In general there are three main etiopathogenic mechanisms of muscle injury: (1) direct trauma—resulting in contusion; (2) indirect trauma—resulting in strains; and (3) lacerations. The latter mechanism is uncommon in sport related lesions, and is excluded from this chapter. The quadriceps muscle is subject to both direct and indirect energy trauma. It has, however, been shown that the mechanism of injury usually depends on which muscle within the quadriceps is involved: vastus medialis and lateralis muscles are more often exposed to direct trauma while lesions of the RF usually result from indirect energy trauma. Indirect trauma to the RF causes the muscle to rupture at its weakest point, which is the myotendinous junction in adults (strains) and the apophyseal growth cartilage in children [8, 9].

The RF has all the characteristics of frequently injured muscles. It is biarticular, spanning the hip and knee and thus responsible for decelerating both; it has a high proportion of type II muscle fibers which generate more tension on contraction; and it acts in an eccentric manner [6, 10–12]. The incriminating role of kicking has been well described [13]. In the final portion of the backswing phase, the hip is hyperextended and the knee is flexed, stretching the biarticular RF muscle and putting it in passive insufficiency, its weakest position, especially at the proximal third. At the onset of the forward swing there is massive eccentric muscular recruitment of the quadriceps and iliopsoas muscles. Forceful muscular contraction in a stretched quadriceps muscle can lead to sprain [6, 9, 13]. This is what typically happens when, for example, soccer or rugby players unexpectedly encounter irregular or slippery turf as they are about to kick the ball, and they try to compensate by extending the hip [8, 14]. It also occurs when one loses one’s footing during abrupt deceleration, an event that is common in all sports that involve running.

The causes of muscle strain injury are multifactorial. Past muscle strain injury is perhaps the most recognized risk factor [15, 16]. Other proposed risk factors include low muscle strength, muscle fatigue, age, lack of warm-up, muscle temperature, and poor flexibility [15, 17]. For quadriceps muscle strains in Australian Rules footballers, Orchard found that both recent (less than 8 weeks) and remote quadriceps strain injury, recent hamstring strain, the dominant kicking leg, short stature, and ground hardness were all associated with increased risk [16]. Orchard described these clinical strains over a 7-year period in the national competition; the injuries were not routinely assessed by MRI, and therefore we do not know what patterns might have been revealed.

Cross sectional imaging, MRI and ultrasound are commonly indicated for professional and/or elite amateur athletes when both the athlete and others (coach, trainer, manager) need accurate diagnosis and prognosis.

Avulsion fractures can involve any of the three osseous insertions of the RF: the AIIS for the direct head, the superior lateral acetabular ridge for the indirect head, and the patellar sleeve for the distal insertion. Unlike strains, which most commonly involve the indirect head, avulsion fractures usually involve the AIIS, the site of insertion of the direct head [9, 21–23]. This type of fracture is also called “sprinter’s fracture” [24]. In children the presence of growth cartilage at the tendon insertion makes the bone-tendon junction more vulnerable to indirect energy trauma than the myotendinous junction. The patient reports a sudden pain and eventually feeling a break when kicking the ball. Pain is usually worst at the site of fracture, which is frequently at the AIIS, or the lateral upper thigh when the indirect head is involved.

Avulsion fractures are readily detected on conventional radiographs. Anterior-posterior x-rays of the pelvis, as well as the oblique alar view, will clearly reveal the avulsion of the cartilage (Fig. 16.3). The degree of retraction and the size of the bone fragment can be evaluated. X-rays taken in athletes years after avulsion fractures show hypertrophy of the AIIS, which appears as a large ossification that projects into the inferior soft tissue. Ultrasound confirms the diagnosis. The avulsed fragment is seen as a hyperechoic structure of variable size lying at some distance from the AIIS [8]. On MRI avulsion fractures appear as hypointense linear abnormalities on T1-weighted imaging of bone underlying the origin of the RF. The osseous fragment and donor site are also evident, as is any hematoma interposed between the avulsed fragment and adjacent bone. The signal intensity of the hematoma depends on the age of the injury, although in the acute phase the hematoma is likely to show decreased signal intensity on T1-weighted images and increased signal intensity on T2-weighted images [9, 25] (Fig. 16.4). Treatment is usually conservative (short period of bed rest followed by progressive weight bearing with crutches) with rapid resolution of pain and return to playing condition in a relatively short period (6 weeks) [26]. Surgery is indicated for long standing symptomatic proximal avulsion after failure of nonoperative treatment [27, 28].

The clinical diagnosis of myotendinous injuries is usually based on a three-point scale: (1) for mild; (2) for partial; and (3) for a complete tear [29]. Mild injuries have no discernible loss of strength or motion restriction. Partial tears demonstrate some loss of strength or motion that is not complete, unlike type 3 injuries [2]. Strain injury is associated with inflammation, edema, and sometimes hemorrhage with proliferation of inflammatory cells and fibroblastic activity in the first 24–48 h [2]. Histological animal models of muscle stretch injury have shown that myotendinous injury results in inflammation, bleeding and muscle fiber necrosis initially. This destructive phase is followed by a concomitant repair and remodeling phase involving recruitment of progenitor cells, scar formation, and remodeling of organized tissue [30].

Clinically, the patient may present with sharp pain associated with movement and impaired mobility. The injured area can be located with precision by the patient and verified by a careful examination showing maximal tenderness over the midline of the thigh [3]. Injuries of the origin of the reflected tendon may mimic hip pain or a lesion of the tensor fascia lata. The patient reports a sensation that something in the hip was displaced during the trauma [8]. When the lesion involves disruption of the distal muscle fibers from the posterior tendon of insertion, however, proximal retraction of the entire muscle belly is observed, resulting in a mass that migrates proximally to the groin with muscle activation. This mass is sometimes mistaken for a soft-tissue neoplasm [6, 25, 31]. Other clinical findings include localized swelling, loss of knee extension, thigh asymmetry and a palpable defect with a retracted mass (in complete ruptures). Prompt diagnosis (within a few days of the muscle trauma) is essential to ensure timely and complete healing and to reduce the likelihood of recurrence. Most important at the time of diagnosis is to differentiate benign injuries from serious injuries that may require protracted rehabilitation. Unfortunately, making the distinction is difficult by clinical examination alone, especially in the first week after injury. Imaging studies however show a significant relationship between initial findings and prognosis [3].

RF injuries are commonly classified on MRI as:

There are also ultrasound classifications for RF injuries [8, 32]. The following were reported by Petroons et al. and modified by Balius et al. [5, 32]: