Fig 1.
Sawbones were designed to represent nonpedicle (A, left side), semi-pedicle (B, left side), and intact pedicle (A and B, right side). The designed computer simulated Sawbones models were shown in the upper row; the real designed Sawbones models were in the lower row.
Fig 2.
Photographs showing the three types of poly-axial screws used in the study.
Left to right: 6.0 mm × 45 mm, 6.0 mm × 50 mm, and 6.5 mm × 45 mm (diameter × length). The length was measured from the point of the bottom of the hub to the screw tip. The diameter was defined as the outer diameter. All screw types had a thread pitch of 1 mm and a thread depth of 0.8 mm.
Fig 3.
Schematic drawing showing the designation of synthetic vertebrae instrumented with various pedicle screws.
Different-sized screws were chosen and randomly implanted into each pedicle of the vertebrae by an experienced surgeon, 6.0 mm × 45 mm, 6.0 mm × 50 mm, and 6.5 mm × 45 mm screws were inserted sequentially into the semi-pedicle (left side of A, B, C) and nonpedicle (left side of D, E, F). The screws were implanted into the right side pedicle as the intact pedicle group (A-F).
Fig 4.
Photograph showing the experimental setup of the screw pullout test.
Sawbones were potted in specific epoxy resins. The prepared specimens were mounted onto a material testing machine (Bionix 858; MTS Systems Corp., MN, USA) to conduct the axial pullout tests with the (A) intact pedicle screw, (B) semi-pedicle screw, and (C) nonpedicle screw.
Fig 5.
Photographs showing the highlighted area of bone that was embedded into the screw thread using radiographic imaging of inserted screws and ImageJ analysis software.
(A) Intact pedicle group, (B) semi-pedicle group, and (C) nonpedicle group. The bone volume embedded into the screw thread was quantified and defined as , where “A” denotes the calculated area of bone embedded into the screw thread from the 2-D radiographic image, and “D” and “d” denote the outer and inner diameter of the pedicle screw, respectively.
Fig 6.
Axial X-ray images show that the 6.0 mm × 45 mm, 6.0 mm × 50 mm, and 6.5 mm × 45 mm screws were inserted sequentially into the semi-pedicle (left side of A, B, C), nonpedicle (left side of D, E, F), and intact pedicle (right side of A-F). An appropriate screw trajectory and insertional depth were confirmed using axial X-ray imaging prior to the pullout test. In the axial view, all screws were convergently inserted into the vertebral body, and the additional 5 mm of depth was clearly noted in the 6.0 mm×50 mm group (B,E) when compared to the other two groups (A, C, D, F).
Fig 7.
Sagittal X-ray images show that the 6.0 mm × 45 mm, 6.0 mm × 50 mm, and 6.5 mm × 45 mm screws were inserted sequentially into the semi-pedicle (left side of A, B, C), nonpedicle (left side of D, E, F), and intact pedicle (right side of A-F). All screws were inserted in a slightly downward orientation and both screws in the same specimen were parallel, consistent with current surgical techniques. No fractures or defects were detected across all vertebrae.
Fig 8.
Mean ultimate pullout forces for various sizes of screws in each group.
The vertical lines represent the standard deviations. Pullout forces of various sizes were not significantly different from each other in the intact and semi-pedicle groups. In contrast, the pullout force of the longer and larger screws were significantly higher in the nonpedicle group (p < 0.05).
Fig 9.
Maximum pullout force percentage for each screw size in the three groups.
Significantly higher pullout strengths were demonstrated for all three screw sizes in the intact pedicle group when compared to the semi-pedicle or nonpedicle groups (p < 0.05). Higher pullout strengths were also measured in the semi-pedicle group when compared to the nonpedicle group for all sizes (p < 0.05).
Fig 10.
Typical force-displacement curves for the intact pedicle, semi-pedicle, and nonpedicle groups when instrumented with various sizes of pedicle screws.
Fig 11.
Embedded bone volumes quantified by ImageJ software using axial X-ray.
In the IP group, the 6.0 mm × 45 mm, 6.0 mm × 50 mm, and 6.5 mm × 45 mm screws had a mean embedded bone volume of 880.88 ± 42.51 mm3, 1008.62 ± 46.6 mm3, and 925.34 ± 50.04 mm3, respectively, compared to 605.71 ± 30.53 mm3, 801.88 ± 40.62 mm3, and 662.59 ± 40.51 mm3, respectively, in the SP group, and 496.71 ± 22.94 mm3, 662.06 ± 30.58 mm3, and 529.12 ± 38.01 mm3 in the NP group.
Fig 12.
Embedded bone volume percentages of the three groups for each screw size.
The value is significantly higher for the IP group than for the other groups and there is no statistically significant difference between the SP group (68–76%) and the NP group (58–65%). The distribution of these values is quite similar to those of the ultimate pullout forces (Fig 9).