Atlas fractures represent the second most common bony lesion of the upper cervical spine. They represent about 25% of all cranio-cervical injuries; 2%–13% of all cervical spine injuries and around 1%–3% of all spine fractures. Furthermore, 3.5% of all pediatric spine fractures and dislocations are atlas fractures.1 Concomitant fractures of the cervical spine are common with C1 fractures, for example, odontoid fractures combined with atlas fractures occur in approximately 40%–44% of cases.
Sir Astley Cooper, a surgeon from the United Kingdom who worked in London and Paris, was the first to describe a fracture of the first cervical vertebra in an autopsy report in 1823. In 1908, Vincenzo Quercioli, an Italian surgeon from Siena2 described 4 symmetrical fracture lines of the atlas in an autopsy. He also emphasized the importance of the integrity of the atlantoaxial ligaments, especially the transverse ligament for the stability of the atlas ring. In 1920, Sir Geoffrey Jefferson, a British neurosurgeon who worked in London and British Columbia, developed an initial classification system of atlas fractures by evaluating 42 cases in the literature and 4 of his own cases. Jefferson again described Quercioli's 4-part burst fracture due to a vertical compression of the atlas, which is nowadays known as “Jefferson-Fracture.” Barker et al3 was the first to describe in 1976 a bony avulsion of transverse atlantal ligament (TAL) by describing an uncommon fracture entity associated with a fracture of the medial wall of the lateral mass of C1. By an unknown fracture mechanism, he postulated that “The bone fragment is produced by a combination of TAL stretching and pressure on the lateral mass due to contraction of the neck muscles.” Twenty years later, Dickman et al4 evaluated lesions of the TAL and their relevance for the stability of atlas fractures.
Despite originally being descried almost 200 years ago, and the relative frequency of the injury, there is still a paucity of literature available to guide treatment of these fractures. A comprehensive PubMed search from 1966 to June 2016 about atlas fracture treatment identified only 13 relevant clinical case studies addressing the topic of osteosynsthesis of C1 fractures (Table 1). Furthermore, not only are there very few studies, the quality of studies on this topic is poor.5,6 No randomized studies are available, and detailed comparison of studies is difficult because of a substantial lack of uniform diagnostic criteria, inhomogeneous classification systems, and incomparability of clinical or radiological evaluation methods.
Clinical presentation of atlas fractures was described in much detail by Jefferson.7 Patients suffering from a traumatic fracture of the atlas may complain of pain in the upper cervical spine. Muscle tenderness, muscle spasms, and decreased motion of the upper cervical spine, especially with rotation, are common. Rarely, the patient may present with torticollis, or a “cock-robin” deformity from a C1 fracture. A neurological deficit due to a spinal cord injury linked to an atlas fracture has been described, but it is rare.8 In addition, a posttraumatic lesion to the lower 4 cranial nerves (IX–XII) called “Collet-Sicard syndrome” may occur.9 Furthermore, because of a potential unilateral or bilateral vertebral artery lesion or posttraumatic thrombosis, symptoms associated with a hypoperfusion of the basilar supply territory like nausea, vomiting, tinnitus, impaired vision, and drop attacks are possible. The differential diagnosis includes predominantly congenital anomalies like hypoplasia of the anterior and posterior atlas arch, bipartite atlas, or posttraumatic nonunion.
There are several classification systems for atlas fractures available (Table 2). In clinical studies, the 3 most commonly used are the Jefferson classification7 followed by the Landells and Van Peteghem10 and the Gehweiler et al11 classification. Although the Jefferson classification is commonly used in America and Asia, the Gehweiler classification is wide spread in Europe.
Jefferson7 distinguished in his original publication between 5 different fracture types (anterior arch, posterior arch, burst, lateral mass, and lateral mass plus posterior arch) without numbering them (Table 2). Yet, several authors12 have added numbers to this description defining posterior alone and anterior alone fractures as Jefferson type I. A fracture of both arches (4-part burst fracture) was described as type II and a lateral mass fracture with or without a posterior arch fracture as type III. Type I is the most common followed by type III and type II.1
The Landells10 system is based on the Jefferson classification and defines a type 1 fracture as a fracture of the anterior or posterior arch. Type 2 fracture involves the anterior and posterior arch, whereas type 3 involves the lateral mass.
Gehweiler11 divided atlas fractures into 5 subgroups (Fig. 1). A type 1 atlas fracture is an isolated fracture of the anterior arch, whereas a type 2 atlas fracture is an isolated, predominately bilateral fracture of the posterior atlas ring. The combined injury of the anterior and posterior arch of the atlas, the “Jefferson-fracture” corresponds to a Gehweiler 3 fracture. It is important to note here the distinction between stable and unstable injuries. In stable injuries, the TAL is intact (so called type 3a). If these fractures are associated with a lesion of the TAL, they are classified as unstable (so called type 3b).13 The type 4 fractures are fractures of the lateral mass. Type 5 fractures are isolated fractures of the C1 transverse process.
The integrity of the TAL is important for atlas stability. Dickman4 presented a classification (Fig. 2) which distinguishes between an intraligamentous rupture and a bony avulsion of the ligament. The intraligamentous rupture (type 1) involves a central lesion (type 1a), whereas a lesion close to the lateral mass is referred to as a type 1b injury. The bony avulsion of the transverse ligament from the lateral mass (type 2) can be differentiated into an isolated bony avulsion (type 2a) and a bony avulsion due to a fracture of the lateral mass (type 2b). Another important factor is the degree of separation or dislocation of the bony avulsion.
The anatomical configuration of the atlas with the superior articular facet inclined in a mediocaudal direction while the inferior articular facet is orientated in a mediocranial direction resulted in the fact that axial loads to the occiput translate into distraction of the lateral masses resulting in ring tension or ring fractures. The predominant area of fractures is the weakest point of the atlas that corresponds to the attachment of the anterior and posterior arch in the lateral mass.
An interesting study from Skold14 reported on 30 atlas fractures identified in autopsies. He identified that forehead injuries associated with extension typically resulted in posterior arch fractures, whereas anterior and posterior ring injuries (Jefferson fractures) were typically associated with an impact to the vertex. Lateral blows when they were associated with compressions resulted in his study in anterior arch fractures. In addition, Panjabi et al15 demonstrated that isolated transverse ligament lesions are possible even without bony injury of the atlas, and Oda et al16 showed that a low velocity trauma is often associated with lateral mass fractures. By contrast, a high velocity trauma typically results in burst fractures of the atlas ring.
Imaging and Criteria for Instability
Using conventional cervical spine radiography, nondisplaced atlas fractures might often be overlooked. In case of an atlas burst fracture with severe dislocation (Fig. 3), the open mouth anterior-posterior x-ray (odontoid view) might show a unilateral or bilateral overhang of C1 lateral mass over C2 superior articular process. In case of a C1 overhang, the distance between the lateral border of C1 and the lateral border of C2 should be measured. If the dissociation is bilateral, both data should be summed. According to the “rule of Spence,” instability is given if the overhang of C1 is more than 6.9 mm. However, it was proposed by Heller et al17 that this number should be adjusted to 8.1 mm because of radiographic magnification factors. Today, the “rule of Spence” has been shown to be inaccurate for assessing the integrity of the TAL, with Radcliff et al reporting little correlation between bony displacement on computed tomography (CT) and the integrity of the TAL. Instead, the authors proposed using magnetic resonance imaging (MRI) for evaluating the integrity of the TAL. Dickman18 also demonstrated that 60% of patients with a TAL rupture would not have met radiographic criteria for a TAL injury. An additional criterion for instability that has been proposed is the widening of the anterior atlantodental interval to more than 3 mm in functional lateral x-rays. To evaluate the integrity of the atlas ring in detail and to classify a potential atlas fracture, a CT is always necessary (Fig. 3). Especially, the axial CT slices should be carefully reviewed to detect a bony avulsion of the transverse ligament as a criterion for a potential instability. If a dislocation of the C1 lateral mass is obvious and the CT was unable to detect a bony avulsion of the transverse ligament, an MRI is recommended to evaluate the integrity of the transverse ligament.18 This is important to distinguish between stable burst fractures (Gehweiler type 3a) and unstable burst fractures (Gehweiler type 3b). Trauma to the upper cervical spine might compromise many vascular structures. However, the vertebral artery is most at risk, especially in Gehweiler type 5 lesions and in lateral mass fracture involving the foramen of the vertebral artery. Especially in these seldom cases, an Angio-CT or an Angio-MRI has to be performed to exclude lesions of the vertebral artery within the foramen of transverse process of C1.19
The treatment of C1 fractures remains controversial and is typically influenced by the presence of other cervical spine injuries. No internationally accepted treatment standards are available. The therapeutic algorithm presented in this review is focused on isolated atlas fractures in adults and is based on the Gehweiler classification. While conservative treatment is the method of choice is most cases, widely accepted indications for surgery are an atlas fracture associated with atlanto-occipital instability, an intraligamentous rupture of the transverse ligament (TAL), and an “unstable” atlas fracture. Instability was defined by Hein20 as a fracture of the anterior and posterior arch of the atlas associated with a rupture of the TAL and an incongruence of the atlanto-occipital and the atlantoaxial facet joints.
Gehweiler Type 1, 2, and 5 Fractures
Atlas fractures type 1, 2, and 5 are stable. These fractures can be treated with cervical spine immobilization for 6 weeks by a soft cervical collar. In type 5 fractures, a vertebral artery lesion has to be excluded.
Gehweiler Type 3 Fractures
In stable atlas fractures, type 3a according to Gehweiler, a conservative therapy in a Philadelphia collar is possible. However, these patients should be carefully reviewed regarding further dislocation, nonunion, and/or signs for atlantoaxial instability.
For unstable type 3b fractures with minimal displaced bony avulsion of the transverse ligament (Fig. 3), a direct osteosynthesis of the atlas or halo-traction for 6–12 weeks is recommended. However, nowadays, more surgeons prefer the surgical management of type 3b lesion regarding the potential discomfort, complication rate, and nonunion rate while using halo-traction for these injuries. An isolated atlas osteosynthesis is not recommended in old patients, because of the age-dependent reduced capability of bony healing and in type 3b fractures with severe dislocated bony avulsion of the transverse ligament (type Dickman II). Although a temporary fixation of the atlantoaxial complex is an alternative in young patients, a definitive atlantoaxial fusion is currently the treatment of choice for the old patients suffering from unstable type 3b atlas fractures.
For displaced type 3b atlas fractures and intraligamentous rupture (type I according to Dickman), an atlantoaxial fusion is recommend because of the assumed unlikelihood of ligamentous healing and potential posttraumatic translational atlantoaxial instability. Depending on the anatomical situation and the ability of intraoperative reduction, a posterior atlantoaxial fusion according to the Magerl/Gallie or Goel/Harms are viable treatment options.
Gehweiler Type 4 Fractures
Most of the type 4 fractures are minimally displaced and can be treated conservatively in a soft cervical collar. In the rare case of a primary or secondary significant dislocation of the fractured lateral mass, resulting in an incongruence of the atlanto-occipital and atlantoaxial joint, a reduction under traction and retention for 6–12 weeks in a halo-fixator is recommended. Axial traction in most of the cases leads to an adequate lateral mass realignment by ligamentotaxis and an adequate bony healing might be achieved. However, after initial reduction and after 3, 6, and 12 weeks under halo-traction, a CT evaluation is necessary to confirm an adequate joint realignment and bony healing. In case of inadequate initial reduction under halo-traction or early redislocation, surgery might be indicated to maintain adequate reduction. Primary surgery might also be indicated in cases where a unilateral lateral mass sagittal split fracture is evident.21 This fracture type is characterized by a large lateral fragment of the lateral mass that is completely separated from the anterior and posterior arch. Recent studies have indicated that this fracture type, if treated conservatively, often results in a lateral displacement of the lateral fracture fragment accompanied by a subluxation of the occipital condyle, clinically associated with nonradiating neck pain, head malposition (cock-robin deformity) and diminished head rotation.21
Because of the potential challenges in achieving a good screw fixation within the fractured lateral mass of C1 and the affection of the atlantoaxial and atlanto-occipital joint, a temporary occipito-cervical stabilization without fusion is recommended by the German treatment guidelines; however, there are some regional variations in treatment. As an alternative, a unilateral lag screw osteosynthesis has been described in oblique fracture patterns without comminution.22 In temporary occipito-cervical stabilizations, the implants should be removed after bony healing of the atlas fracture to allow for regaining motion. In case of an inadequate healing and an occipito-atlantal or atlantoaxial joint incongruity under halo-traction resulting in a symptomatic nonunion, an occipito-cervical fusion is recommended to prevent the patient from persistent pain caused by posttraumatic arthritis.
Most of the patients with stable atlas fractures do well under conservative treatment. Because of the fact that there is no evidence suggesting the superiority of any form of external immobilization (Halo, hard collar, and soft collar) over another, the one with the lowest complication rate might be the best. Specifically in elderly patients, the use of a halo-vest should be avoided, as it has been demonstrated to have high rates of complications in geriatric patients.
There are only small case series available about successful conservative management of unstable atlas burst fractures. However, some authors from recent reviews advocate 6–12 weeks of halo-fixator traction to manage these unstable atlas fractures predominantly with Dickman type II lesion. The advantage of halo-traction is seen in the external stabilization without need for surgery with a potentially surgical complication.23 Another advantage might be the avoidance of atlantoaxial fusion. However, halo-traction is an invasive form of conservative fracture management not without risks for the patient. Complications of halo-traction are documented in a publication of Strohm et al,24 who evaluated halo-fixator treatment in 41 patients with upper cervical spine fractures. The authors described several complications like fracture redislocation (20%), screw loosening (15%), pin screw infection (10%), skin necrosis (5%), and intracranial screw penetration (2.5%).24 Furthermore, the patients were asked about the treatment comfort of halo-traction, with 58% rating it intolerable and only 10% tolerable.24
Because of the discomfort and potential complication of halo-vest treatment and the availability of modern operative techniques, surgical management is recommended for unstable atlas fractures. The exceptions are displaced Gehweiler type 4 fractures especially in young patients; there, the morbidity of atlanto-occipital fixation/fusion has to be balanced individually with the risks and benefits of halo-vest treatment.
Osteosynthesis of the Atlas
Until today, the primary indication for an osteosynthesis of the atlas is an unstable burst fracture (Gehweiler type 3b) with bony avulsion of the transverse ligament (Dickman type II) (Fig. 3). However, up until now, only mild dislocated bony avulsion fragments have been addressed by this surgical technique; otherwise, a good fragment reduction associated with bony healing of the ligament avulsion might be difficult to achieve (Fig. 3). Based on current knowledge, potential consequences of nonhealing of the bony avulsion might be a posttraumatic insufficiency of the transverse ligament with chronic pain due to translatory atlantoaxial instability.
An osteosynthesis of the atlas can be performed by anterior–transoral, isolated posterior or by combined posterior/anterior-transoral approach (Fig. 4). For each kind of approach, case series only with a few patients are available (Table 2).
For isolated posterior atlas osteosynthesis, a standard midline approach to the upper cervical spine with bilateral lateral mass screw placement has to be performed. The trajectory of drilling is given by the anatomy of the lateral mass and the fracture lines, which have to be carefully analyzed before surgery.25 Atlas reposition is the key point of the osteosynthesis and might be achieved by a combination of monoaxial screws, dedicated reposition tools, and manual bilateral external neck compression. The final fixation is achieved by a screw-and-rod connection. It is highly recommended to establish bicortical lateral mass screws because their pull-out strength is significantly higher compared with monocortical C1 lateral mass screws.26 Ma et al27 compared the pull-out resistance for C1 pedicle screws and C1 lateral mass screws biomechanically in vitro. By equal pull-out resistance for monocortical C1 pedicle screws and bicortical C1 lateral mass screws, they recommended monocortical screw placement in case of C1 pedicle screw usage.27
The anterior–transoral atlas osteosynthesis was described by Ruf28 and is performed by a standard transoral approach. As an alternative, a transnasal endoscopic approach has also been described.29 Anteriorly inserted screws should be placed in the “Safe zone.” Detailed anatomical investigations are available regarding these ideal entry points.30
The posterior–anterior atlas osteosynthesis described by Böhm31 combines a posterior screw fixation using intentionally longer bicortical C1 lateral mass screws with an additional transoral wiring between the screw tips to close the C1 ring anterior and posterior. This combined procedure enables a perfect C1 ring reduction. However, because of the increased risk of 2 approaches, this procedure should not be the treatment of first choice. Only one case series is available describing this combined approach (Table 1).
Posterior Atlantoaxial Fusion
Today, the indication for posterior atlantoaxial fusion is given in case of unstable atlas burst fractures with intraligamentous lesion of the transverse ligament (type Dickman I). It is also indicated if translatory atlantoaxial instability develops after conservative management of Dickman type II lesions or failed isolated atlas osteosynthesis.32 Atlantoaxial fusion can be performed either by transarticular C1-2 screw fixation according to Grob and Magerl33 or by posterior screw-and-rod fixation according to Goel/Harms.34
Previously, it was assumed that most of the patients with stable atlas fractures do well under conservative treatment. However, the outcome for patients with conservative management of Jefferson fractures, compared with the general health situation before injury, is suboptimal. Dvorak et al35 have described a worse long-term outcome (Short Form 36 and pain score) for patients with more than 7 mm displaced Jefferson fractures compared with minor displaced Jefferson fractures. Lewkonia et al36 have performed a literature review regarding the outcome of conservative management of C1 burst fractures. They described a rate of 8%–20% with complaints about stiffness in the neck, a rate of 14%–80% with mild pain and a rate of 34% of patients with limitations of their activities.36 Furthermore, a substantial number of malunions and nonunions have been described.
Currently, there are case series with a maximum of only 22 patients available, describing the outcome after operative management of unstable atlas fractures. Hu et al37 treated 12 patients by isolated posterior C1 osteosynthesis. A good relief of visual analog scale (VAS) neck pain (7.52 ± 3.2 → 1.80 ± 2.12) and a physiological cervical range of motion were described. Especially the atlantoaxial rotation, measured by functional CT, was 62 degrees on average (36–75 degrees).37 These findings were similar to the clinical results of several other authors, which are shown in Table 1. However, patients with atlantoaxial fusion also do well, although their head rotation is reduced significantly. Elliot et al38 have compared the outcome of transarticular screws (Magerl procedure) and screw-rod-constructs (Goel/Harms procedure) for C1/2 fusion. A higher incidence of vertebral artery injury (4% vs. 2.0%), a higher rate of screw malposition (7.1% vs. 2.4%) and a slightly lower fusion rate were evident for the use of transarticular screws.38
Shift of Paradigm: A Plea for Atlas Osteosynthesis
An atlas osteosynthesis has several advantages compared with atlantoaxial fusion. With an atlas osteosynthesis, the stabilization is limited to the atlas, the motion can be preserved, and there is no need for bone graft harvesting; hence, the operating room (OR) time, surgical trauma, and potential complications can be reduced.
Unstable atlas fractures have historically been treated by atlantoaxial or occipito-cervical fusion. Until today, an atlantoaxial fusion was recommended for type 3b atlas fractures either with significant dislocated bony avulsion of the TAL or with intraligamentous (midsubstance) ruptures of the TAL. For example, Dickman4 reported that all type 1 TAL injuries require atlantoaxial fusion. The recommendation is based on the paradigm that the lesion of the TAL always results in a translatory atlantoaxial instability. However, recent studies have indicated that even in these cases, an atlas osteosynthesis might be a valid surgical alternative to atlantoaxial fusion39 because “atlas osteosynthesis is not primarily contingent on TAL integrity.” Shatsky et al39 also mentioned that a “TAL incompetence is not a contraindication to primary osteosynthesis of the atlas.”
Although the Rule of Spence is not accurate enough to detect TAL rupture, this diagnostic rule has defined treatment modalities (conservative vs. surgical) for decades. Further in the pre-MRI area, it was not possible to detect TAL ruptures adequately. Hence, it can be assumed that multiple patients with overlooked TAL ruptures due to atlas fractures have been treated conservatively in the past. However, hardly any case is available in the literature that has documented translatory or rotatory instability after this treatment. This might highlight the limited relevance of a TAL rupture in compression fractures of the atlas regarding translatory instability.
Our understanding of the ligamentous translatory atlantoaxial instability is predominantly based on patients with rheumatoid arthritis. However, in these patients, not only is the TAL affected by the rheumatoid inflammation process, all other secondary stabilizers such as the alar ligaments, the longitudinal part of the cruciate ligament, and the tectorial membrane are also involved. Similar disruption of secondary stabilizers can be assumed in traumatic flexion extension injures due to shear forces with combined atlas and axis injuries. That, however, is different in isolated traumatic compression injuries of the atlas. Although the TAL is ruptured with or without an atlas burst fracture, the secondary stabilizers, which are not endangered by compression forces, might remain completely intact, providing enough stability to avoid translatory atlantoaxial instability. Furthermore, atlas osteosynthesis is able to reconstruct the occipito-atlanto-axial tension band (especially, the tension of the alar ligaments and the longitudinal part of the cruciate ligament) through reduction, and in particular, height restoration. That is also different in patients with rheumatoid arthritis where the cranial settling and the erosion of the occipito-atlanto-axial joints results in a height loss associated with floppy ligaments that are not able to provide translatory stability. In addition, a “healing” or at least the development of a stable scar of the TAL might be possible with atlas osteosynthesis. The concept of a TAL scar providing adequate stability which is able to withstand translatory forces has not been studied at all so far.
Biomechanically, the idea that an atlas osteosynthesis can restore a stable atlantoaxial complex is supported by Koller et al40 They have performed an in vitro biomechanical testing to analyze the mechanical property of a C1 osteosynthesis in the situation of an incompetency of the TAL. Five specimens (C0-2) were tested with an intact C1 ring and intact TAL for atlantoaxial subluxation and load displacement data. After bony osteotomy, simulating a Jefferson fracture, unilateral left capsulotomy, cut of the transverse ligament, and C1 osteosynthesis, the specimens were tested again. The authors evaluated a sufficient biomechanical C1-2 stability under physiological loads after C1 osteosynthesis and concluded that “C1 osteosynthesis might be a valid alternative for the treatment of displaced Jefferson burst fractures in comparison with fusion of C1-2.”40
Clinically, these ideas are supported by the retrospective case–control study. Shatzky et al39 evaluated 12 patients with an atlas fracture who had a type I (n = 5) or a type II (n = 6) TAL injury according to Dickman, which were treated with atlas osteosynthesis. After an average follow-up of 17 months, all patients demonstrated atlantoaxial stability on flexion/extension films. Hence, in this series, atlas osteosynthesis was able to restore atlantoaxial stability even in cases of a midsubstance TAL rupture (type 1).
Finally, from the surgical point of view, performing an atlas osteosynthesis does not burn any bridges. Even if a patient with an atlas osteosynthesis develops secondary translatory atlantoaxial instability during follow-up, either because of an inadequate healing of the TAL itself or the bony avulsion fragment to the lateral mass, or instability of secondary stabilizers, an atlas osteosynthesis can easily be converted to an atlantoaxial fusion. The conversion to a Goel/Harms fixation can be archived by just removing the C1-C1 cross-connector, inserting 2 additional screws in C2 and performing a C1/C2 stabilization with bone grafting.
Based on the abovementioned arguments, the current “state of the art” concept of performing an atlantoaxial fusion in unstable atlas fractures with a rupture of the TAL (Dickman type 1) has to be challenged. Maybe, an isolated motion preserving atlas osteosynthesis might be sufficient in these cases. Further studies have to prove whether atlas osteosynthesis has the potential to shift a paradigm.
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