C₂ Nerve Dysfunction Associated with C₁ Lateral Mass Screw Fixation

Abstract

The C ₁ lateral mass screw technique is widely used for atlantoaxial fixation. However, C ₂ nerve dysfunction may occur as a complication of this procedure, compromising the quality of life of affected patients. This is a review of the topic of C ₂ nerve dysfunction associated with C ₁ lateral mass screw fixation and related research developments. The C₂ nerve root is located in the space bordered superiorly by the posterior arch of C₁, inferiorly by the C₂ lamina, anteriorly by the lateral atlantoaxial joint capsule, and posteriorly by the anterior edge of the ligamentum flavum. Some surgeons suggest cutting the C₂ nerve root during C₁ lateral mass screw placement, whereas others prefer to preserve it. The incidence, clinical manifestations, causes, management, and prevention of C ₂ nerve dysfunction associated with C ¹ lateral mass screw fixation are reviewed. Sacrifice of the C₂ nerve root carries a high risk of postoperative numbness, whereas postoperative nerve dysfunction can occur when it has been preserved. Many surgeons have been working hard on minimizing the risk of postoperative C₂ nerve dysfunction associated with C₁ lateral mass screw fixation.

Introduction

Posterior atlantoaxial fixation is widely used to treat atlantoaxial instability. C₁ screw fixation, which includes the C₁ lateral mass technique and C₁ pedicle screw techniques, is the main technique used for atlantoaxial fixation1, 2, 3, 4. The C₁ lateral mass screw technique was first described by Goel and Laheri in 19445, and popularized by Harms and Melcher, who reported on it in 20016. In 2002, Resnick and Benzel were the first to report C₁ pedicle screw fixation7. And in 2003, Tan et al. introduced the C₁ pedicle screw technique8. Since then, many studies have demonstrated the superiority of the C₁ pedicle screw technique1, 2, 9. However, there is widespread agreement that the height of the C₁ pedicle is the factor that most limits achievement of successful C₁ pedicle screw fixation10, 11, 12. When the height is less than 4.0 mm, the pedicle is not able to accommodate the 3.5 mm‐diameter screw that is usually used for C₁ fixation12. Thus, C₁ pedicle screw fixation is not feasible in 8%–53.8% of patients8, 13, 14, 15, 16, 17, 18. Hence, the C₁ lateral mass screw technique is still widely used. However, C₂ nerve dysfunction may occur as a complication of C₁ lateral mass screw fixation, comprising the quality of life of affected patients19, 20, 21, 22, 23. In addition, whether to cut the C₂ nerve root during C₁ lateral mass screw fixation is still controversial. We here review the topic of C₂ nerve dysfunction associated with C₁ lateral mass screw fixation and related research developments.

Anatomy

The C₂ nerve root is located in the space bordered superiorly by the posterior arch of C₁, inferiorly by the C₂ lamina, anteriorly by the lateral atlantoaxial joint capsule, and posteriorly by the anterior edge of the ligamentum flavum (Fig. 1). The height of the C₂ ganglion is 4.97 ± 0.92 mm on the right side and 4.60 ± 0.84 mm on the left side. The C₂ ganglion occupies approximately 50% of the height of the space in the neutral position and approximately 65% in hyperextension with rotation positions24. A large venous plexus, which can cause bleeding, surrounds the C₂ nerve root in the space. The C₂ nerve root passes inferolateral to the lateral atlantoaxial joint and can be pulled downward during surgical maneuvering25.

Figure 1.

The C₂ nerve root is located in the space bordered superiorly by the posterior arch of C₁, inferiorly by the C₂ lamina, anteriorly by the lateral atlantoaxial joint capsule, and posteriorly by the anterior edge of the ligamentum flavum.

The dorsal ramus of the C₂ nerve emerges between the C₁ posterior arch and C₂ lamina, below the inferior oblique which it supplies, receives a connection from the C₁ nerve dorsal ramus, and divides in to a large medial and smaller lateral branch. The medial branch, termed the great occipital nerve, ascends between the inferior oblique and semispinalis capitis, pierces the latter and the trapezius near their occipital attachments and is then joined by a filament from the medial branch of the third dorsal ramus. Ascending with the occipital artery, it divides into branches that connect with the lesser occipital nerve and supply the skin of the scalp as far anteriorly as the vertex26.

Management of the C₂ Nerve Root during C₁ Lateral Mass Screw Placement

Some surgeons suggest cutting the C₂ nerve root during C₁ lateral mass screw placement27, 28, 29, 30, 31, 32, whereas others prefer to preserve it33, 34, 35, 36, 37.

Cutting the C₂ Nerve Root

In their initial study, Goel et al. used a screw–plate system for atlantoaxial fixation38. To leave enough space for the upper part of the plate, they chose a lower screw entry point than the later Harms technique. Because the screw and plate were placed where the C₂ nerve root and its surrounding venous plexus lie, cutting the C₂ nerve root was unavoidable (Fig. 2).

Figure 2.

The Goel technique uses the screw‐plate system for atlantoaxial fixation; cutting the C₂ nerve root is unavoidable.

Subsequently, cutting the C₂ nerve root during C₁ lateral mass screw placement was generally recommended. It was believed that cutting it simplified surgical maneuvering and resulted in less blood loss, shorter operative time and a lower screw malposition rate. Aryan et al. reported using a modified Harms technique for atlantoaxial fixation that used a screw–rod system and in which cutting the C₂ nerve root was avoidable; however, they still cut it27. In their series, only one of 121 patients developed occipital neuralgia. However, they did not report how many patients had numbness in the region innervated by the C₂ nerve. In their meta‐analysis, Elliot et al. found that sacrifice of the C₂ nerve root resulted more frequently in postoperative numbness (11.6% vs. 1.3%) but less frequently in neuralgia (0.3% vs. 4.7%), was associated with less blood loss (213 mL vs. 417 mL) and shorter operative time (118 min vs. 132 min) than when the C₂ nerve root was preserved2. They concluded that cutting the C₂ nerve root during C₁ lateral mass screw placement resulted in better outcomes, even though 11.6% of the patients have postoperative numbness, an outcome that is unacceptable to some patients. Both Squires and Molinari29 and Hamilton et al.28 reported that, in elderly patients, C₁ lateral mass screw placement with intentional cutting of the C₂ nerve root resulted in satisfactory outcomes, albeit with postoperative numbness. Recently, Patel et al. have reported the clinical outcomes of routinely cutting the C₂ nerve root during C₁ lateral mass screw placement in children. None of their 15 cases C₂ developed nerve dysfunction39.

Preserving the C₂ Nerve Root

If they elect to preserve the C₂ nerve root, surgeons have to face problems such as damage to the C₂ nerve root, severe bleeding from the associated venous plexus and inadequate exposure.

With Harms technique, the entry point for the C₁ screw is the midpoint of the posterior inferior part of the C₁ lateral mass6. For atlantoaxial fixation, they use a screw–rod system that looks like a viaduct and allows the C₂ nerve root to pass through the “arch of the viaduct” (Fig. 3). The C₁ screw they use is a partially threaded one: the 8 mm unthreaded portion of the screw stays above the bony surface of the lateral mass, minimizing the risk of irritation to the C₂ nerve root. Since then, many authors have reported their own modified C₁ screw entry points8, 11, 13, 40. Modifications that heighten the screw entry point minimize the risk of damage to the C₂ nerve root. With these modifications of the original technique, more and more surgeons are tending to preserve the C₂ nerve root during C₁ lateral mass screw fixation.

Figure 3.

Harms C₁ lateral mass screw technique (A) A screw–rod system is used, allowing preservation of the C₂ nerve root. (B) The C₂ nerve root passes through the “arch of the viaduct” of the screw–rod system.

C₂ Nerve Dysfunction Associated with C₁ Lateral Mass Screw Fixation

Incidence

The incidence of C₂ nerve dysfunction after atlantoaxial fixation is not clear, reported rates ranging from 0–33%1, 6, 38, 40. Goel et al. cut the C₂ nerve root during C₁ screw placement; 18/160 patients in their study reported postoperative sensory loss in the distribution of the C₂ nerve38. They did not specifically ask about postoperative numbness in the distribution of the C₂ nerve during follow‐up. It is possible that patients were so satisfied with their limb function that they ignored anesthesia in the occipital scalp. Thus, the incidence of postoperative C₂ nerve dysfunction is likely greater than the reported incidence. In Harms and Melcher's study, no C₂ nerve dysfunction was reported in a cohort of 37 patients6. It may make more sense to discuss the incidence according to the specific circumstances. Elliott et al.'s meta‐analysis showed that 11.6% of patients in whom the C₂ nerve root has been sacrificed experience postoperative C₂ numbness, whereas only 0.3% experience C₂ neuralgia. However, 4.7% of patients who have undergone C₁ screw placement with preservation of the C₂ nerve root experience postoperative C₂ neuralgia and 1.3% experience C₂ numbness2.

Clinical Manifestations

The clinical manifestations of C₂ nerve dysfunction are sensory changes in the distribution of the C₂ nerve, including neuralgia, numbness, dysesthesia and paresthesia1, 2, 20, 27, 30, 38. In addition, some patients report indescribable discomfort in the occipital region28, 30.

Causes of Injury to the C₂ Nerve Root

The mechanism of C₂ nerve dysfunction associated with C₁ lateral mass screw fixation is poorly understood23. However, possible causes are as follows: (i) transection of the C₂ nerve root; however, the effects of this are unclear41; (ii) excessive caudal retraction during exposure of the C₁ lateral mass screw entry point22; (iii) damage to the C₂ nerve root during management of bleeding from the associated venous plexus1; (iv) reduction of C₁ onto C₂ 21, 22; and (v) impingement or irritation from the C₁ lateral mass screw6, 20, 23.

Management and Prognosis

There are currently no clear guidelines for the management of C₂ nerve dysfunction associated with C₁ lateral mass screw fixation. Medication is often used1, 20, 21, 22. If the symptoms are severe and do not respond satisfactorily to medication, repeated C₂ ganglion blocks can be tried21. In cases with severe or persisting symptoms, some authors recommend extraction of the screw19, 23, which can achieve good results23, but sometimes fails to do so22.

Symptoms of C₂ nerve dysfunction often subside spontaneously19; however, in some cases medication, C₂ ganglion block, or even screw extraction are needed21, 23. Of note, even after removal of the instrumentation, the pain still persists in some cases22.

Prevention

Sacrifice of the C₂ nerve root carries a high risk of postoperative numbness, whereas postoperative nerve dysfunction can occur when it has been preserved. Many surgeons have been working hard on minimizing the risk of postoperative C₂ nerve dysfunction associated with C₁ lateral mass screw fixation.

Gunnarsson et al. used partially threaded C₁ screws with smooth shanks to reduce irritation to the C₂ nerve root; however, 3/25 patients still reported postoperative C₂ neuralgia in their case series20.

In their case report, Rhee et al. provided several tips on how to prevent this complication. First, avoid intraoperative hyperextension of the neck21. Second, place the head of the C₁ screw sufficiently dorsally to leave enough space in the foraminal area for the C₂ nerve root. Third, use partially threaded C₁ screws with smooth shanks to minimize the chance of irritation to the C₂ nerve root. Fourth, if the surrounding tissue looks tense around the C₁ screw, perform additional mobilization of the C₂ nerve root. Fifth, use a higher entry point and insert the C₁ lateral mass screw via the posterior arch if it can accommodate it. Finally, carefully place fusion materials on the C₁–C₂ posterior arches.

Pan et al. reported using a 3–5 mm diameter bone wax column to protect both the venous sinus and the C₂ nerve root during surgical maneuvering1. In their study, none of the 22 patients who underwent C₁ lateral mass screw placement with this modified technique developed postoperative numbness, whereas 4/12 patients who underwent screw placement with Harms technique reported postoperative C₂ nerve dysfunction.

In 2013, Lee et al. reported a modification of C₁ lateral mass screw insertion, which is also called the notching technique, designed to avoid postoperative C₂ nerve dysfunction11. They insert the C₁ screw at the junction of the C₁ posterior arch and the midpoint of the posterior inferior portion of the C₁ lateral mass with a notch at the entry point to facilitate screw insertion. The notch allows the screw to be placed farther away from the C₂ ganglion than with the Harms technique and provides a screw trajectory that is less cranially tilted. Only 1/12 cases had mild postoperative unilateral C₂ neuralgia, which had resolved 6 weeks after surgery.

Recently, Huang et al. proposed a preoperative measure, the height for screw index, as a predictor of C₂ nerve dysfunction in patients who undergo C₁ lateral mass screw fixation3. The height for screw index is defined as the difference in height between C₂ ganglion and its corresponding foramen and is measured on CT images. This is the first detailed preoperative evaluation designed to prevent C₂ nerve dysfunction; however, this evaluation is not feasible in approximately 46% of patients because of failure to distinguish the C₂ ganglion on CT images. Another recent study by Huang et al. showed that if there is a medullary canal in the C₁ pedicle, it is possible to finish C₁ pedicle screw fixation in the atlas, the pedicle height of which is less than 4 mm; this finding has changed the traditional view that C₁ pedicle fixation can be performed only when the C₁ pedicle is more than 4 mm high4. In these patients, C₁ pedicle fixation rather than C₁ lateral mass screw fixation can be performed to avoid postoperative C₂ nerve dysfunction.