Subchondral bone cysts are fluid-filled or tissue-filled cavities that form in the bone just beneath the cartilage surface of a joint, and they are closely tied to osteoarthritis. They show up in roughly 30 to 50 percent of people with knee osteoarthritis, depending on the study and imaging method used, and they signal that the joint is under significant structural stress. Although the cysts themselves can be painless, their presence generally marks a more advanced stage of joint disease and faster cartilage loss, which makes understanding them more than an academic exercise.
What Causes Subchondral Cysts to Form
Researchers have debated the origins of these cysts for decades, and the honest answer is that no single explanation accounts for every case. Two main theories dominate the discussion, and the evidence suggests both play a role at different stages.
The first theory centers on mechanical stress. When cartilage thins or develops defects, the bone underneath loses its cushion and absorbs forces it was never designed to handle alone. Finite element studies show that cartilage defects create stress peaks in the subchondral bone, and these peaks correspond to the exact locations where cysts tend to appear clinically.1PubMed. The cause of subchondral bone cysts in osteoarthrosis: a finite element analysis Repeated loading under those conditions leads to microfractures, and the damaged bone starts remodeling in ways that can hollow out a cavity. Computational models also show that once a cyst exists, it roughly doubles the stress on the bone immediately surrounding it, creating a feedback loop in which the cyst promotes its own growth.2Bone. Subchondral cysts create increased intra-osseous stress in early knee OA: A finite element analysis using simulated lesions
The second theory involves pressurized synovial fluid. In joints with damaged cartilage, fluid from the joint space can be forced into the bone through cracks and fissures. One computational study found that this pressurized fluid actually decreases the load on surrounding bone tissue, which triggers the body to resorb bone rather than build it, effectively hollowing out a cavity from the inside.3PubMed. The role of pressurized fluid in subchondral bone cyst growth That same study found that pressurized fluid can also kill osteocytes, the cells embedded in bone that sense mechanical loads, and osteocyte death independently drives cyst growth. When both mechanisms operate together, the cyst grows faster. However, other biomechanical work suggests that shear stresses from mechanical trauma are the more plausible initiator, because fluid pressure would need to reach levels higher than what the joint likely produces in real life to actually damage bone on its own.4PubMed. Effects of Internal Fluid Pressure on Stresses in Subchondral Bone Cysts of the Medial Femoral Condyle
What most researchers now accept is that the two mechanisms are not mutually exclusive. Mechanical overload likely initiates the damage, microfractures open a path for fluid intrusion, and the pressurized fluid accelerates the cavity’s expansion. Joint malalignment, like the bowed-leg (varus) deformity common in knee osteoarthritis, amplifies the whole process by concentrating force on one side of the joint.
Where They Tend to Appear
Subchondral cysts are most commonly associated with the knee and hip, but they can form in any joint affected by osteoarthritis. In the knee, they frequently develop on the medial (inner) side, which bears more weight in people with varus alignment. One study of patients undergoing total knee replacement found cysts on X-ray in about 31 percent of cases, compared with near-universal findings of joint space narrowing and bone spurs.5Bentham Open (The Open Orthopaedics Journal). The truth behind subchondral cysts in osteoarthritis of the knee. That same study noted a higher proportion of women among patients with cysts and a greater degree of varus deformity preoperatively, suggesting the cysts track with more severe mechanical imbalance.
In the hip, cysts tend to cluster in the femoral head and acetabulum. Finite element modeling of the hip reveals something interesting: when a cyst forms on the femoral side, it creates a stress peak on the opposing acetabular surface, and cysts on the two sides can appear in a “kissing” pattern, facing each other across the joint.1PubMed. The cause of subchondral bone cysts in osteoarthrosis: a finite element analysis Acetabular cysts also showed higher stress at their edges than femoral cysts did, which may explain why some cysts grow faster than others depending on location.
Interestingly, cysts do not always form directly under areas of heavy load. Research on knee specimens found that cysts appeared in the intercondylar fossa, a part of the knee not covered by cartilage and not a primary weight-bearing zone, in about 45 percent of cases examined.6PubMed Central. Subchondral Bone Cyst Development in Osteoarthritis: From Pathophysiology to Bone Microarchitecture Changes and Clinical Implementations This observation complicates the pure mechanical-overload story and suggests that local bone quality and fluid dynamics matter as much as raw force.
Symptoms and What Cysts Mean for Your Joint
Many subchondral cysts produce no symptoms on their own. You can have one or several and feel nothing beyond the baseline aching of osteoarthritis. The cysts are often discovered incidentally on MRI ordered for general joint pain or swelling. Because of this, some clinicians view them as a radiological marker of disease severity rather than a standalone pain generator.
That said, cysts are not benign bystanders. Their presence is consistently associated with worse structural damage. In a longitudinal study of people with knee osteoarthritis, those with bone cysts had the fastest rate of medial cartilage loss, at about 9 percent per year, compared with roughly 6 percent per year in people who had bone marrow lesions but no cysts, and about 3 percent per year in those with neither.7PubMed Central. The association between subchondral bone cysts and tibial cartilage volume and risk of joint replacement in people with knee osteoarthritis: a longitudinal study Baseline cyst presence was also associated with lower cartilage volume at the start of the study, suggesting these patients were already further along in their disease.
Cyst volume itself correlates with cartilage degradation scores. Larger cysts track with worse cartilage grading, and areas near cysts tend to show replacement of normal cartilage with fibrocartilage, a less elastic tissue that does not hold up as well under load.8Osteoarthritis and Cartilage. Bone turnover and articular cartilage differences localized to subchondral cysts in knees with advanced osteoarthritis The expansion of cysts also promotes sclerotic bone formation around their borders, which stiffens the subchondral plate and further degrades the elastic properties of the overlying cartilage, feeding the cycle of joint deterioration.6PubMed Central. Subchondral Bone Cyst Development in Osteoarthritis: From Pathophysiology to Bone Microarchitecture Changes and Clinical Implementations
When multiple cysts are present, the picture worsens. Simulations show that having several cysts in the same area dramatically increases both the stress on the surrounding cancellous bone and the stress on the articular cartilage above, with larger cysts producing a more pronounced focal effect.9PubMed Central. Multiple Subchondral Bone Cysts Cause Deterioration of Articular Cartilage in Medial OA of Knee: A 3D Simulation Study In practical terms, the presence of subchondral cysts may identify patients at higher risk of eventually needing joint replacement.
How They Are Diagnosed
Standard X-rays can reveal subchondral cysts as rounded lucencies in the bone near the joint surface, but they are easy to miss on plain radiographs, especially when small. MRI is far more sensitive and is the preferred imaging tool. On MRI, cysts appear as well-defined fluid-signal cavities within the subchondral bone, often sitting within or adjacent to larger areas of bone marrow edema (sometimes called bone marrow lesions).
A large imaging study found subchondral cysts in about 5 percent of subregions examined in osteoarthritic or at-risk knees. When contrast dye was given intravenously, over 94 percent of detected cysts showed full enhancement, meaning their walls were actively supplied with blood, and over 91 percent had enhancing bone marrow lesions immediately adjacent.10PubMed Central. Contrast-enhanced MRI of subchondral cysts in patients with or at risk for knee osteoarthritis: The MOST study The close relationship between cysts and bone marrow lesions on imaging supports the idea that cysts often grow out of pre-existing marrow edema. One detail worth noting from that study: in nearly half of the subregions with cysts, there was no adjacent full-thickness cartilage loss, which means cysts can form before the cartilage above them has completely worn away.
When researchers have examined cysts at the tissue level after joint replacement surgery, they find a mix of contents rather than the simple fluid-filled sac the name “cyst” implies. Histological analysis of hip cysts revealed fibrous tissue, cartilage, bony fragments, and fatty tissue inside, with the specific mixture varying by location within the femoral head.11Osteoarthritis and Cartilage. Structural features of subchondral bone cysts and adjacent tissues in hip osteoarthritis Fibrous cysts contained abundant blood vessels and nerve fibers, confirmed by immunostaining, which may help explain why some cysts contribute to pain and others do not. The presence of nerves inside these lesions is an underappreciated detail; the subchondral bone is richly innervated, and remodeling activity around cysts appears to increase local nerve growth factor expression.12PubMed Central. Increased nerve growth factor expression and osteoclast density are associated with subchondral bone marrow lesions in osteoarthritic knees
Conservative Management
Because subchondral cysts are closely tied to the underlying osteoarthritis, the first line of management is the same as for OA in general: weight management, physical therapy to strengthen the muscles supporting the joint, activity modification, and anti-inflammatory medications for pain control. None of these approaches target the cyst directly, but they address the mechanical and inflammatory drivers that make cysts grow.
One area of pharmacological research focuses on bone-remodeling drugs. Osteoclast inhibitors like bisphosphonates and denosumab have shown some ability to reduce joint pain and shrink bone marrow lesions in people with knee osteoarthritis. Zoledronic acid, a potent bisphosphonate, reduced both knee pain and bone marrow lesion size in osteoarthritis patients, though a meta-analysis of randomized controlled trials did not consistently support the pain-relieving effects of bisphosphonates in knee OA as a class.12PubMed Central. Increased nerve growth factor expression and osteoclast density are associated with subchondral bone marrow lesions in osteoarthritic knees The current thinking is that targeting osteoclasts may provide meaningful pain relief specifically in cases where subchondral bone marrow lesions are the primary pain driver, rather than as a blanket treatment for all OA pain. This makes biological sense: if the pain is coming from active bone remodeling around a cyst, slowing that remodeling should help, but if the pain is from cartilage loss or synovitis, a bone-remodeling drug will not address it.
Corticosteroid or hyaluronic acid injections into the joint can provide temporary symptom relief but do not affect the cyst itself. Bracing or unloader braces, particularly for knees with varus malalignment, can shift some of the mechanical load away from the compartment where the cyst sits, potentially slowing progression.
Subchondroplasty and Bone Substitute Injections
A newer minimally invasive option called subchondroplasty has gained attention over the past decade. The procedure involves injecting a calcium phosphate bone cement directly into the cyst or bone marrow lesion through a small needle, guided by fluoroscopy. The cement acts as a scaffold that the body gradually remodels into normal bone, restoring structural support to the subchondral region.13PubMed. Complications Following Intraosseous Injections of Calcium Phosphate Bone Cement in Subchondroplasty
Published results have been generally promising. In one case report, a patient undergoing arthroscopic debridement combined with subchondroplasty of the medial femoral condyle achieved good pain relief and remained complication-free after two years. The procedure is positioned as a temporizing measure, especially in younger patients, that may delay the need for joint replacement while retaining function.14PubMed Central. Knee subchondroplasty for management of subchondral bone cysts: a novel treatment method Animal and lab data suggest calcium phosphate cement performs well adjacent to cartilage, behaving similarly to untreated controls in terms of cartilage health while being considered histologically superior to some alternatives.15PubMed. Effects on articular cartilage of subchondral replacement with polymethylmethacrylate and calcium phosphate cement
The procedure is not without risks. A review of published complications found that avascular necrosis (loss of blood supply to bone) was the most frequently reported concern, particularly in the talus (ankle bone). Post-surgical pain, infection, and continuous wound drainage due to bone substitute leaking into the joint or surrounding soft tissue have also been documented.13PubMed. Complications Following Intraosseous Injections of Calcium Phosphate Bone Cement in Subchondroplasty No cases of permanent disability or death have been reported. The technique is still being refined, and there is a recognized need for better standardization of how much cement to inject and how to prevent extravasation.
Bone Grafting and Core Decompression
For larger cysts, particularly in the hip, surgical options extend beyond cement injection. Core decompression involves drilling small channels into the femoral head to relieve pressure within the bone, improve blood flow, and encourage healing. When combined with bone grafting, the drilled channels and cyst cavities are packed with donor bone or synthetic graft material. The rationale is that reducing intracapsular pressure enhances circulation, and the blood released during the decompression nourishes the transplanted bone, supporting its integration.16PubMed Central. Arthroscopic Core Decompression With Allogeneic Bone Grafting for Femoral Head Osteonecrosis and Femoral Head-Neck Junction Cysts
Case reports of arthroscopic bone grafting for hip subchondral cysts have shown encouraging individual results. In one case, a patient’s pain score dropped from 7 out of 10 before surgery to 1 out of 10 at 18 months, with the hip function score improving substantially and imaging showing that the graft had consolidated with the surrounding bone.17Hip & Pelvis. Arthroscopic Treatment of Subchondral Bony Cyst in Early Osteoarthritis of the Hip Joint Using Allogeneic Bone Graft: A Report of Two Cases These are small case series, not large trials, so the evidence is still limited. But the results demonstrate feasibility, especially for younger patients where preserving the joint is a high priority.
When a cyst is massive and joint replacement is already on the table, surgeons sometimes address both in a single operation. One reported case involved a large femoral condyle cyst that occupied 30 to 40 percent of the distal femoral bone cut. The team packed the void with a mix of the patient’s own bone from the surgical cuts and a synthetic calcium sulfate/calcium phosphate graft, then performed a robotic-assisted total knee replacement. At one year, the patient reported no pain, no activity limitations, and imaging showed integration of the graft with no bone loss around the implant.18Journal of Orthopaedic Case Reports. Management of a Massive Solitary Femoral Condyle Bone Cyst at the Site of Knee Osteoarthritis with a Synthetic Bone Graft and Primary Robotic-assisted Cementless Total Knee Arthroplasty: A Case Report
Regenerative Approaches With Stem Cells
The newest frontier in subchondral cyst treatment involves mesenchymal stem cells (MSCs), usually harvested from the patient’s own fat tissue or bone marrow and often combined with platelet-rich plasma (PRP). The idea is that MSCs provide cellular signals that encourage bone and cartilage regeneration while also reducing inflammation locally.
In one reported human case, a patient with patellofemoral bone marrow edema and subchondral cysts received a single injection of autologous fat-derived expanded mesenchymal stem cells with PRP. The patient’s symptoms resolved, and follow-up MRI showed improvement in joint inflammation, dramatic reduction in a baker cyst, and complete resolution of both the bone marrow edema and the subchondral cysts. Existing meniscus tears and cartilage thinning did not progress.19Advances in Orthopedics and Sports Medicine. Resolution of Patellofemoral Bone Marrow Edema and Subchondral Cysts on MRI Post a Single Autologous Fat-Derived Expanded Mesenchymal Stem Cell Therapy
Animal research tells a similar story. In a horse with a subchondral cyst in the femoral condyle, arthroscopic delivery of adipose-derived mesenchymal stem cells with PRP gel promoted bone regeneration within the cyst cavity.20PubMed Central. Arthroscopic Treatment of a Subchondral Bone Cyst via Stem Cells Application: A Case Study in Equine Model and Outcomes Horses are a valuable model for this kind of research because their joints are large, weight-bearing, and develop subchondral cysts naturally.
These are still individual case reports and early-stage studies, not randomized trials. The results are exciting enough to justify further research, but it would be premature to call stem cell therapy a proven treatment for subchondral cysts. Regulatory frameworks around expanded stem cell products vary widely between countries, and the procedures themselves can be expensive and not covered by insurance. For now, this approach remains investigational, though it represents one of the few strategies that aims to rebuild the damaged bone rather than just fill the void or manage symptoms.
What Subchondral Cysts Are Actually Made Of
The term “cyst” is a bit misleading. Most people hear the word and picture a hollow sac full of fluid, but subchondral bone cysts are more structurally complex than that. When researchers have cut open cysts removed during hip replacement surgery, they found multiple tissue types packed inside: fibrous tissue, islands of cartilage, fragments of bone, and in some cases adipose (fatty) tissue. The fibrous-only type was most common in certain regions of the femoral head, but in other locations, cysts containing cartilage and bone along with fibrous tissue were equally prevalent.11Osteoarthritis and Cartilage. Structural features of subchondral bone cysts and adjacent tissues in hip osteoarthritis
The fibrous cysts were especially notable for containing a high density of blood vessels and nerve fibers, confirmed through immunostaining. Thin bony spicules also appeared inside some cysts, shorter and thinner than the surrounding trabecular bone, as if the body had attempted to rebuild and partially succeeded. The walls of the cysts showed dense trabecular bone with living cells on the bone surfaces, indicating active remodeling rather than dead tissue. This complexity matters clinically because it means subchondral cysts are not simply pockets of destruction. They are sites of ongoing, if disorganized, biological activity, which is part of why they respond to treatments that modulate bone remodeling and why regenerative approaches show promise.