Classification of Muscle Lesions

Imaging grading

MRI finding

US findings

I (strain)

Less than 5% of fibre disruption; feathery oedema-like pattern, intramuscular high signal on the fluid-sensitive sequences

Normal appearance, focal or general increased echogenicity; no architectural distortion

II (partial tear)

Oedema and haemorrhage of the muscle or MTJ may extend along the fascial planes, between muscle groups Fibres, which are disorganized and thin, are surrounded by haematoma and perifascial fluid. If haemosiderin or fibrosis is present, T2-weighted images have low signal intensity. The small calibre of the fibres at the site of injury may be also expression of incomplete healing. In high-performance athletes, MRI findings, particularly the measure of the cross-sectional area of injury, are relevant to define the rehabilitation

Muscle fibres are discontinuous; the disruption site is hypervascularized and altered in echogenicity in and around, with no perimysial striation of the area adjacent to the MTJ

III (complete tear)

Complete discontinuity of muscle fibres, haematoma, and retraction of the muscle ends

Comparable with MRI

Table 9.2

Proposed classification system

Site of lesion
Proximal MTJ
Muscle	• Proximal • Middle • Distal	• Intramuscular • Myofascial • Myofascial/perifascial • Myotendinous • Combined
Distal MTJ

This classification identifies muscle injuries according to their anatomical site. Most practitioners are able to diagnose relevant injuries based on physical examination and to plan an appropriate management. Imaging can give important information which may form the basis for longitudinal studies on the evolution of such injuries.

9.4.1 Classic Classification System

Grade I injury (strain): the tear involves few muscle fibres; swelling and discomfort are evident with maintenance or minimal impairment of strength and function.

At MR imaging, a classic ‘feathery’ oedema-like pattern visible on fluid-sensitive sequences may be associated with some fluid in the central portion of the tendon and, at times, along the perifascial intermuscular region (De Smet and Best 2000), with no discernible muscle fibre disruption or architectural distortion (Kneeland 1997).

US is often normal and may show the presence of focal or general increased echogenicity (Koh and McNally 2007). Perifascial fluid is present in almost 50% of patients.
Grade II injury (partial tear): some continuity of fibres is maintained at the injury site. Less than one-third of muscle fibres are torn in low-grade injuries, from one-third to two-thirds in moderate ones and more than two-thirds in high-grade injuries (Connell et al. 1999). Muscle strength and high-speed/high-resistance athletic activities are usually impaired, with marked loss of muscle function.

At MRI, the appearance of the lesion changes with both intensity and severity of the partial tear; changes are time dependent, and oedema and haemorrhage of the muscle or MTJ may extend along the fascial planes, between muscle groups. Fibres, which are disorganized and thin, are surrounded by haematoma and perifascial fluid (El-Khoury et al. 1996; Palmer et al. 1999). The MRI findings may be used as an estimate of time for rehabilitation (Bianchi et al. 2002; Cross et al. 2004; Slavotinek et al. 2002), and they can sometimes predict how much time high-performance athletes will be away from play (Pomeranz and Heidt 1993; Taylor et al. 1993).

At US, muscle fibres are discontinuous, the disruption site is hypervascularized, and echogenicity is altered in and around the lesion (Lee and Healy 2004), with no perimysial striation of the area adjacent to the MTJ (Koh and McNally 2007). Intramuscular fluid and a surrounding hyperechoic halo may also be appreciated (Koh and McNally 2007; Lee and Healy 2004).
Grade III injury (complete tear): at US and MR imaging, these injuries show complete discontinuity of muscle fibres, haematoma, and retraction of the muscle ends (Lee and Healy 2004); at clinical assessment, muscle function is lost (Palmer et al. 1999; El-Khoury et al. 1996; Agre 1985; Connell et al. 2004).

When extensive acute oedema and haemorrhage fill the defect between the torn edges, it is difficult to distinguish partial from complete tears, whereas real-time dynamic US imaging may be helpful (Koh and McNally 2007). If complete tears are not treated surgically, the ends of the muscle can become rounded and may tether to adjacent muscles or fascia (Koh and McNally 2007).

9.4.2 New Classification System Proposed

Anatomically, muscles have an origin, proximal and distal tendons, proximal and distal MTJs, one or more muscle bellies, and an insertion. Since injuries may involve all these sites, it is proposed to distinguish muscular, MTJ (proximal and distal), and tendon injuries (proximal and distal). Muscular lesions can be further classified as intramuscular, myofascial, myofascial/perifascial, musculotendinous, or a combination. According to the site of injury, muscle injuries are classified as proximal, middle, and distal.

Some studies suggest that the extent of a muscle injury is a prognostic factor for recovery time (Slavotinek et al. 2002; Connell et al. 2004) and variables such as the percentage cross-sectional area of abnormal muscle (typically measured on fat-suppressed images in the transverse plane), the cranio-caudal length of muscle abnormality adjacent to the MTJ (obtained from longitudinal images), and the approximate volume of muscle injury have all been proposed to estimate severity.

9.5 Imaging

US and MRI are the main methods to perform imaging assessment, relate them to patient’s clinical features, and identify possible comorbidities and any history of a previous sport injury.

Ultrasound Scanning: it can be used as a first-level diagnostic tool, and it is useful to monitor the healing process of the lesion. US allows to diagnose a structural injury of the muscle 36 to 48 h after the trauma, as the peak of haemorrhagic oedematous collection is observed after 24 h when it starts to decrease (Lee and Healy 2004). US monitoring can be performed 2, 4, or 5 days after the trauma.

Dynamic US examination is useful for the assessment of both elongation and dislocation of tertiary bundles and the extent of the lesion (Koh and McNally 2007).

Colour Doppler and power Doppler can be used to visualize the course of arteries and veins and to show that the hypervascularity within the scar tissue of the lesion is unstable.

Magnetic Resonance Imaging: it is a multiparametric diagnostic tool, used for detection of also minimal changes (Ehman and Berquist 1986). It has a 92% sensitivity for nonstructural injuries (Kneeland 1997). MRI allows wide evaluation of deeper muscles than to US examination (Koh and McNally 2007). Gadolinium can be useful to monitor the stability of the scar tissue after structural injury.

Indications are:

Prognosis of nonstructural injuries
Exclusion of a structural injury when clinical and US finding are discordant
Assessment of muscles which are difficult to examine at US
In subtotal or complete muscle lesions with suspect tendon involvement or bone-tendon avulsions