(Radiographics. 2000;20:S103-S120.)
© RSNA, 2000
Traumatic Musculotendinous Injuries of the Knee: Diagnosis with MR Imaging1
Jenny T. Bencardino, MD,
Zehava S. Rosenberg, MD,
Robert R. Brown, MD,
Alvand Hassankhani, MD,
Elizabeth S. Lustrin, MD and
Javier Beltran, MD
1 From the Department of Radiology, Long Island Jewish Medical Center, New Hyde Park, NY (J.T.B., A.H., E.S.L.); the Department of Radiology, Hospital for Joint Diseases/Orthopaedic Institute, New York, NY (Z.S.R., R.R.B.); and the Department of Radiology, Maimonides Medical Center, Brooklyn, NY (J.B.). Recipient of a Certificate of Merit award for a scientific exhibit at the 1999 RSNA scientific assembly. Received February 1, 2000; revision requested March 13 and received April 20; accepted April 26. Address correspondence to J.T.B., Department of Radiology, Massachusetts General Hospital, 15 Parkman St, WACC 515, Boston, MA 02114 (e-mail: bencardi@hotmail.com).
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Abstract
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Magnetic resonance (MR) imaging is the imaging modality of choice for evaluation of acute traumatic musculotendinous injuries of the knee. Three discrete categories of acute injuries to the musculotendinous unit can be defined: muscle contusion, myotendinous strain, and tendon avulsion. Among the quadriceps muscles, the rectus femoris is the most susceptible to injury at the myotendinous junction due to its superficial location, predominance of type II fibers, eccentric muscle action, and extension across two joints. Among the muscles of the pes anserinus, the sartorius is the most susceptible to strain injury due to its superficial location and biarticular course. The classic fusiform configuration of the semimembranosus along with a propensity for eccentric actions also make it prone to strain injury. MR imaging findings associated with rupture of the iliotibial tract include discontinuity and edema, which are best noted on coronal images. The same mechanism of injury that tears the arcuate ligament from its fibular insertion can also result in avulsion injury of the biceps femoris. The gastrocnemius muscle is prone to strain injury due to its action across two joints and its superficial location. Injuries of the muscle belly and myotendinous junction of the popliteus are far more common than tendinous injuries.
Index Terms: Knee, anatomy, 45.92 • Knee, injuries, 45.485 • Muscles, injuries, 45.485 • Tendons, injuries, 45.485
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LEARNING OBJECTIVES FOR TEST 3
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After reading this article and taking the test, the reader will be able to:
- Recognize normal versus abnormal MR imaging appearances of the musculotendinous units about the knee.
- Identify the MR imaging characteristics of muscle contusions, myotendinous strains, and avulsion injuries occurring after acute knee trauma.
- Describe the role of MR imaging in demonstration of clinically occult muscle trauma.
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Introduction
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Magnetic resonance (MR) imaging, with its exquisite soft-tissue contrast resolution, is the imaging modality of choice for evaluation of acute traumatic musculotendinous abnormalities. MR imaging allows optimal assessment of damage to all components of the musculotendinous unit including the muscle belly, myotendinous junction, and tendon insertion site (1–3). In addition, the capability of MR imaging for demonstrating occult muscle trauma makes it a useful tool for assessment of this condition (4).
This article reviews the anatomy and acute traumatic injuries of the extensor, flexor, and internal and external rotator musculotendinous units of the knee. The review is organized according to the four compartments of the joint: anterior, medial, lateral, and posterior. The MR imaging characteristics of muscle contusions, myotendinous strains, and avulsion injuries are emphasized. Comprehensive drawings and MR images of normal anatomy supplement the images of traumatic lesions.
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Categories of Musculotendinous Injury
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Acute musculotendinous injuries involving the knee occur as the result of blunt direct trauma or secondary to indirect trauma from excessive tension. Three discrete categories of acute injuries to the musculotendinous unit can be defined based on the mechanism of trauma and the site of involvement: muscle contusion, myotendinous strain, and tendon avulsion.
Muscle contusions are secondary to direct trauma, usually by a blunt object (5). They usually occur deep in the muscle belly and tend to be less symptomatic than strains, since the latter are commonly located in the superficial muscle layers (4). Myotendinous strains are induced by indirect trauma from excessive stretching (5). They are usually the result of a single major traumatic event. Strain injuries affect the myotendinous junction, the weakest point of the musculotendinous unit (6). Several factors make a muscle more susceptible to strain, including (a) composition of type II fibers (fast contracting), (b) extension across two joints, (c) eccentric action, and (d) fusiform shape (2,3,5,7). Myotendinous strains are clinically classified as first degree (stretch injury), second degree (partial tear), and third degree (complete rupture) based on absent, mild, or complete loss of muscle function, respectively (1). Acute avulsion injury at the tendon insertion site results from a forceful, unbalanced, and often eccentric muscle contraction (8). Loss of function and severe tenderness are frequent symptoms in this condition.
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MR Imaging of Musculotendinous Injuries
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The extent of musculotendinous injuries and associated architectural distortion is examined by using axial, coronal, and sagittal images oriented along the longitudinal and short axes of the involved musculotendinous unit (1).
Alterations in water content in the affected musculotendinous units are common to all forms of acute traumatic injuries (1–3). T1-weighted images are relatively insensitive to these changes. However, they provide information regarding the presence of subacute hemorrhage, atrophy, and fatty infiltration (1). Short inversion time inversion-recovery (STIR) or fat-suppressed T2-weighted sequences enable detection of edematous changes in the musculotendinous unit and are of paramount importance for diagnosis of these lesions (2,3).
The MR imaging appearance of muscle contusions varies according to the severity of the lesion. Diffuse edematous changes of the muscle manifest as a feathery appearance on STIR and T2-weighted images (2). Architectural changes other than increased muscle girth are absent. Intramuscular hematomas can develop with more severe trauma (4).
Myotendinous strains can be classified at MR imaging on the basis of the extent of disruption (3). First-degree strains imply a minor degree of fiber disruption. Interstitial edema and hemorrhage are present at the myotendinous junction and extend into the adjacent muscle fascicles, producing a feathery appearance on MR images (2,3) (Fig 1). Second-degree injuries are characterized by a partial tear without retraction. A hematoma at the myotendinous junction is frequently observed, as are perifascial fluid collections (Fig 2). Third-degree strains involve a complete rupture of the myotendinous unit; this diagnosis is usually made on clinical grounds. MR imaging is useful when preoperative assessment of the extent of retraction is needed. In some cases, hematomas, which may require percutaneous drainage, can also be pinpointed at MR imaging (4).
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Figure 1a. First-degree myotendinous strain. Axial (a) and sagittal (b) T2-weighted fast spin-echo MR images (repetition time msec/echo time msec = 4,000/80) show diffuse interstitial edema of the medial head of the gastrocnemius muscle as a feathery pattern (arrowheads), consistent with a first-degree strain. A small intramuscular hematoma in the soleus muscle is also seen (arrow).
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Figure 1b. First-degree myotendinous strain. Axial (a) and sagittal (b) T2-weighted fast spin-echo MR images (repetition time msec/echo time msec = 4,000/80) show diffuse interstitial edema of the medial head of the gastrocnemius muscle as a feathery pattern (arrowheads), consistent with a first-degree strain. A small intramuscular hematoma in the soleus muscle is also seen (arrow).
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Figure 2a. Second-degree myotendinous strain. (a) Axial T2-weighted fast spin-echo MR image (5,200/100) shows a hematoma (*) in the myotendinous junction of the medial head of the gastrocnemius muscle with associated perifascial edema. (b) Coronal fat-suppressed proton-density-weighted MR image (5,000/17) shows the hematoma (*) and thickening and heterogeneity of the Achilles tendon (arrows).
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Figure 2b. Second-degree myotendinous strain. (a) Axial T2-weighted fast spin-echo MR image (5,200/100) shows a hematoma (*) in the myotendinous junction of the medial head of the gastrocnemius muscle with associated perifascial edema. (b) Coronal fat-suppressed proton-density-weighted MR image (5,000/17) shows the hematoma (*) and thickening and heterogeneity of the Achilles tendon (arrows).
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Acute avulsion injury manifests on MR images as a hematoma and periosteal stripping at the tendon attachment site (9). Waviness and retraction of the torn end of the tendon along with a fragment of bone or cartilage (10) may also be found.
The MR imaging characteristics of an intramuscular hematoma may be affected by its age (1). Acute hematomas are often isointense to skeletal muscle on T1-weighted images and hypointense on T2-weighted or STIR images. Subacute hematomas demonstrate increased signal intensity on T1-weighted images and heterogeneous signal intensity on T2-weighted images due to the presence of extracellular methemoglobin. As the lesion ages, the wall of the hematoma may become hypointense from hemosiderin deposition and fibrosis (11).
Differential diagnosis between muscle contusion and myotendinous strain is difficult based on the MR imaging findings alone (4). Whereas contusions produce injury at the point of impact, strains lead to injury at or near the myotendinous junction. Contusions frequently have a diffuse feathery appearance with increased signal intensity on T2-weighted and STIR images, similar to a first-degree myotendinous strain (2). If fiber disruption occurs, contusions may manifest as an intramuscular hematoma. Contusion hematomas are often located deeper in the muscle belly than hematomas associated with second- and third-degree myotendinous strains (4).
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Anterior Compartment
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The quadriceps muscle group extends the knee joint. It is composed of four muscles: the vastus lateralis, vastus medialis, and vastus intermedius, which arise from the proximal femoral shaft, and the rectus femoris, which originates from the anterior inferior iliac spine (12) (Fig 3). The quadriceps tendon has a trilaminar configuration, being formed by a superficial layer from the rectus femoris, a middle layer from the vastus lateralis and medialis, and a deeper layer from the vastus intermedius (13,14) (Fig 4). The medial and lateral patellar retinacula are composed of fibers from the vastus medialis and lateralis, respectively (15). The patellar tendon is mainly composed of rectus femoris fibers that course over the patella (16).
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Figure 3. Quadriceps muscle group. Drawing shows the anterior inferior iliac spine (AIIS), patellar tendon (PT), quadriceps tendon (QT), rectus femoris (RF), vastus intermedius (VI), vastus lateralis (VL), and vastus medialis (VM).
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Figure 4. Normal quadriceps tendon. Sagittal T1-weighted MR image (466/16) shows the trilaminar configuration of the quadriceps tendon, with fat interposed between the tendon components of the rectus femoris (arrowheads), vastus lateralis and medialis (open arrow), and vastus intermedius (solid arrows).
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Among the quadriceps muscles, the rectus femoris is the most susceptible to injury at the myotendinous junction (3). This susceptibility is due to its superficial location, predominance of type II fibers, eccentric muscle action, and extension across two joints (5). Complete disruption of the quadriceps musculotendinous unit at the tendon level often occurs as the end result of repetitive microtrauma or secondary to weakening by other underlying conditions (gout, diabetes, hyperparathyroidism, collagen vascular diseases). However, strong deceleration may result in acute partial or complete tears of the quadriceps tendon a few centimeters from the upper patellar pole (17) (Figs 5, 6). In addition, sports injuries can result in acute avulsions at the insertion of the patellar tendon onto the inferior patellar pole (Fig 7) and less commonly at its tibial insertion (17).
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Figure 5a. Partial tear of the quadriceps tendon. Sagittal T2-weighted MR image (2,000/80) (a) and axial fat-suppressed proton-density-weighted fast spin-echo MR image (4,000/36) (b) show swelling and heterogeneity of the quadriceps tendon (arrows in b). The rectus femoris layer is edematous and discontinuous (arrows in a).
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Figure 5b. Partial tear of the quadriceps tendon. Sagittal T2-weighted MR image (2,000/80) (a) and axial fat-suppressed proton-density-weighted fast spin-echo MR image (4,000/36) (b) show swelling and heterogeneity of the quadriceps tendon (arrows in b). The rectus femoris layer is edematous and discontinuous (arrows in a).
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Figure 6. Complete tear of the quadriceps tendon. Sagittal T2-weighted MR image (2,000/80) shows a complete rupture of the quadriceps tendon with retraction of the torn ends (arrows).
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Figure 7. Avulsion injury of the patellar tendon. Sagittal T2-weighted MR image (2,100/80) shows complete avulsion of the tendon from the patella (arrow). The patella is superiorly displaced (*).
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The vastus medialis obliquus (VMO), a muscular slip of the vastus medialis muscle, plays a fundamental role as a medial patellar stabilizer. The VMO arises from the adductor magnus tendon and inserts at the medial border of the patella (14). The VMO aponeurosis is tightly adherent to the underlying medial patellofemoral ligament (MPFL), which in turn inserts on the adductor tubercle (18). VMO injury is almost inevitable after acute lateral patellar dislocation. At surgical exploration, the VMO muscle remains attached to the MPFL after dislocation, stripping the entire complex anteriorly from the adductor tendon and tubercle (19) (Fig 8). MR imaging findings include disruption of the MPFL-VMO complex with increased signal intensity at the level of the adductor tubercle on T2-weighted images, increased signal intensity within the VMO muscle on T2-weighted images in association with anterior and superior displacement of its belly, interstitial and perimuscular edematous changes of the adductor magnus, increased signal intensity in the patellotibial ligament near the tibial insertion on T2-weighted images, and bone bruises of the lateral femoral condyle or medial patella (17) (Fig 9).
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Figure 8. VMO avulsion injury during lateral patellar dislocation. Drawing shows the VMO stripped from the adductor magnus tendon (AM) and the MPFL avulsed from the adductor tubercle (AT). (Adapted and reprinted, with permission, from reference 19.)
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Figure 9a. VMO avulsion injury. (a) Sequential axial T1-weighted MR images (466/18) (presented from inferior to superior) show the normal anatomy of the VMO-MPFL complex. The VMO (short solid arrow) descends adjacent to the adductor magnus tendon (long solid arrow) and blends with the MPFL (open arrow), inserting together into the adductor tubercle (*). (b-d) Axial fat-suppressed proton-density-weighted fast spin-echo MR images (3,816/40) obtained through the supracondylar region (b) and patellofemoral joint (c) and coronal T2-weighted fast spin-echo MR image (4,000/114) (d) show edema adjacent to the adductor tubercle (curved arrow), anterior displacement of the VMO (white open arrow), thickening and waviness of the MPFL (black open arrow), a fluid collection (short straight solid arrows) surrounding the belly of the vastus medialis (*), and edema around the adductor magnus (long straight solid arrow).
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Figure 9b. VMO avulsion injury. (a) Sequential axial T1-weighted MR images (466/18) (presented from inferior to superior) show the normal anatomy of the VMO-MPFL complex. The VMO (short solid arrow) descends adjacent to the adductor magnus tendon (long solid arrow) and blends with the MPFL (open arrow), inserting together into the adductor tubercle (*). (b-d) Axial fat-suppressed proton-density-weighted fast spin-echo MR images (3,816/40) obtained through the supracondylar region (b) and patellofemoral joint (c) and coronal T2-weighted fast spin-echo MR image (4,000/114) (d) show edema adjacent to the adductor tubercle (curved arrow), anterior displacement of the VMO (white open arrow), thickening and waviness of the MPFL (black open arrow), a fluid collection (short straight solid arrows) surrounding the belly of the vastus medialis (*), and edema around the adductor magnus (long straight solid arrow).
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Figure 9c. VMO avulsion injury. (a) Sequential axial T1-weighted MR images (466/18) (presented from inferior to superior) show the normal anatomy of the VMO-MPFL complex. The VMO (short solid arrow) descends adjacent to the adductor magnus tendon (long solid arrow) and blends with the MPFL (open arrow), inserting together into the adductor tubercle (*). (b-d) Axial fat-suppressed proton-density-weighted fast spin-echo MR images (3,816/40) obtained through the supracondylar region (b) and patellofemoral joint (c) and coronal T2-weighted fast spin-echo MR image (4,000/114) (d) show edema adjacent to the adductor tubercle (curved arrow), anterior displacement of the VMO (white open arrow), thickening and waviness of the MPFL (black open arrow), a fluid collection (short straight solid arrows) surrounding the belly of the vastus medialis (*), and edema around the adductor magnus (long straight solid arrow).
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Figure 9d. VMO avulsion injury. (a) Sequential axial T1-weighted MR images (466/18) (presented from inferior to superior) show the normal anatomy of the VMO-MPFL complex. The VMO (short solid arrow) descends adjacent to the adductor magnus tendon (long solid arrow) and blends with the MPFL (open arrow), inserting together into the adductor tubercle (*). (b-d) Axial fat-suppressed proton-density-weighted fast spin-echo MR images (3,816/40) obtained through the supracondylar region (b) and patellofemoral joint (c) and coronal T2-weighted fast spin-echo MR image (4,000/114) (d) show edema adjacent to the adductor tubercle (curved arrow), anterior displacement of the VMO (white open arrow), thickening and waviness of the MPFL (black open arrow), a fluid collection (short straight solid arrows) surrounding the belly of the vastus medialis (*), and edema around the adductor magnus (long straight solid arrow).
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Medial Compartment
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Pes Anserinus
The sartorius is a long, ribbonlike muscle that winds across the anterior and medial surfaces of the thigh, arising from the anterior superior iliac spine and inserting on the medial surface of the tibia below the tibial tuberosity (12). The gracilis is a thin, straplike muscle on the medial surface of the thigh. It originates from the inferior rami of the pubis and ischium. The semitendinosus arises from the ischial tuberosity close to the long head of the biceps. These three muscles flex and assist in medial rotation of the knee joint (20). At their tibial insertion, the sartorius, gracilis, and semitendinosus tendons are closely associated, constituting the pes anserinus (Figs 10, 11). The sartorius is most susceptible to strain injury due to its superficial location and biarticular course (5) (Fig 12).
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Figure 10. Normal pes anserinus. Drawing shows the gracilis (G), sartorius (S), and semitendinosus (ST) tendons at their conjoined insertion on the anteromedial tibia. VM = vastus medialis.
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Figure 11. Normal pes anserinus. Sagittal proton-density-weighted spin-echo MR image (2,100/20) through the most medial aspect of the knee shows the gracilis (arrowheads), sartorius (straight arrow), and semitendinosus (curved arrow) tendons.
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Figure 12. Second-degree strain of the sartorius and gracilis tendons. Axial fat-suppressed proton-density-weighted fast spin-echo MR image (4,000/34) shows swelling and increased intrasubstance signal intensity in the sartorius (open arrow) and gracilis (straight solid arrow) tendons. Interstitial muscle edema and a perifascial fluid collection (*) are also noted. An associated partial tear of the medial collateral ligament is seen (curved arrow).
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Semimembranosus Muscle
The semimembranosus muscle originates from the ischial tuberosity, crossing deep to the semitendinosus and long head of the biceps (12). The semimembranosus and semitendinosus muscles are referred to as the medial hamstrings and participate in flexion and medial rotation of the knee joint (20). The semimembranosus is fusiform and, unlike most muscles, has a tendinous origin and insertion (5). This classic fusiform configuration along with a propensity for eccentric actions make it prone to strain injury (Fig 13). Partial tears at the origin of the medial head of the gastrocnemius can also be associated with semimembranosus strain injury (Figs 14, 15).
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Figure 13a. Second-degree strain of the semimembranosus tendon. (a) Coronal fat-suppressed T2-weighted MR image (4,000/14) shows a fluid collection (*) surrounding the myotendinous junction of the semimembranosus (SM). BT = biceps femoris, LGH = lateral gastrocnemius head, MGH = medial gastrocnemius head. (b) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (3,816/40) shows the semimembranosus tendon (SM) surrounded by high-signal-intensity fluid or hematoma.
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Figure 13b. Second-degree strain of the semimembranosus tendon. (a) Coronal fat-suppressed T2-weighted MR image (4,000/14) shows a fluid collection (*) surrounding the myotendinous junction of the semimembranosus (SM). BT = biceps femoris, LGH = lateral gastrocnemius head, MGH = medial gastrocnemius head. (b) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (3,816/40) shows the semimembranosus tendon (SM) surrounded by high-signal-intensity fluid or hematoma.
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Figure 14. Normal insertions of the semimembranosus tendon and medial head of the gastrocnemius tendon. Sagittal T1-weighted MR image (500/18) shows the medial gastrocnemius head tendon (MGH) inserting in the medial supracondylar line (*). The main arm of the semimembranosus insertion site (SM) in the posterior medial tibia is also shown. Arrowheads = semimembranosus tendon.
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Figure 15a. Partial tears of semimembranosus and gastrocnemius tendons. (a) Sagittal T2-weighted MR image (2,000/80) shows increased signal intensity within the tendons of the medial head of the gastrocnemius (black arrows) and semimembranosus (white arrows). (b) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (4,000/36) obtained at the level of the knee shows an intrasubstance partial tear of the semimembranosus tendon (arrow). (The injury of the proximal medial head of the gastrocnemius tendon is also shown in Fig 22.)
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Figure 15b. Partial tears of semimembranosus and gastrocnemius tendons. (a) Sagittal T2-weighted MR image (2,000/80) shows increased signal intensity within the tendons of the medial head of the gastrocnemius (black arrows) and semimembranosus (white arrows). (b) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (4,000/36) obtained at the level of the knee shows an intrasubstance partial tear of the semimembranosus tendon (arrow). (The injury of the proximal medial head of the gastrocnemius tendon is also shown in Fig 22.)
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The semimembranosus has a rather complex insertional architecture (21). The main arm of insertion goes to the infraglenoid tubercle of the posteromedial tibial plateau. Other attachment sites include the tibia beneath the medial collateral ligament, the posteromedial capsule, the oblique popliteal ligament, and the superficial fibers of the medial collateral ligament. Avulsion injury of the main arm of insertion of the semimembranosus tendon resulting from a valgus stress to the knee is associated with tears of the anterior cruciate ligament and posterior horn of the medial meniscus (22).
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Lateral Compartment
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Iliotibial Tract
The iliotibial tract is a strong band of deep fascia composed of the fusion of the aponeurotic coverings of the tensor fascia lata, gluteus maximus muscle, and gluteus medius muscle (23) (Fig 16). Above the knee, the iliotibial tract sends insertion arms to the supracondylar tubercle of the lateral femoral condyle and blends with the intermuscular septum (24). The main insertion site is the Gerdy tubercle (lateral tubercle of the tibia) (Fig 17). Other attachment sites include the patella and the patellar ligament. The iliotibial tract stabilizes the knee but does not contribute to movement of the joint (20).
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Figure 16. Normal iliotibial tract and the long head of the biceps femoris muscle. Drawing shows the fibula (F), Gerdy tubercle (G), gluteus maximus (GM), iliotibial tract (ITT), long head of biceps femoris (LHB), and tensor fascia lata (TFL).
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Figure 17. Normal iliotibial tract. Coronal T1-weighted MR image (450/18) shows the iliotibial tract as a linear hypointense structure coursing down the anterolateral aspect of the distal thigh (arrowheads) to insert on the Gerdy tubercle (*).
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Although considered a principal stabilizer of the lateral side of the knee, the iliotibial tract was found to be intact in 15 of 17 patients with acute posterolateral rotatory instability (25). In this series, midsubstance tears of the iliotibial tract were encountered at surgery, rather than avulsion injuries at the Gerdy tubercle (25). The MR imaging findings associated with rupture include discontinuity and edema of the iliotibial tract, which are best seen on coronal images (Fig 18). Bone bruises involving the anterior aspect of the medial femoral condyle may also be seen in patients with acute posterolateral instability or iliotibial tract rupture.
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Figure 18. Midsubstance tear of the iliotibial tract. Coronal T2*-weighted gradient-echo MR image (550/18, 20° flip angle) shows a complete midsubstance tear of the iliotibial tract (black arrow) in association with extensive edematous changes involving the fatty layer distal to the vastus lateralis muscle (small *) and adjacent subcutaneous plane (large *). Bruising of the medial femoral condyle (white arrow) is also noted.
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Biceps Femoris Muscle
The long and short heads of the biceps femoris (lateral hamstring) flex and laterally rotate the knee joint (20). The long head arises from the ischial tuberosity along with the semitendinosus. It is joined above the knee by the short head of the biceps femoris, which originates medial to the linea aspera of the distal femur and from the lateral intermuscular septum (12). The main insertion site is the head and styloid process of the fibula (Figs 16, 19). Nevertheless, both the short and long head have several tendinous and fascial insertion components (24). Fascial extensions of the biceps femoris that insert on the posterior edge of the iliotibial tract account for the occasional concomitant occurrence of traumatic injuries of these two structures (24).
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Figure 19. Normal biceps femoris tendon. Coronal T1-weighted MR image (800/12) shows the short head of the biceps femoris muscle (*) and the biceps femoris tendon (arrowheads) inserting on the lateral aspect of the fibular head (arrow).
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In patients with anterolateral-anteromedial instability secondary to acute trauma, injuries to the biceps femoris muscle complex were encountered in 72% of cases (26). Most of these lesions (96.3%) involved the short head of the biceps. These findings confirm the important role of the biceps femoris muscle complex as a dynamic and static stabilizer of the lateral side of the knee. The same mechanism of injury that tears the arcuate ligament from its fibular insertion can also result in avulsion injury of the biceps femoris (Fig 20). A characteristic bone bruise on the anterior aspect of the medial femoral condyle is present in patients with complete posterolateral disruptions (27).
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Figure 20. Avulsion of the biceps femoris tendon. Coronal T1-weighted MR image (494/20) shows a thickened, heterogeneous biceps femoris tendon that is proximally retracted (arrows) from its insertion on the fibular head. Associated tears of the iliotibial tract and avulsion of the femoral insertion of the popliteus muscle were also present (see Figs 18, 27).
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Posterior Compartment
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Gastrocnemius Muscle
The gastrocnemius is the most superficial muscle of the calf. It arises via medial and lateral heads from the posterior surface of the femur just proximal to the corresponding condyles (12) (Fig 21). These two heads unite to form the main bulk of the muscle extending halfway down the calf. The gastrocnemius ends in a flat tendon that inserts along with the soleus on the calcaneus. The main action of this muscle is plantar flexion of the foot, but it also flexes the knee in the non–weight-bearing state (20). The gastrocnemius muscle is prone to strain injury due to its action across two joints and its superficial location.
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Figure 21. Normal gastrocnemius and plantaris muscles. Drawing shows the Achilles tendon (A), lateral head of gastrocnemius (LHG), medial head of gastrocnemius (MHG), and plantaris (P).
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Strain injuries of the medial head of the gastrocnemius at the midleg or proximal leg can be isolated or occur in combination with rupture of the soleus and plantaris muscles (Figs 1, 2). These injuries are referred to as "tennis leg" due to their common association with this sport (28, 29). MR imaging findings include edematous changes at the myotendinous junction with a feathery pattern, intramuscular hematoma, perifascial fluid collections, and complete rupture with retraction of the tendon (29). Injuries of the medial head of the gastrocnemius can also occur at the level of the knee. In patients with posteromedial instability, these injuries may be isolated (Fig 22) or associated with tears of the semimembranosus tendon (Fig 15). Similarly, tears of the lateral head of the gastrocnemius occur in patients with posterolateral complex injuries. These injuries are frequently associated with tears of the popliteus tendon (Fig 23), biceps tendon, and plantaris muscle (Fig 24).
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Figure 22a. Partial tear at the origin of the medial head of the gastrocnemius muscle. (a) Sagittal T2-weighted MR image (2,000/80) shows increased signal intensity and swelling of the medial head of the gastrocnemius tendon (arrows) and proximal muscle belly (*). (b) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (4,000/36) shows an intrasubstance tear of the medial head of the gastrocnemius tendon (arrow) in association with a peritendinous fluid collection (*).
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Figure 22b. Partial tear at the origin of the medial head of the gastrocnemius muscle. (a) Sagittal T2-weighted MR image (2,000/80) shows increased signal intensity and swelling of the medial head of the gastrocnemius tendon (arrows) and proximal muscle belly (*). (b) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (4,000/36) shows an intrasubstance tear of the medial head of the gastrocnemius tendon (arrow) in association with a peritendinous fluid collection (*).
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Figure 23a. Partial tear at the origin of the lateral head of the gastrocnemius muscle. (a) Sagittal T2-weighted MR image (2,000/80) shows a partial tear at the origin of the lateral head of the gastrocnemius (*). There is fluid around the myotendinous junction of the popliteus (arrow). (b) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (3,816/40) obtained in another patient shows intramuscular high signal intensity (*) involving the lateral head of the gastrocnemius, an appearance consistent with a first-degree strain injury.
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Figure 23b. Partial tear at the origin of the lateral head of the gastrocnemius muscle. (a) Sagittal T2-weighted MR image (2,000/80) shows a partial tear at the origin of the lateral head of the gastrocnemius (*). There is fluid around the myotendinous junction of the popliteus (arrow). (b) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (3,816/40) obtained in another patient shows intramuscular high signal intensity (*) involving the lateral head of the gastrocnemius, an appearance consistent with a first-degree strain injury.
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Figure 24a. Plantaris tear. (a) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (3,816/40) shows increased signal intensity involving the plantaris muscle and surrounding fluid or hematoma (long arrow). Edema in the lateral head of the gastrocnemius is also noted (short arrows). (b) Follow-up axial gradient-echo MR image obtained through the supracondylar line 1 year later shows absence of the plantaris muscle and fatty replacement (arrow). (c) Follow-up coronal T1-weighted MR image (500/18) shows an empty plantaris muscle site in the supracondylar fossa (arrow). (d) Coronal T1-weighted MR image (550/18) obtained in another patient shows a normal plantaris (arrow) and lateral head of the gastrocnemius for comparison.
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Figure 24b. Plantaris tear. (a) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (3,816/40) shows increased signal intensity involving the plantaris muscle and surrounding fluid or hematoma (long arrow). Edema in the lateral head of the gastrocnemius is also noted (short arrows). (b) Follow-up axial gradient-echo MR image obtained through the supracondylar line 1 year later shows absence of the plantaris muscle and fatty replacement (arrow). (c) Follow-up coronal T1-weighted MR image (500/18) shows an empty plantaris muscle site in the supracondylar fossa (arrow). (d) Coronal T1-weighted MR image (550/18) obtained in another patient shows a normal plantaris (arrow) and lateral head of the gastrocnemius for comparison.
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Figure 24c. Plantaris tear. (a) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (3,816/40) shows increased signal intensity involving the plantaris muscle and surrounding fluid or hematoma (long arrow). Edema in the lateral head of the gastrocnemius is also noted (short arrows). (b) Follow-up axial gradient-echo MR image obtained through the supracondylar line 1 year later shows absence of the plantaris muscle and fatty replacement (arrow). (c) Follow-up coronal T1-weighted MR image (500/18) shows an empty plantaris muscle site in the supracondylar fossa (arrow). (d) Coronal T1-weighted MR image (550/18) obtained in another patient shows a normal plantaris (arrow) and lateral head of the gastrocnemius for comparison.
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Figure 24d. Plantaris tear. (a) Axial fat-suppressed proton-density-weighted fast spin-echo MR image (3,816/40) shows increased signal intensity involving the plantaris muscle and surrounding fluid or hematoma (long arrow). Edema in the lateral head of the gastrocnemius is also noted (short arrows). (b) Follow-up axial gradient-echo MR image obtained through the supracondylar line 1 year later shows absence of the plantaris muscle and fatty replacement (arrow). (c) Follow-up coronal T1-weighted MR image (500/18) shows an empty plantaris muscle site in the supracondylar fossa (arrow). (d) Coronal T1-weighted MR image (550/18) obtained in another patient shows a normal plantaris (arrow) and lateral head of the gastrocnemius for comparison.
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Plantaris Muscle
The plantaris is a small muscle that arises from the lateral supracondylar line just above the attachment of the lateral head of the gastrocnemius (Fig 21). Its slender muscle belly, which is approximately 2–4 inches (5–10 cm) long, lies deep to the lateral head of the gastrocnemius (12). The thin and long plantaris tendon crosses down between the medial head of the gastrocnemius and soleus muscles. It inserts on the calcaneus anteromedial to the Achilles tendon or directly into the Achilles tendon. The plantaris is absent in 7%–10% of the population.
Forceful contraction of the plantaris muscle may result in rupture (29). Plantaris muscle strains have been noted in association with traumatic tears of the anterior cruciate ligament, arcuate ligament complex, and posterolateral corner muscles (lateral head of the gastrocnemius and popliteus) (29). A posterior compartment syndrome, which requires surgical decompression, is a potential complication of plantaris or medial gastrocnemius ruptures.
The appearance on MR images depends on the severity of the injury. MR imaging findings include the following: (a) abnormally increased signal intensity in the injured plantaris muscle (Fig 24) or myotendinous junction on T2-weighted images, (b) myotendinous rupture with proximal retraction of the muscle and masslike appearance between the popliteus tendon and lateral head of the gastrocnemius, (c) associated partial tear of the lateral head of the gastrocnemius muscle, (d) fluid collections between the medial head of the gastrocnemius and soleus muscle, (e) associated tears of the anterior cruciate ligament and arcuate complex, and (f) bone contusions in the lateral compartment (29).
Popliteus Muscle
The popliteus muscle arises from the posteromedial aspect of the proximal tibial metaphysis, forming part of the floor of the popliteal fossa (12). Its tendon extends from the muscle belly through the popliteal hiatus and attaches to the lateral femoral condyle at the end of the popliteal sulcus (Figs 25, 26). Two additional insertion sites are the posterior fibular head (popliteofibular ligament) and the posterior horn of the lateral meniscus (30) (Fig 25). Electromyographic studies have determined that the popliteus is the primary internal rotator of the tibia on the femur in the non–weight-bearing state (31). In the weight-bearing state, it externally rotates the femur on the leg.
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Figure 25. Normal popliteus musculotendinous unit. Drawings show the fibular collateral ligament (FCL), popliteus muscle (P), popliteofibular ligament (PFL), popliteomeniscal fascicles (PM), and popliteus tendon (PT). Arrowheads = popliteal sulcus.
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Avulsion injuries at the femoral insertion of the popliteus tendon have been reported (32) (Fig 27). However, injuries of the muscle belly and myotendinous junction of the popliteus are far more common (33). These lesions most likely occur secondary to a direct blow to the anteromedial aspect of the proximal tibia with hyperextension of the knee or from indirect external rotation–hyperextension type of trauma. Enlargement of the muscle and increased signal intensity on T2-weighted images can be demonstrated at MR imaging (Fig 28). MR imaging is also useful for differentiation of partial interstitial tear from complete muscle rupture, since the latter produces retraction and clumping.
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Figure 27. Avulsed popliteus tendon. Coronal fat-suppressed proton-density-weighted fast spin-echo MR image (3,315/30) shows a retracted popliteus tendon (arrow) and empty popliteal sulcus (*), findings consistent with avulsion. Associated avulsion of the fibular collateral ligament is also noted (arrowheads).
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Figure 28a. Popliteus strain injury. (a) Axial fat-suppressed proton-density-weighted MR image (3,800/40) shows intramuscular linear fluid collections (solid arrow) and perifascial fluid (open arrow), findings consistent with a second-degree strain of the popliteus. (b) Sagittal T2-weighted MR image (2,000/90) shows a swollen and edematous popliteus myotendinous junction (arrow).
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Abstract
LEARNING OBJECTIVES FOR TEST...
