Mechanical evaluation by patient-specific finite element analyses demonstrates therapeutic effects for osteoporotic vertebrae.

Osteoporosis can lead to bone compressive fractures in the lower lumbar vertebrae. In order to assess the recovery of vertebral strength during drug treatment for osteoporosis, it is necessary not only to measure the bone mass but also to perform patient-specific mechanical analyses, since the strength of osteoporotic vertebrae is strongly dependent on patient-specific factors, such as bone shape and bone density distribution in cancellous bone, which are related to stress distribution in the vertebrae. In the present study, patient-specific general (not voxel) finite element analyses of osteoporotic vertebrae during drug treatment were performed over time. We compared changes in bone density and compressive principal strain distribution in a relative manner using models for the first lumbar vertebra based on computer tomography images of four patients at three time points (before therapy, and after 6 and 12 months of therapy). The patient-specific mechanical analyses indicated that increases in bone density and decreases in compressive principal strain were significant in some osteoporotic vertebrae. The data suggested that the vertebrae were strengthened structurally and the drug treatment was effective in preventing compression fractures. The effectiveness of patient-specific mechanical analyses for providing useful and important information for the prognosis of osteoporosis is demonstrated.

Finite-element analysis on closing-opening correction osteotomy for angular kyphosis of osteoporotic vertebral fractures.

BACKGROUND: Closing-opening correction (COC) osteotomy is a useful procedure for severe angular kyphosis. However, there is no previous research on the reconstructed vertebrae with kyphotic malalignment in the presence of osteoporosis. Finite-element (FE) analysis was performed to estimate the biomechanical stress with both osteoporotic grades and corrective kyphotic angles during COC osteotomy for osteoporotic angular kyphosis. METHODS: FE models of COC osteotomy were created by changing three major parameters: (1) grade of osteoporosis; (2) kyphotic angle; and (3) compensated posture when standing still. Osteoporosis was graded at four levels: A, normal (nonosteoporotic); B, low-grade osteoporosis; C, middle-grade osteoporosis; D, high-grade osteoporosis. The kyphotic angle ranged from 0 degrees as normal to 15 degrees and 30 degrees as moderate and severe kyphosis, respectively. FE analyses were performed with and without assumed compensated posture in kyphotic models of 15 degrees and 30 degrees . Along each calculated axis of gravity, a 427.4-N load was applied to evaluate the maximum compressive principal stress (CPS) for each model. RESULTS: The CPS values for the vertebral element were the highest at the anterior element of T10 in all FE models. The maximum CPS at T10 increased based on the increases in both the grade of osteoporosis and the kyphotic angle. Compensated posture made the maximum CPS value decrease in the 15 degrees and 30 degrees kyphotic models. The highest CPS value was 40.6 MPa in the high-grade osteoporosis (group D) model with a kyphotic angle of 30 degrees . With the normal (nonosteoporotic) group A, the maximum CPS at T10 was relatively low. With middle- and high-grade osteoporosis (groups C and D, respectively), the maximum CPS at T10 was relatively high with or without compensated posture, except for the 0 degrees model. CONCLUSIONS: Lack of correction in osteoporotic kyphosis leads to an increase in CPS. This biomechanical study proved the advantage of correcting the kyphotic angle to as close as possible to physiological alignment in the thoracolumbar spine, especially in patients with high-grade osteoporosis.

The transmission of stress to grafted bone inside a titanium mesh cage used in anterior column reconstruction after total spondylectomy: a finite-element analysis.

STUDY DESIGN: A finite-element study of posterior alone or anterior/posterior combined instrumentation following total spondylectomy and replacement with a titanium mesh cage used as an anterior strut. OBJECTIVES: To compare the effect of posterior instrumentation versus anterior/posterior instrumentation on transmission of the stress to grafted bone inside a titanium mesh cage following total spondylectomy. SUMMARY OF BACKGROUND DATA: The most recent reconstruction techniques following total spondylectomy for malignant spinal tumor include a titanium mesh cage filled with autologous bone as an anterior strut. The need for additional anterior instrumentation with posterior pedicle screws and rods is controversial. Transmission of the mechanical stress to grafted bone inside a titanium mesh cage is important for fusion and remodeling. To our knowledge, there are no published reports comparing the load-sharing properties of the different reconstruction methods following total spondylectomy. METHODS: A 3-dimensional finite-element model of the reconstructed spine (T10-L4) following total spondylectomy at T12 was constructed. A Harms titanium mesh cage (DePuy Spine, Raynham, MA) was positioned as an anterior replacement, and 3 types of the reconstruction methods were compared: (1) multilevel posterior instrumentation (MPI) (i.e., posterior pedicle screws and rods at T10-L2 without anterior instrumentation); (2) MPI with anterior instrumentation (MPAI) (i.e., MPAI [Kaneda SR; DePuy Spine] at T11-L1); and (3) short posterior and anterior instrumentation (SPAI) (i.e., posterior pedicle screws and rods with anterior instrumentation at T11-L1). The mechanical energy stress distribution exerted inside the titanium mesh cage was evaluated and compared by finite-element analysis for the 3 different reconstruction methods. Simulated forces were applied to give axial compression, flexion, extension, and lateral bending. RESULTS: In flexion mode, the energy stress distribution in MPI was higher than 3.0 x 10 MPa in 73.0% of the total volume inside the titanium mesh cage, while 38.0% in MPAI, and 43.3% in SPAI. In axial compression and extension modes, there were no remarkable differences for each reconstruction method. In left-bending mode, there was little stress energy in the cancellous bone inside the titanium mesh cage in MPAI and SPAI. CONCLUSIONS: This experiment shows that from the viewpoint of stress shielding, the reconstruction method, using additional anterior instrumentation with posterior pedicle screws (MPAI and SPAI), stress shields the cancellous bone inside the titanium mesh cage to a higher degree than does the system using posterior pedicle screw fixation alone (MPI). Thus, a reconstruction method with no anterior fixation should be better at allowing stress for remodeling of the bone graft inside the titanium mesh cage.

Reconstruction after total sacrectomy using a new instrumentation technique: a biomechanical comparison.

STUDY DESIGN: The stresses exerted on the instrumentation and adjacent bone were evaluated for three reconstruction methods after a total sacrectomy: a modified Galveston reconstruction (MGR), a triangular frame reconstruction (TFR), and a novel reconstruction (NR). OBJECTIVE: To perform finite-element analysis of reconstruction methods used after a total sacrectomy. SUMMARY OF BACKGROUND DATA: When a sacral tumor involves the first sacral vertebra, a total sacrectomy is necessary. It is mandatory to reconstruct the continuity between the spine and the pelvis after a total sacrectomy. However, no previous reports have described a biomechanical study of the reconstructed lumbosacral spine. METHODS: A finite-element model of the lumbar spine and pelvis was constructed. Then three-dimensional MGR, TFR, and NR models were reconstructed, and a finite-element analysis was performed to account for the stresses on the bones and instrumentation. RESULTS: With excessive stress concentrated at the spinal rod in MGR, there is a strong possibility that the rod between the spine and the pelvis may fail. Although there was no stress concentration on the instruments in TFR, excessive stress on the iliac bones around the sacral rod was above the yield stress of the iliac bone. Such stress may cause a loosening of the sacral rod from the iliac bone. In NR, excessive stress concentration was not detected in the rod or the bones. This reconstruction has a low risk of instrument failure and loosening. CONCLUSIONS: If the patient were to stand or sit immediately after MGR or TFR instrumentation, failure or loosening may occur. The NR has a low risk of instrument failure and loosening after a total sacrectomy.

Biomechanical evaluation of reconstructed lumbosacral spine after total sacrectomy.

When a sacral tumor involves the first sacral vertebra, total sacrectomy is necessary. It is mandatory to reconstruct the continuity between the spine and the pelvis after total sacrectomy. In this study, strain and stress on the instruments and the bones were evaluated for two reconstruction methods: a modified Galveston reconstruction (MGR) and a triangular frame reconstruction (TFR). Compressive loading tests were performed using polyurethane vertebral models, and a finite element model of a lumbar spine and pelvis was constructed. Then three-dimensional MGR and TFR models were reconstructed, and finite element analysis was performed to account for the stress on the bones and instruments. With MGR, excessive stress was concentrated at the spinal rod, and there was a strong possibility that the rod between the spine and the pelvis might fail. Although there was no stress concentration on the instruments with TFR, excessive stress on the iliac bone around the sacral rod was more than the yielding stress of the iliac bone. Such stress may cause loosening of the sacral rod from the iliac bone. If the patient were to stand or sit immediately after MGR or TFR, instrumentation failure or loosening might occur.