Thoracic Pedicle Screws: Pedicular Approach

Pedicular Approach

Archibald H. von Strempel

Goals of Surgical Treatment

To stabilize the thoracic spine; to correct a kyphotic deformity.

Diagnosis

Instability of the thoracic spine can be caused by fracture, tumor, or spondylodiscitis. Kyphotic deformity can caused by delayed fracture, Scheuermann’s disease, or other reasons for a hyperkyphotic thoracic spine. The diagnosis is made by physical findings and a standing anteroposterior (AP) and lateral x-ray of the whole spine completed by a lateral view of the thoracic spine in supine position with traction (Fig. 17–1).

Indications for Surgery

Thoracic spine instability, painful hyperkyphosis, significant thoracic cosmetic deformity due to hyperkyphosis, thoracic pedicular approach for bone biopsy of the vertebral body.

Contraindications

1. Children or small adults in whom the pedicle size does not allow screw placement with a diameter of 5 or 6 mm.

2. Severe osteoporosis.

Advantages

1. No implant contact to neural structures of the spinal canal.

2. High stability; pedicle screw can loaded by three-dimensional correction forces.

Disadvantages

1. Medial screw misplacement can lead to severe neurologic deficits.

2. Thoracic pedicle diameter (mostly the transverse diameter) can be too small for 6-mm screws even in normal adults.

Procedure

The patient is placed in a prone position on a frame or pillows with no pressure on the abdomen. The arms are positioned cranially with anteversion of the shoulders. Lateral C-arm control is helpful, but image quality can be poor in the upper thoracic area due to ribs and shoulder. The surgeon should be able to do the thoracic pedicular approach even without Carm control. If the following rules are respected, the risk of medial misplacement of the screws is very low. Attention must be given to the correct entry point of the thoracic pedicle. If the pedicle seems to be too small on an AP x-ray (pedicle size is limited by transverse diameter), a computed tomography (CT) scan should be done in the levels that are to be instrumented. We do not recommend pedicle screws with an outer diameter less than 6 mm in adults or adolescents because of the risk of breakage. In the pediatric population, we recommend 5-mm screws.

The medial wall of the thoracic pedicle is thicker than the lateral, and the length of the pedicle is shorter compared with the lumbar pedicle. Even if the transverse pedicle diameter is not much bigger than the screw diameter, the stability of the inserted screw is sufficient, because a greater part of the screw is inserted in the thoracic vertebral body compared with the lumbar situation. In the following technique the screw can cut the thinner lateral wall but not breach the more important medial wall of a pedicle, which is not much bigger than the screw. With an oblique screw orientation a lateral pedicle fracture can be avoided. If the anatomic conditions do not allow the implantation of a 6-mm-diameter screw, we recommend the use of hooks, claws, or wires to fix the internal fixateur to the spine.

Exposure

A midline incision is made one spinous process above the most cranial vertebra down to the spinous process of the most caudal vertebra. The extensor muscles are dissected laterally to the tips of the transverse processes. The inferior facet is resected except in the most cranial vertebra, where the capsule is excised only (Fig. 17–2

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