Posterolateral corner injuries are most often associated with combined ligament injuries (especially the PCL), but have been reported in isolated cases. The essential structures in the posterolateral corner include the biceps tendon, iliotibial band, lateral collateral ligament, popliteus musculotendinous unit, popliteofibular ligament, and posterolateral capsule.
MRI Features
The posterolateral corner of the knee is anatomically complex with several distinct and separate anatomic structures that are responsible for providing stability. These structures include the posterior capsule, arcuate ligament, popliteofibular ligament, popliteomeniscal fascicles, popliteus tendon, fibular collateral ligament, biceps femoris tendon, and the conjoined tendon (Fig. 3, a, b, and c)23. Injury to one or more of these structures can result in posterolateral knee pain, buckling into hyperextension during weight-bearing, and instability of the knee on physical examination.
The posterolateral corner structures are best evaluated with use of T2-weighted sequences, and all three imaging planes (axial, sagittal, and coronal) are required to provide an accurate assessment of each of these individual structures (Fig. 3, d, e, and f). Each anatomic structure should be described individually when injury is present. A 3-point grading system24 is used to describe injury to each individual structure: Grade I indicates strain; Grade II, a partial-thickness tear; and Grade III, a complete disruption.
Arthroscopic Findings
Arthroscopic findings of a posterolateral corner injury include excessive opening of the lateral compartment. This has been described by LaPrade25 as the positive "drive-through" sign (Fig. 3, g and h). Posterolateral corner injuries require open treatment. This is best done with primary repair supplemented by free graft augmentation. The preference of the senior author (M.D. Miller) is a semitendinosus allograft. Anatomic placement of this graft has shown to be important for good long-term results25.
The presence of a meniscal tear is the most common indication for knee arthroscopy, and MRI has evolved into an accurate means of detecting meniscal abnormalities prior to arthroscopy. Understanding tear patterns and MRI features of menisci can help in surgical planning, especially with regard to possible meniscal repair versus partial meniscectomy.
Bucket-Handle Tear of the Meniscus (Fig. 6)
MRI Features
The accuracy of MRI in detecting a meniscal tear ranges between 90% and 95%, and the experience level of the reader is one of the most important factors in maximizing diagnostic accuracy of the MRI. It is critical to assess the meniscus in both the sagittal and coronal imaging planes. Short TE (echo time) sequence imaging (proton density or T1-weighted) is most sensitive for detecting meniscal tears, while abnormalities seen on T2-weighted images are very specific36.
The menisci are composed of fibrocartilage and appear dark on all MRI pulse sequences. On the sagittal images, the peripheral portion of the menisci have been described as demonstrating a "bow-tie" configuration, while more centrally, the meniscus demonstrates a triangular appearance tapering toward the free edge. The anterior and posterior horns of the lateral meniscus are nearly equivalent in size, while the posterior horn of the medial meniscus is nearly twice the size of the anterior horn37.
Identifying a displaced meniscal fragment requires a thorough knowledge of the normal MRI appearance of the menisci in both the sagittal and coronal imaging planes (Fig. 6, a through d). Direct signs of a meniscal tear include (1) unequivocal surfacing signal, (2) missing meniscal tissue (in the absence of prior surgery), and (3) displaced meniscal fragment. Several MRI signs have been described with regard to the identification of a bucket-handle tear of the menisci. The double PCL sign indicates a buckle-handle tear of the medial meniscus displaced into the intercondylar notch (Fig. 6, c). The double anterior horn sign indicates a bucket-handle tear of the lateral meniscus that has flipped into the anterior aspect of the lateral compartment of the knee (Fig. 6, d). The absent "bow-tie" sign indicates that a meniscal fragment has been displaced and no longer sits in it normal anatomic position38-40.
Arthroscopic Findings
The classic arthroscopic appearance of a bucket-handle meniscal tear includes displacement of the torn meniscus into the notch (Fig. 6, e). Bucket-handle tears most commonly occur in the medial meniscus. The meniscus should be reduced back into the medial compartment when found in the notch to allow visualization of the residual peripheral meniscus to decide if a repair is possible. With the meniscus reduced, it is easier to remove if the tear pattern is not amenable to repair and partial meniscectomy is chosen for treatment. If the tear is amenable to repair, the gold standard remains outside-in suture repair that necessitates a separate posteromedial incision for suture removal and protection of the saphenous nerve and vein (Fig. 6, f and g). Newer all-inside repair technology and techniques continue to evolve.
Meniscal Tear with Displaced Fragment
MRI Findings
In addition to the bucket-handle tears, several unstable meniscal flap fragments can occur and involve either the medial or lateral meniscus. It is important when evaluating the meniscus on MRI to look for displaced fragments within the various gutters and recesses of the knee to aid in the arthroscopic evaluation of the meniscus. Unstable meniscal flap fragments can be displaced into several locations37,41. In the medial compartment, meniscal fragments are most commonly seen along the medial joint line with the fragments displaced into either the superior or inferior recesses. The pattern is more variable in the lateral compartment of the knee, but most fragments are noted in the posterior aspect of the lateral compartment.
Arthroscopic Findings
Both the lateral and medial menisci can be visualized on arthroscopy through standard inferomedial and inferolateral portals. Often it is helpful and sometimes necessary to establish posteromedial and posterolateral portals to ensure that the complete meniscectomy is accomplished and to ensure that there are no residual fragments. Meniscal tears are often classified by their tear pattern, which can be best assessed at the time of arthroscopy. Patterns include bucket-handle longitudinal, radial, oblique, parrot-beak, and complex tear patterns. The location of the tear, particularly a longitudinal tear, should be assessed visually at arthroscopy for its proximity to the periphery as this influences healing rates and can determine whether the tear is repairable. The meniscal roots should also be assessed for tears as these tear patterns have been shown to dramatically change the biomechanics of the meniscus42. Often the posteromedial and posterolateral portals are vital to fully visualizing the meniscal root.
Osteochondritis dissecans refers to a lesion of the articular cartilage and underlying bone typically occurring in adolescents or young adults. Lesions are described as juvenile osteochondritis dissecans if the growth plates are open or as adult osteochondritis dissecans if the growth plates are closed. Juvenile osteochondritis dissecans usually has a better prognosis. In the knee, osteochondritis dissecans most often occurs in the lateral aspect of the medial femoral condyle.
MRI Features
The primary role of imaging is to detect the presence of an osteochondritis dissecans lesion and to determine the stability of the fragment, which guides the need for surgical or nonsurgical treatment. The osteochondritis dissecans should be evaluated in both the sagittal and coronal imaging planes, and the most common sequences used for evaluating the fragment include T2-weighted images with fat saturation or proton density images with fat saturation. The articular cartilage, cortex, and underlying marrow signal all need to be evaluated. The MRI grading system43 uses a 4-point scale, with Grade I indicating subchondral bone marrow edema with intact overlying cortex and articular cartilage; Grade II, a partially detached osteochondral lesion; Grade III, a completely detached fragment in situ (Fig. 7, a, b, and c); and Grade IV, a completely detached and displaced osteochondral fragment. MRI signs indicating an unstable fragment include (1) linear bands of high T2 signal at the interface of the fragment and the underlying bone measuring >5 mm in length, (2) the presence of a subchondral cyst measuring ≥5 mm in diameter underlying the osteochondral fragment, and (3) a focal chondral defect measuring >5 mm in diameter. The presence of bright T2 signal undermining the osteochondral fragment is the most common finding indicating an unstable fragment, and sometimes it is difficult to differentiate granulation tissue from fluid undermining the fragment with use of conventional MRI. The presence of contrast completely surrounding the fragment indicates an unstable fragment when magnetic resonance arthrography is used43,44.
Arthroscopic Findings
MRI assessment of stability is important for preoperative planning and, combined with arthroscopic findings and assessment of stability, will guide treatment. Understanding the location of the lesion is important for antegrade or retrograde drilling and for being prepared at the time of surgery for potential fixation or salvage options such as osteochondral grafting or autologous chondrocyte implantation. The classic location for osteochondritis dissecans is on the lateral portion of the medial femoral condyle and often bordering the PCL femoral insertion. It is important to know and recognize the location of the lesion at the time of arthroscopy because it can be the only portion of fluid communication underneath an unstable lesion where the remaining cartilage surface appears intact.
In adults with osteochondritis dissecans or other articular cartilage lesions, it is important to know the location of the lesion from the preoperative MRI and whether a loose body might be present. Knee arthroscopy is also crucial in deciding on treatment options for these lesions as their size and depth can be assessed along with whether the other articular cartilage surfaces are involved and whether there is diffuse osteoarthritis.
The use of MRI and arthroscopy of the knee has evolved substantially over the last several decades, and the advancement in surgical treatment for pathological conditions of the knee has been improved because of both technologies. The astute orthopaedic surgeon must be able to associate the findings on MRI in the decision-making before and during arthroscopy. An accurate understanding of the surgical anatomy and pathology found on both clinical examination and preoperative imaging will help the surgeon to improve the surgical technique at the time of arthroscopy and ultimately improve patient outcomes.