Anatomy of the distal interphalangeal joint of the mature horse: relationships with navicular suspensory ligaments, sensory nerves and neurovascular bundle.

The research article elucidates the complex anatomical structure of the distal interphalangeal (DIP) joint in grown horses, particularly its connections with navicular suspensory ligaments, sensory nerves, and the neurovascular bundle. The study shows that anesthetic injected into this joint can desensitize much of the horse’s foot, thus assisting in diagnosing foot pain.

Anatomy of the Distal Interphalangeal Joint

• The research focuses on the anatomy of the distal interphalangeal (DIP) joint in adult horses. Using polymer plastic injections to visualize the joint, the scientists noticed a complex relationship with the proximal suspensory or collateral sesamoidean ligament (CSL) of the navicular bone, which is an essential part of a horse’s hoof.
• The synovial cavity of the DIP joint forms cranial (towards the head) and caudal (towards the tail) compartments around the CSL abaxially (outside the axis of the body).

Presence of Sensory Nerves

• In addition to the CSL, sensory nerves were identified in the dorsal (upper side) and palmar (lower side) parts of CSL, the distal sesamoidean impar ligament (providing a supportive connection for the joint), and the periarticular connective tissues (around the joint).
• These nerves were identified through peptide immunocytochemistry (a staining technique for the identification of proteins in cells) and silver/gold chloride axonal impregnation (a method used to stain nerve fibers).

Implications of the Findings

• The findings provide evidence that when local anesthesia is injected into the DIP joint, it does not only affect the joint surfaces. Instead, it desensitizes an extended area of the navicular bone, the navicular suspensory apparatus, and the proximal intramedullary (inside the marrow) segments of the distal phalanx (a bone located in the foot).
• Though the DIP joint injection is not selective solely for the joint surfaces, the researchers believe it to be beneficial in diagnosing pain originating from the navicular bone, suspensory apparatus, and proximal aspects of the distal phalanx.
• These findings are then discussed through correlation with local anesthesia’s potential effects injected into the DIP joint with known clinical observations.