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

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.