C1 Lateral Mass and C2 Pedicle Screw Fixation

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Chapter Synopsis	Although many different fixation techniques have been developed to manage atlantoaxial instability, C1 lateral mass–C2 pedicle screw fixation has emerged as a preferred treatment, based on its comparative biomechanical strength and modest risk of neurologic or vascular injury, as well as the ability to perform intraoperative reduction.
Important Points	Surgical indications include atlantoaxial instability caused by trauma, tumors, infection or inflammatory disease, or congenital abnormalities. Alternative fixation techniques include halo vest immobilization, Brooks or Gallie wiring procedures, and Magerl transarticular screw fixation. The C1 lateral mass–C2 pedicle screw technique is typically used for “stand-alone” fixation, but it may be supplemented by rigid cervical collar immobilization in the early postoperative period.
Clinical and Surgical Pearls	Reduction of the C1-C2 joint is not required before screw placement and can be subsequently achieved through repositioning of the patient’s head or direct manipulation of the C1 and C2 instrumentation before rod placement. C1 lateral mass screws typically have a 10-mm proximal smooth shank, which increases mechanical strength and theoretically decreases the risk of C2 nerve root neuralgia. C1 lateral mass and C2 pedicle screws are placed with the use of both direct visualization and fluoroscopic guidance. Because of its modularity, C1 lateral mass–C2 pedicle screw fixation can be extended cranially to the occiput or caudally to the subaxial spine, if needed.
Clinical and Surgical Pitfalls	Preoperative evaluation of atlantoaxial bony anatomy with advanced imaging (computed tomography or magnetic resonance imaging, or both) is crucial to ensure that the C1 lateral masses and C2 pedicles can safely accommodate screw placement. Bleeding from the epidural venous plexus near the C1-C2 joint is common, but it can be controlled with bipolar cautery, FLOSEAL, and cottonoid patties. C2 pedicle screw placement poses the greatest risk of vertebral artery injury, but a superior and medial trajectory decreases this risk.

Atlantoaxial instability can result from multiple disorders, including trauma, tumors, infection and inflammatory conditions, and congenital abnormalities. Regardless of the pathologic process, surgical treatment is often indicated. Surgeons have tended away from halo vest immobilization, which has been associated with significant morbidity and poor patient tolerance, and toward internal fixation techniques performed through a posterior approach. Multiple techniques have been described, including two different posterior wiring procedures described by Brooks and Gallie and C1-C2 transarticular screw fixation described by Magerl.

Although posterior wiring techniques are, in some ways, technically less difficult, they do involve insertion of wires or cables into the spinal canal, a maneuver that poses a risk of spinal cord injury. These techniques also require the use of structural allograft to improve stability and achieve fusion. Even with the addition of halo vest immobilization, however, the rate of pseudarthrosis is up to 30%, as a result of the inferior biomechanical properties of this construct.

The transarticular screw technique provides more stability, based on biomechanical studies, and is also associated with a very high rate of fusion. However, this technique requires reduction of the C1-C2 facet joints bilaterally before screw placement. Additionally, anomalous vertebral artery anatomy, seen in up to 20% of patients, increases the risk of vascular injury and may even preclude the use of this technique.

The most contemporary technique for atlantoaxial fixation was first described by Dr. Jürgen Harms. This technique involves individual screws placed in the lateral masses of C1 and the pedicles of C2 bilaterally connected with rods. The advantages of this technique, also known as the Harms technique, include ability to perform intraoperative reduction and fixation of C1-C2, increased biomechanical strength, and minimized risk of injury to the spinal cord and vertebral artery compared with other fixation techniques. Since its first description in 2001, the Harms technique has been extensively validated in the spinal literature and is now widely used for atlantoaxial fixation.

Preoperative Considerations

History

Although atlantoaxial instability is most commonly seen as a result of trauma, it can also be noted inpatients with infection and inflammatory processes, malignant disease, or congenital anomalies. The history of present illness can often help distinguish among these causes. Undoubtedly, all patients who have sustained high-energy injuries require cervical immobilization until atlantoaxial or cervical instability can be excluded. Unfortunately, other causes of atlantoaxial or cervical instability are not as obvious and, without thorough evaluation, can easily be missed.

Signs, Symptoms, and Physical Examination

Signs and symptoms of atlantoaxial instability differ depending on the chronicity of the instability. Patients with acute instability as a result of high-energy trauma may present with only neck pain, but they may also have signs of spinal cord injury. These signs can include upper or lower extremity loss of sensation or motor strength. More importantly, patients may have difficulty breathing because of the high neurologic level of injury. As a consequence, some patients may not survive the injury if medical assistance does not arrive in time or if Advanced Trauma Life Support (ATLS) protocols are not followed.

Patients with chronic atlantoaxial instability typically report neck pain and often describe pain radiating from the upper part of the neck posteriorly into the occipital area, also known as an occipital headache. This pain is caused by irritation or impingement of the C2 or greater occipital nerve root or roots. Because atlantoaxial instability may cause significant stenosis, the patient may also present with symptoms of myelopathy including poor balance, upper or lower extremity weakness, hyperreflexia, or other upper motor neuron findings.

Imaging

As a part of the initial evaluation, imaging studies to define the extent of the injury and amount of instability are required. Plain static radiographs help to define the overall spinal alignment, and dynamic radiographs may help quantify instability in patients with chronic atlantoaxial instability. In patients with instability caused by acute injury, a fine-cut computed tomography (CT) scan may delineate the extent of the bony injury in greater detail. This modality may also be useful in patients with chronic instability in whom plain radiographs are inadequate.

In either acute or chronic instability, a magnetic resonance imaging (MRI) study is very useful for multiple reasons. It can help determine the severity of soft tissue or ligamentous injury in traumatic conditions. It can also help distinguish between spinal infection or inflammatory conditions and malignant disease as a cause of atlantoaxial instability. Additionally, MRI helps establish the severity of nerve or spinal cord compression. Finally, advanced imaging modalities such as fine-cut CT and MRI help to detail the patient’s bony and vascular anatomy, to determine whether the C1 lateral mass–C2 pedicle screw technique can be safely used.

Indications and Contraindications

Indications for surgical stabilization of atlantoaxial instability include acute, progressive neurologic compromise from any origin that causes instability at the C1-C2 level. Other indications include an anterior atlantodens interval (AADI) of greater than 3 mm in adults and greater than 4 to 5 mm in children, as defined by flexion radiographs. Persistent neck pain or occipital headache related to C2 nerve root irritation or impingement that is a result of chronic atlantoaxial instability (i.e., odontoid pseudarthrosis) is another indication for surgical stabilization.

Contraindications to use of the C1 lateral mass–C2 pedicle screw technique include bony or vascular anomalies that prohibit safe screw placement. One such example is the presence of a ponticulus posticus, or arcuate foramen at C1, which may be identified by radiographs in approximately 15% of pateints. This bony bridge, through which the vertebral artery courses posteriorly, may not be easily identified intraoperatively and can lead to vertebral artery injury if C1 lateral mass screw placement is attempted.

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