We now use castor oil, rather than gel or petroleum jelly as the medium between the shockwave equipment and the patient for many of our shockwave procedures. We have found that the little pain that our patients feel is reduced when we use castor oil and we believe that the shockwaves reach the areas needing treatment more effectively.
The positive effect of using castor oil can be explained by its cavitation-free quality, i.e. the absence of bubbles in pure castor oil allows the shockwave equipment to work more effectively and not have to disperse tiny air bubbles in gels or jelly before it gets to work.
The research that introduced us to this change in procedure is as follows:
In a prospective single-blind study the contact media ultrasound gel, Vaseline and castor oil were examined for their effect on surface pain caused by
extracorporeal shock waves used for tendinosis calcarea (n = 25), radiohumeral epicondylitis (n = 23) and plantar heel spur (n = 12). A total of 60 patients was divided into six groups. Using a Compact S shockwave source (Dornier MedTech), an energy flux density up to 0.12 mJ/mm2 was applied three times within 3 weeks. Independent of the diagnosis, there was a statistically significant influence of the contact medium on the intensity of application pain. In this comparison castor oil was best. For the diagnosis of tendinosis calcarea and plantar heel spur, castor oil was significantly better than the other two contact media, while for epicondylitis there was no significant difference. Castor oil was found to have an advantage over ultrasound jelly and Vaseline in all indications used with regard to application pain. The positive effect of castor oil can be explained by its cavitation-free quality. 
Castor oil decreases pain during extracorporeal shock wave application.