Bone Marrow Edema of the Knee

What is it?

The interior of a bone contains a lattice of fine struts with marrow filling the spaces between. Around the knee, this marrow is predominantly fatty in adults.

Bone marrow edema describes an increase in fluid within this marrow. MRI distinguishes fat from water, and sequences that suppress the fat signal make fluid-containing areas appear bright against the darker background of normal marrow.

The term is imprecise. What appears as edema may be genuine fluid, bleeding, inflammatory cells, fibrous tissue replacing fat, new bone formation, or dead marrow being cleared away. These cannot be distinguished on standard imaging, which is why the broader term bone marrow lesion is increasingly used. The finding describes a state of the marrow rather than naming a cause.

This matters because bone marrow edema is a signal, not a diagnosis. The question is always what produced it, and around the knee the answer is usually one of three things.

Traumatic bone bruising is the most common cause in younger people, and the knee is where this finding is at its most informative.

When a knee is injured, the bones strike one another, fracturing the microscopic struts within the marrow without the bone breaking as a whole. The resulting bleeding and fluid produce marrow edema, and because the bones collide in a predictable way for each type of injury, the pattern of bruising reveals the mechanism.

The clearest example involves the anterior cruciate ligament. When this ligament tears, the shin bone shifts forward and rotates relative to the thigh bone before snapping back, and in doing so the outer part of the thigh bone strikes the back of the outer part of the shin bone. The resulting bruises at these two specific points are characteristic enough that a radiologist seeing them will look carefully at the cruciate ligament even if the tear itself is not obvious. Other injury patterns produce their own distinctive distributions—a direct blow to the outer knee, a dislocated kneecap, and an injury to the inner ligament each leave a recognisable signature.

Bone bruising heals, but slowly. Resolution typically takes six weeks to several months, considerably longer than the ligament or meniscal injury that accompanied it. This mismatch explains a common experience: a knee that has been reconstructed or has recovered from a sprain but continues to ache deeply on loading. The bruise, not the repair, is frequently responsible.

Degenerative marrow lesions are the most common cause in older people, and at the knee they carry particular significance.

These occur in the bone immediately beneath a damaged joint surface, concentrated in the compartment bearing most load—usually the inner side, which is where knee arthritis predominantly begins. They are not simply a byproduct of cartilage loss. Research has consistently shown that these lesions correlate with knee pain better than the cartilage findings do, that they fluctuate in size over time in parallel with symptoms, and that their presence predicts both progression of arthritis and the likelihood of eventually needing joint replacement. Their distribution also reflects alignment, being found in the inner compartment of bowed knees and the outer compartment of knock-kneed ones.

For a person with knee arthritis, the practical implication is that marrow lesions indicate a more active and symptomatic phase, and that reducing load through the affected compartment—by weight management, strengthening, and sometimes footwear or bracing—addresses the mechanism producing them.

Subchondral insufficiency fracture is a specific and important cause in older people, particularly women over 60 with reduced bone density. A small fracture occurs in the bone immediately beneath the joint surface, usually on the inner side of the thigh bone, without any significant injury. It presents with sudden severe knee pain, often with a precise moment of onset that the person can recall, and produces extensive marrow edema. This condition was previously called spontaneous osteonecrosis of the knee, a name now understood to be misleading, since the primary event is a fracture rather than loss of blood supply, with bone death occurring secondarily if the fracture fails to heal. Recognising it matters: with protected weight bearing many heal, while continued loading can lead to collapse of the joint surface and rapid progression to arthritis requiring replacement.

Stress injury produces marrow edema as its earliest sign, appearing weeks before any X-ray change. Around the knee this affects the upper shin bone and lower thigh bone, typically in runners, military recruits, and those who have increased training sharply.

Avascular necrosis of the knee occurs, most often in people taking corticosteroids or with other risk factors, and affects the ends of the thigh and shin bones. It is distinguished from insufficiency fracture by the clinical context and by specific imaging features.

Inflammatory arthritis produces marrow edema where inflamed joint lining meets bone, and is an early indicator of disease activity.

Infection produces extensive marrow edema with surrounding changes, and is a consideration after surgery or in anyone with fever and a hot swollen knee.

Transient marrow edema can occur at the knee, resolving spontaneously over months, though this is more commonly described at the hip.

Marrow edema is also seen after knee surgery, around metalwork, and following cartilage repair procedures, where it is expected rather than concerning.

The symptoms have a characteristic quality.

Pain is typically deep, aching, and poorly localised. People frequently describe it as coming from inside the bone and struggle to point to one spot, which distinguishes it from the joint line tenderness of a meniscal tear or the front-of-knee pain of a kneecap problem.

It is usually worse on weight bearing, stairs, and prolonged standing, and may persist at rest or at night where the change is substantial.

A common pattern is persistent deep aching after a knee injury that has otherwise recovered, which frequently prompts the MRI that identifies the finding.

An important caveat applies. Marrow edema is present in some people with no knee symptoms, so its presence does not automatically explain pain. Its location should correspond to where the person hurts before it is accepted as the cause, and a finding in a different part of the knee should prompt a search for another explanation.

Diagnosis depends on MRI.

Bone marrow edema is invisible on X-ray. A normal knee X-ray provides no reassurance about this finding, and this explains the common sequence of persistent pain after injury, normal X-rays, and an eventual MRI showing substantial bone change.

On MRI, the location and pattern frequently indicate the cause directly. Bruising at characteristic impact points suggests a specific ligament injury. Change beneath a worn compartment suggests degenerative disease. A focal area beneath the joint surface with a subtle line suggests insufficiency fracture. Diffuse change at a stress site suggests stress injury.

Associated findings matter: ligament tears, meniscal damage, cartilage loss, joint effusion, and any collapse of the joint surface all contribute.

X-ray remains necessary for assessing alignment and joint space narrowing and should be taken standing, since the joint space appears falsely preserved when the knee is unloaded.

CT contributes where a fracture line or collapse of the joint surface needs demonstrating.