Applied Radiology

Avulsion fractures about the knee usually result from excessive muscular contraction during athletic activities. Occasionally they are caused by severe blunt trauma. Because these injuries often are associated with internal derangement and instability of the knee, early diagnosis is essential. This pictorial essay shows the range of conventional radiographic findings in avulsion fractures about the knee. MR imaging findings in these fractures and in associated ligamentous, tendinous, and meniscal injuries are reviewed.

Avulsion fractures involving the cruciate ligaments

Anterior cruciate ligament (ACL) -Avulsion fractures of the ACL occur almost exclusively from the tibial spine or intercondylar eminence near the attachment site of the ACL; these injuries are more common in children than in adults. ^1,2 In children, avulsion fractures of the tibial spine commonly occur as isolated injuries, whereas in adults these fractures usually are associated with other injuries. Kendall et al ¹ reviewed avulsion fractures of the tibial spine in 19 adults and 12 children. In adults, they found that 37% of injuries were associated with tears of the menisci (5%), posterior cruciate ligament (5%), collateral ligaments (11%), or other fractures (32%); in children, 8% were associated with other injuries. ¹ The usual mechanism of injury for avulsion fractures of the ACL is hyperflexion of the knee with tibial internal rotation, or hyperextension of the knee. ¹ Patients with ACL avulsions have pain associated with knee flexion, and signs of hemarthrosis.

The appearance of these fractures on conventional radiographs varies with the degree of displacement of fracture fragments (figures 1,2). A classification system described by Meyers and McKeever ² for fractures of the intercondylar eminence of the tibia often is used in the selection of treatment options. Undisplaced or minimally displaced (types I and II) isolated avulsion fractures of the tibial spine are treated by closed means. Surgical reattachment generally is reserved for fractures that are widely displaced, completely detached, or rotated (type III).

FIGURE 1. A 28-year-old woman with an avulsion fracture of the anterior cruciate ligament (ACL). Anteroposterior (A) and oblique (B) radiographs of the right knee show a completely detached avulsion fracture (arrows) of the tibial spine. (C) A sagittal, fatsuppressed, T2-weighted MR image shows an intact ACL attached to an avulsed fracture fragment (arrow). (D) A coronal, fat-suppressed, T2-weighted MR image shows an avulsion fracture and an associated bone bruise on the lateral femoral condyle (arrow).

FIGURE 2. A 15-year-old girl with an avulsion fracture of the anterior cruciate ligament (ACL). (A) A lateral radiograph of the right knee shows a completely detached avulsion fracture of the tibial spine (arrow). (B) Sagittal and (C) coronal proton density MR images show an intact ACL attached to an avulsed fracture fragment (arrows).

Although conventional radiographs typically allow detection of the avulsion fracture, MR imaging may be particularly useful in defining the extent of fracture (figures 1,2). In addition, MR imaging is ideal for showing associated soft-tissue injuries about the knee that are associated with avulsion fractures, specifically in adults with ACL avulsions (figure 3), in whom such injuries are more likely to result in poor outcomes including pain, locking, decreased range of motion, or chronic ACL deficiency. ^1,2 In children, MR imaging is less often used because associated injuries are rare and outcome is generally good.

Posterior cruciate ligament (PCL) -Avulsion fractures of the posterior tibial eminence at the attachment of the PCL are far less common than ACL avulsions. ³ The usual mechanism of injury for avulsion fractures of the PCL is sudden hyperextension of the knee, or a violent posterior displacement of the tibia while the knee is in flexion. ³ Patients present with pain and swelling of the knee, inability to bear weight, and ecchymosis or abrasions of the overlying skin and soft tissue. ^3,4

FIGURE 4. An 18-year-old man with an avulsion fracture of the posterior cruciate ligament (PCL). (A) Lateral and (B) anteroposterior radiographs of the left knee show the fracture fragment in the posterior intercondylar space (white arrow) and a nondisplaced fracture of the lateral femoral epicondyle (black arrowheads). (C) This sagittal, protondensity MR image shows a buckled PCL attached to a proximally displaced fracture fragment (white arrow). Note lipohemarthrosis (black arrowheads). (D) Axial T1-weighted and (E) coronal proton-density- weighted MR images show a combined incomplete avulsion of the fibular collateral ligament from its femoral attachment (arrow) and a vertical fracture of the posterolateral tibial rim (arrowheads).

FIGURE 5. An 18-year-old man with avulsion fractures of the posterior cruciate ligament (PCL) and distal patellar tendon. (A) A lateral radiograph of the right knee shows a fracture of the tibial tuberosity (curved black arrow) and of the posterior tibia (white arrow). (B) The sagittal T2-weighted MR image shows a partially avulsed fracture fragment attached to the patellar tendon, and an underlying bone bruise (arrow). (C) A sagittal, proton-density MR image shows a displaced avulsion fracture of the PCL from its tibial attachment (open arrow).

Conventional radiographs may show a bone fragment in the posterior intercondylar space (figure 4); the fragment is often comminuted. Because avulsion fractures of the PCL are rarely isolated injuries, their presence should prompt a search for associated ligamentous and meniscal tears. ³ Associated ACL tears are reported in 44 to 100% of cases, midportion PCL injuries in 16 to 76%, medial collateral ligament injuries in 46 to 74%, fibular collateral ligament injuries in 7 to 42%, and meniscal tears in 29 to 83%. ^4-6 MR imaging is ideal for showing injuries that are associated with PCL avulsions (figures 4,5) due to its features such as the ability to detect soft-tissue injury.

Although avulsion fractures involving the femoral attachments of the cruciate ligaments sometimes occur, they occur much less frequently than tibial avulsion fractures. ³

Avulsion fractures involving the collateral ligaments

Medial collateral ligament (MCL) -Avulsion fractures at attachment sites of the MCL are uncommon. ⁷ However, a cortical fragment may be avulsed by the MCL from the medial femoral condyle (figures 6,7). After a tear of the MCL, calcification or ossification of the MCL may occur adjacent to the proximal margin of the medial femoral condyle; this finding is known as Pellegrini Stieda disease. ⁸ Patients with this disorder usually have painful swelling about the medial femoral condyle; associated injuries of the cruciate ligament and meniscus are common in those with high grade MCL injuries. Injuries of the ACL occur in 58 to 96% of cases of Pelle grini-Stieda disease, injuries of the PCL in 0 to 39%, fibular collateral ligament injuries in 0 to 78%, medial meniscal tears in 39%, lateral meniscal tears in 5 to 35%, and bone contusions in 96%. ^9,10 MR imaging is ideal for detection of MCL avulsions and associated injuries of the cruciate and meniscus (figure 6,7).

FIGURE 6. A 35-year-old man with an avulsion fracture of the medial collateral ligament (MCL). (A) An anteroposterior radiograph of the right knee shows a vertical fracture fragment (arrow) avulsed from the medial femoral condyle. This was surgically proved to represent avulsion of the proximal attachment of the MCL. (B) A coronal, T2-weighted MR image shows avulsion of the femoral attachment of the MCL, and an underlying bone bruise (arrow).

FIGURE 7. A 35-year-old man with an avulsion fracture of the medial collateral ligament (MCL). (A) The anteroposterior radiograph of the patient's left knee shows an avulsion fracture of the medial femoral condyle (arrows). (B) Coronal T2-weighted and (C) axial gradient-echo MR images show an avulsed fracture fragment attached to the MCL (black arrows), and a bone bruise of the lateral femoral condyle. An irregular focus of increased signal intensity within the PCL (white arrow) suggests partial injury of the PCL. Arthroscopy confirmed a partial tear of the proximal PCL.

Medial capsular ligament -Avulsion fractures of the medial capsular ligament can occur at its tibial attachment on the medial rim of the medial tibial plateau (figure 8). The usual mechanism of injury for this type of fracture is valgus stress with external rotation of a flexed knee. This mechanism is the reverse of that which causes the Segond fracture. ¹¹

Medial capsular ligament avulsion fractures may be associated with tears of the PCL or medial meniscus. These fractures are the mirror-image of associated injuries seen with Segond fractures. ¹¹ Therefore, the finding of medial capsular ligament avulsion fractures on conventional radiographs should prompt the use of MR imaging to exclude associated tears of the PCL or medial meniscus.

FIGURE 8. A 23-year-old woman with an avulsion fracture of the medial capsular ligament. (A) This anteroposterior radiograph of the patient's left knee shows a small fracture fragment along the medial tibial rim (arrow). (B) The coronal gradient-echo MR image shows a small avulsed fracture fragment of the medial proximal tibial rim attached to the meniscotibial portion of the medial capsular ligament (arrow).

Lateral capsular ligament-A small vertical avulsion fracture that occurs at the attachment of the midportion of the lateral capsular ligament on the proximal lateral tibia (just distal to the plateau) is referred to as the Segond fracture or lateral capsular sign. This injury usually occurs during a fall and is due to internal rotation of the knee with varus stress. ^12-15 These same forces often result in associated ACL and meniscal tears. Patients often describe a painful "giving-way" sensation and have anterolateral rotational instability, but walking usually is unimpaired. ^13,14

The appearance of a Segond fracture on the anteroposterior or tunnel views of the knee is fairly consistent (figures 9,10). The fracture fragments are small and ovoid and generally are oriented in a vertical or oblique direction; displacement is minimal. ¹⁴ Segond fractures are associated with ACL tears in 75 to 100% of cases and with meniscal tears in 42 to 67% of cases. ^13-15

FIGURE 9. A 33-year-old man with an avulsion fracture of the lateral capsular ligament, Segond fracture. (A) An anteroposterior radiograph of the right knee shows a small elliptical fracture fragment adjacent to the lateral tibial rim (arrow). This finding is typical of Segond fracture. (B) Coronal T2- weighted MR images show a small avulsed fracture fragment attached to the lateral capsular ligament (straight arrows). Note the location of Segond fractures posterior and superior to the tibial insertion of the iliotibial band on the Gerdy's tubercle (curved arrow). (C) A sagittal, fatsuppressed protondensity MR image shows discontinuity at the femoral attachment of the ACL (arrow), consistent with an ACL tear.

FIGURE 10. A 14-year-old boy with a Segond fracture of the right knee and avulsion of the anterior cruciate ligament attachment. (A) Anteroposterior and (B) lateral radiographs show a small vertical fracture fragment along the lateral tibial rim (arrows) and an avulsion fracture of the ACL with vertical extension into the physis.

Additional osseous injuries have been described in association with Segond fractures. These include avulsions at the tibial eminence, Gerdy's tubercle, and the fibular head (figure 10). Patients whose conventional radiographs show a Segond fracture should undergo MR imaging to exclude associated injuries (figure 9).

Fibular collateral ligament (FCL) -Avulsion fractures involving the FCL usually occur at the site of attachment of the FCL and biceps femoris tendon on the proximal pole or styloid process of the fibula. The usual mechanism of injury for this type of fracture is force directed against the anteromedial tibia with the knee in extension; the result is posterolateral subluxation of the knee. ¹⁶ This injury can be subtle both clinically and radiographically. ¹⁶ Clinically, pain and disability may be minimal. ¹⁷

Radiographs may show a small avulsed fibular head fragment ranging in size from a tiny fleck of bone to a fragment of bone several millimeters in diameter (figures 3,11,12). On conventional radiographs, this fracture has been called the "arcuate sign". ¹⁶ The fragment often signifies disruption of the posterolateral ligamentous complex (also known as the arcuate complex) and the presence of the posterolateral instability. ¹⁶ In patients with FCL avulsions, MR imaging is used to exclude associated injuries of the iliotibial band or cruciate ligaments (figure 3).

FIGURE 3. A 23-year-old woman with avulsion fractures of the iliotibial band (ITB), anterior cruciate ligament (ACL), and fibular collateral ligament (FCL) is shown. (A) Anteroposterior and (B) oblique radiographs of her right knee show a large fracture fragment anterior, superior, and lateral to the proximal tibia (long white arrow), an irregular small fracture fragment superior to the tibial spine (curved black arrow), a crescent fracture fragment superior and lateral to the fibular head (short white arrow), and a depressed fracture of the medial tibial plateau (straight black arrow). (C) A coronal proton-density MR image shows an avulsion fracture of the iliotibial band (arrow). (D) A coronal T2-weighted MR image shows a fracture fragment avulsed from the fibular head and attached to the proximally retracted conjoined tendon of the FCL and biceps femoris (arrow). (E,F) Sagittal proton-density MR images show avulsion of the ACL at its tibial attachment (E, open arrow) and a tear of the proximial posterior cruciate ligamnet (F, solid arrow).

FIGURE 11. A 30-year-old man with avulsion fractures of the fibular head in both knees. (A) An anteroposterior radiograph of the left knee shows a fracture fragment adjacent to the lateral margin of the tibia (solid arrow), another fracture fragment along the medial tibial rim (open arrow), and widening of the lateral femorotibial joint space. (B) The coronal T2-weighted MR image shows an avulsed fracture fragment of the fibular head attached to the proximally retracted fibular collateral ligament and biceps femoris tendon (arrow). (C) This coronal T2-weighted MR image shows disruption of the lateral capsule (arrow) and depressed fracture of the medial tibial plateau. (D,E) Anteroposterior (D) and lateral (E) radiographs of the right knee in the same patient show a large fracture fragment avulsed from the fibular head (solid arrow) and fracture of the anterior proximal tibia (open arrow).

FIGURE 12. A 49-year-old man with an avulsion fracture of the fibular collateral ligament (FCL). (A) This anteroposterior radiograph shows a small triangular fracture fragment superior to the fibular head (arrow). (B) A sagittal proton-density-weighted MR image shows an arcshaped cortical fracture fragment avulsed from the fibular head (arrow). (C) A midsagittal T1-weighted MR image shows a tear of the posterior cruciate ligament at its midsubstance (arrows). Other MR images for this patient (not shown) showed a tear of the popliteal muscle and medial meniscus.

Iliotibial band (ITB) -Avulsion fracture of the ITB usually occurs at its tibial insertion on Gerdy's tubercle of the proximal tibia. ¹² Conventional radiographs may show a small bone fragment along the anterolateral aspect of the proximal tibia. The fracture should be differentiated from a Segond fracture, the location of which is usually superior and posterior to Gerdy's tubercle (figure 3). However, Gerdy's tubercle avulsion may coexist with a fracture of the fibular head or a Segond fracture. ^12,13 The finding of ITB avulsion fracture on conventional radiographs should suggest the presence of associated lateral collateral ligament injury. ¹² MR imaging is ideal for showing associated injuries of the lateral collateral ligament and the lateral meniscus (figure 3).

Avulsion fractures involving the extensor mechanism

Patellar retinacula -Avulsion fractures of the patella may involve any portion of the patellar periphery. Four patterns have been described: superior, inferior (the most common), medial (usually caused by lateral dislocation of the patella), and lateral. ¹⁸ These injuries usually result from excessive sudden contraction of the quadriceps muscle with the knee flexed during running, jumping, or kicking. ¹⁹ Clinically, the injury is suggested by the sudden onset of pain, at times accompanied by an audible sound, and the absence of a direct blow. ¹⁸ Medial and lateral retinacular avulsion fractures are best seen on the sunrise view of the knee, whereas superior and inferior tendon avulsions are best seen on the lateral view (figures 13-15). ¹⁹ In children, these avulsion fractures may be missed because the osteocartilaginous fragment may be undetectable radiographically. ^18,20 MR images enable assessment of the extent of cartilaginous injury, displacement of fracture fragments, and associated retinaculear injuries, and thus help determine the need for surgery.

FIGURE 13. A 22-year-old man with an avulsion fracture of the patellar tendon. (A) A lateral radiograph of the right knee shows a small, thin fracture fragment inferior to the patella (arrow). (B) A sagittal proton-density-weighted MR image shows a small cortical fracture fragment attached to the patellar tendon (arrow).

FIGURE 14. A 52-year-old man with an avulsion fracture of the quadriceps tendon. (A,B) Lateral (A) and internal (B) oblique radiographs of the left knee show a comminuted fracture of the upper pole of the patella (arrow). (C) Sagittal T2-weighted and (D) coronal T2-weighted MR images show two fracture fragments avulsed from the upper pole of the patella (arrows) and attached to the partly retracted quadriceps tendon.

FIGURE 15. A 36-year-old man with an avulsion fracture of the lateral patellar retinaculum. (A) The sunrise view of the right patella shows a thin fracture fragment along the lateral margin of the patella (arrow). (B) This axial T2-weighted MR image shows a small cortical fragment attached to the lateral patellar retinaculum (arrows). Note the underlying bone bruise within the patella.

Distal patellar tendon -Avulsion fractures of the tibial tuberosity usually result from a violent active extension of the knee or a violent passive flexion of the knee against a tightly contracted quadriceps; ^21,22 most are sports-related. ²¹ Typical symptoms are pain and swelling of the knee, and tenderness directly over the tuberosity. ²¹ These avulsions are commonly classified into three types, based on their radiographic appearance (figure 5): type I-fracture of the distal portion of the tuberosity; type II-fracture of the entire tuberosity; and type III-fracture of the entire tuberosity extending vertically to include a portion of the anterior tibial epiphysis. ²² Type III fractures are more common in older adolescents (15 to 17 years), whereas type I and II fractures are most frequently found in younger adolescents (12 to 14 years). ²² MR imaging is used to exclude associated injuries of the ACL, MCL, and meniscus (figure 5). ²³ Additionally, Osgood-Schlatter disease may be related to avulsion fractures of the tibial tuberosity. ²²

Avulsion fractures involving tendons

Semimembranosus tendon -Avulsion fractures of the semimembranosus tendon usually occur at its tibial insertion (figure 16). The probable mechanism of injury is external rotation and abduction of a flexed knee, a mechanism frequently implicated in tears of the ACL. ²⁴ Conventional radiographs may show a small fracture fragment along the posteromedial corner of the tibial plateau. ²⁴ MR imaging can be used to show associated tears of the ACL and medial meniscus. ²⁴

FIGURE 16. A 17-year-old man with an avulsion fracture of the semimembranosus tendon at its insertion. (A) A sagittal, fatsuppressed proton-density MR image shows a small avulsion fracture at the posteromedial corner of the medial tibial plateau (arrow) attached to the distal semimembranosus tendon. (B) A sagittal T2-weighted MR image shows marked anterior subluxation of the tibia resulting from an ACL tear. Other MR images taken of this patient showed a bucket-handle tear of the medial meniscus and a partial tear of the PCL.

Biceps femoris tendon -Avulsion fractures of the biceps femoris tendon may occur at its insertion on the fibular head (figure 11).

Popliteus tendon -Rarely, the insertion of the popliteus tendon avulses from the femur. This injury results from posterior subluxation of the tibia with respect to the femur and is commonly associated with tears of the menisci, PCL, or lateral capsular structures. ²⁵

Conclusion

Most avulsion fractures about the knee are easily detected on conventional radiographs. However, MR imaging can help to detect some fractures that are missed on routine views and is useful for evaluating the extent of injury, as well as confirming coexisting ligamentous, tendinous, or meniscal injuries. AR