Atlantoaxial instability—decision options in the modern era

Nonoperative vs operative management of atlantoaxial instability

The first step in managing atlantoaxial instability is deciding whether nonoperative care is a reasonable option.³ In select cases, typically stable fractures without neurological deficit, patients may be managed with a rigid collar or halo, though nonunion is a frequent concern, especially in elderly or osteoporotic individuals.^4–6 When there is clear instability, progressive deformity, or neurological compromise, surgery is the more reliable path (Figure 1).

Historical evolution of C1–2 fixation techniques

The evolution of C1–2 fixation has been marked by steady progression toward constructs able to both reliable and overall greater stability⁷. Early wiring techniques such as those described by Gallie and Brooks were straightforward to perform but had significant biomechanical limitations (e.g. poor rotational control) leading to high rates of nonunion.⁸ A recent global survey of AO Spine members revealed that 0/24 (0%) routinely utilize wiring techniques.

Transarticular screw fixation (Magerl Technique): benefits and risks

The introduction of Magerl’s posterior transarticular screw fixation in the 1980s was a significant advancement to wiring techniques. Fusion rates of 95–100 percent were reported, and the technique provided a construct strong enough to obviate the need for external immobilization^9,10. However, notably Magerl’s method requires anatomical reduction of C1–2 before placement of screw, and anatomical variations limit its use.¹¹ Specifically, ~18–22% of patients are not candidates due to a high-riding or anomalous vertebral artery.^12,13 Overall, the ~5% risk of vascular injury has certainly tempered enthusiasm for its routine application.¹⁴

Goel–Harms Construct: the modern workhorse for C1–2 fusion

The Goel–Harms construct, involving C1 lateral mass screws connected to C2 pedicle or pars screws by rods, has since emerged as the workhorse technique.^15–17 It allows for intraoperative reduction of malalignment, avoiding the most potentially dangerous transarticular trajectories, and has demonstrated excellent fusion rates with fewer vertebral artery complications.¹⁸ Although exposure can be technically demanding and average blood loss is somewhat higher compared with transarticular fixation, the safety margin and versatility of this approach have made it the preferred option in many centers worldwide.^19,20

Alternative posterior fixation strategies in complex anatomy

Other posterior constructs retain important roles in select cases. On the one hand, the C2 laminar screw technique provides a safe alternative in patients with small pedicles or high-riding arteries, with biomechanical properties sufficient for reliable fusion when paired with C1 fixation.^21,22 When C1 is destroyed or unsuitable for instrumentation, or when instability extends cranially, occipitocervical fusion remains the recommended option.²³ 

On the other hand, for patients with acute, reducible type II odontoid fractures and good bone quality, anterior odontoid screw fixation remains a potential option, and is the only method preserving motion at the C1–2 segment.^24,25 Unfortunately, this option is limited to younger patients with favorable anatomy, outcomes are poorer in older patients, diagnosis of osteoporosis, or with a chronic fracture nonunion.^26,27

Decision-making framework for C1–2 surgical fixation

In practice, decision-making for C1–2 fixation follows a relatively consistent sequence. Surgeons must first determine whether motion preservation is possible, which is feasible only in the small subset of patients amenable to odontoid screw fixation. 

In most cases, posterior fusion is required, and the choice between transarticular and Goel–Harms fixation depends on vertebral artery anatomy, reducibility of the joint, and surgeon preference.²⁸  Once operative indication has been determined, listed below is one potential decision-making algorithm regarding surgical technique (Figure 2).

Author’s preferred surgical strategy: Goel–Harms fixation technique

In our practice, the Goel–Harms construct has become the standard method for addressing atlantoaxial instability.^15,19,20 This configuration (C1 lateral mass screws connected with C2 pariscle (pars/pedicle) or laminar screws using short rods) provides excellent fixation and avoids potentially dangerous transarticular trajectory that carries a small risk of vertebral artery injury. Although in certain instances we consider use of transarticular screws with ideal anatomy and an easily-obtainable reduction, in our hands the Goel–Harms technique offers the safest most reliable solution for the vast majority of patients.

Preoperative planning: vertebral artery assessment and imaging

A preoperative CT angiogram or MRI is required to trace the course of the vertebral arteries (V3) and determine whether C2 can accommodate parsicle screws. Intraoperatively, the patient is positioned prone in a Mayfield head holder on a radiolucent Jackson table, maintaining the neck in neutral or slight extension. After midline incision from the subocciput to C3 spinous process, a subperiosteal exposure is carried caudal to the posterior arch of C1 and to lamina of C2, preserving soft-tissue attachments when possible.  

C1 lateral mass screw placement: surgical technique

Starting point for C1 lateral mass is determined by palpating the medial border of the C1–2 joint with a key elevator and moving roughly two burr-widths lateral corresponding to the center of the lateral mass and aligning with the C2 screws below (see below).²⁹ When considering visualization of C1 lateral masses, C2 nerve roots are identified and can be gently mobilized.  Of note, if there is substantial bleeding from paravertebral/pararadicular venous complex, sacrificing C2 roots may improve visualization and access without long-term effects.

As mentioned, there should be preoperative knowledge of vertebral artery course, which typically travels laterally through the foramen transversarium before entering the sulcus arteriosus on the superior surface of the posterior arch. Thus, surgical dissection and the screw starting point must remain below the C1 posterior arch, with knowledge of potentially anomalous courses.

Bleeding from the paravertebral/pararadicular venous plexus is very common and is controlled with lateral suction while avoiding unnecessary exposure toward the laterally based vertebral artery. A Penfield #4 dissector can retract the C2 root medially during drilling and tapping.

Fluoroscopic guidance is always used throughout for cranial/caudal angulation. On the lateral view, the screw is aimed toward the anterior tubercle of C1, with trajectory angled 5-10 degrees medially toward midline. A 1.7 mm burr can be used to open the cortex/make starting point, advancing under fluoro, ball-tip probe to palpate walls/floor of pilot channel followed by a 3.0 mm conical tap under fluoroscopy and ball-tip probe again. Typically, a 3.5 mm × 30 mm screw (20 mm threaded, 10 mm smooth) is placed.

C2 parsicle screw fixation: hybrid strength and safety

Our preferred technique for C2 fixation is the parsicle screw, a combination of the pars and pedicle screw.³⁰ We believe this hybrid construct combines the biomechanical strength of a pedicle screw with the safety (more medial starting) profile of a pars screw.

The starting point is located just cranial to the C2–3 facet joint and 4 mm inferior and medial to the traditional pedicle screw entry site. The cortex is breached with a 1.7 mm burr, and a Penfield 4 is placed medially to be aware of the location of the spinal canal. The trajectory is typically angled 15-20 degrees medially and 30-45 degrees cranially, always being sure to remain in the dorsolateral quadrant of the pedicle to maximize the distance from the vertebral artery.

The channel is tapped with a 3 mm tap, and walls are palpated with a ball-tip probe before inserting a 3.5-4.0 mm screw (20–26 mm length). Optimal purchase occurs after about 10 mm, when the screw crosses the lamina and engages the dorsal cortex of the pedicle.

The parsicle screw offers excellent cortical contact with overall biomechanical stability, while typically maintaining excellent alignment with the C1 screws above. To this end, it is especially valuable tool in cases of high-riding vertebral artery, narrow pedicle corridor, or revision surgery where standard pedicle screws may be unsafe.

Final C1–2 construct assembly and fusion strategy

After screw placement, short rods are contoured and secured to C1/C2 screws, allowing compression across the grafted joints to achieve reduction and firm bone contact. We then decorticate the C1 posterior arch and C2 lamina and apply additional bone graft along these surfaces. A rigid collar is worn postoperatively for comfort, but rigid internal fixation alone provides sufficient stability for fusion.