17 Additional and Adjacent Level Fractures after Vertebral Augmentation
Vertebral compression fractures (VCFs) are common as are the occurrence of additional vertebral fractures that happen after the incident level fracture. There are several reasons that predispose patients to additional vertebral fractures including the anatomy of the fracture and the use of corticosteroids but the primary determinate factors are low bone mineral density and spinopelvic imbalance. Prior vertebral compression fractures also increase the risk of an additional fracture substantially with an increased risk up to 75 times higher for patients with three or more fractures. There is ample data showing no increased risk of additional fractures after vertebral augmentation but certain procedural features may predispose to adjacent level fractures such as cement extravasation into the disk.
Compression fractures of the spine are a large problem, affecting between 700,000 and a million persons in the United States annually, and 25% of women in their lifetime. 1 , 2 Treatment of these compression fractures should not only focus on the index fracture but also on preventing further fractures in the future. To better understand how to prevent future fractures, it is imperative that we know what puts patients at risk for developing new fractures.
17.2 Risk Factors for Additional or Adjacent-Level Fractures
There are several risk factors associated with the development of additional or adjacent-level fractures. These risk factors are similar to the risk of initial fractures discussed in Chapter 4. Decreased bone mineral density (BMD) is the primary determinant in the development of additional or adjacent fractures whether they are treated or untreated. 3 – 15 In fact a prominent decline in BMD of 2 SD is associated with a fourfold to sixfold increase in risk of additional fracture. 15 , 16 In addition, the use of chronic corticosteroids has been associated with a propensity for developing VCFs and recurrent compression fractures 11 , 17 via their effects promoting osteoclastic activity, inhibiting osteoblastic activity, and the interference on the small intestine’s ability to absorb calcium. The presence of prior fracture is also predictive of future fractures. A single compression fracture increases the risk of another fracture by 3.2 to 5 times, the presence of two or more fractures increases the risk of another fracture by a multiple of 10 to 12, and the presence of three or more fractures increases the risk of another fracture by a very substantial 23 to 75 times. 15 , 18 , 19 A combination of low BMD and more than two prior fractures increases the risk of a new vertebral fracture to at least a factor of 75-fold, relative to women with a BMD in the top 67th percentile and no prior fracture. 15
However, it is not just the presence of a prior fracture that increases the risk of future fracture; there are biomechanical effects directly related to the index fracture that are associated with the increased risk of future fracture. End plate fracture itself affects the biomechanics of the spine by disrupting the ability of the intervertebral disk to pressurize, which increases the compressive loading on the anterior wall of the adjacent vertebrae, predisposing it to fracture. 20 The correction of end plate deformity by reducing the end plate and stabilizing it with polymethyl methacrylate (PMMA) decreases this force and is an important factor in decreasing adjacent-level fractures. 20 An even more important factor in causing additional and adjacent-level fractures is the kyphotic angle formed by the index fracture. This kyphotic deformity causes the body mass from above the fracture to deviate anterior to the typical center of balance, thereby increasing the effective pressure on adjacent vertebrae. 21 Additionally, to maintain equilibrium, the paraspinal muscles must supply a force equal and opposite to this anterior mass movement in direct proportion to the lever arm. So, as the kyphotic deformity increases, so do the downward forces on the spine especially on the anterior portion of the vertebral body.
Clinically, these biomechanical factors have been shown to be directly related to the occurrence of additional and adjacent-level fractures. Lunt et al 19 showed that the shape of the fracture (▶Table 17.1) strongly influences the risk of future fracture, with more kyphotic deformity increasing the risk of fracture by a factor of 5.9 as compared with a compression fracture without kyphotic deformity that increases the risk of future fracture by a factor of only 1.6. In addition to the focal anatomic changes caused by a VCF, Baek et al 22 further defined the relationship between postfracture spinopelvic balance and the development of new fractures. The presence of a segmental kyphotic angle (SKA; ▶Fig. 17.1) of over 11 degrees is associated with increased risk of fracture regardless of whether the vertebral body has undergone augmentation. Other spinopelvic parameters associated with increased fracture risk include a sagittal vertical axis (SVA; ▶Fig. 17.2) of greater than 6 cm, sacral slope (SS; ▶Fig. 17.3) less than 25 degrees, and a lumbar lordosis (LL) of less than 25 degrees.