Sudden severe medial knee pain and nocturnal pain may be spontaneous osteonecrosis of the knee — how to distinguish it from knee osteoarthritis, and where stem cell conditioned media joint injection can and cannot help, explained by Dr. Moriwaki2026.07.21

“I woke up one night with a sharp pain in my inner knee that I’ve never experienced before.” We see this kind of presentation fairly often in middle-aged and older patients at our clinic. Many such cases are diagnosed as “acute exacerbation of knee osteoarthritis,” but they may in fact be an entirely different condition: spontaneous osteonecrosis of the knee (SONK). Spontaneous osteonecrosis of the knee is an osteonecrotic lesion arising in the subchondral bone of the medial femoral condyle, and its pathogenesis and treatment strategy differ from those of knee osteoarthritis. In this article, Dr. Moriwaki at AVAN TOKYO Ginza honestly explains the key clinical points for telling the two apart, and what stem cell conditioned media injection into the knee joint can — and cannot — reasonably be expected to do.

Key Points of This Article

・Spontaneous osteonecrosis of the knee is an osteonecrotic lesion of the subchondral bone at the medial femoral condyle, and is a distinct condition from knee osteoarthritis in both pathogenesis and treatment strategy

・The typical presentation is sudden-onset severe nocturnal pain and weight-bearing medial knee pain; plain X-rays are often unrevealing early on, making MRI the cornerstone of diagnosis

・Stem cell conditioned media joint injection is not a treatment that regenerates the osteonecrotic lesion itself — its role is oriented toward modulating the intra-articular inflammatory environment

・Depending on lesion size and whether subchondral collapse has occurred, surgical options such as total knee arthroplasty may need to be considered

・The safest and most honest treatment sequence is to first obtain orthopedic imaging evaluation and staging before considering injection therapy

What Is Spontaneous Osteonecrosis of the Knee?

This condition is an osteonecrotic process in the subchondral bone of the medial femoral condyle (the inner end of the femur at the knee) that occurs without any clear history of trauma. It is most common in women over 60, and typically presents with sudden, severe medial knee pain.

How It Differs from Knee Osteoarthritis

Knee osteoarthritis is a chronic, progressive disease in which articular cartilage wears down gradually over many years, accompanied by osteophyte formation, joint space narrowing, and synovitis. In contrast, this condition is fundamentally different in that it (1) begins suddenly, (2) is often invisible on imaging early on, and (3) primarily affects the subchondral bone rather than the cartilage. Because both present as “pain on the inner side of the knee,” they are easily confused at the initial visit, which can delay the necessary imaging workup and appropriate surgical decision-making.

When, Where, and How the Pain Occurs

Typical complaints include “a sharp pain suddenly shot through the inner side of my knee one day,” “I can’t sleep at night due to the pain,” and “I can no longer walk without a cane.” The classic triad is: weight-bearing medial knee pain, nocturnal pain, and focal tenderness over the medial femoral condyle.

Commonly Overlooked Diagnostic Clues in Spontaneous Osteonecrosis of the Knee

This condition often shows little to no findings on plain X-rays for several weeks to months after onset, which is a major reason it gets misdiagnosed as “acute exacerbation of knee osteoarthritis.”

Lesions That Only Become Clear on MRI

MRI is central to diagnosis. T1-weighted images show a low-signal band in the subchondral bone of the medial femoral condyle, and T2-weighted or fat-suppressed images reveal high signal consistent with bone marrow edema. Since prognosis and treatment planning vary by staging (e.g., Koshino or Aichroth classification), early MRI is strongly recommended when this condition is suspected. For general information on joint disorders, the Japanese Orthopaedic Association also provides useful reference material.

Who Should Raise Suspicion

In women over 60 who present with sudden medial knee pain and nocturnal pain without any clear history of trauma or overuse, this condition should always be included in the differential. When there is a history of long-term steroid use or heavy alcohol consumption, secondary osteonecrosis of the femoral condyle must also be considered.

What Stem Cell Conditioned Media Knee Joint Injection Can Aim For in Spontaneous Osteonecrosis of the Knee

The first thing we want to make clear is that stem cell conditioned media knee joint injection is not a treatment that regenerates “the osteonecrotic lesion itself.” The core pathology of this condition is subchondral bone necrosis, and this layer is histologically distinct from what an intra-articular injection can directly reach.

Its Role: Modulating the Intra-Articular Inflammatory Environment

In this condition, secondary synovitis, joint effusion, and progressive cartilage degeneration develop as a consequence of the necrotic lesion, and these secondary changes account for a substantial portion of the pain and functional impairment. Knee joint injection is positioned as an intervention that targets this secondary inflammatory environment via the anti-inflammatory action and tissue repair modulation attributed to the various cytokines and growth factors it contains. That said, this is a symptomatic and complementary approach aimed primarily at easing pain and inflammation — it does not halt progression of the osteonecrotic lesion itself, and we always convey this honestly to our patients. Individual response varies, and no definitive outcome guarantee can be made.

What to Expect and What Not to Expect

Realistic goals include suppressing intra-articular inflammation, improving pain scores, and preserving or improving range of motion. On the other hand, bony repair of the necrotic lesion itself, reversal of subchondral collapse, and reconstruction of the joint surface in advanced or end-stage disease all lie beyond what knee joint injection alone can offer.

Where Surgery and Conservative Care Diverge — Orthopedic Evaluation Comes First

Treatment strategy for this condition depends heavily on lesion size, the presence or absence of subchondral collapse, and the degree of joint surface deformity. When the lesion is small and no collapse is seen, conservative management with weight-bearing restriction, analgesics, and intra-articular injections may be reasonable. However, when the lesion is large with progressive subchondral collapse, or when end-stage joint destruction has occurred, surgical options such as high tibial osteotomy (HTO), unicompartmental knee arthroplasty (UKA), or total knee arthroplasty (TKA) enter the picture.

For this reason, before considering stem cell conditioned media knee joint injection, it is a safe and honest sequence to first obtain orthopedic imaging evaluation and staging, and to confirm that surgical intervention is not indicated. Even the same complaint of “knee pain” can call for completely different best options depending on the underlying condition. At our clinic, we always confirm imaging findings at the initial visit and prioritize referral to a partner orthopedic clinic when appropriate. Please also see this page for more on stem cell conditioned media joint injection.

Frequently Asked Questions

Q. I was diagnosed with spontaneous osteonecrosis of the knee. Can stem cell conditioned media knee joint injection alone treat it?

Knee joint injection is not a treatment that regenerates the osteonecrotic lesion itself. The top priority is to obtain orthopedic staging first and determine whether surgical intervention is indicated. At our clinic, we position this therapy as a complementary conservative option aimed at easing intra-articular inflammation.

Q. I was told my plain X-ray was normal, but the pain is severe and I can’t sleep. What should I do?

Early in its course, this condition often shows little on plain X-rays. If the classic triad — sudden onset, nocturnal pain, and focal medial tenderness — is present, we recommend obtaining an MRI through an orthopedic clinic.

Q. What happens if the disease has already progressed?

When subchondral collapse or joint surface deformity has advanced, surgical options such as high tibial osteotomy, unicompartmental knee arthroplasty, or total knee arthroplasty are considered. The best option varies case by case, so consultation with an orthopedic surgeon is essential.

Q. Can this condition be prevented?

Because its cause is not fully understood, there is no established method of definite prevention. Practical measures include managing osteoporosis, avoiding sudden excessive load on the knee, and seeking orthopedic evaluation promptly rather than tolerating sudden knee pain.

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[Supervising Physician] Shin Moriwaki, MD

Member, Japan Society of Aesthetic Surgery (JSAS) / Member, American Academy of Aesthetic Medicine

ECFMG Certificate holder

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