Crosstrees Percutaneous Vertebral Augmentation

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42 Crosstrees Percutaneous Vertebral Augmentation

Philip S. Yuan, Huilin Yang, Dewei Zou

KEY POINTS

• Osteoporosis is typically a silent disease that can first manifest with vertebral compression fractures (VCFs).
• VCFs can lead to kyphosis and functional decline.
• Treatment of VCFs with bedrest, bracing, and narcotic medications is often not effective.
• The Crosstrees system for percutaneous vertebral augmentation uses a removable pod to initially contain the polymethylmethacrylate, allowing for safer injection and preventing complications from cement extravasation.
• The Crosstrees pod also allows for more controlled height restoration and fracture reduction than that possible with kyphoplasty or vertebroplasty.

Introduction

Osteoporosis is a major public health problem affecting an estimated 55% of people over 50 years of age. Every year in the United States more than 700,000 people suffer from vertebral compression fractures (VCFs), with osteoporosis being the main cause. Osteoporosis, the most common metabolic bone disorder, is typically a silent disease, but has the potential to cause debilitating back pain when VCFs occur. Other causes of vertebral fracture include trauma, benign lesions (e.g., hemangioma), and malignant lesions (e.g., multiple myeloma and metastatic cancer). Osteoporosis is characterized by decreased bone mineral density.

In a normal person, the vertebral bodies are composed of a porous structure, called trabecular or cancellous bone, encapsulated within a thin external cap of cortical (dense) bone. In a person with osteoporosis, the trabeculae that form the central porous bone become thinner and weaker. When this occurs, the vertebra can fracture and become deformed. This deformation of the vertebral bodies is classified into three types according to the shape: wedge, biconcave, and crush. As the vertebral bodies collapse, the natural curvature of the spinal column changes. These changes have mechanical effects on the paraspinal musculature and nerves, resulting in a wide range of symptoms, including pain, decreased sensitivity, tingling, and weakness. Multiple VCFs can produce kyphotic deformity, pulmonary dysfunction, loss of appetite, depression, and functional decline.

Until recently, the options for treatment of vertebral fractures were limited. Patients were confined to bed for prolonged periods and were given large doses of analgesics. Bracing was used but was usually not well tolerated by these typically elderly patients and has fallen out of favor. These palliative treatments do not restore the anatomy of the patient’s vertebral column to the alignment and morphology it had before the fracture. Treatment success, defined as relief of pain symptoms, depended on the individual’s capacity to heal the fracture. This physical change, along with forward angulation, can cause persistent deformity.

The traditional surgical techniques used to treat vertebral fractures or to maintain spinal stabilization are not as effective in the setting of osteoporosis, because the weakened bone is often not strong enough to support the metallic rods and screws. Because of the debilitating nature of the disease, many different procedures have been attempted. Among these, the procedure that has been the most successful is the injection of polymethyl methacrylate (PMMA) bone cement into the vertebral body to stabilize it. This procedure, known as vertebroplasty or kyphoplasty (when a balloon is first used to create a space in the vertebra) is performed in patients with painful fractures that fail to respond to conservative treatment.

A potentially devastating complication of vertebroplasty is the accidental escape (or leakage) of PMMA from the vertebral body, a problem known as cement extravasation. This problem can damage the vital structures, such as the spinal cord, or can contribute to the formation of emboli as a result of the flow of cement to the venous plexus. This can result in serious neurological complications or even death. Kyphoplasty was developed to help minimize cement extravasation by first introducing a balloon tamp to create a space for the cement and compact the surrounding bone. However, cement leakage is still possible with kyphoplasty, because the cement is only injected after removal of the balloon.

The Crosstrees PVA (percutaneous vertebral augmentation) pod is a device designed to percutaneously provide well-controlled delivery of PMMA during vertebral augmentation. The Crosstrees PVA System (Crosstrees Medical, Boulder, Colo.) is designed for use with Mendec Spine PMMA manufactured by Tecres S.p.a. (Verona, Italy), which is marketed with approved indications for use in the treatment of pathologic vertebral fracture. The pod device consists of a catheter for administering the cement into a releasable closed fabric barrier. Following delivery of a known volume of PMMA and expansion of the pod to a defined size, the fabric barrier is opened and removed from the vertebral body, leaving only the PMMA within the bony structure. A final volume of highly viscous PMMA can be added to the center of the initial bolus to provide additional interdigitation of PMMA to the cancellous bone. The system is novel in providing the ability to control the delivery of PMMA to the vertebral body and maintaining fracture reduction without the need for a permanent implant to remain within the patient.

Indications and Contraindications

Surgical treatment of VCFs with the Crosstrees PVA pod is indicated when debilitating back pain persists despite nonsurgical therapies (Table 42-1). MRI, the imaging study of choice for diagnosing VCF, typically shows increased signal on short T1 inversion recovery (STIR) sequences when a VCF is acute/subacute or if there is residual bony edema indicating incomplete healing. A nuclear medicine study (bone scan) is particularly useful when a MRI cannot be performed (e.g., when pacemaker is present).

TABLE 42-1 Relative Indications

1. Confirmed acute pain and tenderness over the spine at or near the level of x-ray compression deformity and positive MRI or bone scan evaluation

2. Not more than three vertebral compression fractures located between T4 and L5
3. Painful fracture with a loss of 0% to 60% of the height of the vertebral body compared with the height of an adjacent vertebral body that is normal, as determined by the radiological evaluation
4. Confirmation of fracture by MRI including T1-, T2-, and STIR-weighted sequences to determine the type and presence of fracture(s); or nuclear medicine study (bone scan) when MRI cannot be performed (e.g., because of pacemaker)
5. Adequate vertebral body height and geometry for insertion of the access instruments of 5.2 mm outside diameter
6. Minimum vertebral body anterior height of 6.0 mm

In cases of chronic fracture, vertebral augmentation is not indicated. Other absolute contraindications to Crosstrees or any other PVA technique include pregnancy, coagulopathy, osteomyelitis, spinal instability, known allergy to PMMA, and previous augmentation with PMMA (Table 42-2). Relative contraindications (Table 42-3) include neurologic deficit (i.e., burst fracture with significant bony retropulsion, or fracture extending to the posterior cortical wall) and pathologic vertebral fracture related to primary or metastatic cancer; however, because the Crosstrees pod fully contains the cement during implantation and has a defined shape, it may be used more safely in these cases than current vertebroplasty or kyphoplasty techniques. Another relative contraindication is vertebra plana or greater than 60% loss of height.

TABLE 42-2 Absolute Contraindications

1. Chronic fracture
2. Spinal instability
3. Known or suspected allergy to PMMA
4. Pregnancy
5. Irreversible coagulopathy or bleeding disorder
6. Active or local infection
7. Previous cement injection or augmentation at the fractured level

TABLE 42-3 Relative Contraindications

1. Comminuted or high-energy fracture with extension to posterior wall
2. Burst fracture or pedicle fracture
3. Vertebra plana or significant vertebral collapse, defined as >60% of the original height of the vertebral body, as measured against the nearest normal vertebral body
4. Significantly compromised spinal canal or bony retropulsion, especially in setting of neurologic injury
5. Spinal stenosis
6. Pathologic fractures, both benign and malignant (e.g., myeloma, metastatic lesions)
7. Presence of more than three acute VCFs

Description of the Device

The Crosstrees pod device (Figure 42-1) was developed to provide a percutaneous method of delivering a specific volume of bone cement to the surgical site in orthopedic procedures. The device is designed such that the woven fabric pod is inserted into the intravertebral space, and a predetermined volume of bone cement is delivered into it, thus reducing the likelihood of extravertebral cement leakage. The pod expands to a defined shape with a broad surface area as the cement is injected, elevating the endplates and restoring height to the fractured vertebra. Following delivery of the bone cement, the pod is opened and withdrawn from the vertebra. Additional cement can be delivered to the center of the cement bolus, to provide interdigitation with the bone. PMMA is delivered to the pod by a threaded injection syringe, the Crosstrees CDrive cement dispenser (Figure 42-2), designed to measure and deliver the volume required for the selected pod.

image

FIGURE 42-1 Crosstrees pod device.

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FIGURE 42-2 Crosstrees CDrive cement dispenser.

Principles of Procedure

The Crosstrees pod device is composed of a woven fabric mounted on the end of a stainless steel cement delivery shaft. The cement delivery shaft is housed within an additional stainless steel insertion sleeve which can be positioned along the axial length of the cement delivery shaft and pod such that the pod is contained within the insertion sleeve or exposed before PMMA fill. The proximal end of the delivery shaft is bonded to a Y-adaptor with Luer connector fittings on each arm of the Y-adaptor.

The pod is designed with a nylon release cord that is attached with a conventional stitch to the pod. The release cord runs the length of the device within the delivery shaft and through the straight leg of the Y-adaptor, providing access to the release cord at the proximal end of the device. A polymer cap is bonded to the proximal end of the release cord and secured by Luer thread to the Y-adaptor. The cap can be removed from the Y-adaptor and used to apply tension to the release cord. Following delivery of a defined volume of PMMA to the pod, tension is applied by the user to the release cord, pulling the stitch from the wall of the pod, opening the distal pod end.

Using a stylet and 5.2-mm-diameter access cannula, the paraspinal musculature is traversed to access the vertebral body via the pedicle. Either a transpedicular or extrapedicular approach can be used. The stylet and cannula can be advanced to the bony site and withdrawn slightly to create a location for placement of the Crosstrees pod within the bony site. As an alternative method, the stylet can be withdrawn from the cannula and a bone drill advanced through the access cannula into the vertebra and then withdrawn creating a space for placement of the Crosstrees pod within the bony site. Often bone fragments that remain on the drill can be sent for pathologic examination.

The Crosstrees pod is advanced through the lumen of the access cannula and, under fluoroscopic guidance, placed at the desired location within the vertebral body. Placement in the bone is confirmed by radiographic imaging. On confirmation of positioning, the insertion sleeve is withdrawn to expose the pod. The fabric component of the pod will fill to a known and predictable cubic geometry, aligned such that the maximum surface area is oriented parallel to the vertebral endplate. This geometry will provide the optimal surface area for lifting of the compressed bone and restoring vertebral body height.

Mendec Spine cement (PMMA) is prepared according to manufacturers instructions. The PMMA is loaded into the Crosstrees CDrive and attached to the pod by Luer taper distal connection fitting. Using fluoroscopic guidance, PMMA is advanced from the CDrive cement dispenser to the delivery shaft and injected into the pod. PMMA delivery continues until a maximum pod capacity is contained within the pod located in the bone. After filling to maximum pod capacity, the release cord cap is removed from the straight leg of the Y-adaptor and tension applied to the release cord. The release cord is withdrawn from the Crosstrees pod, removing the stitch from the distal pod end. The release cord is fully withdrawn from the Crosstrees pod assembly. Following removal of the release cord and opening of the distal pod end, the Crosstrees pod is withdrawn from the access cannula. The open pod is removed from the patient, leaving no implant other than the specific PMMA. Withdrawal is controlled by rotation of the threaded extractor mechanism at the proximal end of the pod assembly, resulting in linear proximal movement of the fabric pod component to a position within the access cannula. Withdrawal of the pod will decrease the pod fabric diameter on entry to the access cannula, leaving the PMMA within the bone. Following removal of the Crosstrees pod, the access cannula may be used for further access to the bone to deliver a final PMMA bolus, and it is removed from the patient on completion of the surgical procedure.

Background of Scientific Testing and Clinical Outcomes

It is well established in the published medical literature that the effectiveness of vertebral augmentation is evident almost immediately after the procedure. Numerous authors have reported significant pain relief within 24 to 48 hours, with stable results preserved at subsequent follow-up in a majority of patients.1 Studies in vertebral augmentation have generally evaluated efficacy principally based on pain relief, because pain is typically the reason patients seek treatment. Pain relief assessment by VAS is widely reported, with patients reporting significant relief at 24 hours and later following the procedure. Functional outcomes have also been reported using multiple assessment methods, but is typically secondary to evaluation based on pain relief.2–4 Ledlie et al5 showed the stability of outcomes with respect to pain relief post procedure. The study included 117 consecutive subjects undergoing vertebral augmentation procedures. The authors observed rapid relief in pain, with substantial improvement within 1 week postprocedure and relatively stable results from 1 through 24 months postoperative.

In a 2006 review, Hulme et al4 reported that complication rates are in the range of 1% to 2% for osteoporotic fractures and 5% to 10% for metastatic lesions.6 Complications specifically related to cement leakage can include increased local pain, symptomatic pulmonary embolus, radiculopathy, and cord compression and are estimated to occur in approximately 1% to 3% of cases.7 Of the complications potentially associated with vertebroplasty and kyphoplasty, all but new vertebral fractures occur during or immediately following the procedure. Thus the majority of complications can be identified within a very short period following the procedure.

Clinical literature in vertebral augmentation reports on the incidence of additional fractures as the primary focus of longer term follow-up. Leakage of PMMA from the vertebra has been associated with incidence of new fracture, with average time to new fracture of 48 days for levels adjacent to PMMA extravasation and 98 days absent PMMA extravasation.8 There are reports of the incidence of fracture adjacent to and remote from treated levels with half of new fractures occurring adjacent to treated levels within 3 months follow-up. A majority of subsequent vertebral fractures appear to occur within the first 30 days following a vertebroplasty procedure. Lin et al9 reported that in a series of 38 patients treated with vertebroplasty, new fractures occurred in 14 patients. When cement leakage occurred, the average time to new fracture was 48 days. It was 98 days in patients who did not have any cement leakage.

The time to observation of cement leakage occurrence is similar across studies. The existence of cement leakage is generally identified during or soon after the vertebroplasty procedure. Thus, although cement leakage appears to be the most commonly occurring complication of vertebroplasty, the existence of such an event would be identified well within a 30-day follow-up period.

In addition to evaluation of safety and pain relief, vertebral augmentation studies reported in the literature have often included an assessment of vertebral body morphology. There is disagreement in the literature on the efficacy of current treatments in the restoration of vertebral height and the clinical importance of vertebral height restoration.4,7

Procedural characteristics including the volume of cement used are also reported in the literature. Clinical literature reports variability in the volume of PMMA required for procedure success.8,10 The Crosstrees system delivers an initial bolus of known volume of PMMA, with device size selection determined by the investigator, based on vertebral level, degree of vertebral collapse, and physician assessment of device placement strategy. Pain relief is often immediate and sustained as noted in the literature review noted previously. If complications occur, they should become apparent early in the postoperative period.

Operative Technique

Anesthesia

General anesthesia is usually preferred, because it prevents the patient from feeling any discomfort and allows a controlled environment for safe passage of the cannulas down the pedicles. The procedure can be safely performed using local anesthesia if the patient is medically too unstable to undergo general anesthesia.

Position

The procedure is always performed with the patient prone on a radiolucent frame, such as the Jackson table. All attempts should be made to extend the spine, in an attempt to restore height to the fractured vertebra using ligamentotaxis.

Surgical Procedure for the Crosstrees System

Position the patient prone on a radiolucent table. Drape and prep according to standard surgical technique. Position two C-arms to achieve biplanar fluoroscopy capability as shown. If only one C-arm is available, the radiolucent table used must allow the C-arm to freely complete its arc as it moves from the anteroposterior (AP) to the lateral imaging position and back again.

Transpedicular Approach

Make a skin incision slightly lateral and superior (varies per level) to the intersection of the superior and lateral edges of the pedicle as determined under fluoroscopic guidance. Insert the 11-gauge needle into the incision and anchor it in bone, gently tapping it with a mallet if necessary. Confirm its location with fluoroscopy (AP view). Continue tapping the 11-gauge needle into place, confirming the location of the tip periodically with both AP and lateral fluoroscopic views. To avoid the spinal canal, make sure the tip of the needle does not pass medial to the medial border of the pedicle before entering the posterior vertebral cortex. Once the 11-gauge needle has crossed the posterior wall of the vertebral body, remove the inner stylet and replace it with the Indexed Guide Pin. Advance the guide pin anteriorly and medially into the vertebral body. Use the proximal most visible sizing indicator to select the appropriate size pod. With the guide pin approximately halfway across the vertebral body on the lateral view, remove the 11-gauge needle cannula and insert the blunt cannulated assembly over the guide pin. Attach the strike plate to the strike plate extension. Insert the tines of the strike plate into the mating feature of the blunt cannulated stylet and use the mallet to gently tap the stylet until its tip is just past the posterior vertebral wall on the lateral view.

Remove the inner stylet and guide pin and leave the access cannula in place. Note: The wings of the access cannula should be oriented in a cephalad-caudad position at this point if the primary geometry of the pod is being used. For an alternate geometry, the cannula wings should be parallel to the vertebral endplates. Under fluoroscopic guidance, use the cannulated drill to create a space in the bone before the placement of the pod and injection of PMMA into the pod. Advance the cannulated drill under fluoroscopic observation, avoiding contact with the anterior wall of the vertebra.

Extrapedicular Approach (Usually Recommended in Thoracic Spine)

Make a skin incision a few centimeters lateral and slightly superior to the intersection of the superior and lateral edges of the pedicle. Insert the 11-gauge needle into the incision and anchor it in bone, gently tapping it with a mallet if necessary. The correct entry point to the vertebral body is the costovertebral junction. Confirm the location with fluoroscopy (AP view). Under fluoroscopic guidance, use the cannulated drill to create a space in the bone before the placement of the pod and injection of PMMA into the pod. Advance the cannulated drill under fluoroscopic observation, avoiding contact with the anterior wall of the vertebra.

Use the laser markings on the drill shaft to confirm drill depth relative to the distal end of the access cannula. Do not drill more than 20 mm beyond the distal end of the access cannula. Under fluoroscopic guidance, insert the pod through the access cannula, and slide the extractor collar connector over the cannula wings to seat. The Y-adaptor will be oriented laterally and parallel to the vertebral endplates if the primary pod geometry is being used.

Delivery of PMMA

Remove the insertion sleeve lock from the pod assembly and set aside. Expose the pod membrane by pulling back on the wings of the device assembly. Repeat this process on the contralateral side. Under fluoroscopic guidance, turn the handle on the CDrive to inject spine resin into the pod membrane. Inject the entire contents of the CDrive by rotating the handle until it is flush with the CDrive sleeve. Repeat this process on the contralateral side. Confirm PMMA delivery to the pod with fluoroscopy in AP and lateral views. Remove the luer cap and release cord by withdrawal proximal from the pod assembly, opening the distal end of the pod fabric. Withdraw the pod from the vertebra by rotation of the extractor nut wings clockwise to the limit of the thread travel length. Withdraw the pod assembly from the cannula. Repeat this process on the contralateral side.

Insert the filler placement cannula (FPC) through the access cannula, advancing just far enough to place the tip of the FPC into the center of the cement bolus. Use fluoroscopy and the marks on the FPC cannula to confirm position. Manually dispense additional PMMA under continuous fluoroscopic guidance to achieve interdigitation (Table 42-4).

TABLE 42-4 Timing for the Preparation and Application of the PMMA (Mendec Spine Resin, 68° F)

image

Postoperative Care

Patients usually notice immediate relief of back pain and often do not require any narcotic medication postoperatively. Patients can be discharged home the same day or monitored overnight to watch for other medical comorbidities if necessary. Often these patients are weak because they have been bedridden and may benefit from the overnight stay and a session with a physical therapist.

Conclusions and Discussions

In 2005, osteoporosis-related fractures were responsible for an estimated $19 billion in costs. Osteoporosis is a disease characterized by low bone mass, leading to bone fragility and an increased susceptibility to fractures, especially of the spine, hip, and wrist, although any bone can be affected. VCFs are a frequent cause of pain and disability among the elderly population.

ADVANTAGES AND DISADVANTAGES

The Crosstrees device enables controlled implantation of PMMA into a fractured vertebra, while filling to a known shape and minimizing the chance of cement leakage. It does not require insertion and removal of a balloon tamp before the insertion of cement, rather the pod is inserted, reduces the fracture, and is removed after cement injection and reduction.

Despite the advantages, the Crosstrees procedure may be more technically challenging, because placement of the pod is essential for a good result. The pods need to be placed near the center of the vertebral body because they will not necessarily seek the path of least resistance, as in balloon procedures for VCF. The cement will only fill the pod in a defined shape. With that being said, once the pod is removed, there is the possibility of back filling additional cement if so desired, but the risk of leakage is possible if that path is chosen.

Both vertebroplasty and kyphoplasty have proved to be effective in relieving pain related to VCF. A potentially serious complication of these procedures is cement extravasation. The Crosstrees Medical PVA System for percutaneous vertebral augmentation consists of instruments designed to deliver the cement to the vertebral body in a controlled manner preventing extravasation without the requirement for an implant device. This device is designed to decrease the risk of leakage of bone cement (PMMA) into the spinal canal and the venous plexus, thereby preventing the complications associated with extravasation. The Crosstrees pod also has the added benefit of achieving and maintaining fracture reduction during cement injection, whereas in kyphoplasty the balloon tamp can achieve reduction but is then removed before cement insertion, and reduction is lost in many cases. The Crosstrees pod adds to the surgeon’s armamentarium for treatment of VCFs.

References

1. Bono C., Kauffman C.P., Garfin S., Herkowitz H., editor. Surgical options and indications: kyphoplasty and vertebroplasty in the lumbar spine. Lippincott Williams, 2004.

2. Mathis J.M. Percutaneous vertebroplasty or kyphoplasty: which one do I choose? Skel. Radiol.. 2006;35:629-631.

3. Gill J.B. Comparing pain reduction following kyphoplasty and vertebroplasty for osteoporotic vertebral compression fractures. Pain Physician. 2007 Jul;10(4):583-590.

4. Hulme P.A. Vertebroplasty and kyphoplasty: a systematic review of 69 clinical studies. Spine. 2001;31(17):1983.

5. Ledlie J.T. Kyphoplasty treatment of vertebral fractures: 2-year outcomes show sustained benefits. Spine. 2006;31(1):57-64.

6. Eicholz K.M., O’Toole J.E., Christie S.D., Fessler R.G. Vertebroplasty and kyphoplasty. Neurosurg Clin N Am. 2006;17:507-518.

7. Talmadge K. Vertebral compression fracture treatments. In: Kurtz S.M., Edidin A.A., editors. Spine technology handbook. Elsevier Academic Press; 2006:371-396.

8. Lin E.P. Vertebroplasty: cement leakage into the disc increases the risk of new fracture of adjacent vertebral body. AJNR Am J Neuroradiol. 2004 Feb;25(2):166-167.

9. Lin E.P., Ekholm S., Hiwatashi A., Westesson P.L. Vertebroplasty: cement leakage into the disc increases the risk of new fracture of adjacent vertebral body. AJNR Am J Neuroradiol. 2004;25:175-180.

10. Frankel B.M. Percutaneous vertebral augmentation: an elevation in adjacent level fracture risk in kyphoplasty as compared with vertebroplasty. Spine J. 2007 Sept-Oct;7(5):575-582.

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