BAL­ANC­ING ACT

EQUUS - - Conversati­ons -

Bis­pho­s­pho­nates were first syn­the­sized in 1897 by chemists in Ger­many for industrial ap­pli­ca­tions. Named for the two phos­phate groups they con­tain, bis­pho­s­pho­nates were used to pre­vent cal­cium car­bon­ate scal­ing on ma­chin­ery and plumb­ing. It wasn’t un­til nearly a cen­tury later that the chem­i­cals were con­sid­ered for clin­i­cal ap­pli­ca­tions in hu­man medicine based on their ef­fects on bone re­mod­el­ing.

The con­tin­u­ous break­down and re­pair of bone tis­sue, re­mod­el­ing helps bone adapt to stresses, be­com­ing stronger and/or re­pair­ing dam­age. “Bone re­mod­el­ing is why you can break your arm and it heals,” says Kyle Creech, DVM, equine ve­teri­nary ser­vices manager for Ceva An­i­mal Health, the mak­ers of Til­dren. The re­mod­el­ing pro­cesses is a marvel of bi­ol­ogy, but it can be ex­plained fairly sim­ply:

Cells called os­teo­clasts con­tin­u­ally roam bone sur­face, re­sorb­ing old or dam­aged bone. In their wake, dif­fer­ent cells known as os­teoblasts ar­rive to lay down new bone. At times of bone stress, such as dur­ing ex­treme phys­i­cal ac­tiv­ity or af­ter a trau­matic in­jury, this process is ac­cel­er­ated, with os­teo­clasts work­ing faster to tear down the dam­aged bone. Gen­er­ally, os­teoblasts can keep up the in­creased pace, lay­ing down new, stronger bone quickly, but the process isn’t in­stan­ta­neous.

“There can be a lag,” says Jill Stohs, DVM, tech­ni­cal ser­vices vet­eri­nar­ian with Dechra Ve­teri­nary Prod­ucts, the mak­ers of Osphos. “In nor­mal bone re­mod­el­ing, gen­er­ally it takes about three weeks for os­teo­clasts to fin­ish their job of re­mov­ing the dam­aged and stressed bone. Once the os­teo­clasts have com­pleted their job, they sig­nal and re­cruit os­teoblasts to the area of re­moved bone. Os­teoblasts may take up to three months to com­plete their process of adding new bone. By that time, though, the un­der­ly­ing stress has been re­lieved, so os­teo­clasts have slowed down their ef­forts and the os­teoblasts can catch up to them.” In cases of nav­ic­u­lar syn­drome, how­ever, the stress on the nav­ic­u­lar bone is con­tin­ual, mean­ing os­teo­clasts are con­tin­u­ally work­ing to re­move stressed bone tis­sue and os­teoblasts do not have a chance to catch up, and degra­da­tion of the bone oc­curs. This, say Creech and Stohs, is where bis­pho­s­pho­nates can help.

Bis­pho­s­pho­nates re­store bal­ance to the bone re­mod­el­ing process by in­hibit­ing the re­sorp­tion of bone. Bis­pho­s­pho­nates be­come at­tached to cal­cium in the bone. Then they are in­gested by os­teo­clasts and dis­rupt the meta­bolic process of the bone-eat­ing cells, which causes them to die. With fewer os­teo­clasts at work tear­ing down bone, the os­teoblasts can then keep up with the job of lay­ing down new bone, lead­ing to fewer le­sions. “It’s all about re-es­tab­lish­ing that bal­ance be­tween the os­teo­clasts

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