Threaded polyaxial locking screw plate assembly

The present invention is a polyaxial locking screw plate assembly for immobilization of vertebral bones, via fixation to surfaces thereof. The invention includes a plate, having an upper portion and a lower portion, each of which has a pair of holes having a threaded upper portion and a tapered lower portion. Coupling elements, including slideably joined socket and cap portions, are mounted about the semi-spherical heads of bone screws, which are screwed through the holes in the plate and into the bone. The heads of the screws are polyaxially mounted in the socket portions and as such may be inserted into the bone at a variety of angles. The socket portions of the coupling elements have slots in them which permit crush locking of the heads of the screws once the sockets seat and are forceably driven into the tapered portions of the corresponding holes. The cap portions are threaded so they may be advanced into the upper portions of the corresponding holes, thereby further locking the coupling elements into the holes, and applying an additional driving force against the corresponding socket portions to crush lock the screw heads at the selected angle relative to the plate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to a spinal implant assembly for holding 
adjacent vertebral bones fixed. More particularly, this invention relates 
to a novel assembly of bone screws and plates for use in surgical 
procedures for stabilizing the relative motion of, or permanently 
immobilizing, vertebral bodies, wherein the screws form a polyaxial 
coupling of the plate to the bone, and which maintains a flush exterior 
plate surface through a wide range of entrance angulation. 
2. Description of the Prior Art 
The bones and connective tissue of an adult human spinal column consists of 
more than 20 discrete bones coupled sequentially to one another by a 
tri-joint complex which consist of an anterior disc and the two posterior 
facet joints, the anterior discs of adjacent bones being cushioned by 
cartilage spacers referred to as intervertebral discs. These more than 20 
bones are anatomically categorized as being members of one of four 
classifications: cervical, thoracic, lumbar, or sacral. The cervical 
portion of the spine, which comprises the top of the spine, up to the base 
of the skull, includes the first 7 vertebrae. The intermediate 12 bones 
are the thoracic vertebrae, and connect to the lower spine comprising the 
5 lumbar vertebrae. The base of the spine is the sacral bones (including 
the coccyx). The component bones of the cervical spine are generally 
smaller than those of the thoracic spine, which are in turn smaller than 
those of the lumbar region. The sacral region connects laterally to the 
pelvis. While the sacral region is an integral part of the spine, for the 
purposes of fusion surgeries and for this disclosure, the word spine shall 
refer only to the cervical, thoracic, and lumbar regions. 
Referring now to FIGS. 1 and 2, a typical vertebral body is shown in a top 
view and a side view. The spinal cord is housed in the central canal 10, 
protected from the posterior side by a shell of bone called the lamina 12. 
The lamina 12 has three large protrusions, two of these extend laterally 
from the shell and are referred to as the transverse process 14. The third 
extends back and down from the lamina and is called the spinous process 
16. The anterior portion of the spine comprises a set of generally 
cylindrically shaped bones which are stacked one on top of the other. 
These portions of the vertebrae are referred to as the vertebral bodies 
20, and are each separated from the other by the intervertebral discs 22. 
Pedicles 24 are bone bridges which couple the anterior vertebral body 20 
to the corresponding lamina 12 and posterior elements 14, 16. 
The spinal column of bones is highly complex in that it includes over 
twenty bones coupled to one another, housing and protecting critical 
elements of the nervous system having innumerable peripheral nerves and 
circulatory bodies in close proximity. In spite of these complications, 
the spine is a highly flexible structure, capable of a high degree of 
curvature and twist in nearly every direction. 
Genetic or developmental irregularities, trauma, chronic stress, tumors, 
and disease are a few of the causes which can result in spinal pathologies 
for which permanent immobilization of multiple vertebrae may be necessary. 
A variety of systems have been disclosed in the art which achieve this 
immobilization by implanting artificial assemblies in or on the spinal 
column. These assemblies may be classified as anterior, posterior, or 
lateral implants. As the classification suggests, posterior implants are 
attached to the back of the spinal column, generally hooking under the 
lamina and entering into the central canal, attaching to the transverse 
process, or coupling through the pedicle bone. Lateral and anterior 
assemblies are coupled to the vertebral bodies. 
The region of the back which needs to be immobilized, as well as the 
individual variations in anatomy, determine the appropriate surgical 
protocol and implantation assembly. The use of screw plate assemblies for 
stabilization and immobilization via lateral or anterior entrance is, 
however, common. 
Because the spine is routinely subject to high loads which cycle during 
movement, one of the primary concerns of physicians performing spinal 
implantation surgeries, as well as of the patients in whom the implants 
are placed, is the risk of screw pull-out. Screw pull-out occurs when the 
cylindrical portion of the bone which surrounds the inserted screw fails. 
A bone screw which is implanted perpendicular to the plate is particularly 
weak because the region of the bone which must fail for pull-out to occur 
is only as large as the outer diameter of the screw threads. It has been 
found that for pull-out to occur for a pair of screws which are angled 
inward, "toe nailed", or ones which diverge within the bone, the amount of 
bone which must fail increases substantially as compared to pairs of 
screws which are implanted in parallel along the axis that the loading 
force is applied. It has, therefore, been an object of those in the art to 
provide a screw plate assembly which permits the screws to be entered into 
the vertebral body at angles other than 90 degrees. 
A great concern, however, with screws being implanted in the anterior 
portion spine, most particularly in the cervical spine, is that their are 
important internal tissue structures which, because of their proximity to 
the implant, may be damaged by a dislocated screw. In the cervical spine, 
the esophagus is located directly in front of the anterior surface of the 
vertebral body, and therefore, in potential contact with an implanted 
cervical plate. Breaches of the esophageal wall permit bacterial 
contamination of the surrounding tissues, including the critical nerves in 
and around the spinal cord. Such contamination can be fatal. Because screw 
pull-out represents one of the largest risks of esophageal perforation, it 
has been an object of those in the art to produce a cervical screw plate 
design having a locking means which couples, not only the plate to the 
bone, but locks the screw to the plate. In such a design, it is intended 
that, even if the bone holding the screw fails, the screw will not 
separate from the plate. 
In addition to pull-out, however, it has been observed that if the screw 
plate design includes screw heads which protrude beyond the exterior 
surface of the plate, long term wearing of surrounding tissues may occur, 
leading to the development of abscesses and holes, which, once again, can 
have grave consequences. With respect to cervical plates, which are 
necessarily thin, on the order of a few millimeters, unless the system is 
designed to specifically accommodate non-perpendicular screw-in 
directions, the heads of the screws which are desirably toe-nailed in are 
a considerable risk. 
Similar concerns exist in the thoracic and lumbar regions with respect to 
anterior and lateral fixation implants as their are proximally located 
organs as well as a plurality of major blood vessels which may be 
compromised by either catastrophic screw pull-out and/or long term wearing 
of non-flush surface protrusions. 
One screw plate design which has been offered to provide physicians and 
patients with a reduced risk of pull-out or damage to proximal tissues is 
the Orion (Reg. Trademark) Anterior Cervical Plate System of Sofamor Danek 
USA, 1800 Pyramid Place, Memphis, Tenn. 38132. The Orion.sub.TM system 
teaches a plate having two pair of guide holes through which the screws 
are inserted to fix the plate to the vertebral body. The plate further 
includes external annular recessions about each of the guide holes which 
are radially non-symmetric in depth. More particularly, the annular 
recessions serve as specific angle guides for the screws so that they may 
be inserted non-perpendicularly with respect to the overall curvature of 
the plate. In addition, the Orion.sub.TM plate includes an additional 
threaded hole disposed between each of the pairs of guide holes so that a 
corresponding set screw may be inserted to lock the bone screws to the 
plate. 
Although the Orion.sub.TM system achieved certain advantages over prior 
cervical screw plate assemblies, it is not without failures. Specifically, 
a given plate can accommodate only one screw-in angulation per hole, 
preferably in accordance with the angle of the annular recession. This is 
undesirable, in that physicians often must inspect the vertebral bodies 
during the implantation procedure before making the decision as to which 
screw-in angle is the ideal. By forcing the physician to chose from a 
limited set of angles, it is unavoidable that physicians will be forced to 
implant plates having screws which were positioned non-ideally. While 
providing a variety of plates having different angle guide holes and 
annular recession orientations is possible, the complexity and expense of 
providing a full spectrum of plates available in the operating room for 
the surgeon to choose from is undesirable. It is a failure of the system 
that one plate cannot accommodate a variety of different screw-in angles. 
It is further a failure of the Orion.sub.TM system that an extra set screw 
is required to lock the screw to the plate. Plates for use in the cervical 
spine are very thin, and if the screw head already rests in an annular 
recess, and there is to be enough room for the head of the set screw to 
rest on top of the head of the bone screw, the thickness of the remaining 
plate must be reduced even further. The thinner the plate is at the load 
bearing points--the guide holes--the weaker the plate is overall. 
While the preceding discussion has focused on a specific cervical screw 
plate system and its failures, the same failures apply to the art of 
vertebral immobilizing screw plate systems which are presently available 
as well. There are no presently available screw plate assemblies which 
present a flush surface and provide for means of preventing both screw 
pull-out from the bone and screw backout from the plate, while 
simultaneously providing for a wide range of angulation for the bone 
screws. 
An additional concern for physicians who implant screw plate assemblies for 
spinal fixation is proper alignment for pre-drilling of the holes into 
which the bone screws are driven to hold the plate. As suggested above 
with respect to the angulation of the annular recesses of the Orion.sub.TM 
system, the process of forming the holes generally involves placing the 
plate against the appropriate vertebral bodies and using a guide to hold 
the proper angle with respect to the plate and bone as a drill is used. 
The difficulty in this process involves slippage at the interface between 
the unsecured plate and the bone. To avoid slippage, the surgeon is 
generally required to use, simultaneously, a plate holding mechanism, 
which may be removable affixed to the plate, to maintain the plate in its 
proper position, a drill guide to set the desired angulation (which is set 
by the thread angle of the plate), and the drill itself. It is understood 
that simultaneous manipulation of these three tools by the surgeon is 
tedious and difficult. 
It is therefore, an object of the present invention to provide a new and 
novel cervical, thoracic, and/or lumbar screw plate design having a 
polyaxial coupling of the screw to the plate, whereby a single plate is 
compatible with a wide range of screw-in angles. 
It is also an object of the present invention to provide a screw plate 
design having a flush exterior while being fixed to the vertebral bodies 
which it immobilizes; having no screw head protrusion despite 
non-perpendicular angulation. 
It is also an object of the present invention to provide a spinal insert 
assembly which is more sturdy and more versatile than previous designs. 
Further, it is an object of the present invention to provide a screw plate 
design which provides the surgeon with the greatest freedom to choose the 
most desirable angle to direct the bone screw. 
It is also an object of the present invention to provide an orthopedic 
screw plate assembly which has a simple and effective locking mechanism 
for locking the bone screw to the plate. 
It is also an object of the present invention to provide a screw plate 
assembly which has a simple and effective means of holding the plate in 
position for the pre-drilling of screw holes. 
Other objects of the present invention not explicitly stated will be set 
forth and will be more clearly understood in conjunction with the 
descriptions of the preferred embodiments disclosed hereafter. 
SUMMARY OF THE INVENTION 
The preceding objects of the invention are achieved by the present 
invention which is a flush locking polyaxial screw plate assembly for use 
in stabilizing and immobilizing vertebral bodies. The assembly comprises a 
plate having a set of holes which are threaded at one end and tapered at 
the other, bone screws having a semi-spherical top portion, and a two-part 
coupling element. The top piece of the coupling element has an external 
threading for locking with the threaded end of the holes in the plate, and 
also has an interlocking mechanism for joining with the lower piece of the 
coupling element. The lower piece of the coupling element has a 
semi-spherical interior volume, which is expandable and contractable, for 
receiving the semi-spherical head of the screw therein. The bottom portion 
of the lower piece of the coupling element further includes an exterior 
taper which mates with the tapered end of the corresponding hole in the 
plate, so that there is an inwardly directed force applied to the lower 
piece, thereby causing the interior volume of the coupling element to 
contract and lock to the head of the screw. 
The present invention has a preferred embodiment which is summarized 
hereinbelow. The plate is a flat metal element, having a generally planar 
shape with rounded corners, contoured to the curved cylindrical surface of 
the vertebral bodies to which it is to be secured. There are four holes 
disposed in pairs at distal ends of the plate. The holes extend through 
the plane of the plate and are positioned so that they are aligned in 
pairs with the vertebral bodies to which the plate is to be attached. The 
top portions of the holes have a constant diameter and are threaded. The 
lower portions of the holes are tapered inwardly and are smooth. 
The shaft portion of the bone screws may be of a variety of standard 
designs, or a particular design which may be found more secure than 
standard ones. The head, however, is not standard in that it comprises a 
semi-spherical section. For the purposes of inserting the screw into the 
bone, the head comprises a recessed region, such as a slot, cross 
(phillips), star, or hexagon, which is ideally suited for mating to an 
appropriate screwdriving tool. The recess, however, shall not alter the 
exterior radially semi-spherical shape of the head. 
The two part coupling element comprises and socket portion and a cap 
portion. The socket portion is designed with an interior semi-spherical 
volume, so that it may receive the semi-spherical head of a corresponding 
bone screw. The interior volume of the socket portion is open at both 
axial ends thereof. The exterior surface of the socket portion, at the 
bottom thereof, includes a first set of slots which extend upwardly from 
the opening so that the interior semi-spherical volume may be expanded or 
contracted by the application of a radial force. In addition, the exterior 
surface at the bottom is tapered so that it is narrower at the bottom than 
at a midpoint. 
The upper exterior surface of the socket portion comprises a second set of 
slots, directed axially along the element to the midpoint, such that the 
upper opening of the socket element may expand and contract in accordance 
with the application of a radial force thereon. The exterior surface of 
this upper section of the socket portion is not tapered and is narrower 
than the widest taper position of the bottom of the socket portion. The 
upper section, however, does further include an outwardly extending 
annular lip at the uppermost axial position. This upper section is 
designed to be inserted into, and joined with, the cap portion of the 
coupling element. 
The cap portion has a generally cylindrical shape, having an open bottom. 
The open bottom is inwardly tapered, forming an inwardly extending annular 
lip, so that as the upper end of the socket portion is inserted, its upper 
slots are narrowed. Once axially inserted beyond this taper, the upper 
section of the socket portion expands outward over the inwardly extending 
annular lip. The inwardly extending annular lip engages the outwardly 
extending lip of the socket portion so as to prevent disengagement of the 
two pieces. The socket portion is then permitted to slide into the cap 
portion, until the larger diameter of the tapered lower portion of the 
socket contacts the entrance of the cap portion. 
The exterior surface of the cap portion is threaded, so that it may engage 
the threading of the upper portion of the corresponding hole. In addition, 
the top of the cap includes an opening so that a screw driving tool may 
directly engage the top of the screw. 
The first step in the process of implanting this embodiment of the 
invention is to assemble the parts described above. (It is intended that 
this assembly occur at the manufacturing site, and not be the 
responsibility of the surgical staff.). The semi-spherical head of the 
screw is inserted into the interior volume of the socket portion, and held 
in place by the interference fit of the maximum diameter of the head with 
the unexpanded openings at the top and bottom of the element. The head is 
inserted from the bottom, and more specifically by applying a pressure 
which causes the bottom opening to expand to receive the head into the 
volume. 
Once the screw head is inserted, the upper end of the socket portion is 
inserted into the opening of the cap portion. This insertion causes the 
top of the socket portion to contract inward slightly until the annular 
lips of each portion engage one another. The socket portion and the cap 
portion are thereby joined loosely so that each may slide and rotate 
relative to one another. 
The surgeon then positions the plate against the vertebral bodies and 
aligns the entry points for the screws. The next step is to pre-drill the 
holes into the bones at the desired angle, into which the screws will be 
inserted. With the plate in place, the screws (with the corresponding 
socket and cap portions in place on the head of each screw) are inserted 
through the holes of the plate, and into the vertebral bodies. The 
coupling element provides access to the screw head for driving it into the 
bone. At all times during this insertion, the head of the screw is loosely 
retained within the coupling element, so that the coupling element may 
polyaxially angulate relative to the screw, within a range of angles 
defined by the diameter of the neck of the screw and the bottom opening of 
the socket portion. 
As the screw is inserted into the bone, at the desired angle, the socket 
portion of the coupling element angulates so that the tapered bottom 
portion thereof seats into the tapered bottom of the hole in the plate. 
Continued driving of the screw into the bone, and therefore the socket 
portion of the coupling element deeper into the tapered hole, causes the 
first set of slots in the bottom end of the socket portion to narrow, thus 
causing the head of the screw to be crush locked to the coupling element. 
The cap portion of the coupling element is then threadably inserted into 
the hole, locking the coupling element in the hole, and further driving 
the socket portion into the hole. 
In a preferred variation of this embodiment, the interior surface of the 
cap portion includes a slight narrowing taper so that as the cap is 
threaded downward into the hole in the plate, the upper slots of the 
socket portion are also narrowed, further increasing the crush locking 
effect on the head of the screw.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the present invention will be described more fully hereinafter with 
reference to the accompanying drawings, in which particular embodiments 
are shown, it is to be understood at the outset that persons skilled in 
the art may modify the invention herein described while achieving the 
functions and results of this invention. Accordingly, the descriptions 
which follow are to be understood as illustrative and exemplary of 
specific structures, aspects and features within the broad scope of the 
present invention and not as limiting of such broad scope. Like numbers 
refer to similar features of like elements throughout. 
Referring now to FIG. 3s a plate which is an element of the present 
invention is shown in a perspective view. The plate 100 may be constructed 
of any suitably biocompatible material which has the structural strength 
and durability to withstand the cyclical loading associated with long term 
fixation to the spine. Materials which would be suitable for such 
applications include titanium alloys and steels. A specific titanium 
material which has been utilized in implants of the prior art include ASTM 
F-136 titanium alloy (Ti 6AL-4V). This material has enhanced mechanical 
properties including fatigue endurance and tensile strength, as compared 
with pure titanium. 
The plate 100 comprises upper and lower portions 102, 104 respectively, 
which are connected by a narrower region 106, therein being generally 
I-shaped. It shall be understood that a variety of conformations may be 
utilized in the alternative, the I-shape being the most illustrative for 
the purpose of description. The plate 100 also has a top surface 108 and a 
bottom surface (not shown). A slight curvature is imparted to the plate 
100 so that it may grossly conform to the cylindrical morphology of the 
vertebral bodies which it couples. As shown in FIG. 3a, the top surface 
108 is the convex surface, the bottom surface (not shown) is concave. 
Two pairs of holes 110 and 112, which extend fully through the plate, from 
the upper surface 108 through the lower surface, are disposed in the upper 
and lower portions 102, 104 respectively. Each of the holes 110, 112 is 
ideally suited for receiving therethrough a bone screw for affixing the 
plate to the vertebral bodies. 
Referring now to FIG. 3b, in which a side cross-section view of a hole, 110 
or 112, is shown, the interior conformation of the hole is illustrated. 
Extending from the top surface 108 of the plate, into the hole 110 or 112, 
to a position which is above the bottom surface 103 is a threading 111. 
The diameter of the hole 110 or 112 does not vary in the threaded region, 
however, the bottom section of the hole 110 or 112 includes an inward 
taper 113. 
Referring now also to FIG. 4, a screw of a type which is ideally suited for 
coupling the plate 100 to vertebral bones is shown in a side view. The 
screw 120 comprises a head portion 122, a neck 124, and a shaft 126. In 
FIG. 4, the shaft 126 is shown as having a tapered shape with a high pitch 
thread 128. It shall be understood that a variety of shaft designs are 
interchangeable with the present design. The specific choice of shaft 
features, such as thread pitch, or shaft diameter to thread diameter 
ratio, or overall shaft shape, etc. should be made be the physician with 
respect to the conditions of the patient's bone, however, this invention 
is compatible with a wide variety of shaft designs. 
The head portion 122 of the screw 120 comprises a semi-spherical shape, 
which has a recess 130 in it. It is understood that the semi-spherical 
shape is necessarily is a section of a sphere, greater in extent than a 
hemisphere, and exhibits an external contour which is equidistant from a 
center point of the head. In a preferred embodiment, the major 
cross-section of the semi-spherical head 122 (as shown in the two 
dimensional illustration of FIG. 4) includes at least 270 degrees of a 
circle. 
The recess 130 defines a receiving locus for the application of a torque 
for driving the screw 120 into the bone. The specific shape of the recess 
122 may be chosen to cooperate with any suitable screwdriving tool. For 
example, the recess 130 may comprise a slot for a flat-headed screwdriver, 
a crossed recess for a phillips head screwdriver, or most preferably, a 
hexagonally shaped hole for receiving an allen wrench. It is further 
preferable that the recess 130 be co-axial with the general elongate axis 
of the screw 120, and most particularly with respect to the shaft 126. 
Having the axes of the recess 130 and the shaft 126 co-linear facilitates 
step of inserting the screw 120 into the bone. 
The semi-spherical head portion 122 is connected to the shaft 126 at a neck 
portion 124. While it is preferable that the diameter of the shaft 126 be 
less than the radius of the semi-spherical head 122, it is also preferable 
that the neck 124 of the screw 120 be narrower than the widest portion of 
the shaft 126. This preferable dimension permits the screw to be inserted 
at a variety of angles while still permitting the coupling element (as 
described with respect to FIGS. 5 and 6) to be screwed into the 
appropriate hole 110 or 112 of the plate 100 and remain coupled to the 
head 122. 
Referring now to FIG. 5, the two portions which form the coupling element 
of the present invention are shown in a side cross-section view. Phantom 
lines show the interior structure of the elements along the diametrical 
cross section. With specific reference to the socket portion 132, the 
coupling element comprises a roughly cylindrical shape having an interior 
volume 134 in which the semi-spherical head 122 of the screw 120 is 
disposed. The interior volume 134 is open at the top 136 of the socket 
portion 132 and at the bottom thereof 138. The lower section 131 of the 
socket portion 132 comprises a set of slots 133 which extend vertically 
from the bottom 138 of the socket portion 132 to a position above the 
maximum diameter of the semi-spherical interior volume 134. These slots 
133 permit the interior volume to expand and contract in accordance with 
the application of a radial force thereon. The external surface 135 of the 
lower section 131 of the socket portion 132 is tapered such that the 
narrowest part of the lower section 131 is at the bottom 138. 
The upper section 139 of the socket portion 132 has a generally constant 
diameter, which is less than the diameter at the uppermost position 137 of 
the taper of the lower section 131. A second set of vertical slots 141 are 
provided in this upper section 139 so that it may also expand and contract 
in accordance with radial forces applied thereto. In addition, the 
uppermost end of this upper section 139 comprises an outwardly extending 
annular lip 140. 
The cap portion 142 of the coupling element comprises an opening 143 in the 
bottom thereof, having an inwardly tapered entrance surface conformation 
144. As the upper section 139 of the socket portion 132 is inserted into 
the opening 143 in the cap portion 142, the taper 144 of the opening 143 
provides an inwardly directed force which causes the upper section 139 to 
contract (causes the slots 141 to narrow). This tapered entrance 144 opens 
to form an annular lip 145 which is useful for engaging and retaining the 
annular lip 140 of the upper section 139 of the socket portion 132. The 
interior surface 146 of the cap portion has a constant diameter, therein 
permitting the inserted upper section 139 of the socket portion 132 to 
slide and rotate relative to the cap portion 142. 
The exterior surface of the cap portion 142 comprises a threading 147 which 
is designed to engage the threading 111 in the upper portion of the 
corresponding hole 110 or 112. In addition, the cap portion 142 comprises 
an axial hole 148 through which a surgeon may insert a screw driving tool 
to access the head of the screw which is positioned in the interior volume 
134 of the socket portion 132. 
More particularly, with respect to the disposition of the head 122 of the 
screw 120 in the socket portion 132, and with reference to FIG. 6, a fully 
assembled coupling element is shown in a side cross-section view. The top 
136 of the socket portion 132 is inserted into the opening in the cap 
portion 142 until the annular lip 140 of the socket 132 seats into the cap 
142. The screw 120 is loosely held within the socket 132, which is, in 
turn, loosely retained within the cap 142. 
Referring now to FIG. 7, in which the fully assembled and implanted plate, 
coupling element, and screw is shown in side cross-section view, the 
implantation of this embodiment is described. The plate 100 is positioned 
against the vertebral bones which are to be immobilized. A drill is used 
to pre-form holes into which the bone screws 120 are to be inserted (at 
the desired angulation). The screw 120 is then inserted and driven 
downward into the bone by use of the appropriate screw driving tool. As 
the screw 120 is driven deeper into the bone, the coupling element mounted 
to the head of the screw begins to enter the hole 110 or 112 of the plate 
100. As the coupling element enters the hole 110 or 112, the tapered 
exterior surface 135 of the socket portion 132 seats against the tapered 
bottom 113 of the hole 110 or 112. Continued driving of the screw into the 
bone causes the slot 133 in the bottom of the socket portion 132 to 
narrow, thus causing the interior volume 134 of the socket portion 132 to 
crush against the head 122 of the screw 120. 
The cap portion 142 of the coupling element may then be threadably advanced 
into the top section of the hole. As it is advanced, the upper annular lip 
140 of the socket portion 132 slides upwardly along the inner surface 146 
of the cap 142 until the bottom tapered opening 144 contacts the widened 
taper position of the socket portion 132. Continued advancement of the cap 
portion 142 provides further advancement of the socket portion 132 into 
the hole 110 or 112, thereby increased locking pressure within the 
interior volume 134 against the head 122. 
Referring to FIG. 8, a variation of the above device is shown in a similar 
cross-section view. In this embodiment, the inner surface 146' of the cap 
portion 142 is tapered inwardly in the vertical direction so that the 
advancement of the cap portion 142 along the threading 111 of the hole 110 
or 112 causes the annular lip 140 to be compressed inwardly. This causes 
the slots 141 of the upper section 139 of the socket portion 132 to 
narrow. This may be utilized to further clamp the interior volume 134 
against the head 122 of the screw 120. Once screwed into the plate 100, 
and locked down, the cap portion 142 of the coupling element and the top 
surface of the plate 108 present a flush external surface. 
While there has been described and illustrated implantation devices for 
stabilizing and immobilizing regions of the spine by affixing a polyaxial 
locking screw plate to the anterior portion of the vertebral bones, it 
will be apparent to those skilled in the art that variations and 
modifications are possible without deviating from the broad spirit and 
principle of the present invention which shall be limited solely by the 
scope of the claims appended hereto.