IMAGING DIAGNOSIS AND CRITICAL REVIEW OF CLASSIFICATION

– Written by Maria Lua Sampaio Gulde and Paulo Victor Partezani Helito, Qatar

INTRODUCTION

The hamstring muscles, composed of the biceps femoris, semitendinosus, and semimembranosus, play a crucial role in lower extremity biomechanics. Each of these muscles originates at the ischial tuberosity and attaches to the knee joint, functioning as powerful hip extensors and knee flexors, particularly during high-speed running, jumping, and explosive changes in direction. Their unique anatomical position, crossing two joints, and function make them highly susceptible to injury, particularly in field-based team sports and activities involving dynamic lower limb movements.

Hamstring injuries (HSIs) are among the most common injuries in sports, accounting for up to 10% of all injuries in field-based team sports¹. Activities such as football (soccer), rugby, and American football report the highest incidence of HSIs due to their reliance on rapid acceleration, deceleration, and directional changes. The estimated injury incidence per 1000 hours of exposure ranges from 0.87 in non-contact sports to 0.96 in in contact sports², with professional male European soccer players experiencing up to 4.1 injuries per 1000 competition hours³.

The burden of HSIs extends beyond their prevalence, significantly impacting both athletic performance and economic costs. In professional European football, hamstring-specific injuries represent a substantial injury burden of 15.4 days of absence per 1000 hours of exposure, with an associated economic impact of €90,367 per 1000 hours⁴. These figures underscore the critical need for effective injury prevention, diagnosis, and management strategies to mitigate time loss and financial repercussions for athletes and their clubs.

Imaging plays an essential complementary role in the evaluation of hamstring injuries, enhancing clinical assessments and functional evaluations. Magnetic resonance imaging (MRI) and ultrasound (US) are the most widely used modalities. These tools are essential for confirming the diagnosis, assessing the extent of muscle damage, and providing prognostic information. Additionally, imaging findings can guide decision-making regarding treatment strategies and timing for safe return to play, ensuring a more tailored approach to athlete care. This article will review the role of imaging in hamstring injuries, focusing on current concepts in diagnosis, classification systems, and strategies for return to play.

IMAGING DIAGNOSIS

Imaging modalities are fundamental in the evaluation, classification, and management of hamstring injuries (HSIs). A systematic imaging approach offers essential insights into the characteristics and severity of injuries, enabling the development of targeted rehabilitation strategies and supporting evidence-based return-to-play (RTP) decisions. This section reviews imaging techniques, protocols, findings, and prognostication in HSIs.

Normal Imaging of Muscles

Imaging plays a crucial role in distinguishing between normal and pathological muscle conditions. In normal imaging, muscles appear as organized structures comprising parallel fibers surrounded by connective tissue. On MRI, muscle fibers exhibit intermediate signal intensity on T1-weighted sequences and intermediate to slightly hyperintense signals on T2-weighted sequences. Connective tissues, including tendons and fascia, are hypointense structures with well-defined, regular morphology (Figure 1). Normal muscle in athletes typically lacks significant fatty content or scar tissue unless there has been prior injury or surgery.

On ultrasound, normal muscle fibers display a characteristic pennate pattern, with hypoechoic fascicles interspersed with echogenic connective tissue septa. Tendons are hyperechoic and fibrillar, converging in a well-aligned structure (Figure 2). Care must be taken to position the probe correctly, as angulation can cause anisotropy, reducing tendon echogenicity without pathological cause.

In both MRI and US, no fluid or hematoma should be present within the muscle or perimuscular planes. A deep understanding of normal anatomy is vital for interpreting imaging, such as the distal T-junction of the biceps femoris or the intramuscular raphe of the semitendinosus. Recognizing these baseline characteristics ensures accurate identification of subtle changes that may indicate injury or disease, facilitating timely and precise diagnosis.

IMAGING FINDINGS OF HAMSTRING INJURIES

Injuries are characterized by:

• Muscle edema: MRI shows diffuse hyperintensity on T2-weighted fat-saturated sequences, while US reveals hyperechoic areas. The edema pattern on MRI typically appears feathery, following the orientation (pennation) of muscle fibers⁵ (Figure 3).
• Architectural tears: Disruption of muscle fibers is seen on MRI as focal hyperintensity with fiber disruption, presenting a signal similar to fluid and a loss of the normal fibrillar pattern, often associated with retraction. Conversely, US demonstrates hypoechoic regions due to localized fluid collections or hematomas⁵ (Figure 4).
• Perimuscular fluid and hematomas: Perifascial or intermuscular fluid often appears as a liquid on imaging, showing high signal intensity on MRI T2-weighted sequences and variable echogenicity on US, typically hypoechoic with posterior acoustic enhancement. This finding is commonly associated with muscle injuries. Hematomas may be sizable and well-defined, presenting as clearly delimited fluid areas. Measuring the volume of these collections aids in determining the need for aspiration (Figure 5).
• Involvement of connective tissue: Tendon disruption is a vital indicator of severe hamstring injuries. On MRI, tendons generally exhibit low signal intensity across all sequences, with injuries presenting as thickened areas accompanied by edematous signal changes (hyperintense on T2W sequences). On US, a normal tendon appears hyperechoic with anisotropy, while an injured tendon shows heterogeneity and thickening. Both modalities depict ruptures as regions of discontinuity. In severe cases, injuries may lead to loss of tension, a wavy tendon appearance, and retraction with clearly visible tendon gaps⁵ (Figure 7).
• Dynamic US: Muscle contraction during US examination enhances sensitivity for detecting ruptures, as gaps in the muscle or tendon enlarge during contraction. The distal portion of the muscle may appear loose and fail to move appropriately (Figure 7).
• Degloving injuries: These injuries, classically occurring in the rectus femoris, result from the inner bipennate muscle surrounding the central tendon being torn from the more superficial unipennate muscle. This creates a separation within the rectus femoris muscle. They are characterized by muscle edema and fluid in the interface, along with possible retraction in larger tears. Although not common in the hamstrings, degloving injuries have been described in the semimembranosus⁶ (Figure 8).
• Injuries at the T-junction of the biceps femoris: This unique anatomy arises from the relationship between the distal myotendinous junctions of the short and long heads, with a tendon in the interface between the muscle bellies (zipper portion), forming a T-shaped tendon⁷ (Figure 9).

CLASSIFICATION SYSTEMS

Classification systems for hamstring injuries (HSIs) serve as essential tools for simplifying communication among healthcare professionals, stratifying injuries according to severity, and guiding management strategies. These systems provide a framework for correlating imaging findings with clinical outcomes and prognosis, aiding in treatment decisions and return-to-play (RTP) planning. However, it is important to acknowledge the limitations of these systems, as some injuries fall outside the defined categories.

Purpose and Benefits of Classification Systems

The primary objectives of classification systems are:

• Standardization of Terminology: By categorizing injuries into standardized groups, communication noise is reduced, particularly in multidisciplinary care settings.
• Prognostication: These systems help stratify patients into groups with predictable recovery timelines, allowing clinicians to provide clearer expectations for rehabilitation and RTP. Additionally, they serve as a foundation for research, facilitating the development of evidence-based approaches to injury management and guiding future innovations in classification methodologies.
• Clinical Decision Support: Classifications can guide the intensity and type of rehabilitation based on the severity and location of the injury.

COMMONLY USED CLASSIFICATION SYSTEMS

1. Modified Peetrons Classification

The Modified Peetrons classification provides a simple categorization of injuries based on imaging findings:

• Grade 0: Normal imaging despite clinical symptoms.
• Grade 1: Edema without structural damage.
• Grade 2: Partial fiber disruption.
• Grade 3: Complete tear with muscle retraction.

While straightforward, this system lacks granularity, particularly in addressing the spectrum of partial disruptions and the involvement of adjacent tissues like tendons.

2. BAMIC Classification

The British Athletics Muscle Injury Classification (BAMIC) system is widely adopted in sports medicine⁸. It categorizes muscle injuries based on:

• Severity: Uses a grading system (0–4) to reflect the extent of muscle damage, ranging from mild edema to complete tears.
• Anatomic Location: Subtypes (a–c) differentiate injuries at the myofascial junction, musculotendinous junction, or within the tendon.

See the Table 1 for a summary of the BAMIC classification.

This system is reproducible, correlates well with recovery times, and is versatile across different muscle groups, including the hamstrings and quadriceps (Figure 10).

3. Other Classifications

Other classifications such as the MLG-R system and the Munich consensus have also established criteria for categorization of injuries, integrating clinical information such as mechanism of trauma and previous injuries, and aiming to further improve communication.

IMAGING-BASED CLASSIFICATION AND PROGNOSTIC VALUE

Classification systems rely heavily on imaging findings for accurate injury grading. MRI is particularly valuable in assessing the extent and location of tissue damage, while ultrasound provides dynamic insights and is useful for follow-up. High-grade injuries are associated with longer recovery times and increased risk of reinjury, emphasizing the importance of precise imaging in injury classification.

Certain findings on imaging are strongly correlated with prognosis⁹:

• Extent of Edema: Larger areas of hyperintensity on T2-weighted MRI sequences often indicate longer RTP timelines.
• Connective Tissue Involvement: Injuries involving the myotendinous junction or intramuscular tendon are associated with slower recovery and higher reinjury rates. Tendon waviness indicating loss of tension also poorer prognosis.
• Complete Tears: These injuries, particularly those with tendon retraction, carry the poorest prognosis and often require surgical intervention.

Modern classification systems, such as the BAMIC and MLG-R systems, aim to integrate imaging findings comprehensively. These systems address factors like the size of the injury (e.g., higher BAMIC grades indicating greater muscle involvement), connective tissue involvement (e.g., BAMIC subtype c capturing tendon damage), and tendon waviness (a predictor of tension loss and poorer prognosis). By incorporating these elements, they enhance prognostic accuracy and facilitate tailored rehabilitation and RTP protocols, ensuring a more individualized approach to patient care.

LIMITATIONS OF CURRENT SYSTEMS

While classification systems provide a valuable framework, they are not without limitations:

• Incomplete Categorization: Degloving injuries or injuries at the distal biceps femoris T-junction may not fit neatly into existing categories. There remains controversy regarding their prognosis, particularly T-junction injuries, which are more common than degloving hamstring tears and have been associated with poorer outcomes and higher rates of re-injury⁷.
• Injuries Across the Spectrum: These systems may group injuries from different parts of the spectrum under the same grade, potentially oversimplifying complex injury presentations (Figure 11).
• Auxiliary Findings: Additional findings, such as hematomas or injuries in adjacent muscles, may be overlooked or inadequately represented.

Therefore, we believe that classification systems are important and have their role in the diagnosis and treatment of sports related hamstring injuries. However, the classification does not replace careful assessment of particularities of each injury. A detailed analysis of the imaging findings and good communication between healthcare and sports professions, rather than relying solely on numerical and letter-based classifications, is a paramount for tailored healthcare and better outcomes in athlete care.

RETURN TO PLAY IMAGING

Return to play (RTP) is an integral consideration in the management of hamstring injuries, aimed at achieving a balance between optimal recovery and minimizing the risk of reinjury. Imaging modalities, particularly MRI, may play a supportive role in RTP decisions by providing detailed insights into the resolution of edema, scar maturation, and structural recovery (Figure 12). These imaging findings complement clinical assessments and functional tests, aiding in determining an athlete’s readiness for reintegration into sports.

Recent studies have explored the role of imaging findings in Return to Play (RTP) decisions, challenging traditional views that imaging has limited utility in this context¹⁰. Although we strongly believe that imaging can offer a complementary role by providing additional insights alongside clinical assessments, a detailed discussion of these studies and their implications is beyond the scope of this paper, which focuses primarily on the diagnostic and classification aspects of imaging. Detailed imaging protocols, selection of patients or athletes, timelines for progressive rehabilitation, and criteria for full reintegration into sports are complex and merit dedicated exploration in specialized guidelines and reviews.

CONCLUSION

Hamstring injuries (HSIs) represent a substantial challenge in sports medicine, with significant implications for athletic performance and economic costs. Imaging modalities, particularly MRI and ultrasound, play a pivotal role in the accurate diagnosis, evaluation, and management of these injuries. They provide essential insights into the extent of tissue damage, support treatment planning, and guide return-to-play (RTP) decisions, thereby enhancing recovery outcomes and minimizing reinjury risks.

Classification systems are valuable tools for standardizing terminology, stratifying injury severity, and facilitating communication among clinicians. However, no classification system can capture the full complexity of every injury. Clinicians, sports staff, and radiologists must view each injury as unique, considering all imaging and clinical findings holistically rather than rigidly adhering to classification categories. This individualized approach ensures the delivery of tailored treatments that optimize recovery and align with the athlete’s specific needs and goals.

Maria Lua Sampaio Gulde M.D.

Sports Imaging Fellow

Paulo Victor Partezani Helito M.D., Ph.D.

Consultant Radiologist

Aspetar Orthopaedic and Sports Medicine Hospital,

Doha, Qatar

Contact: paulo.helito@aspetar.com

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