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Seoul St. Mary’s Hospital, College of Medicine, The Catholic University of Korea, Department of Rehabilitation Medicine1, College of Medicine, The Catholic University of Korea, Department of Anatomy, Institute for Applied Anatomy2 |
Introduction
The teres minor is one of the four boundaries of the quadrilateral space, and it functions as an external rotator of the shoulder. The axillary nerve passes through the quadrilateral space and innervates to the deltoid and the teres minor. It is important to accurately sample the teres minor for the diagnosis of axillary neuropathy by needle electromyography. However, as the teres minor is relatively small in size and acts as an external rotator of the shoulder, it may be difficult to distinguish it from the infraspinatus. The purpose of this study is to propose the optimal needle insertion point of the teres minor for needle electromyography and to compare the accuracy with the conventional method.
Materials and methods
28 shoulders of 14 fresh cadavers were dissected. The relation of the muscle to nearby muscular and bony structures was measured in the prone position with the shoulder abducted at 90 degrees.
The length of the reference line from the acromion to the inferior angle of the scapula (AA-IA) was measured (T). The reference line crossed the teres minor muscle, and the mean distance from the acromion to the proximal (P) and the distal (D) crossing points were measured respectively. We hypothesized the midpoint (X) of proximal and distal crossing points would be the optimal target point for needle insertion (Figure 1). We calculated the mean proportion of X to T (X/T) and simplified it to propose a new target point. As the conventional method by Perotto and Delagi suggested the upper one-third of AA-IA (1/3 T) was the target point of needle insertion in the teres minor, we compared the probabilities that the new target point (simplified mean X/T) and the conventional target point (1/3 T) would be inside the teres minor, between the proximal and distal crossing points.
Results
The mean T was 152.51mm. The mean P was 52.82mm, and the mean D was 88.42mm. The mean X was 70.62mm. The mean X/T was 0.46, and we simplified the proportion to 0.5. We set a new target point of the teres minor as 1/2 T, and compared it to 1/3 T, which was the conventional target point. The probability that 1/2 T would reside between P and D was 24 out of 28 (85.71%). In the rest 4 shoulders, 1/2 T was distal than D, which is assumed to be inside the teres major or the latissimus dorsi. The probability for 1/3 T was 11 out of 28 (39.29%). In the rest 17 shoulders, 1/3 T was more proximal than P, which is assumed to be inside the infraspinatus or the deltoid (Table 1).
Discussion
We proposed a new target point of the teres minor (the midpoint of AA-IA) by dissecting and measuring cadaveric shoulders using surface landmarks and compared the accuracy with the conventional needle insertion method by calculating measured values. The accuracy of the new method seemed relatively high (85.71%), compared to that of the conventional method (39.29%). However, further study to prove the accuracy by actual needle insertion will be needed. |
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Table 1. Needle insertion site for the teres minor muscle in relation to the reference line. (Abbreviation : AA, the acromion of the scapula; IA, the inferior angle of the scapula; F, female; M, male; R, right; L, left; T, length of the reference line; P, distance from the acromion to the proximal crossing point of the teres minor with the reference line; D, distance from the acromion to the distal crossing point of the teres minor with the reference line; X, midpoint of the proximal and distal crossing points of the teres minor with the reference line; *, in between the proximal and the distal crossing point of the teres minor muscle with the reference line)
Figure 1. Topographic anatomy of the teres minor in relation to the reference line. (Abbreviation : AA, the acromion of the scapula; IA, the inferior angle of the scapula; T, length of the reference line from the acromion to the inferior angle of the scapula; P, distance from the acromion to the proximal crossing point of the teres minor with the reference line; D, distance from the acromion to the distal crossing point of the teres minor with the reference line; X, midpoint of the proximal and distal crossing points of the teres minor with the reference line; *, in between the proximal and the distal crossing point of the teres minor muscle with the reference line.)
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