Diagnostic anatomy and diagnostics of enthesal pathologies of the rotator cuff

Abstract

Detailed anatomy, crucial in modern high-definition diagnostics imaging, is a base for understanding diagnostic images and the nature of the diagnosed disease.

The aim of this paper

is presentation of a new anatomical model of the rotator cuff, which includes definition of tendinous and capsule-ligamentous layers as equally important to rotator cuff function understanding. Schematic and diagnostic (ultrasonography, magnetic resonance imaging) anatomy of the rotator cuff based on the core tendon concept is presented. Appropriate tissue layers of the cuff are discussed in detail. In the diagnostics part some enthesal pathologies of the rotator cuff are presented.

Material and methods

New anatomical data was analyzed in the context of rotator cuff layers – their presence, thickness and structure observed on magnetic resonance imaging and ultrasonographic images.

Conclusions

Rotator cuff should be regarded as a multilayer structure consisting of fused fibers of tendons and capsuloligamentous complex. The thickness of these layers is comparable therefore it is important to realize that capsular pathologies may become a serious obstacle to normal shoulder joint function. Understanding of anatomical rotator cuff layers explains the formation of calcific cavities within the rotator cuff. In fact between layers of the cuff. Calcific cavities are a sequelae of chronic enthesopathy/enthesal tear with hydroxyapatite filling and probably delaminating the tendinous layer from the capsuloligamentous one.

Introduction

Many diagnostic imaging specialists as well as orthopedic surgeons think that the rotator cuff is a structure built of tendons. Own investigation show clearly that rotator cuff is a complex which consists of outer tendinous layer and inner capsuloligamentous complex (CLC). My own unpublished data shows that the thickness of the CLC reaches up to 2–3 mm, and that most frequently is a 1/3 to 1/2 of the rotator cuff thickness. State of the art diagnostic imaging tools such as US and MRI as well as newest anatomical data^{(1, 2)} allow us today to precisely and separately diagnose ligamentous and tendinous structures of the shoulder's rotator cuff.

US and MRI images published until now describe all tendinous-ligamentous structures between the scapula and humeral tubercles as tendons.

In fact recent anatomical data published^{(1, 2)} prove beyond any doubt that this a complex structure combined of a ligamentous (superior and anteroinferior complex) and tendinous layers.

The supraspinatus/infraspinatus zone is schematically presented in fig. 1.

Fig. 1.

Purple – subdeltoid-subacromial bursa, bright red/green – muscle/tendon of the supraspinatus, dark blue – superior complex, red/brown – deltoid muscle, yellow – subcutaneous tissue, skin

Superior complex consists of coracohumeral ligament, superior glenohumeral ligament, coracoglenoid and for the first time described by Pouliart in 2006⁽¹⁾ superior posterior glenohumeral ligament. The latter one makes a posterior limb of the superior complex and distally interweaves through a rotator cable with superior glenohumeral ligament and coracohumeral ligament (anterior limb of the superior complex). There are many anatomical variants of the superior complex built⁽¹⁾.

Superior as well as anterior-inferior complex can be depicted by both US and MRI as separate from supraspinatus, infraspinatus and subscapularis tendon layers of tissue with different fiber orientation in dynamic US inspection. US seems much more precise in visualization of the superior complex and rotator cuff tendons⁽³⁾. MRI gets on top when we need to know what happens to the bone underneath the enthesis. The area of tendinous and superior complex fiber fusion layer may be therefore wrongly interpreted as delamination zone within the tendon.

Diagnostic anatomy of the rotator cuff

The key point to understanding rotator cuff anatomy and later pathologies is the structure of muscle and core tendons⁽⁴⁾ of the subscapularis, supraspinatus, infraspinatus and teres minor muscles. Those muscles have vast scapular insertions where the muscle attaches via small core tendons and directly attached bands of endomysium and perimysium.

Their distal core tendons run inside the muscle (pennate) (fig. 2).

Fig. 2.

MRI sagittal oblique section just laterally to the labrum where the superior complex starts to be well visible (blue dotted margins). Red dotted margins – supraspinatus, infraspinatus and teres minor muscles, black dotted margins – intramuscular core tendons of their muscles, grey margins – biceps tendon

Core tendons appear on the inner surface of the muscles (become semipennate) just laterally to the plane of acetabulum (figs. 3, 4). Starting from here their fibers touch the CLC and start to intermingle their fibers to create a tendon-complex fusion. The fusion gradually becomes full towards the enthesis (fig. 5).

Fig. 3.

MRI sagittal oblique section approx. 6–7 mm laterally to the image from fig. 2. Blue dotted margins – superior complex, red dotted margins – supraspinatus and infraspinatus muscles, black dotted margins – intramuscular core tendons of their muscles, grey margin – biceps tendon

Fig. 4.

MRI sagittal oblique section approx 6–7 mm laterally to the image from fig. 3. Blue dotted margins – superior complex, red dotted margins and red double arrow – supraspinatus muscular layer, black dotted margins – intramuscular core tendons of supraspinatus and infraspinatus muscles. At this level tendinous layer and ligamentous complex begin to fuse their fibers

Fig. 5.

MRI sagittal oblique section approx. 6–7 mm laterally to the image from fig. 4 – at the edge of the acromion. Blue dotted margin – superior complex side, black dotted margin – tendinous side of the rotator cuff

As for the US visualization of the cuff any plane can be chosen especially when the images are presented in real-time mode when a different fiber direction of the ligamentous complex and tendons can be much better visualized. Some US images of the cuff are shown in figs. 6–9.

Fig. 6.

US transverse supraspinatus zone rotator cuff layers (refers to sagittal oblique in MRI – compare with fig. 4) just laterally to the acromion. Blue dotted margins – superior complex, red dotted margins – supraspinatus muscular layer, black dotted margins – core tendon/tendon of supraspinatus, purple double arrow – subacromial-subdeltoid bursa, yellow double arrow – deltoid muscle

Fig. 9.

US longitudinal rotator cuff image at the supraspinatus zone. Short arrows – tuberculum major (enthesis of the supraspinatus tendon with some anisotropy artifact), long arrow – collum anatomicum (enthesis of the superior complex), arrowheads – humeral head, double arrow – tendinous layer, measurement dots – thickness of the superior complex

Fig. 8.

US transverse/oblique rotator cuff image at the supraspinatus/ infraspinatus zone. Short arrows – humeral head just medially to collum anatomicum. Double short arrow – superior complex layer, double dashed arrow – tendinous layer

Pathologies of the enthesis zone of the rotator cuff

Magnetic resonance and ultrasonographic imaging are the main two methods used for assessment of the rotator cuff of the shoulder nowadays. Both methods have advantages over the other and used together give the fullest picture of rotator cuff pathologies.

Recent literature helps to explain not only the structure of rotator cuff pathologies but also their nature and to some extent their origin^{(1, 2, 4)}.

Firstly it needs to be stressed that the cuff consists of two comparable layers – tendinous and capsuloligamentous. Secondly the layers have both different and similar anatomical and biomechanical properties. The internal, capsuloligamentous, connects bone to bone (scapular acetabulum to collum anatomicum of the humerus), is passive and less susceptible to elongation. The external – tendinous, connects bone to bone (fossa subscapularis, supraspinalis and infraspinalis to tuberculum minor and major of the humerus) is active and more susceptible to elongation.

The pivotal area is where both layers attach to the humerus. The place where they attach one next to and over another is where the bone undergoes shearing stress and may produce most peculiar pathologies (fig. 10).

Fig. 10.

Purple – subdeltoid-subacromial bursa, bright red/green – muscle/tendon of the suptaspinatus, dark blue – superior complex, red/brown – deltoid muscle, yellow – subcutaneous tissue, skin, thin blue layer between the collum anatomicum/tuberculum maius and the tendon/ligament – fibrocartilage of the enthesis, arrow – the border between layers (shearing forces generator area)

Ultrasound seems more sensitive and convincing as to the rotator cuff and its enthesis pathologies detection⁽³⁾. Since it can show the fibrocartilaginous layer of the cuff's enthesis (fig. 7) it can also show usually subclinical early-stage damage if that layer (fig. 11) and subsequent tears of the cuff within and away from the insertion (figs. 12 and 13).

Fig. 7.

US longitudinal supraspinatus zone. Black layer over the white bone margin – the enthesal cartilage thickness (measurement). Blue double arrow – superior complex layer, black double arrow – tendinous layer

Fig. 11.

Scars after a microtear of the enthesal fibrocartilage at the border between the tendinous and the ligamentous layers (left arrow) of the rotator cuff as well as within the ligamentous layer (right arrow)

Fig. 12.

Longitudinal (left) and transverse (right) US image of the rotator cuff partial tear at the border between the tendinous/ ligamentous layers with a faint erosion of the cortical bone of the enthesis (measurements)

Fig. 13.

Patient from previous figure. Enthesis and delamination tear of the cuff at the border between the tendinous/ligamentous layers with a faint erosion/deformation of the cortical bone of the enthesis

The most characteristic elements of the rotator cuff's humeral enthesis damage/tear are small collagenous and/or mineralized/calcified scars reaching into the elements of the cuff or between its layers. Blurring of the smooth enthesis’ margin, erosions and deep pseudocysts within and under the enthesis. Lesions listed above may be observed in combinations.

The MRI in turn can depict the rest of the disease (figs. 14 and 15), reaching deep into the bone which may in US assessment (figs. 12 and 13) seem small and easy to heal. So despite the superior resolution of the US so useful in depiction minor cuff injuries and scars the MRI seems to be important in confirmation/exclusion of those bony surprises that can alter clinical approach to the cuff pathology.

Fig. 14.

Patient from previous figure. Coronal MRI image at the supraspinatus area shows a 6 mm diameter plus edema zone of the deep bone lesion below a 3 mm diameter faint erosion of the cortex seen on images from figs. 3, 4

Fig. 15.

Patient from previous figure. Transverse MRI image shows a lobulated fibrocystic lesion within the bone at the supraspinatus area below a 3 mm diameter faint erosion of the cortex reaching nearly the center of the humeral head

Of course not only small lesions of the cuff can be visualized by the US examination. The larger ones too (fig. 16). US is sensitive in increased perfusion detection, which indicates the presence of vital tissue. Additionally US can indicate precisely which layer, tendinous or ligamentous, is a most likely pain generator. Frequently sites of cuff injury do not generate pain at pressure.

Fig. 16.

US longitudinal image of the supraspinatus area. Amorphous scar after a chronic partial tear of the superior complex (ligamentous) enthesis (small arrows) with a cortical bone discontinuity and deep erosion/cystic formation within the bone (long arrow). Note that the enthesis of the tendinous layer together with its fibrocartilage is intact

In many cases of rotator cuff tears a dynamic examination (not available in other diagnostic imaging methods) helps to establish a final diagnosis.

A specific form of chronic enthesis damage at the border between the tendinous and capsuloligamentous layers is formation of calcific/hydroxyapatite cavities.

These cavities are a sequelae of enthesis tear/decomposition at the border of the tendinous and capsuloligamentous complex. Chronic delamination shearing forces generated between tendon and capsuloligamentous complex within the layer corresponding to the enthesis tear create a space for a cavity to become (fig. 18). The precise mechanism of the cavity's creation is unknown to author's knowledge.

Fig. 18.

Scheme of calcific cavity (JW) creation. Black dashed arrow indicates the route of hydroxyapatites coming from the bone, through the damaged cortex and enthesal fibrocartillage into the cavity

Fig. 17.

US longitudinal image of the supraspinatus area. A complete detachment of the tendinous layer (green arrows), with a retracted to the line of collum anatomicum stump of the tendon (white arrow). Preserved continuity of the ligamentous complex layer (blue arrows) and no delamination of the tendinous/ligamentous complex secures little retraction of the detached tendon. Note erosions limited to the supraspinatus enthesis area – the most likely reason for the selective contusion

Fig. 19.

A. Image of the calcific cavity (arrowheads) of the supraspinatus zone in longitudinal section. B. Cavity after lavage procedure. Arrowheads indicate the past route of hydroxyapatites from the bone/erosion into the cavity. H – humeral head

Mineral parts besides filling and probably creating a second delaminating force, may make deep incrustations into the structure of both the tendons and the CLC complex building a sort of sealing wall around the cavity. During lavage procedure they sometimes give impression of piercing an egg shell.

Cavities may at times reach over the size of 20×20 mm and thickness that may be greater than the cuff's.

Calcific cavities may have different stages of consistence. It may be a watery toothpaste to hard schoolblackboard chalk. In cases it can be both thanks to gravity which sediments particles low in the lesion and that low area sooner becomes hard chalk. That phenomenon can be seen in the subscapularis zone.

Historical name tendinitis ossificans, or tendinitis calcificans/calcarea is false because the pathology is not related to inflammation but to chronic damage of the enthesis in the specific location. And that means enthesopathy in general sense or enthesal tear in specific sense with peculiar process of filling delaminated spaces and their walls with bone-originated material.