FR2728158A1 - Spinal column prosthesis - Google Patents

Abstract

The spinal prosthetic assembly has a convenient number of elastic intervertebrae prosthetic shock absorbing connectors. These are, according to requirements, either omnidirectional (2a) or directional (2b). The first has two pedicular screw (3a,3b), one fitting the top (4a) and the other the bottom (4b) vertebrae, and anchors (5,6) joined by an omnidirectional dovetail connector (9). The other (2b) has two pedicular screws (14a,14b) fitting the top (15a) and bottom (15b) vertebrae and having anchors (17,18) joined by a directional dovetail connector (18). The omnidirectional prosthetic connecting element has a connector having a hollow cylindrical body cut helicoidally to make it axially elastic and a rectangular spiralled section. The upper and lower body parts have top and bottom dove tails which are fixed, after bending in situ, on the anchors while the body central hollow part is filled at rest with a visco-elastic shock absorbing fluid. All the parameters of the prosthesis elements are determined and fatigue tested before implanting in the patient.

Description

 The invention is in the field of joint prostheses intended in particular to overcome the stabilization deficiencies in joint disorders, particularly intervertebral, in which the disc in cooperation with the articular processes no longer performs its original functions.

 The behavior in stability of the spine after failure of at least one of the three intervertebral support points constituted by, the intervertebral disc and the articular processes, leads to degradations which it is important to correct sometimes surgically.

 To do this, the practitioner has intervertebral connecting devices which are most often rigid but which can sometimes have flexibility characteristics in a search for patient comfort.

The studies or analyzes of intervertebral deformations are very numerous and we can cite in particular the major ones of the
ENSAN Biomecanique Laboratory in Paris: 1 - Three-dimensional biomechanical properties of the human cervical spine in vitro. (Européan spine journal - 1993), 2 - A biomechanical analysis of short segment spinal fixation using a three-dimensional geometric and mechanical model (Spine Vol. 18
Number 5. pp 536-545 -1993), 3 - Three-dimensional geometrical and mechanical modeling of the lumbar spine (Biomehanics Vol.25 NO 10 pp 1164 -1992), 4 - Influence of geometrical factors on the behavior of lumbar spine segments : finished the element analysis. (European spine journal -1994>, 5 - Biomechanical behavior of a posterior vertebra inter-apophyseal spring. Experimental analysis of disc behavior in compression and flexion / extension. (Rachis 1993 -Vol.5 N'2>.

 This last publication brought a considerable advantage compared to rigid or more or less flexible bridges by proposing an elastic vertebral stabilizer.

 Load tests had shown the merits of such an elastic prosthetic device which also presented no difficulty in implantation at the level of the surgical procedure.

 Prior to these tests, a patent (No. 2681525 FR) had already been filed in 1991, proposing a device for dynamic stabilization of the spine comprising a shock absorber cooperating with the two fixed anchoring systems with deficient vertebrae.

 Considering that the elastic properties on the one hand and the damping properties on the other hand had to be implemented jointly in order to satisfy the mechanical conditions of in-vivo operation, studies were undertaken by the applicants in order to define an intervertebral prosthesis of stabilization responding to the problem posed.

 To this end, they defined a pedicle stabilization prosthesis comprising, in connection with adequate anchoring means to the vertebrae, a hollow oblong body of revolution such as cylindrical, helically split in order to make it axially elastic while a viscoelastic product was introduced into the axial space so that the ideal elasticity and damping functions were satisfied jointly.

 All these tests and studies were carried out in the context of static applications or of dynamic approach which answered mentally to the data of the problem of the time.

 However, a more detailed analysis of the phenomenon shows that if the static and / or dynamic concept is important, a fundamental parameter must be taken into account.

 Being an implanted prosthetic device, it must undergo a considerable number of mechanical stresses composed in tension, compression and torsion and the criterion of fatigue resistance then became preponderant.

 To this end, the applicants have been led to reconsider their elastic device with damping in order to satisfy the conditions of mechanical stress close to those encountered in vivo.

 The search for an ideal device presenting elastic and damping properties together with a suitable resistance to fatigue, was going to reveal to the applicants a system of intervertebral connection of substantially cylindrical conformation split transversely which could exceed 106 cycles in tests under tensile load - combined and alternating compression and axial rotation.

 Generally, it is known that all the tests of periodically variable intensity and of different natures, can involve a rupture without significant deformation following a progressive cracking even if the stresses remain constantly lower than the theoretical elastic limit of the material constitutive of the system considered.

 This is particularly the case of a prosthetic device which undergoes many times a day combined and alternating stresses which it is very difficult to quantify but which are very important having even seen on this subject, unexplained ruptures of so-called rigid intervertebral bridges "with plates".

 Symbols, designations and definitions are usually defined as shown in FIG. 1 for, on the one hand, the stress cycles and, on the other hand, the fatigue resistance conditions.

- stress cycles: .6 max - maximum stress which is the greatest algebraic value
during a stress cycle (traction or tension +,
compression or pressure .min - minimum stress which is the smallest algebraic value
during a stress cycle.

. # m - mean stress which is the static component of the
stress and the algebraic mean of the stresses (max etcs mit. # a - amplitude of the stress which is the alternating component of 1
constraint, half algebraic difference between # max and (min.

f - frequency of cycles (min. sec.) - Fatigue resistance (according to ISO.R 373 -1964)
N fatigue endurance or longevity corresponding to the number of
cycles necessary to cause rupture, usually
expressed in multiples of 106.

- ç n - fatigue resistance for N cycles or stress value -
for which the device would have a longevity <
statistically determined) of N cycles.

C f D - fatigue limit, statistically determined, may be illi-
moth.

 3 Z - shear stress.

 The scope of the invention makes it very difficult to mathematically model the phenomenon studied.

 However, numerous laboratory tests can account for many experimental lessons which lead by extrapolation to human or even animal applications.

 The following text will show, in support of the appended drawings, how the applicants were able to model a prosthetic device in accordance with the invention and compatible with the experimental data deduced from the numerous observations made by radiography, scanner, NMR imaging.

In the drawings:
FIG. 1 is a diagram representing the cyclic stresses applied to a solid body and varying according to an approximately sinusoidal law of time,
FIG. 2 is a schematic explanatory view of the analysis of the possible modes of movement of the planes resulting from the deficiencies of the intervertebral support points,
Figures 3-4-5 are schematic views showing in the three views, the configuration of a device according to the invention in test configuration,
Figure 6 shows in perspective, a prosthetic device modeled according to the invention, provided with transverse slots.

 In FIG. 1, the aforementioned signs are materialized relating to the fatigue stress cycles, noting that the + sign applies to wavy stresses in tension and the - sign to wavy stresses in compression.

 FIG. 2 very schematically represents the intervertebral behavior of two vertebrae L4 and L5 taken as a reference because 'undergoing the most significant loads.

 These vertebrae have an interposition disc Di composed of the pulposus nucleus surrounded by the anulus, a fibrous substance and the support points are represented in P2-P3 by the articular processes and in Psi, by the center of pressure applied to the part central nucleus.

 Inside these support points, there is located a virtual point v of application of the compression force and which can be inscribed in an ellipsoid E normally contained in the triangle above delimited but which can deviate therefrom a deficiency in one or more of the support points.

 In the normal ground state, the triangle P1-P2-P3 determines planes constituting the base of the set x-x ', y-y', zz 'horizontally and z-z', x-x ', or yy 'vertically.

 these planes can undergo small rotations such as X around x-x ', Y around y-y' and Z around z-z'ce which can cause their tilting as shown schematically hatched in Figure 2.

 In addition, as was previously mentioned, the cyclic constraints can under load, be concomitant in Cz along the axis z-z ', in Cy along the axis y-y' and in Cx along the axis x-x '.

 It is from the approximate study of all these complex movements and extremely difficult to quantify in-vivo, that the applicants were led to design an elastic device with damping under static and dynamic load, capable of undergoing fatigue stresses and whose lifespan is compatible with the duration of implantation on the patient.

 To this end, the invention proposes, with reference to the drawing according to FIGS. 3 to 6, a device which has the required characteristics in the sense that according to the morphological defects noted on the patient, said device is conformed to the request and then tests on a specific test bench and that after these tests duly configured the model of said device can then be standardized so that its model can be implanted on a patient with the same defects.

 In these figures, the device to be tested marked 1 is inserted and fixed between the pads 2 and 3 of the specialized test bench allowing, as indicated above, the application under load, of alternating cyclic constraints.

 The immobilization of the device tested is ensured by two collies 4-5 fixed by screws on the pads 2-3 of the bench and which grip the tail of said device 1.

 The specialized test strip is designed to allow application under a load F corresponding substantially to the vertebral load, fatigue stress cycles on the axes x-x ', yy' and zz 'and around these axes along X , Y, Z so that the virtual point v can be discussed in ellipsoid E, the limits of which have been predetermined.

 As an example, angular values can be displayed on the specialized test bench: &alpha; 1-4, &alpha; 2-4 -ss 1-4, ss 2-4, i-4k. 0 (2.40- for a number of alternating cycles exceeding 106; all the applied stresses are also recorded by a measurement chain from a three-dimensional sensor 6.

 The model of the device tested then standardized, can be implanted (then on a patient presenting the same morphological defects.

 As a preferred example in the invention, Figure 6 shows an elastic device with damping, holding in fatigue, identified 10 as a whole and represented in the form of a solid cylinder 11 extended by two cylindrical tails 12-13 securely enclosed in collars 14 implanted on each vertebral pedicle 15 ...

(it should be noted that the lower part is not shown for reasons which will be mentioned later).

 The device 10 is taken from a T A6 V4 alloy bar (grade ELI 58/32/3) and has the following dimensional characteristics: (in m.m) d, 12-e, 5-b 11-cl3.

 It is provided with six slots (al) spaced from one another (a2> and their orientation two by two is substantially 120 (# = #).

 The bottom of the slots, also rounded along 16, adjoins the axis of revolution of the cylinder.

 The lower part of the device according to FIG. 6 can be identical to the upper part as regards the fixing means but could be different in the sense that, to correct upper or lower stages of the vertebra, part 13 could be 1 extension of the part 12 thereby creating a continuity of the device 10 and the fixing is then ensured on the common tails 13-12 / 13-12 ... by ad-hoc collars 14 ...

The practitioner thus has at his disposal all the means to act on the parameters of a prosthesis to be implanted while being able to act more during the intervention, on the selontoS orientation when the device has been designed asymmetrically. (case of scoliosis for example)
It is obvious that all the materials compatible with implantation as well as all the combinations of transverse slots which could be otherwise radial and / or helical in all combinations with a cylindrical or substantially cylindrical body hollow or solid, would fit within the framework of the 'invention.

Claims (6)

CLAIMS. 1 - Method for configuring the specific elements of a prosthesis, characterized in that it consists of: a / to be determined by any appropriate means such as radiography, scanner, MR imaging. etc ..., the faults to be corrected in relation to the anomaly noted, b / to analyze these faults to identify the corrective parameters suitable for making it possible to design the specific elements of the prosthesis to be implanted, c / to model the prosthesis in function corrective parameters noted, d / a subject the modeled prosthesis to tests on a specialized fatigue bench, e / to standardize the model of prosthesis tested during the tests in order to be able to reconstruct, according to each case considered, the type of prosthesis suitable for such a surgical application, d / to implant the conforming, standardized prosthesis model. 2 - Parametric prosthesis according to claim 1, characterized in that it comprises a device (1) conformed to the required prescriptions, then tested for fatigue after rigid fixing of its terminal parts by anchoring bridges (4-8) on the parts (2-3) of the specialized measurement bench which applies stress variations under load (F) within the displacement limits of the virtual point (v) inside the ellipsorde (E) which is representative of it - even limit values of angular displacements (0 (1-0 (2, l-2, rî-t2) while said values are read and parameterized by a three-dimensional sensor (6) itself connected to a measuring chain and recording (7). 3 - Prosthesis configured according to claims 1 and 2 camber rise in that the conformation of the body of the device (1) is on: cylindrical and transverse slots are practiced in order to perme its axial elasticity with damping as well as the deflections angular in torsion while being able to undergo alternating stresses in fatigue of at least 106 cycles. 4 - Parameterized prosthesis according to any one of claims 2 or 3 characterized in that the device (1) is normalized after tests and then listed in order to serve as a model for identiql implantable prostheses possibly corrected according to angular orientation (o) 5 - Modeled prosthesis according to any one of claims 1 to 4 characterized in that during a given installation, the device (10) consists of a cylinder (11) extended by two cylindrical tails (12-13) enclosed in collars (14 ..) implanted on the vertebral pedicles (15 ..) and the cylindrical body (10) is provided with six transverse slots (17 ...) spaced apart from each other (a2) and oriented two by two of about 1200 (fS) so that the bottom of said otherwise rounded slots (16) substantially adjoins the geometric axis of the cylinder (10). 6 - Modeled prosthesis according to claim 5 characterized in that the slots are helical. 7 - Modeled prosthesis according to any one of claims 2 to 6 characterized in that it constitutes at least one elementary segment of a more extensive prosthetic system which may interest several vertebral sections and the pedicle connections are ensured on the common tails (13 -12,13-12 ...) by common collars (14 ...).