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Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out
The Spine Journal 1 (2001) 408–414
Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out Bret B. Abshire, MDa, Robert F. McLain, MDb,*, Antonio Valdevit, MSc, Helen E. Kambic, MSc a Division of Neurosurgery, The University of California, San Diego, 200 West Arbor Drive, San Diego, CA 92103-8893, USA Spine Research Laboratory, Department of Orthopedic Surgery, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195, USA c Department of Biomedical Engineering, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195, USA Received April 3, 2001; accepted July 18, 2001
b
Abstract
Background context: Biomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better “fit and fill” of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion. Purpose: To determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion. Study design/setting: This biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format. Methods: Porcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Student’s t test. Results: Pullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P.10) and a 50% increase in initial stiffness ( P.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion. Conclusions: Conical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter. © 2001 Elsevier Science Inc. All rights
reserved. Keywords:
Spine; Spinal instrumentation; Biomechanics; Pedicle screws; Pullout testing
Introduction FDA device/drug status: Approved (pedicle screws). This study was supported in part by a grant from Osteonics Corporation, Allendale, NJ. Additional support was provided by the Avrum Katz Foundation. * Corresponding author. Department of Orthopaedic Surgery/A41, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland OH 44195, USA. Tel.: (216) 444-2744; fax: (216) 444-3328.
Lumbar pedicle screw instrumentation is one of the most commonly used and rapidly growing forms of stabilization for lumbar fusion. Problems associated with pedicle screws include loss of fixation, improper placement, fatigue and bending failure, dural tears, cerebral spinal fluid leaks, nerve
1529-9430/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S1529-9430(01)001 1 9 - X
B.B. Abshire et al. / The Spine Journal 1 (2001) 408–414
root injury and infection [1–4]. Bending or breakage is the most common type of hardware failure [5]. Bending and breakage may be affected by the screw’s minor diameter, screw length, insertional depth, screw orientation and use of cross-link constructs [6–9]. When pedicle screws fail, they often do so just distal to the thread-shaft junction, in the region of the pedicle [10–12]. Conical-shaped screws have an increased minor diameter at the thread-shaft junction, which is the region of maximal strain, and are theoretically less likely to fracture or bend in this “at risk region” compared with comparable cylindrical screws [13]. Therefore, conical screws may be more resistant to screw bending failure or fracture than standard strews with a uniform shaft diameter. Biomechanical studies show that bone-mineral density, osteoporosis, cortical fixation, pedicle morphology, screw orientation and screw thread area all affect pedicle screw pullout [14–16]. Although pure pullout is not the mode of failure seen in clinical situations, pullout testing is thought to be a good predictor of pedicle screw fixation strength. However, almost all of the current research on pedicle screw pullout has been done with cylindrically shaped pedicle screws. Subsequently, conical screws have been introduced that may provide better “fit and fill” of the dorsal pedicle [2,17,18]. The conical screws engage more of the pedicle cortex as well as the cancellous bone at the corticocancellous margin than cylindrical screws. Trabecular bone at the corticocancellous interface is stronger than normal unsupported cancellous bone. Therefore, conical screws may also provide better purchase secondary to increased purchase in the corticocancellous interface of the pedicle [18]. Transpedicular screw placement is technically unforgiving, and insertional technique is important in the placement of any screw. Therefore, there is concern about the use of conical screws with respect to adjustments made during instrumentation. Intuitively, conical screws would be expected to lose fixation strength if they were backed out from full insertion after transpedicular placement. How rapidly such a decrease would occur or if it would be clinically significant is unknown and probably depends on the taper of the screw, the thread design and the degree of back-out. Only one previous study of conical pedicle screw pullout has been reported [12], and no studies have investigated failure after back-out. In the present study we address the issue of pullout failure in conical versus cylindrical screws in a threepart analysis. The three hypotheses tested were that conical pedicle screws provide comparable or improved pullout strength compared with similar cylindrical screws; that neither cylindrical nor conical screws lose significant fixation strength when backed out 180 degrees from full insertion; and that neither cylindrical nor conical screws lose significant fixation strength when backed out 360 degrees from full insertion. Methods Fresh-frozen lumbar vertebrae (L1–L6) from eight skeletally mature mini-pigs weighing 150 –to 200 pounds were
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harvested and stripped of all soft tissues. Specimens were stored at 4 F after harvesting and defrosted immediately before testing. Each individual vertebra was thawed and prepared for pedicle screw placement. The screw entry point was selected in accordance with the parameters established by Weinstein et al. [4] with minor modification to accommodate the porcine vertebra. The cortex was perforated at the junction of the transverse process, inferior margin of the superior facet and the pars interarticularis. With direct access to the pedicle, cranial/caudal and medial/lateral angulation was confirmed by direct visualization. For every pedicle, a 3.5-mm pilot hole 40 mm deep was drilled coaxial with the pedicle. Screws were inserted by hand using a custom driver with a torque calibration system. Each screw was inserted until the hub of the screw abutted the lamina and all threads were fully contained within the bone. At full insertion the screw was oriented so that the rod channel was parallel to the weight-bearing axis of the vertebra. All pedicle margins were inspected to rule out disruption of the cortex and confirm proper screw placement before embedding the vertebrae in polymethymethacrylate (PMMA). Vertebra with misplaced or malaligned screws, or pedicle fractures after instrumentation were excluded from testing. The vertebra were embedded in PMMA using a specially designed jig, with the vertebral body completely embedded but both pedicles free for inspection during testing [19]. The embedded vertebrae, mounted in the testing jig, were placed in a variable axis frame (Fig. 1) attached to the MTS base plate (MTS Systems, Eden Prairie, MN). The hydraulic arm of the MTS machine was attached to the screw head by a universal joint and aligned so that pullout would be co-axial to the screw. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads and displacement were recorded at 0.02-second intervals until failure occurred. Thirty-five porcine vertebrae (L1–L6) were selected for testing. Five vertebrae were excluded before testing because of cortical disruption during screw placement. All specimens were inspected after testing to confirm proper screw placement. No vertebra was excluded after testing. The pedicle screws selected for testing were designed and fabricated by the same manufacturer (Osteonics Corporation, Allendale, NJ). The screws differed specifically in the taper of the major and minor diameters from the hub to screw tip. The cylindrical screws maintained a constant diameter from hub to tip. The conical screws tapered 20%, from 7.5 mm at the hub (major diameter) to 6.0 mm at the tip (Fig. 2). The thread pitch was 1.8 mm for both screws. The thread depth was 1.2 mm, for both screws. The thread contour, a proprietary characteristic, was identical for both screws. To select screws for testing that were comparable in a clinically relevant way, we chose screws that had a similar minor diameter at the level of the pedicle isthmus, the dimension most crucial to screw selection [20]. Because the level of the isthmus falls 1.2 to 1.6 cm distal to the entry point of the screw into the lamina, screws were selected that had an equivalent minor diameter at that point [6,7,9]. At
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Fig. 2. Conical and cylindrical screws used in this study.
gaged in cortical bone. Cylindrical screws were 6.5 by 40 mm and conical screws were 7.5 by 40 mm (both are 6.5 mm in diameter 1.2 cm distal to the hub). The manufacturer (Osteonics Corporation, Allendale, NJ) made both screws with the same thread pitch, contour and thread surface area. Part 2: conical and cylindrical screws fully inserted versus backed out 180 degrees
Fig. 1. Test fixture. Poly-axial platform with mounting vise allowed rigid fixation of specimen blocks directly below the double-axle universal joint to ensure axial pullout.
1.2 cm distal to the screw hub, the 7.5-mm conical and the 6.5-mm cylindrical screws selected for this study presented a comparable minor diameter (4.85 mm versus 4.80 mm) and major diameter (6.62 mm versus 6.54 mm) as measured directly using a digital machinists caliper. A paired screw testing model was used to compare one screw with the other in the opposite pedicle with right/left orientation chosen at random. Each vertebra was assigned to one of five test groups and instrumented with the appropriate pair of pedicle screws. In Part 1 of the analysis, fully inserted conical screws were compared with fully inserted cylindrical screws to determine baseline pullout strength. In Part 2 of the analysis, fully inserted conical and cylindrical screws were tested against screws that were backed out 180 degrees from full insertion. In Part 3, fully inserted conical and cylindrical screws were tested against cylindrical screws backed out 360 degrees. Tests were conducted as follows: Part 1: conical versus cylindrical screws, fully inserted Each vertebra (six vertebrae/12 screws) in this test group was instrumented with one conical and one cylindrical screw with right or left side selection at random. Screws were considered fully inserted when the last thread was en-
Six vertebrae (12 pedicles) were instrumented with 6.5 by 40 mm cylindrical screws to full insertion, then one screw in each vertebrae was randomly backed out 180 degrees before testing. An additional six vertebrae (12 pedicles) were instrumented with 7.5 by 40 mm conical screws to full insertion, with one screw in each vertebrae randomly backed out 180 degrees before testing. Part 3: conical and cylindrical screws fully inserted versus backed out 360 degrees Six vertebrae (12 pedicles) were instrumented with 6.5 by 40 mm cylindrical screws to full insertion, and then one screw in each vertebrae was randomly backed out 360 degrees before testing. An additional six vertebrae (12 pedicles) were instrumented with 7.5 by 40 mm conical screws to full insertion, with one screw in each vertebrae randomly backed out 360 degrees before testing. Maximum insertional torque was recorded for all screws using a specially modified digital torque driver. The maximal insertional torque before back-out was used for the back-out groups. Pullout data were analyzed using a Student’s t test. Significance was defined as P .05. Results Part 1: comparison of fully inserted conical and cylindrical screws In six lumbar vertebrae a conical screw was tested against a paired cylindrical screw (Fig. 3). All screws were
B.B. Abshire et al. / The Spine Journal 1 (2001) 408–414
Fig. 4. Pullout characteristics of fully inserted conical and cylindrical pedicle screws. Paired pullout data: axial tensile stiffness (N/mm) was significantly greater in conical screws (P .01); axial load to failure (N) was greater for conical screws, just missing significance (P .07).
Fig. 3. Bone core surrounding the screw threads after axial pulout.
fully inserted (hub contacting the laminar cortex) before testing. The load to failure, stiffness and total energy to pull the screw out are shown in Table 1 below. Conical screws provided a 17% increase in pullout strength compared with their paired cylindrical screws (P .06) and a 50% increase in initial stiffness (P .01; Fig. 4). Part 2: fully inserted conical and cylindrical screws versus screws backed out 180 degrees In this group each vertebra was instrumented with two identical conical screws or two identical cylindrical screws. Lumbar vertebrae from three different hogs were used for testing. One screw from each vertebra, side selected at random, was backed out exactly 180 degrees from full insertion before pullout testing. There was no significant loss of stiffness, force or energy to pull out among the screws backed out 180 degrees in either cylindrical or conical screw groups (Fig. 5). A small but consistent increase in load to failure was seen in the backed out screws compared with their matched, fully inserted controls, but the difference was not significant. Part 3: fully inserted conical and cylindrical screws versus screws backed out 360 degrees Each vertebrae was instrumented with two conical or two cylindrical screws as above. One screw in each vertebra was backed out 360 degrees after full insertion. Six lumbar vertebrae were used to test each group. A small but consistent decrease in pullout loads and stiffness was seen in the backed-out conical screws, but this decrease was not signif-
Table 1 Characteristics of fully inserted screws
Stiffness (N/mm) Force (N) Energy (J)
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Cylindrical
Conical
P value
1703.9 2256.5 4.74
2690.7 2634.1 4.22
.011 .066 .32
icant. Failure loads in these screws were still comparable to those recorded for fully inserted cylindrical screws in Part 1 above and to loads recorded for fully inserted and backedout cylindrical screws in this arm of the study. There was no significant loss of stiffness, force or energy to pullout in the conical screws backed out 360 degrees (Fig. 6). There was no loss of stiffness, force or energy to pullout among cylindrical screws backed out 360 degrees. Insertional torques ranged from 15 to 65 inch·pounds in these young, skeletally mature pigs. There was no correlation between insertional torque and pullout strength in these specimens. Discussion Lumbar transpedicular instrumentation for fusion is a well-established and widely accepted surgical technique. Complications include infection, neural injury, screw breakage, bending failure, loss of screw purchase, surgical misplacement, dural rents and cerebral spinal fluid leaks, and rod fracture [2,3,11,12]. Currently, the most common type of hardware failure is screw bending or breakage [21]. When pedicle screws fracture, they commonly do so distal to the thread-shaft junction, in the intrapedicular portion of the screw [6,9,21]. Conical screws have been introduced in an attempt to improve purchase, and decrease breakage and bending failure in the “at-risk” portion of the screw. Because conical pedicle screws provide a larger minor diameter at the thread-shaft junction and intrapedicular portion of the screw, they decrease the chance for breakage or bending failure in this high-stress area. In theory, conical screws should also improve pullout strength and initial fixation strength through a better “fit and fill” of the dorsal portion of the pedicle. By matching the bell shape of the normal pedicle, conical screws are thought to provide more screw thread engagement of pedicle cortical bone and cancellous
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Fig. 5. Pedicle screws backed out 180 degrees. (Left) Characteristics of conical screws tested at full insertion and after back-out 180 degrees from full insertion. Axial stiffness and pullout force both increased slightly after initial back-out (not significant). (Right) Characteristics of cylindrical screws tested at full insertion and after 180 degrees back-out. Axial stiffness and pullout force were unchanged after back-out (not significant).
bone at the cancellous–cortical interface, which provides better purchase than normal cancellous bone [17,18,22]. Conical screws also progressively compress surrounding bone with each turn of the screw during insertion, which should provide increased purchase but which also could result in a rapid loss of fixation if the screw were backed out during instrumentation. Because minor adjustments of insertion depth are often necessary during screw placement in clinical situations, the effect of screw back-out on fixation strength is a significant concern to surgeons selecting pedicle screw instrumentation. To date, most testing of pedicle screw fixation has been done with standard cylindrical screws [23–25]. Previous tests of fully inserted conical pedicle screws has demonstrated an increased pullout strength compared with cylindrical screws [12,16]. Our review of the literature found only one paper directly comparing pullout strengths of conical and cylindrical pedicle screws and none on the effects of back-out on pullout or fixation strength [12]. Our testing methods had to account for a number of un-
controlled variables and inherent asumptions. A paired screw testing array was used to limit the effect of interindividual variability of bone density and pedicle morphology. We chose screws with the same thread pitch and surface area to eliminate the effect of these variables on pullout testing. A decision had to be made as to what diameter screws to compare, recognizing that any selection would have potential biases in one direction or the other. Screw selection was based on the single variable that would also determine screw selection in the clinical arena: pedicle diameter at the pedicle isthmus. We selected the largest screw that would fit the cancellous domain within the pedicle isthmus without penetrating the cortex or expanding the pedicle in these very uniform specimens. Preliminary testing showed that the 6.5mm cylindrical screw was the best fit for the majority of specimens and that the 7.5 mm conical screw provided essentially the same diameter at the pedicle isthmus. That the 7.5-mm conical screw would have a heavier shaft in the region of common bending failure and might have enhanced purchase dorsal to the pedicle isthmus was the design ad-
Fig. 6. Pedicle screws backed out 360 degrees. (Left) Characteristics of conical screws tested before and after back-out 360 degrees from full insertion. Axial stiffness and pullout force both decreased slightly after initial back-out (not significant). (Right) Characteristics of cylindrical screws after 360 degrees backout. Axial stiffness and pullout force were essentially unchanged after back-out (not significant).
B.B. Abshire et al. / The Spine Journal 1 (2001) 408–414
vantage we were seeking to verify. The screw diameters were measured by hand using a digital machinists caliper. The slight difference between the dimensions provided by the manufacturer and those measured directly from the screw were the result of a combination of usual manufacturing tolerances and the method of measurement. To measure the minor diameter in intact screws, there is always a slight off-axis error introduced by measuring between the threads. These small variations were not large enough to materially alter the thread area or pitch, and they did not introduce a bias into the data acquired. In this investigation we found that conical screws provided a 17% increase in the pullout strength compared with their paired cylindrical screws (P.10) and a 50% increase in initial stiffness (P.05). Pullout loads were comparable to those seen in healthy human vertebrae and higher than in osteoporotic human vertebrae [16]. The increased pullout strength and stiffness in conical screws probably reflect improved fit of the conical screw with the conical shape of the dorsal segment of the pedicle, so that screw threads engage more cortical bone and bone at the corticocancellous junction in the pedicle. We found no difference in strength, stiffness or energy to pullout between fully inserted conical screws and paired conical screws that were backed out 180 or 360 degrees. In fact, the force required to pull out the conical screws backed out 180 degrees was actually higher than for the fully inserted conical screws. The mean stiffness and energy was higher in the 180-degree back-out group, but this was not significantly different. Conical screws backed out 360 degrees had no significant decrease in stiffness, force or energy on pullout testing compared with their paired fully inserted screws. Even after a slight decrease in pullout strength (not statistically significant) in the conical screws that were backed out 360 degrees, the mean pullout loads were still higher than those seen in fully inserted cylindrical screws. Cylindrical screws provided consistent pullout resistance, even after back-out of 180 and 360 degrees. Cylindrical screws backed out 180 or 360 degrees showed no significant decrease in the force, stiffness or energy in pullout testing, as expected. Conical screws placed in cortical bone are known to be very sensitive to screw back-out, losing initial fixation strength and becoming clinically loose in a relatively short period of time. The fact that conical screws did not lose initial fixation strength in this study of transpedicular fixation does not suggest that clinical loosening could not occur over time, but does imply that the mechanical interface between screw and bone is different than in cortical bone. Three factors could contribute to the persistent fixation strength seen in screws backed out up to 360 degrees from full insertion. First, the trabecular bone compressed between the screw shaft and pedicle cortex deforms elastically to a certain point, and will re-expand to maintain the bone thread contact as the screw is backed out. Second, the pedicle itself may expand slightly as the conical screw is inserted and may main-
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tain bone thread contact by contracting as the screw is backed out. Finally, the ample thread diameter in this specific screw design holds a considerable amount of cancellous bone under compression without crushing it, as would happen with a screw with a larger minor diameter and smaller thread area. We suggest that, through these mechanisms, enough elastic deformation occurs at the bone–screw interface within the pedicle to compensate for small reductions in screw diameter as the screw is backed out. While the in vivo loading conditions of the human vertebra differ from those in this experimental model, there is no known clinical, biomechanical, or physiological difference between the human and the selected experimental model that could significantly alter the relative performance of the implants studied in this controlled, paired pullout test. That the absolute values for pullout strength were comparable between these specimens and previous studies with healthy human spines further supports the validity of this model. Nonetheless, caution should be exercised in extrapolating these data to the clinical environment. Porcine specimens have a denser trabecular matrix than do healthy humans and may accommodate screw back-out slightly differently than would osteoporotic patients. Similarly, these were pullout failure studies, and cyclic, toggle testing was not done. It is possible that, through trabecular fatigue or some other unrecognized mechanism, conical screws might perform less well than cylindrical screws under this testing mode. Conclusions Conical pedicle screws provide equal or increased purchase when compared with cylindrical pedicle screws of the same thread surface area and thread pitch. Conical screws can be backed out 180 or 360 degrees without significant loss of stiffness or initial fixation strength in pullout. Our results suggest that it is safe and reasonable to adjust the conical screws we tested up to one full turn out from full insertion in order to gain optimal screw alignment intraoperatively.
References [1] Esses SI, Sachs BL, Dreyzin V. Complications associated with the technique of pedicle screw fixation. A selected survey of ABS members. Spine 1993;18(15):2231–8. [2] Ransom N, La Rocca SH, Thalgott J. The case for pedicle fixation of the lumbar spine. Spine 1994;19(23):2702–6. [3] Schwab FJ, Nazarian DG, Mahmud F, Michelsen CB. Effects of spinal instrumentation on fusion of the lumbosacral spine. Spine 1995; 20(18):2023–8. [4] Weinstein JN, Spratt KF, Spengler D, Brick C, Reid S. Spinal pedicle fixation: reliability and validity of roentgenogram-based assessment and surgical factors on successful screw placement. Spine 1988;13(9):1012–8. [5] Pfeiffer M, Hoffman H, Goel VK, Weinstein JN, Griss P. In vitro testing of a new transpedicular stabilization technique. Eur Spine J 1997;6(4):249–55. [6] McKinley TO, McLain RF, Yerby SA, Sarigul-Klijn N, Smith TS. The effect of pedicle morphometry on pedicle screw loading in unstable burst fractures: a synthetic model. Spine 1997;22(3):246–52.
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[7] McLain RF, McKinley T, Sarigul-Klijn N, Smith T, Yerby S. Effects of cancellous bone quality on pedicle screw loading in axial instability: a synthetic model. Spine 1997;22(13):1454–60. [8] Yerby SA, Ehteshami JR, McLain RF. Offset laminar hooks decrease bending moments of pedicle screws during in situ contouring. Spine 1997;22(4):376–81. [9] Yerby SA, Ehteshami JR, McLain RF. Loading of pedicle screws within the vertebra. J Biomechanics 1997;30:951–4. [10] Davne SH, Myers SL. Complications of lumbar spinal fusion with transpedicular instrumentation. Spine 1992;17(Suppl):S184–9. [11] Dickman CA, Fessler RG, MacMillan M, Haid RW. Transpedicular screw-rod fixation of the lumbar spine: operative technique and outcome in 104 cases. J Neurosurg 1992;77:860–70. [12] Kwok AWL, Finklestein JA, Woodside T, Hearn TC, Hu RW. Insertional torque and pull-out strengths of conical and cylindrical pedicle screws in cadaveric bone. Spine 1996;21(21):2429–34. [13] Yamagata M, Kitahara H, Minami S, et al. Mechanical stability of pedicle screw systems for the lumbar spine. Spine 1992;17(Suppl): S51–4. [14] Pfeiffer M, Gilbertson LG, Goel VK, et al. Effect of specimen fixation method on pullout tests of pedicle screws. Spine1996;21:1037–44. [15] Soshi S, Shiba R, Kondo H, Murota K. An experimental study on transpedicular screw fixation in relation to osteoporosis of the lumbar spine. Spine 1991;16(11):1335–41. [16] 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.
[17] 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(2):139–45. [18] Halawa M, Lee AC, Ling RM, Vangala SS. The shear strength of trabecular bone from the femur, and some factors affecting the shear strength of the cement–bone interface. Arch Orthop Trauma Surg 1978;92:19–30. [19] McLain RF, Fry MF, Moseley TA, Sharkey NA. Lumbar pedicle screw salvage: pull-out testing of three different pedicle screw designs. J Spinal Dis 1995;8(1):62–8. [20] Zindrick MR, Wiltse LL, Doornik A, et al. Analysis of the morphometric characteristics of the thoracic and lumbar pedicles. Spine 1987;12(2):160–6. [21] McLain R, Sparling E, Benson D. Early failure of short-segment pedicle instrumentation for thoracolumbar fractures: a preliminary report. J Bone Joint Surg 1993;75A:162–7. [22] Misenhimer GR, Peek RD, Wiltse LL, Rothman SLG, Widell EH. Anatomic analysis of pedicle cortical and cancellous diameter as related to screw size. Spine 1989;14:367–72. [23] Ashman RB, Galpin RD, Corin JD, Johnston CE. Biomechanical analysis of pedicle screw instrumentation systems in a corpectomy model. Spine 1989;14:1398–405. [24] Chapman JR, Harrington RM, Lee KM, Anderson PA, Tencer AF, Kowalski D. Factors affecting pullout strength of cancellous bone screws. J Biomech Eng 1996;118:391–8. [25] Krag MH, Beynnon BD, Pope MH, Decoster TA. Depth of insertion of transpedicular vertebral screws into human vertebrae: effect upon screw–vertebra interface strength. J Spinal Disord 1998;1:287–94.
Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out Bret B. Abshire, MDa, Robert F. McLain, MDb,*, Antonio Valdevit, MSc, Helen E. Kambic, MSc a Division of Neurosurgery, The University of California, San Diego, 200 West Arbor Drive, San Diego, CA 92103-8893, USA Spine Research Laboratory, Department of Orthopedic Surgery, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195, USA c Department of Biomedical Engineering, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195, USA Received April 3, 2001; accepted July 18, 2001
b
Abstract
Background context: Biomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better “fit and fill” of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion. Purpose: To determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion. Study design/setting: This biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format. Methods: Porcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Student’s t test. Results: Pullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P.10) and a 50% increase in initial stiffness ( P.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion. Conclusions: Conical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter. © 2001 Elsevier Science Inc. All rights
reserved. Keywords:
Spine; Spinal instrumentation; Biomechanics; Pedicle screws; Pullout testing
Introduction FDA device/drug status: Approved (pedicle screws). This study was supported in part by a grant from Osteonics Corporation, Allendale, NJ. Additional support was provided by the Avrum Katz Foundation. * Corresponding author. Department of Orthopaedic Surgery/A41, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland OH 44195, USA. Tel.: (216) 444-2744; fax: (216) 444-3328.
Lumbar pedicle screw instrumentation is one of the most commonly used and rapidly growing forms of stabilization for lumbar fusion. Problems associated with pedicle screws include loss of fixation, improper placement, fatigue and bending failure, dural tears, cerebral spinal fluid leaks, nerve
1529-9430/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S1529-9430(01)001 1 9 - X
B.B. Abshire et al. / The Spine Journal 1 (2001) 408–414
root injury and infection [1–4]. Bending or breakage is the most common type of hardware failure [5]. Bending and breakage may be affected by the screw’s minor diameter, screw length, insertional depth, screw orientation and use of cross-link constructs [6–9]. When pedicle screws fail, they often do so just distal to the thread-shaft junction, in the region of the pedicle [10–12]. Conical-shaped screws have an increased minor diameter at the thread-shaft junction, which is the region of maximal strain, and are theoretically less likely to fracture or bend in this “at risk region” compared with comparable cylindrical screws [13]. Therefore, conical screws may be more resistant to screw bending failure or fracture than standard strews with a uniform shaft diameter. Biomechanical studies show that bone-mineral density, osteoporosis, cortical fixation, pedicle morphology, screw orientation and screw thread area all affect pedicle screw pullout [14–16]. Although pure pullout is not the mode of failure seen in clinical situations, pullout testing is thought to be a good predictor of pedicle screw fixation strength. However, almost all of the current research on pedicle screw pullout has been done with cylindrically shaped pedicle screws. Subsequently, conical screws have been introduced that may provide better “fit and fill” of the dorsal pedicle [2,17,18]. The conical screws engage more of the pedicle cortex as well as the cancellous bone at the corticocancellous margin than cylindrical screws. Trabecular bone at the corticocancellous interface is stronger than normal unsupported cancellous bone. Therefore, conical screws may also provide better purchase secondary to increased purchase in the corticocancellous interface of the pedicle [18]. Transpedicular screw placement is technically unforgiving, and insertional technique is important in the placement of any screw. Therefore, there is concern about the use of conical screws with respect to adjustments made during instrumentation. Intuitively, conical screws would be expected to lose fixation strength if they were backed out from full insertion after transpedicular placement. How rapidly such a decrease would occur or if it would be clinically significant is unknown and probably depends on the taper of the screw, the thread design and the degree of back-out. Only one previous study of conical pedicle screw pullout has been reported [12], and no studies have investigated failure after back-out. In the present study we address the issue of pullout failure in conical versus cylindrical screws in a threepart analysis. The three hypotheses tested were that conical pedicle screws provide comparable or improved pullout strength compared with similar cylindrical screws; that neither cylindrical nor conical screws lose significant fixation strength when backed out 180 degrees from full insertion; and that neither cylindrical nor conical screws lose significant fixation strength when backed out 360 degrees from full insertion. Methods Fresh-frozen lumbar vertebrae (L1–L6) from eight skeletally mature mini-pigs weighing 150 –to 200 pounds were
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harvested and stripped of all soft tissues. Specimens were stored at 4 F after harvesting and defrosted immediately before testing. Each individual vertebra was thawed and prepared for pedicle screw placement. The screw entry point was selected in accordance with the parameters established by Weinstein et al. [4] with minor modification to accommodate the porcine vertebra. The cortex was perforated at the junction of the transverse process, inferior margin of the superior facet and the pars interarticularis. With direct access to the pedicle, cranial/caudal and medial/lateral angulation was confirmed by direct visualization. For every pedicle, a 3.5-mm pilot hole 40 mm deep was drilled coaxial with the pedicle. Screws were inserted by hand using a custom driver with a torque calibration system. Each screw was inserted until the hub of the screw abutted the lamina and all threads were fully contained within the bone. At full insertion the screw was oriented so that the rod channel was parallel to the weight-bearing axis of the vertebra. All pedicle margins were inspected to rule out disruption of the cortex and confirm proper screw placement before embedding the vertebrae in polymethymethacrylate (PMMA). Vertebra with misplaced or malaligned screws, or pedicle fractures after instrumentation were excluded from testing. The vertebra were embedded in PMMA using a specially designed jig, with the vertebral body completely embedded but both pedicles free for inspection during testing [19]. The embedded vertebrae, mounted in the testing jig, were placed in a variable axis frame (Fig. 1) attached to the MTS base plate (MTS Systems, Eden Prairie, MN). The hydraulic arm of the MTS machine was attached to the screw head by a universal joint and aligned so that pullout would be co-axial to the screw. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads and displacement were recorded at 0.02-second intervals until failure occurred. Thirty-five porcine vertebrae (L1–L6) were selected for testing. Five vertebrae were excluded before testing because of cortical disruption during screw placement. All specimens were inspected after testing to confirm proper screw placement. No vertebra was excluded after testing. The pedicle screws selected for testing were designed and fabricated by the same manufacturer (Osteonics Corporation, Allendale, NJ). The screws differed specifically in the taper of the major and minor diameters from the hub to screw tip. The cylindrical screws maintained a constant diameter from hub to tip. The conical screws tapered 20%, from 7.5 mm at the hub (major diameter) to 6.0 mm at the tip (Fig. 2). The thread pitch was 1.8 mm for both screws. The thread depth was 1.2 mm, for both screws. The thread contour, a proprietary characteristic, was identical for both screws. To select screws for testing that were comparable in a clinically relevant way, we chose screws that had a similar minor diameter at the level of the pedicle isthmus, the dimension most crucial to screw selection [20]. Because the level of the isthmus falls 1.2 to 1.6 cm distal to the entry point of the screw into the lamina, screws were selected that had an equivalent minor diameter at that point [6,7,9]. At
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Fig. 2. Conical and cylindrical screws used in this study.
gaged in cortical bone. Cylindrical screws were 6.5 by 40 mm and conical screws were 7.5 by 40 mm (both are 6.5 mm in diameter 1.2 cm distal to the hub). The manufacturer (Osteonics Corporation, Allendale, NJ) made both screws with the same thread pitch, contour and thread surface area. Part 2: conical and cylindrical screws fully inserted versus backed out 180 degrees
Fig. 1. Test fixture. Poly-axial platform with mounting vise allowed rigid fixation of specimen blocks directly below the double-axle universal joint to ensure axial pullout.
1.2 cm distal to the screw hub, the 7.5-mm conical and the 6.5-mm cylindrical screws selected for this study presented a comparable minor diameter (4.85 mm versus 4.80 mm) and major diameter (6.62 mm versus 6.54 mm) as measured directly using a digital machinists caliper. A paired screw testing model was used to compare one screw with the other in the opposite pedicle with right/left orientation chosen at random. Each vertebra was assigned to one of five test groups and instrumented with the appropriate pair of pedicle screws. In Part 1 of the analysis, fully inserted conical screws were compared with fully inserted cylindrical screws to determine baseline pullout strength. In Part 2 of the analysis, fully inserted conical and cylindrical screws were tested against screws that were backed out 180 degrees from full insertion. In Part 3, fully inserted conical and cylindrical screws were tested against cylindrical screws backed out 360 degrees. Tests were conducted as follows: Part 1: conical versus cylindrical screws, fully inserted Each vertebra (six vertebrae/12 screws) in this test group was instrumented with one conical and one cylindrical screw with right or left side selection at random. Screws were considered fully inserted when the last thread was en-
Six vertebrae (12 pedicles) were instrumented with 6.5 by 40 mm cylindrical screws to full insertion, then one screw in each vertebrae was randomly backed out 180 degrees before testing. An additional six vertebrae (12 pedicles) were instrumented with 7.5 by 40 mm conical screws to full insertion, with one screw in each vertebrae randomly backed out 180 degrees before testing. Part 3: conical and cylindrical screws fully inserted versus backed out 360 degrees Six vertebrae (12 pedicles) were instrumented with 6.5 by 40 mm cylindrical screws to full insertion, and then one screw in each vertebrae was randomly backed out 360 degrees before testing. An additional six vertebrae (12 pedicles) were instrumented with 7.5 by 40 mm conical screws to full insertion, with one screw in each vertebrae randomly backed out 360 degrees before testing. Maximum insertional torque was recorded for all screws using a specially modified digital torque driver. The maximal insertional torque before back-out was used for the back-out groups. Pullout data were analyzed using a Student’s t test. Significance was defined as P .05. Results Part 1: comparison of fully inserted conical and cylindrical screws In six lumbar vertebrae a conical screw was tested against a paired cylindrical screw (Fig. 3). All screws were
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Fig. 4. Pullout characteristics of fully inserted conical and cylindrical pedicle screws. Paired pullout data: axial tensile stiffness (N/mm) was significantly greater in conical screws (P .01); axial load to failure (N) was greater for conical screws, just missing significance (P .07).
Fig. 3. Bone core surrounding the screw threads after axial pulout.
fully inserted (hub contacting the laminar cortex) before testing. The load to failure, stiffness and total energy to pull the screw out are shown in Table 1 below. Conical screws provided a 17% increase in pullout strength compared with their paired cylindrical screws (P .06) and a 50% increase in initial stiffness (P .01; Fig. 4). Part 2: fully inserted conical and cylindrical screws versus screws backed out 180 degrees In this group each vertebra was instrumented with two identical conical screws or two identical cylindrical screws. Lumbar vertebrae from three different hogs were used for testing. One screw from each vertebra, side selected at random, was backed out exactly 180 degrees from full insertion before pullout testing. There was no significant loss of stiffness, force or energy to pull out among the screws backed out 180 degrees in either cylindrical or conical screw groups (Fig. 5). A small but consistent increase in load to failure was seen in the backed out screws compared with their matched, fully inserted controls, but the difference was not significant. Part 3: fully inserted conical and cylindrical screws versus screws backed out 360 degrees Each vertebrae was instrumented with two conical or two cylindrical screws as above. One screw in each vertebra was backed out 360 degrees after full insertion. Six lumbar vertebrae were used to test each group. A small but consistent decrease in pullout loads and stiffness was seen in the backed-out conical screws, but this decrease was not signif-
Table 1 Characteristics of fully inserted screws
Stiffness (N/mm) Force (N) Energy (J)
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Cylindrical
Conical
P value
1703.9 2256.5 4.74
2690.7 2634.1 4.22
.011 .066 .32
icant. Failure loads in these screws were still comparable to those recorded for fully inserted cylindrical screws in Part 1 above and to loads recorded for fully inserted and backedout cylindrical screws in this arm of the study. There was no significant loss of stiffness, force or energy to pullout in the conical screws backed out 360 degrees (Fig. 6). There was no loss of stiffness, force or energy to pullout among cylindrical screws backed out 360 degrees. Insertional torques ranged from 15 to 65 inch·pounds in these young, skeletally mature pigs. There was no correlation between insertional torque and pullout strength in these specimens. Discussion Lumbar transpedicular instrumentation for fusion is a well-established and widely accepted surgical technique. Complications include infection, neural injury, screw breakage, bending failure, loss of screw purchase, surgical misplacement, dural rents and cerebral spinal fluid leaks, and rod fracture [2,3,11,12]. Currently, the most common type of hardware failure is screw bending or breakage [21]. When pedicle screws fracture, they commonly do so distal to the thread-shaft junction, in the intrapedicular portion of the screw [6,9,21]. Conical screws have been introduced in an attempt to improve purchase, and decrease breakage and bending failure in the “at-risk” portion of the screw. Because conical pedicle screws provide a larger minor diameter at the thread-shaft junction and intrapedicular portion of the screw, they decrease the chance for breakage or bending failure in this high-stress area. In theory, conical screws should also improve pullout strength and initial fixation strength through a better “fit and fill” of the dorsal portion of the pedicle. By matching the bell shape of the normal pedicle, conical screws are thought to provide more screw thread engagement of pedicle cortical bone and cancellous
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Fig. 5. Pedicle screws backed out 180 degrees. (Left) Characteristics of conical screws tested at full insertion and after back-out 180 degrees from full insertion. Axial stiffness and pullout force both increased slightly after initial back-out (not significant). (Right) Characteristics of cylindrical screws tested at full insertion and after 180 degrees back-out. Axial stiffness and pullout force were unchanged after back-out (not significant).
bone at the cancellous–cortical interface, which provides better purchase than normal cancellous bone [17,18,22]. Conical screws also progressively compress surrounding bone with each turn of the screw during insertion, which should provide increased purchase but which also could result in a rapid loss of fixation if the screw were backed out during instrumentation. Because minor adjustments of insertion depth are often necessary during screw placement in clinical situations, the effect of screw back-out on fixation strength is a significant concern to surgeons selecting pedicle screw instrumentation. To date, most testing of pedicle screw fixation has been done with standard cylindrical screws [23–25]. Previous tests of fully inserted conical pedicle screws has demonstrated an increased pullout strength compared with cylindrical screws [12,16]. Our review of the literature found only one paper directly comparing pullout strengths of conical and cylindrical pedicle screws and none on the effects of back-out on pullout or fixation strength [12]. Our testing methods had to account for a number of un-
controlled variables and inherent asumptions. A paired screw testing array was used to limit the effect of interindividual variability of bone density and pedicle morphology. We chose screws with the same thread pitch and surface area to eliminate the effect of these variables on pullout testing. A decision had to be made as to what diameter screws to compare, recognizing that any selection would have potential biases in one direction or the other. Screw selection was based on the single variable that would also determine screw selection in the clinical arena: pedicle diameter at the pedicle isthmus. We selected the largest screw that would fit the cancellous domain within the pedicle isthmus without penetrating the cortex or expanding the pedicle in these very uniform specimens. Preliminary testing showed that the 6.5mm cylindrical screw was the best fit for the majority of specimens and that the 7.5 mm conical screw provided essentially the same diameter at the pedicle isthmus. That the 7.5-mm conical screw would have a heavier shaft in the region of common bending failure and might have enhanced purchase dorsal to the pedicle isthmus was the design ad-
Fig. 6. Pedicle screws backed out 360 degrees. (Left) Characteristics of conical screws tested before and after back-out 360 degrees from full insertion. Axial stiffness and pullout force both decreased slightly after initial back-out (not significant). (Right) Characteristics of cylindrical screws after 360 degrees backout. Axial stiffness and pullout force were essentially unchanged after back-out (not significant).
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vantage we were seeking to verify. The screw diameters were measured by hand using a digital machinists caliper. The slight difference between the dimensions provided by the manufacturer and those measured directly from the screw were the result of a combination of usual manufacturing tolerances and the method of measurement. To measure the minor diameter in intact screws, there is always a slight off-axis error introduced by measuring between the threads. These small variations were not large enough to materially alter the thread area or pitch, and they did not introduce a bias into the data acquired. In this investigation we found that conical screws provided a 17% increase in the pullout strength compared with their paired cylindrical screws (P.10) and a 50% increase in initial stiffness (P.05). Pullout loads were comparable to those seen in healthy human vertebrae and higher than in osteoporotic human vertebrae [16]. The increased pullout strength and stiffness in conical screws probably reflect improved fit of the conical screw with the conical shape of the dorsal segment of the pedicle, so that screw threads engage more cortical bone and bone at the corticocancellous junction in the pedicle. We found no difference in strength, stiffness or energy to pullout between fully inserted conical screws and paired conical screws that were backed out 180 or 360 degrees. In fact, the force required to pull out the conical screws backed out 180 degrees was actually higher than for the fully inserted conical screws. The mean stiffness and energy was higher in the 180-degree back-out group, but this was not significantly different. Conical screws backed out 360 degrees had no significant decrease in stiffness, force or energy on pullout testing compared with their paired fully inserted screws. Even after a slight decrease in pullout strength (not statistically significant) in the conical screws that were backed out 360 degrees, the mean pullout loads were still higher than those seen in fully inserted cylindrical screws. Cylindrical screws provided consistent pullout resistance, even after back-out of 180 and 360 degrees. Cylindrical screws backed out 180 or 360 degrees showed no significant decrease in the force, stiffness or energy in pullout testing, as expected. Conical screws placed in cortical bone are known to be very sensitive to screw back-out, losing initial fixation strength and becoming clinically loose in a relatively short period of time. The fact that conical screws did not lose initial fixation strength in this study of transpedicular fixation does not suggest that clinical loosening could not occur over time, but does imply that the mechanical interface between screw and bone is different than in cortical bone. Three factors could contribute to the persistent fixation strength seen in screws backed out up to 360 degrees from full insertion. First, the trabecular bone compressed between the screw shaft and pedicle cortex deforms elastically to a certain point, and will re-expand to maintain the bone thread contact as the screw is backed out. Second, the pedicle itself may expand slightly as the conical screw is inserted and may main-
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tain bone thread contact by contracting as the screw is backed out. Finally, the ample thread diameter in this specific screw design holds a considerable amount of cancellous bone under compression without crushing it, as would happen with a screw with a larger minor diameter and smaller thread area. We suggest that, through these mechanisms, enough elastic deformation occurs at the bone–screw interface within the pedicle to compensate for small reductions in screw diameter as the screw is backed out. While the in vivo loading conditions of the human vertebra differ from those in this experimental model, there is no known clinical, biomechanical, or physiological difference between the human and the selected experimental model that could significantly alter the relative performance of the implants studied in this controlled, paired pullout test. That the absolute values for pullout strength were comparable between these specimens and previous studies with healthy human spines further supports the validity of this model. Nonetheless, caution should be exercised in extrapolating these data to the clinical environment. Porcine specimens have a denser trabecular matrix than do healthy humans and may accommodate screw back-out slightly differently than would osteoporotic patients. Similarly, these were pullout failure studies, and cyclic, toggle testing was not done. It is possible that, through trabecular fatigue or some other unrecognized mechanism, conical screws might perform less well than cylindrical screws under this testing mode. Conclusions Conical pedicle screws provide equal or increased purchase when compared with cylindrical pedicle screws of the same thread surface area and thread pitch. Conical screws can be backed out 180 or 360 degrees without significant loss of stiffness or initial fixation strength in pullout. Our results suggest that it is safe and reasonable to adjust the conical screws we tested up to one full turn out from full insertion in order to gain optimal screw alignment intraoperatively.
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