Schedule Of Bone Fusion

This was written by a Veterinarian and professor at one of our country's
most prominent universities...

"Owners and trainers need to realize there's a definite,
easy-to-remember schedule of fusion - and then make their decision as to
when to ride the horse based on that rather than on the external
appearance of the horse. For there are some breeds of horse - the
Quarter Horse is the premier among these - which have been bred in such
a manner as to LOOK mature long before they actually ARE mature. This
puts these horses in jeopardy from people who are either ignorant of the
closure schedule, or more interested in their own schedule (for
futurities or other competitions) than they are in the welfare of the
animal.

The process of fusion goes from the bottom up. In other words, the lower
have fused; and the higher up toward the animal's back you look, the
later. The growth plate at the top of the coffin bone (the most distal
bone of the limb) is fused at birth. What this means is that the coffin
bones get no TALLER after birth (they get much larger around, though, by
another mechanism). That's the first one. In order after that:

2. Short pastern - top & bottom between birth and 6 mos.
3. Long pastern - top & bottom between 6 mos. And 1 yr.
4. Cannon bone - top & bottom between 8 mos. And 1.5 yrs.
5. Small bones of knee - top & bottom on each, between 1.5 and 2.5 yrs.
6. Bottom of radius-ulna - between 2 and 2.5 yrs.
7. Weight-bearing portion of glenoid notch at top of radius - between
2.5 and 3 yrs.
8. Humerus - top & bottom, between 3 and 3.5 yrs.
9. Scapula - glenoid or bottom (weight-bearing) portion - between 3.5
and 4 yrs.
10. Hindlimb - lower portions same as forelimb
11. Hock - this joint is "late" for as low down as it is; growth plates
on the tibial & fibular tarsals don't fuse until the animal is four (so
the hocks are a known "weak point" - even the 18th-century literature
warns against driving young horses in plow or other deep or sticky
footing, or jumping them up into a heavy load, for danger of spraining
their hocks)
12. Tibia - top & bottom, between 2.5 and 3 yrs.
13. Femur - bottom, between 3 and 3.5 yrs.; neck, between 3.5 and 4
yrs.; major and 3rd trochanters, between 3 and 3.5 yrs.
14. Pelvis - growth plates on the points of hip, peak of croup (tubera
sacral), and points of buttock (tuber ischii), between 3 and 4 yrs.and
what do you think is last? The vertebral column, of course. A normal
horse has 32 vertebrae between the back of the skull and the root of the
dock, and there are several growth plates on each one, the most
important of which is the one capping the centrum. These do not fuse
until the horse is at least 5 1/2 years old (and this figure applies to
a small-sized, scrubby, range-raised mare. The taller your horse and the
longer its neck, the later full fusion will occur. And for a male - is
this a surprise? -- You add six months. So, for example, a 17-hand TB or
Saddlebred or WB gelding may not be fully mature until his 8th year
-something that owners of such individuals have often told me that they
"suspected" ).

The lateness of vertebral "closure" is most significant for two reasons.
One: in no limb are there 32 growth plates! Two: The growth plates in
the limbs are (more or less) oriented perpendicular to the stress of the
load passing through them, while those of the vertebral chain are
oriented parallel to weight placed upon the horse's back.

Bottom line: you can sprain a horse's back (i.e., displace the vertebral
growth plates) a lot more easily than you can sprain those located in
the limbs. And here's another little fact: within the chain of
vertebrae, the last to fully "close" are those at the base of the
animal's neck that's why the long-necked individual may go past 6 yrs.
to achieve full maturity). So you also have to be careful - very careful
- not to yank the neck around on your young horse, or get him in any
situation where he strains his neck"

Follow-up piece to Bone Fusion Schedule