United States Patent (19)
`Kuntz
`
`54 INTERVERTEBRAL DISC PROSTHESIS
`76) Inventor: J. David Kuntz, 899 Lahakas Blvd.,
`Kitimat, B.C., Canada
`21 Appl. No.: 159,843
`22 Filed:
`Jun. 16, 1980
`
`.
`
`. .
`
`.
`
`.
`
`. .
`
`.
`
`.
`
`.
`
`.
`
`.
`
`.
`
`.
`
`. .
`
`. .
`
`.
`
`. A61F1/00
`
`51) Int. Cl.3 ......
`
`56)
`
`52 U.S. Cl. ............................................................ 3/1
`58) Field of Search .................... 3/1, 1.9, 1.91, 1911,
`3/1.912, 1913; 128/92 C, 92 CA, 92 R, 92 EC
`References Cited
`U.S. PATENT DOCUMENTS
`2,677,369 5/1954 Knowles ........................... 128/92 R
`3,685,058 8/1972 Tronzo ....................... 128/92 ECX
`3,867,728 2/1975 Stubstad et al. .
`.............. 3/1.91
`3,875,595. 4/1975. Froning ...................................... 3/1
`4,040,131 8/1977 Gristina ....
`... 3/1.91
`4,245,359 1/1981 Stuhmer ...
`... 3/1.9
`4,257,129 3/1981 Volz ..................................... 3/1911
`FOREIGN PATENT DOCUMENTS
`2263842 7/1974 Fed. Rep. of Germany ......... 3/1.91
`2804936 8/1979 Fed. Rep. of Germany ......... 3/1.91
`2372622 6/1978 France .................................... 3/1.9
`
`
`
`11)
`45
`
`4,349,921
`Sep. 21, 1982
`
`Primary Examiner-Clifford D. Crowder
`57
`ABSTRACT
`An intervertebral disc prosthesis intended to replace a
`natural intervertebral disc and to restore the normal
`intervertebral spacing without complete loss of flexibil
`ity of the spinal joint. The prosthesis comprises a body
`of biologically-acceptable material suitably dimen
`sioned and shaped to replace a natural disc. One of the
`longitudinal ends of the prosthesis has suitable means,
`e.g. a raised flange, to facilitate handling of the prosthe
`sis and to prevent penetration to an excessive depth into
`the spinal joint. The other longitudinal end is preferably
`wedge-shaped to facilitate insertion into the interverte
`bral space. The superior and inferior surfaces are prefer
`ably provided with surface characteristics to produce a
`“friction-fit' and are convex to correspond to the adja
`cent vertebral surface. The prosthesis is inexpensive to
`manufacture and can be implanted quite easily with
`little danger to the patient. Moreover, the prosthesis
`maintains at least some of the flexibility of the joint
`while remaining firmly anchored in place.
`
`52 Claims, 15 Drawing Figures
`
`NUVASIVE - EXHIBIT 2005
`Alphatec Holdings Inc. et al. v. NuVasive, Inc.
`IPR2019-00362
`
`

`

`U.S. Patent Sep. 21, 1982
`US Patent
`Sep. 21, 1982
`
`Sheet 1 of 3
`Sheet 1 of 3
`
`4,349,921
`4,349,921
`
`
`
`
`
`

`

`U.S. Patent Sep. 21, 1982
`US. Patent
`Sep. 21, 1982
`
`Sheet 2 of 3
`Sheet 2 of 3
`
`4,349,921
`4,349,921
`
`
`
`
`
`

`

`U.S. Patent
`U.S. Patent
`
`Sep. 21, 1982
`Sep. 21, 1982
`
`Sheet 3 of 3
`Sheet 3 of 3
`
`4,349,921
`4,349,921
`
`
`
`
`
`N_.0_u_
`
`
`
`
`

`

`1.
`
`INTERVERTEBRAL DISC PROSTHESIS
`
`4,349,921
`2
`a porous surface is more prone to harbor bacteria due to
`the large surface area involved. In a porous surface in
`which tissue ingrowth has occurred, there is also in
`creased possibility of repeated injury at the interface
`between the bone and prosthesis due to tearing of fi
`brous tissue or prosthetic fibers with subsequent tissue
`reaction including foreign body rejection.
`A major disadvantage of a porous material is that if
`there is actual tissue ingrowth into the prosthesis, re
`moval could be difficult. Curetting out the prosthesis
`would lead to hemorrhage of a very vascular surface
`area, caused by the tissue ingrowth, with subsequent
`increased danger of cord compression secondary to
`hemorrhage.
`Besides, any porous material allowing tissue in
`growth for stability in a cervical spine would be danger
`ous as the esophagus, which lies anteriorly, could be
`come adhered to the prosthesis with resultant dysphagia
`or difficulty in swallowing.
`There is consequently a need for a prosthesis that
`remains stably in place when implanted but does not
`have the difficulties referred to above associated with
`porous surfaces.
`West German Offenlegungsschrift No. 2,263,842 in
`the name of Hoffmann-Daimler, published on July 4,
`1974 discloses yet another type of disc prosthesis. In its
`simplest form, this prosthesis consists of a circular disc
`having smooth convex faces. The disc may be made of
`a synthetic material. This simple form, however, may
`become displaced when implanted and could possibly
`damage the neural canal.
`The known prostheses are often made of different
`materials bonded together to allow movement within
`the prosthesis, but this weakens the overall loading
`strength of the prosthesis. This is particularly the case
`when the prosthesis is submitted to repeated stresses.
`Bonded materials can become fatigued under stress and,
`due to the very thin spaces involved, any multiple lay
`ered prosthesis would involve quite thin layers of mate
`rial. This would increase the problem of breakage and
`migration of fragments which could have hazardous
`consequences in the spinal area.
`It is believed that each of the prostheses referred to
`above is intended only for the lumbar area of the spine.
`For example, the Stubstad et al. patent describes inser
`tion through an anterior approach which is, in fact, a
`"retroperitoneal' approach. There is therefore a need for
`a disc prosthesis that can be used in the cervical area of
`the spine as well as just the lumbar area.
`Another difficulty of the known prostheses is the
`difficulty of handling them during surgical implanta
`tion. In particular, difficulty is often encountered in
`removing the prostheses once implanted should adjust
`55.
`ment or replacement be required.
`SUMMARY OF THE INVENTION
`It is an object of the present invention to provide an
`improved intervertebral disc prosthesis.
`According to one aspect of the invention there is
`provided an intervertebral disc prosthesis, comprising a
`body of biologically compatible material having a supe
`rior surface, an inferior surface and opposed anterior
`and posterior ends, and means located at one of said
`opposed ends for facilitating holding of the prosthesis
`during its insertion into or removal from an interverte
`bral disc space.
`
`65
`
`20
`
`35
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention relates to an intervertebral disc pros
`thesis and to surgical techniques for implanting the
`prosthesis.
`The vertebrae of the spinal column are connected
`together by intervertebral fibrocartilaginous discs. The
`10
`discs maintain a separation between the vertebrae, but
`occasionally become narrowed so that the interverte
`bral separation is reduced. This reduction in separation
`has a number of painful and unpleasant consequences,
`for example, in the cervical region of the spine, it can
`15
`result in cervical spondylosis, vertebral artery syn
`drome and painful arc syndromes. These symptoms are
`described in more detail below.
`1. Description of the Prior Art
`Procedures have been developed in the prior art for
`alleviating the symptoms resulting from intervertebral
`disc failure.
`One such procedure involves fusing the adjacent
`vertebrae by removing the damaged disc and inserting a
`plug or wedge of bone removed from another part of
`25
`the patient's skeleton.
`An example (known as the "Cloward technique') of
`such a procedure for the cervical vertebrae is described
`in
`“ORTHOPAEDICS-PRINCIPLES AND
`THEIR APPLICATION," Samuel L. Turek, M.D.,
`30
`Lippincott Company, Third Edition, pp. 761-763, in
`which a hole is drilled in the spinal column straddling
`the damaged disc space and including parts of the adja
`cent vertebrae. The hole is then filled with a cylindrical
`plug or dowel of bone in order to fuse the vertebrae
`together.
`Fusion of vertebrae together necessarily results in
`complete loss of flexibility of the spinal column at this
`location and is thus disadvantageous. Accordingly,
`proposals have been made in the past to replace the
`damaged or diseased disc with a disc prosthesis in
`tended to duplicate the function of the natural disc to
`Some extent.
`French patent application publication No. 2,372,622,
`of Bernard Fassio, published June 30, 1978, discloses
`45
`one such disc prosthesis. This consists of a flat circular
`plate having central hemispherical projections on each
`face thereof. The hemispheres allow articulation of the
`joint while the flat plate maintains separation. It is be
`lieved, however, that such a prosthesis would not be
`50
`entirely satisfactory because the hemispheres would not
`correspond closely to the shape of the adjacent verte
`bral surfaces. This could result in crushing of the can
`cellous vertebral bone by the hemispheres and conse
`quent reduction in separation and articulation of the
`joint.
`Another disc prosthesis is disclosed in U.S. Pat. No.
`3,867,728 of Stubstad et al., issued Feb. 25, 1975, as
`signed to Cutter Laboratories Inc. The prosthesis is a
`flattened kidney shaped block of elastomeric synthetic
`resin. The shape of the prosthesis is intended to conform
`closely to the space in a spinal disc from which the
`necleus pulposus has been removed. One disadvantage
`of this type of prosthesis is that elastomeric materials
`have been known to disintegrate in the body and to
`break down under repeated stressing over prolonged
`periods. Moreover, it is disclosed that the surface of the
`prosthesis may be porous to allow tissue ingrowth, but
`
`

`

`5
`
`10
`
`15
`
`4,349,921
`3
`4.
`The prosthesis is designed to restore the normal
`ative bony changes noted on the vertebral bodies and
`width of the disc space. The superior surface of the disc
`the joints of Luschka.
`space is usually slightly convex. This corresponds to the
`Replacement of the cervical discs with disc prosthe
`ses is designed to restore the width of the intervertebral
`slight concavity in the inferior surface of the vertebral
`disc space. This in turn restores the superior-inferior
`body. The inferior side of the disc space is usually more
`nearly planar corresponding to the superior surface of
`diameter or height of the outlet foramen which has been
`diminished with the narrowing of the disc space. It also
`the vertebral body, though it too is often slightly con
`restores the anatomy of the apophyseal joint surfaces
`vex. Accordingly, either both main surfaces of the disc
`posteriorly to a more normal anatomical relationship to
`prosthesis may be made slightly convex, or one may be
`each other. By opening the foramen the compression of
`slightly convex and the other planar. Although it is not
`the nerve root passing through the narrow foramen is
`preferred, both surfaces may be planar if desired.
`subsequently relieved. This in turn leads to relief of the
`Replacement of the defective discs with a disc pros
`symptoms of headache, neck pain, pain radiating to the
`thesis as defined above restores the intervertebral disc
`space and thus immediately corrects many of the prob
`shoulders and arms and associated weakness and numb
`lems producing painful or unpleasant symptoms.
`ness in the arms and hands.
`(B) Vertebral Artery Syndrome
`BRIEF DESCRIPTION OF THE DRAWINGS
`It is noted that the vertebral artery courses through
`FIG. 1 is a plan view of the prosthesis according to a
`the neck carried in the foramen transversarium of the
`first embodiment of the invention;
`transverse processes of the vertebrae. With narrowing
`FIG. 2 is a lateral elevational view of the prosthesis of
`of the disc space it is noted that the space between the
`FIG. 1;
`adjacent transverse process is also narrowed and this
`FIG. 3 is a perspective view of the prosthesis of
`leads to subsequent kinking outward of the accompany
`FIGS. 1 and 2;
`ing vertebral artery in these spaces between the adja
`FIG. 4 is a simplified lateral representation of part of
`25
`cent foramina through which it passes. This leads to
`the spinal column showing natural discs and the pros
`impaired flow within the vessel and this is accentuated
`thesis of FIGS. 1 to 3;
`at various positions of the neck leading to the clinical
`FIGS. 5 to 10 are simplified plan views of an interver
`syndrome of dizziness, blackouts, momentary blurred
`tebral disc space showing the steps in the method of
`vision, occasional tinnitus related to imparied vertebral
`inserting a prosthesis according to a second embodi
`30
`artery flow. The latter two components of this syn
`ment of this invention;
`drome may be related to imparied vertebral artery
`FIG. 11 is a simplified lateral representation of a
`blood flow of the nuclei of the involved cranial nerves
`prosthesis implanted according to the steps shown in
`in the midbrain. With regard to the blurred vision the
`FIGS. 5 to 10;
`impaired flow to the occipital area of the brain involved
`FIG. 12 is a simplified lateral view of a prosthesis
`35
`in vision could also be a factor.
`according to a third embodiment of the invention im
`It has been noted that in restoring the width of the
`planted in an intervertebral space;
`vertebral disc space with the use of a cervical disc pros
`FIG. 13 is a view similar to FIG. 12 showing the
`thesis or intervertebral disc spacer results in obliteration
`prosthesis located in the cervical region of the spine;
`of these associated vertebral artery syndrome com
`and
`plaints of dizziness, blackouts, intermittent blurred vi
`FIG. 14 is a view similar to FIG. 12 showing the
`sion and tinnitus. It is noted that in using the interverte
`prosthesis located in the lumbar region of the spine;
`bral disc prosthesis the distance between the transverse
`FIG. 15 is a cross-sectional view of yet another em
`processes is also restored and hence the kink in the
`bodiment of this invention showing an alternative
`vertebral artery is straightened out as the vessel is now
`means for holding the prosthesis.
`45
`drawn out to its normal length. Hence the turbulent
`As the first step towards putting the present invention
`flow situation is restored to a laminar type flow through
`into practice is for the physician to recognize the symp
`the vessel with subsequent disappearance of the inter
`tons which suggest prosthetic disc replacement, and
`mittent vertebral artery symptoms related to intermit
`since these symptoms are not generally well known in
`tent impaired flow through the vessel with various
`the case of the cervical region of the spine, they are
`50
`positions of the neck.
`briefly discussed in the following.
`(C) Painful Arc Syndromes
`Symptoms Suggesting Prosthetic Disc Replacement
`It is noted that nerve root compression in the outlet
`(I) Cervical Disc Disease (Cervical Spondylosis)
`foramen of the cervical spine leads to muscle paralysis
`55
`This is a clinical syndrome consisting of neck pain
`and weakness distally in the arm in the particular myo
`and stiffness and crepitation on neck movement, with
`tomes supplied by the particular nerve root involved.
`associated posterior occipital and frontal headaches.
`This may result in syndromes of painful shoulder due to
`There are also radicular pains towards the shoulders
`muscle imbalance of the muscle groups crossing the
`shoulder joint. If a particular muscle is paralyzed, the
`and arms associated with numbness and weakness in the
`arms and hands. It is characterized by restricted range
`antagonistic muscle will suddenly work unopposed,
`of movement of the neck especially in extension, and
`creating a muscle imbalance situation in the shoulder
`often is associated with hypoactive reflexes in the af.
`joint with subsequent creation of painful shoulder syn
`fected arm and decreased sensation in the particular
`drome. For instance, in a C4-5 disc degeneration with
`dermatome distribution of the involved nerve root.
`narrowing, the C5 nerve root may be obstructed in the
`65
`The radiological findings are characteristically nar
`outlet foramen. In a C5-6 disc degeneration, the nar
`rowing of the disc space, narrowing of the outlet fora
`rowing and obstruction of the outlet foramen may in
`men at the involved level, and osteophytes and prolifer
`volve the C6 nerve root.
`
`60
`
`

`

`5
`
`15
`
`10
`
`4,349,921
`5
`6
`Since these particular roots supply supraspinatus
`praspinatus tenson and calcific depositis in the tendon
`muscle, which tends to hold the humeral head tightly
`may have to be dealt with later by partial acromionec
`into the glenoid on shoulder abduction movements, the
`tomy or removal of the calcific deposits if these are
`head is not held tightly into the socket when paresis of
`advancaedin degree prior to correction of the cervical
`disc problem.
`this muscle occurs. In addition, the biceps muscle which
`is also supplied by C5 and C6 nerve roots may also be
`DESCRIPTION OF THE PREFERRED
`weakened. Since the long head of the biceps tendon
`EMBOEDIMENTS
`passes over the humeral head to the supraglenoid tuber
`cle, its function in holding the humeral head depressed
`One embodiment of the intervertebral disc prosthesis
`in the socket is subsequently lost. It normally acts as a
`according to the invention is shown at 10 in FIGS. 1, 2
`downward force on the humeral head.
`and 3. The prosthesis 10 is formed of a thin block of
`In constrast, the force of the long head of the triceps
`biologically acceptable material having slightly convex
`muscle, an antagonistic muscle, comes into play. Since it
`superior and inferior surfaces 11 and 12, transverse
`arises from the infraglenoid tubercle and passes across
`grooves 13 in said superior and inferior surfaces and
`the elbow, it acts as a force tending to drive the humeral
`also in both lateral surfaces 14, 14, a flange or lip 15
`head upward toward the acromion. Since triceps is
`raised from said superior and inferior surfaces at one
`supplied by the C7 and C8 nerve roots, its function
`longitudinal end of the prosthesis, and a wedge shaped
`continues unimpaired when the disc degeneration in
`tapering portion 16 at the other longitudinal end.
`volves only the roots of the higher levels as described.
`The prosthesis 10 is designed to replace a damaged
`This leads to a muscle imbalance situation in which the
`intervertebral disc, preferably a cervical disc, in order
`20
`humeral head may ride up superiorly in the glenoid
`to restore the normal intervertebral spacing. The con
`socket. This causes subsequent contact between the
`vexity of the superior and inferior surfaces 11 and 12
`supraspinatus tendon and greater tuberosity under the
`corresponds closely to the slight concavities found in
`acromion as the arm is abducted. This can lead to the
`the inferior and superior surfaces of the vertebrae so
`following:
`that loading at the vertebra/prosthesis interface is
`25
`(1) Sub-acromial bursitis
`spread evenly, resulting in reduced liklihood of damage
`(2) Painful arc syndrome
`to the cancellous vertebral bone structure.
`(3) Rotator cuff degeneration
`The transverse grooves 13 increase the stability of the
`(4) Frozen shoulder or adhesive capsulitis
`prosthesis in the disc cavity both initially and progres
`(5) Shoulder-hand syndrome.
`sively with time. For initial stability the grooves create
`30
`It is noted that in the above list, rotator cuff degenera
`a "friction fit.' It is generally only possible to insert the
`tion is mentioned. It is theorized that this could be cre
`prosthesis when considerable traction is applied in the
`ated in the following manner: If the humeral head is
`operating room. When the traction is released, the fric
`riding high in the socket, the supraspinatus tendon in
`tion fit created by the grooves tends to resist removal of
`sertion is interposed between the two bony surfaces of
`the prosthesis unless longitudinal traction is re-applied.
`35
`head of humerus and the acromion. It is known that
`This is believed to be because the edges of the grooves
`there is a very delicate capillary anastomosis in the
`abut against bony protruberances from the vertebral
`distal half inch of the supraspinatus tendon created by
`surfaces.
`vessels passing through the substance of the tendon
`The grooves 13 also improve the stability of the pros
`from the muscle proximally and the greater tuberosity
`thesis progressively with time as fibrous and fibrocarti
`40
`distally. This anasatomosis occurs within the distal half
`laginous tissue grows into conformity with the configu
`inch of the supraspinatus tendon where it inserts into
`ration of the surfaces of the prosthesis. The tissue does
`the greater tuberosity of the humerus.
`not grow into the structure of the prosthesis, as would
`When the above muscle imbalances occurs, the hu
`be the case if the surface were porous, but rather against
`meral head rides higher in the socket with subsequent
`the surfaces to encapsulate the prosthesis. The prosthe
`45
`pressure between head of humerus and the overlying
`sis becomes encapsulated rapidly due to hypertrophy of
`acromion, local pressure on the interposed tendon
`the pre-vertebral fascia.
`would shut off this delicate capillary anastomosis caus
`This encapsulation process has a further advantage
`ing local necrosis of the interposed tissue. This is analo
`over fibrous growth directly into the structure of the
`gous to pressure necrosis of skin as seen in decubitis
`prosthesis. Removal of a prosthesis encapsulated in this
`50
`ulcers.
`way, for example if it should become infected later on,
`The ischemia of the supraspinatus tendon may lead to
`is relatively simple as the space around the prosthesis
`collagen degeneration, deposition of calcium salt in
`does not become vascular, as is the case with direct
`these tissues as seen in dystrophic calcification, calcific
`tissue ingrowth. After the removal of the prosthesis, the
`tendinitis, and actual supraspinatus tendon rupture sec
`space may then be treated with a cortical cancellous
`55
`ondary to the muscle pull through degenerate tendon.
`bone graft to promote bony fusion as a salvage proce
`Restoration of the diameter of the outlet foramen
`dure.
`allows a return of nerve function to the muscles about
`Although transverse grooves 13 are preferred for
`the shoulder girdle. This leads to correction of the mus
`providing the “friction-fit” because the major deform
`cle imbalance situation with restoration of the normal
`ing forces tend to dislocate the prosthesis in the longitu
`tone to the muscles which determine the position of the
`dinal (anterior-posterior) direction, other surface corru
`humeral head in the socket. This can be readily accom
`gations or projections can alternatively be employed,
`plished through the use of the prosthetic cervical disc
`for example pyramidal, diamond-shape, rasp- or file-like
`spacer as described. With return of normal muscle tone
`projections are suitable. Generally any small projec
`through post-operative shoulder physiotherapy, the
`tions, grooves or corrugations suitable to resist discloa
`65
`muscle bulk is restored, muscle balance is restored and
`tion of the prosthesis, may be employed.
`subsequently the painful arc syndrome can be over
`The wedge-shaped tapering portion 16 allows easier
`come. It is noted that collagen degeneration of the su
`insertion of the prosthesis into the disc space. The
`
`

`

`O
`
`30
`
`4,349,921
`8
`7
`wedge shape separates the vertebrae to the required
`sis. It is important to restrict this lateral movement at
`spacing as the prosthesis is tapped into place. Further
`the diseased level for the following reasons:
`(a) In restoring the width of the disc space, the superior
`more, the wedge shape is also better accommodated at
`inferior diameter or height of the outlet foramen is
`the back of the disc space and it does not interfere with
`increased by the disc prosthesis;
`the outer edges of the vertebrae.
`(b) in diseased segments, the lateral or anterior posterior
`The flange 15 is significant in that it prevents the
`surgeon from inadvertently driving the prosthesis too
`diameter of the canal is narrowed already by the
`osteophytes which form in the outlet foramen at the
`far through the disc space, for example into the neural
`joints of Luschka. These osteophytes are not re
`canal, which could result in quadriplegia. It also pre
`moved at surgery as removal of these would require
`vents migration of the prosthesis after termination of the
`much more extensive and dangerous exposure with
`operation and, in addition to the growth of fibrocarti
`potential problems of hemorrhage due to the proxim
`laginous tissue into grooves 13, helps to anchor the
`prosthesis firmly in position.
`ity of the vertebral arteries,
`(c) relief of the vertebral artery symptoms is obtained
`During a severe flexion extension injury to the spine
`by restoring the artery to its normal length by intro
`(particularly the cervical spine which is susceptible to
`15
`duction of the disc prosthesis.
`this kind of injury) after a disc prosthesis has been im
`planted therein, the flange prevents the prosthesis from
`(d) relief of the nerve root compressions symptoms is
`moving posteriorly into the neural canal as the prosthe
`achieved by gaining some increase in the overall
`dimension of the foramen by the gain in height or
`sis is carried forward with either the vertebra above or
`superior inferior diameter.
`the vertebra below.
`20
`The flange also greatly facilitates gripping of the
`(e) hence lateral bending allowed at the space would
`prosthesis, particularly during insertion into and re
`lose this advantage gained by restoration of the supe
`moval from the disc space. The extreme difficulty of
`rior inferior diameter height of the foramen by the
`prosthesis with subsequent recurrence of symptoms
`gripping a biconvex surfaced prosthesis for removal
`of nerve root compression due to the continuing pres
`from a disc space can well be imagined since any instru
`25
`ment gripping the prosthesis would tend to "squirt' it
`ence of osteophytes which take some of the available
`space in the foramen. The continuing presence of the
`further into the disc space. The disc space would have
`osteophytes in the outlet foramen does not cause any
`to be widened considerably to allow full insertion of the
`symptoms as long as the overall dimension of the
`withdrawal instrument. The presence of the flange 15,
`canal is adequate to allow the passage of the nerve ,
`however, makes removal relatively easy as it remains
`outside the disc space and can easily be gripped.
`roots without any root compression. Hence it is im
`Despite the above advantages of the flange 15, it can
`portant that the prosthesis does not allow lateral
`be omitted if the rear edge of the prosthesis is instead
`movement at the diseased levels since this movement
`provided with screw hole or the like so that the prosthe
`allows a loss of the height of the outlet foramen and
`hence loss of the advantage gained by introduction of
`sis can be attached to a holding instrument during inser
`35
`the disc prosthesis;
`tion or removal. When the prosthesis is properly lo
`(f) the only movement which would be advantageous in
`cated the holding instrument can be removed and the
`design modifications of this prosthesis, as described
`screw hole filled in by means of a cover screw to pre
`vent ingrowth of fibrous tissue. The holder can be de
`later, are the flexion extension movements at this
`space. This can be accomplished simply by a trans
`signed to allow the prosthesis to be inserted into the disc
`40
`verse cleavage plane and a simple hinge using two
`space only to the desired depth.
`As the flange 15 has advantages after the prosthesis
`components of dissimilar materials such as stainless
`steel and high density polyethylene. This design al
`has been inserted, it is generally preferred over the
`alternative suggested above.
`lows flexion and extension movements at this space.
`The flange 15 is shown in the drawing projecting
`It is not essential to make both the superior and infe
`rior surfaces of the prosthesis convex. In particular, the
`from both the superior and inferior surfaces 11, 12. If
`desired, the flange may form a projection at only one of
`superior surfaces of many vertebrae are almost planar,
`so the inferior surface of the prosthesis 10 may similarly
`those surfaces. Moreover, the flange need not extend
`be planar.
`laterally for the whole of the lateral width of the pros
`The required size of the prosthesis correlates directly
`thesis, as shown, but may extend for only part of this
`50
`with the height of the patient. Hence, a small size pros
`width.
`The prosthesis 10 is essentially a spacer and can be
`thesis is used for patients up to about 5'4" in height. For
`fabricated from any biologically acceptable material of
`the patients up to 6' a medium sized prosthesis is used
`suitable strength and durability, for example high den
`and in patients taller than that a large size is required. As
`sity polyethylene, polymethylmethacrylate, stainless
`the size increases, each of the dimensions of the prosthe
`55
`steel, or chrome cobalt alloys. The simplest material for
`sis increase slightly in size.
`fabrication of the prosthesis is a polymer, preferably
`One length is not suitable for all people. The distance
`high density polyethylene, and this may include a radi
`between the front of the vertebra and to the neural
`opaque marker so that the position of the prosthesis can
`canal is less in a small person (below five feet) than a
`be confirmed radiologically. Elastomeric materials are
`larger person (above six feet).
`60
`less preferred because some such materials hve been
`It can be generally stated that a small person has a
`known to disintegrate in the body. Furthermore, it is
`narrower disc than a large person. Similarly the length
`generally preferable to select a material that resists
`of the prosthesis (distance from anterior part of the
`compression or flexing, particularly in the case of a
`vertebral body to neural canal) also increased in the
`cervical prosthesis. It is advantageous in the cervical
`larger person as compared to the smaller person. The
`65
`spine that lateral movement of the neck, as occurs when
`distance between the lateral surfaces also increases as
`tilting the head toward either shoulder, should pur
`the size or height of the patient increases. Hence, in
`posely be restricted by the design feature of the prosthe
`putting a small size prosthesis into a very large person
`
`45
`
`

`

`4,349,921
`10
`the thickness of the prosthesis would be inadequate to
`sternal notch. The level of the incision depends on the
`distract the space sufficiently. The total surface area
`level of the cervical disc or discs involved. The incision
`would also be inadequate to distribute the weight bear
`is placed higher if the disc lesions are located high in the
`ing to the maximum bony surface and the depth or
`neck.
`length would also be inadequate to provide total sup
`The platysma muscle is divided. The sternocleido
`port for the vertebra. Similarly, in attempting to place
`mastoid and the carotid bundle are identified and re
`the large size prosthesis into the small person, the verte
`tracted laterally. The strap muscles, trachea and esoph
`bral body could be crushed as the oversize prosthesis is
`agus are retracted medially. The bulging discs are iden
`driven into a small or narrowed disc space which can
`tified visually and by palpation. A needle marker is
`not accommodate the larger structure. This leads to
`10
`placed in position and a lateral X-ray is taken in the
`wedging of the verbebrae as is seen in a compression
`operating room to confirm the level of the needle and
`wedge fracture. In addition, the oversize prosthesis if
`consequently the level of involved discs. Straps are
`driven into the flange would be driven slightly into the
`placed on the patient's wrists for traction while the
`neural canal with subsequent cord damage.
`X-ray films are taken since this allows better visualiza
`The increased thickness of using the grossly over
`15
`tion of the C6-7 level.
`sized prosthesis in a small person would also lead to
`Once the level of the diseased discs are determined
`instability of the prosthesis since the disc space would
`pre-operatively have been positively identified in the
`be forced into extension (in the neck) and the prosthesis
`operating room, the diseased discs are excised anteri
`would hence be sitting as a wedge which would tend to
`orly and the space is thoroughly curetted out, removing
`cause it to dislocate anteriorly as it would tend to be
`20
`the whole of the disc. The posterior longitudinal liga
`"squirted out' anteriorly.
`ment is not removed and the neural canal is not visual
`In summarily, one length is not suitable for all people.
`ized during the procedure. The periosteal elevator is
`The proper length is equal to