Biomechanical analysis of the effect of ‘intermediate screws’ in short segment posterior fixation of unstable burst fractures of thoracolumbar spine in calf spine model


  • Azad Sait School of Advanced Sciences, Vellore Institute of Technology, Vellore, Tamil Nadu, India
  • Monosha Priyadarshini School of Advanced Sciences, Vellore Institute of Technology, Vellore, Tamil Nadu, India
  • N. Arunai Nambi Raj School of Advanced Sciences, Vellore Institute of Technology, Vellore, Tamil Nadu, India
  • Kenny Samuel David Spinal Disorders Surgery Unit, Department of Orthopaedics, Christian Medical College Vellore, Tamil Nadu, India



Bending moment, Pedicle screw, Load, Displacement curve, Universal testing device, Posterior instrumentation


Background: Efforts in preserving motion levels in unstable thoracolumbar burst fractures steered to short segment fixation. However, short segment spanning fixation in clinical scenario reported high failure rates.  Augmentation of spanning fixation by inserting intermediate screws into the fracture level is proposed to enhance stability. An experimental comparative study was performed to assess the biomechanical role of the ‘intermediate screws.’

Methods: Five calf spine specimens were freshly prepared to record the biomechanical characteristics, range of motion (ROM), and stiffness. CT scan confirmed an unstable burst fracture in each specimen. Each specimen was instrumented with short-segment posterior fixation with an intermediate screw. The same test protocols were repeated with and without intermediate screws.

Results: Intermediate screws contribute to 20.2%, 16.5%, 14.5% and 23% decrease in ROM and 15.4%, 25.6%, 48.3%, and 160.2% increase in construct stiffness.

Conclusions: Intermediate screws significantly increase the construct stiffness and decrease the ROM. 



Wettstein M, Mouhsine E. Operative compared with nonoperative treatment of a thoracolumbar burst fracture without neurological deficit. J Bone Joint Surg Am. 2004;86(3).

Anekstein Y, Brosh T, Mirovsky Y. Intermediate screws in short segment pedicular fixation for thoracic and lumbar fractures: A biomechanical study. J Spinal Disord Tech. 2007;20(1):72-7.

Denis F. The Three Colum Spine and Its Significancve in the Classification of Acute Thoracolumbar Spinal Injuries. Spine. 1983;8:817-31.

Braakman R, Fontijne WPJ, Zeegers R, Steenbeek JR, Tanghe HLJ. Neurological deficit in injuries of the thoracic and lumbar spine-A consecutive series of 70 patients. Acta Neurochir (Wien). 1991;111:1-2:11-7.

Limb D, Shaw DL, Dickson RA. Neurological Injury in Thoracolumbar Burst Fractures. J Bone Joint Surg.1995;774-7.

Kanna RM, Shetty AP, Rajasekaran S. Posterior fixation including the fractured vertebra for severe unstable thoracolumbar fractures. Spine J. 2015;15(2):256-64.

McLain RF. The biomechanics of long versus short fixation for thoracolumbar spine fractures. Spine (Phila. Pa. 1976). 2006;31(11):70-9.

Esses S. Posterior short-segment instrumentation and fusion provides better results than combined anterior plus posterior stabilization for mid-lumbar (L2 to L4) burst fractures: Commentary. J Bone J Surg. 2006;88(10):2311.

Afzal S, Akbar S, Dhar SA. Short segment pedicle screw instrumentation and augmentation vertebroplasty in lumbar burst fractures: An experience. Eur Spine J. 2008;17(3):336-41.

Chiba M, McLain RF, Yerby SA, Moseley TA, Smith TS, Benson DR. Short-segment Pedicle Instrumentation. Spine. 1996;21(3):288-94.

Farrokhi MR, Razmkon A, Maghami Z, Nikoo Z. Inclusion of the fracture level in short segment fixation of thoracolumbar fractures. Eur Spine J. 2010;19(10):1651-6.

Crawford NR, Brantley G, Dickman CA, Koeneman EJ. An apparatus for applying pure noncnstraining moments to spine segments in-vitro. Spine. 1995;20(19):2097-100.

Acosta FL, Buckley JM, Xu Z, Lotz JC, Ames CP. Biomechanical comparison of three fixation techniques for unstable thoracolumbar burst fractures: Laboratory investigation. J Neurosurg Spine. 2008;8(4):341-6.

M. M. Panjabi, “The stabilizing system of the spine.Part II. Neutral zone and instability hypothesis. J Spinal Disorders. 1992;5(4):390-7.

Wilke HJ, Wenger KH, Claes LE. Load-displacement properties of the thoracolumbar calf spine: experimen Load-displacement properties of the thoracolumbar calf spine: experimental results and comparison to known human data. Eur Spine J. 1997;129-37.

Goel VK, Panjabi MM, Patwardhan AG, Dooris AP, Serhan H. Test protocols for evaluation of spinal implants. J Bone J Surg. 2006;88(2):103-9.

Wilke HJ, Wenger K, Claes L. Testing criteria for spinal implants: Recommendations for the standardization of in vitro stability testing of spinal implants. Eur Spine J. 1998;7(2):148-54.

Sekharappa V, Sait A. Simple and economical method to create thoracolumbar burst fracture in a calf spine model. Asian Spine J. 2016;10(1):6-13.

Kahanovitz N, Bullough P, Jacobs RR. The effect of internal fixation without arthrodesis on human facet joint cartilage. Clin Orthop Relat Res. 1984;189:204-8.

Akbarania BA, Crandall DG, Burkus K, Matthews T, Missouri L. Use of Long Rods and a Short Arthrodesis for Burst of the Thoracolumbar spine. J Bone Joint Surg. 1994;76(11).

Panjabi MM, O’Holleran JD, Crisco JJ, Kothe R. Complexity of the thoracic spine pedicle anatomy. Eur Spine J. 1997;6(1):19-24.

Vaccaro AR. Placement of pedicle screws in the thoracic spine: Part II: An anatomical and radiographic assessment. J Bone J Surg. 1995;77(8):1200-6.

Dick W, Kluger P, Magerl F, Woersdörfer O, Zäch G. A new device for internal fixation of thoracolumbar and lumbar spine fractures: The ‘fixateur interne. Paraplegia, 1985;23(4):225-32.

Been HD, Bouma GJ. Comparison of two types of surgery for thoraco-lumbar burst fractures: Combined anterior and posterior stabilisation vs. posterior instrumentation only. Acta Neurochir. 1999;141(4):349-57.

Butt MF, Farooq M, Mir B, Dhar AS, Hussain A, Mumtaz M. Management of unstable thoracolumbar spinal injuries by posterior short segment spinal fixation. Int Orthop. 2007;31(2):259-64.

Yu SW, Fang KF, Tseng IC, Chiu YL, Chen YJ, Chen WJ. Surgical outcomes of short-segment fixation for thoracolumbar fracture dislocation. Chang Gung Med J. 2002;25(4):253-9.

McLain R, Sparling E, Benson D. Early Failure of Short-Segment Pedicle Instrumentation for Thoracolumbar Fractures. J Bone Joint Surg. 1993;75:2.

Lazaro BCR. Biomechanics of thoracic short versus long fixation after 3-column injury: Laboratory investigation. J Neurosurg Spine. 2011;14(2):226-34.

Wang H, Li C, Liu T, Zhao WD, Zhou Y. Biomechanical efficacy of monoaxial or polyaxial pedicle screw and additional screw insertion at the level of fracture, in lumbar burst fracture: An experimental study. Indian J Orthop. 2012;46(4):395-401.






Original Research Articles