A subchondral insufficiency fracture of the knee, or SIFK, is a fracture in the bone just below the joint cartilage. It can cause sudden pain without a major injury. Early X-rays may look normal. If this diagnosis is suspected, assessment and advice about loading the knee take priority over choosing an elective injection.
Why this is different from an arthritis flare
The bone beneath the cartilage helps support the joint surface. An insufficiency fracture develops when that bone cannot tolerate the load placed on it. Some lesions heal with appropriate management, but others progress to collapse of the joint surface and more advanced joint damage.
SIFK has historically been described using the term spontaneous osteonecrosis of the knee, or SONK. Modern reviews explain that a subchondral fracture can be the starting event, with secondary changes developing later. Your clinician should interpret the exact terminology and stage in your report rather than treating every mention of bone marrow oedema as the same condition.
Contact The Wellness about a knee assessment. If your report mentions an insufficiency fracture, subchondral fracture or SONK, say so when you enquire and ask which clinical route is appropriate.
When the pattern deserves another look
A radiology review in the Japanese Journal of Radiology describes sudden knee pain, often without a clear traumatic event, that worsens with weight bearing and can persist at rest or at night. Tenderness and swelling may occur. These features overlap with other conditions, including meniscal injuries, so they do not establish the diagnosis by themselves.
A new inability to bear weight, severe sudden pain or a hot swollen knee with fever or feeling unwell needs urgent assessment. Do not wait for a routine injection appointment to investigate those symptoms.
Why a normal X-ray does not always settle it
X-rays remain useful for assessing bone shape, arthritis and more advanced changes. However, the early fracture line may not be visible. MRI can detect the characteristic subchondral line and associated marrow signal changes when SIFK is suspected.
This does not mean everyone with knee pain needs an MRI. It means persistent or disproportionate symptoms should be reconsidered in their clinical context. Ask what diagnosis is being considered, whether further imaging would change care and what to do while waiting.
Bone marrow oedema is an imaging description, not a single diagnosis. Trauma, arthritis and other disorders can also produce marrow signal changes. The pattern and the rest of the scan matter.
Protecting the knee while a plan is made
Reviews describe protected or restricted weight bearing as part of early management in selected patients. The amount of loading, need for crutches or a brace and the timing of rehabilitation should be set by the treating clinician. Do not copy a fixed timetable from an online article or push through pain because a routine arthritis exercise plan usually encourages activity.
Treatment depends on the lesion, whether the joint surface has collapsed, cartilage condition, alignment and the person's health. Some patients need orthopaedic advice. More advanced disease can require surgery, while an early lesion may be managed without an operation. There is no single treatment course that fits every scan.
What the PRP evidence actually supports
The 2025 Frontiers in Surgery review discusses PRP as an area of research, but says high-quality clinical studies for this condition remain scarce. It also warns against assuming that findings from osteonecrosis of the hip apply directly to the knee. Different joints and disease mechanisms cannot simply be treated as interchangeable evidence.
An injection into the knee joint is also different from an intraosseous procedure directed into bone. A report about one technique does not establish the benefit or safety of the other. Laboratory findings, growth-factor theory and individual case reports do not prove that a standard joint PRP injection heals SIFK or prevents collapse.
If PRP is proposed for coexisting knee osteoarthritis, ask which symptom or diagnosis is being treated. NICE's knee-osteoarthritis guidance describes limited-quality efficacy evidence and special arrangements for governance, consent and audit or research. That guidance is not evidence that PRP repairs an insufficiency fracture.
Questions worth taking to the appointment
- Is SIFK confirmed, suspected or excluded?
- Is the joint surface intact, and is specialist review needed?
- What weight bearing and exercise are appropriate now?
- Are meniscal root damage, alignment or bone-health factors relevant?
- How will healing and symptoms be reviewed, and what would trigger a change of plan?
Bring the images as well as the report if you have them. Explain when the pain began, whether it occurs at night and what you can currently do. Confirm the full fee and whether imaging, specialist review and follow-up are separately charged before booking.
Enquire about the right assessment route at The Wellness. The useful next step is a diagnosis-led plan, not a procedure chosen before the cause of pain is understood.
Sources
- Ochi and colleagues. SIFK: current concepts and radiological differential diagnoses
- Wang and colleagues. SIFK pathogenesis and treatment, Frontiers in Surgery, 2025
- Conservative treatments of bone marrow lesions, 2025 review
- NICE. PRP injections for knee osteoarthritis, HTG497
General information, not a diagnosis or individual treatment advice.