Orthopedic medical devices and cross-sectional imaging:
protocols and artifacts continued
by David Melville, MD
Chronic Post-operative Complications - CT Evaluation
While periprosthetic fractures may occur acutely or chronically following fracture fixation or arthroplasty, CT may be more beneficial in the chronic setting to accurately assess the exact extension of the fracture and residual bone volume, which may be obscured by overlying surgical hardware (Figure 8) (Ohashi, 2009). Further, CT may be helpful to better assess multiple overlapping screws or complex hardware before surgical intervention. Cross-sectional imaging with CT or MRI may also be useful for the assessment of incomplete or stress fractures resulting in chronic post-operative pain.
Fracture healing and joint fusion may be assessed readily with radiographs, but the presence of callus formation alone does not predict progression to complete healing, and the presence of osseous bridging is considered a more reliable indicator of union (Figure 9) (Ohashi, 2009). CT has been found to be more accurate for the determination of osseous bridging, as radiographs may either underestimate or overestimate the extent of bone fusion (Krestan, 2006; Grigoryan, 2003). The presence of exuberant callus formation may obscure the evaluation for osseous bridging on radiographs, and multiplanar CT allows direct visualization of the fracture site. In addition, fine central osseous bridging at the site of small fractures and bone grafts, such as in treated, non-united scaphoid fractures, is better assessed with thin slice CT images (Figure 10). Finally, some articulations are difficult to assess due to overlapping anatomy, such as the posterior subtalar and Lisfranc joints, and CT offers superior delineation of the joint spaces following arthrodesis.
Periprosthetic osteolysis represents a host of conditions, including small-particle disease, mechanical hardware loosening and periprosthetic infection, manifested by bone destruction surrounding implanted hardware (Figure 11). Imaging may not be able to differentiate the underlying cause of osteolysis, but identifying its extent and potential associated soft tissue involvement provides useful information to the referring physician.
In the case of infection, which is uncommon, nuclear medicine studies, aspiration or biopsy may be required to confirm the diagnosis. Small-particle disease results from implant wear, leading to particle shedding and provocation of a histiocytic response, which manifests as bone destruction. The most common cause of small-particle disease is polyethylene wear, and most patients remain asymptomatic until extensive bone loss is present (Ohashi, 2009; Naudie, 2004). Radiographic evaluation of small-particle disease can be limited by location of the lesion, position of the hardware, and patient body habitus. In particular, evaluation of peri-acetabular particle disease can be challenging due to the above factors. In such cases, CT may be indicated to determine the presence and volume of osteolysis prior to hardware revision (Figure 12) (Chiang, 2003; Claus, 2004).
Evaluation of osteolysis requires attention to optimized metal artifact reduction with use of a soft tissue kernel and wide window settings (> 6000) to best visualize periprosthetic lucency (Ohashi, 2009). Focal lucency greater than 2 mm or interval increase compared to prior CT studies typically indicates osteolysis and should prompt clinical evaluation for an underlying cause (Ohashi, 2005; Taljanovic, 2003). It is important to note that comparison of periprosthetic lucency measurements between radiographs and CT may differ and small differences should not be relied upon for diagnosing osteolysis. CT may also be used for prosthesis templating prior to revision arthroplasty in cases of severe bone loss and remodeling.
Radiographs remain the gold standard for assessing the integrity of implanted hardware; however, CT may be useful in particular cases. Prior to complete fracture union, hardware failure poses a significant impediment to fracture healing, and, in fact, may be an indicator of fracture non-union as chronic mechanical stress at the persistent fracture site results in hardware fracture. Consequently, CT examinations for fracture healing should also include a close inspection of surgical hardware, particularly in cases of non-union.
Just as with the immediate post-operative setting, CT is the modality of choice for evaluating complications related to spinal fusion. Screw fracture may occur in up to 25% of patients (Figure 13) (Ohashi, 2009; Lonstein, 1999). As with all cases of hardware failure, the imaging findings should be correlated with patient symptoms as not all hardware complications relate to current clinical symptoms. CT can be useful for the detection of polyethylene dislocation, which may occur in shoulder, hip and knee arthroplasties, and may be difficult to detect on radiographs (Figure 14) (Clarke, 2004).
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