Study by: Shannon K. Reed, DVM, MS; Joanne Kramer, DVM, MS; Lori Thombs, PhD; Jael B. Pitts, DVM, MS; David A. Wilson, DVM, MS and Kevin G. Keegan, DVM, MS, DACVS
This month’s Journal Watch breaks down the largest study to date using body-mounted inertial sensors for the evaluation of lameness and how the information can be used in the clinical evaluation of equine lameness. This study, Comparison of results for body-mounted inertial sensor assessment with final lameness determination in 1224 equids (Reed, et al.), consisted of medical record review of 1,224 horses that presented to the University of Missouri for poor performance or lameness between 2011 and 2017. Information collected included whether or not a final diagnosis could be obtained and, if a final diagnosis was obtained, whether the cause was determined to be in the forelimb(s) only, in the hindlimb(s) only, simultaneously in the forelimb(s) and hindlimb(s), not in the limbs (i.e. neurologic, axial skeletal, or some other cause), or that the horse was normal.
Multiple limb lameness is one of the most common and potentially frustrating presentations veterinarians are faced with in daily practice. Of the over 1200 horses evaluated in this study, 56% exhibited a multiple limb lameness, that is, a combined forelimb and hind limb lameness. There are three reasons a horse might exhibit multiple limb lameness – 1) compensatory lameness, 2) secondary lameness and 3) two or more primary lameness foci.
Much has been studied about compensatory lameness patterns in horses – that which is the appearance of lameness in the opposite half of the body due to weight shifting off of a primary lame limb but is not associated with pain. Compensatory lameness is often confirmed by blocking out the primary lameness and observing that the compensatory lameness resolves as well.
We also know that secondary lameness exists in horses, which is also due to weight shifting off of a primary lame limb but differs from compensatory lameness in that it develops in a chronically overloaded limb and is likely associated with pain. Secondary lameness is not expected to resolve with blocking out the primary lameness. Furthermore, secondary lameness may be caused by pain in entire limb structures (flexor tendons, entire suspensory apparatus, multiple joints, muscle bodies) that may be difficult to block out. Gradual improvement in lameness after successive blocks suggests the limb as a source of secondary lameness. In some cases, it may be simultaneously present with the primary lameness, or may not present itself until after the primary lameness is blocked out. Secondary lameness is much more of a clinical guess, as it is more difficult to study experimentally.
From what has been reported previously in the literature and in veterinary textbooks, compensatory lameness is supposed to follow the law of sides, which has 2 two principles; the ipsilateral principle (apparent lameness in a forelimb and hindlimb on the same side of the body) and the contralateral principle (apparent lameness in a forelimb and hindlimb on opposite side sides of the body). According to the law of sides the ipsilateral principle suggests primary hind limb lameness, and the contralateral principle suggests primary forelimb lameness. However, by measuring lameness with inertial sensors, and using the ability to determine whether lameness is occurring at the beginning of stance (impact lameness) or end of stance (pushoff lameness) in the hind limbs, further information has been gleaned regarding the distribution of lameness and its potential indication of which limb is the primary source. This study looks at the distribution of lameness as measured in a simple straight-line trot and its association with final diagnosis and location of the site of primary lameness after more in depth workups.
The study first describes that all horses presented for lameness can either be measured with no lameness, lameness only in the front limb(s), lameness only in the hindlimb(s), or simultaneous lameness in the forelimb(s) and hindlimb(s). It then defines that simultaneous lameness measured in the forelimb(s) and hindlimb(s), based on the timing of hindlimb lameness (impact or pushoff), can be of 8 potential patterns or types.
The study then associated initial measurement of lameness in the straight-line evaluation with final determination of the site of primary lameness after full workup.
Some of the interesting findings in the study include:
- In horses that present for lameness evaluation, if the horse measures with no lameness in the straight-line evaluation there is a 60% chance that it has no lameness causing abnormality. Considering that this is from a sample of horses where the owner/trainer suspected that the horse had lameness (the reason for presentation), this is actually quite a high percentage. However, in horses presented for lameness evaluation, if the initial straight-line evaluation does not detect lameness, you should conduct other evaluation procedures like lunging, flexion tests, or evaluation under saddle.
- Just over half (59%) of horses presented for lameness evaluation that measure with only forelimb lameness on initial straight-line evaluation will end up with definitive localization of lameness to the forelimb. Smaller percentages will end up with diagnosis of no abnormality (12%), diagnosis, instead, of a hindlimb-only abnormality (16%), or diagnosis of a combined forelimb and hindlimb abnormality (13%).
- Most (65%) horses presented for lameness evaluation that measure with only hindlimb lameness on initial straight-line evaluation will end up with definitive localization of lameness to the hindlimb. Smaller percentages will end up with diagnosis of no abnormality (10%), diagnosis, instead, of a forelimb-only abnormality (18%), or diagnosis of a combined forelimb and hindlimb abnormality (7%).
- Not all horses that measure with single limb lameness during straight line evaluation will have the cause of lameness localized to that limb. Failure to localize lameness to a limb identified during straight line evaluation should prompt the examiner to utilize other activities (lunge, flexion tests, under saddle evaluation) to try to localize the source of lameness.
- Most horses (56%) presented for lameness will measure with a simultaneous forelimb and hindlimb lameness on the initial straight-line evaluation. However, few horses will be diagnosed with a focus of lameness in both a forelimb and a hindlimb.
- If a horse measures with a simultaneous forelimb and hindlimb lameness in the straight-line evaluation it is unlikely that the horse is normal (i.e. naturally asymmetric).
- Simultaneous measurement of forelimb(s) and hindlimb(s) lameness in the initial straight-line evaluation is most frequently the result of compensatory or secondary lameness, not of two separate primary sites of abnormality. Knowing these compensatory and secondary patterns helps one decide the limb that is most likely the source of primary lameness and helps to predict how difficult the subsequent diagnostic process is going to be.
A simple interpretation of the 2 principles of the law of sides (simultaneous ipsilateral = primary hindlimb lameness, simultaneous contralateral = primary forelimb lameness) does not accurately reflect what is found with inertial sensor-based evaluation of lameness in horses.
The 2 principles of the law of sides are correct only when the simultaneous hindlimb lameness during straight line evaluation is wholly or primarily lack of pushoff.
When the hindlimb lameness during the straight-line evaluation is wholly or primarily lack of pushoff, a simultaneous ipsilateral forelimb and hindlimb lameness is most likely a primary hindlimb lameness.
Contrarily, a simultaneous contralateral forelimb and lack of pushoff hindlimb lameness is most likely a primary forelimb lameness.
In a simultaneous forelimb and hindlimb lameness, when the hindlimb lameness is wholly lack of impact, localization of the primary source of lameness will be more difficult.
When the hindlimb lameness during the straight-line evaluation is wholly lack of impact with a simultaneous ipsilateral forelimb lameness, location of the primary source of lameness is equally likely to be in either the forelimb or hindlimb.
Contrarily, when the hind limb lameness is wholly lack of impact, a simultaneous contralateral forelimb lameness is most likely a primary forelimb lameness, but will be more difficult to confirm.
Furthermore, the simplified "law of sides" does not take into account a forelimb lameness with a bilateral hind limb of different types (impact lameness in one hind limb and pushoff lameness in the opposite hind limb).
In a simultaneous forelimb and hindlimb lameness, when the hind limb lameness is ipsilateral lack of impact and contralateral lack of pushoff, this is most likely a primary forelimb lameness.
In a simultaneous forelimb and hindlimb lameness, when the hindlimb lameness is ipsilateral lack of pushoff and contralateral lack of impact, 20% of the cases had a diagnosis of the cause of lameness that was not in the limbs. When this pattern of lameness is seen during straight line evaluation, a cause of lameness that is not in the limbs should be considered more likely.
- There is a logical decision process that can be applied to the 8 possible combinations of multiple limb lameness (considering timing of hind limb lameness as impact or push off type).
- Most pattern combinations are useful to guide us to the primary problem.
- A few patterns help us recognize that this case might be more difficult, or unusual.
- Only by measuring what the horse is doing can we recognize these patterns.