Adverse Local Tissue Reaction After a Metal-on-Metal Total Hip Prosthesis Without Elevated Serum Metal Ion Levels

Case Report

In October 2004, a 70-year-old man with a past medical history of hypertension underwent a left MoM THA for osteoarthritis using a monoblock acetabular component. In October 2005, the acetabular component was revised to a second monoblock acetabular component because of failed ingrowth. The patient then sustained a deep periprosthetic infection and underwent a 2-stage revision with eventual reimplantation of a MoM THA in January 2006 (a 58-mm Magnum monoblock acetabular component, a 13.5×147-mm Taperloc femoral stem, and a 52-mm Magnum head; Zimmer Biomet, Warsaw, Indiana).

In March 2017, the patient presented to a local emergency department for left-sided flank pain and was diagnosed as having renal nephrolithiasis. A computed tomography scan incidentally revealed a large fluid collection in the pelvis and proximal hip musculature. Regarding the left hip, the patient reported worsening lateral swelling during a 6-month period. Erythrocyte sedimentation rate (4 mm/h; reference range, 0–20 mm/h) and C-reactive protein (<0.5 mg/dL; reference range, 0–1.0 mg/dL) were both normal. However, given the patient's history of periprosthetic infection, an intra-articular hip aspiration was performed under fluoroscopic guidance. Cell count and aerobic, anaerobic, and fungal cultures yielded negative results.

In June 2017, the patient presented to the authors' clinic. Radiographs showed a MoM THA with no gross component malposition or loosening (Figures 1A–C). However, there was osteolysis of the ischium (DeLee and Charnley zone 3) and a large amount of periprosthetic femur osteolysis in Gruen zones 1, 2, and 7. Metal artifact reduction sequence magnetic resonance imaging was performed, which revealed a 16-cm fluid collection with irregular wall thickening in the gluteal musculature, along with multiple collections in the iliopsoas, in the hamstrings, and posteriorly displacing the sciatic nerve (Figures 1D–E). Serum metal ion levels were obtained, which revealed minimally elevated cobalt (0.89 ng/mL; reference normal, <0.7 ng/mL) and titanium (0.93 ng/mL; reference normal, <0.7 ng/mL) and normal chromium (0.23 ng/mL; reference normal, <0.3 ng/mL). The patient had normal renal function, with urea nitrogen being 16 mg/dL (reference range, 8–21 mg/dL) and creatinine being 0.96 mg/dL (reference range, 0.75–1.20 mg/dL). Finally, lymphocyte transformation testing revealed normal reactivity to all metal testing.

Figure 1: Preoperative anteroposterior pelvic (A) and left hip (B) and cross-table lateral left hip (C) radiographs showing a metal-on-metal total hip arthroplasty without gross evidence of component loosening. However, focal osteolysis of the ischium (DeLee and Charnley zone 3) and osteolysis around the femoral component in Gruen zones 1, 2, and 7 were noted. Coronal (D) and axial (E) short tau inverted recovery images from a metal artifact reduction sequence magnetic resonance imaging study of the left hip showing a periprosthetic fluid collection with irregular wall thickening involving the gluteal muscles and measuring 16 cm in greatest dimension. Multiple smaller collections were also located in the iliopsoas, in the hamstrings, and posteriorly displacing the sciatic nerve.

On the basis of increased hip swelling and worsening periprosthetic osteolysis, the patient underwent revision THA with the plan of conversion to a dual-mobility bearing and retention of the acetabular component and femoral stem. All revision options, including removal of the monoblock acetabular component (given its composition of cobalt and chromium), and the potential for revision of the acetabular and femoral components if found to be loose intraoperatively were thoroughly discussed with the patient. At revision surgery, a large amount of brownish fluid and inflamed tissue was encountered both superficially and beneath the iliotibial band (Video). The femoral and acetabular components were found to be well fixed, with some corrosion appreciated at the femoral neck taper (Figure 2A). Multiple soft tissue specimens were obtained for analysis; final aerobic, anaerobic, fungal, and mycobacterial cultures yielded negative results. The inflamed tissue sent for analysis grossly had a brown, rubbery appearance with pathology revealing a dense fibrous connective tissue with histiocytes but without granulation tissue or significant acute inflammation (Figure 2B). The MoM bearing was converted to a dual-mobility prosthesis using a 28+6-mm Biolox Delta femoral head (Zimmer Biomet) with an inner titanium revision sleeve and a 28-mm inner and 52-mm outer ultra-high-molecular-weight polyethylene (Stryker, Mahwah, New Jersey) (Figure 3).

Figure 2: Intraoperative photograph showing corrosion at the femoral neck taper (A). Sampled tissue showing dense fibrous connective tissue with histiocytes (arrow). Histiocytes have a moderate amount of vacuolated cytoplasm, round to oval nuclei, uniformly distributed chromatin, and variable nucleoli. There is no granulation tissue or evidence of acute inflammation (hematoxylin-eosin, original magnification ×40) (B).

Figure 3: Postoperative anteroposterior pelvic (A) and left hip (B) radiographs showing conversion of the metal-on-metal bearing to a dual-mobility prosthesis with an inner titanium revision sleeve. The well-fixed acetabular and femoral components were retained.

Postoperatively, the patient was admitted to the orthopedic inpatient unit and allowed to be full weight bearing. He was discharged home on postoperative day 1. The patient returned to the clinic at 3 and 6 weeks postoperatively for follow-up. He had no complications. He was walking unlimited distances without assistance, having no pain on the operative extremity.

Discussion

Metal-on-metal THA grew in popularity in the early 2000s owing to proposed advantages that included decreased volumetric wear, higher fracture toughness, and increased stability with greater range of motion facilitated by larger heads.^2,9 Unfortunately, unique modes of implant failure followed.¹⁰ Adverse local tissue reaction secondary to metal debris is a known complication that can result in massive soft tissue destruction, periprosthethic osteolysis, and associated sequelae including implant loosening, periprosthetic fracture, infection, and late instability.^3–5

The workup of a MoM articulation is multifaceted, involving a thorough history and physical examination, plain radiographs, blood analysis (metal ion levels, erythrocyte sedimentation rate, and C-reative protein), and often, secondary imaging (ie, ultrasound, computed tomography, or magnetic resonance imaging).¹¹ Key factors to consider include patient symptomatology, implant track record, positioning of components, metal ion levels, and alternative diagnoses, particularly periprosthetic joint infection.⁶ However, infection can be challenging to differentiate from the inflammatory reaction generated by metal debris via serum erythrocyte sedimentation rate and C-reactive protein and requires a manual white blood cell count and differential for all synovial fluid samples.¹² The above items all factor into risk stratification algorithms for the diagnosis and management of ALTR recently published in a consensus statement by the American Association of Hip and Knee Surgeons, the American Academy of Orthopaedic Surgeons, and the Hip Society.⁸

Quantification of metal ions in the blood, specifically cobalt and chromium, has proven useful in the diagnosis and treatment of ALTR. Although a direct correlation between metal ion levels and risk of ALTR has yet to be defined, studies have shown that elevated blood metal ion levels are associated with increased wear rates and failed MoM THA.^13,14 Increased metal ion levels have been associated with component malposition (eg, cup abduction angle >50° and/or combined anteversion >40°), larger femoral heads, passage of time, and variations in implant-specific designs (eg, head-to-cup clearance and metallurgy).^13–19 For a MoM articulation, several studies report that a cobalt level above 7 ppb (or 7 ng/mL) is a reasonable threshold for increased concern and pursuit of secondary imaging, whereas other studies suggest a cobalt level of 4.5 ppb.^4,15

To the authors' knowledge, this is the first reported case of a failed MoM implant and ALTR without elevated metal ion levels on testing. Despite visible swelling, progressive radiographic osteolysis, large periprosthetic fluid collections on magnetic resonance imaging, and intraoperative findings of corrosion at the head–neck junction, the patient's metal ion levels were well below the published thresholds for concern. In fact, his cobalt level was lower than the 1-ppb threshold suggested to be concerning for an ALTR in the setting of a metal-on-polyethylene THA.²⁰ This report highlights the fact that metal ion levels are best used in combination with other information and as trends over time rather than as absolute values in isolation.^8,15

Conclusion

There is extensive variability in the presentation of patients with ALTR with MoM THAs. Consideration of the clinical picture as a whole—including patient demographics and symptoms, physical examination, implant history, serial radiographs, metal ion levels, cross-sectional imaging, and other diagnoses—is key to diligent management of individuals with MoM THAs.