The intricate structures present at L5-S1 make any surgery at this level challenging. Before deciding on either an anterior or posterior approach, several anatomical considerations should be addressed, such as the internal iliac artery and vein, middle sacral artery and vein, sympathetic chain, lumbosacral trunk, and sigmoid colon and rectum, all of which lie directly over the sacrum. These structures should be evaluated on preoperative imaging prior to determining a surgical approach.²

The sacrum unites the two pelvic halves, and it has distinctive biomechanical features which enable the transition of axial load from the lumbar spine to the pelvis.³ The widest anterior-posterior diameter of the sacrum is 47 to 50 mm at S1 and it narrows 28 to 30 mm at the S2 level.²,⁴

The sacrum is mostly cancellous bone composed of five fused vertebrae, except the sacral alae and promontory which contain primarily cortical bone. The poor bony architecture of the sacrum makes instrumentation challenging. Instrumentation in this region may result in pseudarthrosis or hardware failure, which makes the lumbosacral junction more difficult to fuse than the rest of the lumbar spine.⁵, ⁶, ⁷

When comparing other aspects of the lumbar spine, the disk heights are similar between L4-L5 and L5-S1; usually, the anterior disk heights are 14 mm at L4-L5 and 13 mm at L5-S1, the posterior disk heights are 5.5 mm at L4-L5 and 4.5 mm at the L5-S1 level.⁸ Additionally, the width of the interlaminar space is the largest at the L5-S1 level, with an average of 31 mm (range: 21 to 40 mm).⁹

Finally, it is crucial to consider the different angles and the balance of curvature in the spine, as an adequate restoration of balance may further improve pain and function after surgery.¹⁰,¹¹

Of the different parameters, pelvic incidence is the most relevant.¹⁰ Pelvic incidence is a fixed anatomical parameter defined as “the angle between the line perpendicular to the sacral endplate at its midpoint and the line connecting this point to the midpoint of femoral heads axis.”¹⁰ Additionally, the pelvic incidence is the sum of the sacral slope and the pelvic tilt.¹⁰,¹² This parameter is closely correlated with lumbar lordosis and pelvic orientation, and is therefore an indicator for sagittal balance of the spine.¹² The most important parameter for spinopelvic balance is that the lumbar lordosis should be within 10 degrees of the pelvic incidence. The spine surgeon should aim to restore the normal pelvic incidence of each particular patient in order to avoid an unbalanced spine after any surgical procedure at the lumbopelvic junction. An excellent review by Mehta et al. ¹⁰ illustrates further the importance of pelvic incidence. Surgery at L5/S1 offers a unique opportunity to restore the patient’s balance if needed, and this should always be taken into consideration as some approaches allow better restoration than others.