|Year : 2018 | Volume
| Issue : 5 | Page : 4-8
Sonoanatomic fundamentals of musculoskeletal ultrasound
Ingrid Moller1, M Isabel Miguel-Perez2, David Bong3, Albert Perez4, Carlo Martinoli5
1 Instituto Poal de Reumatologia; Departament de Patologia i Terapèutica Experimental, Unitat d'Anatomia i Embriologia Humana, Barcelona, Spain
2 Departament de Patologia i Terapèutica Experimental, Unitat d'Anatomia i Embriologia Humana, Barcelona, Spain
3 Instituto Poal de Reumatologia; Departament de cliniques (Enfermeria), Facultat de Medicina i Ciències de la Salut, Campus de Bellvitge, Universidad de Barcelona, Barcelona, Spain
4 Department of Basic Sciences, Universitat Internacional de Catalunya, Barcelona, Spain
5 Department of Health Science, University of Genoa, Ospedale Policlinico San Martino, Genova, Italy
|Date of Web Publication||1-Aug-2018|
Dr. Ingrid Moller
Castanyer 15, Barcelona 08022
Source of Support: None, Conflict of Interest: None
Over the years, anatomists have discerned basic anatomic principles that are applicable to the understanding of the structure and function of the superficial “soft tissues” of the human body including fascia, tendon, muscle, enthesis and nerve. Musculoskeletal ultrasound affords an inexpensive, high-resolution, dynamic, real-time, safe and well-tolerated imaging modality to visualize these tissues. The purpose of this article is to apply these anatomic principles to the ultrasound image of these structures and enhance the fundamental anatomic understanding of practitioners of musculoskeletal ultrasound.
Keywords: Fascia, musculoskeletal, nerves, Sonoanatomy, ultrasound
|How to cite this article:|
Moller I, Miguel-Perez M I, Bong D, Perez A, Martinoli C. Sonoanatomic fundamentals of musculoskeletal ultrasound. Indian J Rheumatol 2018;13, Suppl S1:4-8
| Introduction|| |
In evaluating the so-called “soft tissues” of the human body, a thorough knowledge of anatomy is fundamental to performing an accurate physical examination and developing a tenable differential diagnosis and for the evaluation of imaging of these soft tissues especially in the locomotor system. Imaging utilizing high-resolution musculoskeletal ultrasound (MSKUS) is increasingly becoming a routine part of rheumatologic care. Sonoanatomy refers to the knowledge of anatomy necessary to interpret the complex two-dimensional gray-scale anatomic image obtained of three-dimensional musculoskeletal tissues. Studies have shown that limited knowledge of musculoskeletal anatomy on the part of the rheumatologist presents a significant challenge in assimilating MSKUS and being able to apply it to the problem-solving required in daily practice. Recently, MSKUS and the sonoanatomic images have been employed as a tool in the teaching, the basic principles of musculoskeletal anatomy to health-care professionals at the medical school. All of this have generated the need for continued enhancement of the anatomic knowledge base to stay abreast of scientific advances in the understanding of rheumatic and soft-tissue disorders along with improvements in instrumentation. Yet, it goes beyond simple memorization of more anatomic details. Since MSKUS has a resolution that exceeds other imaging modalities and is dynamic and performed in real time, mastering the anatomy of the musculoskeletal system involves understanding the functions, biomechanics, and interrelationships between the different components that occupy the space between the subcutaneous tissue and the bone. The anatomic and functional organization of skeletal muscle, for example, is governed by general principles, among which the following stand out: the disposition of the different structures must obtain maximum performance with the minimum energy consumption while at the same time protecting essential elements. This article is intended to consider the importance of the elemental aspects of the musculoskeletal system related to the ultrasound image as follows: fascia including retinacula, muscle and tendon, and finally, the enthesis. It has been organized from superficial structures to deep and begins with the fasciae owing to their ubiquitous multifunctional nature and overall importance in their biomechanical contribution to the musculoskeletal system.
| Fascia|| |
Fasciae are soft connective tissue that Benjamin, in his review, described “as a body-wide mechanosensitive signaling system with an integrating function analogous to that of the nervous system.” According to its location and function, the fascia can be divided into superficial and deep, although this distinction can be difficult in some locations of the locomotor system.
The superficial fascia is the adipose tissue [Figure 1] or areolar connective tissue beneath the skin, which transports blood vessels from nerves to the skin while facilitating or restricting movement between it and the underlying tissues. It gives the skin ability to slip and then recover to its initial position as noted on the dorsum of the hand or foot. In other locations, it is firmly fixed to deep planes (palm of the hand or sole of the foot) restricting the movement in favor of a gripping function; in places such as the interphalangeal folds, it attaches directly to the flexor tendon sheaths with the consequent risk of tendon sheath infection when there are puncture wounds at that level [Figure 2]. The collagen fibers maintain a certain direction in the dermis that gives rise to some skin marks called Langer lines whereas the Kraissl lines show the maximum tension of the skin when the muscles contract. However, superficial and deep fasciae can be combined and be affected jointly in fibromatosis such as Dupuytren''s disease or Ledderhose's disease. The deep fascia is the best organized and “bundles” muscles and tendons of the locomotor system.
|Figure 1: Superficial fascia: the superficial fascia (a) is just under the skin (b) in a dissection of the arm|
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|Figure 2: The transversal cut of a finger shows the close relations of the adipose tissue (black arrow) with the flexor tendon sheath (*)|
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The deep fascia surrounds the muscles under the superficial fascia and divides extremities into the different anatomic and functional muscular compartments [Figure 3]. A classic example of clinical interest is the fascia of the psoas muscle that originates on T12 to L5 vertebrae. A localized infection in one of these vertebral bodies can spread distally to the inguinal region transmitted through this fascia. Fasciae vary according to their location and adapting to the function they perform. These modifications may be of interest when interpreting ultrasound or magnetic resonance imaging of the locomotor system. In certain retinacula, which are adaptations of the deep fasciae or in the plantar fascia subjected to significant load, the fibroblasts have a chondrocytic phenotype producing fibrocartilage in response to mechanical stress. This also happens in tendons subjected to the friction stress, such as the posterior tibialis, in its trajectory around the medial malleolus. In the sole of the foot or the palm of the hand, the characteristic tissue of the superficial fascia will be fibroadipose to adapt to the pressures encountered in these areas. Modifications of the fibroadipose tissue of the sole of the foot should be taken into account in the differential diagnosis of biomechanical foot pain.
|Figure 3: This transversal cut of the leg shows the deep fascia how makes the different compartments for the anterior, lateral, and posterior muscles. In the posterior compartment also, there is a different compartment for the superficial and deep muscles|
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The retinacula are specializations of the deep fascia destined to create tunnels that facilitate the sliding and direction of the tendons maintaining them in their correct position. It is interesting to know that the tendons located under the retinacula will always have synovial sheaths. When creating closed spaces, they can cause pathologies related to entrapment. The classic example is the flexor retinaculum of the wrist through which passes not only flexor tendons of the fingers but also the median nerve. Under the retinaculum, the median nerve can be compressed producing carpal tunnel syndrome. “Functional” retinacula are exemplified by the sagittal bands that prevent the subluxation of the extensor tendons of the fingers at the metacarpophalangeal joints [Figure 4] or by the pulleys that on the palmar aspect of the fingers prevent bowstringing of the flexor tendons of the fingers. The retinacula of the wrist, the ankle or the flexor pulleys of the fingers are composed of three different layers of tissue adapted to their function with a sliding internal surface composed of cells that secrete hyaluronic, a thicker intermediate layer with support function made up of collagen fibers, fibroblasts, and elastin, and an outer layer of loose connective tissue which contains vascular channels. Variations in both composition and thickness either of the retinaculum or its content can generate pathologic alterations such as trigger finger or stenosing tenosynovitis.
|Figure 4: In the dorsum of the hand, it is possible to visualize the sagittal bands that keep the extensors tendon (black arrows)|
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| Muscle and Tendon|| |
Skeletal muscles in the human body provide mobility, stability, and posture. They can be classified according to their shape, size, direction, and function (fast and slow contraction). According to the function they perform, the muscles distribute their fibers in different ways. Those muscles in which the alignment of the sarcomeres is oblique with respect to the longitudinal axis of the muscle/tendon are called penniform [Figure 5]. The angle of penance is a variable influenced by factors such as genetics, age, and muscular training. In general, it can be said that the greater the angle of penance, the lower the force load generated toward the corresponding tendon. A muscle maintains its complex relationship with the neurovascular system from embryonic development. Perimysium and endomysium, the connective tissue that surrounds and packages muscle fibers contribute to the maintenance and transmission of muscle-tendon-bone tension.
|Figure 5: This muscle show a penniform of this fiber that inserts in a central tendon|
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The tendons, being of fibroelastic composition, transmit mechanical forces from the muscle to the bone to generate movement while controlling the speed at which is performed. Wood Jones highlighted the importance of the union between muscle and surrounding fascia (“myofascial unions”) as a form of force transmission to surrounding tissues and compares it with the exoskeleton that nonvertebrate animals possess. In the human being, these “myofascial unions” are important in creating a stable column in the lower extremities that maintain the erect position., For each muscle, there will be at least two tendons, proximal and distal, a myotendinous junction and an enthesis. The tendon is continuous with the periosteal tissue of the bone. Many tendons reinforce the joint capsules forming part of the capsule and thus contributing to capsular function. An example of this is the extensor digitorum complex that forms the dorsal capsule of the proximal phalangeal joint. The extensor digitorum complex also exemplifies the functional collaboration that may exist between various muscles and tendons. In this case, the interossei and lumbricals that join the extensors of the fingers in addition to reinforcing the capsule of the phalangeal metacarpal joint are also contributing to its delicate function. Further morphologic and functional adaptations of tendons include widening and shortening if force generation is required, as opposed to lengthening and thinning if it performs delicate movements as in the fingers. In general, the tendons of muscles of greater volume in the extremities give origin to their tendon before reaching the hand/wrist or foot/ankle to avoid dysfunction by attempting to slide the muscle mass into the confined space created by the retinaculum. Internal tendon architecture facilitates movement between the different fascicles of the tendon in different planes. The endotendon is a thin sheath of connective tissue that surrounds each fiber and connects it with neighboring fibers and facilitates interfascicular displacement while directing the vasculonervous bundle. This intratendinous ability of different fibers and fascicles (bundles of fibers) to slide independently is an important property of the tendon from a functional point of view and can be altered in various tendon pathologies. Tendons may be surrounded by a synovial sheath. Tendons, such as the peroneus longus, only have synovial sheaths intermittently at points they encounter the most friction. Some tendons lack a synovial sheath, such as the Achilles tendon, and are surrounded by a paratenon, a loose areolar connective tissue that functions as an elastic cover that enhances free movement of the tendon about the surrounding tissues. Finally, sac-like synovial structures, i.e., bursae may be interposed between tendons and points of frictions such as in the area where the distal biceps tendon runs between the proximal radius and ulnar during supination and pronation of the forearm. In the aging process or secondary to trauma or recurrent microtrauma, the tendon may become smaller as the size of the tendon collagen fibers decreases.
The tendon is less vascularized than its corresponding muscle and occurs mainly at the level of the myotendinous junction. The vessels, after penetrating the tendon, run parallel to the tendinous fibers and are scarce in the areas where the tendon changes direction such as in the tibialis posterior tendon as it curves around the medial malleolus of the ankle or the flexor tendons of the fingers as they run through a pulley. In places where the tendon is more ischemic, there is a greater predisposition to degenerative processes and/or ruptures.
The fat pads associated with the tendons have several functions as follows: they distribute the synovial fluid in intratendinous bursae, they protect the blood vessels as they enter the tendon, and they act as an immune organ containing lymphocytes, granulocytes, and macrophages with their interleukins, cytokines, growth, and adipokines.
| Enthesis|| |
The area where tendon, ligament, and joint capsule or retinaculum attached to the bone is called an enthesis. To dissipate the significant forces in these regions and enhance the strength of the junctions on the bone, the enthesis may have fascial expansions that link them to different neighboring structures, for example, the bicipital aponeurosis which in addition to contributing to the supinator function of the biceps brachii, stabilizes it by inserting it into the deep fascia of the forearm in the dorsal ulna. Similar to the posterior tibialis which inserts into all of the tarsal bones of the medial and plantar midfoot, the semimembranosus muscle has multiple insertions located throughout the knee region such as in the posterior capsule of the knee, in the proximal/middle third of the tibia, in the patella, and in the medial femoral condyle. These multiple insertions improve the medial stabilization of the knee joint. The proximal and distal insertions of the tendons of the rectus femoris muscle, the only part of the quadriceps that crosses two joints, allows the rectus femoris to be not only extensor of the knee but also a flexor of the hip. These functions are facilitated by having entheses in different directions that align with the force vectors of each movement. Thus, in the clinical evaluation and the imaging of these entheses, one must bear in mind their potential anatomical complexity.
The so-called “enthesis organ” consists of connection of tendon, ligament, and joint capsule or retinaculum to the bone (the enthesis itself) along with the fibrocartilage contained within the tendon, ligament, joint capsule or retinaculum, plus the adjacent trabecular bone, and associated bursa and fat pad. At times, the deep fascia can also integrate with this “organ.” Entheses are normally avascular in their fibrocartilaginous component but may undergo vascularization in response to pathological stimuli.
Thus, the enthesis organ and its interdependent components can manifest two very different pathologic expressions as follows: microtrauma/degeneration and inflammation, with distinct lesions identified by imaging techniques such as MSKUS.
| Nerves|| |
Nerves are round or flattened cords, with a complex internal structure made of myelinated and unmyelinated nerve fibers, containing axons and Schwann cells grouped in fascicles. The architecture of peripheral nerves is made up of an external sheath – the outer epineurium – which surrounds the nerve fascicles. Each fascicle is invested by an individual connective sheath – the perineurium – which contains a variable number of nerve fibers and is responsible for the “blood–nerve” barrier. Each nerve fiber is then invested by the endoneurium. Along the course of the nerve, fibers can traverse from one fascicle to another fascicle, and fascicles can split and merge. The stromal tissue intervening between the outer nerve sheath and the fascicles is commonly referred to as the interfascicular epineurium (internal epineurium), as opposed to the outer epineurium which envelopes the nerve trunk as a whole. The amount of connective tissue of the epineurium is more abundant in large multifascicular nerves and in regions in which the nerve is mobile across joints. It provides cushioning for the nerve, and therefore, more resistance to compression injury. Externally, the outer (external) epineurium is in continuity with the loose areolar tissue of perineural tissues. Nerves have a prominent vascular supply formed by an interwoven system of perineural vessels that course longitudinally in the external epineurium and branch among the fascicles (endoneurial vessels).
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