Transpedicular screw system and method of use

An apparatus is provided for the internal fixation of the spine. The apparatus comprises two sets of implants (8) each consisting of a rod (18) and a plurality of vertebral anchors (16). The rod (18) is secured to the vertebral elements by the vertebral anchors (16). The anchor (16) includes a transpedicular screw (21) which is secured to a vertebrae. The anchor (16) further includes an anchor seat (23) which captures the screw (21) and permits micromotion between the anchor seat (23) and screw (21). This seat (23) has a rod-receiving channel (51,52) which captures the rod (18). A cap (25) cooperates with the seat (23) to secure the rod (18) in the anchor (16). A nut (27) screws down from the top of the assembly onto the seat (23) to cause rod (18) receiving flanges (46,47) in the cap (23) to apply a compressive force to the rod (18 ). A method of therapy is presented in which the present implants (8) are inserted surgically into a patient.

This invention relates generally to an apparatus for immobilization of the 
spine, and more particularly, to an apparatus for posterior internal 
fixation of the spine as well as to a method of therapy which utilizes the 
device. 
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 method 
of treatment, however, as surgical techniques have become more 
sophisticated, various methods of internal and external fixation have been 
developed. 
Internal fixation refers to therapeutic 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, spondylolisthesis 
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 Steffee 
plate). 
U.S. Pat. No. 4,805,602 to Puno, et al presents a system sharing advantage 
of both the wired implants and the plate. Specifically, that screw and rod 
system provides a rigidity which is intermediate between the wired implant 
and the plate systems and may be contoured to any plane. 
The present invention represents an improvement in the technology and in 
the therapy advanced in U.S. Pat. No. 4,805,602. in particular, this 
invention greatly reduces the time required to perform the spinal 
operation as compared to the prior invention. As an example of such a 
reduction, the time for inserting the anchors may be cut from hours to 
around an hour. 
Such a time saving represents a significant reduction in the risk 
associated with a surgical procedure. Further, the new design may be 
easier to use as the chances of cross-threading the nut unto the anchor 
are reduced and the nut is more accessible for tightening. This is cf 
particular significance in the bloody environment which obscures the 
spinal surgeon's access to the fixation device. The present device 
achieves this accessibility and attendant time savings without sacrificing 
the mechanical benefits of the earlier design. In particular, the anchor 
is designed so that it is not overly obtrusive. More specifically, the nut 
is thin and further is chamfered to reduce bulk and yet includes a thread 
design to achieve sufficient compression on the rod. The anchor system 
presents a flush upper surface and the total system is elegant and 
effective. Each anchor seat is secured by a cancellous screw which 
cooperates through a sloped bore in the anchor seat so as to provide a 
limited ball and socket motion. The design of the present invention 
incorporates a method of therapy for treating a spinal indication 
utilizing this internal fixator. 
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 done, 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 devices for posterior spinal fixation are discussed above as 
including the Steffee plate and the Luque System. A complete discussion of 
various internal devices are included in L. Wiltse, "Internal Fixation of 
the Lumbar Spine," Clinical Orthopaedics and Related Research, February 
1986, No. 203, pp. 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 lamina 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 laminas 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 
reamed canal generally positioned perpendicular to the longitudinal axis 
of the spine and horizontal or parallel to the transverse plane of the 
vertebral body. 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, 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 neural elements (i.e., spinal cord and or nerve 
roots) is reduced. On the other hand, the use of plates with the screws 
rigidly linked results in the direct transfer of loads at the bone-screw 
interface which is the weakest link in the fixation spine construction. 
This can result in breakage of the screw or failure of the bone-screw 
interface prior to achieving fusion. In addition, the current plate 
designs are bulky and leave little surface for bone grafting and they 
cannot be contoured to account for lateral curvature of the spine (i.e., 
scoliosis). 
The present invention utilizes a rod and vertebral anchors which holds the 
rod in position. 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 riskfree 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 coronal plane curvature; it allows for safe and 
relatively riskfree insertion; and it permits sufficient area for bone 
grafting. 
Further, the present invention presents an improvement over the previous 
rod and anchor system as it streamlines the surgical procedure and 
increases the ease of insertion while maintaining the favorable attributes 
of the other system. Specifically, one less part is required and less time 
is required in preparation of the bony surface to receive the implant. 
In order to achieve these advantages, the present design utilizes two 
implant sets on either side of the spinous processes. Each implant set 
includes a 0.25 inch diameter 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 lamina and the 
transverse process. The rod is held in position by a stainless steel 
vertebral anchor which captures the rods. The anchor has a seat member 
which is secured to the vertebrae by a stainless steel transpedicular 
screw. The screw is separate from the anchor seat and thus provides for 
limited motion between the anchor seat and the vertebrae. In addition, 
this aspect of the design acts as a "shock-absorber" to prevent direct 
transfer of load from the rod to the bone-screw interface prior to 
achieving bony fusion, thereby decreasing the chance of failure of the 
screw or the bone-screw interface prior to achieving bony fusion. This 
greatly facilitates the surgical procedure and therapy incorporating this 
device. 
In the preferred embodiment, the anchor comprises three members; an anchor 
seat having a bore which receives the screw and a rod-receiving channel 
transverse to the screw; a cap which mates with the anchor seat to capture 
the rod between the rod receiving channel and the cap; and an internally 
threaded collar or nut which engages external threads on the anchor seat 
to tighten the cap into position on the rod support as it is screwed 
downward into position on the anchor seat.

The anchor screw and rod system 10 of the present invention includes two 
implant sets 8 on either side of the spinous processes. 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 vertebra 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 
posterior to the vertebra. 
The rod 18 is generally made of quarter inch stainless-steel rod (316L), 
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. 
The vertebral anchor 16 comprises a transpedicular screw 21, an anchor seat 
23, a cap 25, and a nut 27. The various anchor parts 16 can be made of any 
suitably strong biocompatible material such as stainless steel. The screw 
21 which is shown is a standard stainless steel cancellous screw with 6.5 
mm thread diameter. It is available in various lengths. The anchor 16 was 
designed for use with this screw since the screw is readily available, and 
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 posterior neural 
arch. 
The anchor seat 23 is comprised of a hollow cup portion 49 which receives 
the screw and which includes opposing channels 51,52 to receive the rod 
18. The cup 49 has a stepped central longitudinal opening 40 having an 
upper inner diameter section of about 0.358 and a smaller lower diameter 
section which slightly exceeds the diameter of the head 30 of the screw 
21. This step eliminates unwanted motion between the screw 21 and the 
anchor 23. This lower diameter section is about 0.323 of an inch. The 
screw 21 passes through the two sections of the opening 40 within the rod 
support 25 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 has an internal surface at an angle of about 120 degrees and 
defines an opening 43 which has a diameter that exceeds the diameter of 
the shank 32 but which is smaller than that of the head 30 of the screw 
31. The diameter of the opening at the flange is about 0.27 of an inch. 
The internal surface of the detaining flange 42 represents 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 anchor seat 23, a limited 
ball-and-socket joint is formed which permits freedom of movement between 
the rod support 23 and the screw 21. 
The anchor seat 23 has two opposing channel 51,52 of the proper diameter to 
cradle the rod 18. The channels 51,52 form a rod-receiving cradle which is 
about 0.37 of an inch long. 
The height of the anchor seat 23 generally determines the amount that the 
anchor 26 project posterior of the vertebrae. This height ranges from 0.66 
to 0.84 inches. However, if necessary, one or two washers may be added. 
These washers are smooth round washers having an outer diameter which 
corresponds to the diameter of the anchor seat, i.e., 0.5 inch, and height 
of 0.063 inch. The washer fits around the screw 21 and is positioned under 
the seat between the bone and the seat 23. The washers are useful in 
indication of where the patient is heavy or severely deformed. 
On its external surface, the anchor seat 23 includes a threaded area 76. 
This area is 0.27 inch deep to the thread runout. A 45 degree chamfer is 
included at the top to facilitate threading the nut on the seat 23. The 
threads are at a count of 20 thread per inch. The nut 25 has a height of 
0.19 inch and includes a chamfered area 81 on its top surface. This 
chamfered area 81 blunts the edges of the nut and eliminates sharp edges 
which could otherwise irritate the soft tissues post-operatively, two 
opposing clamping flanges 46,47 which each extend about 0.13 inch beyond a 
larger diameter area 46 of the cap 25. Two such larger diameter areas 48 
exist and form opposing buttressing curves where the flanges 46,47 flow 
into the cup portion 59 of the cap 25. These two areas 48 mate with the 
channels 51,52 so that the seat 23 and the cap 25 complement each other to 
form a cylindrical unit into which the nut 27 is threaded. On its bottom, 
the cap 25 includes an arch 72 transverse to the longitudinal axis of the 
cap 25. 
The nut 27 includes internal threads 83 which engage the external threaded 
area 76 on the anchor seat. The nut 27 is a hex nut which can be tightened 
relative to the seat 25. 
As the nut 27 is rotated about the anchor seat 25, it cooperates with the 
top side of the flange 46,47 to tighten the clamp 25 in relation to the 
rod support 23. The rod 18 is gasped in the tunnel 84 formed between the 
rod-receiving channel 54 of the anchor seat 23 and the arch 72 of the cap 
25. 
As a further part of this invention, a cross-link 110 may be used to 
stabilize the rod members 18 against torsional rotation. The crosslink 110 
may be used with this implant device or with any spinal implant which 
utilizes rods for longitudinal stability such as the Marrington rod 
system. It is preferable that two crosslinks are used to form a 
rectangular construct. Each crosslink 110 comprises two clamps 112, each 
secured to the main rods 18. Specifically, each clamp 112 includes a rod 
receiving channel 113 which accommodates the rod 18 and is locked into 
position relative thereto by a first set screw received in bore 114. The 
clamp further includes a link opening 118 which has a well 119 to 
accommodate a link 117 axially transverse to the main rod 18. This link 
117 may be, for example a 4 mm Steinmann pin. The link 117 is locked into 
position by a second set screw which biases the link 117 into the well 
119. The set screws 115 include a hexagonal opening 120 to receive a 
corresponding screwdriver. The screws 115 further include a terminal bevel 
at a 45 degree angle to facilitate locking the rod and link, respectively. 
A method of therapy for use of the present device is described as follows: 
Initially, the area of implantation is surgically approached. A 
longitudinal posterior midline incision is made over the spine. The 
incision is carried through the subcutaneous tissue and the fascia to the 
tips of the spinous processes. Subperiosteal dissection is performed over 
the laminas and transverse processes. The facet capsule and articular 
cartilage are removed in preparation for fusion. 
The pedicle is located using an awl 80. The awl 80 is used to make a hole 4 
mm deep at the intersection of a line drawn transversely through the 
midportion of the transverse process and a line drawn longitudinally 
through the lateral margin of superior articular facet. 
A pedicle hole is made using a pedicle probe 85. The pedicle probe is 
inserted into the hole initially created by the awl 80 and rotated back 
and forth in a 90 degree arc of motion with a very gentle downward 
pressure. The surgeon feels a relatively soft gritty sensation of the 
cancellous bone within the pedicle and vertebral body during this 
procedure. The shaft of the probe 85 should end up at an angle of 10 to 15 
degrees from the midline of the spine when used in the lumbar region. 
Great care should be taken not to penetrate the anterior cortex of the 
vertebral body with the probe 85. 
The depth of the hole is determined by using the graduated markings on the 
pedicle probe 85. The appropriate size screw is then chosen for that 
particular pedicle. The same technique is repeated for the remaining 
pedicles that need to be instrumented. Roentgenographic assistance using 
plain radiographs or fluoroscopy may be recommended for proper insertion 
of the pedicle probe 85 and screw into the pedicle. Both 
anterior-posterior and lateral views are taken with metal markers in the 
holes of the pedicles to assure proper hole direction prior to insertion 
of the screws. 
After the hole has been created, one of four sizes of anchor seats is then 
selected depending on the height needed for the rod to rest above the 
fusion bed. Trial anchors 91 may be inserted on rods 92. Washers are 
provided if additional height is needed. 
The surgeon sequentially inserts an appropriate transpedicular screw 21 and 
anchor 23 seat assembly into each pedicle being instrumented. This is 
accomplished by using a hexagonal screwdriver 97. At the same time, the 
seat holder 98 grips the seat, thereby preventing rotation when the screw 
21 is finally tightened. 
After all the screws and anchor seats are in place, an appropriate length 
of 6.35 mm rod is chosen and contoured with a French bender to fit the 
seats. The rod 18 is placed using a rod holder 100 and secured on the 
seats with caps which are placed over the rod using a rod holder 101 and 
nuts which are tightened down over the cap with the use of a T-wrench 105. 
The procedure is repeated on the other side of the spine over the same 
number of vertebral levels. 
Finally, the crosslinks 110 may be applied for added torsional stability. 
The crosslink is composed of two clamps 112, each of which is secured to 
one of the two main rods with set screws 115. The clamps are then bridged 
together by a 4 mm Steinmann pin which acts as a crosslink 117 which is 
cut to the length equivalent to the distance between the clamps. The 
Steinmann pin is secured to the clamp 117 with a second set screw. It is 
recommended that at least two sets of crosslinks are used to provide a 
more stable construct. 
In the case of arthrodesis, the fusion portion of the procedure is carried 
out in standard fashion. However, it is recommended to place some of the 
bone grafts in the lateral gutter after making the pedicle hole prior to 
screw insertion. The presence of the instrumentation can block the 
visualization of the fusion bed necessary for the proper placement of the 
graft. The remainder of the bone grafts are placed on the fusion bed after 
the instrumentation is completed. 
While in accordance with the Patent Statutes, the best mode and preferred 
embodiment has been set forth, the scope of the invention is not limited 
thereto, but rather by the scope of the attached claims.