Digital Synostosis of the Foot: A Preliminary Radiographic Observation

<title>Abstract</title> <p> <bold>Background</bold> Digital synostosis of the foot — the osseous fusion of the middle and distal phalanges of the lesser toes producing a biphalangeal digit — is a radiographic finding that has received remarkably little systematic attention in the podiatric and orthopedic literature. When identified on routine foot radiographs it is most commonly dismissed as an incidental observation. No large-scale prospective prevalence study with formal demographic and statistical analysis has previously been published in a general podiatric patient population. <bold>Methods</bold> A prospective radiographic database was assembled over eighteen months across three podiatric clinic locations in north-central Illinois. A total of 182 consecutive patients underwent weight-bearing digital radiographic evaluation. Synostosis was defined as osseous fusion of the middle and distal phalanges with the proximal phalanx uninvolved. Variables recorded included age, sex, race, height, weight, BMI, and digit-level synostosis status bilaterally. Statistical analysis included chi-square testing, independent samples t-tests, one-way ANOVA, and Pearson correlation coefficients. <bold>Results</bold> Digital synostosis was identified in 97 of 182 patients (53.3%). The 5th digit was overwhelmingly the most commonly affected (51.6% left, 50.5% right), with prevalence decreasing in a consistent mediolateral gradient: 4th digit 5.5%, 3rd digit 2.7%, 2nd digit 0.5%, and 1st digit 0.0%. No patient demonstrated synostosis of any medial digit without concurrent 5th digit involvement. Bilateral presentation was dominant — 91.8% of 5th digit cases were bilateral, with 100% bilaterality at all more medial digits. Prevalence was equal between sexes (female 51.4%, male 55.1%; χ²=0.111, p=0.739) and unrelated to age (p=0.936) or BMI (p=0.277). Height was the only significant demographic correlate — patients with synostosis were taller on average than those without (170.2 ± 9.5 cm vs 165.8 ± 14.6 cm; p=0.016), with a positive dose-response correlation between stature and number of digits affected (r=0.168, p=0.023). Two novel clinical observations are introduced: the Synostotic Sombrero Sign — a characteristic circumferential bony flaring at the fusion zone with potential clinical implications for heloma mollae and heloma durum formation — and a preliminary hypothesis regarding osseous-etiology angular deformities of the lesser digits arising from delta phalanx morphology in the setting of synostosis. <bold>Conclusions</bold> Digital synostosis of the foot is a common, predominantly incidental constitutional anatomical variant present in over half of a consecutive general podiatric patient population. Its mediolateral digit gradient, near-perfect bilateral symmetry, equal sex distribution, and positive height correlation are consistent with a constitutional trait of developmental and potentially genetic origin. This is an ongoing study; these findings are hypothesis-generating and will be substantiated through expanding data collection and subsequent dedicated analyses. </p>