Bone Marrow Edema of the Pelvis and Hips
Bone marrow edema describes excess fluid within the marrow of a bone, appearing as a bright area on certain MRI sequences. It is a signal rather than a diagnosis, and at the hip it carries more weight than almost anywhere else in the body. Two conditions produce an almost identical appearance in the femoral head: one resolves completely within months and leaves no trace, while the other progresses to collapse of the bone and destruction of the joint. Distinguishing them is among the more consequential judgements in musculoskeletal imaging. The pelvis adds its own considerations, since it is a common site for insufficiency fractures in older people and for inflammatory changes that define a distinct group of arthritis conditions.
What is it?
The interior of a bone contains a lattice of fine struts with marrow filling the spaces between. The pelvis and the upper thigh bone retain red, blood-forming marrow into adult life, unlike the limbs where marrow is predominantly fatty.
Bone marrow edema describes an increase in fluid within this marrow, appearing bright on MRI sequences designed to show fluid while suppressing the signal from fat.
The term is imprecise—what appears as edema may be fluid, bleeding, inflammatory cells, fibrous tissue, new bone, or dead marrow being resorbed—so the broader term bone marrow lesion is increasingly preferred. The finding describes a state of the marrow rather than naming a cause.
At the hip, the central clinical question is one of the most consequential in musculoskeletal imaging, because two conditions produce marrow edema in the femoral head with very different outcomes.
Avascular necrosis occurs when the blood supply to the femoral head is interrupted and bone dies. Marrow edema is an early feature, appearing before any structural change. If the process progresses, the dead segment loses its mechanical integrity, the surface fractures beneath the cartilage, the head flattens, and the joint is destroyed. Treatments aimed at preserving the joint work before collapse and largely do not work afterwards, so the period during which marrow edema is the only abnormality is precisely the window in which intervention is possible.
Transient marrow edema of the hip, sometimes called transient osteoporosis of the hip, produces an apparently similar appearance but behaves entirely differently. It causes sudden or rapidly progressive groin pain without any injury, lasts several months, and then resolves completely, leaving the bone and joint normal. It affects middle-aged men most commonly and women in the third trimester of pregnancy, and its cause is not established. It is self-limiting, and the correct management is protected weight bearing and analgesia rather than surgery.
Distinguishing these matters enormously, and several features help.
In transient marrow edema, the change is typically diffuse, extending from the femoral head into the neck and sometimes beyond, without any focal lesion or demarcating line. In avascular necrosis, there is usually a focal segment within the head demarcated by a dark line separating dead from living bone—the hallmark of the condition—and the surrounding marrow edema is a reaction to it rather than the primary change.
Clinical context contributes substantially. Corticosteroid use, excess alcohol, sickle cell disease, previous hip fracture or dislocation, lupus, and transplantation all point toward avascular necrosis. Pregnancy in the third trimester or a middle-aged man with no risk factors points toward transient marrow edema.
The distinction is not always immediately certain, and repeat imaging after an interval is sometimes used, since transient marrow edema resolves while avascular necrosis persists and progresses.
Several other causes of marrow change occur in this region.
Insufficiency fractures of the sacrum and pelvis are common in older people, particularly women with osteoporosis, and in those who have had pelvic radiotherapy. They occur without injury or after a trivial event, produce marrow edema on MRI, and are frequently invisible on X-ray because the sacrum is obscured by bowel gas and complex anatomy. They are commonly misdiagnosed as arthritis or ordinary back pain for weeks. Fractures frequently occur at more than one site in the pelvic ring, so finding one should prompt a search for others.
Subchondral insufficiency fracture of the femoral head is a distinct entity occurring in older people with reduced bone density, producing sudden severe groin pain with extensive marrow edema and a subtle fracture line beneath the joint surface. Like its counterpart at the knee, it can heal with protected weight bearing or progress to collapse if loading continues, and it is frequently mistaken for avascular necrosis.
Inflammatory sacroiliac disease produces marrow edema in the bone adjacent to the sacroiliac joints, and this finding is a defining diagnostic feature of axial spondyloarthritis—a group of inflammatory conditions including ankylosing spondylitis. It typically affects younger adults, produces buttock pain that may alternate between sides, with prolonged morning stiffness that improves with movement rather than worsening. Recognising it matters because early treatment with appropriate medication substantially alters the course of the disease, and because the symptoms are frequently attributed to mechanical back pain for years. An important caveat is that marrow edema at the sacroiliac joints also occurs in athletes, in women after childbirth, and in mechanical conditions, so the pattern and clinical context determine its significance.
Stress injury affects the femoral neck, the pubic bones, and the sacrum, particularly in runners and military recruits. Femoral neck stress injury deserves specific emphasis: it can progress to a complete fracture with displacement, which in a young person risks avascular necrosis and is a serious injury. Groin pain in a runner should not be assumed to be a muscle strain.
Degenerative marrow change occurs beneath worn joint surfaces in hip arthritis, and at the junction of the head and neck in impingement, where repeated contact damages the bone.
Infection produces extensive marrow edema with surrounding changes and is a consideration in anyone with fever and hip pain, particularly after surgery or injection.
Cancer deposits produce marrow change, and the pelvis is among the commonest sites for cancer to spread to bone, so this is a more prominent consideration here than in the limbs. Features suggesting it include a well-defined lesion replacing marrow, cortical destruction, a soft tissue mass, or multiple areas of involvement.
Marrow edema after hip surgery or around metalwork is expected rather than concerning.
The symptoms have a recognisable character.
Deep groin pain is typical of femoral head change, often severe, worse on weight bearing, and frequently radiating into the thigh and knee in the pattern characteristic of hip joint problems. Pain felt over the outer side of the hip usually arises from the tendons there rather than from bone.
Onset is frequently relatively sudden and without injury in both transient marrow edema and avascular necrosis.
Buttock, low back, or groin pain in an older person, worse on weight bearing and relieved by lying down, suggests insufficiency fracture.
Buttock pain alternating between sides, with morning stiffness lasting more than half an hour and improving with movement, suggests inflammatory sacroiliac disease.
Night pain occurs where the change is substantial.
Diagnosis rests on MRI, with one point worth emphasising.
The conditions producing marrow edema at the hip and pelvis are frequently invisible on X-ray at the stage when treatment matters most. Early avascular necrosis, transient marrow edema, sacral insufficiency fractures, and early inflammatory sacroiliac disease all produce normal X-rays. A normal X-ray therefore provides no reassurance, and persistent unexplained hip or pelvic pain in the presence of relevant risk factors warrants MRI rather than observation.
On MRI, the distinguishing features described above—the presence or absence of a demarcating line, the distribution of change, the sacral pattern, and the location relative to joints—together with the clinical context establish the diagnosis in most cases.
CT demonstrates fracture lines and subtle collapse of the femoral head better than MRI and contributes where these are in question.
Repeat imaging after an interval resolves cases where transient marrow edema and avascular necrosis cannot initially be separated.
Important to Know
Bone marrow edema at the hip and pelvis requires explanation rather than treatment in itself, and the explanation carries unusually high stakes here. Care is managed by orthopaedic, rheumatology, and bone health services according to the cause.
The most important question is which condition is producing the finding, and at the hip this frequently comes down to distinguishing transient marrow edema from avascular necrosis.
Where transient marrow edema is diagnosed, the outlook is good. It resolves completely over several months, leaving the joint normal. Management is protected weight bearing with crutches to reduce load and the risk of fracture, adequate analgesia, and patience. Medications affecting bone turnover are used by some clinicians to shorten the course, though the evidence is limited. The condition occasionally migrates to another joint afterwards, which is recognised and does not indicate anything more serious.
Where avascular necrosis is diagnosed, prompt specialist assessment is appropriate, because the window for joint-preserving treatment closes once the femoral head collapses. Core decompression and related procedures work before that point and largely do not afterwards. The opposite hip should also be imaged, since both are affected in a substantial proportion of cases and the second is frequently silent.
Where the distinction is not initially clear, protected weight bearing is sensible while the situation is clarified, and repeat imaging after an interval is informative.
For insufficiency fractures of the sacrum and pelvis, treatment centres on adequate pain relief and early mobilisation. Prolonged bed rest in an older person causes rapid muscle loss, further bone loss, pneumonia, blood clots, and loss of independence, and the harm from these frequently exceeds that from the fracture. The part of management that changes the future is assessment and treatment of the underlying bone weakness, since these fractures indicate substantially increased risk of further ones including hip fracture. A bone density scan, correction of vitamin D deficiency, consideration of osteoporosis treatment, and falls assessment should all follow.
For inflammatory sacroiliac disease, rheumatology referral is appropriate, and the timing matters. Early treatment substantially alters the course of these conditions, while the symptoms are frequently attributed to mechanical back pain for years. The features that should prompt referral are buttock or back pain beginning before the age of 45, lasting more than three months, with morning stiffness over half an hour, improving with exercise rather than rest, waking the person in the second half of the night, and responding well to anti-inflammatory medication.
For femoral neck stress injury, the location within the neck determines management. Injury on the under-surface is generally stable and treated with rest, while injury on the upper surface risks progression to complete fracture and displacement, and may require protected weight bearing or surgical fixation. Groin pain in a runner warrants proper assessment rather than being attributed to a muscle strain.
Degenerative marrow change is managed as part of treating the hip arthritis or impingement producing it.
Infection and suspected cancer deposits require urgent assessment along their respective pathways.
Follow-up imaging is used purposefully here more than at other sites, since the distinction between conditions that resolve and conditions that progress frequently depends on how the appearance changes over time.
A note on symptoms is useful. Pain felt in the groin usually indicates the hip joint or the bone. Pain over the outer side of the hip usually arises from the gluteal tendons and bursa, a separate condition treated quite differently. Pain in the buttock may arise from the sacroiliac region, the spine, or the hip. Describing precisely where the pain is felt genuinely helps, since these distinctions direct both the imaging and its interpretation.
Care is managed by orthopaedics, rheumatology, and bone health services, with the MRI pattern, the clinical context, and the risk factors interpreted together. No single feature is sufficient alone, and at this site the history frequently decides between diagnoses that look similar on imaging.
Patient education plays an important role. Understanding that marrow edema is a signal rather than a diagnosis, that X-rays are normal in the conditions that matter most here, that transient marrow edema resolves completely while avascular necrosis may not, that insufficiency fractures indicate bone weakness requiring treatment, and that inflammatory back pain has a distinctive pattern warranting early referral all contribute to appropriate care.
Red flag symptoms include new or worsening hip or groin pain in anyone taking corticosteroids or with sickle cell disease, lupus, or a transplant, which warrants MRI rather than reassurance from a normal X-ray; sudden severe groin pain with inability to bear weight; fever with hip pain, suggesting infection; groin or thigh pain in a runner, which may indicate a femoral neck stress injury; buttock or back pain in an older person after minimal injury, suggesting insufficiency fracture and warranting bone health assessment; buttock pain before the age of 45 with prolonged morning stiffness improving with exercise, suggesting inflammatory disease and warranting rheumatology referral; a known history of cancer alongside new pelvic or hip pain; and unexplained weight loss or night sweats. These warrant prompt medical evaluation.