Horse Limbs Function Like Pogo Sticks

If you tried to build a horse limb with children’s blocks, the limb would collapse, because it would be impossible to build the fetlock joint in a hyperextended position. However, in a live limb the digital flexor tendons and ligaments on the flexor side of the limb prevent failure. Tendons have elastic properties and act like rubber bands or springs, providing resistance against which the limb presses when it comes under load, thus resisting further hyperextension and preventing collapse (Figure 26-1). The tendons are stretched at the same time, thus storing elastic strain energy, which can be returned in elastic recoil. During each step energy is stored, and it is returned when the limb leaves the ground. Energy is carried forward from one step to the next, thus reducing work the muscles have to do and saving metabolic energy. Indeed, the muscles associated with the main contributors of this system—the suspensory ligament (SL), deep digital flexor tendon (DDFT), and superficial digital flexor tendon (SDFT)—either are not present at all (the SL and the accessory ligament of the DDFT [ALDDFT]) or are very short in relation to the tendons (e.g., the average length of the deep digital flexor muscle-tendon unit in a Thoroughbred is 77 ± 5 cm, more than 60% of which is tendon. The flexor muscles, being highly pennate and having short muscle fibers (1 cm in length), have limited capacity to change the length of the muscle-tendon unit when contracting. Approximately 7% of the energy stored in the tendons is released as heat, and during gallop the tendons of a galloping horse reach about 45°C.⁴ We have reasoned that increase in temperature may account for core lesions in equine SDFTs but found that although 45°C resulted in death of some cells, tendon cells may be resistant.⁵ Core lesions may result from hyperthermic damage of matrix components.⁵

Fig. 26-1 Anatomical drawing of an equine forelimb (left) and the equine forelimb modeled as a spring-system (right); superficial digital flexor tendon (SDFT); deep digital flexor tendon (DDFT); accessory ligament (AL) of the DDFT; and AL of the SDFT. Limbs function like pogo sticks: the hyperextended fetlock joint is kept from collapsing by the springlike digital flexor tendons on the palmar aspect of the limb. When the limb comes under load during the stance phase of the stride, it compresses by further extending the fetlock joint while stretching the digital flexor tendons. This enables the digital flexor tendons to store elastic energy, which is released for propelling the limb into the swing phase. MCP, Metacarpophalangeal; PIP, proximal interphalangeal; DIP, distal interphalangeal.

Like any springlike structure, the limb itself has a certain stiffness, which is a measure of how much it shortens for a given load. A whole equine limb changes length mainly as a result of fetlock extension and length changes in the digital flexor tendons.⁶ People can adjust their leg stiffness by muscle contraction to suit the softness or hardness of the ground on which they are walking or running.⁷ Horses cannot adjust their leg stiffness because of the limited ability of the flexor muscles/tendons to change length, and this may be the reason why some horses cope better than others on different goings (footings). The flexor muscles do, however, damp vibrations, which otherwise would be likely to cause injury to musculoskeletal tissues.⁸

Bones

To enable a pogo-stick design, the bones in the equine limb are reduced compared with other animals: the radius and ulna are fused, and the horse bears weight only on the third metacarpal bone (McIII) or third metatarsal bone and the digit. Fewer bones allow lengthening of the limb and tendons and reduction of the mass of the distal aspect of the limb. This increases the energy storage capabilities of the digital flexor tendons and also results in a lighter limb that can be swung more rapidly and with less energetic cost, which is of benefit in locomotion. Fewer bones is thus an adaptation for maximum strength with minimum weight, because the bending strength is much higher for one large bone than for the same amount (and hence weight) of material arranged as several smaller bones.² However, a reduction in bone mass for energy efficiency results in an increase in fracture risk, and this is reflected in the fact that the distal limb bones are at higher risk for fracture than the proximal limb bones.⁹

Synovial Joints

Joints fulfill two main mechanical functions: they allow the movement of limb segments in relation to each other, and they act as shock absorbers. Limb movement is limited to the sagittal plane by anatomical adaptations of the phalanges and the fusion of bones, and only small, out-of-plane movements, such as adduction and abduction or rotation, are possible. Movement is further limited by the anatomical features of some of the articular surfaces, such as the interlocking configuration of trochlear and sagittal ridges and matching grooves, and is enforced by collateral ligaments. Without the need for muscular control, these features restrict joint movement to the sagittal plane, thus further decreasing metabolic costs.

Although all joints provide movement and shock absorption to a certain degree, some joints allow only a small range of movement and have the primary function of shock absorption, whereas other joints have a large range of movement and a primary function of movement. These joints can be functionally classified as low-motion or high-motion joints, respectively. The fetlock joint, a good example of a high-motion joint, goes through 90 degrees of movement during the stance phase of gallop, whereas the proximal interphalangeal joint has a range of motion of only 5 to 10 degrees. In complex joints such as the carpus and tarsus, there is a division in function: both the antebrachiocarpal and tarsocrural joints are high-motion joints, but in the more distal joints of the carpus and tarsus there is less movement; the least movement is in the carpometacarpal and tarsometatarsal joints. There is also a difference in occurrence and clinical severity of skeletal disorders between high- and low-motion joints. Osteochrondrosis is much more common in high-motion joints, and osteoarthritis (OA) is usually of greater clinical significance than in low-motion joints.

The Foot as Interface to the Ground

The foot provides the interface between the horse and the ground. It is a complex modification of integument surrounding, supporting, and protecting structures in the distal limb of the horse. The hoof capsule encases within a confined space three bones, a series of ligaments and tendons, two synovial structures, a digital cushion, cartilages of the foot, blood vessels, and nerves. Although the horny hoof capsule provides protection of the internal structures, it does not allow for expansion through swelling; therefore any swelling that does occur as a result of injury leads to an increase in pressure and thus stimulation of pain receptors. Mechanically the foot has three main functions: shock absorption when the foot comes into contact with the ground, support and grip when the limb is bearing weight, and propulsion when the limb leaves the ground. It must also resist excessive abrasion and protect sensitive structures lying internally, while, in wild horses, allowing sufficient natural wear of the wall to maintain hoof shape.¹⁰

The foot has a series of “built-in” protective mechanisms that absorb part of the concussive forces and damp vibrations during impact (Figure 26-2):

Fig. 26-2 Schematic drawing illustrating two of the shock-absorbing features of the equine foot. First, the shape of the solar surface with the frog in the middle and the frog grooves on either side allows the heels to move sideways and distally on ground contact, while the toe retracts. Second, the suspension of the distal phalanx within the horn capsule allows forces to be transferred from the distal phalanx across the laminar junction to the hoof and to the ground via the distal border of the hoof wall. GRF, Ground reaction force.

Structural and Material Properties

When a force is applied to a structure, the structure usually responds by deforming. The greater the force applied to a structure, the greater the deformation. The ability of a structure to resist deformation is expressed as stiffness and represents the slope of a structure’s force-length relationship. The force-deformation relationship of biological structures behaves in a roughly linear elastic manner until the applied load causes nonreversible deformation and ends in failure. The force-deformation relationship describes the structural properties and depends on the material properties of a tendon and on its dimensions. Larger structures are able to cope with larger forces: imagine two ropes made of the same material, but double the cross-sectional area (approximately 1.41 times the diameter): the thicker rope will be able to withstand double the force of the thinner one.

It is often of interest to know about the properties of the material per se, independent of size. This is achieved by dividing the force acting on a structure by its cross-sectional area. When normalized for cross-sectional area, a force is called stress (σ; common units would be mega Newtons per square meter [MN/m²] or Newtons per square millimeter [N/mm²]). The resulting deformation of an applied stress is expressed as strain (ε), the ratio of the change in size to the original size. Being a ratio, strain does not have a dimension; however, it is often expressed as a percentage—for example, ε = 0.1 = 10%. If we are interested in the property of a material rather than structure, we can express this relationship as a stress-strain curve. The slope of this curve is the ratio between the tensile stress and strain and is called the elastic modulus or Young’s modulus (E). The elastic modulus defines whether a material is “rigid” or “compliant.” Rigid materials have a very high elastic modulus and deform very little under load, whereas compliant materials have a low elastic modulus and require less load to deform. Bone, for example, is relatively rigid, whereas articular cartilage is more compliant and thus is able to act as an excellent shock absorber by undergoing considerable deformation when under load. Figure 68-1 shows the stress-strain curves for the digital flexor tendons of a Thoroughbred racehorse.

The ability of structures to deform when loaded and return to their original length when the load is removed allows them to store energy. The amount of energy per unit volume is the area under the linear portion of the stress-strain curve. The capacity of a material to absorb and release energy is often referred to as elastic resilience. The energy a material can absorb before failure defines whether it is “brittle” or “tough”: tough materials are able to absorb considerable elastic energy before failing, whereas brittle materials absorb very little. The digital flexor tendons of a horse are able to store and return a considerable amount of energy during locomotion. Load-deformation curves of tendons are different between loading and unloading, forming a “hysteresis” loop. The area of the loop presents the loss of energy, largely in the form of heat, that occurs during stretch and release (see Figure 68-2). It has been shown that the heat produced by repetition of this mechanism leads to an increase in core temperature, which may contribute to the pathogenesis of tendon injury through thermal damage.⁴ A detailed description of the pathobiology of tendon injuries is given in Chapter 68.

From an injury perspective, stress is probably the most informative mechanical measure in addition to strain. Equine digital flexor tendons, for example, have different elastic moduli and experience different strain rates in vivo, but they also have different cross-sectional areas. Both the SL and the SDFT experience high strains (up to 16%) during locomotion, but the stress in the SDFT is much higher than in the SL because its cross-sectional area is only about a third of that of the SL.¹¹ This corresponds to the fact that the SDFT is the most commonly injured tendon or ligament (see Chapter 68).

Changes in Structural Properties

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