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Description  |
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TECHNICAL FIELD
This disclosure relates to implants for the vertebral comumn, and
specifically to an implant capable of alternately selectively achieving
arthrodesis (fusion) or arthroplasty (joint formation) between adjacent
vertebrae.
BACKGROUND OF THE INVENTION
The vertebral column is a flexible spinal column, extending along the back
of vertebrate animals, which includes a series of bones termed the
vertebrae. The major function of the vertebral column is protection of the
spinal cord. It also provides stiffening for the body and attachment of
the pectoral and pelvic girdles and many muscles. In humans an additional
function is to transmit body weight in walking and standing. Each
vertebra, in higher vertebrates, consists of a ventral body, or centrum,
surmounted by a Y-shaped neural arch. This arch extends a spinous process
or projection downward and backward that may be felt as a series of bumps
down the back, and two transverse processes, one to either side, which
provide attachment of muscles and ligaments. The centrums are separated by
cartilaginous intervertebral disks, which help cushion shock in
locomotion.
In general, these disks have two functions: to allow movement between pairs
of vertebrae and to act as buffers against the shocks of running, jumping,
and other activities that apply stresses to the spine. The disks also
cooperate with paired synovial joints, one on each side of the vertebral
arch, to facilitate complex multidimensional relative movement between
adjacent vertebrae.
Surgical intervention is often required following degeneration of
intervertebral disks. In many instances, spinal fusion is the desired
result. The usual rationale for fusion is prevention of progressive
apposition of the vertebral bodies with consequent subluxation of the
facet joints, narrowing of the nerve route foramina, and development of
arthrosis. For the most persistent cases of disabling back pain, direct
bony fusions are often performed to stop the painful motion between
vertebrae by permanently locking them to one another. However, in many
cases it might be advisable to allow continued movement between adjacent
vertebrae, which helps to prevent mechanical breakdown at nearby bone
segments.
When dealing with ruptured disks, one surgical remedy is to partially
remove the disk. Such removal satisfies only one particular part of the
problem by removing the displaced disk material that has created pressure
on a spinal nerve, but it does not restore the disk to its normal
configuration. The result is a flattened and degenerative disk, which is
usually accompanied by arthritis of the facet joints and might result in
persistent low back pain. This might also lead to recurrent pressure on a
nerve because the disk thinning associated with degeneration might lead to
formation of a spur that results in renewed pressure on the nerve. Current
fusions operations or bony decompressions are utilized, but they are only
relatively satisfactory procedures because of the magnitude of any fusion
operation and because spurs tend to recur when removed without an
arthrodesis. Accordingly, there is a tendency for stenosis of the spinal
canal or neural foramen to occur.
The present invention was developed in an effort to provide a surgeon with
an element of control over the end result of such operations following
disk removal. The desired result can be either arthrodesis (fusion) or
arthroplasty (joint formation). Furthermore, recognizing that all surgical
procedures will fail to achieve their intended results in a statistical
percentage of situations, the present invention assures that an acceptable
result will be achieved as an alternative to that desired. Briefly, where
arthrodesis is the intended result, arthoplasty will be the alternative
result, and where arthroplasty is the intended result, arthrodesis will be
the alternative result. Thus, even though a planned fusion does not occur,
the patient will be provided with a useful prosthetic joint between the
affected vertebrae and, conversely, where joint formation was intended and
not achieved, an acceptable solid fusion will be achieved in its place.
U.S. Pat. No. 4,501,269, issued on February 26, 1985, discloses an
apparatus and process for immediate stabilization and subsequent promotion
of fusion in bone joints. The disclosed implant is a metal cylinder or
basket having fenestrations arranged about its cylindrical surface. Bone
fragments are utilized within the basket to promote bone growth through
the basket. While a cylindrical implant has limited usefulness in spinal
operations where fusion is desired, the placement of a cylindrical basket
is difficult because of the amount of surrounding bone that must be
removed in order to properly insert the cylindrical object between
adjacent vertebrae.
Recognition of the limitations relating to fusion procedures has led to
identification of a need for an immediate artificial disk or fusion
operation that can be done in the process of removing the disk to prevent
further disk space narrowing and degeneration. It is this narrowing or
loss of a shock absorber between adjacent vertebrae that leads to facet
joint hypertrophy, spur development, etc. Furthermore, the required
surgery should be of lower magnitude than present fusion surgery and
should result in more predictably acceptable results. For example, present
fusion operations require an additional one to two hours for carrying out
the surgery after removal of the disk, and also require additional
incisions to collect the required bone, unless a bone bank is utilized.
Six to 12 months' time is needed for the fusion to become solid and this
operation carries a 10 to 20 percent failure rate wherein pseudoarthrosis
(evidenced by an unplanned, false joint) is the end result.
The present development is designed to meet the needs of a surgeon
intending to produce either a multidirectional joint or a bony fusion, and
provides the benefit of an acceptable alternative surgical result where
the intended surgery fails. It assures immediate spacing of the vertebrae
to remove verve pressure and an implant that can promote either
arthrodesis or arthroplasty as indicated by the situation of a specific
patient. This is accomplished by spreading of the vertebrae and
implantation of a hollow spherical member having an interior cavity that
can be filled with bone fragments and having an exterior wall with
multiple fenestrations leading to the cavity. By selective choice of
surgical procedures, the surgeon can promote fusion through the implant or
tissue growth about it to form a defined joint. While one of these results
may be the intended goal of surgery; the other, if it occurs, can be an
acceptable alternative.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiment of the invention is illustrated in the
accompanying drawings, in which:
FIG. 1 is an enlarged perspective view of a spherical implant;
FIG. 2 is a cross sectional view through the spherical implant taken along
line 2--2 of FIG. 1;
FIG. 3 is a diagrammatic side view of a pair of adjacent spinal vertebrae;
FIG. 4 illustrates formation of bony recesses in the vertebrae;
FIG. 5 illustrates initial spread of the vertebrae during placement of the
spherical implant;
FIG. 6 illustrates continued movement of the spherical implant toward the
receiving bony recesses;
FIG. 7 illustrates placement of the spherical implant in the bony recesses;
FIG. 8 is a diagrammatic view illustrating the geometry of the spherical
implant and bony recesses;
FIG. 9 is a diagrammatic posterior view illustrating selection of locations
for placement of spherical implants to correct a scoliotic curve; and
FIG. 10 schematically illustrates placement of spherical implants to
correct the curve shown in FIG. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The following disclosure of the invention is submitted in compliance with
the constitutional purpose of the Patent Laws "to promote the progress of
science and useful arts" (Article 1, Section 8).
The present disclosure pertains to a spherical implant and to a method of
utilizing the spherical implant by placement between opposing vertebrae in
a spine to selectively promote arthrodesis (a fused connection) or
arthoplasty (a joint). In humans, the device is designed to be used mainly
in the lumbar spine. Such usage is normally dictated by the existence of a
ruptured disc or degenerative disc pathology. Usage of the disclosed
spherical implant is also applicable to the thoracic and cervical areas of
the spine.
The implant comprises a hollow sphere 10 having an outer wall 12
surrounding an interior cavity 13. A plurality of fenestrations 11 are
formed through the outer wall 12 of the sphere 10 in open communication
with its cavity 13.
The design of sphere 10 provides the surgeon with control over which end
result (arthrodesis or arthroplasty) is to be encouraged.
If an arthrodesis or fusion is more desirable, then a relatively larger
implant should be used when compared to the prepared bed, thus stretching
the annulus to an extreme for better immobilization. Under those
circumstances, as much bone as is available is inserted into the implant
as the increased amount of bone graft material is a stronger stimulus for
osteoblastic activity and therefore bone fusion. Post-operatively the
patient would be treated with a formidable support to the back to assist
in immobilizing it and therefore encouraging the fusion process. Time of
fusion would have to be determined by x-ray.
If an arthroplasty is more desirable, then a smaller implant relative to
the bed size is used, resulting in minor or only physiological stretch of
the annulus and little or no bone is placed in the implant.
Post-operatively a light brace is used for discomfort only and time of
fusion less critical as far as follow-up x-rays are concerned.
Statistically, some vertebrae will fuse and some will not, regardless of
the techniques carried out.
The hollow sphere 10 can be made of metal or another material, such as
ceramics, which is biocompatible with the adjacent body tissue and
sufficiently rigid to withstand the compressive forces between adjacent
spinal vertebrae. The exterior surface areas of sphere 10 that surround
the fenestrations 11 are smooth and otherwise uninterrupted. The diameter
of sphere will vary widely, typical sizes being in the range of 11, 13,
15, and 17 mm.
An enlarged window or opening 14 is formed through the wall 12 of sphere 10
in open communication with its cavity 13. A typical size diameter for
opening 14 is approximately 5 mm. Opening 14, which might be threated, is
used as an attachment means to releasably mount one end of a driver or
handle 15, that serves as a tool to facilitate placement of sphere 10.
While a straight handle 15 is illustrated in FIG. 1, other handle
configurations can be utilized as required for specific surgical needs.
For instance, a curved handle might be required in order to properly work
the sphere 10 into position through the bone structure adjacent to the
surgical site.
The opening 14 is also utilized as an enlarged aperture leading to the
interior cavity 13 for the introduction of bone chips or other materials
into cavity 13 within sphere 10. It might further be used in association
with a removable of threaded cover, but normally will remain in an open
state after surgical implantation.
The cavity 13 within sphere 10 is defined by a spherical inner surface 16
that is concentric with its outer surface 17. The difference in diameters
between the outer surface 17 and inner surface 16 is substantially less
than the diameter of the outer surface 17 (see FIG. 2).
The fenestrations 11 formed through sphere 10 are preferably circular
openings centered about axes intersecting the center of the sphere. The
diameter of each opening is substantially less than the diameter of the
sphere. A typical diameter size for fenestrations 11 is approximately 2-3
mm.
The sphere 10 is adapted to be located between bony recesses 18 formed in
opposed facing surfaces of a pair of adjacent spinal vertebrae 20
following removal of a ruptured, flattened or degenerated disc (not
shown). The exterior diameter of sphere 10 is slightly greater than the
diameter of the bony recesses 18 between which it is to be located (see
FIG. 8).
The present method for selectively promoting arthrodesis or arthroplasty
between vertebrae 20 can best be understood from a study of FIGS. 3-7.
First, the operating surgeon must partially or completely remove a
ruptured, flattened or degenerated disc (not shown) located between the
centrums of a pair of adjacent vertebrae having opposed facing surfaces
(FIG. 3). The illustrated separation between the opposed facing surface 22
in FIG. 3 is less than it should be due to disc degeneration which has
occurred.
Next, the surgeon must separate the vertebrae, which will typically be
accomplished by use of a lamina spreader (FIG. 4). The surgeon then
selects a desired center of motion between the opposed facing surfaces on
the pair of vertebrae 20. In FIGS. 4-7, the selected center of motion is
visually indicated at 21.
The next step involved in the present method requires drilling and/or
reaming the facing surfaces of the pair of vertebrae to form opposed body
recesses centered about the selected center of motion 21. The bony
recesses 18 are illustrated in a partial section shown in FIG. 4.
FIGS. 5, 6 and 7 illustrate sequential placement of sphere 10 in a position
coincident with the selected center of motion 21. This is accomplished by
progressively moving sphere 10 into a location between the bony recesses
18. Such placement will require spreading of the vertebrae 20 in order to
facilitate entrance of the sphere 10 between them.
While not shown in FIGS. 5-7, a driver or handle attached to sphere 10 will
normally be utilized to facilitate its placement. Handle 15 serves as a
driving tool to assist in controlling movement of sphere 10 prior to its
being seated between the bony recesses 18.
The implanted sphere 10 will be held in place between the bony recesses 18
by the surrounding fibers and muscle (not shown) that complete the spinal
structure. Due to the relative sizes of the spherical implant and bony
recesses, the sphere 10 separates the adjacent vertebrae 20 by a distance
greater than their relative positions prior to surgery. In all instances,
the diameter of the bony recesses 18 (shown as X in FIG. 8) will be less
than the exterior diameter of the outer surface 17 about sphere 10 (shown
as D in FIG. 8). The difference in diameter will result in spreading of
the vertebrae 20 from their original pre-operative positions. This causes
the elastic nature of the surrounding tissue and muscles to maintain the
sphere 10 in compression between the vertebrae 20.
When being used to promote arthrodesis, sphere 10 should be packed with
bone chips or fragments (not shown) to encourage bone growth through the
fenestrations 11. Additional bone fragments should be packed about the
exterior of the implanted sphere 10. The bone fragments are preferably
autogenic, since those produced during the forming of the bony recesses 18
will be most effective in promoting subsequent bone growth. The use of
autogenic bone fragments in a related surgical procedure is described in
U.S. Pat. No. 4,510,269, which is hereby incorporated into the present
invention disclosure by reference. The application of this technique to
the present spherical implant will be readily understood by those skilled
in this field.
It is preferably to place some bone fragments within sphere 10 even when an
arthroplasty is anticipated. While the presence of bone materials within
the implant is primarily to promote arthrodesis, it will result in a
process of healing in the vicinity of the spherical implant that creates a
different type of arthroplasty (pseudoarthrosis) than if no healing had
taken place in the total absence of bone within the implant. It has been
noted, in situations where attempted fusions have failed, that the
operational area sometimes fills in with scar tissue instead of bone or
will have less scar tissue than planned. This results in production of a
synovial lining having fluid in it, which more closely resembles a true
joint. Simply placing an empty metal implant into the body would normally
be expected to produce no stimulation for a healing process and no bodily
reaction leading to arthroplasty or arthrodesis. Such implantation would
therefore be less likely to be physically desirable and the end result is
less likely to resemble a true joint.
A novel aspect of this invention is the ability given to the surgeon to
selectively promote either arthrodesis or arthroplasty between adjacent
spinal vertebrae.
When arthrodesis (fusion) is the desired result, the sphere 10 and the
available open spaces about it should be packed with bone chips and/or
other materials that tend to promote bone growth into and through the
spher, such as hydroxyapatite. When arthroplasty (joint) is desired, the
interior of sphere 10 should contain less bone material and fibrous union
or healing will usually take place about the outer spherical surface areas
between the fenestrations 11 of the sphere 10.
The difference between the exterior diameter of the sphere 10 and the
diameter of the bony recesses 18 between which it is to be located should
be substantial when arthrodesis is the desired result, and should be
minimal when arthroplasty is desired. It is to be noted that the less
spread or difference in diameter of bony recess compared to the outside
diameter of the implant, the better the congruity and facilitation of
motion about the center of sphere 10. Conversely, discongruity will
encourage fusion. As an illustrative example only, when utilizing a sphere
that is 17 mm in diameter, a reamer diameter of 14 mm might be used to
promote arthrodesis, while a reamer diameter of 16 mm might be used when
arthroplasty is the desired result. In both instances the compressive
forces of the surrounding tissues and muscles will cause the bony recesses
to spread sufficiently to properly seat the spherical implant between them
in surface-to-surface contact.
The post-operative regimen of the patient will also affect the end result
desired in a particular situation. Where arthrodesis is desired, the
affected ares must be immobilized for a time sufficient for fusion of bone
to occur. This might require wearing of a brace for six months.
Conversely, arthroplasty is more likely when joint movement is encouraged
by minimizing support of the operative area and encouraging gradual
increase in normal activities.
It should be recognized that in any implant situation, instances of
"failure" will always occur. With the present implant, such "failure" will
result in an acceptable bodily condition. If bone mass grows through the
sphere 10 in situations where arthroplasty was the desired result, the
patient will have a fusion between the adjacent vertebrae which will
immobilize the vertebrae and not require subsequent treatment or surgery.
Similarly, if bone growth does not occur through the sphere 10 where
arthrodesis was the desired result, sphere 10 will be covered by fibrous
growth to produce a spherical socket and a definable spherical joint
between the adjacent vertebrae which will maintain their relative
positions during limited movement or the vertebrae.
Thus, the implant has the ability to turn "failure" of intended surgical
procedures to a "successful" result that does not require corrective
steps. The end result of such an implant is not truly random in nature,
because the implant and its method of use provides the surgeon with
parameters that can be selected to promote arthrodesis or arthroplasty as
the intended result of implantation, even though the alternative result
will be acceptable in most instances.
A suitable surgical technique for utilizing the spherical implant will be
detailed below. It is subject to substantial variation, depending upon the
many variables encountered in surgical situations. Accordingly, this
discussion should be viewed as merely exemplary.
When using a conventional posterior approach, the patient is placed prone
on an appropriate frame for carrying out low back surgery under general
anesthesia. A mid-line incision is made dorsally and the usual approach
for laminectomy and disc removl is carried out. A lamina spreader is
utilized and the center of disc motion is approached with a probe,
including use of an x-ray check on the table.
A bed is drilled and/or reamed and the bone chips harvested. The selection
of a spherical reamer will require one of appropriate size which would be
approximately equal to the size of the utilized implant for encouraging
arthoplasty and a slightly smaller reamer (when compared to the spherical
implant) when encouraging arthrodesis. The overall operation for
arthrodesis should be of larger magnitude and size. The forming of the
bony recesses 18 should be developed by use of progressively enlarged
reamers to approach the final size, with care being taken to assure that
the reaming takes place equally in each adjacent vertebra. The drills
and/or reamers can be rigid or flexible, as well as straight or curved,
depending upon specific surgical needs in each instance of use.
Bone chips and fragments produced during drilling and reaming should be
separated from the cartilage and disc material and the bone only returned
to the implant. Bone may also be utilized from the lamina or spinous
process if needed, as well as from external sources, such a bone bank.
A special driver or handle 15 is screwed into the receiving threaded window
or opening 14, which might be approximately 5 mm. in diameter, and is
tapped into place. This larger single fenestration is also utilized for
filling the implant with bone. After an appropriate check for position and
hemostasis the driver or handle 15 is unscrewed to remove it and the wound
should be irrigated and closed in the usual fashion, taking local and
general measured for antisepsis.
An alternative surgical route is anterior implantation. This will require
abdominal access (intra or extra peritoneal) for the lumbar region,
transthoracic access for the thoracic vertebrae, and anterior lateral
access for the cervical levels. The anterior lumbar approach may be used
when the posterior approach is contra-indicated, and would be the usual
approach in thoracic or cervical applications. Anterior implantation makes
possible the use of larger spherical implants, but the above-described
implantation procedures will otherwise remain unchanged. One advantage of
the anterior approach in the lumbar area is that proper centering of the
implant might be easier, but in most instances the posterolateral
procedure will be more practical.
FIGS. 9 and 10 schematically illustrate usage of the implant to correct a
scoliotic curve between adjacent vertebrae 30. The scoliotic curve is
illustrated by dashed line 31. Correction can be accomplished by selecting
locations for the center of motion 32 between the adjacent vertebrae 30
which are laterally offset from the curve 31. Illustrative selected
centers of motion 32 are shown for bony recesses 33 formed in the opposing
surfaces of vertebrae 30 as previously described. FIG. 10 illustrates the
vertebrae 30 after placement of implanted spheres 10 between them, the
center of each sphere being coincident with the previously selected
centers of motion 32. The eccentric placement of the spheres 10 has
laterally straightened the vertebrae 30 along the center line shown at 34.
In compliance with the statute, the invention has been described in
language more or less specific as to structural features. It is to be
understood, however, that the invention is not limited to the specific
features shown, since the means and construction herein disclosed comprise
a preferred form of putting the invention into effect. The invention is,
therefore, claimed in any of its forms or modifications within the proper
scope of the appended claims appropriately interpreted in accordance with
the doctrine of equivalents.
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