The Piriformis: Getting to the Bottom of It

The piriformis muscle is an important piece of the puzzle to rule out whenever assessing low back, hip, and/or lower extremity pain. When indicated, a correctly placed acupuncture needle into the piriformis can promote positive clinical outcomes. In many patients, palpation of this muscle can prove to be difficult. Precise needle depth is required to achieve the desired benefit when performing acupuncture locally to this muscle.

As a general rule, if you can palpate the piriformis clearly it is likely taut and hyperactive (1). Unfortunately, it is not always that simple. On occasion, body habitus can make efforts to isolate the piriformis seem trivial. There are many landmark techniques that have been reported in the literature which can be helpful in pinpointing this muscle correctly. A couple of examples have been listed below.

A) Draw a line from the PSIS to the greater trochanter of the femur. Reference points will lie at a distance of 1/3 and 2/3 along this line. Insertion points are located 1-3 cm below this reference line (2).

B) Follow the line connecting the PSIS and the superior aspect of the greater trochanter to locate the superior portion of the piriformis muscle belly. Divide the PSIS and sacral horn distance into thirds. The junction of the middle and lower thirds will be used to draw a line to the greater trochanter to locate the inferior portion of the piriformis muscle belly (3).

(Blue lines indicate landmark reference lines and purple arrows indicate piriformis location)

When needling trigger points in the piriformis muscle, appropriate depth of the acupuncture needle is essential. This is especially true when needling the medial portion of the muscle belly which lies deep to the gluteus maximus.

The gluteus maximus is approximately 0.9-1.0" thick in a neutral resting position (4,5). The subcutaneous tissue overlying the glutues maximus varies greatly between individuals. In overweight to severely obese individuals, the subcutaneus tissue can range anywhere from 4.6 - 7.8 cm in thickness. In essence, you would need 2.7 - 3.9" of needle length to penetrate the gluteus maximus to reach the piriformis in this patient population (7). Therefore, a 75-100 mm length needle is required.

With this in mind, a 75 mm can accurately target the piriformis in a majority of patients and may be the ideal needle length for most people. In fact, a recent cadaver study demonstrated that a 75 mm needle could accurately penetrate the piriformis without adversely puncturing the underlying sciatic nerve 84% of the time when using the landmark technique suggested by Reinchart above (3,6). A 100+ mm needle is appropriate for severely obese patients who demonstrate thicker layers of subcutaneous tissue. A 50 mm is effective in reaching the piriformis accurately in individuals with a BMI in the normal range or lower with less subcutaneous tissue in the gluteal region.

While crunching the numbers on needle depth may seem trivial, it is necessary for acupuncturists to exercise surgical precision in regards to needling technique. The classics of Chinese Medicine advise practitioners to avoid needling too deep or superficial to the target tissue to avoid unnecessary injury to underlying or overlying structures. With respect to the piriformis, needling too deep may inadvertently strike the sciatic nerve causing unnecessary paresthesia or the possibility of neuropraxia. Needling too superficial poses little risk other than being ineffective.

1. Janda V., Muscle Function Testing. Butterworths; 1983; p. 237.

2. T. O’Conner et al. Atlas of Pain Injection Tecniques, 2nd ed. 2014

3. Reichert R. Palpation techniques: Surface anatomy for physical therapists. Stuttgart: Thieme; 2011; p. 229–231.

4. Jeong Ju-Ri et al.Reliability of rehabilitative ultrasound imaging for measuring the gluteus maximus muscle at rest and during contraction. Phys Ther Rehabil Sci 2017, 6 (1), 7-13

5. T. Ikezoe, et al. Atrophy of the lower limbs in elderly women: Is it related to walking ability? Eur J Appl Physiol.2011

6. Kearns G, et al. Accuracy and safety of dry needle placement in the piriformis muscle in cadavers. Journal of Manual & Manipulative therapy, 2018 vol. 26, no. 2, 89–96.

7. A. Zayback, et al. Examination of subcutaneous tissue thickness in the thigh site for intramuscular injection in obese individuals. J Ultrasound Med 2015; 34:1657-1662