Integrating weightbearing CT findings into evaluation of conventional radiographs in progressive collapsing foot deformity.

Background: Progressive collapsing foot deformity (PCFD) remains challenging to treat. Surgical planning depends on the amount and complexity of the deformity, which requires accurate differentiation through precise imaging. Weightbearing CT (WBCT) imaging has enhanced the three-dimensional (3D) assessment of PCFD. However, it remains unclear how PCFD findings on WBCT are related to the evaluation of conventional weightbearing radiographs. Therefore, we aimed to (1) compare specific X-ray measurements to corresponding WBCT measurements; (2) evaluate the reliability of X-ray measurements of interest; and (3) investigate whether X-ray measurements can infer osseous impingement in the subtalar joint region identified through WBCT.

Methods: Two clinically established measurements were assessed on standardized weightbearing radiographs (manually) as well as on the WBCT datasets (auto-generated): (1) talo-calcaneal overlap (TCO, mm) and (2) talo-navicular coverage (TNC, °). In addition to the measurements, osseous impingement in the subtalar joint region was assessed on WBCT using three criteria, compared both inter- and intra-rater: (1) joint obliteration; (2) cyst formation; and (3) signs of secondary instability. Two of the criteria needed to be fulfilled to confirm subtalar impingement.

Results: While no significant difference between X-ray and WBCT measurements was evident for TCO, significant differences were found for TNC. Inter- and intra-observer reliability was with an intraclass correlation coefficient > 0.9 excellent for both measurements on X-rays. The mean bias of measurement (between X-ray and WBCT) was 0.2 mm for TCO and -22 degrees for TNC. Cohen's Kappa for inter- and intra-rater reliability to assess patients for subtalar instability was > 0.9. The probability to infer subtalar impingement was ≥ 0.85 if TCO was > 15 mm or TNC was > 25 degrees on X-ray.

Conclusion: The examined imaging parameters are reliably assessable through conventional radiographs (TCO/TNC) or WBCT (osseous subtalar impingement). In situations where WBCT is unavailable, X-ray-assessed TCO and TNC can serve as predictors for osseous sinus tarsi impingement. This finding plays a pivotal role in evaluating PCFD patients, aiding in the surgical decision-making process between joint-preserving interventions (e.g., osteotomies) and joint-sacrificing procedures (e.g., realignment fusion).

Level of evidence: Level IV, observational study.