Posterior Arthrodesis of Atlantoaxial Joint in Congenital Atlantoaxial Instability Under 5 Years of Age: A Systematic Review.

INTRODUCTION

Atlantoaxial instability (AAI) occurs in a variety of congenital and syndromic conditions which usually present with neck pain, limitation of motion, or spinal cord compression. These may also present incidentally when radiographs are obtained for some other reason. The instability usually occurs due to ligamentous laxity of the transverse odontoid ligament. It leads to anterior luxation of C1 over C2 leading to spinal cord impingement anteriorly by odontoid process and posteriorly by C1 posterior arch. The peculiarities of the atlantoaxial joint are its complex anatomy, immature and small bony structures in pediatric spine as well as the poor healing associated in these conditions.[1]

Traditionally, AAI has been treated by atlantoaxial arthrodesis with the use of semi-rigid fixation with halo immobilization in younger children and by rigid fixation in older children above the age of 10 years.[2] Rigid fixation techniques like transarticular screw fixation, C1 lateral mass and C2 pedicle screw, and O-C2 fusion is well described for various etiologies in adults with little mention in pediatric age group.[3456]The literature currently lacks any systematic review on the same in children below 5 years of age with congenital AAI. The aim of this literature review is to obtain a concise knowledge regarding the outcomes of posterior arthrodesis in congenital AAI in children below 5 years of age.

MATERIALS AND METHODS

A literature search was carried out using the online databases PubMed, EMBASE, Google Scholar, and the Cochrane database for all studies published in the English language before July 2020. Four separate searches were carried out using the following phrases: (a) “atlantoaxial fusion,” (b) “atlantoaxial subluxation,” (c) “atlantoaxial dislocation,” (d) “atlantoaxial rotatory fixation,” (e) “atlantoaxial rotatory dislocation,” and (f) atlantoaxial rotatory subluxation. The search was carried out without using any limits. The inclusion criteria of the study were (a) study describing surgical outcome of posterior arthrodesis, (b) study subjects less than 5 years, and (c) congenital etiology of AAI in the study subjects. The studies with the following criteria were excluded (a) case reports, (b) non-English literature, (c) reviews, (d) description of non-congenital etiology, (e) studies describing an additional anterior transoral procedure, and (f) surgical techniques. All the articles were reviewed by two authors (NG, SKB) and matched with the current study criteria as described earlier. Full texts of the eligible studies were retrieved and data were extracted in Microsoft Excel sheets. References of all the studies included were hand searched for any other eligible study. A flow chart using the PRISMA format (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) is presented [Figure 1].

Figure 1

PRISMA chart depicting the systematic review study criteria

RESULTS

Details of studies included

The initial search led to the screening of 12,338 abstracts and after evaluation, 12 articles were included in the systematic review [Figure 1]. All the studies included were retrospective and Level IV evidence studies. The total number of patients included in this review is 23 with a mean age of 3.7 ± 1.1 years (1.7–5 years) [Table 1]. The ratio of males to females was 1.5:1. The mean follow-up of the cases was 48.4 months (median 28.1 months, range 9–195 months). Owing to the absence of any randomized controlled trial and the small sample size, any quantitative analysis of the data obtained could not be done.

Table 1

Details of the studies included in the review

Author (Year)	Level of Evidence	Age (Years)	Sex	Follow-Up (Months)	Diagnosis	ADI (mm)	Procedure/ Construct	Fusion Time (months)	Complications
Shikata (1987)[11]	4	5	F	-	Down Syndrome	9	C1-C2 arthrodesis by Sublaminar Wiring	-	None
Shikata (1989)[12]	4	5	F	45	Down Syndrome	9	C1-C2 arthrodesis by Sublaminar Wiring	-	None
	4	4	F	12	Down Syndrome	-		-	None
Toyama (1994)[16]	4	4	F	24	AARF	-	C1-C2 arthrodesis by Sublaminar Wiring	-	Swan Neck Deformity
Doyle (1996)[8]	4	3.2	M	102	C1-C2 Rotatory Subluxation	-	C1-C2 arthrodesis by Sublaminar Wiring	-	Late C2-C3 Instability Subluxation
	4	5	F	72	Atlantoaxial Instability	-		-	Early Loss Of Reduction and Required Halo Readjustment
	4	5	M	45	Fixed Increased Atlantodens Interval	5	In situ C1-C2 arthrodesis	-	None
Wang (1999)[17]	4	4	F	9	AARF IIB	-	C1-C2 Transarticular Fixation with sublaminar wiring	3	None
Nader (2005)[10]	4	3	F	-	Down Syndrome	5	C1-C2 arthrodesis by Sublaminar Wiring	-	None
Anderson (2006)[3]	4	1.7	M	48	Down Syndrome	-	C1-C2 Transarticular Fixation	4.8	None
	4	4.6	F	12	Congenital	-		4.8	None
Tauchi (2012)[13]	4	4	F	20	Down Syndrome	-	C1 Lateral Mass and C2 Pedicle	-	C1 Posterior Arch Fracture, Non Union
Ping Yi (2014)[7]	4	5	F	23.8	AARF	10	C1 Lateral Mass and C2 Pedicle	6	None
Park (2017)[9]	4	4	F	19	Atlantoaxial Instability of Unknown Cause	-	C1-C2 Posterior Arthrodesis	-	None
Zhang (2017)[14]	4	5	M	44	Congenital	-	C1 and C2 pedicle screws	3	None
	4	2.7	F	98	Dens Aplasia	8.3		4.2	None
	4	4	M	39	Axis Abnormality	6.7		4.2	Screw Loosening
	4	2	M	19	Cerebral Palsy	10.3		4.2	None
	4	2.1	F	18	AARF	6.5		4.2	Allograft Absorption
	4	3	M	15	Transverse Ligament Laxity	7.7		4.2	None
	4	2.5	M	12	Down Syndrome	11.4		4.2	None
Grover (2020)[15]	4	2.2	M	195	Marfan Syndrome	-	C1-C2 arthrodesis by Sublaminar Wiring	-	None
	4	3.6	F	145	Down Syndrome	-		-	None

Preoperative data

The patients commonly presented with complaints of neck pain with stiffness and torticollis and on examination had signs of myelopathy like gait disturbance and neurological deficit.[7891011121314] Down and Marfan syndrome were commonly associated in these patients.[10131415] Preoperatively, all the patients underwent dynamic flexion extension radiographs, computed tomography (CT), and magnetic resonance imaging (MRI) of the cervical spine. Patients were also assessed by preoperative angiography in one study to ascertain the location of vertebral artery.[14] The radiographic diagnosis of the patients was one of the following: (1) atlantoaxial rotatory fixation (AARF), (2) atlantoaxial rotatory instability, (3) AAI, and (4) fixed atlantodens interval (ADI). Instability was diagnosed by the presence of increased ADI which ranged from 5 to 14 mm in the various series.[78,1011,14] Fixed instability was diagnosed by the CT scans. Two studies measured the diameter of C2 pedicle preoperatively (mean 4.8 mm, range 2.4–6.7 mm).[1415]

Surgical details

The atlantoaxial joint was reduced preoperatively under fluoroscopic guidance followed by halo application and second stage surgery performed for posterior arthrodesis.[15] Rest of the studies relied on intraoperative reduction of the joint. The patients underwent posterior C1-C2 arthrodesis with use of implants like sublaminar wire or pedicle screw or both. Sublaminar wires were used either by Gallie’s or Brook’s technique.[812,1516] Pedicle screws were either used in the form of C1 lateral mass and C2 pedicle screws (Goel Harms construct) or transarticular screws (Magerl technique).[310,1314,17] Autogenous bone graft was used from one of the following sites: calvarium, iliac crest, and rib.[37,810,1115,17] Intraoperative neuromonitoring was used in two studies during placement of screws.[713] Postoperatively patients were immobilized in a hard cervical collar or a halo brace for a period ranging from 4 to 24 weeks.[78,1113,1416,17] Two of the studies using transarticular screws as the construct did not use any postoperative immobilization.[310] The arthrodesed spine showed solid fusion in a range of 3 to 6 months.

Outcome

Anatomic reduction was defined as postoperative ADI < 3 mm, partial reduction as ADI > 3 mm with no spinal cord compression and failure of reduction as ADI > 3 mm with spinal cord compression.[14] In the same study, all the patients achieved anatomic or partial reduction. Japanese Orthopedic Assessment score, Neck Disability Index, ASIA score, and Ranawat score were the objective scores to grade the outcomes.[710,14] All the scores showed a statistically significant improvement. A mean vertical growth of 34% (range 17–50%) was noted in one study after a mean follow-up of 28 months.[3] No deaths were recorded in any of the patients included in this study.

Complication

Loss of reduction, subaxial spine instability, C1 arch fracture, pseudoarthrosis, screw loosening, bone graft resorption, and swan neck deformity of the cervical spine were the common complications noted.[813,1416] Bone graft donor site morbidity was noticed when graft was obtained from the iliac crest.[17] The presence of instability of the subaxial spine at follow-up was determined using criteria described by White and Panjabi: measured on resting lateral or flexion–extension radiographs, the horizontal displacement of one vertebra in relation to an adjacent vertebra is greater than 3.5 mm and the angulation is greater than 11° in relation to that of either adjacent vertebra.[3]

DISCUSSION

Clinical presentation

Even in the presence of radiological AAI, a limited number of children present with signs and symptoms pertaining to it. This is evident from this study in view of the extremely smaller number of cases of AAI below 5 years of age who required surgical intervention. A large number of cases remain asymptomatic by the age of 5 years. This should lead the surgeon to be extra cautious when managing a syndromic case with regards to the atlantoaxial joint and prompt its radiological screening. The incidence of radiological AAI in syndromic cases varies from 14 to 90% and that of symptomatic AAI is around 1%.[1018]

Cases of congenital AAI usually present with torticollis, progressive spinal cord compression along with neurological and respiratory symptoms. Less commonly, they may present acutely after an episode of trauma. In milder cases, they should be suspected if the child is unable or unwilling to turn his head from one side to another and the clinical examination is not consistent with other common causes of torticollis like congenital muscular torticollis or retropharyngeal infection.

Diagnosis of AAI in children

There are numerous quantified parameters described in literature to diagnose AAI in children. Lateral cervical radiographs and MRI of the cervical spine are currently the important modalities for diagnosis.

An ADI of more than 5 mm on lateral radiographs indicates instability.[19]

In extension lateral radiographs, overriding of anterior arch of atlas over the odontoid may be seen in up to 30% of cases.[20]

The space available for cord (SAC) is another important parameter to diagnose in congenital AAI in children. It is the distance between posterior aspect of dens and anterior aspect of posterior ring of atlas and SAC less than 13 mm have higher risks of neurological deficit.[21]

The Steel’s rule of thirds: one-third cord, one-third odontoid, and one-third safe space, remains constant throughout the growth of cervical spine and is an important guide on transverse cuts of MRI.[22]

The tip of odontoid lies below the basion in both flexion and extension of the cervical spine. Any translation of more than 4 mm is indicative of AAI.[23]

Swischuk line drawn from posterior ring of C1 to anterior cortex of posterior ring of C3 passing 2 mm or more behind the anterior cortex of posterior ring of C2 is suggestive of AAI.[24]

Associated anomalies in congenital AAI

Congenital AAI is associated with basilar invagination in up to 50% of cases. This may lead to nystagmus due to cerebellar compression and brainstem ischemia due to vertebral artery compression. Unilateral absence of C1 may be noted which may be due to aplasia of the lateral mass or complete absence of hemiatlas leading to torticollis.[25] Aplasia of the odontoid may be complete or partial and is usually an incidental finding.[21] Os odontoideum may be seen but its etiology is currently disputed, whether being traumatic or congenital.[26] Other rarer anomalies include occipitalization of atlas, bifid C1, ossiculum terminale, fusion of vertebral bodies, and hypoplasia of vertebral bodies.[2728] Involvement of other musculoskeletal structures may be noted due to the inherent ligamentous laxity in syndromic cases.

Classification

AAI refers to loss of stable articulation between the atlas and axis and may be classified according to their etiology, that is, congenital, inflammatory, traumatic, or idiopathic.[29] Congenital causes may include syndromes like Down’s, Goldenhar, Morquio, various dysplasias affecting the musculoskeletal system, and osseous abnormalities in the occipitocervical region due to altered embryology. The initial classification of AAD was given by Greenberg: consisted of 2 groups, reducible and fixed.[30] The current widely used classification is the one devised by Wang et al. It classifies atlantoaxial dislocation into four types: instability (type I), reducible dislocation (type II), irreducible dislocation (type III), and bony dislocations (type IV).[18]

Syndromic AAI

Numerous syndromes like Down’s, Goldenhar, Morquio, and various dysplasias affecting the musculoskeletal system are seen to be associated with AAI. Basilar invagination and ligamentous laxity is frequently associated within these syndromic cases.[3132] Conversely, in patients with basilar invagination, AAI should be suspected if any of the following features are present—bifid posterior arch of atlas, os odontoideum, occipitalized atlas, C2-3 or C5-6 fusion, and syringomyelia.[33] Aplasia or hypoplasia of the occipital condyles, odontoid, and atlas should also be looked for in these cases. The AAI in syndromic cases is usually reducible and have lesser severe myelopathy as compared with non-syndromic AAI.[32] These patients are usually associated with higher mortality rates due to cervicomedullary compression and also have narrowed spinal diameter at C1-2 level further aggravating the basilar invagination.[34] In these cases, realignment and reduction may be effective in decompression from posterior approach alone if it is reducible but anterior approach may be needed in case the reduction is not possible from a posterior approach.[33]

Challenges in children less than 5 years

Management of congenital AAI differs significantly than in adults with diagnosis itself being difficult due to incomplete ossification of bones till 9 years of age.[35] Further challenges are due to softer, smaller, and deformed bones in children as well as the fact the fusion may lead to affection of normal growth as well as subaxial misalignment and instability.[36] This misalignment may occur due to factors such as damage to occipitocervical muscles and ligaments during surgery, suboptimal reduction of the joint, compensatory change in subaxial spine secondary to C1-2 fusion, and malposition occurring during postoperative immobilization.[16] The smaller and softer bones of children lead to increased incidence of cortical breach, pedicle fracture, and vertebral artery injury.[15] These difficulties can be better managed by careful preoperative evaluation of the radiographs, CT and MRI which help in better planning with regard to surgical technique being used. For C1 pedicle screw, it is recommended to displace C2 nerve root and transect if necessary, and direct identification of the C1 lateral mass.[37] Similarly, the medial wall of the C2 pedicle exposure is useful for pedicle screw placement.[37] Children usually have a wide space between C1-2 arches, due to which a proper adequate size graft needs to be used.[17] Lateral angulation of atlantoaxial joints, basilar invagination, or spondyloptosis can be seen in up to 35% of children in congenital AAI further complicating the surgical management. Osseous abnormalities like bifid C1 or butterfly C2 can also be seen.[36] The presence of pseudofacets or supernumerary joints also obscure the view of the true joints but these may aid in the reduction of the joint as well as provide increased surface area for fusion.[38] Anomalous vertebral artery maybe present in 5% of patients which makes the surgery more risk prone.[39] These anomalies may pose difficulties to screw placement as they pass directly dorsal to C1 lateral mass. There is also increased incidence of pseudoarthrosis in these children due to postulated factors such as inherent collagen defect and altered immunological response in the initial inflammatory period.[10]

Difficulties during patient positioning and intubation in the operation theater are anticipated challenges which can affect the final outcome. General anesthesia leads to increased joint mobility due to loss of protective reflexes during awake state.[40] To avoid excessive neck movement during intubation, laryngeal mask airways have seen to be a valuable tool aiding in intubation of anesthetized patients.[41] Some authors also suggest use of an endotracheal tube which is two sizes smaller than what normally would have been used.[42] Fiberoptic intubation as well as inline stabilization of the neck can aid in successful intubation with reduced chances of cervical spine injury.[42] Prolonged periods of immobilization may be required in these cases owing to ligamentous laxity.[12] Last but not the least, surgeries for posterior C1-2 arthrodesis involves a lot of fluoroscopic images intraoperatively, which can have a debilitating long-term effect on the child.[14]

Role of conservative management

Conservative management may be considered in symptomatic children without any neurological deficit. These usually consist of cervical halter traction in supine position for 24–48 h followed by orthotic immobilization of the cervical spine with active range of motion exercises with regular follow-up to detect any radiological or clinical progression.[43] Children with conditions predisposing to AAI should have their cervical spine screened at 3 to 5 years and advanced imaging considered in case of any discrepancies on radiographs.[44] Controversy exists regarding the participation of asymptomatic children in sports with the Paralympics committee restricting participation by children with radiological instability.[45] It is still unclear whether the susceptible children with ADI greater than 4.5 mm are at increased risk of neurodeficit.

Indications of surgery

Surgery is indicated in symptomatic AAI to prevent respiratory failure, progressive neurodeficit, and death, but guidelines are not clear regarding management in asymptomatic children. Apart from neurodeficit, the literature suggests operative intervention in children if there is persistent ADI greater than 4 mm or persistent deformity for more than 3 months or persistent radiological or clinical signs even after 6 weeks of conservative management.[18]

Role of traction and anterior procedures

In cases of AAI presenting as dislocation of the atlantoaxial joint, it is imperative to reduce the joint first before proceeding to posterior arthrodesis. Traction can be used for reduction because it lengthens and relaxes muscles leading to reduction of the joint into its normal anatomical position. It is either applied preoperatively or more commonly, intraoperatively. Brief skeletal traction for a period of about 10 min under general anesthesia can be used before proceeding to the definitive procedure in the same setting.[46] It should begin initially with 7–8% of body weight which can be increased to a maximum of 7 kg and monitored by serial lateral imaging. Preoperative traction without general anesthesia is less likely to be successful and usually takes longer time, even if successful.[46] The results of traction for the reduction of atlantoaxial dislocation varies from success rates of 80% to failure rates of up to 80%.[18]

In cases of irreducible atlantoaxial dislocation, odontoidectomy via anterior approach is necessary for complete decompression of the spinal canal. Anterior techniques like transoral anterior release and transoral anterior reduction plate have shown good results in numerous series.[18] Recent advances in endoscopic spinal surgery have enabled odontoidectomy by transnasal, transoral, or retropharyngeal approach. Transoral anterior reduction plate technique is one stage procedure which decompresses the spinal canal as well as fuses the vertebral column anteriorly, making the posterior procedure unnecessary. Studies have shown 100% anatomical reduction and 73.3% improved spinal cord function by this technique.[47] Anterior transarticular fixation is another easier method of reducing irreducible atlantoaxial dislocation with lesser complication rates when compared with anterior plating.[48] However, concerns persist regarding safe screw placement.

Methods of posterior arthrodesis

The earliest method of stabilization of atlantoaxial joint which was semi-rigid involved the Gallie or Sonntag or Brook’s method of wiring along with bone grafting to achieve fusion. The success of these techniques depend hugely on the structural bone graft which provide support to the fixation as well as acts as substrate for fusion.[15] This has fallen out of favor due to need for prolonged immobilization postoperatively, high nonunion rates of upto 30%, and risk of damage to neural and ligamentous structures.[49] Posterior wiring is commonly used today in conjunction with other rigid fixation of atlantoaxial joint in cases requiring revision surgery.[50]

The use of transarticular C1-2 (Magerl construct), C1 lateral mass to C2 pedicle screw (Goel-Harms construct) and C1 pedicle to C2 pedicle constructs are being widely reported in pediatric patients now.[13] They have been used in patients as young as 1.7 years.[3] These techniques do not require rigid immobilization postoperatively and high success rates of fusion when compared with the wiring techniques. They also promote fusion at just one intervertebral level without restricting motion at other cervical spine segments. The advantage of Goel Harms construct is that it is less dependent on the course of vertebral artery when compared with the Magerl technique. The anatomic infeasibility of vertebral artery precluding Magerl technique ranges from 7 to 23% of pediatric patients which can be predicted preoperatively by a CT angiogram.[1417] The other advantage is that, reduction of the joint can be achieved after placement of screws in Goel Harms technique, whereas in Magerl technique, the joint has to be anatomically reduced first before safe placement of the screws. On the other hand, the disadvantage of Goel Harms technique are its increased cost due to more number of implants being used and proud profile of the implant. Limited published literature also show good outcomes in unilateral Goel Harms constructs.[17] The venous plexus between C1, C2 lateral mass, and C2 nerve root is inevitably damaged during exposure of C1 lateral mass, which may lead to serious bleeding which is particularly important in children owing to their low blood volume.[14]

Occipitocervical fusion has a role in the management of congenital AAI. It is indicated in cases of concomitant occipitocervical instability and AAI and in cases which require an anterior release for reduction of the atlantoaxial joint.[10] Relative indication for it also includes concomitant basilar invagination, osseous anomaly of C1 and failure of previous atlantoaxial fusion. The construct for this can either be a custom made occipitocervical construct which can be used along with Magerl or Goel Harms technique or screws and rod separately for the occiput.[51]

Choice of graft

The commonest site of graft harvested in the series noted was either from iliac crest or rib. Series reporting on posterior wiring commonly used grafts from these sites. Graft site complications occur in 10–40% of cases in the pediatric population below 5 years.[52] Rigid fixation techniques like Magerl or Goel Harms construct do not usually require any structural graft. These procedures usually use local bone along with graft harvested from the calvarium. Series have also reported the use of allograft or synthetic bone graft material but they are usually expensive and associated with high rates of pseudoarthrosis.[53]

Complications

Complications arising out of the procedure can be related to the age of the patient as well as the site of surgery. In view of the pediatric population, fractures of atlas or axis may occur during instrumentation. Graft resorption and loss of reduction due to implant loosening can lead to nonunion at the site which necessitates a revision surgery.[713] Down syndrome has seen to be a significant risk factor for complications in such cases.[18] Long-term possible complications would comprise instability of the subaxial spine and loss of lordosis of the cervical spine.[816] Apart from these, complications like vertebral artery injury, spinal cord injury, infection, and donor site morbidity may occur. Respiratory distress defined as single breath count <10 and single breath hold <10 s is one of the single most important factor affecting the morbidity and mortality in congenital AAI.[35] Poor nutritional status, compromised diaphragmatic function and weak respiratory muscles also add to the morbidity and mortality.[5455]

Implications on growth of cervical spine

Maintenance of good spinal alignment and vertical growth can be expected after successful atlantoaxial fusion.[4] Although some studies have noted an increase in cervical lordosis at the rate of 1 degree per fused level per year till skeletal maturity.[56] Subaxial kyphosis may be noted in the postoperative period which usually remodels with growth, called as Toyama remodeling.[57] The growth of C2 vertebrae is lower when compared with other vertebrae.[13] The growth of disc height in the fused area is also lesser when compared with the non-fused area.

Limitations and Conclusion

The major and compelling limitation of this systematic review is the limited number of cases who underwent posterior arthrodesis at atlantoaxial joint. This study only deals with congenital causes, leaving out the infective or inflammatory or traumatic or conditions like os odontoideum, the etiology of which has been in controversy in the recent published literature.[58] Further, the studies included in this review were heterogeneous to a large extent in terms of age and etiology of the AAI. But this study is the first comprehensive review of posterior atlantoaxial arthrodesis in children less than 5 years of age due to any congenital cause.

Patients with congenital cause of AAI may present below 5 years of age with neck pain, limited neck motion, torticollis, or signs of myelopathy. Susceptible syndromic asymptomatic patients should undergo cervical spine screening after 3 years and an increased ADI of more than 5 mm should caution the treating surgeon. Apart from neurodeficit, the literature suggests operative intervention in children if there is persistent ADI greater than 4 mm or persistent deformity for more than 3 months or persistent radiological or clinical signs even after 6 weeks of conservative management. Numerous challenges persist with regard to the pediatric age group in the concerned area. Newer rigid fixation techniques with the use of screws are gaining popularity over semi-rigid fixation using wires.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.