Reliability of a new dynamic ultrasound test for quantifying first-ray mobility.

Background: First-ray hypermobility (FRHM) is a documented abnormality whose pathogenic role is controversial. FRHM has been suggested to participate in many common disorders such as paediatric hallux valgus and recurrence after hallux valgus surgery. The controversy is due to lack of functional data on the first tarso-metatarsal joint (TMT1) in real-life situations, to its major anatomical variability, and to the absence of simple investigation methods. The objective of this study was to assess the feasibility and the inter- and intra-observer reproducibility of a new dynamic ultrasound test that quantifies TMT1 mobility and is simple to use provided a good-quality ultrasound machine is available.

Hypothesis: The new ultrasound TMT1 mobility test is reproducible.

Material And Methods: The 32 feet of 16 consecutive patients whose first ray was considered normal were included. Ultrasonography was performed at rest and during a stress test consisting in causing TMT1 gaping by applying a distal dorsal drawer movement to the first metatarsal. The two plantar bony prominences on either side of the TMT1 were identified, and the distance between them was measured at rest and during the stress test. The stress/rest ratio was computed. Each foot was tested twice by two different examiners, for a total of 128 tests.

Results: Mean TMT1 gaping distance was 1.38mm (range, 1.01-2.02mm) at rest and 1.67mm (range, 1.12-2.95mm). The mean stress/rest ratio was 1.21 (range, 1.02-1.62). Both inter- and intra-observer reliability was strong for all measured parameters.

Discussion: A simple and reproducible ultrasound test for measuring TMT1 mobility is described for the first time. The good reproducibility confirms the working hypothesis. This preliminary study was designed to validate the new test. The measured values need to be assessed in various disorders including FRHM, for which the test was designed. Should the results prove conclusive, the TMT1 gaping test may become a pivotal diagnostic tool.

Level Of Evidence: IV, diagnostic study.