Introduction

Historically, radiology has been the imaging modality of choice to diagnose musculoskeletal conditions, and therefore, ultrasound for this system is relatively underused.¹ These two modalities are complementary with osseous structures and abnormalities thereof readily noted on radiographic studies, but diagnosing soft tissue changes can be unrewarding or equivocal on radiographs.^1,2 Ultrasound is routinely used in human medicine to investigate soft tissue musculoskeletal-related injuries and is widely utilised in equine medicine and surgery.¹ Injuries of the tendons and muscles, changes in the cortical surface of the bone and soft tissue masses in the region of osseous structures and joints can be evaluated ultrasonographically. The concurrent use of Doppler during ultrasound examination can add valuable information to the study in certain instances.³ Another benefit of ultrasound is the ability to conduct ultrasound-guided fine-needle aspirates and arthrocentesis.³

Although magnetic resonance imaging is the diagnostic imaging modality of choice to investigate musculoskeletal soft tissue disorders, its availability, need for general anaesthesia and in some instances, the cost thereof, are often prohibitive.^1,2 Ultrasonography is more cost effective and easily accessible and can be as rewarding as MRI for many conditions.¹

It is imperative, however, possibly more so than with other diagnostic imaging procedures, that a thorough knowledge of the anatomy is known.^1,2 In order to increase the likelihood of identifying abnormalities and to aid in a comparison, it is advisable to prepare and scan the contralateral limb/join/region of interest.

Basic ultrasonographic principles apply. The limb/region of interest should be clipped free of hair and liberal amounts of acoustic coupling gel will facilitate obtaining diagnostic images.^1,2 A high-frequency transducer is needed for two reasons: the musculoskeletal soft tissue structures are often very superficial, and to obtain excellent resolution of the structures. Linear transducers are preferred to ensure that the beam is perpendicular to the soft tissue structures being imaged.^1,2 Always obtain orthogonal images.

Patient positioning will depend on the region or joint being imaged as well as the comfort or degree of pain experienced by a patient. Dynamic studies of, in particular, tendons can also be conducted which can provide further information on the degree or severity of certain injuries.¹

Finally, correlate all findings with those of the clinical and musculoskeletal examination as well as with the findings of radiographic studies.

Muscles

In longitudinal, normal muscles appear hypoechoic with fine, obliquely orientated, hyperechoic striations. These represent the fasciculi. The muscle belly is surrounded by a thin hyperechoic epimysium or fascia.³

Trauma to the muscle can involve a complete or partial rupture. The ultrasonographic appearance of muscle trauma will vary, depending on the severity of the injury as well as the age of the injury.³ More acute complete ruptures will show a loss of normal muscle architecture and very often the site has an accompanying hypoechoic focus of haemorrhage.³ As chronicity progresses, the site will appear more heterogeneously hyperechoic as the haematoma organises. The ends of the muscle at the site of rupture can appear club-like, well defined, and more echogenic than the surrounding tissue.^1,3

Partial rupture will result in incomplete loss of the normal fibre alignment and may be subtle. Comparison with the contralateral limb and surrounding muscle groups may be necessary to adequately identify such sites.^1,2

Muscle fibrosis, due to certain fibrotic myopathies, results in loss of normal architecture with a hyperechoic, often heterogenous, muscle belly. This is in comparison to muscle atrophy due to nerve degeneration. Such muscles too become hyperechoic but tend to retain their normal architecture.³

Tendons and Ligaments

Normal tendons appear as linear hypoechoic structures with a thin hyperechoic peritenon surrounding it. At the site of transition between the muscle and the tendon, it is hypoechoic in appearance, and this should not be confused with pathology.^2,3

Tenosynovitis is the word used to describe inflammation of the tendon and the tendon sheath. In these cases, the tendon sheath (between the tendon and peritenon) becomes distended with anechoic (acellular) or hypoechoic (cellular) material. The sheath can also appear thickened.^2,3

Tendinitis due to partial tears leads to the identification of anechoic to hypoechoic clefts within the tendon and the tendon usually shows an increase in the cross-sectional area. In the acute inflammatory stage, there may be a concomitant increase in blood flow as demonstrated on Doppler studies. Chronic tendon injuries lead to a thick, inhomogenous tendon with or without areas of calcification (dystrophic).^2,3

Complete tendon tears are suspected based on the absence of a normal tendon on ultrasound, with anechoic fluid in the region of the torn tendon. The end of the tendon is often retracted and "curled" on itself and more hyperechoic. One should also always investigate the area of tendon origin or insertion (close to the site of tendon tear) to exclude the presence of avulsion injuries.^2,3

If calcifying tendinopathies are suspected, then be on the lookout for hyperechoic, irregular foci within the muscle or tendon which cast distal acoustic shadows. Muscle fibre orientation and heterogeneity is largely maintained, differentiating this from chronic partial tendon ruptures.³

Bones

Although not the best imaging modality to visualise bones, ultrasound does have some use when investigating osseous pathology. Due to the acoustic impedance mismatch between soft tissue and bone, there is a lot of refection and absorption of sound waves at the surface of bone.^1,2 Thus bone appears as a markedly hyperechoic linear surface with distal acoustic shadowing. There are some exceptions. At the site of tendon or ligament origin and insertion, the surface of the bone is irregularly delineated. In juvenile patients, the physes are identified as focal defects in the cortical surface. Hyaline cartilage is anechoic to hypoechoic in appearance due to the high water content and abuts the thin hyperechoic cortical articular surface of bone. Fibrocartilage on the other hand (menisci and annulus fibrosis of intervertebral discs) is more heterogeneously hyperechoic.^2,3

Ultrasound is a good imaging modality to try and detect fissure or stress fractures not identified on radiographs and early periosteal changes associated with early osteomyelitis. With such a condition, anechoic to hypoechoic lines are noted adjacent to the cortex and one may see subtle subperiosteal cortical erosions.^2,3

The benefit of ultrasound with osseous neoplasia, often strongly suspected after radiographic investigations, is to detect any effect or invasion into the surrounding soft tissue or involvement of adjacent bones or involvement of the joint. It also aids in identifying a cortical defect to guide fine-needle aspirates. Fine-needle aspirates are an alternative to bone biopsies, proving to be less invasive, they give same-day results and are often diagnostic in up to 50% of the time. It is important to use 22-gauge needles to prevent haemodilution which may affect the efficacy of the biopsy.^2,3

Studies have investigated the use of ultrasound to assess fracture healing. Although radiography is required for a complete assessment, ultrasound proves particularly useful in cases of delayed or non-union and to show neovascularisation in callus formation.²

More targeted studies have been well documented in the literature describing the normal ultrasonographic appearance of several joints, tendons and nerves, as well as investigating the effect of certain surgeries on tendinous structures and a plethora of specific musculoskeletal injuries. Further examples and case studies are beyond the scope of this brief communication and will be highlighted during the oral presentation.

References

1.  Nyland TG, Matoon JS. Small Animal Diagnostic Ultrasound. 2nd ed. Philadelphia: Saunders; 2002.

2.  Barr F, Gaschen L, eds. BSAVA Manual of Canine and Feline Ultrasonography. 1st ed. Gloucester: British Small Animal Veterinary Association; 2011.

3.  Barr FJ, Kirberger RM, eds. BSAVA Manual of Canine and Feline Musculoskeletal Imaging. 1st ed. Gloucester: British Small Animal Veterinary Association; 2006.