Biomechanics of the spine. Part II: Spinal instability

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 indication 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.