The treatment of vertebral compression fractures is commonly performed using vertebroplasty or kyphoplasty techniques. Many variations of both techniques exist including different types of needles, balloons, and channel creating devices as well as different types of cements. Regardless of the technique used, excellent pain relief and a very low complication rate have been reported.

Background and biomechanics

More specifically, multiple new devices have been recently introduced for vertebral augmentation, the so-called third generation of vertebral augmentation. Those devices are permanent implants that are introduced percutaneously with the intend of restoring the vertebral height and the sagittal balance of the spine. The spinal sagittal alignment is normally erect and vertical. It is characterized by a virtual line, which passes from the mid-body of the C7 vertebral body through the superior endplate of S1 within 2 cm of the posterosuperior corner of the S1 vertebral body. This is called the C7 plumb line. In the normal erect position, the patient’s center of gravity is anterior to the spine, creating a slight forward bending moment, which are mainly counterbalance by muscles and ligaments. Mechanically, the normal sagittal balance is to allow transmitting the load through the axial skeleton, and more particularly mostly through the anterior column.

Abnormal balance risks producing abnormal forces, adversely affecting both the neural and bony structures. Specifically, vertebral fractures, particularly in the thorax region, are often referred to as kyphotic fractures for being associated kyphotic spinal angulation. These structural changes inherently lead to the patient’s center of gravity being shifted more anteriorly, increasing the lever arm of the forces and the forward bending moments on the already fragile spine [1]. These changes often result in a further compression of the fractured vertebral but also put adjacent vertebrae at a higher risk of developing new fractures. Kyphotic fractures also result in the posterior column transmitting higher percentage of the spinal compression load and as such often result in excessive stresses in the facet joints and posterior spinal structures.

We conducted a PubMed review of the existing literature on the vertebral augmentation implants using the following terms: “implantable vertebral augmentation device,” “SpineJack vertebral augmentation,” “Kiva vertebral augmentation,” and “vertebral body stenting.” We selected those publications particularly covering long-term follow-ups and comparisons to the commonly used BKP. We included all pertinent randomized trials published on the aforementioned techniques. We excluded literature describing implants currently not yet available to treat patients. The results of the review were synthesized and summarized (Table 1).

Table 1 Summary of main articles, design, and key points used to review data for each vertebral implant
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Vertebral body stenting

The device

The vertebral body stenting system (VBS) is an expandable, metallic scaffold stent made of cobalt chromium alloy that is placed in the compressed vertebral body for percutaneous augmentation (VBS; Synthes, GmbH, Solothurn, Switzerland). The stent is first introduced into the vertebral body bilaterally with a bipedicular approach under fluoroscopy guidance [2]. A balloon is inserted inside the deflated stent and then expanded into the vertebral body and can achieve high expansion ratio. After achieving a reasonable size, the balloon is then deflated and removed without risk of partial height loss because the stent remains expanded inside the vertebral body.

VBS is indicated in osteoporotic vertebral compression fractures without involvement of the posterior wall. So, it can be used in Type A1 and some A2 fractures Magerl’s classification with relative contraindication for advanced posterior wall involvement. Multiple biomechanical studies demonstrated a significant reduction in height loss after balloon deflation when using the VBS as compared with balloon kyphoplasty (BKP) [3, 4]. VBS was able to maintain the pre-fracture height and avoid the loss of height, which was achieved with balloon deflation in kyphoplasty (Fig. 1).

Fig. 1
figure 1

A 72-year-old woman with old compression fracture of T10 and new fracture of T12. A and B Sagittal reconstruction CT examination shows almost vertebra planna of T12 with intra-spongiosum vacuum. The VBS before (C) and after (D) full deployment inside the VB. E and F Sagittal reconstruction CT examination shows restoration of boy height, cement and stent inside vertebral body. Patient also underwent a prophylactic VP at T11 and L1 levels

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Clinical outcome

Clinical and radiological six- and twelfth-month follow-up was reported by Muto et al. [5] The height in the fractured vertebral body was increased in 12 of the 20 Vertebral compression fractures (VCFs) included in the study by an average of 1.5 mm. No vascular, foraminal or epidural leakage or other adverse events were observed. In the 12-month clinical follow-up, the authors reported a reduction of four points in the VAS and a 40% reduction in the ODS scores compared with the pre-treatment values. Vertebral stents were radiologically stable at 12-month follow-up with no reported new adjacent vertebral fractures.

In a 2-year retrospective study of patients with traumatic thoracolumbar incomplete burst fractures [6], VBS restored the vertebral kyphosis by 3.2° and segmental kyphosis by 5°. A minor degree of loss of height was observed at follow-up, losing 0.8° vertebral kyphosis and 2.1° segmental kyphosis correction with reported clinical outcomes comparable with BKP. In another prospective study of Type A1.3 or A3.1 (incomplete burst fractures) [7], vertebral body stenting results in a satisfactory improvement in pain, function, and kyphosis correction. Anterior spinal column, especially the fragmented superior endplate, was fairly reconstructed by the stent.

In a randomized control trial between VBS and BKP, Werner et al. [8] reported no beneficial effect of vertebral body stenting over balloon kyphoplasty in painful osteoporotic vertebral fractures with regard to kyphotic correction, cement leakage, radiation exposure time, or neurologic sequelae. They also reported that vertebral body stenting was associated with more device-related complications.

The Kiva system

The device

The Kiva system (IZI Medical, Owings Mills, MD USA) consists of a coiled implant made of PEEK material, mixed with 15% barium sulfate to render the implant radiologically visible. The Kiva cannula is inserted over a guide pin using a traditional transpedicular approach. One implant is used for each vertebral body. For this reason, the access needle and the guide pin should aim for the upper outer corner of the vertebral body to allow the implant to be deployed in the center of the compressed vertebra.

A pre-shaped nitinol Kiva coil is deployed inside the vertebral body through the access canula. Up to 4 or 5 coils can be stacked in the vertebral body depending on the size. The Kiva PEEK implant is then deployed over the nitinol coil loops, which is then retracted slowly. The PEEK implant is now centered inside the compressed vertebra with multiple small holes alone the inner surface. When PMMA cement injected, it flows centrally through those holes with the intent to keep the cement inside the implant and decreases chances of leakage. The implant is then separated from the cannula, and the delivery device is removed (Fig. 2).

Fig. 2
figure 2

A 77-year-old woman with an acute L1 compression fracture. A Sagittal T1-weighted image showing the fracture line. B PEEK coiling after removal of the coil. C AP view after cement injection inside the PEEK coil.

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Clinical outcome

The KAST study [9] was a large prospective, multicenter, randomized, controlled trial designed to demonstrate non-inferiority of Kiva implant to BKP. A mean improvement from baseline to 12 months (primary end point) of 70.8 and 71.8 points in VAS score and 38.1 and 42.2 points in ODI score was noted in the Kiva and Balloon Kyphoplasty arms, respectively. No device-related complications occurred. Analysis of secondary endpoints revealed an advantage of the Kiva device with respect to the amount of cement used and rate of cement extravasation (less in Kiva group). Also, a reported positive trend in adjacent level fracture was observed. An analysis of the peer protocol population showed that 13.8% (16/116) of the Kiva group and 20.2% (23/114) of the BK group had experienced a new adjacent level fracture.

Otten et al. [10] duplicated the KAST study results and reported better pain relief in the patients treated with the Kiva compared with BKP at 6-month follow-up. The authors also reported significantly lower adjacent level fracture rates with the Kiva device when compared with BKP.

Korovessis et al. [11] studied both Kiva and BKP devices and reported that both devices provided similar vertebral body height restoration; however, only the Kiva group provided some vertebral body wedge deformity correction. In cases of osteolytic metastatic lesions, BK and Kiva both provided equally significant pain relief [12]. The authors also suggested that the reduced cement leakage in the Kiva cases, although lower viscosity PMMA is typically used, increases the safety of the implant in augmenting severely destructed vertebrae.

Implantable titanium vertebral augmentation device: the SpineJack

The device

The concept of the Spine Jack (Stryker, Kalamazoo, MI USA) implant is somewhat different from the other described devices. It allows only craniocaudal expansion of the device compared with the round expansion of other implants. This helps to focus most of the applied forces during deployment only on the vertebral end plates, and in that way, height restoration can be obtained without centrifugal movement of bone fragments. This device is reported by some authors as a favorable option in cases of posterior wall disruption; in fact, reduction of the bone fragments is potentially possible in presence of an intact posterior longitudinal ligament [13].

A bilateral transpedicular access is used to insert the SpineJack device, preferably parallel to the endplates. The implants are then deployed by gradual expansion into the appropriate height using a mechanical handle. Bone cement can be injected through a dedicated bone filler device. (Fig. 3). SpineJack has a “self-locking security mechanism” [14]. This avoids extreme expansion and possibility of endplate injury. The device is clinically approved for treatment of benign and malignant compression fractures as well as traumatic fractures (A1, A2, A3, A4, and type B in selected cases, according to AO classification) [15].

Fig. 3
figure 3

A 44-year-old man with traumatic A1 fracture of L1 that failed conservative treatment for 6 months. A Sagittal reconstruction CT image showing the fracture line and about 50% decreased height. B Shows the SpineJack device inserted before deployment. C Deployment of the device with subsequent elevation of the endplate in (arrows). D Lateral view after cement injection with maintained endplate elevation

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In biomechanical studies comparing SpineJack with standard BKP, Krüger et al. [16, 17] were able to demonstrate the advantage of this new technique concerning height restoration and height maintenance. The studies showed that height restoration was significantly better in the SpineJack group compared with the BKP group resulting in better restoration of the sagittal balance and a reduction of the kyphotic deformity.

Jacobson et al. [18] suggested two key biomechanical advantage of the SpineJack in treating osteoporotic vertebral fractures that result in reducing the incidence of adjacent level fractures and deformity. First, the broader support provided to the lateral and anterior parts of the fractured and the depressed superior endplate minimize the anterior shift of the center of gravity that is commonly seen with anterior fracture deformity and kyphosis. Second, correcting kyphotic angulation and maintaining height correction. These changes result in restoring the superior adjacent disc pressure closer to normal. This in turn adds support to the adjacent vertebra, and the incidence of adjacent fractures is significantly decreased. Studies are needed to support these assumptions and related advantages over other devices.

Clinical outcome

In a prospective multicenter study of 107 patients and a 1-year follow-up [19], the authors demonstrated a median decrease in pain intensity (VAS) of 81.5% as a primary endpoint. They also reported a significant reduction in analgesic intake, improvements in disability (91.3% decrease in ODI score), and in quality of life (increase 21.1% of EQ-VAS score) when measured at 3 and 12 months after vertebral augmentation. The study demonstrated a reduction and maintenance of the kyphotic angulation maintained at 12 months postoperatively. No reported adverse events or implant-related complications.

When SpineJack compared with BKP in a prospective randomized trial [20], both techniques showed long-lasting (12 months) decrease in pain levels (94% for SpineJack versus 82% for BKP) and decrease in functional disability (94% for SpineJack and 90% for BKP). Jiann-Her et al. [21] reported significantly more efficient height restoration and kyphotic angle correction with SpineJack compared with BKP. In a 3-year follow-up [22] of both SpineJack and BKP patients, displayed very good long-term clinical efficiency and safety in patients with osteoporotic compression fractures with vertebral body height restoration/kyphosis correction seemed better with the SpineJack procedure.

A prospective multicenter randomized trial of 141 patients comparing SpineJack and BKP (SAKOS) [23] confirmed non-inferiority of the SpineJack to BKP at 6- and 12-month period. SpineJack group was superior to BKP group in VB height restoration at 6- and 12-month period and in pain relief at 1- and 6-month period after surgery. Adjacent fractures were significantly lower after the SpineJack procedure when compared with the BKP patients (12.9% vs. 27.3%).

Future applications of vertebral body implants

Many studies are underway to evaluate the applications of the vertebral implants in different types of spinal fractures, mostly in burst fractures [24]. Traditionally Magerl A1 fractures represented the primary indication for VP or BKP. These fractures can also be treated with non-surgical management. Fracture types A1 and A2 (with the exception of A2.3) can be regarded as stable fractures. Burst fractures and fractures involving the posterior wall lack sufficient anterior column integrity and are classified as unstable. Surgical intervention with anterior column reconstruction is necessary with unstable A fractures and some flexion-distraction (Magerl type B) fractures. In lieu of pedicle screw and rod augmentation and anterior column reconstruction, a vertebral implant like the SpineJack implant can potentially be used in some of these cases to reduce the fracture and provide additional anterior column support to what is possible with cement alone. The SpineJack implant does not seem subject to some of the limitations of BKP including the loss of restored height once the balloon is removed [25, 26]. Worth mentioning in that regard is the difficulty in treating Magerl A2 fractures because of a sagittal or coronal split with a balloon that expanded outwardly [27]. The SpineJack implant may be also capable of producing ligamentotaxis thereby better reducing complex burst fractures and pulling the displaced fracture fragments back to better alignment. Overall the SpineJack implant had been reported to be successfully used on traumatic burst fractures even without the use of additional posterior pedicle screw and rod instrumentation [28].

Treatment of traumatic burst fractures is a controversial subject and ranges from simple conservative treatment to extensive surgical repair. A multicentre registry of 103 traumatic burst fractures treated with Spinejack was reported by Noriega et al.19 as early as 2015. The authors reported significant decrease in pain and analgesic use. They also reported 3- and 12-month improvements in disability scores and quality of life. Postoperative reduction in kyphotic angle was maintained up to 12 month follow-up.

Regarding the use of SpineJack in different types of burst fractures, Kerschbaumer et al.xxix [29] reported that comminuted type A3 fractures with more wedge angles will usually show a more gain than A1 type fractures with less comminution, as comminuted fractures are usually easier to reduce. On the other hand, these comminuted fractures are more at risk of secondary loss of reduction when compared with less comminuted fractures. Vanni et al. [30] used the SpineJack for anterior and middle column reconstruction as a valid alternative to the corpectomy in a limited number of patients with vertebra plana, especially in elderly patients and those with high operative risk. All cases were performed with posterior instrumentation and fusion.

Recently, a stent-screw-assisted internal fixation (SAIF) technique xxxi [31] has been described as a minimally invasive option for VB reconstruction in advanced benign and malignant compression fractures. The procedure is designed to allow a 360° non-fusion form of vertebral fixation. This can be accomplished by placing 2 vertebral body stents (VBS) followed by percutaneous insertion of fenestrated, cement-augmented pedicular screws. This procedure can potentially prove to be useful to avoid more invasive procedures like corpectomy.

Both SpineJack and vertebral body stenting (VBS) using the same (SAIF) technique had shown potential in treatment of burst fractures with posterior wall retropulsion, and no neurologic deficit, armed kyphoplasty [32] yields fracture reduction, internal fixation, and indirect central canal decompression.

Malignant osteolytic spinal metastasis with destructive wall involvement had been treated with the Kiva peek implant aiming at decreasing risks of cement extravasation. In a series of 40 patients [33], the median pre-treatment VAS of 10 (range 6–10) significantly (P < 0.001) dropped to one (range 0–3), with all patients achieving a clinically relevant benefit on pain at 1 month. Differences in pre- and post-treatment analgesic therapy were significant (P < 0.001).

Very little experience is reported for the use of vertebral implants after radiofrequency ablation of metastatic lesions of the spine. Theoretically, placement of a vertebral implant after radiofrequency ablation of a tumor mass can help to add more stabilization of the fracture and maintain kyphotic angle especially with extensive destructive lesions that are critically unstable. Implants could potentially help avoid surgical treatment and also help reducing the amount of cement injected thus decreasing the extravasation rate and extent of cement leakage, a more common problem in malignant lesions. The Kiva system is potentially beneficial in lesions with absent portions of the posterior wall where the cement is contained inside the implant or in focal lesions to add stabilization. The vertebral body stenting with the (SAIF) technique could provide an advantage in cases of retro-pulsed fragments. The SpineJack implant can be used in mechanically unstable fractures (Fig. 4). SpineJack and stents could potentially produce artifacts in MRI imaging more than the PEEK Kiva implant. This should be considered if MRI imaging will be needed in the future.

Fig. 4
figure 4

A 75-year-old man with biopsy proven unusual multiple myeloma T9 and T10 levels crossing the disc space. Sagittal (A) and axial (B) enhanced T1-weighted images with fat suppression technique show complete involvement of the vertebral body with enhancing tumor mass. Axial image (B) taken at T10 level. C Final post procedure lateral images after radiofrequency ablation, SpineJack deployment, and cement injection. Immediate post procedure sagittal (D) and axial (E) at T10 enhanced T1-weighted images with fat suppression technique showing almost complete absence of enhanced tumor tissue with minimal linear artifact showing the implant position

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Summary and conclusion

The third-generation percutaneous vertebral augmentation systems are an important addition to the armamentarium of devices available for the treatment of spinal fractures. Those devices provide multiple advantages over traditional vertebroplasty and kyphoplasty and have demonstrated a significant short- and long-term improvement in the amount of vertebral height restored during the procedure. The kyphotic angle correction is also more obvious with the SpineJack implant due to the direct craniocaudal expansion provided by the device. In addition, there may be better pain relief and significantly less adjacent level fractures as compared with BKP. There is a consistent tendency to use less cement than in BKP that has also been associated with less cement extravasation.

The reported incidence of less adjacent level fractures has surprisingly improved on the natural native incidence of VCFs. Some additional research is required to understand the overall impact of these new observations.

Most of those implants still require a relatively large access cannulas and are not suitable for all vertebral levels, in particular the high thoracic ones. More research will be needed to justify the benefits of implants compared with either other forms of vertebral augmentation, spinal surgery, and non-surgical treatment.