Depending on the stage of your medical training, the sacroiliac joint is variably mobile.

Way back in the anatomy lab, I remember being taught that this large joint was practically immobile. The sheer strength of the interosseous ligament together with the accessory ligaments and the roughened articular cartilage ensured maximal stability. The bulky metal bolts holding the joints together in the skeleton suspended with a watchful eye socket over our laborious studies only served to reinforce the message: sacroiliac joint—a strong stable joint.

All was to change on the orthopaedic ward, however, where we were led to believe that all joints—natural or replaced—were mobile and, …