Please cite this article in press as: Izzo R, et al. Biomechanics of the spine. Part II: Spinal instability. Eur J Radiol (2012), http://dx.doi.org/10.1016/j.ejrad.2012.07.023 ARTICLE IN PRESS G Model EURR-6096; No. of Pages 12 European Journal of Radiology xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect European Journal of Radiology jo ur n al hom epage: www.elsevier.com/locate/ejrad Review Biomechanics of the spine. Part II: Spinal instability Roberto Izzo a,∗ , Gianluigi Guarnieri a , Giuseppe Guglielmi b , Mario Muto a a Neuroradiology Department, “A.Cardarelli” Hospital, Napoli, Italy b Department of Radiology, University of Foggia, Foggia, Italy a r t i c l e i n f o Article history: Received 3 July 2012 Received in revised form 20 July 2012 Accepted 21 July 2012 Keywords: Spine Biomechanics Spinal instability Spinal degeneration Spinal trauma Spinal tumours CT, MR a b s t r a c t Spine stability is the basic requirement to protect nervous structures and prevent the early deterioration of spinal components. All bony and soft spinal components contribute to stability, so any degenerative, traumatic or destructive lesion to any spinal structure gives rise to some degree of instability. Degenerative instability is considered a major cause of axial and radicular pain and is a frequent indica- tion for surgery. Nevertheless the assessment of instability remains difficult in both clinical and imaging settings. All static imaging modalities, even conventional MR, the most accurate technique, are unreliable in assessing instability and chronic pain due to degenerative spine. Dynamic-positional MR is considered the most sophisticated imaging modality to evaluate abnormal spinal motion and instability. In spinal traumas, as multi-detector CT yields high-resolution reconstructions in every spatial plane, it will detect even the tiniest fractures revealing potentially unstable lesions, often avoid the routine use of MR. Nevertheless, MR remains the only modality that will directly and routinely assess soft tissue changes. Unfortunately the objectivity of MR in assessing the integrity of ligaments is not rigorously defined and its use in routine protocols to clear blunt spinal injuries remains controversial. There are no evidence-based guidelines currently available to assess the risk of spinal instability in the setting of neoplastic spinal disease, so predicting the risk of a pathological fracture or the timing of a collapse remains challenging even when the lesions are well-characterized by neuroimaging. Diagnostic difficulties lead to controversy in the choice of the best treatment in all forms of spinal instability. © 2012 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Traumatic, neoplastic and degenerative instability are impor- tant cause of spinal pain and disability. Degenerative instability, in particular, is considered a major cause of axial and radicular acute and chronic pain and is a fre- quent indication for surgery. Nevertheless, an accurate definition of instability and the best diagnostic approach remain matters of debate and have given rise to controversy in the choice of the best treatment. Even though MR is the most accurate imaging modality to study the degenerative spine, conventional recumbent imaging is often not reliable in assessing instability and the source of acute and chronic pain [1]. Positional-upright MR is the most ∗ Corresponding author at: Neuroradiology Department, “A.Cardarelli” Hospital, Viale Cardarelli 9, 80131 Napoli, Italy. Tel.: +39 0817473116. E-mail addresses: roberto1766@interfree.it (R. Izzo), gianluigiguarnieri@hotmail.it (G. Guarnieri), g.gugliemi@unifg.it (G. Guglielmi), mutomar@tiscali.it (M. Muto). sophisticated imaging tool to assess instability but the high cost involved hampers its widespread availability so that conventional dynamic radiographs remain the simplest and most widely used diagnostic reference [2]. Traumatic spinal injuries affect a complex structure consist- ing of soft and bony components having different susceptibility and healing potential: this complexity contributes to difficulties in classifying traumas and in assessing instability and the efficacy of various treatments. Conventional radiology is inadequate to assess the stabil- ity of fractures [3]. As multi-detector CT yields high-resolution reconstructions in every spatial plane, it will detect even the tiniest fractures revealing potentially unstable lesions. While conventional radiology and CT can only indirectly evalu- ate lesions affecting discs and ligaments and are of limited value for prognosis and therapy, MR is the only imaging modality that directly and routinely assesses changes in liga- ments. Nevertheless, the objectivity of MR in assessing the integrity of ligaments and predicting mechanical instability remains contro- versial [4,5]. 0720-048X/$ – see front matter © 2012 Elsevier Ireland Ltd. 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