Motor Points for the Neuromuscular Blockade of the Subscapularis Muscle

Methods 

Twenty subscapular muscles were dissected from 10 cadaveric bodies. The material was obtained under the UK Anatomy Act of 1984. The muscles were skeletally mature and preserved with embalming fluid (methylated spirit, 10% phenol, glycerol, and formalin). Six cadavers were women and 4 were men. The mean age of death ± standard deviation was 74.7±5.8 years (range, 65−84y). There was no evidence of previous injury or surgical procedure around the shoulder joint. The cause of death had not involved any pathologic process that could have affected the innervation pattern of the subscapularis muscle. Cadavers were placed in the prone position, and skin and superficial fascia were removed. The latissimus dorsi and rhomboid major and minor were divided. Serratus anterior was divided at its attachment to the medial border of the scapula and reflected to reveal the subscapularis muscle. The arm was internally rotated, and the subscapular nerves were traced from their origins on the posterior cord of the brachial plexus to their macroscopic entry into the muscle (the motor point).

A metal pin was hammered through the scapula at each motor point and this dorsal position marked. The acromial tip, the medial end of the spine of the scapula, and the inferior angle of the scapula were identified. The dorsal positions of the motor points and surface anatomy positions were recorded onto a transparency overlay and then transferred directly to graph paper.

An x axis was devised such that the vertex of the inferior angle was point (0,0) and the acromial tip always lay on the x axis (fig 1). Coordinates for the medial spine and the motor points were recorded. These coordinates were translated into proportions by dividing the values by the distance along the reference line from the inferior angle of the scapula to the acromial tip. This allowed comparison between specimens. For each dissection, graphical representation of the motor points generated a line of best fit, and the equation of this line was calculated. The average of these 20 lines was then found. The mean position of the medial spine was also calculated. All calculations and graphs were made by using Excel.a

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Fig 1. Graphical representation of data from the 20 subscapularis dissections. The graph is shown as an overlay to a pen drawing of a scapula. The axes are thus shown with the vertex of the inferior angle as point (0,0) and the acromial tip on the x axis. All data are plotted as proportions relative to the distance between the inferior angle and the acromial tip. The motor points are displayed as a scatter plot (red diamonds), and the line of best fit is shown as a solid black line (y=1.48x–0.743). The line of best fit transects the x axis at .50. The position of the medial spines in each dissection is also shown (blue diamonds), with the mean position shown by the black diamond.

Results 

The number of motor nerve branches and hence motor points for each subscapularis muscle varied between 3 and 6 with a median value of 5. The distance between the inferior angle and the acromial tip had a mean value of 18.9cm (range, 16.2−21.1cm). The scatterplot of all of the motor points from the 20 dissections showed clustering of the motor points in a band (see fig 1). A line of best fit was calculated; this bisected the hypothetical line connecting the inferior spine and the acromial tip (x=.50). We believe that this line of best fit represents the optimal path for the injection of botulinum toxin into the subscapularis muscle (fig 2).

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Fig 2. Mock-up of proposed injection technique. Abbreviations: AT, acromial tip; IA, inferior angle; MS, medial spine. Legend: —, imaginary line connecting IA with AT; ---, the path of the injection needle bisects this and aims for the mid-point between the MS and AT.

Discussion 

This study has identified the anatomic motor points of the multipennate subscapularis muscle and related these in terms of proportional distances to 3 fixed bony landmarks. By using this information, we are able to suggest a method for injecting botulinum toxin close to its site of action at the endplate zone within the subscapularis muscle.

Botulinum toxin is taken up by and acts on the terminal nerve endings that form the neuromuscular junction with the motor endplate where it blocks the release of acetylcholine, thus interfering with transmission.5 Identifying motor endplate zones in large adult human muscles is difficult and involves staining cryosectioned fresh whole muscle. Although in unipennate muscles the motor endplate zones have been shown to cluster in a narrow band around the mid point of the muscle fibers, their location in multipennate muscles such as the subscapularis is less clear.14, 15, 16 Studies of the gastrocnemius complex in humans have shown that the motor endplate zone lies just distal to the motor points, the proximal limit of the motor endplate zone coinciding with the motor points.9, 13 Similarly, a study of human facial muscles12 found the motor endplate zone in the immediate vicinity of the nerves’ entrance point. This confirms current thinking that the terminal arborization of the motor nerve occurs just distal to its point of entry in the muscle. Therefore, it has been suggested that the location of the motor endplates corresponds with the location of the motor points, and hence the motor points are a good surrogate site for injection.6 It is for these reasons that we elected to identify the motor points.

Botulinum toxin has been shown to be most effective when injected close to the motor endplates.10 It is able to spread easily along the muscle length and through muscle fascia17 to produce a gradient of chemodenervation, the magnitude and extent of which depends on the dose.18 Saturation of a motor endplate zone may occur, and in such cases diffusion of the toxin may be more obvious clinically with effects at distant sites. Hence, most side effects occur when high total doses of botulinum toxin are used.19

The subscapularis is the strongest of the internal rotator muscles of the shoulder and is implicated in spastic pathology as a cause of pain and/or loss of function. The aim of this study was to discover if the motor points of the subscapularis lie in a definable region, which would provide a site for botulinum toxin injection in clinical practice.

Our dissection confirmed the variable nerve supply to the subscapularis as described by Kato.20 The superior subscapular nerve usually arises as 2 separate branches (the superior and middle subscapular branches) from the posterior divisions of the upper 2 trunks; these nerves supply the bulk of the muscle. The inferior subscapular nerve gives a branch to the axillary portion or subscapularis and then terminates in and supplies the teres major.20

Despite the variability in the origin and number of nerve branches supplying the subscapularis, our results show that the motor points lie in a constant band across the muscle. The line of best fit shows what we believe to be the optimal path along which to inject botulinum toxin into the subscapularis. Our line of best fit bisects a hypothetical line between the inferior angle of the scapula and its acromial tip; therefore, the needle should be inserted at this point. The angle of injection relative to the line is 56°. This can be achieved by directing the needle to a point approximately half way along the spine of the scapula (see fig 2). To maximize coverage of these motor points, the toxin should be injected from the lateral border of the scapula up until the level of the spine of the scapula.

This technique for injection of the motor points of the subscapularis is best achieved from a posterior axillary fold approach. Chiodo et al21 compared 3 methods for needle insertion into the subscapularis (a posterior axillary fold approach, an apical approach, and a medial/vertebral approach) and found the posterior axillary fold approach to be the most successful in terms of needle placement within the muscle and least associated risk of damaging other structures.

Motor innervation of muscles is complete in early childhood, and the localization of endplates within adult human muscle has been shown to correlate with the localization of endplates in the child’s muscles.14, 15, 16 The use of proportional distances allows the application of this method of injection to both the adult and pediatric populations affected by a variety of neuromuscular conditions. This should lead to an increased efficacy and decreased side-effect profile in clinical practice, although clinical trials will be needed to confirm this. The subscapularis is the strongest of the internal rotators. Further research is needed to define an optimal injection technique for the other internal rotator muscles of the shoulder.

Conclusions 

Identification of the motor points for the subscapularis muscle in relation to surface anatomy landmarks has allowed us to propose a method for the neuromuscular blockade of this muscle that is applicable to both children and adults alike. If injections are delivered more accurately to their site of action, we assume that they will be more clinically effective and that the risk of side effects will be reduced.