Medical Devices of the Neck & Spine...cervical spine
by Tim B Hunter, MD, MSc and Mihra S Taljanovic, MD, PhD
Cervical Spine Instrumentation
Anterior and Posterior Cervical Plates
Approximately 1.2 million spine surgeries are performed annually in the United States. General indications for spinal surgery are degenerative disease, trauma, spinal tumors, and spinal infection.
Surgical fusion is a common treatment for cervical spine disk disease not amenable to more conservative therapy. Modern techniques with fixation stabilization hardware and disk replacement are designed to reduce the development of adjacent segment degeneration which often occur after spinal fusion (North, 2012; Pescavage-Thomas, 2014). Goals are to restore and maintain disk space height, to decompress the spinal canal and neural foramina, to maintain normal lordosis and increase the stability of the involved segment. New instrumentation has also been designed to reduce bony non-union and hardware slippage or breakage.
Anterior and posterior cervical fusion plates are
used in cervical spine surgery for trauma, tumor,
degenerative disease, and inflammatory conditions (Bohler, 1980; Gore, 1986; Crockard, 1990; Larson, 1975; de Oliveira, 1987; Cooper, 1988; Churney, 1991; Lesoin, 1985; Stauffer, 1988; Tippets, 1988). In most instances, anterior cervical fusion
plates are used in conjunction with supporting
bone grafts (plugs) and dowels as interbody disk
spacers, so-called anterior cervical (spine) diskectomy and fusion (ACDF) (figure: anterior cervical fusion plate; figure: anterior cervical disk fusion at C5-6).
The best-known anterior cervical
fusion plates are the Caspar plate (Tippets, 1988) and the Orion plate. The term Caspar plate is sometimes
used generically to describe any type of anterior cervical fusion plate.
There are also zero-profile ACDF systems using a low profile spacer anchored by fixation screws into vertebral bodies above and below the diskectomy level (figure: zero-profile ACDF; figure: DePuy Synthes Zero low profile ACDF). They may produce a lower amount of postoperative dysphagia compared to the more traditional ACDF plates. In general, zero-profile ACDF systems are indicated for single level use only in the anterior cervical spine from C2 to T1. They may have tantalum radiopaque markers to verify placement (figure: Prevail cervical interbody device).
Anterior cervical plates are
designed to span two or three vertebral bodies. They are anchored to the underlying vertebral
bodies with screws, which should enter the anterior
cortex of each vertebral body and be seated in
the posterior cortex without impinging on the
cord. Ideally, the screws should not enter an adjacent
end plate and should be at least 2 mm from
the superior and inferior end plates.
Posterior cervical plates are less common than
anterior cervical fusion plates but are used commonly
in trauma patients (figure: occipital strut and posterior cervical plates). If there is posterior
compression of the thecal sac and resection of
posterior elements is required, posterior stabilization
with a plate and screw provides an excellent
means of achieving spinal stability.
plates limit both extension and flexion, and they
are usually attached to the underlying vertebrae
by screw fixation to the articular masses. Currently,
two major types of plates or rod systems
are used posteriorly in the cervical spine: those
that are attached through screws placed in the
pedicles of the cervical vertebrae and those that
are attached through screws placed in the lateral
mass of each cervical vertebra.
In the C2 vertebra
the pedicles are used for screw placement; in
the C3 to C6 vertebrae, lateral mass screws are preferred. The smaller pedicle diameter at these levels is associated with an increased potential risk of pedicle perforation and potential for vertebral artery injury. On imaging lateral mass screws have an upward orientation on lateral radiographs and outward orientation on AP images without extension into the pedicle. Pedicle screws extend more anterior to the lateral mass and into the pedicle with a more horizontal orientation on lateral imaging and inward orientation on AP images (figure: lateral mass and pedicle screws).
The C7 and T1 vertebrae are
most suited for placement of pedicle screws (figure: odontoid screw (nail) fixation and posterior cervical plates; figure: anterior and posterior cervical spine fusion; figure: posterior cervical spine fusion from occiput with halo brace). Sometimes more exotic posterior cervical spine fixation may be seen, such as a posterior fixation clamp (figure: cervical spine clamp).
Anterior and posterior cervical fusion plates
often consist of Vitallium, an alloy of cobalt, chromium, and
molybdenum. Vitallium is less corrosive
than stainless steel. It is more compatible with
MR imaging than stainless steel, although it will
still produce a marked magnetic susceptibility
artifact. Cervical fusion plates are not a contraindication
to MR imaging, but they can produce
troublesome artifacts limiting the usefulness of an
examination (Stradiotti, 2009).
Back to Top
Posterior Cervical Spine Wiring
Use of posterior cervical spine wiring is now less
common than fixation with anterior cervical fusion
plates (Johnson, 1981). Posterior cervical spine wiring is
very good for limiting flexion of the spine, and it
is less complicated than anterior cervical fusion
and plating (figure: posterior cervical spine wiring; figure: odontoid fracture fixation and sublaminar wiring; figure: posterior cervical wire figure of 8). It is poor for preventing spinal
rotation and for treating patients who have
anterior compression on the thecal sac. Twenty-gage stainless steel wire is used most frequently
for posterior cervical spine wiring. There are
many variations in the wiring technique. These
include wires under the lamina, over the lamina,
and through holes drilled in the facets or spinous
processes. In addition, intralaminar or spinous
bone grafts are sometimes placed to supplement
Odontoid Fracture Fixation Devices; Atlantoaxial Subluxation; Traumatic C1-2 Fracture Dislocation
Type 1 odontoid fractures occur at the tip of the odontoid and are stable and heal with conservative treatment. Type 3 odontoid fractures involve the vertebral body of C2 below the level of the odontoid (figure: type III odontoid fracture). They are usually stable and heal adequately. Type 2 odontoid fractures run transversely at the base of the odontoid. They are considered to be unstable and sometimes do not heal adequately with simple external fixation (a halo vest). For these types of fractures, internal fixation may be performed, especially when reduction of the odontoid is needed.
Reduction is generally required if the odontoid fracture fragment is displaced more than 4 mm anteriorly on the body of C2. Posterior cervical fixation wires are commonly used for treating type 2 odontoid fractures (figure: odontoid fracture fixation and sublaminar wiring). They usually achieve satisfactory odontoid fusion, but they may limit neck rotation. Because of this, odontoid fracture fixation may use an odontoid compression screw or nail (Esses, 1991) running caudal to cephalad through the body of C2, the odontoid fracture line, and into the body of the odontoid (figure: odontoid fracture fixation and sublaminar wiring; figure: odontoid screw). On rare occasions, two odontoid fixation screws may be used (figure: two odontoid fixation screws).
Chronic atlantoaxial subluxation, most commonly seen in patients with advanced rheumatoid arthritis, ankylosing spondylitis, or Downs syndrome, is difficult to treat. There is often poor bone stock, and many patients are not good operative risks due to their chronic disease and the possibility of grievous neurological injury during intubation and general anesthesia. Sometimes, the subluxation is treated with close observation and conservative therapy with neck braces as well as aggressive treatment of the underlying disease. At other times surgical intervention is applied, particularly if there are neurological symptoms. This usually involves posterior stabilization of the upper cervical spine with rods and screws that extend from the upper cervical spine to the occiput (figure: occiput strut). Or, there is C1-2 posterior transarticular fixation with screws and connecting rods frequently with C1 lateral mass and C2 pedicle screws sometimes with associated posterior bone grafting (Xie, 2009).
Traumatic bilateral atlantoaxial dislocations are very rare and less common than hangman fractures at C2-3. There may be a traumatic C2 spondylolisthesis along with a C1-2 rotary subluxation. Patients are initially treated with in a halo crown and vest and then followed by posterior surgical C1, C2, and C3 reduction and fixation (Chaudhary, 2015).
Intervertebral Disk Cages; Interbody Fusion Cages; Spine Cages; Titanium Interbody Spacers
The term spine cage or disk cage (spacer) applies to a variety of spinal devices found most commonly in the lumbar spine but also found in the cervical and thoracic spine. The original cages were typically hollow, cylindrical implants placed into a disk space to restore disk height and allow bone growth. The most frequent cages were porous and composed of stainless steel or titanium with autologous bone chips placed inside the cage (figure: lumbar spine disk spacers). It should be noted spine cages (disk cages) can be stand alone or combined with spinal fusion plates and rods. It should also be noted there is considerable overlap in terms and inconsistency in their usage.
In the cervical spine, a bone plug from autograft or allograft material, such as a small cylindrical piece of rib or fibula, may be placed in a disk space and combined with an anterior cervical fusion plate (figure: anterior cervical fusion; figure: anterior and posterior cervical fusion with bone struts).
Vertebral disk cages are nowadays also composed of polyether ether ketone (PEEK) mixed with autologous bone graft (Petscavage-Thomas, 2014) (figure: cervical disk cages; figure: anterior cervical disk fusion at C5-6; figure: anterior cervical fusion C5-7 and PEEK cages; figure: cervical spine PEEK disk cages) or poly (L-lactide-co-D, L-lactide) (PLDLLA), a radiolucent resorbable biopolymer. PEEK has an elasticity closely resembling that of cortical bone, possibly having more load sharing and better stress distribution with more strength than many metals. It does not cause significant artifact on MRI or CT.
Spine cages are most commonly used to maintain foraminal height and spinal decompression whether stand alone or combined with other fixation apparatus (figure: cervical spine intervertebral disk fusion cage; figure: cervical spine artificial disk; figure: cervical spine total disk replacement; figure: Brantigan interbody vertebral cage). The zero-profile systems described above are a form of disk cage with fixation screws for insertion into vertebral bodies above and below the cage (figure: DePuy Synthes Zero low profile ACDF).
Vertebral disk cages like all other spinal fixation apparatus have a potential for complications including non-union, cage migration, infection, foraminal compression, nerve root compression, cord compression, and not uncommonly adjacent segment degeneration from loss of cervical or lumbar lordosis and shift of spine movement to levels where there is no spine fusion (figures: disk cage and ACDF complications). The disk cages may also cause considerable artifacts on MRI and CT imaging (figure: cervical spine MRI disk susceptibility artifact; figure: metal-on-polyethylene cervical disk).
Malignant disease, severe infectious disease, or major trauma to the spine can destroy one or more vertebral bodies. This destruction may be treated with vertebral body resection (corpectomy) and bone grafting combined with placement of a vertebral body “cage” (figure: Harms vertebral cage). The cage may be freestanding or associated with a lateral, anterior, or posterior fixator to give the reconstructed area more strength (figure: cervical corpectomy at C4-5 from diskitis).
These cages are generically called titanium interbody spacers. They are usually manufactured from titanium, which has good strength and good biocompatibility and also produces fewer artifacts on MR images compared with other metals such as stainless steel. These types of vertebral cages have a hollow, threaded, cylindrical structure with teeth on both sides for fixation to vertebral end plates superiorly and inferiorly. The hollow center is usually filled with autograft or allograft bone material to strengthen the fixation and provide later fusion. Many designs exist for the interbody fusion devices, and the designs are evolving. For example, there are newer expandable corpectomy devices for treatment of severe compression fractures or vertebrae collapse due to tumor. The expansion cage along with posterior stabilization screws and rods stabilize the spine to protect the spinal cord. They also restore a degree of vertebral height loss (figure: expandable corpectomy device; figure: expandable corpectomy cage with shift).
Some of the more popular interbody fusion devices (spacers) used in the thoracic or lumbar spine are the Bagby and Kuslich cage, the Ray threaded fusion Cage, the Harms cage, and the Brantigan cage (figure: Harms vertebral cage; figure: Brantigan interbody vertebral cage; figure: cervical spine fusion cage). The Brantigan cage is designed
to be placed through a posterior approach or posterior lumbar interbody fusion. Brantigan cages
are composed of high-density carbon fiber, and two cages are placed side by side in a disk space.
They are contiguous with bone graft material that is interposed between the vertebral end plates and the cages. These interbody cages are purposely designed to be radiolucent so that the interface between the bone graft and the vertebral end plate is well visualized and not hidden by the supporting cage. The cages are identified on radiographs by a small metallic marker within each cage.
Back to Top
Back to Top