Where to needle – part 3

…in teres minor and other SG muscles.

Stimulated by Yi et al 2023.[1]

SG – shoulder girdleTrP – trigger point (referring to a myofascial trigger point)
SG – shoulder girdleTrP – trigger point (referring to a myofascial trigger point)
EPZ – endplate zone (the area of a muscle where motor nerves connect to muscle fibres)

key to acronyms

So, I was clearly premature in selecting a title with just the year in it when I last highlighted an anatomical paper using the Sihler’s staining technique – see Where to needle 2023.

Infraspinatus, the scalenes, and levator scapulae were mentioned last time, and in 2022, when I first came across the technique, the papers that caught my eye focussed on quadratus lumborum and supraspinatus – see Where to needle 2022.

This time it was teres minor – a relatively simple rotator cuff muscle that works alongside, or should I say, just inferior to, infraspinatus. Well, it is useful to include this little muscle, and I have seen one case of primary myofascial pain in this muscle that responded to a one needle treatment (the ultimate test of TrP needling).

The case involved a man in his middle years who managed to overstrain teres major when carrying a bundle on long thin branches. The bundle was on his right shoulder, held in place with his right arm abducted to 90 degrees and balanced with some downward pressure from his right hand and forearm resting on the bundle just in front of his shoulder. Teres minor is on moderate stretch in this position, but not active. As he tilted the bundle forward to put these long branches into an upright bin, he was forced to suddenly slow their rotation by pressing on the bundle farther back behind the point of balance on his shoulder. This last manoeuvre (which is rather like the Hornblower test) was achieved by engaging a powerful external rotation of the humerus whilst it was still in 90 degrees of abduction. This resulted in an overstrain of the diminutive teres minor. I was intrigued by the mechanism of injury and gratified to have such a simple and satisfying response to a single needle.

I used to see a lot of one needle wonder cases in the military, but they are somewhat rarer in civilian primary care and exceptionally rare in secondary care. Whilst we are always on the lookout for them, we must avoid just seeing them everywhere and being oblivious to the bigger picture – A Tale of Two Fridays.

I have consulted all my anatomical reference texts as well as searching online to try to clarify precise details of the anatomy of teres minor. The origin is from the dorsal aspect of the lateral border of the scapula about midway from the angle to the glenoid. But most anatomical texts refer to the upper 2/3 of the lateral border of the scapula. I think this must be a copying error because the long head of triceps attaches to this edge between teres minor and the glenoid. I consulted one of my big versions of Gray’s Anatomy (I have both the 38^th and 39^th editions) and was intrigued to see two attachments labelled for teres minor taking up just over a third of the entire dorsolateral border of the scapula. Teres major took up the lower third and the long head of triceps attached to the upper third, but was just less than a third of this border. It seems to me that a better description of the attachment might be to refer to the middle third of the lateral border of the scapula, rather than the upper 2/3.

The humeral attachment is described as tendinous to the greater tubercle of the humerus with direct muscle attachment below this. This will be relevant when I show you the images in the paper on the blog webinar.

The EPZs based on the neural density after Sihler’s staining appear to follow a diagonal path from top to bottom of the muscle, but closer to a sagittal plane than the angle of the outer border of the scapula where the muscle attaches. This would be consistent with the mid-section of contractile elements of the bulk of the muscle, but not with the lower fibres, where the EPZs are likely to be more lateral ie closer to the humerus. As well as originating from the scapula, teres minor also originates from fascial planes between it and teres major and infraspinatus.

The bottom line is that TrPs are most likely to be found in the middle of the muscle and slightly closer to the scapula attachment than the humeral one. It is best palpated against the lower lateral edge of the scapula and needling directed towards this edge. Bear in mind the proximity of the circumflex artery of the scapula, which winds around this edge between teres minor and major.

It is a small muscle and a relatively short paper, so I thought I might search for similar papers using the term ‘Sihler’s staining’. Well, I got 167 citations on PubMed, and I read all the titles to find the ones I was most interested in. I now have 39 papers in my EPZ folder, but I will not list them all here… I’ll save some for another blog.

There was a recent one on subscapularis showing a quite different distribution of fast and slow twitch fibres between the superior and inferior compartments of the muscle.[2] The superior compartment appeared to be predominantly (80%) fast twitch and the inferior compartment predominantly (80%) slow twitch. The authors were mainly concerned with optimising injection of botulinum toxin and concluded that two injection sites were needed because the injectate did not easily cross the thick intercompartment septum in the muscle.

The same lead author as the teres minor paper has also published on pectoralis major and serratus anterior,[3,4] so I will show you the images for those too at the webinar.

Finally, I found an older paper looking at latissimus dorsi,[5] presumably driven by the interest in using vascularised muscular flaps for surgical repairs. As well as the staining procedure for nerves, the large thoracodorsal vessels were injected with silicone rubber so they could be identified. The EPZs were widely distributed in the midsection of the muscle with a greater density relatively closer to the humeral attachment than the thoracolumbar origin.

References

1          Yi K-H, Kim S-B, Lee K, et al. Intramuscular neural distribution of the teres minor muscle using Sihler’s stain: application to botulinum neurotoxin injection. Anat Cell Biol Published Online First: 19 July 2023. doi:10.5115/acb.23.087

2          Cho T-H, Hong J-E, Yang H-M. Neuromuscular compartmentation of the subscapularis muscle and its clinical implication for botulinum neurotoxin injection. Sci Rep 2023;13:11167. doi:10.1038/s41598-023-38406-0

3          Yi K-H, Lee J-H, Kim H-M, et al. The botulinum neurotoxin for pain control after breast reconstruction: neural distribution of the pectoralis major muscle. Reg Anesth Pain Med 2022;47:322–6. doi:10.1136/rapm-2021-102653

4          Yi K-H, Lee J-H, Kim H-J. Intramuscular Neural Distribution of the Serratus Anterior Muscle: Regarding Botulinum Neurotoxin Injection for Treating Myofascial Pain Syndrome. Toxins 2022;14:271. doi:10.3390/toxins14040271

5          Takahashi N, Watanabe K, Koga N, et al. Anatomical Research of the Three-dimensional Route of the Thoracodorsal Nerve, Artery, and Veins in Latissimus Dorsi Muscle. Plast Reconstr Surg Glob Open 2013;1:1–7. doi:10.1097/GOX.0b013e3182948534

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Declaration of interests MC