Intra-operative screw trimming in direct screw fixation of the odontoid process fracture--technical note

A radiologic determination of the different screw cutting patterns in cut and uncut orthopedic cortical screws using a novel imaging technique

OBJECTIVE

We hypothesize that cut screws will deform in a manner that increases the core and outer diameters of the screw hole compared to uncut controls, and effects will be more pronounced in titanium screws.

MATERIALS AND METHODS

We used biomechanical polyurethane foam blocks to simulate cortical bone. We organized four groups of stainless steel and titanium cut and uncut screws. Blocks were fitted with a jig to ensure perpendicular screw insertion. We imaged the blocks using digital mammography and measured them using PACS software. Power analysis determined a power of 0.95 and an alpha error of 0.05.

RESULTS

Highly statistically significant differences in core diameter were found after cutting stainless steel and titanium screws. Cutting stainless steel screws increased core diameter by 0.30 mm (95% CI, 0.16 to 0.45; p < .001). Titanium screws' core diameter increased by 0.45 mm (95% CI, 0.30 to 0.61; p < .001). No significant differences were found in the outer diameters of stainless steel and titanium screws after cutting.

CONCLUSION

Titanium and stainless steel screw tracts demonstrated screw core diameter and screw thread pattern deformation after cutting. Titanium screws demonstrated more significant effects.

[Morphology of the surgically treated dens fracture]

The peculiarities within the microarchitecture of the base of the dens have to be assumed as one of the underlying causes for type II and III fractures of the dens according to Anderson and D'Alonzo. As can be demonstrated in a section of the dens of a patient after primary osteosynthesis using two small fragment AO cancellous screws this leads to multiple local trabecular fractures and destruction of the cancellous microarchitecture. Further more compression by the screws results in a higher density of the bone mass in the region of fracture. Apart from the fact that two AO screws guarantee rotational stability, this type of osteosynthesis seems to favour intracorporal bony fusion by increased trabecular bone volume and induction of microcallus within the base of the dens. In a patient with a small intracortical transverse diameter of the dens the available space is completely filled by the screws threads. Partial penetration of the threads through the cortical bone seems to be of no clinical and biomechanical relevance and can be avoided using screws of smaller diameter.