What does an indentation over the knee bone indicate and how should it be evaluated and managed?

Indentation on Knee Bone: Evaluation and Management

An indentation over the knee bone most likely represents either osteochondritis dissecans (OCD) requiring imaging evaluation, or indicates cartilage softening/degeneration that warrants arthroscopic assessment with mechanical testing.

Differential Diagnosis and Initial Evaluation

The primary concern with a visible or palpable indentation over the knee bone is osteochondritis dissecans, a focal lesion involving both cartilage and underlying subchondral bone 1.

Imaging Approach

• Obtain plain radiographs first for any patient with knee symptoms and suspected bone/cartilage pathology to identify OCD lesions 1
• Follow with MRI if radiographs show lesions, to further characterize the OCD lesion depth, stability, and identify other knee pathology 1
• This stepwise imaging approach represents the AAOS weak recommendation based on systematic review of available evidence 1

Cartilage Softening and Mechanical Assessment

If the indentation represents cartilage surface changes rather than bony lesions:

Arthroscopic Indentation Testing

• Arthroscopic indentation devices can detect cartilage softening as an early mechanical sign of degradation not yet visible to the naked eye 2
• Normal cartilage stiffness varies by location: lateral femoral condyle is stiffest (5.6 ± 1.2 N), while medial tibial plateau is softest (2.4 ± 0.8 N) 3
• Indentation values on healthy-appearing cartilage adjacent to damaged areas are significantly lower than truly healthy surfaces, indicating subclinical degeneration 2

Ultrasound Indentation for Early Degeneration

• Ultrasound reflection coefficient (R_US) shows high sensitivity (0.77) and specificity (0.98) for detecting early cartilage degeneration without requiring site-matched reference values 4
• This makes ultrasound indentation particularly useful for diagnosing early osteoarthritis before visible changes occur 4

Clinical Context and Timing

ACL-Deficient Knees

• The number of cartilage lesions increases with time from initial ACL injury (r = 0.356, P = 0.011), though cartilage stiffness itself does not correlate with time from trauma 2
• Cartilage on damaged joint surfaces shows reduced indentation values even when appearing healthy 2

Management Implications

The detection of an indentation should trigger:

1. Immediate plain radiographs to rule out OCD 1
2. MRI if radiographs positive for lesion characterization 1
3. Consider arthroscopic evaluation with mechanical indentation testing if imaging is inconclusive but clinical suspicion remains high for early cartilage degeneration 2, 4
4. Quantitative assessment using small cylindrical indenters (1-mm diameter producing 300 μm deformation) for standardized stiffness measurement 3

Critical Caveats

• Indenter geometry matters: spherical indenters produce uniform deformation but slide easily, while cylindrical indenters create deformation gradients but provide better surface contact 5
• Small porous indenters evaluate proteoglycan matrix, while large solid indenters assess collagen network integrity 5
• Site-specific reference values are essential for mechanical testing, as femoral cartilage is generally stiffer than tibial or patellar cartilage 3
• Early collagen degradation may be the first sign of degeneration, making mechanical assessment particularly valuable before visible changes 5