The potential benefits 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 identification 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.

On the other hand, the disadvantage to the convex approach is the inherent difficulty in approaching multiple disc spaces, worsening of the deformity with positioning and blind opening of the contralateral annulus. Given the trajectory of the disc spaces on the concave side, if one were to truly approach them perpendicularly, the required site of skin entry would be prohibitive. Thus, one must anticipate the degree of correction when planning the skin entry. Also, the surgeon cannot take advantage of the table break in correcting the deformity as the break actually worsens the coronal imbalance when approaching from the convex side.

Finally, the contralateral annulus (the concave annulus) must be completely opened to achieve adequate alignment. The concave annulus can only be opened blindly from the convex side with progressively wider dilators. Thus, the coronal correction could be hampered by an inadequate annulotomy on the concave side.

Many surgeons will simply approach the spine from the side on which L4/L5 can be accessed. This is often the most challenging level in the sense that the risk of neural injury is the greatest, the psoas muscle is the thickest, the pelvis is often in the way, and it is the most distal segment to be treated. The L4/L5 disc space is often asymmetric in cases of lumbar degenerative scoliosis. Thus, one can choose to approach the side that is most open and approachable given the relationship of the spine to the pelvis (Figs. 31.2 and 31.3).

The lateral surgeon must pay particular attention to anatomy not normally considered in spinal surgery. The size, shape, and location of the psoas muscle are important as the surgical corridor classically traverses this structure. In addition, the lumbosacral plexus is typically invested in this muscle, and avoidance of injury to this muscle also minimizes the risk of thigh numbness, dysesthesias, pain, and weakness. The finding of an anteriorly located muscle, called the “Mickey Mouse sign” or “rising psoas sign,” should dissuade the surgeon from taking a direct lateral approach. In these instances and at these spinal levels, an anterior, oblique, or posterior approach is preferable (Figs. 31.4 and 31.5).