United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,645,596
`
`Kim et a1.
`
`[45] Date of Patent:
`
`Jul. 8, 1997
`
`USOOS645596A
`
`[54] CERAMIC VERTEBRAE PROSTHESIS
`
`[75]
`
`Inventors: Phyo Kim; Masao Suzuki, both of
`Tokyo, Japan
`
`[73] Assignee: Asahi Kogaku Kogyo Kabushiki
`Kaisha, Tokyo, Japan
`
`[21] Appl. No.: 268,103
`
`[22] Filed:
`
`Jul. 6, 1994
`
`[30]
`
`Foreign Application Priority Data
`
`Jul. 7, 1993
`Nov. 12, 1993
`
`[JP]
`[JP]
`
`Japan .................................... 5-037185
`Japan .................................... 5-060953
`
`Int. Cl.6 ........................................................ A61F 2/44
`[51]
`
`[52] US. Cl.
`....... 623/17
`
`[58] Field of Search .................................... 623/1, 11, 16,
`623/17, 18; 606/60—63
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`........................... 623/17
`2/1975 Stubson et a1.
`3,867,728
`4,713,076 12/1987 Draenert ................... 623/23
`
`7/1988 Bueliner-Janz et a1. .......... 623/17
`4,759,766
`4,798,585
`1/1989 Inoue et a1.
`.
`
`.
`
`.................................. 623/17
`
`9/1989 Manson .................................... 623/17
`4,863,477
`8/1990 Hirayama et 211..
`4,946,378
`11/1990 Ojima.
`4,969,913
`5/1991 Hirayama et a1.
`5,017,518
`7/1991 Ogawa et a1.
`.
`5,030,611
`5,064,436 11/1991 Ogiso et al. .
`5,082,803
`1/1992 Sumita.
`5,123,926
`6/1992 Pisharodi
`5,137,534
`8/1992 Sumita.
`5,147,361
`9/1992 Ojima et a].
`5,158,756 10/1992 OgaWa et a1.
`5,171,720 12/1992 Kawakami .
`5,215,941
`6/1993 Yasukawa .
`.............................. 623/17
`5,306,307
`4/1994 Senter et al.
`
`5,306,309
`4/1994 Wagner et ale
`623/17
`5,314,478
`5/1994 Oka et a].
`................................. 623/18
`
`.
`
`.
`
`Primary Examiner—Robert A. H. Clarke
`Attomey, Agent, or Firm—Greenblum & Bernstein, P.L.C.
`
`[57]
`
`ABSTRACT
`
`A ceramic vertebrae prosthesis to be inserted in a cavity
`defined between upper and lower vertebra bodies with a
`removed intervenebral disk or disks is provided with upper
`and lower convex contact surfaces that come into contact
`with the upper and lower vertebra bodies.
`
`21 Claims, 5 Drawing Sheets
`
`[10
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`NUVASIVE1110
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`US. Patent
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`Jul. 8, 1997
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`Sheet 1 of 5
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`5,645,596
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`Fig. I
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`X
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`Z
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`Y?»
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`10
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`12
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`Fig.2
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`A
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`<25
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`12
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`US. Patent
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`Jul. 8, 1997
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`Sheet 2 of 5
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`5,645,596
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`Fig.3
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`US. Patent
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`Jul. 8, 1997
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`Sheet 3 of 5
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`5,645,596
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`F i g . 5
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`L
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`32
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`Z
`33 by
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`Fig. 5
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`US. Patent
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`Jul. 8, 1997
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`Sheet 4 of 5
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`5,645,596
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`32
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`US. Patent
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`Jul. 8, 1997
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`Sheet 5 of 5
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`5,645,596
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`Fig . 8
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`5,645,596
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`1
`CERAMIC VERTEBRAE PROSTHESIS
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates to a ceramic vertebrae
`prosthesis which utilized as a substitute for one or more
`intervertebral disks that have been resected in a resection
`operation.
`2. Description of the Related Art
`If a nerve root is subject to a pressure due to an injury of
`an intervertebral disk, such as a cervical vertebrae, various
`neurological symptoms often arise therefi‘om. To cure this
`problem, the injured portion is usually subject to local rest
`and fixation thereof and a conservative curing process, such
`as an extensive curing process. Notwithstanding, if pain is
`not remitted, or the injury is too severe to be restored by such
`a curing procedure, a patient undergoes a surgical operation.
`There are two surgical operation methods in which the
`injured intervertebral disk is removed in front of the body of
`the vertebra or the vertebral arch is partly resected in the rear
`of the body of the vertebra so as to deliver a hernia. The first
`operation method in which the injured intervertebral disk is
`resected in front of the body of the vertebra is referred to as
`a forward adhesion in which a bone is implanted and
`adhered in a cavity formed between the upper and lower
`bodies of vertebra.
`
`In such a forward adhesion, in many cases, a self—fliac
`bone is used as a bone to be implanted. However, a patient
`suffers from heavy burden and physical load as a result of to
`a secondary invasive procedure necessitates by the extrac—
`tion of the iliac bone. To this end, prostheses, such as
`ceramics have been recently used and implanted instead of
`the self-iliac bone. In general, the ceramics are stable in and
`highly biocompatible, with the organ in which the prosthesis
`is to be grafted
`However, the ceramics are brittle and hard, and accord-
`ingly are weak with respect to physical shock It is necessary
`for a vertebra prosthesis to have a mechanical strength
`strong enough to resist the compression load exerted to
`upper and lower vertebra bodies. In addition thereto, the
`vertebra prosthesis must be brought into direct contact with
`or directly adhered to the bone without an intervening soft
`tissue between the prosthesis and the bone while no bone
`resorption occurs between the upper and lower vertebra
`bodies.
`
`The shape of the prosthesis is such that the portion of the
`bone to be resected is as small as possible, since the
`prosthesis must be partly removed in accordance with the
`shape of the vertebrae prosthesis. In general, a known
`prosthesis (prosthesis other than ceramics) is subject to a
`certain stress (pressure) due to repeated absorption and bone
`resorption with respect to a sound bone to keep a balanced
`state. However in particular, the ceramic cervical vertebrae
`prosthesis in which no bone resorption takes place must be
`shaped so as to disperse or uniformly distributes the com-
`pression stress. Nevertheless, the conventional ceramic ver-
`tebrae prosthesis has been focused mainly on the strength of
`the material of which the vertebrae prosthesis is made. to
`increase the resistance to the compression load but little or
`no consideration of the shape have been taken into account.
`Moreover, little or no attempts to reduce the amount of the
`bone to be resected have been made.
`
`is necessary to expand the distance
`it
`Furthermore,
`between the upper and lower vertebra bodies so as to insert
`
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`the vertebrae prosthesis therebetween. However, the con—
`ventional vertebrae prosthesis cannot, by itself. expand the
`distance between the upper and lower vertebra bodies.
`Accordingly, it is necessary to use a special expanding tool
`or device, thus resulting in an increase in the time for the
`operation. In particular, a conventional elongated prosthesis,
`which is to be inserted to substitute for a plurality of
`removed vertebra bodies. is usually in the form of a circular
`or angular post linearly extending in the vertical direction.
`Accordingly, it does not conform to the profile of the
`continuously curved vertebra bodies. Consequently, the con—
`ventional elongated prostheses were morphologically
`unnatural.
`
`SUMMARY OF THE INVENTION
`
`The primary object of the present invention is to provide
`an improved prosthesis having an increased resistance to the
`compression stress, wherein no bone resorption occurs; the
`amount of the vertebra body to be resected upon operation
`can be minimized; the prosthesis can be easily inserted; and,
`the inserted prosthesis is stably held in the inserted position.
`Another object of the present invention is to provide an
`improved ceramic prosthesis for a plurality of vertebra
`bodies having a morphologically natural shape.
`These and other objects of the present invention will be
`appreciated more fully after the following descriptions con-
`cerning preferred embodiments of the present invention.
`One of the most significant features of the present inven—
`tion is addressed to the shape of the ceramic vertebrae
`prosthesis. Namely, the inventors have found that the contact
`surface of the vertebra body with the intervertebral disk is
`macroscopically a concave surface. Consequently, if the
`vertebrae prosthesis, which substitutes for the intervertebral
`disk, is provided with a convex surface corresponding to the
`concave contact surface,
`technical advantages can be
`achieved Particularly an increase in resistance to achieved
`compression load, the prevention of a bone resorption, a
`minimization of the amount of vertebral bone to be resected,
`ease of insertion, and an increase in stableness after
`insertion, etc., can be all achieved.
`Namely, according to an aspect of the present invention,
`there is provided a ceramic vertebrae prosthesis to be
`inserted in a cavity defined between upper and lower ver-
`tebra bodies, by removal of an intervertebral disk or disks,
`wherein the vertebrae prosthesis is provided with upper and
`lower outwardly convex contact surfaces that come into
`contact with the upper and lower vertebra bodies.
`The outwardly convex contact surfaces of the vertebrae
`prosthesis provide an increased resistance to the compres-
`sion stress exerted by the upper and lower vertebra bodies.
`Since the contact surfaces of the vertebra bodies that come
`into contact with the prosthesis are macroscopically concave
`surfaces, the amount of bone of the vertebra bodies to be
`resected, depending on the convex surfaces of the prosthesis,
`can be minimized. The engagement of the concave contact
`surfaces of the vertebra bodies and the convex contact
`surfaces of the prosthesis provides a high resistance to the
`compression stress not only in the vertical direction
`(longitudinal direction of the vertebrae) but also in the
`lateral direction slightly deviated from the longitudinal
`direction, since the stress is substantially uniformly distrib—
`uted or dispersed. The distribution of the stress prevents the
`bone of the vertebrae from being resorbed by the prosthesis,
`that is, no bone resorption takes place. Consequently, no
`sinking of the prosthesis results. so that the prosthesis
`appropriately and stably functions as a substitute for the
`
`7
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`5,645,596
`
`3
`removed vertebrae for a long time, without causing a defor-
`mation of the remaining vertebrae.
`The contact surfaces of the prosthesis with the vertebrae
`can be made of either a single curvature of or a composite
`curved surface consisting of different curvatures of crrrved
`surfaces in combination. For instance, in the composite
`curved surface. the curvature thereof in a section perpen—
`dicular to the insertion direction of the prosthesis is different
`from the curvature in a section parallel with the insertion
`direction.
`
`Preferably, the width of the prosthesis gradually decreases
`toward the front end thereof as viewed in the insertion
`direction thereof, so that the prosthesis can be easily inserted
`in a cavity or space between the upper and lower vertebra
`bodies. Namely, for ease for insertion of the prosthesis, the
`latter is preferably of a wedge-shape. Meanwhile, it should
`be recalled that the prosthesis is preferably provided with
`curved surfaces as a whole, as mentioned above. In view of
`these two different requirements, it is preferable that each of
`the curved contact surfaces of the prosthesis in a cross
`section parallel with the insertion direction thereof is made
`up of a front curved surface having a small curvature and a
`rear curved surface opposite thereto having a large
`curvature. so as to satisfy the two requirements.
`Another feature of the present invention is directed to an
`improved cervical prosthesis. The cervical vertebrae are in
`the form of a forwardly convex arch. Accordingly,
`the
`cervical prosthesis, to substitute for the cervical vertebrae,
`should be correspondingly curved into an arch-shape so as
`to provide a morphologically natural form. Furthermore, if
`the upper and lower contact surfaces of the cervical
`prosthesis, with the upper and lower vertebra bodies, are
`made of convex surfaces, the compression stress can be
`effectively dispersed or distributed.
`Namely, according to another aspect of the present
`invention, there is provided a ceramic cervical vertebrae
`prosthesis for a multiple vertebra bodies to be inserted in a
`cavity formed by resecting a plurality of continuous cervical
`vertebrae, wherein the vertebrae prosthesis is provided with
`an outwardly arched front surface, corresponding to front
`surfaces of the removed vertebra bodies, and upper and
`lower convex contact surfaces that come into contact with
`the corresponding upper and lower vertebra bodies.
`In the present invention, there is no limitation on the
`radius of curvature of the front surface of the ceramic
`
`cervical vertebrae prosthesis, but preferably, the radius of
`curvature is 100 mm to 200 mm. The radius of curvature of
`the rear surface of the ceramic cervical vertebrae prosthesis
`can be the same as or different from that of the front surface
`thereof. If the radius of curvature of the rear surface of the
`ceramic cervical vertebrae prosthesis is smaller than the
`radius of curvature of the front surface thereof, the ceramic
`cervical vertebrae prosthesis can be easily and advanta-
`geously inserted from the front of the vertebrae.
`If the upper and lower contact surfaces of the cervical
`vertebrae prosthesis are made of outwardly convex surfaces.
`the resistance to the compression stress exerted by the upper
`and lower vertebra bodies, can be increased. Since the
`contact surfaces of the vertebra bodies that come into
`contact with the prosthesis are macroscopically concave
`surfaces, the amount of bone of the vertebra bodies to be
`resected, depending on the convex surfaces of the prosthesis,
`can be minimized. The engagement of the concave contact
`surfaces of the vertebra bodies and the convex contact
`surfaces of the prosthesis provides a high resistance to the
`compression stress not only in the vertical direction (i.e..
`
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`direction connecting the two adjacent vertebrae bodies) but
`also in the lateral direction slightly deviated from the
`longitudinal direction, since the stress is substantially uni-
`formly distributed or dispersed. The distribution of the stress
`prevents the bone of the vertebrae from being resorbed by
`the prosthesis, that is, no bone resorption takes place.
`Consequently, no sinking of the prosthesis is caused, so that
`the prosthesis appropriately and stably functions as a sub-
`stitute for the removed vertebrae for long time without
`causing a deformation of the remaining vertebrae.
`The contact surfaces of the prosthesis with the vertebrae
`can be made of either a single curvature or a composite
`curved surface consisting of different curvatures of curved
`surfaces in combination. For instance, in the composite
`curved surface, the curvature thereof in a section perpen-
`dicular to the insertion direction of the prosthesis is different
`from the curvature in a section parallel with the insertion
`direction.
`
`The ceramic of which the vertebrae prosthesis (including
`the cervical vertebrae prosthesis) is made can be selected
`from alumina, zirconia, or calcium phosphate ceramic, etc.
`Among them, in particular, the calcium phosphate ceramic
`is advantageous and preferable since a spontaneous adhesion
`of the calcium phosphate ceramic vertebrae prosthesis to the
`associated vertebrae takes place, resulting in a firm connec-
`tion therebetween. As the calcium phosphate ceramic cal—
`cium phosphate compound whose Ca/P ratio is 1.0 to 2.0,
`such as hydroxyapatite, tricalcium phosphate, tetracalcium
`phosphate, calcium hydrogen phosphate. can be advanta-
`geously used. These substances can be used singly or as a
`mixture or compound.
`More preferably, the vertebrae prosthesis of the present
`invention is made of porous calcium phosphate ceramics
`whose porosity (true porosity) is 20 to 55%, and preferably
`30 to 45%. In case of porous ceramics, an accelerated
`adhesion thereof to the vertebra bodies can be expected
`owing to the penetration of the osteoblast into the pores at
`the contact surface of the vertebrae prosthesis and the
`vertebra bodies. This expedites the recovery of function of
`the vertebrae. If the (true) porosity is below 20%, the
`number of the pores is too small to expect the above—
`mentioned advantages. Conversely, if the porosity is above
`55%. the mechanical strength of the prosthesis is too small
`to resist the compression stress. The (true) porosity refers to
`a percentage of a total volume of the closed pores (cells) and
`the open pores (cells) to a unit volume.
`The ceramic vertebrae prosthesis can be produced as
`follows. In the following discussion, hydroxyapatite ceram-
`ics are used to make the ceramic vertebrae prosthesis by way
`of example.
`Phosphate salt and calcium salt are synthesized in a wet
`composition process per se known to obtain a hydroxyapa-
`tite slurry. The slurry thus obtained is dried by use of arotary
`drum type drier or the like to obtain a hydroxyapatite
`powder. The powder is pressed. for example, by use of a dry
`type static hydraulic press into the shape of the ceramic
`vertebrae prosthesis. The compact member thus obtained is
`heated at 1000° C. to 1200° C. in an electric furnace to
`obtain vertebrae prostheses (including cervical vertebrae
`prostheses). Alternatively, it is possible to employ a slurry
`casting process, an injection molding process, or to produce
`a green compact member which is machined by a lathe into
`a predetermined shape.
`The methods mentioned above are useful in producing a
`dense vertebrae prosthesis whose porosity is less than 10%.
`To produce a porous vertebrae prosthesis whose porosity
`is more than 20%, a foaming agent and water are added to
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`

`5,645,596
`
`5
`and mixed with the hydroxyapatite powder to obtain a
`foamed slurry which is then dried. The dried slurry is
`machined by a milling machine or the like into a predeter-
`mined shape of vertebrae prosthesis which is thereafter
`heated at 1000° to 1200° C. in an electric furnace to obtain
`a final product, i.e., a vertebrae prosthesis. In the process
`mentioned above, it is alternatively possible to add a powder
`of thermally disappearing substance, so that the mixture is
`formed and heated to produce a vertebrae prothesis. A
`thermally disappearing substance refers to any of various
`plastic or wood powders that disintegrate when heated at
`high temperatures,
`Furthermore. the vertebrae prosthesis according to the
`present invention can be made of ceramics having a dense
`center portion and a porous circumferential portion. The
`dense center portion mainly contributes to an increase in the
`mechanical strength as a whole, and the porous circumfer-
`ential portion mainly contributes to an accelerated adhesion
`of the vertebrae prosthesis to the vertebra bodies. To produce
`such a ceramic vertebrae prosthesis having a dense center
`portion and a porous circumferential portion, for example, a
`dried dense material corresponding to the center portion is
`fitted in a dried porous material corresponding to the cir-
`cumferential portion and the assembly is heated. or the dried
`dense material is adhered to the dried porous material by an
`apatite slurry and the assembly is heated.
`The subject of the present invention is directed to a
`structure or shape of a vertebrae prosthesis and not to the
`production process thereof. Accordingly, the manufactlning
`process is not limited to those mentioned above.
`The present disclosure relates to subject matter contained
`in Japanese utility model application Nos. 5-37185 (filed on
`Jul. 7, 1993) and 5-60953 (filed on Nov. 12, 1993) which are
`expressly incorporated herein by reference in their entirety.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention will be described below in detail
`with reference to the accompanying drawings, in which:
`FIG. 1 is a plan view of a ceramic vertebrae prosthesis
`according to an embodiment of the present invention;
`FIG. 2 is a front elevational View of FIG. 1;
`FIG. 3 is a right side elevational view of FIG. 1;
`FIG. 4 is a schematic view of a vertebrae prosthesis
`shown in FIGS. 1 through 3. inserted between vertebra
`bodies;
`
`FIG. Sis a cross sectional plan view of a ceramic cervical
`vertebrae prosthesis according to an embodiment of the
`present invention;
`FIG. 6 is a front side elevational View of FIG. 5;
`FIG. 7 is a left side elevational view of FIG. 5;
`FIG. Sis a perspective View of FIG. 5: and.
`FIG. 9 is a schematic view of a ceramic cervical vertebrae
`prosthesis shown in FIGS. 5 through 8. inserted between
`vertebra bodies.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`FIGS. 1 through 3 show a ceramic vertebrae prosthesis 10
`according to the present invention by way of example. The
`ceramic vertebrae prosthesis 10 is substantially rectangular
`in plan View corresponding to the cross sectional shape of
`the vertebra body (cervical vertebrae or
`thoracolumber
`vertebrae). The ceramic prosthesis 10 is has a symmetrical
`shape with respect to the center axis (median plane) in the
`
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`vertical direction Z. The ceramic prosthesis 10 is provided
`with a center portion 11 of wedge-shape which is provided
`on the upper and lower surfaces thereof with curved contact
`surfaces 12 which are in the form of a convex surface.
`
`The wedge-shape center portion 11 has a width which
`gradually decreases toward the front end thereof, in a section
`in a direction parallel with the direction “A” (i.e., z—x plane)
`of insertion of the ceramic prosthes is between the vertebra
`bodies, as can be seen in FIG. 2.
`In a section in a direction parallel with the insertion
`direction “A” (i.e., z—x plane), the contact surfaces 12 are
`each provided with a front small curvature surface portion
`120 (having a large radius of curvature) and a rear large
`curvature surface portion 12b (having a small radius of
`curvature) in combination. The curved surface in each cross
`section in z—y plane perpendicular to the insertion direction
`“A” is comprised of a curved to surface having one curva-
`ture as shown in FIG. 3.
`
`The ceramic prosthesis 10 as constructed above is inserted
`between the vertebra bodies 20 with the front small curva—
`ture surface portion 12a being the leading end, as can be
`seen in FIG. 4. Since the small curvature surface portion 12a
`has a curvature smaller (radius of curvature larger) than the
`large curvature surface portion 12b, and the wedge—shape
`portion 11 reduces the thickness thereof at the front end, the
`ceramic prosthesis 10 can be inserted between the vertebra
`bodies 20 while expanding the space defined between the
`vertebra bodies, due to the wedge effect thereof as a whole.
`When the insertion is completed, the contact surfaces 12
`which are in the form of a convex surface consisting of the
`small curvature surface portion 12a and the large curvature
`surface portion 12b substantially uniformly distribute or
`disperse the compression stress exerted by the vertebra
`bodies 20. and no bone resorption takes place. so that the
`ceramic prosthesis can be stably held between the vertebra
`bodies 20. Moreover, the amount of the vertebra bodies 20
`to be cut—away can be as small as possible, since the convex
`surface 12 correspond to the surfaces of the associated
`vertebra bodies 20 that are macroscopically concave sur—
`faces.
`
`In the illustrated embodiment, although the central por-
`tion 11 is in the form of a wedge which contributes, together
`with the convex contact surfaces 12, to an easy insertion of
`the ceramic prosthesis, as mentioned above, the central
`portion 11 may be in the form of a parallel—sided plate.
`Moreover, in the illustrated embodiment, the vertebrae pros—
`thesis 10 is of a symmetrical shape with respect to the center
`axis in the vertical direction, and accordingly, the prosthesis
`can be inserted with either side (upper or lower side in FIGS.
`2 and 3) located up. Alternatively, the ceramic vertebrae
`prosthesis 10 can be of an asymmetrical shape.
`The following discussion will be adressed to examples of
`a production process of the ceramic vertebrae prosthesis 10
`according to the present invention.
`EXAMPLE 1
`
`A hydroxyapatite slurry was prepared by a wet composi—
`tion process. The slurry thus obtained was dried by a rotary
`drum type drier produced by Nishimura Tekkou Seisakusho
`Co. Ltd., to obtain a hydroxyapatite powder. The powder
`was calcined at 800° C. in an electric furnace for three hours
`for an easy handling thereof.
`The calcined powder was pressed by a metal press to
`obtain a green compact in the form of a cylinder whose
`diameter and length were 35 mm and 35 mm, respectively.
`The green compact thus obtained was pressed at a hydraulic
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`5 ,645,596
`
`7
`
`pressure of 1 ton/cm2 by a static hydraulic press to obtain a
`cylindrical green compact having a mechanical strength
`strong enough to machine the same. The cylindrical green
`compact thus obtained was machined by an NC milling
`machine to produce different sizes of products whose shape
`is as shown in FIGS. 1 through 3, taking into account a
`contraction of the green compact caused during a heating
`process thereof. The products were then heated at 1100° C.
`in an electric furnace for two hours to obtain different sizes
`
`of vertebrae prostheses. The vertebrae prostheses were
`implanted in patients suflering from an intervertebral disk
`hernia or the like by the forward adhesion process. As a
`result. good results were obtained.
`EXAMPLE 2
`
`A hydroxyapatite slurry which was prepared by a wet
`composition process was dried by a rotary drum type drier
`produced by Nishimura Tekkou Seisakusho Co. Ltd.. to
`obtain a hydroxyapatite powder. The powder was calcined at
`800° C. in an electric furnace for three hours for an easy
`handling thereof.
`100 g of albumen albumin was added to and gradually
`mixed with 200 g of the calcined powder by a dry type ball
`mill. Thereafter, 500 g of water was added to the mixed
`powder and foamed for 15 minutes by a hand mixer.
`Thereafter, the foamed mixture was transferred onto a glass
`Petri dish having 20 cm diameter and 5 cm depth. and dried
`at 80° for 24 hours in a drier to obtain a dried porous body.
`The porous body was cut into parallelepiped shapes and
`machined by an NC lathe and milling machine to produce
`diiferent sizes of products whose shape is as shown in FIGS.
`1 through 3. taking into account a contraction thereof caused
`during a burning process thereof. The products were then
`heated at 1200° C. for three hours in an electric furnace to
`
`obtain diiferent sizes of vertebrae prostheses. The vertebrae
`prostheses were implanted in patients sufiering from an
`intervertebral disk hernia or the like by the forward adhesion
`process. As a result. good results were obtained.
`FIGS. 5 through 9 show a ceramic cervical vertebrae
`prosthesiac 30 for multiple vertebra bodies according to the
`present invention. by way of example.
`The ceramic cervical vertebrae prosthesis 30 is in the
`form of an arch corresponding to the shape of the cervical
`vertebrae. so that when the prosthesis 30 is implanted and
`adhered. the surface of the prosthesis 30 corresponding to
`the front surface of the cervical vertebrae is made of a
`convex surface 31. The upper and lower surfaces of the
`prosthesis 30. that are brought into contact with the upper
`and lower vertebra bodies 20. are formed as convex surfaces
`32
`
`Note that the radius of curvature of the front surface 31 of
`the prosthesis 30 corresponding to the front surface of the
`cervical vertebrae is not necessarily identical to and can be
`difierent from the radius of curvature of the rear surface 33
`of the prosthesis 30 opposite to the front surface 31. For
`instance. if the radius of curvature of the convex front
`surface 31 of the prosthesis 30 is larger than the radius of
`curvature of the concave rear surface 33 thereof. the pros—
`thesis 30 can be more easily inserted into a cavity formed
`when a plurality of vertebra bodies have been resected. In
`the illustrated embodiment. the radii of curvature of the
`convex surface 31 and concave surface 33 are 160 mm and
`150 mm. respectively.
`When the ceramic cervical vertebrae prosthesis 30 is
`inserted. the convex front surface 31 is a leading end. as
`shown in FIG. 9. Consequently. the cervical vertebrae pros-
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`15
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`20
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`25
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`30
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`35
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`45
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`50
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`55
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`8
`thesis 30 has a natural curve corresponding to the cervical
`vertebrae. When the insertion of the prosthesis 30 is
`completed, the convex contact surfaces 32 that come into
`contact with the vertebra bodies 20 substantially uniformly
`distribute or disperse the compression stress from the ver—
`tebra bodies 20 due to the convexity thereof, so that the
`prosthesis 30 can be stably held between the vertebra bodies
`20 without a bone resorption. Moreover. the amount of the
`vertebra bodies 20 to be resected is minimized since the
`
`surfaces of the vertebra bodies 20 are microscopically
`concave surfaces, as mentioned above.
`Examples of production process of the ceramic cervical
`vertebrae prosthesis 30 according to the present invention
`will be discussed below.
`
`EXAMPLE 3
`
`A powder of tricalcium phosphate marketed by Taiheiyou
`Kagaku Co. Ltd, was calcined at 800° C. for three hours in
`an electric furnace. It was identified by an X-ray analizer
`that the calcined powder had a hydroxyapatite crystal struc-
`ture. The calcined powder was pressed by a metal press to
`obtain a green compact in the form of a prism of 25 mmx25
`mmx80 mm (height). The green compact thus obtained was
`pressed at a hydraulic pressure of 1 ton/cm2 by a static
`hydraulic press to obtain a prism-shape of green compact
`having a mechanical strength strong enough to machine the
`same. The green compact was then machined by an NC
`milling undergo machining to produce different sizes of
`products whose shape is as shown in FIGS. 5 through 9.
`taking into account a contraction of the green compact
`caused during a heating process thereof. The products were
`then heated at 1100° C. for two hours in an electric furnace
`to obtain difl’erent sizes of cervical vertebrae prostheses. The
`cervical vertebrae prostheses were implanted in patients
`sufiering from an intervertebral disk hernia or the like by the
`forward adhesion process. As a result, good results were
`obtained after the lapse of six months.
`
`EXAMPLE 4
`
`50 g of albumen albumin was added to and gradually
`mixed with 200 g of calcined hydroxyapatite powder
`obtained by the same process as Example 3. mentioned
`above, by a dry type ball mill. Thereafter. 500 g of water was
`added to the mixed powder and foamed for 15 minutes by a
`hand mixer. Thereafter, the foamed mixture was transferred
`onto a glass Petri dish having 20 cm diameter and 5 cm
`depth, and dried at 80° C. for 24 hours in a drier to obtain
`a dried porous body. The porous body was cut into paral-
`lelepiped shapes and machined by an NC milling machine to
`produce different sizes of products whose shape is as shown
`in FIGS. 5 through 9. taking into account a contraction
`thereof caused during a heating process thereof. The prod—
`ucts were then heated at 1200" C. for three hours in an
`electric furnace to obtain diiferent sizes of cervical vertebrae
`prostheses. The cervical vertebrae prostheses were
`implanted in patients suffering from an intervertebral disk
`hernia or the like by the forward adhesion process. As a
`result. good results were obtained after the lapse of seven
`months.
`
`As can be seen from the above discussion, according to
`the present invention. a vertebrae prosthesis having an
`increased resistance to the compression stress can be
`obtained. wherein no bone resorption of the prosthesis
`occurs; the amount of the bone of the vertebra body to be
`resected during the surgical operation can be minimized; the
`vertebrae prosthesis can be easily inserted between the
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`5,645,596
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`9
`vertebra bodies; and, the vertebrae prosthesis can be stably
`held between the vertebra bodies.
`Furthermore, according to the present invention, a cervi-
`cal vertebrae prosthesis for a multiple vertebra bodies can be
`advantageously implanted in a cavity by a forward adhesion
`process after a plurality of vertebra bodies are removed. In
`the cervical vertebrae prosthesis, the compression stress
`exerted by the associated upper and lower vertebra bodies
`can be dispersed or uniformly distributed. thus resulting in
`an increased resistance of the prosthesis to the compression
`stress. In addition to the foregoing, in the cervical vertebrae
`prosthesis according to the present
`invention, no bone
`resorption takes place;
`the amount of the bone of the
`vertebra body to be resected during the surgical operation
`can be minimized; the cervical vertebrae prosthesis can be
`easily inserted between the vertebra bodies; the vertebrae
`prosthesis can be stably held between the vertebra bodies for
`long period; the prosthesis serves as a morphologically
`natural substitute, so that an operator or a patient can be
`remitted from burden or physical load.
`We claim:
`1. A ceramic vertebra prosthesis to be inserted in a cavity
`defined between upper and lower vertebra bodies with a
`removed intervertebral disk or disks, said vertebra prosthesis
`comprising a ceramic