OBJECTIVE: The purpose of this study was to examine the mechanical behaviour of the female sacroiliac joint and the effects of its two major ligaments to joint stability. DESIGN: A cadaveric model was used to study the mechanical behaviour of the sacroiliac joints, and sequential dissection was performed to examine the contribution of the anterior and posterior sacroiliac ligaments in joint stability. BACKGROUND: Instability of the sacroiliac joints have been suspected as a possible cause of low back pain. Despite several investigations on joint anatomy and joint mobility, its stabilising mechanism is still not clear. METHODS: Four fresh cadaveric specimens of the female pelvis were tested on an Instron material testing machine. Eccentric compressive force of 60% of the subject's body weight was applied to the pelvis through the sacrum. Relative three-dimensional six-degree-of-freedom movement at the left sacroiliac joints was recorded with a specially designed motion tracking device. The device has an accuracy of 0.01 mm and is compact enough to be mounted across the joint. The test was repeated after sequential selective dissection of the bilateral anterior, and then posterior sacroiliac ligaments. RESULTS: Rotation up to 1.2 degrees and translation up to 0.9 mm were measured from the intact specimens. Lateral rotation, which tended to open the top portion of the joint, and sacral nutation were the primary rotations. On average, the rotation angles increased 10% when either the anterior or posterior ligaments were cut, and 30% when both ligaments were cut. CONCLUSIONS: Lateral rotation and nutation rotation of the sacrum were found to be the predominant motion, though the values were limited to less than 1.2 degrees. Both the anterior and posterior sacroiliac ligaments were found to play an important role in resisting rotations at the joints.