Extract
The PROFEMUR Z modular total hip arthroplasty component (Wright Medical Technology, Arlington, Tennessee) is a primary total hip replacement system that offers surgeons the ability to alter the length and version of the femoral neck after the femoral stem has been implanted. The modularity is achieved through taper interfaces at the distal and proximal ends of the femoral neck. The proximal femoral neck taper engages with the femoral head, and the distal taper engages with the femoral stem.
The PROFEMUR Z modular total hip arthroplasty component (Wright Medical Technology, Arlington, Tennessee) is a primary total hip replacement system that offers surgeons the ability to alter the length and version of the femoral neck after the femoral stem has been implanted. The modularity is achieved through taper interfaces at the distal and proximal ends of the femoral neck. The proximal femoral neck taper engages with the femoral head, and the distal taper engages with the femoral stem.
In this report, we present the case of a patient who twice underwent total hip arthroplasty revision surgery. The first revision was due to a fractured ceramic femoral head. The second revision was due to a complete fracture of the modular femoral neck. This report will focus on the second revision, and, specifically, the failure of the modular femoral neck at the junction of the neck to the femoral stem.
The patient was informed that data concerning the case would be submitted for publication, and he consented.
A sixty-two-year-old man (height, 181 cm; weight, 84 kg; body mass index, 25.6 kg/m2) presented with severe primary osteoarthritis of the left hip. He underwent uneventful primary total hip replacement surgery in November 2006. The components implanted were a 58-mm Lineage cup fixed with two screws; a size-5 PROFEMUR Z modular stem with a modular, long, 8° retroverted neck; a 32-mm ceramic femoral head with a +3.5-mm offset; and a ceramic Transcend liner (Wright Medical Technology). The patient recovered well with no postoperative complications.
In February 2007, the patient presented to the emergency department with a painful left hip. Radiographs revealed superior migration of the femoral head, and a fracture of the ceramic head was suspected. The patient was hospitalized, and revision total hip arthroplasty was performed. During the procedure, the head was found to be fractured, and the ceramic liner was heavily scored. The femoral neck was removed and replaced with an identical modular, long, 8° retroverted neck component. The fractured femoral head was replaced with a 32-mm cobalt chromium femoral head with a +7-mm offset, and the ceramic liner was replaced with a 32-mm non-cross-linked polyethylene Lineage liner (Wright Medical Technology). Both tapers were cleaned with a dry sponge before engagement. Appropriate soft-tissue débridement was performed.
In March 2009, the patient presented to the emergency department in pain and unable to walk or bear weight on the left lower extremity. The patient reported that he had been walking normally when his left hip collapsed underneath him. Radiographic investigations showed that the distal portion of the femoral neck component had fractured (Fig. 1). The patient was admitted, and revision surgery was performed for the second time. During the procedure, the freely floating neck and head were removed. The stem was well fixed both proximally and distally, and an extended trochanteric osteotomy was performed to remove the component. The polyethylene liner was replaced with a 36-mm non-cross-linked polyethylene liner. The revision component, a 16.5-mm fully porous-coated VerSys stem with a 36-mm femoral head with a +7-mm offset, was inserted (Zimmer, Warsaw, Indiana). The patient recovered well and has had no further complications. Due to the previous ceramic-head failure, this patient will be followed closely for polyethylene wear.
Analysis Methods
The fractured neck and the stem that were extracted during the second total hip arthroplasty revision were inspected visually on explantation. The components were then cleaned ultrasonically to remove debris from the surfaces. Light microscopy was used to identify areas of interest. The modular neck was then prepared for scanning electron microscopy, and images were obtained of the fracture surface and of the interface surfaces of the neck-stem taper. Energy dispersive x-ray spectroscopy (S-4700 Electron Microscope with EDS; Hitachi, Chiyoda, Tokyo, Japan) was performed on numerous regions both at and away from the fracture site to analyze the chemical composition of the modular neck.
Results
Gross visual inspection of the component revealed a fracture at the distal end of the modular neck. A fragment, approximately 1 cm in size, of the distal modular neck remained firmly engaged with the taper of the stem. So-called beach marks indicative of fatigue failure1 were evident on visual inspection of the fracture surface of the modular neck (Fig. 2).
Scanning electron microscopy images of the fracture surface showed characteristic beach marks1 at low magnification (Fig. 3). An analysis of the beach marks indicated that the origin of the fracture was at the lateral anterior corner of the modular neck. Further images were made of this region, and these images revealed that a subsurface crack had propagated through the material before weakening it to the point of catastrophic failure (Fig. 4).
Both the light microscopy and scanning electron microscopy images of the interfacial surface of the neck-stem taper in the area of the fracture origin showed pitting and scratching of the interfacial surface consistent with damage occurring during the impaction of the neck-stem taper (see Appendix). On further examination of this area, large cracks were visible near the origin of the fracture (see Appendix).
Energy dispersive x-ray spectroscopy showed that the composition of the material was approximately as stated by the manufacturer (i.e., Ti-6Al-4V), with titanium being the dominant phase and aluminum and vanadium also being present. In some regions, there appeared to be carbon content as well, but it could not be definitively determined if this was a carbide phase of the material or if this was organic material stuck to the implant.
Modular necks for total hip arthroplasty offer the surgeon a variety of options for altering the offset and version of the component after the femoral stem has been inserted. This can be advantageous when dealing with difficult primary arthroplasty procedures as well as revision procedures in which the final position of the femoral stem is suboptimal. However, any advantage that may be gained by using these modular components is negated if their use compromises the mechanical strength of the prosthesis2.
The neck of a hip prosthesis is known to be a mechanical weak point. Previous studies have found a high rate of failure in femoral necks that had design weaknesses3,4. The neck of the prosthesis is under extensive cyclic bending loads during activities of daily living. The region of the neck under tensile loading is predisposed to the formation and propagation of cracks.
Mechanical taper interfaces have been used extensively and with great success at the proximal end of total hip replacements to attach the femoral head. The PROFEMUR Z component makes use of a taper interface for both the proximal and distal interfaces of a modular neck. There are major mechanical loading differences between the proximal and distal ends of the neck that are critically important when assessing the appropriateness of the use of the taper interface. At the proximal junction, the taper lies at the center of rotation of the hip and is thus subjected to almost pure compression and shear loading. At the distal end of the neck, the long moment arm of the applied force produces a bending moment that puts the lateral edge of the distal taper into tension and the medial edge into compression. When long necks with offset heads are used, the moment arm, which is proportional to the length of the neck plus the head offset, is relatively large and has the effect of providing substantial bending stresses to the distal end of the neck component during loading. The magnitude of the resultant bending stress is further increased with the application of a modular neck that is retroverted, as a retroverted neck further increases the length of the moment arm and localizes the stresses to the anterior lateral corner of the neck5.
In the case of our patient, the distal third of the modular neck remained well fixed in the stem after the fracture. It is possible that a poor geometric shape match between the neck and the stem caused the contact to occur at the distal end of the taper, making it the only region of the neck that was fully engaged. This would have the dual effect of increasing the length of the moment arm and simultaneously decreasing the length of the interface, thus greatly increasing the stress in the component and possibly contributing to the failure.
The alloy used in the manufacture of this prosthesis (Ti-6Al-4V) is a commonly used high-strength orthopaedic alloy. In an unnotched state, Ti-6Al-4V possesses a tensile fatigue strength that is sufficient for total hip arthroplasty components. In a notched state, however, the fatigue strength of Ti-6Al-4V is lowered to the point at which failure may occur. There are two possible sources of this surface damage. When the taper interface is impacted into engagement, the contact between the interfacial surfaces of the neck and the stem may cause plastic deformation and defects in the surface of the neck, thus compromising the strength of the implant. This situation may have been exacerbated by the replacement of the neck at the time of the first revision. Even though both surfaces were cleaned and dried at the time of revision, the procedure did present an opportunity for debris to be introduced to the taper interface. The other possible source of the surface damage is micromotion at the taper interface over time. Biomechanical studies have shown that this process leads to surface damage in modular necks6. The surface defects greatly reduce the fatigue strength of the material, and, in combination with the tensile loading, contribute to crack propagation through the material under loading.
The occurrence of fretting and corrosion at the modular neck interface has previously been investigated in simulation studies7,8 as well as in a multicenter retrieval study9. The simulation studies found minimal wear, but the retrieval study found severe fretting corrosion in a substantial number of necks. There was no visible corrosion on the interface of the implant retrieved from our patient, but, given the material selection and the relatively short time that the component remained in situ, this is perhaps not surprising.
The findings of this case report indicate that the long modular neck, in combination with the short length of the distal taper interface and the surface damage caused by the engagement of the taper interface and micromotion over time, led to detrimentally high cyclic tensile moments acting on a compromised surface and to subsequent early fatigue failure of the component. The contributory effect of exchanging the modular neck at the time of the first revision cannot be determined at this time. It is unknown whether the first modular neck had early signs of premature failure, as it was not retained or investigated. It is possible that this exchange introduced debris to the interfacial surface, which caused the original surface defects leading to the failure. Caution should therefore be used when exchanging modular necks. More in-depth metallographic testing might have provided insight into any metallurgical reasons for the early failure. However, we were unable to perform any destructive testing on the failed component because we were requested to send it in an intact state for study by Health Canada.
It is difficult to extrapolate the results from one patient beyond the case studied, but the mode of failure, the lack of obvious materials problems, and the speed of failure are of great concern. It is suggested that long-necked modular stems, particularly when used with offset heads, should be used with caution, especially in heavier patients, who place higher functional demands on the prosthesis.
Additional light microscopy and scanning electron microscopy images of the fracture surface are available with the electronic version of this article on our web site at jbjs.org (go to the article citation and click on "Supporting Data").
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