Characterisation of Tendon Tissue Used As Autograft In Reconstructive Knee Injuries: What causes the high failure rate in younger patients?

Abstract

Anterior cruciate ligament (ACL) tears are one of the most common ligamentous knee injuries. In New Zealand, the average number of cases is 1,650 per year, and the annual cost to the health system is estimated at NZ$18M. Arthroscopic surgical ACL reconstruction is a common treatment option for ACL tears, and a hamstring tendon autograft is often used. However, re-rupture rates reported globally for this surgical procedure are as high as 3.6%. A local review of ACL reconstructions performed between 2009-2012 (UniSports Clinic, Auckland) showed that younger patients (<20 years old) had a re-rupture rate of 10.1% compared to 2.5% in older patients (≥20 years old). The reasons for the higher re-rupture rate in younger patients are still unknown. Our study aim was to characterise the properties of hamstring tendon samples from patients of different ages. Identifying age-related differences in tendon properties could help explain the higher re-rupture rate of hamstring autografts in younger patients. Excess tissue of hamstring tendons harvested for ACL surgical reconstruction was collected for our study. The properties of tendons were compared between two age groups: patients under 20 year-old and patients who were 20 years or older. Biomechanical testing was performed using Instron 5543, and sections of the tendons were imaged by scanning and transmission electron microscopies and investigated by quantitative image analysis. The profile of genes expressed in tenocytes was analysed using Affymetrix GeneChip® PrimeView™ Arrays. A number of parameters were significantly different between the two age groups. Biomechanical testing showed that Young’s modulus (a measure of stiffness) of the tendons decreased with age. Electron microscopy demonstrated that younger patients had higher proportion of large collagen fibrils (>90nm), and greater length of the period banding structures (D-period) of the collagen fibers. Approximately 140 genes were differentially expressed between the groups (≥1.5 fold change, P<0.05). Eleven genes encoding proteins involved in adhesion were differentially expressed, with the proteoglycans (decorin, asporin and lumican) and the glycoprotein (fibronectin) found at higher levels in the younger group. Further investigations will assess the contribution of the age-related differences identified in our study to the propensity of tendons to re-rupture when used as autografts.