Objective
To study the effect of pregnancy on knee joint contracture in the rat, because the laxity of peripheral joint increases during pregnacy.

Method
We evaluated the difference of contracture between the pregnant and non-pregnant female rats after 3 week of knee immobilization using a wire. The femorotibial angle at immobilization status was 30^o. After immobilization, the femorotibial angle were measured using X-ray after removal of wire at 50 g and 100 g weighted state on the tibia. To study the change of fibroblast in immobilized knee joint ligament, immunohistochemical staining for actin was performed using α-smooth muscle actin antibody (DAKO, Denmark).

Results
The femorotibial angles were 116.7⁑9.2^o and 97.3⁑18.0^o after wire removal, in the immobilized pregnant and nonpregnant rats respectively. The femorotibial angles at 50 g and 100 g weighted state were 136.8⁑7.2^o and 144.7⁑3.8^o, respectively in the immobilized pregnant rats and 129.4⁑12.7^o and 136.3⁑8.9^o in the immobilized non-pregnant rats. The angles of pregnant group were significantly larger than those of non-pregnant group (p＜0.05). In immobilized pregnant and non-pregnant rats, the α-smooth muscle actin was moderately expressed in fibroblast of anterior and posterior cruciate ligaments of the knee by immunohistochemistry, while there was no expression of α-smooth muscle actin in fibroblast of the ligaments in the non-immobilized knee.

Conclusion
Joint contracture develops to a lesser degree in pregnant rats than in non-pregnant rats and α-smooth muscle actin is expressed in fibroblast of contractured knee ligaments.