Anular incision technique on the strength and multidirectional flexibility of the healing intervertebral disc

Study Design This study used a sheep model to biomechanically analyze the healing strength of the anulus fibrosus after two types of anular incisions. Objective This study evaluated whether the type of anular incision made at the time of lumbar discectomy plays a role in the subsequent healing strength of the anulus and the biomechanical flexibility of the corresponding motion segment. Methods Two types of anular incision, a full thickness removal of a box or window of anulus and a full thickness straight transverse slit through the anulus, were made in the intervertebral discs of 18 adult sheep. After healing times of 2, 4, and 6 weeks, the intervertebral discs were tested versus control levels for strength of anular healing and biomechanical flexibility of the corresponding motion segment. Results The box incised discs showed a significantly greater loss in strength during the early healing phase (2 to 4 weeks) and a longer response before recovering anular strength when compared with the slitincised discs. The type of incision also affected the multidirectional flexibility of the motion segments in a differentiated manner. Larger amounts of motion were seen with the box incision when compared with the slit incision at all time periods and in all pure moments. Conclusion The technique of anular incision plays a definite role in the timing and strength of subsequent anular healing. The box incision through the anulus led to significantly weaker healing than did the slit incision in the early healing phase (2–4 weeks). Also, larger amounts of motion were seen in the vertebral motion segments of those discs undergoing box incision when compared with slit or control levels.