To determine the incidence and the risk factors of adjacent segment degeneration（ASD）after
L4-5 fusion with posterior lumbar interbody fusion （PLIF）or transforaminal lumbar interbody fusion （TLIF）.
Of the 355 patients who underwent spinal fusion at the L4-5 level, the ASD occurred in 42 patients（54 segments），including 3 patients at L1-2 （5%）, 13 at L2-3 （24%）, 29 at L3-4 （54%）and 9 at L5S1 （17%）, associated with a total incidence of 11.8%. In term of involved segment, the L3、4 was the most susceptible level（P<0.001）. Although The incidences of ASD in the PLIF was higher than the TLIF, there was not statistical difference between them （14% versus 9.5%, P=0.189）. The cephalic ASD was statistically more common than the caudal ASD in both the PLIF （P<0.001）and TLIF（P<0.001）. The average elapsed time to ASD was 2 years after L4-5 fusion, however, no a statistical difference was noted between the PLIF and TLIF in time of ASD happened based on a Kaplan-Meier analysis （P=0.1559）.
The cumulative incidence of ASD requiring reoperation is 12% over a mean period of 2 years with the most susceptible level at the L3-4. Both the PLIF and TLIF have similar incidences of ASD.