Biomechanical Properties of Small-Size Hamstring Autografts

Abstract

Purpose To evaluate small-size hamstring (HS) autografts for biomechanical properties and determine a threshold diameter necessary for appropriate reconstruction. Methods In a controlled laboratory setting, biomechanical testing was performed upon 15 hamstring autografts. The grafts were divided into three groups by diameter, with five grafts each at diameter sizes of 6, 7, and 8 mm. Testing of the specimens was performed using an MTS 858 (Materials Testing System, Eden Prairie, MN). We determined load to failure by looking at the maximum load as well as the stiffness of the graft. Statistical analysis was performed via analysis of variance (ANOVA) testing with Tukey's post-hoc test and P-values set at 0.05. Results There was a significant difference in ultimate tensile strength for the different size grafts: 1990 +/- 302.42 N for 6 mm grafts (n=5), 2179 +/- 685.36 N for the 7 mm grafts (n=5), and 3074 +/- 781 N for 8 mm grafts (n=5). This was statistically significant for the group overall (p=0.039), as well as between the 6 mm and 8 mm grafts (p=0.044). Graft stiffness for the 6 mm grafts was 317 +/- 85 N (n=5), 288.6 +/- 66 for 7 mm grafts (n=5), and 428.053 +/- 83 for 8 mm grafts (n=5). This achieved statistical significance for the group overall (p =0.037) as well as between the 8 mm and 7 mm grafts. Conclusions The biomechanical data presented here demonstrate that graft diameter is highly correlated with ultimate tensile strength and stiffness. Clinical relevance When viewing this biomechanical data in conjunction with prior clinical data, consideration should be given for the supplementation of an HS autograft as the size decreases below 8 mm.

Keywords: acl repair; biomechanical properties; hamstring autografts.

Figure 1. MTS 858 used for graft testing

Testing of the specimens was performed using an MTS 858. The hamstring graft specimens were secured by the free end with clamps. They were then frozen into the clamps with dry ice before final tightening. Pictured is the block of dry ice holding the tendon secured to the hook. MTS 858: Materials Testing System, Eden Prairie, MN

Figure 2. Close-up of testing device construct

The hamstring graft specimens were secured by the free end with clamps. They were then frozen into the clamps with dry ice before final tightening. Pictured is a close-up of the block of dry ice holding the tendon secured to the hook.

Figure 3. Maximum tensile load vs graft diameter

Graph depicting the maximum tensile load identified versus the graft diameter

Figure 4. Graft stiffness by diameter

Graph depicting graft stiffness by diameter