A biomechanical perspective of sacro-iliac pain

Pain in the sacroiliac region is often referred to by the non-specific term sacro-iliac syndrome. Quite often there is dull pain accompanied with local tenderness over the sacrum and posterior superior iliac spines. The hypothesis of this work is that the pain may result from surprisingly large stresses within the extensor aponeurosis and other extensor forces directly transmitted to the sacro-iliac region. The forces in 48 muscles were estimated from a detailed, anatomic computer model of the trunk. Forces in muscles that originate from the sacral and posterior superior iliac spine region were summated. For sagittal plane lifts involving relatively light loads of 27·3 kg, nearly 7 kN of force was transmitted to the extensor origins of the sacro-iliac region. A large proportion of this force was produced by the longissimus thoracis pars lumborum and iliocostalis lumborum pars lumborum complex. These predicted high stresses are discussed as possible factors in clinically observed sacroiliac trauma and pain.