Acute muscle strain injuries: a proposed new classification system

Transcription

1 DOI /s 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 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 n.maffulli@qmul.ac.uk 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).

2 Table 1 Present classification system and relationship with imaging features of muscle injuries Imaging grading I (strain) II (Partial tear) III (Complete tear) Radiological findings MRI Less than 5 % of fibre disruption; feathery oedema-like pattern, intramuscular high signal on the fluid-sensitive sequences Oedema and haemorrhage of the muscle or MTJ may extend along the fascial planes, between muscle groups 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 Complete discontinuity of muscle fibres, haematoma and retraction of the muscle ends US Normal appearance, focal or general increased echogenicity; No architectural distortion 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 Comparable with MRI 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, 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

3 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

4 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 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

5 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 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 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 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

6 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 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. 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