The distal sesamoidean impar ligament: comparison between its appearance on magnetic resonance imaging and histology of the axial third of the ligament.
Authors: Dyson S, Pool R, Blunden T, Murray R
Journal: Equine veterinary journal
Summary
# Editorial Summary: MRI interpretation of the distal sesamoidean impar ligament Accurate diagnosis of navicular syndrome relies partly on interpreting MRI changes in the distal sesamoidean impar ligament (DSIL), yet there has been limited validation of what these imaging alterations represent histologically. Dyson and colleagues examined 50 limbs from 28 horses using high-field MRI alongside detailed histological examination of the axial third of the DSIL, grading both for structural integrity, fibre organisation, collagen quality and vascularity. Key findings showed that specific MRI features—particularly cystic lesions in the distal third of the navicular bone, distal border fragments, and increased signal intensity at the DSIL insertion on the distal phalanx—correlated significantly with histological degradation of the ligament body itself. Strong associations also emerged between navicular pathology patterns; for instance, cystic lesions were consistently accompanied by distal border fragments or new bone formation at the insertion, whilst DSIL swelling on MRI corresponded with either border irregularities or increased signal intensity. For practitioners interpreting navicular cases, these validated MRI markers provide greater confidence that observed imaging changes reflect genuine structural damage rather than imaging artefact, making them more reliable indicators of ligamentous degeneration and therefore more useful for clinical decision-making and prognosis.
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Practical Takeaways
- •When MRI shows a cystic structure in the distal navicular bone or increased signal at the DSIL insertion, expect significant internal ligament damage even if external signs are subtle—this guides prognosis and treatment decisions
- •Distal border fragments and navicular bone cysts rarely occur in isolation; finding one should prompt careful evaluation of the entire DSIL, navicular bone, and distal phalanx attachment
- •MRI swelling of the DSIL body and increased signal intensity in fat-suppressed sequences are reliable indicators of histological deterioration and should influence rehabilitation timelines and workload management
Key Findings
- •Significant correlations exist between MRI findings (cystic structures in navicular bone, distal border fragments, increased signal intensity at DSIL insertion) and histological grade of the DSIL body
- •Cystic structures in the distal third of the navicular bone on MRI are associated with distal border fragments and entheseous new bone formation at the DSIL insertion
- •MRI abnormalities including DSIL swelling and increased signal intensity in fat-suppressed images correlate with cystic navicular bone lesions
- •Distal elongation of the flexor border of the navicular bone is significantly associated with the presence of one or more distal border fragments