3-DIMENSIONAL SIMULATION OF HARRINGTON DISTRACTION INSTRUMENTATION FOR SURGICAL-CORRECTION OF SCOLIOSIS

Harrington distraction rod surgery on six female patients with idiopat hic scoliosis was simulated in three-dimensional osseoligamentous fini te element models with individual geometry taken from preoperative ste reo roentgenographic reconstructions of the spine and ribcage and comp ared with the measured outcome. Boundary conditions at the ends of the spine were used to maintain pelvis and head alignment. Published mate rial and flexibility properties were used. The amount of hook distract ion was calculated from measured changes in the distance between the h ook sites (range, 13-27 mm). Initial simulations underestimated the Co bb angle correction by an average 6%. They underestimated the spinal e longation by 36% and predicted an average 12-degrees increase in kypho sis angle compared with an actual 10-degrees average decrease. Agreeme nt for sagittal plane changes improved in five cases when the beams re presenting the motion segments were displaced posteriorly. In the sixt h case (with the rod applied over a lordotic spinal region), agreement was improved with the motion segment beams displaced anteriorly. The amount of the beam displacement that gave the best agreement was varia ble, and we were not able to predict it for each individual. Both meas ured and simulated changes in vertebral transverse plane rotations and in rib angulations were small. The greatest source of errors in these simulations appeared to be inadequate representation of in vivo motio n segment behavior by in vitro measured stiffness properties.