SPINAL IMPLANT

Abstract: A spinal implant adapted to replace a disc between adjacent vertebrae having a predetermined thickness and profile chosen to match the space between said vertebrae and characterized by elongated protuberances (5) adapted to be located in grooves drilled in said vertebrae and a porous coating (7) on at least a portion of its surface. A method and apparatus for placement of said spinal implant are also disclosed.

Description: This invention relates to a spinal implant, particularly an implant adapted to encourage bone ingrowth resulting in stabilization of adjacent vertebrae and reduction of back pain. The invention is also concerned with a method of implantation and an apparatus for placement of the spinal implant between adjacent vertebrae. Many types of intervertebral implants are known in the art. For example, United States Patent No. 3,867,728 discloses a prosthesis for spinal repair which comprises a laminated core element made from an elastomer, such as silicone rubber or polyurethane, and fabric-reinforced elastomer. United States Patent No. 4,309,777 discloses an artificial intervertebral disc comprising upper and lower disc portions having a plurality of springs positioned therebetween and spikes extending outwardly from said disc portions for engagement with adjacent upper and lower vertebrae. United States Patent No. 4,349,921 discloses an intervertebral disc prosthesis comprising a shaped body of substantially rigid, non-porous, biologically compatible material. The surfaces of the disc have characteristics to produce a "friction-fit". United States Patent No. 4,479,491 discloses an intervertebral stabilization implant including an elongated central portion and a pair of reverse wings each disposed at an angle with respect to the elongated central portion. EP-A-0179695 describes a vertebral prosthesis comprising a rigid solid body having first, second and third surfaces defining, in one embodiment, a substantially D-shaped profile, the curved portion of which is adapted to conform to the outer profile of the vertebrae between which it is to be fitted. Other types of spinal stabilisers, for example fixator plates and rods, are also known in the art, but these are not intervertebral implants as that term is used herein. It has now been found that improved stabilisation is achieved by an implant which is adapted to encourage bone ingrowth and thereby stabilises adjacent vertebrae by fusing said vertebrae to the implant. In accordance with the present invention there is provided a spinal implant comprising a rigid solid body having a first surface, a second surface and a third surface of predetermined thickness between and perpendicular to said first and second surfaces, each of said first and second surfaces being defined by a substantially D-shaped profile the curved portion of which conforms to the outer profile of the vertebrae between which the implant is adapted to be implanted, said predetermined thickness being chosen to correspond to the space between said vertebrae, said implant being characterised in that each of said first and second surfaces has an elongated protuberance of substantially semi-circular cross-section extending the full width of the surface parallel to the straight side of the D, and in that at least a portion of each first and second surface has a porous coating thereon. Preferably, said porous coating is applied to each of said protuberances. In a preferred embodiment of the invention a threaded hole and an adjacent locking hole are located in said third surface parallel to the straight side of the D, for holding said implant and placing it in position between vertebrae. The rigid solid body of the implant according to the invention preferably is made from a high strength, biocompatible, corrosion-resistant, cobalt-chromium molybdenum alloy and the porous coating comprises two layers of substantially spherical particles of the same or similar alloy as the body. A particularly suitable alloy of this type is that known by the registered Trade Mark "Vitallium". The invention will be more particularly described with reference to a preferred embodiment as illustrated in the accompany drawings, in which: Figure 1 is a perspective view of a spinal implant according to the invention; Figure 2 is an end elevation of the implant of Figure 1; Figure 3 is a side elevation of the implant of Figure 1; and Figure 4 is a perspective view showing two implants of the invention in place between two adjacent vertebrae. Referring to the drawings, Figures 1-3 illustrate a preferred spinal implant according to the invention. Said implant comprises a rigid solid body having a first surface 1 and a second surface 2. A third surface 3 of predetermined thickness is between and perpendicular to said first and second surfaces. Each of said first and second surfaces is defined by a substantially D-shaped profile whose curved portion 4 conforms to the outer profile of the vertebrae between which the implant is adapted to be implanted (see Figure 4). The pre determined thickness of the third surface 3 is chosen to correspond to the space between the vertebrae in which the implant is to be implanted, said thickness corresponding substantially to that of the healthy disc which originally occupied said space. When a disc between adjacent vertebrae is removed prior to placement of an implant according to the invention, the resultant space between the vertebrae is carefully measured and an implant of appropriate thickness is chosen for the subsequent implantation. Each of the first and second surfaces 1, 2 of the implant has an elongated protuberance 5 of substantially semi-circular cross-section extending the full width of said surface parallel to the straight side 6 of the D. In the preferred embodiment illustrated in Figures 1-3 the surface of each of said protuberances 5 has a porous coating 7 thereon. The rigid solid body of the preferred embodiment, i.e. the body defined by the above described surfaces, is made from a high strength, biocompatible, corrosion-resistant cobalt-chromium-molybdenum alloy, for example "Vitallium", and said porous coating preferably comprises two layers of substantially spherical particles of the same or similar alloy as the body. A particularly suitable porous coating is that disclosed in commonly assigned U.S.Patent No. 4,550,448, the disclosure of which is incorporated herein by reference. In alternative embodiments of the invention, the porous coating may extend over all or part of the first and second surfaces 1, 2 in addition to the surface of the protuberances. However, it has been found that a porous coating over the protuberances only (as illustrated in the drawings) is normally sufficient to provide adequate bone ingrowth to stabilize the vertebrae. The third surface 3 does not come into direct contact with bone and is not porous coated. A prime object of the porous coated spinal implant of the invention is to eliminate back pain associated with loss of joint space secondary to a back fusion procedure. The implant is designed to stabilize the vertebrae following debridement of a disc. After placement of the implant between adjacent vertebrae tissue/bone ingrowth into the porous coating provides long-term stability. The various thicknesses of the implants are chosen to reconstitute the disc space between the vertebrae, thus preventing pressure from developing about the dura and/or nerve roots relative to the collapse of said disc space. The protuberances, which extend anterior to posterior, provide rotational stability. This is clearly illustrated in Figure 4 which shows two implants placed in side by side relationship in the space formed by removal of a disc between two adjacent vertebrae 8,8. The use of separate implants allows replacement of the disc without interfering with the spinal cord. The space 9 between the implants will eventually be filled with ingrowth of tissue or bone.

Claims: 1. A spinal implant comprising a rigid solid body having a first surface (1), a second surface (2) and a third surface (3) of predetermined thickness between and perpendicular to said first and second surfaces, each of said first and second surfaces being defined by a substantially D-shaped profile the curved portion (4) of which conforms to the outer profile of the vertebrae between which the implant is adapted to be implanted, said predetermined thickness being chosen to correspond to the space between said vertebrae, said implant being characterised in that each of said first and second surfaces has an elongated protuberance (5) of substantially semi-circular cross-section extending the full width of the surface parallel to the straight side of the D, and in that at least a portion of each first and second surface has a porous coating (7) thereon. 2. An implant according to claim 1, characterised in that said porous coating is applied to each of said protuberances. 3. An implant according to claim 1, characterised in that a threaded hole (28) and an adjacent locking hole (29) are located in said third surface (3) parallel to the straight side (6) of the D, for holding said implant and placing it in position between vertebrae. 4. An implant according to claim 1, characterised in that the rigid solid body is made from a high strength, biocompatible, corrosion-resistant, cobalt-chromium-molybdenum alloy and the porous coating comprises two layers of substantially spherical particles of the same or similar alloy as the body.

Inventors: KENNA, ROBERT VINCENT
Applicants: PFIZER HOSPITAL PRODUCTS GROUP, INC.
Application year: 1988
Main classification: A61F2
IPC codes: A61F2, A61B19, B22F7, A61B17