Clinical Anatomy 27:789–797 (2014)

REVIEW

The Fibular Collateral Ligament of the Knee: A Detailed Review YOUNG-BIN SONG,1 KOICHI WATANABE,1 ELIZABETH HOGAN,2 ANTHONY V. D’ANTONI,3 ANTHONY C. DILANDRO,3 NIHAL APAYDIN,4 MARIOS LOUKAS,2 MOHAMMADALI M. SHOJA,1 AND R. SHANE TUBBS1,2* 1

Pediatric Neurosurgery Children’s of Alabama, Birmingham, AL Department of Anatomical Sciences, St. George’s University, Grenada 3 Division of Pre-Clinical Sciences, New York College of Podiatric Medicine 4 Department of Anatomy, Ankara University Faculty of Medicine 2

The fibular collateral ligament (FCL) is one of the larger ligaments of the knee. The FCL, along with the popliteus tendon, arcuate popliteal ligament, and joint capsule, make up the posterolateral corner of the knee. Recently, there has there been an increased awareness and research on the structures of the posterolateral corner of the knee, particularly the FCL. Studying the detailed structure of the FCL may provide a better understanding that can lead to better diagnosis and treatments following injury. Therefore, this article reviews the FCL, which appears to be the primary restraint to varus rotation but is poorly oriented to resist external rotation of the knee. Clin. Anat. 27:789–797, 2014. VC 2013 Wiley Periodicals, Inc. Key words: anatomy; leg; knee; thigh; ligaments; injury; surgery; biomechanics

ANATOMY The fibular collateral ligament (FCL; Fig. 1) is located in the deepest layer of the lateral structures of the knee, which also include the fabellofibular and arcuate popliteal ligaments (Espregueira-Mendes and da Silva, 2006). The fabellofibular ligament is a thickening of the capsular arm of the biceps femoris as it runs distally to the fibula. The FCL is described as a well-defined, “pencil-like” bundle that is outside of the knee joint capsule (Brinkman et al., 2005; Espregueira-Mendes and da Silva, 2006). Others have described it as being “cord-like” (Hollinshead and Rosse, 1985; Agur and Lee, 1991; Soames, 1995; Miller, 1998; Surgita and Amis, 2001) (Figs. 2–4). A cross-sectional view of the FCL demonstrates an elliptical shape in the midline and the ligament fans out distally near its attachment onto the fibula (Meister et al., 2000). The FCL travels inferiorly and posteriorly from the area between the lateral epicondyle and the supracondylar process of the femur to the head of the fibula. Meister et al. (2000) found the mid-point between its femoral and fibular attachments had the smallest cross-sectional area.

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For comparison, the tibial collateral ligament is broad and flat band attached proximally to the medial condyle of femur and distally it is inserted onto the medial surface of the body of the tibia about 2.5 cm below the level of the condyle. This ligament resists forces that would push the knee medially, which would otherwise produce valgus deformity. Some researchers have shown that the FCL is not directly attached to the lateral epicondyle, but rather is proximal and posterior to the lateral epicondyle (Seebacher et al., 1982; Watanabe et al., 1993; Tria, 1995; Terry and LaPrade, 1996; Meister et al., 2000; LaPrade et al., 2003; Brinkman et al., 2005; Espregueira-Mendes and da Silva, 2006; LaPrade,

*Correspondence to: R. Shane Tubbs, Children’s Hospital, Pediatric Neurosurgery, 1600 7th Avenue South, Lowder 400, Birmingham, Alabama 35233. E-mail: [email protected] Received 30 May 2012; Revised 3 May 2013; Accepted 26 June 2013 Published online 19 November 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/ca.22301

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Fig. 1.

Schematic drawing of the FCL.

2006). On average, LaPrade et al. (2003) and Brinkman et al. (2005) reported the attachment to be proximal (1.4 and 1.3 mm, respectively) and posterior (3.1 and 4.6 mm, respectively) to the lateral epicondyle. The location of the femoral attachment of the FCL is in a depression with various authors using terms such as fovea, or “saddle” to describe this location (Terry and LaPrade, 1996; Meister et al., 2000; LaPrade et al.,

2003; Espregueira-Mendes and da Silva, 2006) with the average cross-sectional area measured at 0.48 cm2 by LaPrade et al. (2003) and 0.52 cm2 by Brinkman et al. (2005). From its femoral attachment site, the FCL is posteriorly oriented as it travels distally to its fibular attachment site (Tria, 1995; Meister et al., 2000; Surgita and Amis, 2001; Espregueira-Mendes and da Silva, 2006; LaPrade, 2006) (Fig. 2). Along the distal quarter of the FCL, studies have found a bursa present between the FCL and the biceps femoris muscle (Terry and LaPrade, 1996; LaPrade and Hamilton, 1997; Espregueira-Mendes and da Silva, 2006). The bursa is located around the distal aspect of the FCL and has an appearance of an inverted “J” shape around the lateral, anterior, and anteromedial portions of the FCL (LaPrade and Hamilton, 1997). The FCL is attached to the lateral aspect of the fibular head (Terry and LaPrade, 1996; Meister et al., 2000; LaPrade et al., 2003; Brinkman et al., 2005; Espregueira-Mendes and da Silva, 2006; LaPrade, 2006). Both LaPrade et al. (2003) and Brinkman et al. (2005) have found the attachment to be posterior to the anterior margin of the fibular head. With respect to the tip of the fibular styloid process, the attachment has been reported to be anterior by 11.7 mm (Brinkman et al., 2005) and 10 mm (Espregueira-Mendes and da Silva, 2006) and distal (LaPrade et al., 2003) to it. Meister et al. (2000) have described the attachment location as a “V-shaped plateau,” either having a depressed, level, or elevated attachment sites. The average cross-sectional area of the fibular attachment has been measured to be 0.35 cm2 (Brinkman et al., 2005) and 0.43 cm2 (LaPrade et al., 2003). Both Meister et al. (2000) and Mendes and

Fig. 2. Cadaveric sample illustrating the FCL with the knee in extension. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Fig. 3. Cadaveric sample demonstrating the FCL with the knee in flexion. ACL 5 anterior cruciate ligament. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Silva (2006) measured the mean proximal–distal dimensions to be longer: 12.9 mm (Meister et al., 2000), 10.9 mm (Espregueira-Mendes and da Silva, 2006) than the mean anteroposterior dimensions 8.4

mm (Meister et al., 2000) and 8.7 mm (EspregueiraMendes and da Silva, 2006). In addition, the attachment site forms a reinforcement between the FCL and surrounding muscles.

Fig. 4. Posterolateral view of the FCL from a cadaver noting the surrounding relationships. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Song et al. 1996; Crum et al., 2003; LaPrade et al., 2003, 2006; Griffith et al., 2007). In addition, a biomechanical study on canine knees has shown that the FCL acts as the primary restraint to varus rotation at all flexion angles (Griffith et al., 2007), which has also been shown in human knees (Grood et al., 1981; Nielsen et al., 1984; Gollehon et al., 1987; Grood et al., 1988; Markolf et al., 1993; LaPrade et al., 1999; Meister et al., 2000; Surgita and Amis, 2001; LaPrade et al., 2004b; Ciccone et al., 2006; Espregueira-Mendes and da Silva, 2006; LaPrade, 2006; Coobs et al., 2007). The FCL of the canine had a limited effect on internal rotation, also similar to the FCL in human knees (Nielsen et al., 1984; Veltri et al., 1995; Griffith et al., 2007). However, there are some unique anatomical features that are different among canine, rabbit, goat, and human knee structures. In both rabbits and goats, the tibia and fibula are fused together to form a tibiofibula (Crum et al., 2003; LaPrade et al., 2006). In rabbits, the fibular head is distal to the lateral tibial plateau surface (Griffith et al., 2007) and in goats, the fibular head is fused to the proximal edge of the lateral tibial plateau and forms a bony prominence (Crum et al., 2003). The fibula of a canine knee is closely associated with the tibia distally, but is not fused (Griffith et al., 2007).

EMBRYOLOGY

Fig. 5. Relationships of the FCL (disconnected proximally and pierced by needle) and the biceps femoris distal tendon in a cadaver. CFN 5 common fibular nerve.

The FCL reinforces the fibularis longus muscle’s fascia and is directly attached to the lateral aponeurotic expansions of the short head of the biceps femoris muscle (Terry and LaPrade, 1996; Espregueira-Mendes and da Silva, 2006; LaPrade, 2006). The biceps femoris muscle overlays the FCL attachment site (Terry and LaPrade, 1996; Meister et al., 2000; EspregueiraMendes and da Silva, 2006; LaPrade, 2006) (Fig. 5).

Many studies have been devoted to the development of the human knee joint, but not to the development of its ligaments. Merida-Velasco et al. (1997) have found the organization of the FCL during O’Rahilly stage 23: the FCL was independent from the knee joint capsule. Throughout weeks 9 and 10, the FCL maintains its independence from the joint capsule and remains superficial to the popliteus tendon  rida-Velasco et al., 1997). Gardner and O’Rahilly (Me (1968), on the other hand, have seen the initial development of the FCL during O’Rahilly stage 19, and by stage 20, they noted, the FCL is clearly present. Gray and Gardner (1950) identified the mesenchymal cells that give rise to the FCL in 7-week-old embryos, and by 14 weeks, the FCL resembled that of the adult knee.

Comparative Anatomy

Histology

Anatomic studies on animal knees have been performed to better understand the posterolateral aspect of the human knee. Qualitative and quantitative measurements of the anatomy and biomechanics of this region have been studied in rabbits (Crum et al., 2003; LaPrade et al., 2004c; LaPrade et al., 2006), goats (LaPrade et al., 2006), and more recently, canine knees (Griffith et al., 2007). Overall, structures of the FCL in rabbits, goats, and canines resemble the human FCL (Crum et al., 2003; LaPrade et al., 2006; Griffith et al., 2007). The course and the attachment site of the FCL are very similar between these animals and humans: the femoral attachment is posterior and proximal to the lateral epicondyle (Terry and LaPrade,

Histological studies of the FCL show that it is composed of elongated fibroblasts, which parallel their matrix (Espregueira-Mendes and da Silva, 2006). The matrix, which dominates in volume compared to the cells, is made up of collagen fibers that are grouped into bundles with what Espregueira-Mendes and da Silva (2006) described as an “undulating waviness that allows the ligament to elongate or shorten slightly in an accordion-like fashion to adapt to external stresses.” Studying the attachment site of the FCL shows the interdigitation of the collagen fibers and rigid bone tissues, mediated by the zone of fibrocartilage and mineralized fibrocartilages (EspregueiraMendes and da Silva, 2006). Mendes and da Silva

The Fibular Collateral Ligament of the Knee (2006) believed that the insertion of the ligament onto the bone is direct, after observing that collagen fibrils pass directly from the ligament into the bone cortex. Overall, the FCL is relatively avascular (De Avila et al., 1989; Espregueira-Mendes and da Silva, 2006). However, both Espregueira-Mendes and da Silva (2006) and De Avila et al. (1989) identified blood vessel and nerve penetration between fibrils. After studying the mechanoreceptor innervation of the FCL, De Avila et al. (1989) found the largest fascicles to be 75–100 lm in diameter, consisting of both myelinated and unmyelinated axons. They (De Avila et al., 1989) also identified a small number of “non-Paciniform” nerve endings.

Biomechanics and Kinematics The FCL plays a crucial role in the static stability of the posterolateral corner of the knee. Many studies have shown that the FCL is the primary varus (adduction) stabilizer of the knee (Grood et al., 1981; Nielsen et al., 1984; Gollehon et al., 1987; Grood et al., 1988; Markolf et al., 1993; LaPrade et al., 1999; Meister et al., 2000; Surgita and Amis, 2001; LaPrade et al., 2004b ; Ciccone et al., 2006; EspregueiraMendes and da Silva, 2006; LaPrade, 2006; Coobs et al., 2007). Sequential cutting studies have shown that isolated sectioning of the FCL results in a significant increase in varus rotation at every angle of knee flexion. Particularly, isolated sectioning of the FCL brought the greatest amount of varus instability at 30 of knee flexion (Nielsen et al., 1984; Gollehon et al., 1987; Grood et al., 1988; LaPrade et al., 1999; Surgita and Amis, 2001; Coobs et al., 2007). From 30 of knee flexion to the full flexion, varus rotation

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decreases following sectioning of the FCL (Gollehon et al., 1987; Grood et al., 1988; Coobs et al., 2007). In addition, Grood et al. (1988) and Gollehon et al. (1987) have observed an increase in varus angulation, by subsequent sectioning the posterolateral part of the knee capsule, popliteus tendon and posterior cruciate ligament. Previous studies have indicated that the FCL plays a role in stabilizing external rotation of the tibia (Nielsen et al., 1984; Gollehon et al., 1987; Grood et al., 1988; Markolf et al., 1993; Veltri and Warren, 1994a; Veltri et al., 1995; LaPrade et al., 2004b; Coobs et al., 2007). Sectioning the FCL results in increases in external tibial rotation, although the differences in the amount of increased external rotation are relatively small, according to Coobs et al. (2007). Coobs et al. (2007) also noted that the FCL plays its greatest stabilizing role at 30 of knee flexion and higher, correlating with past studies that show significant increase in external rotation at greater angles of knee flexion (Nielsen et al., 1984; Gollehon et al., 1987; Grood et al., 1988). However, different authors give different results and conclusion. Surgita et al. (2001) pointed out that, although FCL does resist external rotation at 0 of knee flexion, the stabilizing role of the FCL is greatly compromised when the knee is flexed. The FCL becomes slack or relaxed with knee flexion (Brantigan and Voshell, 1941; Edwards et al., 1970; Wang and Walker, 1973; Grood et al., 1981; Meister et al., 2000; Surgita and Amis, 2001) (Fig. 6). At 90 of knee flexion, nearly 40 of external tibial rotation is required before it becomes taut; therefore, it is poorly oriented to resist external rotation (Surgita and Amis, 2001). On the other hand, Surgita et al. (2001) noted that the popliteofibular ligament complex, a ligamentous structure that descends from the

Fig. 6. The FCL (first image) relaxed in flexion (middle image) allowing rotation of the leg in either direction (first and last image) thereby checking movement (Brantigan and Voshell 1941). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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musculotendinous junction of the popliteus to the posterosuperior prominence of the fibular head, retains a reasonable orientation and does not slacken significantly throughout knee flexion. Therefore, the popliteofibular ligament complex plays an important role in stabilizing external rotation at all angles of knee flexion. This ligament connects the popliteus muscle at its musculotendinous junction to the posterior and medial portion of the fibular head. Additionally, LaPrade et al. (2004b) also pointed out that the FCL and popliteus muscle complex have complementary roles as stabilizers during external rotation of the knee. By applying forces at different angles of knee flexion, LaPrade et al. (2004b) found that the greatest mean load response for the FCL was at 30 . The mean load response of the FCL decreased with increasing knee flexion, whereas the mean load response of the popliteus complex increased with increasing knee flexion. Therefore, LaPrade et al. (2004b) concluded that the FCL has a more important role in primary restraint to external rotation near extension, whereas the popliteus complex plays a more important primary role with increasing knee flexion (LaPrade et al., 2004b). The FCL plays a small role in stabilizing internal tibial rotation, although there is a high variability among which flexion angles are important. Coobs et al. (2007) suggested that the FCL plays an important primary role in stabilizing internal rotation throughout the entire range of flexion angles based on their data that showed increasing internal rotation at all flexion angles. Both Markolf et al. (1993) and Nielsen et al. (1984) noted that FCL plays a role in stabilizing internal rotation only at increased flexion angles. LaPrade et al. (2004b) have found a large standard deviation in their data, concerning the FCL load response to internal rotation, and concluded that there is a large variation in the FCL’s role in stabilizing internal rotation. As noted above, many researchers have reported that flexion of the knee causes relaxation or slackening of the FCL (Brantigan and Voshell, 1941; Edwards et al., 1970; Wang and Walker, 1973; Grood et al., 1981; Meister et al., 2000; Surgita and Amis, 2001). The amount of slackening depends on the type of tibial rotation. Neutral and manual external rotation of the tibia show similar results—at full flexion, the separation of the femoral and fibular attachment points decreases to a mean of 88% of the length measured in full extension (Meister et al., 2000). During manual internal rotation of the fibula, on the other hand, the

FCL maintains its original length throughout flexion, maintaining a mean of 97.7% of that measured in full extension (Meister et al., 2000).

MECHANISM OF INJURY AND HISTORY The typical and usual mechanisms for posterolateral knee injury are due to athletic injuries, falls, and motor vehicle accidents (Hughston et al., 1976; LaPrade and Terry, 1997). These accidents show a direct blow to the anteromedial knee, varus force, or hyperextension of the knee (Hughston et al., 1976; LaPrade and Terry, 1997; Ross et al., 1997). After reviewing 71 patients who had surgery to treat posterolateral knee injuries, LaPrade and Terry (1997) saw the most common mechanisms of injury to be twisting (30%) and noncontact and contact hyperextension (21 and 15% respectively). Isolated FCL injuries then can lead to instability of the posterolateral knee, further leading to functional limitations, a varus thrust gait pattern, and increased compressive forces at the medial tibiofemoral compartment (LaPrade and Terry, 1997). LaPrade and Terry (1997) also showed that the majority of FCL injuries presented with other ligamentous injuries around the knee—only 23% were found to have isolated grade III posterolateral corner knee injuries (Table 1). The typical injuries to posterolateral knee structures occur in combination with injuries to the cruciate ligaments (Veltri et al., 1995; LaPrade and Terry, 1997; Ross et al., 1997; Meister et al., 2000; Buzzi et al., 2004). Compared to other ligamentous injuries, injuries to the FCL are relatively uncommon (Ross et al., 1997; Latimer et al., 1998; Meister et al., 2000; Ciccone et al., 2006). Combining different physical examination findings, classification of lateral and posterolateral knee injuries can be determined. Physical examinations that are used consist of a posterolateral drawer test, Dial’s external rotation tests at 30 and 90 , and varus stress testing.

IMAGING Compared to the conventional anteroposterior radiographs of the knee, radiographic tests that include anteroposterior weight-bearing views in both 45 of flexion and extension produce more accurate and sensitive radiographs of the posterolateral instability

TABLE 1. Classsification of Posterolateral Knee Injuries Grade

Sprain type

Grade I

Minor

Grade II

Moderate

Grade III

Severe

Reference: LaPrade (2006)

Clinical findings Minimal increase in varus translation, external rotation at 30 and 90 , and posterolateral drawer at 90 . An increase of varus less than 1 cm (compared with the contralateral side) with a palpable end point; an increase of external rotation at 30 less than 10 ; an increase in posterolateral drawer no more than one grade compared with the contralateral side. Greater than 1 cm of varus opening at 30 ; a 10 increase of external rotation at 30 compared with the contralateral side; a one to two grade increase in the posterolateral drawer test at 90 compared with the contralateral side.

The Fibular Collateral Ligament of the Knee (Rosenberg et al., 1988). Additionally, the varus thrust standing anteroposterior, or varus stress radiograph, is very useful in situations when finding the neutral axis point during a clinical situation is hindered by ligament injuries, significant varus movements, or medial compartment pseudolaxity (LaPrade, 2006). In these situations, LaPrade noted (2006), the varus thrust standing anteroposterior radiograph can precisely portray the extent of the opening to the lateral compartment. This radiograph is done with the patient standing while applying the varus stress to the knee. According to LaPrade et al. (2000), MRI in the axial, coronal, and coronal-oblique views produce the best images of the FCL. In addition, MRI of the axial and coronal planes should be more focused, when injuries to the FCL are suspected (LaPrade et al., 2000). LaPrade et al. (2000) have suggested a specific MRI protocol, when evaluating a posterolateral knee injury. In addition to the thin-slice (2 mm) proton density coronal oblique images of the FCL, evaluating images of the complete fibular head and styloid process are performed. The reason is that the most common injury patterns observed are soft tissue avulsion at the insertion on the femur and a bone avulsion associated with an arcuate fracture at the fibular head (LaPrade et al., 2000). The arcuate fracture is an avulsion of the fibular head and styloid process at the fibular attachment that is associated with injuries of the FCL and tendon of the biceps femoris muscle (Capps and Hayes, 1994; LaPrade and Terry, 1997; Juhng et al., 2002; Huang et al., 2003). An arcuate fracture can be visualized on standard radiographs, but MRI in the coronal and sagittal views is recommended, due to the extent of associated ligaments attached to that area (LaPrade et al., 2000).

SURGERY Because the FCL is one of the main structures that provide static stability to the lateral and posterolateral aspect of the knee, it is very important that any surgical procedure to repair the injured FCL aims to restore its natural anatomy and biomechanics (Fanelli et al., 2011). In addition, since FCL and posterolateral injuries rarely occur in isolation and occur commonly with anterior cruciate or posterior cruciate ligament injury (Veltri et al., 1995; LaPrade and Terry, 1997; Ross et al., 1997; Meister et al., 2000), proper diagnosis and treatment are critical for the success of both the FCL and cruciate ligament repair. There are two posterolateral corner reconstructive techniques: non-anatomic and anatomic (Schorfhaar et al., 2010). The non-anatomic techniques include biceps femoris tenodesis, Larson’s technique, and Stannard’s modified two-tailed technique; anatomic techniques include using autogenous and allograft hamstring tendons (LaPrade et al., 2003; Buzzi et al., 2004; LaPrade, 2006; Coobs et al., 2007; Schorfhaar et al., 2010). Both techniques attempt to reconstruct either an acute or a chronic posterolateralinjured knee and establish the reconstruction by creating restraints from either the fibular head or the posterolateral corner of the tibia to the lateral femoral epicondylar region (Schorfhaar et al., 2010).

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Anatomic reconstruction of the FCL is described by LaPrade (2006), using autogenous and allograft hamstring tendons. The tendon of the long head of the biceps femoris muscle is harvested and used as the FCL graft (LaPrade, 2006). After identifying the femoral attachment site of the FCL, 20- to 25-mm tunnel is drilled into the attachment site and tapped, using a 7-mm reamer. The graft is then passed into the femur and fixed using a bioabsorbable screw. A second tunnel is drilled through the lateral aspect of the fibular head and the posterior aspect of the fibular styloid process, using a 6- or 7-mm cannulated reamer. The graft is then passed under the superficial iliotibial band and the lateral aponeurosis to the long head of the biceps femoris and passed through the fibular tunnel and pulled back upon itself to the lateral epicondyle. The knee is positioned in 30 of knee flexion and neutral rotation while a valgus force is applied to reduce any varus opening. The graft is then secured using bioabsorbable screws and is tested for varus instability at 30 of knee flexion. If the varus instability at 30 of knee flexion is eliminated, the graft is tied back on itself at the lateral fibular location, using 0Vicryl or non-absorbable sutures. An anatomic FCL reconstruction using an autogenous semitendinosus graft can significantly restore normal stability to knees with an injury. One study shows that static varus and external rotation stability in cadaveric knees with simulated grade III injury of the posterolateral structures of the knee are restored to near normal range, after a reconstruction surgery (LaPrade et al., 2004a). Another study, after comparing the anatomic reconstructed FCL graft to the sectioned FCL, shows that the reconstructed FCL is significantly improved in varus, external, and internal rotations compared to the sectioned FCL (Coobs et al., 2007). Thus, a normal knee range of motion and stability can be achieved through reconstructive grafting of the FCL. However, surgery of the FCL has potential complications. The interference fixation of a FCL graft to the fibular head is dramatically weaker and less stiff, compared to an intact FCL (Ciccone et al., 2006). The weak property of the interference fixation can then lead to a graft slipping out of the fixation site, further requiring surgeries and rehabilitation (Ciccone et al., 2006). In addition, risk for infection and injuries to the common fibular nerve are possible (LaPrade, 2006). Finally, disruption of the normal dynamics of knee motion can result from recurrent laxity and instability of the knee (Meister et al., 2000; LaPrade, 2006). A comprehensive understanding of the FCL will aid the physician who treats, interprets imaging, and operates the posterolateral knee. It is our hopes that this article will provide clinicians with an overview of this anatomy.

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