Image Quiz

A Two-Week History of Hip Pain Ten Years after Total Hip Arthroplasty1 (continued)

Answer: Fracture of the stem.

Magnetic resonance imaging showed a fracture involving the lateral (tensile) side of the femoral component (Fig. 2). Changes that were consistent with bone-remodeling along the medial aspect of the cement-bone interface were also seen in the trabecular pattern of the femur (Fig. 3).


Fig. 2		 Fig. 2 A coronal fast-spin-echo magnetic resonance image with a surface coil placed over the stem demonstrates a subtle offset (arrow) at the tensile side of the stem, defining the fracture line.

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Fig. 3		 Fig. 3 An axial fast-spin-echo magnetic resonance image made just cephalad to the fracture depicts alteration of the trabecular pattern adjacent to the medial aspect of the stem, indicating stress remodeling.

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On the basis of this information, an uncomplicated revision of the fractured femoral component was performed. The fractured stem is shown in Figure 4.


Fig. 4		 Fig. 4 The retrieved specimen demonstrates complete fatigue fracture through the stem in the same location as that noted on the magnetic resonance image (Fig. 2).

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Discussion

Magnetic resonance imaging has not been considered a useful diagnostic tool in the evaluation of a total hip replacement because the extensive metal artifact adversely affects the evaluation of the components and the surrounding soft tissues. The artifact occurs because of frequency shifts generated by the focal magnetic field disturbances that are caused by the proximity of the easily magnetized metallic components and the poorly magnetized soft-tissue envelope.

The intensity of the artifact is a function of several factors: the alignment of the components with the external magnetic field, which runs parallel to the long axis of the body in a closed magnetic resonance unit; the relative ferromagnetism of the components; and the shape of the components. Titanium is less ferromagnetic and therefore causes less artifact than cobalt-chromium alloy does. Components that are oriented parallel to the long axis of the external magnetic field produce less artifact than do those that are oriented perpendicular to the external magnetic field. Thus, the soft-tissue structures surrounding the femoral component, which is parallel to the external field, are depicted better than those around the acetabular component, which has a screw or cement mantle that is oriented obliquely relative to the external magnetic field. The shape of the components also affects the intensity of the artifact. A spherical metallic head creates more frequency shifts and misregistration to the adjacent soft tissues than do more linear components such as a femoral stem.

The pulse sequences used in this study are all commercially available and may be duplicated in any closed high-field unit. The artifact reduction was achieved by the alteration of pulse-sequence parameters to reduce the frequency shift that disturbs the appearance of regional anatomic structures on magnetic resonance imaging. These parameters include the use of wider receiver bandwidths and/or commercially available modifications of the radiofrequency profile. Fast-spin echo techniques use 180° refocusing "pulses," which act to limit signal loss secondary to diffusion and thereby increase overall signal-to-noise ratio. In addition, alignment of the long axis of the metallic components parallel to the axis of the frequency-encoding gradient, which is an option at the time of image acquisition, also helps to diminish this artifact. Additional frequency-selective fat-suppression techniques are not recommended because of the local field disturbance that occurs in the presence of metal, and fast inversion recovery sequences are a suitable substitute.

Magnetic resonance imaging is currently used at our institution to detect component loosening, to measure the size of osteolytic lesions, and to evaluate periprosthetic soft-tissue abnormalities, including regional tendon disruption, nerve compression, and neuroma formation. By accurately determining the size of osteolytic lesions preoperatively, the surgeon can more effectively assess the type and quantity of bone graft or substitute that will be needed. We are also investigating the use of magnetic resonance imaging technology to determine the intra-articular load of particulate matter that leads to the development of osteolysis. Unlike computerized tomography, magnetic resonance imaging does not involve ionizing radiation and can be used to obtain direct multiplanar images without the need for reformations. To our knowledge, we are the first to report on the use of magnetic resonance imaging to directly evaluate the integrity of the metallic components rather than the adjacent soft tissues.

Reference

1. Cook SM, Pellicci PM, Potter HG. Use of magnetic resonance imaging in the diagnosis of an occult fracture of the femoral component after total hip arthroplasty: a case report. J Bone Joint Surg Am. 2004;86:149-53.