Where to needle 2023

…in infraspinatus and other muscles.

Stimulated by Lee et al 2023.[1]

SS – supraspinatus
QL – quadratus lumborum
IS – infraspinatus
EPZ – endplate zone (the area of a muscle where motor nerves connect to muscle fibres)
MEP – motor endplate (the connection between a motor nerve and a muscle fibre)
MEZ – motor endplate zone (another acronym for EPZ)
TrP – trigger point
MSK – musculoskeletal
US – ultrasound

key to acronyms

Last year I came across a couple of anatomical papers that used a rather complicated staining procedure in order to visualise the EPZs of supraspinatus (SS) and QL: Where to needle 2022.[2,3] At the time I remember thinking that I would have preferred to see infraspinatus (IS) rather than SS, since clinically relevant myofascial pain seems to be more common in the latter muscle and tendinopathy more common in the former.

Well, last week my wish was granted with a paper devoted to the EPZs of IS. These papers are all aimed at finding the best injection sites for botulinum toxin, but they are also the most likely area to find TrPs, which are the target for practitioners of dry needling and Western medical acupuncture.

The staining method is called modified Sihler’s staining, and it is a seven-step process. It seems to be a bit quicker than described in the previous papers I highlighted, with the time duration of the process reducing from 3 months in the papers I highlighted in June 2022 to about 5 weeks in Lee et al 2023.[1] I note that the papers all come from the same group in Seoul, South Korea.

The staining method colours both nerves and muscle fibres, but then desaturates the muscle so that the nerves can be seen through the resulting semitranslucent muscle using a medical film viewer (something like an x ray film box, if you are old enough to remember those ;-)…). The nerve entry points and the EPZs were mapped out from 20 different specimens of the IS muscle (12 male and 8 female).

The nerve entry points were quite central with the majority distributed around SI11, but the EPZs were more proximal in the muscle (more medial in the standard anatomical position). I was pleased to see the EPZs mapping out a C shape around SI11, which matches our clinical findings of the most frequent positions of TrPs in IS.

The same group have also published similar papers focussed on levator scapulae and the anterior and middle scalene muscles.[4,5] Unfortunately, I cannot access the full text of the paper on levator scapulae, but the abstract describes the position of the EPZs as being between 30% and 70% of the length of the muscle if 0% and 100% represent the origin and insertion. This central location of EPZs is consistent with muscle innervation in general and makes sense in functional physiological terms; however, when teaching I often find MSK clinicians focussing on the scapula attachment of levator scapulae because this is a frequent site of tenderness. My guess is that the tenderness here comes from trapezius being compressed onto the upper ala of the scapula and not from the tendinous attachment of levator scapulae. I am happy to find that this paper is consistent with my educated guess on the subject.

The paper on the anterior and middle scalene muscles uses diagnostic US in live human subjects as well as the modified Sihler’s staining of cadaveric specimens of the muscles in an attempt to find the best locations for safe injection of these muscles in thoracic outlet syndrome. 15 anterior scalene muscles and 13 middle scalene muscles were obtained from 8 cadavers. The nerve endings were most frequently seen in the inferior half of these muscles, specifically the lower third quarter and fourth quarter. Since the density of innervation appeared greater in the lower third quarter and the dome of the pleura is just behind the lower fourth quarter, the authors recommend an injection site about 3cm (2 adult finger breadths) above the clavicle at the lateral margin of the clavicular sternocleidomastoid muscle. They describe recommended depths for each muscle based on US measurements in a sample of 10 adult volunteers. The depth of the surface of the muscles at 3cm above the clavicle was 5–14mm for the anterior scalene and 7–15mm for the medial scalene.

References

1          Lee H-J, Lee J-H, Yi K-H, et al. Anatomical analysis of the motor endplate zones of the suprascapular nerve to the infraspinatus muscle and its clinical significance in managing pain disorder. J Anat Published Online First: 29 March 2023. doi:10.1111/joa.13868

2          Yi K-H, Lee K-L, Lee J-H, et al. Guidance to trigger point injection for treating myofascial pain syndrome: Intramuscular neural distribution of the quadratus lumborum. Clin Anat N Y N 2022;35:1100–6. doi:10.1002/ca.23918

3          Lee H-J, Lee J-H, Yi K-H, et al. Intramuscular Innervation of the Supraspinatus Muscle Assessed Using Sihler’s Staining: Potential Application in Myofascial Pain Syndrome. Toxins 2022;14:310. doi:10.3390/toxins14050310

4          Lee J-H, Lee K-W, Yi K-H, et al. Anatomical analysis of the intramuscular distribution patterns of the levator scapulae and the clinical implications for pain management. Surg Radiol Anat SRA Published Online First: 4 May 2023. doi:10.1007/s00276-023-03146-3

5          Lee K-L, Lee J-H, Huh H-W, et al. Novel Ultrasound-guided Injection Method for Thoracic Outlet Syndrome Based on Anatomical Features: A Cadaveric Study. Pain Physician 2023;26:E163–9.

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Declaration if interests MC