Knee Surg Sports Traumatol Arthrosc (2012) 20:2356–2362
DOI 10.1007/s00167-012-2118-z
SPORTS MEDICINE
Acute muscle strain injuries: a proposed new classification system
Otto Chan • Angelo Del Buono • Thomas M. Best
Nicola Maffulli
•
Received: 29 March 2012 / Accepted: 18 June 2012 / Published online: 7 July 2012
Ó Springer-Verlag 2012
Abstract
Purpose To better define and classify acute muscle strain
injuries.
Methods Historically, acute muscle strains have been
classified as grade I, II and III. This system does not
accurately reflect the anatomy of the injury and has not
been shown to reliably predict prognosis and time for
return to sport.
Results We describe an imaging (magnetic resonance or
ultrasound) nomenclature, which considers the anatomical
site, pattern and severity of the lesion in the acute stage. By
site of injury, we define muscular injuries as proximal,
middle and distal. Anatomically, based on the various
muscular structures involved, we distinguish intramuscular,
myofascial, myofascial/perifascial and musculotendinous
injuries.
Conclusions This classification system must be applied to
a variety of muscle architectures and locations to determine
O. Chan
Department of Radiology, The London Independent Hospital,
1 Beaumont Square, London E1 4NL, UK
A. Del Buono
Department of Orthopaedic and Trauma Surgery, Campus
Biomedico University of Rome, Via Alvaro del Portillo,
Rome, Italy
T. M. Best
Division of Sports Medicine, Department of Family Medicine
The OSU Sports Medicine Center, The Ohio State University,
Columbus, OH, USA
N. Maffulli (&)
Centre for Sports and Exercise Medicine, Barts and The London
School of Medicine and Dentistry, Mile End Hospital,
275 Bancroft Road, London E1 4DG, UK
e-mail: [email protected]
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its utility; additional studies are therefore needed prior to
its general acceptance.
Level of evidence V.
Keywords Terminology Muscle Injury Imaging Classification
Introduction
The risk of muscle strain injuries increases in high-demand
sports [31] and accounts for a high percentage of all acute
sports injuries [22, 30]. The most commonly injured muscles are the hamstrings, rectus femoris and medial head of
the gastrocnemius, all with a greater percentage of type II
fibres, a pennate architecture, crossing 2 joints and typically
injured during the eccentric phase of muscle contraction
[3, 23, 27]. It is often difficult to predict both short-term
outcome and long-term prognosis following a muscle strain
[6], although these injuries may have a significant impact on
the athletes and their teams. Although the diagnosis is
usually clinical, imaging tools are often advocated to better
understand extent and site of lesion, the relevant prognostic
factors predictive of recovery time, return to pre-injury sport
activity and risk of recurrence [5, 16, 33, 36]. Acute muscle
injuries are commonly classified as strains (Grade I), partial
tears (Grade II) and complete tears (Grade III) [10, 24, 43].
The traditional classification system described earlier does
not take into account the exact location of the injury, which,
with the advent of MRI and ultrasound imaging, can now be
exactly identified. Therefore, to stress the concept that an
ideal classification system should inform on extent, size and
exact location of a muscle injury [2], we propose a system
that takes into account imaging (based on MRI and US)
features of acute muscle strain injuries (Tables 1, 2).
Knee Surg Sports Traumatol Arthrosc (2012) 20:2356–2362
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Table 1 Present classification system and relationship with imaging features of muscle injuries
Imaging
grading
Radiological findings
MRI
US
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
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
Fibres, 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 crosssectional area of injury, are relevant to define the
rehabilitation
III
(Complete
tear)
Complete discontinuity of muscle fibres, haematoma and
retraction of the muscle ends
Table 2 Proposed classification system
Site of lesion
1. Proximal MTJ
2. Muscle
A. Proximal
a. Intramuscular
B. Middle
b. Myofascial
C. Distal
c. Myofascial/perifascial
d. Myotendinous
e. Combined
3. Distal MTJ
MTJ musculo-tendinous junction
Materials and methods
Traumatic muscle injuries, varying on the directions and
angle movements of forces applied, may be broadly
divided into contusions, strains or lacerations [22, 30].
Contusions and strains account for more than 90 % of all
sports-related skeletal muscle injuries, while lacerations are
relatively uncommon [30]. Contusions are frequent in
contact or combat sports as a result of large compressive
forces applied directly on the muscle. Muscle strains, very
common in sprinters and jumpers [13, 22], usually arise
from an indirect insult, from application of excessive tensile forces. In acute injuries, rectus femoris, hamstrings and
gastrocnemius [13, 22] are the most commonly injured
muscles, usually at the MTJ [42]. Passive injuries are
secondary to tensile overstretch of the muscle in the
absence of contraction, whereas active injuries usually
result from eccentric muscle actions [21]. Muscle lacerations, rare in athletes, arise from direct blunt trauma to the
epimysium and underlying muscles [35].
In Grade I injury (Strain) (Table 1), the tear involves a
few muscle fibres, swelling and discomfort are complained,
Comparable with MRI
with conservation or minimally impairment of strength and
function. US findings, often normal, may indicate the
presence of focal or general increased echogenicity [35],
and perifascial fluid is present in almost 50 % of the
patients. Some authors consider ultrasonography not as
accurate as MR imaging, given the difficulty to depict the
normal hyperechoic intramuscular portion of the tendon
after injury [37]. At MR imaging, a classic ‘feathery’
oedema-like pattern visible on fluid-sensitive sequences
may be often associated with some fluid in the central
portion of the tendon and, at times, along the perifascial
intermuscular region [16], with no discernible muscle fibre
disruption (Fig. 1) or architectural distortion [34].
Grade II Injury (Partial Tear): Macroscopically, some
continuity of fibres is maintained at the injury site
(Table 1). Based on injury severity, 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 [11]. Muscle strength and high-speed/
high-resistance athletic activities are usually impaired, with
marked loss of muscle function (ability to contract). At US,
muscle fibres are discontinuous, the disruption site is hypervascularized, and echogenicity is altered in and around
the lesion [37], with no perimysial striation of the area
adjacent to the MTJ [35]. Intramuscular fluid and a surrounding hyperechoic halo may also be appreciated
[35, 37]. At MRI, appearance varies with both the acuity
and the severity of the partial tear, changes are timedependent, and oedema and haemorrhage of the muscle or
MTJ may extend along the fascial planes, between muscle
groups (Fig. 2a, b). Fibres, disorganized and thin are surrounded by haematoma and perifascial fluid [20, 43]. In
general, MRI findings, particularly the length and crosssectional area of injury, may be used as an estimate of time
for rehabilitation [7, 14, 48] and can sometimes be
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muscle fibres, haematoma (Fig. 3a, b, c, d) and retraction
of the muscle ends (Table 1) [37]; at clinical assessment,
muscle function is lost [1, 19, 20, 43]. 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 (Fig. 4) [35]. If complete tears are not treated
surgically, the ends of the muscle can become rounded and
may tether to adjacent muscles or fascia [35].
Site of muscle injury and anatomy
Fig. 1 Grade I—Coronal T1 STIR—of Rectus femoris with measurement of tear image of feathery oedema-like pattern with
intramuscular high signal on the fluid-sensitive sequences, with no
discernible muscle fibre disruption (arrow) and adjacent to distal
quadriceps tendon (arrowhead)
predictive of the time high-performance athletes will be
away from play [44, 49].
Grade III Injury (Complete Tear): At US and MR
imaging, these injuries show complete discontinuity of
The weak link in the muscle–tendon–bone chain varies
with age [9]. In children, the biomechanical weakness of
the apophyseal growth plates may lead to apophyseal
avulsion fractures when excessive tensions are applied on
the muscle–tendon–bone chain. In young adults, mechanical failure usually occurs at the muscle tendon interface; in
older adults, coexistent tendinopathy and overload of the
musculotendinous unit may contribute to the tearing process [44]. Overall, strains and complete tears occur most
often at the MTJ, the weakest link within the muscle tendon unit [16, 24], where the tendon emerges from the
muscle belly (musculotendinous junctions) and myo-tendinous junction (Fig. 5a) [35]. As observed in eccentric
Fig. 2 a, b Grade II tear of BF (Sag STIR) oedema and haemorrhage of the muscle or MTJ extending to the fascial planes of biceps femoris. In
the traditional classification system, this would have been a grade II injury. In the newly proposed system, this is a 2.B.b injury
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Fig. 3 a–d Grade III tear (Cor T1 and STIR and axial STIR showing
BF muscle and avulsed MTJ from fibula head) of BF with complete
avulsion of musculotendinous junction and associated large amount of
Fig. 4 US—muscle haematoma with hypoechoic fluid collection and
debris. In the traditional classification system, this would have been a
grade III injury. In the newly proposed system, this is a 2.B.a injury
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oedema with complete interruption of muscle fibres and associated
haematoma
muscle actions, when muscle tension increases suddenly,
the damage may occur in the area beneath the epimysium
and the site of muscle attachment to the periosteum
[21, 35]. On the other hand, epimysial fascia and the
muscle belly are less commonly damaged. In fascial injuries, common in the medial calf and biceps femoris, differential contractions of adjacent muscle bellies are
suspected to stretch the intervening fascia and may produce
aponeurotic distraction injuries [36]. Hamstring strain
muscle injuries, the most widely studied, typically occur in
the region of the MTJ, a transition zone organized in a
system of highly folded membranes, designed to increase
the junctional surface area and dissipate energy [28]. The
region adjacent to the MTJ is more susceptible to injury
than any other component of the muscle unit, independently from type and direction of applied forces and muscle
architecture [20]. In this area, even a minor strain, by
inducing an incomplete disruption, evident only at
microscopy, may weaken it, and predispose to further
injury. At microscopy, haemorrhage is immediately seen at
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pre-injury level. There is evidence that muscle strains
involving a free tendon may prolong the recovery time over
injuries to the muscle/muscle–tendon junction. A study on
hamstring injuries in sprinters has showed that the size and
position of the injury in relation to the ischial tuberosity
(more or less cranial) are predictive of good recovery, with
better prognosis for patients with distal lesions than those
with cranial involvement [4].
New concepts
Fig. 5 a MRI image (axial STIR) of myotendinous involvement with
myofascial fluid. In the traditional classification system, this would
have been a grade I or II injury. In the new proposed system, this is a
1.d injury. b MRI image (axial STIR) of myofascial tear. In the
traditional classification system, this would have been a grade I or II
injury. In the newly proposed system, this is a 1.c/d injury
Aside from traditional clinical features, novel classification
systems should rely on early clinical assessment of range of
motion and muscle function, which have a direct bearing
on management and outcome [40]. A classification system
has been introduced in acute hamstring injuries, the most
often injured muscle group, based on imaging findings and
clinical exam (active range of motion) [40]. The same
principles [39, 40] could be extended to other muscle
groups; however, this remains a topic beyond the scope of
the current article given the limited data in this area. We
therefore propose an imaging classification scheme which
more precisely defines muscular injuries by anatomical
site. There is no doubt that, based on physical examination,
most practitioners would be able to diagnose the relevant
injury and plan appropriate management, but imaging does
convey important information which may form the basis
for longitudinal studies on the evolution of such injuries.
We further suggest that imaging (US and MRI) assessment
is not only helpful for severely injured patients or highlevel athletes candidate to undergo surgery, but it could
also be used to better assess injury severity and predict the
time to return to sport activity.
Generalities of imaging
the disruption sites (\24 h after disruption), whereas an
inflammatory reaction is evident later, usually after 2 days
[46]. Laying down of fibrous tissue and scar tissue starts
after 7 days [22, 41] and becomes visible as early as
14 days following the initial insult [25]. After 2 weeks, the
muscle has regained over 90 % of its function. However,
the presence of retracted fibrous tissue alters the muscle’s
optimal length, may impair maximal contraction and predispose to further injuries [32]. In muscle–tendon complex
of the long head of biceps femoris, a clinical assessment of
the point of highest pain on palpation, within 3 weeks from
the injury, is predictive of recovery time [4]. Since palpation alone cannot distinguish between tissues involved,
MRI findings showing the involvement of the free proximal
tendon have been associated with longer time to return to
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In early or low-grade injuries, the focal muscle swelling on
US is secondary to oedema and haematoma. A muscle
haematoma appears as a hypoechoic fluid collection and
may contain debris [37] (Fig. 5). At times, an intramuscular haematoma is assessed at MRI between 2 days and
5 months from injury [17, 18]. T1- and T2-weighted images are hyperintense if methemoglobin levels are increased
[15], while the serous-appearing fluid may produce an
intramuscular pseudocyst [26]. In patients with an equivocal or remote history of trauma, imaging is advised, as it
may help to better define a soft tissue mass if a neoplastic
mass is clinically suspected [29, 43, 45, 50, 51]. Pseudotumors within the rectus femoris, semimembranosus or
semitendinosus may occur after a muscle strain. In patients
with uncertain clinical and imaging features, the
Knee Surg Sports Traumatol Arthrosc (2012) 20:2356–2362
administration of contrast material may help to differentiate a simple haematoma from a haemorrhagic neoplasm. If
the lesion shows no enhancement, the diagnosis of neoplasm is improbable; conversely, an enhancing nodule
induces greater suspicion of neoplasm than haematoma
[38].
Imaging assessment nomenclature (Table 2)
The advent of new technological advances in imaging has
improved both diagnosis and prognosis of musculoskeletal
disorders. However, the diagnosis of muscle strain injury is
most often a clinical one. US is increasingly used because
of its lower costs and portability, particularly in experienced hands [8]. MRI, very sensitive for contrast resolution, anatomic detail, and reproducibility [47] may be
helpful when patient’s symptoms, physician’s findings and/
or US are discrepant [16, 30].
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
each of the above observed sites, we propose to distinguish
muscular, MTJ (proximal and distal) and tendon injuries
(proximal and distal). Considering the anatomy, muscular
lesions can be further classified as intramuscular, myofascial (Fig. 5b), myofascial/perifascial, musculotendinous or
a combination. With regard to the site of injury, we classify
muscular injuries as proximal, middle and distal. The
severity of the muscular and musculotendinous injuries is
classified according to a 3-grade classification system from
MRI and US [35].
Some studies suggest that the extent of the muscle injury
is a prognostic factor for recovery time [12, 48], and
variables such as the percentage cross-sectional area of
abnormal muscle, the cranio-caudal length of muscle
abnormality adjacent to the MTJ, and the approximate
volume of muscle injury have been proposed as well to
estimate severity. The percentage cross-sectional area of
abnormal muscle is typically measured on fat-suppressed
(FS T2-weighted or STIR) images in the transverse plane.
On the image showing the maximal extent of injury, a
region of interest is drawn around the region of abnormal
T2 signal (injury extent) and around the whole muscle
belly (total area of muscle belly). The ratio of extent of
injury to the total area of the muscle belly estimates the
percentage cross-sectional area, which reflects the proportion of myofibrils within a given muscle that have been
disrupted at the level of measurement. The cranio-caudal
extent of injury, obtained from longitudinal (coronal or
sagittal) images, is perhaps the most simple and reproducible measurement once MR images have been obtained
[48]. Intramuscular or intermuscular haematoma should be
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differentiated: in the first instance, given the action of the
intact muscle fascia which compresses the intramuscular
vessels, the increased compartment pressure reduces
bleeding and limits the size of the haematoma; in the
second instance, when the fascia surrounding the muscle is
torn, blood spreads into the interstitial and interfascial
spaces, with no significant increase in pressure within the
muscle [31]. An inevitable weakness of this article is that it
reports an evidence-based but nevertheless subjective
opinion. Prior to its general acceptance, this system must
be assessed in several different muscles, and well planned
and powered clinical investigations should be performed to
determine whether the classification proposed in this article
can be applied in clinical practice and be of greater value
than the present system.
Conclusion
Clinical assessment, site of injury and pattern of the lesion
can all provide prognostic information regarding convalescence and recovery time following both an acute and
recurrent muscle strain injury [31]. We describe a comprehensive system to classify all muscle injuries, on the
basis of exact anatomical site involved, and severity at
imaging assessment (Tables 1, 2). We define muscular
injuries by site as proximal, middle and distal, as intramuscular, myofascial, myofascial/perifascial, and musculotendinous. We propose a new terminology for muscle
injuries, a proposal of which will undergo appropriate
validation and reliability studies and will also be used for
prognostic studies.
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