Dry needling (Myofacial Trigger Point Dry Needling) is the use of either solid filiform needles (also referred to as acupuncture needles) or hollow-core hypodermic needles for therapy of muscle pain, including pain related to myofascial pain syndrome. Dry needling is sometimes also known as intramuscular stimulation (IMS). Acupuncture and dry needling techniques are at times identical, depending on the style of practice of the practitioner. Chinese style tendinomuscular acupuncture relies on careful palpation of what are called "Ah Shi" points, which often correspond to both trigger points and/or motor points in the myofascial tissue. Chinese acupuncture tends to use the lower gauge needles neccessary for puncturing contraction knots with a high degree of precision. On the other hand, lighter styles of acupuncture, such as Japanese style, or many American styles, require very shallow insertions of higher gauge needles, as well as a detailed knowledge, not of western anatomy, but of the channel networks and connections. Thus, while some forms of acupuncture are not at all the same as dry needling, the term dry needling can refer quite specifically to what is now called Myofascial Acupuncture or some versions of Sports Acupuncture.
The origin of the term “dry needling” is attributed to Janet G. Travell, M.D. In her book, 'Myofascial Pain and Dysfunction: Trigger Point Manual', Dr. Travell uses the term "dry needling" to differentiate between two hypodermic needle techniques when performing trigger point therapy. The two techniques she described are the injection of a local anesthetic and the mechanical use of a hypodermic needle without injecting a solution (Travell, Simons, & Simons, 1999, pp. 154–155). Dr. Travell preferred a 22-gauge, 1.5-in hypodermic needle for trigger point therapy and used this needle for both injection therapy and dry needling. Dr. Travell never used an acupuncture needle. Dr. Travell had access to acupuncture needles but reasoned that they were far too thin for trigger point therapy. She preferred hypodermic needles because of their strength and tactile feedback: “A 22-gauge, 3.8-cm (1.5-in) needle is usually suitable for most superficial muscles. In hyperalgesic patients a 25-gauge, 3.8-cm (1.5-in) needle may cause less discomfort, but will not provide the clear “feel” of the structures being penetrated by needle and is more likely to be deflected by the dense contraction knots that are the target… A 27-gauge needle, 3.8-cm (1.5-in) needle is even more flexible; the tip is more likely to be deflected by the contraction knots and it provides less tactile feedback for precision injection” (Travell, Simons, & Simons, 1999, p. 156).
The use of a hypodermic needle for dry needling was described by Dr. Chang-Zern Hong in his research paper on "Lidocaine Injection Verses Dry Needling to Myofascial Trigger Point”. In his research, he describes the procedure for trigger point injection and dry needling by using a 27-gauge hypodermic needle 1 ¼-in long (Hong, 1994). Both Travell and Hong used hypodermic needles for dry needling. Dr. Hong, like Dr. Travell, did not use an acupuncture needle for dry needling.
Although dry needling originally utilized only hypodermic needles due to the concern that solid needles had neither the strength or tactile feedback that hypodermic needles provided and that the needle could be deflected by "dense contraction knots", those concerns have proven unfounded and many healthcare practitioners who perform dry needling have found that the acupuncture needles not only provides better tactile feedback but also penetrate the "dense muscle knots" better and are easier to manage and caused less discomfort to patients. For that reason both the use of hypodermic needles and the use of acupuncture needles are now accepted in dry needling practice. Ofttimes practitioners who use hypodermic needles also provide trigger point injection treatment to patients and therefore find the use of hypodermic needles a better choice. As their use became more common, some dry needling practitioners without acupuncture in their scope of practice, started to refer to these needles by their technical design term as "solid filiform needles" as opposed to the FDA designation "acupuncture needle."
The "solid filiform needle" used in dry needling is regulated by the FDA as a Class II medical device described in the code titled "Sec. 880.5580 Acupuncture needle" as "a device intended to pierce the skin in the practice of acupuncture."  Per the Food and Drug Act of 1906 and the subsequent Amendments to said act, the FDA definition applies to how the needles can be marketed and does not mean that acupuncture is the only medical procedure where these needles can be used. Also the FDA definition does not mean that the FDA or any US Regulatory agency defines Dry Needling as a form of Acupuncture or that the two terms are interchangeable. Dry needling using such a needle contrasts with the use of a hollow hypodermic needle to inject substances such as saline solution, botox or corticosteroids to the same point. Such use of a solid needle has been found to be as effective as injection of substances in such cases as relief of pain in muscles and connective tissue. Analgesia produced by needling a pain spot has been called the needle effect.
Dry needling for the treatment of myofascial (muscular) trigger points is based on theories similar, but not exclusive, to traditional acupuncture; both acupuncture and dry needling target the trigger points, which is a direct and palpable source of patient pain.