30 “Fixed” (Irreducible) Atlantoaxial Dislocation

10.1055/b-0034-81407

30 “Fixed” (Irreducible) Atlantoaxial Dislocation

Goel, Atul, Muzumdar, Dattatraya

Among other causes, atlantoaxial dislocation can result from trauma, inflammation, and congenital anomalies. Although shorter-duration dislocations often reduce on extension of the neck, long-standing dislocations may become fixed with time. The incidence of fixed atlantoaxial dislocation is relatively low, and the literature on the subject is available only as isolated reports. The treatment protocol in such cases has not yet been fully agreed upon.1–11

This chapter will review the nomenclature, pathogenesis, and treatment options available for fixed atlantoaxial dislocation, with a special emphasis on the utility of joint manipulation and distraction in such patients. According to our current hypothesis, based on observations in surgically treated cases, we now believe that all patients with “fixed” dislocations have pathologically abnormal movements that cause micromotion and cord compression. All patients labeled as having fixed dislocations and neurological deficits can be candidates for manipulation of the atlantoaxial joint, craniovertebral realignment, and reduction and fixation of the dislocation.

Nomenclature

An atlantoaxial dislocation can be described as either mobile or fixed. In mobile dislocation, there is complete radiographic reduction of the dislocation on full extension of the neck or after institution of cervical traction. The dislocation is described as fixed when no radio-graphic reduction occurs even on full neck extension or after institution of cervical traction. In a select group of patients, a dislocation considered fixed on awake traction may reduce when traction is applied under general anesthesia. Therefore, some authors attempt reduction by traction under general anesthesia before labeling an entity as fixed.

Etiopathogenesis

Atlantoaxial dislocation may be congenital in nature or secondary to trauma to the region. It may also occur secondary to rheumatoid or degenerative osteoarthritis of the atlantoaxial joints. Irreducibility of atlantoaxial dislocation is probably the result of a gradual and progressive process rather than an acute episode. Various causes of irreducibility have been suggested. Some authors speculate that prolonged atlantoaxial dislocation results in contraction of the anterior muscles, ligaments, and capsules of the atlantoaxial joint and subsequent scar formation. Scar tissue around the dens and ossification may also be seen in this type of contraction. The development of scar tissue in these cases prevents reduction by nonoperative methods, such as skull traction. Entrapment of the transverse ligament after fracture at the base of the odontoid process, fusion anomalies of the region, and other such causes have also been implicated. Subin et al. believed that, in chronic fixed atlantoaxial dislocation, granulation tissue, callus around the dens, scarring between the dens and the axis, and locking of the atlantoaxial lateral joints are the major reasons for the irreducibility.11 Congenital os odontoideum and fracture at the base of the odontoid process are frequent accompaniments of fixed atlantoaxial dislocation, and in many of these cases, an exact differentiation between the congenital or traumatic etiology may not be possible.

Surgical Options

A variety of methods to treat reducible atlantoaxial dislocation have been described in the literature. Procedures involving screw implantation into the lateral masses have been popular and include either the transarticular method described by Grob and Magerl in 198712 or the interarticular method of fixation described by us in 1988.13 Both the transarticular and interarticular methods of fixation are technically challenging operations, but they provide remarkable stability to the region.

The treatment protocol for fixed atlantoaxial dislocation is, however, not adequately streamlined in the literature. Various authors have suggested a transoral bony decompression followed by a posterior fixation as the safest method of treatment of this complex anomaly. Wang et al. theorized that the dislocation in these patients might be reduced by anterior transoral atlantoaxial joint release without the need for odontoid resection.14 Treatment by posterior decompressive procedures has also been reported but may be associated with a high complication rate.2 Other authors have reported success with stand-alone transoral decompression of the region, without the need for subsequent posterior fixation.

In a substantial number of patients, the irreducibility is due to partial fusion of the atlas–axis joint space; in these patients, attempts at reduction with techniques that do not involve opening of the joint usually fail. Our method of joint distraction followed by interarticular lateral mass plate and screw fixation involves wide exposure of the atlantoaxial facet joint after sectioning of the C2 ganglion. The articular cartilage of the joint is widely removed, the joint space is distracted, and bone graft is introduced in the joint space. This direct atlantoaxial facet distraction facilitated by traction and radiographic control results in reduction of the fixed dislocation in a significant number of cases. Thus, we observed that even in cases where a fusion was evident on imaging, an attempt can be made to manipulate and reduce the joint. Cervical traction followed by occipitocervical fixation may not be effective in a large percentage of these cases. On the other hand, direct opening of the joint and manual distraction are a far more effective procedure. We observed during several operations that in these cases the region is not entirely fixed and is instead abnormally mobile. The excessive and unnatural mobility of the joint is probably the cause of compressive symptoms. It was also observed that fixation, even if it was in an incompletely reduced position, resulted in symptomatic recovery.

Intraoperative Joint Distraction for Reduction of Fixed Atlantoaxial Dislocation

Surgical Technique

The basic steps of the surgery are the same as discussed in our articles on similar subjects.14–18 The patient is placed prone, with the head end of the table elevated to ~35°. The head-high position provides countertraction and helps reduce venous engorgement in the operative field. Cervical traction is instituted prior to anesthesia and is continued. The atlantoaxial facet joints are opened up after excising the capsule and exposed on both sides widely after sectioning of the large C2 ganglion. Large veins in the region of the C2 ganglion and lateral masses frequently cause troublesome bleeding. The venous bleeding can be controlled by employing judicious use of diathermy and packing of the extradural space and the space lateral to the facets with Surgicel and/or Gelfoam. The articular cartilage is widely removed using a microdrill. The joints on both sides are distracted using an osteotome. The distraction is maintained by placement of bone graft pieces harvested from the iliac crest. Whenever necessary, the distraction of the joint can be maintained with the help of specially designed multiholed titanium blocks used as spacers or strut grafts that are placed or impacted into the joints ( Fig. 30.1 ). The size of the spacers used depends on the space available within the distracted joint space. In our initial cases, hydroxyapatite blocks were used, which were later replaced by titanium spacers in subsequent cases. Spacers measure 12 mm in length, 10 mm in breadth, and 4 mm in height. These customized titanium spacers have multiple small holes and are tapered at one end for easier placement during insertion in the joint space. Holes in the spacers allow bone incorporation across the prepared joint space. Morcellized bone graft harvested from the iliac crest is inserted into the distracted joint space on all sides and into the holes of the spacer. Plate and screw fixation of the region is subsequently performed by the interarticular technique. A two-holed stainless steel plate is used measuring 15 to 20 mm in length. The screws measure 2.4 to 2.8 mm in diameter and 20 to 28 mm in length. Screws are passed bilaterally through the holes in the plate into the lateral mass of the atlas and pars of the axis ( Figs. 30.2, 30.3, 30.4, 30.5, and 30.6 ). Axis screws are tightened first, then the atlas screws are tightened simultaneously on both sides. This maneuver, which is akin to reduction of lumbar spondylolisthesis using pedicle screws and plates/rods, helps to reduce the forward slip of the atlas over the axis. The reduction is assessed on the table by intraoperative fluoroscopy. Our experience with neuronavigation suggests that this technology can help to make the procedure significantly safe as regards the vertebral artery and in selecting the best trajectory for screw implantation. Postoperatively, the traction is discontinued, and the patient is placed in a four-poster hard cervical collar for 3 months; furthermore all of the patient’s physical activities involving the neck are restrained during this period.