The effect of skeletal maturity on functional healing of the anterior cruciate ligament

Abstract

Background: The effects of skeletal maturity on functional ligament healing are unknown. Prior studies have suggested that ligament injuries in skeletally mature animals heal with improved mechanical properties. In this study, we hypothesized that skeletally immature animals have improved functional healing compared with skeletally mature animals.

Methods: Twenty-one Yucatan minipigs (eight juvenile, eight adolescent, and five adult animals) underwent bilateral anterior cruciate ligament transection. On one side, the ligament injury was left untreated to determine the intrinsic healing response as a function of age. On the contralateral side, an enhanced suture repair incorporating a collagen-platelet composite was performed. Biomechanical properties of the repairs were measured after fifteen weeks of healing, and histologic analysis was performed.

Results: Anterior cruciate ligaments from skeletally immature animals had significantly improved structural properties over those of adult animals at three months after transection in both the untreated and repair groups. Use of the enhanced suture technique provided the most improvement in the adolescent group, in which an increase of 85% in maximum load was noted with repair. The repair tissue in the adult tissue had the highest degree of hypercellularity at the fifteen-week time point.

Conclusions: Functional ligament healing depends on the level of skeletal maturity of the animal, with immature animals having a more productive healing response than mature animals.

Fig. 1

Schematic diagram depicting the primary suture repair with the collagen-platelet composite scaffold in place. Sutures were fixed proximally with an EndoButton device. The sponge was threaded onto four of the trailing suture ends (purple), which were then passed through the tibial tunnel and tied over a button to provide initial knee stability. The remaining two suture ends (green) were tied to the sutures in the tibial stump of the anterior cruciate ligament.

Fig. 2

Gross appearance of the healing anterior cruciate ligament in the juvenile, adolescent and adult age groups in both the untreated knees (top row) and the knees treated with suture repair augmented with a collagen-platelet composite (CPC) (bottom row). For the knees treated with the collagen-platelet composite, organized collagenous tissue was visible in the region of the anterior cruciate ligament and appeared more robust and less lax than that in the untreated contralateral knees.

Fig. 3

The maximum load in each age group as normalized by the maximum load of the intact anterior cruciate ligament for that age group. *The juvenile animals had a threefold greater normalized maximum load than the adult animals in the untreated (no Tx) group (p < 0.01). †For ligaments treated with collagen-platelet composite (CPC), both the juvenile and adolescent animals had higher normalized maximum loads than the adult group (p < 0.01 for both comparisons). §The addition of the collagen-platelet composite resulted in an 85% increase in maximum load in the adolescent group (p < 0.01).

Fig. 4

Histologic sections of representative areas of the repair tissue in the juvenile (A), adolescent (B), and adult (C) groups after fifteen weeks of healing. Note the increased cell density in the adolescent and adult specimens and the larger nuclei of the cells in the juvenile repair tissue (hematoxylin and eosin, ×400).