CT-RSA with induced displacement used for assessment of aseptic loosening of distal femur reconstructions with megaprostheses in patients reporting pain

Aims: The aim of this study was to evaluate the relationship between experienced pain and findings of suspected pathological movement on CT-radiostereometric analysis (RSA) with induced displacement. Methods: Patients with reported pain after distal femur reconstruction with a megaprosthesis between January 2000 and December 2022 indicated the pain they experienced at rest and during activity using a visual analogue scale (VAS) and a pain drawing. Two consecutive CT scans with induced displacement were performed for each patient. The translational and rotational movement between bone and implant in the tibia and femur were calculated and compared with reported pain location and type at rest and during activity. Results: A total of 12 patients were examined; ten had measured movement above the preset cutoff values for pathological movement in either femur, tibia, or both. A large inter-patient variability in translation and rotation was found. The movements for the femur components were up to 3.46 mm/1.14° (translation/rotation). For the tibia components, these values were 2.16 mm/2.95°. Overall, six of 12 patients reported a severe type of pain. All six had a suspected loose implant at the location marked on their pain drawing. Six of 12 patients had a > 30 mm difference on VAS between rest and activity; five of those six had a suspected loose implant according to CT-RSA. Conclusion: CT-RSA with induced displacement appears useful for diagnosing suspected pathological movements in megaprosthesis implants. Pain locations on drawings seem to match detected suspected loose implants. Further studies with repeated CT-RSA examinations are needed to establish reliable cutoff values and assist decisions on revision surgery.