The effect of relaxin on the female anterior cruciate ligament: Analysis of mechanical properties in an animal model.
BACKGROUND
The peptide hormone relaxin, found in pregnant and non-pregnant females, has been shown to have collagenolytic effects on ligamentous tissue. Relaxin receptors have recently been identified on the human female anterior cruciate ligament (ACL). Relaxin may affect the load bearing properties of the female ACL and contribute to non-contact ACL injuries.


OBJECTIVE
The administration of recombinant relaxin+/-estrogen will lead to a significant decrease in ACL integrity in the guinea pig model.


METHODS
Controlled laboratory study.


METHODS
Adult female guinea pigs were divided into three experimental groups. Group 1 (n=4) was administered only 20 microg/h of recombinant porcine relaxin for 3 weeks. Group 2 (n=4) was administered 20 microg/h of recombinant porcine relaxin and 5 microg/h of estradiol for 3 weeks. Group 3 (n=4) served as both a normal control before surgical transection of the ACL and a positive ACL tear control after transection. All hormones were administered using separate implanted osmotic pumps. ACL laxity was tested by implanting radio-opaque markers in the femur and tibia of each leg. After applying a standard anterior force (22 N), the distance between markers was measured radiographically at day 0 and day 21. The animals were then sacrificed and the ACL's were analyzed for load-to-failure using a material testing machine.


RESULTS
Load-to-failure testing indicated that animals treated with relaxin only had significantly weaker ACL's (micro=40.4 N, p=0.001) compared to controls (micro=64.1 N). The relaxin+estrogen group (micro=32.7 N) was also significantly weaker than controls (p=0.007). There were no statistical differences between relaxin and relaxin+estrogen groups. Both relaxin only and relaxin+estrogen groups showed an increase in anterior translation of the tibia after 3 weeks of infusion, but it did not achieve statistical significance.


CONCLUSIONS
Relaxin significantly alters the mechanical properties of the ACL in an animal model.


CONCLUSIONS
The effects of relaxin, possibly in conjunction with estrogen, may contribute to a comprehensive etiology for human female non-contact ACL injuries.
Dragoo
Jason L
JL
Department of Orthopaedic Surgery, Stanford University, Palo Alto, CA 94305, United States. jasondragoo@yahoo.com
Padrez
Kevin
K
Workman
Rosemary
R
Lindsey
Derek P
DP
D000818:Animals; D016118:Anterior Cruciate Ligament / Q000502:physiology*; D000070598:Anterior Cruciate Ligament Injuries; D001696:Biomechanical Phenomena; D004195:Disease Models, Animal; D004958:Estradiol / Q000008:administration & dosage / Q000502:physiology; D005260:Female; D006168:Guinea Pigs; D007261:Infusions, Intra-Arterial; D007593:Joint Instability / Q000503:physiopathology*; D011994:Recombinant Proteins; D012065:Relaxin / Q000008:administration & dosage / Q000502:physiology*
D011994:Recombinant Proteins; D004958:Estradiol; D012065:Relaxin