# Analysis of femoroacetabular impingement by a triade of label-free optical spectroscopy techniques

## TL;DR

This paper explains how non-invasive optical techniques reveal biochemical changes in hip joint disease, linking them to red cartilage staining.

## Contribution

Combining three optical spectroscopy techniques to non-invasively identify biochemical markers of femoroacetabular impingement.

## Key findings

- Elevated amide I levels indicate bone damage in FAI-affected cartilage.
- Increased tyrosine levels correlate with red staining due to pheomelanin formation.
- Optical spectroscopy reveals active bone regeneration and hardening in FAI.

## Abstract

Understanding the biochemical mechanisms of hip joint diseases remains challenging. Femoroacetabular impingement (FAI) is a condition where developmental deformities of the hip joint reduce mobility and cause tissue damage, with characteristic red staining observed in affected cartilage. Here we show how combining three complementary optical analysis techniques- laser-induced breakdown spectroscopy, Raman spectroscopy, and diffuse reflectance spectroscopy- reveals tissue alterations without invasive procedures. Our analysis identifies elevated amide I levels indicating bone damage, reduced hydroxypyroline showing active bone regeneration, and increased mineral components (phosphate and carbonate) reflecting bone hardening. These bone healing processes elevate tyrosine levels in the affected tissue. When exposed to air, this excess tyrosine rapidly oxidizes through enzymatic pathways to form pheomelanin, producing the distinctive red staining. This biochemical model explains both the compositional changes and visual characteristics of FAI-affected tissue, demonstrating how non-invasive optical techniques can elucidate complex disease mechanisms in joint pathologies.

FAI hip joint deformities cause red-stained cartilage. Using LIBS, Raman, and reflectance spectroscopy, we found bone damage/healing processes produce excess tyrosine. Upon air exposure, tyrosine oxidizes to pheomelanin, causing red staining.