The effects of initial end contact on medial collateral ligament healing: A morphological and biomechanical study in a rabbit model

Abstract

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.

Healing ligaments have decreased cyclic modulus compared to normal ligaments and immobilization further compromises healing ligament response to cyclic loading

“…Scars were tested at a 30'%, stress-strength ratio, in the linear-region of the MCL scar stress-strain curve [4,13], thus, quite probably loading the complete cross-section of the scars. Equivalent stresses, however, were only in the toe-region of the normal MCL stress-strain curve [15], where not all fibres in the cross-section might be recruited [17].…”

“…0 f 0 . 5 mm gap created in the midsubstance of the right MCL [4]. The resulting unilateral MCL scars were harvested at 3 weeks 0 7 = 8).…”

“…To compare the creep properties of these scars that have increasing failure strength with healing [4,13], creep test stresses were a constant 30'%1 of the unilateral scar failure strength. We had previously confirmed that the failure strength of unilateral and bilateral MCL gap scars that had no immobilization were similar [14].…”

Strength of medial structures of the knee joint are decreased by isolated injury to the medial collateral ligament and subsequent joint immobilization

“…All groups underwent the same mechanical testing protocol, shown previously to characterize the mechanical behaviour of injured and uninjured control ligaments [6]. After joints were mounted at 70" flexion in an Instron 1122 materials testing machine (Instron, Canton, MA, USA), two cycles between 5 N compression and 2 N tension at 1 mm/min were then performed, determining the position where the medial tissues first resisted tensile distraction.…”

“…Studies of the rat medial collateral ligament (MCL) used healing times on the order of 2 weeks typically, but some were as short as 1 week [2,4,7,13,14,16,22]. Studies of the rabbit MCL covered a large range of healing times from 3 to 104 weeks (2 years) [3,5,6,[8][9][10][11]. Likewise, studies of the canine MCL spanned 6 to 48 weeks (1 year) [12,15,21].…”