Keywords

8.1 Introduction

The “classification” of muscle injury refers to the process of describing or categorising a muscle injury according to type. Approaches to the classification of muscle injuries have utilised the site of injury (proximal versus distal), mechanism of injury (contusion versus non-contact), predominant tissue involved (tendon versus muscle), nature of onset (overuse versus acute) and imaging findings (magnetic resonance imaging (MRI) positive/negative). The effective classification of muscle injury is of relevance when determining the most appropriate management, informing patients of the injury nature, and when trying to evaluate the efficacy of different diagnostic or treatment approaches. By contrast, to “grade” a muscle injury is to provide an indication of the injury severity [1]. Severity may be determined by the injury history or mechanism, significance of the described symptoms and clinical signs and extent of imaging findings. However, from the perspective of athletes and coaches, the most important measure of injury severity is the length of time taken to return to full sports participation following injury.

Despite (or perhaps because of) numerous muscle injury classification and grading systems existing, international consensus on the most appropriate approach remains divided. The following chapter aims to provide an overview of existing (historical and current) classification and grading approaches to acute muscle injury.

8.2 History of Muscle Injury Classification and Grading

In the first half of the twentieth century, increasing participation in sporting activities led to clinical interest in the recognition and management of musculoskeletal injuries. Muscle injuries were recognised as a frequent occurrence and from an early stage were identified and described in terms of their various symptoms, signs and recovery. Reflecting this, in 1966, the American Medical Association (AMA) published a categorical system for clinically classifying and grading muscle injuries [2]. Despite lacking an evidence base, the approach of the AMA formed the basis of subsequent approaches to grading muscle injuries (see Table 8.1).

Table 8.1 AMA system for muscle injury grading
Full size table

In the 1990s, the availability of MRI and ultrasound (US) allowed for the visualisation of underlying muscle structure and injury. Radiologists correlated clinical observations in injured patients with observed imaging characteristics and established early radiological grading approaches [3,4,5,6]. However, the initial radiology literature was limited by small sample sizes and as a result lacked any substantive evidence of a relationship between imaging appearance and prognosis [5,6,7].

In the early twenty-first century, using larger cohorts, researchers attempted to correlate MRI findings with clinical outcome [3, 8, 9]. Consistently, it was found that injures that were MRI negative for any observable abnormality had a significantly better prognosis than all other grades of injury [10,11,12,13,14,15]. Using a large sample of elite footballers with hamstring injuries, researchers also observed a statistically significant difference in clinical outcome between MRI-determined grades 1 and 2 muscle injury [8]. However, the wide variance observed in reported return to play (RTP) durations appears to limit the clinical utility of this finding [16]. Concurrently, researchers assessed MRI severity as a continuous variable determined by length, cross-sectional area and estimated volume of imaging abnormality and correlated these findings with RTP duration [11,12,13, 17, 18]. Methodological constraints and limitations in the data have meant that this approach has had limited success in predicting RTP duration [19]. Finally, disruption to the intramuscular tendon has been proposed as a key predictor of RTP duration, but further evidence is required to determine its true significance [20,21,22,23].

In recent years, there has been increased attention directed at developing standardised and practical approaches to muscle injury classification and grading [1, 21, 24,25,26,27,28]. The following section will briefly consider the strengths and weaknesses of the proposed systems.

8.3 Modern Approaches to Muscle Injury Classification and Grading

8.3.1 MRI-Based Muscle Injury Scoring Scale for Return to Play—Cohen et al. [25]

Nature:

Grading

Study sample:

National Football League (American football)

Muscle:

Hamstring

Cohen et al. proposed a novel scoring system based on age and a range of MRI variables including the number of muscles involved (1–3), location (proximal, middle, distal), insertional involvement (yes/no), cross-sectional percentage of muscle involvement, amount of muscle retraction (cm) and long axis muscle involvement. Each variable was allocated a score and the total score considered for severity assessment [25].

Analysing 43 National Football League injuries over a 10-year period for a relationship between “total MRI score” and the number of games lost to injury, the authors concluded that a rapid RTP was more likely in those injuries with an MRI score of less than 10, compared to a score of greater than 10. Indicators of a poor prognosis included multiple muscle involvement, a higher percentage (>75%) of transverse muscle involvement, more than 10 cm of craniocaudal involvement and muscle retraction [25]. However, its 10-year retrospective nature, limited subject numbers, lack of detail regarding the RTP process and failure to be reproduced have limited this study’s impact [29].

8.3.2 MRI- and US-Based Acute Muscle Strain Classification System—Chan et al. [24]

Nature:

Classification

Study sample:

N/A

Muscle:

General

Chan et al. described a three-layered image-based classification system for non-contact muscle injury, with the novel aspects pertaining to the detailed MRI-based description of the injuries’ anatomical locations. Initially, radiologically classified as proximal musculotendinous junction (MTJ), muscle or distal MTJ, the injury is then further subclassified as proximal, middle or distal, before being defined by the principle tissue involved, specifically, intramuscular, myofascial, myofascial/perifascial, myotendinous or combined [24].

As a radiological classification, there is no published reliability or validity. This system has a narrow focus, having no inclusion for a primary tendon injury and no approach to more than one muscle being involved, and is unclear in terminology and taxonomy [24].

8.3.3 The Munich Consensus Statement—Mueller-Wohlfahrt et al. [26]

Nature:

Classification and grading

Study sample:

Professional football

Muscle:

General

The Munich consensus approach addresses both the classification of muscle injury and the grading of severity for non-contact muscle injury using both clinical and radiological information. As such, it is unique with respect to the modern era approaches by including a combination of clinical and radiological findings to define the nature of muscle injury.

Taxonomically, the classification distinguishes direct (contusion and laceration) from indirect muscle injury. Indirect muscle injuries are then further classified as either functional or structural injuries, sub-classified further into a type of injury, and finally divided into either a diagnostic group (e.g. fatigue-induced muscle disorder, DOMS, muscle- or spine-related neuromuscular disorder) or severity grade (minor partial, moderate, subtotal, complete or avulsion). Each classification/grade is defined and described with expected symptoms, signs and imaging findings. Research on UEFA footballers supported the observation that structural injuries (largely determined by those that are MRI positive for muscle damage) have a greater time loss than functional injuries and that moderate and subtotal/total injuries have a worse prognosis than minor partial muscle tears [30].

While comprehensive in approach, elements of the construction of the Munich classification and grading system are based on principles that are not universally accepted. For example, the term “functional” in this classification has a specific meaning, distinct to its use in other areas of medicine [31]. While the use of the term “functional injuries” may be clinically appealing, there remains only limited basis upon which to base functional diagnoses such as “spine-related neuromuscular muscle disorder” and “muscle-related neuromuscular muscle disorder” [32]. It could be argued that functional injuries actually reflect either microscopic (structural) damage below the current resolution of imaging or a combination of factors.

With regard to non-contact muscle injuries, the Munich consensus utilises a mixed approach of three classifications [26]. Minor and moderate partial tears (type 3A and 3B) are differentiated taxonomically from (sub)total muscle tears/tendinous avulsions (type 4). There is no distinction based on the specific tissue involvement (such as the intramuscular tendon); rather, the separation is based on the extent of the injury as determined by imaging and clinical appearance, with many similarities to the nomenclature of the 1960s [2].

Efforts to validate the Munich classification system [30] revealed a wide range in RTP durations for minor partial (3a), moderate (3b) and (sub)total tears (3–132, 8–111 and 52–61 days, respectively). This observation suggests the system may have limited utility in predicting the RTP duration for these distinctly classified muscle injuries.

8.3.4 British Athletics Muscle Injury Classification—Pollock et al. [27]

Nature:

Classification and grading

Study sample:

Track and field

Muscle:

Hamstring

Based on hamstring injuries in elite track and field athletes, this approach specifically addresses non-contact injuries [27]. The approach grades injury severity from 0 to 4 based on a combination of clinical and MRI features, before refining grades 1–3 to reflect the predominant structure involved (myofascial, muscle tendon junction or intra-tendinous). Grade 0 injuries are those that are MRI negative, and an additional differentiator of “N” may be applied to any grade when there is a clinical “suspicion of a neural component” to the injury. The authors have illustrated substantial levels of intra- and inter-rater reliability [33] and, in a subsequent retrospective study of 65 hamstring injuries, assessed the time to return to full training (TRFT) and recurrence rate versus the grade and classification of injury [21]. MRI-negative (grade 0) injuries were associated with a shorter TRFT than all other injury grades, but there was no difference in prognosis between grades 1 (small tear) and 2 (moderate tear) or between myofascial and MTJ injuries. There was also a significant difference in both RTP and re-injury rate for injuries that involved the intramuscular tendon [21]. While an independent prospective study applying this approach in a football cohort found an overall effect for severity grading and anatomical sites [33], they accounted for only 7.6–11.9%, respectively, of the total variance in time to RTP. Subsequently, while the length of time to RTP on average was greater for higher-grade injuries, we urge clinicians to look beyond the average values and to consider the implications of the overlap (variance) between the injury categories when considering its prognostic utility.

The degree of anatomical detail provided by this approach is enticing and may be relevant for determining best practice treatment modalities. The approach has been shown to be reliable and is based upon the available evidence of prognostic elements involved in muscle injury. Contusions are ignored in this approach to muscle injury due to their limited relevance in track and field.

8.3.5 The MLG-R Muscle Injury Classification System—Valle et al. [28]

Nature:

Classification and grading

Study sample:

N/A

Muscle:

General

Driven by the experiences of the Barcelona Football Club and in collaboration with international colleagues, a muscle injury classification and grading approach based on four taxonomic layers was proposed [28]. This classification system for muscle injuries is based on a four-letter initial system: MLG-R, respectively, referring to the mechanism of injury (M), location of injury (L), grading of severity (G) and number of muscle re-injuries (R). Based on clinical history, the first identifier distinguishes the mechanism of injury as either direct (D) or indirect (I), with indirect injuries additionally identified as sprinting or stretch related. The second and third major identifiers are MRI variables describing the anatomical location and grade of the injury, respectively. The grade of the injury is determined by specific features of oedema and haemorrhage and the cross-sectional area of signal hyper-intensity. The final identifier (R) relates to the re-injury status.

Unique to this system is the incorporation of re-injury status into the grading. The presence of re-injury may influence rehabilitation progression and RTP decisions and may therefore be relevant in a classification paradigm. There is currently no reliability or validity study on the potential of this system to provide distinguishing prognoses.

While the detailed approach to the injury description supports the effective understanding of the injury nature, it includes a complex nomenclature which may limit the appeal of the system to the broader clinical and sporting community.

8.3.6 Grading Based on Connective Tissue Injury—Prakash et al. [34]

Nature:

Grading

Study sample:

Various sports

Muscle:

Calf

Prakash et al. grade injury severity 0–3 based on connective tissue involvement on MRI in calf injuries. They define grade 0 injury as oedema or fluid adjacent to an intact tendon/aponeurosis/epimysium without myofibril detachment; grade 1 injury as myofibril detachment without tendon/aponeurosis/epimysium change; grade 2 injury as myofibril detachment with adjacent tendon/aponeurosis/epimysium increased signal, delamination or defect, but no retraction; and grade 3 injury as myofibril detachment with adjacent tendon/aponeurosis/epimysium retraction indicating failure.

In a retrospective analysis in 100 patients with calf injuries, they found a correlation between higher grade and longer time to RTP. Although the time to RTP on average was greater for higher-grade injuries, there is an overlap between the grades which limits prognostic effectiveness in any individual athlete. Furthermore, to what extent these results can be generalised to hamstring injuries remains unknown.

8.4 Conclusion

In the last decade, traditional means of categorically describing and grading muscle injury have been challenged. This has been largely driven by the high rate of muscle injury and the desire of athletes and coaches to have an accurate prognosis for RTP. An ongoing challenge to any consensus on muscle injury nomenclature is a consistent approach to language. While there are broad similarities in the description of the classic non-contact sprinting injury to muscle, there remains variability in the language utilised (e.g. tear, strain, injury). The Munich consensus argued against the use of strain to describe non-contact muscle injury due to its confusing history and implied aetiology and suggests preferentially utilising the term ‘tear’. The use of “tear” is also the preferred approach of British Athletics, while Chan et al. continue to utilise strain, and both Valle et al. and Cohen et al. utilise the neutral term “muscle injury”. These differences may be superficial, but there remain other disagreements in language that may not be so easily reconciled. This includes the use of non-standardised terminology such as functional, muscle-related neuromuscular disorder and other diagnoses, which potentially reflect the range of experience of clinicians involved in the classification development and other factors which may challenge consensus.

While increased availability of imaging and larger research cohorts has led to increased understanding of relevant features in determining RTP, ultimately, injury rehabilitation is influenced by a myriad of pathophysiological, social and psychological factors. As a result, the clinical and radiological appearance of any given injury at a single time point in an injury process is unlikely to provide more than a small part of the prognostic picture [35, 36]. This may also partly explain the limited scientific evidence for the prognostic efficacy of such approaches.

Ultimately, a clear diagnosis, based on the accumulated evidence from a careful history, examination and imaging, should assist in the prescription of an appropriate rehabilitation programme. This should be the goal of any clinician. When classifying and grading muscle injury using any of the current approaches for the purposes of providing a prognosis, this should be done while recognising the limitations incumbent in that system. In essence, more evidence on the clinical utility of these systems is needed prior to clinical practice being able to rely solely on a particular classification and grading system.