The effects of in situ freezing on the anterior cruciate ligament. An experimental study in goats
Jackson, D.W.; Grood, E.S.; Cohn, B.T.; Arnoczky, S.P.; Simon, T.M.; Cummings, J.F.
Journal of Bone and Joint Surgery. American Volume 73(2): 201-213
1991
ISSN/ISBN: 0021-9355 PMID: 1993715 Document Number: 369989
We developed an in situ freeze-thaw model designed to simulate an ideally placed and oriented autogenous graft of the anterior cruciate ligament. In this model, the anterior cruciate ligament was exposed, and the femoral insertion, tibial insertion, and body of the anterior cruciate ligament were frozen in situ with specially designed freezing probes. Freeze-thaw cycles were repeated five times. We used the technique in thirty-three mature goats to study the biological and biomechanical outcomes of the devitalized anbd devascularized anterior cruciate ligament at zero, six, and twenty-six weeks after treatment. Thus, the collagen fibers of the simulated autogenous graft remain in normal anatomical position and the simulated graft is fixed under physiological tension. At twenty-six weeks, no statistically significant difference were noted between treated and contralateral control (untreated) ligaments relative to anterior-posterior translation, maximum force to rupture, stiffness in the linear region of the force-length curve, modulus of elasticity in the linear region, strain to maximum stress, or maximum stress. The only statistically significant difference was an increase in cross-sectional area of the ligament. This increase was 22 and 42 per cent greater than that in the control ligaments at six weeks and six months. At six months, the ligaments in the control group had an average mid-cross sectional area of 17.7 .+-. 1.2 square millimeter and the ligaments in the experimental group, 25.2 .+-. 3.1 square millimeters. Changes in the size and density of the collagen fibrils also were demonstrated at six months. These observations are in sharp contrast to our previous studies of replacement of the anterior cruciate ligament, in which an allograft of the ligament or an allograft supplemented with a 3M ligament augmentation device (LAD; 3M, St. Paul, Minnesota) was used. In those studies, and average reduction in maximum strength of 75 per cent for the allografts and 50 per cent for the allografts that had a ligament-augmentation device was found at one year. We concluded that devitalized, devascularized anterior cruciate ligaments do not lose strength if the anatomical position and the orientation of the collagen fibers are not altered. Clinical Relevance: The loss in strength that occurs postoperatively in allografts of the anterior cruciate ligament may not be the natural sequale of the revascularization and healing process, but, rather, the consequence of improper orientation and tensioning of the graft. The specific events that are involved in the loss of strength need to be clarified. These events may range from simple stress-strain rupture of the allograft to the more complex initiation of an immune response in the host by the allograft. Techniques of implantation that precisely provide proper orientation and tensioning of the graft may minimize the loss of strength.