Pullout strength of misplaced pedicle screws in the thoracic and lumbar vertebrae - A cadaveric study

Abstract

Background: The objective of this cadaveric study was to analyze the effects of iatrogenic pedicle perforations from screw misplacement on the mean pullout strength of lower thoracic and lumbar pedicle screws. We also investigated the effect of bone mineral density (BMD), diameter of pedicle screws, and the region of spine on the pullout strength of pedicle screws.

Materials and methods: Sixty fresh human cadaveric vertebrae (D10-L2) were harvested. Dual-energy X-ray absorptiometry (DEXA) scan of vertebrae was done for BMD. Titanium pedicle screws of different diameters (5.2 and 6.2 mm) were inserted in the thoracic and lumbar segments after dividing the specimens into three groups: a) standard pedicle screw (no cortical perforation); b) screw with medial cortical perforation; and c) screw with lateral cortical perforation. Finally, pullout load of pedicle screws was recorded using INSTRON Universal Testing Machine.

Results: Compared with standard placement, medially misplaced screws had 9.4% greater mean pullout strength and laterally misplaced screws had 47.3% lesser mean pullout strength. The pullout strength of the 6.2 mm pedicle screws was 33% greater than that of the 5.2 mm pedicle screws. The pullout load of pedicle screws in lumbar vertebra was 13.9% greater than that in the thoracic vertebra (P = 0.105), but it was not statistically significant. There was no significant difference between pullout loads of vertebra with different BMD (P = 0.901).

Conclusion: The mean pullout strength was less with lateral misplaced pedicle screws while medial misplaced pedicle screw had more pullout strength. The pullout load of 6.2 mm screws was greater than that of 5.2 mm pedicle screws. No significant correlation was found between bone mineral densities and the pullout strength of vertebra. Similarly, the pullout load of screw placed in thoracic and lumbar vertebrae was not significantly different.

Keywords: Misplaced pedicle screw; pullout strength; vertebra.

Figure 1

(a) Vertebra with pedicle screw inserted in standard trajectory, (b) corresponding X-ray

Figure 2

(a) Vertebra with pedicle screw inserted in medial trajectory, (b) corresponding X-ray

Figure 3

(a) Vertebra with pedicle screw inserted in lateral trajectory, (b) corresponding X-rays

Figure 4

Bar diagram showing mean pullout load of screw placed in medial trajectory (thoracic vertebra: No. 1-7, 10-11, 16-18 and lumbar vertebra: No. 8-9, 12-15, 19, 20)

Figure 5

Bar diagram showing mean pullout load of screw placed in lateral trajectory (thoracic vertebra: No. 1-2, 4, 7-10, 14-18 and lumbar vertebra: No. 3, 5-6, 11-13, 19, 20)

Figure 6

Bar diagram showing mean pullout load of screw placed in standard trajectory (thoracic vertebra: No. 1-5, 7, 10, 12, 14-15, 17-18 and lumbar vertebra: No. 6, 8-9, 11, 13, 16, 19-20)