Reducing subsidence risk by using rapid manufactured patient-specific intervertebral disc implants
Department of Industrial Engineering, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa. Electronic address: .The spine journal: official journal of the North American Spine Society (Impact Factor: 2.43). 10/2012; 12(11). DOI: 10.1016/j.spinee.2012.10.003
BACKGROUND CONTEXT: Intervertebral disc implant size, shape, and position during total disc replacement have been shown to affect the risk of implant subsidence or vertebral fracture. Rapid manufacturing has been successfully applied to produce patient-specific implants for craniomaxillofacial, dental, hip, and knee requirements, but very little has been published on its application for spinal implants. PURPOSE: This research was undertaken to investigate the improved load distribution and stiffness that can be achieved when using implants with matching bone interface geometry as opposed to implants with flat end plate geometries. STUDY DESIGN: The study design comprises a biomechanical investigation and comparison of compressive loads applied to cadaveric vertebrae when using two different end plate designs. METHODS: Four spines from male cadavers (ages 45-65 years, average 52 years), which had a total of n=88 vertebrae (C3-L5), were considered during this study. Bone mineral density scans on each spine revealed only one to be eligible for this study. Twenty remaining vertebrae (C3-L3) were potted and subjected to nondestructive compression tests followed by destructive compression tests. Custom-made nonfunctional implants were designed for this experiment. Ten implants were designed with matching end plate-to-bone interface geometry, whereas the other 10 were designed with flat end plates. Testing did not incorporate the use of a keel in either design type. I-Scan pressure sensors (Tekscan, Inc., MA, USA) were used during the nondestructive tests to assess the load distribution and percentage surface contact. RESULTS: Average percent contact area measured during nondestructive tests was 45.27% and 10.49% for conformal and flat implants, respectively-a difference that is statistically significant (p<.001). A higher percent contact area was especially observed for cervical vertebrae because of their pronounced end plate concavity. During destructive compression tests, conformal implants achieved higher failure loads than flat implants. Conformal implants also performed significantly better when stiffness values were compared (p<.0001). CONCLUSIONS: One of the main expected benefits from customizing the end plate geometry of disc implants is the reduced risk and potential for subsidence into the vertebral bone end plate. Subsidence depends in part on the stiffness of the implant-bone construct, and with a 137% increase in stiffness, the results of this study show that there are indeed significant potential benefits that can be achieved through the use of customization during the design and manufacture of intervertebral disc implants.
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ABSTRACT: Design methods for medical rapid prototyping (RP) of personalized cranioplasty implants are presented in this paper. These methods are applicable to model cranioplasty implants for all types of the skull defects including beyond-midline and multiple defects. The methods are based on two types of anatomical data, solid bone models (STereoLithography files – STL) and bone slice contours (Initial Graphics Exchange Specification – IGES and StrataSys Layer files – SSL). The bone solids and contours are constructed based on computed tomography scanning data, and these data are generated in medical image processing and STL slicing packages.Rapid Prototyping Journal 08/2003; 9(3):175-186. DOI:10.1108/13552540310477481 · 2.03 Impact Factor
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ABSTRACT: Background Distal humeral hemiarthroplasty results in altered cartilage contact mechanics, which may predispose to osteoarthritis. Current distal humeral hemiarthroplasty prostheses do not replicate the native anatomy, and therefore contribute to these changes. We hypothesized that prostheses reverse-engineered from the native bone shape would provide similar contact patterns as the native articulation. Methods Reverse-engineered distal humeral hemiarthroplasty prostheses were manufactured for five cadaveric elbow specimens based on computed tomographic images of the distal humerus. Passive flexion trials with constant muscle forces were performed with the native articulation intact while bone motions were recorded using a motion tracking system. Motion trials were then repeated after the distal humerus was replaced with a corresponding reverse-engineered prosthesis. Contact areas and patterns were reconstructed using computer models created from computed tomography scan images combined with the motion tracker data. The total contact areas, as well as the contact area within smaller sub-regions of the ulna and radius, were analyzed for changes resulting from distal humeral hemiarthroplasty using repeated-measures analyses of variance. Findings Contact area at the ulna and radius decreased on average 42% (SD 19%, p = .008) and 41% (SD 42%, p = .096), respectively. Contact area decreases were not uniform throughout the different sub-regions, suggesting that contact patterns were also altered. Interpretation Reverse-engineered prostheses did not reproduce the same contact pattern as the native joints, possibly because the thickness of the distal humerus cartilage layer was neglected when generating the prosthesis shapes or as a consequence of the increased stiffness of the metallic implants. Alternative design strategies and materials for distal humeral hemiarthroplasty should be considered in future work.Clinical Biomechanics 09/2014; 29(9). DOI:10.1016/j.clinbiomech.2014.08.015 · 1.97 Impact Factor
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ABSTRACT: A retrospective review of prospectively collected radiographic and clinical data. This study aims to investigate the relationship between cage subsidence and bone mineral density (BMD), and to reveal the clinical implications of cage subsidence. Posterior lumbar interbody fusion (PLIF) has become one of the standard treatment modality for lumbar degenerative disease. However, cage subsidence might result in recurrent foraminal stenosis and deteriorate the clinical results. Furthermore, numbers of osteoporosis patients who underwent PLIF are increasing. Therefore, the information on the correlations between cage subsidence, BMD, and clinical results will be of great significance. A total 139 segments was included in this retrospective study. We examined functional rating index (Visual Analogue Scale for pain, Oswestry Disability Index, Short Form-36 score) preoperatively, and investigated their changes after postoperative 1 year. Correlation between cage subsidence and clinical scores was investigated. Plain anteroposterior and lateral radiograph were taken preoperatively and postoperatively and during follow-up. Preoperative BMD and subsidence measured by postoperative 1 year 3-dimensional computed tomography were achieved and their correlation was assessed. All postoperative clinical scores improved significantly compared with preoperative ones (pain Visual Analogue Scale: 7.34-2.89, Oswestry Disability Index: 25.34-15.86, Short Form-36: 26.45-16.46, all P<0.001). BMD showed significant weak correlation with subsidence (r=-0.285, P<0.001). Severe osteoporotic segments (T score <-3.0) had more risk to develop severe subsidence (>3 mm) compared with the segments in which T score were higher than -3.0 (P=0.012), and its odds ratio was 8.44. Subsidence had no significant correlation with all clinical scores. This study revealed that cage subsidence is relevant to BMD. However, it was demonstrated that subsidence is not related to the clinical deterioration. Therefore, PLIF procedure which is conducted carefully can be a good surgical option to treat lumbar degenerative disease for osteoporotic patients.Journal of spinal disorders & techniques 08/2015; DOI:10.1097/BSD.0000000000000315 · 2.20 Impact Factor
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