General Principles of Spinal Screw Fixation

10.1055/b-0034-84468

General Principles of Spinal Screw Fixation

Curtis A. Dickman and Volker K. H. Sonntag

Internal spinal fixation can be achieved with wire and bone grafts, cables, clamps, screws, plates, rods, or hooks.1–10 The ideal system would provide immediate rigid fixation with few risks. It would be nonfatiguing, promote fusion, and preserve normal motion segments. Screw fixation provides diverse alternatives for fixating the cervical spine. Screws or screw plates are indicated for internal spinal fixation when wire cannot be used, when wire is inadequate to fixate the spine, or when a biomechanically rigid fixation is needed. The rate and strength of spinal fusions are improved quantitatively and qualitatively as the rigidity of implants increases,11 and screws offer the advantage of providing immediate rigid fixation of spinal segments.12–15

In the cervical spine, screws have been used for odontoid fixation, atlantoaxial facet fixation, C2-pedicle fixation, anterior vertebral body screw-plate fixation, posterior lateral mass screw-plate fixation, and posterior occipitocervical screw-plate fixation. Screws are used either to fasten metal plates onto bone or as lag screws to hold together fragments of bone. Both techniques have merit in the cervical spine. This chapter reviews the fundamental principles of screw fixation for the cervical spine.

Types of Vertebral Screws

Surgical bone screws are usually constructed of stainless steel, pure titanium, or alloyed titanium. Screws should be nontoxic, biocompatible, strong, and noncorrosive. Steel consists of a mixture of iron, chromium, and nickel and has traditionally been used for bone implants and screws. Pure titanium, or an alloy of titanium, aluminum, and vanadium, is also popular for implants. Titanium and alloyed titanium are safe, strong, and malleable. Titanium is resistant to corrosion and is inert and biocompatible. Titanium has 90% of the strength of steel.16 Titanium, however, is notch sensitive, cracks when bent, and may fatigue and break more readily than steel. Titanium has a significant advantage over steel—it produces minimal artifact with magnetic resonance imaging (MRI), thereby allowing postoperative assessment of the spinal cord. When using titanium implants, one should consider the mechanical stresses placed on the implants, as well as the possible need for postoperative radiographic evaluation.

Portions of this chapter are reprinted from Dickman CA, Sonntag VKH, Marcotte P. Techniques of screw fixation of the cervical spine. BNI Quarterly 1992;8(2):9–26; 1993;9(4):27–39.

Screw Components

Screws are composed of a head, a tip, a shaft, and a threaded portion. The major diameter refers to the widest diameter of the screw shaft (i.e., diameter of the threads). The minor screw diameter, also called the core or inner diameter, refers to the diameter of the shaft beneath the threads. Screws can be characterized as cortical or cancellous screws, lag screws, fragment screws, self-tapping or non–self-tapping screws, cannulated or noncannulated screws, solid or hollow screws, or locking screws.

Self-Tapping and Non–Self-Tapping Screws

Self-tapping and non–self-tapping screws are differentiated by the design of their threads and screw tips. They require different surgical techniques for insertion ( Fig. 37.1 ). Both types of screws require drilling a pilot hole in the bone to create a tract for the screw. The diameter of the pilot hole should match the minor diameter of the screw. Self-tapping screws have sharp threads and a sharp tip with a distal cutting channel. The sharp self-tapping screws cut their path into the bone as they are inserted. The threads of non–self-tapping screws are less sharp than those of self-tapping screws. The tip is blunt and has no cutting flute. Before non–self-tapping screws are inserted, the screw thread pattern is cut into the bone using a tap, which is inserted into the pilot hole. The width and pitch of the threads of the tap correspond to the thread characteristics of the screw.

Non–self-tapping screws (unlike self-tapping screws) can usually be removed and reinserted without a high risk of inadvertently creating a new tract. Theoretically, the purchase of self-tapping screws in the bone is slightly less secure than that of non–self-tapping screws. However, no proven biomechanical advantage supports tapping a screw site.14,17–19

Cortical and Cancellous Bone Screws

Cortical bone screws are usually non–self-tapping screws, have relatively narrow threads, and are usually threaded along their entire length ( Fig. 37.1 ). They are available in a variety of diameters and lengths, and each size has its corresponding drill bit and tap. The diameter of the drill bit corresponds to the minor diameter of the screw; the diameter of the tap corresponds to the major diameter of the screw.

Cancellous bone screws can be non–self-tapping or self-tapping screws. Their minor diameter is small and their thread is wide and deep. These dimensions increase the holding power of the screw in trabecular bone. Cancellous bone screws have either partially or fully threaded shafts. Fully threaded screws are used for attaching plates to bone or as lag screws; partially threaded screws are used as lag screws. Engaging the tip of the screw in a distal bone cortex increases the holding power of cancellous screws.16,18

**(A)** Screw thread profiles for cortical and **(B)** cancellous bone screws. Cortical and cancellous screws differ primarily in the design of the screw threads and tips and in the methods of insertion. **(C)** Self-tapping screws have a sharp tip with a distal cutting channel and sharp, wide threads. They can be inserted into a pilot hole without tapping the bone. Self-tapping screws are usually used in cancellous bone or in bone with thin cortices. **(D)** Non–self-tapping screws have a dull tip and narrower threads and are usually used for bone with thick, dense cortices. The pilot hole must be tapped to cut threads into the bone before the non–self-tapping screw is inserted. (Reprinted with permission from Barrow Neurological Institute.)

Lag Screws

In the spine, lag screws are used to fixate odontoid fractures and the C1-C2 facets. Lag screws restore structural continuity. They place adjacent bone fragments under compression and thereby facilitate healing. The threads of a lag screw should engage the distal bone but not the proximal bone fragment. When the threads engage only the distal fragment, the bone fragments can be reduced and compressed. As the screw is tightened, the lag effect (i.e., compression) is generated as long as the functioning screw threads do not cross the fracture line ( Fig. 37.2 ).

Screw purchase in the proximal bone is avoided by one of two mechanisms. Either the proximal screw shaft must have no threads, or the hole in the proximal bone must be drilled wider than the major diameter of the screw. Such a wide proximal hole is called a gliding hole ( Fig. 37.2 ). The gliding hole lag technique is used with fully threaded screws.

Techniques for lag screw insertion. **(A)** Self-tapping screws are inserted directly into the pilot hole. **(B)** With fully threaded screws, a pilot hole is drilled and the hole is tapped. The proximal bone is overdrilled to create a gliding hole that is wider than the screw diameter. The screw threads purchase only the distal bone and reduce the fracture. **(C)** Double-threaded compression screws are inserted directly into a pilot hole. No threads cross the fracture line. The pitch of the threads on the proximal and distal ends of the screws differs so that when the screw is inserted, the fracture is reduced and a lag effect is created. (Reprinted with permission from Barrow Neurological Institute.)

**(A)** Lag screws are ideally inserted at right angles to the fracture line. This technique is best for transverse fractures across the base of the dens (i.e., odontoid type II fractures). **(B)** Lag screw reduction of an oblique odontoid fracture can theoretically cause malalignment because shearing forces are generated during reduction. If an oblique odontoid fracture is fixated, the fracture should be anatomically reduced with postural or open techniques prior to inserting a screw. (Reprinted with permission from Barrow Neurological Institute.)

Partially threaded screws with threads confined to the distal portion of the screw provide another method for lag screw fixation. Lag screw fixation can also be achieved by using double-threaded compression screws.20,21 The pitch of the proximal and distal screw threads differs, so that the distal fragments are compressed as the screws are tightened ( Fig. 37.2C ).

Lag screws should be inserted at a right angle to the fracture line. Otherwise, shearing forces can shift the bone fragments when the screw is tightened ( Fig. 37.3 ). Oblique fractures of the odontoid are particularly susceptible to this problem. When possible, fractures should be reduced and alignment restored before a lag screw is placed.

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