'Pseudofacets' or 'supernumerary facets' in congenital atlanto-axial dislocation: boon or bane?

Certain abnormal contact points, appearing like additional joints (pseudofacets) were observed between atlas and axis in a subset of patients with congenital atlantoaxial dislocation (CAAD). The origin, function and bearing on management of such pseudofacets remain largely undetermined. The object is to study 'pseudofacets'or 'accessory joints' in patients with CAAD and to analyze the possible genesis, role and bearing of these on surgery and fusion rates.

35 patients with CAAD were analyzed. Reconstructed images of CT craniovertebral junction passing through these pseudo and true facets were studied. A novel method was devised to measure the faceto-isthmic angle of axis, both in patients with CAAD and normal subjects. Operative details and fusion rates were studied in patients with pseudofacets and compared with those without it.

Eight out of 35 patients (6 Irreducible CAAD and 2 with RCAAD) had pseudofacets. These are seen posterior to the true facets and resemble partially formed joints. The C2 facet was acutely bent over its isthmus in these patients. The direction of these pseudofacets appeared to counter the abnormal mobility at C1-2 true facets. Intraoperatively, they posed a visual hindrance to reach up to true facets for placement of spacers and lateral mass screws, requiring extensive drilling. At the same time, they did help in distraction and increased the surface for fusion between C1 and C2 in cases where sublaminar wiring alone was used. Fusion rates were 100 % in patients with pseudofacets.

Pseudofacets may be a result of genetic aberration and nature's mechanism to restrict abnormal C1-2 mobility in CAAD by imparting some stability. Their presence hinders the visualization making it difficult to reach upto the true facets, thus a bane. They may require extensive drilling when direct posterior approach is used, thereby disrupting the natural restrictive mechanism. However, the flattened surfaces provide an increased area for postoperative bony fusion between C1 and 2, making their presence a 'boon'.