Annular Ligament Syndrome
Also Known As: Annular Ligament Desmitis — ALS
Caused By: Digital Sheath Tenosynovitis – Flexor Desmitis – Superficial Digital Flexor Tendon Injury
Introduction
The palmar/plantar annular ligament is a fibrous band that wraps around the flexor tendons as they pass over the fetlock sesamoid bones on the palmar/plantar surface of the limb.
Annular ligament syndrome (ALS) is thickening of the palmar or plantar annular ligament (PAL) which may or may not also involve the digital sheath and other associated structures.
ALS can be primary or secondary. Primary disease originates from pathology within the PAL itself, usually caused by local trauma or chronic overstretching. Secondary ALS occurs as a result of pathology originating within soft tissue structures associated with the PAL such as the digital tendon sheath or flexor tendons. Both result in constriction and interference with the normal smooth movement of the tendons within the fetlock canal and this stress causes the annular ligament to thicken and enlarge.
The most common cause of ALS in the forelimb is tenosynovitis of the digital sheath, while in the hindlimb most cases are a result of trauma to the PAL causing subsequent primary desmitis.
Distribution
Affects horses worldwide.
Signalment
ALS is most common in heavy breeds including warmbloods. Horses with existing foot pathology and disease will also be predisposed.[1]
Clinical Signs
ALS causes a characteristic notch to form in the palmar/plantar contour of the limb at the proximal margin of the PAL.
Lameness is often chronic and progressive, but its severity will depend on the nature and degree of pathology.
Flexor tendonitis will exhibit its own clinical signs such as bowing of the tendons, heat and swelling.
Diagnosis
Ultrasonography is the most effective and readily available method of diagnosis.
In primary ALS, no pathology of the flexor tendons will be present, but hypoechoic regions will be visible within the PAL, indicating desmitis.
In secondary ALS, lesions may be seen within the bodies of the flexor tendons or effusion and thickening of the digital sheath wall may indicate tenosynovitis as the primary cause. Thickening may be present in the synovial membrane, PAL or subcutaneous tissues. In the clinically normal horse, the annular ligament is very difficult to distinguish from the outer layer of the digital tendon sheath.
An annular ligament with a thickness of >2mm is diagnostic of ALS.[2]
A palmar nerve block placed above the digital sheath will usually improve lameness but not completely resolve it.
Treatment
Annular ligament desmotomy results in resolution (return to work) of 70% of uncomplicated cases of primary PAL thickening.[2]
In secondary ALS, the primary pathology must of course be addressed, all of which require strict rest. Desmotomy of the annular ligament will also reduce constriction, allowing healing and drainage and thus aiding return to soundness if the primary pathology can be controlled.
If adhesions are suspected, a larger incision should be made so as to grant better visualisation. Endoscopy can also be performed. A post-operative exercise program will be required if adhesions are present, to prevent recurrence after resecting them.
Short term improvement may be gained from injections of corticosteroids into the ligament where surgery is not possible for economic reasons but this lasts only 1-3 months.
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References
Robinson, N. E., Sprayberry, K. A (2009) Current Therapy in Equine Medicine 6th Ed. Saunders (Elsevier), Missouri, USA.
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