Radial extracorporeal shockwave therapy (rESWT) for coccydynia: a prospective study of 14 patients

Background: Coccydynia is defined as pain in the coccyx. We investigated the effect of radial extracorporeal shockwave therapy (rESWT) in the management of coccydynia. Methods: In this prospective study, patients (≥18 years) diagnosed with coccydynia at a sports clinic located in Thessaloniki, Greece, were eligible for rESWT treatment when they reported a visual analogue scale (VAS) pain level ≥6. Treatment sessions were once weekly and ended when VAS pain levels decreased to ≤3. Recurrence rates were documented at 3-month and 12-month follow-ups. Results: Fourteen patients were treated using rESWT. The mean age and symptom duration of our cohort was 33.6±7.9 (range: 20–45) years and 9.4±8.5 (range: 3–36) months, respectively. The mean number of treatment sessions per patient was 6.4±1.6 (range: 4–8). The mean device pressure, frequency, and number of pulses was 1.2±0.1 (range: 1–1.4) bar, 5.0±0.1 (range: 5–6) Hz, and 2082±74.8 (range: 2000–2300) pulses, respectively. Treatment alleviated pain in all patients, and no recurrence of symptoms was reported during follow-up. There was a positive correlation between symptom duration and the number of treatment sessions (r=0.701, P=0.005). Pairwise comparison highlighted significant reductions in VAS pain levels between each stage of treatment (P<0.001). Conclusion: Our study affirms the safety and efficacy of rESWT in managing coccydynia.


Introduction
Coccydynia, or coccygodynia, refers to pain in the coccyx (tailbone).The coccyx is a triangular-shaped bone in the lowermost region of the spine and is composed of three to five individual segments [1] .It serves as the attachment site for several pelvic structures and possesses an extensive nerve supply [2] .Coccydynia is five times more common in women than men, although its exact prevalence is unclear [3,4] .Factors associated with the development of coccydynia include obesity, prolonged sitting, and childbirth [1,4] .Degenerative joint disease is also thought to increase risk [1] .The most common presentation is following a traumatic event, typically a backwards fall [1] .
Most cases of coccydynia can be managed conservatively with rest, anti-inflammatory medications, seating aids, and warm baths [3] .Physiotherapy and manipulative therapy are also popular treatment options [5] .Steroid and anaesthetic injections may be useful in patients who do not respond to these methods, but there is controversy surrounding the optimal site for injection [1] .Surgical coccygectomy is performed in treatment-resistant cases, and whilst outcomes are generally good-to-excellent, the incision is made close to the anal canal and can make subsequent care challenging [6,7] .
Extracorporeal shockwave therapy (ESWT) is a physical agent modality that offers an alternative and non-invasive means of treatment for coccydynia [8] .Devices are placed against the skin and deliver impulses of energy to injured tissue at a given pressure (bar) and frequency (Hz) [9][10][11] .The exact mechanism of action of ESWT remains unknown; however, biological responses are proposed to be mediated by mechanotransduction and include tissue regeneration,

HIGHLIGHTS
• Radial extracorporeal shockwave therapy (rESWT) offers an alternative and non-invasive means of treatment for coccydynia.wound healing, angiogenesis, and neovascularisation [12][13][14][15] .Altogether, this can result in significant pain alleviation [12] .There are two types of ESWT: focused (fESWT) and radial (rESWT).These differ marginally in terms of shockwave production and energy delivery [8] .In practice, rESWT devices are smaller, cheaper, and easier to use [8] .Moreover, rESWT has proven to be successful in the management of musculoskeletal disorders such as plantar fasciitis [16][17][18] , calcific shoulder tendinopathy [19,20] , trigger digit [21] , sesamoid osteonecrosis [22] , patellar tendinopathy [23] , medial tibial stress syndrome [24][25][26] , and lateral epicondylitis [27,28] .Previous literature has highlighted the benefit of rESWT for coccydynia [29][30][31][32] .One study found rESWT to be more effective than steroid injections for pain relief over 6 months of observation [32] .A recent systematic review reported that shockwave therapy offered the second-greatest improvements in pain among all available treatment options [33] .However, research on the subject remains limited.Furthermore, most of the available studies adopt a 'one size fits all' approach by treating patients at a single dose over a predetermined number of sessions.The research objectives of this study were: (i) determine whether a pain-adapted protocol of rESWT is effective in treating coccydynia, (ii) assess for a relationship between treatment duration and symptom intensity, and (iii) monitor patients for possible adverse effects of treatment.

Methods
This prospective study was reported in line with the PROCESS (Preferred Reporting Of CasE Series in Surgery) Guideline [34] , Supplemental Digital Content 1, http://links.lww.com/MS9/A206.

Subjects
Patients presenting to a sports clinic located in Thessaloniki, Greece, between January 2017 and December 2020 were diagnosed with coccydynia by a consultant sports physician (N.M.) following focused clinical history, examination, and imaging.Physical examination included inspection of the overlying skin and palpation, which revealed localised focal tenderness in the region of the coccyx [3,35] .Anteroposterior and lateral radiographs of the lumbar spine and coccyx were obtained to assess the position of the coccyx and rule out coccygeal dislocations.Magnetic resonance imaging scans were also performed to exclude other pathologies of the lumbar spinal cord, coccyx, and associated soft tissue.Treatment options, including the use of rESWT, were discussed with patients if they reported a visual analogue scale (VAS) pain level ≥ 6. Patients aged ≥ 18 years were eligible for the study.Exclusion criteria were: local or systemic neurological disease, rheumatological disease, malignant disease, previous spinal surgery or spinal disease, coagulopathies, pregnancy, recent ( ≤ 6 weeks) trauma to the coccyx, and coccygeal dislocations.

Equipment and procedure
Treatment was administered to the maximal point of tenderness exactly over and posterior to the coccyx by the same consultant sports physician, using the Storz Medical Masterpuls MP200 (Storz Medical; Tägerwilen, Switzerland) rESWT device [29,36] .All treatments were performed after verbal and written informed consent had been obtained.The probe was placed on the skin at 90°to the coccyx in the sagittal plane [29] .Ultrasound gel was applied to the skin before treatment to reduce energy loss from the device.Local anaesthesia is known to lower the efficacy of treatment and was therefore not used [37] .Treatment sessions took place once a week to allow for adequate periods of recovery and healing for at least three sessions.Any ongoing treatments for coccydynia were stopped, and patients were advised against the use of non-steroidal anti-inflammatory drugs.
The rESWT device was set to a minimum pressure and frequency of 1 bar and 5 Hz, respectively.Each session included the delivery of at least 2000 pulses.These parameters were set based on the experience of our authors with previous cases of coccydynia and mirror those utilised in similar rESWT research [16,19,21,22] .Pressure, frequency, and the number of pulses were adjusted between sessions in line with patients' pain tolerance.Pain tolerance is the duration of time until an individual can no longer withstand an unpleasant stimulus and reflects the greatest level of discomfort they are willing to endure [38,39] .Tolerance is of particular value in coccydynia, given that pain is the most common presenting symptom and its management is the primary goal of treatment.
VAS pain levels were used to monitor patient response and clinical improvement throughout treatment, with patients ranking the severity of their pain at rest between 'no pain' (0) and 'worst imaginable pain' (10) [40,41] .Patients were asked their pain level before treatment at each session, although this did not form part of our formal data collection.Treatment continued until patients reported a VAS pain level ≤ 3, at which point they were discharged from the clinic [42] .

Study outcomes
The main outcome measure of this study was pain management.Each patient provided a baseline VAS pain level based on their initial symptomology.Follow-up measurements were taken immediately after the cessation of treatment, at 3 months posttreatment, and at 12 months posttreatment.Symptom resolution and disease recurrence were monitored during this period.The mean device pressure and frequency, number of pulses, and number of rESWT sessions were also documented using electronic device records.

Statistical analysis
Descriptive statistical analysis was carried out using Microsoft Excel (Microsoft Corporation, Redmond, Washington, USA), IBM SPSS Statistics Version 27 (IBM Corporation, Armonk, New York, USA), and Jamovi (Version 2.3, Sydney, Australia).The variables analysed did not meet the normality assumption, which was performed using the Shapiro-Wilk test (P < 0.001).The repeated measures ANOVA (analysis of variance) procedure (Friedman) was used to test for significant reductions in VAS pain levels.Pairwise comparisons using Durbin-Conover measured differences in VAS at each stage of follow-up when compared with baseline scores.Spearman's rank correlation coefficient was used to assess the correlation between variables.A predefined confidence level of 95% (α = 0.05) was set for all statistical analyses.
The nonparametric Friedman test of differences rendered a statistically significant chi-square (χ 2 ) value of 38.7 (P < 0.001).The pairwise comparison revealed significant reductions in VAS pain levels between each stage of treatment (P < 0.001) (Table 3).No recurrence of symptoms was reported during the follow-up period.

Discussion
Our study recorded high success rates when treating coccydynia with pain-adapted protocols using rESWT.Mean VAS pain level reductions of 78%, 89%, and 96% were recorded immediately posttreatment, at 3-month, and 12-month follow-up, respectively, compared to baseline.Patients with longer-standing symptoms at presentation generally required more treatment sessions.There was no symptom recurrence reported during the follow-up.
The value of rESWT for coccydynia has been highlighted in a handful of studies [29][30][31][32] .However, across these studies, rESWT device pressure (2-4 bar), frequency (5-21 Hz), number of pulses (2000-3000 pulses), and number of sessions ranged considerably.To our knowledge, this study is the first to deliver a pain-adapted protocol of rESWT.We achieved this by (i) adjusting rESWT device parameters in line with patients' pain tolerance and (ii) continuing treatment until a VAS pain level ≤ 3 was reported.We believe that continuing treatment until a low level of pain is achieved is relevant to the favourable outcome of this study.
Treating patients according to pain tolerance is common with the use of shockwave therapy.Numerous regimes in a 2013 systematic review described making dose alterations 'as tolerated' when treating soft tissue conditions [43] .To illustrate, if a patient found the same dose of treatment to be less painful at a later session compared to an earlier session, then device parameters may be increased.Alternatively, if a patient finds treatment uncomfortable at a particular dose, then device parameters may be lowered.A 2007 study also observed the greatest reductions in pain when patients with chronic heel pain were treated with 'maximum tolerable' energy densities, compared to a fixed protocol [44] .
The mean device pressure in this study increased by a maximum of 0.2 bar (1.1-1.3 bar).Whilst this may be an arguably insignificant change, it is worth noting the positive outcomes observed at lower overall device pressures [19] .Higher device pressures are said to increase pain; therefore, our approach may also make treatment more comfortable for patients [45] .The mean device frequency increased by a maximum of 0.2 Hz (5-5.2Hz), and the mean number of pulses per session increased by a maximum of 8.95% (2000-2179 pulses).An increased number of larger cavitation bubblesand resultant shockwavesare generated at lower frequencies, which may explain the findings of one study, where the device frequency was set to 21 Hz, and increases in pain were reported at the 7-month follow-up [30,46] .In contrast, no symptom recurrence was present in our study.The number of pulses per session increased only between sessions numbers two and six, and with careful consideration of energy accumulation in those who had more than six sessions.
Patients were discharged from the clinic once their VAS pain level reached ≤ 3.This meant that they all underwent at least four, and no more than eight, treatment sessions.In contrast, previous studies administered treatment over a predetermined number of sessions [30,31] .Given the symptom control and lack of recurrence in our cohort, we may argue for the number of treatment sessions to be dictated by patients' pain levels, as this appears to be the most important indicator of successful treatment.Pain levels after treatment and during follow-up were also lower in our cohort compared to others.Yet, inadequate pain relief among patients was observed in another study that continued rESWT until VAS pain levels were ≤ 3 [29] .Equally, we acknowledge that complete symptom resolution (VAS = 0) was not achieved among our entire sample, and, in fact, no patient was completely pain-free immediately posttreatment (VAS 1-3).While the long-term effect of rESWT can be observed during follow-up, four patients remained symptomatic (VAS = 1) at 12 months.This suggests that while treatment may have been successful, it did not always eradicate pain.A lengthier follow-up period may have highlighted further decreases or recurrences in VAS; however, 12 months can be considered adequate, given the nature and conditions of this study.
Our analyses found a strong positive correlation between symptom duration and the number of treatment sessions required (r = 0.701).There were seven patients who underwent either seven or eight sessions, all of whom reported pain for more than 6 months.However, symptom duration did not appear to be reflected in pretreatment VAS.The remaining patients, whose symptomologies ranged from 3-6 months, required no more than six sessions.This suggests that patients with chronic symptoms ( > 6 months) at presentation are likely to require more rESWT sessions.
Despite its poorly understood mechanism of action, rESWT continues to serve as a safe and effective management option for coccydynia [47,48] .Based on our findings, the following treatment protocol may be proposed: 1.2 bar, 5 Hz, and 2000 pulses per session until patients report a VAS pain level ≤ 3.There appears to be value in adjusting treatment doses according to pain tolerance, both in optimising long-term pain management and minimising the risk of recurrence.This approach may also make rESWT more comfortable for patients.However, our research was not comparative; therefore, only tentative conclusions can be drawn.There is a need for randomised controlled trials (RCTs) comparing rESWT with other management options for coccydynia and comparing different rESWT protocols.

Limitations
We acknowledge some limitations of this study.Firstly, the number of patients included was small because coccydynia is a condition of low incidence.Secondly, our analysis must be interpreted with some degree of caution.Although a single modality was delivered, we cannot be certain that there were no alternative explanations for the effects observed.Additionally, VAS pain levels during treatment were not formally recorded.VAS was also the only measure used to assess patients' pain tolerance.Employing a second tool to confirm the decrement of intensity may have strengthened our conclusions regarding the efficacy of rESWT.Other outcomes, such as quality of life and disability, have featured in similar research and assessed using scales such as SF-36 and Oswestry Disability Index, respectively.

Conclusion
We had identified a high success rate and no recurrence of disease at 12-month follow-up when coccydynia patients were treated with a pain-adapted protocol using rESWT.Our results align with the current evidence base recognising rESWT as a conservative management option for coccydynia and also suggest that adjusting the number of treatment sessions and doses during treatment in line with patients' pain tolerance may optimise pain management and lower the risk of recurrence.This may also make treatment more comfortable for patients.More RCTs are needed to compare the effect of rESWT on other surgical and non-surgical treatment modalities.

Table 1
Patient characteristics.

Table 2
rESWT device parameters during treatment.
Figure 1.Box plot displaying median, quartile, minimum, and maximum VAS ratings at each stage of treatment.VAS, visual analogue scale.