Introduction

The Centers for Disease Control and Prevention (CDC) reports that there were 36.8 million U.S. residents older than 65 years in 2005. That number is predicted to grow to over 70 million by 2030.¹ Presently, it is estimated that 25% of women older than 50 years, 40% of women older than 80 years, and 33% of men reaching 75 years of age will sustain an osteoporotic compression vertebral fracture (OCVF).² Although many OCVFs appear to go clinically undetected, up to 30% of symptomatic fractures remain unresponsive to conservative management and are considered candidates for surgical intervention.^3,4 Considering these statistics along with the general effectiveness of vertebral augmentation in treating chronic pain from OCVFs, it is easy to understand the expected steady increase in frequency of procedures performed to treat this disease process.

Originally developed in France during the mid-1980s to treat vertebral hemangiomas and later introduced in the United States in 1994 in the treatment of symptomatic OCVFs, the percutaneous injection of bone cement, polymethyl methacylate (PMMA), has gained widespread use by physicians who treat OCVFs. Vertebroplasty and kyphoplasty have consistently demonstrated the ability to improve patient function and quality of life through excellent pain control and rapid mobilization for this “at risk” group of patients.⁵ However, risks pertaining to cement toxicity, exothermic reactions and tissue damage, embolism, and the potential neurologically devastating complication of cement extravasation outside the vertebral body (spinal canal, neuroforamen) remain.^2,6 Furthermore, concerns have been raised and debated about whether vertebral bodies augmented with PMMA (with or without intradiscal extravasation) are associated with a higher adjacent-level vertebral fracture (ALF) rate than would be expected because of the natural risk of additional fractures. Investigators have challenged the direct causal relationship between PMMA-augmented vertebral bodies and ALF, arguing that ALFs may simply be the result of the natural history of the underlying disease (osteoporosis). However, it is generally agreed that when fractures occur following PMMA augmentation, it is more common to see them occur within the first 3 months after the procedure, and they are more likely to occur at an adjacent level than elsewhere in the spinal column.⁷ In contrast, people with osteoporotic bones with compression fractures who have not had PMMA vertebral augmentation have subsequent vertebral fractures that are distributed more randomly throughout the spine and will occur sporadically throughout the following year. Data such as these do suggest that PMMA augmentation of vertebral bodies at least predisposes adjacent vertebral levels to fracture.^8–12

It has been proposed that the ideal bone cement for vertebral augmentation should be biodegradable and nontoxic, have a low setting temperature, and have a biomechanical profile close to that of human bone.¹³ To avoid some of the drawbacks of PMMA noted previously, and to provide an “ideal” biological cement for vertebral augmentation, a new option was developed that involves the minimally invasive injection of morselized allograft bone into a polyester expandable mesh container (OptiMesh, Spineology Inc., Minneapolis, Minn.). The bone injection procedure generates lifting force for potential fracture reduction, and the resultant bone graft strut is immediately load sharing and has a modulus of elasticity closely approximating that of native bone.

Indications and Contraindications

The primary indications for use of this mesh–bone construct include painful osteoporotic, traumatic, or steroid-induced Vertebral compression fractures (VCFs) from T4 to L5, with or without secondary kyphosis, that has not responded to a reasonable trial of conservative therapy. An additional indication is for benign but symptomatic vertebral hemangiomas. Pain and tenderness should be localized to the fracture level identified on x-ray, CT, MRI, or technetium Tc 99m bone scan. Patients should be medically stable to at least tolerate a percutaneous procedure and be able to assume a prone position for the procedure.

The main contraindication, as with all vertebral augmentation procedures, is the presence of an unstable vertebral fracture with retropulsed fragments causing more than 20% canal compromise (i.e., true burst fracture pattern) or any fracture pattern with neurologic deficit. The author has successfully and safely used this technique to treat compression fractures with “bowing” of the posterior cortex (posterior longitudinal ligament intact) along with fractures demonstrating minimal retropulsion of the superior or inferior endplate into the canal causing less than 20% canal compromise. Of interest, the use of this device as a minimally invasive option to treat true burst fractures with the AO classification of A1 has even been shown effective when combined with short-segment pedicle screw fixation.¹⁴

Because of issues of healing in the presence of adjuvant therapy as well as the inherent biologic aggressiveness of metastatic tumor leading to compression fractures, this device should not be used, when used with allograft bone as a filler material. Other absolute contraindications include comorbid conditions such as uncorrected coagulation or bleeding disorders, osteomyelitis, epidural abscess, and vertebra plana. Finally, the efficacy of prophylactic treatment in patients at high risk for VCFs has not been proved.

Description of the Device

The basic concept behind this technology is that a cavity within a fractured vertebra is created percutaneously through a relatively small access portal via a fluoroscopically guided unilateral, extrapedicular approach. A deflated porous polyester mesh bag (OptiMesh) is inserted and subsequently filled with allograft morsels, allowing a large load-sharing graft and strut to be built within the vertebral body. OptiMesh is designed to contain and reinforce morselized bone graft. The pore size of the mesh is nominally 1500 μm, allowing the mesh to effectively constrain the morselized allograft while also allowing ingrowth of vessels and native bone elements for ultimate graft incorporation and remodeling.

Using the principles of granular mechanics, impaction of bone granules into the mesh bag generates a distractive force, producing the capability for reduction (i.e., height restoration) of the fractured vertebra. Because the bone granules are tightly packed, force chains develop between the bone chips, thus creating a strut capable of supporting the axial loads of the spine.

The bone allograft (Musculoskeletal Transplant Foundation [MTF], Edison, N.J.) is a mixture of lyophilized corticocancellous chips and demineralized bone matrix (DBM), providing both osteoconductive and osteoinductive properties for graft incorporation.

Background of Scientific Testing and Clinical Outcomes

The nonresorbable mesh is knitted from yarn made of polyethylene terephthalate (PET) thread. Animal studies have shown that the mesh does not produce an adverse tissue reaction or create a barrier to bone growth from the host into the graft pack.¹⁵ In vitro biomechanical testing has shown that the mesh filled with bone graft restores intact vertebral body strength in compression. The final construct’s restored strength and stiffness is less than PMMA-augmented vertebral bodies, which may have a desired protective effect to reduce the potential for adjacent level fractures. As the reduction of the fracture (i.e., height restoration) occurs while the device is being deployed and filled, the author has found that this procedure more reliably restored and maintained vertebral body height. This is in contrast to loss of almost two thirds of potential restored height that occurs upon deflation of the kyphoplasty balloon in treating similar “mobile” compression fractures.¹⁶ Finally, the MTF bone mixture has been shown to generate new bone formation equivalent to autograft when placed in the vertebral bodies of sheep.¹⁷

To assess clinical efficacy, the author conducted an INVESTIGATIONAL REVIEW BOARD (IRB)-approved retrospective study to review consecutive patients treated with OptiMesh from March, 2004, to December, 2007. Forty patients were enrolled under the protocol. There were 32 women and 8 men with an average age of 73.4 years (range: 44 to 95 years). Of the 40 patients, 29 patients had osteoporotic compression fractures, 10 patients had trauma, and 1 patient had breast cancer. A total of 48 levels were implanted. More than 50% of the fractures occurred at the thoracolumbar junction (Figure 43-1). Average follow-up was 16 months. There were 27 patients (67.5%) who had greater than 6-month follow-up, with an average of 23 months. Of the patients treated for osteoporotic VCFs, 19 patients had preoperative DUAL X-RAY ABSORPTIOMETRY(DEXA) scores with an average T-score of −2.4.

FIGURE 43-1 Distribution of fracture levels among patients treated with OptiMesh.

Pain and function were assessed using the four-point Odom score (excellent, good, fair, poor). This scale allows the treating physician to combine clinical observations and physical examination with the patient’s global assessment. Only two patients (5%) rated a poor score in the early postoperative period. All of the patients with longer than 6-month follow-up expressed satisfaction with the procedure and were rated an excellent or good Odom score.

Postoperative x-rays were evaluated for new vertebral body fractures and maintenance of restored vertebral body height. Six patients (15%) suffered new fractures, but only two patients (5%) had ALF. The postoperative CT scan on one patient identified a fracture of the medial pedicle wall, without sequelae. One patient with a recurrent glioblastoma was treated for an osteoporotic VCF, but at 3-month follow-up had significant loss of restored vertebral body height on x-ray, although this was asymptomatic. All patients who had a CT scan beyond 12 months postoperatively showed good evidence of graft incorporation within the mesh.

Clinical Presentation and Evaluation

Patent MW: A 69-year-old woman with osteoporosis who suffered T7 and T8 VCFs (T-score: −2.5) secondary to a fall. Fractures were demonstrated by plain x-rays and MRI, and she had severe pain and disability unresponsive to conservative therapy. She underwent a T7 and T8 biologic vertebral augmentation procedure with OptiMesh and morselized allograft during a single session under IV conscious sedation. She reported complete pain relief at 24 hours, and within 48 hours was able to ambulate and perform activities of daily living without assistance. At the 3-year postoperative visit, she continued to be pain free and her 3-year follow-up CT scan showed osseous integration of the graft pack (Figure 43-2).

FIGURE 43-2 CT scans at 36 months showing good osseous integration of bone graft.

Operative Technique

These procedures are performed most commonly with monitored anesthesia control on a radiolucent table. Because the procedure uses a unilateral, parapedicular approach into the vertebral body, biplanar fluoroscopic imaging is required to guide instrument placement, reduce the risk of neural injury, and size and fill the mesh.

Guide pin (i.e., Steinmann pin) placement determines instrument trajectory throughout the entire procedure and is thus the most critical portion of the procedure to perform correctly. The guide pin is passed percutaneously from a paramedian approach (approximately 7 cm from midline, depending on the level being treated). It is ultimately docked and then allowed to penetrate the lateral base of the pedicle at the pedicle–vertebral body junction. The pin is then advanced into the center of the vertebral body with a trajectory that ultimately positions its tip halfway across the silhouette of the vertebral body on both anteroposterior (AP) and lateral fluoroscopic images. Once an appropriate trajectory is obtained, a dilator is introduced over the guide pin and docked onto the pedicle vertebral body junction. The access portal (i.e., working channel), in turn, is passed over the dilator tube and similarly docked onto the vertebral body. The guide pin and dilator tube are then removed.

A cavity is then created within the vertebral body by first drilling obliquely across the vertebral body to within 5 to 6 mm of the contralateral vertebral body cortex with a 6-mm hand drill. An expandable shaper is then used to core out a cavity that leaves 2 to 3 mm of bone between the cavity and the endplates.

Based on the drill depth, shaper usage, neighboring normal vertebral body height, and the stiffness of the fracture (i.e., acute vs subacute), the appropriate size of mesh bag is determined. An acute fracture may require a larger construct to restore vertebral body height, whereas in a nonmobile fracture, the goal may only be to fill the cavity created without height restoration. Each size mesh has a maximum fill volume, and this should never be exceeded to avoid potential rupture and loss of containment of the allograft. Chronic fractures may allow little expansion of the mesh if a cavity is inappropriately undersized. In this situation, the graft pack may achieve load-bearing capabilities at fill volumes less than that recommended.

The mesh bag is attached to a mesh holder and passed into the cavity. The mesh is then filled using prefilled tubes prepared and provided by Musculoskeletal Transplant Foundation. The mesh bag is filled circumferentially utilizing initially the diverted (i.e., angled tip) tubes to fill the bag peripherally followed by a straight (i.e non-angled tip) tube which fills the mesh bag centrally ensuring an even packing of allograft throughout the construct. Upon completion of filling, the mesh is detached from its crimp tip, and then all instruments are removed from the patient (Figure 43-3).

FIGURE 43-3 A, A cavity is created. B, Mesh is inserted empty. C, Mesh is filled and released.

Postoperative Care

These procedures are often done on an outpatient basis with the patients allowed to ambulate following clearance by the anesthesia team. No bracing is necessary and patients can return to light to moderate levels of activity immediately. In patients with osteoporosis, treatment of the underlying disease with best medical management is essential.

Complications and Avoidance

This procedure involves placement of an actual implant in the vertebral body, so prophylactic antibiotics should always be used to avoid the development of spondylitis. As with all percutaneous spinal procedures, adequate visualization with biplanar fluoroscopy and surgeon experience with interpreting these images are mandatory. Though the extrapedicular approach reduces the likelihood of canal violation, nerve root injury, pedicle fracture, spinal cord injury, and misplacement of the device outside the confines of the fractured vertebral body can occur if the landmarks for guide pin placement and trajectory are not well visualized or interpreted.

The potential for retroperitoneal bleeding (for lumbar fracture repair) and for hemothorax or pneumothorax (with thoracic fracture repair) are valid concerns but in the author’s experience rarely occur. To avoid violating the pleural space in treating thoracic fractures, the guide pin should be walked along the dorsal side of the rib cage as it is guided from lateral to medial to the costovertebral junction, and then more ventrally along the lateral aspect of pedicle. Ipsilateral exiting nerve root injury at any level can be avoided by always keeping the guide pin (which defines the subsequent trajectory of all additional instrumentation) within the shadow of the pedicle on lateral fluoroscopic imaging.

Conclusions and Discussion

The clinical study indicates that placement of a load-sharing mesh and bone graft strut into the anterior column creates structural stability and is effective in reducing pain and increasing function in patients with VCFs. Long-term radiographic follow-up shows a low incidence of ALFs, and osseous integration of the graft occurs even in osteoporotic patients. New advancements of this technology currently being developed include new biologic graft materials and a smaller (5.5-mm outside diameter) access portal with fewer steps to the implant procedure.