Supplemental S1 fixation for type C pelvic ring injuries: biomechanical study of a long iliosacral versus a transsacral screw
A single iliosacral screw placed into the S1 vertebral body has been shown to be clinically unreliable for certain type C pelvic ring injuries. Insertion of a second supplemental iliosacral screw into the S1 or S2 vertebral body has been widely used. However, clinical fixation failures have been reported using this technique, and a supplemental long iliosacral or transsacral screw has been used. The purpose of this study was to compare the biomechanical effect of a supplemental S1 long iliosacral screw versus a transsacral screw in an unstable type C vertically oriented sacral fracture model.
Materials and methods
A type C pelvic ring injury was created in ten osteopenic/osteoporotic cadaver pelves by performing vertical osteotomies through zone 2 of the sacrum and the ipsilateral pubic rami. The sacrum was reduced maintaining a 2-mm fracture gap to simulate a closed-reduction model. All specimens were fixed using one 7.0-mm iliosacral screw into the S1 body. A supplemental long iliosacral screw was placed into the S1 body in five specimens. A supplemental transsacral S1 screw was placed in the other five. Each pelvis underwent 100,000 cycles at 250 N, followed by loading to failure. Vertical displacements at 25,000, 50,000, 75,000, and 100,000 cycles and failure force were recorded.
Vertical displacement increased significantly (p < 0.05) within each group with each increase in the number of cycles. However, there was no statistically significant difference between groups in displacement or load to failure.
Although intuitively a transsacral screw may seem to be better than a long iliosacral screw in conveying additional stability to an unstable sacral fracture fixation construct, we were not able to identify any biomechanical advantage of one method over the other.
Level of evidence
Does not apply—biomechanical study.
KeywordsIliosacral screws Transsacral screws Type C pelvic ring injuries
Pelvic fractures account for 1–3 % of all skeletal fractures and comprise a broad spectrum of injuries: from low-energy fractures in osteoporotic patients to high-energy disruptions of the pelvic ring [1, 2]. Type C pelvic ring injuries are vertically unstable due to complete disruption of the posterior arch [3, 4, 5, 6]. This posterior injury is by necessity accompanied by a second injury site in the ring, commonly in the anterior arch of the pelvic ring, and consisting of disruption of the pubic symphysis, and ipsilateral and/or contralateral fractures of the superior and inferior pubic rami [3, 7, 8]. Posterior ring disruption is associated with high morbidity and mortality rates [9, 10]. As shown in multiple studies following treatment of the pelvic injury, residual deformity or associated injuries can create significant problems in functional recovery [11, 12]. Numerous investigators have found that displacement through the weight-bearing arch of the pelvis can lead to long-term problems of pain and inability to regain function and resume previous lifestyle [13, 14, 15, 16, 17]. Regardless of the exact location of the posterior disruption, early restoration of pelvic ring integrity is vital, and surgical management is thought to reduce long-term complications, such as malunion, nonunion, neurologic dysfunction, low-back pain, and gait abnormalities [4, 15, 16, 17, 18, 19, 20, 21, 22].
Many surgical techniques have been described for fixation of the posterior pelvic ring injury, with iliosacral screw fixation into the first sacral body being in common practice [4, 8, 19, 23, 24, 25, 26]. Single iliosacral screw fixation into the S1 vertebral body has been shown to be clinically unreliable for unstable type C vertically oriented sacral fractures . Insertion of a second, supplemental, iliosacral screw into the S1 or S2 vertebral body has been widely used . In 2006, Moed and Geer published data on series of patients reporting safe use of S2 screws. However, they raised the concern about using this type of screw in osteopenic patients and recommended its use only with good bony purchase after instrumentation . More recently advocated is the use of a long iliosacral screw (extending from the external surface of the ilium to just short of the contralateral sacroiliac joint) or a transsacral screw (extending from the external surface of the ilium across the contralateral sacroiliac joint and exiting the ilium) [8, 29, 30]. To our knowledge, no biomechanical study has been performed to differentiate the effect of these two screw lengths on fixation construct stability in type C, zone 2 sacral fracture with a residual gap at the fracture site to mimic the clinical situation of a closed reduction in which an anatomic reduction of the sacral fracture is not attained.
The purpose of this study was to biomechanically compare the effect of a supplemental S1 long iliosacral screw versus a transsacral screw in an unstable type C vertically oriented zone 2 sacral fracture model [3, 5].
Materials and methods
The right superior and inferior pubic rami were osteotomized in a vertical fashion using an oscillating surgical power saw with a thin blade.
An ipsilateral vertical zone 2 sacral fracture was created by making a unilateral cut through the sacral neuroforamina using an oscillating surgical power saw with a thin blade.
The ipsilateral sacrospinous and sacrotuberous ligaments were transected to ensure complete disruption of the sacroiliac complex.
Statistics were calculated using SPSS software (SPSS version 19; SPSS Chicago, IL, USA). The Mann–Whitney U test was used to compare bone density (as noted above), displacement, and load to failure of the two fixation groups. Freidman test was used for displacement comparisons within each group. The level of statistical significance was defined as p < 0.05.
Posterior displacement for the long iliosacral group
Number of cycles
Displacement (in mm)
Posterior displacement for the transsacral group
Number of cycles
Displacement (in mm)
Load to failure for both groups
Load to failure (in N)*
Long iliosacral screws
Comparison between groups
Long iliosacral group
Mean displacement at 25,000 cycles (in mm)
Mean displacement at 50,000 cycles (in mm)
Mean displacement at 75,000 cycles (in mm)
Mean displacement at 100,000 cycles (in mm)
Mean load to failure (in N)
Stabilization of posterior pelvic ring injuries with iliosacral screws inserted into the first sacral body is a commonly used technique [19, 20, 32, 39, 40, 41]. Yinger et al. and van Zwienen et al., in their biomechanical studies, showed that for a completely unstable pelvic ring injury, using two iliosacral screws increases rotational stiffness and load to failure [26, 28]. Consistent with these findings, two iliosacral screws inserted into S1, or one each into the S1 and S2 bodies, are used as a preferred method for fixation for these injuries . However, this two-screw construct is clinically unreliable in some situations, especially with percutaneous fixation of unstable type C, zone 2, vertically oriented sacral fractures in which a residual gap exists at the fracture site .
Matta and Tornetta suggested that longer iliosacral screws might provide better fixation because they have greater resistance to toggle and are more resistant to vertical shear stress . However, data to support this contention are wanting. A number of studies were unable to show any significant differences in fracture stability using different iliosacral screw lengths [8, 30]. Griffin et al., in a study evaluating percutaneous iliosacral screw fixation of 62 unstable type C injuries, used four different screw lengths: into the sacral body, to the level of the contralateral sacral foramen, to the contralateral sacral ala, and across the sacroiliac joint . They reported that a vertical sacral fracture was the only statistically significant risk factor for fixation failure . Tornetta et al. found that a construct using a standard iliosacral screw in combination with a transsacral screw performed no better than a standard two-screw construct . However, Tabaie et al., in a biomechanical study with a design similar to ours, compared standard iliosacral screws to a novel locked transsacral screw construct and reported significantly improved fixation using the transsacral locked method .
The purpose of our study was to assess the potential improvement of fixation using one of two alternative long-screw fixation options: a transsacral or a long iliosacral implant. In order to create an “extreme” condition, osteopenic/osteoporotic pelvic specimens were used, and the anterior fractures were not fixed. This allowed us to focus directly on the posterior fixation in a model at the greatest risk for fixation failure. In addition, to maximize vertical shear and minimize compression across the posterior pelvic arch, a single-limb-stance model was used .
Comparison of load to failure for long iliosacral, transsacral, short iliosacral, and locked transsacral screw constructs
Although there have been a number of studies comparing standard iliosacral screws with longer screw constructs, we know of no study directly comparing these longer screw methods. Tornetta et al. described the concept of different modes of failure , reporting that standard screws cut through the sacrum while long screws bent, indicating that the long screw was better anchored at its distal end . However, in our study, after applying load to failure, none of the screws in either group were bent or broken. Our mode of failure was at the S1 body and alar bone stock. This difference may be related to dissimilarities in design between the two studies: ours using a fracture-gap single-stance model; theirs using anatomic reduction in a bilateral-stance model.
Although intuitively a transsacral screw may seem to be more advantageous than a long iliosacral screw in conveying additional stability to a type C, zone 2, vertically unstable sacral fracture fixation construct, we were not able to identify any biomechanical advantage of one fixation method over the other. Further study with a larger number of samples may be required to more accurately compare screw configuration in these injuries.
Conflict of interest
The authors attest that they have no financial or personal relationships of any kind with other people or organizations (such as employment, consultancies, stock ownership, honoraria, paid expert testimony, patent applications/registrations, and other funding) that could inappropriately influence (bias) this work, entitled “Supplemental S 1 fixation for type C pelvic ring injuries: biomechanical study of a long iliosacral versus a transsacral screw.” However, one of the authors is one of the owners of a patent for a locked transsacral implant that is not commercially available.
At our institution, all human and animal studies must be approved by the appropriate ethics committee and therefore be performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. This study is a biomechanical one involving cadaver specimens. Therefore, it was exempt from formal Institutional Review Board Review, and the need for informed consent was waived by the ethical committee since rights and interests of those who donated their bodies to science would not be violated and their privacy and anonymity would be assured by this study design.
- 4.Sagi HC (2009) Pelvic ring fractures. In: Bucholz RW, Court-Brown CM, Heckman JD, Tornetta PIII (eds) Rockwood and Green’s fractures in adults, 7th edn. Lippincott Williams and Wilkins, Philadelphia, pp 1415–1462Google Scholar
- 6.Tile M, Hearn T, Vrahas M (2003) Biomechanics of the pelvic ring. In: Tile M, Helfet DL, Kellam JF (eds) Fractures of the pelvis and acetabulum, 3rd edn. Lippincott Williams and Wilkins, Philadelphia, pp 32–45Google Scholar
- 9.Egol KE, Koval KJ, Zuckerman JD (2010) Pelvis fracture. In: Egol KE, Koval KJ, Zuckerman JD (eds) Hand book of fractures, 4th edn. Lippincott Williams and Wilkins, Philadelphia, pp 327–343Google Scholar
- 17.Raf LA (1966) Double vertical fractures of the pelvis. Chir Scand 131:298–305Google Scholar
- 23.Dujardin FH, Hossenbaccus M, Duprac F, Biga N, Thomine JM (1998) Long-term functional prognosis of posterior injuries in high-energy pelvis disruption. J Orthop Trauma 12:145–150Google Scholar
- 30.Tornetta P III, Jacofsky DJ, Jaczynski AM (2009) Trans-sacral screw fixation of stable zone 2 sacral fractures. http://www.hwbf.org/ota/am/ota09/otapo/OTP09018.htm. Accessed 20 Oct 2014
- 32.Gorczyca J, Hearn T, Tile M (2003) Biomechanics and methods of pelvic fixation. In: Tile M, Helfet DL, Kellam JF (eds) Fractures of the pelvis and acetabulum, 3rd edn. Lippincott Williams and Wilkins, Philadelphia, pp 116–129Google Scholar
- 33.MacAvoy MC, McClellan RT, Goodman SB, Chien CR, Allen WA, van der Meulen MCH (1997) Stability of open-book pelvic fractures using a new biomechanical model of single-limb stance. J Orthop Trauma 11:590–593Google Scholar
- 36.Tudor-Locke C, Ham SA, Macera CA, Ainsworth BE, Kirtland KA (2004) Descriptive epidemiology of pedometer-determined physical activity. Med Sci Sports Exerc 36:1567–1573Google Scholar
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