Arrowhead Technique for Posterior Atlantoaxial Fusion
Purpose of Study: There are a number of techniques for posterior stabilization of the Atlanto-axial articulation. These techniques have been changed and adapted over the years to yield a number of different modifications. These techniques include the transarticular screw technique, C1 lateral mass plates, Gallie, Brooks, and Sonntag fusions and their modifications.
Background: One of the most popular techniques is the Gallie fusion and it's modifications. This fusion involves placement of a sublaminar wire or cable placed at C1. An iliac crest graft formed in the shape of an H to fit over the spinous process of C2 and rest against the posterior arch of C1. The sublaminar wire is then used to fashion the graft tightly in place by passing over the graft and wrapping around the spinous process of C2. The modifications of this technique involve different placement of the wire, however all modifications consist of the sublaminar wire at C1 and the wire or cable wrapping around the spinous process of C2.
The Sonntag technique is similar to the gallie technique and differs mainly in the shape and placement of the iliac graft. The graft is actually placed between the posterior arch of C1 and C2 and held in place by a sublaminar wire at C1 and again wrapping around the spinous process of C2. The Brooks technique uses sublaminar wires at C1 and C2. Two bone grafts are placed on either side of the midline and fashioned posteriorly using the sublaminar wires.
We are reporting a novel technique not yet described in the literature that is essentially a hybrid of the Sonntag and Brooks fusion. The technique involves the use of the single intralaminar graft used in the Sonntag fusion and the Sublaminar cables at C1 and C2 used in the Brooks fusion.
The surgical approach to the C1/C2 junction posteriorly is done using standard dissection techniques. The C1/C2 junction must be adequately exposed so as to allow passage of the cables under the posterior arch of both C1 and C2. The Iliac area should be prepped and an iliac crest graft should be taken in the same fashion as the Sonntag fusion. The posterior arch of C1 and the spinous process of C2 should be prepared using a burr to decorticate the bone at the fusion sites in order to increase the fusion rate. Once the intralaminar graft is prepared it should be carved to tightly fit over the spinous process of C2 and grooved to fit between the posterior arch of C1 and C2. Once the graft bone is prepared, and the fusion sites have been cleaned and decorticated the sublaminar cables can be placed using commercially available wire/cable passers, or a bent blunt tip needle and some silk suture that is passed sublaminar and used to pull the cable under the posterior arches. The H-shaped graft is then put in place and the sublaminar wire is tightened around the graft.
The modification allows greater strength with the sublaminar wires at C2 than is provided by the spinous process at C2, which in many cases may be damaged or not present in the case of Spina Bifida occulta at this level. The posterior arches and the graft may be notched to provide greater stability by preventing slippage of the cables. This method provides much improved rotational stability when compared to the gallie fusion, and the posterior laminar clamps. This technique also simplifies the graft construction when compared to the brooks fusion and also requires ½ the number of sublaminar passes with the cable or wire. The advantage of this technique over the transarticular screws and C1 lateral mass plates is the same as the other laminar techniques, which is to avoid the vascular complications of the lateral mass techniques.
Possible drawbacks to this technique include an increased risk of spinal cord damage by the placement of the sublaminar wires at C2. There also may be increased A-P slippage of C1 on C2 when compared to the gallie and Sonntag techniques due to the lack of the S-shape curve of the cable lying in the sagittal plane at the inferior surface of the graft, however the amount of force required to cause such a slip would be likely to cause damage to any of the constructs described above.
In conclusion, this technique may provide an alternative to the classic methods of posterior fixation of the Atlanto-axial joint. It may prove to be especially useful in the case of damage to the spinous process of C2 or a congenital absence of the C2 spinous process. By placing a notch in the posterior arch of C1 or C2 this technique could also be used in the case of an incomplete arch at C1 or C2. The whole construct can also be enforced with a Hydoxyapetite paste which may also increase the strength and fusion rate of this procedure.