image What Are You Trying to Measure? 2018 Objective Lameness Academy Recap

Multiple Limb Lameness: Separating Secondary from Compensatory

by Kevin G. Keegan, DVM, MS, DACVS

“Entia non sunt multiplicanda praetor necessitatum”, or Ockham’s Razor, translated (roughly) as “more things should not be used than are necessary”, is attributed to a 14th century friar/philosopher, William of Ockham.  (In this context, “razor” means principle not a sharp cutting tool).  It is also known as “the principle of parsimony” which states a general scientific maxim offered by most great scientists before and after Ockham – including Ptolomy, Newton, and Einstein.

What does this have to do with equine lameness?

We can use this principle to help sort out multiple limb lameness using what we know about compensatory movement or lameness to spare the primary limb, and what we suspect about secondary lameness, or lameness due to chronic overload in a second limb. Recall that Compensatory lameness is a false lameness due to weight-shifting to spare a primarily painful limb, i.e. the horse looks lame in a limb, but has no foci of pain in the limb. Secondary lameness is a true lameness that often is the result of prolonged overloading due to the primary lameness.  There is pain in this limb.

I say we know a lot about compensatory lameness because it is easy to study experimentally (it is easy to suddenly create lameness in a forelimb or hind limb for a short time) and we can see the disappearance of compensatory lameness when a primary lameness is blocked out.   It follows fairly closely the old “Law of Sides” (see below).  Not all horses show compensatory lameness, but most do, and some compensate greatly.

Secondary lameness, on the other hand, is more difficult to study experimentally (you have to create lameness in a horse for long periods of time, which is actually quite difficult), and because of this, we can at best make experiential guesses from our clinical observations.

These secondary patterns make sense when one understands that the majority of weight (>60%) in the horse is borne by the forelimbs.  Not all horses show secondary lameness, but most do.  Of course it takes some time to overload the other limb.  Chronic overload potentially cause pain in the entire limb or in entire support structures that may span great length in the limb’s anatomy, think the entire suspensory apparatus, the entire foot, all flexor tendons, extensor muscle groups.  Because of this, secondary lameness may be very difficult to completely block out with local anesthetic techniques.

In secondary lameness, a primarily lame forelimb will most likely overload the opposite forelimb, less likely overload the opposite hind limb, and unlikely overload the same side hind limb.  A horse with a primary hind limb lameness will equally likely overload the opposite forelimb or opposite hind limb, but will unlikely overload the same side forelimb.

The graphic below is a memory aid that summarizes known compensatory and suspected secondary lameness patterns.

Given these known and suspected patterns, then, what is the least complex and therefore the most likely explanation for multiple limb lameness?  When considering all possible patterns of forelimb plus hind limb lameness, there are actually only 8 possible patterns.  For the sake of enumerating these patterns let us assume that the forelimb lameness is in the right front limb.

RF Lameness with:

1. Ipsilateral hind lack pushoff of and lack of impact. Following the first part of the “Law of Sides” this is most easily explained as a primary RH lameness and a compensatory RF lameness.
2. Ipsilateral lack of pushoff. Following the first part of the “Law of Sides” this is most easily explained as a primary RH lameness and a compensatory RF lameness.

3. Ipsilateral hind lack of impact. This one is tough.  The RF lameness could be compensatory to the RH lameness and the RH lameness could be compensatory to the RF lameness (the hind limb lameness is ipsilateral lack of impact).  Only good physical examination and blocking can sort this out.

4. Contralateral hind lack of impact, ipsilateral hind lack of pushoff. This pattern is uncommon (less than 1% of trials in >1000 cases – unpublished data), but could best be explained as primary RH lameness with compensatory RF lameness and secondary LH lameness.

5. Contralateral hind lack of pushoff and lack of impact. This pattern is most easily explained as a primary RF lameness with compensatory LH pushoff lameness following the second principle of the “Law of Sides”.  The contralateral lack of hindlimb impact could be explained as a secondary lameness.  Another explanation is primary hind limb lameness with secondary forelimb lameness.  Compensatory lameness being more common than secondary lameness tips the balance in favor of the first explanation.

6. Contralateral hind lack of pushoff. This pattern exactly follows the contralateral principle of the “Law of Sides”. A  primary RF lameness with compensatory LH pushoff lameness is the most simple explanation.

7. Contralateral hind lack of impact.This is the other tough one.  There is no compensatory pattern here.  This is either primary and secondary lameness or two primary lameness conditions (lightning struck twice).  This is either a primary RF lameness and a secondary LH impact lameness or a primary LH lameness with a secondary RF lameness.  Physical examination and evaluating the horse in motion in other activities (flexion test, lunging, before and after blocking) are needed to sort this out.

8. Ipsilateral hind lack of impact, contralateral hind lack of pushoff. This follows the contralateral principle of the “Law of Sides” and the easiest explanation is primary RF lameness with compensatory LH push off and RH impact lameness.

Of course, the relevant history, close physical examination, palpation of the limbs, other movement evaluations, and other manipulative techniques may be necessary to figure all multiple limb lameness presentations.  Two primary lameness conditions simultaneously developing is possible but improbable.  Knowledge of the common patterns for compensatory and secondary lameness can help to decide which limb to block first and when to stop blocking (secondary lameness is tough to block out and will not go away when the primary lameness is blocked out).

I should add that I do not think that these patterns are easily discernable without using body-mounted inertial sensors.  I certainly do not recognize them well enough with just my eyeballs.  It is not uncommon for me to see the compensatory or secondary lameness, but miss the primary lameness.  Use of body mounted inertial sensors has saved me from chasing unproductive blocking regimens.  This is especially helpful to me when time to diagnosis is important (this is frequently the case) and when horses are difficult or even dangerous to block.  Time is money and you do not want to waste it chasing the wrong limb.

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