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A 68-year-old woman with hip pain 3 years after primary THA

Issue: September 2013

ByBrandon J. Erickson, MD

ByJoshua J. Jacobs, PharmD

ByRachel M. Frank, MD

ByCraig J. Della Valle, MD

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A 68-year-old woman presented with progressive groin pain following a primary right total hip arthroplasty performed for osteoarthritis 3 years earlier. The index procedure was uncomplicated, and components included a fully porous-coated, diaphyseal-engaging 12-mm cobalt-alloy cementless stem, a 54-mm titanium-alloy cementless cup, a highly cross-linked neutral polyethylene liner and a 36-mm+0 cobalt-alloy femoral head. The patient initially recovered well with resolution of her symptoms and without complications.

On physical examination, the patient was noted to have a mild coxalgic gait favoring the right leg. Her posterior hip surgical incision was intact without surrounding warmth or erythema. Range of motion was unremarkable, but painful at the extremes of range of motion as well as with straight-leg raise.

Diagnostic studies

Radiographs obtained were compared to the 6-week postoperative films (Figure 1) and showed well-fixed femoral and acetabular components with evidence of proximal femoral osteolysis. Erythrocyte sedimentation rate (ESR) was elevated at 99 mm/hr (normal <27) and serum C-reactive protein (CRP) was 199.5 mg/L (normal <8). Given the elevations in the ESR and CRP combined with early osteolysis, the right hip was aspirated and cloudy fluid was obtained. Synovial fluid white blood cell (WBC) count was 322 WBC/uL and cultures showed no growth.

Given the negative evaluation for infection and the clinical history, corrosion at the head-neck junction was suspected. Serum metal ion levels were notable for serum cobalt of 49.8 parts per billion (ppb) (normal <0.7) and serum chromium of 9.8 ppb (normal <0.3). A metal artifact reduction sequence (MARS) MRI revealed several well-circumscribed masses surrounding the hip with moderate surrounding muscular edema (Figure 2).

 

Figure 1. The 6-week postoperative AP radiograph of the hip shows acceptable positioning of the components. Acetabular inclination is 54° (A). The 3-year follow-up AP radiograph of the hip demonstrates well-fixed femoral and acetabular components with evidence of proximal femoral osteolysis best seen in Gruen Zone 7 (B).

 

Figure 2. An axial view of the right hip with metal artifact reduction sequence-MRI demonstrates three relatively well-circumscribed masses. The dominant mass (white arrow) is noted to be heterogenous, approximately 4.1 x 4.4 x 11 cm in size, and residing in the right iliopsoas muscle with moderate surrounding muscular edema.

Images: Frank R and colleagues

What is your diagnosis?

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Corrosion at the modular head-neck junction in primary THA

Given both the elevated serum metal ion levels, and in particular a differential elevation of serum cobalt over chromium, as well as the MRI findings consistent with an adverse local tissue reaction (ALTR), the diagnosis of corrosion at the modular head-neck taper of her metal-on-polyethylene bearing THA was made. The corrosion subsequently resulted in an ALTR in the musculature and soft tissue surrounding the components, which ultimately caused pain and difficulty with ambulation.

The diagnosis of corrosion can be challenging. In this case, the patient presented with symptoms, plain radiographic findings and serum laboratory values that were suspicious for periprosthetic joint infection, however, an aspiration including a synovial fluid WBC count, differential and culture ruled this out. In addition, patients with a metal-on-polyethylene bearing surface are not classically considered to be at risk for ALTR. However, recent case series have suggested that this is a potential cause of pain postoperatively, particularly if a modular neck-stem has been used, although this can occur with a standard femoral component as was seen in this case.

Figure 3. Intraoperative photographs at time of revision total hip arthroplasty (THA) demonstrate caseous-looking material around the hip capsule and surrounding the components (A) and corrosion of the taper between the femoral component and modular femoral head (B).

Serum metal ion levels can be helpful in confirming the diagnosis; a serum cobalt level that is higher than serum chromium is typical. It is important to recognize, however, that the threshold value that is suspicious for a non-metal-on-metal (MoM) bearing is different than that the clinician may use for diagnosis of a problematic MoM bearing. Specifically, any elevation above one part per billion is suspicious for corrosion. The use of MARS-MRI can be used to diagnose ALTR, typically showing an effusion, a cystic or solid mass and in advanced cases, soft tissue destruction. Arriving at the correct diagnosis in this type of case is critical, as substantial soft tissue damage (particularly to the abductor musculature) can occur if treatment is not timely.

Discussion and management

The clinical implications of corrosion in the setting of both MoM and more recently, of metal-on-polyethylene bearings and modular neck femoral components, are becoming increasingly understood. It is well established that the process of mechanically assisted crevice corrosion (MACC), a combination of fretting and crevice corrosion, is associated with modular implants.

Several reports have linked the soluble and particulate debris that results from modular junction corrosion to elevated serum metal ion levels, particle deposition in the periprosthetic tissue and, ultimately, to the development of ALTRs. While some patients with mild corrosion will remain clinically silent, other patients can develop hip pain secondary to an effusion and damage to the soft tissue musculature surrounding the hip. While head size may play a role in the predisposition to MACC, this phenomenon has also been observed in association with 28-mm and 32-mm heads.

 

Figure 4. Intraoperative photograph at time of revision THA demonstrates substantial necrotic-appearing material that was debrided and removed. An intraoperative frozen section contained acellular appearing tissue and was without evidence of acute inflammation.

 

Figure 5. The trunion was cleaned of visible debris, and the femoral head was exchanged to a ceramic femoral head with a titanium sleeve (to remove one source of cobalt form the modular connection). It is important to note that a ceramic femoral head must be used with a metallic sleeve if the taper is to be re-used to reduce the risk of femoral head fracture.

 

When evaluating any painful THA, infection should first be ruled in or out, particularly if failure or pain occurs within the first 5 years postoperatively. Screening begins with an ESR and CRP followed by an aspiration of the hip joint if these values are elevated or if the clinical suspicion for infection is high. Although we have, in general, found the synovial fluid WBC count and differential to be reliable for the diagnosis of infection, in cases where there is a MoM bearing or corrosion, these values can be falsely elevated secondary to cellular debris in the joint. Hence, in these clinical scenarios, we ask the lab to perform a manual count, which is oftentimes more accurate and only rely on samples where a differential can also be performed.

Although our knowledge of treatment in these cases is limited, this patient was successfully treated with a synovectomy, exchange of the modular liner and conversion to a ceramic femoral head that uses a titanium sleeve to remove one source of cobalt (Figures 3-5). Although our initial results with treatment in this fashion have been acceptable at short-term follow-up, it is unclear if a well-fixed stem should be removed if corrosion at the head-neck junction is present. In this case, removal of a well-fixed, diaphyseal engaging stem was felt to be associated with the potential for substantial morbidity.

Postoperatively, the patient recovered uneventfully and her pain resolved. Longer-term follow-up on this patient and similar cases will be required to understand if a modular bearing exchange with the use of a ceramic femoral head and a titanium sleeve is adequate for long-term treatment.