Patent Application
20090012619
The present invention relates to a motion restoring intervertebral prosthesis in the form of an artificial intervertebral disc.
The vertebral bodies forming the spinal column are connected with one another via intervertebral discs which allow the spinal column to move or bend forward and backward, i.e., along the anterior-posterior plane (A/P plane) and sideways along the medial-lateral plane (M/L plane). The column can also pivot at angles intermediate such planes.
A partially or wholly failed natural disc resulting from the imposition of excessive loads or perhaps simply aging can result in considerable back pain for the unlucky individual. The failed disc can be surgically removed and replaced with a rigid solid spacer to maintain the natural spacial separation between the vertebrae; however, such a spacer greatly limits the ability of the vertebrae adjacent the implanted spacer to move relative to each other. Many artificial discs have been proposed in an attempt to restore the natural motion between the vertebral bodies between which a natural disc has been removed. Perhaps the most common type is in the form of a ball and socket. See Hoffmann-Daimler's 1974 article published in Intervertebral Disk Replacement, Vol. 112, No. 4, August 1974. Yuan et al., U.S. Pat. No. 5,676,701 (“'701 patent”), discloses a low wear artificial spinal disc having opposing convex and concave contoured surfaces with a full 360 degree circumference. This allows for rotation but no translation. Nishijima et al, U.S. Pat. No. 5,899,941, discloses a similar device but where the articulating surface is generally non-conforming to allow rotation and translational movement. This is similar to the intervertebral device disclosed in U.S. Pat. No. 6,113,637 by Gill et al. This artificial spinal disc incorporates a ball and trough type articulation with a substantially flat portion. Both the Nishijima and Gill et al. patents describe devices which rely on point contact, a potential source of excessive wear debris. Also see German Patent No. 2,263,842.
The prior art ball and socket intervertebral devices of which I am aware allow for a wide range of motions (ROM) of the order of 20°-22° between the separated vertebrae or +/−10° and 11° bending in all directions. See the Bryan artificial cervical disc. (Hoang Le et al, Neurosurg Focus 17 (3)) and the Prestige II cervical disc (Porchet et al, Neurosurg Focus 17(3)). Also see the '701 (“'701 patent”) in which the inventors state that a natural healthy disc experiences a limit of about 11° motion in the A/P plane, i.e., bending forward/backward and about 3°-5° in the M/L plane, i.e., bending side to side, and that some skilled artisans considered that artificial discs should have such limitations.
Apparently, the 11° motion in the A/P plane represents the angle deviation from each side of neutral or +/−11° This is consistent with the '701 patent's reference to 20° in flexion in the design disclosed in U.S. Pat. No. 4,759,769 in which the backward angle B (
The large range of motion 20°-22° discussed in the Neurosurg Focus paper and the 20° and advocated in the '701 patent conforms to standard and customary practices for all large joint arthroplasty devices (hips, knees, shoulders). The problem with the outer perimeter or outer rim of an artificial joint contacting is wear debris and implant loosening. This is especially true with hip replacements. If a patient moves his or her leg out past what the artificial hip joint can accommodate, a large moment arm begins to wedge the implant out of position while at the same time generating rim wear from surfaces contacting which were not designed to contact
The problem with transferring the design philosophy of large joint prosthesis to spinal arthroplasty is that the natural disc only moves a small amount, i.e., about 10°-12° total ROM. In addition, natural discs resist excessive motion whereas current articulating type artificial discs do not resist motion above such limits.
Some orthopedic spinal specialists are starting to see a fair amount of induced segmental kyphosis and scoliotic curvatures with both cervical and lumbar disc arthroplasty. See, for example, the October 2006 issue of “J Spinal Disorders and Techniques, pages 466-469.
There is a need for a intervertebral prosthesis which more closely matches the range of motion of a natural disc.
A motion restoring intervertebral prosthesis for placement between adjacent vertebrae, between which a natural disc has been partially or completely removed, in accordance with the present invention includes first and second articulating components forming an intervertebral prosthetic disc. Each component has a vertebral body engaging surface on one side for buttressing against a respective vertebral body and an articulating surface on the other side joined with an encircling rim section. One of the articulating surfaces is in the form of an outwardly projecting convex section forming, for example, a partial spherical surface and the other articulating surface is in the form of a mating inwardly extending concave recessed section forming, for example, a partial spherical surface. The rim sections are arranged to contact each other to limit the angle through which the components can pivot relative to each other in both directions in M/L plane and in a forward direction in the A/P plane to about +/−6° or less, i.e, +/−3°-4°. To inhibit the disc from exiting the space between the separated vertebrae during excessive backwards bending the posterior portions of the rim sections may be chamfered to allow bending in that direction within the range of about 20° to 30°. The prosthesis is basically in the form of a ball and socket, allowing rotation about the spinal axis, with encircling rims which contact each other to limit the bending motion in the M/L and A/P directions. As an option the concave recess can be provided with a nonconforming radius to allow limited translation in the anterior-posterior direction as is explained in some detail in pending PCT Application No. US0616392 assigned to the assignee of this application, the contents of such application are incorporated herein by reference.
Referring now to
Component 10 has an articulating surface 10c in the form of a centrally located generally semispherical convex center section joined to or merged with an encircling rim section 10d. The rim section includes a beveled portion 10e on the posterior side 10f (as installed).
Component 12 has an articulating surface 12c in the form of a generally semi-spherical convex or recessed center section functioning as a socket, for mating with the ball section 10c. The articulating socket 12c is joined to or merged with an encircling rim section 12d, which rim section is formed with a beveled portion 12e along the posterior side 12f. The beveled portions 10e and 12e are slanted upwardly and downwardly, respectfully, as shown in the figures. The actual orientation will depend on which component is located on top.
The encircling rims while illustrated as generally rectangular with rounded corners can be any desired shape.
Looking at the assembled disc along the M/L plane in
There has thus been described a novel ball/socket type intervertebral prosthesis which allows rotational movement about the spinal axis, restricted bending of about +/−6° in each direction in the M/L plane and in the forward direction in the A/P plane. Optionally the prosthesis may accommodate additional bending of about 20° to 30° in the backward direction in the A/P plane.
Various modifications and perhaps improvements to the disclosed embodiments will undoubtedly occur to those skilled in the art without involving any departure from the spirit and scope of the invention as defined by the appended claims.
