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LETTERS TO THE EDITOR

Modified Subgluteal Approach to the Sciatic Nerve

Department of Anesthesiology and Pain Management, John H. Stroger Jr. Hospital of Cook County, Chicago, IL

To the Editor:

We have read with interest the recent article by Sukhani et al. (1) on a modification of the subgluteal approach to the sciatic nerve using muscle identifications as opposed to "bony" landmarks.

We would like to make a few comments:

1. A technique that purposely misses the posterior femoral cutaneous nerve should more appropriately be compared to a popliteal block than to more proximal approaches.

2. We agree that it is usually difficult to identify bony landmarks ("particularly in overweight patients"), as needed with most of the sciatic techniques. However, it is difficult to imagine that palpating the biceps femoris high in the thigh is any easier "particularly in overweight patients."

3. The proposed approach is based on finding the point in the gluteal crease at which the lower border of the gluteus maximus and lateral border of biceps femoris intercept. Throughout their article, the authors equate the gluteal crease with the lower border of the gluteus maximus. This is a common but erroneous belief. As the anatomy literature shows (2), the gluteal crease is a fold of the skin and subcutaneous tissue that "does not correspond to the lower border of the gluteus maximus muscle" (3). In fact, they do not even have the same direction. The inferior border of the gluteus maximus goes from medial to lateral on a steep angle to insert mainly in the iliotibial tract. The gluteal crease on the other hand goes from medial to lateral rather horizontally. Thus the biceps femoris, at the level of the gluteal crease, is under the cover of the gluteus maximus. This is especially true for the lateral border of biceps (see Fig. 1).

View larger version (138K): [in this window] [in a new window]   Figure 1. Right buttock dissection performed on fresh tissue cadaver showing: (1) gluteus maximus muscle, (2) biceps femoris muscle, (B) point of interception of lateral border of biceps and gluteus maximus, (AB) lower border of gluteus maximus, (AC) gluteal crease (projection).

We congratulate the authors for their contribution. When biceps can be palpated, this proposed approach could be an alternative to a popliteal approach.

References

1. Sukhani R, Candido K, Doty R, et al. Infragluteal-parabiceps sciatic nerve block: an evaluation of a novel approach using a single-injection technique. Anesth Analg 2003; 96: 868–3.[Abstract/Free Full Text]
2. Gosling JA, Harris PF, Humpherson JR, et al. Atlas of human anatomy with integrated text. Philadelphia: J.B. Lippincott Company, 1985: 6.11.
3. Snell RS. Clinical anatomy for medical students. 3rd ed. Boston: Little, Brown and Company, 1986: 554.

Response

Department of Anesthesiology, Northwestern University, Chicago, IL

In Response:

We thank Drs. Franco and Tyler for their correspondence and offer the following responses to their suggestions and comments.

1. The purpose of our study was to define a proximal approach to sciatic nerve block (at a site where the nerve has not yet divided into its two neural components—tibial and common peroneal nerves) using easily identifiable soft tissue landmarks (1). There was no attempt to make a comparison of our approach with any other proximal posterior sciatic nerve block approach. Since the posterior cutaneous nerve does not innervate structures below the knee, it does not need to be blocked for ankle surgery, and one may utilize a more distal approach to sciatic nerve block that may be less uncomfortable to the patient than more a more proximal approach.

2. Since the first description of our technique over 5 yr ago, (2) we have used this sciatic nerve block approach in over two thousand cases (400–500 times per year). Our patients have ranged in weight from 40–180 kg (88–396 lb). The biceps femoris is easily palpable in the middle and upper thigh even in obese and morbidly obese individuals. To facilitate the demarcation of the lateral border of the biceps femoris at the level of the gluteal crease, the palpating fingers are walked progressively cephalad while a counter pressure is applied on the calf muscles using the opposite hand, with the patient actively flexing the knee. Typically, the block is performed with minimal sedation (as documented in the study); therefore, patient cooperation has not been an issue.

3. The subgluteal approach described by Di Benedetto et al. (3), and the infragluteal parabiceps approach that we described, aim at approaching the sciatic nerve at a site where there is minimal or no overlying muscle tissue. We agree with Drs. Franco and Tyler that the gluteal crease does not correspond exactly to the lower border of the entire gluteus maximus muscle in all individuals, in that the inferior border of the muscle runs in a steep mediolateral angle. However, for the purposes of our described approach, it is the medial half of the gluteal crease that is of interest. In fact, the impetus for developing our approach was initiated by a review of Grant’s Atlas of Anatomy (4), which states, "The sciatic nerve is most readily accessible deep in the angle between the lower border of Gluteus Maximus and the lateral border of the long head of Biceps." Additionally, in Human Anatomy 1, Frick et al (5) state, "Directly at the lower margin of the gluteus maximus and at the entrance into the popliteal fossa, the nerve (sciatic) lies relatively superficial and close to the fascia lata. For the entire remainder of its course it is covered by the long head of the biceps which passes obliquely over it distally and laterally." We confirmed the anatomical information via a series of cadaver dissections to establish the veracity of these anatomical descriptors. Our clinical experience with this approach supports its simplicity (94% sciatic nerve blocks completed by resident trainees) and high success (100% with evoked motor response inversion, 86% with evoked motor response plantar flexion), attesting to its applicability in the clinical setting.

4. Drs. Franco and Tyler speculate that "the described approach requires one to overcome the difficulty of locating a muscle (biceps femoris) through a second muscle (gluteus maximus) not to mention doing so through adipose tissue." They go on to propose that the technique should be performed several centimeters distal to the gluteal crease. We firmly believe that attention to anatomy will refute this speculation. As we have indicated, the anatomical foundation for our approach was based on the anatomical literature as well as cadaver dissections we personally performed. From our study, the sciatic nerve lies lateral to the biceps femoris for only a brief distance of 3 to 4 cm at the distal edge of the gluteus maximus. Distal to this point, the nerve dives beneath the biceps and is no longer easily accessible. Attempting to locate the nerve several centimeters beneath the gluteal crease would completely defeat the stated objective, that of minimizing the difficulty inherent in locating a nerve through bulky musculature (biceps femoris). Incidentally, this has been one of the greatest limitations of posterior sciatic nerve approaches performed where a needle must pass through the gluteus maximus.

Finally, we would like to briefly comment on the issue of the posterior femoral cutaneous nerve (PFCN) of the thigh. Our approach to sciatic nerve block was never intended to block the PFCN and was so stated. We are unaware of an accepted categorization of nerve blocks of the lower extremities to be considered a sciatic nerve block technique as "sciatic block" only in the presence of PFCN block or "popliteal block" to be a block of the sciatic nerve in the absence of PFCN block. It has been our experience from our cadaver dissections as well as from our review of the literature that the PFCN, distal to the piriformis muscle descends over the biceps femoris muscle at a substantial distance away from the main trunk of the sciatic nerve. While approaches to sciatic nerve block in the vicinity of the piriformis muscle (Pauchet-Labat (6) and Mansour’s (7) Parasacral Approaches) reliably block PFCN, any approach that blocks the nerve at a more distal level may not.

References

1. Sukhani R, Candido KD, Doty Jr. R, et al. Infragluteal-parabiceps sciatic nerve block: an evaluation of a novel approach using a single-injection technique. Anesth Analg 2003; 96: 868–73.
2. Pawlowski J, Sukhani R, Frey K, et al. Infragluteal sciatic nerve block: latency versus evoked motor response. Reg Anesth 1999; 24: 1.
3. Di Benedetto P, Bertini L, Casati A, et al. A new posterior approach to the sciatic block: a prospective, randomized comparison with the classic posterior approach. Anesth Analg 2001; 93: 1040–4.[Abstract/Free Full Text]
4. Grant’s Atlas of Anatomy. 7th ed. Baltimore: Williams & Wilkins, 1978: Section 4–34.
5. Frick H, Leonhardt H, Starck D, eds. Human Anatomy 1. General Anatomy, Special Anatomy: Limbs, Trunk Wall, Head and Neck. New York: Verlag Publishers, 1991: 355.
6. Labat G. Regional anesthesia: its technique and clinical applications. 2nd ed. Philadelphia: WB Saunders, 1922: 45–55.
7. Mansour NY. Reevaluating the sciatic nerve block: another landmark for consideration. Reg Anesth 1993; 18: 322–3.[Medline]

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