New developments for treatment of osteoporotic spine

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Kyphoplasty

Vertebroplasty carries its share of risks and complications, but it does lead to significant pain reduction and improved function. It does not, however, improve the sagittal balance or the kyphosis caused by the fracture. Kyphoplasty is a new technique, which tries to address this issue. Kyphoplasty is similar to vertebroplasty except that it calls for introduction of an inflatable bone tamp into the vertebral body which, when inflated, tries to restore the vertebral body height back to its original height while creating a cavity that can be filled with cement (Fig. 4). This technique is performed via a bipedicular approach for a uniform restoration of the compression. Why might reduction of the kyphosis be important in these patients? It has been shown that patients with spinal osteoporotic fractures have significantly diminished pulmonary function compared to those without fractures. More importantly, the reduction in the pulmonary functions has been shown to correlate significantly with severity of the spinal deformity [55]. Furthermore, it has been shown that, if left untreated, the thoracic compression fracture can lead to worsening of the kyphosis over 3 months and further deterioration at 3 years [14]. If the kyphosis can be corrected, pulmonary functions may improve and further collapse may be avoided.

An ex vivo biomechanical evaluation comparing verte-broplasty to kyphoplasty showed that both techniques result in significantly stronger vertebral bodies relative to the initial fractured state. Kyphoplasty was able to restore vertebral height to 97% of the original height. Vertebro-plasty resulted in a significantly lower restoration of ver-

Fig. 4 The inflated balloon restores vertebral body height, while creating a cavity that can be filled with cement (kypho-plasty)

tebral height, to 30% of the original height [5]. The ability to restore vertebral body height has been shown in other laboratory studies as well [7, 64]. Clinical studies have shown increased vertebral height, but not the level of increase obtained in the laboratory. Lieberman et al. [38] reported on 70 consecutive kyphoplasties performed on 30 patients for painful VCFs with a mean duration of symptoms of 5.9 months. The patients were followed prospectively for 3 months. In 70% of the patients, height was restored to 46.8% of predicted values. In 30% of the patients there was no restoration of height. Pain and physical functional scores significantly improved after kyphoplasty. Although no conclusions could be made with regards to the age of the fracture and the ability to regain height, the authors got the "impression" that they were able to restore height more predictably in fractures less than 3 months old. A balloon failure rate of 20% and cement leakage rate of 8.6% was also reported.

Since the approval of kyphoplasty by the FDA in 1998, a multi-center study in the US has been initiated, with results reported for 2,194 kyphoplasty procedures in 1,439 patients [20]. In fractures less than 3 months old, the average fractured vertebral body height improved from the 71% pre treatment to 92% after treatment. In fractures more than 3 months old, the height improved from 74% pre treatment to 84% after treatment. Ninety percent of the patients had relief of their pain as they returned to the pre-fracture level of pain medication use. There were three thoracic level parapareses related to instrument insertion through the medial wall of the seventh pedicle with cord injuries, and there was one case of epidural hematoma in a patient on anticoagulation medication. These complications occurred in the first 100 fractures treated. Since technique adjustment, no neurological complications have been reported.

Kyphoplasty has the added benefit of less cement leakage. When the balloon is inserted and inflated, it forms a contained cavity that can then be injected. As the cement travels along the path of less resistance, it will then fill this empty cavity rather than flowing into the surrounding osseous or venous structures. In an in vivo comparison of the potential for extravertebral cement leakage after verte-broplasty and kyphoplasty, there were significantly lower rates of leakage of contrast material with kyphoplasty [52]. In the recent US experience, there was only one cement embolus, without breathing consequences [20].

Vertebroplasty using Cortoss

Cortoss is a new synthetic bone void filler that contains bis glycidyl methyl-methacrylate, bisphenol, a polyethylene glycol diether dimethylacrylate, triethyleneglycol di-methylacrylate monomer and bioactive glass ceramic [60]. It is provided in a double lumen cartridge with specially designed tips for mixing. After the composite is expressed

through these tips, polymerization begins and the material is ready for use. The monomer is not volatile and Cortoss polymerizes in a three-dimensional network, which minimizes the chances of leaking. After mixing, the material has the consistency of toothpaste, and stays that way until it polymerizes quickly, in a matter of seconds. This characteristic provides a consistent tactile feedback and allows for an even injection. The polymerization has a much lower exotherm than PMMA (63°C vs 84°C), which reduces the risk of thermal necrosis. The modulus of elasticity of Cortoss is close to that of bone [60]. This composite is bioactive, and in animal studies the cement-bone interface continues to be strengthened over time with bone apposition occurring at the interface without any fibrous interposition. Cortoss cement appears well suited for use in the treatment of VCFs. The aliquot delivery system allows for accurate amounts of cement to be injected directly into the region of interest.

A prospective clinical study has been conducted at our institution with Cortoss [48]. To participate, patients had to have fracture-related pain measuring at least 50/100 on the VAS, which also caused a change in lifestyle or disability. Patients were scheduled for follow-up at 4 days, 1 week, and 1, 3, and 6 months after the procedure. Two metal trocars of 10 G diameter were introduced through the pedicles at each level treated. Twenty-four patients with osteoporotic fractures were enrolled. The average pain scores were 69 preoperatively and 38 at 4 days post-operatively. The scores continued to decrease, to 33 at 1 week and 29 at 1 month, and then returned to 33 at 6 months. This represents a reduction of pain of 46% at 6 months. The quality of life has been evaluated with the short form 1 (SF-12) questionnaire. Ability to ambulate was impaired in 75% preoperatively and in 28% at 6 months postoperatively. Ability to perform activities of daily living improved significantly post PVP. There was a 3% rate of minor complications, and no leakage into the spinal canal. Results indicate that Cortoss addresses the shortcomings of PMMA for vertebroplasty augmentation. This cement is a fixed composition material with less variability than current variations of PMMA, and in conjunction with the Aliquot delivery system can be accurately delivered in incremental doses without excessive material waste.

Bone substitutes in vertebroplasty

As requested by Heini [24], bone substitutes for vertebro-plasty need the following properties: injectability, radiopacity, adapted viscosity, long setting time, good mechanical properties for the load (compressive strength/stiffness), biocompatibility, bioactivity, and slow degradation. Calcium phosphate cement meets these criteria well. In their ceramic form they cannot be used as injectable device. Tetracalcium phosphate with dicalcium hydroxy apatite and amorphous calcium phosphate also meet the criteria and are readily available. They can be injected through a 10- or 11-G needle. The results of animal tests are very promising, and in vitro experimental studies have shown interesting resistance in compression, of around 45 MPa. As reported by Le Huec [35], these resorbable calcium phosphates provide the calcium for local bone formation and are of great interest for the treatment of osteoporotic fractures. Clinical applications on humans are in progress, but the results of these studies have not yet been published. Also yet to be reported on is the effect of combining the use of resorbable calcium phosphates with bone morphogenic protein as a carrier, which is a promising technique to promote bone healing in fracture cases.

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