Introduction

Vertebroplasty is one of the most widely used image-guided minimally invasive vertebral augmentation procedures for treating painful vertebral compression fractures. A percutaneous bipedicular approach is typically used to access the vertebral body. Polymethylmethacrylate (PMMA) bone cement is injected directly into the cancellous bone, stabilizing the fracture and providing virtually immediate pain relief. Relatively low viscosity cement is required for this procedure to achieve adequate fill and interdigitation. The cement flow is uncontrolled, however, and leakage into the vascular system, paravertebral space, or disk is commonly reported. Although most cement leaks are asymptomatic, serious leakage-related clinical complications such as compression of neurologic structures or formation of pulmonary embolus have been reported.

In an effort to achieve fracture reduction and restore sagittal balance, balloon kyphoplasty was introduced. This procedure has proven to be safe and efficacious, and its beneficial effects are sustained according to the most recent clinical studies.¹ However, reproducible and clinically significant fracture reduction has not been clearly demonstrated in these same studies.^2,3 The technique involves the use of inflatable bone tamps to create a cavity through compaction of bone and marrow, followed by high viscosity cement injection using bone filling cannulas. The cumulative volume required to fill the large voids requires the use of multiple cannulas, but provides the surgeon greater control of the cement injection rate and volume compared to vertebroplasty. Cement flow and interdigitation are limited to some extent by the compressed bone lining the cavity walls and by the cement viscosity. This technique is generally reliable and safe, provided cement viscosity is high and the operator includes careful fluoroscopic monitoring.

More recently, new devices and procedures have been designed to achieve fracture reduction and reduce leakage rates. The Shield Kyphoplasty System (Soteira, Inc., Natick Mass.) was developed to better contain and control the flow of cement, reduce cement leakage rate and create biomechanically optimized cement augmentation. In this chapter, the components of the Shield Kyphoplasty System and the associated surgical technique will be described in detail. Mechanical testing of fractured osteoporotic vertebral bodies treated with this system under monotonic and cyclic loading conditions will be discussed. Finally, the primary outcomes from long-term clinical evaluations of this system will be presented, including a randomized multicenter study that compared pain relief and cement leakage for the Shield Kyphoplasty System and conventional bipedicular vertebroplasty.

System Overview

The Shield system features a non–load-bearing, hollow, self-expanding implant that is deployed into a cavity created within the center of the fractured vertebral body. The function of the device is to initially contain injected cement, then to regulate and direct the flow of the cement through engineered openings in the anterior wall of the device. Cement injection into the implant and through the openings creates a mantle of cement in the anterior vertebral body, which spans the endplates and stabilizes the fracture by filling cracks and voids, interdigitating with viable trabecular bone. Placement of the device in a central cavity helps to limit posterior flow of cement via the basivertebral plexus and allows cement to permeate the entire vertebral body using a unipedicular approach.

The Shield system includes a set of single patient use disposable instruments for unipedicular percutaneous access and specially designed instruments for cavity creation, implant deployment, and cement injection, as shown in Figure 39-1. The unique curved design of the cavity creation instrument allows the surgeon to drill a curved path from one pedicle, crossing the sagittal midline, and stopping within the contralateral anterior quadrant of the vertebral body. The cavity creation instrument then converts to a reamer in situ, which is capable of creating a 10-mm diameter cylindrical cavity in the retrograde (proximal) direction that is matched to the implant size. The delivery system subsequently provides a means to insert and deploy the cement directing device within the cavity and facilitates cement injection with a high pressure injection system.

FIGURE 39-1 Components of the Shield Kyphoplasty System. Shown from left to right are the blunt tipped wire, working channel, curved drill/cavity cutter, implant and cement delivery system, and tamp.

The Shield cement director is an elongated 10-mm diameter hollow structure fabricated from braided nitinol wire and other biocompatible textile and polymeric materials. The device is available in three lengths: 15 mm, 20 mm, and 25 mm, a range selected to approximate the anatomic distance between the medial pedicle borders in the thoracic and lumbar spine in the patient population with osteoporosis. The cylindrical wall of the implant is impermeable to bone cement with the exception of small holes located anteriorly-superiorly and anterior-inferiorly on the device, as shown in Figure 39-2. The implant is supplied preloaded onto a delivery device and collapsed within a sheath to facilitate placement into the cavity through the working channel. After placement, the sheath is retracted to deploy the self-expanding implant in the prepared cavity.

FIGURE 39-2 The Shield implant, shown in the expanded state. The holes (3 of 6 holes shown, arrows) are positioned to direct cement flow in the anterior, superior and inferior directions. There are no holes on the posterior surface of the implant to prevent cement flow in this direction.

Indications

The Shield Kyphoplasty System is intended for use in the treatment of osteoporotic vertebral body compression fractures in the adult spine in levels T4-L5. It is intended to be used with a PMMA bone cement cleared for use in vertebral body fixation. Up to three levels with osteoporotic compression fractures may be treated during one operative session regardless of fracture age, excluding vertebral plana, unstable fractures, or suspected infection.

Contraindications

The Shield Kyphoplasty System should not be used when the following conditions are present: