Poor outcomes associated with anterior cruciate ligament reconstruction in pediatric patients, especially using the semitendinosus tendon (ST), are a major concern. Quadriceps tendon (QT) grafts have become more popular in the recent years due to the shortcomings with ST grafts. The tendons become mechanically stronger and harder with growth owing to increased collagen content, collagen cross-linking in the extracellular matrix of the tendon, and changes in the collagen fibril diameter distribution, as observed in animal models. There are only a few studies on tendon growth in humans and many unknowns. We hypothesized that one of the reasons for this is that aging changes the microstructure of the tendon and these changes vary with tendons. This study aimed to examine the effects of growth on the microstructural properties of the ST and QT.
The small amounts of ST and QT were harvested at the time of several knee surgeries in patients of various ages. We divided the samples into three groups based on the state of the epiphyseal plate at the distal femur of the patients as follows: the immature group (patients with open epiphyseal plates (of >1.5 mm thickness); the young group (patients <20 years of age, with closed epiphyseal plates); and the adult group (patients =20 years of age, with closed epiphyseal plates).
The samples were examined at ×8000 magnification under a transmission electron microscope.
Minimum collagen fibril diameters were measured from these cross-sections using Image J software. At least four slides from each sample were evaluated, and more than 100 collagen fibrils on each slide were evaluated.
The intra-observer reliability of the measurements for the collagen fibrils of tendon tissue, determined using the intraclass correlation coefficient (ICC), was 0.849 and 0.955, respectively. The inter-observer reliability was 0.949 and 0.991. All measurements were analyzed using IBM SPSS Statistics for Windows, version 24.0. This study was approved by the ethics committee at our hospital, and consent was obtained from the patients after being informing that data from their case would be submitted for publication.
The average age of the ST and QT groups was 11.8±3.7, 12.3±0.8 years in the immature group, 15.7±1.5, 15.8±1.0 years in the young group, and 37.7±5.3, 29.8±11.3 years in the adult group.
The average collagen fibril diameters of the ST and QT groups were 73.1±10.5, 89.7±14.4 nm (P = 0.04) in the immature group, 90.8±15.0, 94.8±16.4 nm (P = 0.67) in the young group, and 105.8±14.9, 107.2±12.1 nm (P = 0.85) in the adult group. The average fibril diameter in the immature group was significantly smaller compared with the adult groups in both tendons (ST: P=0.002, QT: P=0.001).
Both ST and QT had thickened collagen fiber diameter with age. QTs in the immature group had significantly thicker fiber diameters than STs.