Fig 1.
Schematic drawings showing four types of expandable screws: (A) 4-slit with 16-mm EEL, (B) 4-slit with 22-mm EEL, (C) 6-slit with 16-mm EEL, and (D) 6-slit with 22-mm EEL.
The EEL was defined as the length from the point of diameter change of the internal hole to the screw tip. All of the screws had the same outer diameter of 6 mm, a length of 40 mm from hub to tip, a thread pitch of 2 mm and a thread depth of 0.8 mm. The lengthwise slit length from the screw tip was 27 mm for all screws. All of the expansive screws had an internal hole 3 mm from the screw head, which was connected to a smaller hole (1.5 mm in diameter) that extended to the screw tip.
Fig 2.
Photographs that show expansive screws with 6 (left) and 4 (right) separated lengthwise slits (A), and expansive screws with different EELs: (B) 16-mm EEL (with an inner pin of 44-mm in length) and (C) 22-mm EEL (with an inner pin of 38-mm in length).
The insertion pin was inserted into the interior of the screw to open the fins at the tip of the expansive screws. The difference in EEL affects the range of screw expansion following pin insertion.
Table 1.
Allocation of the specimens to experimental groups.
Fig 3.
Radiological photograph showing the procedure for an expansive screw with different preparations.
For screws without cement augmentation (top): (A) Insertion of the expansive screw into the test block. (B) Insertion of inner pin to achieve screw expansion. For screws with cement augmentation (bottom): (C) insertion of the expansive screw into the test block. (D) Cement injection after screw insertion (prior to expansion). (E) Insertion of the inner pin to achieve screw expansion.
Fig 4.
Photographs showing the pedicle screws after expansion: (A) 6-slit screw with 16-mm EEL and (B) 6-slit screw with 22-mm EEL.
A pedicle screw with a 22-mm EEL has a larger expansion range (top).
Fig 5.
Photographs showing various cemented screws after the pullout tests. Left to right: solid, 4-slit with 16-mm EEL, 4-slit with 22-mm EEL, 6-slit with 16-mm EEL, and 6-slit with 22-mm EEL.
A larger expansion range was observed for screws with 22-mm EELs compared with screws with 16-mm EELs and that cement infiltration into the open cell of the test block led to the formation of a cement/bone composite structure. Regardless of the number of slits (4 or 6), the cement/bone composite structure was distributed closer to the screw head for screws with a larger expansion range (22-mm EELs).
Fig 6.
Typical force-displacement curve for various types of expansive screws with or without cement augmentation.
Fig 7.
The mean ultimate pullout strengths of various types of expansive screws.The vertical lines represent the standard deviations. Horizontal lines above the bars indicate groups that were not significantly different from each other.
Regardless of the screw design (screws with 4 or 6 slits and EELs of 16 or 22 mm, or a solid screw), screws with cement augmentation demonstrated significantly higher pullout strengths than did pedicle screws without cement augmentation (p < 0.001).