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Musculoskeletal
Ultrasound - Overview

Quick Image Reference


Figure 1: Bone appears as a distinct bright echogenic
line with no visible structures beneath.

Figure 2: Patellar tendon originating from the inferior patella.

Figure 3: A) longitudinal orientation with a 13 MHz linear transducer. B) Transverse view of muscle, fascia, and median nerve in the forearm using a linear transducer.

Figure 4: A high-resolution linear transducer (7-12 MHz or higher) is recommended for most musculoskeletal applications.

Video clip 1: Tendon range of motion – technique:

Video clip 2: Tendon range of motion imaging:

Video clip 3: Shoulder evaluation technique:

Video Clip 4: Posterior shoulder internal and external rotation.

Video clip 5: Anispotropy: angling the transducer

Video clip 6: Anisotropy artifacts:

Video clip 7: Anisotropy demonstrated on the median nerve in the forearm

Video clip 8: Transducer tilting resulting in anisotropic alteration of the median nerve appearance.

Figure 5: Longitudinal view of a clavicle.

Figure 6: A Colles fracture of the distal radius.

Video clip 9: Sternal fracture moving with respiration.

Figure 7. A knee effusion seen with a linear transducer. Orientation marker is proximal.

John P. Gullett, M.D. and Christopher Raio M.D., RDMS

I. Introduction and Indications

Clinical bedside ultrasonography offers the emergency physician (EP) numerous useful applications for diagnosing musculoskeletal pathology and guiding related procedures. [1-17] In its ideal role, bedside ultrasound is rapid, focused, accessible, portable, real-time, and interactive. It is therefore well suited for assessing acute musculoskeletal pathology in a busy emergency department (ED). This modality empowers the clinician to advance care in the ED with less dependence on other departments to acquire studies and then await the results. That being said, it is important to be clear that ED clinician-performed ultrasound is approached from the standpoint of limited focused studies to answer specific questions rather than a complete and in depth analysis that might be performed by a consultative service such as radiology or rheumatology.

There are some simple applications of bedside musculoskeletal (MSK) ultrasound that require minimal training and are within the scope of emergency ultrasound. Some applications of bedside ultrasound may be superior to conventional radiographs such as fractures of the ribs and sternum.[1,2] Furthermore, its use in austere environments such as disaster and military scenarios is increasingly appreciated [16], as well as in medical facilities in countries without resources to support a large costly radiology department.

Clinically, indications for emergency department MSK ultrasound include pathology such as fractures, effusions, bursitis, dislocations, and tendon injuries. It can also be used to guide hematoma blocks of fractures, fracture reduction, peripheral nerve blocks, foreign body removal, and arthocentesis.

Clinical Suspicion for:

Fracture
Joint Dislocation
Joint Effusion
Tendon Tear/Laceration
Ligament Injury

Procedure Guidance for:

Fracture and Joint Relocation
Arthrocentesis
Fracture Hematoma Block

 

II. Anatomy

Sonographic anatomy, as with all radiology, is simply gross anatomy in black and white. The appearance of musculoskeletal anatomy on ultrasound is fortunately usually intuitive. Nonetheless, some points should be made and the sonographer should concentrate on pattern recognition of normal structures so that, even in the absence of a clear diagnosis, abnormal versus normal may be distinguished.

Bones

Bone appears bright on ultrasound because its hard densely calcified cortex reflects virtually all ultrasound waves (highly echogenic or hyperechoic) back to the transducer. As a result, usually only the cortex or surface is visualized (Figures 1,2,4). This is generally adequate because fractures of only a few millimeters displacement can be seen in the cortical margin. Effusions and acute hematomas (fluid) appear as black (i.e. not echogenic or anechoic) collections around the joint or fracture as fluid reflects virtually no ultrasound waves (anechoic) back to the probe to produce an image.

 

Figure 1: Bone appears as a distinct bright echogenic
line with no visible structures beneath.

 

Soft tissue contains many different structures to identify: fat, muscle, tendon, nerves, veins, arteries, connective tissue plains, etc.  Fortunately, the sonographic appearance of these structures is largely intuitive. (Figure 2 and 3) 

Subcutaneous Fat is relatively hypoechoic with thin septations of connective tissue (Figure 1). The thickness of this layer depends on the body habitus of the subject and may require increasing the depth setting of your machine.

Muscle when scanned longitudinally, appears as slabs of irregularly striated hypoechoic tissue contained within the thin hyperechoic lines of fascia. Viewed transversely, muscle striations or septa appear dotted and punctuate, or form short lines. Fascial lines separate muscle compartments (Figure 3).

Tendons have a characteristic narrow densely striped fibrillar appearance of parallel lines when viewed longitudinally.  They are more echogenic and more densely striated than muscle.  Transversely, tendons appear as round or flattened ovals with a punctate interior.  When assessing tendons, it is crucial to understand the artifact of anisotropy, which gives the appearance of dark areas within the tendon and can be mistaken for tears or defects.  This is discussed in the Pitfalls section.  Ligaments attach to bone and appear similar to tendons but tend to be more irregular and difficult to discern. [14]

Synovial Bursa are normally potential spaces that are collapsed and not often visualized. They may generally contain up to 4mm of fluid - more if inflamed or infected.

Hyaline Articular Cartilage appears as a thin hypoechoic rim over a hyperechoic bony cortex.

Tendons have a characteristic pattern of fine parallel lines when viewed longitudinally (Figure 2,3). They are more echogenic and densely striated than muscle. Transversely, tendons appear to be round or flattened ovals with a punctuate interior. When assessing tendons and nerves it is crucial to understand the artifact of anisotropy (see below), which gives the appearance of dark areas within the tendon and can be mistaken for tears or defects.

Ligaments connect bones and appear similar to tendons but tend to be more compact and irregular. They are only a few millimeters in width and difficult to discern, especially in transverse view.[3]


Figure 2: Patellar tendon originating from the inferior patella.

Nerves are visible running along fascial planes, paired with blood vessels, and sometimes within muscles. Longitudinally, they appear as hyperechoic fibrillar cords. They may look similar to tendons but are more hyperechoic and, viewed in transverse orientation, the nerve fascicles give a “honeycomb” appearance and are less densely fibrillar than tendons (Figure 3a and 3b).

 



Figure 3: A) longitudinal orientation with a 13 MHz linear transducer. B) Transverse view of muscle, fascia, and median nerve in the forearm using a linear transducer.

 

III.  Scanning Technique and Normal Findings

When evaluating musculoskeletal and soft tissue structures, it is generally best to use a high frequency linear array transducer (7-12 MHz). (Figure 4)  This probe is useful for showing superficial structures at high resolution and is also best to visualize blood vessels and nerves located near the surface.  Nevertheless, it is entirely possible to use other probes to evaluate deeper structures such as a femur within a large thigh.

One of the advantages of clinician-performed ultrasound is the dynamic nature of exam.   It is often helpful to have patient pinpoint the area of complaint or maximal tenderness to guide the exam.  Furthermore, examination of contralateral normal structures for comparison is helpful.  As with all diagnostic ultrasound, knowing the normal appearance of the sonographic anatomy is essential to assure successful identification of pertinent structures.
The anatomical appearance of complex musculoskeletal structures such as the wrist or joints though can be quite challenging.  In these instances it can be helpful to first identify an anatomical point of reference, such as the hyperechoic cortical lines of expected bony structures, and then subsequently identify adjacent tendons, ligaments and muscles.

Like with all other examinations, the probe marker is pointing to the right of the patient in transverse scanning and has a cranial/proximal orientation in longitudinal axis scanning.
Very superficial structures might be better visualized when scanned with a standoff pad or in a water bath; whatever is feasible in the clinical environment.

IV.  Pathology

Fractures and Abnormal Fluid

There are certain technical points to understand in order to more easily and accurately perform and interpret your exams. When evaluating musculoskeletal and soft tissue structures it is generally best to use a high frequency linear array transducer (7-12 MHz, Figure 4). This transducer is ideal for showing superficial structures at high resolution, including blood vessels and nerves. Nevertheless, it is possible to use other transducers to evaluate deeper structures such as a femur within a large thigh.

Figure 4: A high-resolution linear transducer (7-12 MHz or higher) is
recommended for most musculoskeletal applications.

The first principle of technique is patient positioning. The patient and sonographer should both be in a position of comfort. If the sonologist is in an ergonomically awkward or uncomfortable position, the exam will be an unpleasant experience for all, and likely of poor quality. Generally, a natural anatomic position is best for examining most musculoskeletal structures.

One of the advantages of clinician-performed ultrasound is the ability to perform a dynamic exam. The musculoskeletal system is based on motion and ranging. This should be taken advantage of by sonographers. Observe tendons moving through their range of motion, for example, have the shoulder internally an externally rotate to observe the humeral head in the glenoid when evaluating for dislocation, and so on (Video clip 3,4).


Video clip 1: Tendon range of motion – technique: place the hand in a water bath and, hover a high frequency linear transducer over the tendon in question – in this case a flexor tendon of the hand, have the patient flex the digit or other attached structure while observing the US screen.

Video clip 2: Tendon range of motion imaging: Observe the flexor tendon sliding along its path. Look for discontinuities such as complete tears with a cut end sliding into view, or partial tears as a dark or raged section in a focal area of the tendon.

Video clip 3: Shoulder evaluation technique: This video shows the positioning and technique for observing shoulder internal and external rotation.

Video Clip 4: Posterior shoulder internal and external rotation. This view (corresponding to video clip 3) shows the posterior humeral head rotating in the glenoid, confirming shoulder reduction, and demonstrating the infraspinatus/teres minor complex in motion.

It is often helpful to have patient pinpoint the area of maximal tenderness to guide your exam. As emergency physicians we do not aspire to perform comprehensive exams, but rather focused investigations to identify focal acute pathology. Consider using a copious amount of gel or a standoff pad on a painful area to minimize the need for pressure with the transducer.

Finally, examination of contralateral normal anatomy for comparison is crucial. As with all ultrasound, knowing the normal appearance of the sonographic anatomy is essential. However, even a novice may compare the acute structure to the normal contralateral structure and reasonably deduce the presence and likely, the nature, of the lesion present.

Understanding the scope of the ED amateur sonographer in point of care musculoskeletal ultrasound creates much anxiety and hesitation to use the ultrasound out of fear of doing an inadequate exam. Keep it simple. The limited exam of the ED clinician may be guided by a few simple principles. Most of the pathology the EP cares to find will be marked either by a visible fracture or abnormal fluid or both. Simply identifying abnormal fluid in musculoskeletal structures is almost all that an EP need be able to do to derive the majority of use from point of care musculoskeletal US. Infection, inflammation, effusion, cysts, abscesses, hematomas are all the hallmarks of emergency musculoskeletal pathology and easily identifiable by US. The EP need only locate and recognize this and use his or her clinical observations to progress them onto the next best step. The characterization of abnormal fluid is, in fact, a complex subject that should be left to expert radiologists, however, the emergency physician is certainly capable of locating and recognizing abnormal fluid.

Anisotropy: An important concept to understand is the phenomenon of anisotropy. This is a sonographic artifact that is occurs when viewing tendons and nerves. Anisotropy is the property of being directionally dependent. In terms of ultrasound, the physical properties of nerves and tendons require that the transducer beam must be perpendicular to the axis of the tendon or nerve. When viewing these structures angling the probe back and forth long the axis of the structure will cause the structure to appear darker and less distinct when the transducer is no longer parallel. When viewing a nerve, for instance, in transverse axis, angling the probe back and forth along the nerve’s axis will cause the bright honeycombed pattern to darken and become less discernable. This can make identifying nerves and tendons difficult if the sonographer is unaware of this phenomenon. Nerves and tendons, of course, rarely run a straight course for any significant distance. Thus, the sonographer must continually change the transducers angle to match the axis of the tendon, for instance.

Anisotropic artifacts may appear as a dark area within the length of a tendon or nerve. This may be mistaken for a partial tendon tear, which can look similar. The savvy sonographer should carefully assess and be aware of the transducer angle at all times to maintain clear and accurate views of these particular structures. Anisotropy is unavoidable, and so should not be a source of frustration or discouragement. Rather, these adjustments should be considered simply routine technique.

Video clip 5: Anispotropy: angling the transducer over the flexor tendons of the hand will cause anisotropic derangements of tendon and nerve appearance.

Video clip 6: Anisotropy artifacts: when the transducer is not perpendicular to the tendon or nerve – in this case the flexor tendons of the hand as demonstrated in video 5 – the structure darkens and loses the bright fibrillar/punctate pattern that makes them recognizable.

Video clip 7: Anisotropy demonstrated on the median nerve in the forearm. As the transducer is angled away from being perpendicular to the nerve (Video clip 9) the nerve that is normally bright with a honeycomb-like appearance becomes dark and difficult to distinguish.

Video clip 8: Transducer tilting resulting in anisotropic alteration
of the median nerve appearance. See also video clip 7.

IV. Pathology

Fractures
Ultrasound imaging of fractures is generally very straightforward. The dramatic difference in echogenicity of the bright outer cortex of the bone with the overlying hypoechoic soft tissue makes distinguishing even small cortical disruptions of 1mm relatively easy. Fractures appear as a sharp discontinuity of the bright line of boney cortex. Sometimes hypoechoic hematoma/effusion of the immediately surrounding soft tissue is visible as well (Figure 5,6; video clip 5).



Figure 5: Longitudinal view of a clavicle.



Figure 6: A Colles fracture of the distal radius.


Video clip 9: Sternal fracture moving with respiration.

Ultrasound has been studied for use in identifying numerous anatomical fractures. The role of ultrasound in evaluating acute fractures in the emergency department is that of a point of care test. That is, primarily used to determine whether or not a fracture is present, and how displaced the fracture is. The geometry and comminution of a fracture however, is more difficult to assess with a two-dimensional ultrasound beam. Ultrasound guidance of hematoma blocks for anesthesia of the fracture is another simple and effective application. More advanced uses include detecting occult pediatric fractures, small avulsion fractures, and guiding fracture reductions.

Ultrasound is most unambiguous when evaluating long bone fractures such as distal radius fractures, femur, clavicle, etc. Ultrasound of fractures in or around joints is more difficult to evaluate due to the convoluted surfaces and anatomy involved. Thus, sensitivity for fracture detection in these areas is less reliable. Ultrasound has been shown to have higher sensitivity than plain radiographs for diagnosing rib and sternal fractures.[13-15]

Effusions
Fluid is an ideal ultrasound medium and is therefore easily detected by ultrasound clinically. As it is a good medium for transmitting ultrasound waves, there is little wave reflection, thus fluid appears black (anechoic) on ultrasound. Joint effusions will appear as black collections of fluid within the joint space. These are usually clinically evident, but may be subtle. In such a case ultrasound may be especially useful to guide an arthocentesis. The presence of irregular internal echoes or material may indicate pus, fibrinous material, or a complicated effusion.


Figure 7. A knee effusion seen with a linear transducer. Orientation marker is proximal.

Soft Tissue, Muscle, Tendon and Ligament Injuries
Soft tissue infection, inflammation, effusion, cysts, abscesses, hematomas are hallmarks of emergency musculoskeletal pathology and often readily identifiable by ultrasound because they present with an abnormal amount of fluid.  Often recognition of abnormal fluid combined with physical exam and clinical observations will lead to a diagnosis.  The characterization of abnormal fluid is, however, quite complex at times, and may not need to be the primary concern of the emergency physician sonologist.
Muscle can be evaluated for injuries and inflammatory changes such as partial or full thickness tears, hematoma and edema. Tendons can be evaluated for partial or complete rupture, tendinosis and tenosynovitis. Here the dynamic nature of diagnostic ultrasound can be utilized and muscle contraction and relaxation as well as graded compression can be used to differentiate between specific pathologies.
Real-time ultrasound offers the ability to examine ligaments dynamically.

 

IV. Pearls and Pitfalls

• Failure to compare to normal contralateral anatomy
• Failure to obverse structures throughout their dynamic range when possible
• Failure to set up the exam beforehand so as to be comfortable for both patient and
sonographer.
• Failure to account for anisotropy in the study of tendons and nerves
• Failure to acquire good recognition of normal structures
• Perform the exam based on the focal complaint
• Use copious gel or stand-off pad over painful or topographically convoluted surfaces
• Scan all structures in two planes, i.e. longitudinal and transverse
• Failure to know one’s limits and that of the tools you use

VI.  References

1.   Valley VT, Stahmer SA.
Targeted musculoarticular sonography in the detection of joint effusions. Acad Emerg Med. 2001;8:361-367.

2.   Tayal VS, Antoniazzi J, Pariyadath M, Norton HJ.
Targeted musculoarticular sonography in the detection of joint effusions. Acad Emerg Med. 2001
Prospective use of ultrasound imaging to detect bony hand injuries in adults.
J Ultrasound Med. 2007;26(9):1143-8.

3. Freeman K, Dewitz A, Baker W.
Ultrasound guided hip arthrocentesis in the ED. Am J Emerg Med. 2007;25:80-86.

4. LaRocco BG, Zlupko G, Sierzenski P.
Ultrasound diagnosis of quadriceps tendon rupture. J Emerg Med. 2008;35:293-295.

5.  Roy S, Dewitz A, Paul I.
Ultrasound-assisted ankle arthrocentesis. Amer J Emerg Med. 1999;17:300-301.

6. Canagasabey MD, Callaghan MJ, Carley S.
The Sonographic Ottawa Foot and Ankle Rules Study (the SOFAR Study). Emerg Med J. 2010 Oct 13.

7.  Adhikari S, Blaivas M, Lander L.
Comparison of bedside ultrasound and panorex radiography in the diagnosis of a dental abscess in the ED. Am J Emerg Med. 2010 Apr 30.

8. laton A, Poletti PA, Van Aaken J, Fusetti C, Della Santa D, Beaulieu JY, Becker CD.
Occult fractures of the scaphoid: the role of ultrasonography in the emergency department.
Skeletal Radiol. 2011 Jan 1.

9.  Wiler JL, Costantino TG, Filippone L, Satz W.
Comparison of ultrasound-guided and standard landmark techniques for knee arthrocentesis. J Emerg Med. 2010;39(1):76-82.

10. Costantino TG, Roemer B, Leber EH.
Septic arthritis and bursitis: emergency ultrasound can facilitate diagnosis. J Emerg Med. 2007;32(3):295-7.

11.  Bücklein W.
 [Emergency ultrasound diagnosis of the musculoskeletal system]. Rontgenpraxis. 1999;52(3-4):103-9. Review. German.

12. Emergency Ultrasound Guidelines – 2008. 
American College of Emergency Physicians [ACEP]. ACEP clinical policy approved by the Board of Directors, October 2008. [Accessed January 5th 2011].

13.  Griffith JF, Rainer TH, Ching AS, Law KL, Cocks RA, Metreweli C.
Sonography compared with radiography in revealing acute rib fracture.  AJR Am J Roentgenol, 1999; 173:1603-1609.

14.  Jin W, Yang DM, Kim HC, Ryu KN.
Diagnostic values of sonography for assessment of sternal fractures compared with conventional radiography and bone scans. J Ultrasound Med. 2006;25(10):1263-8.

15. You JS, Chung YE, Kim D, Park S, Chung SP.
Role of sonography in the emergency room to diagnose sternal fractures. J Clin Ultrasound. 2010;38(3):135-7.

16.  McNeil CR, McManus J, Mehta S.
The accuracy of portable ultrasonography to diagnose fractures in an austere environment. Prehosp Emerg Care. 2009;13(1):50-2.

17.  Marshburn TH, Legome E, Sargsyan A, Li SM, Noble VA, Dulchavsky SA, Sims C, Robinson D.
Goal-directed ultrasound in the detection of long-bone fractures. J Trauma. 2004;57(2):329-32.

18. Jacobson, Jon A.
Fundamentals of Musculoskeletal Ultrasound. Philadelphia, PA: Saunders/Elsevier, 2007. Print.

 

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