MRI of popliteo-meniscal fasciculi of the knee: a pictorial review.

The popliteomeniscal fascicules (PMFs) provide the attachment of the lateral meniscus to the popliteus musculotendinous region, forming the floor and the roof the popliteal hiatus. In the second half of 1900's, some anatomic studies claim the important function of the PMF as stabilizers of the lateral meniscus; these anatomical structures work in conjunction with the popliteus musculotendinous unit to prevent excessive lateral meniscal movement and possible meniscus subluxation. A correct diagnosis of the PMFs pathology is crucial to establish the suitable surgical treatment for each patient. MRI is a well-established imaging technique in the musculoskeletal system and the frequency of recognition of normal PMF in the normal knees is high in almost all MRI studies. At day, the gold standard for diagnosis is the arthroscopic evaluation that allows the direct visualization of the popliteo-meniscal ligaments at popliteal hiatus and evaluation of lateral meniscal movements. For this reason if unstable condition of meniscus was suspected, arthroscopic observation with probing into the popliteo-meniscal fascicle area is essential for the identification of the fascicle tears. Despite many treatments have being proposed in literature since now there is high recurrence of knee locking after repair and it is fundamental to develop new surgical techniques in order to achieve better outcome.

Introduction

The popliteomeniscal fascicules (PMFs) provide the attachment of the lateral meniscus to the popliteus musculotendinous region, forming the floor and the roof of the popliteal hiatus.

In the second half of 1900’s, some anatomic studies claim the important function of the PMF as stabilizers of the lateral meniscus; these anatomical structures work in conjunction with the popliteus musculotendinous unit to prevent excessive lateral meniscal movement and possible meniscus subluxation (1-5). More recently it is underlined the role of PMFs for posterolateral stability. Tear of MPFs is the more common lesion involved in the 80% of patients with grade III of posterolateral injuries associated with anterior cruciate ligament (ACL) insufficiency (6-10).

Congenital absence or isolated injuries of the PMFs can result in micro or gross instability of the meniscus producing snapping or locking of the knee (11-15); tear or insufficiency of PMFs associated with ACL tear increases the knee laxity in as many as 25% of patients. Both types of lesions often are difficult to recognize clinically (16-19). Therefore, a correct diagnosis of the PMFs pathology is crucial to establish the suitable surgical treatment for each patient.

Normal Anatomy

The popliteus is a thin and flat muscle that forms part of the floor of the popliteal space. Its tendon inserts on the lateral epicondyle, just anteroinferior to the proximal attachment of the lateral collateral ligament and passes posteriorly to the posterior horn of the lateral meniscus. The popliteus tendon is considered as intra-capsular but extra-articular and extra-synovial.

The popliteo-meniscal fascicles are posterolateral meniscocapsular structures that from body and posterior horn of the lateral meniscus blend inferiorly into the popliteus musculotendinous unit and allow the tendon to pass from an intra-articular to an extra-articular compartment (20-25).

However in literature there is still debate about the number of popliteomeniscal bands (26), most studies described at least two different fascicles: an antero-inferior and postero-superior (2, 27-30). Cohn and Mains described that the superior border of the popliteal hiatus defined the superior popliteo-meniscal ligament while the inferior border defined the inferior PMF (1).

The anteroinferior PMF arises from the body of the lateral meniscus, courses in a postero-inferior direction to form the floor of the popliteal hiatus, and then blends with the musculotendinous unit of the popliteus muscle; the lateral aspect of the anteroinferior PMF fuses with the popliteofibular ligament forming a conjoined attachment to the fibula (31-35) (Fig. 1a-b).

Figure 1 a-b.

MR sagittal proton density weighted with fat saturation (PDw fat sat) sequence (a) and arthroscopy (b) of normal popliteal hiatus with popliteal tendon (arrowheads) and antero-inferior PMF (arrows)

The posterosuperior PMF fascicle arises from the posterosuperior edge of the posterior horn of the lateral meniscus just medial to the popliteus tendon and it blends to the posterior capsule and the popliteus tendon; it forms the roof of the popliteal hiatus (Fig. 2) (31).

Figure 2.

MR sagittal PDw fat sat sequence of normal postero-superior (arrowhead) and postero-inferior PMFs (arrow)

Sussmann et al. reported that the anteroinferior fascicle shows greater control over lateral meniscal mobility compared with the posterosuperior fascicle and they suggested that, during embryologic development, the fascicle provided a vascular supply to the lateral meniscus near to the popliteal hiatus where the meniscus was free of capsular attachment (36).

Several authors reported the presence of a third fascicle (2, 37-41), the posteroinferior PMF, which extends in the medial direction from the medial aponeurotic extension of the popliteus and attaches to the lower margin of the posterior horn of the lateral meniscus immediately under the ligament of Wrisberg (Fig. 2) (31, 42, 43).

MRI is a well-established imaging technique in the musculoskeletal system and the frequency of recognition of normal PMF in the normal knees is high in almost all MRI studies (27, 31, 44-46).

In 1999 Johnson and De Smet reviewed sagittal MR images of 66 consecutive patients who had no evidence of injury to the lateral compartment and founded that PMFs were seen in 64 of the 66 patients with only 3% of knees with absent of PMFs (15, 45, 47-50).

Subsequently Sakai et al founded the anteroinferior PMF in 94.1% and the posterosuperior PMF in 88.2% of subjects; they recommended suitable parameters for better depicting the popliteomeniscal ligaments; in particular they suggested that the optimal parameters for the depiction of the PMF on MRI are proton-density weighting oblique-coronal images and 45° or 50° slice angle with the posterior tibial condyle for depicting both fasciculi at the same time (27, 51-55).

In 2008, Peduto et al. performed MR arthrographic examination of 10 cadaveric knees and identified the anteroinferior and posterosuperior PMFs in all 10 knees and posteroinferior PMF in 4 of the knees; they stated that the posterosuperior popliteomeniscal fascicle was uniform in thickness, while the anteroinferior popliteomeniscal fascicle was variable in thickness (31, 56-58).

The so called “Wrisberg variant” is the third of the three lateral meniscal variant described by Watanabe and Takeda in their classification and it is characterized by the absence of the PMFs as well as meniscotibial coronary ligament and the meniscofemoral ligament of Wrisberg or Humphrey represent as the only posterior stabilizing structure (59)

Pathologic conditions

PMFs Congenital Absence (Wrisberg variant of lateral meniscus)

The “Wrisberg variant” of lateral meniscus is characterized by congenital absence of PMF fascicles and meniscotibial coronary ligament (12, 59, 60). This anatomic condition predisposes the posterior horn to hypermobility with locked or recurrent subluxation of lateral meniscus leading to knee locking or snapping in a young patient (61-64) (Fig. 3).

Figure 3.

Two MR sagittal PDw fat sat consecutive sequences of lateral meniscal instability. The posterior horn of lateral meniscus is dislocated anteriorly (arrowhead) and its anatomic region is replaced by fluid signal (asterisks). Note the PMFs torn (arrow). MR sagittal PDw fat sat sequence of normal postero-superior (arrowhead) and postero-inferior PMFs (arrow)

It is important to differentiate this snapping cause from the other’s one, like subluxations of the biceps femoris (65), discoid lateral meniscus (66), popliteus (67) and semitendinosus snapping (68).

The MRI of nondisplaced discoid Wrisberg variant of lateral meniscus shows the absence of PMF and coronary ligament, with a floating meniscus appearance and fluid signal line behind the posterior horn of meniscus (12, 69, 70) (Figs.4 a-c).

Figure 4 a-c.

MR sagittal (a) and coronal (b) PDw fat sat sequences and arthroscopy (b) of Wrisberg variant of discoid lateral meniscus. The lateral meniscus shows discoid morphology (arrowheads), congenital absence of PMFs (with arrow) and it was stabilized only by the Wrisberg ligament (black arrows)

Even if Wrisberg variant of lateral meniscus be part of classification of discoid meniscus, Wrisberg meniscus may or may not have discoid morphology. In fact the absence of PMF is often associate with normal C-shape morphology of meniscus (12, 59, 71) (Fig. 5).

Figure 5.

Two MR sagittal PDw fat sat consecutive sequences of Wrisberg variant of lateral meniscus with congenital absence of PMFs (with arrow) but normal C-morphology

Some authors hypothesize that a trauma of the popliteomeniscal fascicles could lead to an unstable meniscus mimicking a Wrisberg variant, a so-called post-traumatic Wrisberg variant (61). Arthroscopic criteria have not been described that would distinguish a true Wrisberg variant meniscus from a traumatic injury to the posterior capsule (12) but the young age, the clinical presentation and the non-traumatic history of the patient can help to understand the origin of abnormality of PMFs.

If the patients could voluntarily lock and unlock his knee, the sagittal and coronal MRI sequences acquired before and after the pathologic movements showed the lateral meniscus in its anatomic position in the unlocked-knee position and the anteriorly flipped posterior horn lateral meniscus in the locked-knee position (62, 72) (Fig. 6).

Figure 6 a-d.

MR sagittal T2w (a) and coronal PDw fat sat (b) sequences of Wrisberg variant of lateral meniscus with knee in routine position show the normal position of the meniscus. After a movement that has locked the knee, the MR sagittal PDw fat sat (c) and coronal PDw fat sat (d) sequences show the antero-medial dislocation of posterior horn of lateral meniscus

PMFs tears

The PMFs have a crucial role as a stabilizers of the knee, avoiding the instability of the lateral meniscus during flexion and extension of the knee (11, 16, 37, 73).

Tears of PMFs could be an isolated consequence of trauma, but they have been observed at arthroscopic surgery in high percentage of knee injuries with anterior cruciate ligament tear and associated injuries of the postero-lateral complex (74, 75); the mechanisms of injury may involve a single traumatic event or subacute onset after repeated microtraumas (2).

Although a popliteo-meniscal fascicle tear often cause vague mechanical symptoms, their tears may be associated with a postero-lateral instability and/or knee snapping sensation due to subluxation of the lateral meniscus (63, 69, 76).

Even if few articles have been published articles on the semiotic signs and diagnostic accuracy of MRI in detection of acute and chronic PMFs injury in non-subluxated meniscus, the absence of the continue linear structures referable to PMFs and a water-signal interposed between the posterior horn of lateral meniscus and joint capsule should be considered pathologic (75, 77) (Fig. 7).

Figure 7.

MR sagittal PDw fat sat sequences of postero-superior (arrow) and postero-inferior (arrowhead) PMFs tear

Using the 3D isotropic proton density sequence, Ahn et al found sensitivity and specificity of 58% and 92% for PMF injury (77).

However, the presence of hemarthrosis, which is a common finding in patients with acute tears of the anterior cruciate ligament with particularly related risk of popliteo-meniscal fascicle tears, would be used as contrast media for better depict the anatomical structures (27) (Fig. 8a-b).

Figure 8 a-b.

MR sagittal PDw fat sat sequence (a) and arthroscopy (b) of acute tear of PMFs. The joint effusion help to delineate the lesion of PMFs (white arrows) and lesion of popliteal musculotendinous junction (black arrow)

Suganuma et al, evaluated 238 knee MRI exams and founded abnormal iPMF in 40% and 26% respectively in healthy group; the percentages of abnormality in contralateral knee of group with recurrent subluxation of the lateral meniscus (RSLM) were 100% and 29% respectively while in knee joint with RSLM were 100% and 100%. They finally stated that a significantly high incidence of abnormal sPMFs was found not only in RSLM joint but also in contralateral knees (44).

When the popliteo-meniscal fascicles are disrupted, the normal peripheral hoop tension of the lateral meniscus is lost, and consequently the lateral meniscus could be displaced medially into the joint (78) (Fig. 9).

Figure 9.

MR sagittal PDw fat sat sequence of acute tear of PMFs (arrow) with little antero-medial displacement of posterior horn of lateral meniscus (arrowhead)

Arthroscopy and Treatments

At day, the gold standard for diagnosis is the arthroscopic evaluation that allows the direct visualization of the popliteo-meniscal ligaments at popliteal hiatus and evaluation of lateral meniscal movements (2). For this reason if unstable condition of meniscus was suspected, arthroscopic observation with probing into the popliteo-meniscal fascicle area is essential for the identification of the fascicle tears (79).

In case of PMFs tears, in symptomatic patients, surgery represents the treatment of choice (80). Some different treatments have been reported in literature: meniscectomy, coronary ligament and meniscocapsular repair, and thermal shrinkage of the posterolateral capsule (11, 44, 81). However, 11% to 33% of patients who undergo coronary ligament and meniscocapsular repair or RFE of the posterolateral capsule have high recurrence of knee locking, whereby new surgical techniques are being developed in recent years (76, 82).

Conclusions

Popliteomeniscal fasciculi could be absent or a tear could occur in association with acute anterior cruciate ligament injury and diagnosis is crucial to treat the consequently hypermobility or locking of the meniscus. Even if other studies are required to standardize and validate the semiotic signs, the MRI represents a valid method to help in the PMF abnormalities.

Despite many treatments have being proposed in literature since now there is high recurrence of knee locking after repair and it is fundamental to develop new surgical techniques in order to achieve better outcome.