Altering ligament water content affects ligament pre-stress and creep behaviour

Abstract

The water content of a ligament can be altered by injury and surgical intervention in vivo, and inadvertently or purposely during in vitro tests. We investigated how altering the water content of the rabbit medial collateral ligament (MCL) affected its resulting creep behaviour (defined as an increase in strain from sequential cyclic and static creep tests). The water content of normal MCLs ( n=4) was compared to that of MCLs soaked for 1 h in a sucrose solution ( n=4) or phosphate buffered saline (PBS; n=8). Sucrose exposure decreased hydration and PBS exposure increased hydration. In addition, soaking in PBS caused a shift in “ligament zero” (the position where there was 0.1 N of tension on the ligament). Following the same single solution treatment, additional MCLs were creep tested at 4.1 MPa using a load based on the ligament cross-sectional area measured before solution treatment: sucrose ( n=4), PBS new “ligament zero” ( n=5), and PBS old “ligament zero” ( n=6). Normal MCLs were also tested at 4.1 MPa ( n=7) in a humidity chamber that maintained normal ligament water content. Additional MCLs were treated with both solutions in series ( n=12) to examine the reversibility of the mechanical changes caused by single solution treatment. This was the first investigation to show that ligament creep behaviour was clearly affected by the initial state of hydration: creep decreased with decreased hydration and creep increased with increased hydration. Another unique finding was that ligaments with increased hydration had decreased ligament functional length and increased ligament pre-stress. The creep behaviour of these ligaments was decreased if they were loaded from the pre-stressed state compared to the unloaded state. These results suggest that maintenance of physiological water content is important for in vitro mechanical testing of ligaments and controlling the low-load stress state of ligaments in situ.