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Study finds 3D CT measurements of cup orientation for MoM hips superior to 2D measurements

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Researchers in the United Kingdom have found three-dimensional computed tomography to be more accurate than two-dimensional pelvic radiographs in determining cup orientation accuracy in metal-on-metal hip arthroplasty, according to a recently presented study.

Kinner Davda, MD, presented his team’s findings at the American Academy of Orthopaedic Surgeons 2012 Annual Meeting, in San Francisco.

“[Three-dimensional computed tomography (3D CT)] has been shown to be accurate in determining orientation in total hip and total knee arthroplasty when compared to X-ray, and it has been used increasingly in metal-on-metal sites,” Davda said.

According to the study abstract, the researchers examined 96 hips consisting of 93 resurfacings and three modular components, looking specifically at cup position as measured on anteroposterior pelvic radiographs and low-dose CT scans. For the sake of reliability measurements, 30 hips were randomly selected and measured by independent observers using both imaging modalities.

The study results note that, between the two imaging modalities, the mean difference in inclination values was 0.3° — with errors up to 9°. Version differences were reported to be much greater at 8°, with 2D underestimating by 21° or overestimating by 5.2°.

“A significantly greater number of hips fell within a 10° safe zone of 45° inclination and 20° version, if cup orientation was measured by 3D CT rather than 2D,” the authors wrote in the abstract.

According to the discussion within the study abstract, inter and intra observer reliability of cup version was found to be poorer with 2D analysis than with 3D CT, with retroversion being evident in three hips on 3D CT and none on plain radiographs. The authors suggested in the abstract that 3D CT is a more effective tool in determining cup orientation with “greater accuracy and reliability,” noting that they do not advocate 2D assessment.

“The take-home message is there are substantial differences when determining cup orientation using either 2D analysis or 3D-CT analysis,” Davda concluded. “These differences are critical when investigating the implant’s cup orientation.”

Reference:

• Davda K, Smyth NA, Henckel J, et al. 2D measurements of cup orientation in MoM hips are highly inaccurate compared to 3D CT. Paper #649. Presented at the American Academy of Orthopaedic Surgeons 2012 Annual Meeting. Feb. 7-11. San Francisco.
• Disclosure: Davda has no relevant financial disclosures.

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