Effect of insertional temperature on the pullout strength of pedicle screws inserted into thoracic vertebrae: an in vitro calf study.

STUDY DESIGN

The axial pullout strength of pedicle screws that were at different temperatures when inserted was compared in calf vertebrae.

OBJECTIVE

To determine if insertional temperature of the screw itself affects pullout strength.

SUMMARY OF BACKGROUND DATA

Fixation stability of pedicle screws depend on several factors. The development of alternate insertion techniques and screw designs were used to improve the stability. Polymethylmethacrylate and calcium sulfate augmentation have been shown to be viable options for improving fixation; but have the potential disadvantages.

METHODS

Three cadaveric thoracic calf spines were instrumented between T1-T10 bilaterally with one type of pedicle screws stored at different insertional temperatures. The axial pullout tests were performed at cross head speed of 5 mm/min. Pullout loads and displacement were recorded at 1/20 seconds intervals until failure occurred.

RESULTS

The highest pullout force was obtained with the screws inserted at 4 degrees C. These screws had a 19% increase in pullout strength compared with the screws inserted at 24 degrees C. The highest force/torque proportion was gained in the same group as 0.30 kn/Nm.

CONCLUSION

The technique showed increased pullout force with the screws inserted at 4 degrees C. Using pedicle screws stored at 4 degrees C before instrumentation, seems reasonable in an attempt to obtain a better bone-screw interface.