Abstract
Pedicle screw loosening resulting from insufficient bone-screw interfacial holding power is not uncommon. The screw shape and thread profile are considered important factors of the screw fixation strength. This work investigated the difference in pullout strength between conical and cylindrical screws with three different thread designs. The effects of the thread profiles on the screw fixation strength of cannulated screws with or without cement augmentation in osteoporotic bone were also evaluated. Commercially available artificial standard L4 vertebrae and low-density polyurethane foam blocks were used as substitutes for healthy vertebrae and osteoporotic bones, respectively. The screw pullout strengths of nine screw systems were investigated (six in each). These systems included the combination of three different screw shapes (solid/cylindrical, solid/conical and cannulated/cylindrical) with three different thread profiles (fine-thread, coarse-thread and dual-core/dual-thread). Solid screws were designed for the cementless screw fixation of vertebrae using the standard samples, whereas cannulated screws were designed for the cemented screw fixation of osteoporotic bone using low-density test blocks. Following specimen preparation, a screw pullout test was conducted using a material test machine, and the maximal screw pullout strength was compared among the groups. This study demonstrated that, in healthy vertebrae, both the conical and dual-core/dual-thread designs can improve pullout strength. A combination of the conical and dual-core/dual-thread designs may achieve optimal postoperative screw stability. However, in osteoporotic bone, the thread profile have little impact on the screw fixation strength when pedicle screws are fixed with cement augmentation. Cement augmentation is the most important factor contributing to screw pullout fixation strength as compared to screw designs.
Highlights
Pedicle screw fixation has been widely used to treat spinal instability and other spinal disorders, including degenerative spinal diseases, scoliosis, vertebral fractures, spinal infection and metastatic spinal lesions [1,2,3]
Numerous studies have focused on different pedicle screw designs to prevent screw loosening
Transpedicular screw instrumentation has been regarded as a successful surgical technique in the treatment of spinal disorders because it ensures a three-dimensional structure over the vertebral motion level, achieving a stiff stabilization
Summary
Pedicle screw fixation has been widely used to treat spinal instability and other spinal disorders, including degenerative spinal diseases, scoliosis, vertebral fractures, spinal infection and metastatic spinal lesions [1,2,3]. Numerous studies have focused on different pedicle screw designs to prevent screw loosening These designs include screws with an increased outer diameter or length [8], different thread profiles [9,10,11], a cylindrical or conical core [12,13,14], expanding screws [15,16] and cannulated screws with polymethylmethacrylate (PMMA) cement augmentation [14,17,18,19]. All of these screw designs are continuously studied. Among these screw designs, cannulated screws in particular have an efficient alternative and innovative design for preventing osteoporotic incidents when used with cement augmentation [14,17,19,20,21]
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