The Effect of a Ligament-Augmentation Device on Allograft Reconstructions for Chronic Ruptures of the Anterior Cruciate Ligament, Frank R. Noyes and Sue D. Barber (now Barber-Westin); Journal of Bone and Joint Surgery; August 1992, Volume 74-A, Number 7, pages 960-973. Comments: This landmark study deals with augmentation of natural ACL grafts with a synthetic rope-like strand, but it also provides very penetrating insight into the phenomenon of ACL-graft incorporation, a topic which is central to the ligamentation of a completely natural tendon-type ACL graft. (At the time this study was done, it was already known that a fully synthetic ACL graft [i.e. not just a synthetic reinforcement of a natural tendon graft] was biomechanically inferior to natural grafts. Noyes and Grood had published, in the early 1980s, a detailed study on various synthetic ACL-grafting materials which expounded on the problems engendered by completely artificial knee ligaments.) The authors of this 1992 study, Noyes and Barber (now Barber-Westin), both pioneering visionaries in the field of knee research, were the first to recognize that synthetic graft-augmentation materials, despite yielding promising short-term results, have no place in the knee. In this study, they foresightfully noted that there was absolutely no benefit generated by the use of a ligament-augmentation device. The reason such synthetic reinforcement caused problems was because of the "strain-shielding" effect: the synthetic portion did not have the elasticity of the tendon graft, and because the tendon graft must be subjected to gradually increasing physiological stresses in order to develop the parallel-aligned collagen fibres that are the hallmark of a ligament, the end result was that the implanted tendon graft never actually developed the requisite strength (albeit this unfortunate situation was often not detected until the synthetic ligament-augmentation device failed due to fatigue). The goal of using a synthetic material to augment a natural slice-of-tendon ACL graft was rooted in the desire to protect the natural graft from stretching out due to overly high tensile loadings (e.g. from an overly soon return to knee-demanding activities), and also to address the concern of the natural graft being weakened by necrosis (given that the freshly implanted graft has no blood supply). This means that, for a substantial number of weeks after installation, an ACL tendon graft (whether autograft or allograft) is weak and thus vulnerable to stretching-out if subjected to excessive loadings. Gradually, blood vessels (and later, nerve endings as well, albeit these can take a very long time to grow, and the neurological-connectivity issues are still being researched) grow into the dead-tendon graft. Cells begin to proliferate, as the graft serves as a scaffolding upon which a surrogate ligament gradually grows. This process, known as ligamentization or incorporation or remodelling, progresses rapidly from roughly the second month post-op to about the fifth month post-operatively, then proceeds more slowly for several years afterwards. The exact point at which the graft can be considered meaningfully alive is difficult to discern, because time is required for blood vessels to grow into the dead graft tissue. It is best to think of the dead-tendon-sliver graft as being converted incrementally to a fully living, surrogate ACL. So, there is no exact timeframe during which the graft is weak due to its necrosis. Also, the remodelling process does not occur at the same rate in every person. Typically, by about the 5-6-month mark, satisfactory surrogate-ligament strength has been obtained to enable the person to return to knee-demanding activities, but the exact return-to-sports timeline is something which must be decided on by the surgeon, with input from the physiotherapist. (The importance of the services of a knee-experienced physiotherapist is clear, given that a carefully supervised ramping-up of physiological stressing is needed in order to develop the aforementioned parallel-aligned collagen fibre structure of the surrogate ligament.)

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