The usual posterior spinal implant is composed of anchoring members, longitudinal members, a kind of component-component connecting mechanism to connect the anchoring members to longitudinal members, and the transverse members to cross-link the longitudinal members.

The anchoring member is the part of implant system that grips the bony structure of the spinal column and transmits the force effected by the instrumentation to the spinal column. As this component comes directly in contact with the bone, there forms a bone-component interface. Anchoring members for the posterior spinal instrumentation may be divided into a penetrating type and a gripping type by the form of their bone-component interface.

Penetrating-type anchoring members are those engaging the bone by penetration into the bony structure. They make an important component of the cantilever constructs. By the alternation of the component-component connecting mechanism, they may act as a fixed moment arm, non-fixed moment arm, or an applied moment arm cantilever beam (Fig. 35.2). Penetrating anchors are divided into two groups; those with pullout resistance and those without pullout resistance. However, penetrating anchors without pullout resistance, called the “post”, are seldom, if ever, used alone in posterior implant systems [8]. Penetrating anchors with pullout strength comprise screws and smooth posts that change shape to offer pullout resistance after penetration into the bone. In present practice, screws are the most commonly employed penetrating anchors in posterior instrumentation [10].

Screws are presently used in the posterior instrumentation system for fixation in the pedicle [6, 11], cervical lateral mass [12], sacral ala [13], and iliac wings [14]. They have gained more popularity in recent years as they offer a rigid vertebral grip which is stable immediately after the insertion without need of a force loading and enable reliable fixation in the presence of posterior element defects which preclude the use of gripping types of anchors. But in some situations, employing a penetrating-type anchor may be difficult due to the complex anatomy of the region and risks of causing a major neural element or vascular damage [11, 15].

Screws used for posterior instrumentation may be a cortical or a cancellous type. However, the fact that the parts of the vertebra which engage the screws, including the pedicles, are composed of cancellous bone, the use of cancellous-type screws is more common [16]. A screw is made up of five parts: head, core, thread, tip, and the neck that connects the screw head to the core (Fig. 35.3). The screw head is the part of the screw opposite the tip and functions as the receiving port to the inserting device. Its main biomechanical function is to resist the inward translation force generated by the rotation of the screw at the terminal phase of screw tightening. When the screw is designed to tighten against a metallic implant, for example a plate, the implant will offer a substantial resistance to pull-through and the screw head needs to be just so big so as not to pass through the screw hole. On the other hand, if the screw is designed to tighten against the bone, the screw head must be substantially larger to offer an effective resistance to pull-through.

The screw core is the part of the screw from which the threads arise. Biomechanically, it provides the strength of the screw per se and resists bending and torsion moments acting on the screw. Since screws are frequently subject to bending moments in posterior instrumentation, the bending strengths of screws have significant clinical importance. The bending strength of a screw is proportional to the section modulus (Z) that is calculated as Z = πD³/32, in which D is the core diameter of the screw. As the section modulus changes by the cube of the change in the core diameter, even a slight alternation in the core diameter greatly affects the bending strength of the screw. This implies the importance of using a screw of the largest permissible diameter when using screws as anchoring members [17].

The screw thread is the part of the screw that provides the pullout resistance against a force directed along the long axis of the screw. As the pullout strength of the screw is proportional to the volume of the bone between the threads, the pullout strength is affected by the major diameter of the screw, thread depth, and pitch, which is the distance between two threads (Fig. 35.4). The cancellous-type screws used in posterior instrumentation causes compression of the soft cancellous bone during insertion and increases the density, and hence the amount of bone held within the threads, and are effective in enhancing the pull-out strength of the screws [18]. Screws used in posterior instrumentation have various designs to increase the pull-out strength (Fig. 35.5).

The screw tip is the part of the screw that first enters the bone. Though most of the screws used in posterior instrumentation are cancellous-type screws that do not need pre-tapping, some screws have leading-edge flutes like self-tapping screws to facilitate insertion (Fig. 35.6).

The neck of the screw is the part connecting the head of the screw to the core. As screws are frequently subject to cantilever bending moments, the neck portion receives most of the bending moment acting on the screw and is the most frequent site of fracture [19]. Some screws are designed to have a reinforced neck to offer more effective resistance to the cantilever bending moments concentrated here.

Gripping-type anchors are the components which “grip” the vertebra without penetrating into the bone. Hooks and wires are gripping type anchors most commonly employed in posterior instrumentation. The common sites for application of the gripping-type anchor are lamina, pedicle, spinous process, and the transverse process. The pull-out strength of the gripping-type anchors may be substantial as they contact the hard cortical shell of the vertebra. Biomechanically, the pull-out strength of the gripping-type anchor depends on the surface area under the component and the structural integrity of the bony element to which the anchor is attached. Since the bone has to resist the pull-out force by its inherent mechanical strength, even a minor fracture that weakens the part of the posterior element receiving the anchor substantially decreases the pull-out strength. This fact also limits the use of gripping-type anchors in the osteoporotic spine where cortical bone fails to provide enough resistance to the cut through of the components.

Recently, gripping-type anchors have been decreasing in use and are being replaced by penetrating anchors. The main reason for this substitution is the inferiority of holding power when compared to the screws. Additional reasons are prerequisite of an intact posterior element for a reliable fixation, necessity of preloading that inhibits the unconstrained motion of the spinal column under force, and the necessity of intruding the spinal canal that may increase the risk of neurologic injury.

In today’s modern posterior spinal instrumentation, implants are designed in such a way that several types of anchoring members may be used in the same instrumentation procedure, allowing the surgeon to choose the anchoring component according to the situation. Gripping-type anchors may be used with penetrating-type anchors in the same instrumentation to share the pull-out strength and hence protect the penetrating-type anchors from excessive pull-out stress.

The longitudinal members are the part of the implant to which the anchoring members are attached. The biomechanical function of the longitudinal member is to resist the principle force applied to the instrument. The longitudinal member of the distraction instrumentation has to resist the bending moment created by the weight of the body above the instrument while the longitudinal member of the compression instrumentation has to resist the tension stress.

Longitudinal members in a posterior implant may be a plate or a rod. Plates are very strong and offer a rigid fixation when combined with a constrained bolt. However, they are gradually becoming less popular as contouring of a plate to conform to the curvature of the spine is difficult and is fraught with technical problems. Additional reasons for this trend are lack of versatility that limits the available anchoring component to screws and the relative bulk of the implant that causes greater extent of soft tissue damage during implantation and takes up room for placement of bone graft.

The rod is presently the most common form of longitudinal member employed in the posterior instrumentation. They are easy to contour and allow attachment of both the gripping and the penetrating-type anchoring members. Similar to screws, their strength is proportional to the section modulus (Z). As Z = πD³/32, the strength of the rod is greatly influenced by the diameter. However, as increasing the diameter of the rod also increases the bulk of the implant causing many untoward problems, the manufacturers are trying more and more to produce thinner rods with increased strength which allow the implant to have a low profile [20]. Some are trying to use rods of higher elasticity to prevent mechanical failures, but the ultimate result is still to be clarified. Rods may be sliding types with or without a surface finish, a threaded or a rachetted type (Fig. 35.7).

The sliding type of rod is the most common type used presently. It allows free sliding of the anchoring members along the length of the rod enabling distraction and compression along the rod. It is suitable tar both distraction and compression constructs. While some of these rods are smooth, some have surface alterations to increase the friction between the rod and the component attaching mechanism.

Threaded rods allow powerful, controlled distraction or compression of the anchoring members along the rod. However, as bending of the rod with mechanical benders often results in damage to the threads causing difficulty in tightening of the nuts, they are usually more malleable than the sliding-type rods. This limits their use in posterior instrumentation to compression (tension band) constructs.

Rachetted rods like the one used in the Harrington distraction device are primarily used in distraction constructs. As the rackets act as stress risers, often leading to mechanical failure of the rod, this type of rod is rapidly falling out of favor.

The longitudinal member of the posterior implant is connected to the anchoring members by a component-component connecting mechanism Except for a wire which is tightened around the longitudinal member to offer a grip, all the connection between the components of posterior implants are one or combinations of the following six fundamental locking mechanisms; (1) three-point shear clamp, (2) lock screw, (3) circumferential grip, (4) constrained screw-plate, (5) semiconstrained screw-plate, (6) semiconstrained anchoring component-rod.

The three-point shear clamp is the mechanism of locking employed in the screws in the synergy (Cross Medical, USA) system and the new type DDT of the Cotrel-Dubousset system (Sofamor-Danek, USA). The locking is provided by the force applied at the interface and the friction between the components.

The lock screw mechanism uses the set screw to push the rod to abut the other part of the component and is presently the most common type of locking mechanism used in posterior instrumentation. Examples are the screws and hooks of the Cotrel-Dubousset system (Sofamor-Danek, USA) and the Diapason system (Stryker, USA).

Circumferential grip offers connection between the components by friction effected by two halves of the pincer. An example is the old type DTT in the Cotrel-Dubousset system and the closed clamps in the Colorado system (Sofamor-Danek, USA).

The constrained bolt plate is the type of locking mechanism used in the VSP (Acromed, USA) and connection of the closed clamp with the anchoring components in the Colorado system (Sofamor-Danek, USA). It is very rigid and offers the strongest component-component connection available. However as it needs a perfect contact between the undersurface of the plate with the upper surface of the screw for optimal function constrained bolt plate connections directly between the anchoring member and the longitudinal member pose many problems in practice and are also falling out of favor. Newly developed implants employing this component-component interface usually use this mechanism to connect the anchoring member to a clamp that is again connected to the longitudinal member to facilitate the instrumentation procedure.

The semiconstrained screw plate is the type of connection employed in most of the screw plate systems. As the screws are not rigidly fixed to the plates, this connection allows toggling of the screws on the plate and is unable to achieve a true rigid fixation. Although bicortical fixation of the screws increases the rigidity of this kind of connection, obtaining a bicortical fixation from the posterior side of the spine is often difficult and dangerous. In posterior instrumentation, Roy-Camille plates and the cervical lateral mass plates use this component-component interface.

The semiconstrained component-rod is the type of connection that allows toggling of the anchoring member on the rod. A typical example is the Harrington distraction device. As they do not achieve a true rigid fixation and are prone to mechanical failure at the component-rod interface, they are also gradually fading away.

The transverse member, commonly known as the cross link, is the component that transversely connects two or more longitudinal members of posterior implants to convert the construct into a quadrilateral frame. Its biomechanical function is to enhance the torsion resistance and to resist parallelogram deformation of the construct [21, 22]. Transverse members do not increase mechanical resistance to other stresses (e.g., flexion-extension, lateral bending) [21, 22].

The optimal number of cross linking is at two sites, as adding more transverse connections does not significantly increase the torsion resistance.

The posterior instrumentation maybe classified by the nature of the force imparted by the instrumentation on the spinal column. Though it is sometimes very difficult to define the principle acting force due to the complex three-dimensional nature of the spinal anatomy, the principle forces effected by the posterior instrumentation are distraction, compression, three-point bending, and translation. By the degree of freedom allowed by the instrumentation, they are further divided into rigid and dynamic types. The rigid type is the construct that does not allow motion between the instrumented spinal segments whereas the dynamic types allow some motion in the instrumented segments either by motion at the component-component interface or component-bone interface.