Characterization of Mechanical and Microstructural Properties of Pediatric Anterior Cruciate Ligaments and the Tendons used for Reconstruction

Purpose: A considerable amount of research has been devoted to the biomechanical characterization of knee tendons and ligaments in adults; however, relatively little is known about the pediatric population due to the rarity of these specimens. The clinical need for this data has grown along with the recent increase in diagnosed anterior cruciate ligament (ACL) tears in skeletally immature patients. This study sought to characterize the mechanical and microstructural properties of ACLs and the most common tendons used for reconstruction in the pediatric knee. Methods: Five fresh-frozen knee specimens from separate donors were used in this study (3M, 2F, average age 9.2 years). ACLs, patellar, quadriceps, and semitendinosus tendons were fine dissected from the knee and cut into dog-bone shapes. Cross sectional areas of each specimen were measured using a non-contact laser measurement system. Specimens were tested in a universal testing frame under axial tension with a protocol consisting of preconditioning, stress relaxation, and a ramp to failure. From these data, ultimate stress, ultimate strain, stiffness, Young's modulus, and strain energy density were determined. The ACL and tendons from the contralateral knee of one donor (female, age 9) were selected for histology and ultrastructural analysis. Specimens were stained with hematoxylin and eosin and cell densities were measured under bright field microscopy, while crimp patterns were observed using polarized light microscopy. Collagen fibril diameters were obtained from transmission electron microscopy (TEM) micrographs. Results: The patellar ligament exhibited mechanical properties that were most similar to that of the ACL, particularly for ultimate stress, ultimate strain, Young's modulus, and strain energy density (Table 1). The semitendinosus tendon provided the highest ultimate stress, but also had the lowest ultimate strain, while the quadriceps tendons provided the highest ultimate strain. The ITB was more densely populated by fibroblasts and also appeared to exhibit a higher crimp frequency than the other tendons but was similar to the ACL (Figure 1). Fibril diameters for the patellar ligament, quadriceps tendon, and semitendinosus tendon were distributed between 10 and 200 nm, whereas the fibril diameters for the ITB and ACL were more tightly distributed between 10 and 125 nm. Conclusion: The pediatric specimens studied here exhibited considerably weaker mechanical properties than what has been