A 50-year-old female underwent metal-on-metal (MoM) total hip arthroplasty 1 year ago for osteoarthritis. Her pain initially resolved, however she is now experiencing new onset buttock and thigh pain as well as clicking of her hip. She comes to you with an x-ray performed at an outside hospital and has questions pertaining to whether or not she is at risk of developing cancer and whether the hip implant should be revised to a non-MoM device (Fig. 7–2).

Figure 7–2

Which of the following is TRUE regarding MoM bearing surfaces?

1. Unlike polyethylene-on-metal bearing surfaces, MoM surfaces do not create particulate debris
2. MoM bearing surfaces do generate particulate debris. These particles are larger but fewer in number than those created with polyethylene-on-metal bearing surfaces
3. MoM bearing surfaces are characterized by higher wear during a “running-in” period that occurs during the first 1 to 2 years following MoM THR
4. Serum metal ions generated with MoM devices correlates directly with the patient’s level of activity

Discussion

The correct answer is (C). MoM bearing surfaces have been approved for use in the United States since 1999. The results regarding second-generation MoM implants have been promising and suggestive that there may be less failure due to osteolysis and wear as seen with conventional polyethylene and metal. Although MoM implants are considered a low-wear alternative bearing surface, MoM does generate particulate debris. These particles are smaller but more numerous than polyethylene particles. Hip simulator tests have demonstrated a higher wear rate of MoM implants during a “running-in” period that corresponds to the first million cycles in a simulator or the first 1 to 2 years in vivo which is then followed by steady-state wear. The amount of wear debris in MoM devices has not been found to correlate with a patient’s level of activity.

Which of the following tests should be performed on this patient?

1. Plain radiographs of the pelvis and hip
2. ESR and CRP
3. Blood cobalt and chromium ions levels
4. Cross-sectional imaging (MARS MRI)
5. All of the above

Discussion

The correct answer is (B). In any painful THR a plain film should be obtained to rule out mechanical failure, loosening, or periprosthetic fracture. Deep prosthetic joint infection must also be ruled out, and therefore ESR and CRP are indicated. Blood Co and Cr ion levels should be obtained to determine whether the patient may be experiencing any of the potential reactions to metal ion particulate debris

generated from the MoM bearing surface. Possible sequelae of increased serum metal ions include local tissue reactions, pseudotumors, and hypersensitivity. MRI imaging reveals a region of lymphocytic reaction next to a metal-on-metal prosthesis. MR imaging with “metal artifact-reducing sequences” (MARS) has been used to identify lymphocytic responses manifesting as aseptic lymphocyte-dominated vasculitis-associated lesions (ALVAL). Contrast enhancement can occur in bone, synovial tissue, joint capsule, and at the periphery of a soft tissue mass. A relationship between MRI findings and pain has not been found.

Variations in cobalt/chromium serum levels following MoM THR are associated with all of the following EXCEPT:

1. Femoral head size
2. T-cell counts
3. Lymphocytic changes
4. Retroverted femoral component
5. Cup abduction angle

Discussion

The correct answer is (D). Variations in cobalt/chromium levels following THR are associated with excessive cup abduction angles as well as excessive cup anteversion which results in the contact zone of the bearing surface near or on the rim of the cup. Rim contact increases the rate of wear. Clinically, malposition of the cup has been associated with increased generation of metal ion debris resulting in increased serum metal ion levels. MoM bearings of large diameter result in greater systemic exposure of metal ions than smaller-diameter heads. In addition, lymphocyte reactivity has been found to correlate with increased metal ion exposure.

What is the recommended course of action in patients with asymptomatic MoM implants according to the Food and Drug Administration (FDA)?

1. Revision to a conventional bearing surface if serum cobalt level is greater than 2 μg/L
2. Serial MARS imaging
3. Serial measurement of serum metal ion levels
4. If the implant is functioning properly, the FDA does not recommend serial routine soft tissue imaging or serum ion levels be performed

Discussion

The correct answer is (B). Although the vast majority of MoM implants have been successful, the Medicines and Healthcare Products Regulatory Agency (MHRA) in the UK issued a medical device alert in April 2010 for all MoM hip replacements base upon a trend of early failures. Subsequently on May 6, 2011 the United States Food and Drug Administration issued a postmarket surveillance study of all MoM hip implants. In February 2011 the FDA launched an MoM hip webpage indicating that MoM hip implants pose additional risks in comparison to all other hip implants. On January 17, 2013 the FDA issued an updated public health Safety Communication on MoM hip implants with detailed information regarding the recommended course of action before and after implantation of an MoM hip device. The FDA is currently not recommending revision arthroplasty based upon a specific threshold serum metal ion level and asserts that the decision to revise should be based upon overall clinical assessment and active symptoms.

The FDA has identified risk factors for developing adverse local tissue reactions (ALTR) following MoM hip implantation. These risk factors include all of the following EXCEPT:

1. Corticosteroid use
2. Malalignment of the acetabular or femoral components
3. Renal insufficiency
4. Excessively low BMI
5. Patients with suspected metal allergies

Discussion

The correct answer is (D). The FDA webpage on MoM implants suggests the following risk factors for ALTR following MoM hip replacements:

Patients with bilateral implants

Patients with resurfacing systems with small femoral heads (44 mm or smaller) Female patients

Patients receiving high doses of corticosteroids Patients with evidence of renal insufficiency Patients with suppressed immune systems

Patients with suboptimal alignment of device components

Patients with suspected metal sensitivity (e.g., cobalt, chromium, nickel) Patients who are severely overweight

Patients with high levels of physical activity

According to the FDA, the following systemic problems can occur as a result of MoM implants EXCEPT:

1. A general hypersensitivity/rash
2. Thyroid dysfunction
3. Cardiomyopathy
4. Cognitive impairment
5. Multiple myeloma

Discussion

The correct answer is (E). The FDA webpage regarding MoM implants lists a number of clinical sequela of increased serum metal ions including general hypersensitivity, thyroid dysfunction, cardiomyopathy, cognitive impairment, auditory or visual impairment, and psychological disturbance. Specific neoplasms have not been linked to increased metal ions in the bloodstream according to the FDA.

Helpful Tip:

Metal ion testing may be indicated in patients with symptomatic MoM bearing hips. Not all testing laboratories can accurately detect more than trace levels (more than 10 μg/L) of cobalt or chromium. Many laboratories use different techniques and normal ranges, and therefore it can be difficult and inaccurate to compare results from one laboratory to another. Metal ion concentration values should be interpreted in conjunction with blood urea nitrogen (BUN) and creatinine to concurrently evaluate the patient’s renal function. There is no threshold level of cobalt or chromium that indicates revision arthroplasty.

Objectives: Did you learn...?

The recommended evaluation of a patient with symptomatic and asymptomatic MoM bearing surfaces?

The possible local and system reactions to increased metallic debris as generated by MoM hip implants?