Revision rate high for pseudotumor formation in MOM large head diameter THA

Issue: October 2010

Nearly 7% of patients who underwent a large-diameter head, metal-on-metal total hip arthroplasty ended up with pseudotumor formation, for which early revision was required in all but one of the eight cases, according to a 120-subject, retrospective Belgian study. The average time between the primary and revision procedure was 28.1 months.

Lead author Patrick S. Deprez, MD, an orthopedic surgeon from Brugge, Belgium, and his colleagues undertook the study because of “the uncommon problems we saw during follow-up, like swelling and persistent pain, which we did not see in other bearing couples,” Deprez told Orthopedics Today. “We thought that the problems were due to the metal-on-metal [MOM] components.”

The published literature about metal hypersensitivity “shows similarities to our findings, when infection is excluded,” Deprez said. “However, we were not able to prove this until an independent lab confirmed a metal hypersensitivity. That did not surprise us, but we were quite surprised about the number of patients and the percentage of patients with equal symptoms that appeared during further follow-up. Metal allergy-pseudotumor formation might be underestimated and can cause osteolysis.”

Women more susceptible

Seven of the eight patients with extensive femoroacetabular fluid production, cysts, were women. “It is unclear why some patients develop this phenomenon and others do not. Women seem to be more susceptible,” Deprez said. Nonetheless, all affected patients “described a swelling in the groin and/or had slight to moderate pain.” Infection parameters were normal and there were no signs of loosening, except in one case. “Fluid was odorless and color was milky white or troubled,” Deprez said. In addition, microscopy revealed small amounts of metal particles. “Histology pointed to metal sensitivity,” he said.

One case Deprez presented at the annual meeting of the American Academy of Orthopaedic Surgeons in New Orleans was progressive osteolysis around the acetabulum and a progressive fracture of the greater trochanter.

Some patients had late positive fluid aspiration cultures. “It is possible that this fluid production due to metal hypersensitivity is more susceptible for [secondary] infections, although this could not be proven,” Deprez said. “Based on the clinical expression and blood sample examination, we believe that none of these cases were a primary-grade infection, although symptoms can mimic it. It is clear, though, that deep infection needs to be excluded in every case where similar symptoms appear after using such a device.”

Technique and materials

Two other potential factors to deep metal allergy are component orientation and component manufacturing. “Some authors suggest that cup position might be an important contributing factor,” Deprez said. “Cups oriented in excessive anteversion and/or a lateral opening angle of more than 55° leads to edge loading and high wear, which may cause delayed-type hypersensitivity.” The average inclination in the eight study cases was 57.6°, he noted. “However, this might be an over- or underestimation because measuring methods without adequate software are unreliable on plain X-rays.”

“Be careful in placing MOM total hips in women, and don’t place them if they have a known metal, nickel or chromium [skin] allergy,” said Deprez, who has virtually eliminated using this type of prosthesis. “The only MOM hip prosthesis we still place is the resurfacing prosthesis — and only in men.” To reduce the revision rate in men, “pay attention to cup position and avoid edge loading.” Also important is providing preoperative information to the patient “about the possible side effect of metal hypersensitivity.”

Preoperative screening

Deprez said, “We need some preoperative screening tests for metal hypersensitivity. Pre-, postoperative deep-metal-allergy skin screening tests are not reliable.” He advocates routine postoperative sonography.

Two questions that linger from the study are what causes metal hypersensitivity and why it is so high in this series. “We do not encounter this problem nearly as frequently in our resurfacing patients where the same cup is used,” Deprez said. “This metal hypersensitivity seems to be a toxic reaction by the surrounding soft tissues, more seen in women, which might be triggered by component position and the type of prosthesis, such as metal alloy and tapered connectors between head and stem.” – by Bob Kronemyer

Reference:
Deprez PS, Berghe LV, Demuynck M. High early revision rate due to pseudotumor formation in metal-on-metal large head diameter THA. Paper 011. Presented at the 2010 Annual Meeting of the American Academy of Orthopaedic Surgeons. March 9-13. New Orleans.

Patrick S. Deprez, MD, can be reached at AZ Sint-Lucas ziekenhuis, Sint-Lucaslaan 29, 8310 Brugge, Belgium; 32 50 36 9080; e-mail: patrick_deprez@yahoo.com.

Pseudotumors have been reported since many years and during recent years especially in relation to MoM resurfacing THA (RTHA) with an incidence from 0-8%. The paper by Deprez et al. on large diameter head (LDH) standard stemmed THA showed a pseudotumor incidence of 7% (8/120) after a mean follow-up of 4 years with an average time between the primary and revision procedure around 2 years. This is a surprising result for the LDH stemmed THA and has to my knowledge not been published before. When we acknowledge that pseudotumor in most cases is seen in patients with elevated ion-concentrations the results in the present study might be design related.

Sleeves with modular junctions and an open femoral head design may cause more ion release than bearing surface with a closed head design and RTHA, which could be an explanation.

When evaluating metal ion release from MOM hip arthroplasty, the total metal load from implants should be considered.

As seen in several other studies the majority of cases with pseudotumors were diagnosed in women. Thus female gender also seems to be a risk factor in LDH THA as in RTHA.

— Soren Overgaard,professor, MD
Department of Orthopaedics and Traumatology
Odense University Hospital, Denmark