© Springer-Verlag London 2016
Dominique G. Poitout (ed.)Biomechanics and Biomaterials in Orthopedics10.1007/978-1-84882-664-9_4040. Knee Ligamentoplasty: Prosthetic Ligament or Ligament Allograft?
(1)
Faculté de Médecine Nord, Sce Chirurgie Orthopédique et Traumatologie, Aix-Marseille Université, Centre Hospitalier et Universitaire Marseille North, Chemin des Bourrely, 13015 Marseille, France
(2)
Department of Orthopaedic Surgery and Trauma, Hopital Nord, Chemin des Bourrely, 13015 Marseille, France
Lesions of the central pivot of the knee are responsible for chronic disabling instabilities which, in the long term, lead to irreversible arthrotic destruction of the articular surfaces. Since 1980, we have been using synthetic ligaments and, more recently, since 1986, preserved human ligaments. The principle of ligamentoplasty is quite simply to replace torn ligaments with a prosthesis, aiming to reproduce the anatomical course of the ligament and its functional properties as faithfully as possible.
The main advantages of these techniques are:
Shortening of the time under surgery,
The other anatomical structures of the knee are not damaged,
And the possibility of early rehabilitation.
General Biomechanics
Basic Properties
A mechanical evaluation of an artificial ligament is only useful if it is compared with the properties of the corresponding human ligament.
The Parameters
The parameters evolve along the usual tension–elongation curve which determines the behavior of the ligament. Extreme stresses are only encountered in traumatological circumstances.
The rigidity of the ligament directly affects the function of the joint because if it is too rigid, it requires an intense muscular effort to mobilize the knee and risks causing tearing of the ligament or its attachments; if it is too weak, it will prevent the ligament from playing its stabilizing role.
Apart from rigidity, the main parameters therefore seem to us to be the values of strength and elastic lengthening which, more than the maximum values, represent the true tolerance limit of the ligament as well as the time it takes for the initial length of the ligament to recover after single stretching.
Measuring Methods
Two main methods can be used:
A specimen involving the ligament and the bone consists of removing knees from cadavers, and dissecting them until only the anterior cruciate ligament connects the two articular surfaces.
Another way of testing ligaments consists of attaching them directly to the inside of traction machines, using grips.
The problem with this technique is that with grips there are always phenomena which crush and lacerate the tissue, which can alter its elasticity or its strength, hence the preparation of a cone-shaped device which surrounds the ends of the ligaments fixed in acrylic resin.
Mechanical Properties of the Human Anterior Cruciate Ligament and of Prosthetic Ligaments
As far as the mechanical properties of the anterior cruciate ligament of a cadaver is concerned, it would appear that there is no significant difference between the results obtained on a frozen specimen after being frozen for several months.
On the other hand, the age of the subject displays a difference in the maximum average strength between a group of subjects approximately 60 years of age and a group of subjects approximately 20 years of age with the strength being double.
As far as the rate of elongation is concerned, it seems that the maximum strength of the human anterior cruciate ligament increases with elongation and it is often seen that although a slow rate leads to tearing due to bone avulsion, a rapid rate is more likely to lead to the ligament tearing in its middle part.
The Life of a Ligament
Two series of tests can be used in practice:
Cyclical tensioning tests,
Cyclical deformation tests.
As far as cyclical tensioning is concerned, the variations in tension experienced by the ligament during flexion–extension of the knee are reproduced, at the end of which each ligament has its residual elongation measured and a maximum elongation test is performed to assess the changes in the strength and rigidity of this ligament, compared with a new ligament.
When a cyclical deformation test is used, the resistance to repeated flexion is analyzed, combining constant, fixed-angle flexion of the ligament and continuous axial rotation of this ligament. This test therefore varies the points where maximum tension and compression forces are applied in a homogeneous manner on the periphery. At the end of this test, the ligament is subjected to a maximum elongation test.
Implantation in vivo in animals is certainly the method most used in research on synthetic ligaments. However, the frequency with which premature tears occur in the implant is regret-table as are the problems associated with applying the observations made in animal studies to man.
Biocompatibility and Rehabilitation
Biocompatibility
The Risks
In the case of ligament prostheses, general tolerance at a distance from the implant can take several forms, whether it be problems associated with general cytotoxicity, allergic reactions, toxicity specific to an organ, or teratogenicity.
As for local tolerance, it has to be dissociated from the local inflammatory reaction connected with the introduction of any kind of foreign body into the system. This reaction does not affect biocompatibility, insofar as it ends after a few days or a few weeks to make way for stabilization of the interface.
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