserted.
This insertion can take place at any time, as the interior space 24 of counter 3 is permanently accessible from the outside.
Of course, other print configurations of equivalent function can be used.
In the example shown in the figures, this impression 26, in cooperation with the screwdriver, is the only possibility for the surgeon to rotate the screw 1, in particular when placing the screw 1 on the bone 21. Alternatively, as mentioned above, this function of rotating the screw 3 during its installation can also be provided in a conventional manner by a set of flats provided on the external surface of the pin 3, on which a screwdriver engages.
The inner impression 26 of the pin 3 is used when the surgeon wants to change the depth of the screw 3 in the bone 21, for example to adjust the distance between the surface of the bone 21 and the shaft 20. It is in such circumstances that the invention is useful. Indeed, the permanent access to the impression 26 allows the surgeon to modify this recess at any time, even after the spinal osteosynthesis device has been completely installed, including the installation of the connector 5 and the nut 12 or 22, which then mask any flats of the pin 3 and make them unusable, and the tightening of the nuts 12 or 22 and, when present on the original parts, the breaking along their lines of least resistance 4, 17 of the upper parts of the pin 3 and the nut
12. To this end, the surgeon must often begin by loosening the nut 12 or 22 to a greater or lesser extent in order to reduce the pressure exerted by the rod 20 on the collar 10 of the screw 1 and the tension of the entire osteosynthesis device, but without necessarily releasing the rod 20 from the grip of the hook 8. Thus, the screw 1 regains a certain degree of mobility, which allows the correction of its height position by rotation. After this, the nut 12 or 22 can — be tightened, and the osteosynthesis device is thus once again locked and tensioned in its corrected position, possibly after a similar operation has been performed on one or more other screws 1 of the device. The inner space 24 of the pin 3 can advantageously also be used during the reduction operation, i.e. when the relative positions of the vertebrae are changed by the surgeon. The latter can push a tool into the said inner space 24 of the pin 3, enabling him to manipulate the vertebra concerned and to bring it into the desired position A further advantage of the impression 26 according to the invention is that it makes the screw 1 also manipulable when the break-off part 19 of the pin 3 has accidentally broken off during the installation of the device. As the manipulation of the vertebra during reduction is preferably carried out from the inside of the pin 3 and not from its outside, this risk of premature rupture is reduced compared to the prior art. However, it cannot be completely eliminated, but the presence of the indentation 26 makes it possible to significantly reduce the disadvantages that would be associated with this rupture by preserving a good possibility of engagement with the remaining part of the pin 3. Furthermore, in the case of a nut 12 having a tubular portion 13, the breakable part of the pin 3 is protected as long as said tubular portion 13 has not been separated from the rest of the nut 12, which constitutes a further limitation of the risk of premature rupture of the pin 3. The above also applies to cases where the screws 1 are used for purposes other than spinal osteosynthesis, for example when they are integrated into a device for reducing a long bone fracture.
If a nut 12 with a possibly breakable tubular upper part 13 is used, it is also advantageous to provide, as shown in Figure 6, that the inner space 27 of the nut 12 has flats 28 with which a screwdriver can be engaged when tightening the nut 12.
If it is desired that these flats 28 be usable even after the tubular part 13 of the nut 12 has been cut off, in order to ensure that the nut 12 can be easily gripped and rotated for loosening and retightening, they must be present up to below the line of least resistance 17 so that the nut 12 can be tightened and loosened by a screwdriver at all stages of the installation and use of the osteosynthesis device.
This is not the case in the example shown, see figure 6, where the nut 12 can be loosened by means of the flat 18 on the outer surface of the flange 15 and a tool cooperating with this flat 18.
The devices that have been shown in detail are, of course, only non-limiting examples.
In particular, the connector 5 and the seat 10 of the screw 1 on which it rests may have different shapes, of any known type, as long as they allow free permanent access to the recess 26 located in the lower part of the inner space 24 of the pin 3. It could, for example,
be foreseen that the connector 5 is constituted by a clamp, intended to surround the rod 20 over its entire periphery, without direct contact between the rod and the screw 1, the connector being responsible for transmitting to the seat 10 the forces exerted on the screw 1 by the rod 20. Alternatively, it may be intended that the seat 10 and connector 5 allow for a different orientation of the connector 5 than that shown, where the general orientation of the connector 5 would be perpendicular to the longitudinal axis of the screw 1.
The dimensions of the recess 26 according to the invention may conform to a predetermined standard of shape and size, so as to make it possible to use a common type of screwdriver.
They may also be of a particular type, in which case a suitable screwdriver will usefully be incorporated into a kit also comprising at least one screw 1 - connector 5 -
nut 12 assembly, or even one or more tools enabling the pin 3 and the nut 12 to be broken at their respective breakable parts 4, 17 if they exist.
If necessary, this kit may also include a screwdriver for handling the nut (12, 22).