Ceramic-on-Poly Femoral Head Fractures: What’s Going On?

Abstract

As outlined in our January 16, 2013, editorial, “The Case Report Redefined with JBJS Case Connector,” one important role of JBJS Case Connector is to alert the orthopaedic community about a potentially problematic device or therapy. When two or more such cases with similar mechanisms appear, we will identify the procedure or implant as a “watchable” intervention. While this system is not statistically conclusive and may or may not be supported by other published case reports or registry data, the intention is to sharpen the focus of clinicians on the potential for similar problems and thereby enhance clinical outcomes and patient safety. The “Watch” designation may also encourage others to report related difficulties and enlist the orthopaedic community to either demonstrate that these are isolated, unrelated cases or sharpen the focus further on rigorously evaluating the intervention. Where appropriate, we may identify brand, model, or implant-lot specifics. Introduction In total hip arthroplasty (THA), a fractured ceramic head is a catastrophic but thankfully very rare event. Most case reports of ceramic head fractures involve ceramic-on-ceramic articulations, but this “Watch” describes three cases of femoral head fractures involving ceramic-on-polyethylene bearings. Moreover, the fractures occurred in association with little or no postsurgical trauma. Ceramic hip components are often chosen for younger patients in an effort to minimize long-term wear. On the downside, unlike metal, ceramic components can fracture. Ceramic femoral head fractures arise from four main factors: trauma; material or manufacturing defects; non-compatible, damaged, or contaminated femoral stem/head taper connections; or other suboptimal surgical techniques. The ceramics industry has a more-than-forty-year track record of continuous improvement in hip-component manufacturing. CeramTec, the main U.S. player in this arena, first introduced ceramic hip components in 1974; those first-generation femoral heads had a fracture rate of 0.026%. By 2003, when CeramTec introduced its fourth-generation BIOLOX delta product, reported fracture rates had fallen to 0.001%. These rates may not include all clinical fractures, but they are indicative of a substantially improved trend. As this “Watch” emphasizes, the risk of ceramic-component fracture can be minimized not only by continued engineering and manufacturing improvements, but also by careful component selection and meticulous technique by hip surgeons. A Pain-Free but Clicking Hip In the December 23, 2015, issue of JBJS Case Connector,Pomeroy et al. describe the case of a forty-one-year-old man who underwent THA with a 32-mm BIOLOX delta ceramic head articulating with an ultra-high molecular weight polyethylene liner. The procedure was uncomplicated, and the surgeon reported wiping the femoral stem taper with a clean, dry swab before impacting the head onto the stem taper. Ten days after surgery, the patient noted a clicking sound while walking, but he did not report this anomaly until his routine six-week follow-up appointment. At that time, a radiograph showed a fracture of the femoral head, despite the patient reporting neither trauma nor pain since the surgery. During revision, surgeons removed six main fragments (Fig. 1) and multiple smaller flakes of ceramic material, performed synovectomies around both the femoral and acetabular components, and copiously irrigated and suctioned the tissues to remove as many small ceramic fragments as possible. The polyethylene acetabular liner was damaged but was not breached through to the metal shell. Because surgeons noted no macroscopic defects on the trunnion, they left the stem in place rather than risk morbidity associated with removal of an ingrown component. Instead, they used a taper sleeve trunnion adaptor with the existing stem, along with a BIOLOX delta 32-mm ceramic head with a 16/18 taper. They revised the polyethylene acetabular liner with a ceramic liner out of concern that retained ceramic particles might cause substantial wear to a new polyethylene liner. Six months after the revision surgery, the patient was doing well and radiographs revealed satisfactory component placement and alignment.Fig. 1: The main fragments of the fractured femoral head and the damaged polyethylene liner from the case report by Pomeroy et al.The authors sent the retrieved ceramic fragments and the explanted polyethylene liner to the manufacturer for analysis. The report back said that a definitive cause of failure “was not readily identifiable,” while it raised the possibility that debris at the head-taper interface may have promoted crack formation and propagation leading to fracture or that the head had not been seated correctly. Pomeroy et al. note that “the surgeon involved in [the original] case implants over twenty ceramic-on-polyethylene couples per year” and that the head was reportedly impacted onto the stem “with a single firm strike.” However, this case report does not divulge whether the surgeon used a plastic femoral-head impactor, as most instructions for use strongly recommend, to avoid a metal hammer coming in direct contact with the ceramic component. Ricardo Heros, an independent consultant to CeramTec, could not comment definitively on this case, but he said that “clicking ten days postoperatively certainly indicates something wasn’t right,” and, based on the limited information available, he speculated that a head-stem taper mismatch may have been the root cause. Because this major complication occurred without any associated pain or range-of-motion deficits, the authors recommend that “surgeons have a high index of suspicion in any patients with unusual symptoms following hip arthroplasty with ceramic implants.” They also emphasize that patients receiving such implants should be told to report without delay any postsurgical symptoms such as pain, stiffness, clicking, or grinding, so that, if necessary, prompt revision can be performed to limit complications from ceramic debris. Finally, regardless of surgeon experience with ceramic materials or practice volume, Pomeroy et al. stress “the importance of meticulous care at the time of implantation.” Older-Generation Zirconium Head Fracture In another case of a ceramic-head fracture, Tucker and Acharya1 reported on a sixty-eight-year-old man who received a ceramic-on-polyethylene couple in 1999 to revise a failed metal-on-polyethylene THA. Fourteen years after the revision, the patient presented with acute hip pain not associated with any trauma. A radiograph revealed a fracture of the ceramic femoral head and osteolysis behind the acetabular shell. During revision, surgeons found and removed four main fragments of the fractured head and copiously irrigated and suctioned the area. The acetabular shell was removed, and the large area of osteolysis behind it required bone-grafting. Despite being well fixed, the femoral stem was removed because of macroscopic trunnion damage. As in the Pomeroy et al. case, surgeons used a ceramic-on-ceramic bearing for the revision. Tucker and Acharya noted that ceramic failures usually begin with a hard-to-detect crack that can propagate to fracture without much warning. They advised surgeons and their assistants to “take particular care not to damage the ceramic during implantation” by “preventing other hard materials from coming into contact with the brittle ceramic, especially during the final reduction manoeuvere.” These authors observed that “there is usually a preceding failure of the arthroplasty that begins the process of ceramic failure,” and they recommended active surveillance of all patients undergoing hip replacements with ceramic components. It’s important to note that this case report describes the fracture of a ceramic head made predominantly of zirconium oxide, a biomaterial that has fallen out of use because of its high fracture rate. While today’s more reliable ceramic hip implants are made from alumina composites toughened with zirconia, there are still many patients walking around on zirconium hips. Ambiguous Etiology: Trauma or Taper Incompatibility? The third case was complicated slightly by a history of postsurgical physical trauma. Heiner and Mahoney described the case of a forty-five-year-old woman who underwent THA with a 36-mm BIOLOX delta ceramic head with an 11/13 taper articulating against a polyethylene liner and mounted on an 11/13 neck trunnion. At a follow-up appointment about three months after surgery, radiographs were normal and the patient was doing well, but she mentioned a fall down some steps ten days previously. At a second follow-up, fifteen months after surgery, the patient reported a well-functioning, pain-free hip, and radiographs again were normal. However, eighteen months after surgery, she presented with moderately severe hip pain. She mentioned that she had been in a bicycle accident soon after her fifteen-month follow-up appointment. Radiographs showed radiodense fragments lateral to the femoral head, and a CT scan confirmed that the femoral head was fractured. During revision, surgeons removed visible head fragments (Fig. 2) along with the liner, which had articular-surface damage and embedded ceramic debris. Surgeons removed the femoral stem because the trunnion was also visibly damaged. Revision components were of the same design and size as those used for the index arthroplasty. Seven months after revision, the patient was doing well, even though follow-up radiographs showed residual ceramic fragments superior to the acetabular shell.Fig. 2: Fragments of the fractured femoral head from the case report by Heiner and Mahoney.Head-Stem Taper Compatibility: Easier Said than Done After reviewing Figure 2 from the case report by Heiner and Mahoney, Ricardo Heros noted that the traces of metal apparent at the bottom of the head taper suggest two possible root causes in addition to trauma: a taper mismatch or debris introduced during impaction of the head onto the stem, causing a loose connection. He emphasized the absolute necessity of confirming compatibility between stem and head tapers, but he was quick to add that doing so “is not as easy as it sounds.” That’s because there are more than thirty different stem-taper angles on the market, so not all “11/13” stems, for example, are the same. According to Heros, “the difference between a 3° and 5.42° stem taper angle is not detectable by eye at the time of surgery, and each stem might call for a different head.” Using noncompatible head and stem tapers can change the load-transfer mechanics in both magnitude and location, said Heros, which can increase stress to very high levels. Definitively confirming head-stem taper compatibility often requires looking at approved-combination charts posted on manufacturers’ web sites. A recent visit to the Zimmer site turned up more than 100 stem and eighteen ceramic-head choices, representing more than 1800 possible combinations. According to an eyeball analysis of the charts, Zimmer “approves” fewer than half of them. “I often ask surgeons if they look at those compatibility charts before they do a hip revision,” said Heros, “and no one has yet said ‘yes.’” Despite the hip-replacement marketplace’s demand for individuality via modular design, the implant industry had taken some small steps to slow the proliferation of stem-taper complexity. But according to Heros, imposing meaningful standardization on stem tapers “will require external action from the AAOS, the ASTM/ISO, or the FDA.” He emphasized that the taper-compatibility problem is not restricted to ceramic components. “Head-stem taper mismatches with metal heads cause their own set of complications,” he noted. Conclusion Here are some “golden rules” for lowering the risk of ceramic head fractures following hip replacement: Make absolutely sure that the tapers on the stem and head are compatible in all dimensions. During surgery, protect both stem and head tapers at all times. The slightest damage can create tremendous stress risers. Before placing the head on the trunnion, make sure that both tapers are dry and free from any debris. After placing the head on the trunnion, turn it 90° to make sure that it sits tightly and is properly aligned prior to impaction. When impacting a ceramic head, use the plastic impactor provided by the manufacturer. Do not strike a ceramic head directly with a metal hammer. Ceramic head fractures are more likely to occur from insults during or after implantation than from manufacturing defects. Consequently, these reports emphasize the importance of hip surgeons being fully cognizant of ceramic’s material idiosyncrasies; the crucial importance of careful selection, handling, and placement of ceramic heads on trunnions; and the need for patient education prior to surgery, during which patients should be encouraged to promptly report any and all postsurgical irregularities.