STUDY DESIGN: An in vitro biomechanical comparison of 2 fusion techniques, anterior lumbar interbody fusion (ALIF) and transforaminal lumbar interbody fusion (TLIF), on cadaveric human spines. OBJECTIVE: To compare the immediate construct stability, in terms of range of motion (ROM) and neutral zone, of ALIF, including 2 separate approaches, and TLIF procedures with posterior titanium rod fixation. SUMMARY OF BACKGROUND DATA: Both ALIF and TLIF have been used to treat chronic low back pain and instability. In many cases, the choice between these 2 techniques is based only on personal preference. No biomechanical performance comparison between these 2 fusion techniques is available to assist surgical decision. METHODS: Twelve cadaveric lumbar motion segments were loaded sinusoidally at 0.05 Hz and 5 Nm in unconstrained axial rotation, lateral bending and flexion extension. Specimens were randomly divided into 2 groups with 6 in each group. One group was assigned for TLIF whereas the other group for ALIF. In the ALIF group, there were 3 steps. First, the lateral ALIF procedure with the anterior longitudinal ligament (ALL) intact was performed. Afterwards, the ALL was cut without removing the ALIF cage. Finally, another appropriately sized ALIF cage was inserted anteriorly. Biomechanical tests were conducted after each step. RESULTS: In the ALIF group, the lateral ALIF and subsequent anterior ALIF reduced segmental motion significantly (P=0.03) under all loading conditions. Removing the ALL increased ROM by 59% and 142% in axial rotation and flexion extension, respectively (P=0.03). The anterior ALIF approach was able to achieve similar biomechanical stability of the lateral approach in lateral bending and flexion extension (P>0.05) under all loading conditions. The TLIF procedure significantly reduced the range of motion compared with the intact state (P=0.03). However, no statistical difference was detected between the TLIF group and the ALIF group (P>0.05). CONCLUSIONS: Both ALIF and TLIF procedures combined with posterior instrumentation significantly improved construct stability of intact spinal motion segments. However, there was no statistical difference between these 2 fusion techniques. The 2 ALIF approaches (lateral and anterior) also had similar construct stability even though anterior longitudinal ligament severing significantly reduced stability.