Instability of the pelvis after removal of iliac bone for grafting was demonstrated roentgenographically in six patients. The anatomy of the pelvic ring is such that stresses placed on it by abnormal motion at any one of three points—the two sacroiliac joints and the symphysis—may cause instability. Diagnosis of pelvic instability must rest on a degree of suspicion and must be fortified by the clinical findings of pain with certain stress tests, routine roentgenographic examination of the pelvis, and special roentgenographic techniques.

Treatment should be directed to the offending area, that is, the sacro-iliac joint or the symphysis or both. In the cases presented, four of six patients had sacro-iliac fusions and three were asymptomatic at last follow-up; the fourth was improved but continued to have backache. Two patients had wedge resections of the symphysis pubis; one was asymptomatic at last follow-up and the other had undergone bone-grafting of the sacro-iliac joint too recently to determine the result.

Continued attention should be given to the sacro-iliac joints. In women who are multiparous and who have roentgenographic evidence of a rather short, straight, flat sacro-iliac joint, an iliac graft should be taken from an area where the sacro-iliac ligaments will not be disturbed.