Hypermobility & Hypomobility
Initially, sprained sacroiliac ligaments allow the joint surfaces to separate slightly more than normal, causing hypermobility. The smooth gliding in all directions changes to a pivoting aberrant motion. With the development of the ridge and groove (see Ridge and Groove), the sacrum moves in a distinct pattern, in accordance with their shapes. However, when the joint surfaces separate enough to move out of alignment, the main ridge, and/or smaller ridges, may move out of their respective grooves, and wedge out of position, causing hypomobility. Therefore, one can have both hypomobility and hypermobility within the same joint [1-6].
Vleeming et al.  stated “Under abnormal loading conditions of SI joints with ridges and depressions it is theoretically possible that a SI joint is forced into a new position where ridge and depression are no longer complementary. Such an abnormal joint position could be regarded as a blocked joint.”
Wilder et al.  indicated that, for movement to occur, the presence of matched bumps and depressions on the surfaces forces the sacroiliac joint to open wider than it would if the surfaces were smooth. This widening would stress the ligaments sooner than otherwise and would further limit range of motion.
Cox  suggested that “Upon dissection of a locked joint, it was found that the locking was due to one or more of the roughened projections becoming impinged in a corresponding cavity of the opposite bone.”
Hazle and Nitz  stated that “If movement is available to allow significant deviation from neutral, the presence of that motion must be excessive to precipitate the position of fixations. Thus, the apparent hypomobile joint may actually be an unstable joint…We suggest that that concept of a fixated instability of the sacroiliac joint is a reasonable extrapolation of the available evidence and is consistent with findings reported by clinicians, albeit of a different classification scheme than traditionally accepted for orthopedic disorders.”
Additionally, hypermobility of the sacroiliac joint is accompanied by hypermobility of the symphysis pubis  P174.
- Cox, H., Sacro-iliac Subluxation as a Cause of Backache. Surgical Gynecology & Obstetrics, 1927. 45: p. 637-648.
- Egund, N., et al., Movements in the sacroiliac joints demonstrated with roentgen stereophotogrammetry. Acta Radiologica: Diagnosis (Stockholm), 1978. 19(5): p. 833-46.
- Wilder, D.G., M.H. Pope, and J.W. Frymoyer, The functional topography of the sacroiliac joint. Spine, 1980. 5(6): p. 575-9.
- Vleeming, A., et al., Relation between form and function in the sacroiliac joint. Part II: Biomechanical aspects. Spine, 1990. 15(2): p. 133-6.
- Vleeming, A., et al., An integrated therapy for peripartum pelvic instability: a study of the biomechanical effects of pelvic belts. American journal of obstetrics and gynecology, 1992. 166(4): p. 1243-7.
- Hazle, C.R., Evidence-based assessment and diagnosis of pelvic girdle disorders: a proposal for an alternate diagnostic category. Physical Therapy Reviews, 2008. 13(1): p. 25-36.
- Levangie, P. and C. Norkin, Joint Structure and Function. A Comprehensive Analysis. 2005, Philadelphia, PA: F.A. Davis Company.