Severe Corrosion of a Modular Dual Mobility Acetabular Component
A 55-year-old male patient presents with severe hip pain due to an intraprosthetic dislocation 8 years after total hip arthroplasty. Should the presence of a modular dual mobility construct cause concern for additional issues?
Erik Zachwieja, MD, and Peter F. Sharkey, MD
The use of modular dual mobility (MDM) components during primary and revision total hip arthroplasty (THA) has grown substantially over the past 5 years, [1,2] increasing from 6.7% of primary THAs in 2012 to 12% in 2018 and from 19.5% of revision THAs in 2012 to 30.6% in 2018 in the US alone. 
Increased utilization is likely driven by the enhanced articulation stability provided by dual mobility implants and the increased head sizes they allow. Large femoral head size coupled with motion at 2 articulating surfaces increases the hip’s range of motion prior to impingement, resulting in a low incidence of dislocation. [4-8]
Furthermore, the advent of modularity has facilitated the ease of implantation. The original dual mobility construct utilized a monoblock cobalt-chromium acetabular component that did not allow for supplemental screw fixation or attachment of an insertion handle, causing difficulties with implant insertion. Contemporary dual mobility designs consist of a standard titanium acetabular component and a modular cobalt-chromium articular insert.
However, this combination of dissimilar metals has led to justifiable concerns related to corrosion, elevated blood metal ion levels, and adverse local tissue reaction (ALTR). Two recent publications have described significantly elevated blood metal ion levels in approximately 10% of patients with MDM components. [9,10] Several cases of ALTR were also found in these patient cohorts.
In this article, we present the case of a patient who had severe corrosion of MDM components that was found during revision THA for intraprosthetic dislocation.
A 55-year-old male presented to a hospital emergency department complaining of acute onset of severe left hip pain 8 years following an uncomplicated left THA for primary hip osteoarthritis. A 58-mm Tritanium cup, 46-mm MDM liner, 28-mm cobalt-chromium ball, and size 5 lateral Accolade II stem were used (Stryker Orthopaedics; Mahwah, New Jersey). The polyethylene liner and metal head had been assembled according to the manufacturer’s instructions.
- Height: 6 feet, 1 inch; weight: 195 pounds; BMI: 25.7
- Well-healed anterolateral incision
- Shortened extremity, internally rotated
- Hip range of motion unable to be assessed secondary to pain
- Normal distal lower extremity neurovascular exam
- Serum erythrocyte sedimentation rate: 6 mm/hour
- Serum C-reactive protein: 2.4 mg/L
- Blood cobalt level: 5.8 mcg/L (reference: 1.8 mcg/L or less)
- Blood chromium level: 1.0 mcg/L (reference: 1.2 mcg/L or less)
Radiographs taken on presentation to an outside institution’s emergency department and after attempted closed reduction were obtained (Figures 1 and 2).
Figure 1. Anteroposterior radiograph of the pelvis at the time of presentation to the emergency department. There is an intraprosthetic dislocation of the femoral head posterosuperiorly.
Figure 2. Anteroposterior radiograph of the pelvis after attempted closed reduction reveals persistent subluxation of the femoral head posterosuperiorly. Components appear well fixed.
- Acute intraprosthetic dislocation of a modular dual mobility primary THA with possible corrosion
There were several considerations for the management of this patient:
- Is there ALTR that requires debridement secondary to fretting corrosion, as evidenced by elevated preoperative blood metal ion levels? How will ALTR, if present, affect the hip abductor mechanism and hip stability?
- Is the femoral stem well fixed? Is there trunnion damage? A damaged trunnion will necessitate removal of the stem, even if it well fixed, as trunnion corrosion could lead to metallosis.
- Is the acetabular component well fixed? If so, should a new dual mobility articulation be implanted, or does the acetabular shell allow for conversion to a polyethylene-on-ceramic or polyethylene-on-metal articulation?
We discussed with the patient the need for revision surgery to address the intraprosthetic dislocation. We hoped to only revise the acetabular liner and femoral head, but we knew that full acetabular or femoral revision, along with debridement of any non-viable tissue related to ALTR, was a possibility.
The implant that had initially been utilized (Tritanium cup) allowed for conversion to a standard polyethylene liner. Femoral component revision would be necessary to prevent future metallosis if we found significant trunnion damage. A loose acetabular component would require full acetabular revision.
Instability following revision THA is a significant concern that is often addressed with the use of modular dual mobility components. We elected not to utilize a dual mobility construct, however, due to concerns for metallosis. Instead, we had constrained polyethylene liners available to manage any instability detected intraoperatively. In addition, we had the option to change the femoral or acetabular components to ensure appropriate stability.
- Utilizing the prior incision, dissection was taken down and deep to the fascia.
- An intraprosthetic dislocation was noted. The femoral head had disassociated from the polyethylene liner and had migrated posteosuperiorly, while the liner remained within the acetabular shell.
- Severe and very focal wear of the inner MDM polyethylene liner was identified posterosuperiorly (Figure 3). This suggested impingement from the neck on the polyethylene liner, ultimately leading to dislocation.
Figure 3. Inner modular dual mobility polyethylene liner with focal damage at the posterosuperior aspect is suggestive of impingement.
- There was no evidence of ALTR.
- The 28-mm cobalt-chromium head on the femoral stem was intact without obvious damage. Following removal of the femoral head, careful inspection revealed no corrosion of the head, with minimal damage to the trunnion.
- The modular acetabular liner was inspected and noted to be well fixed and completely inserted into the titanium acetabular shell.
- The MDM components were then disassembled. Inspection showed severe corrosion debris on the backside of the cobalt-chromium liner and inner surface of the titanium shell (Figures 4 and 5).
Figure 4. Backside wear of the modular cobalt-chromium insert. Note the black, chromium-rich surface consistent with mechanically assisted crevice corrosion.
Figure 5. Wear of the titanium acetabular shell superomedially. Note the chromium-rich black surface consistent with mechanically assisted crevice corrosion.
- The femoral and acetabular shell components were well fixed and well positioned.
- The corrosion debris was thoroughly debrided and a 36-mm highly cross-linked polyethylene liner was inserted into the shell.
- A titanium sleeve was placed over the trunnion and a +7.5, 36-mm ceramic head was inserted. This achieved satisfactory limb length and excellent stability.
- Immediate postoperative radiographs are shown in Figure 6.
Figure 6. Immediate postoperative radiograph following head and liner exchange to a ceramic–on–polyethylene articulation.
- The patient was made weight-bearing as tolerated immediately postoperatively. No hip precautions were instituted.
- At the initial postoperative visit, the patient was ambulating without assistive devices and had a well-healed incision.
- The patient will be monitored to assess if his metal ion levels decrease to within the reference range.
Corrosion following articulation of dissimilar metals during THA has been demonstrated to be a serious concern, sometimes leading to significant complications and the need for revision surgery. [11-13] The most common mechanism proposed for this corrosion is mechanically assisted crevice corrosion (MACC). [14,15] Micromotion between 2 dissimilar metals is necessary for MACC to occur. This motion leads to the hallmark findings of chromium-rich black surface deposits and serum cobalt levels significantly higher than chromium levels. [16,17]
Recent retrieval analyses of modular cobalt-chromium acetabular inserts have revealed evidence of corrosion at the titanium shell/cobalt-chromium modular insert interface. [16,18] This is consistent with the case presented here, where obvious corrosion was identified on the backside of the acetabular insert and inner surface of the titanium shell. In addition, elevated serum cobalt levels, independent of chromium levels, is consistent with MACC.
Mechanisms for the induced micromotion is not fully understood, but recent evidence suggests malseating of the cobalt-chromium insert may be a contributing factor. Wright et al  created a crevice environment conducive to erosion by utilizing an electrochemical chamber and MDM liners (2 well-seated liners and 2 liners canted at 6°). After cyclic loading at physiologic levels, the authors found that compared with a well-seated liner, malseating the insert 6° resulted in increased fretting corrosion at physiologic loads.  This is important to note, as the same group identified nearly 6% of MDM constructs implanted over a 2-year period at their institution were malseated on postoperative radiographs. 
Although our patient was not found to have a malseated insert, either intraoperatively or on review of previous radiographs, surgeons utilizing MDM constructs must be cognizant of the potnential to malseat the insert and its resulting consequences.
Another potential mechanism for MACC, which may have occurred in our patient, involves impingement. The contemporary MDM cobalt-chromium liner used in the patient’s index procedure has a lip extending above the titanium acetabular shell. Impingement of the femoral neck on this elevated lip is certainly possible and likely, particularly in younger active patients. Impingement could easily create the interface micromotion needed to facilitate MACC for this modular device.
- Intraprosthetic dislocation of MDM components must be addressed surgically. The surgeon should assess for corrosion at the time of revision. If the femoral and acetabular implants are well fixed, revision to a conventional ceramic-on-polyethylene articulation is acceptable.
- The surgeon must avoid malseating the cobalt-chromium insert when utilizing MDM components.
- Due to the increased use of MDM components in primary and revision THA, surgeons should evaluate patients for metallosis to prevent the potential sequelae of ALTR.
Erik Zachwieja, MD, and Peter F. Sharkey, MD, are from The Rothman Institute at Thomas Jefferson University Hospital, Philadelphia, Pennsylvania.
Adult Reconstruction Section Editor, Rothman Institute Grand Rounds
Disclosures: Dr. Zachwieja has no disclosures relevant to this article. Dr. Sharkey has disclosed that he receives royalties from and is a paid consultant and paid speaker for Corentec Orthopaedics and Zimmer Biomet and that he receives royalties from StelKast. Dr. Courtney has disclosed that he is a paid consultant for Hip Innovation Technology.
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