Patellar luxation is abnormal displacement of the patella from the femoral trochlear
groove. It is seen primarily in small breed dogs and causes pain and limited mobility of
the stifle joint. This study aimed to investigate the relationship among patellar
luxation, skin extension, and skin collagen fibril diameter. Nine dogs with patellar
luxation and five clinically normal dogs were enrolled in the study. We measured the skin
extension and investigated the ultrastructure of the skin and patellofemoral ligament by
histopathology and transmission electron microscopy. The mean skin extension in dogs with
patellar luxation was 18.5 ± 5.5% which is greater than the reference value (14.5%). Mean
skin extension in controls was 8.8 ± 1.7% and was within the normal range. In dogs with
patellar luxation, histopathology of the skin and patellofemoral ligament showed sparse
and unevenly distributed collagen fibers. Transmission electron microscopy identified
poorly organized, irregularly shaped, thin collagen fibrils. Collagen fibril thickness in
dogs with patellar luxation was significantly less than fibril thickness in controls
(P<0.001). There was a significant negative correlation (ρ= −0.863;
P<0.001) between skin collagen fibril diameter and skin extension.
Skin extension was correlated with patellar luxation and disease severity. Dogs with
patellar luxation, joint dysplasia, and hyperextensible skin appear to be pathologically
related. This might represent a phenotype of the Ehlers–Danlos syndrome, a hereditary
connective tissue disorder in humans.
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