Do patterns from MOM hips apply to MOM disks?

Disk arthroplasty is a new technology aimed at avoiding fusion for the treatment of degenerative disk disease. In the United States, two lumbar and three cervical disk prostheses have been approved by the Food and Drug Administration (FDA) for implantation.

The indication for lumbar disk arthroplasty is single-level intractable low back pain that has not responded to nonsurgical therapies. Cervical disk arthroplasty is indicated following diskectomy for the treatment of radiculopathy or myelopathy.

The theoretical advantages of disk arthroplasty include the following:

• diminishing the chance of adjacent segment degeneration
• maintaining segmental motion and spinal function
• shortening recovery times
• avoiding complications such as pseudarthrosis that are related to fusion

Various bearing surface designs have been developed, largely based on experience from joint arthroplasty. Lumbar bearing couples include cobalt-chromium (CoCr) alloy articulating with conventional ultra-high molecular weight polyethylene and metal-on-metal (MOM) CoCr articulations.

The cervical spine experiences smaller loads than the lumbar spine. In addition, the use of postoperative magnetic resonance images (MRI) is greater, leading to use of alternative materials. These include titanium polycarbonate-polyurethane, stainless steel, titanium-carbide, and ceramics. Similar to the experience with total hip arthroplasty (THA), results of cervical and lumbar disk arthroplasty will likely depend on prosthetic bearing couple wear and subsequent inflammatory actions to the wear debris.

Lessons from the hip
Adverse reactions secondary to elevated periprosthetic and systemic metal ion levels were recognized with first-generation MOM THA devices such as the McKee-Farrar prosthesis in the mid 1970s. Improved understanding of MOM bearings, implant manufacturing, metallurgy, and bearing tolerance resulted in the design and use of many large and small contemporary MOM THA articulations from the mid 1990s to the present.

It was hypothesized that these improvements would eliminate the problems encountered with first-generation MOM THA devices and result in minimal articular surface wear, debris generation, and metal ion liberation. The thinking was that these second-generation MOM devices would therefore provide an excellent bearing surface for the young, active THA patient.

Unfortunately, reports detailing rapid and unexpected failures of contemporary MOM resurfacing and THA devices are emerging. In particular, release of a Medical Device Alert by the British Medicines and Healthcare products Regulatory Agency has brought the failure of MOM hip resurfacings and replacements due to adverse inflammatory reactions to the forefront.

Case reports of significant tissue reactions in association with MOM THA and resurfacing devices do exist. Perivascular lymphocytic infiltration, including formation of large pseudotumors, are consistent pathologic findings in cases of resurfacing or THA metal hypersensitivity. The exact incidence of pseudotumor formation and other adverse reactions is not known, but is likely to be greater than 1 percent.

Implant design, position, and size appear to influence the incidence of adverse reactions, serum metal ion concentrations, and the incidence of MOM prosthetic hip bearing failure. Acetabular prosthesis malposition has been shown to cause accelerated wear of MOM resurfacing and THA devices that can lead to early failure of these devices, just as it does in metal-on-polyethylene THA.

Wear, metal ion levels, and failure rates appear to be higher in small one-piece resurfacing MOM acetabular prostheses than in larger cups because of increased edge loading and decreased articular surface contact area. Because midterm follow-up failure rates are similar for some MOM hip bearing designs and metal-on-polyethylene articulations, it should not be assumed that all MOM hip bearings will be associated with an elevated risk of clinical failure.

Similar problems in MOM disks
Similar abnormal inflammatory reactions with MOM disk replacements have been recently reported. A recent report of four patients—three enrolled in FDA trials and one from Australia—discussed presumed metal hypersensitivity and pseudotumor reaction. All disk replacements were CoCr metal-on-metal designs (three lumbar; one cervical).

Initially, all patients had good clinical results; however, increasing pain and neurologic deficits developed. In two patients, a new onset of clicking was noted. Computed tomography (CT) myelography showed neurologic compression due to a large ventral soft-tissue mass in all patients.

When the devices were removed, a thick, yellowish vascular epidural soft-tissue mass was identified. All the devices were found to be stable. Visible black-stained tissue was found in the patient with the cervical implant, and a surface examination of the implant revealed micrometer and submicrometer thick third-body wear. Histologic examination revealed largely avascular hyaline connective tissue with chronic inflammatory cells consisting of lymphocytes; macrophages and eosinophilic granules were dispersed throughout the specimen. No metal debris was observed.

Another report of a MOM cervical disk replacement involved a patient who initially did well but began to have recurrent C6 radicular pain and numbness after 6 months. Other than metal artifact, an MRI showed nothing, and a CT myelogram showed a ventral mass extending from C4-5 to the midbody of C6. When the device was removed, a large thick mass of yellowish necrotic tissue was found to extend into the epidural space. Histologic examination showed a large area of necrotic hyaline debris with chronic inflammatory cells without significant visible metal debris.

Another recently reported case resulted in iliac vein occlusion and epidural cauda equina compression after a L4-L5 MOM total disk replacement. Two years after surgery, the patient began to experience bilateral leg pain consistent with spinal stenosis, and a year later, she returned with acute bilateral leg edema, pain, and leg weakness. Doppler studies showed a deep venous thrombosis of the left iliac vein and vena cava. A CT scan found a large retroperitoneal mass obstructing the left iliac vein and vena cava and extending into the epidural space. Serum inflammatory markers were unremarkable. A complete block of dye at the mid L4 level was seen on CT myelography. After posterior decompression fusion, histologic examination revealed a granulantous mass with diffuse metal wear debris particles.

What these results portend
Currently no CoCr MOM disk implants are approved in the United States; the devices we describe were implanted as part of FDA trials, except for the one lumbar device implanted in Australia. Because we know the number of implantations, we can determine the incidence of reported cases (0.3 percent for lumbar and 0.54 percent for cervical arthroplasty). However, the number of reported problems is likely to increase over time because of increased attention by physicians and longer follow-up. The results of MOM disk implants may parallel the situation in joint arthroplasty; initially, case reports were observed but recently, the number of reported incidences in specific designs has been between 1 percent and 2 percent.

The histology of periprosthetic tissues in the spinal devices is similar to that seen in MOM hip pseudotumors. Large areas of necrotic debris are interspersed with chronic inflammatory cells. In one case, metal debris was noted only with light microscopy; another case involved third-body wear without histologic confirmation of debris. The lack of visible particulate debris is likely a consequence of submicron sized particles not visible on light microscopy. Sophisticated testing of cell markers or inflammatory cytokines from the spinal devices is also lacking.

The etiology of adverse inflammatory reactions is unknown. Excessive wear with production of bioactive particles is thought to play an important role in some cases despite the absence of wear debris on many pathologic specimens. This may be the result of the inability to visualize submicron or smaller metal ions. Pathologically, large areas of necrosis are present suggesting cellular toxicity, secondary to high concentrations of metal ions. Finally, metal hypersensitivity reactions to metal ions may be another explanation. Immunologic studies have identified a type IV T-cell mediated hypersensitivity reaction.

The reports cited in this article confirm that patients with new onset of pain, mechanical sounds, or neurologic changes should be investigated for adverse inflammatory processes. Due to the artifacts created by the CoCr alloy, MRI is not likely to be of value unless the process is extensive. CT-myelography found soft-tissue masses ventral to the device, and extension into the epidural space in all patients. Due to the large size of the pseudotumors, venous, ureteral, and lumbar plexus impairment should be considered and appropriate diagnostic investigations should be performed when indicated.

This adverse inflammatory reaction appears random. Therefore, we believe that all patients having a MOM spinal arthroplasty should have regular follow-up and be provided information regarding warning signs of possible inflammatory complications, such as a new onset of pain, mechanical sounds such as clicking or grinding, lower extremity edema, and neurologic signs of symptoms.

Paul A. Anderson, MD, is a member of the AAOS Biomedical Engineering Committee. He can be reached at anderson@ortho.wisc.edu

Matthew Squire, MD, is a colleague of Dr. Anderson at the University of Wisconsin. He can be reached at squire@ortho.wisc.edu