Recent studies have revealed that following injuries, ligament tissues such as anterior cruciate ligaments (ACL), release large amounts of matrix metalloproteinases (MMPs). These enzymes have a devastating effect on the healing process of the injured ligaments. Although these enzymes are produced following ligament injuries, because of different healing capacities seen between the medial collateral ligament (MCL) and ACL, we were curious to find if the MMP activity was expressed and modulated differently in these tissues. For this purpose ACL and MCL fibroblasts were seeded on equi-biaxial stretch chambers and were stretched in different levels. The stretched cells were assayed using Zymography, Western Blot and global MMP activity assays. The results showed that within 72 h after injurious stretch, production of 72 kD pro-MMP-2 increased in both ACL and MCL. However, the ACL fibroblasts generated significantly more pro-MMP-2 than the MCL fibroblasts. Furthermore we found in ACL pro-MMP-2 was converted more into active form. With 4-aminophenyl mercuric acetate (APMA) treatment, large amounts of pro-MMP-2 were converted into active form in both. This indicates that there is no significant difference between ACL and MCL fibroblasts in post-translational modification of MMP-2. The fluorescent MMP activity assays revealed that the MMP family activities were higher in the injured ACL fibroblasts than the MCL. Since the MMPs are critically involved in extracellular matrix (ECM) turnover, these findings may explain one of the reasons why the injured ACL hardly repairs. The higher levels of active MMP-2 seen in the ACL injuries may disrupt the delicate balance of ECM remodeling process. These results suggest that the generation and modulation of MMP-2 may be directly involved in the different responses seen in ACL and MCL injuries.

In order to determine the appropriate load history for optimal remodeling of an anterior cruciate ligament graft, methods for establishing and measuring the graft force due to an external load could be set to a preselected value in in vivo are required. Our objectives with this study were to (a) develop a method in which the graft force due to an external load could be set to a preselected value in a living animal, (b) show that this force could be maintained after fixation, and (c) determine what happens to the forces after the animal has functioned for as long as 2 weeks postoperatively, when differing levels of load sharing between the segments had been set at surgery. The anterior cruciate ligament was reconstructed in 12 goats with use of a bone-patellar tendon-bone graft and a synthetic augmentation device. The forces in the graft segments were established, at the time of surgical fixation, with use of a force-setting technique. In five animals, the tendon segment was set to carry 90% of the total graft force; in the remaining seven animals, the augmentation segment was set to share 90% of the total graft force. Graft forces were measured, with the use of buckle transducers mounted extra-articularly over the anterior tibia, under a 67 N anterior tibial load at 60 degrees of knee flexion before and after fixation and at 2 weeks postoperatively.(ABSTRACT TRUNCATED AT 250 WORDS)

Prognostic level III. See Instructions for Authors for a complete description of levels of evidence.

It has been hypothesized that load affects the mechanical properties of an anterior cruciate ligament graft while it remodels. The goal of this study was to use an existing goat model to evaluate the effect of intraoperative set force on the postoperative mechanical properties of an autograft that had been augmented with a synthetic segment. The following questions were addressed. Do augmented autografts set with a high force intraoperatively have improved structural and material graft properties and lower anterior-posterior knee laxity at 3 months after surgery, compared with autografts set with a low intraoperative force? How do the structural and material properties of these implanted autografts compare with the mechanical properties of an intact anterior cruciate ligament or an unimplanted control autograft? The anterior cruciate ligament was reconstructed in seven goats with use of a composite graft consisting of a bone-patellar tendon-bone autograft and a synthetic augmentation device. A force-setting technique was used intraoperatively to establish the load-sharing between the autograft and augmentation segments such that the autograft carried either a high (16.5 N in four animals) or low (1.5 N in three animals) level of force, while the total force in the composite graft remained constant. Tensile testing was performed at 3 months after surgery to determine the material and structural properties of the autograft, the intact anterior cruciate ligament from the normal contralateral knee, and a control bone-patellar tendon-bone graft of similar size that was harvested from the contralateral knee at the time of necropsy and had never been implanted in the joint. The structural and material properties of the autografts initially set to high or low loads at surgery were not significantly different after 3 months of implantation. The strength and stiffness of the implanted tendons were an average of 24 and 20% of the strength and stiffness of the normal anterior cruciate ligament and 31 and 62% of the control tendons, respectively. Intraoperative set force in an augmented anterior cruciate ligament graft at the levels chosen in this study did not significantly affect weakening of the autograft at 3 months.

An existing goat model was used to measure in vivo graft forces during walking, to determine if the forces set at surgery change over time under the same external load and if the forces in the graft during in vivo function can be dictated by the forces set at surgery. The anterior cruciate ligament was reconstructed in 12 goats with use of a composite graft consisting of a bone-patellar tendon-bone autograft and a synthetic augmentation segment. The forces in the graft segments were established intraoperatively by a force-setting technique. In five animals, the tendon segment was set to carry 90% of the total graft force, and in the seven other animals, the augmentation segment was set to carry 90% of the total force. The total graft force was the same in all animals. Graft forces due to anterior tibial loads of 67 N were measured before and after fixation and 6 weeks after surgery with the use of buckle transducers mounted extra-articularly over the anterior tibia. They were also measured during straight, level walking at 6 weeks. The forces changed significantly from just after surgery to 6 weeks later, such that the initially set load-sharing was eliminated by 6 weeks. At 6 weeks, a relatively smooth gait had been achieved, and the maximum total graft force in each animal during walking averaged 35 N and was of similar magnitude to forces generated by the anterior tibial loads of 67 N with the animal anesthetized. After fixation, forces in the tendon graft segments were significantly different between the group with high set forces and that with low set forces. At 6 weeks, when functional joint loads were approaching normal levels, the graft segment forces for the two groups were not significantly different. Load-sharing between tendon and augmentation segment and load in the tendon segment at 6 weeks could not be dictated at surgery.