Dickman Retractor

74/M

R hemiparesis, neck pain

Metastatic colon cancer, pathologic C2 burst fracture, spinal cord compression

Occipitocervical fusion to C5 with polyaxial screw/rod system, bilateral C1 LMS

9 days

Died from pneumonia

LMS = Lateral mass screw, TAS = transarticular screw, F/E = flexion/extension, VA = vertebral artery.

LMS = Lateral mass screw, TAS = transarticular screw, F/E = flexion/extension, VA = vertebral artery.

Fig. 6. Patient 1. Preoperative lateral cervical radiograph demonstrating anteriorly displaced type II odontoid fracture.

Illustrative Case: Patient 1

A neurologically intact 16-year-old girl was referred for management of a type II odontoid fracture. She had sustained the fracture 2 months earlier in a motor vehicle accident and had been treated with external immobilization by the referring physician. She was referred when follow-up radiographs showed a 1-cm subluxation at C1-C2 (fig. 6). After 3 days of halo traction, follow-up radiographs demonstrated minimal reduction of the subluxation, and she was taken to the OR for internal fixation. Intraoperative attempts to reduce the fracture under fluoroscopy were unsuccessful. Internal fixation was achieved using a polyaxial screw-rod system with C1 lateral mass screws and C2 pars interarticularis screws, supplemented with interspinous iliac crest autograft and sublaminar cable. The patient was mobilized postoperatively in a hard cervical collar. Postoperative radiographs revealed solid fixation at C1-C2 (fig. 7). The patient is neurologically intact at 6 months' follow-up.

Discussion

We have described a technique to achieve solid fixation of the C1 lateral mass that can be utilized in a variety of instrumentation constructs for varying

Fig. 7. Patient 1. Postoperative AP and lateral radiographs demonstrating C1-C2 construct with polyaxial screw-rod system and interspinous cable and autograft.

indications. In our experience, the most common indication for the use of C1 lateral mass screws is atlantoaxial instability. Although a variety of techniques exist to treat atlantoaxial instability, certain anatomic factors may preclude their application in specific situations. C1-C2 interspinous fusion techniques using either sublaminar cables or interlaminar clamps in combination with iliac crest autograft require the presence of intact posterior elements. These techniques cannot be applied when the C1 arch or C2 laminae have been disrupted by trauma, neoplasm, or other pathologic processes, or when resection of these elements is necessary to achieve neural decompression. C1-C2 transarticular screw fixation is likewise precluded by a variety of factors. In up to 20% of patients, a medially located or 'high-riding' vertebral artery will preclude safe passage of C1-C2 transarticular screws unilaterally. In 3% of patients, vertebral artery anatomy will preclude passage of screws bilaterally [9, 10]. Irreducible C1-C2 subluxation will likewise preclude optimal placement of C1-C2 transarticular screws. In this case, a screw trajectory traversing the articular surfaces of C1 and C2 cannot be achieved. Severe cervicothoracic kyphosis may preclude C1-C2 transarticular screw placement by obstructing the trajectory of the instruments used to insert the screws. Destruction or erosion of the osseous substrate for screw fixation by trauma, neoplasm, or other pathologic processes will likewise preclude transar-ticular screw placement. In these situations occipitocervical fusion may be considered as an alternative means to treat atlantoaxial instability. Occipitocervical fusion may be avoided by using C1 lateral mass screws to achieve atlantocervi-cal fixation. By avoiding occipitocervical fixation, patients avoid the risk of intracranial bleeding which may occur with placement of occipital hardware [11]. Additionally, range of motion at the atlantooccipital joint is maintained, reducing morbidity from craniocervical malalignment that may occur following occipitocervical fusion. Clinical studies have also suggested that avoidance of occipitocervical fusion may decrease the incidence of delayed subaxial subluxation [12, 13]. In this series we achieved atlantocervical fixation in 7 patients who demonstrated various anatomic characteristics that precluded traditional methods of atlantoaxial fixation. These patients were all mobilized in hard cervical collars, avoiding postoperative halo vest immobilization.

It is likely that C1 lateral mass screws will also prove to be an extremely useful technique for occipitocervical fixation. C1 lateral mass screws provide additional fixation points for occipitocervical constructs, possibly increasing resistance to construct failure. This additional construct integrity is achieved without fusing additional cervical levels, thus preserving cervical motion segments. In the one patient in this series who underwent occipitocervical fixation, we were able to achieve solid C1 fixation and integration into the occipitocervical construct using C1 lateral mass screws. As the use of polyaxial screw-rod systems for occipitocervical fixation becomes more widespread, we anticipate that C1 lateral mass screws will be used more frequently, since the contourable rods used in these systems will allow C1 screws to be easily incorporated into occipitocervical constructs.

In 6 of the 9 patients in this series who underwent atlantocervical fixation, constructs using C1 lateral mass screws were supplemented with a posterior C1-C2 fusion using sublaminar cable and interspinous autograft. In recent case series, Dickman et al. [4] reported an 86% fusion rate after interspinous fusion with cables and autograft, while Farey et al. [5] reported a 58% fusion rate. In two smaller series, fusion rates of 100% were achieved using the Brooks method of interspinous fusion, but all patients were immobilized in a halo vest for 3 months [14, 15]. It is not clear from this series whether C1 lateral mass screw constructs will increase fusion rates when applied in addition to interspinous fusion techniques, but it seems likely that the additional rigidity conferred by these constructs should result in improved outcomes. The ability of unilateral C1 lateral mass screw constructs to increase fusion rates when applied together with contralateral C1-C2 transarticular screws is also unclear. Song et al. [16] reported a 95% fusion rate after unilateral transarticular screw placement combined with posterior inter-spinous fusion in a group of patients with high-riding vertebral arteries. In the present series, 1 patient had a unilateral C1 lateral mass screw construct in combination with a contralateral transarticular screw and interspinous fusion. Again, it seems likely that the supplemental fixation provided by the C1 lateral mass screw construct will increase rigidity and result in higher fusion rates.

It is also unclear whether C1 lateral mass screw constructs can be used as a stand-alone method for achieving atlantocervical fusion in the absence of interspinous fusion or unilateral transarticular screw fixation. Harms and

Melcher [2] suggest that temporary C1-C2 constructs using C1 lateral mass screws may be used in selected cases, including rotatory subluxation and young patients with displaced odontoid fractures, allowing preservation of rotation at C1-C2 after instrumentation removal. In addition, they state that C1-C2 fixation with C1 lateral mass screws eliminates the morbidity associated with passage of C1 sublaminar cables. In this series we treated 3 patients with displaced odontoid fractures, adding interspinous cable and autograft to increase construct rigidity and provide additional substrate for bony fusion. We feel that odontoid fractures with irreducible subluxation are best treated with C1-C2 interspinous arthrodesis in addition to instrumentation, to provide optimal rates of long-term fixation. In our experience, passage of C1 sublaminar cables can be performed with minimal morbidity when neural compression is not present. In this series 3 patients were treated with stand-alone constructs because absence of the C1 or C2 laminae precluded interspinous arthrodesis. Two patients had stable radiographs at follow-up, and rod-screw separation occurred in 1 patient with a unilateral construct, although there was no overt radiographic instability. When stand-alone constructs are used, it is important to achieve lateral arthrodesis by decorticating the lateral masses and C1-C2 joint space, with placement of cancellous autograft laterally. In the future, the decision to employ a C1 lateral mass screw construct without interspinous fusion or contralateral transarticular screw fixation should be considered on an individual basis in the context of the pathologic process causing instability, bone quality, and other comorbidities influencing bone fusion, as well as the potential morbidity of the alternative treatment, occipitocervical fusion. Larger studies with long-term follow-up will be necessary to determine the safety and efficacy of C1 lateral mass screw constructs.

To date, only one study has examined the biomechanical characteristics of atlantoaxial constructs using C1 lateral mass screws. Lynch et al. [17] evaluated an atlantoaxial construct with C1 lateral mass screws and C2 pedicle screws with and without supplemental interspinous cable and graft, and compared this construct with atlantoaxial transarticular screws. The C1 lateral mass screw construct was most resistant to lateral bending and axial rotation, and less resistant to flexion and extension. Adding a posterior cable and graft reduced motion slightly. Compared to transarticular screws, the C1 lateral mass screw construct was slightly less rigid, allowing an average of 0.6° more motion. This study suggests that C1 lateral mass screw constructs are a reasonable alternative to transarticular screws for achieving atlantoaxial stabilization.

Harms and Melcher [2] used a specially modified screw at C1 with an unthreaded proximal shaft, in order to reduce the risk of greater occipital nerve irritation as well as screw breakage. In our series we used standard screws with threads along the entire shaft. We did not observe any cases of occipital neuralgia or screw breakage. These results indicate that standard screws may be used at C1. We believe the risk of greater occipital nerve irritation is small as long as there is adequate space caudal to the C1 screw for passage of the nerve. In 1 patient not included in this series, placement of C1 lateral mass screws was planned. However, intraoperatively the C2 nerve roots were found to be much larger than usual, occupying the entire space between the inferior aspect of the C1 dorsal arch and the superior aspect of the C2 pars. In this case, we elected not to place C1 screws, since the risk of C2 irritation was unacceptably high. In this case the alternative would be to place C1 screws directly into the dorsal aspect of the C1 arch, rather than into the inferior surface of the arch. This would place the screw shafts away from the C2 nerve roots. However, this can only be considered when the rostrocaudal dimension of the C1 arch is large enough to accommodate a screw. In addition, the vertebral artery in the C1 groove must be retracted rostrally during drilling, increasing the risk of embolic complications or direct injury to the vertebral artery by the drill, retractor, or other instruments.

The risk of vertebral artery injury must always be assessed when placement of lateral mass screws or transarticular screws is planned. In this small case series, there were no vertebral artery injuries. To minimize this risk, preoperative assessment of the path of the vertebral artery using CT scanning is mandatory prior to placement of C1 lateral mass screws. Magnetic resonance angiography or catheter angiography may be performed to provide additional information concerning the path and patency of the vertebral arteries, although in our experience we have not found this to be necessary. The surgeon must note that the trajectory of the C1 lateral mass screw is very different from that of lateral mass screws placed in the subaxial cervical spine. Particularly important is that the C1 screw is placed with a slight medial angulation to avoid the vertebral artery laterally and the spinal canal medially. We consider the use of intraoperative fluoroscopy or CT-based image guidance mandatory to safely place C1 lateral mass screws. Fluoroscopy allows safe placement of bicortical screws under direct visualization, while CT-based image guidance provides additional three-dimensional information about the vertebral artery and spinal canal. Virtual fluoroscopy may also prove to be a useful adjunct to screw placement.

Conclusion

The placement of C1 lateral mass screws provides a useful alternative method to achieve atlantocervical fixation when anatomic factors preclude the placement of atlantoaxial transarticular screws. This method achieves immediate rigid stabilization of the atlantoaxial joint and obviates the need for halo vest immobilization. This technique may be used in certain cases as an alternative to occipitocervical fusion, and may also be used to increase construct stability when occipitocervical fixation is employed. Evaluation of the course of the vertebral artery with preoperative CT scanning and use of intraoperative fluo-roscopy or image guidance are mandatory when using this technique. Placement of C1 lateral mass screws is a technically demanding procedure that may result in grave complications from vertebral artery injury if improperly performed. We thus advocate that this procedure only be performed by surgeons who are highly experienced in the treatment of atlantoaxial instability, and who have an intimate understanding of the anatomy of the region. The uninitiated surgeon can minimize the possibility of complications during C1 lateral mass screw placement by first performing this procedure in a cadaveric setting. Further biomechanical analysis of this technique should be performed to quantify the strength of constructs employing C1 lateral mass screws as compared with other fixation methods. Further clinical studies should be performed to determine the safety and efficacy of this technique.

References

1 Fiore AJ, Haid RW, Rodts GE, Subach BR, Mummaneni PV Riedel CJ, Birch BD: Atlantal lateral mass screws for posterior spinal reconstruction: Technical note and case series. Neurosurg Focus 2002:12/1:article 5.

2 Harms J, Melcher RP: Posterior C1-C2 fusion with polyaxial screw and rod fixation. Spine 2001;26:2467-2471.

3 Stokes JK, Villavicencio AT, Liu PC, Bray RS, Johnson JP: Posterior atlantoaxial stabilization: New alternative to C1-2 transarticular screws. Neurosurg Focus 2002;12/1:article 6.

4 Dickman CA, Sonntag VK, Papadopoulos SM, Hadley MN: The interspinous method of posterior atlantoaxial arthrodesis. J Neurosurg 1991;74:190-198.

5 Farey ID, Nadkarni S, Smith N: Modified Gallie technique versus transarticular screw fixation in C1-C2 fusion. Clin Orthop 1999;35:126-135.

6 Dickman CA, Sonntag VK: Posterior C1-C2 transarticular screw fixation for atlantoaxial arthrodesis. Neurosurgery 1998;43:275-281.

7 Haid RW, Subach BR, McLaughlin MR, Rodts GE, Wahlig JB: C1-C2 transarticular screw fixation for atlantoaxial instability: A 6-year experience. Neurosurgery 2001;49:65-70.

8 Moskovich R, Crockard HA: Atlantoaxial arthrodesis using interlaminar clamps. Spine 1992;17:261-267.

9 Madawi AA, Casey AT, Solanki GA, Tuite G, Veres R, Crockard HA: Radiological and anatomical evaluation of the atlantoaxial transarticular screw fixation technique. J Neurosurg 1997;86:961-968.

10 Paramore CG, Dickman CA, et al: The anatomic suitability of the C1-2 complex for transarticular screw fixation. J Neurosurg 1996;85:221-224.

11 Vale FL, Oliver M, Cahill DW: Rigid occipitocervical fusion. J Neurosurg 1997;91(suppl 2):144-150.

12 Clark CR, Goetz DD, Menezes AH: Arthrodesis of the cervical spine in rheumatoid arthritis. J Bone Joint Surg Am 1989;71:381-392.

13 Kraus DR, Peppelman WC, Agarwal AK, DeLeeuw HW, Donaldson WF: Incidence of subaxial subluxation in patients with generalized rheumatoid arthritis who had previous occipital cervical fusions. Spine 1991;16(suppl):486-489.

14 Boden SD, Dodge LD, Bohlman HH, Rechtine GR: Rheumatoid arthritis of the cervical spine. A long-term analysis with predictors of paralysis and recovery. J Bone Joint Surg Am 1993;75:1282-1297.

15 McCarron RF, Robertson WW: Brooks fusion for atlantoaxial instability in rheumatoid arthritis. South Med J 1988;81:474-476.

16 Song GS, Theodore N, et al: Unilateral posterior atlantoaxial transarticular screw fixation. J Neurosurg 1997;87:851-855.

17 Lynch JJ, Crawford NR, Chamberlain RH, Bartolomei JC, Sonntag VK: Biomechanics of lateral mass/pedicle screw fixation at C1-2. Proceedings of the 2002 Annual Meeting of the American Association of Neurological Surgeons, Chicago, 2002.

Department of Neurosurgery, Emory University

1365B Clifton Road, NE, Ste. 6400, Atlanta, GA 30322 (USA)

Tel. +1 404 778 5770, Fax +1 404 778 4472, E-mail [email protected]

Haid RW Jr, Subach BR, Rodts GE Jr (eds): Advances in Spinal Stabilization. Prog Neurol Surg. Basel, Karger, 2003, vol 16, pp 142-153

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