Ultrasound of Muscle Injury

Introduction

Muscle is probably one of the most difficult tissues in the musculoskeletal system to assess with ultrasound. There are a variety of reasons for this, including the complex anatomy of the structures involved, the isotopic nature of muscle, the different functional bundles present within muscles and compartments, and the varied site and appearance of muscle lesions.

Once the examining sonographer appreciates the level of difficulty involved, muscle ultrasound becomes a more viable clinical tool. Increasing levels of exercise in the general population and an emphasis on imaging of sports-related injuries in athletes has resulted in an increase in requests for muscle ultrasound. Developments in ultrasound technology have also resulted in the development of more mobile ultrasound services with many professional clubs now employing diagnostic ultrasound as part of their regular medical services.

Technique

Key Point

An ultrasound scan is part of a clinical assessment and therefore each examination should begin with a brief history and sometimes even a physical examination of the patient.

The history obtained with regards to muscle injury is important as the appearances of haematoma and muscle tears vary with time postinjury. The nature of the injury is also important in determining the exact sites that need to be most carefully assessed. It is important for the examiner to differentiate between direct contusion and indirect muscle injury. These two mechanisms damage the muscle in different sites; muscle ultrasound requires the examiner to focus the examination at the site of injury most frequently affected and often with comparison to the contralateral side required to demonstrate subtle changes.

Practice Tip

The presence of bruising is important; the site of muscle damage frequently lies immediately proximal to the site of subcutaneous ecchymosis.

The sonologist should use the highest-frequency probe to give sufficient depth of penetration to allow assessment of the entire muscle under interrogation. In general high-frequency linear array probes (for example, 7–13 MHz and 5–17 MHz) can be used to assess muscle injury even in large compartments such as the thigh or hamstring. This is achieved by a combination of lowering the frequency of the probe to give sufficient penetration and by using electronic curvilinear field of view to give greater tissue coverage. Extended field of view scanning is useful in muscle ultrasound as it gives the examiner an appreciation of the overall muscle architecture and the degree of damage present. This is different to the use of extended field of view scanning in other areas where frequently it is only of value in demonstrating abnormalities for clinicians; in muscle ultrasound it helps with the grading and identification of muscle tears.

The focus of this chapter is limited to ultrasound imaging of muscle injury. Inflammatory conditions of muscles are, in the opinion of the author, best assessed with MRI and have not been considered in this chapter.

Muscle Anatomy

Muscle has a hierarchical internal structure with hypoechoic myofibrils separated by thin internal septa. This structure produces the typical ultrasound appearances of muscle tissue with hypoechoic tissue separated by thin hyperechoic parallel lines. This parallel orientation of the myofibrils renders muscle anisotropic, resulting in differing ultrasound properties dependent on the incident angle of the ultrasound beam. In transverse this produces the classic starry sky appearance with bright septa seen as dots within the hypoechoic dark surrounding myofibrils (Fig. 33.1). The echogenicity of the surrounding myofibrils varies with probe angle, either accentuating or decreasing this appearance. When examined in the longitudinal plane, muscle anisotropy is easier to appreciate by angling the probe or beam steering (Fig. 33.2). Within any single muscle there are usually separate functional bundles, especially in large muscles crossing two joints more prone to injury. This results in fibres tracking in different directions within muscles themselves, complicating even further the ultrasound assessment of muscle.

Figure 33.1 Transverse sonogram of the rectus femoris showing the typical starry sky appearance of the muscle septa with the surrounding myofibrils.

Figure 33.2 Image of the gastrocnemius (G) and soleus (S) muscles of the calf. With no beam steering (A) the gastrocnemius is slightly hypoechoic relative to soleus. Beam steering towards the feet accentuates this (B), whereas cranial beam steering (C) placed the gastrocnemius fibres more parallel to the probe, rendering them virtually isoechoic to the soleus fibres.

An understanding of myotendinous junction (MTJ) anatomy is vital to successful imaging of muscles (Fig. 33.3).

Figure 33.3 Extended field of view sonogram of the forearm demonstrating the long-central MTJ (arrows) in a bipennate muscle.

Key Point

In young skeletally mature athletes, the MTJ represents the weak point in the kinetic chain most prone to injury.

The anatomy of the MTJ can be complicated and requires detailed assessment. Muscles with multiple heads usually have more complex MTJ anatomy and are unfortunately the most commonly injured muscles requiring imaging.

Practice Tip

The junction between a muscle and a tendon can be either epimysial (at the periphery of the muscle) or via a central aponeurosis with a gradual thickening of the aponeurosis/epimyseum as the rounded tendon forms.

Anatomy of Commonly Injured Muscles

Quadriceps Anatomy

The quadriceps muscle comprises four components, vastus lateralis, vastus medialis, vastus intermedius and rectus femoris. This muscle is responsible for knee extension and is commonly injured during kicking. The vastus intermedius contains a high proportion of slow twitch fibres and, as it originates on the anterior femoral shaft, it only crosses one joint. As such it is rarely injured in exercise-related sprinting injury, though is prone to contusion, mainly as a result of its close proximity to the femoral shaft. Vastus medialis and lateralis also only cross one joint.

The vastus lateralis is the largest part of the quadriceps femoris. It arises by a broad aponeurosis, which is attached to the upper part of the intertrochanteric line and the upper half of the lateral lip of the linea aspera. The distal tendon forms from an aponeurosis on the deep surface of the lower part of the muscle: this aponeurosis forms part of the quadriceps tendon and inserts on the superolateral border of the patella.

The vastus medialis arises from the lower half of the intertrochanteric line, the medial lip of the linea aspera and the tendons of the adductor group. Its fibres attach to an aponeurosis that lies on the deep surface of the muscle and forms part of the quadriceps tendon. The most inferior portion of vastus medialis lies directly adjacent to the femoral shaft and as such is prone to contusion. This inferior portion has a more horizontal fibre orientation and inserts onto the medial retinaculum of the patella. This portion of the muscle is sometimes referred to as vastus medialis obliquus and is prone to injury during patellar dislocation and contusion.

Rectus femoris arises from the two proximal tendons: the direct (or sometimes called the straight) head and the indirect (reflected) head. These merge 2 to 3 cm below their origins to form the conjoint tendon. The direct head arises from the anterior inferior iliac spine with the indirect head arising from the iliac bone just posterior and inferior to the direct head origin. The direct head forms the anterior portion of the tendon and forms an anterior epimysial MTJ terminating in the proximal thigh. The indirect head forms the posterior portion of the conjoint tendon terminating in a central aponeurosis within the muscle, which extends a long way down the muscle. This central septum terminates approximately 10 cm above the superior pole of the patella and overlaps the distal rectus femoris MTJ. As a result it is possible and indeed not unusual to have a proximal MTJ injury in the distal thigh.

Practice Tip

Rectus femoris crosses two joints and contains a high proportion of fast twitch fibres, which is why it is the most commonly injury of the knee extensors during kicking injuries.

This muscle also contains differing functional units with unipennate direct and bipennate indirect components that predispose to longitudinal splitting type of aponeurotic injury most commonly seen in the central septum.

Hamstring Anatomy

Practice Tip

The hamstrings consist of three muscles, all of which cross two joints with high proportions of fast twitch fibres. Like rectus femoris, this combination makes them extremely prone to exercise-related injury. These muscles all arise from the ischial tuberosity.