The proposed benefi ts of approaching from the concave side of the spine include easier access to multiple discs from a single incision, better coronal alignment with positioning (when breaking the table), and a more complete open­ing of the concave annulus. Given the col­lapsed nature of the spine on the concave side and the corresponding favorable trajectory of the disc spaces, multiple discs can be accessed from a single incision on the concave side. Up to three intervertebral disc spaces can be approached from a single incision, while those same discs may not have been approachable from the convex side even with multiple inci­sions. Approaching from the concave side is also benefi cial since improved coronal align­ment can be achieved with bed positioning alone as the table can be broken toward the convexity. This cannot be done when approach­ing the convexity since breaking the table would actually worsen the coronal alignment. In addition, the ipsilateral annulus can be opened readily under direct visualization, thus improving the ipsilateral height increase with serial dilation.

The disadvantages of approaching on the concave side include a more diffi cult access to the disc space, a theoretical higher risk to the lumbar plexus, and a longer depth needed to reach the spine. On the concave side, the disc space is usually completely collapsed making it much more diffi cult to access. When the end­plates are touching, the surgeon must be extremely careful not to fracture into them dur­ing dilation. Greater downward forces are also needed to access the disc, thus increasing the risk of disrupting the endplates. This greater downward force may also increase the risk to the lumbar plexus. In addition, this approach places the surgeon at a greater distance from the spine making visualization and manipula­tion of instruments challenging.

The lumbar plexus is grouped closer together on the concave side and stretched as the patient’s deformity is being corrected. Placing a retractor on these nerves as they are stretched could theoretically lead to increased neurologic compromise versus the concave side where the stretched nerves are being relaxed during correction. Scheer et al. per­formed a retrospective analysis of 32 patients who had a lateral lumbar approach for scoliosis and grouped the patients as convex versus con­cave based on the side of approach [ 17 ]. They had 17 patients approached from the concave side and 15 patients approached from the con­vex side. There was a higher incidence of post­operative neurological defi cit in the concave

The sequence of disc removal can be important when approaching from the concave side. Particularly when spanning the whole lumbar spine, it can be diffi cult to reach the cranial and caudal ends of the construct. If one begins with discectomy and cage placement in the apex (mid- lumbar spine), then the cranial and caudal disc spaces are pushed away from the incision as the spine lengthens. Thus, one should potentially prepare to treat the cranial and caudal discs fi rst. Managing the ends of the construct fi rst allows for the best chance to reach the most diffi cult discs (Fig. 31.1 ).

The potential benefi ts of approaching from the convex side of the curve include more thorough and easier discectomy, uniform distraction of the disc space with downward forces, relaxation of the retracted lumbar plexus during correction, and shallower approach to the spine. The disc space on the convex side is usually readily accessible and lends itself to a more complete discectomy. In turn, less downward force is needed for the dis­traction of the contralateral side. As the curve is corrected, theoretically the ipsilateral lumbar plexus would be relaxed and be less affected by the retractor. Finally, the depth to the spine is shal­lower on the convex side making visualization and manipulation of the instruments less convoluted.

A convex approach also means that entry into the disc space duplicates a natural wedge to open the concavity of the spine. The larger annulus and reduced osteophytic overgrowth on the convexity of the spine also make identifi cation of the start­ing point easier. This minimizes the risk of inad­vertent violation of the endplate, which is a particular concern with osteoporotic patients. As rectangular cages are inserted, the disc space will be distracted in a more natural manner.