  Objective: To explore the ability of polarisation-sensitive optical coherence
tomography (PS-OCT) to rapidly identify subtle signs of tissue degeneration in
the equine joint.
  Design: Polarisation-sensitive optical coherence tomography (PS-OCT) images
were systematically acquired in four locations along the medial and lateral
condyles of the third metacarpal bone in 5 equine specimens. Intensity and
retardation PS-OCT images, and anomalies observed therein, were then compared
and validated with high resolution images of the tissue sections obtained using
Differential Interference contrast (DIC) optical light microscopy.
  Results: The PS-OCT system was capable of imaging the entire equine
osteochondral unit, and allowed delineation of the three structurally
differentiated zones of the joint, that is, the articular cartilage matrix,
zone of calcified cartilage and underlying subchondral bone. Importantly,
PS-OCT imaging was able to detect underlying matrix and bone changes not
visible without dissection and/or microscopy.
  Conclusion: PS-OCT has substantial potential to detect, non-invasively,
sub-surface microstructural changes that are known to be associated with the
early stages of joint tissue degeneration.
