Biomechanics of Spinal Fixation

Melissa Kuhn

Sushil Sudershan

Vijay Goel

Hassan Serhan

Nerve root decompression and stabilization of an unstable spine to relieve back pain and radiculopathy remains one of the main goals of fusion-promoting procedures on the spine. In recent years, there has been significant progress in the surgical treatment of spinal pathologies through improved spinal implants and instruments.¹ Stability and alignment of the spine is achieved using screws, hooks, or wires carefully implanted in exposed posterior elements of the vertebrae and connected with rods and transverse devices to form a stable framework.² Corrective maneuvers for realignment of segmental vertebrae using a variety of techniques can be facilitated using spinal implants.³

Since the early 1990s pedicle screw instrumentation systems have been increasingly used because of their superior biomechanical properties, their relative safety of insertion, and effectiveness in correcting complex deformities.⁴, ⁵, ⁶, ⁷ Other benefits of pedicle screw constructs in general include lower rates of pseudoarthrosis, reduced loss of correction, lower risk of neurologic complications, and preservation of more mobile spinal segments.⁶,⁸ The introduction of polyaxial and uniplanar screws has further reduced the incidents of these complications.⁹

CONSIDERATION OF BIOMECHANICS WITH SPINAL FIXATION

The forces applied to the spinal motion segment include compression, tension, shear, torsion, and bending moment as seen in Figure 11.1.

The combined loading of the spine and its biomechanical impact is critical for both design and clinical application of spinal fixation devices and will be referenced throughout the chapter.

Rigid Fixation

The overall goal of spinal fusion is to eliminate excessive motion of the spine segment to provide stability and increase the chance for solid arthrodesis.¹⁰ Solid bony fusion is achieved by meticulous posterior decortication and/or using interbody fusion devices with bone autograft (local, iliac, or costal) or biologic bone substitutes.¹¹ Fusion devices are those that either directly or indirectly enable bone healing across a previously mobile motion segment to provide rigid fixation.¹² On average, it takes about 3 to 6 months for fusion to occur during which time the loads transfer from the implant to the spinal structures.¹³ Once fusion has occurred, the loss of motion at the operative level results in higher stress and increased motion in the adjacent segments. This increase may accelerate adjacent segment degeneration.

Anterior, posterior, and lateral fixation all utilize a mechanical construct intended to transmit the load away from the fractured or injured body to the superior and inferior vertebral bodies.¹⁴ In all approaches, the design must do the following during the fusion process:

Provide segmental stability¹⁴
Resist expulsion¹⁴
Have enough surface area and be of the proper stiffness to resist subsidence¹⁴

Bending Moment of Rigid Fixation

Rigid pedicle fixation techniques utilize a cantilever beam with a fixed moment arm. Let us review an example of bending moment in relation to two screws and a connecting plate or rod. The
screws may be compressed by the spine under axial loading. Because Moment = Force × Distance, at the very tips of the screws there is little moment (distance = 0), but at the screw-to-plate interface where the distance is greatest, the screws bear substantial cantilever loads. The stress from a fixed moment arm construct may cause failure at the screw-to-plate, or screw-to-rod, interface.

Figure 11.1 3-D visual representation of spine with forces applied to the spinal motion segment identified: (A) Compression; (B) Tension; (C) Shear; (D) Torsion; (E) Bending moment.

The bending strength is critical for screws. It is important to understand that the strength of a screw is proportional to the cube of its diameter (equation: Z = (πdx³)/32). As the diameter increases, the strength increases exponentially to the third power. For example, the difference in strength between a 5 and 6 mm screw is nearly double. As seen in Figure 11.2, as the core diameter of a screw increases, the stress on the implant is reduced using the equation Stress = Bending moment/Z.

Therefore, to reduce the likelihood of screw failure, the largest diameter possible within the anatomic space should be used.¹⁵

Screw Pullout

Various other strategies such as bone cement augmentation with fenestrated and cannulated screws and expanding pedicle screws to increase fixation strength have also been explored, with high importance in cases of osteoporotic bone. The screw pullout test is often the gold standard to categorize the fixation level with higher pullout forces correlated with higher bone purchase. As stated above, one common practice for achieving strong fixation is to insert the largest diameter pedicle screw that is anatomically possible. Length of screws also plays a role, as biomechanical data also show a significant improvement in strength of screws with an
increase in screw penetration depth.¹⁶ Zhang et al. found a linear correlation between screw pullout strength and thread number on the screw.¹⁷ This confirms that the longer the purchase length of screw threads, the greater the pullout strength.

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