AU599464B2 - Vertebral screw for an osteosynthesis device in particular of the lumbar and dorsal rachis - Google Patents

Description

PATENT ACT 1952 COMPLETE SPOCI FICATIOt4

(ORIGINAL)

FOR OFFICE USE 599 46*~ CLASS INT. CLASS Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: Related Art-:- This document contains th B Section 49 and is corr-ect for printing.

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NAME OF APPLICANT: ADDRESS OF APPLICANT: AMES)OF INVENTOR(S) SOCIETE DE FABRICATION DE MATERIEL

ORTHOPEDIQUE

60-62 rue Rothschild 62600 Berk/tier France Yves COTREL 'ADDRESS FOR SERVICE: DAVIES COLLAISOk4. Patent Attorneys I Little Collins Street, Melbourne, 3000.

J A COMPLETE SPECIFICATION FOR THE IUWNTIOM ENTITLED:' "VERTEBRAL SCREW FOR AN OSTEOSYNTHESIS DEVICE IN PARTICULAR OF THE LUMBAR AND DORSAL RACHIS"' The following statesmt Is. a f ul decitioal of this inventioa, including the best ethod Of pedfoig it known to us 'S 4 1A Vertebral screw for an osteosynthesis device in particular of the lumbar and dorsal rachis I r tI It t

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~I *6 16 I ri The present invention relates to a vertebral screw for an osteosynthesis device, in particular of the lumbar and dorsal rachis and of the sacrum, comprising a screw-threaded stem extended by a neck and a terminal body.

In known vertebral screws, the neck has, in axial section, a concavity extending from the end of the screw-threaded stem 10 and terminating in a sharp corner defining the base of the body which is provided with an opening for the passage of a rod which is so dimensioned as to extend along a plurality of vertebrae.

These screws must be capable of being inserted to the 15 maximum extent in the pedicle so as to resist bending stress.

Now, it is found that the maximum insertion which can be obtained with these screws is insufficient to resist bending stress under really satisfactory conditions. Moreover, the screwing and the jamming of the screw in the pedicle create 20 problems owing, in particular, to the size of the body with respect to the neighbouring bone parts (articular and transverse apophyses) which may hinder the introduction of the screw and may possibly be damaged by the body of the latter.

An object of the invention is therefore to overcome these drawbacks.

Aeeardins to the invPntian. tho neck of thP icrnA hars -^1 r '-r 6 i i sl j 2 1 In accordance with the present invention, there is 2 provided a vertebral screw for an osteosynthesis device used 3 in lumbar and dorsal rachis and sacrum regions comprising a 4 screw-threaded stem extended by a shaped neck and a terminal body, wherein, in longitudinal axial section, the neck has, 6 in combination, a concave portion extending from and having 7 a width increasing in the longitudinal direction away from 8 the screw-threaded stem, and a convex portion extending from 9 said concave portion to said body, the concave portion being so dimensioned as to permit the insertion of the screw to 11 about mid-way of the length of the neck without damage to a 12 bone element, into which the screw is driven, or neighboring 13 bone elements, while the convex portion is so dimensioned 14 that insertion of the screw is not hindered by the S 15 neighboring bone elements, said concave portion being so 16 dimensioned that the diameter mid-way along the length of Err-, 17 the neck does not exceed the outside diameter of the screw- 18 threaded stem by about more than 60%, said diameter mid-way 19 along the length of the neck being disposed between the 20 points of inflection of said portions, which are such that 21 the profile of said neck is smooth and continuous, and said 22 neck having a length which exceeds the difference between 23 the radius of said body and the radius of said stem.

C 9-a 24 The insertion of the screw in the pedicle may consequently be substantially more than that of the known 26 screws owing to the particular geometry of the neck, which 27 improves the resistance of the screw.

28 Other features and advantages of the invention will be 29 apparent from the following description with reference to 30 the accompanying drawings which illustrate several 31 embodiments given merely be way of non-limitative examples.

32 In the drawings: 33 Figure 1 is a partial longitudinal elevational view to 34 an enlarged scale of a vertebral screw according to the invention; 36 Figures 2, 3 and 4 are longitudinal elevational views \AL /,37 of three modifications of the screw of Figure 1; 8 :t i: i; i; 1i 900426, dbwspe.006. cablav2. spe, 2 'gf 4 3 t rttt

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I I I II I I II Fig. 5 is a longitudinal elevational view to an enlarged scale of a screw according to the invention inserted in the pedicle of a vertebra and having a metal rod extending therethrough Fig. 6 is an elevational view of two screws inserted in two corresponding vertebrae and having extending therethrough a common rod Fig. 7 is a longitudinal elevational view of a test assembly of a screw according to the invention through which.

10 extends a rod clamped in the body of the screw by two bolts; Fig. 8 illustrates a test assembly for studying the bending strength of a screw according to the invention Figs. 9A and 9B are longitudinal elevational views of a screw according to the invention in two different positions 15 of insertion Fig. 10 is a cross-sectional view in a coronal plane of a dorsal or lumbar vertabra of an osteosynthesis device provided with screws according to the invention.

The vertebral screw according to the invention shown in Fig. 1 to a very enlarged scale is adapted to be part of an osteosynthesis device, in particular of the lumbar or dorsal rachis or of the sacrum.

This screw 1, which has a general axis X-X, comprises a screw-threaded stem 2 whose end, constituted by a cylindrical portion 5, is extended by a shaped neck 3 and a terminal body 4.

The neck 3 extends over a length between the cylindrical N II -4portion 5 and the body 4 and has the following portions, in axial section, and starting at the cylindrical end 5 whose diameter is equal to the diameter of the crests of the screw threads 2a of the stem 2 a) a concave portion 6 whose radius r increases in the direction away from the screw-threaded stem 2 b) a convex portion 7 whose end has a maximum radius R and defines the base of the cylindrical body 4.

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The latter has a throughway bore 8 perpendicular to the axis X-X and adapted to receive a rod 9 (Fig. The concave portion 6 and convex portion 7 have respecti1e inflexion points I 1 and 12. The profile of the concave portion 6 is so determined as to permit the insertion of the C 8* ,screw 1 in the pedicle of a lumbar or dorsal vertebra up to 15 approximately the middle M of the neck without damaging the ha a, bone element, while the convex portion 7 and its radius R are so dimensioned that the screwing is not hindered by the neighbouring bone parts (articular apophysis and transverse apophysis).

According to an advantageous embodiment, the concave portion 6 is so dimensioned that the diameter D_ of the middle M of the neck 6, 7 does not exceed by more than about t he diameter D 2 of the crests of the threads 2a.

By way of a numerical example, if 2 5 mm, D 1 must not exceed about 8 mm., Indeed, a value of D 1 substantially greater than the aforementioned value would render the insertion of the neck in the bone element difficult and could I:

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Furthermore, the length of the neck d exceeds the difference between the radius R of the body 4 and the radius r of the cylindrical portion 5 of the stem 2, as illustrated in Figure 1.

The particular geometry of the neck 3 constituted by the combination of the concave portion 6 and convex portion 7 with the aforementioned features, moreover has the advantage of enabling the screw to be inserted to the maximum extent in the pedicle and thus enabling it to resist b e n d i n q under satisfactory conditions without resulting in damage to the bone element.

The screw 1 is made from a biocompatible material, for example austenitic stainless steel.

Figs. 2 to 4 illustrate three possible embodiments of the vertebral screw according to the invention.

These three screws 12, 13, 14 each comprise a screwthreaded stem 15 and a neck 16 which are identical but differ from one another by their respective bodies 17, 18, 19.

The body 17 (Fig. 2) is closed and provided with bore 21 for the passage of the rod and with an aperture of smaller diameter than the bore 21 extending axially of the screw and in a direction perpendicular to the bore 21 for receiving a clamping bolt 22.

The body 18 of the scre w 13 (Fig. 3) having a rear opening is in the form of two branches 18a extending on each side of the axis of the screw 13 in the direction away from the screw-threaded stem 15. The opening 18b defined by the the branches 18a i~s adapted to receive an intermediate mounting member 23 for immobilizing the screw 13 on a rod by means of two clamping bolts 24 (only one being visible I~1 ~1

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**r-f 6t ir i-i iri-; S~ S It S St £6 1 0t in Fig. 3).

The body 19 of the screw 14 has a lateral opening defined by two branches 19a for receiving the rod, the clamping of which is effected by two bolts 26 (only one of which is visible).

Fig. 5 shows the mounting of a pedicular screw 1 in the bone element 27, 28 of the pedicle up to the median plane of the neck 3 defined by the points M. The rod 9 is clamped by two bolts 29 engaged in apertures provided in the body 4 which extend in a direction parallel to the axis X-X and on each side of the latter, the screw 1 being of the closed body type 4 according to Fig. 2.

As will be explained hereinafter in more detail with reference to Fig. 7, the double clamping of the rod 9 by two bolts 29 considerably reinforces the solidity of the fixing between the screw 1 and the rod 9 relative to the fixing with a single bolt proposed up to the present time.

Fig. 6 shows the implantation of two screws 1 interconnected by a rod 9 in the pedicles 31 of two vertebrae 32, 33 of the lumbar or dorsal rachis. The screws 1 perform a vertebral anchoring function to permit the expansion (also termed "detraction") or the compression of the vertebrae with respect to one another or with respect to the sacrum. These movements of "detraction" and compression are respectively represented symbolically by the arrows F and G. The pedicular screws 1 must above all be capable of resisting forces perpendicular to their axis while the bone screws .:i

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'i a I il:: t"-r I J 7 must resist principally the axial extraction.forces. For the latter, the threads of the screw is the most important element while for the screws according to the invention the body and the neck are the essential elements. It has been found that for a diameter of less than 3.5 mm, the body of the screw is insufficient for resisting the forces usually required for the correction of rachidian deviations.

This is the reason why two adult vertebral screw diameters are usually employed a) an outside diameter of the screw-threaded part of mm with a length of 25 or 40 mm, a screw-thread of 0.75 mm and a core of 3.5 mm; b) an outside diameter of 6 mm for the screw-threaded part, a length of 40 mm, and a screw thread of 0.75 mm.

Before inserting a pedicular screw it is necessary to appreciate the diameter of the pedicle and to ensure that it permits the passage of the screw, the width of the pedir 1 cle indeed varying according to the levels, the individuals and the pathology. Mechanical tests have shown that the resistance to forces perpendicular to the axis of the screw is considerably increased if the latter is inserted up to the half-way"point M of the neck (Fig. for example to a distance of 2.5 mm from the beginning of the concave portion 6. The screwing may therefore be effected to this level, the particular curvature of the neck according to the invention facilitating this penetration.

Fig. 7 illustrates a device for testing the strength of "ii 1 -8 4.44.4 4. 4 t t 4 4 4.44.4 4. 4 4..ce 4.

4..44..4 4. 4 4. .4.4 4.4.4 4. 4.4.

the fixing between the screw 1 and a knurled rod 9 when this fixing is achieved by two screws 30 inserted in the body 4 on each side of the axis X-X.

The body 4 bears against a support block 34 provided with a bore 35 for receiving the rod 9, the support block 34 being rigid so as to avoid any parasite bending effect.

The resistance of the fixing to a force K exerted along the axis of the rod 9 not perpendicular to the axis X-X is measured after the screwing of the screws 30 until fracture 10 occurs, the force K being applied in a progressive manner.

The results, compared to those obtained when the fixing between the screw 1 and the rod 9 is achieved by a single screw, have shown that the fixing with two screws 30 may withstand a force applied on the rod 9 which is greater by about 35 than the force a fixing between a screw and a rod achieved by a single bolt can withstand. By way of an example, when the fixing with a single bolt resists at at the most a force of 160 daN, the fixing with two screws may resist a force of 210 daN.

This superiority of the fixing with two bolts is the result of the increased stability of screw-threaded stems on the diamond points 9a of the knurled rod 9. The aforementioned force of 210 daN is the threshold or limit force before sliding of the rod 9 with respect to the body 4 of the screw.

The device shown in Fig. 8 has for function to test the bending strength of the screw 1 as a function of the extent .4

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rFtTrC C rC C (I Ir r r PP IF ct C t f t 9 to which it is inserted in a rigid support block 36. Formed in the latter is a passageway 37 filled with expoxy resin 38 which simulates an anchoring of the screw 1 in a bone.

A progressive force E is applied along the axis Y-Y of the bore in the body 4 through a ball 39 and a member 41. In order to compensate for the different leverage lengths due to the experimental conditions, the distnace between the axis Y-Y and the geometric plane P of insertion (Figs.9A and 9B) was measured for each test for the purpose of correcting the results for comparison purposes (the real plane of' insertion is slightly spaced away from the geometric plane of insertion owing to the modulus of elasticity of the bone).

The values of the force E applied in each test were noted for a radial deformation of the screw 1 of 0.5 mm measured by 15 means ofa comparator 42. In the position of Fig. 9A, the screw 1 is inserted only up to its cylindrical portion to the exclusion of the neck 3, while in the position of Fig. 9B, the screw has been inserted an additional distance x up to the point M mid-way along the length of the neck 3.

The two following tables give the numerical results of three tests carried out in each case with screws having a diameter

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2 5 mm and D 2 6 mm.

III I screw D 5 mm D 2 6 mm i leverage length (mn) 9.9 11.35 measured force (daN) 67 68 -I .l.J.I obending torque (am.daN) 663 772 i C c:' C f 4 C r t A 10 Foregoing Table 1: Results of tests in the case of the insertion of Fig. 9A.

screw D2 5 mm D 2 6 mm leverage length (mm) 8.03 7.91 'measured force (daN) 157.33 163 bending torque (mm.daiN) 1,253 1,289 f Qr i- Table 2: Results of tests in the case of the insertion of

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The results of these tests show that in the case of the insertion of Fig. 9A, the bending strength of the vertebral screw having a diameter in its screw-threaded portion I D 2 6 mm is about 16% greater than that of the screw having a diameter D 2 5 mm. This can be explained by the difference between the sections of the screws in the region of the insertion plane P.

On the other hand, in the case of Fig. 93 where the screw t V is inserted to an extent x 2 mm which exceeds its insertion in Fig. 9A, up to the midway point M of its neck 3, the results no longer show notable differences in the behavior of the two screws (Table 2).

*These results may be interpreted by the fact that the geometry of the two screws in the region of the insertion plane P in the case of Fig. 9B is substantially the same. In accordance with these results,'it may be concluded that the force supported by the screw before deformation is increased i by 235% for the vertebral screw having a diameter of 5 mm and J l i 4~j by 200% for the screw having a diameter of 6 mm, between an insertion up to the end of the screw thread 2a of the screw (Fig. 9A) and an additional insertion of x 2 mm (Fig. 9F3) under the aforementioned test conditions.

The following points may also be deduced from these resuits: the double bolting must be employed for the fixing between the screw and the rod in order to increase the effectiveness thereof; it is important to arrange that the pedicular screws are screwed at least up to the midway point of the length of their neck 3 in order to obtain an excellent bending s trength; the difference between the bending strengths of a screw having a diameter of 6 mm and a screw having a diameter of 5 mmn decreases as a function of the extent of the insertion .of the screw until it becomes nonexistent.

Fig. 10 shows an embodiment of a vertebral screw 1 according to the invention in an osteosyntiesis assembly of the lumbar or dorsal rachis. The screws 1 are implanted across the two pedicles 31 on the same vertebral body 32 and converge in the forward direction to the bone element 32a, and two knurled rods 9 extend therethrolujh,the screws being fixed together at the rear by a transverse traction device having a screw-threaded bar 43 extending through the bodies 4. The bar 43 is fixed in position by nuts 414 which are tiightened by a tool 45 by bringing the bodies 4 and the two screws 1

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12 closer together at the rear.

This transverse traction device (currently termed "TTD") results in an excellent resistance to extraction owing to the geometry of the assembly. Owing to the fact that the bodies 4 of the two screws 1 are brought closer together at the rear by the "TTD", the forces exerted by the latter being symbolically represented by the arrows T1, the front screw-threaded portions 2 of the screws 1 tend to come into contact, under the effect of the corresponding forces T2 developed in this region in the vertebra with the lateral cortical 32a of the vertebral body 32, which is stronger than the spongy bone so that the stability of the assembly is still further improved.

It will be observed that the necks 3 and the bodies 4 of the two screws 1 may be placed in position to the desired extent of insertion in the pedicles 31 without their penetration being hindered by the apopheses 46.

It must be understood that the assembly of Fig. is given merely by way of a non-limitative example among the various possibilities of the use of the vertebral screws according to the invention in osteosynthesis devices.

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