Hypothesis 3: Developmental Disorder

Hypotheses:
	Developmental disorders  are a possibility  in this filly as the cause of her lameness.  
Osteochondritis  dissecans  is a disorder of the cartilage which results in malformations  of 
the hyaline  cartilage  of the joint.  The cartilage starts eroding and can form flaps which 
produce irritation in the joint, especially  during movement.  These flaps can also break off 
and form "joint mice" which  float free in the joint  space  and cause  further irritation                                                                                                                             .  The 
filly could also be suffering from uneven growth which could cause pain from putting 
undue stress on the structures of her leg.  She might  also have contracture of the flexor 
tendons, which is making it difficult for her to put her leg on the ground.

Researched hypotheses:
	In osteochondritis dissecans, the epiphyseal cartilage separates from the 
underlying bone.  This could be due to nutritional causes, genetics, rapid growth or trauma.  
The subchondral bone becomes  inflamed on its surface as                                                                                            a result of the separation.  The 
injured osteochondral junction causes fissure lines to extend to the surface cartilage , and 
these lines cause cartilage flaps.   The damaged tissue  and broken off flaps (joint mice)  in 
the joint  cause synovial effusion and lameness.   In this filly, the severe  lameness  would 
support this diagnosis, but  her other clinical signs do not.  OCD is usually seen in horses 4 
months to 2 years old, and this filly is only a few weeks  old.  Also, OCD does not usually 
occur  in the fetlock joint, and there  is no joint  effusion present  in this filly.

	Subchondral  bone  cysts are an infolding of retained epiphyseal  cartilage.  They  
occur  commonly  in the fetlock joint.  This disorder  is usually seen in horses that have  
started performing  athletically.   It has been postulated that  the lameness  is due to an 
emptying of the cystic contents into the joint space, or by an interruption of the 
microvasculature  and marrow  fibrosis.  Bone cysts are usually  occur  in horses 6 months 
to 2 years of age.  Again, the filly's lameness and its location support  this data, but her age 
and other clinical signs do not.

	Angular limb deformities  are often a result of incomplete ossification of the carpus 
or tarsus, or asymmetric growth of the metaphysis/epiphysis.    The stresses of normal 
weight bearing on abnormal bones result in varus or valgus  deformities  arising from 
compressed or crooked growth plates and cartilage.  These deformities  are nonpainful, and 
very unlikely to be the cause of the filly's lameness, so they will not be discussed further.

	Congenital contracture  deformity  of the fetlock  is also a possibility.  The foals are 
usually  otherwise healthy.  They may  or may not be able to stand,  may knuckle  over on 
the joint,  or may walk on the dorsum of the joint.  The cause  is unknown, but  there are 
many  etiologies  postulated,  such as malpositioning in the uterus, teratogens, genetics,  and 
nutrition of the dam.  Pain is associated with stretching of the soft tissue structures while 
the deformity is being corrected.  This filly is young, and will not bear weight  on the leg, 
but those are the only pieces  of data that  support  this hypothesis.  It is unlikely that  she 
has a contracture  deformity, especially since  she is already  a few weeks  old and the owner 
had not noticed the problem previously.