Case Reports / Journal of Clinical Neuroscience 15 (2008) 295–301
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Os odontoideum: Report of three cases J.E. Brecknell *, G.M. Malham Department of Neurosurgery, The Alfred Hospital, Melbourne, Victoria, Australia Received 30 March 2006; accepted 14 July 2006
Abstract Os odontoideum is a condition in which a smoothly corticated ossicle exists dorsal to the anterior arch of C1, taking the place of the rostral dens, but with no bony connection to the body of the axis. Three patients presented with this condition: the first with Lhermitte’s phenomenon 10 years after significant trauma, the second as an incidental finding during routine cervical spine imaging following a road traffic accident, and the third with recurrent transient quadriparesis precipitated by falls from a surfboard. Patients had at least 10 mm of sagittal instability on dynamic imaging and the second patient had a minimum sagittal canal diameter of only 11.5 mm. Posterior atlanto-axial fixation was successfully achieved in all cases using polyaxial screws and rods with the assistance of computed tomography-based image guidance. Image guidance provided an invaluable aid to preoperative planning and intraoperative placement of the posterior spinal instrumentation. 2007 Elsevier Ltd. All rights reserved. Keywords: Os odontoideum; Atlanto-axial fixation; Polyaxial screws; Frameless stereotaxy; Image guidance; Biomodels
1. Introduction
2. Case reports
Os odontoideum was described as early as 1886 as a postmortem finding.1 It is a condition in which a smoothly corticated ossicle exists dorsal to the anterior arch of the atlas, with no bony union to the body of the axis yet taking the place of the missing rostral dens. McRae expressed the opinion that it is not possible to distinguish between an old ununited fracture of the odontoid process and a congenital pathology,2 and the precise aetiology of os odontoideum remains unclear. While the condition has been treated with posterior instrumented atlanto-axial fusion for 40 years or more, the nature of the instrumentation has evolved from interlaminar wiring3 to transarticular screw fixation.4 More recently the contemporary technique of polyaxial screw and rod fixation has been applied to posterior instrumented atlanto-axial fusion.5 Here we describe three patients with os odontoideum treated with image-guided posterior instrumented atlantoaxial fusion using polyaxial screw and rod fixation. We use these cases to discuss the association of os odontoideum with trauma, the indications for surgical stabilisation, the choice of operative technique and the role of image guidance.
2.1. Patient 1
* Corresponding author. Present address: Department of Neurosurgery, Queen’s Hospital, Rom Valley Way, Romford RM7 0AG, UK. Tel.: +44 (0) 845 1304204x2087. E-mail address:
[email protected] (J.E. Brecknell).
A 33-year-old woman presented in July 2005 with Lhermitte’s phenomenon and C2 root pain, present since a fall from a ladder 10 years previously, but more severe over the preceding 6 weeks. On examination she held her neck stiffly to minimise her symptoms, but had no neurological deficit. CT examination revealed a dystopic os odontoideum with an interdigitating articulation with the posterior surface of the anterior arch of the atlas. The posterior arch of the atlas was unfused (Fig. 1). Dynamic MRI examination revealed 10 mm of sagittal instability between flexion and extension but no evidence of signal change in the spinal cord (Fig. 2). The minimal canal diameter measured between the postero-superior corner of the body of C2, and the anterior cortex of the posterior arch of C1 in flexion in the sagittal plane was 15 mm. 2.2. Patient 2 A 35-year-old woman was involved in a motor-vehicle accident in May 2005. A plain radiograph of the cervical spine showed an os odontoideum with some minor anterior atlanto-axial subluxation. She had a history of many years of previously uninvestigated occipital pain, but was otherwise well. She had a Hoffman’s reflex on the right and bilateral finger jerks, but a neurological examination detected no other abnormality. Dynamic CT examination revealed a dystopic os odontoideum with 10 mm of sagittal atlanto-axial instability as well as a few millimetres of lateral
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Case Reports / Journal of Clinical Neuroscience 15 (2008) 295–301
Fig. 1. CT obtained pre-operatively for patient 1. (a) Coronal view showing os odontoideum with a blunt tooth C2 configuration. (b) Axial view through C1 showing an unfused dorsal arch. Note the absence of the dens from its usual position. (c) Sagittal view. The ossicle is closely related to the clivus (dystopic) and has an interdigitating articulation with the anterior arch of C1 (jigsaw sign).
Fig. 2. Sagittal images from preoperative dynamic MRI in flexion, neutral and extension for patient 1, who had 10 mm atlanto-axial instability. Note that the ossicle (*) moves with C1 and the occiput and that the cord becomes draped over the dens remnant (#) in flexion.
atlanto-axial subluxation. The minimal canal diameter was 11.5 mm. An MRI showed evidence of increased T2 signal in and wasting of the cord at the atlanto-axial level (Fig. 3). 2.3. Patient 3 A 25-year-old man presented in April 2006 with Lhermitte’s phenomenon and recurrent episodes of transient quadriparesis on falling from his surfboard over a 3-year period following a significant fall while snowboarding. Examination revealed no neurological deficits. Dynamic CT imaging revealed a dystopic os odontoideum with 15 mm of sagittal instability between flexion and extension and a minimal canal diameter of 15.5 mm. An MRI showed no evidence of signal change within the cord. An acrylate biomodel of the patient’s cranio-cervical junction was made to facilitate surgical planning and explanation to the patient (Fig. 4b). 2.4. Surgery For all three patients the CT data were used to plan trajectories for trans-isthmic C1 lateral mass and C2 pedicle screws using the planning station of an image guidance system (iPlan! 1.1, BrainLAB AG, Heimstetten, Germany;
Fig. 4a). Through a standard posterior midline exposure of the occipito-cervical junction a reference fiducial array was clamped to the spinous process of C2. The exposed posterior elements of C2 were then registered to the image guidance system using a pointer-based surface matching technique. The information from the image guidance system was correlated with the local anatomy by feeling the dorso-medial surface of the pedicle with a blunt hook before the entry point was marked with a high-speed drill. The pedicle was then cannulated using a hand drill before 3.5-mm-diameter self tapping, polyaxial, top-loading screws (18–22 mm in length) were placed (Axon, Synthes, Oberdorf, Switzerland). Because of the gross atlanto-axial instability in all three patients the registration of C2 was inadequate to guide instrumentation of C1. Using a cranial reference fiducial array attached to the three-pin head clamp we were unable to register C1 to the image guidance system. However, by localising the isthmus and lateral mass of C1 with a blunt hook and aiming towards the midpoint of the anterior arch as guided by lateral fluoroscopy, five out of six trans-isthmic lateral mass screws (22–24 mm in length) were successfully placed. During placement of the left-sided screw in patient 2 the hand drill broke out of the isthmus inferiorly and medially. As a rescue technique the lateral
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Fig. 3. Preoperative images for patient 2. Sagittal CT images in flexion (a), neutral (b) and extension (c) showing 10 mm atlanto-axial subluxation and dystopic os odontoideum. The minimal canal diameter was 11.5 mm. (d) Axial CT through C1. (e) Coronal CT showing a few millimetres of lateral atlanto-axial subluxation. (f) MRI showing signal change in the high cervical cord.
Fig. 4. (a) 3D reconstruction of the planned preoperative trajectories for patient 2. (b) Posterior view of the acrylate biomodel prepared for patient 3.
mass was captured below the isthmus with a screw 18 mm in length following subperiosteal mobilisation of the C2 nerve root and its associated venous plexus. Screws were secured to longitudinal rods and morselised bone graft harvested from the iliac crest was laid on the decorticated surfaces of the C1 posterior arch and C2 spinous process and laminae.
2.5. Early postoperative course Postoperative imaging revealed that the constructs were satisfactory (Figs. 5 and 6). No patient experienced any complication with surgery, and patients were discharged home 3–6 days after surgery. At 3 months, patients 1 and 3 were free of symptoms and patient 2 remained so.
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Fig. 5. Postoperative CT for patient 1 showing screw placement. (a) Parasagittal view. (b,c) Axial views through C1. (d,e) Axial views through C2.
occult occipito-cervical instability was revealed in any of the cases by the successful atlanto-axial fusion. 3. Discussion 3.1. Aetiology and association with trauma
Fig. 6. Postoperative imaging for patient 2. Lateral radiographs in (a) extension and (b) flexion showing atlanto-axial stability. (c) Parasagittal CT on right showing trans-isthmic C1 lateral mass screw and C2 pedicle screw. (d) Parasagittal CT on left showing direct C1 lateral mass screw and C2 pedicle screw.
Patients 1 and 3 remained neurologically intact and patient 2 no longer had any upper motor neuron signs. Dynamic radiographs showed atlanto-axial stability (Fig. 6). No
At least some cases of os odontoideum are acquired. One series includes nine patients, most with a past history of significant trauma to the head or neck, with normal cervical radiographs taken previously, who subsequently developed os odontoideum.6 Os odontoideum may be orthotopic, when it is more closely associated with and moves as a unit with the atlas, or dystopic, when it is more closely associated with and moves as a unit with the clivus.6 While the dystopic os has been reported to be more likely to be congenital,7 Stevens et al.,8 who distinguished between diagnoses of chronic ununited fracture of the dens and os odontoideum on the basis of a prior history of injury, found no radiological criteria that allowed the two groups to be separated, echoing the comments of McRae from 50 years ago.2 A more recent study suggests that an interdigitating articulation between the os and the anterior arch of the atlas, the jigsaw sign, is a reliable radiological sign of congenital os odontoideum.9 That study, however, contained very few patients with chronic ununited fracture for comparison. Patient 1 in the present study demonstrated a positive jigsaw sign, yet had a history of significant trauma 10 years previously. The association between os odontoideum and dysplasias including Down’s syndrome, Morquio’s disease, pseudoachondroplasia and multiple epiphyseal dysplasia, and other bony malformations of the occipito-cervical junction argues for a congenital aetiology. Nine out of 35 patients in the series of Fielding et al.6 had no history of trauma. There is one report of familial os odontoideum with an autosomal dominant pattern of inheritance.10 David and
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Crockard11 proposed that the mechanism underlying congenital os odontoideum is one of fracture of the cartilaginous dens in utero. Thus it may be true to say that all cases of os odontoideum are traumatic in origin, with the timing of the traumatic event being all that separates congenital from acquired cases. Trauma may also be the precipitant for the development of symptoms from a pre-existing os odontoideum, with 16 of 37 patients fitting this pattern of presentation in one series.12 The presenting traumatic event, while clearly not responsible for the underlying abnormality, may cause an additional soft tissue injury that is sufficient to increase the degree of instability and render the lesion symptomatic. The series of both Spierings and Braakman12 and Fielding et al.6 contain patients, like patients 2 and 3 in the present report, whose os odontoideum was discovered incidentally during routine cervical radiography following trauma. Thus, the association between trauma and os odontoideum is a complex one. 3.2. Indications for surgery Os odontoideum may constitute part of a more generalised abnormality of the cranio-vertebral junction (CVJ).13–15 Analysis of the stability of the entire CVJ prior to surgical intervention is required; atlanto-axial stabilisation can expose previously occult occipito-cervical instability with subsequent neurological deterioration.13 Fixed subluxation of the CVJ with ventral compression of the cervico-medullary junction may require transoral decompression.14,15 Recently, transoral release of the ventral ligaments has been shown to allow reduction of the otherwise fixed subluxation in some patients, obviating the need to proceed with decompression.16 In one report a ventral soft tissue mass associated with os odontoideum was shown to resolve after posterior stabilisation.17 However in the three patients presented here os odontoideum was the only major abnormality of the CVJ, and resulted in reducible atlanto-axial subluxation. Os odontoideum in itself is not an indication for surgical intervention. Twenty of 37 patients in one series12 were treated conservatively and observed for an average of 7 years without evidence of deterioration. Of these 20 patients, 13 were examined with dynamic radiography, of whom all but two had 8 mm or more of sagittal atlanto-axial instability. The series of Fielding et al.6 also included four patients followed for a mean of 2 years without surgical intervention, who remained asymptomatic and neurologically intact, although only one of these patients had radiological evidence of instability. However, with such alarming degrees of atlanto-axial instability, the upper cervical cord is at risk and os odontoideum has a well-established association with myelopathy.18 The minimal canal diameter at the atlanto-axial level has been consistently found to be correlated with myelopathy.12,19,20 In the best available longitudinal study, a minimal canal diameter of less than 13 mm was associated
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with a ‘10 to 1’ risk of the patient developing a permanent myelopathy.12 Although the degree of instability,19 the configuration of the atlanto-axial articulation,21 a dystopic os7 and MRI evidence of cord compression22 have all been reported to be correlated with myelopathy at presentation, they have not been shown to represent risk factors for subsequent damage to the cord. Who then should undergo surgical stabilisation? A recent systematic review4 concluded that patients with os odontoideum, neurological symptoms or signs and atlanto-axial instability may be managed with surgical stabilisation. Fielding et al.6 considered pain alone an indication for surgery, which was effective in alleviating this symptom. After two perioperative deaths, Spierings and Braakman12 considered that local symptoms alone were not a sufficient indication for surgery. Patients with no other indications for surgery, with a minimal canal diameter of less than 13 mm, stand a very high chance of developing a myelopathy and should be offered surgery. The significance of Lhermitte’s phenomenon23 as an indication for surgical stabilisation in os odontoideum has not been previously addressed. McAfee et al.24 considered it evidence of neurological compromise in the setting of cranial settling. For patients 1 and 3 we considered that this symptom represented evidence of impending damage to the cord and thus represented an indication for surgery. 3.3. Surgical technique Posterior atlanto-axial fusion in os odontoideum has been successfully achieved with wiring techniques combined with external bracing or transarticular screw fixation.4 The latest generation of posterior spinal instrumentation allows the independent capture of spinal elements with top-loading polyaxial screws before these in turn are connected and secured to rods. This approach has been applied to atlanto-axial fusion by Harms and Melcher5 and their series included six patients with os odontoideum. Other authors have reported success with the technique,25–27 which has also been applied unilaterally with a transarticular screw contralaterally in a case of os odontoideum.28 While biomechanical studies disagree on whether transarticular screw fixation or the ‘Harms technique’ are superior, both consistently perform better than wiring alone.29–31 A high-riding vertebral artery makes transarticular screw placement impossible in about 20% of patients.5 Non-reducible atlanto-axial subluxation is another contraindication, and the steep angle of insertion may not be available in patients with exaggerated thoracic kyphoses. Wiring methods rely on the posterior elements being intact. The Harms technique is free of all these restrictions. The placement of the axial screw into the supero-medial portion of the pedicle via the pars interarticularis keeps it away from the vertebral artery. Also, the independent capture of axis and atlas makes it more flexible in atlanto-axial subluxation, permitting reduction following capture and prior to securing the screws to the rod.
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The usual entry point for capture of C1 with a lateral mass screw is on the posterior face of the lateral mass at its junction with the isthmus. This is also the entry point used for the atlantic screw in the plate and screw technique described by Goel et al.32 All 104 specimens examined in two anatomical studies were able to accommodate a 3.5mm screw placed in this way.33,34 Exposure of this entry point, however, is frequently accompanied by profuse venous bleeding and requires either inferior mobilisation of the C2 nerve root and dorsal root ganglion or its sacrifice.32 An alternative entry point is on the dorsal surface of the posterior arch of C2, with the trajectory extending through the isthmus into the lateral mass.35 In order to avoid the vertebral artery in its groove on the superior aspect of the isthmus, this entry point lies 2 mm above the inferior edge of the dorsal arch. In the present cases, trans-isthmic atlantic lateral mass screws were planned and five of six were successfully placed. Following the drill breaking out of the isthmus in the sixth, the conventional entry point was successfully employed as a rescue technique (Fig. 6). 3.4. Image guidance The chief difficulty facing the surgeon in posterior spinal instrumentation is with directing a screw through unexposed anatomy. If the screw leaves the safety of its bony trajectory then neurovascular structures are at risk. Although the standard landmarks for entry points are well known, as are standard trajectory angles, pathology may distort the anatomy enough to render these inadequate. Other conventional aids to the safe placement of screws are preoperative imaging, palpation of unseen anatomy (here feeling the isthmus of C1 and the pedicle of C2 with a blunt hook) and intra-operative fluoroscopy. Frameless stereotaxy or image guidance was developed as a useful aid to intracranial localisation.36 It was first applied to spinal surgery to assist in the placement of lumbosacral pedicle screws,37 and has since been applied to anterior and posterior spinal surgery at all levels.38–41 Recently, biomodels42 have been used as a further aid for the safe and accurate placement of spinal instrumentation, and this technology was employed for patient 3 in our report (Fig. 4b). An exact acrylate plastic copy of the patient’s occipito-cervical junction was generated from a CT scan. This biomodel was useful for obtaining consent, surgical planning, intra-operative guidance and teaching. The vertebral artery is particularly at risk during capture of C1 and C2, as well as during the passage of transarticular screws, and this is stressed by many authors.5,25,26,28,30,32,35,40,43,44 Careful preoperative planning is key to avoiding this complication, and routine CT angiography has been suggested.43 Image guidance has been successfully applied to transarticular screw fixation40 using a C2 reference array and a surface-matching technique, as in the present report. We found the image guidance system to be invaluable for trajectory planning (Fig. 4a) and a highly useful adjunct to the anatomical
placement of the C2 screws in these three cases. The use of a fluoroscopy-based technique to register C1 would probably have allowed the intra-operative use of image guidance during placement of the C1 screws in the same way. It is possible that had we employed this technology for patient 2, the optimisation of entry point and trajectory angle may have prevented the drill cutting out of the isthmus. 4. Conclusion Os odontoideum has an interesting relationship with trauma; it is probably the causative agent either ante- or post-natally, but can also cause a pre-existing lesion to become symptomatic, or provoke imaging that reveals the pathology incidentally. To the list of pain, progressive neurological deficit and a minimum canal diameter of 13 mm or less, we would add Lhermitte’s phenomenon as an indication for surgical stabilisation. The contemporary technique of polyaxial screw and rod fixation is a safe and effective way of achieving this. Image guidance provides a useful adjunct to the more traditional methods of achieving a safe trajectory for the placement of the posterior spinal instrumentation. We hope that these observations are of use to other spinal surgeons in treating patients with os odontoideum. References 1. Giacomini C. Sull’ esistenza dell’ ‘‘os odontoideum’’ nell’ uomo. Gior R Accad Med Torino 1886;49:24–8. 2. McRae DL. Bony abnormalities in the region of the foramen magnum. Correlation of the anatomic and neurologic findings. Acta Radiol 1953;40:335–54. 3. Wollin DG. The os odontoideum. Separate odontoid process. J Bone Joint Surg 1963;45A:1459–71. 4. Hadley MN. Os odontoideum. Neurosurgery 2002;50:S148–55. 5. Harms J, Melcher RP. Posterior C1-C2 fusion with polyaxial screw and rod fixation. Spine 2001;26:2467–71. 6. Fielding JW, Hensinger RN, Hawkins RJ. Os odontoideum. J Bone Joint Surg Am 1980;62:376–83. 7. Shaffrey CI, Chenelle AG, Abel MF, et al. Anatomy and physiology of congenital spinal lesions. In: Benzel EC, editor. 2nd ed. Spine Surgery; Techniques, Complication Avoidance, and Management. Philadelphia: Elsevier Churchill Livingstone; 2005. p. 61–87. 8. Stevens JM, Chong WK, Barber C, et al. A new appraisal of abnormalities of the odontoid process associated with atlantoaxial subluxation and neurological disability. Brain 1994;117: 133–48. 9. Fagan AB, Askin GN, Earwaker JWS. The jigsaw sign. A reliable indicator of congenital aetiology in os odontoideum. Eur Spine J 2004;13:295–300. 10. Morgan MK, Onofrio BM, Bender CE. Familial os odontoideum: case report. J Neurosurg 1989;70:636–9. 11. David KM, Crockard A. Early development of the craniovertebral junction and cervical spine. In: Crockard A, Hayward R, Hoff JT, editors. Neurosurgery; the Scientific Basis of Clinical Practice. 2nd ed. Oxford: Blackwell Scientific Limited; 2000. p. 74–86. 12. Spierings ELH, Braakman R. The management of os odontoideum; analysis of 37 cases. J Bone Joint Surg Br 1982;64:422–8. 13. Menezes AH, Ryken TC. Craniovertebral abnormalities in Down’s syndrome. Pedriatr Neurosurg 1992;18:24–33.
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doi:10.1016/j.jocn.2006.07.022
Carotid stenting and cerebral hyperperfusion syndrome Nimeshan Geevasinga b c
a,*
, John G.L. Morris
a,b
, David L. Ross
a University of Sydney, Sydney, New South Wales, Australia Department of Neurology, Westmead Hospital, Sydney, New South Wales, Australia Department of Cardiology, Westmead Hospital, Sydney, New South Wales, Australia
Received 3 April 2006; accepted 19 October 2006
* Corresponding author. Present address: Western Clinical School, Westmead Hospital, Hawkesbury Road, Westmead, New South Wales 2145, Australia. Tel.: +61 404 188 664. E-mail address:
[email protected] (N. Geevasinga).
a,c
