Developing a successful rehabilitation protocol involves first reaching an accurate diagnosis followed by establishing clearly defined rehabilitation goals that consider the biomechanical implications of the original injury. This outline will review various physical modalities commonly employed in equine rehabilitation. Unfortunately, universal recommendations regarding the timing, frequency and specific indications of many of the below-described modalities are still lacking. As further research is able to define specific parameters, significant advancements within the rehabilitation field can be expected.
Laser therapy
Mechanism of Action
Laser therapy is thought to create a photobiomodulation to cellular aerobic respiration and thus can have beneficial effects throughout the stages of injury.1 The light energy is absorbed at a subcellular level that leads to an increase in ATP production stabilizing the cell membrane and increasing DNA activity and the synthesis of RNA and proteins.1,2 Although not all mechanisms are clearly understood, laser therapy is used as a noninvasive procedure to stimulate cell regeneration, increase angiogenesis, decrease inflammation and modulate pain.3 Laser therapy has been to shown to reduce the production of the inflammatory cytokines PGE2 and TNFα in both human in-vitro and clinical studies.3 Multiple animal models have demonstrated laser therapy applied to cell cultures increases cell proliferation, migration, and collagen deposition.4,5 Recently, rat aspirated bone-marrow derived mesenchymal stem cells exposed to laser therapy at 5.0J/cm 2 had a significantly enhanced production of growth factors VEGF and NGF.21 Human cell culture studies have identified that energy doses applied between 0.5 – 5.0 J/cm2 have a stimulatory effect on cellular responses, while higher doses (16 J/cm2) have an inhibitory effect.3 Laser irradiation of equine bone marrow derived mesenchymal stem cells exposed to 1064 nm wavelength irradiation with an energy density of 9.77 J/cm2 and a mean output power of 13.0 W resulted in significant upregulation of IL-10 and VEGF expression.6 While cells in culture demonstrate promising results for healing in response to laser therapy, the behavior of these cells in-vivo has yet to be demonstrated. Surgically created suspensory ligament branch injuries were created in all four lateral suspensory branches in twelve warmblood horses.7 Laser therapy using a class IV laser with a power output of 15 W was applied daily on two of four induced lesions for four consecutive weeks.7 Significant improvements were demonstrated in the fiber alignment, collagen III expression, lesion size, shape and density of nuclei in the laser treated lesions when compared to controls.7
Clinical Application
The exact light wavelength, dosage and treatment frequency needed for optimal treatment of select musculoskeletal diseases is largely unknown. It is recommended to clip and clean the site prior to laser therapy as the presence of hair and dirt reduce laser light penetration through the digital flexor tendon region in horses.8 Typically the lower wavelengths (630nm) are used for wound healing9 with the higher wavelengths (850nm) used for soft tissue inflammation and deeper musculoskeletal conditions. Lower frequencies may be applied for acute conditions and the higher frequency settings have been reportedly used for pain and chronic injuries. Daily application is typically used when treating acute inflammation while tissue regeneration and chronic injuries are typically treated every other day to three times a week. A single study of chronic back pain in horses that applied laser therapy (904 nm, 360Hz) over 9 acupuncture points weekly demonstrated that clinical signs of back pain were alleviated in 10 of 14 horses.10 Most recently, high intensity laser therapy was used to manage horses with distal hock joint osteoarthritis and was found to significantly improve lameness scores five days following completion of 10 total treatments within a 14 day time frame.11 A clinical study looking at 150 horses with various soft tissue injuries from SDF tendinopathies to suspensory ligament desmopathies were treated with class IV laser with a maximum power output of 15,000 mW and with four different wavelengths simultaneously (635, 660, 810, 980).12 Horses were divided into laser therapy only group or laser plus additional therapies ranging from PRP, shockwave and stem cells. Laser application occurred daily for two weeks. 12 Those horses in the laser therapy only group had significantly improved lameness and ultrasound scores four weeks after laser therapy compared to the combined group.12
Pulsed Electromagnetic Field Therapy
Mechanism of Action
Pulsed electromagnetic field (PEMF) therapy uses an electrically-generated magnetic field that is placed around an injured region or adjacent to a body segment of interest. The induced magnetic field produces secondary electrical currents within biological tissues, that stimulate cellular repair.13 PEMF utilizes a low frequency and short phase duration, which results in a current generated within the tissues without heat production.14 PEMF therapy in humans is advocated in the reparative phase of musculoskeletal injury. In addition, to improving blood flow through vasodilation and improved metabolite clearance, pulsed electromagnetic field therapy is reported to increase metabolic rate and enzymatic activity at a cellular level, reduce pain, relieve muscle spasm and improve tissue elasticity.15-18 Of particular interest for fracture healing are the reports of PEMF stimulating bone repair via enhanced osteogenesis.14,18-19 PEMF is approved by the FDA for treatment in humans with long-bone non-union fractures and as an adjunct treatment following lumbar/cervical spine fusion surgery.20 PEMF has been shown to accelerate fracture healing in both skeletally mature normal rats and in rats with induced osteoporosis.21 PEMF significantly improved the success rate in people with delayed union long-bone fractures with 77.4% of the PEMF treated fractures healing compared to 48.1% in the sham group at 4 months.22 PEMF has shown to increase bone volume fraction, trabecular thickness, trabecular number, and suppress trabecular separation in a rat model of induced knee osteoarthritis. In this same study, PEMF was shown to promote WNT gene expression in the subchondral bone. Activation of WNT promotes osteoblast and osteocyte activity while indirectly suppressing bone resorption and osteoclast differentiation.23 In non-equine species, targeted PEMF devices have also been shown to help reduce pain and inflammation, mainly through stimulation of the nitric oxide pathway, which results in vasodilation and enhanced circulation. Recently, a targeted PEMF device (Assisi Animal Health Santa Fe, NM) demonstrated improved functional outcomes and reduced need for systemic pain medications in dogs recovering from hemi-laminectomy, compared to dogs recovering from the same surgery that were treated with a placebo controlled device.24 Devices that produce PEMFs vary by a number of important features, which include frequency, waveform, strength, and types of stimulators. Particular attention should be paid to the parameters utilized in those studies that found PEMF therapy to be efficacious as the positive outcomes are specific to the PEMF signal configuration.24
In horses, initial reports of PEMF use were for treating chronic non-union fractures and stimulating bone healing. Utilizing an equine experimental osseous defect model, Cane et al., demonstrated that a PEMF setting of 75Hz delivered continuously for 30 days resulted in significant increases in the mineral apposition rate and osteoblastic activity during the healing process of defects created in the diaphysis of the third metacarpal bone.25 An earlier study conducted by this same group, however, using the same PEMF parameters demonstrated mixed results among defects created in the metaphysis of the third metacarpal bone.19 In some instances, the results were identical to the positive effects appreciated in the above referenced study and in other metaphyseal samples there was a negative influence of PEMF on bone healing rates.25 A similar equine osseous defect model incorporating cancellous bone grafts demonstrated increased calcification rates compared to controls when using PEMF settings of 1.5Hz for 3 hours/day for up to 240 days post grafting.25 Although healing rates of up to 87% of chronic non-union fractures in people have been reported following PEMF therapy, the efficacy and quantification of its effect on fracture healing is widely varied - similar to what has been appreciated in the horse.
Currently, there are no reports of PEMF use in treating naturally-occurring joint disease in horses. However, in ponies with amphotericin B-induced carpal synovitis, the effect of PEMF on arthritic and nonarthritic joints was measured by comparing synovial fluid parameters, the degree and duration of lameness, carpal joint range of motion, and carpal circumference.26 In treated ponies, there were significant reductions in the severity and duration of lameness, carpal swelling, and the severity of gross pathological and radiographic changes. It was concluded that significant beneficial articular effects were produced when treated with PEMF and that no adverse treatment effects were noted.26 Cleary, more research needs to be conducted to understand the influence PEMF has on bone metabolism and soft tissue healing and to determine the appropriate settings, timing and duration of applied therapy.
Similar to humans, back pain and inflammation of the epaxial musculature is a significant problem in all equine athletes. Treatment of back pain can be challenging and often requires a multimodal approach. In humans, bio-electromagnetic energy regulation therapy (BEMER) has been reported to be effective in pain modulation. Well defined effects for this type of PEMF therapy have also been reported to significantly increase microvessel vasomotion, arteriovenous pO2 difference, number of open capillaries, arteriolar and venular flow volume, as well as enhance flow rate of red blood cells within the targeted microcirculatory area.27 The improvements in blood flow lead to increases in tissue perfusion, metabolic activity, and muscle relaxation thereby facilitating cellular repair and diminishing nociception. 27 A recent study assessed the analgesic responses and biomechanical outcome variables using a bio-electromagnetic energy regulation therapy blanket to evaluate the treatment effects in horses with thoracolumbar epaxial muscle pain.28 In a group of eight horses with objectively diagnosed, naturally-occurring thoracolumbar epaxial muscle pain BEMER blanket therapy significantly reduced pain and improved balance control and spinal flexibility.28 This emerging technology provides the equine practitioner with a medication free bio-solution to modulate thoracolumbar epaxial muscle pain in horses.
Clinical Application
Therapy is typically applied for 30-60 minutes twice daily at a variety of predetermined manufacturer settings. Each device varies greatly in frequency, waveform, strength, and types of stimulators, thus there are no standardized evidence based protocols.
Whole Body Vibration
Mechanism of Action
Whole body vibration therapy (WBV) involves the application of low frequency, low-amplitude mechanical stimulation for therapeutic purposes. The response of the equine musculoskeletal tissues to vibration stimuli is determined by the frequency, direction (vertical versus oscillatory), magnitude (displacement and acceleration), and the duration of therapy. As a result of the large number of vibration factor interactions whole body vibration (WBV) guidelines for enhancing musculoskeletal tissue responses has not been determined in the horse. Multiple studies conducted in rodent and ovine models of fracture repair demonstrate the diversified vibration protocols including variations in frequency, magnitude and duration. Both closed and open fracture repair models in rodents demonstrate that vertical vibration at a frequency of 45Hz negatively impairs fracture healing while vibration frequencies of 35 and 50 Hz enhance fracture healing. In contrast, horizontal oscillatory vibration therapy regardless of frequency demonstrates no positive or negative effects on fracture healing in rodent models.29 Peripheral blood flow and tissue oxygenation in people has been shown to be positively influenced by WBV.30 The mechanical oscillations transferred from the vibration plate to elderly individuals has improved postural control and thus has reduced the risk of falling within the elderly population.31-32 Furthermore, there is modest evidence that supports the use of vibration therapy to reduce pain perception in osteoarthritic individuals.33-34 Lastly, there is an emerging profile for application of vibration therapy as an exercise modality for people. Vibration training for human athletes appears to have a rapid and energy sparring warm-up effect and has demonstrated increases in jump height and muscle power.33-34
A translational comparison within the human literature to the horse becomes very difficult as a number of different WBV platforms are commercially available. Anecdotally, WBV has been applied to horses with various claims of effectiveness. Acute hematologic and clinical effects of horses undergoing alternating horizontal and vertical vibration therapy have been recently described, noting no adverse effects following vibration sessions exposed to a frequency of 15-21Hz.36 Within the rehabilitative setting, there has been recent interest in the effects of prolonged vibration therapy on the cross-sectional area and symmetry of the multifidus muscle. A significant increase in multifidus muscle cross-sectional size and symmetry was found following 60 days of twice daily, 30 minute, 40Hz vertical WBV sessions.37 Known for its role in spinal stabilization and postural muscle acuity, development of the multifidus muscle is thought to have potential as an osteoarthritis deterrent. Only one study has been conducted in the horse evaluating the effects of vibration therapy on bone density in stalled patients. Twelve horses were confined to stalls for 60 days with half the group exercised daily on a mechanical walker and the others underwent vertical WBV therapy at 50Hz for 45 minutes, 5 days a week.38 Whole body vibration therapy in stalled horses maintained the same bone mineral content to that of horses that received daily light exercise and therefore should be considered for horses restricted to stall rest only.38 The rate of hoof growth was measured in ten horses standing on a vertical displacing WBV at 40Hz, for 30 minutes, twice a day, five days a week for eight weeks. The rate of hoof growth did increase by 41% only in the first 30 days, no further increase was noted in the last 30 days. However, to date no direct comparison studies on vibration direction in the horse have been conducted.
Clinical Application
Whole body exercise appears to be a safe method for rehabilitation in the horse, but additional studies are needed to assess efficacy in equine patients for stimulating bone metabolism, soft tissue healing, proprioceptive awareness and motor control mechanisms responsible for joint stability and movement patterns. Manufacturer recommendations for treatment protocols typically involve twice daily 30-minute treatment sessions at varied frequencies. Still largely unknown to the equine community is the evidence-based support for the appropriately combined settings of frequency, direction (vertical versus oscillatory), magnitude (displacement and acceleration), and the duration of therapy for a specific musculoskeletal injury.
Conclusion
The focus of these notes is to provide an overview of different therapeutic modalities available for rehabilitation including the scientific basis, physiologic effects and indications. It is by no means an exhaustive list, but our hope is to provide sufficient information for practitioners to develop rehabilitation protocols specific to the patients’ specific injuries and needs.
References