Kevin G. Keegan, DVM, MS, DACVS
The Problem. Any system or method that measures lameness as asymmetry will not be able to detect with high sensitivity or measure with high precision forelimb or hind limb lameness that is truly bilateral – especially if the lameness severity is distributed evenly between right and left limbs in every stride. This is true for all methods – including body-mounted inertial sensors, line-of-site kinematic analysis (video), and the stationary force plate. It is the method (measuring asymmetry), not the equipment, that produces these results.
Here is an example. A horse with a hypothetical grade 3 lameness in the right limb and a hypothetical grade 2 lameness in the left limb, will be measured with a hypothetical grade 1 (because 3 – 2 = 1) lameness in the right limb. In the practical sense, however, it is very unusual for a horse with a bilateral lameness to have the same amplitude of lameness in both sides on all strides; so detecting the existence of bilateral lameness with methods of asymmetry measurement is generally not an issue – it just requires some diligence.
As discussed in the recent advanced webinar on multiple limb lameness, the following case example was in fact a bilateral forelimb lameness, which was slightly worse in the left forelimb. The report on the left is the baseline straight evaluation before any blocking. The true amplitude of the left forelimb lameness is indeterminable initially. The left forelimb lameness is measurable, but small, partially offset due to the right forelimb lameness that is also present. The left forelimb lameness is suppressed or partially cancelled by the co-existing right forelimb lameness. Only after blocking the left forelimb is the right forelimb lameness revealed (middle report). Also notice that the right hind limb push off lameness initially measured must be mostly compensatory to the initial left forelimb lameness – however small it is – because it switched sides with the change in forelimb lameness after block.
Conventional Methods. Skilled and experienced equine veterinarians know this difficulty and, when presented with a horse with bilateral lameness, they attempt to overcome this difficulty by one or both of two methods; 1) they mentally compare the movement of the horse in question to a database they keep in their head about how a normal horse moves, and 2) they frequently work to “lateralize” the lameness, by inducing it to be worse on one limb compared to the other.
This first method usually concentrates on the amplitude of some body movement, like stride length. Because it is highly dependent on size, age, breed, use of horse, type of movement, and many other factors, the method requires development of experience and is prone to error. It is also usually just the first step; the first “seed” planted in the head of the veterinarian providing the first impression “this horse has a bilateral lameness”. Invariably the second method, lateralizing lameness, dominates the decision process.
Lateralization of the lameness is attempted by moving the horse in circles to change the type and amplitude of forces on limbs, by stressing a body part in one limb and reevaluating (flexion tests), or by selective elimination of lameness in one limb (blocking) and observing the change in amplitude and/or side of lameness. In many clinical presentations, a bilateral lameness is confirmed by first “lateralizing” and then eliminating the “lateralized” lameness. An attempt to measure the amplitude of lameness as asymmetry in a bilateral lameness situation as the summation of lameness in both limbs without first “lateralizing” the lameness is simply not possible. This seems like an unresolvable problem. But is it?
Confounding Data. Here is an example that I remember from reviewing a paper several years ago. The authors were evaluating horses with hind limb distal tarsal arthritis with a stationary force plate and using difference in vertical ground reaction force (in units of % body weight) between right and left limbs as the measure of the amplitude of lameness. A +10% difference was a right hind limb lameness. A -10% difference was a left hind limb lameness. A +10% difference was a worse right hind limb lameness than a +5% difference and vice versa for a left hind limb lameness (i.e. -10% difference worse than -5% difference). In this study a horse with a +10% difference before treatment and a +5% difference after treatment was considered improved! This may not be correct; the horse may not have really improved.
As an example, assume that before treatment a horse had a grade 3 lameness in the right hind limb and a grade 2 lameness in the left hind limb. Let us assume that this was equivalent to a +10% difference in vertical ground reaction forces between the right and left limbs. And then, after treatment, the horse had grade 3 lameness in both hind limbs – a real worsening of the overall lameness. But the grade difference after treatment is 0 and the difference in vertical ground reaction force between right and left hind limbs is less than before treatment. This horse got worse; but, based on asymmetry, it would have been assessed as improved.
The Challenges. How can the methods of measuring bilateral lameness be improved?
Any single measurement cannot be definitive without having an absolute threshold between “lame” and “not lame”. The absolute threshold cannot be based on asymmetry. Given the biological variability in size, shape, movement, weight distribution between front and hind halves of the body, speed of movement, etc. in the equine species, it is highly unlikely, verging on impossible, that such a threshold can be established. This would require many thresholds for many different situations. So, practically speaking, this cannot be remedied for a single evaluation of lameness. But, as in the above situation, when there is an assessment of a change in lameness between two evaluations – like before & after treatment or block – there is a remedy.
Consider the stationary force plate study described above. What if the vertical ground reaction forces on the right limb and left limb before treatment was 80% + 70% = 150%, respectively? And, what if the vertical ground reaction forces on the right limb and left limb after treatment were 70% + 70% = 140%, respectively? This horse had a 10% worsening in total hind limb lameness, not the improvement that was measured using asymmetry! At first glance, though more difficult to use in the clinical situation, the absolute quantification by limb would seem to suggest that the stationary force plate has a technical advantage in measuring bilateral lameness.
The Solution. There is, however, a potential measurement to assess change in bilateral lameness using body-mounted inertial sensors, and this measurement is total vertical excursion. Currently Lameness Locator is using vertical head movement asymmetry between right and left forelimb stance to detect and measure forelimb lameness. Because Diff Max Head and Diff Min Head are considered together as the single value, it is not possible for Lameness Locator to indicate that there is lameness in both the right and left front limbs. The same is true for hind limb lameness, except that lack of impact and lack of pushoff are assessed as separate measurements. A horse can have a pushoff lameness in one hind limb and an impact lameness in the other hind limb, but Lameness Locator cannot report that the horse pushes off less or comes down less on both hind limbs. Therefore, an accurate quantification of the amount of improvement after block or treatment of a horse with bilateral lameness is not provided in our current methods and calculation. However, Lameness Locator, as part of its analysis, separates the frequency components of the continuous, periodic up and down head and pelvis trajectories to assess total expected vertical head and pelvis movement given the state of the horse (the state of lameness) in that particular trial. Imagine a normal horse bounding up and down because it has no pain in the limbs and a bilaterally lame horse trying to not push off very hard and not land very hard on each limb. The “not-lame” horse is bounding, the lame horse is not bounding. This bilaterally lame horse before block would go from not bounding to bounding after block. So, it is possible to measure a change in bilateral lameness with body-mounted inertial sensors by assessing propensity for bounding, or the total expected vertical excursion!
The Future. We are working on this now. What this involves is finding many of the clinical cases in our data base (a few thousand cases) that ended up with a diagnosis of bilateral lameness, regenerating the analysis to report expected vertical excursion of the head and pelvis (i.e. bounding), and assessing any difference before and after block or before and after treatment. For forelimb lameness this will be easy. For hind limb lameness, separating out the contribution of impact and pushoff on bounding will be a little harder.
In the interim, veterinarians will have to suspect bilateral lameness from their clinical examination and from lateralizing the apparent lameness. Expect some results of these investigations in the second half of 2017.
Don’t have time to read the entire article right now? Download for later.