Dr Patricia Dolan BSc, PhD
Reader in Spine Biomechanics,
Centre for Comparative and Clinical Anatomy
Back and neck pain are extremely common and will affect most of us during our lifetime. Symptoms often resolve within a few weeks, but in some cases they can be persistent and debilitating. The reasons why acute back pain becomes chronic in some people are poorly understood although it is now clear that many factors are involved. Age, genetic inheritance and environmental factors (such as mechanical loading) all affect our predisposition to tissue injury, but whether such injury leads to chronic pain is dependent upon factors such as pain sensitisation and psychosocial influences. In recent years, the emphasis within back pain research has been to improve the management of chronic pain either by tackling the psychological factors that have such a strong influence on pain behaviour, or by replacing the painful tissues with prosthetic implants. However, primary prevention requires a better understanding of the events that lead to tissue disruption and the factors that influence the biological response to such injury.
Our current research is investigating the links between muscle function, mechanical loading and tissue injury using both in vivo and in vitro experimentation. Studies in living people are investigating how impaired muscle function may increase the risk of back and neck pain. Electromyographic (EMG) and kinematic assessments enable us to explore how factors such as muscle fatigue, soft tissue creep and altered sensorimotor function influence the way we load our spines in life, and how this might increase the risk of tissue injury. Studies on healthy volunteers have shown that prolonged or repetitive flexion can impair proprioception and delay reflex muscle activation allowing increased loading on the underlying passive tissues of the spine. If such changes occur in an occupational setting then this may increase the risk of strain injury.
Data from our in vivo studies is also used to design in vitro experiments that simulate realistic (physiological) loading conditions. These experiments demonstrate how subtle changes in spinal loading or posture can alter tissue stresses in such a way as to cause pain, degeneration and injury. Using a small pressure transducer which can be pulled through the intervertebral disc, we have shown that prolonged compressive loading, even at low to moderate loads, leads to a loss of pressure in the nucleus and peaks of compressive stress in the annulus of the intervertebral disc. Such stress differentials are even more marked in degenerated discs where they are often accompanied by high levels of neural arch load bearing. These stress concentrations may initiate pain by stimulating nerve endings in the outer annulus and apophyseal joints. They will also alter the mechanical environment of the disc cells and this may inhibit matrix synthesis and limit the disc’s capacity for repair. More recently, we have shown that disc degeneration may also increase the risk of osteoporotic fracture. In these circumstances, increased loading of the neural arch acts to stress shield the anterior part of the disc and vertebral body leading to a loss of bone mineral density anteriorly. Loading of the neural arch becomes even greater after fracture, further exacerbating the effects of disc degeneration. Cement augmentation of the vertebral body using techniques such as vertebroplasty and kyphoplasty, can reverse these effects and restore spinal load-bearing towards pre-fracture values (Figure 1). This may have long-term beneficial effects in slowing the fall in bone mineral density in the anterior vertebral bodies and helping to prevent vertebral deformity.
Figure 1. X-ray above: Vertebral osteoporotic fracture can lead to wedge deformity (arrow). Graph above: Stress profiles show a marked loss of pressure in the central region (nucleus pulposus) of the intervertebral disc following fracture that is associated with concentrations of compressive stress in the periphery of the disc (annulus). These changes in load-bearing are largely reversed by vertebroplasty which acts to repressurise the nucleus and reduce stress concentrations in the annulus.
- Luo J, Skrzypiec DM, Pollintine P, Adams MA, Annesley-Williams DJ, Dolan P (2007). Mechanical efficacy of vertebroplasty: Influence of cement type, BMD, fracture severity, and disc degeneration. Bone 40(4): 1110-9.
- Przybyla A, Skrzypiec D, Pollintine P, Dolan P, Adams MA (2007). Strength of the cervical spine in compression and bending. Spine 32(15):1612-20.
- Skryzpiec D, Pollintine P, Przybyla A, Dolan P, Adams MA (2007). The internal mechanical properties of cervical intervertebral discs as revealed by stress profilometry. European Spine Journal 16:1701-9.
- Skrzypiec D, Tarala M, Pollintine P, Dolan P, Adams MA (2007). When are intervertebral discs stronger than their adjacent vertebrae? Spine 32(22) 2455-61.
- Adams MA, Dolan P (2008). Biomechanics of the musculoskeletal system. In: Gray’s Anatomy (40th Ed), Elsevier/Churchill Livingstone, Edinburgh (pp. 117-125)
- Luo J, Daines L, Charalambous A, Adams MA, Annesley-Williams DJ, Dolan P (2009). Vertebroplasty: only small cement volumes are required to normalise stress distributions on the vertebral bodies. Spine 34(26): 2865-73.
- Adams MA, Dolan P, McNally DS (2009).The internal mechanical functioning of intervertebral discs and articular cartilage, and its relevance to matrix biology. Matrix Biology 28: 384-89.
- Pollintine P, Luo J, Offa-Jones B, Dolan P, Adams MA (2009). Bone creep can cause progressive vertebral deformity. Bone 45(3) 466-72.
- Zhao F-d, Pollintine P, Hole BD, Adams MA, Dolan P (2009). Vertebral fractures usually affect the cranial endplate because it is thinner and supported by less dense trabecular bone. Bone 44(2) 372-9.
- Pollintine P, van Tunen MSLM, Luo J, Brown M, Dolan P, Adams MA (2010). Time-dependent compressive deformation of the ageing spine: relevance to spinal stenosis. Spine 35(4): 386-94.
- Sanchez-Zuriaga D, Adams MA, Dolan P (2010). Is activation of the back muscles impaired by creep or muscle fatigue? Spine 35(5): 517-25.
- Luo J, Bertram W, Sangar D, Adams MA, Annesley-Williams DJ, Dolan P (2010). Is kyphoplasty better than vertebroplasty in restoring normal mechanical function to an injured spine? Bone 46: 1050-7.
- Jiang G, Luo J, Pollintine P, Dolan P, Adams MA, Eastell R (2010). Vertebral Fractures in the Elderly may not always be Osteoporotic. Bone 47: 111-6.
- Adams MA, Dolan P (2010). Biomechanics of the spine. In: Hochberg MC et al. (Eds.) Rheumatology (5th Ed). Elsevier, Philadelphia, USA.
- Adams MA, Stefanakis M, Dolan P (2010). Healing of a painful intervertebral disc should not be equated with reversing disc degeneration. Clinical Biomechanics 25:961-971.
- Luo J, Adams MA, Dolan P (2010). Vertebroplasty and kyphoplasty can restore normal spine mechanics following osteoporotic vertebral fracture. Journal of Osteoporosis doi: 10.4061/2010/729257.
- Al-Rawahi M, Luo J, Pollintine P, Dolan P, Adams MA (2011). Mechanical function of vertebral body osteophytes, as revealed by experiments on cadaveric spines. Spine 36 (10): 770-777.
- Adams MA, Dolan P (2011). Biomechanics of vertebral compression fractures and clinical application. Arch Orthop Trauma Surg 131: 1703-10.
- Cardozo AC, Gonçalves M, Dolan P (2011). Back extensor muscle fatigue at submaximal workloads assessed using frequency banding of the electromyographic signal. Clinical Biomechanics 26: 971-976
- Luo J, Pollintine P, Gomm E, Dolan P, Adams MA (2012). Vertebral deformity arising from an accelerated ‘creep’ mechanism. Euro Spine J 21:1684–1691.
- Adams MA, Bogduk N, Burton K, Dolan P (2012). “Biomechanics of Back Pain” 3rd Edition. Churchill Livingstone, Edinburgh.
- Stefanakis M, Al-Abbasi M, Harding I, Pollintine P, Dolan P, Tarlton J, Adams MA (2012). Annulus fissures are mechanically and chemically conducive to the ingrowth of nerves and blood vessels. Spine (in press).
- Adams MA, Dolan P (2012). Intervertebral disc degeneration: evidence for two distinct phenotypes. Journal of Anatomy (in press).
- Dolan P, Adams MA (2012). Time-dependent mechanisms that impair muscle protection of the spine. In: “Motor control of the trunk.” Editor: P. Hodges. Elsevier. (In press)
- Brumagne S, Dolan P, Pickar J (2012). What is the relation between proprioception and low back pain? In: “Motor control of the trunk.” Editor: P. Hodges. Elsevier. (In press)
- Dolan P, Adams MA (2012).Biomechanics of spinal muscles and ligaments. In: Soft tissue disorders and injuries of the spine. Editors: R. Gunzburg and M Spalszki. (In press)