—NOTE— The Influence of Exercise Intensity on Bucked Shin Complex in

Yoshinari KATAYAMA, Nobushige ISHIDA, Mikihiro KANEKO, Sadao YAMAOKA and Masa-aki OIKAWA* Equine Research Institute, Japan Racing Association, 321-4 Tokami-Cho, Utsunomiya City, Tochigi Prefecture, 320-0856, Japan

We tested the hypothesis that frequency of intense exercise during training would affect the J. Equine Sci. likelihood of bucked shins complex developing in young horses. The relative prevalence of Vol. 12, No. 4 bucked shins were examined in two groups of Anglo-Arabian horses that trainined by pp. 139–143, 2001 different training programs. One group (6 horses) exercised once per week at maximal speed, the other group (7 horses) exercised twice per week at maximal speed. They ran on similar track surface conditions on the same track. Although the group exercised twice per week at maximal speed had a prevalence, there was no significant difference in the incidence of bucked shins between the two groups. It suggest that the number of maximum gallop than the duration of intense exercise. Pathomorphogenetically, there was suggestion that focal radiolucent areas, indicating focal remodeling regions consisting of a focal conglomerate of irregular-sized osteons, at the site of the middle layer in the cortical beneath periosteal bony proliferation might play a role as a precursor lesion for the occurrence of otitis of the third metacarpal bone (Mclll). Key words: bucked shin complex, exercise intensity,

The serios cause of exercise induced in pathological processes behind its occurrence are still young horses in training is associated with bucked shin not fully understood. complex. This condition is particularly common in two We hypothesized exercise intensity are related to years olds during their fast year of training after fast bucked shin problems and the role of structural work on hard ground. Epidemiological survey by the changes in the third metacarpal in this Japan Racing Association (JRA) has indicated a 66% syndrome. incidence of bucked shin complex in racehorse during Experimental horses-Thirteen Anglo-Arabian female their first 8 months of training [3]. Bucked shin horses aged between 2 years 3 months and 2 years 5 complex is broadly considered to be associated with months (2.36 ± 0.07) were used in this study (Table 1). complex factors involving fast work on hard ground [4] Physical examination revealed no abnormalities in any and the stage of skeletal immaturity prior to the onset of the horses. of the syndrome [10], both of which are intricately Training program- In order to analyze the connected. relationship between bucked shin syndrome and There have been a few reports on the influence of frequency of intense exercise, and training regimens, different track surfaces on the occurrence of this the horses were divided into three groups: Horses in problem [6]. However, the influence of exercise Group A (6 horses) were run once a week at maximal intensity on the development of bucked shin complex speed on a dirt course consisting of sand 8–9 cm thick: has not been determined. Furthermore, the Group B (7 horses) were run twice a week at maximal speed on the same dirt course. The horses were initially This article was accepted December 5, 2001. trained for 2 months at a walk, trot and slow canter. *Corresponding author. This was followed by 6 weeks of running at a canter for 140 Y. KATAYAMA, N. ISHIDA, M. KANEKO ET AL.

Table 1. Summaries of cases Group Case No. Breed Sex Age Affected extremities (years & months) by bucked shins A1A-AF2.3R A2A-AF2.3N A3A-AF2.3N A4A-AF2.5R, L A5A-AF2.3R, L A6A-AF2.3N B7A-AF2.3R, L B8A-AF2.4L B9A-AF2.4N B 10 A-A F 2.3 R, L B11A-AF2.5N B12A-AF2.4R B13A-AF2.4L C14A-AF2.4N C15A-AF2.4N A- A: Anglo-Arabian, F: Female, R: Right limbs, L: Left limbs, N: Not found.

Fig. 1. Relationships between the onset of sore shins and exercise period at maximal speed. Clinical signs tended to appear following two or three high speed gallops, so they tended to occur earlier in group B horses. ○ : Horses run once a week at maximal speed (group A). ● : Horses run twice a week at maximal speed (group B). 1) First week.: 200 m at maximal speed. 2) Second week.: 400 m at maximal speed. 3) Third week.: 600 m at maximal speed. 4) Fourth week.: 800 m at maximal speed. 5) Fifth week.: 800 m at maximal speed. ◆ : Mean ± SD in maximal speed galloped (m/s).

1,000 m at 8.3 m/s, then 1,200 to 1,800 m at 10 m/s. week, 600 m in the third week, and 800 m for the next Following this, the horses began a 5 week period in two weeks (Fig. 1). Horses in Group A galloped at which they gallop at maximum speed for increasing maximum speed one time each week, horses in group B distance each week. The horses were run at maximal two times. Otherwise, training programs (distance, speed for 200 m in the first week, 400 m in the second speed, duration) were identical for the two groups. For BUCKED SHIN COMPLEX 141

Table 2. Incidence of bucked shin Group A B Limbs affected by bucked shin 6 (50%) 10 (71%) Contra-lateral limbs of the affected horse without bucked shin 6 (50%) 3 (29%) Limbs of the control horse without bucked shin 0 (0%) 0 (0%) N.S. (χ2 test) Note: Figures in parentheses indicates the ratio of the prevalence of bucked shin.

comparison, two horses aged 2 years 5 months without occurred in 50% (3/6) of group A horses, and 71% (5/ bucked shin resting at stall were used as control (Group 7) of group B horses. Bucked shins occurred C). unilaterally in 33% (1/3) of group A horses, and 60% Measurement of track hardness- The thickness of (3/5) group B horses. Therefore, the prevalence of cushion sand on the dirt course and the mean value of bucked shins in the total preparation of limbs at risk deceleration impact on the sand were measured by a was 42% (5/12) for group A and 50% (7/14) for group self-propelled racetrack hardness measurement system B. The prevalence of bucked shins in this study are throughout this experiment [9]. similar to those reported by Moyer et al. [6]. There Clinical observation- Bucked shin syndrome was were no statistically significant differences of the estimated clinically by a slightly shortened anterior prevalence of bucked shins between group A and group stride, frequent lead changes, changes in training B. The connection between the onset of bucked shin enthusiasm and a reluctance to accelerate. This was syndrome and training schedule appeared to exist. In accompanied by heat and swelling in the dorsal every case, clinical signs of bucked shins were first metacarpal region of the affected limb, and pain observed after the beginning the exercise period. As during digital palpation. Fig. 1 demonstrates, clinical signs tended to appear Pathological observation- Immediately after the following two or three high speed gallops, so they onset of the signs of bucked shin complex, the affected tended to occur earlier in group B horses. horses were killed humanely and unaffected horses Clinically, as the onset of bucked shins developed, were also euthanized and evaluated to verify that they the signs in the affected limbs include heat and had no lesions as well as clinical signs. Afterward, 5-mm swellings on the dorsal metacarpal region, and pain thick horizontal bone slices of the mid-diaphysis of during digital palpation. each of the third (McIII) were cut for At necropsy, the common gross findings of the soft radiography. Soft radiographs were taken at 40 affected area of McIII were distinct congestion and kvp, 5 mA, and 80 s. Following softgrams, the bone edema of the surrounding periosteum and slices were embedded in polyester resin (Rigolac, subcutaneous tissue. Nissin EM Co., Tokyo, Japan) and ground to a The typical soft radiogram and microradiogram of a thickness of 20–30 µm [8]. The ground sections of the cross-section through the mid-shaft of the McIII from calcified bone were taken by micro-radiography at 38 the horse affected with bucked shins show periosteal kvp, 5 mA and 20 min. After micro-radiographic new bone formation on the outer circumferential examination, the ground sections of calcified bone lamella of the dorsal aspect, focal radiolucent area were stained with Cole’s hematoxylin and eosin [8] and beneath the periosteal bone proliferation at the middle then examined microscopically. layer in the cortical bone of the dorsal aspect, The thickness of cushion sand on the dirt course and indicating focal remodeling region consisting of a focal the mean value of impact during a standardized conglomerate of irregular-sized osteon, endosteal bony deceleration during the experiment were 8–9 cm and proliferation on the inner circumferential lamella of 56.70 ± 13.81G (n=24), respectively. These value are the dorsal aspect (Figs. 2, 3). The thin line or fissure lower than those racetracks where JRA have been used which appear in the cortical bone crossing focal [9]. remodeling were occasionally seen (Fig. 2). As There was no significant difference in the prevalence mentioned above, a similar focal remodeling of osseous of bucked shins between groups A and B. Bucked shins architecture were observed in the mid-shaft at middle 142 Y. KATAYAMA, N. ISHIDA, M. KANEKO ET AL.

Fig. 2. Micro-radiography of Mclll affected by bucked shin. Fig. 3. Micro-radiography of Mclll affected by bucked shin. Slight new bone formation on outer circumferential Focal remodeling region consisting of a focal lamella (arrowhead) and on endosteum (arrow) at conglomerate of irregular-sized osteons in the middle the dorsal aspect of Mclll. Thin line or fissure layer in the cortical bone of the dorsal aspect of crossing the local porosity (focal remodeling region) McIII. × 120. in the cortex of the dorsal aspect of McIII. × 40.

layer in the cortical bone of the dorsal aspect of the McIII from the contra-lateral limbs of the horse without bucked shins. Otherwise, these focal remodeling of osseous architecture were not seen in the specimens of non-affected horse as control. Histologically, at the site of periosteal bony proliferation, it was seen subcutaneous edema with vascular microthrombosis in affected horses (Fig. 4). Judging from our results, we suspect that the onset of bucked shin syndrome appear a connection with frequency of intense exercise during training (the number of maximum gallop) rather than the duration of intense exercise (a certain distance gallop at high speed). It is clear that McIII imposed on the dorsal Fig. 4. Subcutaneous edema (arrow 2) with vascular mixed aspect of the cortical bone at the mid-shaft by too much microthrombosis (arrow 1) at the site of periosteal bending with number of high compression during high bony proliferation on the dorsal aspect of Mclll. Cole’s hematoxylin and eosin stain. Bar=50 µm. speed work than the duration of compression. Structural studies of bone indicated that repeated loading causes a progressive loss of stiffness, a decrease in yield strength, an increase in permanent the dorsal aspect of McIII in Group B subjected to deformation and hysteresis [2]. In addition, the dorsal repeated compressive stresses than McIII in Group A region of equine McIII is weaker monotonically, which experienced repeated compressive stresses. suggest that osteonal size and structure implicate Pathomorphologically, periosteal bony proliferation mechanical weak area in the dorsal region [7]. Thus, coexisting with underlying focal bone remodeling is the reason for the difference of the period of time for the common and predominant lesion in bucked shins. the onset of bucked shin syndrome seems that The fact that focal remodeling was observed not only in mechanical fatigue damage accumulates more rapidly horses with bucked shins but also in those without in areas of bone loaded with compressive forces than indicates that focal periosteal and also endosteal bone bone experiencing tensile forces [1], therefore fatigue proliferation might occur as compensatory, adaptative microdamage might be accumulated much more for hyperplasia or healing process to compensate for focal BUCKED SHIN COMPLEX 143 remodeling as a precursor lesion [5]. Although the Rose ed., Post Graduate Committee in Veterinary pathogenesis of focal bone remodeling is not unclear, Science, University of Sydney, Sydney. focal bone remodeling might be formed prior to the 5. Katayama, Y., Ishida, N., Kaneko M., Yamaoka, S., onset of the main clinical symptoms of bucked shins. Oikawa, M., and Yoshihara, T. 1991. Focal inflammation and pain on the dorsal aspect of Pathomorphological observations on ostitis of the McIII as the main symptom of bucked shins might third metacarpal bone in racehorses: soft be induced and/or exacerbated by circuratory radiographical and micro-radiographical disturbance consisting of vascular thrombosis and examinations. Bull. Equine Res. Inst. 28: 1–6. congestion at the site of periosteal bony proliferation. 6. Moyer, W., Spencer, P.A., and Kallish, M. 1991. In coclusion, the difference of exercise intensity Relative incidence of dorsal metacarpal disease in young racehorses training on two might influence on the onset of bucked shins. different surfaces. Equine Vet. J. 23: 161–168. 7. Martin, R.B., Gibson, V.A., Stover, S.M., Gilberg, J.C., and Griffin, L.V. 1996. Osteonal structure in References the equine third metacarpus. Bone 19: 165–171. 8. Oikawa, M., Yoshihara, T., and Kaneko, M. 1989. 1. Carter, D.R. and Hayess, W.C. 1976. Fatigue life of Age-related changes in articular cartilage compact bone-I. Effects of stress amplitude, thickness of the third metacarpal bone in the temperature and density. J. Biomech. 9: 27–34. thoroughbred. Jpn. J. Vet. Sci. 51: 839-842. 2. Carter, D.R. and Hayes, W.C. 1977. Compact bone 9. Oikawa, M., Inada, S., Fujisawa, A., Yamakawa, H., fatigue damage: A microscopic examination. Clin. and Asano, M. 2000. The use of a racetrack Orthop. Relat. Res. 127: 265–274. hardness measurement system. Equine Pract. 22: 3. Japan Racing Association. 1999. Annual Report on 26–29. Racehorse Hygiene. Number of new patients: 24–44. 10. Richardson, D.W. 1984. Dorsal cortical fractures of 4. Jeffcott, L.B. 1990. Wastage due to sore shins. pp. the equine metacarpus. Compend. Contin. Educ. & 93–95. In: Equine Lameness Foot Conditions. Pract. Vet. 6: S248–S254. Refresher Course for Veterinarians. Proceedings. 130. R.