Lumbar interbody fusion is a common procedure for treating lower back pain related to degenerative disc diseases. The Coflex-F is a recently developed interspinous spacer, the makers of which claim that it can provide stabilization similar to pedicle screw fixation. Therefore, this study compares the biomechanical behavior of the Coflex-F device and pedicle screw fixation with transforaminal lumbar interbody fusion (TLIF) or anterior lumbar interbody fusion (ALIF) surgeries. This study used a validated three-dimensional finite element model of the L1-L5 lumbar intact spine. To simulate TLIF surgery, left lateral total facetectomy and partial discectomy were performed at the L3/L4 segment of this intact model. An AVS-TL moon cage (Stryker Orthopaedics) was implanted in the L3/L4 segment and using a Coflex-F device or pedicle screw fixation. To simulate ALIF surgery, partial discectomy and total nuclectomy were performed at the L3/L4 segment of this intact model. A SynCage-Open cage (Synthes Spine, Inc.) was implanted into the L3/L4 segment and combined with a Coflex-F device or pedicle screw fixation. A 400 N follower load and a 10 Nm moment were applied to the intact model to mimic four physiological motions. All four implanted models were subjected to 400 N follower load and specific moments in accordance with the hybrid test method. The results show that the Coflex-F device combined with ALIF surgery can provide stability similar to that of pedicle screw fixation combined with TLIF or ALIF surgery. Also, both the posterior instrumentations (Coflex-F and pedicle screw fixation) combined with TLIF surgery had lower stability than combined with ALIF surgery.