Loading of the rat ulna is an ideal model to examine
stress fracture healing. The aim of this study was to undertake a detailed examination of the histology, histomorphometry and gene expression of the healing and remodelling process initiated by
fatigue loading of the rat ulna. Ulnae were harvested 1, 2, 4, 6, 8, and 10 weeks following creation of a
stress fracture.
Stress fracture healing involved direct remodelling that progressed along the fracture line as well as woven bone proliferation at the site of the fracture. Histomorphometry demonstrated rapid progression of basic multicellular units from 1 to 4 weeks with significant slowing down of healing by 10 weeks after loading. Quantitative PCR was performed at 4 hours, 24 hours, 4 days, 7 days, and 14 days after loading. Gene expression was compared to an unloaded control group. At 4 hours after fracture, there was a marked 220-fold increase (P<0.0001) in expression of
IL-6. There were also prominent peak increases in
mRNA expression for OPG, COX-2, and
VEGF (all P<0.0001). At 24 hours, there was a peak increase in
mRNA expression for
IL-11 (73-fold increase, P<0.0001). At 4 days, there was a significant increase in
mRNA expression for Bcl-2, COX-1,
IGF-1, OPN, and SDF-1. At 7 days, there was significantly increased
mRNA expression of RANKL and OPN. Prominent, upregulation of COX-2,
VEGF, OPG, SDF-1, BMP-2, and SOST prior to peak expression of RANKL indicates the importance of these factors in mediating directed remodelling of the fracture line. Dramatic, early upregulation of
IL-6 and
IL-11 demonstrate their central role in initiating signalling events for remodelling and
stress fracture healing.