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.
Modularity in the heads and stems of total hip prostheses has afforded orthopaedic surgeons the ability to intraoperatively adjust version, limb length, and offset in ways that can optimize hip biomechanics. Modular implants have achieved these important objectives in thousands of patients over the last couple of decades. However, the rewards of modularity come with risks—some of them serious. Reports of these risks seem to have become more prevalent in the recent orthopaedic literature.
In this “Watch,” we bring to the attention of the orthopaedic community several femoral neck fractures in patients with implants that had modular head-neck and neck-stem designs. While some of these designs are no longer available from manufacturers, thousands of such devices have already been implanted. This “Watch” encourages surgeons to be wary about one specific aspect of modular hip designs: long femoral necks.
One of the weakest points in any hip prosthesis is the femoral neck. This region experiences extensive cyclic bending loads during activities of daily living. These mechanical stresses predispose the neck to the formation and propagation of cracks, which can set the stage for fractures.
In general, modularity exacerbates existing weaknesses. In a modular design, the proximal interface between the head and stem is subjected to almost pure compression and shear forces. If there is also a taper interface between the neck and the stem, there are substantial bending stresses due to relatively large moment arms, especially if the neck is long. In addition, whenever the number of mechanical metal junctions in a system increases, as it does with modular hip designs, so does the risk of fretting (micromotion), corrosion, and fracture.
The question is whether the functional advantages gained from modularity outweigh the risk of compromise to the mechanical integrity of the prosthesis. This question gains urgency because a femoral neck fracture is a serious orthopaedic injury, usually requiring an extensive revision procedure. The orthopaedic community should closely follow patients with modular head-neck, neck-stem, or dual modular junctions, and should pause to consider the use of these designs in new patients. Caution is especially recommended, as we shall now see, when the patient has a high body mass index (BMI), when the femoral neck is long, and when the femoral head is skirted.
In the January 8, 2014 edition of JBJS Case Connector, Baratz and Abdeen reported on a femoral neck fracture in a patient whose Meridian (Howmedica) uncemented stem component was coupled with a 32-mm femoral head with a +16-mm skirted neck length. Surgeons had selected the long neck and skirted head to provide soft-tissue tension and stability for the sixty-one-year-old female patient.
After seven years of uneventful, asymptomatic hip function, the patient felt a pop in the hip upon rising from a seated position, and she fell. Radiographs taken at the local emergency department showed a displaced fracture through the femoral neck, with the acetabular component and stem appearing well fixed. Laboratory analysis indicated a low probability of infection.
During revision hip arthroplasty, surgeons discovered that the femoral neck fracture was flush with the interface of the skirted portion of the head; additionally, the deep soft tissues showed metallic debris suggestive of corrosion. The surgeons deemed the proximal part of the femur to be unreconstructable, necessitating a proximal femoral replacing endoprosthesis.
Revision included a cemented stem (11 mm × 127 mm, with 40-mm and 50-mm modular extension segments) and a 32-mm cobalt-chromium head with a +0-mm neck length and no skirt. The immediate postoperative course was uneventful, and two years postrevision, the patient was asymptomatic; radiographs showed no evidence of mechanical complication.
Surgeons were unable to remove the explanted portion of the fractured neck from within the skirted head for additional investigation. Visual inspection of the taper interface revealed discoloration and dullness of the neck adjacent to the fracture, which suggested corrosion. Based on the Goldberg et al.1 criteria for taper corrosion and fretting, the authors assigned a score of 3 (moderate); they did not perform microscopic analysis of the retrieved components.
The authors noted that possible risk factors for femoral component fracture include a patient’s activity level and BMI, component loosening, varus malalignment, stress risers, faulty design, and metallurgical factors. Although the components in the first procedure were matched metallurgically, the authors cite two previous case reports that associated the use of skirted femoral heads with crevice corrosion and fracture risk.
The authors acknowledged that while the patient in this case was obese according to BMI classification, this type of catastrophic failure does not occur in the vast majority of obese patients who undergo hip replacement, so body mass alone does not explain the fracture. Rather, Baratz and Abdeen speculated that the fracture was “related to the use of a skirted modular femoral head with a high neck length, which has been shown previously to result in both a mechanical disadvantage and decreased resistance to corrosion.” They therefore recommend that surgeons avoid the use of skirted femoral heads with a high neck length, especially in obese patients.
The three additional JBJS cases that prompted this “Watch” all deal with the double-modular PROFEMUR Z component (Wright Medical Technology). Long neck length is the common denominator between these cases and the Baratz and Abdeen case. However, in these three cases, the fracture occurred at or near the modular neck-stem interface rather than at the modular head-neck interface.
Wright et al. reported on a forty-nine-year-old obese man who received a hip replacement with use of a dual-tapered rectangular stem with a modular neck, all made from a titanium alloy. Modularity in this case allowed surgeons to select from six neck geometries and two neck lengths. They chose a long varus anteverted neck.
Four years after the surgery, the man slipped on ice and fell squarely on the replaced hip. He didn’t feel pain initially, but did notice new clicks and squeaks from the hip. Three days after the fall, he heard a snap in the hip while tying his shoes, and was unable to bear weight on his lower extremity.
Radiographs revealed a fracture of the modular neck, with the femoral head remaining well fixed to the proximal fragment of the fractured neck and the distal fragment seated in the bore of the femoral stem.
Lab tests prior to revision hip arthroplasty and intraoperative frozen-section evaluation revealed no evidence of infection. Surgeons were unable to remove the distal part of the femoral neck from the stem, so that portion was removed en bloc. They performed an extended trochanteric osteotomy and reconstructed the hip with a 66-mm trabecular metal cup, a 15.2-cm stem, and a 40-mm head.
Light microscopy of the distal fragment of the modular neck showed fretting and corrosion damage with corrosion debris, located primarily on the lateral and anterior surfaces where the neck mated with the stem. Electron microscopy revealed scalloping and pitting of the mating surfaces, and the fracture surface showed the clamshell pattern characteristic of a fatigue fracture.
The authors emphasized the increased potential for fretting corrosion at any metal-on-metal junction, even if the metals are matched. They also noted that the findings in this case align with in vitro studies showing that corrosion and fretting in neck-stem junctions occur primarily at the medial contact point. “We believe that the body habitus of the patient along with the long varus neck contributed to the finding of substantial fretting and corrosion, and subsequent neck fracture,” they concluded.
Wilson et al. reported on a sixty-two-year-old overweight man who underwent two hip arthroplasty revision surgeries with identical PROFEMUR Z modular stems, each with a modular, long, 8° retroverted neck. The first, implanted as part of a total hip replacement in November 2006, was replaced with an identical modular stem in February 2007 due to a fracture of the original 32-mm ceramic femoral head, which had a +3.5-mm offset. The revision femoral head had a +7-mm offset.
In March 2009, the patient collapsed while walking normally and presented with pain and inability to bear weight. Radiographs revealed a fracture of the distal portion of the femoral neck. The patient underwent an extended trochanteric osteotomy and a second revision with a 16.5-mm porous-coated stem and a 36-mm femoral head with a +7-mm offset. No postsurgical complications were reported in this patient.
During extensive analysis of the retrieved components, the authors noted no visible corrosion on the implant interface, but gross visual inspection revealed beach marks on the fracture surface of the modular neck, indicative of fatigue failure. Viewing of the beach-marked area with a scanning electron microscope indicated that the origin of the fracture was at the lateral anterior corner of the modular neck and that a subsurface crack had propagated through the material.
Microscopy also showed pitting and scratching of the interfacial surface, consistent with damage occurring during neck-stem taper impaction. The authors suggested that surface damage might have occurred when the taper interface was engaged through impaction, perhaps from plastic deformation and surface defects. That insult could have been exacerbated by the replacement of the neck during the first revision, which may have introduced debris into the taper interface. Although they could not determine conclusively whether exchanging the modular neck at the time of the first revision had any contributory effect, they reported that “caution should be used when exchanging modular necks.”
The authors concluded that the fracture of the long modular neck in this case arose from a combination of surface damage caused by engagement of the taper interface and micromotion that led to high cyclic tensile moments acting on an already compromised surface. Their bottom-line message: “Long-necked modular stems, particularly when used with offset heads, should be used with caution, especially in heavier patients.”
The third case of a fracture near the neck-stem interface occurred in a thirty-year-old man with rheumatoid arthritis who received a total hip replacement with a PROFEMUR Z stem with a long straight neck and no offset. Atwood et al. reported that the hip performed well for almost two years, at which point the man fell and landed on the implanted hip. Radiographs revealed a fractured femoral stem, and subsequent revision surgery showed that the modular neck had fractured about 2 mm below the edge of the stem.
During revision surgery with an extended trochanteric osteotomy approach, the patient received a bowed, fully porous-coated femoral stem. One year after revision, he reported no pain and had full range of motion and normal abductor strength.
Visual inspection and electron microscopy of the retrieved components revealed pits up to 200 µm in diameter at the site of fracture initiation. A crescent-shaped feature near the lateral-anterior corner of the neck opposite the site of final tearing was determined to be the location where the initial crack had transitioned into catastrophic failure (Fig. 1).
Fracture surface showing the initiation site near a large pit (right) and the transition to catastrophic fracture (left). A region of tearing is seen in the corner opposite the initiation site, where the component was finally fractured into two pieces. Note that the initiation site at the lateral-anterior corner occurred on the tensile side of bending.
The authors proposed that failure in this case began with a sharp crack initiated by crevice corrosion at the neck-stem junction, where burnishing indicative of micromotion and fretting wear was noted. Rather than gradually propagating to failure from high-cycle fatigue, the crack rapidly fractured from the trauma of the fall when mechanical forces transitioned from tensile to shear.
The authors noted that although implants made from titanium (Ti6Al4V) alloy are protected from corrosion by a passive oxide layer on the surface, fretting at the modular junction can wear away the oxide film. Resulting corrosion-induced cracks can lead to catastrophic failure in the face of high stresses.
Also noting that “a long neck will be subjected to higher (tensile bending) stresses than a short neck,” these authors suggested that “long necks may contribute to a greater risk of fracture.”
Orthopaedists cannot be faulted for wanting to maximize a patient’s hip biomechanics and function, and arthroplasty candidates (especially younger, more active ones) may clamor for such improvements. However, if a modular hip prosthesis presents itself as an option, patients and surgeons should carefully consider the trade-offs between high performance and the low but real risk of catastrophic failure.
More specifically, we believe that this newly documented risk of fracture with long-necked modular prostheses should be discussed with patients if such devices are under consideration. Orthopaedists whose patients have a hip implant with a dual modular neck design should closely monitor these patients over time for complications of either corrosion with local metal soft-tissue reactions or catastrophic fatigue failure of the neck.
S. A multicenter retrieval study of the taper interfaces of modular hip prostheses. Clin Orthop Relat Res.