📝 STANDARD DESCRIPTION:
Synovial ginglymus between middle (P2) and distal (P3/coffin) phalanges including navicular bone.
Articular surfaces: Distal P2 articulates with proximal P3; navicular bone (distal sesamoid) lies palmar to joint surface.
Navicular bone suspended by collateral sesamoidean ligaments from P1 and anchored distally by impar ligament to flexor surface of P3.
Deep digital flexor tendon (DDFT) passes over navicular bone enclosed within navicular bursa reducing friction.
Collateral ligaments connect P2 to P3 bilaterally.
Vulnerable to 'street nail' injuries penetrating sole and entering navicular bursa causing septic bursitis—catastrophic lameness requiring surgical drainage.
📚 DETAILED DESCRIPTION:
THE DISTAL ARTICULAR COMPLEX
The coffin joint represents the terminal shock-absorbing articulation, intimately associated with the navicular bone and deep flexor apparatus.
NAVICULAR APPARATUS: The navicular bone (os sesamoideum distale) lies palmar to the joint, suspended within the DDFT pathway. Its proximal border articulates with P2; distal border connected by the impar ligament (ligamentum distale phalangis) to the semilunar crest of P3. Collateral sesamoidean ligaments suspend the navicular from P1, maintaining its position as the DDFT changes angle.
NAVICULAR BURSA: Synovial sac enclosed between DDFT and navicular bone; critical friction-reduction interface. Communicates intimately with coffin joint capsule in most specimens.
BIOMECHANICS: The joint functions as a hinge with angular movement constrained by the navicular-DDFT complex. The navicular bone acts as a 'pulley' altering DDFT direction by approximately 45° to insert on P3 flexor tubercle.
STREET NAIL PATHOLOGY: Nails or sharp objects penetrating the sole/frog frequently pierce DDFT and enter navicular bursa. Septic bursitis produces severe non-weight-bearing lameness; requires surgical windowing ('street nail procedure') through frog and DDFT to drain and lavage bursa. Prognosis better than equine due to cloven anatomy allowing digit amputation alternatives.
// COMPARATIVE ANALYSIS
| HORSE |
Navicular bone larger with distinct navicular syndrome (bone degeneration); single digit makes salvage critical. |
| DOG |
P3 called 'ungual process' with ungual crest; navicular smaller; joints more mobile. |
CLINICAL RELEVANCE
Septic Navicular Bursitis: Catastrophic lameness from penetrating foreign body; requires immediate surgical drainage via street nail procedure.
Coffin Joint Septic Arthritis: Tracks from sole abscesses or white line disease; difficult to treat due to enclosed anatomy.
Impar Ligament Injury: Rare but contributes to navicular bone instability and chronic foot lameness.
