A deformity is simply a variation in the shape of a structure when compared to the typical shape of that structure. A spinal deformity is produced by any combination of curvature and twisting of the spine.
To understand more about the correction of spinal deformities, it helps to first understand a bit about typical spinal anatomy.
The Vertebral Column:
The spine normally has several gentle curves when viewed from the side (see “Vertebral Column” picture). These curves work in harmony to keep the body’s center of gravity aligned over the pelvis. The cervical spine, or spine in the neck, has a gentle lordosis (inward curve). The thoracic spine, or spine in the upper and mid-back, has a gentle kyphosis (outward curve). And the lumbar spine, or spine in the low back, has a gentle lordosis (inward curve) again. Below the lumbar spine is the sacrum. In children, the bones of the sacrum are separate. The bones begin to fuse during puberty, and in adults, the sacrum is a single bone. Together, the curves at these spinal levels keep the spine in balance in the front-to-back direction, or the sagittal plane. But too much thoracic kyphosis or too little lumbar lordosis can force the body’s center of gravity too far forward. This type of imbalance is called sagittal imbalance.
Viewed from behind, the normal spine is straight. It is a mechanically stable structure that provides a maximum of stability with a minimum of effort.
The spine is composed of 33 bones called vertebrae. Each vertebra (single bone) is made up of a vertebral arch and a vertebral body (see image “Anatomy of a Vertebra”). The vertebral arch is an arch-shaped section of bone at the back of the vertebra. Bony projections called processes extend from the back of the vertebral arch. Some of these can be felt as bumps beneath the skin of the back. At the front of the vertebra is the sturdy vertebral body. It is a solid bone, shaped something like a marshmallow, and it helps the spine bear weight. Each vertebral body is connected to its neighbors above and below by intervertebral discs that cushion and connect the bones, allowing the spine to bend and flex. The vertebral arch and vertebral body are connected by strong columns of bone called the pedicles. Together, the vertebral arch, pedicles, and vertebral body form a bony ring around a hollow center.
The Spinal Canal:
Stacked on top of one another in the spinal column, these rings align to form a long, well-protected channel known as the spinal canal. The spinal canal houses the spinal cord, the bundle of nerves connecting brain and body. Nerve roots exit the spinal canal through openings called foramen.
Types of Deformity
Recall that the typical spinal column has gentle curves when viewed from the side, and is straight when viewed head-on.
Deformity in the spinal column causes bending or rotation in one or both directions. Deformity can occur in adults as well as in children.
The natural curve of kyphosis in a typical upper spine, for example, may measure between 20 and 40 degrees. A greater degree of curvature can cause sagittal imbalance. Conditions that produce sagittal imbalance include hyperkyphosis (a great amount of kyphosis), chin-on-chest syndrome, flatback syndrome, and ankylosing spondylitis. Severe sagittal imbalance can produce problems like stooping, fatigue, pain, and difficulty looking ahead and meeting the gaze of others. It can also compress the heart, lungs or other organs.
A side-to-side curvature of 10 degrees or more is called scoliosis. Causes include adolescent idiopathic scoliosis, degenerative scoliosis, neuromuscular imbalances, congenital deformity and spine tumors. A curve in one direction only is called a “C” shaped curve. A curve in both directions is known as an “S” shaped curve. Some forms of scoliosis are not painful, while others are. Severe scoliosis can even interfere with the heart and lungs.
Deformity Correction and Stabilization
Surgical correction is considered for severe curvature that interferes with organ function, causes pain, and/or shows signs of continuing to progress. When forming a treatment plan, a surgeon is also guided by a patient’s age. A surgeon would expect more rapid curve progression in a pediatric patient with a lot of growing left to do than in an adult whose skeletal growth is complete.
In general, surgical treatment is considered for kyphosis of 70 degrees or more and scoliosis of 45 degrees or more. Curves of these magnitudes may interfere with organ function, and tend to continue to progress if not surgically corrected. The goals of surgical correction and stabilization are to:
- reduce pain
- restore ability to stand erect
- relieve pressure on organs such as the heart and lungs
- prevent deformity from progressing
Each surgery has two components: correcting the deformity and re-stabilizing the spine in the new, corrected position. Hardware like screws, rods, plates and cages (special implants) usually hold the spine in its new alignment while it heals. The process of implanting this hardware is called fixation. Bone graft (transplanted bone) may also be placed in the area to encourage the bones to fuse, or permanently grow together. This is called fusion. Fixation provides stability in the short term, but good bony fusion provides long-lasting strength and stability to the area.
Types of Deformity Correction
The main correction and stabilization procedures are osteotomy, pedicle subtraction osteotomy, vertebral column resection, and spinopelvic fixation. These procedures vary in the amount of bone they remove and in the amount of correction they provide.
- The osteotomy (sometimes called a posterior column osteotomy, or PCO) removes some bone from the back of the vertebral arch. It produces about 10-20 degrees of correction at each level and it can be performed at several levels. Therefore it is often used to correct long, gradual curves of kyphosis, like ones that may be produced by Scheuermann kyphosis or ankylosing spondylitis. Of the bone removal procedures, it removes the least bone.
- The pedicle subtraction osteotomy removes more bone than the PCO described above. A PSO removes the vertebral arch and the pedicles that connect the arch to the vertebral body. Part of the vertebral body is removed as well. This procedure produces about 30 degrees of correction, and can be combined with PCOs in other vertebrae to yield greater overall correction. It is especially useful in treating some cases of ankylosing spondylitis, flatback syndrome, and any sharp, angular kyphosis. A surgeon can also correct some C-shaped scoliosis with a pedicle subtraction osteotomy.
- The vertebral column resection (VCR) removes the most bone of all correction and stabilization procedures: it removes the entire vertebra. The vertebra is replaced with bone grafts and implants called cages. A system of screws and rods restore spinal stability while the bone graft heals. This procedure can provide the greatest amount of correction–80 degrees or more. A single vertebra may be removed to correct a sharp curve, or more than one may be removed to correct a broader curve.
- Spinopelvic fixation is a procedure in which screws, rods, or other hardware rigidly connect the base of the spine and the surrounding bones of the pelvis. This procedure reduces the forces of bending and rotation at the junction between the lumbar spine and the sacrum. Since these forces are normally great, a spinopelvic fixation can help take the strain off a healing bone fusion in the lower spine.
Balancing the spine is a complex process that must take into account the mechanics of the spine, the spinal cord and nerve roots, and the nearby organs. Achieving the best outcome requires a surgeon to possess technical skill, a thorough familiarity with orthopedic and neurological considerations, and experience tailoring treatment for individual cases.
Preparing for Your Appointment
Dr. Peter D. Angevine at The Spine Hospital at the Neurological Institute of New York is one expert in complex deformity correction and stabilization. He has performed hundreds of these surgeries, and regularly teaches courses on deformity correction for other neurosurgeons. His background includes fellowship training in both neurological and orthopedic spine surgery. His practice includes adult and pediatric patients.
Drs. Christopher E. Mandigo and Sheng-fu (Larry) Lo at the Spine Hospital at the Neurological Institute of New York also have particular expertise in evaluating and treating disorders of spinal alignment such as scoliosis and sagittal imbalance. Their focus is adult patients.
Dr. Richard C.E. Anderson‘s area of expertise is the pediatric spine. He specializes in the unique considerations of deformity correction in this patient population, including the surgical treatment of scoliosis and/or spinal instability in children.