Background

Acute C1 fractures represent 3–13% of all cervical spine fractures and 1–2% of all spinal injuries [1]. Concerning C1 fractures, they can be generally categorized into three separate groups based on the location and mechanism of injury [2]. Type 1 involves a single arch (31–45%) and is caused by an axial load with a flexion/extension force applied. Type 2 are referred to as “Jefferson” type fractures (37–51%). Type 2 fractures were historically considered four-point burst fractures of C1, however now they incorporate two- or three-point fractures. Comparatively to type 1 fractures, these injuries occur after an axial load is applied, without flexion or extension. Type 3 fractures are lateral mass fractures which occur due to an axial load and rotation make up 13–37% of all fractures [2] (Fig. 1).

Fig. 1
figure 1

Coronal (A and B) and axial (C and D) computed tomography (CT) images of “Patient Y” with Jefferson fracture. Adapted from Gumpert et al. [3] 2021 under the under the Creative Commons Attribution-Non-Commercial License 4.0 (CCBY-NC)

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Acute C1 fractures can lead to atlantoaxial instability and excessive rotational or translational forces, resulting in direct compression of either the spinal cord or neighboring brain-stem, with potentially devastating consequences. Due to this, surgical stabilization is required when atlantoaxial instability is present. Traditionally, this was achieved with posterior fusion techniques, such as those described by Goel, Hahms or Magerl [4,5,6,7]. Ensuring stabilization of C1 arch fractures is paramount as considerable motion occurs at the atlantoaxial joint. The C1-C2 joint contributes approximately 10° of flexion-extension and 40.5° of axial rotation of the cervical spine, while the C0-C1 segment contributes 23–25° of flexion-extension of the skull. Anatomically, stabilizing ligaments act to ensure appropriate non-pathological motion. This is primarily achieved by the transverse atlanto-ligament (TAL) [7]. As such, C1 fractures with concomitant TAL injuries have been typically defined as unstable injuries, requiring C1-C2 or even occiput to C2 fusion [8]. Although fusion techniques adequately stabilize the spine, they do so by sacrificing considerable motion of the cervical spine, which can negatively impact long-term clinical and patient reported outcome measures (PROMs) [9].

Due to the significant decrease in range of movement following fusion, osteosynthesis has been proposed as an alternative management strategy of C1 fractures. C1 osteosynthesis can be undertaken using either a transoral or posterior approach, as described below.

Anatomy

It is important to understand the anatomy of the C1-C2 joint to fully appreciate its contribution to the overall motion of the cervical spine. The atlas is a transitional structure between the rigid skull and flexible cervical spine. The C1-C2 joint is a highly mobile joint which is reinforced anteriorly and posteriorly by the atlantoaxial ligament. The purpose of the joint is to support the occiput while also providing the greatest range of motion and maintaining stability. The odontoid is secured to the anterior arch of the atlas by the TAL. The TAL is a strong ligament and is approximately 3 mm thick, 8 mm in width and 20 mm in length [10, 11] It acts as the primary stabilizer against translation forces at the C1-C2 junction. Capsular ligaments, alar ligaments, apical ligaments, tentorial membrane and atlantoaxial membranes act as secondary stabilizers against translation forces.

Several classification systems exist for C1 fractures. The Levine classification is based solely on bony anatomy, while Dickman examines injury to the TAL and Gehweller examines both bony anatomy and TAL injury [12]. Reliable classification systems can help guide treatment algorithms. Lauubach et al. [13] examined the interobserver reliability of the Gehweiler classification and found a moderate interobserver reliability for the Gehweiler classification (k = 0.50), with the lowest kappa value observed for 3 A fracture types.

The AO trauma group developed a more comprehensive classification system that incorporates both bony and ligamentous factors which also examines the neurological status among other factors [14]. The use of proven classification systems is hugely beneficial as they can help categorize the injury and aid decision making to determine the optimal treatment strategy for a particular patient. The differentiation between a type AO IIA (a stable bony injury with no significant ligamentous, tension band or disc injury on the C1 ring and/or C1-C2 joint) versus a type AO IIB (tension band ligamentous injury with/without bony injury, without complete separation of anatomic integrity, and considered stable or unstable depending on injury specifics) typically predominates as an indicator for definitive fixation with instrumentation to stabilize the fracture.

It is essential to establish if a C1 fracture is stable or unstable as this will ultimately determine the management strategy. It can be difficult to assess C1 on plain film radiographs due to interference from the occiput. Plain film radiographs should include anterior-posterior lateral and open-mouth odontoid view [12]. Definitive diagnosis of a C1 fracture often requires CT scanning and MRI for the assessment of ligamentous structures which are crucial for determining management [15]. Disruption to the ligamentous structures can be determined by examining the atlantodental interval (ADI) and also assessing for the presence of lateral mass dislocation or atlantoaxial subluxation. An ADI < 3 mm represents normal anatomy, 3–5 mm suggests an injury to the TAL while > 5 mm indicates an injury to TAL and secondary stabilizers and surgical intervention is generally required.

Fusion techniques

The first atlantoaxial stabilization procedure was carried out by Mixter and Osgood in circa 1910 using silk thread for stabilization of the spinous processes [16, 17]. The patient was a 15-year-old boy who had fallen from a tree over six months previously and who had failed multiple attempts conservative treatment. The procedure was completed in the ventral position and pressure on the anterior arch through the pharynx coupled with traction on the posterior arch achieved realignment of the atlas (which was anteriorly displaced) before suturing in place with a heavy silk thread. They patient made a full and uneventful recovery with the use of a leather cuirass for two months post-operatively [16, 17].

While there have been significant advancements in the treatment, currently, no consensus exists regarding the most efficacious surgical strategy for the management of C1 fractures [18]. Injury characteristics such as TAL injury, single-level vs. multi-level injury and patient related factors must be considered in order to optimize surgical management strategies. If there is an isolated fracture, then fusion or single-level C1 osteosynthesis may be indicated. Multi-level injuries are common as the incidence rate of concurrent C2 fractures is between 41 and 44% and often requires traditional screw-rod fusion constructs [18]. Historically C1-C2 fusion is associated with a high arthrodesis rate for primary fusion, reported as > 90% in the majority of studies, with low rates of complications [4, 19]. Nevertheless, C1-C2 fusion is a technically demanding operation and can have devastating complications owing to its proximity to the craniovertebral junction and the associated neurovascular structures. The most common fusion techniques used today are the Goel technique, Harms technique and trans articular screw (TAS) first described by Magerl and Seeman [4,5,6].

The Goel technique was first reported in 1994 which described a new method of posterior fusion involving a C1 lateral mass and C2 pars screw with posterior cervical plates. Fusion rates of 100% were reported [4]. Initially, the C1-C2 facet joint is exposed from a posterior approach. The C1 lateral mass screw is placed through the center of the lateral mass in a medial to lateral direction to avoid medial wall penetration. One major disadvantage of the Goel technique is that it requires ligation of the C2 nerve root for appropriate placement of the cervical screw and can lead to post-operative posterior scalp numbness, as previously reported in 11.6% of patients [20, 21].

The Harms technique was first described in 2001, and is largely a modification of the Goel technique. It utilizes C1 lateral mass screws and C2 pars screws connected by rods and similarly reports fusion rates as high as 100% [5]. The Harms technique has the additional benefit of preserving the greater occipital nerve. In this technique the entry point in the C1 lateral mass is in the center of the junction between the posterior arch of C1 and the lateral mass. Typically, a 3.5 mm poly-axial screw is then inserted into the lateral mass and achieves bi-cortical purchase. The C2 pars screw is then inserted. The entry point is at the superior and medial aspect of the lateral mass. The trajectory of the screw is approximately 20–30° medially and superiorly. Another 3.5 mm poly-axial screw is then inserted. A rod is then used to connect the screws to complete the construct. Spinal fusion or arthrodesis has proved an effective and popular technique of stabilizing fracture segments since its inception in 1911 and is often supplemented with bone grafting, typically autologous iliac crest or synthetic alternatives such as bone morphogenetic protein (BMP) or demineralized bone matrix (DBM) among others [4, 19].

The TAS technique was first described in 1987 by Magerl and Seeman [6] In this technique, fusion was achieved using threaded screws that initially entered the C2 pars crossing the atlantoaxial facet joints with the points of the screws positioning the C1 lateral mass. The C2 entry point was 3 mm medial and 3 mm superior to the medial aspect of the C2-C3 facet joint. The screws were inserted using intra-operative fluoroscopy. They were aimed towards the C1 anterior arch and then directed 0–10° medially. The fixation can also be supplemented by the presence of sublaminar or a C1 hook [22]. Length of fusion with this technique can be from occiput to C2/C3 or C1-C2 [23].

Although aforementioned approaches offer excellent stability to C1 fractures, they do so at considerable sacrifice in relation to retained range of motion. As previously noted, the C0-C1 segment contributes 23–25° of flexion and extension of the skull. Atlantoaxial motion provides a further 10–22° of movement. Comparatively, C1-C2 fusion has been reported to result in loss of 40–60° of axial rotation, and represents the major disadvantage of traditional fusion techniques as this loss of motion can negatively impact long-term clinical outcomes and PROMs. One of the proposed theoretical benefits of osteosynthesis compared to fusion is the preservation of motion segments [23]. This is achieved maintaining axial rotation of the atlantoaxial complex and flexion-extension of the cervical spine. Osteosynthesis in theory should also reduce the concurrence of adjacent segment disease and degeneration. The proposal of C1 osteosynthesis as an effective surgical strategy for C1 arch fractures was met with initial apprehension. The main worry concerned the efficacy of osteosynthesis if TAL was injured or disrupted, as TAL presents the primary stabilizer of the atlantoaxial complex. Furthermore, there were also concerns regarding the high rates of complication particularly via a transoral approach which were reported to be as high as 75% in older ENT studies [24].

The emergence of C1 osteosynthesis and challenge of traditional theories

Ruf et al. [25] first described osteosynthesis of unstable C1 fractures through a transoral approach in 2004, in a retrospective study of 6 patients. The transoral approach was performed using a technique first described by Schmelzle and Harms [25]. Initially the soft plate was split, then the posterior mucosa of the pharynx was incised to create a flap, followed by splitting of longitudinal muscles. The anterior arch of C1 and the anterior aspect of the lateral mass were then able to be visualized. Reduction was performed by applying traction to the head using a Crutchfield clamp. In the first three cases, osteosynthesis was achieved using a compression plate. In the later three cases, osteosynthesis was achieved using a screw-rod construct. The cases were followed up for a mean of six years and five months.

In one case there was a partial subluxation of the lateral masses, and this occurred due to loosening of the screw rod construct. Since its initial description, certain authors report modifications of the technique described by Ruf et al. In 2010 Jo et al. [26] described a technique for osteosynthesis through a posterior approach to minimize the complications associated with a transoral approach through a C1 lateral mass screw construct and provided adequate stability with preservation of spinal motion.

More recently, Zhang et al. [27] described a new technique for the treatment of unstable atlas fractures using a posterior mono-axial screw construct. In this study, only isolated atlas fractures were included. Patients were placed prone in the reverse Trendelenburg position and in traction. A posterior incision was made from occiput to the C3 spinous process. The posterior arch of C1 was then exposed subperiosteally. Two monoaxial 3.5 mm screws were placed through the lateral mass of C1 through the posterior arch using the notching technique. The entry point, length of screws and trajectory were decided based on pre-operative CT imaging. The monoaxial screws were partially threaded and used as lag screws for coronal split fractures. These monoaxial screws were then connected to a titanium rod which was pre-contoured. The authors reported satisfactory reduction was confirmed using intra-operative fluoroscopy. Spinal navigation has found widespread adoption within the field of spinal surgeries, and is renowned for its application in scenarios of challenging anatomy or injury patterns. Since its first use in 1995 by Kalfas et al. [28], intraoperative navigation has evolved to the use of perioperative CT scanning which provides live feedback with the use of a reference field placed in the surgical field. The application of intraoperative CT-scanners such as the O-arm® (Medtronic, MN, USA) has shown comparative or improved efficacy over conventional methods of pedicle screw placement for both open and percutaneous procedures [29, 30] (Fig. 2).

Fig. 2
figure 2

Adapted from Gumpert et al. [3] 2021 under the under the Creative Commons Attribution-Non-Commercial License 4.0 (CCBY-NC)

Coronal (A and B) and axial (C and D) postoperative computed tomography (CT) images of “Patient Y” with screw-rod osteosynthesis construct placed via posterior approach.

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As previously mentioned, one of the reported concerns with osteosynthesis is that an acute C1 arch fracture injury cannot be stabilized with osteosynthesis if the TAL is injured, as it acts as the primary stabilizer of atlantoaxial complex. However, Koller et al. [31] challenged this assumption. In a cadaveric study of 5 patients, cervical-thoracic (C0-T2) sections were instrumented with a screw-rod construct (2 × 3.5 mm screw, 40 mm length rod), and subjected to incremental loading forces, with TAL intact and TAL disrupted. Koller defined a translational load of 100 N as the upper limit of physiological loading. An acceptable difference in ADI between TAL intact and TAL disrupted was reported at forces of 10–100 N. Furthermore, all five specimens (TAL disrupted) succeeded a force load greater than 150 N, 4/5 succeeded a force greater than 250 N, and 3/5 succeeded a force greater than 300 N. The study showed that complete TAL disruption resulted in a C1-2 load transfer of approximately 50% through the posterior capsule, which acted as secondary stabilizers. Puttlitz et al. [32] corroborated these findings and stated that alar and capsular ligaments functioned as strong secondary stabilizers to sagittal plane translation. Atlantoaxial subluxation did not have pronounced levels (defined as anterior ADI > 6 mm) until the alar and/or capsular ligament stiffness were reduced by 75%, in addition to TAL rupture. Therefore, these two studies ultimately questioned if TAL disruption is of major concern as previously believed, once secondary stabilizers are intact.

Comparative outcomes

To date there are ten studies published in the literature pertaining to osteosynthesis for the management of acute C1 fractures [3, 26, 28, 31,32,33,34,35,36,37,38,39]. Case reports were not included for analysis. Types of fracture per respective study are outlined in Table 1.

Table 1 Type of C1 fractures reported in studies
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Comparative outcomes in terms of fusion rates, preservation of motion segments, deformity correction, PROMs and complications are described below.

Fusion rates

In regards to fusion rates, eight studies reported fusion rates of 100%, while two studies reported “no instability on follow-up imaging” (Table 2). Follow-up X-rays were performed as the sole form of follow-up imaging in two studies. Three studies employed CT as the only follow-up imaging, while five studies employed both forms of imaging in the follow-up period.

Table 2 Fusion rates of C1 osteosynthesis in respective studies
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Preservation of motion segments

One of the major potential benefits of C1 osteosynthesis versus fusion for C1 fractures is the preservation of motion segments, which has been assessed by six studies to date [3, 24, 32,33,34, 38]. Four of the studies implemented a transoral approach, while two employed a posterior navigated approach. Osteosynthesis was successful at maintaining axial rotation at the atlantoaxial complex with a postoperative mean range of 39–74° between the studies. The results (outlined in Table 3) demonstrate that the range of motion (ROM) is retained after undergoing osteosynthesis.

Table 3 C1 osteosynthesis outcomes in terms of preservation of motion segments
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Deformity correction

Another concern regarding C1 osteosynthesis is that it would not be able to correct the resultant deformity. Deformity of C1 can be evaluated by assessing the presence of lateral mass dislocation (LMD) and by measuring the atlantodental interval (ADI). Seven studies in total examined reduction of deformity with regards to C1 osteosynthesis [25, 27, 32, 33,34,36, 38]. All of these studies reported a notable reduction in deformity markers, as reported in Table 4. Four of the studies showed a statistically significant reduction in deformity parameters while one demonstrated complete reduction, as shown in Fig. 3 [27, 29, 33, 34].

Fig. 3
figure 3

Comparative pre- vs. post-operative values of lateral mass dislocation (LMD) in respective studies. (@) = reported as “completely reduced”. (*) = p < 0.01. (**) = p < 0.001. (***) = p < 0.0001

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Table 4 C1 osteosynthesis outcomes in terms of deformity correction
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Patient reported outcomes measures

Patient reported outcomes capture surgical efficacy and quality from a patient’s perspective, complementing traditional markers or parameters of surgical quality. Six studies highlight the overall impact C1 osteosynthesis has on patient reported outcomes (PROMS), as shown in Table 5 [27, 32, 33, 36,37,38]. The two parameters reported across studies were the visual analogue score (VAS) and the neck disability index (NDI). A statistically significant improvement in PROMs was evident in three studies (Fig. 4) [32, 36, 37].

Fig. 4
figure 4

Comparative pre- vs. post-operative values of visual analogue scale (VAS) score in respective studies. (*) = p < 0.01. (**) = p < 0.001

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Table 5 C1 osteosynthesis outcomes in terms of patient reported outcome measures
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Complications

Nine studies examined complications associated with C1 osteosynthesis [25, 27, 33,34,35,36,37,38]. The complications of C1 osteosynthesis ranged between 0 and 22%. An important complication to note is that re-dislocation was reported in four cases across studies, highlighted in Table 6.

Table 6 C1 osteosynthesis outcomes in terms of complications reported
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Comparative studies

Yan et al. [39] is the only study to date that examines C1 osteosynthesis versus C1-C2 fusion in the treatment of unstable C1 fractures. This was a prospective, multicenter randomized controlled study with a five-year follow up. Overall, 72 patients were included in the study. It showed that C1 ring osteosynthesis outperformed C1-C2 fusion (P < 0.001) in terms of NDI (13.1 ± 2.5 vs. 16.3 ± 3.3) and VAS score at follow-up, with reduced operative time (75.6 ± 11.3 vs. 189.5 ± 15.6), reduced radiation dose (0.62 ± 0.14mSv vs. 1.04 ± 0.29mSv), reduced length of stay (4.5 ± 0.6days vs. 6.2 ± 0.7days) and cost (29,590.7 ± 100.5RMB vs. 37,129.4 ± 118.5RMB). Additionally, C1 osteosynthesis reported improved flexion-extension (82.3° ± 9.4° vs. 70.0° ± 8.4°) and axial (153.3° ± 10.0° vs. 101.5° ± 8.7°) rotation. The authors concluded that C1-ring osteosynthesis is a safe and reliable surgical technique.

Limitations and future outlook

There are several limitations to the current evidence to support C1 osteosynthesis in the treatment of unstable C1 fractures. The vast majority of studies are retrospective and non-comparative in nature, with the exception of Yan et al. [38]. Retrospective studies may be influenced by inherent selection bias by the surgeon. Additionally, there are currently no studies reporting the relationship between radiographic improvements on deformity reduction and PROMS. It is also important to incorporate changing population dynamics. As there is an increasingly geriatric population, many of whom are frail and have associated conditions such as sarcopenia. Employment of intraoperative navigation could aid singular screw placement in these challenging, as the efficacy of intraoperative navigation for cervical screw placement has been reported in the literature [40, 41]. Nevertheless, Long term efficacy of C1 osteosynthesis in a population with potentially inherent low bone quality is currently unknown, and should serve the premise of further robust prospective studies with long-term follow-up data.

Choice of Procedure and decision algorithm

There is no consensus on a single best surgical technique for Type 3a/3b C1 arch fractures in the literature, however, there are numerous possible methods for surgical stabilization reported [1, 2, 13, 39, 42]. There is currently a paucity of internationally recognized treatment algorithms available [2]. Although Koller et al. [29] challenged the assumption that C1-arch fracture injury cannot be stabilized with osteosynthesis if the TAL is injured (provided secondary stabilizers are intact), there is little clinical evidence currently available to change practice and therefore more research is required before this can be implemented in practice. Therefore, TAL certainly plays a role in the decision-making process as to which procedure might be most appropriate in individual cases.

Exploring this in further detail, Kandziora et al., state that C1 osteosynthesis is only a useful alternative to C1-C2 fusion in Gehweller type 3a injuries following failed repositioning and stabilization using a halo fixator [42]. Type 3a injuries are Jeffersson fractures with intact TAL. A study by Landellis et al. agrees with this approach, however, Mead et al. disagrees, and suggests that in patients with type 3a fractures who fail conservative stabilization, C1-C2 internal fixation should be performed [1, 2].

In relation to type 3b fractures (those with TAL disruption or bony avulsion of ligament with intact TAL), Kandziora et al. state that C1-C2 fusion is the method of choice due to the possible presence of a translational atlantoaxial instability in addition to the unstable fracture. Therefore, TAL status is an important consideration in decision-making regarding choice of surgical treatment [42]. Interestingly, Kandziora et al. also suggest that in unstable type 3b injuries with a slightly dislocated bony avulsion of the TAL, isolated stabilization of the atlas can be performed to preserve cervical mobility, and that C1-C2 fusion should be avoided. They also recommend Goel/Harms stabilization for type 3b injuries, however, this requires subsequent implant removal to release the atlantoaxial motion segment which is a drawback to this procedure. Numerous authors recommend the Goel/Harms technique as a useful stabilization technique, however, the requirement for a second procedure to regain mobility is a significant drawback to the use of this technique, if it can be avoided [1, 2, 13, 39, 42].

Finally, it is described by Kandziora et al. that in unstable type 3b injuries with severely dislocated bony avulsion of the TAL, C1-C2 fusion is indicated as it cannot be assumed that the bony avulsion will heal, effectively ruling out management by osteosynthesis in this patient cohort [42]. A 2022 paper by Yan et al. disagreed with this approach. Here, it was found that C1 osteosynthesis was superior to C1-C2 fusion in unstable arch fractures in terms of long-term neck pain relief and preservation of mobility [39]. They also found advantages in relation to operative time, intraoperative blood loss, post-operative length of stay, radiation dose to the patient and cost when osteosynthesis is performed compared with C1-C2 fusion. Laubach et al. agreed with this assessment and went on to state that ‘the rationale for this surgical management may be related to the fact that isolated osteosynthesis of the atlas in cases of violated TAL integrity was not associated with atlantoaxial stability on flexion/extension films in Dickman type 1 and type 2 injuries’ [13]. Laubach et al. went on to explain that, historically, the requirement for C1-C2 fixation for intraligamentous TAL lesions was due to ligamentous translational atlantoaxial instability in patients suffering from rheumatoid arthritis and who had rheumatoid processes which involved multiple ligaments of the upper cervical spine [13].

Mead et al. indicated that when a patient presents with further sub-axial cervical vertebrate injuries conferring instability, then C1-C2 internal fixation should be undertaken, and fixation should span to the most inferior affected vertebral level, as this will confer significantly greater overall stability across the cervical spine than C1 osteosynthesis alone [1].

Regarding a decision algorithm, there is currently no international consensus on the best approach to these injuries, particularly type 3b injuries. There are some sub-cohorts of patients of whom the decision surrounding operative management can be made with more conviction, such as those with concomitant sub-axial injuries and those with C1 arch fractures with heavily dislocated bony fragment who will require C1-C2 fusion. Type 3b fractures with only slight displacement of bony avulsions can be considered for osteosynthesis [1, 2, 42].

Conclusion

In conclusion, it appears C1 osteosynthesis offers a safe and efficacious alternative option for the surgical treatment of C1 fractures, albeit, in limited circumstances. Some authors suggest that C1 osteosynthesis may be inappropriate in patients who have ongoing pain or persistent instability following failed conservative management of stable 3a Jefferson fractures. However, this is contradicted by others, and some data suggests that osteosynthesis is a suitable alternative and may be beneficial over C1-C2 fusion in certain trauma patients with isolated C1 arch fractures. However, it is also suggested that C1 osteosynthesis is not the operative method of choice in patients with further concomitant cervical spine injuries. In the circumstances where C1 osteosynthesis is deemed appropriate, it has the potential to reduce deformity, increase ROM, improve PROMs and has complication rates comparable with those of fusion techniques. However more robust prospective evidence is required to elucidate the correct patient cohorts for this procedure and to ensure optimal outcomes.