The pain-free function of the spine depends upon its proper alignment. The lordosis (“sway-back”) in the lumbar (low back) and cervical (neck) regions must balance the kyphosis (“hunchback”) in the thoracic (mid-spine) to place the body’s center of gravity over the hips and pelvis. When the curves are harmoniously balanced, erect posture requires minimal muscular effort as the skeleton resists the gravitational force.
Disruption of the normal sagittal alignment can occur due to one of several conditions, including post-traumatic kyphosis, degenerative disc disease, or after spinal fusion. If the lumbar lordosis is insufficient (“flatback”) the patient’s center of gravity is shifted forward relative to the hips and pelvis. The result is that, to stand upright, the patient must contract his or her back muscles, and possibly flex at the hips and knees. While these compensating maneuvers may help to normalize posture temporarily, over time severe back pain can result. Once the process begins patients often notice that the problem is progressive and they feel more and more “bent over” over time.
Non-operative management can, in some cases, help to ameliorate patients’ symptoms. Physical therapy, including gait and posture training, can improve flexibility and function. Sagittal imbalance, particularly after a lumbar fusion, often is most effectively treated by surgery to directly address the structural abnormality.
Because the causes of sagittal balance vary from patient to patient, no two procedures are identical. The overall goal of these operations, however, is the same: to reestablish overall sagittal spinal balance and place the spine into a physiologically advantageous alignment. As the general problem is that there is either insufficient lumbar lordosis or too much thoracic kyphosis, surgery is generally planned to add lordosis or reduce the kyphosis.
Surgery to correct sagittal imbalance can be thought of in three parts. Usually all three component parts are performed during one operation. The first part is the correction of the deformity, the second is the spinal fixation, and the third is the spinal fusion, or arthrodesis.
In sagittal imbalance the correction of the deformity is critical. In order to restore a more advantageous spinal alignment it may be necessary to perform one or more osteotomies. The osteotomy procedure, of which there are several types, loosens the spine by removing bone. The extent and location of bony removal depends on several factors, including whether the patient has previously undergone a spinal fusion procedure, the location and type of spinal deformity present, and the goals of surgery.
After the correction has been performed the spine is rigidly fixed into its new alignment. This is often done with the placement of multiple metal screws and hooks (fixation). These hooks and screws, each of which attaches to one vertebra, are then connected with long rods that are contoured to match the new spinal alignment.
The final part of the surgery is a fusion procedure. Bone is often harvested from the patient’s pelvis and may be supplemented with bone from a bone bank or with biological substances that promote bone growth. The long term outcome of surgery for sagittal imbalance depends on this bony fusion, which is inhibited from forming by smoking (or nicotine from other sources).