30.1 Biomechanics and Objective Assessment

The stabilization of a fractured vertebra is for the most part an intuitively sound endeavor. Most chapters in this book have assessed the open surgical approach to spine stabilization. This chapter, however, focuses on a minimally invasive approach, vertebral augmentation.

Vertebral augmentation procedures can be categorized into two groups: (1) stand-alone vertebral body filler techniques (i.e., vertebroplasty) and (2) vertebral body expansion and filler techniques (i.e., kyphoplasty and related techniques). Both theoretically strive to achieve the acquisition of stability and deformity correction. Regarding the latter goal, kyphoplasty would theoretically be expected to be more effective regarding deformity (kyphosis) correction. The evidence regarding vertebral augmentation strategies as a treatment for vertebral compression fractures is not strong, but it does support their use. ¹⁴ Again, conflicting reports do exist. ⁴

Vertebroplasty, as a technique, essentially fills pores. This is illustrated in Fig. 30.1. As such, it would not be expected to significantly increase vertebral body height or correct deformity (e.g., kyphosis). Of concern with all vertebral techniques is the potential for leakage of liquid acrylic into the extravertebral spaces, particularly the spinal canal. This may be most commonly expected when breaches of the dorsal vertebral cortex are present.

The term kyphoplasty is derived from Greek roots: kyphos (“hump”) and –plasty (“plastic surgery,” from plassein, “to form”). Like a mud jack, it should be able to elevate the vertebral height and correct deformity by expanding the intravertebral space (Fig. 30.2). This is not often achieved, most likely because of the relatively excessive forces required to reduce the fracture and the relatively soft platform (vertebral body side of the end plates) to which the force must be applied. In addition, the stiffness of a vertebra increases rapidly following fracture. Hence, a delay of weeks to months following fracture radically affects the chance for correcting a fracture-related deformity.

Syringe and syringe design, the fluid mechanics associated with injection, and the amount of material injected affect deformity correction and stability acquisition, as well. Syringe characteristics have been shown to affect the rate and volume of an injection. The viscosity of the injectate affects the rate and volume of the injection, as well. Viscosity changes rapidly following initiation of the polymerization process with polymethylmethacrylate (PMMA). This and bone porosity are the major factors affecting injection volume. Flow rate and flow volume (penetration into bone) are described by laws of physics, including the Hagen-Poiseuille law and Darcy’s law, respectively. ^{1 ,​ 15 – 17}

Alternatives to kyphoplasty have been devised as strategies to create a void in bone and expand the marrow spaces. In one such technique, the cement is introduced into a bone void–filling container. This theoretically reduces the chance for cement extravasation outside the confines of the vertebral body. ¹⁸

Vertebral body stiffness is theoretically augmented by vertebral augmentation techniques. Other parameters, such as strength, are also affected. Although stiffness is augmented, it is not restored to preinjury values. ^{1 ,​ 19 ,​ 20} Stresses applied to bone following augmentation vary, depending on the type of bone. Hence, the efficacy of a vertebral augmentation procedure may be related more to the biomechanical characteristics of the bone than to the actual procedure selected or the injectate volume. ^{1 ,​ 21} Higgins et al confirmed these findings and observed that vertebral body strength is increased following vertebroplasty, but that the location of cement placement does not affect strength. They also observed that augmentation of the upper thoracic vertebrae is not associated with an increase in strength, as is the case in the low thoracic and lumbar vertebrae. Finally, they observed that specimens with low bone mineral density show greater strength improvement following vertebroplasty. ⁹ These factors obviously affect deformity correction, as well. Conflicting reports prevail. ^{22 – 24} Kayanja et al demonstrated that multilevel stiffness and strength are not affected by vertebral augmentation of an intermediate vertebra. They concluded that augmentation of vertebral compression fractures by kyphoplasty does not alter the stiffness or strength of the multilevel segments. ⁵ The implications of these findings are that compression fractures that occur subsequent to vertebral augmentation may not be related to the augmentation procedure itself, but rather to progression of disease and possibly deformity.

Adjacent-level fractures following vertebral augmentation procedures are relatively common. This phenomenon is related to several factors. First, the patient and the patient’s bone are susceptible because of structural and load-bearing characteristics. Second, if a kyphotic deformity exists, excessive stresses are applied to adjacent levels. Third, the vertebral augmentation procedure itself stiffens the treated segment and in turn causes greater forces to be applied to adjacent segments. Regardless, controversy also prevails in this arena, with conflicting reports refuting the aforementioned notion that stiffness augmentation transmits increased loads to adjacent segments. ^{25 – 29}

Ahn et al theorized mechanisms for both adjacent and nonadjacent fractures following vertebral augmentation procedures. They suggested that a direct transmission of forces via stiffening of the treated segment is at least in part a causative factor associated with adjacent-level fractures, with the transmission of loads directly to the adjacent segments (Fig. 30.3), whereas nonadjacent fractures may result when the pillar effect is not prominent because of the immobility of the adjacent segment (Fig. 30.4). ³ In this situation, fractures may be related to deformity (kyphosis), the immobility of the adjacent segment, and the mobility of the nonadjacent segment.

The material employed for vertebroplasty or kyphoplasty does not seem to affect pain reduction or vertebral body geometry related to fracture treatment. ³⁰ Also, injectate volume does not seem to be affected by injectate type. ³¹

Vertebral augmentation is often employed for vertebral bodies with cancer involvement. The tumor for which vertebral augmentation is most often performed is multiple myeloma, although other cancers may be amenable to such treatment. Oakland et al studied two donor spines, one affected with multiple myeloma and the other with metastatic bladder cancer. Following augmentation, they observed a significant increase in failure strength. Patterns of tumor infiltration affected fracture strength. ⁸ Such observations are critical to our appreciation and understanding of regional variations in vertebral body strength and to the planning of case-specific treatment strategies.