Why Does My Back Hurt? The Biomechanics of back pain explained

Your ‘backbone’ or spine comprises a series of separate bones (vertebrae) joined together by deformable intervertebral discs, which are tough gristle-like pads of cartilage. Adjacent vertebrae are also connected by small sliding joints, the apophyseal, or facet, joints (Figure 1), and by ligaments which are tough fibrous bands. All of these structures, including the discs, have nerves that are capable of signalling pain.

Unfortunately, the spine lies so deep in the body that it is difficult to put your finger directly on the pain source. However, several research studies have managed to do just this: the treating clinician has prodded and poked inside their patients’ back until their exact pain is reproduced. These heroic diagnostic studies, which sometimes involved expensive imaging techniques such as MRI, have shown that severe and chronic back pain most often comes from intervertebral discs, and from the apophyseal joints. Often, the MRI images reveal abnormalities in structures that turn out to be unrelated to the chronic pain! In contrast, back pain that flares up quickly and is gone in a week appears to come from overstretched back muscles or ligaments.

Because of the pain and expense of diagnostic tests, and because the tests are not 100% reliable, most clinicians do not use them with each individual patient. Instead they use information from the research studies to estimate where the pain is probably coming from in each patient. If they suspect there is no serious underlying problem, the clinician may perform no invasive tests, and simply refer to the patient as having ‘non-specific back pain’. This just means that the patient has back pain, that the pain is of unknown origin, and that it is expected to clear up quickly. It can be frustrating for the patient not to know precisely where their pain comes from, but uncertainty is often better than invasive and possibly misleading diagnostic tests.

A great deal of non-specific back pain is probably just ‘functional’ in the sense that it arises from the manner in which the patient uses their back, without there being any underlying injury or degeneration. For example, standing for long periods in a lordotic hollow-back posture can give rise to high compressive and shear forces being concentrated in the apophyseal joints (Figure 1), or in parts of the disc. This can cause backache for as long as the posture is maintained, but the pain soon fades when the patient adopts another posture, such as sitting.

Patients can also be frustrated if they are told (or otherwise find out) that there is a suspected ‘psycho-social’ component to their pain. But this does not mean that the pain is not real, or ‘all in the head’. It simply means that the clinician believes that aspects of the patient’s behaviour (such as the decision to seek treatment, or to report that they are feeling better, or not) are significantly influenced by psychological factors such as fear of injury, or by social factors such as a physically-demanding job. And why wouldn’t they be!

Clinicians must treat whole patients and not specific symptoms, and recognising the involvement of psycho-social factors may help to prevent inappropriate or unnecessary treatments.

Fig 1 This side view of the lumbar spine (lower back) shows five vertebrae, separated by intervertebral discs which are shaded blue. The surface of the back is shown on the right, in orange, and the region occupied by a typical apophyseal joint is shown by the red circle. S       and C denote the direction of compressive and shear forces acting on the spine. Figure      adapted from: “The Biomechanics of Back Pain” by Adams, Bogduk, Burton and Dolan. Publishers: Churchill Livingstone (3rd Edition 2013).