C1 Lateral Mass Fixation

Amory J. Fiorea, Barry D. Birchb, Regis W. Haid, Jr.a a Department of Neurosurgery, The Emory Clinic, Atlanta, Ga., and b Department of Neurosurgery, The Mayo Clinic, Scottsdale, Ariz., USA

A variety of techniques exist for fixation of the upper cervical spine. Recently, several authors have published case series reporting the use of C1 lateral mass screws for posterior cervical fixation [1-3]. In this chapter we describe our surgical technique for placement of C1 lateral mass screws, including indications and results from our experience.

C1 lateral mass screws may be used to provide additional fixation points in occipitocervical constructs, possibly increasing resistance to construct failure in the cervical spine without increasing the number of cervical levels fused. Additionally, C1 lateral mass screws may be used as a supplement to or substitute for other forms of atlantocervical fixation. Techniques for achieving atlantocervical fusion include posterior interspinous fusion with sublaminar cables and iliac crest bone graft [4, 5], C1-C2 transarticular screw fixation [5-7], and interlaminar clamp fixation [8]. While each of these methods has been successfully employed to achieve atlantocervical fusion, anatomic factors may exist in certain situations that preclude their use. Interspinous fusion at C1-C2 with sublaminar cables or interlaminar clamps cannot be performed if the posterior elements of C1 or C2 are absent or disrupted. C1-C2 transarticular screws cannot be placed successfully in the presence of a medially located vertebral artery, irreducible subluxation, severe cervicotho-racic kyphosis, or destruction of the C2 pars interarticularis. In these cases constructs employing C1 lateral mass screws may be used to achieve fixation. We present a small case series in which C1 lateral mass screws were used to achieve atlantocervical fixation when anatomic characteristics precluded the use of traditional fixation methods. Also included in this series is one case where C1 lateral mass screws were used to provide additional fixation points for occipitocervical fusion.

Methods

Ten patients with cervicomedullary compression or atlantoaxial instability were treated surgically between February 1998 and February 2002. Preoperative diagnoses included C2 metastasis in 2 patients, irreducible odontoid fracture in 3 patients, atlantoaxial subluxation in 2 patients, and transverse ligament synovial cyst in 3 patients. Posterior atlantocervical fixation was planned in 9 patients, one of whom had a transoral resection of the odontoid prior to posterior fusion. In 1 patient with a C2 metastasis and pathologic fracture, an occip-itocervical fusion was planned due to the high degree of instability of the atlantoaxial complex. Atlantoaxial screw fixation was chosen as the initial fixation procedure when pre-operative imaging studies did not reveal anatomic factors precluding screw placement. All procedures were performed with intraoperative lateral fluoroscopy or CT-guided frameless stereotaxic navigation (Stealth Station, Sofamor Danek, Memphis, Tenn., USA). Autogenous iliac crest bone graft was harvested via a separate posterior iliac crest incision and used for arthrodesis.

Surgical Technique

The patient is positioned prone using a Mayfield head holder (OMI, Cincinnati, Ohio, USA). The neck is kept neutral with the head in the 'military tuck' position. The arms are tucked at the sides and the shoulders retracted caudally using tape. A midline incision is made extending from the inion to the spine of C3 if atlantoaxial fixation is planned. The incision is extended inferiorly as indicated by the planned procedure. A bilateral sub-periosteal dissection of the paraspinal musculature is performed to expose the lateral margins of the facet joints at the C2-C3 level. Dissection is continued laterally over the dorsal arch of C1, exposing the vertebral artery in the vertebral groove on the C1 arch. Bipolar cautery and hemostatic agents such as gelfoam and fibrillar collagen are used to control bleeding from the perivertebral venous plexus. The C2 nerve root is identified and mobilized inferiorly. The lateral mass of C1 inferior to the C1 arch is exposed. The medial wall of the lateral mass is identified using a forward angle curette to identify the medial limit of screw placement. The medial aspect of the transverse foramen can also be identified and serves as the lateral limit for screw placement. The entry point for screw placement is identified 3-5 mm lateral to the medial wall of the lateral mass, at the junction of the lateral mass and inferior aspect of the C1 arch (fig. 1). The entry point may be varied depending on the distance between the medial wall of the lateral mass and the C1 transverse foramen. A high speed drill with a 3-mm round burr is used to remove a small portion of the inferior aspect of the C1 arch overlying the entry point, to create a recess for the screw head and plate or rod (fig. 2a). An assistant retracts the C2 nerve inferiorly and protects the vertebral artery with Penfield dissectors or similar instruments during drilling and screw placement. Using fluoroscopy or image guidance a 3-mm drill bit and guide are used to drill a hole with 10-15° of medial angulation to penetrate the anterior cortex of C1 (fig. 2b, 3). On lateral fluoros-copic imaging the drill is aimed toward the anterior tubercle of C1, so that the drill penetrates the ventral cortex of the lateral mass midway between the superior and inferior facets of C1 (fig. 2c, 4). The hole is tapped with a 3.5-mm tap. If lateral mass plates are used, an appropriate-sized plate is selected and contoured, after which a 3.5-mm screw is placed through the plate (fig. 5). Caudal fixation points are then finalized. If a polyaxial screw-rod

Fig. 1. Intraoperative photograph demonstrating entry point for C1 lateral mass screw, vertebral artery in vertebral groove of C1, inferior aspect of C1 transverse foramen, and C2 nerve root retracted inferiorly.

system is used, all screws are placed after which an appropriate sized and contoured rod is secured (fig. 2d). In both cases an appropriate screw length is selected to achieve bicortical fixation.

Once instrumentation placement is complete, decompression is performed if necessary. Finally, arthrodesis is performed. Posterior arthrodesis with sublaminar cable and inter-spinous bicortical autograft is preferred if the laminae of C1 and C2 are preserved. Otherwise, lateral arthrodesis is performed by carefully decorticating the exposed surfaces of the C1-C2 joints with a high-speed drill, and then packing cancellous iliac crest autograft over these joints. A Hemovac drain is placed prior to wound closure.

Results

Seventeen C1 lateral mass screws were placed in 8 patients (table 1). These screws were incorporated into several different constructs using lateral mass plates (Axis, Sofamor Danek) or a polyaxial screw-rod system (Vertex, Sofamor Danek) to achieve atlantocervical fixation in 9 patients and occipitocervical fixation in 1 patient. There were no intraoperative complications and no vertebral artery injuries. One patient died on postoperative day 9 from complications of aspiration pneumonia. The remaining patients were immediately mobilized postoperatively in hard cervical collars worn for 3 months. Immediate rigid fixation was achieved in all patients. Follow-up ranged from 9 days to 18 months (mean 6.6 months). Osseous fusion was documented in 2 patients on 9- and 18-month

Laterol Mass Screws

Fig. 2. a Recess for the screw head is created on the inferior aspect of the C1 arch. b Axial drawing shows medial screw angulation and relationship to transverse foramina. c Lateral view of C1-C2 construct with C1 lateral mass screw, C2 pedicle screw, and plate, demonstrating screw trajectory. d Posterior view of completed C1-C2 construct using polyaxial screw and rod system.

Fig. 2. a Recess for the screw head is created on the inferior aspect of the C1 arch. b Axial drawing shows medial screw angulation and relationship to transverse foramina. c Lateral view of C1-C2 construct with C1 lateral mass screw, C2 pedicle screw, and plate, demonstrating screw trajectory. d Posterior view of completed C1-C2 construct using polyaxial screw and rod system.

Fig. 3. Axial CT images showing medial angulation of C1 screws and relationship to transverse foramina.

Fig. 4. Intraoperative fluoroscopic image demonstrating C1 screw trajectory toward midpoint of anterior C1 tubercle.

Fig. 5. Completed C1-C2 construct using lateral mass plates and screws.

follow-up radiographs, and 6 patients had delayed postoperative flexion/extension radiographs demonstrating construct stability. In 1 patient a unilateral C1 lateral mass screw was placed after the contralateral lateral mass fractured during drilling. In this patient the polyaxial screw was connected with a rod to a C2 sublaminar hook for atlantoaxial fixation. On follow-up radiographs the screw was noted to be disconnected from the rod, but there was no instability on flexion-extension films and the patient was clinically improved.

Table 1. Case representations

Patient

Age/sex

Presentation

Diagnosis

Treatment

Follow-up

Outcome

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