Before screw placement, an awl was used to create a pilot hole in the pedicle, but no drilling or tapping of the holes was performed (10). All screws were inserted by a single surgeon (P.V.M.) to a length of 30 mm according to the standard surgical method of Magerl (20). All screws were inserted parallel to the long axis of the pedicle, and no screws penetrated either the cortex of the pedicle or the cortex of the anterior vertebral body (Fig. 1). Insertional torque was recorded during screw placement using a modified torque screwdriver (Transducer Techniques, Temecula, CA, U.S.A.).
After the two screw types were placed into the 21 paired specimens, radiographs were again taken to confirm proper screw placement. The 42 vertebral body halves were wrapped in latex and embedded in a plastic cup with bone cement (Bosworth Fastray, Skokie, IL, U.S.A.) with the tops of the screws protruding out of the cups (21). Care was taken to prevent the bone cement from coming into contact with the protruding pedicle screws. The cups were then mounted in a vise on a servohydraulic testing frame (MTS, Eden Prairie, MN, U.S.A.). The screws were gripped by a chuck attached to the hydraulic actuator. The long axis of each screw was aligned co-linearly with the hydraulic actuator to minimize bending. Each screw was then pulled fully from the pedicle using a constant displacement rate of 1 mm/s) (Fig. 1) (6,10). Load was measured using a 1,115 N (112.5 kg) load cell (AMTI, Watertown, MA, U.S.A.), and displacement was measured internally from the linear variable displacement transducer (MTS).
Three output parameters were defined from the resulting force-deformation curves: maximum pullout force, energy-to-failure (area under the force-deformation curve), and stiffness (slope of the initial portion of the force-deformation curve) (Fig. 3) (11). These output parameters were then compared statistically between the two screw types using a paired t test. Regression analysis was also performed for the two screw's pullout strengths (Microsoft Excell 98, Microsoft Corporation, Redmond, WA, U.S.A.).
The biomechanical performance of the double-threaded screw was evaluated. The mean ± SD values of insertional torque for the double-threaded screw was 6.02 ± 2.69 Nm [85.3 ± 38.1 inch pounds (in-lb)], ranging from 1.41 to 11.44 Nm (20–162 in-lb). For maximum pullout strength, the mean ± SD values was 567 ± 238 N, ranging from 128 to 980 N. For energy-to-failure, the mean ± SD values were 1.04 ± 1.12 J, ranging from 0.07 to 3.72 J. Finally, the stiffness values for the double-threaded screw had a mean ± SD of 292 ± 121 N/mm, ranging from 110 to 563 N/mm (Table 2).
Four parameters of the performance of the two screw types were then compared. First, the mean ± SD values of insertional torque for the single-threaded screw was 6.89 ± 3.35 Nm [97.6 ± 47.5 in-lb), ranging from 1.48 to 13.70 Nm (21–194 in-lb). The insertional torque values were higher for the single-threaded screw in 15 of the 21 matched pairs, and this difference was statistically significant (p = 0.04) (Table 2). Second, for the single-threaded screw, the mean ± SD values for maximum pullout strength was 614.67 ± 261.77 N, ranging from 168 to 1,042 N. The maximum pullout strength values were higher for the single-threaded screw in 14 of 21 of the matched pairs. This difference was not statistically significant (p = 0.12) (Table 2). Regression analysis of the maximum pullout strengths of the single-threaded screw versus the double-threaded screw revealed that the intercept of the regression line was not statistically different from zero (p > 0.34) and the slope was not significantly different from unity (p > 0.5) (Fig. 4). Third, the energy-to-failure for the single-threaded screw had mean ± SD values of 0.88 ± 0.96 J, ranging from 0.11 to 4.30 J. The energy-to-failure was higher for the double-threaded screw in 13 of 21 cases, but this difference was not statistically significant (p = 0.29) (Table 2). Finally, mean ± SD stiffness for the single-threaded screw was 317.05 ± 165.18 N/mm, ranging from 72 to 588 N/mm. Stiffness was higher for the single-threaded screw in 11 of the 21 paired specimens, but this too was not statistically significant (p = 0.54) (Table 2).
The goal of this study was to test the hypothesis that a double-threaded pedicle screw would have greater bone purchase and pullout strength in elderly bone than would a typical clinically used single-threaded screw. During the pullout tests, both screw types pulled straight out of the pedicle, stripping the interface between the screw threads and the trabecular bone in a similar fashion. The major diameter of the screw determines the amount of bone sheared off during pullout (14). The minor screw diameter has been shown in previous studies to be a relatively less important variable for screw pullout (14), and we have found that a second lower-height inner thread is also a less significant variable. A post-hoc statistical power analysis indicated that we could detect a 15% statistically significant difference between the groups in maximum pullout strengths using our 21 paired specimens. We found that the mean maximum pullout strength was 7% higher for the single-threaded screw, but this difference was too small to be statistically significant. We conclude, therefore, that the second smaller thread on the double-threaded screw is not likely to significantly increase either bone purchase or pullout strength in elderly vertebrae.
Regardless of any possible systematic errors in our experiment, because we implanted both screw types in the same fashion into paired pedicles, the interscrew comparison should remain valid. Because we used paired pedicles for our comparison, we did not need to perform dual X-ray absorptiometry to assess for interspecimen variation in bone mineral density. Prior studies have shown that there is minimal variation in pullout strength because of differences in bone mineral density between paired pedicles (13).
When interpreting our results, it is appropriate to realize a number of caveats. First, we chose to perform only transverse axial screw pullout in our test because the change in thread design was most likely to affect transverse shear across the screw surface. However, pedicle screws in vivo are exposed to complex multiaxial cyclic loads (7,12). To more closely simulate in vivo loads, some researchers prefer to perform axial pullout tests after first cyclically loading the screws in the sagittal plane, whereas others subject pedicle screws to simultaneous sagittal and axial cyclic loads to create a rotational moment resulting in screw pullout (7,12). It remains unclear how closely these various methods of cyclic loading actually simulate in vivo fatigue loads on the pedicle screws. Cyclic loading of the two screw types was not performed in our study, and it remains to be determined whether a significant difference exists between the two screws' performances after cyclic loading. However, it is unlikely that a major difference in screw performance will exist during complex cyclic loading when no significant difference can be found during simple axial screw testing. Second, our experiment was conducted on embalmed elderly spine specimens. Because of lower costs and greater availability, embalmed spines have been used to conduct several recent pedicle screw pullout studies (9,22–25). McElhaney et al. (26) have shown that embalmed beef long bones have decreased compressive strength but unchanged tensile strength when compared with fresh beef long bones. However, in the pullout performance of an implant, any embalming effects appear to be minor. Prior studies have shown that screws placed into embalmed human spines have pullout strengths similar to those placed into osteoporotic fresh human spines (24). Consistent with this, our pullout values are similar to those found for 6.5-mm Steffee pedicle screws implanted in fresh elderly osteoporotic vertebrae (13). Consequently, we believe that embalmed spines are an adequate testing platform for biomechanical screw testing.
Our study is unique in that it addressed the biomechanical performance of a double-threaded screw with two parallel threads of differing heights throughout the screw shaft. Prior studies focused only on the clinical applications of screws with two, different, nonparallel threads on opposite ends of the screw either to reduce displaced bone fragments or to reduce lumbar spondylolisthesis (16–19). Future alterations in pedicle screw designs will merit further biomechanical testing of new screw types in the years to come.
A second, smaller, inner thread on a double-threaded pedicle screw does not translate into either increased bone purchase or higher pullout strengths in elderly vertebral bone.
The authors thank Pamela Derish for editorial assistance. We also thank Lippincott Williams & Wilkins (Philadelphia, PA, U.S.A.) for permission to modify and use Figure 1. This work was supported in part by an Unrestricted Research Grant from Medtronics Sofamor Danek (Memphis, TN, U.S.A.). The authors have no financial interest in the materials or devices described in this article. This work was supported by University of California, San Francisco; University of California, Berkeley; and an Unrestricted Research Grant from Medtronics/Sofamor Danek.
1. Masferrer R, Gomez CH, Karahalios DG, et al. Efficacy of pedicle screw fixation in the treatment of spinal instability and failed back surgery: a 5-year review. J Neurosurg 1998; 89:371–7.
2. McCormick PC. Selection criteria for degenerative lumbar spine
instability. Clin Neurosurg 1997; 44:29–39.
3. Daftari TK, Horton WC, Hutton WC. Correlations between screw hole preparation, torque of insertion, and pullout strength
for spinal screws. J Spinal Disord 1994; 7:139–45.
4. Moran JM, Berg WS, Berry JL, et al. Transpedicular screw fixation. J Orthop Res 1989; 7:107–14.
5. Soshi S, Shiba R, Kondo H, et al. An experimental study on transpedicular screw fixation in relation to osteoporosis of the lumbar spine
. Spine 1991; 16:1335–41.
6. Willett K, Hearn TC, Cuncins AV. Biomechanical testing of a new design for Schanz pedicle screws
. J Orthop Trauma 1993; 7:375–80.
7. Zdeblick TA, Kunz DN, Cooke ME, et al. Pedicle screw pullout strength
. Correlation with insertional torque. Spine 1993; 18:1673–6.
8. Halvorson TL, Kelley LA, Thomas KA, et al. Effects of bone mineral density on pedicle screw fixation. Spine 1994; 19:2415–20.
9. Hirano T, Hasegawa K, Takahashi HE, et al. Structural characteristics of the pedicle and its role in screw stability. Spine 1997; 22:2504–10.
10. Kwok AW, Finkelstein JA, Woodside T, et al. Insertional torque and pull-out strengths of conical and cylindrical pedicle screws
in cadaveric bone. Spine 1996; 21:2429–34.
11. Skinner R, Maybee J, Transfeldt E, et al. Experimental pullout testing and comparison of variables in transpedicular screw fixation. A biomechanical study. Spine 1990; 15:195–201.
12. Wittenberg RH, Lee KS, Shea M, et al. Effect of screw diameter, insertion technique, and bone cement augmentation of pedicular screw fixation strength. Clin Orthop 1993; 296:278–87.
13. Zindrick MR, Wiltse LL, Widell EH, et al. A biomechanical study of intrapeduncular screw fixation in the lumbosacral spine. Clin Orthop 1986; 203:99–112.
14. Krag MH, Beynnon BD, Pope MH, et al. An internal fixator for posterior application to short segments of the thoracic, lumbar, or lumbosacral spine. Design and testing. Clin Orthop 1986; 203:75–98.
15. Schatzker J, Sanderson R, Murnaghan JP. The holding power of orthopedic screws in vivo. Clin Orthop 1975; 108:115–26.
16. Herbert TJ, Fisher WE. Management of the fractured scaphoid using a new bone screw. J Bone Joint Surg [Br] 1984; 66:114–23.
17. Sammarco GJ, Scioli MW. Metatarsal osteotomy using a double-threaded compression screw: an adjunct to revision forefoot surgery. Foot Ankle 1989; 10:129–39.
18. Aota Y, Kumano K, Hirabayashi S, et al. Reduction of lumbar spondylolisthesis using a CDI pedicle screw system. Arch Orthop Trauma Surg 1995; 114:188–93.
19. Sijbrandij S. Reduction and stabilisation of severe spondylolisthesis. A report of three cases. J Bone Joint Surg [Br] 1983; 65:40–2.
20. Magerl FP. Stabilization of the lower thoracic and lumbar spine
with external skeletal fixation. Clin Orthop 1984; 189:125–41.
21. Pfeiffer M, Gilbertson LG, Goel VK, et al. Effect of specimen fixation method on pullout tests of pedicle screws
. Spine 1996; 21:1037–44.
22. Hasegawa K, Takahashi HE, Uchiyama S, et al. An experimental study of a combination method using a pedicle screw and laminar hook for the osteoporotic spine. Spine 1997; 22:958–62.
23. Lynn G, Mukherjee DP, Kruse RN, et al. Mechanical stability of thoracolumbar pedicle screw fixation. The effect of crosslinks. Spine 1997; 22:1568–72.
24. Seybold EA, Baker JA, Criscitiello AA, et al. Characteristics of unicortical and bicortical lateral mass screws in the cervical spine. Spine 1999; 24:2397–403.
25. Xu R, Ebraheim NA, Ou Y, et al. Anatomic considerations of pedicle screw placement in the thoracic spine. Roy-Camille technique versus open-lamina technique. Spine 1998; 23:1065–8.
26. McElhaney J, Fogle J, Byars E, et al. Effect of embalming on the mechanical properties of beef bone. J Appl Physiol 1964; 19:1234–6.
Keywords:© 2002 Lippincott Williams & Wilkins, Inc.
Biomechanics; Lumbar spine; Pedicle screws; Pullout strength