Comparing MRI and arthroscopic appearances of common knee pathologies: A pictorial review

Teaching point: Descriptions of partial ACL tears vary from bleeding at the femoral origin to complete rupture of the anteromedial (AM) or posterolateral (PL) bundle.^[6] This variability in the degree of ligament disruption makes the definitive diagnosis of partial ACL tear very difficult, even with MRI.^[6-9] The variable MRI appearance of partial ACL rupture results in low sensitivity and specificity in diagnosis, with reported 55% of sensitivity and 75% of specificity when using primary MRI signs of partial ACL tear.^[10] Therefore, arthroscopy is used in combination with history, physical examination, and MRI to unequivocally diagnose, in the setting of a partial ACL tear, a functionally ACL deficient knee, and develop an appropriate treatment plan [Figure 1].^[4,6,9]

Teaching point: Clinically, a high-grade partial rupture of the autograft can be difficult to diagnose without significant instability.^[7] With well-positioned grafts, the same principles apply when evaluating ACL integrity for non-operative versus operative decision-making [Figures 2 and 3].^[9,10]

Teaching point: The absence of ACL femoral insertion at the intercondylar notch (“empty wall/notch sign”) is an indication of complete rupture [Figure 4].^[7]

Teaching point: The normal orientation of the ACL is typically parallel to the intercondylar roof (Blumensaat’s line) or slightly steeper. A lack of parallelism of ACL fibers and Blumensaat’s line indicates ACL rupture [Figure 5].^[7]

Teaching point: The acutely disrupted ACL commonly presents with associated characteristic injuries.^[7] Hemorrhagic effusion, “coup-contrecoup” bone bruising, and “kissing” bone contusions from the pivot shift or hyperextension mechanism are some signs detected radiologically.^[7,11,12] It is important for radiologists to make the distinction between acute and chronic tears. ACL reconstruction in the acute setting (1–3 weeks) can lead to arthrofibrosis and permanent loss of knee motion in some patients. The radiologist can further improve MR interpretation of ACL injuries by including findings such as anterior tibial translation, uncovering of the posterior horn of the lateral meniscus, and buckling of the posterior cruciate ligament (PCL). These particular findings are important in diagnosing an unstable ACL tear [Figure 6].^[5,13-15]

Teaching point: ACL ganglion cysts are rare and currently of unknown etiology.^[16-19] It is believed that they may develop from a congenital abnormality, previous injury, mucoid deterioration, or proliferation of pluripotential mesenchymal stem cells.^[16-18] The relevance of ACL ganglion cysts is a factor when they become symptomatic, particularly with chronic knee pain, mechanical symptoms, limited range of motion, or instability.^[18] When interpreting an MRI of this lesion, a broad differential diagnosis must be formed because a para-ligamentous or intra-ligamentous ganglion cyst can be mistaken for a tear [Figure 7].^[7,15-18]

Teaching point: The cyclops lesion is a type of localized arthrofibrosis in the form of a fibrous nodule found at the anterior intercondylar notch.^[20-23] It typically forms at the tibial insertion of the reconstructed ACL graft resulting in a mechanical block to extension.^[7,21] The etiology is unknown, but several theories explaining its formation are described. Repetitive trauma of graft against bone from anterior graft placement resulting in impingement and graft hypertrophy with fibrous proliferation is a reported explanation.^[22,23] An undebrided ACL stump and poorly debrided drilling debris of the tibial tunnel could also cause fibrous nodular development.^[22,23] Arthroscopically assisted debridement is the accepted treatment of symptomatic patients [Figure 8].^[7,20,21,23]

Teaching point: It is well established that there are two distinct bundles of the ACL, anteriomedial (AM), and posterolateral (PL), with separate femoral and tibial attachment sites.^[23-27] The bundles are named by their relative insertional positioning at the ACL footprint of the tibia.^[24] Using the clock face over the frontal view of the knee, AM and PL bundles are reported to originate on the posterior part of the medial wall of the lateral femoral condyle at the 10:30/1:30 (left/right knee) and 9:30/2:30 (left/right knee) positions, respectively. Although they have different rupture patterns, with the AM bundle commonly failing at the femoral origin and the PL bundle rupturing at the mid-substance or tibial insertion,^[26] the normal separation between the bundles should not be mistaken for a tear on MRI [Figure 9].^[23,25]

Teaching point: Tibial intercondylar eminence fractures are most commonly seen in the pediatric population aged 8–14.^[5,7,26,28] Similar to adults, a non-contact pivot shift mechanism of injury causes avulsion fracture of the tibia intercondylar eminence and resultant compromised ACL function.^[28] The following modified Meyers and McKeever classification describes the varying degree of injury that can occur and guides treatment. Type I, non-displaced; Type II, displaced anteriorly with an intact posterior cortical hinge; Type III, completely displaced and void of all bony contact; and Type IV, comminuted.^[28] This injury can be identified on initial radiographic workup, but MRI is important to determine the extent of damage and comminution [Figure 10].

Teaching point: Radiological and arthroscopic studies have confirmed that parameniscal cysts arise from synovial fluid extrusion through underlying meniscal tears.^[29-31] A Horizontal cleavage tear is the most commonly described tear pattern associated with cyst formation.^[29-31] The goal of treatment is to treat the tear with either meniscal debridement or repair and cyst decompression.^[30,31] Of note, there is a reported high frequency (>95%) of medial parameniscal cysts with medial meniscus tears and lateral parameniscal cysts with tears of the body and posterior horn of the lateral meniscus.^[29] More recent literature found that 36% of underlying meniscal tears were absent, with cysts adjacent to the anterior horn or 1/3 of the lateral meniscus.^[29] If parameniscal cysts exist anterior to the lateral meniscus with no identifiable meniscal tear, other causes should be considered, such as Hoffa fat pad ganglia, intermeniscal ligament tears, or degeneration [Figures 11 and 12].^[29]

Teaching point: PVNS is a proliferative disease of synovial joints but most commonly affects the knee in young adults.^[7] It occurs as a single nodule (focal), a cluster of synovial nodules, or diffuse, boggy proliferative synovitis.^{[5,7,30,32,33]} Initially, diagnosis can be challenging because the most common clinical presentation in the knee is a painless or mildly painful effusion. The nodules can cause mechanical symptoms and mimic meniscal tears, loose bodies, or patellofemoral disorders.^[7] PVNS also frequently involves the posterior compartment.^[7] When evaluating a knee effusion with an insidious onset, the astute radiologist should be suspicious of PVNS because treatment is surgical.^[7,32,33] Current recommendations favor arthroscopic synovectomy.^[7,32,33] Lesions in the posterior compartment must be identified on MRI for appropriate preoperative planning, as nonstandard posterior arthroscopic portals are made to access the posterior knee joint and complete the synovectomy [Figure 13].

Teaching point: Synovial chondromatosis is a rare tumor-like disorder characterized by cartilaginous metaplasia of the intimal layer of the synovium, including synovial structures outside of the joint (i.e., tendon sheaths and bursa).^[7,34,35] The cartilaginous nodules can detach from the synovium resulting in loose bodies that may undergo secondary calcification and ossification.^[14] When viewed arthroscopically, the accumulated nodules resemble a “snowstorm” in the knee joint. Due to its high recurrence rate (>25%) after arthroscopic synovectomy, it is important for radiologists to identify among other benign synovial disorders that can mimic its MRI appearance, such as tumoral calcinosis and PVNS [Figures 14 and 15].^[7,34-36]

Teaching point: The altered meniscal anatomy can make diagnosing a retear in the postoperative meniscus challenging.^[37,38] MR arthrography can increase the sensitivity of meniscal retear detection^[7,37,38] but conventional MRI may still provide some utility. Even in a previously debrided or repaired meniscus, abrupt change in the contour of a postoperative meniscus should be met with reasonable suspicion for reinjury [Figure 16].^[37]

Teaching point: A meniscal flap tear is the result of a short-segment, horizontal meniscal tear with either superior or inferior displacement. MRI findings are subtle, but this tear pattern can be clinically significant, warranting arthroscopic resection. Careful examination of the triangular meniscal shape on sagittal images should be performed to detect abnormalities [Figure 17].

Teaching point: Flap tears of the meniscus can continue propagating without treatment and become large floating pieces in the joint, potentially causing mechanical symptoms.^[7] Again, abrupt changes in the thickness of the meniscus suggest tears and detailed descriptions of the locations of torn fragments should be made to facilitate arthroscopically identification [Figure 18].^[38]

Teaching point: Acute radial tear of the mid-body of the lateral meniscus is often a result of the pivot shift mechanism and is associated with acute ACL rupture^.[7,39,40] This particular tear pattern has detrimental effects on the contact pressures experienced by the articular cartilage secondary to the disruption of the circumferential collagen fibers of the meniscus.^[41] Identifying meniscal tears with concomitant ACL ruptures is important because if repairable, there is a greater healing potential of the meniscal repair due to the release of growth factors when drilling bone tunnels for ACL reconstruction [Figure 19].^[7]

Teaching point: Being able to see the meniscal body on three or more consecutive MR sagittal images is suggestive of discoid meniscus.^[1]

Arthroscopically, a thick meniscal rim is present even after cauterization [Figures 20 and 21].

Teaching point: Degenerative tears arthroscopically can have a frayed appearance with correlating increased intrasubstance signal intensity on MRI.^[7]

Isolated anterior horn lateral meniscus tears are rare but do occur [Figure 22].

Teaching point: The “Double PCL sign” is a characteristic finding of bucket handle medial meniscus tears.^[7] This tear can cause significant mechanical symptoms [Figure 23].

Teaching point: Swelling and softening of articular cartilage are much more prone to further damage and degradation. Commenting on areas of chondromalacia allows the arthroscopist to understand how fragile the articular surface is before surgery [Figure 24].

Teaching point: Outerbridge grade II and III cartilage lesions can be difficult to differentiate, but size dimensions should be reported to allow the surgeon to prepare for a microfracture technique or simple debridement of the unstable portions of the lesion [Figure 25].^[7]

Teaching point: Malalignment, ligamentous laxity, and location of full-thickness defects of cartilage are deciding factors in the surgical treatment of cartilage defects in the knee. Correction of alignment and/or ligamentous instability would precede any cartilage restorative procedures or be performed concomitantly.^[7] Thus, it is important for radiologists to specify the location of high-grade lesions and other knee derangements for pre-operative planning [Figure 26].

Teaching point: Subchondral depression from a pivot shift injury is technically a fracture and should be treated as such. Findings may be subtle and can be mistaken for a bone bruise, but any depression or loss of the normal distal femoral contour should be appreciated and reported. Chondral impaction fractures should be treated with a period of non-weight bearing in addition to addressing potential concomitant injuries from the pivot shift mechanism, that is, ACL and meniscal tears [Figure 27].