FR2745706A1 - Prosthetic vertebral support device - Google Patents

Abstract

The support has a flexible central rod (6) entirely surrounded coaxially and helically with multiple flexible rods (71,72...). The rods are connected and solidly fixed at each end by a cap (81b,82b...) and allows the support to be fixed to the pedicular screws in the standard manner. The rods can be fixed into the caps by laser or electron beam welding. The support can be formed of biocompatible materials and can have a surface which is mirror polished or matt.

Description

 The present invention relates to intervertebral prostheses and relates more particularly to: that secured to the vertebrae by pedicle screws.

Prostheses of this kind consist almost entirely of rigid rods such as those shown for example in many patents. (î)
Experience shows that ruptures occur prematurely due to the high number of alternative constraints applied from various origins (pounding due to walking, etc.) exceeding 10,000,000 cycles, which very often requires replacing the prosthesis.

 According to the application of the fatigue rules for materials, the elastic limit of a given material progressively decreases when said material is subjected to a number of alternating stresses producing rupture after a period of germination has appeared in the metal. priming then cracks. The axial stress applied to a rigid rod will vary according to the same cycle, oscillating between two values #max positive, stress in tension and #min negative, stress in compression; this stress being the symmetrical alternating stress of axial fatigue, the average stress: # m = #max + #min being zero in this case.

 2. # Min is equal to - 6 max in this case, according to the representation in accordance with Figure 1 below.

 The actual stresses exerted on a stem prosthesis, in tension / compression / flexion / torsion are in fact, even more restrictive.

(1)
FR - 2693365/2692471/2687561/2684866/2682280/2680461/2676354/2659546 /
2659225/2658414 / 26584l3 / 2657775 / 256l992 / 26 50173/2645732/2645427 /
2642642 / 26404g3 / 2638e32 / 2636227 / 26a3177 / 2631 54o / 22472O / 2623390 /
2621476/2515095.

PCT- 93-13722 / 93-12737 / 93-10726 / 93-10717 / 93-07823 / 89-08431 / 89-00028
88-06024.

EP - 578320/565149/564046/558883/487895/487830/348581/330881/284559.

 Rigid intervertebral prostheses with rods, although they risk being ruptured prematurely, nevertheless had an indisputable advantage linked to the fact that they ensured the conservation of the geometry of the intervertebral conjugation hole thereby preventing compression of the nerve roots.

 In order to avoid the risks of premature ruptures and to improve patient comfort, we had the idea of replacing the rigid rods with a damping device which can be elastic.

 We find for this purpose, numerous patents and publications (2) as well as an own previous request of the applicant (EP NO 95 40 0558), in which elastic systems with damping, either directional in order to correct certain specific defects, were provided. such as scoliosis for example, is still omnidirectional.

 Such flexible systems undoubtedly improved the patient's comfort, practically eliminated the risks of rupture but introduced on the other hand, a defect inherent in their very flexibility which was to restrict the geometry of the conjugation hole then compressing the nerve roots.

 It remained to design a system which had the advantages of rigid prostheses, with respect to the geometry of the conjugation hole and flexible prostheses, their mechanical strength and the comfort provided to the patient.

(2) - Three-dimensional properties of the human cervical ... (European spine journal 1993) - A biomechanical analysis of short segment spinal ... (Spine Vol 18 N. 5
PP 536/545 - 1993) - Tfree dimensional geometrical and mechanical ... (Biomechanical Vol. 25 NO 10 PP 1164/1992) - Influence of geometrical factors ... (European spine journal - 1994j
- Bio-mechanical behavior of the spring ir.ter-apophysa: -e vertebral (spine 1993 - Vol.5 NO 2) -Clinical Bionechanical Vol. 7 NO 4 Nov '992.

- Patents: F 2681525/2683445/2674264 / EP 516567/576379/538133/188954
The invention will be better understood in the following text which will show in support of the appended drawings, an embodiment of this invention.

 FIG. 1 conventionally represents the graph of the stresses appearing on a rigid part subjected to alternating traction / compression forces.

FIGS. 2 and 3 represent, in conventional section and lateral view, the conformation of two vertebrae such as the lumbar for example,
FIG. 4 schematically represents a rigid rod undergoing a traction / compression work, FIG. 5 a helical spring undergoing an equivalent work or it is known that it is subjected to a shear stress and FIG. 6, the device according to the invention which works this time in bending.

 Figure 7a shows in side view, one of the two devices according to the invention disposed between two vertebrae and Figure 7b is a cross section along a-a 'of said device.

 Figure 8 is a cross section along b-b 'of the upper vertebra of Figure 7a.

Figures 2 and 3 show respectively, a sectional view at level, a normal interface between a pulposus nucleus and two vertebrae above and below jacent and the view in lateral projection according to
F of this vertebral unit.

According to Figure 2 we can first of all distinguish in a known manner: the vertebral body identified CV, the nucleus pulposus NP, the spinous process AE, the yellow ligament LJ, the hard mother DM, the epidural space
EE, then the spinal cord ME, giving birth to the spinal ganglia GR then to the left roots RAg and right RAd by the anterior roots RAa and posterior RAp. The corbugation holes TGg (left) and TSd (right) normally ensure the passage of the roots.

 According to Figure 3 we can recognize all the elements identified above with in addition, the anterior articular apohhyses AA, the transverse process AT, the upper vertebral body CVs, the lower vertebral body CVi; the nucleus pulposus NP being represented in the presumed normal position.

 A deterioration noted DET of this nucleus pulposus NP and represented in dotted lines in FIG. 3, can compress the roots hA and in any event require the fitting of a prosthesis.

 It is in such cases that recourse is made to conventional rigid prostheses or to more recent flexible prostheses, as has already been mentioned previously.

 In Figures 7a and 7b we see the conventional positioning of the pedicle screws noted 1-2 on the upper vertebra and 3 4 on the lower vertebra determining the positioning 1-3 in the right part and 2-4 in the left part.

 These pedicle screws include their collars and their clamping nuts in a known manner.

 Each device according to the invention identified 5 as a whole and represented in FIGS. 7 and 8, comprises a central flexible rod 6 surrounded coaxially and helically, with a plurality of flexible rods 71, 72 of lower section; all of said flexible rods being attached to upper caps 81 and lower caps 82.

 In a preferred embodiment given purely by way of example; the central rod 6 has a diameter of 2.2 mm and each peripheral rod 7 ... 8 in number, has a diameter of 1.8 mm leading to an outer diameter of the cap 8 of 7 mm in diameter for a thickness of 0.6 mm wall; said caps being terminated by a spherical cap.

 the pitch of the propellers of the rods 7 is of the order of 2 mm and the fixing of all the rods to their caps is ensured by welding with an electron beam or laser.

 The height of each device thus formed, which is of the order of 46 mm or more for 2 stages, 3 stages etc ..., is compatible with the usual spacings between pedicle screws of lumbar vertebrae and can of course be adapted for all other intervertebral fixations.

 It is noted that the flexible intervertebral device according to the invention can replace all existing rigid or elastic systems falling under a method of fixing by pedicle screws with collars tightened by screws as shown in the different figures.

 It does not have any of the aforementioned drawbacks either of rupture for rigid prostheses or of compression of the roots for elastic or flexible prostheses; this resulting from its particular working mode in bending ensuring sufficient rigidity associated with a certain flexibility in all the movements exerted by the patient.

 The materials used are biocompatible and the external appearance is mirror or matt polished.

 Fatigue tests carried out by the National Testing Laboratory have shown that the fatigue limit of 10 cycles had been exceeded, without rupture, on a device according to the invention, which characterizes the fact that, statistically in accordance with the curve of Wohler, no structural changes should appear during the continuation of the test.

 The system offers progressive rigidity in the movements allowed (the greater the range of motion, the greater the bending force).

 Finally, bone grafts (9) implanted at the time of the surgical intervention in the bone interstices, coat the prosthesis so as to thus reconstitute an intervertebral bridge of consolidation.

Claims (5)

1 - Prosthetic device with pedicle screws characterized in that it comprises a central flexible rod (6) surrounded coaxially and helically with a plurality of flexible rods (71,72.); said rods being joined together and fixed integrally at each of their ends by a cap (81b, 82), which allows said device to be fixed to said pedicle screws in the usual way. 2 - prosthetic device with pedicle screws according to claim 1 characterized in that the method of fixing the rods (6-71,72 ...) in the caps (81,82) is achieved by laser welding or electron beams. 3 - Prosthetic device according to any one of claims 1 or 2 characterized in that the device is made of biocompatible materials and has a mirror or matt polished surface state. 4 - Prosthetic device according to claim 1 characterized in that bone grafts (9) are implanted in the interstices and coat the prosthesis so as to consolidate the intervertebral bridge. 5 - Prosthetic device according to any one of claims 1 to 4 characterized in that it may interest several vertebral stages.