Knee Subchondroplasty: MRI Findings of Bone Marrow Lesions, Medial Meniscus Root Tear, and Osteoarthritis

How to Recognize Subchondral Bone Marrow Lesions and Identify Postoperative Calcium Phosphate Injection on Knee Imaging

Edited by ScholarGen MediAI Team

Executive Answer

Knee subchondroplasty is a minimally invasive procedure designed to treat symptomatic subchondral bone marrow lesions associated with knee osteoarthritis and selected early insufficiency fractures. MRI is essential for identifying the target lesion, particularly subchondral bone marrow edema-like signal on fluid-sensitive sequences. In this case, right knee imaging demonstrates medial compartment osteoarthritis, a medial tibial plateau bone marrow lesion, a radial tear near the posterior root of the medial meniscus, and a Baker cyst. Following subchondroplasty, injected calcium phosphate appears as dense subchondral sclerosis on radiographs. Recognizing this expected postoperative appearance is critical to avoiding confusion with metastatic disease or other sclerotic bone lesions.

1. Why Knee Subchondroplasty Matters in Modern Musculoskeletal Imaging

Knee osteoarthritis is not exclusively a disease of articular cartilage. It is a whole-joint disorder involving cartilage degeneration, subchondral bone remodeling, meniscal dysfunction, synovial changes, and alterations in joint biomechanics.

Among these abnormalities, subchondral bone marrow lesions (BMLs) are particularly important because they are associated with pain and structural progression in patients with knee osteoarthritis.

A patient may present with persistent knee pain even when conventional radiographs demonstrate only relatively mild degenerative changes. In such situations, MRI can reveal abnormalities within the subchondral bone that are not visible on routine radiographs.

One treatment approach developed for selected patients with symptomatic subchondral bone abnormalities is knee subchondroplasty.

Subchondroplasty involves the image-guided delivery of a synthetic calcium phosphate bone substitute into the affected subchondral region. The intended purpose is to provide structural support to weakened subchondral bone while promoting subsequent bone remodeling.

For radiologists, understanding this procedure requires more than recognizing an injection-related opacity on postoperative radiographs. It requires the ability to:

Identify the original bone marrow lesion on MRI.
Evaluate associated osteoarthritis and meniscal abnormalities.
Understand the relationship between meniscal root dysfunction and subchondral mechanical stress.
Recognize the expected postoperative appearance of calcium phosphate.
Differentiate injected bone substitute from other causes of bone sclerosis.
Identify complications related to extraosseous or intra-articular material leakage.

This case illustrates how these findings should be integrated into a coherent musculoskeletal imaging assessment.

2. Clinical Presentation and Imaging Protocol

A woman in her mid-sixties presented with right knee pain and underwent radiographic and MRI evaluation.

The imaging examinations included:

Anteroposterior radiograph of the right knee.
Coronal T2-weighted fat-suppressed MRI.
Sagittal proton density fat-suppressed MRI.

Follow-up anteroposterior and lateral radiographs were obtained approximately six months after the initial evaluation and treatment.

The subsequent intraoperative fluoroscopic images demonstrate the subchondroplasty procedure, including cannula placement and injection of bone substitute material into the medial proximal tibia.

The clinical information does not provide detailed laboratory results, a quantified pain score, or a long-term functional outcome.

The principal imaging abnormalities are:

Medial tibiofemoral compartment joint-space narrowing.
Small marginal osteophytes.
Subchondral bone marrow edema-like signal in the medial tibial plateau.
Radial tear at the posterior horn–posterior root junction of the medial meniscus.
Displaced meniscal fragment.
Large Baker cyst.
Postprocedural dense calcium phosphate material in the medial proximal tibia.

These findings provide the basis for understanding both the original pathology and the subsequent postoperative imaging appearance.

3. Initial Radiographic Findings: Recognizing Medial Compartment Osteoarthritis

FIGURE 1 · RIGHT KNEE ANTEROPOSTERIOR RADIOGRAPH

Figure 1. Anteroposterior radiograph of the right knee demonstrating medial tibiofemoral joint-space narrowing and small marginal osteophytes.

The initial anteroposterior radiograph demonstrates several features of medial compartment osteoarthritis.

3.1 Medial Tibiofemoral Joint-Space Narrowing

The medial tibiofemoral joint space is narrowed compared with the lateral compartment. This finding reflects loss of articular cartilage thickness, although radiographic joint-space width is also influenced by meniscal position and integrity.

In this case, the medial meniscal abnormality identified on MRI provides an additional explanation for the altered joint mechanics.

3.2 Marginal Osteophyte Formation

Small marginal osteophytes are present along the joint margins. These represent a characteristic radiographic feature of osteoarthritis and reflect chronic remodeling of the joint.

3.3 Why Radiographs Are Not Sufficient

Conventional radiography is useful for assessing:

Joint-space narrowing.
Marginal osteophytes.
Subchondral sclerosis.
Subchondral cyst-like changes.
Advanced structural deformity.

However, radiographs cannot reliably characterize the extent of bone marrow lesions.

A patient with relatively modest radiographic osteoarthritis may have substantial subchondral abnormalities on MRI. Therefore, persistent focal knee pain should not automatically be attributed to the degree of joint-space narrowing.

Radiology insight: When symptoms appear disproportionate to radiographic osteoarthritis, MRI can provide important information about the subchondral bone and meniscal structures.

4. MRI Findings: The Critical Role of Subchondral Bone Marrow Lesions

FIGURE 2 · CORONAL T2 FAT-SUPPRESSED AND SAGITTAL PD FAT-SUPPRESSED MRI

Figure 2. MRI of the right knee demonstrating the principal abnormalities involving the medial compartment, subchondral bone, and posterior knee.

MRI is the key imaging modality for evaluating symptomatic subchondral abnormalities in patients with knee osteoarthritis.

The most important finding in this case is a region of increased signal intensity within the subchondral bone of the medial tibial plateau on fluid-sensitive MRI.

4.1 What Is a Bone Marrow Lesion?

A bone marrow lesion is an MRI finding characterized by altered signal intensity within the bone marrow, usually adjacent to an articular surface in patients with osteoarthritis.

On fluid-sensitive sequences, including T2-weighted fat-suppressed imaging and proton density fat-suppressed imaging, these lesions typically appear as areas of increased signal intensity.

On T1-weighted images, they commonly demonstrate decreased signal intensity relative to normal fatty marrow.

The term bone marrow edema-like lesion is often preferred because the MRI appearance does not necessarily represent simple interstitial edema.

Histopathologically, these lesions may contain a combination of:

Microtrabecular fractures.
Bone marrow fibrosis.
Vascular alterations.
Necrotic changes.
Reactive bone remodeling.
Local inflammatory changes.

The precise histopathologic composition varies among patients and cannot be determined from MRI signal alone.

4.2 MRI Appearance in This Case

FIGURE 3 · CORONAL T2-WEIGHTED FAT-SUPPRESSED MRI

Figure 3. Coronal T2-weighted fat-suppressed MRI demonstrating increased signal intensity in the subchondral region of the medial tibial plateau, consistent with a bone marrow lesion.

The coronal T2-weighted fat-suppressed image demonstrates a conspicuous hyperintense area in the subchondral medial tibial plateau.

This abnormality is located immediately beneath the articular surface and corresponds to the region of medial compartment degenerative disease.

The imaging pattern is consistent with a subchondral bone marrow lesion.

The important diagnostic considerations are:

The abnormality is centered in the subchondral bone.
Its location corresponds to the mechanically stressed medial compartment.
It is visible on a fluid-sensitive MRI sequence.
It occurs in association with medial compartment osteoarthritis.
It is accompanied by a medial meniscal root-region tear.

These findings support a mechanically related subchondral abnormality in the setting of degenerative knee disease.

4.3 Why Bone Marrow Lesions Matter Clinically

Bone marrow lesions are clinically important because they are associated with pain and structural deterioration in knee osteoarthritis.

Articular cartilage is aneural, whereas the subchondral bone contains sensory nerve fibers. Changes in subchondral bone structure, intraosseous pressure, and local inflammatory activity may contribute to pain generation.

Consequently, cartilage degeneration alone does not explain every patient's symptoms.

The subchondral compartment deserves particular attention when evaluating patients with persistent weight-bearing knee pain.

4.4 MRI Sequences for Bone Marrow Lesion Assessment

MRI sequence	Typical appearance	Diagnostic contribution
T1-weighted	Decreased signal	Demonstrates altered marrow composition
T2-weighted fat-suppressed	Increased signal	Sensitive to fluid-sensitive marrow abnormalities
Proton density fat-suppressed	Increased signal	Demonstrates marrow and associated soft-tissue abnormalities
STIR	Increased signal	Useful for detecting marrow edema-like abnormalities

Fat suppression is important because it reduces the high signal from normal fatty marrow, improving the conspicuity of abnormal fluid-sensitive signal.

Practical point: A systematic review of the subchondral bone on fluid-sensitive sequences should be part of every knee MRI examination performed for persistent unexplained pain.

5. Medial Meniscus Posterior Root Tear: A Major Biomechanical Contributor

One of the most important associated findings in this case is a radial tear at the junction of the posterior horn and posterior root of the medial meniscus.

FIGURE 4 · CORONAL T2-WEIGHTED FAT-SUPPRESSED MRI

Figure 4. Coronal T2-weighted fat-suppressed MRI demonstrating a radial tear near the posterior root attachment of the medial meniscus. The displaced meniscal fragment is also indicated.

5.1 What Is a Meniscal Root Tear?

The meniscal roots are the attachment sites that anchor the anterior and posterior horns of the menisci to the tibial plateau.

The posterior root of the medial meniscus plays an essential role in maintaining normal meniscal biomechanics.

An intact meniscus distributes axial loading through circumferential hoop tension. This mechanism helps convert compressive forces into circumferential tensile forces, reducing focal stress on the articular cartilage and subchondral bone.

A radial tear close to the posterior root attachment can interrupt this mechanism.

The functional consequences may resemble those of a meniscus-deficient knee.

5.2 Why Posterior Root Tears Increase Subchondral Stress

When the posterior root is disrupted, the medial meniscus may lose its ability to maintain normal circumferential tension.

This can lead to:

Meniscal extrusion.
Reduced effective contact area.
Increased focal contact pressure.
Greater loading of the medial tibial plateau.
Accelerated cartilage degeneration.
Increased mechanical stress on subchondral bone.

The relationship between meniscal root dysfunction and subchondral bone abnormalities is particularly important in older adults with medial compartment osteoarthritis.

A root tear should therefore not be considered an isolated meniscal finding.

It may be part of a larger mechanical process involving the meniscus, articular cartilage, and subchondral bone.

5.3 The Ghost Meniscus Sign

FIGURE 5 · SAGITTAL PROTON DENSITY FAT-SUPPRESSED MRI

Figure 5. Sagittal proton density fat-suppressed MRI demonstrating the absence of the normal meniscal body at the expected location of the posterior root-region tear, producing the ghost meniscus appearance. A large Baker cyst is also demonstrated.

The sagittal image demonstrates an important MRI finding known as the ghost meniscus sign.

Normally, the meniscus appears as a low-signal triangular structure on sagittal MRI images.

When the posterior root is disrupted, the normal meniscal structure may no longer be visible in its expected position on the sagittal image.

This produces an apparent absence of the meniscus, resembling a ghost.

The ghost meniscus sign is an important indirect indicator of a posterior root tear.

However, it should be interpreted together with coronal and axial images, because partial-volume effects, meniscal degeneration, and displaced fragments can complicate interpretation.

5.4 Displaced Meniscal Fragments

In this case, a displaced meniscal fragment is also identified on the coronal image.

The combination of:

A radial tear near the posterior root.
Displacement of a meniscal fragment.
Abnormal meniscal morphology.
Associated subchondral bone marrow signal abnormality.

provides a more complete explanation of the structural abnormalities affecting the medial compartment.

Radiologist's insight: When a medial tibial plateau bone marrow lesion is identified, the posterior root of the medial meniscus should be examined carefully. Focusing exclusively on the subchondral abnormality may overlook an important associated mechanical contributor.

6. Baker Cyst: An Additional Finding That Should Not Be Overlooked

The sagittal proton density fat-suppressed image demonstrates a large Baker cyst in the posterior aspect of the knee.

A Baker cyst, also known as a popliteal cyst, represents fluid distention of the gastrocnemius-semimembranosus bursa.

It is commonly associated with intra-articular knee abnormalities, including:

Osteoarthritis.
Meniscal tears.
Joint effusion.
Synovitis.

In patients with degenerative knee disease, increased intra-articular fluid can communicate with the gastrocnemius-semimembranosus bursa through a communication between the joint and bursa.

MRI Appearance

A Baker cyst typically demonstrates:

Fluid signal intensity.
A location between the semimembranosus tendon and medial head of the gastrocnemius.
A well-defined fluid-containing appearance.
Variable size depending on the degree of distention.

In this case, the cyst is visible as a substantial fluid collection in the posterior knee.

Clinical Significance

A Baker cyst may be asymptomatic or contribute to posterior knee discomfort and swelling.

Large cysts can occasionally rupture, producing fluid tracking into the calf and symptoms that may resemble deep venous thrombosis.

The cyst should therefore be documented, particularly when it is large or symptomatic.

However, the presence of a Baker cyst does not independently establish the cause of the patient's primary knee pain.

7. Integrated MRI Diagnosis and Clinical Interpretation

The imaging findings should be interpreted collectively rather than as unrelated abnormalities.

Imaging finding	Interpretation	Clinical significance
Medial joint-space narrowing	Osteoarthritis	Degenerative joint disease
Marginal osteophytes	Osteoarthritis	Chronic joint remodeling
Medial tibial plateau hyperintensity	Bone marrow lesion	Potential contributor to pain
Posterior root-region radial tear	Medial meniscal root injury	Impaired hoop-stress transmission
Displaced meniscal fragment	Meniscal structural disruption	Altered compartment mechanics
Ghost meniscus sign	Indirect sign of root-region tear	Important MRI diagnostic clue
Large Baker cyst	Popliteal cyst	Associated intra-articular pathology

The central diagnostic concept is the coexistence of medial compartment osteoarthritis, a subchondral bone marrow lesion, and medial meniscal root dysfunction.

This combination provides a biomechanical context for the patient's symptoms and explains why treatment may be directed toward the subchondral compartment.

8. Subchondroplasty: What Is the Procedure?

Subchondroplasty is a minimally invasive image-guided procedure intended to treat selected symptomatic subchondral bone marrow lesions.

The procedure uses a synthetic calcium phosphate bone substitute injected into the affected subchondral cancellous bone.

The material is designed to provide temporary structural support and serve as an osteoconductive scaffold during bone remodeling.

It is important to distinguish subchondroplasty from other orthopedic procedures involving bone cement.

8.1 Calcium Phosphate Bone Substitute

The material used in subchondroplasty is a synthetic calcium phosphate bone substitute.

It is not polymethyl methacrylate (PMMA).

The distinction is clinically and radiologically important.

Material	Typical clinical application
Synthetic calcium phosphate	Subchondroplasty and selected bone defects
Polymethyl methacrylate	Vertebral augmentation and selected orthopedic applications
Autologous bone graft	Bone reconstruction and fusion procedures
Glass polyalkenoate	Primarily dental applications

Calcium phosphate is selected for its bone-substitute properties and capacity to support bone remodeling.

8.2 Basic Procedural Technique

Subchondroplasty is performed using fluoroscopic guidance, generally with arthroscopic assessment as part of the operative approach.

The basic procedural sequence includes:

Preoperative identification of the target bone marrow lesion on MRI.
Planning of the access trajectory.
Percutaneous placement of a cannula into the target subchondral region.
Fluoroscopic confirmation of cannula position.
Injection of synthetic calcium phosphate bone substitute.
Assessment of material distribution and possible leakage.
Postprocedural imaging assessment.

The injection should be directed toward the previously identified abnormal subchondral region.

The volume and distribution of the material depend on the lesion and procedural planning.

The goal is not to fill the entire bone marrow compartment but to treat the selected target region.

9. Intraoperative Fluoroscopy: Recognizing Calcium Phosphate Injection

FIGURE 6 · INTRAOPERATIVE FLUOROSCOPY

Figure 6. Intraoperative fluoroscopic image demonstrating cannula placement within the medial proximal tibia during subchondroplasty.

FIGURE 7 · INTRAOPERATIVE FLUOROSCOPY

Figure 7. Intraoperative fluoroscopic appearance of the injected calcium phosphate bone substitute within the medial proximal tibia.

Fluoroscopy is essential for confirming the location of the delivery cannula and monitoring the distribution of the injected material.

In this case, the cannula is positioned in the medial proximal tibia, corresponding to the region of the previously identified bone marrow lesion.

The injected material appears relatively radiopaque compared with the surrounding cancellous bone.

Why Fluoroscopic Guidance Matters

Accurate localization is important for several reasons:

The injection must correspond to the target lesion identified on MRI.
The cannula must be positioned within the intended subchondral region.
Material distribution should remain within the planned osseous compartment.
Unintended extension toward the joint surface or surrounding soft tissues should be avoided.

Two complications of particular interest to radiologists are extraosseous leakage along the cannula tract and intra-articular leakage.

These findings should be actively assessed during procedural imaging and postoperative follow-up.

10. Postoperative Radiographs: The New Sclerotic Lesion

FIGURE 8 · SIX-MONTH FOLLOW-UP RADIOGRAPHS

Figure 8. Follow-up anteroposterior and lateral radiographs of the right knee demonstrating new dense sclerosis in the medial proximal tibia following subchondroplasty.

One of the most important educational points in this case is the appearance of the injected material on postoperative radiographs.

The follow-up examination demonstrates a newly developed dense sclerotic region in the medial proximal tibia.

This appearance is related to the calcium phosphate bone substitute injected during the procedure.

10.1 Why Does the Injected Material Appear Sclerotic?

Calcium phosphate bone substitute is relatively radiopaque.

After injection, it appears as an area of increased radiographic density within the treated subchondral bone.

On MRI, the injected material generally demonstrates low signal intensity on both T1-weighted and T2-weighted sequences.

The radiologist should recognize this expected postoperative appearance and correlate it with:

The original MRI lesion.
The location of the injection.
The intraoperative fluoroscopic images.
The timing of the procedure.
Previous radiographs.

The distribution of the radiopaque material should approximately correspond to the original MRI-defined bone marrow lesion.

This spatial correlation is one of the strongest clues to the correct interpretation.

10.2 Radiographic Appearance in This Case

FIGURE 9 · POSTPROCEDURAL RIGHT KNEE AP RADIOGRAPH

Figure 9. Postprocedural anteroposterior radiograph demonstrating dense calcium phosphate bone substitute within the medial proximal tibia, with associated medial compartment osteoarthritic changes.

The important finding is the dense subchondral opacity in the medial proximal tibia.

The opacity is located in the same general region as the original bone marrow lesion.

This finding, together with the known procedural history, supports the diagnosis of postoperative calcium phosphate deposition following subchondroplasty.

The radiologist should avoid interpreting this appearance as a newly developed primary bone lesion or metastatic deposit without reviewing the surgical history and prior imaging.

11. Differential Diagnosis of a New Sclerotic Bone Lesion After Knee Surgery

A new sclerotic lesion in an older adult requires careful interpretation.

Although postoperative calcium phosphate deposition is the correct explanation in this case, several other conditions can produce increased bone density.

The differential diagnosis includes:

11.1 Osteoblastic Metastasis

Osteoblastic metastases can produce focal or multifocal areas of increased bone density.

Common primary malignancies associated with osteoblastic skeletal metastases include prostate cancer and breast cancer.

Features that may support metastatic disease include:

Multiple skeletal lesions.
Distribution unrelated to a previous surgical target.
Progressive abnormalities on serial imaging.
Additional suspicious findings in patients with known malignancy.

However, imaging appearance alone may not always distinguish metastatic disease from postoperative material.

The surgical history and comparison with preoperative studies are essential.

11.2 Bone Island (Enostosis)

A bone island is a benign focus of compact bone within cancellous bone.

It usually appears as a homogeneous sclerotic lesion with characteristic thorny or brush-border margins.

Bone islands are generally stable over time.

A newly appearing, relatively dense opacity at a known subchondroplasty injection site is less likely to represent an incidental bone island.

11.3 Reactive Subchondral Sclerosis

Reactive subchondral sclerosis is a common feature of osteoarthritis.

It develops in response to chronic mechanical loading and remodeling of the subchondral bone.

Unlike the localized distribution of injected bone substitute, degenerative sclerosis typically follows the subchondral articular surface.

The location and morphology of the opacity are useful distinguishing features.

11.4 Osteoid Osteoma

Osteoid osteoma is a benign osteogenic tumor that typically occurs in younger individuals.

It commonly demonstrates a small nidus surrounded by reactive sclerosis.

The presence of a characteristic nidus, rather than a broad region of injected subchondral material, helps distinguish this condition.

11.5 Osteosarcoma

Osteosarcoma is a malignant primary bone tumor with variable radiographic appearances.

Aggressive features may include:

Cortical destruction.
Periosteal reaction.
Soft-tissue extension.
Aggressive mixed lytic and sclerotic changes.

The clinical context and imaging characteristics are substantially different from the expected postoperative appearance of subchondroplasty.

11.6 Polymethyl Methacrylate Injection

PMMA is another radiopaque bone cement used in selected orthopedic procedures.

It is particularly familiar in vertebral augmentation.

A history of prior cement injection is important because PMMA may also appear as a dense postoperative opacity.

The distinction between PMMA and calcium phosphate should be based on the actual procedure and material used, not radiographic density alone.

Differential Diagnosis Comparison

Diagnosis	Typical imaging clue	Important distinction
Calcium phosphate injection	Dense opacity at the treated subchondral site	Corresponds to prior MRI lesion and procedure
Osteoblastic metastasis	Focal or multifocal sclerosis	Clinical malignancy history and lesion distribution
Bone island	Homogeneous sclerosis with thorny margins	Usually stable over time
Reactive sclerosis	Subchondral density along joint surface	Associated with degenerative changes
Osteoid osteoma	Small nidus with surrounding sclerosis	Typically occurs in younger patients
Osteosarcoma	Aggressive bone destruction and/or sclerosis	Cortical and soft-tissue involvement may be present
PMMA injection	Dense cement-like material	Depends on the documented procedure

Key diagnostic principle: A new sclerotic focus in a patient with a recent orthopedic intervention should prompt a review of the procedural history before a neoplastic diagnosis is considered.

12. Complications of Knee Subchondroplasty: What Radiologists Should Monitor

Although subchondroplasty is minimally invasive, complications can occur.

Radiologists have an important role in detecting postoperative abnormalities and distinguishing expected procedural appearances from complications.

12.1 Calcium Phosphate Leakage Into Soft Tissues

One recognized complication is extravasation of calcium phosphate along the cannula tract into adjacent soft tissues.

This may appear as an unexpected radiopaque deposit outside the intended osseous compartment.

Extraosseous material may subsequently be associated with local irritation or heterotopic ossification.

Importantly, postoperative soft-tissue calcium phosphate deposition should not automatically be interpreted as dystrophic calcification or an independent soft-tissue mass.

The procedural history and distribution of the material are essential.

12.2 Intra-Articular Leakage

Another important complication is leakage of calcium phosphate into the joint.

This can occur if the injected material extends beyond the intended subchondral region.

Potential consequences include mechanical irritation and damage to adjacent articular structures.

The radiologist should carefully assess whether the injected material remains confined to the intended bone compartment.

12.3 Persistent Bone Marrow Abnormality

Persistent pain or abnormal MRI findings after subchondroplasty require careful reassessment.

Possible considerations include:

Persistent or recurrent bone marrow lesion.
Incomplete treatment of the target region.
Progression of underlying osteoarthritis.
Associated meniscal pathology.
Subchondral insufficiency fracture.
Other causes of postoperative knee pain.

Not every persistent MRI signal abnormality indicates procedural failure. Clinical correlation and comparison with baseline imaging are necessary.

12.4 Progression of Osteoarthritis

Subchondroplasty does not directly restore damaged articular cartilage or repair a torn meniscal root.

Therefore, underlying degenerative disease may continue to progress.

The procedure should not be described as a definitive cure for knee osteoarthritis.

Radiologists should continue to evaluate joint-space narrowing, osteophytes, subchondral remodeling, and other structural changes on follow-up examinations.

13. Clinical Indications and Patient Selection

The central indication for subchondroplasty is a symptomatic subchondral bone marrow lesion in an appropriately selected patient.

Potential clinical settings include:

Painful bone marrow lesions associated with knee osteoarthritis.
Selected early subchondral insufficiency fractures.
Persistent symptoms despite appropriate conservative management.

However, the presence of bone marrow edema on MRI alone is not sufficient to establish an indication for surgery.

13.1 Appropriate Imaging Evaluation

Before considering the procedure, the imaging assessment should establish:

Whether a definite subchondral bone marrow lesion is present.
Whether the lesion corresponds anatomically to the patient's symptoms.
Whether the lesion is associated with osteoarthritis.
Whether a subchondral insufficiency fracture is present.
Whether associated meniscal pathology contributes to abnormal loading.
Whether there is advanced structural joint destruction or another competing diagnosis.

13.2 Conditions That Require Particular Caution

Subchondroplasty is not intended as a treatment for every abnormality producing increased marrow signal.

Acute traumatic fractures, aggressive lytic bone lesions, and asymptomatic marrow abnormalities are not routine indications.

Advanced joint destruction may also influence whether a joint-preserving procedure is appropriate.

The ultimate treatment decision requires orthopedic assessment, clinical correlation, and consideration of alternative treatment options.

14. What Does Current Evidence Tell Us About Subchondroplasty?

The clinical literature on subchondroplasty has expanded, but its interpretation requires attention to study design and patient selection.

The original case-based discussion of this procedure should be considered alongside more recent clinical evidence.

14.1 Radiologist-Oriented Foundational Review

Agten and colleagues published an important review addressing the imaging characteristics and procedural considerations of subchondroplasty.

Their article emphasized the role of radiologists in identifying bone marrow lesions, recognizing postoperative calcium phosphate, and detecting complications.

Reference:

Agten CA, Kaplan DJ, Jazrawi LM, Burke CJ. Subchondroplasty: What the radiologist needs to know. AJR American Journal of Roentgenology. 2016;207(6):1257–1262. doi:10.2214/AJR.16.16521.

14.2 Prospective Clinical Evidence

A multicenter prospective observational study evaluated subchondroplasty in patients with symptomatic knee bone marrow lesions.

The study reported improvements in pain and functional outcome measures during follow-up.

However, conversion to knee arthroplasty remained an important outcome, demonstrating that subchondroplasty does not eliminate the possibility of subsequent joint replacement.

Reference:

Prospective Evaluation of Clinical Outcomes of the Subchondroplasty Procedure for Treatment of Symptomatic Bone Marrow Lesions of the Knee. 2024.

14.3 Systematic Review and Meta-Analysis

A systematic review and meta-analysis published in 2026 evaluated 12 studies involving 829 patients.

The authors reported improvements in patient-reported pain and functional outcomes following subchondroplasty.

However, they also emphasized important limitations, particularly relatively short follow-up periods and the absence of adequate control groups in the available literature.

Calcium phosphate leakage was the most frequently reported complication category in their analysis.

Reference:

Klincke V, Demoor B, Dehouwer F, Tampere T, Van Onsem S, Vermue H. Subchondroplasty for bone-marrow lesions in the osteoarthritic knee improves pain and function: a systematic review and meta-analysis. The Knee. 2026;61:104441. doi:10.1016/j.knee.2026.104441.

14.4 How Should These Findings Be Interpreted?

The available clinical evidence suggests that subchondroplasty may improve pain and function in selected patients with symptomatic bone marrow lesions.

Nevertheless, several questions remain important:

Which patients benefit most?
How should lesion size and location influence patient selection?
How should associated meniscal root tears be managed?
Does the procedure alter the long-term progression of osteoarthritis?
Does it consistently delay total knee arthroplasty?
Which MRI findings best predict treatment response?

These questions require further prospective comparative research.

The distinction between symptomatic improvement and modification of the natural history of osteoarthritis is particularly important.

A treatment may reduce pain without necessarily preventing cartilage loss or structural joint deterioration.

15. AI in Musculoskeletal Radiology: Can Artificial Intelligence Help Detect Bone Marrow Lesions?

The imaging findings in this case illustrate an area in which medical AI may eventually provide meaningful support: the detection and quantitative assessment of subtle musculoskeletal abnormalities.

MRI interpretation of knee osteoarthritis involves multiple interacting structures.

A radiologist must evaluate:

Articular cartilage.
Meniscal morphology.
Meniscal root attachments.
Subchondral bone.
Ligaments.
Joint effusion.
Synovium.
Popliteal structures.

This complexity creates opportunities for AI-assisted image analysis.

15.1 Automated Bone Marrow Lesion Detection

Deep learning models could potentially identify regions of abnormal marrow signal on fluid-sensitive knee MRI sequences.

A clinically useful system would need to distinguish between:

Subchondral bone marrow lesions.
Normal marrow heterogeneity.
Vascular channels.
Subchondral cysts.
Fracture-related abnormalities.
Other marrow-replacing processes.

An automated detection system could highlight suspicious regions for radiologist review.

However, increased signal intensity alone is not sufficient to establish a definitive diagnosis.

Clinical and anatomical context remain essential.

15.2 Meniscal Root Tear Detection

AI-assisted MRI interpretation could also help identify:

Radial tears.
Posterior root discontinuity.
Meniscal extrusion.
Displaced meniscal fragments.
The ghost meniscus sign.

Automated detection may be especially useful when subtle abnormalities are present across multiple imaging planes.

However, false-positive findings may occur when degenerative meniscal signal is mistaken for a definite tear.

15.3 Quantitative MRI and Longitudinal Monitoring

Another potential application is quantitative assessment of bone marrow lesions.

A future AI workflow could:

Segment the subchondral region.
Identify areas of abnormal fluid-sensitive signal.
Estimate lesion volume.
Compare the lesion with previous MRI examinations.
Assess changes following treatment.
Generate structured imaging measurements for clinical review.

Such tools could improve consistency in longitudinal assessment.

However, quantitative MRI measurements require standardized acquisition protocols, reproducible segmentation, and clinical validation before they can be used as reliable outcome measures.

15.4 AI Failure Modes

AI interpretation of postoperative knee MRI has specific challenges.

Potential AI failure	Clinical implication
False-positive marrow lesion detection	Normal or nonspecific signal may be labeled abnormal
False-negative detection	Small or subtle lesions may be missed
Incorrect anatomical localization	A lesion may be assigned to the wrong compartment
Confusion with postoperative material	Calcium phosphate may be misclassified
Domain shift	Performance may deteriorate across scanners and MRI protocols
Automation bias	Clinicians may over-rely on automated results

The most important principle is that AI should support, not replace, the radiologist's interpretation.

15.5 Integration Into PACS and Radiology Workflow

A practical AI-assisted workflow could be organized as follows:

MRI Acquisition

DICOM knee MRI sequences

PACS / AI Orchestration

Image routing and processing

AI-Assisted Analysis

Bone marrow lesion detection · Meniscal tear assessment · Segmentation

Radiologist Review

Image verification and clinical interpretation

RIS / EHR Reporting

Structured findings and clinical communication

The potential value of this workflow is not simply faster image interpretation.

It is the ability to standardize the identification, localization, and longitudinal documentation of abnormalities that may influence treatment decisions.

Any implementation should be evaluated for diagnostic accuracy, false-positive burden, interoperability, workflow integration, and human oversight.

Importantly, no AI performance claim can be made from this individual clinical case.

16. Diagnostic Risk: Why This Case Can Be Misinterpreted

This case illustrates several potential diagnostic pitfalls in musculoskeletal radiology.

Pitfall 1: Attributing All Symptoms to Osteoarthritis

Radiographic joint-space narrowing may encourage the reader to attribute knee pain entirely to degenerative joint disease.

However, an associated bone marrow lesion may contribute substantially to the clinical presentation.

Pitfall 2: Missing the Medial Meniscus Root Tear

A subtle radial tear near the posterior root may be overlooked if the examination focuses primarily on cartilage and bone marrow signal.

Careful evaluation in multiple planes is essential.

Pitfall 3: Misinterpreting the Ghost Meniscus Sign

Failure to recognize the absence of the normal meniscal structure on sagittal images may result in an incomplete assessment of meniscal root integrity.

Pitfall 4: Misdiagnosing Postoperative Calcium Phosphate

Dense subchondral opacity on postoperative radiographs may be mistaken for a new sclerotic bone lesion.

Reviewing the procedural history and preoperative MRI is essential.

Pitfall 5: Overlooking Material Leakage

The radiologist should not assume that all postoperative calcium phosphate remains within the intended subchondral region.

Extraosseous and intra-articular leakage must be considered.

Pitfall 6: Failing to Compare Previous Studies

Comparison with baseline imaging is particularly valuable in distinguishing pre-existing abnormalities from expected postoperative changes.

The most reliable interpretation combines the current imaging findings, procedural history, and longitudinal imaging comparison.

17. Practical Radiology Pearls

1. Bone marrow lesions are best demonstrated on fluid-sensitive MRI sequences. Coronal T2 fat-suppressed, proton density fat-suppressed, and STIR images are particularly useful.

2. Always evaluate the medial meniscus posterior root. A radial tear near the root can substantially alter medial compartment biomechanics.

3. Recognize the ghost meniscus sign. The absence of the expected meniscal structure on sagittal MRI can indicate a posterior root-region tear.

4. Do not overlook displaced meniscal fragments. Their identification adds important information about the severity and morphology of meniscal injury.

5. A Baker cyst is an associated finding, not necessarily the primary source of knee pain. Document its location and size when clinically relevant.

6. Calcium phosphate appears radiopaque after subchondroplasty. Dense subchondral sclerosis at the known injection site is an expected postoperative appearance.

7. Use spatial correlation. The postoperative material should approximately correspond to the original MRI-defined bone marrow lesion.

8. Look for extraosseous leakage. Soft-tissue deposition along the cannula tract is an important potential complication.

9. Assess for intra-articular leakage. Material extending into the joint should not be overlooked.

10. Do not equate symptom improvement with prevention of osteoarthritis progression. Subchondroplasty targets the subchondral compartment rather than directly restoring cartilage or repairing meniscal root integrity.

18. Frequently Asked Questions

Q1. What is knee subchondroplasty?

Knee subchondroplasty is a minimally invasive procedure that delivers synthetic calcium phosphate bone substitute into selected symptomatic subchondral bone marrow lesions. Its intended purpose is to provide structural support and promote bone remodeling.

Q2. What is the most important MRI finding before subchondroplasty?

The principal finding is a subchondral bone marrow lesion demonstrating increased signal intensity on fluid-sensitive sequences such as T2 fat-suppressed, proton density fat-suppressed, or STIR MRI.

Q3. What material is used in subchondroplasty?

A synthetic calcium phosphate bone substitute is used. It is different from polymethyl methacrylate, which is commonly associated with vertebral augmentation procedures.

Q4. How does calcium phosphate appear on postoperative radiographs?

It typically appears as a relatively dense radiopaque region within the treated subchondral bone. Its location should correspond approximately to the original MRI-defined bone marrow lesion.

Q5. What is the ghost meniscus sign?

The ghost meniscus sign is an indirect MRI sign of a meniscal tear, particularly a posterior root-region tear. It describes the apparent absence of the normal meniscal structure on the expected sagittal image.

Q6. Why is a medial meniscus posterior root tear important?

A posterior root tear disrupts normal circumferential meniscal tension and reduces the ability of the meniscus to distribute joint loading. This can increase focal stress on the medial tibial plateau and contribute to cartilage and subchondral bone abnormalities.

Q7. What are the major complications of subchondroplasty?

Important complications include calcium phosphate leakage into adjacent soft tissues and intra-articular leakage. Persistent symptoms and progression of underlying osteoarthritis also require postoperative assessment.

Q8. Can subchondroplasty prevent total knee replacement?

It may improve symptoms and potentially delay arthroplasty in selected patients, but it cannot guarantee avoidance of joint replacement. Long-term comparative evidence remains limited.

Q9. Can MRI distinguish postoperative calcium phosphate from metastatic disease?

MRI and radiographs provide useful information, but interpretation requires correlation with the operative history, original lesion location, material distribution, and previous examinations. Clinical context is essential.

Q10. Can AI detect knee bone marrow lesions?

AI-based MRI analysis has potential for automated lesion detection, segmentation, and longitudinal comparison. However, clinical implementation requires external validation, assessment of false-positive and false-negative findings, and radiologist oversight.

19. Educational Radiology Quiz

Test your understanding of the major imaging findings and procedural principles.

Question 1. Which MRI finding is most characteristic of a subchondral bone marrow lesion in this case?

A. Low signal intensity on T2-weighted fat-suppressed images

B. Increased signal intensity in the subchondral medial tibial plateau on T2-weighted fat-suppressed images

C. Diffuse cortical thickening

D. Multiple intraosseous gas collections

Question 2. Which associated meniscal abnormality is demonstrated?

A. Horizontal tear of the lateral meniscus

B. Bucket-handle tear of the lateral meniscus

C. Radial tear near the posterior root of the medial meniscus

D. Discoid lateral meniscus

Question 3. What is the most likely explanation for the newly developed dense opacity in the medial proximal tibia on follow-up radiographs?

A. Osteoblastic metastasis

B. Bone island

C. Postoperative calcium phosphate deposition

D. Osteoid osteoma

Question 4. Which material is used in knee subchondroplasty?

A. Polymethyl methacrylate

B. Synthetic calcium phosphate

C. Glass polyalkenoate

D. Metallic bone cement

Question 5. Which postoperative complication should the radiologist specifically evaluate?

A. Calcium phosphate leakage into surrounding soft tissues

B. Intra-articular calcium phosphate leakage

C. Both A and B

D. Neither A nor B

Answer key and explanations

Q1 — B: The medial tibial plateau demonstrates increased signal intensity on fluid-sensitive MRI, consistent with a subchondral bone marrow lesion.
Q2 — C: The coronal MRI demonstrates a radial tear at the posterior horn–posterior root junction of the medial meniscus.
Q3 — C: The newly developed dense opacity corresponds to the region treated with calcium phosphate during subchondroplasty.
Q4 — B: Synthetic calcium phosphate bone substitute is injected into the affected subchondral bone.
Q5 — C: Both extraosseous soft-tissue leakage and intra-articular leakage are important potential complications.

20. Key Takeaways

Knee subchondroplasty provides an important example of how musculoskeletal imaging can guide minimally invasive orthopedic intervention.

The essential diagnostic and procedural lessons from this case are:

Knee osteoarthritis involves the entire joint, including subchondral bone and meniscal structures.
MRI is essential for identifying subchondral bone marrow lesions that conventional radiographs may not show.
A medial meniscus posterior root tear can substantially alter load distribution within the medial compartment.
The ghost meniscus sign is an important indirect indicator of injury to the posterior root region.
A Baker cyst may coexist with degenerative intra-articular pathology.
Subchondroplasty involves injection of synthetic calcium phosphate into selected subchondral lesions.
The injected material produces a characteristic dense radiographic appearance.
Comparison with preoperative MRI and procedural imaging is critical when evaluating postoperative sclerosis.
Extraosseous and intra-articular calcium phosphate leakage are important complications to monitor.
AI may eventually improve the detection and quantitative assessment of musculoskeletal abnormalities, but human radiologist oversight remains essential.

Ultimately, the value of musculoskeletal radiology lies not simply in identifying individual abnormalities but in understanding their anatomical, mechanical, and clinical relationships.

In this case, the combination of osteoarthritis, meniscal root dysfunction, and subchondral bone marrow abnormality provides a coherent explanation for the imaging findings and the subsequent decision to perform subchondroplasty.

The radiologist's responsibility extends beyond preoperative diagnosis to include accurate recognition of postoperative material, identification of complications, and appropriate longitudinal imaging assessment.

References

[1] C. A. Agten, D. J. Kaplan, L. M. Jazrawi, and C. J. Burke, “Subchondroplasty: What the radiologist needs to know,” AJR American Journal of Roentgenology, vol. 207, no. 6, pp. 1257–1262, 2016. DOI: 10.2214/AJR.16.16521.

[2] S. B. Cohen, C. Hajnik, G. J. Loren, S. Akhavan, P. J. DeMeo, D. J. Wyland, T. Youm, L. M. Jazrawi, R. J. Daley, J. Farr, P. Reischling, and J. E. Woodell-May, “Prospective evaluation of clinical outcomes of the subchondroplasty procedure for treatment of symptomatic bone marrow lesions of the knee,” The Journal of Knee Surgery, vol. 38, no. 6, pp. 290–299, 2025. DOI: 10.1055/a-2501-0910.

[3] V. Klincke, B. Demoor, F. Dehouwer, T. Tampere, S. Van Onsem, and H. Vermue, “Subchondroplasty for bone-marrow lesions in the osteoarthritic knee improves pain and function: A systematic review and meta-analysis,” The Knee, vol. 61, Art. no. 104441, 2026. DOI: 10.1016/j.knee.2026.104441.

[4] Y. Tran, R. Pelletier-Roy, G. Merle, C.-É. Aubin, and M.-L. Nault, “Subchondroplasty in the treatment of bone marrow lesion in early knee osteoarthritis: A systematic review of clinical and radiological outcomes,” The Knee, vol. 39, pp. 279–290, 2022.
DOI: 10.1016/j.knee.2022.10.004.

[5] B. Di Matteo, G. Anzillotti, P. Conte, P. Angele, P. Emans, J. Minguell-Monyart, J. Woodell-May, M. Correa-Tapia, and E. Kon, “Subchondroplasty® (SCP) provides resolution of symptoms and functional improvements in mild-to-moderate knee osteoarthritis with persistent bone marrow lesions: 12-month follow-up results from a multicentric open-label prospective clinical trial,” Cartilage, vol. 16, no. 2, pp. 181–189, 2025. DOI: 10.1177/19476035241264011.

[6] S. B. Cohen and P. F. Sharkey, “Subchondroplasty for treating bone marrow lesions,” The Journal of Knee Surgery, vol. 29, no. 7, pp. 555–563, 2016.
DOI: 10.1055/s-0035-1568988.

[7] A. S. Levy and K. Cousins, “The rationale for and efficacy of subchondroplasty in the injured worker,” Journal of Orthopaedics, vol. 22, pp. 48–52, 2020.
DOI: 10.1016/j.jor.2020.03.047.

[8] P. Randelli et al., “Efficacy of subchondroplasty in the treatment of pain associated with bone marrow lesions in the osteoarthritic knee,” Orthopaedic Journal of Sports Medicine, vol. 11, no. 5, 2023. DOI: 10.1177/23259671231163528.

[9] J. Ochi, T. Nozaki, A. Nimura, T. Yamaguchi, and N. Kitamura, “Subchondral insufficiency fracture of the knee: Review of current concepts and radiological differential diagnoses,” Japanese Journal of Radiology, vol. 40, no. 5, pp. 443–457, 2022.
DOI: 10.1007/s11604-021-01224-3.

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