Forcefully engaging rods into tulips with gap discrepancy leading to pedicle screw loosening-a biomechanical analysis using long porcine spine segments.

BACKGROUND CONTEXT: Long-segment pedicle screw instrumentation is widely used to treat complex spinal disorders. Rods are routinely precontoured to maximize assistance on the correcting side of the deformity, but there often exists a residual gap discrepancy between the precontoured rods and screw tulips. No previous research has investigated the diminished pullout strength of the most proximal or distal pedicle screw resulting from a mismatched rod in long-segment pedicle screw instrumentation. PURPOSE: The present study aimed to investigate the decreased pullout force of pedicle screws affected by the gap discrepancy when forcefully engaging a mismatched rod into a tulip in a normal-density porcine spine. STUDY DESIGN: The pedicle screw fixation strength under axial pullout force was compared among three different gap discrepancies between rods and tulips using long porcine spine segments. METHODS: Twelve porcine lumbar vertebrae (L3-L6) were implanted with pedicle screws and rods. Screws on one side had no gap between the tulip and rod (0-mm group), while the most proximal screw on the other side had an intentional gap of 3 mm (3-mm group) or 6 mm (6-mm group). Three hours after forcefully engaging the rod into the tulips at room temperature, the set screws in all specimens were loosened, and each specimen was dissected into individual vertebrae for subsequent pullout testing. RESULTS: The control group exhibited significantly greater pullout strength (1987.68 ± 126.80 N) than the groups from different rod-tulip configurations (p<.05), with significantly greater strength in the 3-mm group (945.62 ± 97.43 N) than the 6-mm group (655.30 ± 194.49 N) (p<.05). Only 47.6% and 33.0% of the pullout strength was retained in the 3-mm and 6-mm groups, respectively, compared to the control group. CONCLUSIONS: Gap discrepancies between rods and tulips can significantly reduce pedicle screw pullout strength, with a correlation between decreased strength and increased gaps. Surgeons should avoid forcefully engaging mismatched rods and consider well-fitted contoured rods in spinal surgery to minimize the risk of screw loosening. CLINICAL SIGNIFICANCE: The gap discrepancy between rod and tulip significantly affected pullout strength, with greater gaps leading to reduced strength. Forcefully engaging mismatched rods into tulips in degenerative spinal surgery should be avoided to minimize the risk of early screw pullout.

Influence of various pilot hole profiles on pedicle screw fixation strength in minimally invasive and traditional spinal surgery: a comparative biomechanical study.

Despite advancements in pedicle screw design and surgical techniques, the standard steps for inserting pedicle screws still need to follow a set of fixed procedures. The first step, known as establishing a pilot hole, also referred to as a pre-drilled hole, is crucial for ensuring screw insertion accuracy. In different surgical approaches, such as minimally invasive or traditional surgery, the method of creating pilot holes varies, resulting in different pilot hole profiles, including variations in size and shape. The aim of this study is to evaluate the biomechanical properties of different pilot hole profiles corresponding to various surgical approaches. Commercially available synthetic L4 vertebrae with a density of 0.16 g/cc were utilized as substitutes for human bone. Four different pilot hole profiles were created using a 3.0 mm cylindrical bone biopsy needle, 3.6 mm cylindrical drill, 3.2-5.0 mm conical drill, and 3.2-5.0 mm conical curette for simulating various minimally invasive and traditional spinal surgeries. Two frequently employed screw shapes, namely, cylindrical and conical, were selected. Following specimen preparation, screw pullout tests were performed using a material test machine, and statistical analysis was applied to compare the mean maximal pullout strength of each configuration. Conical and cylindrical screws in these four pilot hole configurations showed similar trends, with the mean maximal pullout strength ranking from high to low as follows: 3.0 mm cylindrical biopsy needle, 3.6 mm cylindrical drill bit, 3.2-5.0 mm conical curette, and 3.2-5.0 mm conical drill bit. Conical screws generally exhibited a greater mean maximal pullout strength than cylindrical screws in three of the four different pilot hole configurations. In the groups with conical pilot holes, created with a 3.2-5.0 mm drill bit and 3.2-5.0 mm curette, both conical screws exhibited a greater mean maximal pullout strength than did cylindrical screws. The strength of this study lies in its comprehensive comparison of the impact of various pilot hole profiles commonly used in clinical procedures on screw fixation stability, a topic rarely reported in the literature. Our results demonstrated that pilot holes created for minimally invasive surgery using image-guided techniques exhibit superior pullout strength compared to those utilized in traditional surgery. Therefore, we recommend prioritizing minimally invasive surgery when screw implantation is anticipated to be difficult or there is a specific need for stronger screw fixation. When opting for traditional surgery, image-guided methods may help establish smaller pilot holes and increase screw fixation strength.

Finite element analysis of optimized novel additively manufactured non-articulating prostheses for cervical total disc replacement.

Ball-and-socket designs of cervical total disc replacement (TDR) have been popular in recent years despite the disadvantages of polyethylene wear, heterotrophic ossification, increased facet contact force, and implant subsidence. In this study, a non-articulating, additively manufactured hybrid TDR with an ultra-high molecular weight polyethylene core and polycarbonate urethane (PCU) fiber jacket, was designed to mimic the motion of normal discs. A finite element (FE) study was conducted to optimize the lattice structure and assess the biomechanical performance of this new generation TDR with an intact disc and a commercial ball-and-socket Baguera®C TDR (Spineart SA, Geneva, Switzerland) on an intact C5-6 cervical spinal model. The lattice structure of the PCU fiber was constructed using the Tesseract or the Cross structures from the IntraLattice model in the Rhino software (McNeel North America, Seattle, WA) to create the hybrid I and hybrid II groups, respectively. The circumferential area of the PCU fiber was divided into three regions (anterior, lateral and posterior), and the cellular structures were adjusted. Optimal cellular distributions and structures were A2L5P2 in the hybrid I and A2L7P3 in the hybrid II groups. All but one of the maximum von Mises stresses were within the yield strength of the PCU material. The range of motions, facet joint stress, C6 vertebral superior endplate stress and path of instantaneous center of rotation of the hybrid I and II groups were closer to those of the intact group than those of the Baguera®C group under 100 N follower load and pure moment of 1.5 Nm in four different planar motions. Restoration of normal cervical spinal kinematics and prevention of implant subsidence could be observed from the FE analysis results. Superior stress distribution in the PCU fiber and core in the hybrid II group revealed that the Cross lattice structure of a PCU fiber jacket could be a choice for a next-generation TDR. This promising outcome suggests the feasibility of implanting an additively manufactured multi-material artificial disc that allows for better physiological motion than the current ball-and-socket design.

Biomechanical evaluation of pedicle screw stability after 360-degree turnback from full insertion: effects of screw shape, pilot hole profile and bone density.

Intraoperative pedicle screw depth adjustment after initial insertion, including both forward and backward adjustments, is sometimes necessary to facilitate rod application and ensure that the screw is in the correct position, which is determined by intraoperative fluoroscopy. Adjusting the screw with forward turns has no negative influence on the screw fixation stability; however, screw turnback may weaken the fixation stability. The aim of this study is to evaluate the biomechanical properties of screw turnback and demonstrate the reduction in the fixation stability after the screw is turned 360° from its full insertion position. Commercially available synthetic closed-cell polyurethane foams with three different densities simulating various degrees of bone density were utilized as substitutes for human bone. Two different screw shapes (cylindrical and conical) together with two different pilot hole profiles (cylindrical and conical) were tested. Following specimen preparation, screw pullout tests were conducted using a material test machine. The mean maximal pullout strength between full insertion and 360-degree turnback from full insertion in each setting was statistically analyzed. The mean maximal pullout strength after 360-degree turnback from full insertion was generally lower than that at full insertion. The reduced mean maximal pullout strength after turnback increased with decreasing bone density. Conical screws had significantly lower pullout strength after 360-degree turnback than cylindrical screws. The mean maximal pullout strength was reduced by up to approximately 27% after 360-degree turnback when using a conical screw in a low bone density specimen. Additionally, specimens treated with a conical pilot hole presented a less reduction in pullout strength after screw turnback as compared to those with a cylindrical pilot hole. The strength of our study was that we systematically investigated the effects of various bone densities and screw shapes on screw stability after turnback, which has rarely been reported in the literature. Our study suggests that pedicle screw turnback after full insertion should be reduced in spinal surgeries, particularly procedures that use conical screws in osteoporotic bone. Pedicle screw secured with a conical pilot hole might be beneficial for screw adjustment.

Biomechanical evaluation of position and bicortical fixation of anterior lateral vertebral screws in a porcine model.

Although an anterior approach with anterior lateral screw fixation has been developed for stabilizing the thoracolumbar spine clinically, screw loosening still occurs. In this novel in vitro study, we attempted to elucidate the optimal screw position in the lateral lumbar vertebra and the effect of bicortical fixation. A total of 72 fresh-frozen lumbar vertebrae from L1-6 were harvested from 12 mature pigs and randomly assigned to two modalities: bicortical fixation (n = 36) and unicortical fixation (n = 36). Six groups of screw positions in the lateral vertebral body in each modality were designated as central-anterior, central-middle, central-posterior, lower-anterior, lower-middle, and lower- posterior; 6 specimens were used in each group. The correlations between screw fixation modalities, screw positions and axial pullout strength were analyzed. An appropriate screw trajectory and insertional depth were confirmed using axial and sagittal X-ray imaging prior to pullout testing. In both bicortical and unicortical fixation modalities, the screw pullout force was significantly higher in the posterior or middle position than in the anterior position (p < 0.05), and there was no significant differences between the central and lower positions. The maximal pullout forces from the same screw positions in unicortical fixation modalities were all significantly lower, decreases that ranged from 32.7 to 74%, than those in bicortical fixation modalities. Our study using porcine vertebrae showed that screws in the middle or posterior position of the lateral vertebral body had a higher pullout performance than those in the anterior position. Posteriorly positioned lateral vertebral screws with unicortical fixation provided better stability than anteriorly positioned screws with bicortical fixation.

Biomechanical Comparison of Different Numbers and Configurations of Cross-Links in Long-Segment Spinal Fixation-An Experimental Study in a Porcine Model.

STUDY DESIGN: Biomechanical study. OBJECTIVE: Cross-links are a type of common clinical spinal instrumentation. However, the effects of the position and number of cross-links have never been investigated in long-segment spinal fixation, and the variables have not been optimized. We conducted an in vitro biomechanical study by using a porcine long-segment spinal model with 5 different crosslink configurations to determine the optimal construct for clinical practice. METHODS: Five modalities with paired segmental screws from T15-L5 were tested in 20 porcine spines. The spines without cross-links composed the control group, Group A; those with a single cross-link from L2-3 composed Group B; those with 2 cross-links from L1-2 and L3-4 composed Group C; those with 2 cross-links from T15-L1 and L4-5 composed Group D; and those with 3 cross-links from T15-L1, L2-3 and L4-5 composed Group E. Spinal stiffnesses in flexion, extension, lateral bending, and axial rotation were compared among 5 different cross-link configurations in 5-level porcine spinal units. RESULTS: Flexional, extensional and lateral bending stiffnesses did not significantly change with an increasing number of cross-links or positions in the construct. Axial stiffness was significantly increased with 2 cross-links compared to one (P < 0.05) and with placement more distant from the center of the long spinal fixation construct (P < 0.05). CONCLUSIONS: Two cross-links individually placed proximal and distal from the center of a construct is an optimal and efficient configuration to achieve biomechanical stability in non-rigid lumbar spines undergoing long-level fixation.

The role of counter-torque holders in tightening of pedicle screw-rod constructs: a biomechanical study in a porcine model.

BACKGROUND CONTEXT: Pedicle screw-rod assembly procedures following pedicle screw insertion include contouring and placing rods into screw tulips, introducing set screws into the tulip along the screw thread, applying a counter-torque holder and tightening all the set screws clockwise. Even if an appropriate pedicle screw is implanted, screw dislodgement after tightening of the tulip and set screw is not uncommon. Pedicle wall violation resulting from excessive rotational force due to inadequate use of a counter-torque holder might be the reason. However, the strain change in the pedicle during tulip-set screw tightening and the role of counter-torque have never been investigated. PURPOSE: This study determined differences in the strain change in the outer and inner pedicle walls during tulip-set screw tightening; additionally, the influence of counter-torque on pedicle wall violation was elucidated. STUDY DESIGN: A controlled biomechanical study; the strain values of outer and inner pedicle walls in cadaveric porcine L4-L5 vertebrae during tulip-set screw tightening with or without a counter-torque holder were measured. METHODS: Twelve L4-L5 fresh-frozen porcine lumbar vertebrae were implanted with screw-rod constructs; the set screw was randomly locked into the tulip in the right L5, right L4, left L5 and left L4 testing groups. The maximal values from eight strain gauges (P-R-O: outer cortex of right pedicle in proximal vertebra; P-R-I: inner cortex of right pedicle in proximal vertebra; D-R-O: outer cortex of right pedicle in distal vertebra; D-R-I: inner cortex of right pedicle in distal vertebra; P-L-O: outer cortex of left pedicle in proximal vertebra; P-L-I: inner cortex of left pedicle in proximal vertebra; D-L-O: outer cortex of left pedicle in distal vertebra; D-L-I: outer cortex of left pedicle in proximal vertebra) for each specimen during tightening to 12 Nm were measured. RESULTS: The maximal strain values of the ipsilateral strain gauges in all testing groups were almost significantly higher when a counter-torque holder was not used than when one was used. The strain values in the adjacent pedicle of specimens without a counter-torque holder were significantly increased: P-R-O and P-R-I in the right L5 group; D-R-I in the right L4 group; P-L-I and P-L-O in the left L5 group; D-L-O and D-L-I in the left L4 group. CONCLUSIONS: The constraint effect of counter-torque during tulip-set screw tightening is necessary. Clockwise rotational force with a fragile lateral pedicle wall suggests that caution is required when using a counter-torque holder to tighten the right L5 and left L4 constructs. CLINICAL SIGNIFICANCE: A counter-torque holder is important during tulip-set screw tightening; improper use may lead to adjacent pedicle wall violation, sequentially resulting in pedicle screw loosening.

Biomechanical comparison of pedicle screw fixation strength among three different screw trajectories using single vertebrae and one-level functional spinal unit.

Three key factors are responsible for the biomechanical performance of pedicle screw fixation: screw mechanical characteristics, bone quality and insertion techniques. To the best of the authors' knowledge, no study has directly compared the biomechanical performance among three trajectories, i.e., the traditional trajectory (TT), modified trajectory (MT) and cortical bone trajectory (CBT), in a porcine model. This study compared the pullout strength and insertion torque of three trajectory methods in single vertebrae, the pullout strength and fixation stiffness including flexion, extension, and lateral bending in a one-level instrumented functional spinal unit (FSU) that mimics the in vivo configuration were clarified. A total of 18 single vertebrae and 18 FSUs were randomly assigned into three screw insertion methods (n = 6 in each trajectory group). In the TT group, the screw converged from its entry point, passed completely inside the pedicle, was parallel to the superior endplate, was located in the superior third of the vertebral body and reached to at least the anterior third of the vertebral body. In the MT group, the convergent angle was similar to that of the TT method but directed caudally to the anterior inferior margin of the vertebral body. The results of insertion torque and pullout strength in single vertebrae were analyzed; in addition, the stiffness and pullout strength in the one-level FSU were also investigated. This study demonstrated that, in single vertebrae, the insertion torque was significantly higher in CBT groups than in TT and MT groups (p < 0.05). The maximal pullout strength was significantly higher in MT groups than in TT and CBT groups (p < 0.05). There was no significant difference in stiffness in the three motions among all groups. The maximal pullout strength in FSUs of MT and CBT groups were significantly higher than the TT groups (p < 0.05). We concluded that either MT or CBT provides better biomechanical performance than TT in single vertebrae or FSUs. The lack of significance of stiffness in FSUs among three methods suggested that MT or CBT could be a reasonable alternative to TT if the traditional trajectory was not feasible.

Predicting pullout strength of pedicle screws in broken bones from X-ray images.

Pedicle screw fixation is one of the most common procedures used in spinal fusion surgery. The screw loosening is a major concern, which may be caused by broken pedicles. In vitro pullout tests or insertion torque are the main approaches for assessing the stability of the screw; however, direct evidence was lacking for clinical human spines. Here, we aim to provide a model that can predict the pullout strengths of pedicle screws in various pedicle conditions from X-ray images. A weighted embedded bone volume (EBV) model is proposed for pullout strengths prediction by considering the bone heterogeneity and confinement of the screw. We showed that the pullout strength is proportional to the EBV for homogeneous bone and the weighted EBV for layered composite bone. The proposed weighted EBV model is validated with in vitro Sawbones® pullout experiments. The results show that the model has better accuracy than the simple EBV model, with a coefficient of determination of 0.94. The proposed weighted EBV model can help assess the stability of a pedicle screw in a broken pedicle by simply examining 2D X-ray images.

Improved fixation stability for repairing pedicle screw loosening using a modified cement filling technique in porcine vertebrae.

Polymethylmethacrylate (PMMA) has been applied clinically and biomechanically repair loose pedicle screws. Controversies have arisen over data due to uncontrolled cement properties, various locations and sizes of fenestrated holes in repair screws, irregular holes and different bone densities of specimens. In this study, the pullout strength was compared for two techniques, the modified technique to use PMMA to augment a threaded hole and the traditional technique with retrograde injection of a PMMA filling, for standard loose screws in porcine vertebrae. Both techniques provided statistically significant results for sufficiently randomized specimens and experimental procedures. The difference in the pullout strength between conical and cylindrical screws for the aforementioned cement augmentation techniques was also investigated. Twenty-four single-level fresh-frozen lumbar vertebrae from L1 to L6 were harvested from four mature pigs. A total of 0.8 ml of PMMA was retrograde injected into screw holes with a 5.5 mm diameter, followed by insertion of a 5.0 mm diameter repair screw in the traditional group (n = 12). A stiff threaded PMMA hole was created with a 4.5 mm tapping screw before insertion of repair screws in the modified group (n = 12). Two screw geometries were randomly assigned as cylindrical (n = 6) and conical (n = 6) in each group. The correlations between filling techniques, screw geometries and axial pullout strength were analyzed. An appropriate screw trajectory and insertion depth were confirmed using X-ray imaging prior to pullout testing in both groups. For a given screw geometry (cylindrical or conical), the pullout force of the modified group was significantly higher than that of the traditional group. There was no significant difference in the pullout force between the screw geometries for a given filling technique. The cement augmentation technique is far more influential than the screw outer geometry. The modified PMMA technique created a greater anchor force than the traditional method and could be an alternative for revision of pedicle screw loosening.

Quantitative imaging of ultrasound backscattered signals with information entropy for bone microstructure characterization.

Osteoporosis is a critical problem during aging. Ultrasound signals backscattered from bone contain information associated with microstructures. This study proposed using entropy imaging to collect the information in bone microstructures as a possible solution for ultrasound bone tissue characterization. Bone phantoms with different pounds per cubic foot (PCF) were used for ultrasound scanning by using single-element transducers of 1 (nonfocused) and 3.5 MHz (nonfocused and focused). Clinical measurements were also performed on lumbar vertebrae (L3 spinal segment) in participants with different ages (n = 34) and postmenopausal women with low or moderate-to-high risk of osteoporosis (n = 50; identified using the Osteoporosis Self-Assessment Tool for Taiwan). The signals backscattered from the bone phantoms and subjects were acquired for ultrasound entropy imaging by using sliding window processing. The independent t-test, one-way analysis of variance, Spearman correlation coefficient rs, and the receiver operating characteristic (ROC) curve were used for statistical analysis. The results indicated that ultrasound entropy imaging revealed changes in bone microstructures. Using the 3.5-MHz focused ultrasound, small-window entropy imaging (side length: one pulse length of the transducer) was found to have high performance and sensitivity in detecting variation among the PCFs (rs = - 0.83; p < 0.05). Small-window entropy imaging also performed well in discriminating young and old participants (p < 0.05) and postmenopausal women with low versus moderate-to-high osteoporosis risk (the area under the ROC curve = 0.80; cut-off value = 2.65; accuracy = 86.00%; sensitivity = 71.43%; specificity = 88.37%). Ultrasound small-window entropy imaging has great potential in bone tissue characterization and osteoporosis assessment.

Use of longer sized screws is a salvage method for broken pedicles in osteoporotic vertebrae.

Screw loosening due to broken pedicles is a common complication resulting from the insertion of screws either with inadequate diameters or into an osteoporotic pedicle. In this novel in vitro study, we tried to clarify the contribution of the pedicle to screw fixation and subsequent salvage strategies using longer or larger-diameter screws in broken pedicles. Sixty L4 fresh-frozen lumbar vertebrae harvested from mature pigs were designed as the normal-density group (n = 30) and decalcified as the osteoporosis group (n = 30). Three modalities were randomly assigned as intact pedicle (n = 30), semi-pedicle (n = 15), and non-pedicle (n = 15) in each group. Three sizes of polyaxial screws (diameter × length of 6.0 mm × 45 mm, 6.0 mm × 50 mm, and 6.5 mm × 45 mm) over five trials were used in each modality. The associations between bone density, pedicle modality and screw pullout strength were analyzed. After decalcification for 4 weeks, the area bone mineral density decreased to approximately 56% (p < 0.05) of the normal-density group, which was assigned as the osteoporosis group. An appropriate screw trajectory and insertional depth were confirmed using X-ray imaging prior to pullout testing in both groups. The pullout forces of larger-diameter screws (6.5 mm × 45 mm) and longer screws (6.0 mm × 50 mm) were significantly higher (p < 0.05) in the semi- and non-pedicle modalities in the normal-density group, whereas only longer screws (6.0 mm × 50 mm) had a significantly higher (p < 0.05) pullout force in the non-pedicle modalities in the osteoporosis group. The pedicle plays an important role in both the normal bone density group and the osteoporosis group, as revealed by analyzing the pullout force percentage contributed by the pedicle. Use of a longer screw would be a way to salvage a broken pedicle of osteoporotic vertebra.

Biomechanical comparison of pedicle screw fixation strength in synthetic bones: Effects of screw shape, core/thread profile and cement augmentation.

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.

Biomechanical study of the fixation stability of broken pedicle screws and subsequent strategies.

Pedicles are often broken when screws are inserted into hard pedicles with small diameters or when the diameter of the screw itself is inadequate. However, there is a lack of biomechanical literature that addresses screw loosening as a result of broken pedicles or the resulting salvage of those screws. We performed a novel in vitro study to compare the pullout strength of screws between intact pedicles and two different types of broken pedicles; strategies to prevent screw loosening were also compared. Thirty L4 Sawbones were designed to represent intact pedicles, semi-pedicles, and nonpedicles and were prepared for screw insertion. Three sizes of polyaxial screws (diameter × length dimensions of 6.0 mm × 45 mm, 6.0 mm × 50 mm and 6.5 mm × 45 mm) were independently and randomly distributed into the intact-pedicle group (IP group, n = 30), the semi-pedicle group (SP group, n = 15), or the nonpedicle group (NP group, n = 15). The experiments were conducted across a minimum of five trials for each of the chosen screw sizes. We then analyzed the results of the imaging, pullout testing, and embedded bone volume. Any fractures or defects of the vertebrae caused by screw insertion were excluded from the study. The appropriate screw trajectory and insertional depth were confirmed using axial and sagittal X-ray imaging prior to screw pullout testing. A pullout strength of only 41% to 45% for the SP group and 29% to 39% for the NP group was retained following a broken pedicle. The use of longer or larger-diameter screws appears to be an effective salvaging procedure for the NP group (p < 0.05). The embedded bone volume percentage analysis indicated that, compared to the IP group, 68% to 76% of effective bone embedded into the screw threads in the SP group, and 58% to 65% in the NP group. There was no direct correlation between the pullout strength and the embedded bone volume; however, less effective embedded bone volume was associated with lower pullout strength. This study describes the evolution of the well-established screw pullout test being applied to the broken pedicle Sawbone testing model. The pedicle plays an important role in determining the pullout strength of a screw. However, a salvage procedure that utilizes a longer or larger-diameter screw might be a reliable clinical approach to address broken pedicles.

Application of two-parameter scoliometer values for predicting scoliotic Cobb angle.

BACKGROUND: Adolescent idiopathic scoliosis, in which obvious curves are visible in radiographic images, is also seen in combination with lumps in the back. These lumps contribute to inclination, which can be measured by a scoliometer. To the authors' knowledge, there are no previous formulas combining thoracic and lumbar scoliometer values simultaneously to predict thoracic and lumbar Cobb angles, respectively. This study aimed to create more accurate two-parameter mathematical formulas for predicting thoracic and lumbar Cobb angles. METHODS: Between Dec. 2012 and Jan. 2013, patients diagnosed with idiopathic scoliosis in an outpatient clinic were enrolled. The maximal trunk rotations at the thoracic and lumbar regions were recorded with a scoliometer. Right asymmetry hump was deemed positive (+), and left asymmetry hump was deemed negative (-). The Cobb angles were measured with a Picture Archiving and Communication System. Statistical analysis included Pearson's correlation coefficient, multivariate regression and Bland-Atman analysis. RESULTS: One-hundred and one patients were enrolled in our study. The average thoracic curve (TC) was 23.3 ± 1.8°, while the average lumbar curve (LC) was - 23.3 ± 1.4°. The thoracic inclination (TI) and lumbar inclination (LI) were 4.5 ± 0.7 and - 5.9 ± 0.6, respectively. The one-parameter formula for the thoracic curve was TC = 2.0 TI + 14.3 (r = 0.813); for the lumbar curve, it was LC = 0.9 LI - 16.9 (r = 0.409). By multivariate regression, the two-parameter formulas for the thoracic and lumbar curves were TC = 2.6 TI - 1.4 LI (r = 0.931) and LC = - 1.5 TI + 2.0 LI (r = 0.874), respectively. The two-parameter formulas were more accurate than the one-parameter formulas. CONCLUSIONS: Based on the results of these two-parameter formulas for thoracic and lumbar curves, the Cobb angles can be predicted more accurately by the readings of the scoliometer. Physicians and other healthcare practitioners can thus evaluate patients with scoliosis more precisely than before with a scoliometer.

Characterization of a novel caudal vertebral interbody fusion in a rat tail model: An implication for future material and mechanical testing.

BACKGROUND: Of the proposed animal interbody fusion models, rat caudal discs have gained popularity in disc research due to their strong resemblance to human discs with respect to geometry, composition and mechanical properties. The purpose of this study is to demonstrate an efficient, repeatable and easily accessible animal model of interbody fusion for future research into mechanical testing and graft materials. METHODS: Twelve 12-week-old female Sprague-Dawley (SD) rats underwent caudal interbody fusion of the third and fourth coccygeal vertebrae of the tail. Serial radiological evaluation, and histological evaluation and manual palpation after sacrifice were performed to assess the fusion quality. Mechanical testing of functional units (FUs) of non-operated and operated segments was compared using a three-point bending test. RESULTS: At postoperative 12 weeks, callus formation was observed at the fusion sites in all rats, with the mean radiological evaluations of 2.75/3 according to the Bransford classification. Newly formed bone tissue was also observed in all rats with the mean histological score of 5.85/7, according to the Emery grading system. No palpable gaps and obvious change of bending stiffness was observed in the operated segments. The mean bending stiffness of the FUs was statistically higher than that of the control FUs (26.57 ± 6.71 N/mm vs. 12.45 ± 3.21 N/mm, p < 0.01). CONCLUSION: The rat caudal disc interbody fusion model proved to be an efficient, repeatable and easily accessible model. Future research into adjuvant treatments like growth factor injection and alternative fusion materials under conditions of osteoporosis using this model would be worthwhile.

Effects of Strontium Ranelate on Spinal Interbody Fusion Surgery in an Osteoporotic Rat Model.

Osteoporosis is a bone disease that afflicts millions of people around the world, and a variety of spinal integrity issues, such as degenerative spinal stenosis and spondylolisthesis, are frequently concomitant with osteoporosis and are sometimes treated with spinal interbody fusion surgery. Previous studies have demonstrated the efficacy of strontium ranelate (SrR) treatment of osteoporosis in improving bone strength, promoting bone remodeling, and reducing the risk of fractures, but its effects on interbody fusion surgery have not been adequately investigated. SrR-treated rats subjected to interbody fusion surgery exhibited significantly higher lumbar vertebral bone mineral density after 12 weeks of treatment than rats subjected to the same surgery but not treated with SrR. Furthermore, histological and radiographic assessments showed that a greater amount of newly formed bone tissue was present and that better fusion union occurred in the SrR-treated rats than in the untreated rats. Taken together, these results show significant differences in bone mineral density, PINP level, histological score, SrR content and mechanical testing, which demonstrate a relatively moderate effect of SrR treatment on bone strength and remodeling in the specific context of recovery after an interbody fusion surgery, and suggest the potential of SrR treatment as an effective adjunct to spinal interbody fusion surgery for human patients.

Elution and Mechanical Strength of Vancomycin-Loaded Bone Cement: In Vitro Study of the Influence of Brand Combination.

Antibiotic-loaded bone cement (ALBC) is widely used in orthopaedic surgery for both prevention and treatment of infection. Little is known about the effect of different brand combinations of antibiotic and bone cement on the elution profile and mechanical strength of ALBC. Standardized specimens that consisted of one of the 4 brands of bone cement and one of the 3 brands of vancomycin were fashioned, producing 12 combinations of ALBC. Two dosages of vancomycin in 40g bone cement were used to represent the high (4g vancomycin) and low (1g vancomycin) dose groups. Concentrations of vancomycin elution from ALBC was measured for up to 336 hours. The ultimate compression strength was tested at axial compression using a material testing machine before and after elution. In both high-dose and low-dose groups, Lyo-Vancin in PALACOS bone cement resulted in the highest cumulative elution and Vanco in Simplex P bone cement resulted in the lowest elution (458% and 65% higher in high- and low-dose groups, respectively). The mechanical strength was not significantly compromised in all groups with low dose vancomycin (range: 70.31 ± 2.74 MPa to 87.28 ± 8.26MPa after elution). However, with the addition of high dose vancomycin, there was a mixed amount of reduction in the ultimate compression strength after cement aging, ranging from 5% (Vanco in Simplex P, 81.10 ± 0.48 MPa after elution) to 38% (Sterile vancomycin in CMW, 60.94 ± 5.74 MPa after elution). We concluded that the selection of brands of vancomycin and bone cement has a great impact on the release efficacy and mechanical strength of ALBC.

Modification of Mechanical Properties, Polymerization Temperature, and Handling Time of Polymethylmethacrylate Cement for Enhancing Applicability in Vertebroplasty.

Polymethylmethacrylate (PMMA) bone cement is a popular bone void filler for vertebroplasty. However, the use of PMMA has some drawbacks, including the material's excessive stiffness, exothermic polymerization, and short handling time. This study aimed to create an ideal modified bone cement to solve the above-mentioned problems. Modified bone cements were prepared by combining PMMA with three different volume fractions of castor oil (5%, 10%, and 15%). The peak polymerization temperatures, times to achieve the peak polymerization temperature, porosities, densities, modulus and maximum compression strengths of standard (without castor oil), and modified cements were investigated following storage at ambient temperature (22°C) or under precooling conditions (3°C). Six specimens were tested in each group of the aforementioned parameters. Increasing castor oil content and precooling treatment effectively decreased the peak polymerization temperatures and increased the duration to achieve the peak polymerization temperature (P < 0.05). Furthermore, the mechanical properties of the material, including density, modulus, and maximum compression strength, decreased with increasing castor oil content. However, preparation temperature (room temperature versus precooling) had no significant effect (P > 0.05) on these mechanical properties. In conclusion, the addition of castor oil to PMMA followed by precooling created an ideal modified bone cement with a low modulus, low polymerization temperature, and long handling time, enhancing its applicability and safety for vertebroplasty.

Modified fixations for distal femur fractures following total knee arthroplasty: a biomechanical and clinical relevance study.

PURPOSE: Distal femur fractures adjacent to total knee arthroplasty are a rare yet complex problem. Recently, extramedullary locking plate and retrograde intramedullary nail fixations have become popular options, but the complication rates associated with these procedures are 15-20 %. Modified fixations were assessed in an effort to reduce complications from unstable periprosthetic fractures. METHODS: Using experimental and finite element methods, this study compared the construct behaviours of a locking plate, a retrograde intramedullary nail, and their modifications (a spiral-blade supplemented in an intramedullary nail or a locking plate/allograft hybrid) when subjected to various fracture types, locations, loading conditions, and bony strength. The implanted models were used to assess construct stiffness, fracture micromotion, and implant stress under different osteoporotic conditions. Finally, we collected 40 cases for radiological analysis to indicate the appropriate procedure for treating periprosthetic fractures following total knee arthroplasty. RESULTS: Regardless of the fracture type, femoral constructs fixed with a conventional or spiral-blade supplemented intramedullary nail exhibited higher axial but lower torsional stiffness than those fixed with a locking plate. Torsional deformation occurred if the lower-positioned fracture had no medial support. The locking plate/allograft construct exhibited the highest stiffness and the least micromotion. A review of 40 clinical cases confirmed the above findings regarding the locking plate/allograft construct. CONCLUSION: The spiral-blade supplement of retrograde intramedullary nail and locking plate/allograft modified constructs significantly stabilizes the unstable fractured gaps. The locking plate/allograft is recommended for the periprosthetic fractures with deficient bone stock and severe osteoporosis to improve alignment and healing potentials.