DOI: 10.1148/rg.241035185
(Radiographics. 2004;24:257-285.)
© RSNA, 2004
Medical Devices of the Head, Neck, and Spine1
Tim B. Hunter, MD,
Mark T. Yoshino, MD,
Robert B. Dzioba, MD,
Rick A. Light, DDS and
William G. Berger, MD
1 From the Departments of Radiology (T.B.H., W.G.B.) and Orthopaedic Surgery (R.B.D.), University of Arizona College of Medicine, 1501 N Campbell Ave, PO Box 245067, Tucson, AZ 85724-5067; Arizona State Radiology, Tucson, Ariz (M.T.Y.); and Rick Light, DDS, Tucson, Ariz (R.A.L.). Received August 22, 2003; revision requested September 2 and received October 13; accepted October 13. Address correspondence to T.B.H. (e-mail: tbh@3towers.com).
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Abstract
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There are many medical devices used for head, neck, and spinal diseases and injuries, and new devices are constantly being introduced. Many of the newest devices are variations on a previous theme. Knowing the specific name of a device is not important. It is important to recognize the presence of a device and to have an understanding of its function as well as to be able to recognize the complications associated with its use. The article discusses the most common and important devices of the head, neck, and spine, including cerebrospinal fluid shunts and the Codman Hakim programmable valve; subdural drainage catheters, subdural electrodes, intracranial electrodes, deep brain stimulators, and cerebellar electrodes; coils, balloons, adhesives, particles, and aneurysm clips; radiation therapy catheters, intracranial balloons for drug installation, and carmustine wafers; hearing aids, cochlear implants, and ossicular reconstruction prostheses; orbital prostheses, intraocular silicone oil, and lacrimal duct stents; anterior and posterior cervical plates, posterior cervical spine wiring, odontoid fracture fixation devices, cervical collars and halo vests; thoracic and lumbar spine implants, anterior and posterior instrumentation for the thoracic and lumbar spine, vertebroplasty, and artificial disks; spinal column stimulators, bone stimulators, intrathecal drug delivery pumps, and sacral stimulators; dental and facial implant devices; gastric and tracheal tubes; vagus nerve stimulators; lumboperitoneal shunts; and temperature- and oxygen-sensing probes.
© RSNA, 2004
Index Terms: Bones, grafts, 30.46 • Catheters and catheterization, 20.46, 30.46 • Feeding tubes, 30.46 • Radiations, protective and therapeutic agents and devices, 20.46, 30.46 • Spine, fixation devices, 30.46 • Stents and prostheses, 20.46, 30.46
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Introduction
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Many medical devices are used in the treatment of head, neck, and spinal diseases and injuries, and new devices are constantly being introduced. Most devices found in everyday practice, however, have been used for a number of years. Many of the newest devices are variations on a previous theme. Knowing the specific name of a device is not important; in fact, it is sometimes impossible, because a slight modification of a device design usually leads to a new name. It is important, however, to recognize the presence or absence of a device (if the patient’s clinical history suggests one is present), to understand its function, and to identify the complications associated with its use. The referring physician knows what device he or she has installed, but he or she relies on the radiologist to determine the postoperative location of the device and its integrity. The article discusses the most common and important devices of the head, neck, and spine.
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Brain and Skull
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Cerebrospinal Fluid Shunts and the Codman Hakim Programmable Valve
Cerebrospinal fluid (CSF) drainage procedures to relieve elevated CSF pressure have been performed for at least 100 years. The commonly used drainage procedures drain excess fluid either to the peritoneum or, sometimes, to the right atrium. The present shunts are used to drain CSF past areas of intracerebral obstruction and to decrease CSF pressure (1–3). Each works through the use of associated pressure valves and antisiphon devices. In practice, any location within the lateral ventricle is an acceptable position for the shunt, although the ideal location is near the foramen of Monro. The most common shunts are ventriculoperitoneal. On plain radiographs, they are not well visualized, being composed mainly of relatively radiolucent materials, although they may have metallic tips and connectors (Fig 1). Adequate visualization may require fluoroscopically guided oblique radiographs obtained with a soft-tissue technique.
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Figure 1. Standard CSF shunt. Lateral radiograph of the skull (see also Fig 11) shows a typical CSF shunt (arrows). There are also four aneurysm clips (black arrowheads) near the circle of Willis, scalp (skin) staples (white arrowheads), craniotomy fixation plugs (*), and dental fillings.
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An important relatively new shunt is the programmable valve, which provides a noninvasive way to control CSF drainage. The Codman Hakim programmable valve is the prototype for these valves (4) (Fig 2). One of the main challenges in ventriculoperitoneal shunt drainage is to control the CSF pressure by draining the proper amount of fluid and preventing over- and underdrainage in patients with hydrocephalus. The valve’s pressure settings can be changed or "programmed" by placing an external programming device near the patient’s skull. Precise positioning of the valve is important, because it is required for correct programming. The valve position should be assessed by noting the position of the valve’s right-hand-side indicator on properly obtained radiographs of the patient’s skull. If the valve is correctly positioned, its small right-hand-side indicator will be on the right-hand side of the larger, central marker of the valve. Fluoroscopy is particularly useful for confirming that the valve is examined en face (ie, with the observer looking down the barrel) and is thus accurately read.
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Figure 2a. Programmable shunt. (a) Lateral radiograph of the skull shows a Codman-Hakim programmable shunt (arrows). (b) Close-up view of the shunt shows its programmable valve (arrow).
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Figure 2b. Programmable shunt. (a) Lateral radiograph of the skull shows a Codman-Hakim programmable shunt (arrows). (b) Close-up view of the shunt shows its programmable valve (arrow).
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Ventriculoperitoneal and other types of CSF drainage shunts are subject to multiple complications, including improper shunt placement, shunt blockage or obstruction, infection in the central nervous system or in the peritoneal cavity, and subdural or intraparenchymal hematoma formation. One serious complication described with the Codman Hakim valve is the resetting of its opening pressure if the valve is placed in a strong magnetic field. Because there is a considerable risk that MR imaging can reset the opening pressure of the valve, it is mandatory to obtain a properly oriented radiographic study of a patient after MR imaging to determine whether the valve positioning and pressure settings are correct.
Subdural Drainage Catheters, Intracranial Electrodes, Subdural Electrodes, Deep Brain Stimulators, and Cerebellar Electrodes
Most acute symptomatic subdural hematomas are evacuated via craniotomy, because most of them are gelatinous clots that are difficult to extract via burr holes. Chronic subdural hematomas may be liquified and can be more easily drained by means of burr holes and subdural lavage. The use of subdural drainage catheters placed through burr holes is not common (5–8). When they are used, they are placed to diminish hematoma reaccumulation and allow the brain to reexpand. These catheters are typically straight tipped and use gravity drainage.
Intracranial (recording) electrodes placed in the brain tissue or placed in an extraaxial location are not common (Fig 3). One type of intracranial electrode is the intracranial pressure monitor (Fig 4). Intracranial pressure monitors are used to measure the pressure surrounding the brain in patients who have sustained head trauma, brain hemorrhage, or conditions in which the brain may swell. If pressure surrounding the brain becomes too high, it can decrease the blood flow to the brain and lead to brain damage.
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Figure 3. Cross-table postoperative lateral radiograph of the skull shows recording electrodes (arrows) that lie on the brain surface bilaterally. Scalp (skin) staples and postoperative air can also be seen anteriorly under the frontal bone.
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Intracranial electrodes may be used for recording purposes only, or they may be used to treat intractable partial epilepsy or other disorders (9). Subdural extraaxial recording electrodes are the most common type of intracranial electrode (10) (Fig 5). They can be recognized as strips of four to six electrodes or a matrix of electrodes embedded in a Silastic sheet. Depth electrodes are placed directly in the brain tissue, and they consist of 0.5–3.0-mm wires usually made from polyethylene, stainless steel, platinum, or platinum-iridium alloys. They may be placed bilaterally at up to eight sites on each side.
Deep brain stimulators are similar in appearance to intracranial recording electrodes but serve a different purpose (11) (Fig 6). These devices have one or more insulated electrodes that are implanted into the thalamus for the treatment of essential tremor and symptoms of Parkinson disease. Small electrical impulses block the brain signals that evoke the tremors. The stimulation battery pack is implanted beneath the skin in the upper chest. A stimulating electrode on each side can control tremors on both sides of the body.
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Figure 6a. (a) Lateral radiograph of the skull shows bilateral deep brain stimulators implanted to treat Parkinson disease. (b, c) CT (b) and MR (c) images of the same patient show the stimulators in the thalamus on each side.
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Figure 6b. (a) Lateral radiograph of the skull shows bilateral deep brain stimulators implanted to treat Parkinson disease. (b, c) CT (b) and MR (c) images of the same patient show the stimulators in the thalamus on each side.
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Figure 6c. (a) Lateral radiograph of the skull shows bilateral deep brain stimulators implanted to treat Parkinson disease. (b, c) CT (b) and MR (c) images of the same patient show the stimulators in the thalamus on each side.
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In selected patients, cerebellar stimulation may be used to diminish spasticity associated with cerebral palsy. The cerebellum is stimulated by placing an electrode array on the cerebellar surface, commonly a strip of eight electrodes over the cerebellar hemispheres (Fig 7).
Occasionally, metal plates may be placed in the skull to close a large cranial defect (Fig 8). They are probably most common in elderly patients with remote war injuries. Cranial plates may also be used for stabilization of the skull after surgery for craniosynostosis (Fig 9).
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Figure 8. Cranial plate in a 77-year-old man who suffered extensive head wounds from a mortar in the Philippines in 1944. Scout CT image of the cranium shows a metallic cranial plate that was placed emergently at a battlefield hospital. It has given the patient no trouble since the Second World War.
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Coils, Balloons, Adhesives, and Particles
In selected medical centers where there are facilities and personnel available for high-risk interventional neuroradiologic procedures, patients with arteriovenous malformations are frequently treated by means of embolic occlusion of the vessels that feed and drain the malformations (12–15). Stainless steel and platinum coils are often used for this type of therapy (Fig 10a, 10b). Platinum coils, although very expensive, have the advantage of being able to pass through microcatheters, in addition to being biocompatible, MR compatible, and thrombogenic.
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Figure 10a. Intracranial embolic materials. (a) Frontal radiograph of the skull shows a platinum coil (arrow) in a small, distal middle cerebral artery branch. It does not look circular, because it has conformed to the shape of the artery. (b) Massive hemorrhage not controlled by surgery in a 61-year-old man who had undergone radiation therapy for unresectable squamous cancer of the neck. Frontal view from angiography shows that the right external carotid artery has been occluded by multiple platinum coils. (c) Lateral view from angiography shows cyanoacrylate glue (arrow) that occludes an arterial venous malformation. (Courtesy of Gary Duckwiler, Los Angeles, Calif.) (d) On a CT scan, polyvinyl alcohol is difficult to see. If the patient undergoes imaging immediately after embolization, as in this case, the clot produced by polyvinyl alcohol may be seen as a linear area of increased attenuation (arrow).
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Figure 10b. Intracranial embolic materials. (a) Frontal radiograph of the skull shows a platinum coil (arrow) in a small, distal middle cerebral artery branch. It does not look circular, because it has conformed to the shape of the artery. (b) Massive hemorrhage not controlled by surgery in a 61-year-old man who had undergone radiation therapy for unresectable squamous cancer of the neck. Frontal view from angiography shows that the right external carotid artery has been occluded by multiple platinum coils. (c) Lateral view from angiography shows cyanoacrylate glue (arrow) that occludes an arterial venous malformation. (Courtesy of Gary Duckwiler, Los Angeles, Calif.) (d) On a CT scan, polyvinyl alcohol is difficult to see. If the patient undergoes imaging immediately after embolization, as in this case, the clot produced by polyvinyl alcohol may be seen as a linear area of increased attenuation (arrow).
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Figure 10c. Intracranial embolic materials. (a) Frontal radiograph of the skull shows a platinum coil (arrow) in a small, distal middle cerebral artery branch. It does not look circular, because it has conformed to the shape of the artery. (b) Massive hemorrhage not controlled by surgery in a 61-year-old man who had undergone radiation therapy for unresectable squamous cancer of the neck. Frontal view from angiography shows that the right external carotid artery has been occluded by multiple platinum coils. (c) Lateral view from angiography shows cyanoacrylate glue (arrow) that occludes an arterial venous malformation. (Courtesy of Gary Duckwiler, Los Angeles, Calif.) (d) On a CT scan, polyvinyl alcohol is difficult to see. If the patient undergoes imaging immediately after embolization, as in this case, the clot produced by polyvinyl alcohol may be seen as a linear area of increased attenuation (arrow).
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Figure 10d. Intracranial embolic materials. (a) Frontal radiograph of the skull shows a platinum coil (arrow) in a small, distal middle cerebral artery branch. It does not look circular, because it has conformed to the shape of the artery. (b) Massive hemorrhage not controlled by surgery in a 61-year-old man who had undergone radiation therapy for unresectable squamous cancer of the neck. Frontal view from angiography shows that the right external carotid artery has been occluded by multiple platinum coils. (c) Lateral view from angiography shows cyanoacrylate glue (arrow) that occludes an arterial venous malformation. (Courtesy of Gary Duckwiler, Los Angeles, Calif.) (d) On a CT scan, polyvinyl alcohol is difficult to see. If the patient undergoes imaging immediately after embolization, as in this case, the clot produced by polyvinyl alcohol may be seen as a linear area of increased attenuation (arrow).
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In a similar fashion, percutaneously placed detachable balloons can also be used to occlude arteries and treat a variety of fistulas, such as those between the internal carotid artery and the cavernous sinus. Balloons are well suited for permanent vascular occlusion, because while the balloon is in place and still attached to the introducer device, its effect on the patient can be judged if the patient is sedated but able to be aroused. Thus, if there are acute ill effects from the balloon placement, the balloon can be rapidly deflated and repositioned. Balloons are usually made of silicone and can be filled with contrast material to increase their radiologic visibility.
Liquid adhesive (butyl cyanoacrylate), or glue, is also used to treat arteriovenous malformations, high-flow fistulas, and sometimes tumors (Fig 10c). The butyl cyanoacrylate is mixed with contrast material and tantalum powder to give it maximal contrast. This mixture polymerizes quickly on contact with the blood, but it can pass far distally into small vessels, which it then permanently occludes, forming a "cast" of the vascular abnormality. Unfortunately, all of the glue deposited in the vascular spaces may not be visible on radiographs, because the mixture has a tendency to settle unequally with uneven distribution of the tantalum powder.
Like the above-mentioned substances, various particulate agents, such as gelatin powder (Gelfoam; Upjohn, Kalamazoo, Mich) and polyvinyl alcohol, can be used as embolic agents to treat a variety of intracranial lesions. Particulate agents are minimally radiopaque and difficult to see at radiography or computed tomography (CT). Patients often receive particulate emboli before a definitive surgical procedure. The thrombus caused by the particles may be seen on CT scans, however, where it has high attenuation, which is the typical appearance of any other intravascular thrombus (Fig 10d).
Aneurysm Clips
Cerebral aneurysm clips are used to clamp off the neck or base of an aneurysm, depriving it of its blood supply and thereby preventing or stabilizing a serious intracranial hemorrhage (16) (Fig 11). Many aneurysm clips are applied to the side of the vessel facing the surgeon so as to require as little brain manipulation as possible. To enable the clip to be applied to the side of the vessel, these clips are designed as curved or straight, parallel flattened wires fastened together by a hinge mechanism. Other aneurysm clips are round or oval because they are designed to encircle the parent vessel. This design allows the surgeon to clip aneurysms on the side of the artery opposite that directly visible to the surgeon.
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Figure 11. Aneurysm clips (same patient as in Fig 1). Frontal radiograph of the skull shows four aneurysm clips (black arrowheads) near the circle of Willis. There are also a right-sided CSF shunt (black arrows), scalp (skin) staples (white arrowheads), left craniotomy fixation plugs (*), and dental fillings.
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Most cerebral aneurysm clips used to be made from stainless steel, steel alloys, or tungsten. These metals are reasonably biocompatible and are radiodense so that the clips are readily visible on radiographs. Unfortunately, clips of this type are ferromagnetic and prone to torquing in strong magnetic fields, such as those produced by MR imaging. The presence of one of these clips is an absolute contraindication for any type of MR imaging study. Currently, many cerebral aneurysm clips are made from nonferromagnetic materials, such as tantalum, but these clips are indistinguishable by appearance from ferromagnetic clips. Thus, the presence of such a clip(s) is not necessarily a contraindication for an MR imaging study, but the radiologist and referring physician must be absolutely certain that the clip has no ferromagnetic properties before any MR imaging is performed. If this fact cannot be ascertained, all MR imaging is contraindicated.
Radiation Therapy Catheters, Intracranial Balloons for Drug Installation, and Carmustine Wafers
Malignant primary brain tumors and metastatic disease to the brain remain difficult challenges. Over one-half of patients with intracranial metastatic disease die of these metastases. Surgery, chemotherapy, and radiation therapy are the main treatment options for brain malignancy. Chemotherapy, administered orally or intravenously, has a limited effect against brain malignancy, because most chemotherapeutic drugs do not cross the blood-brain barrier in sufficient concentration to be effective. Radiation therapy is the primary nonsurgical treatment for brain tumors. Although external beam whole-brain radiation is potentially effective in controlling cerebral malignancy, it is severely limited by its toxic effect on adjacent normal brain tissue.
Total whole-brain radiation doses are limited to approximately 5,500 rad. To supplement whole-brain radiation, lower "boost" doses are administered by means of finely collimated external beams or implanted radioactive seeds (brachytherapy) (Fig 12). Typically, the seeds consist of iodine-125. I-125 has the advantage of ready availability and low-energy gamma radiation with limited tissue penetration. This limits the volume of normal brain tissue subjected to irradiation and diminishes the radiation exposure to other persons. The radioactive seed may be placed in the brain temporarily or permanently. Temporary seeds are left in place 5–8 days; they are introduced and removed through catheters, which are percutaneously placed in the lesion with CT guidance. The catheters are made of Teflon or plastic, and on radiographic or CT examination, they appear as tubular structures extending from a burr hole into tumor. If the iodine seeds are present in the catheters, the tip of each catheter will appear very dense.
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Figure 12. CT scan shows intracranial brachytherapy catheters (arrow). They have a low radiodensity because they have not yet been loaded with radioactive seeds and are filled with air.
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Balloons may also be surgically implanted into the brain tissue for the later introduction of radioactive material or chemotherapeutic agents (17) (Fig 13). The balloons have a thin channel, which rests under the scalp and acts as a subcutaneous port through which medication can be introduced into or removed from the balloon. Polymer wafers impregnated with carmustine (BCNU) are also used for treatment of recurrent glioblastoma multiforme and anaplastic astrocytoma (18). The wafers are designed so that the chemotherapeutic agent (carmustine) is slowly released into the tumor surgical cavity over a 2–3-week period. The wafers are implanted in the tumor cavity after surgery to treat tumor recurrence. At CT, the wafers have a linear low attenuation. There is also transient gas in the surgical bed for up to 3 weeks after initial placement of the wafers. The wafers have little contrast enhancement, and contrast enhancement in the tumor bed should be considered recurrent tumor or tumor necrosis.
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Figure 13a. Intracranial balloon system in a 41-year-old woman treated for glioblastoma multiforme. Lateral radiograph of the skull (a) and CT scan (b) of the cranium show an intracranial balloon system (GliaSite balloon; Proxima Therapeutics, Alpharetta, Ga) for the installation of radioactive iodine. In a, note the scalp (skin) staples and the cranial bone staples from recent intracranial surgery. In b, note the polymer wafer (arrow) impregnated with carmustine (BCNU), which shows up only as a thin white line because of its small size (1.45-cm diameter, 1-mm thickness).
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Figure 13b. Intracranial balloon system in a 41-year-old woman treated for glioblastoma multiforme. Lateral radiograph of the skull (a) and CT scan (b) of the cranium show an intracranial balloon system (GliaSite balloon; Proxima Therapeutics, Alpharetta, Ga) for the installation of radioactive iodine. In a, note the scalp (skin) staples and the cranial bone staples from recent intracranial surgery. In b, note the polymer wafer (arrow) impregnated with carmustine (BCNU), which shows up only as a thin white line because of its small size (1.45-cm diameter, 1-mm thickness).
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Hearing Aids, Cochlear Implants, and Ossicular Reconstruction Prostheses
Hearing aids are very common, and as the population ages, they will be even more common (19). They are often seen on skull, mandible, and cervical spine studies as incidental devices (Fig 14). If they are not recognized as hearing aids, they may be confused with an intracranial device, and sometimes earrings and jewelry may be confused with hearing aids or other important medical devices. Hearing aids usually consist of a behind-the-ear electronic battery-powered unit and an ear mold that is fitted into the external auditory canal. The electronic unit consists of a microphone, one or more small batteries, a tiny speaker, and controls for on/off and volume. The ear mold is usually made of acrylic material. Hearing aids only amplify sounds and often deliver it to a damaged sensory system. The listener receives a louder sound, but it still may not be interpretable.
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Figure 14a. Hearing aids. (a) Lateral radiograph of the cervical spine shows a hearing aid with a behind-the-ear compartment containing a battery (arrow), microphone, and amplifier. The ear mold has been removed. (b) Oblique radiograph of the skull shows a right cochlear implant. Receiver/stimulator (arrows) may look like an external hearing aid at radiography. (c) Cranial CT scan of the same patient as in b shows the cochlear implant. The receiver/stimulator (large arrow) lies in a groove cut into the mastoid portion of the temporal bone. The wire leaving the receiver/stimulator (small arrow) passes into the cochlea and forms the electrodes. The metallic portion of the cochlear apparatus is not seen because it lies in a different plane. (Courtesy of Steve Bessen, MD, Las Vegas, Nev.)
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Figure 14b. Hearing aids. (a) Lateral radiograph of the cervical spine shows a hearing aid with a behind-the-ear compartment containing a battery (arrow), microphone, and amplifier. The ear mold has been removed. (b) Oblique radiograph of the skull shows a right cochlear implant. Receiver/stimulator (arrows) may look like an external hearing aid at radiography. (c) Cranial CT scan of the same patient as in b shows the cochlear implant. The receiver/stimulator (large arrow) lies in a groove cut into the mastoid portion of the temporal bone. The wire leaving the receiver/stimulator (small arrow) passes into the cochlea and forms the electrodes. The metallic portion of the cochlear apparatus is not seen because it lies in a different plane. (Courtesy of Steve Bessen, MD, Las Vegas, Nev.)
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Figure 14c. Hearing aids. (a) Lateral radiograph of the cervical spine shows a hearing aid with a behind-the-ear compartment containing a battery (arrow), microphone, and amplifier. The ear mold has been removed. (b) Oblique radiograph of the skull shows a right cochlear implant. Receiver/stimulator (arrows) may look like an external hearing aid at radiography. (c) Cranial CT scan of the same patient as in b shows the cochlear implant. The receiver/stimulator (large arrow) lies in a groove cut into the mastoid portion of the temporal bone. The wire leaving the receiver/stimulator (small arrow) passes into the cochlea and forms the electrodes. The metallic portion of the cochlear apparatus is not seen because it lies in a different plane. (Courtesy of Steve Bessen, MD, Las Vegas, Nev.)
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Cochlear implants, however, have the potential to provide improved speech understanding (20–23). They bypass the tympanic membrane, the ossicles, and the oval window. Cochlear implants use eight to 24 stimulators to stimulate the cochlea directly. They consist of a microphone-stimulator located on or just under the skin next to the ear. From the microphone, information travels to a microcomputer speech processor, which then sends a signal by radio waves to the internal portion of the cochlear implant. Underneath the microphone-stimulator is an internal receiver-stimulator or implantable cochlear stimulator, which consists of a computer chip, a receiving coil, and stimulating electrodes placed in Silastic housing. The signal from the external portion of the implant is received by the coil and decoded by the internal microcomputer chip. The decoded signals are sent to the cochlea via the stimulating electrodes. As a general rule, cochlear implants should be considered a contraindication for MR imaging. Newer designs may permit an MR imaging study, but the radiologist, the patient, and the referring physician must be certain the patient’s implant is specifically compatible with MR imaging.
Patients with middle-ear injuries or destruction from chronic otitis media, cholesteatoma formation, congenital malformations, or unusual tumors may be candidates for an ossicular prosthesis (24). Functional middle-ear structures may be constructed from autograft material, homograft material from cadavers, or synthetic middle-ear bone prostheses. Synthetic prostheses have become more popular. Autografts are difficult to construct, and homografts are difficult to obtain. The prostheses used for ossicular reconstruction are best evaluated with thin-section CT.
Orbital Prostheses and Intraocular Silicone Oil
Custom-made orbital prostheses are used to fill in the orbital socket in patients who have undergone enucleation. A mold of the orbital cavity is made, and then a silicone implant is fabricated to fit into the socket with minimal discomfort. For a maximal cosmetic result, the implant is tinted to match the patient’s skin. Next, an ocular prosthesis is placed into the orbital prosthesis or directly into a remnant of the globe in the case of evisceration (Fig 15a, 15b). Ocular prostheses are made of either glass or acrylic resin. Ocular prostheses are custom made for the individual patient to match the patient’s iris. Sometimes a plastic scleral shell may be used to treat patients with phthisis bulbi or microphthalmia. Most orbital, ocular, and scleral prostheses are custom made, and they vary in radiologic appearance. Usually they are easily recognized, because the associated surgical defects are obvious, as is the asymmetry when the appearance is compared with that of the opposite side.
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Figure 15a. Orbital prostheses and silicone oil. (a) Radiograph shows an hydroxyapatite orbital prosthesis. The prosthesis is designed for the insertion of a glass eye, and the intraocular muscles can be attached to provide proper motion of the prosthetic system in concert with the motion of the other eye. (b) CT scan shows a right ocular prosthesis. (c) CT scan shows silicone oil installed in the left eye for treatment of retinal detachment.
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Figure 15b. Orbital prostheses and silicone oil. (a) Radiograph shows an hydroxyapatite orbital prosthesis. The prosthesis is designed for the insertion of a glass eye, and the intraocular muscles can be attached to provide proper motion of the prosthetic system in concert with the motion of the other eye. (b) CT scan shows a right ocular prosthesis. (c) CT scan shows silicone oil installed in the left eye for treatment of retinal detachment.
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Figure 15c. Orbital prostheses and silicone oil. (a) Radiograph shows an hydroxyapatite orbital prosthesis. The prosthesis is designed for the insertion of a glass eye, and the intraocular muscles can be attached to provide proper motion of the prosthetic system in concert with the motion of the other eye. (b) CT scan shows a right ocular prosthesis. (c) CT scan shows silicone oil installed in the left eye for treatment of retinal detachment.
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Vitrectomy and silicone-oil tamponade may be used to treat retinal detachment or to prevent recurrent detachment in patients with acquired immunodeficiency syndrome and retinitis (25). Silicone is relatively lucent at radiography but will appear hyperattenuating relative to the extraocular muscles at CT (Fig 15c). It is less dense than the orbital bones and has an attenuation similar to that of the lens on CT scans and appears as a high-attenuation mass in the posterior segment of the globe in an appropriately treated patient.
Lacrimal Duct Stents
Epiphora is the abnormal outflow of tears onto the cheek, usually caused by blockage or stricture of the lacrimal passage. Surgery has been the conventional treatment for severe cases of epiphora, but placement of polyurethane and nitinol stents is a nonoperative way to treat this condition (26). Balloon dilation of the lacrimal system is another nonoperative treatment method. The stents can be removed if they become blocked, and they are relatively nontraumatic, but their success rates vary, and the long-term patency rates are not as encouraging as for surgical intervention. Such stents are small and easily overlooked on radiographic and CT studies. They are readily visible if looked for, particularly when an appropriate clinical history is furnished.
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Cervical Spine
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Anterior and Posterior Cervical Plates
Anterior and posterior cervical fusion plates are used in cervical spine surgery for trauma, tumor, or degenerative or inflammatory conditions (27–36). In most instances, anterior cervical fusion plates are used in conjunction with supporting bone grafts (plugs) and dowels as interbody disk spacers (Fig 16). The best-known anterior cervical fusion plates are the Caspar plate (36) and the Orion plate. The term Caspar plate is sometimes used generically to describe any type of anterior cervical fusion plate. Anterior cervical plates are designed to span two or three vertebral bodies, and 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.
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Figure 16a. (a) Anterior cervical fixation plate with prefabricated bone allograft that failed to fuse. Lateral radiograph shows radiolucency around the allograft. The somewhat anterior position of the allograft and plate is typical for these plates, and it is not subluxed from its original placement position. (b) Oblique radiograph of a 27-year-old woman who had a tumor resected from the upper thoracic spine demonstrates Steffee plates and screws that were placed for spinal stabilization. Gold foil (arrows) was inserted to protect the spinal cord when the patient underwent radiation therapy.
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Figure 16b. (a) Anterior cervical fixation plate with prefabricated bone allograft that failed to fuse. Lateral radiograph shows radiolucency around the allograft. The somewhat anterior position of the allograft and plate is typical for these plates, and it is not subluxed from its original placement position. (b) Oblique radiograph of a 27-year-old woman who had a tumor resected from the upper thoracic spine demonstrates Steffee plates and screws that were placed for spinal stabilization. Gold foil (arrows) was inserted to protect the spinal cord when the patient underwent radiation therapy.
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Posterior cervical plates are less common than anterior cervical fusion plates but are used commonly in trauma patients. 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. Posterior 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 C-2 vertebra the pedicles are used for screw placement; in the C-3 to C-6 vertebrae, the lateral mass screws are preferred; and the C-7 and T-1 vertebrae are most suited for placement of pedicle screws.
Anterior and posterior cervical fusion plates often consist of Vitallium (Howmedica Osteonics, Allendale, NJ), an alloy of cobalt, chromium, and molybdenum, because 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.
Posterior Cervical Spine Wiring
Use of posterior cervical spine wiring is now less common than fixation with anterior cervical fusion plates (37). Posterior cervical spine wiring is very good for limiting flexion of the spine, and it is less complicated than anterior cervical fusion and plating (Fig 17). It is poor for preventing spinal rotation and for treating patients who have anterior compression on the thecal sac. Twenty-gauge 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 the wires.
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Figure 17a. Posterior cervical spine wiring. Frontal (a) and lateral (b) radiographs of the cervical spine show posterior cervical wires from C-4 to C-7 after laminectomy. The lateral view was obtained after cervical myelography.
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Figure 17b. Posterior cervical spine wiring. Frontal (a) and lateral (b) radiographs of the cervical spine show posterior cervical wires from C-4 to C-7 after laminectomy. The lateral view was obtained after cervical myelography.
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Odontoid Fracture Fixation Devices
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 C-2 below the level of the odontoid. 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 C-2. Posterior cervical fixation wires are commonly used for treating type 2 odontoid fractures (Fig 18). They usually achieve satisfactory odontoid fusion, but they may limit neck rotation. Because of this, odontoid fracture fixation may use an odontoid compression screw (38) running caudad to cephalad through the body of C-2, the odontoid fracture line, and into the body of the odontoid (Fig 18a).
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Figure 18a. Odontoid fracture fixation. (a) Lateral radiograph of the cervical spine obtained after surgery shows fixation of an odontoid base fracture by an odontoid screw and a sublaminar wire between C-1 and C-2. Note the surgical drain in the posterior aspect of the neck, the gown snaps, and dental fillings. (b, c) Frontal (b) and lateral (c) radiographs of the cervical spine in a patient with rheumatoid arthritis show an occipital-spinal strut with posterior wiring used to stabilize the cervical spine and subluxation of C-1 to C-2. There is vertebral body ankylosis (arrows in c) from the rheumatoid arthritis.
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Figure 18b. Odontoid fracture fixation. (a) Lateral radiograph of the cervical spine obtained after surgery shows fixation of an odontoid base fracture by an odontoid screw and a sublaminar wire between C-1 and C-2. Note the surgical drain in the posterior aspect of the neck, the gown snaps, and dental fillings. (b, c) Frontal (b) and lateral (c) radiographs of the cervical spine in a patient with rheumatoid arthritis show an occipital-spinal strut with posterior wiring used to stabilize the cervical spine and subluxation of C-1 to C-2. There is vertebral body ankylosis (arrows in c) from the rheumatoid arthritis.
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Figure 18c. Odontoid fracture fixation. (a) Lateral radiograph of the cervical spine obtained after surgery shows fixation of an odontoid base fracture by an odontoid screw and a sublaminar wire between C-1 and C-2. Note the surgical drain in the posterior aspect of the neck, the gown snaps, and dental fillings. (b, c) Frontal (b) and lateral (c) radiographs of the cervical spine in a patient with rheumatoid arthritis show an occipital-spinal strut with posterior wiring used to stabilize the cervical spine and subluxation of C-1 to C-2. There is vertebral body ankylosis (arrows in c) from the rheumatoid arthritis.
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Cervical Collars and Halo Vests
There are many kinds of cervical spine immobilization devices, including cervical collars, braces, and halo vests. Cervical collars are ubiquitous and are commonly placed on trauma patients in the emergency department. The most frequent cervical collar design is the Philadelphia collar, which is molded from plastic and has chin and occipital supports. Although it is effective in stabilizing the neck to prevent harmful motion, it is uncomfortable, and patients want to remove it as soon as possible. A more comfortable collar for patients is a soft foam collar covered by cotton. Although this collar is ineffective in controlling neck motion, it is useful as a reminder to the patient that neck motion must be avoided. No neck collar provides adequate long-term neck stabilization for unstable cervical spine fractures.
Unlike cervical collars, cervical braces are designed for the long-term treatment of cervical spine fractures. They consist of chin and occipital supports that are connected to a thoracic vest by metal rods. Cervical braces do provide good prevention against harmful flexion, but they are not as effective in preventing harmful extension.
The halo vest contains a metallic ring (the halo) that is attached to the outer table of the skull by screws (39–41) (Fig 19). The halo is connected to a padded fiberglass or plastic thoracic cast by metal rods (the struts), which hold up the patient’s head. Although halo vests involve placement of screws into the skull and are a major undertaking for both the patient and the physician, they provide the best long-term fixation of the cervical spine. They are especially indicated for unstable fractures and dislocations and work best in the upper cervical spine.
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Figure 19a. Halo vest and brace. (a, b) Frontal (a) and lateral (b) views of a halo vest and brace. Four metallic uprights connect the halo ring around the skull to the vest. (c, d) Frontal (c) and lateral (d) radiographs of the skull show head tongs used to stabilize the head and neck in a patient with a cervical spine fracture. One or more screws penetrate the outer table of the skull on each side. They are connected to each other by horizontal or vertical bars on each side that are attached to an external traction device.
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Figure 19b. Halo vest and brace. (a, b) Frontal (a) and lateral (b) views of a halo vest and brace. Four metallic uprights connect the halo ring around the skull to the vest. (c, d) Frontal (c) and lateral (d) radiographs of the skull show head tongs used to stabilize the head and neck in a patient with a cervical spine fracture. One or more screws penetrate the outer table of the skull on each side. They are connected to each other by horizontal or vertical bars on each side that are attached to an external traction device.
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Figure 19c. Halo vest and brace. (a, b) Frontal (a) and lateral (b) views of a halo vest and brace. Four metallic uprights connect the halo ring around the skull to the vest. (c, d) Frontal (c) and lateral (d) radiographs of the skull show head tongs used to stabilize the head and neck in a patient with a cervical spine fracture. One or more screws penetrate the outer table of the skull on each side. They are connected to each other by horizontal or vertical bars on each side that are attached to an external traction device.
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Figure 19d. Halo vest and brace. (a, b) Frontal (a) and lateral (b) views of a halo vest and brace. Four metallic uprights connect the halo ring around the skull to the vest. (c, d) Frontal (c) and lateral (d) radiographs of the skull show head tongs used to stabilize the head and neck in a patient with a cervical spine fracture. One or more screws penetrate the outer table of the skull on each side. They are connected to each other by horizontal or vertical bars on each side that are attached to an external traction device.
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Thoracic and Lumbar Spine
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Spinal Implants
Spinal implant surgery is performed to correct severe congenital or developmental spinal deformities or defects. It is also used to stabilize the spine and repair damage to bone and ligaments resulting from trauma, typically motor vehicle accidents, gunshot wounds, and diving accidents (42–56). The main congenital disease indications are severe scoliosis, kyphosis as in Scheuermann disease, spinal stenosis, and spinal dysraphism.
Posterior Spinal Instrumentation.—
Posterior spinal instrumentation is preferred over anterior spinal instrumentation in the thoracic and lumbar regions, because it allows easier decompression and visualization of neural elements (42–56). The posterior surgical approach is much simpler and safer in general than an anterior approach going through the neck, chest, or abdomen. On the other hand, the anterior approach is generally preferred for spinal surgery in the cervical region for two reasons. First, posterior instrumentation in the cervical spine is usually limited, because the vertebrae in the cervical spine are smaller, as is the volume of the soft tissue. Second, the substantial implants used posteriorly in the thoracic and lumbar regions are too large to be used in the cervical spine. Mechanically, they would not safely fit in the cervical region, and the degree of support they provide for the thoracic and lumbar regions is not needed in the cervical spine.
Surgery to fuse the lumbar spine has been performed for more than 100 years. The techniques used for the earliest procedures involved only bone grafting in situ, and they are still applicable today. Currently, it is quite common to combine bone grafting with the use of spinal fixation apparatus to improve fusion rates. Successful bone grafts are necessary for the ultimate success of spinal surgery. If the bone grafting and eventual fusion are not successful, the spinal fixation apparatus will ultimately fail. Spinal implants are very useful, however, for they often permit less extensive surgery, providing immediate spine stability, improved fusion rates, and rapid patient mobilization and rehabilitation.
One of the first practical applications of spinal fixation apparatus was the use by Luque of sublaminar wires attached to anchor rods for thoracic and lumbar spine posterior stabilization (Fig 20). Although this technique provided very secure fixation, it had a relatively high risk of associated neurologic complications. In Luque’s original technique, the wires were "blindly" passed under the lamina at surgery with no direct visualization of the spinal dura neurologic elements. The original technique has for the most part been replaced with use of segmental wires passed through the base of the spinous processes. The new technique is easier for the surgeon, and it avoids injury to the spinal cord and its nerve roots. In a further modification of the original sublaminar wire technique, rectangular fixation devices (Luque and Hartshill rectangles) or metallic buttons (Wisconsin posterior spinal wiring) are used with sublaminar wires designed to hold the metallic rectangles or buttons to adjacent vertebrae (Figs 21, 22).
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Figure 20. Bilateral Luque rods. Frontal radiograph shows bilateral Luque rods that were placed to stabilize the spine for treatment of scoliosis. Note the rod fractures at the T-12 level.
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Figure 21. Hartshill rectangle used to treat degenerative disease in the lower lumbar spine. Frontal radiograph shows a Hartshill rectangle with surrounding bone fusion. The wires through the sacral lamina are broken, which is not a problem if there is solid bone fusion.
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Figure 22a. Frontal (a) and lateral (b) radiographs of the lower thoracic spine show Wisconsin posterior spinal wiring. The wires go under the lamina and are connected to buttons that sit external to the posterior elements of the spine.
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Figure 22b. Frontal (a) and lateral (b) radiographs of the lower thoracic spine show Wisconsin posterior spinal wiring. The wires go under the lamina and are connected to buttons that sit external to the posterior elements of the spine.
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Harrington rods (flanged ends) and Knodt rods (threaded rods) have hooks along the rods (Fig 23). They are designed to either distract the spine or compress it, depending on the direction in which the hooks are placed. The distraction or compression is used to treat a variety of congenital and developmental deformities, such as marked adolescent scoliosis. Sometimes, the deformity is stabilized so that it will not worsen as the patient grows older. Sometimes, the deformity can actua
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Abstract
Introduction
