Occipitocervical plate

[0001] Claim of Priority: Pursuant to the provisions of 35 U.S.C. §119(e), this application claims the benefit of the filing date of provisional patent application Ser. No. 60/574,282, filed May 25, 2004, for “OCCIPITOCERVICAL PLATE,” the contents of which are incorporated by reference herein.

[0002] The present invention relates generally to apparatus and methods for spinal fixation. More specifically, the present invention relates to apparatus and methods for providing internal support and spinal fixation for patients suffering from occipital-cervical instability.

[0003] Spinal fixation is a well known and frequently used medical procedure. Pedicle, lateral, and oblique mounting devices may be used to secure corrective spinal instrumentation to a portion of the spine that has been selected to be fused by arthrodesis. Fixation of the skull to the cervical spine may be used to treat trauma to the neck, degenerative diseases such as rheumatoid arthritis, and congenital instability. Many current implantable devices designed to immobilize the skull with respect to the upper cervical spine are assemblies of several components which are not designed specifically for fusing the cervical spine to the skull, but instead are assembled from multiple components designed for other applications. Such assembly may prolong and complicate the implantation procedure.

[0004] A typical spinal fixation system includes corrective spinal instrumentation that is attached to selected vertebrae of the spine by screws, hooks, and clamps. Various types of screws, hooks, and clamps have been used for attaching such corrective spinal instrumentation to selected portions of a patient's spine. Examples of pedicle screws and other types of attachments are illustrated in U.S. Pat. Nos. 4,763,644; 4,805,602; 4,887,596; 4,950,269; and 5,129,388. Each of these patents is incorporated by reference as if fully set forth herein. Examples of such multipart spinal fixation systems include U.S. Pat. Nos. 5,360,429 and 5,542,946, the disclosure of each of which is incorporated by reference.

[0005] With respect to occipital-to-cervical spinal fixation systems, contoured loop and wire constructs, rod constructs, rod and plate constructs and pre-contoured “U-loop”-type constructs have been used. An example of such a device is the OMI “U loop” device manufactured by Ohio Medical Instruments. However, such devices have a number of limitations, including the lack of appropriately sized loops for children under five years of age; the extensive modification and bending of the loops required during surgery, which can lead to failure of the device even before installation; cumbersome methods of coupling the devices to the anchor screws; and the lack of an option for installing a posterior cervical screw, which can be an urgent need for patients with missing lamina or inadequate laminar bone quality.

[0006] With these multi-piece systems, a number of problems may occur. For example, pressure necrosis may occur at the points of hook or wire fixation, leading to failure. Supplementation of such systems with halo vests often then fails to prevent micro-motion leading to non-union of the arthrodesis. Additionally, the time for surgery may be extended by the need to build and install a multi-piece assembly from separate components.

[0007] A few occipital-cervical spinal fixation systems, such as that disclosed in U.S. Pat. No. 6,146,382, the disclosure of which is incorporated by reference herein, attempt to simplify the implantation of the system by reducing the number of parts. A single plate attaches to an attachment site on the skull, and arms extend down from the plate to the cervical vertebrae. The arms are coplanar with the plate and bend at the tips where a separate connection member is attached. The separate attachment member is then attached at the top surface of the C2 vertebra. A cable is then attached by a hook system to the plate to a vertebra posterior to the arms, in order to retain a bone graft material in place. The '382 device thus still includes a number of parts that are assembled in situ, retaining the issues described with multi-piece systems.

[0008] Approximately 500 surgical cases of pediatric occipito-cervical fusions are performed in North America each year on children suffering from occipital-cervical instability. Current occipital-cervical fixation devices, such as the '382 device, are designed for adults and are therefore typically too large for use in children. Additionally, as the relationship of a child's head to the body differs from that of an adult due to allometric growth, devices designed for adults may not sustain the correct relationship of the head and neck for children. Surgical concerns are magnified when treating children, due to their smaller physical size, the abnormal anatomy that may be caused by craniovertebral anomalies, and their growth potential.

[0009] Previously, graft/wire constructs were reported to be associated with a nonunion rate as high as 30% for C1-C2 fusion; however, this incidence improves considerably with the use of a halo orthosis. Transarticular screw placement creates immediate atlantoaxial joint stability and, in contrast to previous posterior wiring/graft constructs, does not require postsurgical brace therapy. However, such procedures require surgical precision because serious potential risks are associated with improper screw placement. Thus, many spine surgeons are reluctant to perform such a procedure.

[0010] Accordingly, an occipital-cervical spinal fixation system that operated as a single plate, not requiring the use of additional component plates, hooks or rods would be an improvement in the art. Such a system that is configured for use in children or small adults would be an additional improvement in the art.

[0011] One aspect of the present invention relates to occipital-cervical spinal fixation systems. In some embodiments the system includes a plate configured for attachment to the occipital bone, with two arms that extend out from either side of the plate, with the distal end of the arms turning downwards parallel to one another. A bend is placed in the arms, such that the arms extend down from the occipital bone upon installation, behind the spinous process of the C1 and C2 vertebrae. A second bend is placed in the arms, allowing attachment to the C2 vertebrae. Some embodiments are configured in appropriate dimensions for installation in a child for pediatric applications.

[0012] Other aspects of the present invention include methods of spinal fixation of the C1 and C2 vertebrae to the occiput. A one-piece spinal fixation system is attached to the C2 vertebra with transarticular screws that extend through the C2 vertebra into the C1 vertebra. The system extends from the occipital bone, behind the spinous process of the C1 and C2 vertebrae. The transarticular screws may be headless screws requiring only a single emplacement. The system is attached to the occipital bone by one or more attachment screws extending through a top plate. A bone graft material may be held in place between the cervical vertebrae and the skull by installing a cable to the installed system to retain the bone graft material in place.

[0026] FIGS. 1, 2 and 3 depict an illustrative embodiment of an occipital-cervical spinal fixation system 10. An attachment plate 12 may be configured for attachment to the occipital bone. Attachment plate 12 may include an enlarged area 13, with one or more attachment holes 16, through which attachment screws 160 (FIG. 6) may be placed to attach the plate to the occipital bone. Each of attachment holes 16 may include beveled edges, allowing an attachment screw placed therein to lie flush with the plate surface (by being countersunk therein). As depicted, the plate 12 may be planar in conformation. The plate 12 may be contoured as desired to fit the surface of the occiput prior to installation.

[0027] At opposite sides of plate 12, two arms 14A and 14B extend out from the plate 12 in opposite directions. Each arm 14 then extends downwards becoming generally parallel to one another and generally sharing a common plane throughout their length. It will be appreciated that while the arms 14 may be generally parallels, some variation for individual patients may be required, based on the patients anatomy. Viewed from the front or back (as in FIG. 2), the relationship of the plate 12 and arms 14A and 14B may generally resemble a horseshoe.

[0028] Each arm 14A or 14B contains a bend 20A or 20B in the length thereof, at a distance from the plate. The bends 20A and 20B are generally parallel to one another in the respective arms 14A and 14B, such that the arms 14A and 14B remain generally parallel. Again, it will be appreciated that while the bends 20A and 20B may be generally parallel to each other and arms 14A and 14B may be generally parallel to each other, some variation for individual patients may be required, based on the patient's anatomy. The angle A of bends 20A and 20B is selected to ensure that, upon installation, the arms 14A and 14B extend down from the occipital bone, behind the spinous process of the C1 and C2 vertebrae (as best depicted in FIG. 8). In one embodiment, an angle of about 115 to about 135 degrees, as depicted in FIG. 3 as about 127 degrees, may be used. It will be appreciated that other angles may be used based upon the anatomy of the individual patient.

[0029] A second bend 30A and 30B may be placed in each arm, 14A and 14B, respectively, at a point distal to the first bend 20A or 20B. The second bend 30A or 30B positions a distal end of each respective arm such that it may be attached to the lower surface of the spinous process of the C2 vertebrae (as best depicted in FIG. 8). In one embodiment, the angle B of the second bend 30A or 30B may be from about 140 to about 160 degrees, as depicted in FIG. 2 as about 151 degrees, although it will be appreciated that other angles may be used, based upon the anatomy of the patient. Attachment may be accomplished by insertion of a fastener, such as bone screw 20 (FIG. 6) through a fastener hole 18A or 18B located near the distal end of the arm 14. Fastener hole 18A or 18B may include beveled edges, allowing a screw placed therein to lie flush with the plate surface (by being countersunk therein). Long screws 40 that extend through both the C1 and C2 vertebrae may be used, where desirable for the procedure.

[0030] A headless bone screw 20, together with a retaining nut 22, depicted in FIGS. 4A, 4B and 4C, may be used to attach system 10 to the lower surface of the spinous process of the C2 vertebrae. Headless bone screw 20 includes an elongated shaft S, extending from a distal tip T, which may be configured for penetrating bone, to a retaining end R. Bone threads 24 are disposed around the shaft S, extending from near the distal tip T back along the shaft S allowing for secure rotational placement of the bone screw 20 into bone, such as the C1 and C2 vertebrae. It will be appreciated that the depicted bone threads 24 are illustrative only and any spacing and size of thread sufficient to retain the bone screw 20 in place may be used. Retention threads 26 are located on the shaft S near retaining end R, and are configured to rotatably connect with threads 28 disposed on the internal channel 23 of the retaining nut 22. Retention threads 26 are typically smaller in size and more closely spaced than bone threads 24. However, it will be appreciated that the depicted retention threads 26 are illustrative only and any spacing and size of thread sufficient to retain a fastener to the bone screw 20 may be used. A non-threaded area of shaft S, which may be the thickness of the threads 24 or 26, may be disposed between the two sets of threads.

[0031] Retaining end R includes structures allowing headless bone screw 20 to be rotatably inserted into bone. As depicted in FIG. 4C, a socket 30 may be formed in retaining end R, into which a tool can be inserted to rotate the headless bone screw 20 by interaction with the sides thereof. It will be appreciated that, although depicted in FIG. 4C as hexagonal, socket 30 may have any desired shape that allows a tool to be inserted therein, examples include square, triangular, irregular, and radially patterned sockets for customized tools. It will also be appreciated that non-socket tool interconnection structures may be used, such as one or more slots for a screwdriver tip disposed on the retaining end R or within a shallow recess disposed thereon.

[0032] As depicted, retaining nut 22 has a rounded lower surface 27 for interacting with system 10, and an upper end 29 configured for attachment to a connection tool, such as a wrench. As depicted, upper end 29 is hexagonal shaped, although any desired shape that allows a tool to be removably attached thereto may be used, such as, for example square, triangular, irregular, and radially patterned upper ends for interacting with customized tools.

[0033] Fixation system 10 may be configured in dimension for installation in a child. Such configuring may account for both the smaller size of a child's head and neck, as compared to an adult, and for the different allometric relationship of the head to the neck in a child. For example, in one illustrative embodiment, the length from the top of plate 12 to the first set of bends 20A and 20B, depicted by line 100 (FIG. 3.) and including the plate and the upper section of arms 14A and 14B may be around 32 mm. A second length between the two bends 20A and 20B and 30A and 30B, as represented by line 102 (FIG. 3) may be around 22 mm. A third length between the second bend 30A or 30B and the distal end of the respective arms 14A and 14B may be around 6 mm as depicted by line 104. A ratio of 16:11:3 may thus be realized. It will be appreciated that different lengths may be used for each of these features, depending on the age and anatomy of the patient. Accordingly, different ratios may be used for children at different stages of development.

[0034] Table I below provides some illustrative examples of measurements (in millimeters) that may be used on some embodiments in accordance with the principles of the present invention.

[0035]

Embodiment	1	2	3	4	5	6	7	8
Plate thickness	3	3	3	3	3	3	3	3
Coronal Width	26	28	30	32	34	36	38	40
Distance Bend A	18	20	22	24	26	28	30	34
to Bend B
Distance Bend B	30.4	31.2	32	32.8	33.6	34.4	35.2	36
to Coronal tip
(generally
variable between
30 and 36,
dependent on
patient need)
Plate width	7	7	7	7	7	7	7	7
Accepts Screw	3.5	3.5	3.5	3.5	3.5	4	4	4
Size

In such embodiments, tab length may be between 6 and 10, plate thickness may be around 7 and arm width may be around 8.6. All measurements are given in mm and are merely illustrative. Embodiments have different measurements are contemplated within the scope of the present invention. For example, larger embodiments of a system may be constructed for use in adult patients. For smaller adult patients or patients with anatomical variations, different embodiments may be constructed having different dimensions as appropriate for the patient.

[0036] The curve of the arms 14 when extending from plate 12 may also be expressed as a ratio. As depicted in FIG. 3, the inside curve of the bend may have a radius of approximately 12.5 mm (as depicted by arrow 106), while the outer curve may have a radius of about 22 mm (as depicted by arrow 108), giving a ratio of 12.5:22. Each arm 14 may have a width of around 7.8 mm and be spaced apart from each other in a common plane by about 30 mm measured from a centerline axis of each arm 14. In such a sized embodiment, the system may have a thickness of around 2 mm and the edges of the system may be rounded to about 1 mm. Again, it will be appreciated that these measurements are for one merely illustrative pediatric embodiment and that embodiments having different measurements are contemplated within the scope of the present invention. For example, larger embodiments of a system may be constructed for use in adult patients, which could maintain a similar ratio for these curves.

[0037] Once being made aware of the system 10, those of ordinary skill in the art will be readily able to make the components thereof with the use of conventional materials. For example, the plate 12 may be formed from any suitable biocompatible material, including metal, such as titanium, stainless steel, cobalt-chromium-molybdenum alloys, titanium-aluminum vanadium alloys, other metals and alloys, or even a biocompatible plastic, such as an ultra high molecular weight polyethylene. Arms 14A and 14B may be constructed from similar suitable materials. It will also be appreciated that bone screws 20, retaining nuts 22 and attachment screws 160 may be constructed of similar materials.

[0038] FIGS. 5 through 9 show the installation and use of one embodiment of a system 10. As best depicted in FIG. 9, upon the completion of installation, the plate 12 is fastened to the occiput O with at least one attachment bone screw 160 passing through one of the attachment holes 16. Where desirable, multiple attachment bone screws 160 may be used in the different attachment holes 16. Where possible, it may be desirable that at least one bone screw 160 be placed into the midline keel of the occiput O. Multiple attachment holes 16 may allow such placement with minimal adaptation of the system 10. A transarticular headless bone screw 20 passes through fastener hole 18 in each arm 14 and into both the C2 and C1 vertebrae, attaching the system 10 thereto in conjunction with the retention nut 22 rotatably retained thereon. It will be appreciated that a bone screw having a head, rather than a headless bone screw 20 may be used in conjunction with system 10, as will be explained further herein.

[0039] Typically, prior to installation, a detailed image of the patient's occipital-cervical region is performed, such as a thin-cut CT scan or other appropriate imaging technique, to allow selection of an appropriate system 10 and guidance through the procedure. The placement of the system 10, and the attachment screws may be performed under intraoperative fluoroscopy. For placement, the system 10 may be placed in position, the attachment and fastener hole positions marked. The system 10 may then be removed and the attachment sites prepared for insertion of bone screws, which may involve placement or partial emplacement thereof. As depicted in FIG. 5, bone screws, such as headless screws 20 may be placed transarticularly passing angularly upwards through the C2 vertebra into the C1 vertebra (as depicted in FIG. 8) using an emplacement tool E. Where appropriate, paths for the insertion of the bone screws may be drilled prior to insertion. Where headless bone screws 20 are used, the screws may then be left in place and the system 10 mounted with the retention end R of each screw protruding through an attachment hole 18A or 18B, as depicted in FIG. 6. Where headed bone screws are used, the screws may be entirely or partially emplaced, then removed, the plate positioned and the screws re-emplaced to attach the system 10. In some embodiments, fastener hole 18 may have a keyhole 50 extension that allows the system to be emplaced around a partially inserted transarticular screw, as shown in FIG. 11A. Fastener hole 18 may also be formed as a recessed slot 52 (FIG. 11B) allowing for adjustment of the system during emplacement, following insertion of the transarticular bone screws.

[0040] Attachment is completed by the attachment of the plate 12 the occiput O with at least one attachment bone screw 160 passing through one of the attachment holes 16. Multiple attachment bone screws 160 may be used in the different attachment holes 16. Where possible, at least one bone screw 160 may be placed into the midline keel K of the occiput O. Multiple attachment holes 16 may allow such placement with minimal adaptation of the system 10. Where appropriate, paths for the insertion of the bone screws may be drilled prior to insertion.

[0041] A bone graft material G, such as iliac crest or rib, (FIG. 9) may be placed between the occiput O and the vertebrae to promote fusion therebetween. The system 10 retains the bone graft material G in proper position and under compression to facilitate fusion between the occiput and the vertebrae, using a cable C, which is wrapped around the system 10 to retain the bone graft B in place. From the foregoing discussion, it will be apparent that use of a system or method in accordance with the present invention may lead to short surgery times and stronger systems than current procedures.

[0042] It will be appreciated that systems and methods of the present invention may be used to treat craniocervical junction instability through fusion of the occiput-C2 region, where the instability results from any cause, so long as patient's is sufficiently healthy to undergo implantation surgery and the patient's anatomy will allow successful implantation. Examples of causes of such instability that may be treated with the systems and methods of the present invention include trauma, os odontoideum, congenital anomaly (such as Down syndrome, Stihl disease, metatrophic dwarf, Morquio syndrome, Klippel-Feil syndrome, axis assimilation, or skeletal dysplasia), neoplasm, rheumatoid arthritis, or chronic instability.

[0043] FIG. 10 depicts a back view of another embodiment of a system 10A in accordance with the teachings of the present invention. Similar to system 10, system 10A differs by having an enlarged plate 12A with an additional attachment hole 16A, that extends down between the arms 14 of the system. Such a system 110A may facilitate placement attachment bone screw 160.

[0044] FIGS. 11A and 11B depict alternative embodiments of fastener holes 18. For example, fastener hole 18 may have a keyhole 50 extension that allows the system to be emplaced around a partially inserted transarticular screw having a head, as shown in FIG. 11A. Fastener hole 18 may also be formed as a recessed slot 52 (FIG. 11B) with a shelf 51 therein allowing for adjustment of the system during emplacement, following insertion of the transarticular bone screws.

[0045] In order to facilitate use of a spinal fixation system in accordance with the present invention, a kit is included within the scope of the present invention. Such a kit may include a system in accordance with the present invention (such as a system 10 or 10A), transarticular screws or headless bone screws 20 and retaining nuts 22, attachment bone screws 160, drill guides, cable (such as a titanium cable), taps and calipers, all sized appropriately for use together. Such a kit may be provided in sterile form in a sealed sterile container. Individual kits may be selected based on the size of the system 10 or 10A appropriate for the intended usage.

[0046] Data used in this example was also used in preparing the paper, Gluf and Brockmeyer, Atlantoaxial transarticular screw fixation: a review of surgical indications, fusion rate, complications, and lessons learned in 67 pediatric patients, J. Neurosurg Spine 2:164-169, 2005, the disclosure of which is incorporated herein by reference.

[0047] 23 pediatric patients were treated for occipital cervical instability by occipitocervical fusion performed with implants in conjunction with C1-C2 transarticular screws. While older patients were treated using Ohio Medical Instruments U-loop, rod/plate devices, patients from the ages of 2 to 5 years of age were treated using a system in accordance with the present invention, referred to as the Avery Brockmeyer-Thiokol plate, sized to fit patients 2 to 5 years of age.

[0048] A preoperative imaging protocol including plain cervical flexion-extension radiography and thin-cut (1-mm) CT scanning of the occiput-C3 region with sagittal and coronal reconstructions was performed. However, no presurgical flexion-extension x-ray films were obtained in traumatically injured patients with obviously unstable atlantoaxial or craniovertebral fractures. Craniovertebral magnetic resonance imaging was found to be helpful in some cases but not critical in the planning phase for placing C1-C2 transarticular screws. Image analysis of the thin-cut CT scan was performed on a high-speed CT workstation with multiplanar reconstructions in the screw trajectory pathway being generated to determine the best path for screw placement.

[0049] In general, 3.5-mm-outer-diameter screws were used for transarticular placement in patients 4 years of age and younger and 4-mm-outer-diameter screws for patients older than the age of 4 years. However, as each case is individualized, however, appropriate screw diameter for each patient was determined only after careful study of the reformatted CT scans.

[0050] Before surgery, an understanding of the anatomical landmarks for determining the starting point for screw trajectory was obtained. The screw starting point is measured in millimeters from the midportion of the C2-C3 facet joint and the screw trajectory is defined in number of degrees from both the parasagittal plane and the dorsal or ventral position of the screw relative to the VA foramen in C-2. Where a patient's anatomy was unsuitable for using C1-C2 transarticular screws on either side, a pars screw was placed, which was found to be usually sufficient to anchor the chosen type of occipitocervical hardware.

[0051] Once the patient was intubated (either with axial traction or with a hard collar in place), he or she was lifted and turned carefully into the prone position. Mayfield three-point fixation was routinely used in all patients, even those as young as 2 years of age. Gentle axial traction was applied (except in cases of occipitoatlantal dislocation) under direct lateral fluoroscopic vision and the patient's neck flexed and posteriorly translated into the so-called military tuck position. During occipitocervical fusion, the patients were realigned into a neutral position prior to placing the rigid occipito cervical instrumentation to avoid the problem of fusion occurring in a nonanatomical flexed position.

[0052] Postoperative imaging demonstrated successful fusion in all patients. Postoperative imaging included plain radiographs obtained at 1 and 2 months and fine-cut CT scans of the occiput-C3 region reconstructed two dimensionally 4 months after surgery. If solid fusion was not evident on the 4-month CT reconstruction, monthly repeated CT scanning involving the same protocol used in the 4-month study was performed until confirmation of fusion. Fusion was defined as continuous, bridging trabecular bone seen between the occiput and C-2 on two-dimensional sagittal reconstructions of thin-cut CT scans indicated fusion.

[0053] It will be apparent that details of the apparatus, processes, and methods herein described can be varied considerably without departing from the concept and scope of the invention.