An approximate model for cancellous bone screw fixation

This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ 2013 Taylor & Francis.This paper presents a finite element (FE) model to identify parameters that affect the performance of an improved cancellous bone screw fixation technique, and hence potentially improve fracture treatment. In cancellous bone of low apparent density, it can be difficult to achieve adequate screw fixation and hence provide stable fracture fixation that enables bone healing. Data from predictive FE models indicate that cements can have a significant potential to improve screw holding power in cancellous bone. These FE models are used to demonstrate the key parameters that determine pull-out strength in a variety of screw, bone and cement set-ups, and to compare the effectiveness of different configurations. The paper concludes that significant advantages, up to an order of magnitude, in screw pull-out strength in cancellous bone might be gained by the appropriate use of a currently approved calcium phosphate cement

Pull-out strength of patient-specific template-guided vs. free-hand fluoroscopically controlled thoracolumbar pedicle screws: a biomechanical analysis of a randomized cadaveric study

PURPOSE To assess the pull-out strength of thoracolumbar pedicle screws implanted via either a patient-specific template-guided or conventional free-hand fluoroscopically controlled technique in a randomized cadaveric study, and to evaluate the influence of local vertebral bone density, quantified by Hounsfield units (HU), on pedicle screw pull-out strength. METHODS Thoracolumbar pedicles of three spine cadavers were instrumented using either a free-hand fluoroscopically controlled or a patient-specific template-guided technique. Preoperative bone density was quantified by HU measured on CT. Pedicle perforation was evaluated on postoperative CT scans by an independent and blinded radiologist. After dissected vertebrae were embedded in aluminum fixation devices, pull-out testing was initiated with a preload of 50 N and a constant displacement rate of 0.5 mm/s. Subgroup analyses were performed excluding pedicle screws with a pedicle breach (n = 47). RESULTS Pull-out strength was significantly different with 549 ± 278 and 441 ± 289 N in the template-guided (n = 50) versus fluoroscopically controlled (n = 48) subgroups (p = 0.031), respectively. Subgroup analysis limited to screws with an intrapedicular trajectory revealed a tendency toward a higher pull-out strength in the template-guided (n = 30) versus fluoroscopically controlled screws (n = 21) with 587 ± 309 and 454 ± 269 N (p = 0.118), respectively. There was a trend toward a higher pull-out strength (709 ± 418 versus 420 ± 149 N) in vertebrae with a bone density of (>171 HU) versus (<133 HU), respectively (p = 0.061). CONCLUSIONS There was a significantly higher pull-out strength of thoracolumbar pedicle screws when inserted via a patient-specific template-guided versus conventional free-hand fluoroscopically controlled technique, potentially associated with screw trajectory

Revision strategies for complications and failure of vertebroplasties

Percutaneous vertebroplasty is widely discussed in the management of osteoporotic spinal compression fracture, but few reports are available concerning salvage procedures after failure of this technique. We studied 22 percutaneous vertebroplasty patients who required revision surgery upon presentation of new symptoms postoperatively. The indications for revision surgery included recurrent intractable back pain with no response to medical treatment, infectious spondylitis, cement leakage with neurologic deficit, and cement dislodgement and/or fragmentation. Five patients underwent repeated percutaneous vertebroplasty of the initially cemented vertebrae. Seventeen patients underwent anterior, posterior, or combined anterior and posterior surgery. Four patients required a third surgical procedure because of poor augmentation with cement, subsidence of the anterior bone graft, or pullout of the instrumentation. Finally, four (18%) patients underwent repeat vertebroplasty, two (9%) patients underwent anterior surgery only, one (5%) patient underwent posterior surgery only, and 15 (68%) patients underwent combined anterior and posterior surgery; all but one regained ambulatory status equivalent to that prior to surgery. In conclusion, percutaneous vertebroplasty is a simple and effective, but not risk- or complication-free procedure for the treatment of osteoporotic spinal compression fracture. The spine surgeon should be familiar with varied approaches and techniques for revision surgery. Combined anterior and posterior surgery seems to be the most secure salvage method to treat severely osteoporotic patients in whom percutaneous vertebroplasty initially failed