Posterior Instrumentation Surgery for Craniocervical
From Head-neck-joints instability conditions
From: Neurol Med Chir (Tokyo) 45, 439¿447, 2005
Posterior Instrumentation Surgery for Craniocervical Junction Instabilities: an Update
Joji INAMASU,DanielH. KIM, and Arnett KLUGH
Department of Neurosurgery, Stanford University School of Medicine, Stanford, California, U.S.A.
Contents |
Abstract
The surgical treatment of craniocervical junction (CCJ) instability has recently undergone significant development and change. Posterior instrumentation surgery has been the mainstay of treatment of CCJ instability, and is the focus of this review. For the treatment of atlantoaxial instability, C1-2 transarticular screw fixation has shown good stability, and has been regarded as the ``gold standard procedure. Because of potentially hazardous complications including vertebral artery injury, however, C-1 lateral mass–C-2 pedicle screw fixation is gaining popularity. For treatment of atlantooccipital instability, occipitocervical fixation using screw constructs (combined with either rods or plates) has shown more stability than sublaminar wiring techniques, and has been utilized more frequently. Both innovation in material engineering and in vitro biomechanical studies have contributed significantly to the development of more rigid internal fixation devices, and as a result, many patients who would have been treated conservatively with external orthosis are treated nowadays with instrumentation surgery, resulting in earlier ambulation, shortened hospital stay, and earlier recovery into social activities. New surgical techniques and instruments, however, need to stand the test of time to see whether they are free from long-term adverse events. The rapid turnover of new surgical techniques and hardware has made it difficult for less experienced surgeons to keep up with the latest developments. Conventional techniques can be safer and less technically demanding than newer techniques for those who are not familiar with them.
Key words: atlantoaxial joint, atlantooccipital joint, craniocervical junction instability, posterior instrumentation, fixation surgery
Introduction
Surgical treatment of craniocervical junction (CCJ) instability has undergone significant development and change within the last decade, owing to a greater understanding of the surgical anatomy and the biomechanical characteristics of the CCJ, development of better fixation hardware and surgical instruments, and computer systems used for intraoperative image guidance.42,51,80) There has been much controversy about the treatment options for the diseases affecting the CCJ, because of its rarity and technical difficulty in approaching the region. The authors aim to provide an update on recent progress made within the last decade and on trends among North-American spine surgeons with regards to surgical treatment of CCJ instability. Posterior instrumentation has been the mainstay of treatment of CCJ instability, and is the focus of this review. Comprehensive review of the literature published between 1994 and 2004 was conducted with Pubmed search, using key words such as C-1, C-2, atlantoaxial, atlantooccipital/occipitoatlantal, craniocervical, craniovertebral, fixation, instrumentation, instability, occipital, posterior, and surgery.
Indication
Although etiologies of CCJ instability are quite variable, they can be arbitrarily classified as 1) traumatic, 2) congenital/pediatric, and 3) proliferative/destructive (e.g., due to rheumatoid arthritis, infection, and metastatic tumor).47,52,53) Recent trends in posterior fixation surgery for CCJ instability are discussed for each etiologic category. CCJ instability is usually subdivided into atlantoaxial instability and occipito-atlantal (atlantooccipital) instability.52,53,77) Surgical fixation techniques are described separately for atlantoaxial instability and occipitoatlantal instability.
I. Traumatic
Due to the anatomic complexity of the CCJ, trauma to this region can result in various fracture and/or ligamentous injury patterns.28,29,38,51,55) Furthermore, the quality and degree of anatomical/mechanical instability will vary significantly in each patient, depending on which portion of the key anatomical structure has been destroyed. Detailed classification schemes and treatment strategy for all the craniocervical or upper spinal column injuries is beyond the scope of this review. Readers should refer to the listed references for additional reading materials.28–30,51,55,84) Posterior atlantoaxial fixation has been a treatment of choice for type II and some type III odontoid fractures, the most common upper cervical spine fracture.33,34,52,67) Spine surgeons from North America have favored a posterior surgical approach for fixation of type II and III odontoid fractures.8,11, 14,58,87) In contrast, surgeons from Europe and Japan frequently perform and prefer the anterior odontoid screw fixation technique.5,9,37,56) Although there has been little consensus regarding when these two approaches (anterior vs. posterior) are indicated for patients with fresh type II or III odontoid fracture, 29,45) the presence or absence of concomitant transverse atlantal ligament injury, reducibility of the fracture, direction of the fracture line in the odontoid process, timing of clinical presentation after injury, age of the patient, and surgeon's preference are influential determinant factors.9,29,56,72) Posterior atlantoaxial fixation for odontoid fracture may be best indicated for those patients with an irreducible fracture, ruptured transverse ligament, concomitant C-1 fracture, or non-union.9,28,29,72) Traumatic injuries to the atlantooccipital joint, such as occipital condyle fractures or atlantooccipital dislocation, are usually associated with more extensive ligamentous damage and instability than the isolated odontoid fracture.25) Although isolated unilateral occipital condyle fracture can be treated conservatively,35) patients with bilateral occipital condyle fractures or survivors of atlantooccipital dislocation will require rigid and long segmental fixation, i.e., occipitocervical fixation.25,75,79,83) Recently, a new posterior transarticular screw fixation technique for atlantooccipital dislocation has been reported, with favorable results found in both a biomechanical study24) and in a few clinical cases.26) This technique will preserve the rotatory motion of the atlantoaxial joint, which is inevitably abolished by occipitocervical fixation.24,26) The safety of the procedure as well as its long-term outcome is to be reported.
II. Congenital/pediatric
Congenital malformations causing CCJ instability are diverse and include ligamentous hypermotility associated with Down's syndrome, Chiari malformation, achondroplasia, and Klippel-Feil syndrome. 7,10,46,73,78,88) In addition to translational atlantoaxialinstability, they often cause vertical dislocation of the odontoid process into the posterior fossa, resulting in basilar invagination and impingement of the brainstem between the tip of the odontoid process and the posterior arch of the atlas.10,88) For such vertical dislocation, occipitocervical fixation with or without anterior removal of the odontoid process is usually performed for decompression of the brainstem and restoration of the anatomical alignment.6,73,78,88) The pediatric population will frequently display distinct patterns of fracture and/or ligamentous injury of the CCJ. Os odontoideum, a congenital anomaly (although some authors favor its traumatic origin), is characterized by a small corticated ossicle separated from the base of the odontoid, and is a major cause of atlantoaxial instability in young children.13) The adult population is also frequently affected.13) Posterior atlantoaxial or occipitocervical fixation has been a successful treatment for patients with symptomatic os odontoideum.13,31,54) Ligamentous laxity in some children occasionally leads to the pathological fixation of the atlas on the axis in a rotated position called atlantoaxial rotatory subluxation following a relatively minor trauma.12,48) Posterior atlantoaxial fixation has been shown to be an effective treatment for patients with irreducible or recurrent rotatory subluxation.7,46,48)
III. Proliferative/destructive
Rheumatoid arthritis (RA) commonly causes inflammation of synovial joints, including the occipitoatlantal and atlantoaxial joints.70) Thirty-nine percent and 11% of RA patients have been reported to suffer from translational atlantoaxial subluxation (anterior translation of the atlas on the axis) and basilar invagination, respectively.50) These CCJ instabilities can cause not only kyphotic deformity, myelopathy, or intractable neck pain, but also sudden death.50,70) The poor, osteoporotic bone quality of RA patients, who often take large doses of steroids or immuno-suppressants, has made fixation surgery difficult and risky.50,71,74) Posterior atlantoaxial fixation has been indicated for patients with translational atlantoaxial subluxation.47) For the smaller number of patients who present with vertical atlantoaxial subluxation with basilar invagination or cranial settling, occipitocervical fixation is usually required.1,50,57) Anterior decompression of the odontoid process is not performed routinely, because posterior fixation can reduce the pannus formation and anterior compression in the majority of cases.1,50,74) However, decompression is necessary for patients who have marked anterior compression of the neuraxis due to irreducible vertical atlantoaxial subluxation, so transoral resection of the anterior arch of C-1, the odontoid process, and pannus is performed in such patients.57) Although odontoidectomy itself (in the absence of vertical atlantoaxial subluxation) does not fully destabilize the spine, occipitocervical fixation is always necessary in RA patients who undergo odontoidectomy. Occipitocervical fixation is performed following the transoral procedure, usually under the same anesthetic.57) Occipitocervical fixation has also been used as a salvage surgery for those who have progression of the instability despite prior treatment with posterior atlantoaxial fixation.1,50,74) Infection of the spinal column (spondylitis) has become a rare disease in industrialized countries. However, tuberculosis is still a common cause of morbidity and mortality in developing nations. In several series in India, tuberculous atlantoaxial dislocation was frequently associated with anterior compression of the brainstem from massive tuberculous granuloma, necessitating removal of the granuloma via an anterior approach followed by a posterior atlantoaxial fixation, or an occipitocervical fixation for those in whom the posterior arch of the atlas was destroyed.2,44,76) Neurological recovery and significant pain relief was achieved in most cases.2,44,76) Metastatic tumor rarely involves the CCJ, but can cause progressive instability in the region. Osteolytic destruction of atlantoaxial spine elements can result in pathological fractures. The anterior elements (C1-2 lateral mass and vertebral body, and odontoid) are frequently involved.3,19) The most common presentation is severe neck pain secondary to spinal instability. Treatment strategy depends on patient condition, the degree and region of bone destruction, and neurological status. In cases resistant to radiation therapy or chemotherapy, or cases with rapidly progressive neurological deficits, studies from several large cancer centers have shown beneficial effects of posterior instrumentation surgery in restoring stability, reversing neurological deficits, reducing intractable pain, and improving the quality of life of these patients.3,19) Radical removal of the tumor mass is apparently not curative, and has not been attempted in most cases. The levels of instrumentation depend on the integrity of the posterior elements, the ability to reduce the atlantoaxial subluxation, and the degree of subaxial cervical spine involvement.3,19) Preemptive occipitocervical fixation is preferred by the authors, because of the unpredictable nature of progression from the destructive neoplastic process.3,19)
Surgical Techniques
I. Atlantoaxial fixation
Atlantoaxial transarticular screw fixation technique supplemented by sublaminar wiring and bone graft, as reported by Grob and Magerl27) in 1987 (Figs. 1 and 2), has many advantages over previously performed atlantoaxial sublaminar wire fixation procedures such as Gallie's, Brooks', or modified Sonntag's, or with an interlaminar clamp technique, and has been considered a gold standard for posterior atlantoaxial fixation.14,21,34,51,52) It affords biomechanical stability immediately after surgery, particularly against rotational motion and lateral bending, and has obviated the need for prolonged, postoperative use of the Halo vest in the majority of cases.59) Unlike standalone sublaminar wiring techniques, the transarticular screw fixation technique can also be used for those with disrupted posterior elements. Fusion rate has been very high, exceeding 90% in most reported series.14,34,51,52) The technique has been, however, associated with various complications, including hypoglossal nerve injury16,87) and atlantooccipital joint injury.87) Injury to the vertebral artery (VA) is the most serious complication. 47,87) Insertion of a transarticular screw through C-2 pars to C1-2 facet joint can injure the VA, because of the proximity of the VA in the bony groove and the ``blinded passage of the guiding wire or screw through these structures.47,65,87) In a large retrospective survey among North American neurosurgeons, VA injury during the procedure had occurred in 4.1% of the 1318 patients surveyed.87) Radiological analyses using a reformatted thinslice computed tomography images have shown that in about 20% of the patients, placement of an atlantoaxial transarticular screw was thought to be unsafe, secondary to the aberrant course of the VA or thin C-2 pars/pedicle.65) Meticulous preoperative evaluation of the relevant anatomy using axial, sagittal, and coronal computed tomography reformatted sequences is essential for visualization of the course of the VA. The sagittal angle of the screw insertion against the cervical column is acute, and this technique is contraindicated for patients with marked kyphotic deformity, morbid obesity, or barrel chest. Moreover, atlantoaxial subluxation should be reduced before screw insertion. Detailed anatomical studies of the atlas and the axis using cadaveric material have contributed to better understandings of the ideal screw trajectory. 17,22,39–41) Recent development of image guidance systems has increased the percentage of successful transarticular screw insertion and decreased the rate of complications associated with screw malposition. 4,20,60,86) Recently, Goel et al.23) and Harms and Melcher36) independently reported a novel technique for posterior atlantoaxial fixation. Instead of a C1-2 transarticular screw, screws to the C-1 lateral mass and C-2 pedicle were placed separately and connected with either a plate23) or a rod36) (Figs. 3 and 4). The biomechanical stability following this technique may be superior to that of the transarticular fixation technique, as previously reported by the senior author.43)
The C-1 lateral mass–C-2 pedicle screw technique seems to be gaining popularity among spine surgeons. 36,43,68,69,81) At the time of writing, no serious complications caused by this technique, including VA injury, have been reported. The more perpendicular sagittal angle of screw trajectory into C-1 lateral mass and C-2 pedicle, compared to that in the transarticular technique, make this technique less likely to hit the VA.36,68,81) However, long-term outcome of this new technique has yet to be determined, and it is not certain whether this technique would eventually replace the transarticular screw technique. Other posterior atlantoaxial fixation techniques have recently been reported, including atlantal translaminar screw,18) atlantoaxial interlaminar hook/rod,61) or intraarticular screw techniques combined with interlaminar clamps.82) They have been used less frequently with a limited number of cases, and may have less biomechanical stability than transarticular screw fixation. However, they are virtuallyriskfree fromVAinjuryand maybe beneficial in selected cases. Use of titanium mesh cage as an adjunct to the transarticular screw technique has been reported, with promising preliminary results.49)
II. Occipitocervical fixation
The goal of occipitocervical fixation is directed not only at restoring sagittal alignment, but also at reversing cranial settling and basilar invagination. Optimal alignment of the CCJ and upper cervical spine must be restored before fixation surgery. Contoured loops or rods connected with sub-or interlaminar wires or cables have been the mainstay of occipitocervical fixation devices.53,77,85) Wire fixation mechanically does not provide enough rigidity, and thus long segment fixation down to the lower cervical spine together with prolonged postoperative use of external orthotic devices has been mandatory.62,66) Wire loosening or pullout has been encountered. Recent biomechanical studies have shown that screw construction is more rigid than wire or laminar hook construction (Fig. 5).62,66) The number of thelower cervical vertebraetobefixed to gain sufficient stability can be reduced with screw fixation. Another advantage of these techniques is avoiding sublaminar instrumentation that can injure or compress the neural tissue. The key element in providing maximum immobilization at the CCJ is screw purchase (as opposed to wire fixation) into both the occiput and the atlantoaxial complex, largely independent of the coupling device.62,66) Several occipitocervical fixation devices using screw/rod or screw/plate have been manufactured, without demonstrable statistical difference in biomechanical stability (Fig. 6).1,6,75) For an occipital screw, bicortical purchase provides better stability than unicortical purchase.15,32) However, bicortical purchase has a higher risk of damaging the underlying structures, i.e., cerebellar or venous sinus injury. Anatomical studies of the occipital bone have shown the relative ``safety zone for screw insertion of variable lengths; the occiput is thickest in the midline ridge and at the foramen magnum with thickness dropping dramatically out laterally.15,32) The average thickness is 14 mm (10–18 mm) at the midline.15) Screws should be placed near the midline crest and not far laterally. Recent biomechanical testing showed that unicortical fixation at the midline ridge approached the pullout strength of bicortical fixation at other anatomic locations in the occiput.32) In pediatric cases, the occipital bone is not thick enough to accept the occipital screw, and cables or wires are still the instruments of choice. To avoid unexpected screw penetration, some authors have developed inside-outside stabilization techniques.63,71) Although apparently safer than bicortical screw placement, it has not been shown whether this technique is biomechanically as stable and durable as for bicortical screw placement. Both C1-2 transarticular and C-2 pedicle screw placement as a lower (caudal) end of the instrumentation have been shown to have sufficient stability, and incorporation of the lower cervical spine is unnecessary in many cases.62,66)
Conclusion
Surgical treatment of CCJ instability has undergone a significant change within the last decade. Many patients who would have been treated with conservative management with external orthotic devices such as the Halo vest are treated nowadays surgically with rigid internal fixation devices, resulting in earlier ambulation, shortened hospital stay, and earlier recovery into social activities.51) There have been remarkable advances in the material and structural design of internal fixation devices, and in vitro biomechanical studies have made significant contributions to the development of more rigid fixation systems.42) Accumulation of knowledge derived from many anatomical studies together with development of image guidance technology has made these potentially hazardous procedures safer. Despite recent enthusiasm for minimally invasive spine surgery,43) however, posterior instrumentation surgery for CCJ instability still remains a ``maximally invasive procedure which requires a wide anatomic exposure, and a very steep learning curve for spinal surgeons. The rapid turnover of new surgical techniques and hardware has made it difficult for less-experienced spine surgeons to keep up with the latest developments.14) Conventional techniques, such as use of sublaminar wiring or interlaminar claw or hook, although associated with lower fusion rate and longer periods of external immobilization, may be safer and less technically demanding than transarticular screw or suboccipital screw techniques for those who are not familiar with the procedures.64,85) Interlaminar claw or hook fixation is a good alternative to pedicle screw fixation when the anatomical configuration of the pedicle does not allow screw insertion. No matter how external fixation devices are modernized, they still need to be supplemented with bone graft as they only provide stabilization until bony fusion has developed.
New surgical techniques and instruments, although they seem to be associated with more favorable short-term outcome, need to stand the test of time to see whether they are free from long-term adverse events, such as accelerated spondylitic degeneration of the adjacent levels observed in the lowercervicalspine afteranteriorfixationsurgery.
Acknowledgment
Dr. Joji Inamasu received a research fellowship from Pfizer International Fellowship Award for members of Japanese Association of Acute Medicine (JAAM) and a clinical fellowship from Synthes Spine. No benefits in any form have been received from commercial parties related directly or indirectly to the subject of this manuscript.
We wish to thank Dr. Judith Murovic of the Department of Neurosurgery at Stanford University Medical Center for editorial assistance.
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Address reprint requests to: J. Inamasu, M.D., Department
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e-mail: jinamasu＠hsc.usf.edu