Subchondroplasty is a minimally invasive procedure that treats knee pain caused by bone marrow lesions, which are areas of damaged, fluid-filled bone just beneath the cartilage surface. A surgeon injects a synthetic bone substitute into these damaged areas to provide structural support and encourage the bone to heal. It’s primarily used for people with mild to moderate osteoarthritis who haven’t responded to conservative treatments like physical therapy, injections, or anti-inflammatory medications.
How Bone Marrow Lesions Cause Knee Pain
Bone marrow lesions are pockets of damaged tissue inside the bone that sits directly under your joint cartilage. They show up on MRI scans as bright white areas on certain image sequences, indicating swelling and structural breakdown within the bone itself. These lesions are common in osteoarthritis and are strongly associated with the deep, aching knee pain that doesn’t respond well to typical treatments targeting cartilage or soft tissue.
The pain from bone marrow lesions differs from general arthritis pain because it originates in the bone rather than the joint surface. When the internal structure of the bone weakens, it can no longer support the loads placed on it during walking or standing. This creates a cycle where ongoing stress worsens the lesion, which worsens the pain.
What Happens During the Procedure
Subchondroplasty is performed using real-time X-ray guidance (fluoroscopy). The surgeon inserts a thin, hollow needle through the skin and into the bone at the exact location of the lesion identified on your MRI. Through this needle, a flowable calcium phosphate compound is injected into the spaces between the weakened bone structures.
Once injected, the calcium phosphate crystallizes through a cooling reaction and hardens into a porous scaffold that mimics the structure of natural bone. This scaffold serves two purposes: it immediately provides mechanical support to the weakened area, and its porous structure is designed to allow your body’s bone cells to gradually move in, break down the scaffold, and replace it with new living bone over time. The procedure typically takes under an hour and can be done as an outpatient surgery.
Who Is a Candidate
The procedure is generally considered for people between 40 and 75 years old with localized knee pain that has persisted for at least six months despite conservative treatment. The key diagnostic requirement is an MRI confirming that a bone marrow lesion exists in the subchondral region of the knee, specifically in the tibial plateau or femoral condyle, and that the lesion’s location matches where the patient feels pain.
Subchondroplasty is not intended for people with severe, bone-on-bone arthritis. It targets a specific source of pain in patients who still have some cartilage remaining but whose bone marrow lesions are driving their symptoms. If your pain is primarily from cartilage loss, meniscus tears, or ligament damage rather than bone marrow lesions, this procedure is unlikely to help.
Pain Relief and Clinical Results
Studies show substantial short-term pain relief. In one study, patients’ average pain scores dropped from 7.6 out of 10 before surgery to 1.8 at six weeks, a reduction of nearly six points. These improvements held at final follow-up, with average pain scores remaining around 1.6 to 1.9. A drop of more than five points on a ten-point pain scale is considered clinically meaningful.
The procedure also appears to delay or prevent knee replacement for many patients. Five-year data shows that 73% of patients who underwent subchondroplasty had not gone on to need a knee replacement. Among those who did eventually convert to knee replacement, the average time between subchondroplasty and the later surgery was about 28 months. For patients hoping to buy time before a major joint replacement, or avoid one altogether, these numbers offer a reasonable picture of what to expect.
What Recovery Looks Like
After the procedure, you’ll typically be restricted to toe-touch weight bearing with crutches. Physical therapy usually doesn’t begin until at least two weeks after surgery, giving the injected material time to fully harden and integrate. Your surgeon or physical therapist will guide you on when to wean off crutches, often transitioning to a single crutch on the opposite side before walking unassisted.
Because the procedure is minimally invasive (no large incisions, no hardware), recovery is significantly faster than a knee replacement. Most patients return to daily activities within weeks rather than months, though the full bone remodeling process takes considerably longer.
Risks and Open Questions
The most commonly reported complication is extravasation, where some of the injected material leaks outside the intended area, sometimes into the joint space. This risk increases when the bone marrow lesion is large, when there are existing stress fractures in the area, or when the lesion is located in the femoral condyle (the rounded end of the thighbone) because the needle path passes through a more complex area to reach it. In some cases, leaked material needs to be removed with a follow-up arthroscopy, though some patients with minor leakage recover without additional intervention.
A more nuanced concern involves how well the calcium phosphate actually gets replaced by living bone. The manufacturer states that the material is gradually resorbed and replaced with new bone over time, and animal studies have supported this, showing the cement reduced to just 4% of its original volume within six months in goats. However, a detailed case study of a human patient who later underwent knee replacement four years after subchondroplasty found a different picture. Microscopic and imaging analysis of the removed bone showed the calcium phosphate material was still fully intact, with no signs of resorption, bone remodeling, or new bone growing into the scaffold. This suggests that the replacement process in humans may be much slower than animal models predicted, or may not occur in all patients.
This doesn’t necessarily mean the procedure fails. Pain relief can still occur even if the material persists, likely because the mechanical support itself resolves the symptoms. But it does raise questions about the long-term biological behavior of the implanted material.
Insurance Coverage
Coverage for subchondroplasty remains limited. Major insurers, including Cigna, classify the procedure as experimental, investigational, or unproven. This means many patients face out-of-pocket costs or need to navigate prior authorization and appeals. The procedure is billed under several CPT codes (including 0707T and 0869T), which are designated as temporary “tracking” codes for newer procedures that haven’t yet received standard billing status. If you’re considering subchondroplasty, confirming coverage with your insurer before scheduling is essential, as denial of claims is common.