Discussion

While the most common indication for musculoskeletal ultrasonography is the investigation of tendon or ligament injury, ultrasound can also be used for evaluating a number of other structures, including:
- joints
- bone
- muscle
- wounds

This presentation will review these areas with specific examples.

Joints:
Ultrasonography is useful to identify the following:
1) Joint effusion, where the nature of the effusion can be determined (e.g. floccular in the case of joint sepsis).
2) Synovial hypertrophy - especially evident in joint sepsis.
Villonodular synovitis can be identified on the dorsal aspect of the metacarpophalangeal joint.
3) Articular cartilage defects - many of the joints of the limb have very thin cartilage and the resolution of ultrasound limits its usefulness to identify subtle cartilage lesions. However, the technology is advancing rapidly and better definition is being achieved. Younger animals have thicker cartilage and abnormalities such as osteochondrosis of the stifle are easily identified, because of the thicker cartilage in this region, with or without underlying subchondral bone defects.
4) Chip fractures of the margins of the joints.
5) Abnormalities of associated soft tissue structures:
a) Collateral ligaments - such as avulsions, desmitis, with or without communication with the underlying joint. Many collateral ligaments ascribe a curved path around the joint, so that the majority of the ligament will be "off- incidence" and dark.
b) Menisci - the menisci of the stifle is particularly easy are imaged longitudinally from the medial and lateral aspects of the femoropatellar joint and are fine-stippled triangular shaped structures with their apex directed towards the centre of the joint. Meniscal tears, most frequently affecting the medial meniscus, can be identified in areas of the meniscus not visible arthroscopically, allowing better assessment of the extent of the tear.
c) Intra-articular ligaments e.g. cruciate ligaments of the stifle. These ligaments can be identified with some practice if the limb is maintained in a flexed position and lower frequency sector or curvilinear transducers applied to the cranial and caudal aspects of the stifle. The transducer is directed caudodistally towards the tibial plateau for the cranial cruciate, or craniodistally from the popliteal region for the caudal cruciate. However, the sensitivity and specificity for ultrasonography is detecting lesions of these structures has not been determined.

Bone:
Because the bone surface of the distal limb is usually very superficial, it is easily injured. This, together with the sensitivity of ultrasound to changes on the surface of bone, ultrasonography is a useful way of investigating bony abnormalities, especially where radiographical appraisal is limited:
1) Enthesiophytosis at the insertion of tendons, ligaments, and fibrous joint capsules.
2) Fractures - small chip fractures area readily identified ultrasonographically where their size and location can be accurately determined preoperatively. In the upper limb and body, where large body mass allows few radiographical views with scatter compromising definition, ultrasonography is particularly useful, as in fractures of the wing of the ilium, ribs, deltoid tuberosity, and third trochanter. However, in large or multiple fractures, multiple shadowing artefacts make appreciation of the exact fracture configuration difficult.
3) Osteitis/osteomyelitis - the presence of anechogenic fluid adjacent to the surface of bone is diagnostic for septic osteitis. There is usually also an irregular surface to the underlying bone. Sequestra can be identified via there thin bone surface echo (BSE) and usually lie in an incomplete involucrum. Their length can be recorded over time to determine if they are resorbing under conservative management.
4) Callus formation - the amount of callus and extent of bridging of the fracture gap can be identified ultrasonographically. Immature callus does not cause acoustic shadowing so that active callus on the surface of a bone shows a thickened BSE. Once the callus has matured, the BSE returns to normal width.

Muscle:
Ultrasonography is particularly useful for the diagnosis of muscle abscessation and haematomata. Acute intramuscular haemorrhage is usually echogenic, so that the only apparent change is a loss of the normal coarse striated pattern of muscle. Once the haemorrhage organises into a haematoma, anechoic areas are visible as a multiloculated structure with jelly-like consistency when ballotted. Subsequently, the haematoma will either resorb or become a seroma, characterised ultrasonographically as an anechoic area within the muscle, with or without loculations. A muscle abscess has a similar appearance except that it usually has an obvious echogenic fibrous capsule and floccular fluid. Most seromas and abscesses cause acoustic enhancement deep to them. If the abscess contains large numbers of white cells and debris that settle in the most dependent part.
Post anaesthetic myopathy results in increased echogenicity, enabling ultrasound to differentiate this from neuropathy. Other rare abnormalities which can be detected ultrasonographically include neoplasia such as haemangiosarcoma and radbomyosarcoma.



Wounds:
Ultrasonography has particular benefits in the assessment of wounds and discharging tracts as many penetrating equine wounds involve soft tissues +/- the surface of bone. However, it is not a substitute to radiography and both are used together to obtain a full evaluation.
The tract of the penetrating wound, as well as more established discharging tracts, can be imaged and followed to their limit. This can indicate the cause of the discharging tract (e.g. sequestration, foreign body) and demonstrate if any adjacent
structures have been damaged (e.g. joints, underlying bone).

Ultrasound is a sensitive detector of foreign bodies, including radiolucent foreign bodies such as wood, all of which cast acoustic shadows or reverberation artefact. While the acoustic shadowing is helpful in locating the foreign bodies, it also obscures underlying structures. Hence, the ultrasonographic examination should involve any many planes as possible around the foreign body. Air also casts an acoustic shadow with a variable amount of reverberation artefact which can obscure detail and therefore, large gaping wounds with or without aspirated air under the skin are not good candidates for ultrasonographic examination. In these cases, digital palpation, contrast radiography, or surgical exploration are better alternatives.