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This study was carried out to test the hypothesis that improvements in ligament scar mechanical behavior during healing may be related, in part, to increases in collagen fibril diameters. Forty-eight adult female New Zealand White rabbits had standardized midsubstance gap injuries created in their right medial collateral ligaments (MCLs) and were allowed normal cage activity until sacrifice in groups of 12 at 3, 6, 14 or 40 weeks post-injury. Eight animals in each group had both MCLs tested biomechanically while 4 animals had transmission EM investigation of midsubstance collagen fibril diameters by a standardized protocol. Results of mechanical tests showed a three- to fourfold increase in scar strength and stiffness over the intervals of healing studied while there was no change in collagen mean fibril minimum diameters. These results demonstrate no correlation between material or structural properties of scar and collagen fibril diameters in this model of healing and suggest that other mechanisms for scar mechanical improvement under these conditions must be investigated.

In this investigation, the effect of initial end contact on medial collateral ligament (MCL) healing was studied in the rabbit model. Sixty-eight 1-year-old New Zealand white rabbits were separated into two groups. In one group, a 4 mm saggital Z-plasty was performed in the right MCL midsubstance (contact group), and in the other group, an analogous 4 mm midsubstance segment was removed (gap group). Left knees were unoperated to serve as internal contralateral MCL controls. Animals had unrestricted cage activity until sacrifice in groups of eight at 3, 6, 14, and 40 weeks postoperatively. Early results demonstrated that contact and gap injuries healed with what appeared to be scar tissue both morphologically and biomechanically. In both groups, laxities recovered to their contralateral values within 6 weeks and biomechanical viscoelastic behaviors recovered to 68-92% of contralaterals by 14-40 weeks. Despite these similarities, contacts showed morphological and biomechanical evidence of improved healing over gaps. Contact scars remodeled more quickly, recovered laxity more quickly, and were generally closer to contralaterals than gaps in terms of their structural strength, stiffness, and material behaviors, after 40 weeks of healing. With the exception of appearances and failure stress, all measured properties of contact healing MCLs were statistically indistinguishable from contralateral MCLs at 40 weeks of healing. These advantages of contact healing in this model support speculations that there are differences in the early rate and possibly in the later quality of ligament healing when cut rabbit MCL ends are in proximity. Longer-term studies to define end points and mechanisms of healing are required.

This study evaluated microscopic flaws in the healing rabbit medial collateral ligament and their significance in terms of the material properties of this ligament during healing. A gap injury was created in the midsubstance of the medial collateral ligament in the right hindlimb of 15 skeletally mature (12 months old) New Zealand White rabbits. At postoperative intervals of 3, 6, or 14 weeks, histomorphometric analysis of the flaws was carried out in subgroups of animals. The medial collateral ligaments from four of the left hindlimbs (randomly selected) were used as uninjured contralateral controls. In one histologic section of each area of scar tissue and the analogous area in the controls, specified tissue flaws (blood vessels, fat cells, hypercellular areas, loose matrix, disorganized matrix, or a combination of these) were measured by four independent and blinded observers. The results showed that the mean total area of the flaws, as a percentage of the total section, and the mean area of the largest flaw decreased with healing time in each healing group but did not achieve control values by 14 weeks. Because it was not possible to test the healing medial collateral ligaments mechanically prior to measurement of the flaws (due to the destructive nature of failure testing), the data on the flaws were compared with the material strength and stiffness of a separate series of similarly injured and mechanically tested medial collateral ligaments (data published previously). A maximum likelihood statistical analysis showed a very strong functional association between the mean area of the largest flaw and the stress at failure (p < 0.004) and between the mean flaw area as a percentage of the total section area and the elastic modulus (p < 0.001). This study therefore demonstrates that it is possible to quantify material flaws in scar tissue in rabbit medial collateral ligaments, that these flaws become smaller with healing time as the scar remodels, and that flaws are functionally associated with the material properties of the ligament in this model (larger flaws with less tensile strength and more flaws with less stiffness).

In this study, the short-term effects of immobilization on joint damage and medial collateral ligament (MCL) healing were investigated in unstable, anterior cruciate ligament (ACL)-deficient knees in rabbits. Forty-six 12-month-old female New Zealand white rabbits were separated into three groups. Animals from each group had surgery on their right knees: group I, sham controls (n = 9); group II, complete transection of the ACL and removal of a 4 mm segment (gap injury) of MCL midsubstance with no immobilization of the limb (n = 19); and group III, same injuries to the ACL and MCL (as group II) but with immobilization of the limb (n = 18). No surgical repair of disrupted ligaments was performed. Left knees served as unoperated contralateral controls. All animals were allowed unrestricted cage activity until sacrifice in subgroups at 3, 6, and 14 weeks of healing when biomechanical properties of all MCLs were measured. All knee joints were systematically examined for gross evidence of damage to articular cartilage, menisci, and periarticular soft tissues. To monitor relative in vivo loads on injured limbs during healing, hindlimb weight bearing was assessed at biweekly intervals. Results indicated that animals in both groups II and III bore relatively lower loads (compared to preinjury values) on their injured hindlimbs. Mechanical testing of MCLs showed only minor changes in sham controls, while group II and III healing MCLs demonstrated significantly lower force and stress at MCL complex failure compared to contralateral controls. In specific comparisons of group III to group II animals, we noted that immobilization prevented joint damage over the early intervals studied. In addition, immobilization resulted in MCL laxity similar to contralateral control values but inhibited development of structural strength and stiffness in healing MCLs. These results suggest that in the rabbit, short-term immobilization of an ACL-deficient knee offers some advantages to the joint and to certain low load behaviors of the healing MCL, but it also results in a smaller quantity of scar tissue that is less able to resist higher loads. Longer-term studies involving remobilization are necessary before the effects of brief immobilization on joint damage and MCL healing in this ACL-deficient model can be fully defined.

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