Rotator cuff anatomy has been traditionally viewed as a series of single muscles with each muscle responsible for a different action. For instance it is commonly known the origin and insertion of each of the four rotator cuff muscles and that their tendons splay out and interdigitate to form a common, continuous insertion on the humerus. But what of the anatomy and function of the different parts of each one of these rotator cuff muscles, for example the anterior and posterior parts of the supraspinatus? Or the fascial anatomy of these muscles in relation to different pain presentations or proprioceptive function?
A more detailed look at the anatomy of the rotator cuff muscles and the deltoid reveal:
- A ‘single’ muscle can be anatomically divided into different sections.
- Each one of these sections has different functions.
- Fibrous septa can exist within different parts of the muscle to give structural support and help in soft tissue repair. This structural support gives mechanical strength to the muscle preventing it from deforming and migrating during contraction.
- Regional anatomy of the fascia not only dictates different symptoms but mechanically has a proprioceptive function in the shoulder.
- Entrapment neuropathies can exist in the rotator cuff muscles.
- Anatomical anomalies may account for different presentations.
The article looks at the anatomy and function of the different muscles, including its fascia, of:
- Teres Minor.
- Rotator Interval.
Lastly it reviews:
- Fascia's proprioceptive function in the shoulder.
Origin: medial two thirds of the supraspinous fossa and supraspinatus fascia.
Insertion: greater tuberosity from the bicipital groove to the top of the bare area (Curtis et al 2006).
The tendon is contiguous with the transverse humeral (subscapularis tendon --> greater tubercle) and coracohumeral ligaments and the infraspinatus. It blends into the articular capsule in the distal 1cm. The tendon is enveloped by a thick sheet of fibrous tissue derived from the coracohumeral ligament (Clark and Harryman 1992).
Gates et al (2010) divided the supraspinatus into anterior and posterior sections.
Anterior section: has a larger muscle with ‘a very strong’ fibrous frame within the muscle (Gagey, 1993). It produces 71% of the total force of the muscle. The tendon of the anterior part of the supraspinatus is thicker and more tubular. It extends further medially from its insertion on the greater tuberosity to branch into a fibrous framework derived from the coracohumeral ligament (Clark and Harryman 1992). Clark & Harryman (1992) found the biceps tendon was ensheathed by interwoven fibers derived from the anterior part of the supraspinatus (and subscapularis) tendons.
Posterior section: has a smaller muscle with a wider thinner tendon. It produces 29% of the total force. The posterior tendon does not have the fibrous organization of the anterior tendon (Gates et al 2010).
Gates et al (2010) found the greatest amount of rotation occurred at lower degrees of abduction and at the highest supraspinatus load.
Anterior section: internally or externally rotates the humerus depending on if the shoulder is internally or externally rotated respectively.
Posterior section: externally rotates or does not induce rotation. Nishishita et al (2018) found the supraspinatus most effectively stretched in internal rotation.
Ihashi et al (1998) found external rotation was induced with increased abduction. Conversely internal rotation occurred with decreased abduction. These authors didn’t differentiate between anterior and posterior portions of the muscle.
Entrapment neuropathy of the suprascapular nerve
Duparc et al (2010) found an entrapment neuropathy of the suprascapular nerve in the:
- Suprascapular notch.
- Spinoglenoid notch.
- Supraspinatus fascia: within the supraspinatus fascia in the supraspinous fossa between the suprascapular notch and spinoglenoid notch. Bektas et al (2003) found in cases of an absent spinoglenoid ligament a septum formed by the thickening of the fascial cover of the distal third of the supraspinatus and infraspinatus muscles. It originated from the spinoglenoid notch and extended into the posterior capsule. They named this structure the spinoglenoid septum. The suprascapular nerve passed between the bony margin of the spinoglenoid notch and the medial concave margin of the spinoglenoid septum. This septum may be a cause of dynamic compression of the suprascapular nerve.
- Between the deep fascia of the Subscapularis and the superior transverse scapular ligament (STSL) (Tasaki et al 2015). Refer to ‘Subscapularis’.
- Anterior coracoscapular ligaments (Sahu et al 2012).
- Hypertrophied Infraspinatus (Sahu et al 2012).
- In the common tendon of the omohyoid and subclavius posticus or chondroscapularis: The subclavius posticus, that is not often present, originates from the postero-superior side of the costocoracoid membrane and costoclavicular ligament and inserts onto the superior boarder of the scapula and on the superior transverse scapular ligament Grigorita et al (2016). As the suprascapular nerve runs above the superior transverse scapular ligament and perforates the subclavius posticus it can be prone to entrapment. As this muscle can share a common tendon with the inferior belly of the omohyoid muscle could this be the entrapment site of the suprascapular nerve by the omohyoid mentioned by Sahu et al (2012)?
Origin: infraspinous fossa and infraspinatus fascia.
Insertion: greater tuberosity. The tendon runs anteriorly to attach to the supraspinatus and then continues to run anteriorly over the rotator interval to the subscapularis. The tendon also runs continuous with the teres minor.
The infraspinatus fascia is a tough sheet of connective tissue that covers the infraspinatus fossa of the scapula and the muscle (Moccia et al 2016). Tsaki et al (2015) found the Infraspinatus split into three muscular partitions: superior, middle and inferior partitions. Each partition is bounded and separated by fascia.
Fabrizio and Clemente (2014) gave the following description of the three partitions of the muscle:
Superior muscle partition: arises from the inferior surface of the scapular spine and the deep surface of the infraspinatus fascia at the attachment of the fascia to the spine of scapula. The fibers attached distally to the posterior aspect of the greater tuberosity. Gagey (1993) found the superior part of the infraspinatus contained a significant fibrous reinforcement.
Middle muscle partition: arose from the medial boarder of the scapula and from the posterior surface of the infraspinous fossa. The fibers course superolaterally toward to the glenoid cavity deep to the superior and inferior partitions. The middle partition blended with the tendons of the superior and inferior partitions and attached on the posterior aspect of the greater tuberosity. Tsaki et al (2015) found the middle partition of the Infraspinatus strongest to posteriorly stabilise the shoulder.
Inferior muscle partition: arose from the inferior one-third of the medial boarder of the scapula, the infraspinous fossa, and the deep surface of the infraspinatus fascia. The fibers coursed superior-lateral parallel to the lateral border of the scapula. The fibers attached to the greater tuberosity.
The fascia of the Infraspinatus has six components (Moccia et al 2016):
- Medial band: a band of fascia extending from the midspine of the scapula towards the inferior angle.
- Superomedial band: a band of fascia extending from the medial border of the scapula, near the origin of the spine of the scapula, toward the lateral border. In the three divisions of the Infraspinatus noted by Fabrizio and Clemente (2014) this band of fascia maybe the one cited in the article as separating the superior from the middle portion of the Infraspinatus.
- Inferomedial band: a band of fascia extending from the inferior angle of the scapula toward the scapular neck and glenoid fossa. The Infraspinatus fibers originate most strongly from the deep aspect of the inferomedial band. In the three divisions of the Infraspinatus noted by Fabrizio and Clemente (2014) this band of fascia maybe the one cited in the article as separating the middle and the inferior portions of the Infraspinatus.
- The insertion of the posterior belly of the deltoid muscle is inserted into the infraspinatus fascia inferior to the scapular spine.
- A band of fascia extending transversely from the posterior deltoid to anchor near the teres minor and teres major muscles.
- A retinacular sheet of fascia deep to the posterior deltoid and superficial to the infraspinatus and teres minor muscles as they approach the proximal humerus. Cooper et al (1993) claimed this fascia ran continuous with the clavipectoral fascia around the lateral aspect of the proximal humerus. This was verified by Nauta & Landsmeer (1948) who found a continuous sheet of fascia extending from posteriorly over the infraspinatus to continue laterally as the subdeltoid fascia and to terminate anteriorly as the clavipectoral fascia.
Myofascial trigger points are located in the medial band, inferomedial band,and the superomedial band,respectively. Referred pain patterns to the anterior and middle deltoid regions might be a result of the infraspinatus fascia acting as an intermediary between the two muscles (Moccia et al 2016).
This arrangement of the fascia lends it prone to causing a compartment syndrome involving the infraspinatus and teres minor muscles (Moccia et al 2016).
Additional bands of this fascia include (Moccia et al 2016):
(1) A loose, superficial fascia between the superior aspect of the latissimus dorsi muscle across the infraspinatus.
(2) A thin sheet of superficial connective tissue extending between the superior boarder of the latissimus dorsi muscle to the spine of the scapula.
Middle: Tsaki et al (2015) found the middle partition of the Infraspinatus posteriorly stabilises the shoulder.
Inferior: Hughes et al (2014) found isometric external rotation of the shoulder is most active in this part of the muscle irrespective of shoulder position.
Origin: upper two thirds on the dorsal aspect of the scapula and the two aponeurotic laminae seperating it from the teres major and infraspinatus.
Insertion: greater tubercle and a humeral attachment between the greater tubercle and triceps attachment and the joint capsule.
Gagey (1993) found the teres minor contained significant fibrous re-enforcement.
Action: lateral rotator and adductor.
Chafik (2013) found the fascia of the Teres Minor had two distinct characteristics:
(1) A stout, inflexible fascial compartment enveloping the teres minor muscle. This compartment may be the potential site of compression and tethering of the primary motor nerve to teres minor.
(2) A continuous fascia enveloping both the infraspinatus and teres minor muscles.
Origin: costal surface of the scapula, aponeurosis that covers the subscapularis and separates it from the long head of triceps and teres major. It is contiguous with the Teres Major and Latissimus Dorsi.
Insertion: extrarticular and intraarticular.
(a) Anterior capsuloligamentous complex (ACLC) (Kordasiewicz et al 2016). Recognised as a single structure it comprises the anterior joint capsule and Middle and Inferior Glenohumeral Ligaments.
(b) Lesser tubercle (Kordasiewicz et al 2016).
(2) Intraarticular: this explains the presence of synovial tissue around the tendon (Abe et al 2014).
The Middle and Inferior Glenohumeral Ligaments develop postnatally in response to mechanical demands from the Subscapularis and Biceps. The Inferior Glenohumeral Ligament development is more closely associated with the Bicep muscle attachment. The Middle Glenohumeral Ligament develops postnatally to cover the large opening (subcoracoid bursa) below the Superior Glenohumeral Ligament and to assist the intra-articular course of the Subscapularis.
Wickham et al (2014) divided the Subscapularis into superior and inferior segments. These segments were not only structurally different but exhibited differential activity depending on shoulder position. They hypothesised this could indicate different roles for each segment of the subscapularis in shoulder movement.
Superior portion of the Subscapularis: this part of the muscle is thicker. Gagey (1993) found a strong fibrous framework within the upper part of the subscapularis.
Inferior portion of the Subscapularis: is considered mechanically weaker and more susceptible to injury. It enhances dynamic stability of the joint with injury to this segment resulting in greater functional limitations (Whickham et al 2014).
Clark & Harryman (1992) found the biceps tendon was ensheathed by interwoven fibers derived from the subscapularis (and supraspinatus) tendons. This has an effect of stabilising the biceps tendon. Arai et al (2010) found stabilisation of the biceps tendon relied on tension in the superior glenohumeral ligament and the most superior insertion point of the subscapularis from behind this ligament.
The subscapularis fascia is the thinnest of the different fascia surrounding the muscles of the scapula. Laterally it continues with the axillary and infraspinatus fascia and superiorly with the supraspinatus fascia.
Primary action of the upper and lower subscapularis
The lower portion of the subscapularis primary function is to stabilise and centralise the humeral head during movement. It therefore activates before the superior portion of the subscapularis to counteract any shearing movement of the humeral head. For instance in abduction deltoid contraction can cause a superior shear of the humeral head so the lower subscapularis counteracts this by pulling down and produces an inferior shear. It also resisting anterior shear of the humeral head in abduction and external rotation.
- Internal rotation at mid and high levels of abduction. Rathi et al (2017) found the primary role of the upper subscapularis was as an internal rotator.
- Forward punch/push up plus exercises (Whickham et al 2016).
Due to its stabilising function in maintaining the humeral head in a neutral position the inferior subscapularis is particulary active during abduction and flexion.
- Humeral head depression (inferiorly depresses the humeral head to counteract the superior pull of the deltoid).
- In mid ranges of abduction contributes the majority of resistance to anterior translation.
Accessory Subscapularis (aka accessory subscapularis-teres-latissimus muscle)
This muscle is not always present and anatomically is highly variable:
Origin: anterior surface of the subscapularis (Pires et al 2017) on its superior lateral aspect (Breisch 1986), lateral margin of the scapula and the latissimus dorsi (Kameda 1976).
Insertion: into the shoulder joint (Pires 2017) and lesser tubercle (Breisch 1986).
Entrapment neuropathies from the Subscapularis
(1) Axillary and Inferior Subscapular nerve.
(2) Radial nerve.
(3) Suprscapular Nerve.
(1) Axillary and Inferior Subscapular nerve
Between the Subscapularis and the accessory subscapularis passes the axillary nerve (Pires et al 2017) and inferior subscapular nerve (Breisch 1986) that run through a myotendinous tunnel (Breisch 1986).
(2) Radial nerve
Kasmeda (1976) found an anomalous muscle passing through the brachial plexus designated the 'accessory subscapularis-teres-latissimus muscle'.
This muscle arose near the lateral margin of the scapula, either from the surface of the subscapularis muscle or from the latissimus dorsi tendon or from both of those sources. It ran obliquely upward to fuse with the insertion of the subscapularis.
Depending on its anatomy the muscle can separate the radial nerve into two roots.
(3) Suprascapular nerve
The deep fascia of the Subscapularis attaches posteriorly to the costal surface of the scapula. At the superior end of this between the base of the coracoid and the medial end of the scapula notch runs the superior transverse scapular ligament (STSL). Between the deep fascia of the subscapularis and the STSL is a space that the suprascapular nerve runs through (Tasaki et al 2015).
The rotator cuff interval: a triangular space between the subscapularis and supraspinatus and coracoid process. Jost (2000) found it composed of and represents a complex interaction of the supraspinatus, subscapularis, coracohumeral ligament, superior glenohumeral ligament, and glenohumeral joint capsule.
The rotator interval capsule: is the anterosuperior aspect of the glenohumeral joint capsule (Petchprapa et al 2010). It forms the roof of the rotator cuff interval linking the subscapularis and supraspinatus tendon. It is reinforced externally by the coracoacromial veil (ligament between the coracoacromial ligament and rotator interval capsule, Rothenberg et al 2017), coracohumeral ligament and the spiral glenohumeral ligament; internally it is reinforced by the superior glenohumeral ligament.
The fibers of the coracohumeral ligament cannot be separated from those of the anterior supraspinatus and superior subscapularis tendons with which it interdigitates or from the rotator interval capsule.
Lesions of the rotator interval may result in (Arai et al 2010):
- Glenohumeral joint contractures.
- Shoulder instability.
- Lesions to the long head of the bicep tendon. The bicep tendon is ensheathed by interwoven fibers derived from the subscapularis and supraspinatus tendons (Clark & Harryman 1992).
Origin: lateral third of clavicle, acromion (from 4 to 5 tendinous insertions, Moatshe et al 2018) and spine of scapula.
Insertion: deltoid tuberosity. Lorne et al (2001) found eight half-cone shaped distal fibrous structures that merged together into the distal tendon of the deltoid. The intermediate fibers of the deltoid contained four deep fibrous intramuscular bands that glide inside these. The anterior and posterior parts of the deltoid muscle lacked such bands.
Anterior fibers: The anterior part of the deltoid muscle lacked fibrous bands (Lorne et al 2001).
Intermediate fibers: the intermediate fibers have a short fiber length, complex multipennate structure, and high cross sectional area (Peterson & Rayan 2011). From 4 to 5 tendinous insertions (Moatshe et al 2018) four fibrous intramuscular bands descend from the acromion to interdigitate with three septa ascending from the deltoid tubercle. The septa are connected by short muscle fibers that provide powerful traction. Lorne et al (2001) proposed the role of the fibrous bands was not only to help repair the muscle but to stop migration and deformation of the muscle during strong contraction.
Posterior fibers: The posterior parts of the deltoid muscle lacked fibrous bands (Lorne et al 2001).
Audenaert and Barbaix (2008) proposed the deltoid to be divided into at least seven functional (not anatomical) muscle segments all of which have the potential to be independently coordinated by the central nervous system.
Fascial relations of the deltoid
Superficial & deep deltoid fascia
The superficial deltoid fascia continues with the fascia covering the trapezius muscle. The deep deltoid fascia inserts into the spine of scapula and clavicle.
Subdeltoid fascia (Nauta & Landsmeer 1948)
The subdeltoid fascia is a tough sheet of connective tissue extending posteriorly from the infraspinatus/teres minor fascia; laterally being stretched out over the greater tubercle and surgical neck of the humerus; anteriorly extending to the acromion and clavipectoral fascia (and costocoracoid membrane).
It receives attachments from the:
- Supraspinatus fascia.
- Subscapularis fascia.
- Deltoid: connects to the subdeltoid fascia proximally to prevent formation of folds in the fascia during abduction.
- Short head of biceps.
- Coracoacromial ligament.
Rispoli et al (2009) found the deltoid tendon and fibrous aponeurosis was in continuity with the lateral intermuscular septum posteriorly and the lateral aspect of the brachialis and deep brachial fascia anteriorly. Stecco et al (2008) found the posterior deltoid inserted fibers directly into the posterior portion of the brachial fascia.
Anterior fibers: shoulder flexion and adduction (with clavicular portion of the pectoralis major, Barberini 2014) and rotates arm medially (with pectoralis major).
Intermediate fibers: abductor. In the initial phase of abduction it translates the humeral head superiorly (with the Supraspinatus). Hereter et al (2014) found the tightening and elevation function of the intermediate fibers increased from its posterior to its anterior segment. This indicates a greater stabilizing effect, and tightening of, the anterior segment of the intermediate fibers in for example rotator cuff tears.
Posterior fibers: external rotator and extensor (with Latissimus Dorsi and teres major)
Variations in anatomy
Kamburogu et al (2008) found continuation and fusion of the fibers of the deltoid muscle into the trapezius, pectoralis major; vertebral border of the scapula, infraspinous fascia, and the axillary boarder of the scapula.
Kamburogu et al (2008) found the posterior fibers of the deltoid can be enclosed in a distinct fascial sheet and arise from the middle 1/3 of the medial boarder of the scapula.
The posterior part of the deltoid can be enclosed within a separate fascial sheath (Kayikçioglu, 1993).
Coracobrachialis (Maiti and Bhattacharya 2018)
Origin: coracoid process with a conjoint origin of the short head of biceps. It is formed of two fused heads.
Insertion: antebrachial fascia and the medial epicondyle of the humerus.
Variations include the coracobrachialis brevis. This muscle can insert proximally to the capsule of shoulder joint, root of coracoid process or conoid ligament of clavicle. It can insert distally into the medial intermuscular septum, medial supracondylar ridge, medial epicondyle and ligament of Struthers.
Entrapment neuropathies of the coracobrachialis:
(1) Musculocutaneous nerve
In human beings, the upper two heads of the coracobrachialis are usually fused while taking origin from the coracoid process. They enclose the musculocutaneous nerve in between the two fused heads. This gives the impression that the musculocutaneous nerve pierces the coracobrachialis muscle.
(2) Median nerve
The lower head of the coracobrachialis which is usually suppressed in human beings is sometimes present as the ligament of Struthers. The Median nerve and brachial artery passes deep to this ligament and are vulnerable to compression by being entrapped between the ligament and the bony surface.
Fascia's proprioceptive function in the shoulder
Stecco et al (2007, 2008) hypothesised the role of muscular insertions in pulling on the fascia to stimulate the proprioceptors in fascia.
Examples of the myofascial continuity of the fascia around the shoulder includes:
1. Brachial fascia
The brachial fascias myofascial connections include the:
- Fascia over the clavicular part of the pectoralis major runs from the clavicle (deep layer of pectoralis fascia) and the superficial lamina of the deep cervical fascia (superficial layer of pectoralis fascia) to attach to the anterior brachial fascia. Barberini (2014) found the clavicular part of the pectoralis major a later evolutionary development designed to optimise stabilisation of the upper limb to the thorax.
- Fascia from the costal part of the pectoralis major runs from the sternum (deep layer), contralateral pectoral fascia (superficial layer), rectus abdominis muscle and contralateral external oblique fascia to attach to the medial brachial fascia and medial intermuscular septum.
- Fascia from the latissimus dorsi sends fibrous lamina to the tricpes brachial fascia.
- Posterior deltoid muscle fibers attached to the posterior portion of the brachial fascia.
- Rispoli et al (2009) found the deltoid tendon and aponeurosis was continuous with the deep brachial fascia anteriorly.
2. Infraspinatus fascia (Moccia et al 2016)
The Infraspinatus fascia has musclar connections to the:
- Posterior deltoid.
- Teres Minor.
- Teres Major.
3. Lateral intermuscular septum:
Lateral intermuscular septum separates the anterior and posterior compartments of the arm. It extends from the greater tuberosity to the lateral epicondyle and annular ligament. Its connections are:
- Proximally it's in continuity with the the deltoid tendon and its fibrous aponeurosis (Rispoli et al 2009).
- The midsection is in continuity with the lateral aspect of the brachialis and deep brachial fascia anteriorly and gives attachment to the triceps posteriorly.
- Distally it gives attachment to the brachioradialis and extensor carpi radialis anteriorly and is confluent with the annular ligament encircling the radial head.
These muscular attachments pull on the fascia that stimulate the mechanoreceptors in the fascia. The complexity of this model also includes that one muscle attaches onto to various different fascial sheets and if one muscle e.g. levator scapula, pulls on another muscle e.g. supraspinatus could this pull on the supraspinatus fascia and alter proprioceptive function?
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