Extracellular matrix content of ruptured anterior cruciate ligament tissue Kate Young a, ⁎, Tom Samiric b , Julian Feller c , Jill Cook d a Epworth Hospital Richmond, VIC 3121, Australia b Division of Health Studies, LaTrobe University, Bundoora, VIC 3086, Australia c LaTrobe University Medical Centre, Bundoora, VIC 3086, Australia d School of Exercise and Nutrition Sciences, Deakin University, 221 Burwood Highway, Burwood, VIC 3125, Australia abstract article info Article history: Received 2 December 2009 Received in revised form 10 May 2010 Accepted 13 May 2010 Keywords: ACL Rupture Collagen Proteoglycans Anterior cruciate ligaments (ACLs) can rupture with simple movements, suggesting that structural changes in the ligament may reduce the loading capacity of the ligament. We aimed to investigate if proteoglycan and collagen levels were different between ruptured and non-ruptured ACLs. We also compared changes in ruptured tissue over time. During arthroscopic knee reconstruction surgery 24 ruptured ACLs were collected from participants (10 females; 14 males; mean age 24 years). Four non-ruptured ACLs were obtained from participants undergoing total knee replacement surgery (one female, three males; mean age 66 years). Western blot analysis was used to characterise core proteins of aggrecan, versican, decorin and biglycan and glycosaminoglycan assays were also conducted. Collagen levels were measured by hydroxyproline (OHPr) assays. Significantly lower levels of collagen, were found in ruptured ACL compared to non-ruptured ACL (p = 0.004). Lower levels of both small and large proteoglycans were found in ruptured than non-ruptured ACLs. No correlation was found between time since rupture and proteoglycan or collagen levels. Ruptured ACLs had less collagen and proteoglycans than non-ruptured ACLs. These changes indicate either extracellular matrix protein levels were reduced prior to rupture or levels decreased immediately after rupture. It is possible that the composition and structure of ACLs that rupture are different to normal ACLs, potentially reducing the tissue's ability to withstand loading. An enhanced understanding of the aetiology of ACL injury could help identify individuals who may be predisposed to rupture. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Rupture of the anterior cruciate ligament (ACL) is common in active people; the incidence has been reported to be as high as 1 in 3000 people [1]. Sudden deceleration with a change of direction or single limb landing that results in valgus collapse of the knee places large strains on the ACL and can rupture the ligament [2]. Although large strains to the ACL are the common cause of its rupture, microscopic damage or ‘microtrauma’ to ligament tissue has been shown to occur at relatively low levels of strain [3]. Microscopic changes such as cellular damage and alterations to the organisation of the extracellular matrix modify the mechanical properties of ligaments. When strain is applied to a ligament with microstructural irregularities, rupture can occur [3]. The concept that microtrauma can exist in ACLs prior to rupture arises from the response of tendons to repeated strain. Both tendons and ligaments are dense, regularly arranged connective tissues [4] and tendinopathy, like ACL rupture, is a common problem within the active population. A central feature of tendinopathy is elevated levels of proteoglycans and collagen degradation [5]. Tendon studies show ruptured tendons are always histopathologically altered [6,7]. It is possible that athletes' ACLs that are under repeated stress exhibit similar histopathological changes prior to rupture. On a cellular level, repeated mechanical forces imposed on ligaments determine the level of specific proteoglycans present within the extracellular matrix of the tissue [8]. Both normal and injured ligaments are constantly remodelling in response to loading [9]. The composition of ligaments is affected by this remodelling, which in turn determines the ligaments susceptibility to failure. Surprisingly no studies were identified that investigated levels of collagen and proteoglycans in ruptured ACL tissue. Therefore, the primary aim of this study was to compare the total proteoglycan and collagen content of non-ruptured and ruptured ACLs. In addition, we aimed to compare the proteoglycan species present in non-ruptured and ruptured ACLs. Based on the tendinopathy model, it was hypothesised that the ruptured ACL tissue would have greater amounts, and different types, of proteoglycans than non-ruptured ACL tissue, and that collagen content would be adversely affected by the change in proteoglycans. The Knee 18 (2011) 242–246 ⁎ Corresponding author. 5/402 Toorak Rd, Toorak, VIC 3142, Australia. Tel.: + 61 3 0438 627 273. E-mail address: kateayoung@yahoo.com.au (K. Young). 0968-0160/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2010.05.008 Contents lists available at ScienceDirect The Knee