The influence of proximal anchors on the risk of proximal junctional fracture in the osteoporotic spine: biomechanical comparison between pedicle screws and transverse process hooks.

STUDY

DESIGN

Biomechanical comparison of the risk of proximal junctional fracture (PJF) after multilevel spinal instrumentation using pedicle screws or transverse process hooks on the top of a pedicle screw construct.

OBJECTIVE

To compare the loads leading to PJF when using pedicle screws or transverse process hooks at the proximal level after multilevel spinal instrumentation using pedicle screws.

SUMMARY OF BACKGROUND DATA: With stronger spinal fixation techniques, there is increased risk of PJF, especially in the osteoporotic spine. The use of transverse process hooks over pedicle screws at the proximal level of multilevel pedicle screw constructs has been proposed to decrease the incidence of PJF. However, there is no biomechanical study evaluating this concept.

METHODS

Twenty-four segments of 4 vertebrae from 6 human cadaveric spines were evaluated after instrumentation of the distal 3 vertebrae using pedicle screws, except at the top of the construct where either pedicle screws (AP group) or transverse process hooks (PTPH group) were placed. The proximal vertebra was left uninstrumented. Quantitative computed tomography scan was used before instrumentation in order to assess the bone density for each specimen. Cyclic compression loading between 50 and 1000 N at 1 Hz was applied to each of 24 instrumented spinal segments until obtaining a PJF.

RESULTS

Bone density was similar between the AP and PTPH groups. A PJF occurred in 22 of 24 tested specimens. The number of cycles required to produce the PJF ranged between 2 and 1002. The number of cycles required to produce the PJF was similar between the AP construct group (median: 3; interquartile range: 3-7) and the PTPH construct group (median: 4; interquartile range: 3-5).

CONCLUSIONS

The current study failed to observe a significant impact of using transverse process hooks over pedicle screws on top of multilevel pedicle screw construct to decrease the risk of PJF.