Transpedicular screw and rod system

An apparatus is provided for the internal fixation of the spine. The apparatus comprises two sets of implants each consisting of a rod and a plurality of vertebral anchors. A set of implants is positioned on the spine on either side of the spinous process spanning the portion of spine to be immobilized. The rod is secured to the vertebral laminae by the vertebral anchors. The anchor includes a transpedicular screw member which is secured to a vertebrae. A rod support includes a cup which captures the screw and optionally permits micromotion between the rod support and screw. The rod support also includes a rod-receiving channel which captures the rod. A clamp is provided to secure the rod in the channel.

BACKGROUND OF THE INVENTION 
The invention relates generally to an apparatus for immobilization of the 
spine, and more particularly, to an apparatus for posterior internal 
fixation of the spine. 
Various methods of spinal immobilization have been known and used during 
this century in the treatment of spinal instability and displacement. The 
preferred treatment for spinal stabilization is immobilization of the 
joint by surgical fusion, or arthrodesis. This method has been known since 
its development in 1911 by Hibbs and Albee. However, in many cases, and in 
particular, in cases involving fusion across the lumbosacral articulation 
and when there are many levels involved, pseudoarthrosis is a problem. It 
was discovered that immediate immobilization was necessary in order to 
allow a bony union to form. Early in the century, post operative external 
immobilization such as the use of splints and casts was the favored 
methods of treatment, however, as surgical techniques have become more 
sophisticated, various methods of internal and external fixation have been 
developed. 
Internal fixation refers to methods of stabilization which are wholly 
internal to the patient and include commonly known devices such as bone 
plates and pins. External fixation in contrast involves at least some 
portion of the stabilization device which is external to the patient's 
body. Internal fixation is now the favored method of immobilization since 
the patient is allowed greater freedom with the elimination of the 
external portion of the device and the possibility of infections, such as 
pin tract infection, is reduced. 
Some of the indications treated by internal fixation of the spine include 
vertebral displacement and management such as kyphosis, spondylolishtesis 
and rotation; segmental instability, such as disc degeneration and 
fracture caused by disease and trauma and congenital defects; and tumor 
diseases. 
A common problem with spinal fixation is the question of how to secure the 
fixation device to the spine without damaging the spinal cord. The 
pedicles are a favored area of attachment since they offer an area that is 
strong enough to hold the fixation device even when the patient suffers 
from osteoporosis. Since the middle 1950's, methods of fixation have 
utilized the pedicles. In early methods, screws extended through the 
facets into the pedicles. More recently, posterior methods of fixation 
have been developed which utilize wires that extend through the spinal 
canal and hold a rod against the lamina (such as the Luque system) or that 
utilize pedicular screws which extend into the pedicle and secure a plate 
which extends across several vertebral segments (such as the Steffe 
plate). 
The present invention presents a new system sharing advantages of both the 
wired implants and the plate. Specifically, the screw and rod system of 
the present invention provides a rigidity which is intermediate the wired 
implant and the plate systems. While the screw and rod system of the 
present invention retains the stability provided by the plate and screw 
system, the present invention may be contoured to any plane. 
In particular, the present invention is viewed as having an application in 
the stabilization of the thoracolumbar, lumbar, and sacral spine. There 
are problems of fixation unique to this area of the spine such as the fact 
that the lumbar spine is normally lordotic and this lordosis must be 
preserved. In addition, indicated spinal decompression often requires a 
destabilization of the spine posteriorly. This may result in instability 
unless fusion is used, and fusion will often fail to become solid unless 
effective internal fixation is used. Finally, the points of sacral 
fixation are the weakest point of fixation. These problems are addressed 
by the present invention. 
Prior Art 
Prior art devices for posterior spinal fixation are discussed above as 
including the Steffe plate and the Luque System. A complete discussion of 
various internal fixation devices are included in L. Wiltse, "Internal 
Fixation of the Lumbar Spine", Clinical Orthopaedics and Related Research, 
Feb. 1986, No. 203, p.p. 2-219. Known implant configurations include facet 
screws, double distraction systems, compression distraction systems, 
springs, spinous process plates, wired implants and transpedicular screw 
and plate systems. 
Common distraction and compression systems utilize a threaded rod and hooks 
which engage selected transverse processes of the vertebrae. Examples of 
such systems include the Harrington distraction system sold by Zimmer USA, 
Inc., the Keene system shown in U.S. Pat. No. 4,269,178 and the 
Lewis-Greenlaw System illustrated in U.S. Pat. No. 4,085,744. U.S. Pat. 
No. 3,648,691 to Lumb et al. shows the use of spinous process plates. 
Wired implants are favored by some orthopedic surgeons because of the 
flexibility of the system. Dr. Eduardo Luque has developed a wired implant 
system where two L-shaped rods are secured along their long sides to the 
vertebral laminae by means of wires which pass through the vertebral 
foramina. The short legs of the rods extend across the vertebrae between 
the spinous process. A similar wired implant is shown in U.S. Pat. No. 
4,604,995 to Stephens et al. 
Transpedicular screw and plate systems rely on a screw threaded into a 
reemed canal generally positioned perpendicular to the longitudinal axis 
of the spine and horizontal or parallel to the anterior/posterior plane of 
the vertebral body. Methyl methacrylate is sometimes used to secure the 
screw in the canal, particularly if osleoporesis is a problem. The screws 
engage a plate which has been bent to conform to the normal curvature of 
the spine or to the points of desired reduction. One screw and plate 
system which has been used with significant success is the Steffee system. 
In this system, the screws are inserted first, the spine plates are then 
inserted over the pedicle screws and then posterior tapered nuts are 
screwed on. The screws are tightened bilaterally until the plate is locked 
between two nuts. 
While the wired implants have the advantages of facilitating vertebral 
alignment, permitting variation of the device to allow for variations in 
individual spines, and decreasing rigidity, this method of fixation 
includes the increased risk of damage to the neural structures. This risk 
can be countered by the use of transpedicular screws and plates. The 
pedicle presents an area for fixation of sufficient size and depth, that 
under careful conditions, the risk of damage to the nerve chord is 
reduced. On the other hand, the use of plates with the screws is more 
rigid than the wired implants and the tension and compression of the plate 
on the screw can cause dislocation or even shearing of the screw. In 
addition, the current plate designs are bulky and leave little surface for 
bone grafting and they cannot be contoured to any lateral curvature of the 
spine. 
SUMMARY OF THE INVENTION 
The present invention utilizes a rod and vertebral anchors which holds the 
rod in position against the vertebral lamina. Each anchor is secured to 
the vertebrae by a transpedicular screw member. 
The screw and rod system of the present invention combines favorable 
attributes discussed above of wire implants and of screw plate systems. In 
particular, the present invention has an object of providing a fixation 
system which adequately immobilizes the lumbosacral area, allows 
relatively simple and risk-free insertion and provides adequate area for 
bone grafting. 
Thus, the present invention combines advantages of the known devices as it 
provides suitable immobilization, in particular of the lumbosacral region; 
it allows for adaptation to individual patient characteristics such as 
degree of sagittal and/or cornonal plane curvature; it allows for safe and 
relatively risk-free insertion; and it permits sufficient area for bone 
grafting. 
In order to achieve these advantages, the present design utilizes two 
implant sets on either side of the spinous process. Each implant set 
includes a 0.25 inch stainless steel (316L) rod which spans the vertebrae 
to be immobilized. Generally, an implant set is used on each side of the 
spinous process on the posterior side of the laminae and the transverse 
process. The rod is held in position by a stainless steel vertebral anchor 
which captures the rods. The anchor is secured to the vertebrae by a 
stainless steel transpedicular screw. The screw is preferably separate 
from the anchor but could also form an integral part of the screw. 
In the preferred embodiment, the anchor comprises three members; a rod 
support having a cup which contains the screw and a rod-receiving channel 
transverse to the screw; a clamp having a central arch which mates with 
the rod support to capture the rod; and an internally threaded collar or 
nut which engages external threads on the clamp to tighten the clamp into 
position on the rod support. The collar includes holes so that a crimping 
tool can be inserted to partially strip the clamp threads to prevent 
loosening but still allow removal of the nut with the use of a wrench.

DETAILED DESCRIPTION OF THE DRAWINGS 
The screw and rod system 10 of the present invention includes two implant 
sets 12, 14 on either side of the spinous process. Each set is comprised 
of a plurality of vertebral anchors 16 and a rod 18 which is of sufficient 
length to span the length of spine to be immobilized. 
Each anchor 16 is positioned on the dorsal side of the transverse process 
and in general, a separate anchor 16 is used for each vertebrae comprising 
the length of spine to be stabilized. The rod 18 is held by the anchors 16 
inside the curve of the transverse process posterior to the vertebral 
laminae. 
The rod 18 is generally made of quarter inch stainless-steel rod (316 L), 
but could be made of any material which has suitable biocompatibility and 
material strength characteristics. The rod should be able to withstand 
lateral bending forces and torsion since the system may be used to correct 
spinal displacement and curvature. On the other hand, it is important that 
the rod 18 can be bent to a certain extent so that the rod can be bent to 
the proper curvature for the individual application. 
In the preferred embodiment, the vertebral anchor 16 comprises a 
transpedicular screw 21, a rod support 23, a clamp 25 and a collar 27. The 
anchor 16 can be made of any suitably strong biocompatible material such 
as stainless steel. The screw 21 which is shown is a standard 6.5 
millimeter stainless steel cancellous screw. The anchor 16 was designed 
for use with this screw since the screw is readily available, it has a 
proven record in fracture fixation; and the size can be accommodated by 
the average adult pedicles of the lower thoracic, lumbar and the upper two 
sacral segments vertebrae. 
The screw 21 includes a head 30 which accommodates a hex driver. The screw 
21 includes a smooth shank 32 of 2-4 millimeters length which joins the 
rounded rear shoulder 34 of the head 30. After insertion, the screw 21 
extends from the curve formed on the dorsal side of the transverse process 
into the bony area above the neural arch. 
The rod support 23 is comprised of a hollow cup portion 49 which receives 
the screw, and two opposite transverse brackets. The cup 49 has a central 
longitudinal opening 40 having an inner diameter which slightly exceeds 
the diameter of the head 30 of the screw 21. This diameter is about 0.33 
of an inch. The screw 21 passes through the opening 40 within the rod 
support 23 until the rear shoulder 34 of the screw 21 encounters a 
detaining flange 42 within the central opening 40 of the rod support 23. 
The flange 42 defines an opening 43 which has a diameter that exceeds the 
diameter of the shank 32. The diameter of the opening at the flange is 
about 0.27 of an inch. The detaining flange 42 has a sloped shoulder 44 
which forms a socket for the rear shoulder 34 of the screw head 30. Thus, 
when the screw 21 engages the rod support, 23, a limited ball-and-socket 
joint is formed which permits freedom of movement between the rod support 
23 and the screw 21. 
The rod support 23 has two brackets 46, 47 which extend laterally from the 
cup portion 49 of the rod support 23. The brackets 46, 47 each have a 
groove 51, 52 on their upper surface of the proper diameter to cradle the 
rod 18. The grooves 51, 52 form a rod-receiving channel 54 which is about 
0.69 of an inch long. 
The bottom side of the brackets 46, 47 form buttressing curves 56, 57 where 
the brackets 46, 47 flow into the cup portion 49 of the rod support. 
The cup 49 is reinforced in the area under the curve of the brackets 46, 47 
so that the cup 49 includes two opposite reinforced portions 59, 60 having 
a larger external diameter (about 0.43 of an inch) which are separated by 
two curved walls 62, 63 having a smaller external diameter (about 0.35 of 
an inch). 
The depth of the rod support 23 and the clamp 25 determines the amount that 
the anchor 16 projects beyond the lamina of the vertebrae. The rod support 
23 has a total depth ranging from about 0.35 to about 0.52 of an inch 
including the brackets 46, 47 which extends about 0.11 of an inch above 
the cup 49. The rod-receiving channel 54 has a radius of about 0.13 of an 
inch and a depth of about 0.01 of an inch. The clamp 25 has a full height 
ranging from about 0.58 to about 0.74 of an inch, while the arch 72 has a 
height ranging from 0.51 to 0.67 of an inch. 
On its bottom side, the cup 49 of the rod support 23 has an annular groove 
65 so that the cup 49 includes annular teeth 66 which will bite into the 
bone as the rod support 23 is tightened into position. 
The U-shaped clamp 25 includes a hollow central arch 72 defining two 
opposite, curved, externally-threaded walls 74, 75 having an outer 
diameter of about 0.50 of an inch. The arch is about 0.28 of an inch wide 
and has an inner radius at the top of about 0.14 of an inch. The walls 74, 
75 are both grooved to form curved inner surfaces 78, 79. The inner 
surfaces 78, 79 of the walls 74, 75 are of the proper dimensions to engage 
the smaller diameter areas 62, 63 of the rod support 23, while the 
brackets 46, 47 and the reinforced portions 59, 60 of the rod support 23 
fit within the arch 72 of the clamp 25. The reinforced areas 59, 60 of the 
rod support cup 49 complements the clamp 25 to form a hollow cylinder 
which captures the screw 21 along its longitudinal axis and which captures 
the rod 18 along its longitudinal axis transverse to the longitudinal axis 
of the screw. 
The annular collar 27 includes internal threads 83 which engage the 
external threads 76 on the clamp 25. The collar 27 includes a plurality of 
holes 83 so that the collar can be tightened relative to the clamp 25. The 
clamp 25 likewise includes two holes 77 which permit the surgeon to use a 
holder to grasp the clamp 25. The collar 27 has holes 84 which permit the 
surgeon to use a crimping tool to partially strip the clamp threads 76 to 
prevent loosening of the clamp 25 but still allow removal of the collar 27 
with a wrench. 
As the collar 27 is rotated about the clamp 25, it cooperates with the 
underside of the brackets 46, 47 to tighten the clamp 25 in relation to 
the rod support 23. The rod 18 is grasped in the tunnel 84 formed between 
the rod-receiving channel 54 of the rod support 23 and the arch 72 of the 
clamp 25. 
The anchor 16 was designed to deliver a high compressive force on the rod 
18 when tightened with a special spanner wrench. This compressive force 
increases the limiting friction to decrease the possibility of rod 
slippage. This can be expressed by the formula below: 
EQU F.sub.L =.mu..sub.S R 
where: 
EQU F.sub.L =limiting friction 
.mu..sub.S =coefficient of static friction 
R=force applied to the surface. 
This is an additional manner in which the current design is an improvement 
over prior art devices. 
In the second embodiment of the present invention shown in FIG. 7, the rod 
support 116 includes a U-shaped head 117 which defines a rod-receiving 
channel 118 having a depth which exceeds the diameter of the rod 18. The 
rod 18 engages the channel 118 and is secured within the channel 118 by 
wires 120 which extend through bores 122 transverse to the longitudinal 
axis of the rod 18. The head 116 includes on its underside, two fillets 
124 which act to grip the bone. In the second embodiment, the rod support 
116 is countersunk 123 to accommodate the screw head 30. 
In both embodiments, the screw may be an integral portion of the anchor. It 
is preferable however, that the screw is a separate member having the 
previously described freedom of movement. This allows for easier insertion 
without having to back out the screw in order to achieve proper alignment 
of the rod-receiving channels of the anchors. Further, this provides a 
bone-implant interface where micro-motion can occur upon fixation and 
dampen some of the load being transferred directly from the implant to the 
bone as well as decrease the degree of stress-shielding by the implant. 
This helps to compensate for the increased transfer of load to the 
bone-implant interface caused by the use of a more rigid implant which 
increases the likelihood of bony errosion by the screws. 
While the invention has been shown and described with respect to a 
particular embodiment thereof, this is for the purpose of illustration 
rather than limitation, and other variations and modifications of the 
specific embodiment herein shown and described will be apparent to those 
skilled in the art all within the intended spirit and scope of the 
invention. Accordingly, the patent is not to be limited in scope and 
effect to the specific embodiment herein shown and described nor in any 
other way that is inconsistent with the extent to which the progress in 
the art has been advanced by the invention.