The Hidden World of Cartilage

The Invisible Healer

Imagine a surgical light that sees into cartilage, guiding repairs at the cellular level. At 365 nanometers (nm)—a specific wavelength of ultraviolet A (UVA) light—scientists are unlocking the structural secrets of articular cartilage, the slippery tissue cushioning our joints ¹ . Unlike its cousin 395 nm UV (which emits a violet glow), 365 nm penetrates deeper with minimal visible light interference, making it ideal for probing biological tissues ^{5 7} . This article explores how researchers harness this "invisible healer" to understand cartilage degeneration and design next-generation therapies for osteoarthritis, a condition affecting over 595 million people globally ⁹ .

Why 365 nm? The Science of Selective Light

The UVA Advantage

Ultraviolet light spans 100–400 nm, divided into UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm). UVA—particularly at 365 nm—offers unique benefits:

• Deeper Penetration: It scatters less in biological tissues than shorter wavelengths, enabling it to reach subsurface structures ^{1 6} .
• Biochemical Safety: As the least cytotoxic UV type, it minimizes damage to living cells during medical procedures ¹ .
• Precision Fluorescence: Many biomaterials and cartilage components absorb 365 nm optimally, triggering diagnostic fluorescence without excessive heat ^{5 7} .

Cartilage Under the UV Microscope: Structure Meets Light

A Layered Fortress

Articular cartilage resembles a multi-layered shield:

1. Superficial Zone: A dense collagen "armor" resisting shear forces.
2. Middle Zone: Proteoglycans (PGs) trap water for shock absorption.
3. Deep Zone: Collagen fibers anchor cartilage to bone ^{1 3} .

Light's Journey Through Cartilage

When 365 nm UV enters cartilage, three factors dictate its path:

1. Collagen Scattering: Tightly packed surface collagen scatters light, reducing penetration.
2. Proteoglycan Absorption: PGs absorb UV, converting it to heat.
3. Water Content: Swelling (e.g., in arthritis) dilutes the extracellular matrix (ECM), increasing light transmission ^{1 3} .

Cartilage Zones

The three distinct layers of articular cartilage each interact differently with UV light.

UV Light Analysis

Researchers use specialized UV equipment to study light transmission through cartilage samples.

The Decisive Experiment: Mapping UV's Path Through Cartilage

Methodology: Slicing, Swelling, and Shining Light

In a landmark 2020 study, researchers investigated 365 nm transmission in bovine knee cartilage ¹ :

1. Sample Prep: 100 μm-thick cartilage slices were cut from superficial, middle, and deep zones.
2. Swelling Simulation: Sections were soaked in phosphate-buffered saline (PBS) to mimic arthritic tissue expansion.
3. UV Exposure: A 365 nm LED light source irradiated samples while sensors measured transmitted light.
4. ECM Analysis: Collagen, PG, and water content were quantified post-exposure.

Results: Depth and Hydration Matter

• Depth Dependency: UV transmission rose from superficial to deep zones. Attenuation coefficients (measuring light loss) were 3× higher at the surface due to collagen density ¹ .
• Swelling Effect: PBS-soaked cartilage transmitted 40% more UV than native tissue. Water diluted ECM components, reducing scattering ^{1 3} .
• Scattering vs. Absorption: Scatter coefficients dominated, being 10× higher than absorption coefficients—confirming collagen's role as the primary "light blocker" ¹ .

The Scientist's Toolkit: Essentials for Cartilage-UV Research

Key reagents and tools enabling these insights:

From Lab to Operating Room: Medical Breakthroughs

Photocrosslinkable "Band-Aids" for Cartilage

Injectable hydrogels like GM-HPCH + TGFβ1 use 365 nm to form instant scaffolds in defects. In rats, this combo:

• Recruited stem cells 2.5× faster than natural healing.
• Reduced inflammation by shifting macrophages to regenerative (M2) type ⁴ .

3D-Printed Cartilage Organoids

Bioprinted constructs (e.g., DNA-SF hydrogel) are cured with 365 nm to create living cartilage mimics. These organoids:

• Express 300% more collagen II (healthy cartilage's main protein) than traditional grafts.
• Integrate seamlessly with host tissue in 8 weeks ⁹ .

UV-Guided Surgeries

Emergent techniques use 365 nm endoscopes to:

1. Detect Early Degeneration: Degraded collagen fluoresces brighter than healthy tissue.
2. Monitor Biomaterial Curing: Real-time tracking of hydrogel integration ^{3 6} .