Been-Der Yang

Prophylactic vertebroplasty may reduce the risk of adjacent intact vertebra from fatigue injury: an ex vivo biomechanical study.

Study Design. In vitro biomechanical study using human spine specimens. Objective. To find the biomechanical consequences of prophylactic vertebroplasty post fatigue loading. Summary of Background Data. Percutaneous vertebroplasty man be an effective treatment for osteoporotic vertebral compression fracture. One frequently observed complication post surgery is the adjacent vertebral failure (AVF). The prophylactic vertebroplasty was proposed to prevent the AVF. The vertebroplasty is, nevertheless, an invasive intervention. More scientific proves are needed for the application of this surgery on a still intact vertebra. Methods. Fourteen 5-level fresh human cadaveric thoracic motion segments were divided into standard and prophylactic group. Both ends of the specimen were mounted, leaving the center 3 vertebrae free. The lower level of free vertebrae was artificially injured and cement augmented. The center level vertebra of standard group remained intact and nonaugmented. The center level vertebra of prophylactic group also remained intact, but augmented with bone cement. The specimen was applied with a 2-hour, 5-Hz, 630-N (mean) compressive fatigue loading. Impulse test and CT scanning were conducted both before and after fatigue loading to find the variance of strain compliance of cortical shell and height of vertebral body. Results. The strain compliance of cortical shell is generally not statistically significantly affected by the fatigue loading, cement augmentation and vertebral level (All P > 0.05). The only exception is that the cortical strain compliance of augmented vertebrae tentatively decreased post fatigue loading (P = 0.012 for tensile strain compliance, and P = 0.049 for compressive strain compliance). The height loss of intact vertebra adjacent to a 2-level augmented (or intact-augmented) vertebra is significantly lower than the one adjacent to a 1-level augmented (or injury-augmented) vertebra (P = 0.014). For an osteoporotic vertebra, neither cortical strain compliance nor vertebral height loss is connected with bone mineral density (all P > 0.05). Conclusion. The strain compliance of cortical shell is generally not a sensitive indicator to predict risk of fatigue injury if the fatigue loading is mild. The prophylactic augmentation strengthens the osteoporotic vertebrae, decreases the progression of vertebral height loss, reduces the anterior body shift, and hence protects the adjacent intact vertebra from elevated flexion bending. It can be cautiously suggested that if the vertebra is osteoporotic and adjacent level is located at pivot or lordotic level of spinal column, the prophylactic augmentation may be an option to prevent the AVF.

Mechanism of fractures of adjacent and augmented vertebrae following simulated vertebroplasty

Percutaneous vertebroplasty (VP) is a minimally invasive procedure that is used to treat osteoporosis-induced vertebral compression fractures (OVCFs). Frequently observed complications are fractures of adjacent and augmented vertebrae. In the present work, mechanisms for these fractures are presented. Fresh 4-level osteoporotic thoracic motion segments were tested. Both ends of the specimen were mounted. The lower level of the free vertebra was compressively fractured and followed by an injection of a 3.5 mL of a PMMA bone cement. Three steps of fatigue loading (5 Hz for 5 h) were incrementally and vertically applied on the specimens from 650 N to 950 N to 1150 N. Specimens of intact, compressively fractured, cement augmented and post-fatigued loading were radiographed for the measurement of deformations of the vertebra, the canal, and the foramen. At the end of fatigue loading, the vertebrae were sliced for micro morphologic analysis. The largest height loss after fatigue loading was at the posterior region of the augmented vertebra. In the augmented vertebra, fissures were found along the bone-cement interface. These fissures split the cement and the trabeculae and propagated into the vertebrae and the endplates. The compactness ratio of the trabeculae region of the adjacent cranial vertebra was higher than that for intact and adjacent caudal ones. We attribute the fracture of the augmented vertebra, following simulated VP, to the initiation of fissures along the cement-bone interface, which, in turn, may be due to uneven deformation of the vertebra. Fracture of the adjacent cranial vertebra is attributed to collapse of its trabeculae.

A miniature patient-mount navigation system for assisting needle placement in CT-guided intervention

CT‐guided intervention is routinely performed in an iterative fashion that often leads to lengthy operation and high X‐ray exposure to patients. To streamline the workflow, we develop a patient‐mount navigation system for assisting needle placement in CT‐guided interventions.

Interference in intradiscal pressure measurement using a needle‐type pressure transducer

Abstract The invasive needle‐type pressure transducer is widely used in the measurement of intradiscal pressure (IDP). However, the protocol for measuring IDP is not a standard procedure. Most of the in vitro studies employ an anterior insertion of the transducer while most of the in vivo studies employ a lateral insertion. The interference between transducer and disc needs to be understood before comparing the results of the two different protocols. Sixteen porcine one‐motion segments (T9–10, T11–12) were used. The transducers were inserted into the disc from either the anterior site (N = 9) or the lateral site (N = 7). All specimens had axial compressive force applied on eight locations along the mid‐sagittal line on the top of the cast to simulate flexion, neutral, and extension bending moments. The effects of bending moment and alignment of pressure transducers on the IDP were analyzed. Both the alignment of the transducer and bending moment affect the measurement of IDP. The IDP measured by an anterior‐inserted‐transducer (ant‐IDP) declined gradually from flexion through neutral to extension. The IDP measured by a lateral‐inserted‐transducer (lat‐IDP) during flexion and extension is higher than the IDP measured during the neutral condition. The ant‐IDP is higher than the lat‐IDP during flexion and neutral, but lower during extension. The ant‐IDP may be overestimated, compared to lat‐IDP, during flexion and neutral but underestimated during extension. We suggest that lateral insertion of transducer may be a better choice for both in vivo and in vitro IDP measurement.

FAST AND HIGH PRECISION DIGITALLY RECONSTRUCTED RADIOGRAPH GENERATION ON GRAPHIC PROCESSING UNIT

Digitally reconstructed radiograph (DRR) from CT volumetric data has been used in numerous medical applications such as 3D treatment planning and CT-to-fluoroscopic alignment. The poor efficiency of the DRR generation is the main problem in such applications. Many researches have been attempted to accelerate the DRR calculation. However, the performance and precision cannot be achieved without the sacrifice of one or the other. In this study, a fast and high precision DRR generation method is proposed on a consumer PC platform. Instead of using CPU, the method takes the advantages of the powerful parallel computation and flexible programming capability of the graphic processing unit (GPU) to reach almost interactive rendering rate while maintaining 12-bit precision of the original CT data. This method can generate DRR images at 4.6 frames per second using 512 × 512 × 261 dataset in the 512 × 512 view port, and its precision is compatible to that generated by the CPU-based method. Besides, in order to simulate clinical radiograph images, a compensation filter is implemented in the DRR generation to compensate varying thickness of bone structures. The additional compensation filter can achieve a DRR image with more uniform optical density but takes no obvious performance overhead.

MECHANISM AND RISK FACTORS OF ADJACENT VERTEBRAL FAILURE POST PERCUTANEOUS VERTEBROPLASTY - A STRAIN ENERGY DENSITY APPROACH

Abstract Percutaneous vertebroplasty (PV) is an effective treatment procedure for compression fracture of osteoporotic vertebra; however, adjacent vertebral failure (AVF) is a frequently observed complication of PV. The mechanism and risk factors of AVF are not yet clear and this problem has attracted a lot of research effort in the medical community. The purpose of this study is to analyze the mechanism and risk factors of AVF post cement augmentation using a strain energy density (SED) approach. Fresh porcine spine specimens (L1‐L5) were used. The effect of cement augmentation on the SED of vertebral bodies (VB), including the damaged VB and both adjacent cranial and caudal VBs, was analyzed. The result showed that the SED of the adjacent VBs did not increase after cement augmentation; however, the accumulated energy in the VBs of the whole spine motion segment decreased significantly after cement augmentation. This globally reduced energy is speculated to be absorbed by the disc between VBs. It is reasonable to believe that AVF may be initiated from the stressed disc. This study also suggests that poor bone quality and flexion compression are the two risk factors for AVF.

Strain energy density distribution of vertebral body of two motion segment model under combined compression and sagittal bending moment – an in vitro porcine spine biomechanical study

Abstract The purpose of the current study is to find the strain energy density (SED) distribution of a vertebral body during different compression loadings, combined with sagittal bending moments. The combined flexion and extension, which are generated by applying an eccentric pointed loading on the motion segment, is to mimic different postures of trunk and loading on the spine. Two strain gage rosettes were applied at an anterior site and a posterior site of a vertebral body. The total SED, deviatoric SED and dilatation SED were obtained from the measurements of the two rosettes. Three major phenomena are observed in the current study; first, the anterior site on the vertebra is at higher risk compared to the posterior site on the vertebra when the motion segment is in compression combined with extreme flexion and extension. Second, the SED is minimal when the loading is applied along the trajectories of the spinal canal and joint facets. Third, the major contribution to SED is from the deviatoric SED. The distribution of SED within the vertebral body during different loading conditions can serve as the baseline for treatment to protect the vertebral body from the risk of compression fracture.