Plantar fasciitis is the most common cause of heel pain in adults with a lifetime incidence of 10%. It accounts for approximately 1 million patient visits per year in the US, with most patients (60%) being seen initially in a primary care setting. Plantar fasciitis is associated with a variety of sports, being reported in recreational and elite runners with an incidence of 5-10% [14].
A variety of treatment options are available including physical therapy with stretching and strengthening, foot orthoses, heel padding, ice massage, NSAIDS, decreased weight bearing activity, taping, dry needling/acupuncture, night splints, extracorporeal shock wave therapy, steroid injections, and surgery. These approaches have been reviewed [1,3]. However, most treatment modalities are not supported by sufficient evidence to recommend one strategy over another [1,9]. The majority (over 75%) of patients with PF will improve with conservative non- operative treatment within 12 months [8], but during the interim patients often suffer from significant impairment of recreational and activities of daily living, as well as lost time from work [10].
The anti-inflammatory and anti-nociceptive actions of a variety of essential oils has been receiving increased scrutiny [15,16], including rosemary oil [17], clove oil [18], eucalyptus [19], tea tree oil [20], and peppermint [21] among others. The mechanism of action of a complex mixture of oils such as the current formulation is difficult to elucidate, however the three main anti-inflammation properties of essential oils include inhibition of arachidonic acid metabolism, cytokine production, and pro-inflammatory gene expression [11]. Camphor is known to desensitize the pain-mediating TRP1A receptor [33], eugenol (a small phenolic compound and not strictly a terpene) is best known for its original use in dentistry with local analgesic and antiseptic effects [34], 1,8-cineole from eucalyptus is a natural antagonist of human TRPA1 that has analgesic and anti-inflammatory effects possibly due to its inhibition of TRPA1 [35]. Menthol from wintergreen oil is known as a topical analgesic postulated to work in part as a counterirritant [36] and, more recently has been shown to inhibit the production of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) [37]. Vanillin (also a small phenolic compound) is the archetypic agonist for the TRPV (transient receptor potential channel subclass vanilloid) from which the classification derives its name, and which plays a central role in pain transduction [38]. Tea tree oil also has reported topical analgesic properties [39], although no effect was seen by itself in this study. Given the complexity of the current formulation it is not possible to determine a precise mechanism of action, nor if efficacy is due solely to the known active agents as discussed or to a complex synergy with other oil components. The simplicity of the current approach, however, is its novel application of a mixture of agents with an established history of safety to achieve a result superior to earlier combinations.
Terpenes are the main constituents of essential oils and consist of isoprene (C5H8) units. Terpenes of natural origin have a ‘Generally Regarded As Safe’ (GRAS) status with the Food and Drug Administration [22,23], confirming the topical safety of the oils in this current formulation [23] and making their use low-risk. The major constituents of the formulation used in this study include agents known to have topical therapeutic effects for pain relief as discussed, and in addition each of these agents is FDA approved for topical use OTC with long track records of safety.
It is likely that skin permeation enhancement by limonene [28,29], rosemary oil [30], and DMSO [31] contributed to the efficacy seen in this study. The clinical use of pharmaceutical-grade DMSO as a penetration enhancer is supported by the robust data that have accumulated over the past 3 decades demonstrating the favorable safety and tolerability profile as well as its anti-inflammatory actions [31]. Diclofenac in DMSO has recently been FDA approved for topical use for osteoarthritis with a gel containing 45% DMSO [32]. The concentration in the current formulation of 15% delivers approximately 300ul of DMSO per day when 1ml of the formula is applied twice daily, a very modest amount.
Limitations of this study include the lack of racial diversity (87% white), relatively small sample size, and lack of patient stratification based on activity level. However, given these limitations, the results appear generalizable and clinically applicable to large numbers of those who suffer from PF.
The need exists for a reliable, effective, and rapid treatment for plantar fasciitis. In this study we evaluated the efficacy of a solution containing essential oils with concentrations of known therapeutic agents for topical use, formulated with the skin permeation enhancers limonene and small amounts of DMSO in the treatment of plantar fasciitis. After 10 days of twice daily application 78% of patients had an 85% or greater reduction in their pain scores. No adverse events were reported.