We present two patients with swelling of the groin following metal-on-metal total hip replacement without radiological signs of component loosening. MRI in both patients showed a round shaped intrapelvic lesion ventral to the femoral head. During surgery we found cystic structures filled with fluid and necrotic masses. After resection the metal head and insert were changed to a ceramic head and a polyethylene insert.

Although two different kinds of CoCrMo alloy were used in the metal-on-metal THR (Sikomet: low carbon content-Metasul: high carbon content) histopathological analysis in both cases showed typical morphological signs of hypersensetively determined inflammation. Despite the distinct soft tissue reaction bony component integration was unaffected.

In our opinion open resecion of the cystic lesion and changing of the metal-on-metal articulation is the treatment of choice.

As we do not have any reliable testing for clinical use to predict a hypersensitive reaction to metal wear after implantation of metal-on-metal THR this articulation surface should not be used in cases where allergy to metal is suspected.