Original Article Pullout strength of misplaced pedicle screws in the thoracic and lumbar vertebrae - A cadaveric study
Shyam K Saraf, Ravindra P Singh, Vakil Singh, Ashish Varma
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.
Key words: Misplaced pedicle screw, pullout strength, vertebra
Introduction of the construct decreased significantly after the pedicles were fractured.1 However, the literature available regarding osterior pedicle screw fixation of the thoracic and the effect of pedicle injuries on screw pullout strength is lumbar spine provides rigid stabilization, however, limited.2‑4 inserting thoracic pedicle screws is technically P 3 demanding, with high rates of inaccurate placement, which In 1991, George et al. compared the pullout strength of can lead to pedicle fractures and construct failure. In a thoracic pedicle screws inserted after different techniques study on the multidirectional flexibility of pedicle screw/rod were used to prepare the holes. Unintentional cortical instrumentation after iatrogenic pedicle injuries, stability breaches occurred in 5 of the 16 pedicles studied. The pullout strength of screws in the fractured pedicles decreased Department of Orthopaedics, Institute of Medical Sciences, Banaras Hindu University, 11% compared with the intact pedicles; however, the Varanasi, India sample size used in this study was small to obtain statistically Address for correspondence: Dr. Shyam K Saraf, significant results. Department of Orthopaedics, Institute of Medical Sciences, Banaras Hindu University, Varanasi - 221 005, India. E‑mail: [email protected] Wittenberg et al.,5 found that the axial pullout force of Schanz screws increased with a 1‑mm increase in screw Access this article online diameter. Hirano et al.,6 observed that in osteoporosis, a Quick Response Code: Website: larger diameter screw does not enhance screw stability and www.ijoonline.com may break the thin cortex. The use of a larger screw does not increase the stability of the construct and may result in 7 DOI: cortical cutout of the pedicle. Misenhimer et al., found that 10.4103/0019-5413.111502 pedicle screws do not gain purchase in cortical bone within the pedicle because the pedicle deforms first; therefore, a
Indian Journal of Orthopaedics | May 2013 | Vol. 47 | Issue 3 238 Saraf, et al.: Pullout strength of misplaced pedicle screws pedicle screw with a diameter larger than the endosteal diameter of the pedicle will fracture the pedicle.
The primary objective of our work is to analyze the effects of iatrogenic pedicle perforations from screw misplacement on the pullout strength of thoracic and lumbar pedicle screws, the diameter of screw and the effect of osteoporosis.
Materials and Methods
The present study was done in collaboration with a b Department of Forensic Medicine, Department of Radiology, Figure 1: (a) Vertebra with pedicle screw inserted in standard trajectory, and Department of Metallurgy in the year 2011-2012. (b) corresponding X-ray
Isolation of vertebra from the fresh human cadaver Sixty fresh human thoracic vertebrae (T10–L2) were harvested as five‑vertebra segments from nine male and three female adult cadavers of average age 48 years. The cadavers were obtained as and when available. The soft tissue attachments to the vertebra were carefully dissected. On visual and radiographic inspection, none of the specimens had evidence of spinal pathologies. Bone mineral density (BMD) of the whole vertebral segment was determined by dual‑energy X‑ray absorptiometry (DEXA) scan. Although the harvested vertebrae were intended to a b be subjected to pullout testing immediately after isolation Figure 2: (a) Vertebra with pedicle screw inserted in medial trajectory, (b) corresponding X-ray from the cadaver, however, because of technical reasons when they could not be tested immediately, we preserved them in deep freezer at −20°C after wrapping in plastic bag which was tested later within 6-12 h from isolation. Before the pedicle screw insertion, one vertebra from each cadaver was subjected to CT scan study to document the density of medial and lateral pedicle wall. Similarly, after the testing procedure, one vertebra from each cadaver was subjected to the histological examination of uninjured pedicle to document the trabecular arrangement of medial and lateral wall of pedicle. a b Figure 3: (a) Vertebra with pedicle screw inserted in lateral trajectory, Placement of pedicle screws (b) corresponding X-rays Standard anteroposterior (AP) and lateral X‑ray views cortex. Pedicle screws were placed under direct vision with of each vertebra was taken prior to the placement of the the aid of C‑arm image intensifier. Tapered pedicle screws screws. The specimens were divided equally into three (SYNTHES Titanium alloy) (TiAl N ) of size 5.2 and 6.2 groups according to the trajectory of screw used: a) standard 16 7 trajectory means no cortical perforation [Figure 1a and b]; mm were inserted one in each vertebra of length 40 mm. b) medial trajectory are the ones with medial cortical Of the total of 60 vertebrae studied, in 30, 5.2‑mm‑diameter perforation [Figure 2a and b]; and c) lateral trajectory has screws and in another 30, 6.2‑mm‑diameter screws were the lateral cortical perforation [Figure 3a and b]. Vertebra passed. They were equally divided trajectory wise. Although from each cadaver was reallocated to each group to ensure variable length of the pedicle screw of range between 5 consistency in bone quality among the groups and to allow and 10 mm (mean 6 mm) remained outside the entry a paired (repeated‑measures) statistical analysis. The screw point in the pedicle, this had no effect on the final pullout was inserted until approximately 80% of the total trajectory strength as the part of the screw remaining outside the was reached. This strategy was designed to control the fill pedicle had no role to play in the final pullout strength of inside the pedicle and the depth of insertion relative to the the pedicle screws. Medial and lateral cortical perforations size of the specimen, and to avoid perforating the anterior were created by intentionally using a more medial or lateral
239 Indian Journal of Orthopaedics | May 2013 | Vol. 47 | Issue 3 Saraf, et al.: Pullout strength of misplaced pedicle screws trajectory, respectively. Standard AP and lateral X‑rays of was 37.25 kg [Figure 6]. From these observations, we infer each vertebra were taken to confirm the screw placement. that there is significant difference in pullout loads in the Direct visual inspection was used to assess each pedicle three groups (P < 0.001). The pullout load of medial group screw position. We believe this method allows more accurate was 9.4% higher than that of standard group and that of measurements than fluoroscopy or computed tomography lateral group was 47.3% lesser than that of standard group. because it provides real‑time feedback and the specimen The pullout load of screw with medial trajectory was 53% can be assessed in all dimensions. greater than that of screw with lateral trajectory.
The sagittal orientation of the screws within the pedicle and The mean pullout load of pedicle screws in thoracic vertebra vertebral body was determined in each specimen using was 30.97 kg, whereas in lumbar vertebra it was 35.27 kg. visual inspection and fluoroscopy to exclude undesired The pullout load of pedicle screws in lumbar vertebra was variables, such as superior or inferior pedicle breaches and 13.9% greater than that in the thoracic vertebra (P = 0.105), placement of the screws within the endplates. but it was not statistically significant.
Testing procedures The mean pullout load of pedicle screws of diameter 6.2 Pullout strength was measured within 6 h of taking out from mm was 37.2 kg, whereas with diameter of 5.2 mm it was deep freezer using INSTRON Universal Testing Machine 27.96 kg. Thus, the mean pullout load of pedicle screws in Department of Metallurgical Engineering. A steel wire with diameter 6.2 mm was 33% greater than that of screws was wrapped around the vertebral body and attached with diameter 5.2 mm (P < 0.001). There was no significant to lower grip. Similarly, a steel wire was attached to the difference between pullout loads in the vertebra with head of pedicle screws and held in the upper grip. The different BMD (P = 0.901) [Table 1]. pullout load was determined by moving the crosshead of the machine, to which the vertebra is attached through a The histomorphometric analysis of the representative steel wire downward at a constant velocity of 5 mm/min. vertebrae was also done. Computerized Tomography (CT) The crosshead of the machine to which the vertebra was scan of the vertebra revealed that the density of the medial attached through a steel wire was moved downward at a wall of the pedicle was greater than that of the lateral wall. constant velocity till the steel wires attached to vertebra. The mean density of the medial wall of the pedicle was The pedicle screw were tensioned adequately till the pedicle calculated as 969.14 Hounsfield Units, whereas that of the screw was in exact alignment with the axis of piston and lateral wall was 772.00 Hounsfield Units. The histological along the axis of the pedicle of vertebra, and there was no examination of the vertebra revealed no significant slackness in the steel wire. After ensuring the exact alignment difference between the trabecular arrangement among the of the pedicle screw and adequate tension in the steel wire, medial and lateral pedicle walls. the pullout testing was performed. The variation of the load with the crosshead displacement was recorded on chart Discussion continuously. The peak load was taken as the pullout load. Load range taken was 200 kg, chart speed was 2 cm/min, Accurate pedicle screw placement is particularly challenging and crosshead speed was 0.05 cm/min. because of factors like variable anatomy of vertebral body, relatively small pedicle diameter in thoracic vertebra, and Statistical analysis complex three‑dimensional orientation of the pedicle, The statistical analysis was done using statistical particularly in scoliosis cases. Hence, in clinical studies, software SPSS for windows (version 16.0). One‑way screw misplacement and subsequent fracture of the repeated‑measures analysis of variance (ANOVA) followed pedicle is a complication associated with thoracic and by Holm‑Sidak test were conducted to determine whether lumbar pedicle screw instrumentation. Literature on the mean pullout strength differed significantly for each screw biomechanical effects of pedicle fractures is scarce.2‑4 Our technique. Student’s “t” test was used to calculate mean difference of two groups. Probability values (P) < 0.05 were Table 1: The relationship of bone mineral density of vertebra considered statistically significant. with the mean pullout strength BMD (g/cm2) No. of Pullout load F‑value P value vertebrae (Mean±SD) Results 0.500–0.550 6 30.33±9.33 0.193 0.901 0.551–0.600 17 31.88±9.49 The mean pullout load of the pedicle screws with the medial 0.601–0.650 19 32.84±10.95 trajectory was 40.75 kg [Figure 4], with the lateral trajectory 0.651–0.750 18 33.61±9.91 was 19.65 kg [Figure 5], and with the standard trajectory BMD = Bone mineral density
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3XOORXWORDG NJ 9HUWHEUD 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)
3XOORXWORDG NJ 9HUWHEUD 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)
SXOORXWORDG NJ
9HUWHEUD 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) study is an extension of those done by earlier researchers and laterally misplaced pedicle screws had 21% lesser in several important ways. a) We included both lumbar and pullout strength compared to well placed pedicle screws. lower thoracic vertebrae in our study (D10–L2). b) The effects of BMD on the mean pullout strength of standard Our data indicate a significant association between mean and misplaced pedicle screws were observed and we also pullout strength and screw position in the transverse plane. studied the effect of diameter of pedicle screws (5.2 and As the position of the screw shifted from medial to lateral, 6.2 mm) on the mean pullout strength. the mean pullout strength decreased in a linear fashion. In terms of mean pullout strength, pedicle screws malpositioned George et al.3 observed that unintentional pedicle fracture medially were 9.4% stronger than standard pedicle screws reduced the mean pullout strength by 11% compared with and were 53% stronger than pedicle screws malpositioned the screws with intact pedicles. Brasiliense et al.2 observed laterally. These findings can partially be attributed to the that in thoracic human cadaveric vertebra (T6–T11), the internal architecture of the pedicles. According to Kothe et medially misplaced screws had 8% greater pullout strength al.,8 approximately 65% of the diameter of a thoracic pedicle
241 Indian Journal of Orthopaedics | May 2013 | Vol. 47 | Issue 3 Saraf, et al.: Pullout strength of misplaced pedicle screws is formed by cancellous bone in the core. The remaining statistically significant. The reason for the lack of association 35% constitutes cortical bone forming the cortex. The is unclear. Brasiliense et al.2 also found no significant width of the bone in the lateral portion of the cortex is only correlation between BMD and mean pullout strength in a one‑third (33%) that of the medial portion.9 Furthermore, similar experiment on 40 fresh human cadaveric vertebrae. 60% of the fixation strength of pedicle screws derives from Yuskel et al.15 attributed their lack of correlation to their the pedicle.10 Therefore, pedicle screws placed through a small sample and lack of a widely varying range of BMD lateral trajectory will likely result in the loss of about 40% of values among specimens. Burval et al.16 speculated about pullout strength, compared with a medial trajectory. the effect of osteoporosis on pullout strength and suggested augmentation with methyl methacrylate. Most of our Accordingly, Santoni et al.11 proposed a “cortical bone specimens were harvested from spines in the osteoporotic trajectory” as an alternative to a standard pedicle screw range of BMD (mean BMD 0.624 g/cm2). Furthermore, trajectory. They reasoned that a different screw trajectory as noted by Inceoglu et al.,9 the BMD in the pedicle is increased fixation strength because the screw is oriented poorly represented by traditional DEXA measurements of from medially to laterally, so that the amount of cortical the vertebral body. The possible difference in BMD within bone engaged by the screw threads increases. Their data the pedicle and BMD within the vertebral bodies recorded demonstrated that this strategy increased axial pullout by DEXA scans could underline the lack of a significant strength by 30%. We did not study such a medial‑to‑lateral correlation. trajectory. Our study shows that screw misplacement in the medial wall of the pedicle is least likely to loosen and We also studied the effect of diameter of the screws on pullout. For practicing spine surgeons, the current data the mean pullout strength of the pedicle screws and found strongly support the idea that pedicle screw misplacements that the pullout strength with diameter 6.2 mm was 33% fall into two categories. Medially malpositioned screws can greater than that of screws of size 5.2 mm. This difference be considered comparable with standard pedicle screws in is statistically significant. We agree with the explanation of terms of immediate stability and should not raise concern Zhang et al.17 that the force needed for pullout is related about the strength of the construct. Inevitably, however, to the surface area of the cylinder which is determined by there is a risk of spinal canal encroachment, dural tears, and the major diameter of the screw and purchase length and injury to the neurovascular structures. In contrast, screws that the bone experiences serious shear stress at the thread malpositioned laterally eliminate these risks but create a root radius when the screw is pulled out. In another study, suboptimal environment for bone fusion. Fakhouri et al.18 found 37% increase in the pullout strength of 6 mm screws than the 5 mm screws. Barber et al.19 noted Our data also shows that the mean pullout strength of an advantage of screws placed in 30° of convergence as laterally misplaced screws was 47.3% less then that of compared with screws in parallel, as far as pullout strength standard pedicle screws. The trajectory used to place the is concerned. lateral screws closely resembles the extrapedicular technique described by Dvorak et al.12 However, our data contradict We noticed no significant association (P 0.105) between their findings because in their experiment, extrapedicular = the mean pullout load and the vertebra used (thoracic and screws had a slightly better resistance to pullout compared with intrapedicular (standard) screws. In a similar study, lumbar vertebrae). This could be because of not much White et al.13 demonstrated a significant difference between anatomical variations among the lower thoracic and upper intrapedicular and extrapedicular screws, with the latter lumbar vertebrae (D10–L2). providing 70% of the pullout strength of intrapedicular screws. Perhaps these inconsistencies reflect differences in The results of our in vitro study represent the axial pullout screw trajectory and specimen selection. Pedicle fracture force in individual pedicle screws. It should be noted that can be the result of placing too large a screw into a small in vivo scenarios typically involve constructs with four to pedicle, or not placing it along the pedicle axis. Fracturing of six screws and the loss of pullout force with one misplaced a pedicle decreases the strength of fixation, and also carries pedicle screw may have a small effect on the overall strength the risk of injuring the surrounding neural structures;10 of the construct. Secondly, in clinical situations, bending however, Krag14 opined that this may not be true always. and rotational forces are applied on an implant. We only investigated medial and lateral malpositioning of pedicle Interestingly, we did not find any correlation between screws. Future research on the effects of rostrocaudal BMD and ultimate pullout strength. Although there was malpositioning of screws, especially the change in pullout an increase in pullout strength regardless of the position resistance that might occur if the vertebral endplate is of the screw with increasing BMD, however, it was not crossed, should be conducted.
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To conclude, the pullout strength of the pedicle screws (Phila Pa 1976) 2005;30:1485‑90. did not decrease when the medial cortex was perforated. 10. Weinstein JN, Rydevik BL, Rauschning W. Anatomic and However, placement of the screw laterally decreased the technical considerations of pedicle screw fixation. Clin Orthop pullout strength. The larger diameter of screw provides Relat Res 1992;284:34‑46. 11. Santoni BG, Hynes RA, McGilvray KC, Rodriguez‑Canessa G, better pullout strength. The relationship between pullout Lyons AS, Henson MA, et al. Cortical bone trajectory for lumbar strength and osteoporosis was not statistically significant. pedicle screws. Spine J 2008;9:366‑73. 12. Dvorak M, MacDonald S, Gurr KR, Bailey SI, Haddad RG. An References anatomic, radiographic and biomechanical assessment of extrapedicular screw fixation in the thoracic spine. Spine (Phila 1. Roy‑Camille R, Saillant G, Mazel C. Internal fixation of the Pa 1976) 1993;18:1689‑94. 13. White KK, Oka R, Mahar AT, Lowry A, Garfin SR. Pullout strength lumbar spine with pedicle screw plating. Clin Orthop Rel Res of thoracic pedicle screw instrumentation: Comparison of the 1986;203:7‑17. transpedicular and extrapedicular techniques. Spine (Phila Pa 2. Brasiliense LB, Theodore N, Lazaro BC, Sayed ZA, Deniz FE, 1976) 2006;31:E355‑8. Sonntag VK, et al. Quantitative analysis of misplaced pedicle 14. Krag MH. Biomechanics of transpedicle spinal fixation. screws in the thoracic spine: How much pullout strength is In: Weinstein JN, Wiesel SW, editors. The Lumbar Spine. lost? J Neurosurg Spine 2010;12:503‑8. Philadelphia: W B Saunders; 1990. p. 916‑40. 3. George DC, Krag MH, Johnson CC, Van Hal ME, Haugh LD, 15. Yüksel KZ, Adams MS, Chamberlain RH, Potocnjak M, Park SC, Grobler LJ. Hole preparation techniques for transpedicle Sonntag VK, et al. Pullout resistance of thoracic extrapedicular screws. Effect on pull‑out strength from human cadaveric screws used as a salvage procedure. Spine J 2007;7:286‑91. vertebrae. Spine (Phila Pa 1976) 1991;16:181‑4. 16. Burval DJ, McLain RF, Milks R, Inceoglu S. Primary pedicle screw 4. Kote R, Panjabi MM, Liu W. Multidirectional instability of augmentation in osteoporotic lumbar vertebrae: Biomechanical the thoracic spine due to iatrogenic pedicle injuries during analysis of pedicle fixation strength. Spine (Phila Pa 1976) transpedicular fixation. Spine 1997;22:1836‑42. 2007;32:1077‑83. 5. Wittenberg RH, Lee KS, Shea M, White AA 3rd, Hayes WC. Effect 17. Zhang QH, Tan SH, Chou SM. Investigation of fixation and screw of screw diameter, insertion technique, and bone cement pullout strength on human spine. J Biomechan 2004;37:479‑85. augmentation of pedicular screw fixation strength. Clin Orthop 18. Fakhouri SF, Zamarioli A, Wichr CR, Araujo CA, Defino HL, Rel Res 1993;296:278‑87. Shimano AC. Biomechanical Study of the Pullout Resistance 6. Hirano T, Hasegawa K, Takahashi HE, Uchiyama S, Hara T, in Screws of a Vertebral Fixation System. Adv Mech Eng Washio T. Structural characteristics of the pedicle and its role 2011;2011:6. in screw stability. Spine (Phila Pa 1976) 1997;22:2504‑10. 19. Barber JW, Boden SD, Ganey T, Hutton WC. Biomechanical 7. Misenhimer GR, Peek RD, Wiltse LL, Rothman SL, Widell study of lumbar pedicle screws: Does convergence affect axial EH Jr. Anatomic analysis of pedicle cortical and cancellous pullout strength? J Spinal Disord 1998;11:215‑20. diameter as related to screw size. Spine (Phila Pa 1976) 1989;14:367‑72. 8. Kothe R, O’Holleran JD, Liu W, Panjabi MM. Internal architecture of the thoracic pedicle. An anatomic study. Spine (Phila Pa 1976) How to cite this article: Saraf SK, Singh RP, Singh V, Varma A. Pullout strength of misplaced pedicle screws in the thoracic and lumbar 1996;21:264‑70. vertebrae - A cadaveric study. Indian J Orthop 2013;47:238-43. 9. Inceoglu S, Burghardt A, Akbay A, Majumdar S, McLain RF. Source of Support: Nil, Conflict of Interest: None. Trabecular architecture of lumbar vertebral pedicle. Spine
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