Prone positioning is necessary for surgical access to the posterior spine, the posterior fossa, and the posterior aspect of the lower extremities. Spine procedures commonly using the prone position are discectomy, laminectomy, and spinal fusion. Recent studies have shown that the cardiopulmonary effects of prone positioning depend on which surgical frame is used, rather than on patient body mass index. Knowing the various frames therefore will greatly assist anesthesiologists as they care for patients in the prone position.

Many surgical frames are used for posterior spine procedures, and the differences between them can confuse the anesthesiologist. Keep in mind that the surgical frame selected should help expose the surgical field and decrease blood loss, which commonly results from prone positioning. Spine surgeons choose surgical frames based on whether the procedure requires preserving or decreasing lordosis of the cervical and lumbar spines. Preserving lordosis maintains alignment of the spine for fusion procedures. Decreasing lordosis allows better access to the intervertebral space and is commonly used for discectomy after herniation. Understanding the commonly used surgical frames will help the anesthesiologist understand the cardiopulmonary physiology encountered during procedures done on patients in the prone position.

The two frames that maintain lordosis involve using longitudinal bolsters or the Jackson table. Longitudinal bolsters, consisting of two parallel chest rolls commonly made from foam or rolled sheets, are inexpensive and effective. The bolsters extend from just caudad to the clavicle to just beyond the inguinal area, bilaterally, and are used on a conventional surgical table. The Jackson table is an elaborate, free-standing surgical table with 360-degree rotational capabilities and pads to support the thighs, pelvis, and chest, which allows the abdomen to hang freely. Using the Jackson table has been shown to affect ventilatory mechanics and hemodynamics less than does using longitudinal bolsters.

The frames that decrease lordosis are the Wilson frame, the Relton-Hall frame, and various kneeling frames. The Wilson frame involves using two longitudinal, curved pads to provide continuous support to the chest
and pelvis. This frame is used on a conventional surgical table. With space between the two pads, the abdomen is not fully compressed using this frame. The curve in the Wilson frame pads can also be decreased to restore lordosis to permit a fusion to be done. The Relton-Hall frame, consisting of two pairs of V-shaped pads, one pair in the upper thoracic region and one pair along the pelvis, leaves the abdomen hanging free. Kneeling frames come in many varieties including the knee-chest, the modified knee-chest (Andrews), the tuck, and the modified tuck frames. Use of these frames optimizes pulmonary physiology, especially in obese patients, but increases the risk for nerve palsies and hypotension, because these frames cause blood to pool in the lower extremities. Use of the kneeling frames is also contraindicated in patients whose vertebral columns are unstable.

Prone positioning alters pulmonary mechanics and hemodynamics no matter which frame is used. Specifically, prone positioning decreases pulmonary compliance. The magnitude of this decrease varies with the frame used. The main cause of such reduced pulmonary compliance is elevated intra-abdominal pressure resulting from the surgical table or frame compressing the abdominal wall and abdominal viscera. This increased pressure is referred to the diaphragm and, in turn, to the lungs, resulting in a decrease in pulmonary compliance. These changes are manifested clinically as high peak airway pressures during positive pressure ventilation. Pressure control ventilation can be used to limit the high airway pressures. Palmon et al. demonstrated that pulmonary mechanics in the prone position depend on which frame is used and not on body habitus. This study concluded that use of the Jackson table disturbed ventilatory mechanics, as measured by peak airway pressure, less than did use of the Wilson frame or longitudinal bolsters. Several studies have assessed the efficacy of different surgical frames for prone positioning. Use of a surgical frame that allows the abdomen to hang freely in the prone position has repeatedly been shown to decrease pulmonary compliance less than does use of other surgical frames.