Hypothesis 1:  Hyperextension Deformities

Immature foal with lack of development of all muscles that contribute to the 
superficial digital, deep digital, and calcanean tendons.  The following muscles 
contribute to the calcanean tendon:  biceps femoris, gastrocnemius, 
semitendinous, and the pectineus.  The superficial digital and deep digital 
tendons are formed by the superficial digital and deep digital flexors, 
respectively.  The superficial digital flexor originates on the femur and passes 
through the calcaneal bursa of the calcaneal tendon.  The deep digital flexor 
originates on the proximal and caudal tibia and fibia.  The deep digital flexor 
tendon does not pass through the calcaneal bursa or contribute to the calcanean 
tendon.  The superficial digital flexor tendon inserts on P2 and the deep 
digital flexor tendon inserts on P3.    

The flexor muscles have not yet hypertrophied so they are small and inadequate.

Inadequate strength of flexor muscles places sub-optimal amounts of tension on 
the calcaneal, superficial digital and deep digital flexor tendons.  This lack 
of tension permits hyperextension of the fetlock joint.  

Inactivity of the fetus in utero may have predisposed the foal to this 
condition.  Fetal movement in utero are important to promote adequate muscle 
development to support normal tension of flexor tendons. 

The fetlock joint in a hyperextended position is unable to distribute forces 
equally across the articular surface as the foal atttempts to bear weight on it.  
Joint stress caused by the hyperextension damages cartilage and chondrocytes, 
chondrocytes release inflammatory mediators.  These inflammatory mediators 
result in joint effusion which was detected on physical examination.