The Telltale Bone

I review a lot of bone MRI images, toiling alone for hours in my secluded office, in near-darkness. Let me tell you a secret (in a creepy Sixth Sense whisper): sometimes bones talk to me.

What do they say? And how do I hear them?

toy hippo on top dinosaur's head
And am I insane?

Follow the bone marrow edema

Bones don’t literally talk to me, as you probably guessed (though I yell at my spine on occasion). When I see abnormal bone on an MRI, it points me toward the problem and cause of pain. This appearance is often caused by bone marrow edema.

What exactly is bone marrow edema? I think of it as a bruise inside a bone. When injured, a bone will react like most other parts of the body: it will collect fluid (edema=excess fluid) and sometimes blood. A bruise near the skin surface will swell up with fluid, but a “bruise” in a bone collects fluid on the inside, called the marrow cavity. The outer rigid shell called the cortex contains the fluid inside the bone and does not allow swelling.

bone marrow soup
This soup possibly contained bone marrow

Importantly, bone marrow edema can only be seen on MRI. It does not show up on the two other imaging modalities we typically use for bones—CT and X-ray. Without getting too much into the physics of it, MRI is designed to be sensitive to fluid, and bone marrow edema is extra fluid in the bone.

A bone MRI consists of multiple sets of images. Some of these sets (called sequences) are designed to detect fluid in tissues and bone. “STIR” and “T2 fat saturation” are examples of these sequences, on which fluid shows up as bright spots. Radiologists use the term “signal” when describing MRIs, i.e., a bright spot would be described as increased signal.

Bone marrow edema can also be seen on other types of MRI images. A future post on MRI and the associated physics will explore this topic further, probably in more detail than you ever wanted to know.

Why do I have bone marrow edema?

When I talk about bone injury, not only do I mean traumatic injury, but also a multitude of various insults to which the bone reacts with marrow edema. Just within the realm of trauma, it can be caused by the following entities:

  • Fracture
hip fracture on MRI
MRI image of a hip fracture. STIR sequence shows the fracture line (arrow) in the neck of the femur (thigh bone) near the hip joint. Adjacent increased signal in the femur represent bone marrow edema.
  • Bone contusion: Bruising without fracture is called contusion.
bone contusion on knee MRI
MRI image of bone contusion in the knee. Increased signal representing bone marrow edema is present adjacent parts of the femur and tibia. These “kissing contusions” occurred during a hyperextension injury.
  • Stress reaction: Abnormal biomechanics can result in stress on part of a bone, and eventually lead to fracture if not addressed.
  • Tendon or ligament injury: With the tear of a tendon (connecting muscle to bone ) or a ligament (connecting bone to bone), the bone often reacts with marrow edema near the attachment site.
edema in calcaneus from plantar fasciitis
MRI image of plantar fasciitis in the foot. Mild marrow edema is present at the site of attachment of the plantar fascia to the calcaneus bone in the heel of the foot. The plantar fascia (or plantar aponeurosis) is not technically a ligament or tendon, but attaches to the bone in a similar fashion and can cause marrow edema at the attachment site.

Among the additional causes of (non-traumatic) bone marrow edema listed below, osteoarthritis, is probably the most commonly seen.

  • Osteoarthritis (aka degenerative arthritis): Arthritis is a general term meaning pain in the joint. In osteoarthritis, wear and tear on the joints results in cartilage loss and eventual bone-on-bone contact, with reactive bone marrow edema in affected areas.
hip arthritis on MRI
MRI image of hip osteoarthritis. The right hip (arrow) shows marrow edema in the top of the femur (thigh bone) where it forms the hip joint. Several small fluid-filled cysts are also indicative of severe osteoarthritis. Contrast to the normal left hip on the other side.
  • Inflammatory arthritis: Inflammation results in pain, redness, and swelling of the affected joint, and these inflammatory disorders are often grouped together to differentiate them from the more common osteoarthritis (which often exhibits significantly less inflammation). In inflammatory arthritis, MRI can show marrow edema before other evidence of the disease is present on X-ray or CT. Examples of diseases in this category are rheumatoid arthritis and gout.
  • Infection: Bone infection (osteomyelitis) and infection in the joint (septic arthritis) can cause bony erosion and reactive marrow edema.
osteomyelitis of the big toe
MRI illustrating osteomyelitis in the big toe. Top image: Sagittal (from the side) image of a normal big toe is centered near the metatarsophalangeal joint (ball of the foot). Bottom image: Osteomyelitis in the same region of the big toe, with bony destruction and marrow edema adjacent to the joint.
  • Bone tumors: Both benign (not cancer) and malignant (cancer) tumors can destroy parts of the bone and cause adjacent marrow edema.
bone lesion spine
MRI image of a bone lesion in the lumbar spine. In this image, a metastatic (cancerous) tumor has spread to one of the bones, the vertebral body. Some of the abnormal appearance is due to the actual lesion, while some represents bone marrow edema reacting to tumor invasion.
  • “Ischemic” lesions: Ischemia means lack of blood supply, resulting in injury or even death of tissue in the affected region. In the initial stage, injury to the bone manifests as marrow edema. Diseases in which ischemia is thought to play a role in marrow edema include transient bone marrow edema, avascular necrosis, osteonecrosis, and complex regional pain syndrome

What do I do about bone marrow edema?

Bone marrow edema tells us that one of the above underlying abnormalities is causing fluid accumulation in the bone. As you would imagine, the treatment depends on the disease process triggering the marrow edema.

Controversy exists as to whether marrow edema is necessarily the cause of pain. In some situations, such as bone contusion, marrow edema is a direct result of an injury, and pain will resolve when marrow edema resolves. However, in osteoarthritis, for example, marrow edema certainly has a relationship to severity of disease, but may not necessarily be linked with the degree of pain.

After reading this post, maybe you too will learn to “talk” to your bones. It would be nice to know I am not the only bone whisperer out there.

cattle skull
“Howdy”

 

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