Rigid screw fixation of the axis, for either atlantoaxial fixation or for incorporation of C2 into subaxial cervical constructs, provides significant stability and excellent long-term fusion results but remains technically demanding due to the danger of injury to the vertebral artery. Anatomic variability of the foramen transversarium in the body of the axis can preclude safe transarticular C1-C2 screw placement in up to 20% of patients. Although more recent methods of C2 screw fixation with pedicle screws allow safer fixation in a higher number of patients, there remains a significant risk to the vertebral artery with C2 pedicle screw placement. The author describes a novel technique of C2 rigid screw fixation using bilateral, crossing C2 laminar screws, not previously reported in the literature, which does not place the vertebral artery at risk during C2 fixation. This technique has been successfully used by the author in cases of craniocervical and atlantoaxial fixation as well as for incorporation of C2 into subaxial fixations. The technique is illustrated, and the author's initial experience in treating 10 patients with crossing, bilateral C2 aminar screws for indications of trauma, neoplasm, pseudarthrosis, and degenerative disease is reviewed. The possible advantages of C2 fixation with C2 laminar screws are discussed.

Including both known and suspected cases, the risk of VA injury was 4.1% per patient or 2.2% per screw inserted. The risk of neurological deficit from VA injury was 0.2% per patient or 0.1% per screw, and the mortality rate was 0.1%. The choice of management of intraoperative VA injuries was evenly divided between placing the patient under observation and initiating immediate postoperative angiography with possible balloon occlusion.

Object The 847 active members of the American Association of Neurological Surgeons/Congress of Neurological Surgeons (AANS/CNS) Section on Disorders of the Spine and Peripheral Nerves were surveyed to quantitate the risk of vertebral artery (VA) injury during C1-2 transarticular screw placement. Methods This retrospective study elicited the number of patients treated with transarticular screws, the number of screws placed, the incidence of VA injury and subsequent neurological deficit, and the management of known or suspected VA injury. Two hundred thirteen (25.1%) of the 847 surgeons responded. One hundred one respondents (47.4%) had placed a total of 2492 C1-2 transarticular screws in 1318 patients. Thirty-one patients (2.4%) had known VA injuries and an additional 23 patients (1.7%) were suspected of having injuries. However, only two (3.7%) of the 54 patients with known or suspected VA injuries exhibited subsequent neurological deficits and only one (1.9%) died from bilateral VA injury. Other iatrogenic complications included dural tears, screw fractures, screw breakout, fusion failure, infection, and suboccipital numbness. Conclusions Including both known and suspected cases, the risk of VA injury was 4.1% per patient or 2.2% per screw inserted. The risk of neurological deficit from VA injury was 0.2% per patient or 0.1% per screw, and the mortality rate was 0.1%. The choice of management of intraoperative VA injuries was evenly divided between placing the patient under observation and initiating immediate postoperative angiography with possible balloon occlusion.

Bone marrow-derived mesenchymal stem cells are pluripotential cells that have the capacity to differentiate into an osteoprogenitor line. It has been demonstrated that BMP-2 can enhance this differentiation process. In an attempt to prolong the transforming effect of BMP-2, we used an adenoviral vector carrying the human BMP-2 gene to transduce marrow-derived mesenchymal stem cells of New Zealand white rabbits. Assays on tissue culture demonstrated that these cells indeed produced the BMP-2 protein. These transduced stem cells were then autologously reimplanted into the donor rabbits. The cells were placed in the intertransverse process area of five rabbits. In one out of the five rabbits, this resulted in the production of new bone which was demonstrable on both radiographic and histologic examination. We conclude that it is possible to successfully transduce mesenchymal stem cells with the gene for BMP-2 such that these cells will produce the BMP-2 protein in vitro. Further, the transduction results in transformation of these cells into an osteoprogenitor line capable of producing bone in vivo. These data suggest the feasibility of employing gene therapy using recombinant adenoviral vectors as a tool for enhancing spine fusion. Further work to improve the fidelity and longevity of the gene transfer is warranted.

It has been well established that bone morphogenetic protein-2 (BMP-2) can induce bone formation both in vivo and in vitro, although high concentrations (up to milligrams) of BMP-2 have been required to achieve this effect in vivo. Further, clinical applications are usually limited to a single dose at the time of implantation. In an attempt to prolong the transforming effect of BMP-2 we used a recombinant adenoviral vector carrying the human BMP-2 gene (Adv-BMP2) to transduce marrow-derived mesenchymal stem cells (MSC) of skeletally mature male New Zealand white rabbits. The pluripotential MSC were incubated with Adv-BMP2 overnight followed by culture in growth medium for 1 week. Assays on tissue cultures demonstrated that these Adv-BMP2 transduced MSC produced BMP-2 protein, differentiated into an osteoprogenitor line, and induced bone formation in vitro. These MSC had increased alkaline phosphatase activity, increased expression of type I collagen, osteopontin, and osteocalcin mRNA, and induced matrix mineralization compared with both non-transduced cells and cells transduced with a control adenoviral construct. To analyze the osteogenic potential in vivo, Adv-BMP2-transduced MSC were autologously implanted into the intertransverse process space between L5 and L6 of the donor rabbits. The production of new bone was demonstrated by radiographic examination 4 weeks later in areas implanted with cells transduced with Adv-BMP2, whereas no bone was evident at sites implanted with cells transduced with the control adenoviral construct. Histological examination further confirmed the presence of new bone formation. These accumulated data indicate that it is possible to successfully transduce mesenchymal stem cells with a recombinant adenoviral vector carrying the gene for BMP-2 such that these cells will produce BMP-2, differentiate into an osteoprogenitor line, and induce bone formation both in vitro and in vivo. Moreover, incubation of the Adv-BMP2-transduced cells for an additional 7 days in culture before transplantation enhances the success rate in bone formation (three out of three) as compared with our previous report (one out of five, Calcif Tissue Int 63:357-360, 1998).

The majority of specimens can safely accept placement of a laminar screw. This study establishes anatomic guidelines to allow for accurate screw selection and insertion. Preoperative planning is essential for safe screw placement via this technique.

Instability of the atlantoaxial complex may result from inflammatory, traumatic, congenital, neoplastic, or degenerative disorders and often requires surgical stabilization. Initial dorsal wiring techniques allow safe fixation but require rigid external immobilization and have been associated with high fusion failure rates. Rigid screw fixation techniques including transarticular screw fixation and C1-C2 rod-cantilever fixation offer higher fusion rates and less need for rigid immobilization but are more technically demanding. C1-C2 fixation using crossing C2 laminar screws offers rigid fixation but without the technical demands of C2 pars placement. The history and techniques of dorsal fixation of the atlantoaxial complex are reviewed, and the success rates and complications of each are discussed.

The PEG hydrogel spinal sealant evaluated in this study is safe and effective for providing watertight closure when used as an adjunct to sutured dural repair during spinal surgery. This readily available tool is superior to other standard of care technologies commonly used to achieve intraoperative watertight dural closure.