Repeated mechanical loading enhances the expression of Indian hedgehog in condylar cartilage

Indian hedgehog (Ihh) acts as a mechanotransduction mediator that converts mechanical strain into cellular proliferation and cartilage formation in mandibular condylar cartilage. The aim of this study was to examine the effect of repeated mechanical strain on the level of expression of Ihh and type II collagen mRNA in condylar growth. Two hundred and eighty 35 days old Sprague-Dawley rats were divided into 10 experimental and 10 control groups. Repeated mechanical loading was applied by advancing the mandible in a stepwise manner by fitting a stepwise bite jumping appliance. Animals were sacrificed together with their matched controls on 10 different time points. Total RNA was extracted from condylar cartilage immediately after dissection. Ihh and type II collagen mRNA was quantified using real-time PCR. In the experimental group, Ihh mRNA increased significantly on experimental day 7. Upon the second advancement, another peak was elicited 7 days later. Type II collagen showed a significant increase on days 21 and 44 of advancement. This indicated that mechanical loading in a repeated manner, triggers the expression of Ihh which in turn increases the number of replicating mesenchymal cells as well as the amount of the cartilage formed. Taken together these events increase condylar growth.