FROM IMAGING ANATOMY TO DIAGNOSING INJURIES
– Written by Gulraiz Ahmad, UK, Carles Pedret, Spain, Jawad Naqvi, UK, and Waqar Aslam Bhatti, UK
INTRODUCTION
Quadriceps injuries are frequent in sports that involve sprinting and repetitive kicking such as football and rugby, with rectus femoris muscle injuries accounting for up to 4.6% of all professional football injuries1. The rectus femoris is the most frequently injured muscle of the quadriceps compartment and most injuries occur due to excessive tension during muscle contraction, particularly during eccentric contraction2,3. Injuries range from low grade muscle strains to high-grade complete avulsion injuries.
Over recent years, increased knowledge of the anatomy and histological architecture of muscles, as well as advances in imaging, has allowed a greater understanding of potential injuries that can occur and also the prognosis and prediction of return to play (RTP) time in these injuries.
Magnetic resonance imaging (MRI) has traditionally been the gold standard for imaging quadriceps injuries as it allows detailed assessment of soft tissue and osseous injuries and is a repeatable study that can assess temporal healing of injuries.
Ultrasound (US) is also increasingly being used as an initial point of care imaging modality which allows dynamic assessment of soft tissue structures such as muscles and tendons and is readily accessible for clinicians.
This article shall demonstrate the anatomy of the rectus femoris complex and illustrate the wide spectrum of injuries that can occur.
HISTOARCHITECTURE AND ANATOMY OF THE RECTUS FEMORIS COMPLEX
A detailed knowledge of the histological architecture and anatomy of the rectus femoris is essential to understand the different types of injuries that can occur.
Most muscle complexes including the rectus femoris contain three main types of connective tissue; tendons, aponeuroses and fasciae. The differences between these types of connective tissue is based on the histological orientation of the collagen fibres that they are composed of4.
Fascia is defined as fibrous connective tissue that covers muscle and contains loose, disorganized collagen fibres which allows flexibility of the fascia and enables smooth movement of muscle within muscle compartments4.
Tendon is defined as dense connective tissue with well-organized collagen fibres oriented in a compact longitudinal plane, allowing the tendon to withstand high forces and transmit force from muscle to bone to induce movement4.
Aponeurosis is also defined as dense connective tissue, similar to tendons, but the collagen fibres are oriented in a flattened, horizontal plane rather than a longitudinal plane. Aponeuroses are therefore often considered as broad, flattened expansions of tendon and able to withstand similar forces in the horizontal and longitudinal plane4.
The rectus femoris is a long and fusiform muscle that forms the most superficial component of the quadriceps (anterior) muscle compartment and is innervated by the femoral nerve. It is the only quadriceps muscle that crosses the hip and knee joints (biarticular) and contains a high proportion of type II muscle fibres, predisposing to a greater tendency to injure5.
The rectus femoris can be classified into a proximal and distal myoconnective complex.
Proximal myoconnective complex
The rectus femoris originates from 2 main tendons, the direct (straight) head and the indirect (reflected) head. The direct tendon arises from the anterior inferior iliac spine (AIIS) and the indirect tendon originates from the superolateral acetabular ridge and hip capsule. These tendons then merge to form the conjoint, or common tendon of the proximal musculotendinous junction of the rectus femoris (Figures 1, 2, 4). The fibres of the direct head then contribute to the superficial (anterior) aponeurotic expansion of the proximal rectus femoris and eventually fuse with the anterior fascia6.
The indirect head fibres contribute to the deep intramuscular central tendon, also known as the central septum, which arises medially within the proximal rectus femoris muscle belly and thins out distally terminating within the distal third of the inner bipennate muscle, separated from the outer unipennate muscle. The interaction of the direct and indirect heads has resulted in a so-called ‘muscle within a muscle’ configuration, in which the outer unipennate muscle surrounds the inner bipennate muscle.
In 2006, Tubbs et al proposed an anatomical variation of a third head, or capsular head, arising from the indirect tendon, although this is not always visualized on MRI studies7. Mechó et al described that this third tendon can occasionally be visualized along the inferior margin of the indirect tendon8. A case series by Armstrong et al also recently proposed that the posterior most aspect of the intramuscular indirect central tendon (central septum) may have a contribution from the capsular head (third head)9.
Distal myoconnective complex
The distal myoconnective complex arises along the posterior proximal third of the rectus femoris muscle belly as myofascial connective tissue, which is usually not visible on MRI examination due to its thin nature. The myoaponeurotic complex/posterior aponeurosis then arises within the mid third of the rectus femoris, continuing into the distal third and is readily visualised on MRI examination as thickened low signal flattened connective tissue. The posterior aponeurosis then continues as a free tendon forming the superficial layer of the distal quadriceps tendon. The intermediate layers of the quadriceps tendon are formed by the vastus medialis and lateralis and the deepest layer formed by the vastus intermedius. The quadriceps tendon inserts into the superior pole of the patella. (Figures 1, 3, 4).
INJURIES INVOLVING THE RECTUS FEMORIS
Injuries involving the rectus femoris can be classified based on anatomical location and the type of connective tissue. These injuries can be broadly classified into bony avulsion injury at the origin, myofibrillar, myofascial, myotendinous, myoaponeurotic and intratendinous injuries. Several studies have demonstrated that injuries with dense connective tissue involvement such as myoaponeurotic and intratendinous injuries are associated with an increased return to play (RTP) time10,11,12.
Proximal to mid-thigh injuries
In the proximal third of the thigh, injuries may occur at the origins of the direct (AIIS) and indirect (supraacetabular ridge) heads, common conjoint tendon, superficial anterior aponeurosis, lateral or posterior myofascial region, within the muscle belly (myofibrillar) or involving the central tendon (central septum).
Rectus femoris origin
In the paediatric and adolescent population, bony avulsion of the anterior inferior iliac spine apophysis is frequently seen in skeletally immature patients, particularly with sports that involve kicking such as football (Figure 5). These injuries are often treated conservatively and are infrequent above the age of 16, when fusion of the AIIS apophysis occurs. Surgery is infrequently performed to remove heterotopic ossification adjacent to the AIIS in complex cases13.
In adult/skeletally mature patients, injuries may involve the direct and indirect head tendon origins, both individually or combined or the conjoint tendon (Figure 6). Tendon tears may be partial thickness, which can be longitudinal or transverse, or full thickness transverse tears. Full-thickness transverse tendon tears often result in loss of tendon tension and distal retraction of the musculotendinous complex (Figure 6). These full-thickness transverse tears may require surgical repair in professional athletes to ensure optimal rehabilitation14.
Superficial (anterior) aponeurosis of the proximal rectus femoris
A further injury within the proximal third of the thigh is injury involving the superficial aponeurosis. Isolated injuries involving the superficial aponeurosis without involvement of the central intramuscular tendon are usually treated conservatively and have relatively short RTP times relative to other aponeurotic injuries within the rectus femoris (Figure 7)15.
Myofibrillar and myofascial injuries
Injuries involving muscle fibres often produce localised oedema and interfascicular haemorrhage and are classified as pure myofibrillar tears if there is no connective tissue involvement (Figure 8). These injuries may occur in isolation (without fascial, aponeurotic or tendon involvement) at sites of previous injury, where there is fibrotic scar that creates focal tension in the region (more common in veteran athletes).
If there is connective tissue involvement with a muscle tear/myofibrillar disruption, the tear is often classified depending on the connective tissue involved, for example, myoaponeurotic or myofascial tears, if the aponeurosis or fascia are involved respectively.
Myofascial injuries typically occur at the lateral or posterior myofascial surface of the proximal rectus femoris with involvement of the fascia and often concomitant myofibrillar disruption (Figures 9, 10). These injuries usually have a good prognosis as there is no aponeurotic or tendon involvement and are often managed conservatively15. If there is significant myofibrillar disruption/tear, this may produce an intramuscular haematoma which is occasionally aspirated if large (Figure 11), to allow apposition of muscle fibres, promote quicker healing and reduce risk of scar formation.
Injuries involving the central tendon (central septum) and central musculotendinous complex
The central intramuscular tendon or central septum is an intramuscular expansion of the indirect head and is situated within the inner bipennate portion of the rectus femoris and separated from the outer unipennate muscle fibres. Injuries involving the central intramuscular musculotendinous junction are the most common types of rectus femoris injuries that occur in football16.
Injuries may vary from low grade myofibrillar tears seen adjacent to the central tendon (often described paraseptal myofibrillar tears) to the most severe cases, where there is an intratendinous tear involving the central tendon with retraction of the tendon (Figure 12). These intratendinous tears may be treated conservatively depending on the length of tendon involvement but if there is involvement of the proximal tendon, surgical intervention is occasionally considered in high performance athletes17.
Degloving and pre-degloving injuries
Degloving injuries were described by Kassarjian et al as a focal muscle tear involving the distal central musculotendinous junction of the rectus femoris which results in separation and retraction of the inner bipennate muscle from the outer unipennate muscle (Figure 13)18. These injuries often have a better prognosis than central tendon injuries as there is no dense connective tissue involvement and are often treated conservatively with a mean RTP time of 39 days19. Surgical repair is less frequently employed, although there currently remains no consensus on the optimal management of these injuries20.
Pre-degloving injuries have recently been described as injuries where there is myofibrillar disruption or feather-like oedema at the interface between the inner bipennate and outer unipennate muscles of the distal rectus femoris, but without proximal retraction of the inner bipennate muscle, that would typically be seen in true degloving injuries (Figure 14). These injuries are considered to represent a muscle overload injury which is a precursor to degloving injuries and have a good prognosis/short RTP time if managed appropriately8.
EXERCISE RELATED SIGNAL ABNORMALITY
Exercise related signal abnormality (ERSA) lesions were described in 2022 by Kho et al as focal areas of high signal seen within muscle, that have similar intensity to delayed onset muscle soreness (DOMS) oedema but lack the typical feather-like oedema or muscle/connective tissue disruption that is associated with acute muscle strain injuries21. The pattern of distribution allows classification into three types: peritendinous (type A), subfascial (type B) or mixed peritendinous and subfascial (type C)21. ERSA lesions have also been characterised as being distinct from delayed onset muscle soreness (DOMS) type oedema, where there is typically a more diffuse geographic cloud-like oedema within muscle (Figure 15c) rather than subfascial or peritendinous in nature21.
INJURIES INVOLVING THE DISTAL RECTUS FEMORIS – POSTERIOR APONEUROSIS
The most common injury involving the distal rectus femoris myoconnective complex is injury involving the posterior aponeurosis, with anterior myofascial injuries being less common. Significant injuries to the posterior aponeurosis may lead to proximal retraction of muscle fibres and cause localised intermuscular haemorrhage (Figure 16). Low grade myoaponeurotic injuries are often treated conservatively but if there is severe retraction of muscle fibres or involvement of the free aponeurosis, then surgery may be considered15.
ASSESSMENT OF MUSCULOTENDINOUS INJURY
The authors recommend the serial use of MRI examination as gold standard, especially in professional athletes, when assessing the healing of muscle and tendon injuries as it allows objective assessment of findings such as a reduction in muscle oedema and connective tissue healing on serial interval imaging.
MRI features of healing in conservative management
In myofibrillar or myofascial muscle strain injuries, a positive healing response would typically manifest as a reduction in muscle oedema and intramuscular/intermuscular haemorrhage. Chronic sequelae of muscle injuries include focal fat infiltration and scar formation (Figure 17).
The imaging features of healing tendon and aponeurotic injuries often have predictable characteristics on serial interval imaging. Isern-Kebschull et al recently described three phases of tendon healing which involves the destruction phase, characterised by tendon disruption and feathery oedema (phase 1); the repair phase, characterised by immature scar formation and reparative oedema (phase 2) and the remodelling phase, characterised by mature scar formation, fusiform thickening of the tendon and resolution of oedema (phase 3) (Figure 18)22. Adverse prognostic imaging features that may predispose to suboptimal or poor healing, include persistent oedema and tears within the scar tissue or adjacent connective tissue22.
MRI features of healing in post-surgical cases
Surgical repair may be indicated in complete, full-thickness tears involving both the direct and indirect tendon origins or the proximal common tendon, particularly in elite athletes, enabling return to baseline function14,23. To avoid misdiagnosis, an understanding of the post-operative appearances of a primary surgical suture repair and primary muscle suture tenodesis is essential.
Primary surgical suture repair involves restoring the avulsed tendon to its anatomical location/origin to maintain native tension (Figure 19)14. Another surgical technique that has recently been advocated is primary muscle suture tenodesis which involves resection of the proximal avulsed tendon and any ossific fragments and end-to-end suture fixation to the muscle belly (Figure 20)24. This technique has been shown to have a reduced risk of injury recurrence compared to primary surgical repairs24.
Close follow-up imaging is often useful to assess for progressive healing and complications such as post-surgical seromas and haematomas, scar formation and denervation oedema or muscle atrophy. The post-operative appearance of repaired tendon typically results in a slightly thickened appearance to the tendon, often with localised susceptibility artifact from suture material (Figure 19).
CONCLUSION
The rectus femoris is a complex biarticular muscle and is the most commonly injured quadriceps muscle. An accurate understanding of the anatomy and histological architecture of the rectus femoris is essential to understand the different types of injuries that can occur and therefore plan appropriate rehabilitation and management plans. Dense connective tissue involvement is usually associated with longer RTP times. MRI imaging is an excellent screening tool to assess the different structures affected and enable prognosis of an injury.
Gulraiz Ahmad M.D., Ph.D.
Consultant Musculoskeletal Radiologist
Manchester University
Hospital NHS Foundation Trust and Manchester Metropolitan University
Carles Pedret M.D., Ph.D.
Sports Medicine and Imaging
Clinica de Medicina Integral Diagonal SLU
Esplugues de Llobregat, Spain
Jawad Naqvi M.D.
Consultant Musculoskeletal Radiologist
Manchester University
Hospital NHS Foundation Trust and Manchester Metropolitan University
Waqar Aslam Bhatti M.D.
Professor
Consultant Musculoskeletal Radiologist
Manchester University Hospital NHS Foundation Trust and Manchester Metropolitan University
Contact: gsahmad0@gmail.com
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