gms | German Medical Science

Objective: Osteoporosis of the axis has been identified as a significant predictor for the development of a pseudarthrosis after screw fixation of type II odontoid fractures. If osteoporosis of the axis is present, the most frequent mode of implant failure after screw fixation is via cut-out through the anterior wall of C2. The concept of polymethylmethacrylate (PMMA) augmentation of the proximal screw shaft could serve as a useful supplement under these conditions. The authors tested a cannulated and perforated lag screw for odontoid fracture fixation in combination with in situ PMMA augmentation in osteoporotic specimens.

Method: The second cervical vertebra of 18 human specimens was harvested (median 86.5 years; 69-98 years). Bone mineral density (BMD) of all specimens was determined by quantitative computed tomography. Two groups (A and B) with no significant difference of age (p = 0.71) and BMD (p = 0.31) were composed. Type II odontoid fracture was generated by a sharp chisel. Osteosynthesis was done by a newly developed cannulated screw, which has perforations in the region of the C2 vertebral body. Cement augmentation was carried out for group A with high viscosity PMMA cement. The position of the screw and the cement distribution were evaluated by computed tomography. A load perpendicular to the dens was applied to test osteosynthesis to failure. The load was to the anterior face of the dens transferred by means of a compression die (r = 1.25 mm) at the middle of the articulation with C1 with a loading rate of 10 mm/min.

Results: Cement distribution in the C2 vertebral body was circumferential around the screw shaft with no leakage into the spinal canal or into the fracture gap. In group A (cement augmented screws) the maximum force to failure was 2.4 times higher (363 ± 94 N, p < 0.001) and the stiffness until a load of 40 N was 1.76 times higher (90 ± 35 N/mm; p = 0.031) in comparison to group B (screws without augmentation). The energy to failure for the cemented specimens was 2.7 times higher than for the non-cemented ones (p < 0.001).

Conclusions: Cement augmentation of the newly developed screw is technically easy and safe under in vitro conditions. The biomechanical tests demonstrate a clear superiority of the cement-augmented screws in the axis with reduced bone quality compared to the non-cemented screws. The technique appears promising with regard to the surgical treatment of elderly patients with osteoporotic odontoid fractures.