A stepwise approach decoding edema-like marrow signal intensity around knee joint: A pictorial review

Subchondral bone ELMSI

Chondromalacia patellae (CMP)

CMP [Figure 2] is a common disorder involving the patellar articular cartilage, especially in young athletes and adolescents.^[3] Grade IV CMP is associated with characteristic subchondral ELMSI and overlying cartilage defects, which are best picked up on axial sections in either medial or lateral patellar facet. The coronal and sagittal sections are not ideal for identifying the changes due to the near parallel orientation of the articular cartilage and subchondral bone.

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Figure 2:

Chondromalacia patellae grade IV-A 32-year-old female patient presenting with anterior knee pain. (a) Axial proton density-weighted, fat-suppressed section at the level of patella shows focal subchondral edema-like marrow signal intensity (ELMSI) (red arrow) with overlying hyperintensity and loss of patellar articular cartilage (white arrow). (b) Mid sagittal section shows the same focal ELMSI with the tiny subchondral cyst (Red arrow).

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Osteochondritis dissecans

It is an uncommon idiopathic disorder of the subchondral bone plate and is commonly seen involving the distal femur and femoro-trochlear groove.^[4,5] On axial and coronal mid-sections, the isolated ELMSI in non-weight-bearing antero-lateral aspect of the medial femoral condyle in the adolescent age group is seen in osteochondritis dissecans [Figure 3]. On sagittal medial sections, similar findings are seen with a better depiction of the subchondral fracture.

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Figure 3:

Osteochondritis dissecans of femoro-trochlear groove – A 17-year-old male patient presenting with vague knee pain. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of patella shows edema-like marrow signal intensity (ELMSI) in the anterior aspect in the subchondral aspect of the medial trochlear groove (yellow arrow) with loss of the overlying trochlear cartilage. (b) Mid sagittal PDFS section shows the extent and depth of ELMSI (yellow arrow).

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Subchondral insufficiency fracture

It is commonly seen in elderly patients involving the weight-bearing surface of the medial femoral condyle and occurs due to subchondral plate overload and vascular insufficiency. It may be seen with or without osteonecrosis, and subchondral insufficiency fracture with osteonecrosis was previously called spontaneous osteonecrosis of the knee.^[6] Subchondral hyperintensity with disproportionate ELMSI in weight-bearing surface [Figure 4] is seen, best picked up on the coronal mid and sagittal medial sections.^[7-14] The extent and temporal characteristics of the ELMSI in these cases have been found to be associated with clinical severity and pain evolution.^[6]

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Figure 4:

Subchondral insufficiency fracture – A 45-year-old female patient presenting with medial joint pain. (a) Mid coronal proton density-weighted, fat-suppressed section shows a localized edema-like marrow signal intensity (yellow arrow) involving the weight-bearing surface of the medial femoral condyle. (b) Corresponding T1-weighted coronal image depicts the associated convex hypo-intense line (yellow arrow) without any separation.

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Chronic regional pain syndrome (CRPS)

CRPS is clinically diagnosed with allodynia and hyperparesthesia post-trivial trauma, and the severity of pain generally outweighs the degree of trauma.^[7] Subchondral ELMSI is best seen on sagittal sections widely distributed in all the sections, involving the patella, femur, and tibia without any changes of arthritis, absence of erosions, and minimal reactive synovitis. [Figure 5] shows the imaging diagnostic features in CRPS, helping in differentiating from arthritis.

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Figure 5:

Chronic regional pain syndrome – A 30-year-old male patient presenting with post-trivial trauma diffuse knee joint pain. (a) Medial, (b) mid, and (c) lateral sagittal proton density-weighted, fat-suppressed sections show diffuse subchondral edema-like marrow signal intensity involving the patella (yellow arrows in b and c), femur (red arrows in a and b), and tibia (white arrow in c).

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Osteonecrosis/bone infarction

It occurs due to cell death secondary to ischemia and can be a cause of knee pain. Most of the cases have an underlying systemic cause resulting in bilateral involvement.^[8] Localized serpiginous ELMSI is seen either in metaphysis or meta-diaphysis [Figure 6]. The presence of multiple similar lesions, areas of internal fat intensity, and serpiginous outlines helps in making the correct diagnosis.

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Figure 6:

Osteonecrosis/bone infarction – A 54-year-old female patient presenting with diffuse knee joint pain. (a) Lateral sagittal proton density-weighted, fat-suppressed section shows a serpiginous localized edema-like marrow signal intensity (yellow arrow). (b) Lateral proton density-weighted fast spin-echo section shows fat intensity in the central part (yellow arrow).

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Patellar fractures

Patellar fractures can cause a localized or diffuse pattern of ELMSI. On axial sections, ELMSI is seen on the anterior aspect of patella. On coronal and sagittal sections, the inferior patella is usually involved in patellar fractures [Figure 7] or patellar tendon avulsion fractures. A more generalized involvement may be seen in complex patellar fractures.

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Figure 7:

Patellar fracture – A 23-year-old male patient presenting with knee swelling post-trauma. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of patella shows localized edema-like marrow signal intensity (ELMSI) in the inferior pole of patella (yellow arrow). (b) Axial PDFS section at the level of tibial condyle focal ELMSI in the anterior aspect of the medial tibial plateau (curved yellow arrow) corresponding to the associated meniscofemoral ligament injury. (c) Mid sagittal proton density-weighted, fat-suppressed section shows ELMSI in the inferior pole (red arrow). (d) Mid sagittal proton density-weighted sections show the associated thin hypointense curvilinear line (yellow arrow).

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Tibial fracture

Plateau fracture

Medial or lateral tibial plateau fractures are a common cause of post-traumatic ELMSI, commonly seen extending to the articular surface [Figure 8]. On axial sections, localized ELMSI involving the either plateau in a post-trauma case should raise suspicion with a better depiction of the ELMSI and fracture line on coronal and sagittal sections.

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Figure 8:

Tibial plateau fracture – A 35-year-old female patient presenting with knee swelling post-trauma. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of tibial plateau shows localized edema-like marrow signal intensity (ELMSI) in the posterior tibial plateau (red arrow). (b) Medial sagittal and (c) posterior coronal PDFS sections show the ELMSI (red arrow in b and c) along the fracture line representing the callus and fibrotic stage of fracture healing.

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Anterior cruciate ligament (ACL) avulsion fracture

Two bundles of the ACL arise from the anterior aspect of the intercondylar eminence and course superomedially to insert on the lateral surface of the medial femoral condyle. On axial sections, localized ELMSI in the anterior part of the intercondylar eminence in a case of knee twisting is highly suggestive of ACL avulsion fracture injury. On mid sagittal and coronal sections, ELMSI in the anterior aspect of the intercondylar eminence, along with the exact size and displacement of the avulsed fractured fragment, is well appreciated [Figure 9].

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Figure 9:

Anterior cruciate ligament avulsion fracture – A 32-year-old male patient presenting with twisting injury of knee. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of femoral condyle shows edema-like marrow signal intensity (ELMSI) in the mid part of the lateral femoral condyle (red arrow) and mid part of medial femoral condyle (red arrowhead). (b) Axial PDFS section at the level of tibial plateau shows ELMSI in anterior aspect of the tibial intercondylar eminence (red arrow). (c) Oblique anterior coronal PDFS image section shows non-displaced avulsion fracture of the anterior cruciate ligament with adjacent ELMSI and mild hyperintensity along the bundle fibers (red arrow). Tear of the meniscofemoral ligament (curved red arrow) is seen as well with ELMSI in the subjacent mid part of medial femoral condyle (red arrow head). (d) Mid sagittal PDFS section shows the minimally displaced anterior cruciate ligament avulsion fracture (red arrow) associated with localized ELMSI. (e) Mid sagittal T1-weighted fast spin-echo section depicts the extent of fracture line (red arrow).

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Posterior cruciate Ligament (PCL) avulsion fracture

PCL bundles arise from the posterior aspect of the intercondylar eminence. On axial sections, localized ELMSI involving the posterior part of the intercondylar eminence is seen. On sagittal and coronal sections, localized ELMSI in the posterior aspect of the intercondylar eminence, along with a displaced or non-displaced avulsion fracture fragment, is seen [Figure 10].

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Figure 10:

Posterior cruciate ligament avulsion fracture – A 25-year-old female patient presenting with knee injury. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of tibial condyles shows localized edema-like marrow signal intensity (ELMSI) (red arrow) in the mid posterior aspect of the tibial intercondylar eminence. (b) Posterior coronal PDFS section shows localized ELMSI (red arrow) in the posterior aspect of the tibial intercondylar eminence with a displaced avulsion fracture (yellow arrow) of the posterior cruciate ligament. (c) Mid sagittal PDFS paramedian section shows ELMSI (red arrow) in the posterior aspect of the tibial intercondylar eminence. (d) Sagittal PDFS midline section shows a displaced avulsion fracture (yellow arrow) of the posterior cruciate ligament. (e) Oblique mid-sagittal PD section better depicting the displaced avulsion fracture (yellow arrow) of the posterior cruciate ligament.

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Anterolateral ligament (ALL) avulsion injury

ALL is a capsular condensation along the anterolateral aspect of the knee joint consisting of femoral, meniscal, and tibial components. In case of injury to the tibial component or its avulsion, a subchondral ELMSI [Figure 11] may be seen involving the mid part of the lateral tibial plateau on axial sections and mid coronal sections, midway between the head of fibula and Gerdy’s tubercle.

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Figure 11:

Anterolateral ligament injury – A 23-year-old female patient with known post-traumatic anterior cruciate ligament injury. Mid coronal proton density-weighted, fat-suppressed section shows a localized edema-like marrow signal intensity involving the subchondral lateral tibia (yellow arrow) associated with tear of the tibial component of anterolateral ligament (red arrow) in a case of high-grade anterior cruciate ligament tear.

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Complex fractures

Complex fractures of the tibia can also present with a generalized ELMSI on axial, coronal as well as sagittal sections. The fracture lines are better appreciated on the coronal and sagittal planes [Figure 12].

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Figure 12:

Complex fracture – A 31-year-old female patient presenting with road traffic accident. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of femur shows edema-like marrow signal intensity (ELMSI) involving the lateral femur (red arrow) and extensive associated soft tissue and ligament injury (red star). (b) Axial PDFS section at the level of tibia and head of fibula shows ELMSI in the head of fibula (red arrow) and extensive associated soft tissue and ligament injury (red star). (c) Mid coronal section shows extensive ELMSI involving lateral femoral condyle (red arrows), lateral tibial plateau (LTP) (yellow arrow), and medial tibial plateau (white arrow) along with and extensive associated soft tissue and ligament injury (red star). (d) Mid coronal T1 turbo spin echo section shows curvilinear fracture lines in the lateral femoral condyle (red arrows), LTP (yellow arrow), and medial tibial plateau (white arrow) along with and extensive associated soft tissue and ligament injury (red star).

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Stress reaction

Stress reaction and fracture occur as a result of increased loading in an underlying normal bone.^[15] On axial sections, a more localized ELMSI can be seen along the medial or lateral cortex. On coronal sections, ELMSI with characteristic involvement of a single cortex can be seen [Figure 13].

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Figure 13:

Stress reaction – A 45-year-old female patient presenting with medial joint pain. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of tibial condyles shows extensive edema-like marrow signal intensity (ELMSI) involving the medial tibial plateau (orange star). (b) Medial sagittal PDFS section shows the curvilinear hypointense line (red arrow) with extensive ELMSI. (c) Mid coronal T1 turbo spin echo section shows the unicortical hypointense fracture line (red arrowhead).

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Femoral fractures

Meniscofemoral ligament injury

It is the deep component of medial collateral ligament complex and is commonly injured in high-velocity trauma. On axial and mid coronal sections, ELMSI on the mid aspect of the medial femoral condyle is seen [Figure 14], which is generally associated with other high-grade collateral and cruciate ligament injuries.

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Figure 14:

Meniscofemoral ligament injury – A 36-year-old male patient presenting with post-traumatic medial joint pain. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of the femoral condyles shows edema-like marrow signal intensity (ELMSI) in mid part of medial femoral condyle (red arrow) with hyperintensity along the meniscofemoral ligament fibers (yellow arrow). ELMSI is also seen in mid part of lateral femoral condyle (blue arrow). (b) Mid coronal PDFS section shows ELMSI in mid part of medial femoral condyle (red arrow) with hyperintensity along the meniscofemoral ligament fibers, mid part of lateral femoral condyle (blue arrow), anterior part of intercondylar eminence (red star), and tear of the lateral meniscus (red oval). These findings are highly suggestive of meniscofemoral ligament injury in the background of anterior cruciate ligament and lateral meniscus injury.

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Fibular fractures

Fibular collateral ligament avulsion injury

Fibular collateral ligament is part of the lateral collateral ligament complex, which is attached to the lateral surface of the head of the fibula along with the biceps femoris tendon. On coronal and sagittal sections, a localized relatively larger ELMSI [Figure 15b and c], involving the head of the fibula, with a larger avulsed fragment, is suggestive of fibular collateral ligament avulsion injury.

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Figure 15:

A 38-year-old male patient presenting with anterior cruciate ligament tear. Arcuate ligament avulsion injury (a) Coronal proton density-weighted, fat-suppressed (PDFS) section shows more localized, smaller-sized edema-like marrow signal intensity (ELMSI) (red arrow) involving the tip of head of fibula. Fibular collateral ligament avulsion injury, (b) Coronal PDFS section shows a localized relatively larger ELMSI (yellow arrow) involving the head of the fibula. (c) Coronal T1-weighted image section shows the fibular collateral ligament avulsion fracture line (yellow arrows).

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Lateral patellar dislocation

This is an underdiagnosed cause of anterior knee instability, especially in young athletes and adolescents, which can be traumatic or non-traumatic. The patella is dislocated laterally, hitting the anterolateral surface of lateral femoral condyle.^[16] On axial sections, the combination of ELMSI involving the medial patellar facet of the patella and anterior aspect of the lateral femoral condyle is diagnostic. On coronal anterior sections, ELMSI in the lateral aspect of lateral femoral condyle may be seen. Many a time, especially in cases of chronic instability, the patella may relocate back into the trochlear groove and the presence of this characteristic ELMSI combination is the only clue for diagnosis [Figure 16].

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Figure 16:

Lateral patellar dislocation (LPD) relocation sequence with osteochondral injury. A 14-year-old female patient presenting with post-traumatic knee instability. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of patella shows edema-like marrow signal intensity (ELMSI) in the medial patellar facet with osteochondral defect (yellow arrow). (b) Sequential axial PDFS image at the level of inferior patella shows similar changes in the anterolateral aspect of the lateral femoral condyle (Straight yellow arrow and curvilinear yellow arrow [n]) with Hoffa’s fat pad edema (curvilinear arrow). (c) Axial proton density-weighted, fat-suppressed (PDFS) image at the level of patella in an another case of LPD shows medial facet (blue arrow) and anterolateral lateral femoral condyle (yellow arrow) ELMSI with displaced osteochondral fragment (red arrow) deep to the medial patellofemoral ligament. (d) Anterior coronal PDFS section shows similar findings in the knee of a 10-year-old child with ELMSI in anterior sections (yellow arrow) and displaced osteochondral fragment (red arrow) in lateral gutter. (e) Anterior coronal PDFS section shows isolated ELMSI in (Yellow arrow) lateral femoral condyle. (f) Mid to lateral sagittal PDFS section shows the changes in anterior lateral femoral condyle (yellow arrow).

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Pivot shift injury

Pivot shift injury occurs due to biomechanical valgus stress on the knee joint in a semi-flexed position with external rotation of the tibia.^[17] On axial sections, localized ELMSI involving the mid part of the lateral femoral condyle and posterior lip of the lateral tibial plateau is diagnostic of a high-grade ACL injury. On lateral sagittal sections, similar ELMSI can be seen along with deepening of the lateral sulcus. Associated osteochondral fractures are also well seen on sagittal images [Figure 17].

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Figure 17:

Pivot shift injury – A 19-year-old male patient presenting with twisting knee injury. (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of femoral condyles shows edema-like marrow signal intensity (ELMSI) (yellow arrow) in the mid portion of the lateral femoral condyle. (b) Axial PDFS section at the level of tibial plateau shows ELMSI in the posterior part of the lateral tibial plateau (yellow arrow). (c) Lateral sagittal PDFS section shows ELMSI in the mid part of lateral femoral condyle with subchondral depressed fracture in lateral sulcus (yellow arrow) and posterior lip of lateral tibial plateau (red arrow). Associated arcuate avulsion fracture of the tip of fibula (curved red arrow) is seen as well. These findings are near pathognomic of high-grade anterior cruciate ligament injury.

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Degenerative arthritis

Patellofemoral arthritis

Patello-femoral arthritis can occur either in isolation or along with tibio-femoral arthritis in degenerative knee disease.^[18] On axial sections, subchondral localized ELMSI is seen involving the medial and lateral patellar facets, more on the medial aspect with reduced joint space [Figure 18a]. On sagittal sections, similar findings can be seen either on medial or lateral sections.

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Figure 18:

Degenerative arthritis – a 54-year-old female patient presenting with knee pain: Patellofemoral (a) and tibiofemoral (b). (a) Axial proton density-weighted, fat-suppressed (PDFS) section at the level of patella shows subchondral multifocal edema-like marrow signal intensity (yellow arrow) with reduced joint space and cartilage thinning (red arrow). (b) Mid coronal PDFS section shows subchondral marrow edema in medial tibiofemoral compartment (yellow arrow) with osteophytes (white arrow) and medial meniscal degenerative tear (red oval). (c) Medial sagittal PDFS section shows subchondral marrow edema in medial tibiofemoral compartment (yellow arrows) with osteophytes (white arrow) and articular cartilage thinning (red arrows).

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Inflammatory arthritis

Inflammatory arthritis can be both infective and non-infective. In axial sections, the presence of multiple areas of localized ELMSI in the correct clinical setting could point toward inflammatory arthritis. On coronal and sagittal sections, the presence of erosions and synovitis, along with the ELMSI, can establish the diagnosis [Figure 19]. Involvement of the deeper marrow (more than one-third thickness) points toward underlying associated osteomyelitis.^[19]

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Figure 19:

Inflammatory infective arthritis – A 25-year-old male patient presenting with knee joint pain and swelling for 1 year. Tuberculosis knee – (a) Mid coronal proton density-weighted, fat-suppressed (PDFS) section shows multifocal areas of edema-like marrow signal intensity (ELMSI) in both femoral condyles (red arrows), tibial condyles (yellow arrows) with associated erosions and marked synovitis (yellow stars). (b) Mid sagittal PDFS section shows multifocal areas of ELMSI in both femoral condyles (red arrows), tibial condyles (yellow arrows), and patella (red dot) with associated erosions and marked synovitis (yellow star) extending into suprapatellar bursa (black star) and Hoffa’s fat pad (curved yellow arrow).

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Physeal injuries

The physeal injuries are best characterized on sagittal and coronal sections. The presence of localized ELMSI with focal widening of the physis is seen in Salter–Harris type I fracture [Figure 20a]. Careful examination should be done to look for extension of the injury into epiphysis or metaphysis.

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Figure 20:

A 10-year-old female patient presenting with vague knee joint pain. (a) Physeal injury grade I – Posterior coronal proton density-weighted, fat-suppressed (PDFS) section shows focal areas of physeal widening with adjacent edema-like marrow signal intensity (ELMSI) (red arrows) as compared to normal physis (yellow arrow). A 12-year-old female patient presenting with joint pain. (b) Focal periphyseal edema-lateral sagittal PDFS section shows star-shaped ELMSI around the normal physis (yellow circle). (c) Red marrow rest – Mid coronal PDFS section shows localized periphyseal flame-shaped ELMSI (red arrow) adjacent to normal physis (yellow arrow).

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Focal periphyseal edema

Focal periphyseal Edema aka FOPE are focal areas of ELMSI along the physis best seen on sagittal and coronal sections, extending into the epiphysis and metaphysis. Characteristic starburst shape and absence of any associated physeal widening are suggestive of FOPE [Figure 20b], differentiating them from Salter–Harris type I injuries.

Red marrow rests

Red marrow rests are seen in the growing skeleton, appearing as tiny focal areas of ELMSI. Their characteristic flamed shape on coronal and sagittal sections, classical sub-articular and periphyseal location, along with T1-weighted signal higher than the adjacent skeletal muscle, help in reaching the correct diagnosis [Figure 20c].

Irregular femoral ossification

Irregular femoral ossification, a variation in the maturation of the femoral epiphseal cartilage, may present as a focal localized area of ELMSI seen on sagittal lateral sections, involving the non-weight-bearing surface of the lateral femoral condyle [Figure 21].

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Figure 21:

Irregular femoral ossification – An 11-year-old female patient presenting with posterior knee joint pain. (a) Lateral sagittal proton density-weighted, fat-suppressed section shows focal subcartilaginous edema-like marrow signal intensity (yellow arrow) in lateral femoral condyle. (b) Lateral sagittal T1 turbo spin echo section shows the areas of irregular femoral ossification (yellow arrow).

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Osteochondrosis

Osteochondrosis is an abnormality of the epiphysis and its equivalents with trauma, or vascular etiology hypothesized as the underlying cause. Sinding–Larsen–Johansson syndrome and Osgood-Schlatter disease are the two common osteochondrosis seen around the knee joint, with the associated localized ELMSI best seen on mid-sagittal sections, involving the inferior pole of the patella and tibial tuberosity, respectively [Figure 22].

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Figure 22:

Sinding Larsen syndrome – A 17-year-old male patient presenting with anterior knee joint pain. Mid sagittal proton density-weighted, fat-suppressed shows focal areas of localized edema-like marrow signal intensity (ELMSI) in inferior pole of patella showing irregularity (red arrow) with adjacent Hoffa’s fat pad ELMSI (yellow arrow).

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Non-aggressive tumors

The common non-aggressive bone tumors associated with ELMSI around the knee joint are osteoid osteoma and chondroblastoma.

Aggressive tumors

Most of the aggressive bone tumors present with associated disproportionate ELMSI. The common aggressive bone tumors associated with ELMSI seen around the knee joint are sarcomas, lymphomas, and giant cell tumors.