Metastatic Spine Tumors: Fracture Risk Analysis And Prophylactic Treatment

Part 1:Fracture Risk Analysis of Metastatic Spine Tumors--Analysis of Fracture Risk by Combining QCT and Analyze MD softwareBackground and ObjectiveAt least one million new cases of cancer are diagnosed each year in the United States, skeletal metastases occur in 2/3 of them, most commonly to the spine. Failure of the spine's structural integrity from metastatic disease can lead to both pain and neurologic deficit. Fractures that require treatment occur in over 30% of bony metastases.Fracture risk analysis and prophylactic stabilization of spine are the hot topics recently. Prophylactic cement augmentation of a vertebral body infiltrated with tumor is an entirely different entity. An osteoporotic vertebral compression fracture does not cause spinal cord compression, whereas a vertebral body that fractures due to tumor infiltration often causes neurologic symptoms and signs with potential devastating consequences. Therefore, it is vital important to exactly predict the fracture risk of the metastatic spine and apply prophylactic spinal stabilization so that patients could remain ambulant and continent. Methods to predict fracture risk in metastatic vertebral disease must measure changes in both the material properties and the bone geometry within the vertebral body. Quantitative computerized tomography is able to delineate the density and geometry of both the tumor and the remaining bone and accurately predict, via composite beam theory, the vertebra's load carrying capacityCurrently, manual input of the QCT image-based strength and modulus data, and their subsequent analysis is a time consuming and user dependent process. The development of an automated, user friend, and reproducible software package would enable the vertebral load carrying capacity determination to occur in a clinical setting. The image analysis software program, Analyze MD, developed at the Mayo Clinic College of Medicine's Biomedical Imaging Resource (BIR) includes a functional data-base management system that recognizes digital imaging and communications in medicine (DICOM) and QCT images. Application-specific interfaces can be developed and optimized for specific clinical applications. We have chosen Analyze as our software platform for the automation of bone mineral density and bone structure determination, then calculate the load-bearing capacity of the lesion spine, and finally calculate the Fracture Risk Value (FRV). The risk for impending fracture was defined as the theoretic normal axial rigidity divided by the axial rigidity of lesion vertebra. The risk of fracture increases with the increase of FRV.Finally, we retrospectively studied 308 existing Mayo cancer patients with spinal metastatic disease and evaluated the significance of FRV calculated from Analyze MD. This is a creative study to predict the fracture risk of metastatic spine lesion by combining the QCT results and novel Analyze MD software. Materials and Methods1. Study CohortThe appropriate IRB approval has been accomplished. Then we retrospectively studied 308 existing Mayo cancer patients with spinal metastatic disease between 2001 and 2005. Patient inclusion criteria include: (1) patient had two consecutive spine CT scans done in less than a six month period; (2) we only investigated the metastatic lesion of thoracic and lumbar spine, metastatic lesions of the cervical and sacral spine were not counted in; (3) no pathologic fracture was detected on the first CT images; (4) the investigated spinal lesion was a separated lesion whose adjacent upper and lower segment were relatively normal on radiographic images; (5) after the diagnosis of spinal metastasis from the first radiographic images, patient didn't have some further operative treatment (like open surgery and internal fixation or percutaneous vertebroplasty) other than subsequent radiotherapy or chemotherapy for the primary tumor; no conservative treatment had been provided other than observation by the treating physician; patient they had used crutches or were advised to substantially decrease their activities for more than a few weeks were excluded, since this modification inactivity alone would have decreased the fracture risk.The initial CT scans will be compared to the follow-up CT scans for the presence of new pathologic fractures, a fracture will be defined by criteria used for osteoporotic vertebral fractures or vertebral endplate fractures. These criteria include either a 15% loss of height from one side of the vertebra compared to the other in the frontal or sagittal plane, or a 15% loss of vertebral height compared to adjacent vertebrae. Finally, all the eligible cases were divided into two groups:fractured group and non-fractured group.2. FRV analysis with QCT and Analyze MD After confirmation of all the eligible cases, all the radiographic images of the patients were transferred into the Analyze MD system by radiology department of Mayo Clinic. The author opened the radiographic data of the patient from the Analyze MD system, and selected the interested metastatic vertebral body together with its adjacent upper and lower spinal segments. Once the vertebral body was selected, the Analyze MD system can automatically calculate final FRV of the metastatic vertebral body through some empirical formulas.3. Statistical analysisUnivariate analysis, with the Student t test and chi-square test as appropriate, was used to compare demographic data, including age, gender, primary tumor diagnosis, site of the defect, and affected segment, between the fracture and non-fracture groups. Calculated values of FRV were assessed for normality with use of the Kolmogorov-Smirnov test, and no significant skewness was detected. Therefore, the two-sample Student t test was used to compare the fracture and non-fracture groups. Sensitivity, specificity, and accuracy were calculated to determine the diagnostic performance of FRV. For all comparisons, a two-tailed P< 0.05 was considered significant. The data were analyzed with use of the SPSS software package (version 13.0; SPSS, Chicago, Illinois).ResultsIn out study,73 eligible cases in total were included. Thirty nine patients were male and thirty four were female, and their ages ranged from thirty one to eighty two years (mean and standard deviation,58.7±10.9 years). Twelve patients had a primary tumor of renal cell cancer, ten lung cancers,9 breast cancers,7 prostate cancers and 35 others. Fifty seven lesions were located in the thoracic spine; fifty nine in the lumbar spine. The results of the univariate comparisons of the demographic data between the fracture and non-fracture groups showed no significant difference between these two groups (P>0.05).FRV derived with quantitative computed tomography and Analyze MD system differed significantly (P<0.01) between the fracture and non-fracture groups. At the cut-off value of 1.45, FRV attained the 92.7% sensitive and 64.0% specific and 74.1% accuracy for predicting fracture.ConclusionBy combining the QCT and the powerful Analyze MD software image analysis system, we can easily accommodate current clinical CT imaging and calculate the FRV by validating an automated program. Our retrospective study indicated that the calculated FRV is an excellent independent criterion to predict the fracture risk of a metastatic spinal lesion especially in thoracic and lumbar spines with high sensitivity and specificity. We believe that this method can provide accurate objective criteria for planning treatment of metastatic spinal lesions and monitoring treatment response.Part 2:The Experimental Study of Prophylactic Treatment of Spinal Metastasis--Synthesis, Characterization, and Crosslinking Properties of a Novel Injectable Biomaterial Background and ObjectiveProphylactic treatment of a vertebral body infiltrated with tumor is very necessary. Great progresses have been made in the treatment of spinal metastasis recently. Treatments include chemotherapy, radiotherapy, surgery and combined therapy. Chemotherapy and radiotherapy are useful to decrease the symptoms, however, they can not increase the stability of spine. Surgery has many complications, and the effects of conservative treatments are in issue, and they can not be used to increase the stability of spine, or halt the progress of disease.Vertebroplasty is a minimal invasive treatment guided under radiographic images. PMMA bone cement was injected into the lesion vertebral body to relieve pain and stabilize the vertebral body. It has several strong points of less invasive, early recovery and great clinical effect. It is especially useful in the treatment of most spinal metastases which are not eligible for open surgery. PMMA is the most common used injectable bone cement especially in some load-bearing areas. After injection, PMMA polymerizes in situ and provides instant mechanical support. However, several drawbacks also exist in PMMA. PMMA has a significantly higher modulus than trabecular bone and is non-degradable. Additionally, PMMA has a relatively short working time for preparation and injection, and exhibits high exothermic heat release during curing that is not ideal for use as injectable bone substitute.A promising candidate material of this type is poly(propylene fumarate) (PPF), an unsaturated linear polyester that can be modified or cross-linked through its fumarate double bonds. The curing time has ranged from 1 to 121 min, depending on the ratio of initiator, monomer or macromer, and PPF. The maximum temperature during PPF cross-linking has been 48℃, which is much less than that of PMMA. Although many efforts have been made to explore the applications of PPF-based materials, there are still many important limitations of this material. The propylene glycol in each repeating unit provides only one free rotating carbon-carbon bond that contributes to the rigidity of the PPF polymer chain. In addition, a cross-linker is needed to form cross-linked PPF networks via redox initiation, which may lead to cytotoxicity associated with unreacted cross-linking monomers.Polycaprolactone (PCL) is a FDA-approved biodegradable polymer with excellent biocompatibility and flexibility. A variety of copolymers based on PCL have been made to enhance the applications and crosslinking properties of such material.In an attempt to combine the favorable properties of PPF and PCL, our laboratory recently designed and synthesized a new injectable copolymer PPF-co-PCL composed of PPF and PCL. The chemical and physical properties of the uncrosslinked copolymers have been characterized. In this study, we further evaluated the handling and mechanical properties of the crosslinked copolymer by varying parameters. Maximum crosslinking temperature, gelation time, mechanical properties, cytotoxicity and in vitro degradation of the crosslinked copolymers were evaluated. This is a creative study to characterize and evaluate the novel polymerized copolymer as an injectable bone substitute.Materials and Methods1. MaterialsPCL diols with nominal molecular weights of 530,1250 and 2000 g/mol were purchased from Aldrich (Milwaukee, WI). All the other chemicals in the present study were also purchased from Aldrich.2. Experimental design The three variables we investigated were:PPF Mn, PCL precursor Mn, and PCL feed ratio. Eighteen copolymers were synthesized and characterized. The copolymers were crosslinked by adding benzoyl peroxide and dimethly toluidine. The maximum crosslinking temperature, gelation time, mechanical properties and biocompatability were measured. All experiments were conducted in triplicates, and data is expressed as means +/- standard deviations.3. Synthesis of PPFPPF was synthesized as described previously. The polymerization reaction was run at 150℃for 1 and 5 h, producing PPF with Mn around 700 and 2000 g/mol, respectively, as measured by gel permeation chromatography (GPC).4. Synthesis of PPF-co-PCLThe PPF-co-PCL copolymer was synthesized from PPF and PCL with antimony trioxide added as a catalyst. The resulting PPF-co-PCL copolymer was purified by solution precipitation forming a viscous melt or wax-like solid.5. Copolymer Characterizations5.1 GPCThe uncrosslinked copolymer's molecular weight and molecular weight distribution were characterized by gel permeation chromatography (GPC).5.2 FT-IRFourier transform infrared spectroscopy (FT-IR) spectra were obtained on a Nicolet 550 spectrometer.5.3 NMR Proton nuclear magnetic resonance (1H-NMR) spectra were acquired on a Varian Mercury Plus NMR spectrometer (1H=300.1 MHz).5.4 DSCDifferential scanning calorimetry (DSC) was measured on a TA Instruments Q1000 differential scanning calorimeter. The glass transition temperature (Tg) was determined by using the midpoint temperature of the glass transition process.6. Crosslinking of PPF-co-PCLAs related previously, a typical crosslinking procedure was performed. Additionally, two other different amounts of accelerator solution were also used when crosslinking copolymer 1.7. Maximum temperatureThe temperature profile was recorded through a thermocouple. Time zero was defined when accelerator was added into the mixture.8. Gelation timeThe gelation time was measured on a rheometer. Time zero was defined when accelerator was added into the mixture, and the gelation time was defined when the viscosity of the composite suddenly increased as shown on rheometer.9. Mechanical propertiesThe crosslinking cylinder specimens were analyzed using a 312 Materials Testing System mechanical testing machine. The compressive modulus was calculated as the slope of the initial linear portion of the stress-strain curve. The compressive toughness was calculated as the area under the stress-strain curve before sample failure.10. In vitro Cytotoxicity evaluationThe in vitro cytotoxicity of parts of our PPF-co-PCL formulations together with PPF and PMMA were investigated using MTS Assay. Three time points (1 day,3 days, and 7 days) were chosen.11. In vitro DegradationThe crosslinked specimens were dipped into the PBS solution. Three time points (1 day,3 days, and 7 days) were chosen to check the weight loss of the specimen. Specimens were also analyzed using a 312 Materials Testing System mechanical testing machine.12. Statistical analysisAll experiments were conducted in triplicate and the data were expressed as means±standard deviation (SD). One or three factors analysis of variance (ANOVA), followed by Duncan's multiple range test, were performed with SAS version 9.1.3 software to identify statistical difference at a significance level of P<0.05.ResultsCopolymer CharacterizationsPPF-co-PCL Mn ranged from 3141±48 to 6607±144 g/mol. The polydispersity ranged from 2.6±0.2 to 4.0±0.3. FT-IR of copolymer showed a combination of spectra from its two components. PCL composition in the copolymer was calculated in 1H NMR and showed a good correlation with the actual PCL feed ratio. All the copolymers have a single glass transition temperature which fell between -39.7±2.4 and -15.0±1.6℃. They decreased significantly with increasing PCL feed ratio (P<0.01).Maximum crosslinking temperatureThe maximum crosslinking temperature of crosslinked copolymer fell between 38.2±0.3 and 47.2±0.4℃. It increased with increasing PCL precursor's molecular weight.Gelation timeGelation time of the crosslinked copolymer fell between 4.2±0.2 and 8.5±0.7 min. It decreased with increasing PCL precursor's molecular weight. All the gelation times of crosslinked copolymers were longer than that of PMMA, and significantly shorter than those of their PPF precursors (P<0.01). When decreasing accelerator, the gelation time greatly increased (from 6 min to over 60 min).Mechanical propertiesThe compressive modulus ranged between 44±1.8 and 142±7.4 MPa. It increased with increasing PPF precursor's molecular weight and decreasing PCL feed ratio. Copolymer 13 had a much greater compressive toughness than its PPF precursor (PPF2000).The compressive toughness of all crosslinked copolymers and PPF (PPF700 and PPF2000) fell between 4.1±0.3 and 17.1±1.3 KJ/m3.CytotoxicityNo cytotoxic response was demonstrated from the different formulations of PPF-co-PCL and PMMA bone cement. On the 7 days time point, there was a slight decrease trend of cell viability in PPF samples compared with the control group. However no statistical difference was shown. In vitro DegadationOur in vitro degradation experiment showed that the copolymer degraded faster with increasing PCL feed ratio. No significant difference of weight loss was found from different formulations varying in PPF or PCL precursor's molecular weights. The compressive modulus of all degraded copolymers was tested at different time points after crosslinking. The compressive modulus of all these copolymers kept increasing the end of the fourth week after setting. It subsequently became stable.ConclusionOur results show that the maximum crosslinking temperature, the gelation time, the mechanical properties and degradation rate of crosslinked copolymer PPF-co-PCL could be modulated by varying its composition for different applications. Incorporation of PCL in the copolymer composition led to a fully crosslinked copolymer network. By integrating both virtues of its PPF and PCL precursors, PPF-co-PCL appeared to be an optimal injectable, in situ polymerizable biomaterial with a more favorable characteristic.