Effect of bone material properties on effective region in screw-bone model: an experimental and finite element study

BioMedical Engineering OnLine Effect of bone material properties on effective region in screw-bone model: an experimental and finite element study Shuai Liu 0 2 3 Wei Qi 0 1 2 3 Yang Zhang 0 2 3 Zi-Xiang Wu 0 2 3 Ya-Bo Yan 0 2 3 Wei Lei 0 2 3 0 Department of Orthopaedics, Xijing Hospital, Fourth Military Medical University , Xi'an 710032, Shaanxi Province , P.R. China 1 Surgery Department of 520th Hospital of PLA , Mian yang , China 2 Department of Orthopaedics, Xijing Hospital, Fourth Military Medical University , Xi'an 710032, Shaanxi Province , P.R. China 3 Authors' information SL: Department of Orthopaedics, Xijing Hospital, Fourth Military Medical University , Xi'an , China; WQ: Surgery Department of 520th Hospital of PLA , Mian yang , China; YZ: Department of Orthopaedics, Xijing Hospital, Fourth Military Medical University , Xi'an , China; ZXW: Department of Orthopaedics, Xijing Hospital, Fourth Military Medical University , Xi'an , China; YBY: Department of Orthopaedics, Xijing Hospital, Fourth Military Medical University , Xi'an , China; WL: Department of Orthopaedics, Xijing Hospital, Fourth Military Medical University , Xi'an , China. Shuai Liu, Wei Qi and Yang Zhang are the co-first authors Background: There have been numerous studies conducted to investigate the pullout force of pedicle screws in bone with different material properties. However, fewer studies have investigated the region of effect (RoE), stress distribution and contour pattern of the cancellous bone surrounding the pedicle screw. Methods: Screw pullout experiments were performed from two different foams and the corresponding reaction force was documented for the validation of a computational pedicle screw-foam model based on finite element (FE) methods. After validation, pullout simulations were performed on screw-bone models, with different bone material properties to model three different age groups (<50, 50-75 and >75 years old). At maximum pullout force, the stress distribution and average magnitude of Von Mises stress were documented in the cancellous bone along the distance beyond the outer perimeter pedicle screw. The radius and volume of the RoE were predicted based on the stress distribution. Results: The screw pullout strengths and the load-displacement curves were comparable between the numerical simulation and experimental tests. The stress distribution of the simulated screw-bone vertebral unit showed that the radius and volume of the RoE varied with the bone material properties. The radii were 4.73 mm, 5.06 mm and 5.4 mm for bone properties of ages >75, 75 > ages >50 and ages <50 years old, respectively, and the corresponding volumes of the RoE were 6.67 mm3, 7.35 mm3 and 8.07 mm3, respectively. Conclusions: This study demonstrated that there existed a circular effective region surrounding the pedicle screw for stabilization and that this region was sensitive to the bone material characteristics of cancellous bone. The proper amount of injection cement for augmentation could be estimated based on the RoE in the treatment of osteoporosis patients to avoid leakage in spine surgery. Biomechanics; Pedicle screw; Cancellous bone; Finite element analysis - Osteoporosis is a common skeletal disorder of the spine and hip in the elderly population. Spine surgeons often encounter patients with osteoporotic spines that require spinal decompression and management with surgical instrumentation due to degenerative and traumatic spinal diseases [1,2]. Pedicle screw fixation is a routine tool for spine stabilization, with the screw providing rigid bony secured points on internal fixation devices. However, it is a challenge for spine surgeons to perform pedicle screw instrumentation surgery on osteoporotic spines to prevent many potential complications, such as screw loosening, migration or back-out [3]. It has been reported that the mechanical strength of the bone-screw interface is adversely affected by low bone density in patients with osteoporosis [4,5]. Different methods have been used to enhance the short- and long-term stability of implanted screws in the osteoporotic lumbar spine [6,7]. The in situ injection of biomaterials, such as calcium phosphate cement (CPC), calcium sulfate cement (CSC), and polymethyl-methacrylate (PMMA), into the screw hole is a common option for enhancing pedicle screw fixation strength [8,9]. Although pedicle screw augmentation with cement is an attractive option for improving screw fixation, there exist risks of excessive cement leakage beyond the confined target bone, affecting the spinal cord and resulting in nerve compression [10]. A smaller-volume cement injection might not enhance the augmentation screw performance. Although there have been many experimental screw-bone interaction studies, it is technically difficult to determine the region of effect (RoE) by observing the screwbone interaction during pullout testing. Sources in the literature have reported that the pullout strength (...truncated)