Physical treatment of the equine athlete

Introduction

Sports medicine is a highly important area in the equine world, not only for elite sport horses, but also for the average horse in training. It includes different types of physical modalities for the treatment of musculoskeletal disorders. In this chapter, techniques such as thermotherapy, therapeutic ultrasound, extracorporeal shock wave therapy, electrical stimulation, laser therapy and magnetic field therapy will be described.

The techniques are described by providing background information, indications and contraindications, mechanism of action, a general description of treatment protocols, and finally a short overview of the outcome measures documented in the literature. There are very few validated treatment protocols for horses. Therefore, additional focus has been made to describe the mechanism of action. Techniques that lack clinical documentation might still be regarded as promising if there is a well-documented mechanism of action. The following information is mainly based on equine practice, but if there was a lack of scientific documentation regarding horses, examples are given from studies of other species. Therefore the text includes both case studies in horses and meta-analyzes in humans, studies that obviously do not have the same scientific nor clinical significance, but can be valuable to read.

Many therapies are claimed to stimulate the regeneration process, often by a reduction of inflammation and swelling, an increase in blood circulation and alleviation of pain. Depending on the aim of the treatment and its specific target tissue, its efficacy may be measured with a variety of assessment tools, often with higher intra-rater reliability than inter-rater reliability. For example, pain perception can be assessed with palpation ¹, pain scales and questionnaires², muscle tenderness (mechanical nociceptive threshold) with an algometer^3,4, the degree of swelling and muscle bulk can be measured with diagnostic ultrasound, but also with simpler tools like measuring tape and calipers. The joint range of motion may be assessed with goniometer (Fig. 64.1).⁵ Lameness can be evaluated by ordinary clinical lameness examination, but also with different forms of motion analyzes such as force plates, pressure mats, and high-velocity filming.⁶

Fig 64.1 Goniometer used to measure tarsal joint range of motion. Courtesy of Y. Liljebrink.

Thermotherapy

Cryotherapy

After an injury, reduced hemorrhage and subsequent swelling is explained by a reduction in blood perfusion due to vasoconstriction. A significant decrease in soft tissue perfusion of the equine digit has been demonstrated scintigraphically in feet subjected to cryotherapy for 30 minutes.⁹ It is well recognized that cryotherapy can prevent swelling from occurring, but it remains debatable whether it may help reduce edema that is already present.^7,8 A decrease in inflammation is also explained by decreased local metabolism and enzyme activity. It is suggested that tissue temperature needs to be around 10°C for a period of about 15 min in order to reduce skin metabolic rates¹¹ and a joint temperature of 30°C to slow the cartilage degrading enzymes.¹²

An analgesic effect is due to a reduction in nerve conduction velocity, and a reduction in muscle spasm caused by the influence on muscle spindle fiber activity.¹³ It is shown that cold water immersion had the greatest effect on human tissue temperature and sensory nerve conduction.¹⁴ Further, it is hypothesized that cryotherapy is working through the gate theory with intense stimulation of cold sensitive receptors at spinal level. There are also indications that cryotherapy reduces muscle force and affects proprioception, through a reduction in firing of muscle spindles and Golgi tendon organs. As a negative effect, it is recognized that collagen becomes less pliable with cold, along with a subsequent temporary reduction in mobility.⁷

Therapeutic protocol

Cryotherapy should be started immediately after the injury and continued until the response to trauma is stabilized, usually 24–72 hours. The general recommendation is that cold should be applied for at most 20 minutes, every two hours. Based on review articles in humans, recommendations range from 20 minutes 2–4 times per day, up to 30–45 minutes every two hours.¹⁵ LaVelle & Synder suggest that 10 rather than 20 minute intervals achieve the same skin temperature, but with less ‘hunting response‘ (an axonal reflex causing a secondary vasodilatation that is believed to occur after 20–30 minutes of cold treatment).¹⁶ However, the question of a hunting response is still debated, and probably requires target areas that involve muscles. There are indications that cryotherapy does not create hunting responses in the treatment of equine distal limbs.^9,17,18 Using shorter repeated cold applications allows the skin temperature to return to normal between treatments, thus maintaining the low muscle temperature without endangering the skin.

In horses, the thermal response to cryotherapy has been studied in skin,17,19 tendons^20,21 and hoof.^9,18 Cold water immersion is generally recommended at a temperature of 14–16°C. Ice water immersion (0°C) may be a more effective way of cooling the distal limb than cold packs, cooling splints and crushed ice. In humans, application of a pack of frozen peas produced skin temperatures adequate to induce skin analgesia in 10 minutes, and reduced metabolic enzyme activity to clinically relevant levels in 20 minutes. Finally, the application of cold should be monitored carefully to avoid ice burn. A damp barrier (like a wet towel) can be used between a contact cold source and the skin.

Outcome measures

There are studies in horses that support the clinical efficacy of cryotherapy. Studies indicate positive effects of 5–9°C cold water treatment in horses with digital flexor tendon damage and suspensory ligament injury ²², as well as a positive effect on laminitis.²³ Another study showed a temperature decrease of 21°C after the application of cold, reaching a minimum tissue temperature of 10°C.²¹ Finally, ice water immersion of the distal limb, at 3°C for 48 hours, did not adversely affect the tissue.¹⁸

Most studies on cryotherapy in humans have used pain as an outcome measure. However, the efficacy of cold on relieving pain in clinical settings is uncertain. A Cochrane review reports that that ice alone seems to be more effective to reduce pain and swelling than control.¹⁵ Further, the application of ice immediately before a rehabilitation program significantly decreased pain and increased weight bearing.²⁴ Ice therapy has showed benefit on pain relief, edema, joint motion and function, compared with control.¹⁰

Heat therapy

Background

Heat therapy is generally used to alleviate pain, reduce joint stiffness, muscle spasm and edema. It can be applied as superficial heat, affecting superficial tissues (heat wraps, hot packs, hot water bottles, warm water immersion, infrared light, electrically heated pads or blankets, high effect laser) and deep heat, which involves deeper layers of tissue like muscles (therapeutic ultrasound, high effect laser).

Indications and contraindications

Heat therapy is seldom applied in acute injuries, due to the vasodilatory effect and an increased risk of tissue swelling. Heat therapy is more commonly used for treatment of chronic tendinitis, joint diseases, joint stiffness and secondary muscle tenderness. It can also be used as a pre-treatment before passive joint motion or muscle stretching to increase the plasticity of connective tissue or as a comforting, sedative complementary treatment during general malaise.

Contraindications for heat therapy include infection, inflammation and neoplasia. As with cryotherapy, a contraindication is treatment of areas with insufficient blood circulation and impaired skin sensitivity. With a contact heat source, it is important to use enough insulation between the source and the skin. With infrared light and solariums, the animal should be placed at a safe distance from the heat emitting source and should not be close to something metallic that may hold excessive heat.

Mechanisms of action

It is proposed that by increasing tissue temperature, heat therapy can reduce pain, muscle spasm, joint stiffness and edema. The analgesic effect is explained by a direct influence on nerve conduction.7,8 Although superficial heat may only reach the superficial parts of tissues (approximately 1 cm from the skin surface), there is a possibility that it affects superficial nerves and thus have both a sedative and pain-relieving effect explained by the gate control theory. A reduction in muscle spasm is explained by the influence of heat on muscle spindle fiber activity, breaking the pain-muscle spasm-pain cycle. It is claimed that temperatures over 42°C slow the signaling rate of muscle spindles, but increases the signaling of Golgi tendon organs.⁷ Heat also reduces the viscosity of soft tissues and increases the elasticity of scar tissue, as well as reducing joint stiffness.²⁴

Reduction of edema is explained by a vasodilatation effect, through spinal reflexes leading to a decreased smooth muscle tone of vessels. The increase in tissue temperature also creates an increase in metabolism. Enzymatic reactions are believed to increase with temperatures over 45°C, and a rise of 2–5°C is claimed to increase the action of phagocytic cells and cause an increase in blood flow with a reduction in swelling.7,8,12

Therapeutic protocol

Superficial heat can be applied with different types of heat packs covered in a moistened towel. The heat packs usually stay warm for approximately 30 minutes. Temperatures in superficial tissue can increase approximately 5°C after six minutes of superficial heat, and it takes approximately 15–30 minutes for deeper tissues to reach therapeutic levels. After approximately nine minutes of hot water hose therapy at a temperature of 38–41°C, the skin temperature over the equine metacarpal region was raised to 39.5–41°C and the subcutis to 39–40°C.²⁰ Further, tissue temperatures return to baseline approximately 15 minutes after end of treatment. As previously mentioned, heating of connective tissues before stretching facilitates joint motion. To maintain tissue flexibility the temperature must be raised to 40–45°C for 5–10 minutes. Thereafter, the tissue must be put under stress.²⁴

The application of heat should be monitored carefully to avoid inflammatory reactions and burns. Contact heat sources warmer than 45°C should be wrapped in moist towels before application to the skin. Finally, the tissue temperature should be allowed to regain its normal temperature before new heat is applied, since there is an increased risk of burns with repeated heat administration.

Outcome measures

Studies in horses have shown a rise in tissue temperature after treatment with gel wraps, warm water hose and warm water bath.9,17,20 A 25 % increase in blood flow was reported after a 30 minute, 47°C water bath.⁹ A non-controlled study on 102 horses with various injuries reported a positive effect of dry whirlpool therapy, and that the temperature of the subcutis was raised 5°C.²⁵ In humans, there are no conclusive data on superficial heat therapy for either swelling or pain.¹⁵

Therapeutic ultrasound

Background

Therapeutic ultrasound has been used for injuries in the human musculoskeletal system since the 1950s. It is claimed to reduce pain and muscle spasms, and increase blood circulation and induce micro-massage at the cell-level, thus promoting tissue healing. However, there is an ongoing debate on its clinical efficacy.

The ultrasound machine contains a piezoelectric element which, when activated with an electric current, starts to vibrate. The vibration results in a sound wave, with a frequency dependent on the magnitude of the electric current. The sound waves are transmitted to the tissue through a transducer head that is put in contact with the skin of the horse. The wave is reflected in air-tissue interfaces, thus one needs an ultrasound coupling media between the transducer head and the skin. The absorption rate is higher in tissues that contain protein than fat. When the waves hit bone, they are reflected, which might lead to an increase in temperature and a pain reaction. The waves can either come in pulsations or continuously, thus emitting different levels of energy per unit of time.

Indications and contraindications

Therapeutic ultrasound is applied primarily in tendinitis, desmitis, sprains, lacerations, and myositis, but also in chronic conditions such as joint disease. Heating tissues prior to passive joint motion increases soft tissue extensibility facilitating treatment of contractures. Further, it is claimed to reduce adhesions, calcifications, and scar tissue, reduce nerve root irritation and speed bone healing. Besides these indications, therapeutic ultrasound can be used for phonophoresis, a means of delivering pharmaceuticals through the skin.

The contraindications for treatment are acute injuries and inflammatory disease, infection and neoplasia. Treatment should be avoided over areas with insufficient blood circulation, recent hemorrhaging, pregnant uterus, gonads, contaminated wounds and epiphyseal plates.

Therapeutic ultrasound has the potential to cause tissue damage if not handled properly, such as overheating and inflammation. Pain response to treatment can be observed as a withdrawal or stomping of the limb. Treatment of animals that are sedated or with impaired skin sensitivity should be restricted since they may not have a proper avoidance reaction. In humans, pain reactions, burns, and subperiosteal damage have been reported. The risk for so called ‘standing waves‘ can be reduced by continually moving the transducer head.

Mechanisms of action

The mechanisms of actions for therapeutic ultrasound are divided into thermal and non-thermal effects.²⁶ The thermal effects include an increase in blood circulation,²⁷ and a direct influence on the nerve through a prolonged nerve transmission velocity.⁷

A non-thermal effect is explained by vibrations causing ‘micro-massage at the cell level‘, which is thought to increase the cell membrane potential, permeability, and transport mechanisms. The vibrations are also believed to create micro-currents of liquid and cavitation (the formation of tiny gas bubbles in the tissues as the result of ultrasound vibration), which may cause tissue destruction.⁷ The eventual effect of therapeutic ultrasound on bone healing is explained by an enhancement of the endochondral portion of fracture healing. Other proposed non-thermal effects are the acceleration of the inflammatory phase, stimulation of fibroblast proliferation and decreased pain.^7,27,28

Therapeutic protocol

It is important that the electric apparatus and cords are functioning and protected from damage. Before using the ultrasound, one should make sure that it works properly by covering the transducer head with gel (or put it in a water bath) and increase the wattage to check if ‘bubbles‘ are created. It is also important to calibrate the instrument regularly, since studies have shown a variability of 30% in output intensity.²⁸

Treatment parameters

The settings of the apparatus often offer choice of frequency, intensity and duty cycle. The protocol should also include a specified treatment area, treatment duration, and speed of transducer head movement and treatment schedule.

The frequency determines the depth of wave penetration; 1 MHz heats to a depth between 2–5 cm, 2 MHz 1–3 cm and 3 MHz to 0.5–3 cm.

Intensity is the amount of energy delivered per unit area (W/cm²). Intensity is set between 0.25–5 W/cm²depending on treatment goals. The higher the intensity, the faster is the rise in tissue temperature. Higher intensities are used to heat tissues before motion treatment and lower, non-thermal intensities are often used when treating pain and muscle spasm.

The duty cycle can be set as either continuous or pulsed ultrasound; the thermal effects are reduced if the sound waves are delivered in pulses. The most commons ratio settings are 1 : 4, 1 : 5 or 1 : 1. The ratio 1 : 5 is often chosen when treating areas over superficial bone.

The treatment area should not exceed three times the transducer head area.

The dosage can be calculated as: energy (J =W/cm²) x applicator size (cm²) x time (sec).

Treatment times usually vary between 5–10 minutes.

Treatment regimes

Place the animal in a secure place. In order to create the best possible sound wave transmission, the hair should be clipped and a generous amount of acoustic gel applied between the transducer head and the skin. It is important that the ultrasound head is moved during treatment. It is possible to treat an area with an irregular surface by immersing area (such as a limb) into water.

In horses, treatment is usually performed at 1.5 W/cm² for 10 minutes, once or twice daily for 10–14 days. The thermal effects of ultrasound are difficult to predict because of the varying rates of absorption by different tissues. A lower temperature rise was seen in the treatment of dogs with an intact hair coat, compared to dogs with clipped hair. However, there was a great temperature increase within the hair. Temperature increase of 0.6–3.5°C has been shown at 1–10 cm depth in canine muscles after treatment with 1–2 W/cm² and 1–3 MHz.²⁶

Outcome measures

Lippiello and Smalley draw the conclusion that bone healing in equines was stimulated by ultrasound therapy.²⁹ There are divergent results from studies on muscle and tendon regeneration in horses and dogs.^30–34 In a clinical study without case controls, Lang treated 53 horses and 143 small animals. Based on owner evaluation, 63 % of the treatments were successful.³⁵ One study failed to demonstrate any positive effects of ultrasound therapy on the healing of surgically severed Achilles tendons in dogs (1 MHz, pulse ratio1 : 4, 0.7–1.0 W/cm2, 10 minutes, three times a week, 13 treatments). In human systematic reviews authors conclude that there is limited evidence to support the use of active ultrasound therapy for treating people with musculoskeletal disorders.^36,37