When PRSS is applied in the spine, asymmetrical stress is created by the lateral sideway push of the plate rod, compressive stress is exerted over the convex side, while tensile stress is exerted over the concave side of the curvature which is expressed in the form of change of the width of the disc spaces (Fig. 49.4a, b). In this manner, asymmetrical growth over both sides of the vertebral cartilaginous end plate occurs, wedging is corrected, and the vertebrae are remodeled to normal shape (Fig. 49.4a, c) [17].

Photoelastic study was used to show the mechanical properties of PRSS [18]. A model of scoliosis and correcting clamp was built on the base of similar principle (Fig. 49.5a).

According to the factual situation, the in situ stress was measured by using photoelastic and strain gage method. ANSYS (a finite element analysis software) method was also applied to simulate the experiment process and evaluate the reliability of measurement.

The results indicated that a small lateral curvature of the spine can produce asymmetrical spinal loading; the relational changes and variation of stress on both sides of scoliotic spine were demonstrated by the change on the color band in the photoelastic test. Color stripe showed more color stripe on the concave side than the convex side in the scoliotic spine (Fig. 49.5b at the top site) indicating that more compress stress in this side, and it was increased when 20 kg vertical load was given (Fig. 49.5b at the middle site); when the lateral push was applied over the convex side, more concentration of color stripe appeared on the convex side indicating compressive stress increase on this side and compressive stress decrease, and tensile stress was produced on the concave side (Fig. 49.5b at the bottom site). It was also shown that the stress value was in proportion to the correcting load. The asymmetrical stress data were expressed in a linear formulation of green line as change of compressive stress on the concave side and red one on the convex side (Fig. 49.5c, d), indicating PRSS has a significant efficiency to alter the asymmetrical mechanical loading on both sides of the scoliotic spine.

Type X collagen has also been studied to express the therapeutic mechanism. Type X collagen is used to reflect chondrogenesis, subchondral bone formation, and cartilage degeneration [19]. Type X collagen was studied as growth mark of cartilage end plate using semiquantitative RT-PCR method. The fact that more type X collagen was expressed on the convex side (Fig. 49.6-CL) than concave side (Fig. 49.6-CR) suggests that compressive stress leads to increase earlier cartilage degeneration of the convex side end plate, correlating as well with decreased growth of the end plate of this side and resulting in maximum spinal realignment.

The patient is placed in the prone position. A standard posterior approach and exposure as for Harrington rod procedure is made. But exposure of the subperiosteal laminae is limited to the hook sites. In this way, the unwanted spontaneous posterior ankylosis or “autofusion” is minimized.

Hook insertion: The upper and lower hooks are inserted on the lamina of the selected end vertebrae, fixed with a screw and linked with the connector (see Fig. 49.7a, b), which forms two attached points to accept the rod and plate rod. An elastic prebent plate rod is first placed on the convex side by way of the lateral sideway push. This maneuver provides asymmetrical stress on both sides of the end plate of the vertebrae all along the apical region. The lower end of this plate rod is first passed through the hole of the lower connector to an appropriate length, and free from the inside of the hole, the upper end of the plate rod is then introduced into the open head of the upper connector and fixed with the setscrew. The rod (5.5 mm in diameter) is placed on the concave side after contouring to fit the profile of the spine. This forms a frame-like spinal construct (Fig. 49.7c). The upper and the lower intermediate hooks are inserted in place in a manner similar to that of CD system. The hooks are linked with the interconnector and fastened with the rod and plate rod using the U-shaped ring. This allows additional correction of scoliotic curve. The screw caps of the intermediate screw hook on the concave side are tightened to bring the screws and its linking lamina backward for derotation of the vertebral bodies over the apical area (Fig. 49.8a, b).