History of Spinal Instrumentation: The Modern Era

The Harrington Rod Area

In 1975, the Harrington rod represented the state of the art spinal instrumentation. The rod system, initially developed by Paul Harrington for the correction of spinal deformities, was soon used in the treatment of traumatic injuries ^, ( Fig. 3.1 ), degenerative disease, and metastatic disease. ^, The system provided distraction rods as well as compression rods and hooks. Over the years, however, their widespread use led to the recognition of their limitations. The use of a distraction system provided excellent correction of coronal plane deformities (scoliosis). Unfortunately, the use of distraction as the sole correction tool resulted in the loss of normal sagittal plane alignment. The loss of normal lordosis was associated with “flatback syndrome.” ^, Hook dislodgment and rod breakage also proved to be troublesome complications. ^, In addition, casting or bracing was generally required in the postoperative period, which proved to be complicated or impractical for some patients and surgeons.

The Birth of Segmental Spinal Fixation

In response to difficulties with the Harrington Rod, Eduardo Luque advanced a major concept in the mid-1970s that quietly pushed forward the future direction of spinal instrumentation: segmental spinal fixation. The issue of bracing was of particular importance to Luque. Practicing in the warm climate of Mexico City, it was difficult for Luque from a practical standpoint to use the postoperative casting required in Harrington rod instrumentation.

Besides, a large number of his patients of low socioeconomic status would travel a great distance to seek treatment and would not comply with bracing, or became lost to follow-up.

Luque popularized the use of a 3/16-inch steel rod secured at each spinal level with sublaminar wires ( Fig. 3.2 ). He reasoned that increasing the number of fixation points along a construct would reduce the force placed upon each point at the bone–implant interface and obviate the need for a postoperative cast or brace. Additional beneficial effects of segmental fixation were that it increased the potential corrective power of instrumentation, reduced the potential for construct failure, and resulted in improved fusion rates.

The concept of segmental fixation to a contoured rod was widely embraced because it produced greater construct rigidity and allowed improved control of the sagittal plane.

A hybrid form of Paul Harrington’s technique (from Texas) and Eduardo Luque’s technique (from Mexico) was sometimes referred to as the “Tex-Mex” operation, and could lead to possibly severe neurological complications. ^, Revision surgery to alter sublaminar wiring was problematic because of scarring, which may preclude the passage of new wires at the same laminae. In response to these concerns, Drummond et al. developed a method for segmental fixation using a button-wire implant passed through the base of the spinous process. This technique did not provide as strong fixation as sublaminar wires. It did, however, avoid passing anything into the spinal canal, and thus reduced the risk of direct neurological injury. The compromise between increased fixation and decreased risk of neurological injury was seen as a prudent choice by many surgeons operating on healthy, neurologically normal adolescents with idiopathic scoliosis.

The Comeback of Hook-Rod Systems and the Rod Rotation Revolution

Increasingly sophisticated multiple hook-rod systems appeared in the 1980s that provided much of the strength of wire fixation but with greater flexibility to address deformities in both the sagittal and coronal planes. The Cotrel–Dubousset (CD) system was introduced to the United States in 1986, using a 1/4-inch rough-surfaced rod. Multiple hooks allowed spinal surgeons to apply compression and distraction over different areas within the same rod. The multiple-hook design applied the principles of segmental fixation without the need for sublaminar wires. Significantly, the system provided for a unique mechanism for deformity correction: rod rotation. Rod rotation proved to be a powerful force in the correction of scoliosis. Further stability was provided by cross-linking the two parallel rods together.

The advantages of the CD system were partially offset, however, by the difficulty of removing it. The locking mechanism of the hooks was irreversible without destroying the hook or cutting the rod. The Texas Scottish Rite Hospital (TSRH) system was a design advance that addressed the issue of revision surgery. It was similar to the CD system but allowed the removal of the system’s components if necessary. Although the features of the TSRH system simplified revision surgery, the top-loading side-tightened system was not universally appreciated. After maturation of the fusion mass, the side-tightened system was difficult to access in the case of revision surgery. The following decade saw the introduction of numerous similar dual-rod systems, such as Moss-Miami and Isola. ^, The major variations revolved around the leading and locking mechanism: side loading, top-loading, side tightening, or top tightening.

Pedicle Screws

A major advance was the exploitation of the pedicle as a site for segmental fixation. This innovation is generally credited to Roy-Camille of Paris. Roy-Camille performed his first operation in 1963 but did not publish the results until 1970. Pedicle screws are biomechanically superior as a point of fixation compared with hook- or wire-rod constructs and can be placed into the sacrum, an area in which fixation is otherwise difficult. In addition, they can be used even after laminectomy has been performed and can be positioned without entering the spinal canal.

This advantage allowed for the massive proliferation of spinal instrumentation in the field of degenerative spinal disorders. Before the advent of pedicle-screw instrumentation systems, there had been only sporadic reports of the use of instrumentation for degenerative spinal disorders.

Arthur Steffee popularized the use of pedicle screws in the United States in 1984, using a contourable plate. At about the same time, a screw-rod system, developed by Yves Cotrel of France, was in use in Europe that became incorporated into the “universal” CD system. Controversy soon followed, with both the screw-plate and screw-rod constructs developing a group of proponents. Advocates of plates noted that plates were stronger than the screw-rod construct. However, most surgeons were ultimately attracted to rods because their use provides greater flexibility, reduces encroachment upon the adjacent facet joints, and leaves more surface area for fusion. The marriage of long dual-rod constructs to lumbar pedicle screws was a significant development that enhanced the surgeon’s ability to accomplish increasingly challenging and complex spinal reconstructions.

The use of the pedicle screw has further advanced the ease of spinal reconstructions and corrections. Suk et al. ^, started to use pedicle screws in the thoracic spine for adolescent idiopathic scoliosis (AIS) correction in 1988. As more experience has been gained, this procedure is today widely accepted across the globe.

An improved ability to derotate the apical vertebrae in thoracic AIS allows for better corrections, particularly of the thoracic “hump.” Decoration maneuvers and the desire for more aggressive corrections resulted in more widespread adoption of thoracic pedicle screw constructs. ^, Biomechanical studies have found that pedicle screws provide superior stability, improved deformity correction, less pseudarthrosis, and fewer implant failures in comparison with hook and wire instrumentations.

Monoaxial screws provide superior derotation and restoration of thoracic symmetry compared with polyaxial screws. ^{, , ,} However, monoaxial screws may make rod-screw engagement more difficult. Recently, a third type of pedicle screw head design has been introduced whose head pivots in only one sagittal plane: the uniplanar screw. This screw may combine the advantages of polyaxial and monoaxial screws. Liu et al. found that the difference in rotational correction between monoaxial and uniplanar screws is smaller than that between polyaxial and uniplanar screws. On the other hand, the study showed that pedicle strain in the uniplanar screw was lower than that in the monoaxial screw. Yaszay et al. compared postoperative apical vertebral rotation between uniplanar and polyaxial pedicle screws. Their results showed that the uniplanar screw group had better apical vertebral derotation than the polyaxial screw group.