Overview of the back and spine

Surgical surface anatomy

Surface anatomy plays an important role in achieving adequate surgical exposure of the spine but its limitations must be appreciated. In the setting of revision surgery, previous scars or even palpable hardware may be enough to localize the starting point of the selected approach. When using apparently reliable landmarks, it may be reasonable to confirm localization with fluoroscopic imaging prior to making an incision or widening an established exposure. There are surface landmarks for nearly every cervical vertebra. The prominent spinous pro­cesses of C2 and C7 (vertebra prominens) can be palpated about the posterior midline ( Fig. 30.1 ). The hyoid bone approximates the level of C3, the upper border of the thyroid cartilage is used to indicate C4, and the cricothyroid space approximates the C5–C6 disc space. Chassaignac's tubercle can be palpated on the anterior transverse process of C6. For the thoracic spine, the suprasternal notch and the sternal angle can be used to approximate T2–T3 and T4–T5, respectively. Posterior­ly, and with the arms resting at the sides, palpation of the scapular spine and the inferior angle of the scapula may help to localize T3 and T7, respectively. The spine of T7 overlies the level of the body of T8, and is at the level of the T9–T10 spinal cord segment (see Fig. 30.1 ).

The lumbar spine has several useful surface anatomy landmarks, although individual variations should always be borne in mind. The midline median furrow indicates the insertion of latissimus dorsi on to the supraspinous ligaments between the spinous processes; it runs the entire length of the spine and is deepest in the mid-lumbar spine. The furrow flattens out at the start of the natal cleft, which approximates the S3 vertebral level. Each spinous process, and the intervening interspinous space between two adjacent processes, can be palpated in the midline. Starting in the paramedian plane, the posterior superior iliac spines (PSIS) can be palpated on either side as the most medial aspects of the iliac crests: they usually correspond to approximately the S1 or S2 spinous process. From these points the iliac crests can be palpated along their course on each side laterally. At their most superior aspects, the iliac crests constitute an important skeletal landmark (see Fig. 30.1 ). An imaginary line drawn between them, the intercristal (Tuffier's) line, corresponds to the L4–L5 interspace when studied radiographically but can commonly fall at the L3–L4 interspace by palpation, particularly in females and in patients with higher body mass indices. The greater trochanter of the femur can be palpated over the lateral hip. The imaginary line between this point and the ipsilateral PSIS is often used to indicate a corridor for safe passage of instrumentation within the iliac wing.

Important surface anatomy landmarks for the direct lateral approach to the lumbar spine are the lower ribs, the iliac crests and the greater trochanter of the hip. The lowest two ribs, usually originating from T11 and T12 vertebral levels, are typically ‘false’ or ‘floating’ ribs that do not articulate anteriorly with the sternum. Due to their obliquity, they can often pass over the L1–L2 and L2–L3 disc spaces, respectively, although table angulation can elevate their position. These ribs are flexible and can usually be displaced sufficiently for surgical access during the approach without necessitating rib resection. The greater trochanter of the hip can be palpated and is an important landmark for positioning on the operating table: the patient should be placed on the table in a position such that the greater trochanter lies just distal to the hinge of the table angular breakpoint. These surface landmarks will provide the general region of the incision but lateral radiographs are used to localize to the middle, anterior border and posterior border of the targeted intervertebral disc space prior to incision.

To achieve a successful approach to the anterior lumbar spine, it is critical for the surgical incision to be optimally placed, based on either preoperative fluoroscopic or radiographic imaging with a marker placed on the skin, or on the patient's surface anatomy, according to surgeon preference and experience. Since the original description of the anterior approach to the lumbar spine for the excision of an intervertebral disc and interbody fusion by Paul Harmon in 1963, this approach has been a widely used technique by spine surgeons. It provides maximal access to the intervertebral disc space and can be utilized for tumour resection, debridement of infection and interbody fusion for degenerative processes; in many countries it remains the only available surgical approach for the implantation of a lumbar artificial disc replacement. The retroperitoneal approach is most commonly utilized, usually with the aid of a vascular surgeon. Depending on which spinal level is targeted, either the approach can pass between the common iliac veins and arteries, or these vessels must be mobilized and transposed for the duration of the procedure. It is important to know that the aorta bifurcates at the level of the L4 vertebral body in approximately 70% of patients, at the L4–L5 disc space level in 12% of patients, and at the L5 vertebral body level in 18% of patients. Access to the L1–L4 levels usually requires a peri-umbilical incision and the surgeon may need to mobilize the abdominal aorta to gain access to the midline spinal structures. The lumbosacral junction (L5–S1 disc space) can usually be accessed via an incision that starts immediately superior to the pubic symphysis, which can be readily palpated regardless of body habitus. If the operation is planned for only the L5–S1 level, then either a vertical (median or paramedian) or transverse (Pfannenstiel) incision can be used within the hypogastric quadrant. A major advantage of a median vertical incision is that it passes through the relatively avascular plane of the linea alba and is muscle-splitting.

The sacral vertebrae are small and difficult to palpate.

Clinical anatomy