The Use of High-Resolution Magnetic Resonance Imaging
Object: Interbody fusion is a gradual process of graft resorption and tissue formation, ideally resulting in a bone bridge between two adjacent vertebral bodies. Initially, fibrous tissue and cartilage are formed, which subsequently are replaced by bone through the process of endochondral ossification. When cages and/or their contents are made of resorbable polymers like lactic or glycolic acids, there is a simultaneous process of implant degradation, which is eventually accompanied by reactions in the surrounding tissues. The purpose of this study was to explore the use of high-resolution magnetic resonance (MR) imaging for monitoring tissue differentiation, spinal fusion, cage degradation, and eventually tissue reactions as a function of time.
Methods: Lumbar vertebral segments obtained in 14 goats with 3, 6, and 12 months of follow up (three, four, and seven animals, respectively) were available from a study of the feasibility of poly(L,D-lactic acid) cages for spinal fusion. Plain x-ray films, MR images, and histological sections were used to evaluate spinal fusion and cage resorption. The first follow-up tests revealed that MR imaging noninvasively provided three-dimensional information on cage placement, cage degradation and bone formation, and that it has potential to differentiate between the various soft tissues.
Conclusions: Although the magnetic field strength and thus the resolution used were higher than normal in clinical practice, MR imaging appears to be a promising modality for the noninvasive clinical follow up of patients who undergo fusion with resorbable cages. Tissue reactions were not encountered in this study, and thus could not be evaluated.
Spondylosyndesis is a surgical procedure that aims to correct spinal deformities, decompress the nerves, and fix unstable vertebral segments resulting from disc degeneration, fracture, spondylolysis, and/or spondylolisthesis. Ideally, a biological and mechanical environment is created that results in an osseous fusion of adjacent VBs. Interbody fusion is essentially a biological process of tissue differentiation, involving (among others) mesenchymal stem cells and directed by (among others) growth factors, vascularization, and oxygen. The mechanical environment is involved in two different ways. First, as in fracture repair, mechanical stability is a prerequisite for bone formation: excessive motion leads to cartilagenous or fibrous tissues (pseudarthrosis). Second, connective tissues need mechanical deformation to transport nutrients and waste products to and from the cells through the extracellular matrix, and probably also to proliferate and differentiate; stiff implants relieve the load on living tissues and thus may retard or even inhibit interbody fusion (stress shielding). Therefore, the design and material of the spinal instrumentation largely determine clinical success.
The introduction of the intervertebral cage revolutionized the surgical treatment of degenerative spinal distortions with excellent clinical results. The cage allowed surgeons to restore the sagittal plane alignment and the load-bearing capacity of the anterior column accurately, and introduced proper mechanical stability into the treated segment. With the relatively small size of the cages, minimally invasive techniques could be applied, thus limiting soft-tissue damage and the risk of infections. As a result, patients could be mobilized earlier, which shortened the rehabilitation process. The first-generation cages, however, had important disadvantages. First, the metal cages were much stiffer than the surrounding tissues, thus retarding or even inhibiting interbody fusion. Second, the metal cages obscured the intervertebral space on neuroimaging examination; it became practically impossible to determine if interbody fusion had occurred. Third, the cages are foreign bodies within the bone bed; if infection or immune rejection occurs, removal of the device is almost impossible. Newer cage materials like carbon fibers or polyethylethylketone reduced the first two problems but did not solve the third.
Polymer-based bioabsorbable materials like polylactic acids have recently been shown to be promising cage materials for spinal fusion: they have demonstrated strength and resorption characteristics commensurate with the physiological and biomechanical requirements of the human spine. Histological analysis has also demonstrated successful and timely resorption, accompanied by bone replacement and remodeling in an animal model. The radiolucent nature of these materials improves image assessment of spinal fusion, and their resorption characteristics allow controlled dynamization. Over time, the devices are resorbed through natural pathways, thereby reducing load sharing and consequently stress shielding of the surrounding tissues. Nevertheless, the composition of polymers is very diverse, showing a wide variety of resorption characteristics and evoking tissue reactions of various grades of severity. Also, the polymer degradation rate depends on the implant dimensions, sterilization procedure, implant site, mechanical loading conditions, and host tissue.
To follow the process of interbody fusion, as well as the degradation of the bioabsorbable material, a noninvasive tool for clinical evaluation is required. We think that MR may be ideal for this purpose, because it has been shown to be sensitive to changes in tissues over time, for example, during differentiation, inflammation, and edema. Moreover, MR images are able to demonstrate the infiltration of tissues into the implants as well as the materials' degradation over time in vivo. Specifically, MR imaging can be helpful for the evaluation of the fusion zone within the cage and for the assessment of a sentinel sign at the anterior side of the segment. The 3D coverage can provide information on the position of the cage after surgery. There is an additional advantage in the analysis of specimens in animal models: whereas histological and histomorphomic studies will be limited to one or more sections of a specimen at one point in time (after planned death), MR imaging can be used to evaluate the complete 3D content of a cage in a longitudinal follow-up study.
To explore the potential benefits of high-resolution MR imaging, without incurring the practical problems that arise when testing large animals in vivo, we performed an ex vivo investigation by using specimens originating from a related study on resorbable cages in goats. Qualitative and selected quantitative analyses were performed on treated vertebral segments obtained in goats at 3 to 12 months of follow up. The focus of the research was on placement and degradation of the cage, tissue differentiation within the cage, tissue reaction, and assessment of the sentinel sign.
Object: Interbody fusion is a gradual process of graft resorption and tissue formation, ideally resulting in a bone bridge between two adjacent vertebral bodies. Initially, fibrous tissue and cartilage are formed, which subsequently are replaced by bone through the process of endochondral ossification. When cages and/or their contents are made of resorbable polymers like lactic or glycolic acids, there is a simultaneous process of implant degradation, which is eventually accompanied by reactions in the surrounding tissues. The purpose of this study was to explore the use of high-resolution magnetic resonance (MR) imaging for monitoring tissue differentiation, spinal fusion, cage degradation, and eventually tissue reactions as a function of time.
Methods: Lumbar vertebral segments obtained in 14 goats with 3, 6, and 12 months of follow up (three, four, and seven animals, respectively) were available from a study of the feasibility of poly(L,D-lactic acid) cages for spinal fusion. Plain x-ray films, MR images, and histological sections were used to evaluate spinal fusion and cage resorption. The first follow-up tests revealed that MR imaging noninvasively provided three-dimensional information on cage placement, cage degradation and bone formation, and that it has potential to differentiate between the various soft tissues.
Conclusions: Although the magnetic field strength and thus the resolution used were higher than normal in clinical practice, MR imaging appears to be a promising modality for the noninvasive clinical follow up of patients who undergo fusion with resorbable cages. Tissue reactions were not encountered in this study, and thus could not be evaluated.
Spondylosyndesis is a surgical procedure that aims to correct spinal deformities, decompress the nerves, and fix unstable vertebral segments resulting from disc degeneration, fracture, spondylolysis, and/or spondylolisthesis. Ideally, a biological and mechanical environment is created that results in an osseous fusion of adjacent VBs. Interbody fusion is essentially a biological process of tissue differentiation, involving (among others) mesenchymal stem cells and directed by (among others) growth factors, vascularization, and oxygen. The mechanical environment is involved in two different ways. First, as in fracture repair, mechanical stability is a prerequisite for bone formation: excessive motion leads to cartilagenous or fibrous tissues (pseudarthrosis). Second, connective tissues need mechanical deformation to transport nutrients and waste products to and from the cells through the extracellular matrix, and probably also to proliferate and differentiate; stiff implants relieve the load on living tissues and thus may retard or even inhibit interbody fusion (stress shielding). Therefore, the design and material of the spinal instrumentation largely determine clinical success.
The introduction of the intervertebral cage revolutionized the surgical treatment of degenerative spinal distortions with excellent clinical results. The cage allowed surgeons to restore the sagittal plane alignment and the load-bearing capacity of the anterior column accurately, and introduced proper mechanical stability into the treated segment. With the relatively small size of the cages, minimally invasive techniques could be applied, thus limiting soft-tissue damage and the risk of infections. As a result, patients could be mobilized earlier, which shortened the rehabilitation process. The first-generation cages, however, had important disadvantages. First, the metal cages were much stiffer than the surrounding tissues, thus retarding or even inhibiting interbody fusion. Second, the metal cages obscured the intervertebral space on neuroimaging examination; it became practically impossible to determine if interbody fusion had occurred. Third, the cages are foreign bodies within the bone bed; if infection or immune rejection occurs, removal of the device is almost impossible. Newer cage materials like carbon fibers or polyethylethylketone reduced the first two problems but did not solve the third.
Polymer-based bioabsorbable materials like polylactic acids have recently been shown to be promising cage materials for spinal fusion: they have demonstrated strength and resorption characteristics commensurate with the physiological and biomechanical requirements of the human spine. Histological analysis has also demonstrated successful and timely resorption, accompanied by bone replacement and remodeling in an animal model. The radiolucent nature of these materials improves image assessment of spinal fusion, and their resorption characteristics allow controlled dynamization. Over time, the devices are resorbed through natural pathways, thereby reducing load sharing and consequently stress shielding of the surrounding tissues. Nevertheless, the composition of polymers is very diverse, showing a wide variety of resorption characteristics and evoking tissue reactions of various grades of severity. Also, the polymer degradation rate depends on the implant dimensions, sterilization procedure, implant site, mechanical loading conditions, and host tissue.
To follow the process of interbody fusion, as well as the degradation of the bioabsorbable material, a noninvasive tool for clinical evaluation is required. We think that MR may be ideal for this purpose, because it has been shown to be sensitive to changes in tissues over time, for example, during differentiation, inflammation, and edema. Moreover, MR images are able to demonstrate the infiltration of tissues into the implants as well as the materials' degradation over time in vivo. Specifically, MR imaging can be helpful for the evaluation of the fusion zone within the cage and for the assessment of a sentinel sign at the anterior side of the segment. The 3D coverage can provide information on the position of the cage after surgery. There is an additional advantage in the analysis of specimens in animal models: whereas histological and histomorphomic studies will be limited to one or more sections of a specimen at one point in time (after planned death), MR imaging can be used to evaluate the complete 3D content of a cage in a longitudinal follow-up study.
To explore the potential benefits of high-resolution MR imaging, without incurring the practical problems that arise when testing large animals in vivo, we performed an ex vivo investigation by using specimens originating from a related study on resorbable cages in goats. Qualitative and selected quantitative analyses were performed on treated vertebral segments obtained in goats at 3 to 12 months of follow up. The focus of the research was on placement and degradation of the cage, tissue differentiation within the cage, tissue reaction, and assessment of the sentinel sign.
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