Fig. 17.1
(a) Axial T1-weighted MRI shows the long head of the biceps femoris arising together with the semitendinosus tendon (red) on the inferomedial aspect of the ischial tuberosity (IT). Note also the semimembranosus tendon (yellow) on the superolateral aspect of the IT. F femur. (b) In the plan distal to A, the long head of the biceps femoris (pink), semitendinosus (green), and semimembranosus (yellow) tendons are visible. Note that the semitendinosus muscle becomes bulbous distally in (c). F femur, IR ischial ramus. (d) Axial plan of the proximal middle third of the thigh shows the muscle bellies of the long head of the biceps femoris (pink) and semitendinosus (green). Note the semimembranosus muscle belly (yellow) distally in (e). F femur. (f) The three muscle bellies have about the same areas in the middle third of the thigh, and the short head of the biceps femoris tendon (blue) arises from the lateral lip of the linea aspera of the femur (*). The belly of this muscle is visible distally in (g). Note also the muscle bellies of the long head of the biceps femoris (pink), semitendinosus (green), and semimembranosus (yellow). F femur, MMC medial muscular compartment. (h) Bellies of the short (blue) and long (pink) heads of the biceps femoris, semitendinosus (green), and semimembranosus (yellow) muscles in the distal third of the thigh. Note the semitendinosus tendon (green) distally in (i) and (j). F femur, P patella, AMC anterior muscular compartment, QT quadriceps tendon, SPMCL superficial posterior muscular compartment of the leg. (k) Axial plan of the knee shows fused fibers of the long and short heads of the biceps femoris (light blue), semitendinosus (green), and semimembranosus (yellow) tendons. PT patellar tendon. (l) Axial plan of the knee shows the insertions of the semimembranosus tendon (direct arm; yellow) on the medial tibial condyle. The insertions of the biceps femoris (light blue) on the lateral side of the fibular head (FH) and the anterior arm of the semimembranosus tendon (yellow) are visible in (m). T tibia, DPMCL deep posterior muscular compartment of the leg. (n) Axial plan of the proximal leg shows the insertions of the semitendinosus tendon (green) on the superomedial surface of the tibia (T). At its insertion, the semitendinosus tendon lies posterior to the tendon of the sartorius and inferior to that of the gracilis. These three tendons form the pes anserinus. F fibula, AMCL anterior muscular compartment of the leg, LMCL lateral muscular compartment of the leg
On ultrasound, the same two rounded areas at the ischial tuberosity appear as echogenic structures. Distally, the muscle bellies present a fibrillar pattern like that of other skeletal muscles. The distal insertions of the biceps femoris and, especially, the semitendinosus muscles can be identified by ultrasound. This modality is also suitable for evaluation of the pes anserinus tendons. As with MRI, the multiple insertions of the semimembranosus muscle tendons are not well distinguished by ultrasound.
17.6 Acute HMC Lesions
17.6.1 Proximal Lesions
HMC lesions typically occur in the region of the musculotendinous junction (MTJ), a 10–12 cm zone of transition that is the weakest point in the bone-tendon-muscle complex in adults. In children and adolescents, the apophysis is the weakest biomechanical link in the musculotendinous unit (Fig. 17.2) [2]. Injuries to the hamstring origin usually occur after extreme, unbalanced, and often eccentric muscle contraction [19], and the risk of such injury can be increased by any condition that affects muscular function, such as fatigue, poor flexibility, and previous injury [20].
Fig. 17.2
A 17-year-old professional male gymnast with apophsitis. Coronal (a) and axial (b) CT images show widening and irregularity of the physis. Coronal (c) and axial (d) fat-suppressed proton density-weighted MRI also show physeal widening associated with bone marrow edema
17.6.1.1 Ischiogluteal Bursitis
The ischiogluteal bursa is a variable anatomical structure located between the gluteus maximus and ischial tuberosity [21]. Ischiogluteal bursitis is an uncommon disorder that can cause referred pain in the posterior thigh and resemble clinical manifestations of injury to the hamstring muscle.
This lesion was historically encountered in weavers due to irritation of or intermittent pressure on the ischial tuberosity from prolonged sitting, and was thus called “weaver’s bottom.” Currently, it predominantly affects athletes engaging in sports that cause acute or chronic shearing force on the ischial tuberosity, such as canoeing, horseback riding, and wheelchair racing in paraplegic patients [21, 22].
MRI findings usually include hyperintensity on T2-weighted sequences, with enlarged bursae that may contain blood-fluid levels and septations [22]. Enhancement of the bursal wall, synovial proliferation, and mural nodules can be seen following contrast injection [22, 23].
MRI and advanced imaging techniques also play an important role in differential diagnosis. Intrabursal synovial proliferation can present as a solid mass, making the exclusion of other conditions, such as pigmented villonodular synovitis, synovial sarcoma, synovial hemangioma, and non-calcified synovial chondromatosis, crucial [21, 22].
17.6.1.2 Proximal Hamstring Tendinopathy
High hamstring tendinopathy is a less common overuse injury that manifests clinically as the subacute onset of deep buttock or thigh pain exacerbated by repetitive activity, such as long-distance running, and often aggravated by sitting. MRI findings include increased tendon size, peritendinous edema with a distal feathery pattern, ischial tuberosity edema, and increased internal T1 and T2 signal intensity (Figs. 17.3 and 17.4).
Fig. 17.3
A 21-year-old male soccer player with proximal tendinopathy of the semimembranosus. Axial T1-weighted MRI (a) shows subtle areas of increased intratendinous signal intensity and axial STIR image (b) shows edema surrounding the proximal semimembranosus tendon
Fig. 17.4
A 33-year-old male professional soccer player. Coronal (a) and axial (b) STIR MRI show partial rupture of the right hamstring tendons from the ischial tuberosity at their origin. Note also proximal tendinopathy of the left hamstring tendons associated with mild bone marrow edema. (c) Posterior longitudinal ultrasound image used to guide the injection of platelet-rich plasma shows retraction of the hamstring muscle insertions. (d) Ultrasound examination performed 30 days after platelet-rich plasma injection reveals reduction of the tendinous gap and partial recovery of the fibrillar pattern
17.6.1.3 Avulsion Injuries
Complete avulsion of the HMC from the ischial tuberosity is uncommon in adults. It is often caused by falling or slipping involving forceful forward hip flexion while the knee is extended, resulting in violent overstretching of the hamstring muscles [24]. This kind of injury has been documented predominantly in cross-country and downhill skiers, water skiers, runners, and athletes who commonly perform splits, such as ballet dancers and gymnasts [24–26]. The conjoint tendon is most commonly affected, usually in association with complete or partial tearing of the semimembranosus muscle [7].
The early diagnosis of tendon avulsion is important because acute surgical intervention has a better outcome than do chronic repairs, defined as operative reapproximation of avulsed tendons four weeks after the index trauma [26, 27].
Avulsion injuries at the ischial tuberosity are more common than distal avulsion and usually occur without an osseous fragment in adults [28, 29]. MRI typically demonstrates low signal intensity of the injured and retracted tendon, surrounded by high signal intensity of the proximal muscle belly due to edema or hemorrhage in acute cases (Fig. 17.5) [28]. Ultrasound has limited use in the evaluation of proximal avulsion because of the depth of injury. In young patients, as the apophysis is the weakest link, a displaced osseous fragment can be detected by radiography (Fig. 17.6).
Fig. 17.5
A 66-year-old woman with complete proximal hamstring avulsions after trauma. (a) Coronal STIR MRI shows torn and retracted hamstring tendons. Note the presence of a fluid-filled gap at the ischial tuberosity, indicating the avulsion site. (b) Axial STIR MRI reveals a large intermuscular hematoma
Fig. 17.6
A 13-year-old boy with apophyseal avulsion fracture of the ischial tuberosity. Coronal MRI (a) and 3D reformatted CT image (b) demonstrate the presence of a displaced fragment of the inferolateral portion of the right ischial tuberosity. Sagittal (c) and coronal (d) fat-suppressed proton density-weighted MRI show the retracted and torn hamstring tendons. Note also the associated hematoma on axial fat-suppressed proton density (e) and T1-weighted (f) MRI. (g) Anteroposterior radiograph of the right hip taken after surgical treatment reveals internal fixation of the ischial apophysis
Subacute or chronic avulsion may mimic infectious or neoplastic process, and functional MRI may help to exclude conditions such as osteosarcoma or osteochondroma in the differential diagnosis. Muscular atrophy and fatty replacement can also be seen in chronic injuries.
17.6.2 Hamstring Strain
Acute hamstring strains are common in individuals engaging in various sports that involve acceleration, deceleration, jumping, and rapid changes of direction, such as track and field, soccer, and American football [30–32]. Such injuries occur most commonly in the MTJ, where they can be confused with pure intramuscular lesions. They also occur commonly at the myofascial junction, mainly in the long head of the biceps femoris, along the aponeurotic interface with the short head.
Muscles that undergo eccentric contraction, that cross two joints, and that contain high proportions of fast-twitch fibers are at greatest risk of strain. The biceps femoris is by far the most commonly strained muscle, followed by the semimembranosus [33]. Muscle strains are usually caused by overly forceful muscle contraction. A different type of hamstring strain has been identified in dancers, in which the mechanism involves excessive slow-speed stretching combined with hip flexion and knee extension, as opposed to rapid cutting maneuvers. The semimembranosus muscle is most frequently involved in this type of injury [33, 34].
The types of injury and sports activity influence recovery time; for instance, rehabilitation times are shorter in soccer and American football players than in elite sprinters [30, 32, 33, 35]. This difference is related to the level of exertion; a member of a soccer team playing at 85 % of full capacity will not influence the competitiveness of the team overall, whereas sprinting at only 85 % capacity will make a difference in the final results of competition [33].
Hamstring strain has a high rate of recurrence [36]. Inadequate rehabilitation, early return to sports activity, and factors related to modifications after initial muscle injury, such as weakness, presence of scar tissue, and biomechanical alterations [36, 37], may be responsible for reinjury, which may cause a longer absence from normal activities than the initial injury [30, 38, 39].
17.6.2.1 Grading Muscular Injuries
The degree of strain can be classified into three grades according to the spectrum of injury to facilitate communication among physicians and guide patient management. Alternative classification schemes that consider the site of injury (proximal MTJ, muscle belly, or distal MTJ) and muscular structures involved (intramuscular, myofascial, perifascial, or myotendinous) have been proposed [40].
Grade 1 (Figs. 17.7, 17.8, and 17.9)
This grade describes microscopic injuries to the muscle or tendon, in which 0 % to <5 % of muscle fibers are ruptured. The most common MRI findings are focal or diffuse areas of high signal intensity on fluid-sensitive sequences due to edema and hemorrhage, centered at the main MTJ, surrounding the intramuscular part of the tendon, or in the periphery of the muscle at the myofascial junction [31, 41–43], with intact but potentially distorted myotendinous fibers. The edema track along muscle fascicles may produce a feathery appearance [44]. Associated edema extending into the muscle belly or deep fascia is common and may be extensive, but the injury is classified as grade 1 in the absence of gaps within muscle fibers [45].
Fig. 17.7
A 27-year-old male professional soccer player with proximal myotendinous junction strain. He presented with pain in the posterior thigh after a match. Coronal (a) and axial (b, c) STIR MRI show feathery intramuscular edema at the proximal myotendinous junction of the biceps femoris, consistent with a grade 1 strain
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