ACL graft constructs: In-vitro fatigue testing highlights the occurrence of irrecoverable lengthening and the need for adequate (pre)conditioning to avert the recurrence of knee instability
Issue title: Papers from the Regensburg Applied Biomechanics Symposium, June 2005
Guest editors: Joachim Hammerx and Michael Nerlichy
Affiliations: [a] Biomechanics Laboratories, Department of Materials and Medical Sciences, Cranfield University, Shrivenham, UK | [b] Gloucester Royal Hospital & Cranfield University Postgraduate Medical School, Gloucester, UK | [x] Mechanical Engineering Faculty, Laboratory for Materials Technology, University of Applied Science, Regensburg, Germany | [y] University Clinic, Department of Traumatology, Regensburg, Germany
Correspondence:
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Address for correspondence: Dr.~Peter Zioupos, Dept of Materials & Medical Sciences, Cranfield University, Shrivenham, SN6 8LA, UK. Tel.: +44 1793 785932; Fax: +44 1793 783076; E-mail: p.zioupos@cranfield.ac.uk.
Abstract: The performance of ACL grafts in both the short and long term is only as good as the condition of the graft at the time of surgery. If the graft lengthens under load at the two fixation ends incorporation will take longer to occur. Previous studies have shown that the various grafts currently used are strong enough. However, data on strength came primarily from quasistatic single pull to failure tests with, in some cases, modest cycling to precondition the grafts. The present study examined the in-vitro biomechanical behaviour of model ACL grafts, which have been fatigue cycled to failure over a wide range of loads in physiological ambient conditions. Load/deformation curves and the stretch of the grafts was continuously recorded until final rupture. The grafts demonstrated typical creep-rupture like behaviour with elongation (non-recoverable stretch) and loss of stiffness leading to gradual failure. Some of the graft designs were consistently shown to elongate up to 20 mm in length within the first 2000 cycles at moderate physiological loads and a further 10 mm of elongation occurred between the initial preconditioned state and just prior to complete rupture. Not enough attention has been paid previously to the likely long term elongation patterns of ACL grafts post-surgery and even after the usual empirical preconditioning has been performed by the surgeon. Increased graft dimensions may result in recurrent knee instability and may also lead to failure of the graft to incorporate. Preconditioning in-vitro may still be a way to remove some slack and prepare the graft for its operational environment by stiffening in particular the tissue/fixation interface for those grafts that use soft polymer fixation ends.
