Low-load behaviour of the patellar tendon graft and its relevance to the biomechanics of the reconstructed knee

Background Although the linear elastic (high-load) properties of patellar tendon graft used for anterior cruciate ligament (ACL) reconstruction are studied extensively, the low-load properties of the graft, associated with the toe region of its load–deformation curve, are largely neglected. The objectives of this study were to (i) determine the properties of the patellar tendon in the low-load region and compare to that of the intact ACL, (ii) assess the extent of variability in these properties and relate them to donor factors, and (iii) establish the theoretical importance of low-load properties to the function of the reconstructed knee. Methods The patellar tendons of 20 unpaired fresh frozen human knees (10 males and 10 females) were harvested. The central portions were trimmed, measured, and tested in tension to failure. The stress–strain curves were fitted with a bi-linear model and the graft properties were measured. Findings The low-load region modulus of elasticity, E0, of the patellar tendon, ranged from 49 MPa to 276 MPa in males and 63 MPa to 279 MPa in females. The strain at which the graft transitions from its low-load region to its linear elastic region (ε* not, vert, similar 4.6%) is on average above the strain levels experienced by the ACL during daily activities (ε not, vert, similar 4%). Donor sex, height and body mass were found to significantly affect some of the mechanical properties of the low-load region. Interpretation The results suggest that the low-load region properties of the patellar tendon, not the linear elastic properties, govern the biomechanics of the tibiofemoral joint during the early healing period. Also, a wide variability of graft properties in the low-load region exists partly due to certain donor factors. Clinicians should be aware of the low-load region behaviour of the graft and its influence on time = 0 behaviour of the reconstructed knee.