32.1 Introduction

Vertebral augmentation (VA) has been extensively used for pain palliation and stabilization of vertebral body (VB) fractures due to trauma, osteoporosis, and tumors. ^{1 – 3}

32.2 Osteoporotic Fractures

Osteoporotic vertebral fractures can occur spontaneously or due to trauma, generally a compressive load injury mechanism involving the VB. ⁴ Both the anterior and middle columns together support about 80% of the overall spinal load (i.e., muscle forces and body weight) and are most commonly affected. The spectrum of severity ranges from mild and stable compression fractures, ⁵ affecting the disk–end plate region and leading only to minor deformity to unstable fractures with a high-degree of osseous fragmentation, collapse deformity, middle-column involvement, pediculo-somatic junction fracture, and kyphotic deformity ^{6 , 7} (▶Fig. 32.1a, b). The underlying poor bone quality surely represents a risk factor ^{8 , 9} and might prevent osseous healing, potentially evolving toward the creation of osteonecrotic clefts ¹⁰ and, together with the detrimental effect of the increased bending momentum due to kyphosis at the fracture level, ¹¹ might be responsible for fracture progression (▶Fig. 32.1c, d).

Cement VA is widely used to treat fragility fractures, palliate pain, restore axial load capability of the VB, and arrest fracture progression. ¹²

Ideally, VB reconstruction, height restoration, and homogeneous cement augmentation should be obtained with bone cement filling the two anterior thirds of the VB from superior to inferior disk end plates on both sides of midline, especially for the most severe of these fractures.

In reality, vertebroplasty is not intended to restore VB structure or height, and balloon kyphoplasty (BKP) has not been proven to guarantee sufficient height restoration, either due to the fact that the balloon tamps expand following the path of least resistance, or due to the deflation effect between balloon removal and cement injection. Moreover, the polymethyl methacrylate (PMMA) cement does not have adhesive properties to ensure stability in highly fragmented osseous structures, and the cement might distribute into the fractured VB in a heterogeneous and unpredictable manner. The cement may extend around the trabeculae and into intra-osseous clefts and it is not always possible to safely augment the entire VB, especially the bone adjacent to the intervertebral disk and end plates. Cement injection, usually monitored with fluoroscopy or CT imaging, is halted if cement tends to leak outside of the VB, into veins or if it approaches the posterior third of the VB. The termination of the cement injection is for the purpose of minimizing the risk of epidural leakage.

Following VA, refracture of the treated VB is a well-known and reported event. ^{13 – 17} The refracture usually implies subsidence of the non-augmented portions of the VB around the cement cast ¹⁸ (▶Fig. 32.2). This occurrence might be asymptomatic or be accompanied by pain recurrence. The refracture may be simply a minimal remodeling of the VB around the cement augmentation or it may manifest as a more prominent collapse of the non-augmented portions of the VB.

A less frequent event is the refracture of the middle column, at the junction between middle and posterior third of the VB where most commonly the junction between cement-augmented and non-augmented VB is located. ¹⁹ These fractures are characterized by involvement and retropulsion of the posterior wall and eventually result in catastrophic splitting and separation between augmented anterior portion of the VB and middle column (▶Fig. 32.3). This may also be accompanied by a kyphotic deformity at the incident level. Such fractures are not frequent and are largely unreported but when they do occur they pose a real therapeutic challenge. ^{20 , 21} These fractures may be repaired by surgical decompression of the central canal, corpectomy and grafting, and posterior stabilization, but this is a very invasive surgical procedure that is associated with significant morbidity and mortality risk, especially in elderly patients with scarce bone quality. ²²

In our anecdotal experience these refractures occur mostly, although not exclusively, in the lumbar region, likely due to a combination of higher loads (both forces and bending moment) ^{23 , 24} that are accentuated by the kyphotic shape of the fractured vertebra. ¹¹

The importance of the middle-column stability might be indeed largely underestimated since the load-bearing capacity of the vertebra is usually referred to the anterior column as a whole structure, totally neglecting the important role of the middle column. Furthermore, the middle column, with the posterior third of the VB, the posterior wall, and the pediculo-somatic junctions, remains relatively non-augmented, even after satisfactory cement augmentation due to the safety measure to avoid cement dispersion too close to the posterior wall. The middle column, after cement augmentation, if observed on an axial post-procedure CT image, might be regarded as a “bare area,” not reinforced, and therefore a potential point of weakness of the vertebra (▶Fig. 32.4). Finally, some authors have reported the cement augmentation of the pedicles and pediculo-somatic junction, the so-called pediculoplasty, ²⁵ but it should be considered that main stress forces at the level of the pedicles and pediculo-somatic junction are tensile, and PMMA is known to have optimal resistance to compressive loads rather than to tensile ones. ²⁶

Another aspect to consider is the transition between the augmented anterior portion of the vertebra that is completely filled with high-modulus bone cement, and the middle column, which is much weaker, particularly when osteoporosis is present. The result is a load intensification effect on the middle column, possibly explaining the collapse seen in clinical practice. Standard VA might therefore represent an under-treatment in osteoporotic fractures with middle-column involvement.