Cause of Pelvic Crest Pain After Fusion
In this study, orthopedic surgeons tried to see if patients having low back spine fusion using their own bone graft have more pain than those who don't have a bone graft. Those who did not have bone graft to help with the fusion were given a bone substitute instead.

The bone graft does not require harvesting bone from the patient. After surgery, each patient was tested for tenderness over the incision site as well as over the back of each iliac crest (posterior right and left).

The iliac crest is the area of the pelvic bones where bone graft is taken most often. You can feel this bone when you put your hands just below your waist. The authors were surprised to find out that just as many patients without a bone graft had tenderness over the bone in this area.

Fifty-seven per cent (57%) of the bone graft group had tenderness or the posterior iliac crest on one or both sides. This compared with 51 per cent in the bone graft substitute group.

And oddly enough, patients in both groups had pain over both iliac crests (not just the donor side and not just in patients who had their own bone harvested for the fusion site). The severity of pain (as rated by each patient) was no different between the two groups.

Of course they wondered what could be causing this type of pain response in patients who have not had any bone harvested from the iliac crest. The presence of pain in both groups may suggest something else besides bone harvesting from this site is causing the tenderness.

One possibility is referred pain from new lumbar spine pathology. Other sources of iliac crest pain following lumbar spine fusion include inflammation, nerve root irritation, or muscle scarring from the fusion procedure. And it's possible that the iliac crest tenderness was really caused by residual pain from the low back area.

The authors did not test patients in either group for the presence of iliac crest tenderness before surgery. It may be they all had iliac crest pain/tenderness that didn't go away after surgery.

One other unanswered question has to do with the patients in both groups who did not have any posterior iliac crest pain or tenderness after surgery. This group was made up of nearly half of all the patients. Future studies to determine any factors that might predict who will or won't have post-operative iliac crest pain may be helpful in preventing this problem.
Jennifer M. Howard, MPH, et al. Posterior Iliac Crest Pain After Posterolateral Fusion With or Without Iliac Crest Graft Harvest. In The Spine Journal. June 2011. Vol. 11. No. 6. Pp. 534-537.