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.