More Than Meets the Eye? Evaluating 3D Printing for Progressive Collapsing Foot Deformity Classification.

Background: Progressive collapsing foot deformity (PCFD) is a complex, multiplanar condition involving the ankle, hindfoot, midfoot, and forefoot. The 2020 classification system introduced a structured framework, distinguishing two stages (flexible vs. rigid) and five deformity classes. Diagnosing certain classes, particularly Class D, remains challenging. Three-dimensional (3D) printing provides a novel tool for anatomical assessment. This study evaluated the inter- and intraobserver reliability of PCFD classification using 3D-printed models.

Methods: A retrospective analysis was performed on 60 patients (37 females, 23 males; mean age 53 ± 17.2 years; body mass index (BMI) 31.9 ± 7.0) who underwent WBCT prior to surgical correction of PCFD. WBCT data were segmented using Bonelogic™ software, and 3D-printed models were created at 80% scale. Five fellowship-trained foot and ankle surgeons, blinded to clinical data, independently assessed the presence of PCFD classes A-E. Classifications were repeated after several weeks to assess intra-observer reliability. Percent agreement and reliability were analyzed using Fleiss' and Cohen's Kappa.

Results: The most frequent class combinations were ABCD (30%) and ABC (23%). Intra-observer reliability was highest for Class A (Kappa = 1.00) and lowest for Class B (Kappa = 0.40). Interobserver reliability ranged from slight (Class B, Kappa = 0.10) to fair (Class D, Kappa = 0.38), with perfect agreement for Class A.

Conclusions: 3D-printed models demonstrated moderate-to-perfect intraobserver but variable interobserver agreement in PCFD classification. While they enhance spatial understanding, their use without WBCT may limit diagnostic consistency. Further studies should explore the additive value of 3D printing alongside WBCT.

Level of evidence: IV, retrospective case series.