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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a device for fixing adjacent vertebrae to each
other using a rod and unique hollow screws.
2. Description of the Related Art
Fixation (or fusion) of vertebral columns with bone material or rods and
plates is a common, long practiced surgical method for treating a variety
of conditions. Many of the existing procedures involve components that
protrude outwardly that may contact and damage a body part, such as the
aorta, the vena cava, the sympathetic nerves, the intestine and the
ureter. Also, many constructions involve components that may loosen and
cause undesirable problems. A Dunn device was on the market until pulled
by the U.S. Food & Drug Administration because of problems with delayed
rupture of the aorta secondary to the device being so bulky as to contact
the aorta, erode its surface and lead to fatal hemorrhage in several
cases.
U.S. Pat. No. 5,152,303 issued to Allen on Oct. 6, 1992 relates to an
anterolateral spinal fixation system including a cannulated screw threaded
into a vertebra and a rod attached to the screw. The process involves
threading the cannulated screw into a pilot hole drilled into the
vertebral body portion and fastening a rod at its lower and upper ends to
the vertebral body by the cannulated screws. (Col. 3, lines 62-64; Col. 4,
lines 5-8).
U.S. Pat. No. 4,059,115 issued to Jumashev et al. on Nov. 22, 1977 relates
to a surgical instrument for operation of anterior fenestrated
spondylodesis in vertebral osteochondrosis. The instrument includes a
hollow cylindrical cutter with a cutting edge, and a handle. By rotation
of the handle accompanied with slight pressure the cutter is worked into
the bodies of the adjacent vertebras (abstract, Col. 6, lines 56-58).
U.S. Pat. No. 5,015,247 issued to Michelson on May 14, 1991 relates to a
method of performing internal stabilization of a spine. The method
involves seating a drill sleeve into the two vertebrae and drilling the
vertebrae with the drill installed through the drill sleeve. Bagby U.S.
Pat. No. 4,501,269 is mentioned. (Col. 6, lines 27-30, Col. 7, line 68,
Col. 9, lines 22-25, 39).
Current devices have substantial deficiencies when osteoporotic bone is
encountered. The soft, decalcified bone in such patients has poor pull-out
strength for screws. Bone screws are known to have very little holding
power in osteoporotic bone and loosen readily, severely limiting the
holding power and fixation ability of current devices.
Some devices have designs that include hollow screws or screws with
transversely drilled holes, presumably to improve holding power and allow
bone to grow therethrough. These devices are all relatively small screws
which are not capable of large surface area fixation.
The art described in this section is not intended to constitute an
admission that any patent, publication or other information referred to
herein is "prior art" with respect to this invention, unless specifically
designated as such. In addition, this section should not be construed to
mean that a search has been made or that no other pertinent information as
defined in 37 C.F.R. .sctn.1.56(a) exists.
SUMMARY OF THE INVENTION
The invention provides a method and device for fixing two or more
vertebrae. The process is elegantly simple and solves many of the problems
attendant with previous devices.
Each vertebra to be joined is prepared by forming a partial annular cut, as
by a hole saw, preferably leaving the core plug of bone in place. A hollow
screw is threaded into the annular ring recess thus formed. A channel is
cut in the vertebral bone between each of the screws to accommodate a rod
that is placed over each screw. A locking cap over each screw secures the
rod to the screws and thereby fixes the spine as desired.
The method and device provide many advantages. The hollow screws are
exceptionally strong, having greater holding surface area than
conventional solid screws. The rod is held in the screw between two widely
spaced slots. The rod is also held firmly by a third point by a dimple on
the locking cap. The rod is secured to the screws by at least three points
of fixation over a much greater distance than traditional systems. This
provides a linkage which is significantly greater in terms of mechanical
stability over the prior art.
Holes in the side walls of the hollow screws allow for bone ingrowth to
further strengthen the connection. Since the bone plug is not removed, the
screw's wall is very thin, bone can grow through the screw rapidly, thus
securely fusing the screw to the vertebra and provides a better anchor to
the vertebral bone. Additionally, as the bone grows through the holes in
the screw, the bond becomes stronger with time. Prior art devices use
screws that may slowly become less secure with age and the inevitable
micromotion that occurs between the screw and the vertebral bone.
By varying the cross-section geometric structure and diameter of the rod,
various degrees of stiffness may be imparted. Also, by varying the
geometric cross-section structure of the rod, stiffness may be imparted
selectively in the appropriate plane of motion. For instance, if increased
flexion-extension stability is desired, the rod can be oriented in the
flexion-extension plane and elongated such that it will provide greater
stiffness in flexion-extension than in lateral bending. Such a feature
will allow the surgeon to define the plane of stiffness necessary to match
the pathology encountered.
The rods within the cut channels avoid the cantilever effect of prior art
devices where the load is carried far from the center of the spine. Thus,
the rod acts more like an intramedullary rod in the vertebrae. This is far
preferable in that a rod nearer the center of the axis of rotation does
not have the cantilever effect of prior art systems. This also presents no
protrusions that may abut against vital body components.
The process is very simple, requiring only the drilling of a single hole
saw cut in each vertebra, formation of channels therebetween and
installation of the hollow screws, placement of the rod and securement
with the locking caps.
BRIEF DESCRIPTION OF THE DRAWINGS
A detailed description of the invention is hereafter described with
specific reference being made to the drawings in which:
FIG. 1 is a perspective view of the device of the invention securing
vertebrae together;
FIG. 2 is an exploded view of the rod, screw, cap and prepared vertebrae;
FIG. 3 is a cross-sectional view taken through line 3--3 of FIG. 1;
FIG. 4 is a cross-sectional view taken through line 4--4 of FIG. 3;
FIG. 5a is a cross-sectional view taken through line 5--5 of FIG. 4 showing
the rod in cross-section;
FIG. 5b is a cross-sectional view taken through line 5--5 of FIG. 4 showing
an alternate rod in cross-section;
FIG. 5c is a cross-sectional view taken through line 5--5 of FIG. 4 showing
an alternate rod in cross-section;
FIG. 5d is a cross-sectional view taken through line 5--5 of FIG. 4 showing
an alternate rod in cross-section;
FIG. 5e is a cross-sectional view taken through line 5--5 of FIG. 4 showing
an alternate rod in cross-section;
FIG. 6 is a cross-sectional view similar to FIG. 3 with an overcap design;
FIG. 7 is an end view of a corpectomy block partially cut away;
FIG. 8 is a tool for threading in the device into the annular ring recess
in the vertebral body;
FIG. 9 is a perspective view of the corpectomy block of FIG. 7;
FIG. 10 is a perspective view of a wedge placable between adjacent
vertebrae;
FIG. 11 is a cross-sectional view of the wedge of FIG. 10 through line
11--11;
FIG. 12 is a cross-sectional view of the wedge of FIG. 10 through line
12--12;
FIG. 13 is an alternative wedge in which a top is not planar;
FIG. 14 shows a section of a spinal column in need of realignment;
FIG. 15 shows the spinal column of FIG. 14 with a correcting wedge in
place;
FIG. 16 shows a spinal column in need of alignment, such as in scoliosis;
and
FIG. 17 shows the spinal column of FIG. 16 in cross-section, realigned with
a correcting wedge.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With specific reference to FIGS. 1 and 2 it will be seen that an anterior
spinal fixation system 10 may join adjacent vertebrae together. The system
includes an elongated rod 12 and at least two hollow cylindrical screws
14. Each screw 14 includes external bone engaging threads 16, internal cap
engaging threads 28, an upper rim 18 and a lower rim 20. The internal
threads 28 need only be as deep as the locking cap. A smooth inner wall is
preferred to avoid placing torque on the remaining bone core during
insertion of the screw. As shown in FIG. 2, upper rim 18 is broken by two
opposing rod fixation slots 22, 24 that are sized such that rod 12 may
pass into the slots 22, 24 as shown. Preferably, the screws 14 include a
plurality of bone ingrowth openings 36 through the side walls which allow
bone to grow therethrough.
The rod 12 is held to the screws by a locking cap 26. As shown, locking cap
26 may be disc-shaped, having threads 30 about the circular periphery. Top
32 of the cap 26 may have a pair of spaced holes 34 to which a tool (not
shown) may connect to insert said cap 26 into a hollow, threaded screw 14.
Cap 26 may be threaded into said screw 14 such that no part of said cap
projects beyond the screw 14. The cap may be porous and may have holes to
allow bone ingrowth and increase the blood supply to the interior.
Alternatively, as shown in FIG. 6, the cap may be designed as an overcap 72
which engages with threads 74 on the outer surface of the bone screw 14.
As shown, overcap 72 includes a cap projection 42 which abuts against rod
12. In the case of an overcap, some bone may be removed to accommodate the
overcap as shown.
Rod 12 is preferably made of a biocompatible, malleable metal such as
titanium. A rod of titanium has an advantage of having a modulus of
elasticity similar to natural bone. In any case, the rod is bent by the
surgeon to attain the correct configuration desired for the patient. As
shown in the Figures, the rod 12 may have a plurality of spaced dimples 40
which may be round or elongated. The dimples 40 interface with a mating
projection 42. In the case of a round dimple 40, the projection 42
engagement serves to prevent slippage of the rod relative to the screw 14.
An elongated dimple 40 allows limited slippage which is sometimes
desirable.
FIGS. 5a-e show that the cross-section of rod 12 may be nearly any shape
other than round. Although a round cross-section rod would work, any
non-round rod provides better torsion control. The size of the rod may be
selected depending on the individual patient's size. As stated previously,
the cross-sectional shape of the rod may be altered to provide stability
in the proper axis of motion for a particular patient.
Installation of the device is straight forward. The surgeon exposes the
vertebra 46 anteriorly and drills a cylindrical opening 50 in the
vertebral bone as shown in FIG. 2. Preferably, a hole saw is used to form
the opening 50, since a hole saw will leave the core 52 in place. For ease
of illustration, FIG. 2 does not show a bone core 52, although the bone
core 52 is shown in FIGS. 3 and 4. If a bone core 52 is not left in place,
the opening may be packed with bone or bone substitute. It is noted that
the bone opening 50 may be threaded if the drill used is self-tapping or
it may be tapped by an additional tool used after drilling.
Preferably, the implant screws 14 are slightly larger in external diameter
than the external diameter of the hole saw cut, thus providing a high
friction secure linkage to the vertebral body. This also provides a screw
with an internal diameter slightly larger than the outer diameter of the
bone core, thus reducing the possibility of torquing the core during
placement of the screw. Torquing the core in the process of insertion may
destroy the blood supply to the core at its posterior surface. This would
be undesirable since it may lead to delayed incorporation of the bone
locking ability through the holes in the screw.
It may also be possible to use a bone screw 14 of the device to cut its own
opening 50. In such a case, the screw 14 is then left in place after fully
inserted.
Each of the vertebra adjacent another bone opening 50 is then cut, as by a
chisel or router tool, to form a channel 54 of a depth to hold at least
half the diameter of the rod 12. Although the vertebrae may be connected
without the channel using the device and methods of this invention, many
of the advantages are lost if at least most of the rod is not in a channel
54.
The bone screws 14 are then screwed into the bone openings 50 with threads
28, 16 on the interior and exterior of the bone screws 14 engaging
vertebral bone. The screws are positioned such that the rod fixation slots
22, 24 line up with the bone channels 54. A tool 58 as shown in FIG. 8 may
be used to screw the bone screws 14 into bone openings 50. As shown, tool
58 includes a shaft 60 with a handle 62 on one end a screw engaging head
64 on the other end. The screw engaging head 64 includes a pair of tabs
66, 68 that engage with slots 22, 24. The head 64 closely fits into the
interior of the screw 14. No part of tool 58 projects beyond the outer
circumference of the bone screws 14.
A rod 12 of the required length is then bent to the required shape and
inserted into each screw via the rod fixation slots 22, 24 and into the
formed bone channels 54. The rod 12 may be removed to adjust the curvature
of the spine that will be defined by the installed system as needed. Once
the degree of correction has been achieved, the rod is captively held in
place by securing a locking cap 26 over each screw 14 thereby trapping the
rod 12 in place.
The bone screws 14 are preferably placed into the vertebral bone quite
deep, leaving a safety zone of about 3 mm. Depending on the size of the
vertebrae, the screw diameters may range from 1.5 to 3.5 cm. Preferably,
the diameter of the screws 14 is sufficient to cut into the harder, outer
bone of the vertebra. The screws 14 may have a relatively thin cylindrical
wall and still provide great strength and holding power.
The installed system of the invention provides a spinal correction with
many important advantages. Since the cylindrical screws have far greater
surface area than a conventional solid screw, the holding power is much
higher. The installed system is entirely contained within the confines of
the vertebral bone. Nothing projects outwardly that may contact adjacent
body structures. The rod 12 is much closer to the center of the vertebra
meaning that undesirable cantilever effects as in the prior art devices is
greatly reduced.
The system of the invention may be used to stabilize many or only two
vertebrae. It may be used to provide corrections due to rumor, fracture,
degenerative disease, deformity or infection. The non-round rod used in
most cases provides longitudinal rotational control. The normal healing
process of the body will cause bone growth around the screws, rods and
caps to lock the system even more securely to the vertebra. The screws 14
may include perforations throughout the length of the cylinder to allow
bone ingrowth which may increase holding power.
FIGS. 1, 7 and 9 show that the invention may be used between two or more
adjacent vertebrae and may be used in conjunction with a corpectomy block
80 which functions as a spacer for a removed vertebral body. A corpectomy
block 80 is used when a large portion of the vertebral body has been
removed, such as to remove tumor, fractured bone or in cases of massive
bone loss. Most of the vertebra is removed anteriorly forming a gap
between the remaining vertebrae. The usual prior art solution is to
provide a number of fill plates with rods or a large ceramic block
anchored with plates and conventional screws. U.S. Pat. No. 5,192,327
shows a suitable corpectomy block which merely needs to be designed with a
slot through which the rod 12 may pass.
In FIG. 7 and 9 are corpectomy block 80 is shown in which the block is
substantially hollow and is formed from a body compatible material such as
titanium or ceramic. The block may be porous or at least roughened at the
ends to allow bone ingrowth. A fill port 82 may be built into the block 80
to allow addition of bone graft. The block 80 includes a lengthwise slot
84 through which rod 12 may pass. In use, a block 80 of the appropriate
size is fitted between the remaining vertebrae after re-section and is
filled with bone graft. The rod 12 is placed through the slot 84 and is
tightened at each bone screw 14 by end caps 26 or 72. This firmly holds
the corpectomy block 80 in place to allow bone fusion to the adjacent
vertebrae as shown in FIG. 1.
The block should allow for vascular ingrowth by having at least porous end
plates 86, 88. The block 80 may be porous titanium or a ceramic with
roughened end plates.
FIG. 1 shows that the invention will function even if one or more vertebra
are resected and replaced with a corpectomy block. In all forms of the
invention, the hollow screws 14 provide greater holding power and allow
for an intramedullary rod that eliminates the cantilevered structures
found in prior art pedicle screw systems such as in U.S. Pat. No.
5,324,290 that issued Jun. 28, 1994. The present invention directs the
forces from nearer the center of the vertebrae and therefor the axis of
forces and motion.
In some cases, the spinal column is in need of realigning, front to back,
side to side, or both. FIGS. 10 through 17 show means of correcting
alignment while using the spinal fixation device 10 of the invention.
In FIGS. 10-13 wedges are shown which may be inserted between the vertebrae
in place of a removed disc. The wedge 90 of FIGS. 10-12 may be a solid
block of ceramic, may be a titanium wedge or any other body implantable
material that could replace a disc. The anterior side 92 of wedge 90 is
higher than the posterior edge 94. A slot 96 is formed to allow the rod 12
to pass thereby.
The wedge 100 of FIG. 12 includes a similarly formed body, includes a slot
96 but no directional wedge. Rather, a surface 102 of wedge 100 is rounded
or otherwise non-planar. In this manner, wedge 100 with surface 102
against a vertebra may allow rotational or angular correction of
deformity. Wedges 100 may be inserted to replace the disc, forming a
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