A biomechanical comparison of a cement-augmented odontoid screw with a posterior-instrumented fusion in geriatric patients with an odontoid fracture type IIb.

PURPOSE: Possible surgical therapies for odontoid fracture type IIb include odontoid screw osteosynthesis (OG) with preservation of mobility or dorsal C1/2 fusion with restriction of cervical rotation. In order to reduce material loosening in odontoid screw osteosynthesis in patients with low bone density, augmentation at the base of the axis using bone cement has been established as a suitable alternative. In this study, we compared cement-augmented OG and C1/2 fusion according to Harms (HG). METHODS: Body donor preparations of the 1st and 2nd cervical vertebrae were randomized in 2 groups (OG vs. HG). The range of motion (ROM) was determined in 3 principle motion plains. Subsequently, a cyclic loading test was performed. The decrease in height of the specimen and the double amplitude height were determined as absolute values as an indication of screw loosening. Afterward, the ROM was determined again and loosening of the screws was measured in a computed tomography. RESULTS: A total of 16 were included. Two groups of 8 specimens (OG vs. HG) from patients with a median age of 80 (interquartile range (IQ) 73.5-85) years and a reduced bone density of 87.2 (IQ 71.2-104.5) mg/cc dipotassium hydrogen phosphate were examined for their biomechanical properties. Before and after exposure, the OG preparations were significantly more mobile. At the time of loading, the OG had similar loading properties to HG decrease in height of the specimen and the double amplitude height. Computed tomography revealed similar outcomes with regard to the screw loosening rate (62.5 vs. 87.5%, p = 0.586). CONCLUSION: In patients with an odontoid fracture type IIb and reduced bone density, cement-augmented odontoid screw yielded similar properties in the loading tests compared to the HG. It may, therefore, be considered as a primary alternative to preserve cervical mobility in these patients.

Numerical Prediction of the Mechanical Failure of the Intervertebral Disc under Complex Loading Conditions.

Finite element modeling has been widely used to simulate the mechanical behavior of the intervertebral disc. Previous models have been generally limited to the prediction of the disc behavior under simple loading conditions, thus neglecting its response to complex loads, which may induce its failure. The aim of this study was to generate a finite element model of the ovine lumbar intervertebral disc, in which the annulus was characterized by an anisotropic hyperelastic formulation, and to use it to define which mechanical condition was unsafe for the disc. Based on published in vitro results, numerical analyses under combined flexion, lateral bending, and axial rotation with a magnitude double that of the physiological ones were performed. The simulations showed that flexion was the most unsafe load and an axial tensile stress greater than 10 MPa can cause disc failure. The numerical model here presented can be used to predict the failure of the disc under all loading conditions, which may support indications about the degree of safety of specific motions and daily activities, such as weight lifting.

Influence of Complex Loading Conditions on Intervertebral Disc Failure.

STUDY DESIGN: High resolution imaging investigation of the failure of ovine lumbar intervertebral discs under complex loading. OBJECTIVE: To investigate how different loading combinations influence the mechanism and extent of intervertebral disc failure. SUMMARY OF BACKGROUND DATA: Even though there has been extensive research on how an intervertebral disc fails under various conditions, failure mechanisms remain unclear. In addition, the influence of different loading directions on the mode and extent of failure under complex loading was never systematically investigated. METHODS: Thirty ovine lumbar spinal segments were loaded in a newly developed, dynamic, 6-degree-of-freedom (6-DOF) disc loading simulator under five combinations of the following loading parameters: 0°-13° flexion, 0°-10° lateral bending, 0°-4° axial rotation, 0-800 N axial compression. A total of 1000 cycles at 2 Hz were done. After testing, imaging of the discs was performed in an ultra-high field magnetic resonance imaging (11.7 T) scanner and with a micro-computed tomography scanner. RESULTS: A total of 13 large endplate junction failures (EPJFs) occurred, of which all but one maintained an intact cartilaginous endplate. Ten out of 13 EPJFs occurred caudally. Four solely annulus failures occurred affecting only the outer posterior annulus. A herniation was not observed. The maximum moments measured in any group (median) were 52.5 N ·â€Šm flexion, 16.5 N ·â€Šm lateral bending, and 14.0 N ·â€Šm axial rotation. CONCLUSION: Complex loading protocols could lead to EPJFs (76%) and annulus failures (24%) in vitro. The combination of flexion, lateral bending, axial rotation, and axial compression bears the highest risk for caudal EPJF. Flexion without lateral bending and vice versa has the lowest risk for failure. Both axial compression and axial rotation seem to have a smaller influence than flexion and lateral bending. It seems that a herniation requires an additional failure of the cartilaginous endplate, likely initiated by further axial compressive load. LEVEL OF EVIDENCE: 4.

A new dynamic six degrees of freedom disc-loading simulator allows to provoke disc damage and herniation.

PURPOSE: The cause of disc herniation is not well understood yet. It is assumed that heavy lifting and extreme postures can cause small injuries starting either in the inner anulus or from the outside close to the endplate. Such injuries are accumulated over years until its structure is weakened and finally a single loading event leads to a sudden failure of the last few intact lamellae. This paper describes a novel, custom-developed dynamic 6-DOF disc-loading simulator that allows complex loading to provoke such disc damage and herniations. METHODS: The machine's axes are driven by six independent servomotors providing high loads (10 kN axial compression, 2 kN shear, 100 Nm torque) up to 5 Hz. A positional accuracy test was conducted to validate the machine. Subsequently, initial experiments with lumbar ovine motion segments under complex loading were performed. After testing, the discs were examined in an ultra-high field MRI (11.7 T). A three-dimensional reconstruction was performed to visualise the internal disc lesions. RESULTS: Validation tests demonstrated positioning with an accuracy of ≤0.08°/≤0.026 mm at 0.5 Hz and ≤0.27°/≤0.048 mm at 3.0 Hz with amplitudes of ±17°/±2 mm. Typical failure patterns and herniations could be provoked with complex asymmetrical loading protocols. Loading with axial compression, flexion, lateral bending and torsion lead in 8 specimens to 4 herniated discs, two protrusions and two delaminations. All disc failures occurred in the posterior region of the disc. CONCLUSION: This new dynamic disc-loading simulator has proven to be able to apply complex motion combinations and allows to create artificial lesions in the disc with complex loading protocols. The aim of further tests is to better understand the mechanisms by which disc failure occurs at the microstructural level under different loading conditions. Visualisation with ultra-high field MRI at different time points is a promising method to investigate the gradual development of such lesions, which may finally lead to disc failure. These kinds of experiments will help to better investigate the mechanical failure of discs to provide new insights into the initiation of intervertebral disc herniation. This device will also serve for many other applications in spine biomechanics research.

Intervertebral disc lesions: visualisation with ultra-high field MRI at 11.7 T.

PURPOSE: Tears and fissures in the intervertebral disc are probably influencing spinal stability. Discography investigations with the aim of fissure detection have been criticised and are discouraged. Therefore, alternative imaging methods, such as MRI, must be investigated. METHODS: A custom-made device was used to insert six needles with different diameters (0.3-2.2 mm/30-14 G) into the annulus of six bovine tail discs (Cy2-Cy3). Directly after removal of the needles, the discs were scanned in an 11.7 T MRI (Res.: 0.059 × 0.059 × 0.625 mm(3), tscan: 31 min), in a 3 T MRI with a clinical and additionally with two experimental protocols (exp_HR: Res.: 0.3 mm(3), tscan: 97 min/exp_LR: Res.: 0.5 mm(3), tscan: 13.4 min). The obtained images were analysed for lesion volume and lesion length using a 3D-reconstruction software. RESULTS: At 11.7 T, all lesions were visible along with the lamellar structure of the annulus. In the clinical 3 T images, no lesions were visible at all. The 3 T experimental protocols revealed 4 (exp_HR) and 2 (exp_LR) of the 6 lesions. The reconstructed lesions did not have an ideal cylindrical shape. The measured volumes of the lesions ranged from 0.7 to 13.9 mm(3) (11.7 T), 0.1-11.4 mm(3) (exp_HR) and 0.0-12.4 mm(3) (exp_LR) and correlated, but were smaller than the corresponding needle size. The lengths of all needle lesions ranged from 2.9 to 12.3 mm (11.7 T), 0.8-9.7 mm (exp_HR) and 0.0-9.7 mm (exp_LR). CONCLUSIONS: Ultra-high field MRI at 11.7 T is a non-invasive tool to directly visualise annular lesions in vitro, while a 3 T MRI, even with experimental protocols and longer scanning times, demonstrates limited ability. In vivo, it is problematic with the clinical systems available today.

Pedicle screw loosening: a clinically relevant complication?

PURPOSE: Literature studies showed a very wide range of pedicle screw loosening rates after thoracolumbar stabilization, ranging from less than 1 to 15 % in non-osteoporotic patients treated with rigid systems and even higher in osteoporotic subjects or patients treated with dynamic systems. Firstly, this paper aims to investigate how much this complication is affecting the success rate of pedicle screw fixation, in both non-osteoporotic and osteoporotic patients, and to discuss the biomechanical reasons which may be related to the variability of the rates found in the literature. The secondary aim was to summarize and discuss the published definitions and conventions about screw loosening from a clinical and radiological point of view. METHODS: Narrative literature review. RESULTS: Screw loosening appears to be a minor problem for fixation and fusion of healthy, non-osteoporotic bone. Pedicle screw fixation in osteoporotic bone is believed to be at risk of loosening, but clinical data are actually scarce. Both expandable and augmented screws may be a viable option to reduce the risk of loosening, but clinical evidence is missing. Posterior motion-preserving implants seems to have a significant risk of screw loosening. Standardization appears to be lacking regarding the radiological assessment. Marked differences in the loosening rates found based either on planar radiography or on CT scanning were observed. CONCLUSIONS: Reported loosening rates primarily depended on the protocol used for the clinical examination during follow-up and on the conventions used for the radiological assessment. Aiming to a better comparability of published data, we recommend the authors of clinical studies to describe which criteria were used to assess a loosened screw, as well as the protocol of the clinical follow-up examination. Low-dose CT should be used for the assessment of screw loosening whenever possible.

Cement augmented anterior odontoid screw fixation is biomechanically advantageous in osteoporotic patients with Anderson Type II fractures.

STUDY DESIGN: A biomechanical human cadaveric study. OBJECTIVE: The authors tested a cannulated and perforated lag screw and compared in situ polymethylmethacrylate (PMMA) augmentation against nonaugmentation for fixation of osteoporotic type II odontoid fractures. SUMMARY OF BACKGROUND DATA: Osteoporosis has been identified as a strong predictor for pseudarthrosis after screw fixation of type II odontoid fractures with cut-out through the anterior wall of C2 as the most frequent mode of implant failure. The concept of PMMA augmentation of the proximal screw shank could serve as a useful supplement in this context. METHODS: A total of 18 fresh-frozen human cadaveric C2 vertebrae were harvested (median 86.5 y; range, 69-98 y). Reduced bone quality was verified by quantitative computed tomography. Type II odontoid fractures were created and repaired with a cannulated lag screw, which has perforations in the proximal screw shank. Additional PMMA augmentation was carried out for 9 specimens. The position of the screw and cement distribution were evaluated by computed tomography. Values for maximum force to failure, energy to failure, and stiffness were statistically compared between cement augmented and nonaugmented screws. RESULTS: Cement distribution in the C2 vertebral body was circumferential around the screw shank without leakage into the spinal canal or into the fracture gap in all 9 specimens. The cement augmented screws showed a 2.4 times higher maximum force to failure (363±94 N, P<0.001), a 2.7 times higher energy to failure (1300±698 mJ, P<0.001), and a 1.76 times higher stiffness (90±35 N/mm, P=0.031) in comparison with the nonaugmented screws. CONCLUSIONS: Cement augmentation for fixation of osteoporotic type II odontoid fractures showed biomechanical advantages. It was also shown that cement augmentation of the newly developed screw is technically easy and safe under in vitro conditions. The technique might be useful with regard to the surgical treatment of elderly patients with osteoporotic odontoid fractures.

Increase or decrease in stability after nucleotomy? Conflicting in vitro and in vivo results in the sheep model.

Nucleotomy is a common surgical procedure to treat disc herniations. The potential occurrence of segmental instability after surgery, however, is suspected to necessitate re-operation and fusion. Although in vitro studies support the theory of destabilization after nucleotomy, a prior, in-house animal study contrarily revealed an increase in stability after surgery. To identify which structural compartment of the motion segment is decisive for increased stability after nucleotomy in vivo, the flexibilities of ovine motion segments were measured after different stepwise reductions at the anterior and posterior spinal column. Different test groups were used in which nucleotomy had been performed during surgery in vivo and under isolated in vitro conditions, respectively. In accordance with expectations, in vitro nucleotomy on ovine motion segments significantly increased flexibility. By contrast, nucleotomy significantly decreased flexibility 12 weeks after surgery. After removal of the posterior structures, however, the differences in flexibility diminished. The present results thus suggest that it might not exclusively be the trauma to the intervertebral disc during surgery which is decisive for post-operative stability, but rather adaptive mechanisms in the posterior structures. Therefore, care should be taken to minimize the damage to the posterior structures in the course of the surgical approach, which more likely compromises stability.