Biomechanical effects of transthoracic microdiscectomy

Study Design. Nondestructive flexibility testing was performed to quantify biomechanical parameters of human cadaveric thoracic spines before and after microdiscectomy. Objectives. To assess the biomechanical differences between the normal thoracic spine and the thoracic spine after microdiscectomy and to determine whether microdiscectomy results in spinal instability. Summary of Background Data. Previous studies have investigated thoracic disc properties and the biomechanical effects of thoracic ligament or bone trauma. No studies were found assessing the effects of thoracic discectomy. Methods. Eight motion segments (T4-T5 to T11-T12) from five human cadaveric thoracic spines were studied before and after microdiscectomy. Three-dimensional motion was recorded in response to nondestructive, nonconstraining pure moments. Parameters measured included the neutral zone, elastic zone, range of motion, rotational flexibility, and instantaneous axis of rotation. Results. The neutral zone, elastic zone, and range of motion increased a small but significant (average P = 0.02 for range-of-motion increase) amount in all directions after thoracic microdiscectomy (mean bilateral range of motion increase, 2.1 degrees; range, 0.5-4.2 degrees). Flexibility increased slightly during lateral bending and flexion. The instantaneous axis of rotation location usually did not change, but sometimes shifted slightly away from the discectomy site after microdiscectomy. Conclusions. Thoracic microdiscectomy had small effects on the immediate mechanics and kinematics of the thoracic spine and did not overtly destabilize the motion segments.