Page 972 - Adams and Stashak's Lameness in Horses, 7th Edition

938 Chapter 8

2–3 times for a tissue temperature increase of 10 °C. been shown to increase the effectiveness of isokinetic
161
Soft tissues may be stretched more effectively when they exercise for functional improvement in knee osteoar-
VetBooks.ir tissue elasticity. Low‐load, prolonged stretching of tis- when a pulsed duty cycle was used. However, thera-
thritis (range of motion, walking speed), particularly
are warm. Heat decreases tissue viscosity and increases
84
sues heated between 40 and 45 °C results in increased
peutic ultrasound showed no significant reduction in
extensibility of tendons, joint capsules, and muscles. 73,132 pain when used for conditions affecting the lower
There are two forms of thermotherapy: superficial limb. Thermal application of therapeutic ultrasound
175
(heating tissues at a depth of 1 cm) and deep. Deep ther- was shown to increase tissue temperature by 5 °C in the
motherapy is the application of modalities that cause a triceps at 3 MHz. This temperature increase, however,
41
rise in tissue temperatures beyond the level of the skin. was short‐lived, lasting 3.3 minutes. Therapeutic ultra-
Mechanisms to heat the deeper tissues include use of con- sound has been used with mixed success to stimulate
tinuous wave therapeutic ultrasound and radiofrequency bone growth and fracture healing in both animal models
diathermy. Both modalities have been reported in non‐ and human clinical trials. In humans, low‐intensity
equine species to therapeutically heat tissues and thus pulsed therapeutic ultrasound has been approved by the
modulate pain at a depth of 3–5 cm. Methods of superfi- FDA to treat acute and nonunion fractures. Low‐intensity
cial heat application include heating blankets and packs, pulsed therapeutic ultrasound targeting fracture healing
or other materials that can be heated in a microwave and has established an optimal intensity of 30 mW/cm at
2
affixed to the patient. Although clinical effectiveness for 1.5 MHz for 20 minutes in both human and nonhuman
superficial heating has yet to be demonstrated, it is often models. Success rates of low‐intensity pulsed ultrasound
recruited in both the training and rehabilitation settings for the treatment of human delayed unions and nonunions
prior to exercise as mounted heating lamps (solarium). range from 67% to 90%. 94,207
Therapeutic Ultrasound Evidence in Horses
Mechanism of Action A study performed in 2013 evaluated the ability of
Therapeutic ultrasound converts mechanical energy therapeutic ultrasound to heat the SDFT, DDFT, and
into sound waves through the piezoelectric effect – passing epaxial musculature to therapeutic levels. The mean tem-
an electric current through a crystal. The sonic waves perature rise was 3.5 °C in the SDFT and 2.5 °C in the
2
produced are transmitted by propagation through molec- DDFT after 10 minutes of application using 1.0 W/cm at
ular collision and vibration, which results in generation of 3.3 MHz with a continuous wave. No significant increase
heat if a continuous pulse wave is applied. 142,188 The ther- in heat production occurred when the intensity was
2
mal effects of therapeutic ultrasound are dependent on the increased to 1.5 W/cm . Cooling of the SDFT and DDFT
134
amount of tissue heating that occurs. Increasing tissue occurred at 2.5 and 1.5 minutes, respectively. Similar
temperature by 1 °C from baseline tissue temperatures will results were found in a canine study, which showed that
increase the regional tissue metabolic rate, a 2–4 °C rise in cooling of the caudal thigh muscle after therapeutic
120
tissue temperature reduces pain and inflammation and ultrasound occurred in less than 10 minutes. Therefore,
increases blood flow, and a tissue elevation >3 °C is neces- if stretching is to be performed following heating of the
sary to improve tissue elasticity. 36,78,114 Nonthermal effects tissues, it should be performed immediately after appli-
also occur, which include induction of cavitation and cation of therapeutic ultrasound.
acoustic microstreaming. These effects alter cell mem- A 20‐minute treatment was applied to the epaxial
2
brane structure and function, resulting in stimulation of musculature of horses using an intensity of 1.5 W/cm at
tissue repair, fibrinolysis, collagen synthesis, tissue regen- 3.3 MHz. Tissue temperature was measured at three dif-
eration, and bone healing. Other nonthermal effects dem- ferent depths – significant differences in mean tempera-
onstrated in vitro include earlier resolution of inflammation, ture rise were seen when comparing 1‐cm versus 4‐cm
heightened fibroblast recruitment, accelerated angiogene- depths and 1‐cm versus 8‐cm depths. No difference was
134
sis, and increased tissue tensile strength. 24,188 Nonthermal seen between 4‐cm and 8‐cm depths. While therapeu-
effects occur during both application of a continuous and tic ultrasound was ineffective at heating the deeper
pulsed duty cycle application. Penetration and absorption epaxial muscle, the use of a frequency of 3.3 versus
of the ultrasound waves are dependent on the intensity 1 MHz likely resulted in a lack of penetration of the
applied and the frequency used. Use of 1 MHz results in ultrasound waves, as acknowledged by the authors.
absorption at a depth of 3–5 cm, while use of 3 MHz results Therefore, further studies are needed to determine
in absorption at 1–2 cm. Low absorption of ultrasound if therapeutic ultrasound can heat deeper tissues.
waves occurs in tissues of high water content, such as fat, Additionally, although a statistically significant differ-
while high absorption occurs in tissues rich in protein, such ence was seen between 1‐cm and 4‐cm depths, the rise
188
as skeletal muscle. Therapeutic ultrasound can also be in tissue temperature at 1 cm was only 1.6°F, and
used to delivery low molecular weight drugs transdermally, therefore unlikely to be clinically significant. 134
a process known as phonophoresis. 141,188
Radiofrequency Diathermy
Evidence in Humans
Mechanism of Action
Therapeutic ultrasound has been shown to be effec-
tive at reducing pain (average reduction of 47%) and Microwave and shortwave radiofrequency radiation
increasing functional scores in patients with knee osteo- is used to heat tissues situated deep in the body while
arthritis, when compared with placebo. It has also minimizing a rise in skin temperature. Microwave
142