Parameters of vibration stimulation for the relief of pain of different origins and locations

Introduction: The use of vibration stimulation in alleviating pain is an issue appearing more and more often in modern rehabilitation. Aim of study: The aim of the work was to review literature regarding the therapeutic usage of vibrations in analgesic activity. Material and methods: Searches based on the effect of alleviating pain with the help of vibrotherapy in the therapy of diseases of various origins and localizations, as well as with various physical therapy treatments and modalities. The most important parameters were determined, such as: frequency, amplitude, places of application, and exposure time, and were described on the basis of selected tests. Both the Polish and English-language literature has been discussed. Most of the studies were English-language (over 93%). The searches were carried out among the following databases: Medline, Embase, Cochrane, ScienceDirect, PubMed, IEEE Xplore, Wiley Online Library. The key words used were: pain, vibration, WBV, analgesics. The mentioned sources were qualified on the basis of: availability, data verification, selection of research material and regularity of update. Summary and conclusions: While acute pain is, by definition, a short and self-limiting process, chronic pain begins to dominate life and concerns the patient and his/her family. In addition to significant deterioration in quality of life, chronic pain imposes a large financial burden. The use of vibrotherapy can be a solution. Therefore, it is proposed to further analyse the parameters of vibrotherapy in analgesic activity, which may lay the foundation for “personalized pain relief medicine”, which will clearly define the standards of vibrational therapy parameters. Słowa kluczowe ból, wibracja, WBV, przeciwbólowy Streszczenie Wstęp: Wykorzystanie stymulacji drganiami w łagodzeniu dolegliwości bólowych różnego pochodzenia i lokalizacji jest zagadnieniem pojawiającym się coraz częściej we współczesnej rehabilitacji. Cel pracy: Celem pracy było dokonanie przeglądu piśmiennictwa, dotyczącego terapeutycznego wykorzystywania wibracji w działaniu przeciwbólowym. Materiał i metody: Poszukiwania oparto na wynikach, dotyczących łagodzenia bólu przy pomocy wibroterapii w terapii schorzeń różnego pochodzenia i lokalizacji oraz przy rozmaitych zabiegach fizykoterapeutycznych/leczniczych. Opisano najważniejsze parametry takie jak: częstotliwość, amplituda, miejsca aplikacji do ciała, czy czas ekspozycji, które zostały opisane na podstawie wybranych testów. Rozpatrywano piśmiennictwo w języku polskim i angielskim. Większość znalezionych opracowań stanowiły artykuły anglojęzyczne (ponad 93%). W celu zgromadzenia danych przeszukano następujące bazy danych: Medline, Embase, Cochrane, ScienceDirect, PubMed, IEEE Xplore, Wiley Online Library. Słowa kluczowe: ból, wibracja, WBV, przeciwbólowy. Wymienione źródła zostały zakwalifikowane na podstawie: dostępności, weryfikacji danych, wyboru materiału badawczego i systematyczności aktualizacji. The individual division of this paper was as follows: a – research work project; B – data collection; C – statistical analysis; D – data interpretation; E – manuscript compilation; F – publication search Article received: 2 May. 2017; Accepted: 2 Oct. 2018 Please cite as: Pasterczyk-Szczurek A., Pogwizd P., Bigosińska M. Parameters of vibration stimulation for the relief of pain of different origins and locations. Med Rehabil 2018; 22(2): 20-29. DOI: 10.5604/01.3001.0012.6875 Internet version (original): www.rehmed.pl Medical Rehabilitation (Med Rehabil) 2018, 22 (2): 20-29 DOI: 10.5604/01.3001.0012.6875 eISSN 1896–3250 © AWF Krakow 21 Podsumowanie i wnioski: Podczas, gdy ból ostry jest z definicji krótkim i samo ograniczającym się procesem, ból przewlekły zaczyna dominować w życiu wielu chorych oraz ich rodzin. Oprócz znacznego pogorszenia jakości życia, ból przewlekły narzuca duże obciążenia finansowe. Wykorzystanie stymulacji drganiami może stanowić rozwiązanie. Dlatego proponuje się dalsze analizy parametrów wibroterapii w działaniu przeciwbólowym, które mogą stworzyć podwaliny do „spersonalizowanej medycyny łagodzenia bólu”, która jasno określi standardy parametrów terapii wibracyjnej.


INTRODUCTION
The year 2018 was defined by the International Association for the Study of Pain (IASP) as a year of global improvement in pain education. The society is an organization that deals with the scientific, practical and educational aspects of the pain problem, which proposed one of the most frequently cited definitions of pain. According to IASP, pain is a: subjectively unpleasant and negative sensory and emotional sensation arising under the influence of stimuli, damaging the tissue or threatening damage. The feeling of pain should primarily be perceived as a system that notifies us of irregularities and allows the creation of appropriate forms of behaviour. Nevertheless, pain is a subjective, often immeasurable and incomparable sensation. Many ill people are unable to live with it, and that is why it is everything and anything the patient perceives this way, regardless of objective symptoms associated with it 1 .
Mechanisms of muscle pain, such as e.g. a stiff neck or lumbago, largely remain a mystery, although this type of pain is one of the key reasons for anxiety of patients, clinic visitors and hospitals with different profiles. Musculoskeletal pain is so common that some types are considered a normal part of life. Spinal pain syndromes are a serious problem due to their frequent occurrence, as well as costs incurred by the health service for hospitalization of patients and are the main cause of incapacity to work (frequent absenteeism). Many published studies show that up to 39.5% of adolescents suffer from low back pain, and more and more people above the age of 30 suffer from pain in the course of discopathy of a different location 2 . In many cases, it is associated with great discomfort and a feeling of pain covering the whole body. According to Debono et al. 3 , in an EBM review (evidence-based medicine), pain is de-fined as the most common indication for medical consultation. This is confirmed, among others, by the Eurostat survey from 2017, showing that almost half of the European population suffer from pain of various localization, origin and clinical expression almost every day 4 .
Often, contra-stimulation treatments are used in widely understood physiotherapy. We use them unconsciously, for example, when we hit our hand, we wave it to relieve pain, or tap a place other than where the pain occurs. And on the basis of similar experiments it was found possible to eliminate pain discomfort by means of contra-stimulation in the form of mechanical vibration 5 . One of the first to study this issue were Gammon and Starr from the University of Pennsylvania in Philadelphia 5 . The authors used stimulus at frequencies from 50 to 60 Hz, because on the basis of their patients' opinions, they observed that this frequency was the most effective in analgesic activity. They also noted that the best results are obtained if the vibratory stimulus is used on the exact site of pain 5 . Another publication that described pain relief using vibrations is an article published in "Perceptual and Motor Skills" during the mid-1960s by Blitz et al. 6 . The authors of this report showed that vibration stimulation significantly increases pain threshold and, as a result, suppresses the pain itself. Exactly 20 years later, i.e. in 1984, Lundeberg et al. published the results of research on relieving pain with vibration stimulation.
Today, we already know that vibration with strictly defined parameters activates nerve fibers with large diameters, interacting with the impulses transmitted on pain pathways, which leads to alleviation of pain 8 . Following Moayedi and Davis 9 , in 1965, Ronald Melzack presented a theory called the gate theory of pain. It assumes that if there is another stronger impulse on the path of the pain pulse, the first one may not reach the brain and be omitted. This theory can be explained by the reduction in pain when massaging one place of the body and resolving it in another (trigger pain). In 2014, Imtiyaz et al. 10 proved that vibration stimulates muscle spindles and increases their afferent functions. Both vibration and TENS (transcutaneous electrical nerve stimulation) reduce the perception of pain via the mechanism referred to in the gate theory of pain. This leads to increased background activity and synchronization of motor system functions in muscles subjected to vibrations 10 . Vibration stimulates skin-surface as well as deep sensation mechanoreceptors that are sensitive to vibrational stimulation. Lamellar receptors highly sensitive to vibrations include lamellar bodies and annulo-spiral ring ends of muscle spindles. The same authors believe that pain relief is the result of the activation of lamellar corpuscles in the connective tissue, ligaments or joints, as well as the endings of the annulo-spiral ring spindles of the muscles. Activation of other receptors in the skin, subcutaneous tissues and bones can also affect the feeling of pain relief 10 . Another consequence of the use of vibration stimulation is the weakening of motor neuron excitability, which innervate the antagonistic muscle by way of retardation. This phenomenon may explain the reduction of pain in patients undergoing vibrational therapy in the area of the antagonistic muscles. Interestingly, regular vibration stimulation lasting from 30 to 45 minutes may cause redness of the skin and a feeling of warmth in the area of its application. Therefore, it cannot be ruled out that the effect of pain relief may be partly due to an autonomic reaction 8 . Numerous studies also indicate that vibration stimulates both superficial-ly and deeply located receptors. Subsequent activity in myelinated sensory axons may interact with nociceptive processing at various levels of the nervous system, including the spinal cord. One of the effects of this state is prolonged increase in pain threshold, which causes it to be less experienced 11 . Vibrations are a non-pharmacological therapeutic technique used to relieve pain by triggering superficial and deep mechanoreceptors. Vibrational stimuli are transmitted through the Meissner body, which is sensitive to a frequency of about 40 Hz, and through the Vater-Pacini body at above 100 Hz. It was also found that high vibration frequencies ( 100 Hz) are more effective than lower ones, which leads to the suggestion that afferents of the Vater-Pacini body play a special role in the modulation of pain. Among others, Bushnell and Casey 12 , Pertovaara 13 and Mendes et al. 14 suggest that a key role in the so-called vibrational elimination of pain is played by the Vatera-Pacini body, and this particular possibility of modulation explains the greater efficiency of high frequencies.
Alsuhibni et al. 15 and Hollins et al. 16 decided to explore this possibility. They used high-frequency vibrations on the face and lips, where it is impossible to prove the existence of afferents of Vater-Pacini bodies, as well as the functioning of the Vater-Pacini body canal. In spite of the absence of Vater-Pacini bodies, high frequency vibration stimulation was still effective in reducing pain 15,16 . Therefore, it can be assumed that the Vater-Pacini body canal does not play a significant role in the vibration mechanism of pain relief, at least not in the area of the face. The most probable explanation for greater effectiveness of high frequencies is the intensity of stimulus sufficient enough to engage mechanoreceptive afferents, where the dependence of discharges of those afferents increases directly with the function of the vibration-touch frequency. Given the above, more continuous and greater activity can be triggered to a greater extent by high frequency vibrations than by those which are lower. Thanks to this, they interfere more effectively with nociceptive sig-nals within the Central Nervous System (CNS) 17 . Under the influence of rhythmic shocks, the excitability of pain receptors increases while the excitability of proprioceptors reduces. The reflex route is affected by changes in the nervous system and increased vascular flow 18 . Vibration therapy can improve the functioning of the musculoskeletal system. Vibration stimuli cause increased neuronal discharges, initiating an increase in the excitability of neuromuscular spindles, which leads to elevated muscle fibre activity, providing improved muscle strength and increased range of motion, while abating painful pain sensations 14 . Hollins et al. 19 believe that vibrations primarily distract attention from the pain caused by the detrimental stimulus.

METHODOLOGY
In order to collect data for the review, it was decided to conduct searches among the largest databases indicated in medical directives for clinical evaluation, such as: Medline, Embase and PubMed. At the same time, databases such as Cochrane, Science-Direct, IEEE Xplore and Wiley Online Library were also searched. The search process was carried out by one author during the period from 15 th to 20 th April, 2018. The following keywords were used: pain, vibration, WBV, analgesics, and their combinations. The first stage of the search consisted in assessing the relevance of the results in terms of their compliance with the topic. 89 titles were qualified for the next stage, in which subjects were exposed to therapeutic vibrations with a significant analgesic effect. The types of tests qualified for review were the following: prospective clinical trials, pilot studies, preliminary reports, case studies, meta-analyses and reviews. The next step was to assess the content value according to the inclusion criteria in line with the PICO standard. Individuals, regardless of race, sex, age and diseases, were indicated as the population. Interventions, i.e. treatments using mechanical vibrations, were determined by the authors as thera-peutic -not as a part of the vibration sensation test. The control group was assumed as acceptable at the level of all types of controls, including other therapeutic procedures. In the results, the trials for which the endpoints indicated an effective and safe analgesic effect of vibration were selected. Those with incomplete data on the intervention or study population were rejected. The length of the follow-up period was not taken into account due to insufficient data. The results had to be characterized by access to full texts. If possible, the authors were contacted in order to obtain information on the full content of the publication. The main aim was to show the characteristics of the most important parameters regarding vibrational therapy stimulus, effective and safe in reducing pain. The literature was considered in Polish and English. No weight values were assigned to individual items due to the descriptive nature of the work.

RESULTS
After initial evaluation of the content of the articles, 89 publications were identified. 55 positions were indicated as irrelevant. According to the inclusion criteria, 30 tests were identified that met the criteria of this review and 4 were used to describe the general technical state. The subjects were adults aged 18-65, complaining of pain with various etiologies. The most frequently assessed interventions concern local vibrations with a probe or using vibrating surfaces such as a cushion or blanket. All the individuals tolerated vibrational therapy well. No study provided information about adverse events. Based on 43 publications, the 4 most important parameters of vibro-therapy are described: frequency, amplitude, area of application and duration of treatment. The oldest study was from 1941, and the most current, 2018. The results of the review are described below. Each study was aimed at assigning one of the categories of stimulus characteristics to which it was most appropriate and with a unique parameter value. Only two literature items were simultaneously classified into two groups. Most often, by providing information about the stimulus, the authors describe frequency. Amplitude is a difficult parameter to determine, especially in local therapy. The authors of only 3 of the qualified studies provided this data. The location of stimulus application was determined in most of the studies. 9 such positions were qualified. The duration of treatments was evaluated on the basis of 6 publications.
Frequency is the most important parameter described during treatments using vibration stimulation. Vibrotherapy in analgesic activity most often uses vibrations in the form of constant stimulus, unchanged over time. The range used is wide − from 0.5 Hz to even 18 kHz, using local vibration. The frequency most commonly applied in clinical practice is within the range of 20 Hz to 100 Hz. The study by Kessler and Hong 20 confirms the effective application of the lower vibration limit in pain reduction. These authors have shown that the levelling of pain after applying the vibration continues directly after each session for an average of 3 hours. They used a vibrating platform that works on the basis of socalled whole-body vibration (WBV). In addition, the results of this study confirm the results of previous deliberations, indicating the effectiveness of whole-body vibration as potential treatment for diabetic peripheral neuropathy 20 . Mechanical stimulation is also used as a local anaesthetic during cosmetic or dental procedures. In those cases, vibrations with frequencies up to 100 Hz are usually used. Vibration anaesthesia can potentially help in a variety of procedures, including Botulin Type A injections in patients treated for excessive sweating, subcutaneous introduction of so-called filling substances or injection with steroids, laser therapy of the varicose veins, laser ablation of tattoos, epidermal needle excision, patients afraid of subcutaneous and intramuscular injections, incisions and thermoablation of warts on the face [21][22][23][24] . Topical vibrations at 60 Hz turned out to be an effec-tive manner of pain reduction during scar treatment with a CO 2 fractional laser. As it can be seen from the research by Song et al. 25 , the average VAS score (visual analogue pain scale) before application of vibration was 6.11 points, while after using vibration, it was 4.6 points 25 . Biomechanical changes that arise after surgical resection of breast cancer increase the need for new rehabilitation programmes. The aim of the Mendes et al. 14 study was to assess the mean and long-term effects of vibration therapy on the intensity of pain, range of motion, myoelectrical effect and muscle strength after oncological surgery. This time, it was proved that vibration therapy up to 40 Hz was effective in reducing the intensity of pain in women after breast resection surgery with a simultaneous increase in the range of arm movement. Using a vibratory stimulus on the postoperative site leads to an increase in muscle fibre activity, providing improved muscle strength, increased range of movement and reduction in pain 14 . Many authors have shown that vibrations alleviate muscle soreness after damage due to intense exercise. The frequency range used in these procedures is between 12 and 120 Hz. Reduction of pain after exercise especially important in novice athletes − is not negligible. Among others, it is necessary to create quick possibility for further exercises, facilitating the return to proper training, increasing their effectiveness and reducing the risk of injury [27][28][29][30][31] .
The applied amplitudes range from a hundredth of a millimeter to even 8 mm. The most sensitive ends of the main muscle spindles could be activated at a vibration amplitude of <10 µm. As it can be seen, the range of parameters is very vast, and both micro amplitudes and higher ones are successfully used in pain reduction therapy with a similar effect. Maddalozzo et al. 6 used amplitudes ranging from 0.6 mm to 1.2 mm at a frequency of 20 Hz to 50 Hz in the treatment of non-specific chronic back pain in the lower spinal segment. In the group of patients who underwent vibrations, better clinical results were achieved in the area of the NRS (numeric rat-ing scale) and disability assessment, which is caused by pain in the lumbar region of the spine (ODI − Oswestry Disability Index) 26 . However, the use of a 3 mm amplitude at 12.5 Hz allowed to improve the quality of life indices in women with fibromyalgia, previously unable to perform physical efforts 32 . Amplitudes within the range of 0.0001 to 0.05 mm and frequencies up to 18,000 Hz are used, inter alia, in patients suffering from osteoarthritis of the knee joints 33 . Based on the reported results, it should be assumed that various amplitude ranges are used to treat various types of disorders using vibrations.
Vibrotherap y is used, inter alia, to reduce the sensation of muscular-skeletal, chronic and neuropathic pain. Among others, Gammon and Starr 5 and Naro et al. 34 argue that it works best when used on the painsite or within its immediate vicinity. In patients with pain in the cervical spondylosis or lumbar-sacral spine (LS), local application of vibrations reduced pain not only in this place, but also in other, distant locations. This position is shared, among others, by Lundeberg et al. 7 and Dahlin et al. 11 . The authors believe that the best method for pain reduction is the usage of vibrations directly on its location, but on remote ones as well. Dahlin et al. 11 also suggest that a beneficial analgesic effect is achieved by using vibration at a height of no more than two segments of the spine lower than its location or in the same dermatome on the opposite side. Vibration stimulates both superficial and deeply located receptors. Subsequent afferent activity in myelinated sensory axons may interact with nociceptive processing at various levels of the nervous system, including the spinal cord. One of the effects is a prolonged increase in pain threshold 11 . Ruan et al. 35 demonstrated that after 3 months of using vibro-therapy, there was a definite reduction in chronic back pain. Similar conclusions were drawn, inter alia, by Boucher et al. 36 . The authors' research shows that local muscle vibrations lead to significant improvement in the neuromuscular control of the trunk in these patients. Mus- Table 1 The results qualified into particular categories (frequency, amplitude, place of stimulus application, time)  Local muscle vibrations led to significant improvement in neuromuscular control of the trunk in patients with chronic back pain in the lumbar region before and after the muscle fatigue protocol.
Prospective clinical trial with control group P None Vibrational therapy has the potential to rehabilitate patients who report acute inflammation of soft tissues and injuries resulting from non-use or immobilization of the organ. After three weeks of treatment (3 / week), the symptoms decreased. The patient no longer reported pain during any daily or resting activities, and the VAS score decreased by 50% (6 / 10-3 / 10). The patient no longer complained of weakness or feelings of instability in the elbow.

None
It was found that after applying a series of massages, the average level of pain was statistically significantly reduced (2.8 -which means "moderate pain") in relation to the level before therapy (4.6which means "severe pain") assessed subjectively on the 11-point VAS, where "0" means "no pain" and "10": "unbearable pain".     33 and Beinert et al. 38 , reduced the joint pain and cervical spondylosis, which have a positive effect on mobility of the arms. Exposition to vibration proposed by many authors ranges from 80 seconds to even 60 minutes. It can consist of several, 1-minute stimulations with intervals or 1 continuous session. This is confirmed, among others, by Chochowska et al. 39 , who, in the years 2009-2011 subjected patients with chronic spinal pain to vibrations under outpatient conditions, and compared the results with those of classical massage. According to the authors of this report, the use of vi-brations in the amount of 10 sessions, lasting 12 minutes each, provides better results compared to those obtained in patients who underwent classical massage. Alleviation of pain in the group of patients undergoing vibrations with the above-mentioned characteristics took place earlier than in the case of patients from the second group 39 . Similar observations are reported by, among others, Stamboliev et al. 40 , using this method in patients diagnosed with diabetic neuropathy. After 8 weeks, they noted significant improvement in postural stability, reduced paraesthesia of the limbs and in pain 40 . In the study by Mendes et al. 14 , applying a vibration blanket for 10 days, in sessions lasting 15 minutes each, caused a long-lasting decrease in pain, an increase in muscle strength and mobility of the glenohumeral joint. Bily et al. 41 used vibrations in patients following total knee arthroplasty (TKA) in 12 group sessions, which resulted in significant pain reduction. Yang and Seo 42 , however, applying vibrations 3 times a week for 6 weeks in sessions lasting 5 minutes each among patients suffering from chronic back pain (spinal), noted a reduction in pain, especially in the distal part 42 . Therefore, it is difficult to adopt a unified duration of applying vibrations to overcome pain-related problems of various origins and locations, which seems to require further observation and research 43 . Table 1 presents detailed results of tests selected for the review (Table 1).

SUMMARY AND CONCLUSIONS
Living with specific pain is primarily a struggle with disease, but also with oneself, based on continuous compromises. Often, it causes the necessity to give up professional dreams and ambitions, as well as being a threat of elimination from social activity or any other activity. This struggle is accompanied by support from analgesic pharmacology, surgical treatment and broadly understood physiotherapy. We are convinced that in this area, there is also room for vibro-therapy. The use of vibrations in analgesic therapy is an extremely interesting and still developing subject. However, it should be borne in mind that only vibration with properly correlated parameters will ensure the safety of patients and its effectiveness. Therefore, it is proposed to create standardized prognostic methods that would take many factors into account directly. By creating "personalized pain relief medicine" (PPRM) identifying a unique patient profile, one could maximize the potential for beneficial responses to treatment using vibration. This provides much room for more insightful research, especially since many vibration functions in reducing pain remain undiscovered. It is necessary to investigate which vibration parameters best relieve pain of a given type for a given patient's characteristics, in addition to developing the most optimal application method, position and duration of exposure. In addition, many studies lack information on the duration of pain relief after using vibro-therapy. This makes it difficult to develop a potential schedule or stimulus dose that could be tailored to the specificity of pain. What also needs to be taken into account is that vibration can be harmful or an initiator of pain. There is a modulation of the response direction to the stimulus from hypoalgesia to hyperalgesia. This change suggests that at a neuronal level, there is the possibility of switching from inhibition to excitation when the signals become strong enough. Therefore, it is very important that the stimulus parameters in vibrational therapy be thoroughly tested and controlled. Despite the fact that the pain complaints analysed in the cited scientific publications were characterized by different location and etiology, and also related to different populations, they were joined by a common element, which is the use of vibrations in their treatment. It is difficult not to notice the clear and beneficial effect among patients with the presented ailments.
At this point, it should be recalled that the authors of the presented reports determine and characterize the vibration parameters in the research criteria, the duration of exposure and location, as well as the frequency of treatments and the number of sessions. The vibrational devices used to treat these patients had different technological characteristics ( Table 2).
The subject of applying vibration in the treatment of certain diseases certainly requires further research, but it is difficult to resist the belief that it is definitely justified.