Head Movement Pattern in Horses With Forelimb and Hindlimb Lameness

=WM= IN-DEPTH: LAMENESS IN MOTION Head Movement Pattern in Horses With Forelimb and Hindlimb Lameness Kevin G. Keegan, DVM, MS, Diplomate ACVS The vertical head movement pattern in horses with forelimb lameness may contain information useful for determining the instant of peak pain within the stride cycle. This information may be helpful to the practitioner in isolating lameness within the affected limb. Author's address: E. Paige Laurie Endowed Program in Equine Lameness, College of Veterinary Medicine, University of Missouri, Columbia, MS 65211; e-mail: keegank@missouri.edu. 2005 AAEP. 1. Introduction Most equine practitioners look at how the horse's head moves during a trot to help them diagnose lameness." It is a common and well-accepted maxim that horses with forelimb lameness will show a "head nod" (or "head bob"), but descriptions of the "head nod" are incomplete, varied, and somes conflicting. Objective and precise measurements of head movement in lame horses have been made in experimental studies." The observations from these studies tell much about what the "head nod" really is and how close evaluation of it may tell us something about the specific lameness being evaluated. This paper will first try to document what is written in the most popular equine lameness textbooks about the "head nod" and then compare these descriptions to what has been determined by controlled objective analysis of the head movement patterns in sound and lame horses. In the latter part of this paper, I will comment on head movement patterns in horses with primary hindlimb lameness, and finally, I will introduce a hypothesis that the specific head movement patterns observed help the evaluator determine the type of lameness (i.e., when the pain is occurring during the stride). 2. What the Textbooks Say "As a result of lameness in a forelimb, the head will drop when the sound foot lands and rise when weight is placed on the unsound foot or limb." 1 "The head and neck elevate or rise when the lame forelimb is bearing weight or hits the ground and nod down or fall when the sound forelimb hits the ground... it is immediately obvious that the elevation of the head and neck is much easier to see than the head nod down... the horse appears to be elevating the head and neck just before the lame limb hits the ground, and then, during the later part of the support or stance phase, the head and neck nod down." 2 "Forelimb lameness is usually evident as a head bob: the head rises immediately prior to and during weight bearing of the lame limb. Conversely, the head drops as the sound limb contacts the ground and bears weight." 3 Each of these descriptions are similar in that they state clearly that the head moves upward during the weight-bearing phase of the lame limb and downward during the weight-bearing phase of the sound NOTES 114 2005 / Vol. 51 / AAEP PROCEEDINGS

Sound in Forelimbs -10- Head height -4-Right forelimb foot height_ IN - DEPTH: LAMENESS IN MOTION timo Fig. 1. Pattern of vertical head movement in a sound horse. limb. Indeed, this is what actually happens in horses with severe forelimb lameness and what appears to happen to the naked eye during the trot in horses with mild to moderate forelimb lameness. The rapid movement of the limbs in a horse even at a slow trot (4 m/s) and the limited temporal resolution of the human eye cause it to appear as such. However, they are also somewhat different in their varied emphases on whether it is the upward or downward movement of the head that is easiest to see. In general, they are also somewhat simplified and, in so being, incomplete. In the next sections, I will describe the objective evidence for how the head moves during forelimb lameness using data generated with high-speed cameras and computer-assisted kinematic evaluation during controlled conditions. The more complete description of head movement with lameness allows one to see that somes it is the downward movement and somes the upward movement of the head that is important for the detection of forelimb lameness. Quite possibly, a more complete and discriminating evaluation of head movement can help us isolate lameness within the lame limb. 3. Objective Analysis of Head Movement in the Sound Horse and in the Horse With Unilateral Forelimb Lameness In a sound horse at the trot, the head moves up and down twice during one complete stride (Fig. 1). At the beginning of stance, the head is already moving down along with the rest of the torso and following the contralateral suspension or swing phase. It continues to move down, reaching a nadir or minimum position at full weight bearing when the limb is perpendicular to the ground at midstance. It then begins to move upward during the caudal one-half of stance, continuing its upward movement during breakover and pushoff and into the beginning of the swing phase of the stride when the horse is suspended in mid-air with no limbs on the ground. It reaches a maximum position a short before impact of the contralateral limb. After impact of the contralateral limb, this same sequence of downward and then upward head movement occurs again. The key observations in this sequence of events are the downward followed by upward head movement during stance and upward followed by downward head movement during the swing phase of the stride. In the sound horse, the maximum and minimum vertical he.fd posaicas-durifig each one-half e of _the. stride. r e_equ al. In the horse with a mild to moderate unilateral forelimb lameness, this sequence of downward and upward head movement is the same. There is downward and then upward movement of the head during the stance phases of both the lame and sound limbs, and there is upward and then downward movement of the head after pushoff of both the lame and sound limbs. The difference is in the relative head heights at the midstance positions and after pushoff of the two forelimbs. This relationship breaks down in horses with severe lameness when the movement of the head during stance may be first upward and then downward or when the head maximum position is reached before the end of stance. The purpose of this manuscript is not to describe the head movement patterns in horses with severe forelimb lameness; accurate detection of this lameness is not difficult. Therefore, the relative height of the head during midstance (the downward movement) or after pushoff (the upward movement) determines the side of lameness, but which one is more important? It is not so simple. AAEP PROCEEDINGS / Vol. 51 / 2005 115

IN-DEPTH: LAMENESS IN MOTION Right Forelimb Lameness - pattern 1 762 -a- Head height -4-Right forelimb foot height 303 '700 E E 1650 250 E 200 t a ' CT) V 1600 RS C) 1550 1 150 I); 100 0 50 kv) 1500 Fig, 2. Pattern 1. The head moves down during the stance phase of the lame limb, but the absolute height is higher than during the stance phase of the sound limb. The head moves up after pushoff of the lame limb, but the absolute height is lower than after pushoff of the sound limb. 0 After evaluating many horses with mild to moderate unilateral forelimb lameness, both natural and induced, we have observed the following patterns of movement: (1) The head moves down during the stance phase of the 1 lwibut the absolute height is higheran during the stance prase the kind limb- the head moves up after pushoff of thilatne limb, but the absolute height is lower than after pushoff of the so=l_limb-(fig. 2). (2) The head moves down during the stance phase of the lame limb, but the absolute height is higher than during the stance phase of-the inab; the head moves upward after pushoff of the lame limb, but the-abbulutelreightii the same as after pushoff of the sound limb (Fig. -13). (3) Thse-head7moves_down during-the_ stsn re phase_ of the lame limb, but theabscaute_heigia-is-higherthaadur- Mg the stance phase of the sound limb; the head moves up after pushoff of the Ta me limb, but the absolute height is high-6r Than after, pushoff of the sound limb (Fig 4). (4) -The1iead moves downduring-the stance Right forelimb lameness - pattern 2 --s- Head height Rig_ht forelimb foot height 'GT E E 0 Fig. 3. Pattern 2. The head moves down during the stance phase of the lame limb, but the absolute height is higher than during the stance phase of the sound limb. The head moves upward after pushoff of the lame limb, but the absolute height is the same as after pushoff of the sound limb. 116 2005 / Vol. 51 / AAEP PROCEEDINGS

Right forelimb Lameness - pattern 3 IN-DEPTH: LAMENESS IN MOTION -a- Head height Right forelimb foot height 125, - 0 Fig. 4. Pattern 3. The head moves down during the stance phase of the lame limb, but the absolute height is higher than during the stance phase of the sound limb. The head moves up after pushoff of the lame limb, but the absolute height is higher after pushoff of the sound limb. phase of the_lame_limb, but the absolute height is the same as _dining the--stice _phase of the sound limb; the hea_d_moyes_upafteip - ushofrortfie1 the absolute height is higher than after pushoff of the sokmdlififf-wig. 5). There is an old adage "down on sound," which means that the head moves down to a lower height during the stance phase of the sound limb. Judging from the patterns that we have seen, this statement is not always correct. "Down on sound" would pick the correct limb in the first three patterns described above, but pattern four would be evaluated as sound. The head never moves down to a lower position during stance phase of the lame limb cornpaivitylee sound limb, so if this is observed, the correct -ori 'Rt-Timip-can always be identified. "Down on sound" is generally a good way to observe forelimb lameness. Evaluation of the head movement upward is more difficult but, as we will discuss later, potentially Right forelimb lameness - pattern 4 -a- Head height - - Right forelimb foot height Fig. 5. Pattern 4. The head moves down during the stance phase of the lame limb, but the absolute height is the same as during, the stance phase of the sound limb. The head moves up after pushoff of the lame limb, but the absolute height is higher after pushoff of the sound limb. AAEP PROCEEDINGS / Vol. 51 / 2005 117

IN-DEPTH: LAMENESS IN MOTION very informative for isolating forelimb lameness. Somes the head reaches a maximum position after pushoff of the lame forelimb that is somes higher, somes lower, and somes the same as that after pushoff of the sound forelimb. One could argue that it is the relative excursion of the head rather than the absolute height that one should be watching when evaluating for forelimb lameness. If this is the case, then the four patterns described above are the same but can be rephrased in excursion terminology. (1) The head moves down the same amount during the stance phases of the lame and sound limbs, but it moves up less after pushoff of the lame limb compared with pushoff of the sound limb. (2) The head moves down less during the stance phase of the lame limb than during the stance phase of the sound limb, and it moves up less after pushoff with the lame limb compared with pushoff of the sound limb. (3) The head moves down less during the stance phase of the lame limb than during the stance phase of the sound limb, but it moves up the same amount after pushoff of the lame and sound limbs. (4) The head moves down less during the stance phase of the lame limb than during the stance phase of the sound limb, and the head moves up more after pushoff of the lame limb than after pushoff of the sound limb. These descriptions are more difficult to understand and, as we shall see later, are more problematic for detection of lameness, which may result because of variations in lameness severity and of occurrence. 4. Head Movement in Horses With Primary Hindlimb Lameness It has been well described in textbooks and in objective, controlled studies that horses with primary hindlimb lameness can appear to have ipsilateral forelimb lameness."'6'7-16 This is because the horse uses its head movement to help reduce load on the lame hindlimb, moving it down to a lower position when the lame hindlimb is in stance. Using the "down_ and sound" maidm, it seems that the horse has a forelimb lameness ipsilateral to the true hindlimb lameness. The que tiofils-hol-,y -6M-1-x10-68-this happen and how likely is it for a practitioner to "mistake" a forelimb lameness for an ipsilateral hindlimb lameness. In a recent study of 17 horses with induced hindlimb lameness, it was apparent that rather mild, primary hindlimhlameness frequently caused a more apparent and severe but false ipsilateral forelimb lameness_16 This effect was significant when evaluated over the entire group of 17 horses, but it was also apparent that this effect was variable when looking at the horses individually. Some horses with primary hindlimb lameness had little change in head movement, whereas some had dramatic he movement asymmetry. An individual horse's body conformation may be a discriminating factor. Watching out for ipsilateral, false forelimb lameness is an important consideration during lameness evaluation in horses. This author on occasion has spent days blocking out 118 2005 / Vol. 51 / AAEP PROCEEDINGS suspected forelimb lameness only to find the true primary hindlimb lameness. 5. Types of Forelimb Lameness? Can Head Movement Help Us to Further Localize Lameness Within the Forelimb? A Hypothesis A sound horse, with no extraneous, random, vertical head movement, will move its head up and down twice per stride in a symmetrical pattern such that frequency analysis of the vertical head movement signal will yield a single frequency; this will be equivalent to twice the stride rate.' Frequency analysis of the vertical head movement signal in a horse with unilateral forelimb lameness will yield two prominent harmonic frequencies: one representing the lameness, occurring at the stride rate, and one at twice the stride rate representing the natural, inertially driven, up and down movement of the head, as in the sound horse. 6 Each of the above, previously described four different head movement patterns, although they seem quite different, represent a continuum in the temporal relationship between the two harmonic components that make up vertical head movement in a trotting horse with unilateral forelimb lameness. The frequency amplitude spectrums of each pattern are identical, but the phase differences between the two predomjnant harmonic frequencies vary.,--pa-t-i-6-rit-t4 :_,) the head moves down during the itatitse-thise of the lame limb, but the absolute height is higher than during the stance phase of the sound limb; the head moves up after pushoff of the lame limb, but the absolute height is lower than are in phase (i.e., their phase difference is 0 ). When the phase difference is 0, the peak or maximum of the lameness component, which can be thought of as the of maximum pain or discomfort during the stride, coincides with the beginning of stance. Thus, this is the_nattern_pf_head-movement ex_pect ieness is "felt! by_the horse primarily 1 _ du ring impact of the forelimb (Fig. 6). (attern th-e-held moves down during the stance phaie of the lame limb, but the absolute height is higher than during the stance phase of the sound limb; the head moves upward after pushoff of the lame limb, but the absolute height is the same as are out of phase by 90. When the phase difference is 90, the peak or maximum of the lameness component coincides with midstance. Thus, this is the pattern of head movement expected when lameness is "felt" by the horse primarily during full weight bearing on the limb, which occurs at midstance (Fig. 7). The difference in the downward positions of the head increases and the difference in the upward positions of the head decreases as the phase difference varies from 0-90 or from impact to full weight

IN - DEPTH: LAMENESS IN MOTION Fig. 6. Head-movement components in phase (phase difference = 0 ). The peak of the lameness component occurs at the instant of the right forelimb hoof impact (arrows). Note the less downward and less upward movement of the head during and after stance of right forelimb. Black horizontal bars, right forelimb stance; blue signal, lameness component; red signal, natural vertical head movement component; purple signal, sum of blue and red signals. Fig. 8. Head-movement components out of phase (phase difference between 90 and 180 ). The peak of the lameness component occurs at the end of stance (arrows). Note the less downward movement of the head during stance but greater upward movement of the head at the end of stance. Black horizontal bars, right forelimb stance; blue signal, lameness component; red signal, natural vertical head movement component; purple signal, sum of blue and red signals. bearing in the deceleratory part of the stance phase of the stride. ittern 3: ---jhe head moves down during the stakes-- e of the lame limb, but the absolute height is higher than during the stance phase of the sound limb; the head moves up after pushoff of the lame limb, but the absolute height is higher than are out of phase by 90-180. When the phase difference is between 90-180, the maximum of the lameness component occurs between midstance and the end of stance. Thus, this is the pattern of head movement expected when lameness is "felt" by the horse primarily during theiaiidarone-half or acceleratory -o-f_ftgt'ance phase of the strid-e, des the_ entirety of breakover(fig. 8). the head- moves down during the stance phase of the lame limb, but the absolute height is the same as during the stance phase of the sound limb; the head moves up after pushoff of the lame limb, but the absolute height is higher than are out of phase by 270. When the phase difference is between 180 and 270, the maximum of the lameness component occurs after breakover-at the end of the stance phase of the stride. Thus, this is the patternof-field movement expected when lameness is "felt" by the horse primarily during the initial swing part of the stride when thislimb is not weight bearing (Fig 9). The difference in the downward position of the head decreases and the difference in the upward position of the head increases as the phase difference varies from 90-270 or from full weight bearing through pushoff (in the acceleratory part of the stance phase of the stride) and into the beginning of the swing phase of the stride. The two-component model of vertical head movement for diagnosis of lameness is plausible, because the front end of the horse can be thought of as two linked segments (the torso-leg and the head-neck segments). Movement in the head-neck segment Fig. 7. Head-movement components out of phase (phase difference = 90 ). The peak of the lameness component occurs at midstance (arrows). Note the less downward movement of the head during stance but the same upward movement at the end of stance. Black horizontal bars, right forelimb stance; blue signal, lameness component; red signal, natural vertical head movement component; purple signal, sum of blue and red signals. Fig. 9. Head-movement components out of phase (phase difference = 2701. The peak of the lameness component occurs after the end of stance (arrows). Note that there is no difference in downward movement of the head but greater upward movement of the head after the end of the stance. Black horizontal bars, right forelimb stance; blue signal, lameness component; red signal, natural vertical head movement component; purple signal, sum of blue and red signals. AAEP PROCEEDINGS / Vol. 51 / 2005 119

IN-DEPTH: LAMENESS IN MOTION will always try to reduce force on the torso-leg segment. Because much is already known about the dynamic behavior of many anatomic structures within the equine limb, each a potential focus of lameness, knowledge of the instant of occurrence of peak lameness within the stride cycle may help further isolate the lameness within the limb. For example, limb vibration, as opposed to a total force applied to it at any one instant, is known to cause specific arthritic and other musculoskeletal disorders in humans." In the horse at limb impact, a significant proportion of the vibration occurring to the limb is attenuated or damped within the joint structures. 12 Therefore, arthritic conditions would be expected to a show pattern 1 head motion. Figure 1 depicts the head-motion pattern in a horse with experimentally induced carpal arthritis. Similarly, it is known that at the instant of hoof impact at the beginning of the stance phase, the force on the hoof is small, but it experiences high-frequency vibration. This vibration is damped predominantly by the lamina of the foot. Laminar pathology could, therefore. also be expected to cause maximum pain res onse at hoof impact; Tea rito ii head-movement pattern. Another example is that the naviciita-f- ii pensory ligament is known to be strained maximally during breakover. 13 Injury or lameness to the navicular suspensory ligament could be expected to cause pain duking breakover, leading to head movement like patterh-3. Figure 3 is an actual example of the head-iii-ovement pattern of a horse with calcification of the navicular suspensory ligament seen on radiographic examination of the foot. This horse also "blocked out" (i.e., the vertical head movement pattern became temporally symmetric) after a palmar digital nerve block. Similarly, the inferior check strained maximally during ligament is known to be breakav_er, 14 and injury to the inferior check ligament could be expected to result in a similar headmovement pattern. Other anatomic structures _are known to be maximally stressed at full weight bearing or activated during the swin le d stride, leadin-g-to headnovement patte s 2 an respectively. Knowledge of when pain is maximally occurring during the stride, obtained from close analysis of the specific head movement pattern, would be immeasurably beneficial to equine practitioners attempting to narrow the differential diag- nosis in specific lameness cases. With practice, close visual analysis of the specific head-movement patterns can potentially ascertain these different head movement patterns. References 1. Stashak TS. Examination for lameness. In: Stashak TS, ed. Adams' lameness in horses, 5th ed. Philadelphia, PA: Lippincott, Williams & Wilkens, 2002;113-183. 2. Ross MW. Movement. In: Ross MW, Dyson SJ, eds. Diagnosis and management of lameness in the horse. St. Louis, MO: Saunders, 2003;64-65. 3. Kaneps AJ. Diagnosis of lameness. In: Hinchcliff KW, Kaneps AJ, Geor RJ, eds. Equine sports medicine and surgery: basic and clinical sciences of the equine athlete. St. Louis, MO: Saunders, 2004;247-259. 4. Buchner HHF, Savelberg HHCM, Schamhardt HC, et al. Head and trunk movement adaptations in horses with experimentally induced fore and hind limb lameness. Equine Vet J 1996;28:71-76. 5. Keegan KG, Wilson DJ, Smith BK, et al. Changes in kinematic variables observed during pressure induced forelimb lameness in adult horses trotting on a treadmill. Am J Vet Res 2000;61:612-619. 6. Keegan KG, Pai PF, Wilson DA, et al. Signal decomposition method of evaluating head movement to measure induced forelimb lameness in horses trotting on a treadmill. Equine Vet J 2001;33:446-451. 7. Wilson DA, Keegan KG. Pathophysiology and diagnosis of musculoskeletal disease. In: Kobluk CN, Ames TR, Geor RJ, eds. The horse: diseases & clinical management. Philadelphia, PA: W. B. Saunders, 1995;602-658. 8. Caron JP. Objective and subjective gait analysis techniques. In: White NA II, Moore JN, eds. Current techniques in equine surgery and lameness, 2nd ed. Philadelphia, PA: W. B. Saunders, 1998;501-504. 9. Bucher HHF. Gait adaptation in lameness. In: Back W, Clayton H, eds. Equine locomotion. London: W. B. Saunders, 2001;251-280. 10. Kelmer G, Keegan KG, Kramer J. Computer-assisted evaluation of induced compensatory movements resembling lameness in horses trotting on a treadmill. Am J Vet Res 2005;66:646-655. 11. Bovenzi M, Zadini A, Franzinelli A, et al. Occupational musculoskeletal disorders in the neck and upper limbs of forestry workers exposed to hand-arm vibration. Ergonomics 1991;34:547-562. 12. Willemen MA, Jacobs MW, Schamhardt HC. In vitro transmission and attenuation of impact vibrations in the distal forelimb. Equine Vet J 1999;30(Suppl):245-248. 13. Rooney JR. The lame horse: causes, symptoms and treatment. Hollywood, CA: Melvin Powers Wilkshire Book, 1973. 14. Schamhardt HC, Hartman W, Jansen MO, et al. Biomechanics of the thoracic limb of the horse. Swiss Vet 1991;8: 7-10. 120 2005 / Vol. 51 / MEP PROCEEDINGS