EQUINE VETERINARY JOURNAL
133
Equine vet. J. (1996) 28 (2) 133-138
Tendon strain in the forelimbs as a function of gait and ground characteristics and in vitro limb loading in ponies D. J. RIEMERSMA, A. J. VAN DEN BOGERT, MARGOT 0. JANSEN and H. C. SCHAMHARDT* Department of Veterinary Anatomy, Faculty of Veterinary Medicine, Utrecht University, The Netherlands. Keywords: horse; tendon strain; gait type; ground conditions
Summary Strains in the tendons of the m. flexor digitalis superjicialis (superficial digital flexor, SDFT) and m. flexor digitalis profundus (deep digital flexor, DDFT) tendons, the accessory ligament of the deep digital flexor muscle (inferior check ligament, ICL) and the m. interosseus medius (suspensory ligament, SL) of 5 ponies were recorded at the walk and trot using mercury-in-silastic strain gauges (MISS), on a hard surface (brick pavement) and on sand. The horses were shod with normal, flat shoes. On pavement, strain in the SDFT, DDFT and SL increased significantly from the walk (2.19%, 1.15% and 3.36%, respectively) to the trot (4.15%, 1.70% and 5.78%, respectively), but that in the ICL did not change significantly ( 5 3 6 % at the walk, 4.88% at the trot). Strains in the ICL and SL were higher on pavement than on sand (PeO.1) and strains in the SDFT and DDFT were not significantly different. Tendon strain in the SDFT and SL, but not in the ICL and DDFT, increased (PeO.1) in a pony at the walk on pavement with a rider. Post mortem loading of the same instrumented limbs revealed that the metacarpophalangealjoint could be further extended when the elbow joint was extended. The in vitro tendon strain was different from that in vivo, implying that results from in vitro limb loading tests have only limited value for assessing tendon functioning in vivo. Introduction The aetiology of tendon injuries in horses is rather complicated. One of the possibilities is excessive chronic or acute overloading, causing the tendon to approach, or even to exceed, its mechanical limits. An indirect cause of tendon overloading might result from a redistribution of loads on parallel tendons due to muscle exhaustion or abnormal limb loading conditions on muddy or slippery ground (McIlwraith 1987). Young et al. (1991) found a higher osteodensity of the sesamoid bones, indicating higher loading levels of the interosseus medius muscle (suspensory ligament, SL) in Thoroughbreds trained on a dirt track compared with horses trained on a wood chip track. Indications are present for gait or breed related incidence of specific tendon injuries (Fackelman 1973), but specific data are not available. Stephens er al. (1989) used HALL effect transducers attached to the tendons "Author to whom correspondence should be addressed.
and showed that the strain in the tendon of the m. jlexor digitalis superficialis (superficial digital flexor, SDFT) approximately doubled from the walk to the trot and from the trot to the gallop in ridden horses. The same study showed that the strain in the SDFT at the walk was substantially lower without a rider. The distribution of loads among several tendons in the equine forelimb has been studied extensively in vitro (Moser 1908; Mehrle 1926). Camp and Smith (1942) and Kingsbury et al. (1978) deduced in vivo tendon function from in vitro limb loading experiments. Shoemaker et al. (1991) have recorded an increase in strain in the SDFT after transecting the accessory ligament of the superficial digital flexor muscle (superior check ligament) in forelimbs loaded in vitro. This kind of limb loading experiments is relatively easy to perform and can be carried out on cadaver limb specimens. However, previous studies on tendon loads in isolated equine hindlimbs gave results differing largely from those recorded in vivo (Riemersma and Lammertink 1988). Therefore, it is important to assess the extent to which the results obtained from in vitro experiments can be extrapolated to predict and explain the function of equine tendons in vivo. The present study was undertaken to quantify the in vivo strain in the SDFT, the tendon of the m. flexor digitalis profundus (deep digital flexor, DDFT), the accessory ligament of the deep digital flexor muscle (inferior check ligament, ICL)and the SL in the forelimb of 5 ponies at the walk and at the trot, during locomotion on a hard surface and on a loose sand track. The second aim was to compare the loads in the same instrumented structures in the same limbs while loaded in vitro, with those occurring in vivo.
Materials and methods Ponies Five ponies (weight 165-240 kg, age 2-13 years and height 1.15-1.26 m), were used. All animals were examined for soundness. These ponies were the same as described by Riemersma et al. (1995).
Recordings Both forelimbs of the ponies were shod by an experienced farrier with normal flat shoes to which a small steel lip was welded to the tip of the toe. The lip was used for the attachment of an accelerometer', and of a lightweight black and white painted aluminum bar of 150 mm length which provided visual control of the angle and position of the hoof when the hoof itself was hidden
Tendon strain and in vitro limb loading in ponies
I34
TABLE 1: Mean (s.e.) stride duration ( 8 ) from 5 ponies
Walk on pavement Walk on sand Trot on pavement Trot on sand
0.919 0.975 0.514 0.544
(0.031) (0.030) (0.015) (0.021)
TABLE 2: Changes in mean angle (s.e.) between hoof-sole and horizontal during the stance phase of 5 ponies at the walk on sand. The stance phase Is divided into 3 periods, defined by the percentage of the total stride duration. The tangent of a least squares linear regression line through the data of hoofsole angle during the stance phase is also presented
Subject
Start (0-1 0%)
Pony 1 Pony 2 Pony 3 Pony 4 Pony 5
2.5 (0.9) 14 (0.3) 3.4 ( 1 . 1 ) 1.4 (0.1) 2.3 (0.5)
Stance - .- - ohase Middle End (2&30%) (4&50%)
3 (0.5) 11.2 (0.1) 4.3 (0.8) 1.7 (0.3) 4.2 (0.4)
4.8 (0.6) 12.3 (0.2) 6.2 (0.5) 3.2 (0.2) 5.4 (0.6)
Tangent degr./Yo
0.08 * 0.08 0.21 ' 0.17 * 0.14 *
*=indicates that the tangent was different from zero (P
