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LETTERS TO THE EDITOR

Posterior Approach to Popliteal Block

Department of Anesthesiology and Pain Management, Cook County Hospital, Chicago, Illinois

To the Editor:

We read with interest the work by Hadzic et al. (1) comparing "classical" versus "intertendinous" popliteal block. We agree with the authors that finding the midpoint between biceps and semitendinosus tendons at the popliteal crease is easier and more accurate than trying to palpate the muscle boundaries. In fact, we perform a similar technique.

However, simplifying the technique should not negate the anatomical fact that the sciatic nerve is not located at midpoint between the tendons. This fact is widely recognized (2–7). The partial dissection in their Figure 1 unfortunately does not show the nerve. Our Figure 1 shows that the sciatic nerve is indeed closer to biceps than to semitendinosus. Their MRI image fails, however, to demonstrate this supposed fact. To understand why, the author of this letter underwent a knee MRI. The result of our MRI is shown in Figure 2, and it is in almost every respect similar to the original. Some of the structures identified in the original MRI need clarification. Labeled as "biceps" is the short head of this muscle. The long head, the one we palpate, is shown superficially to it. What is labeled as "semitendinosus" is in fact semimembranosus. Semitendinosus ("half tendon") is seen as a small oval-shaped structure superficial to the semimembranosus.

View larger version (151K): [in this window] [in a new window]   Figure 1. Dissection of the popliteal fossa showing (1) semitendinosus, (2) biceps, (3) semimembranosus (deeper to semitendinosus), (4) sciatic nerve, (5) popliteal vein, (6) popliteal artery.

View larger version (172K): [in this window] [in a new window]   Figure 2. MRI performed on the letter’s author. The numbered structures correspond to (1) long head of biceps, (2) short head of biceps, (3) semitendinosus, (4) semimembranosus, (5) sciatic nerve, (6) popliteal vein, (7) popliteal artery, (8) gracilis, (9) sartorius, (10) femur. Notice that as it ascends from the knee, the femur becomes more lateral (eccentric position), leaving room for the powerful adductor muscles.

View larger version (152K): [in this window] [in a new window]   Figure 3. MRI section through femur (1) and patella (2) shows that it was obtained with the knee in almost perfect neutral position.

View larger version (203K): [in this window] [in a new window]   Figure 4. This is Figure 2 from the original authors’ article. The following has been added to it: (1) segmented line AB tangential to the posterior surface of biceps and semitendinosus. This line forms a 16° angle with the horizontal and (2) two small-segmented lines originating from the surface arrows of the original pictures. These lines form a 13° angle with the original simulated paths and would have been the actual paths had the knee been in neutral position. Notice that the original has some structures that have been mislabeled (compare it with Fig. 2).

We congratulate the authors for helping to popularize a technique that is still underutilized and for their efforts to simplify it. We have been performing a similar technique at 7 cm above the midpoint of the crease for the last 8 years. We direct the needle 45° cephalad, but most importantly, also slightly lateral to compensate for the lateral position of the nerve in the fossa. On the basis of the evidence presented here, this lateral angle should be 10 to 15°. This maneuver accomplishes what the 1-cm correction of the classic approach does while also preventing the needle from traveling through or getting too close to biceps (pain and unwanted twitch).

References

1. Hadzic A, Vloka J, Singson R, et al. A comparison of intertendinous and classical approaches to popliteal nerve block using magnetic resonance imaging simulation. Anesth Analg 2002; 94: 1321–4.[Abstract/Free Full Text]
2. Rorie D, Byer D, Nelson D, et al. Assessment of block of the sciatic nerve in the popliteal fossa. Anesth Analg 1980; 59: 371–78.[Abstract/Free Full Text]
3. Hahn M, McQuillan P, Sheplock G. Regional anesthesia: an Atlas of anatomy and techniques. St Louis: Mosby 1996: 151–6.
4. Macrae W. Lower limb blocks, principles and practice of regional anesthesia, 2nd ed. Edinburgh: Churchill Livingstone, 1993: 189–02.
5. Chelly J. Peripheral nerve blocks: a color atlas. Philadelphia: Lippincott Williams and Wilkins, 1999: 85–6.
6. Brown D. Atlas of regional anesthesia, 2nd ed. Philadelphia: W.B. Saunders, 1999: 125–8.
7. Bridenbaugh P, Wedel D. The lower extremity: somatic blockade, neural blockade in clinical anesthesia and management of pain, 3rd ed. Philadelphia: Lippincott-Raven Publishers, 1998: 373–94.

Response

Department of Anesthesiology, St. Luke’s-Roosevelt Hospital Center, New York, New York

In Response:

We thank Dr. Franco for his comments and interest in our intertendinous approach to popliteal block. However, his critiques on the anatomy and technique based on a single MRI image and one anatomical model can be only anecdotal. In contrast, the recommendations of our study are based on the data obtained on methodically studied extremities (n=20) in 10 volunteers. Each and every extremity studied in our study was carefully clinically examined for landmarks and then studied by MRI to compare how the clinical landmarks compared with the actual anatomy depicted by MRI. A major problem with interpreting anatomical and MRI images in regional anesthesia is that they are taken out of the clinical context. For instance, it is important to realize that in actual patients, it is the tendons and not the muscles that are easily palpable. Both our clinical (1) study and the experimental study using MRI images (2) clearly indicate that perpendicular insertion of the needle between the tendons of the biceps and semitendinosus muscles yields high success rate in localizing the sciatic nerve (70%). A lateral redirection of 10–15° is required in 30% of cases for the needle to reach the sciatic nerve. The intertendinous technique to popliteal block is featured in detail on our regional anesthesia web site (www.NYSORA.com) (3), and the testimony to its merit is exemplified by continuous positive feedback from the readership. We thank Dr. Franko for noticing the mislabeling of the semimembranosus muscle on the MRI image in Figure 2 (2). This obvious oversight, however, is of mere academic interest, and it has no significant practical relevance with regards to the accuracy of the description of the technique or landmarks in Figure 1.

Acknowledgments

The author would like to acknowledge the contribution of Mark Pisaneschi, MD, Department of Radiology, Cook County Hospital.

References

1. Hadzic A, Vloka JD. A Comparison of the posterior versus lateral approaches to the block of the sciatic nerve in the popliteal fossa. Anesthesiology 1998; 88: 1480–6.[Web of Science][Medline]
2. Hadzic A, Vloka JD, Singson R, et al. A comparison of intertendinous and classical approaches to popliteal nerve block using MRI simulation. Anesth Analg 2002; 94: 1321–4.
3. New York School of Regional Anesthesia (www.NYSORA.com).

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