Technique with polymethylmethacrylate

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PVP is performed under fluoroscopic guidance. The patient is under conscious sedation and is positioned prone on a radiolucent table. Adequate and clear pictures must be obtained prior to the start of the procedure, as it is crucial to be able to visualize the cement being injected into the vertebral body. The back is then prepped and local anesthetic is injected over the area of needle placement. Under fluoroscopic guidance, an 11-G bone marrow biopsy needle is introduced into the fractured vertebra via a transpedicular approach (Fig. 1a,b). In the thoracic spine, one can opt to enter the vertebral body extrapedicularly, between the rib head and the lateral aspect of the pedicle. The needle is then advanced to the anterior half of the vertebral body. At this point, an optional intraosseous venogram can be performed to aid in placement of the needle out of the venous flow path to avoid embolization to the lungs. Additionally, the intraosseous venogram can aid in determination of the flow pattern in the vertebral body, which may allow for cement leaks. Once the needle is in the correct position, the cement is injected. The cement should be radio opaque, with addition of barium powder or tungsten powder. Each kit of polymethylmethacrylate (PMMA) cement can be mixed with 5.0 g barium sulfate and 2.0 g tungsten powder [3]. The cement is allowed to achieve a paste-like consistency prior to injection. Using a 1-cc or

Fig. 1 a,b Radiograph of an osteoporotic fracture with a needle in the fractured vertebra. c Computed tomography scan showing needle positioning

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Fig. 2 a Computed tomography scan showing cement filling after bilateral needle injection. b Lateral view radiographic control

3-cc syringe, the cement is injected into the vertebral body under fluoroscopic guidance. Filling of the posterior one-third of the vertebral body should signal the end of the injection to avoid overfilling (Fig. 2). Typical volumes for cement injection are 2-3 cc for thoracic and 3-5 cc for lumbar vertebrae [3]. Usually there is symmetrical filling of the vertebral body, but if it is asymmetrical, then the contralateral pedicle can be used for further delivery of the cement. After the procedure, the patients are allowed to ambulate as tolerated.

fracture levels and that overfilling can increase the stiffness beyond that of the intact state. Overfilling has several other disadvantages: it can cause asymmetrical distribution and lead to single-sided load transfer and toggle, it can lead to leakage of cement into the epidural space [54], and in the long term it can cause increased stress on adjacent vertebrae, leading to increased risk of adjacent level fractures [9].

Whether to perform a bipedicular or unipedicular approach depends on the individual case. In biomechanical controlled studies, no significant difference has been found between the two techniques in terms of strength and stiffness [6, 39]. Further analysis, however, shows that while providing the same strength and stiffness, the use of a uni-pedicular approach leads to a medial-lateral bending motion or toggle toward the untreated side with uniform loading [39]. The clinical significance of this toggle is not known. Clinically, the two techniques have been shown to give similar results. The unipedicular approach can result in filling across the midline in 96% of cases [33]. The mean opacification of the vertebral body did not differ between the groups. More importantly, there was no difference in the amount of pain relief achieved with the two techniques.

Biomechanical considerations

There is a continual effort being made to optimize the technique of PVP. Biomechanical and clinical studies have been performed to determine the characteristics of different cements, the role of cement volume, and differences in the approach used (unipedicular vs bipedicular). Presently acrylic cement such as methylmethacrylate is used most frequently for PVP. Use of cement in a fractured vertebra has been shown to increase vertebral body strength and stiffness [4, 8, 25, 40]. Other materials, like glass-ceramic matrix [4], calcium phosphate [40], and hydroxyapatite [8, 25] have also been compared to methylmethacrylate and have shown similar biomechanical properties. The theoretical clinical benefit of using calcium phosphate or hydroxyapatite is that they are osteoconductive and can undergo remodeling, although the ability of pathologic os-teoporotic bone to regenerate or, for that matter, to remodel is questionable.

The effect of different cement volumes on the biome-chanical properties of the vertebrae depends on the type of cement used. Belkoff et al. [6] showed that when using Orthocomp, thoracic and thoracolumbar vertebrae needed 4 cc and lumbar vertebrae needed 6 cc to restore stiffness to the pre-fracture levels. For simplex P, the volumes needed were 6 cc and 8 cc, respectively. Using anatomically accurate finite-element models, it has been shown that approximately 15% volume fraction or approximately 3.5 cc is needed to restore stiffness of the vertebra to pre-

Clinical results: literature review

The clinical results of PVP from the United States, Europe, and Asia show a 70-95% success rate in relieving pain. Most reports in the literature are retrospective, although a few prospective studies have been published. The main indication for the procedure is pain persisting despite nonoperative treatment of osteoporotic compression fractures. One series bravely included four burst fractures treated with PVP [46]. The majority of the cases are around the thoracolumbar area. The largest retrospective study [18] was a collaboration between seven centers in the US, where 488 consecutive patients underwent PVP for vertebral compression fractures. A telephone questionnaire was conducted with 245 patients at median of 7 months' follow-up. Questions were designed to measure pain, ambulation, and ability to perform activities of daily living. The pain decreased from a mean of 8.9 pre PVP to 3.4 post PVP. Ability to ambulate was impaired in 72% pre PVP and in 28% post PVP. Ability to perform activities of daily living improved significantly post PVP. There was a 4.9% rate of minor complications.

In another study, Barr et al. [3] studied 38 patients with 70 symptomatic fractures who had failed to respond to medical treatment. After undergoing PVP, 63% reported marked to complete relief and 32% had moderate relief of pain. Peh et al. [50] retrospectively studied 37 patients with 48 compression fractures treated with PVP. At a mean follow-up of 11 months, pain relief was complete in 47% and partial in 50%.

More recently, prospective studies have shown similar success with PVP. The largest prospective study [43] reported on 100 patients who underwent PVP for vertebral compression fractures. At final follow-up averaging 21 months, 97% of the patients reported significant pain reduction, with the VAS improving from 8.9 to 2.0. Cortet et al. [13] added to the literature by reporting on 16 patients with 20 VCFs of more than 3 months' duration not responding to medical treatment. They all underwent PVP and showed a statistically significant improvement in VAS pain score immediately after the procedure, which remained at 30, 90, and 180 days after the procedure. Additionally, there was a significant improvement in the general health status as assessed by Nottingham Health Profile, which includes pain, mobility, emotional reaction, social isolation, and energy.

The longest follow-up has been reported by Perez-Higueras et al. [51], who followed 13 patients with VCFs for at least 5 years following PVP. The VAS improved significantly from a score of 9 pre PVP to 2 immediately post PVP, to 1 at 3 months. At 5 years, the VAS was 2.2. Significant improvement after treatment with PVP was also noted on the McGill Questionnaire.

The safety and efficacy of the procedure in the upper thoracic spine was reported by Kallmes et al. [29], who studied 41 patients with 63 vertebral compression fractures from T4 to T8. There was a significant pain reduction, as the mean VAS decreased from 9.7 pre PVP to 1.7 post PVP. There was one case of a pedicle fracture and no cases of pneumothorax.

The issue of timing of vertebroplasty was reviewed by Kaufman et al. [31]. Seventy-five patients with 122 VCFs underwent PVP. The age of the fracture at time of PVP was not independently associated with post PVP pain or activity. The procedure was efficacious in reducing pain and improving mobility in patients, regardless of the age of the fracture. However, the authors found that increasing age of the fracture was independently associated with increased needs of analgesia post PVP. Whether the delay in carrying out PVP leads to tolerance of and dependence on pain medication, leading to higher requirements post PVP, is not known.

Complications

While these clinical studies have shown good success rates in improving pain and function, the procedure is not without risks and complications. Most series report a complication rate of between 4 and 6% [3, 15, 18, 28]. Reported complications associated with the insertion of the needle include rib fractures [28], neuritis [3], pedicle fracture [29], and infection [29]. The most feared complication is the potential for leakage of cement into the spinal canal (Fig. 3) or into the venous system. Cement leakage into the spinal canal has been reported in a small number of

Fig. 3 Cement leakage in the foramen

Fig. 3 Cement leakage in the foramen

patients without causing any clinical symptoms [46], while there have been reports of transient neuropathy [28] and one case of paraplegia associated with PVP of T11 [36]. We have consulted on a patient in whom PVP was performed for burst fracture of L2 with cement leakage into the spinal canal causing symptoms of spinal stenosis. The patient underwent a decompression and removal of cement from the spinal canal.

Leakage of cement into the venous system can have a spectrum of clinical consequences, from being asymptomatic [51], causing pulmonary embolism [27, 47], or causing a paradoxical cerebral artery embolization in a patient with patent foramen ovale [57]. In a recent study [46], 17 patients had CT scans performed immediately after undergoing PVP. Cement in the epidural veins adjacent to the vertebra was found in 48% of the cases, with only one patient developing a transient neuritis. The risk of cement leakage into the spinal canal or venous system is increased with higher volumes of injected cement [54]. This problem is so feared that some have advocated the use of pre PVP venography to assess the risk of cement leakage.

Venography can document sites of potential leakage during cement injection [21, 42, 63]. In one study [42], venog-raphy was performed prior to vertebroplasty, and the results retrospectively reviewed. Venography could predict the flow characteristics of cement within the vertebral body and within the venous structures. While venography could predict cement leakage into endplates or central defects in 100% of cases, it could only predict leakage into the venous structures in 29% of the cases. Another study [63] specifically looked at 205 PVP procedures in 137 patients without antecedent venography, and found only one cement leakage causing symptoms of radiculopathy. The value of antecedent venography will need to be determined with prospective studies.

A topic of interest is the occurrence of new vertebral body fractures after PVP in patients with osteoporosis [2, 9, 62]. This was noted in a follow-up of 25 patients who underwent PVP. The average follow-up was 48 months. The authors found a significantly increased risk of vertebral fractures adjacent to a cemented vertebra, with the odds ratio of 2.27, whereas the odds ratio for sustaining a vertebral fracture next to an uncemented fracture was 1.44

[23]. In another report [62], 177 patients treated with PVP for osteoporotic fractures were followed for a minimum of 2 years. Twenty-two patients (12.4%) developed a total of 36 new vertebral body fractures. Two-thirds (67%) of the new fractures involved a vertebra adjacent to a previously treated vertebra.

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