My friend's horse was recently diagnosed with an atypical case of ringbone. The horse is 16 years old, so not really old in the whole grand scheme of things. The vet thinks the main cause is this horse's hooves are very low in the heels and the broken back pastern angle contributed to the problem. Poor guy now has arthritis and a limited riding career left.
It got me to thinking about how long any horse can go with poor trimming before it causes permanent damage. My own horse had 7 farriers over the past 5 five years before we finally found one who could properly trim a foot. He spent 8 months on average with long toes, then short toes, then underun heels, overly high heel club foot, etc. Are there any studies on how long a healthy horse can have poorly trimmed feet before it starts to cause lasting damage?