Objectives To study the mechanical behavior of the posterior longitudinal ligament (PLL) to retropulsed fragment in spinal canal in different conditions of thoracolumber burst fracture. Methods 5 fresh human thoracolumber spine were tested in following conditions intact specimen injury specimen with lordosis injury specimen with lordosis and axial distraction injury specimen with kyphosis injury specimen with kyphosis and axial distraction. A probe containing a strain gauge tip was mimicked as a retropulsed fragment by passing through L1 body tunnel which was created previously to press PLL in different deepness. Results The indirect reduction strength of PLL without axial distraction was less than with axial distraction (p<0.05). There was not a significant difference statistically between lordosis and kyphosis after axial distraction.There was a good correlation between the degree of canal encroachment and indirect reduction strength of PLL. conclusions Regardless of the sagital plane angulation of the injuried spine the indirect reduction strength of retropulsed fragment in spinal canal by tension of PLL is mainly affected by axial distraction force It is importment to use the distraction device in management thoracolumber burst fracture in order to achieve indirect reduction of retropulsed fragment in spinal canal.