The Plantar Fascia Talar Head Correlation: A Radiographic Parameter with a Distinct Threshold to Validate Flatfoot Deformity and Its Corrective Surgery on Conventional Weightbearing Radiographs
Authors: Merian Marc , Kaim Achim H. 
Published: January 2022

Category: Hindfoot; Midfoot/Forefoot Introduction/Purpose: Corrective surgery for flexible flatfoot deformity (FD) remains controversial, and one of the main reasons for this is the lack of standardised radiographic measurements to define an FD. Previously published radiographic parameters to differentiate between a specimen with and without FD do not have a commonly accepted and distinct threshold. Such a parameter would help validate FDs and the required corrective surgery. The purpose of this study was to assess a new conventional radiographic parameter with a distinct threshold that allowed the differentiation between feet with and without an FD and evaluate its application in corrective surgery for FD. Methods: The plantar fascia-talar head correlation (PTC) with its defined threshold was assessed by measuring the distance between the medial border of the plantar fascia [a connecting line between the medial border of the tibial sesamoid and the calcaneal insertion of the plantar fascia (mPCal)] and the centre of the talar head (DPT) on conventional dorsoplantar and lateral weightbearing radiographs; the authors were blinded to the clinical diagnosis of the 189 patients' first visits. Feet were grouped into specimens with and without an FD based on clinical examination. Then, the effect of surgical correction of FD on PTC was retrospectively evaluated on 38 patients. The intra-class correlation (ICC2) of the intra-observer reliability and the ICC2 of the inter-observer reliability of DPT measurement of 50 randomly selected specimens were calculated. The results of PTC were compared to the measurements of the lateral talar-1st metatarsal angle and the talonavicular uncoverage angle. Results: The intra-class correlation (ICC2) of the intra-observer reliability of DPT measurement was 0.95 (95%-confidence interval CI95[0.92, 0.97]), the ICC2 of the inter-observer reliability was 0.93 (CI95 [0.89, 0.95]). PTC, lateral talar first metatarsal angle and talonavicular uncoverage angle showed a statistically significant difference between the specimens with and without a FD (Wilcoxon test p-values <0.001). However, only the discriminatory power of PTC was distinct. The sensitivity and specificity of PTC was 0.88 (95%CI: 0.77-0.95) and specificity 0.96 (95%CI: 0.92-0.98), respectively, to identify an FD, consistent with the clinical examination. The identification of a sagittal plane FD caused by midfoot osteoarthritis on a lateral conventional weightbearing radiograph before measuring DPT increased the sensitivity for the combined method to 0.98 (95%CI: 0.9-1). Thirty-five of 38 surgeries successfully corrected the FD and PTC comparable to that in subjects without an FD. The unsuccessful corrections did not adequately correct the PTC. Conclusion: PTC is a unique and reliable radiographic parameter with an excellent threshold to confirm an FD, can be used intraoperatively to titrate the required amount for correction of FD under simulated weightbearing fluoroscopic imaging and is therefore useful to monitor and validate sufficient correction of its reconstructive surgery.PTC is an objective radiographic measurement to reliably describe the clinical subjective conclusion of any kind of FD with the exception FDs caused by an explicit midfoot pathology.