Fixation Stiffness Research Articles

Opening Wedge Distal Femur Osteotomy: Biomechanical Study of a Biplane Fixation Using Conventional Implants

Opening Wedge Distal Femur Osteotomy: Biomechanical Study of a Biplane Fixation Using Conventional Implants

Locked Plate Fixation of Proximal Humerus Fractures Using Cannulated Subchondral Support Pegs

Background In severely osteoporotic proximal humerus fractures, the biomechanics of fixed-angle fracture fixation with locked smooth pegs may be improved by using polymethyl methacrylate (PMMA) augmentation. Commercially available smooth pegs were modified (added a central cannula and two distal side ports) to accommodate bone cement injection around the pegs. The failure strength and stiffness of proximal humerus fracture fixation with modified pegs and PMMA augmentation was evaluated. Methods Nineteen fresh-frozen cadaveric humeri were fixated with a locked plate using: (i) smooth pegs (seven specimens), (ii) threaded pegs (six specimens) or (iii) modified smooth pegs with PMMA augmentation (six specimens). The osteotomized humeri constructs were subjected to an axial compressive load to failure at loading rate of 5 mm/minute. The maximum failure load and slope for each specimen was calculated. Results There was no difference ( p > 0.31) between the failure strength of constructs containing smooth or threaded pegs. However, the mean failure strength and stiffness of modified smooth pegs with PMMA augmentation was higher than the constructs containing either the smooth or threaded pegs. Conclusions The results obtained in the present study show that the use of PMMA augmentation along with smooth pegs may benefit humerus fracture patients with severe osteoporosis.

A case for optimising fracture healing through inverse dynamization

The mechanical conditions in the repair tissues are known to influence the outcome of fracture healing. These mechanical conditions are determined by the stiffness of fixation and limb loading. Experimental studies have shown that there is a range of beneficial fixation stiffness for timely healing and that fixation stiffness that is either too flexible or too stiff impairs callus healing. However, much less is known about how mechanical conditions influence the biological processes that make up the sequence of bone repair and if indeed mechanical stimulation is required at all stages of repair. Secondary bone healing occurs through a sequence of events broadly characterised by inflammation, proliferation, consolidation and remodelling. It is our hypothesis that a change in fixation stiffness from very flexible to stiff can shorten the time to healing relative to constant fixation stiffness. Flexible fixation has the benefit of promoting greater callus formation and needs to be applied during the proliferative stage of repair. The greater callus size helps to stabilize the fragments earlier allowing mineralization to occur faster. Together with stable/rigid fixation applied during the latter stage of repair to ensure mineralization of the callus. The predicted benefits of inverse dynamization are shortened healing in comparison to very flexible fixation and healing time comparable or faster than stable fixation with greater callus stiffness.

Midfoot Fusion

Numerous reconstructive techniques for midfoot collapse secondary to Charcot neuroarthropathy have been described, but few have been studied biomechanically. The purpose of this study was to biomechanically compare 2 of the most common techniques. Seven paired below-knee specimens were amputated through the talonavicular and calcaneocuboid joints. The nonligamentous soft tissue was stripped proximal to the metatarsal heads and disarticulated through the tarsometatarsal (TMT) joints. For each paired specimen, the TMT joints were fused by plantar plating or intramedullary screw fixation for the contralateral side. The specimens were mounted, loaded, and cycled, and fixation stiffness was determined. Load versus displacement graphs were used to calculate overall construct stiffness, and data were analyzed by Student t tests. There was no failure of hardware. All failures were at the bone-implant interface. Failure was either by screw pull-out, bone fracture, or a combination of the two. There were no notable differences between the 2 fixation techniques with respect to stiffness or loads to failure. There was a trend toward a stiffer first TMT construct using the plantar plating method. Five of the 7 screw fixations failed by pullout of the base of the first metatarsal and the other 2 by pullout of screws from all MT bases. Seven of the 7 plantar plate fixations failed by separation of the fifth to third MT bases originating at the fifth, and 3 showed fracture of the fifth metatarsal base. There was no notable biomechanical difference between the 2 techniques. There was a trend toward a stiffer construct at the first TMT with plantar plating. This study biomechanically analyzes two common Charcot midfoot reconstruction techniques and highlights the need for further study of both techniques and combinations of these techniques.

Femoral Cortical Suspension Devices for Soft Tissue Anterior Cruciate Ligament Reconstruction

Background: Optimization of anterior cruciate ligament (ACL) fixation is desired to improve graft healing. New soft tissue cortical suspension devices for femoral tunnel fixation should be biomechanically evaluated. Hypothesis: All femoral fixation devices would prevent a clinically significant amount of displacement and support loads significantly larger than in situ forces experienced by the ACL during early rehabilitation. Study Design: Controlled laboratory study. Methods: Four cortical soft tissue ACL graft suspension devices were tested under cyclic and pull-to-failure loading conditions in both an isolated device-only setup and as a complete bone-device-tendon construct in porcine femurs using a tensile testing machine. Results: There were significant differences in the ultimate failure loads among the devices. The highest ultimate failure loads when tested as a construct were observed for the XO Button (1748 N), followed by the Endobutton CL (1456 N), ToggleLoc with ZipLoop (1334 N), and TightRope RT (859 N). Cyclic displacement after 1000 cycles during isolated device testing was less than 1 mm for all devices. Cyclic displacements after 1000 cycles in the porcine construct were 1.88 mm, 2.74 mm, 3.34 mm, and 1.82 mm for the Endobutton, TightRope, ToggleLoc, and XO Button, respectively; all were significantly different from each other except when the Endobutton was compared with the XO Button. The ToggleLoc exceeded the 3.0-mm displacement threshold defined as a clinical failure. The most displacement occurred during the first cycle, especially for the adjustable-length loop devices. Stiffness reapproximated the native ACL stiffness for all constructs. Conclusion: The Endobutton, TightRope, and XO Button have the necessary biomechanical properties with regard to ultimate failure strength, displacement, and stiffness for initial fixation of soft tissue grafts in the femoral tunnel for ACL reconstruction. The ToggleLoc had sufficient ultimate failure strength but crossed our 3.0-mm clinical failure threshold for cyclic displacement. Although this study was not designed to compare fixed and adjustable-length loop devices, it was noted that both fixed-loop devices allowed less cyclic displacement and initial displacement. Clinical Relevance: Adjustable-length loop devices may need to be retensioned after cycling the knee and fixing the tibial side to account for the increased initial displacement seen with these devices.

Biomechanical Comparison of Parallel Versus 90-90 Plating of Bicolumn Distal Humerus Fractures With Intra-Articular Comminution

To compare the biomechanical properties of 90-90 versus mediolateral parallel plating of C-3 bicolumn distal humerus fractures. We created intra-articular AO/Orthopaedic Trauma Association C-3 bicolumn fractures in 10 fresh-frozen matched pairs of cadaveric elbows. We determined bone mineral density of the metaphyseal region with dual-energy x-ray absorptiometry. The matched pairs of elbows were randomly assigned to either 90-90 or parallel plate fixation. We tested anteroposterior displacement at a rate of 0.5 mm/s to a maximum load of ± 100 N for both the articular and entire distal humerus segments. We tested torsional stability at a displacement rate of 0.1 Hz to a maximum torque of ± 2.5 Nm. After cyclical testing, we loaded the specimens in torsion to failure. There was no significant difference in the bone density of the paired specimens. Compared with parallel fixation, 90-90 plate fixation had significantly greater torque to failure load. Both plating constructs were equally sensitive to bone density. Both techniques had the same mode of failure in torsion, a spiral fracture extending from the medial plate at the metaphyseal-diaphyseal junction. There was no significant difference in the stiffness of fixation of the articular fragment or the entire distal segment in anteroposterior loading. This study demonstrated that 90-90 and parallel plating had comparable biomechanical properties for fixation of comminuted intra-articular distal humerus fractures, and that 90-90 plating had greater resistance to torsional loading.

Validation of a finite element model of a unilateral external fixator in a rabbit tibia defect model

In case of large segmental defects in load-bearing bones, an external fixator is used to provide mechanical stability to the defect site. The overall stiffness of the bone–fixator system is determined not only by the fixator design but also by the way the fixator is mounted to the bone. This stiffness is an important factor as it will influence the biomechanical environment to which tissue engineering scaffolds and regenerating tissues are exposed. A finite element (FE) model can be used to predict the system stiffness. The goal of this study is to develop and validate a 3D anatomical FE model of a bone–fixator system which includes a previously developed unilateral external fixator for a large segmental defect model in the rabbit tibia. It was hypothesized that the contact interfaces between bone and fixator screws play a major role for the prediction of the stiffness. In vitro mechanical testing was performed in order to measure the axial stiffness of cortical bone from mid-shaft rabbit tibiae and of the tibia–fixator system, as well as the bending stiffness of individual fixator screws, inserted in bone. μCT-based case-specific FE models of cortical bone and SCREW–BONE specimens were created to simulate the corresponding mechanical test set-ups. The Young's modulus of rabbit cortical bone as well as appropriate screw–bone contact settings were derived from those FE models. We then used the derived settings in an FE model of the tibia–fixator system. The difference between the FE predicted and measured axial stiffness of the tibia–fixator system was reduced from 117.93% to 7.85% by applying appropriate screw–bone contact settings. In conclusion, this study shows the importance of screw–bone contact settings for an accurate fixator stiffness prediction. The validated FE model can further be used as a tool for virtual mechanical testing in the design phase of new tissue engineering scaffolds and/or novel patient-specific external fixation devices.

Enhancing Pedicle Screw Fixation in the Aging Spine With a Novel Bioactive Bone Cement

A paired biomechanical study of pedicle screws augmented with bone cement in a human cadaveric and osteoporotic lumbar spine model. OBJECTIVES.: To evaluate immediate strength and stiffness of pedicle screw fixation augmented with a novel bioactive bone cement in an osteoporotic spine model and compare it with polymethylmethacrylate (PMMA) cement. A novel bioactive bone cement, containing nanoscale particles of strontium and hydroxyapatite (Sr-HA), can promote new bone formation and osteointegration and provides a promising reinforcement to the osteoporotic spine. Its immediate mechanical performance in augmenting pedicle screw fixation has not been evaluated. Two pedicle screws augmented with Sr-HA and PMMA cement were applied to each of 10 isolated cadaveric L3 vertebrae. Each screw was subjected to a toggling test and screw kinematics were calculated. The pedicle screw was subjected to a pullout test until failure. Finally, the screw coverage with cement was measured on computed tomographic images. Screw translations in the toggling test were consistently larger in the Sr-HA group than in the PMMA group (1.4 ± 1.2 mm vs. 1.0 ± 1.1 mm at 1000 cycles). The rotation center was located closer to the screw tip in the Sr-HA group (19% of screw length) than in the PMMA group (37%). The only kinematic difference between Sr-HA and PMMA cements was the screw rotation at 1000 cycles (1.5° ± 0.9° vs. 1.3° ± 0.6°; P = 0.0026). All motion parameters increased significantly with more loading cycles. The pullout force was higher in the PMMA group than the Sr-HA group (1.40 ± 0.63 kN vs. 0.93 ± 0.70 kN), and this difference was marginally significant (P = 0.051). Sr-HA cement covered more of the screw length than PMMA cement (79 ± 19% vs. 43 ± 19%) (P = 0.036). This paired-design study identified some subtle but mostly nonsignificant differences in immediate biomechanical fixation of pedicle screws augmented with the Sr-HA cement compared with the PMMA cement.

Biomechanical efficacy of monoaxial or polyaxial pedicle screw and additional screw insertion at the level of fracture, in lumbar burst fracture: An experimental study

Background:Use of a pedicle screw at the level of fracture, also known as an intermediate screw, has been shown to improve clinical results in managing lumbar fracture, but there is a paucity of biomechanical studies to support the claim. The aim of this study was to evaluate the effect of adding intermediate pedicle screws at the level of a fracture on the stiffness of a short-segment pedicle fixation using monoaxial or polyaxial screws and to compare the strength of monoaxial and polyaxial screws in the calf spine fracture model.Materials and Methods:Flexibility of 12 fresh-frozen calf lumbar spine specimens was evaluated in all planes. An unstable burst fracture model was created at the level of L3 by the pre-injury and dropped-mass technique. The specimens were randomly divided into monoaxial pedicle screw (MPS) and polyaxial pedicle screw (PPS) groups. Flexibility was retested without and with intermediate screws (MPSi and PPSi) placed at the level of fracture in addition to standard screws placed at L2 and L4.Results:The addition of intermediate screws significantly increased the stability of the constructs, as measured by a decreased range of motion (ROM) in flexion, extension, and lateral bending in both MPS and PPS groups (P < 0.05). There was neither any significant difference in the ROM in the spines of the two groups before injury, nor a difference in the ROM between the MPSi and PPSi groups (P > 0.05), but there was a significant difference between MPS and PPS in flexion and extension in the short-segment fixation group (P < 0.05).Conclusions:The addition of intermediate screws at the level of a burst fracture significantly increased the stability of short-segment pedicle screw fixation in both the MPS and PPS groups. However, in short-segment fixation group, monoaxial pedicle screw exhibited more stability in flexion and extension than the polyaxial pedicle screw.

Jones Fracture Fixation

Background: Stabilization of fifth metatarsal Jones fractures with intramedullary screw fixation is the most common method for surgical fixation when operative treatment is indicated. Conventional partially threaded screws of various diameters are routinely used for Jones fracture fixation. Recently, the use of tapered variable pitch screws has become popular, but information regarding their performance in Jones fracture fixation is limited. No previous studies have compared conventional and tapered variable pitch screws in Jones fracture fixation under physiologic cyclic loading conditions. Purpose: To determine whether biomechanical differences exist between appropriately sized conventional partially threaded screws and tapered variable pitch screws under physiologic cyclic loading conditions with regard to Jones fracture fixation. Study Design: Controlled laboratory study. Methods: Simulated Jones fractures were created in 23 matched pairs of fresh-frozen fifth metatarsals. One bone from each pair was stabilized with a conventional partially threaded screw and the contralateral bone with a tapered variable pitch screw. Initial compression, as well as fracture site compression, angulation, and bending stiffness, was compared between groups throughout 1000 physiologic cyclic loads. Results: Conventional partially threaded screws obtained significantly greater initial compression compared with tapered variable pitch screws. Significantly greater compression was maintained throughout cyclic loading with conventional screw fixation compared with tapered variable pitch screws. Fracture site angulation was significantly greater using tapered variable pitch screws from the tenth load cycle through completion of cyclic loading. Despite a trend toward increased fracture site bending stiffness when using conventional screws, no difference in fixation stiffness was demonstrable between the 2 screw types. Conclusion: In this cadaveric Jones fracture fixation model, conventional partially threaded screws provided improved fracture site compression and decreased fracture site angulation but offered no advantage in improving fracture site stiffness compared with tapered variable pitch screws. These results provide empirical evidence to guide implant selection decision making for operative fixation of Jones fractures. Clinical Relevance: While the use of tapered variable pitch screws is a potential alternative for fixation of fifth metatarsal Jones fractures, conventional partially threaded screws may provide better biomechanical stability, the effect of which on fracture healing is unknown.

Biomechanical analysis of screw fixation vs. K-wire fixation of a slipped capital femoral epiphysis model

Previous data have shown that due to the technical ease, low-morbidity, and lower complication rates, the in situ single-implant fixation is the current standard for stabilization of slipped capital femoral epiphysis (SCFE) fixation. Multiple-implant fixation is thought to be combined with a higher incidence of serious complications. The purpose of the current study was to evaluate single- vs. multiple-implant fixation regarding strength and stiffness. Furthermore, different screw designs, including telescopic screw, were evaluated regarding the stiffness, strength, and especially fixation failure. Forty porcine proximal femurs were sectioned through the physeal line using a gigli saw and stabilized with a 7.3-mm stainless steel AO screw, a dynamic telescopic screw, three 1.6-mm Kirschner wires (K-wires), and three 2.0-mm K-wires. The femurs were biomechanically tested to determine failure load (N) and stiffness (N/mm). No significant differences were found regarding failure load and stiffness between the two screw groups. The 2.0-mm K-wire construct was significantly the strongest and stiffest fixation. The 1.6-mm K-wire fixation had the lowest values, but not statistically significant. Regarding the fixation failure, no femoral shaft fracture occurred. SCFE stabilization with three 2.0-mm K-wires leads to increased stability over single-screw fixation and 1.6-mm K-wire fixation. However, none of the two screws seemed to be superior in fixation stability and fixation failure.

Biomechanical comparison of strategies to adjust axial stiffness of a hybrid fixator

To evaluate strategies for increasing the axial stiffness of a hybrid external bone fixator. Type Ia hybrid fixators, consisting of a uniplanar linear component connected to a circular ring, were tested in displacement controlled loading in axial compression. The basic hybrid construct was modified to explore strategies considered to increase fixator stiffness including: decreasing ring diameter, increasing ring thickness, adding pins to the ring fixation, and adding struts between the ring and vertical post components of the device. Stiffness in the initial phase of loading was compared between the groups. The addition of a single diagonal bar between the ring and linear connecting rail did not significantly improve the stiffness of constructs. However, the addition of two half-pins to the ring, the addition of two struts between the ring and linear connecting rail, or decreasing the internal ring diameter from 115 to 85 mm progressively increased the stiffness of the frame. The most effective strategy consisted of increasing the thickness of the ring from 6 to 12 mm, thereby increasing the stiffness of the control frame by 335%. Modulating the ring thickness, adding two struts between the ring and linear connecting rail, and reducing the ring diameter appear to be the most effective, simple, and clinically versatile ways to increase axial stiffness, most likely due to their impact on reducing ring bending.

Effects of in vivo mechanical loading on large bone defect regeneration

Fracture healing is highly sensitive to mechanical conditions; however, the effects of mechanical loading on large bone defect regeneration have not been evaluated. In this study, we investigated the effects of functional loading on repair of critically sized segmental bone defects. About 6-mm defects were created in rat femora, and each defect received 5 µg recombinant human bone morphogenetic protein-2 (rhBMP-2), delivered in alginate hydrogel. Limbs were stabilized by either stiff fixation plates for the duration of the study or compliant plates that allowed transfer of compressive ambulatory loads beginning at week 4. Healing was assessed by digital radiography, microcomputed tomography, mechanical testing, histology, and finite element modeling. Loading significantly increased regenerate bone volume and average polar moment of inertia. The response to loading was location-dependent with the polar moment of inertia increased at the proximal end of the defect but not the distal end. As a result, torsional stiffness was 58% higher in the compliant plate group, but failure torque was not altered. In single samples assessed for histology from each group, a qualitatively greater amount of cartilage and a lesser degree of remodeling to lamellar bone occurred in the loaded group compared to the stiff plate group. Finally, principal strain histograms, calculated by FE modeling, revealed that the compliant plate samples had adapted to more efficiently distribute loads in the defects. Together, these data demonstrate that functional transfer of axial loads alters BMP-induced large bone defect repair by increasing the amount and distribution of bone formed within the defect.

Mechanical regulation of vascular growth and tissue regeneration in vivo.

New vascular network formation is a critical step in the wound healing process and a primary limiting factor in functional tissue regeneration. Like many tissues, neovascular networks have been shown in vitro to be highly sensitive to mechanical conditions; however, the effects of matrix deformations on neovascular network formation and remodeling in engineered tissue regeneration in vivo have not been evaluated. We quantified the effects of early and delayed functional loading on neovascular growth in a rat model of large bone defect regeneration using compliant fixation plates that were unlocked to allow transfer of ambulatory loads to the defect either at the time of implantation (early), or after 4 wk of stiff fixation (delayed). Neovascular growth and bone regeneration were quantitatively evaluated 3 wk after the onset of loading by contrast-enhanced microcomputed tomography and histology. The initial vascular response to bone injury featured robust angiogenesis and collateral vessel formation, increasing parameters such as vascular volume and connectivity while decreasing degree of anisotropy. Application of early mechanical loading significantly inhibited vascular invasion into the defect by 66% and reduced bone formation by 75% in comparison to stiff plate controls. In contrast, delaying the onset of loading by 4 wk significantly enhanced bone formation by 20% and stimulated vascular remodeling by increasing the number of large vessels and decreasing the number of small vessels. Together, these data demonstrate the mechanosensitivity of neovascular networks and highlight the capacity of biomechanical stimulation to modulate postnatal vascular growth and remodeling.

Bone Repair and Regeneration: Editorial Comment

Regenerative therapies for bone and muscle reconstitution require understanding of both biologic and mechanical conditions. Even though this is well known, its implications for clinical therapies are only to a limited degree recognized. The goal of this symposium was to initiate discussion on the cellular, tissue, and organ levels, formulate current concepts in musculoskeletal regeneration, and promote the translation of their techniques into the clinic. Recent advances in both musculoskeletal biomechanics and molecular and cell biologic approaches in tissue engineering have led to high expectations among scientists, clinicians, and patients, especially in joint and bone reconstruction. Mechanobiology plays here a central role in linking mechanotransduction and tissue engineering with macroscopic tissue behavior and characteristics of native and artificial biomaterials. Successful translation to clinical practice additionally requires extensive interaction with physicians. By initiating discussion and interaction between the basic disciplines involved in investigating the regeneration of musculoskeletal tissues, together with clinical partners, we aim to identify strategic areas within and interactions between the disciplines that are essential for bringing new therapies to the clinic (Fig. 1). Fig. 1 Dr. Georg N. Duda is shown. The work from Ball et al. [1] reflects recent developments in bio-inspired nanomaterials and tissue regeneration of bone. Engineering of large volumes of bone in vivo seems applicable; however, a thorough materials analysis of tissue-engineered bone still remains mandatory to come to an understanding of native tissue formation. Willie et al. [7] present an approach to vary fixation stiffness and describe how stiffness affects bone healing in rat osteotomies. Mehta et al. [4] describe delayed bone healing in female aged rats compared to male rats and how mechanical stimulation affects tissue formation and maturation between the genders. Johnson et al. [2] explain the highly metabolic and vascularized bone tissue essential for normal growth and repair. A logical strategy to promote healing is therefore to deliver factors that stimulate increased or accelerated vascular growth. Effective sustained delivery of small-molecule activators of specific pathways open new and exciting therapeutic approaches. Kolar et al. [3] analyzed hematoma in patients with autoimmune diseases or otherwise restricted immune functions. They compared these hematoma samples with ones from otherwise healthy controls and found an upregulation of specific immune cells. Other work [5] has also recently illustrated specific subsets of immune cells apparently relevant for prompt healing. These studies raise the question to what degree the early immune response in healing should be differentiated in a more and in a less beneficial phase to bone healing. If so, treatment modalities should specifically address the distinct aspects of the immune responses after injury. Finally and from a completely different perspective, von Domarus et al. [6] describe the relevance of vitamin D for skeletal health and cancer prevention. Such a link is important to understand since a substantial portion of the global population suffers from a vitamin D deficit. It will be of importance for the future care of patients with fractures to better understand bone regeneration and healing from the standpoint of influencing factors, such as the immune status, bone metabolism, and gender-specific aspects of healing. The comprehensive view of the patient itself and the modalities of treatment currently available present a great chance to come to a realistic understanding of deficits in healing and—in a true sense—a personalized medical approach in musculoskeletal surgery.

In vitro comparison of stiffness of plate fixation of radii from large- and small-breed dogs

To compare in vitro axial compression, abaxial compression, and torsional stiffnesses of intact and plated radii from small- and large-breed dogs. Radii from 18 small-breed and 9 large-breed skeletally mature dogs. 3 groups were tested: large-breed dog radii plated with 3.5-mm limited-contact dynamic compression plates (LCDCPs), small-breed dog radii plated with 2.0-mm dynamic compression plates (DCPs), and small-breed dog radii plated with 2.0/2.7-mm cut-to-length plates (CTLPs). The axial compression, abaxial compression, and torsional stiffnesses of each intact radius were determined under loading with a material testing machine. An osteotomy was performed, radii were plated, and testing was repeated. The stiffness values of the plated radii were expressed as absolute and normalized values; the latter was calculated as a percentage of the stiffness of the intact bone. Absolute and normalized stiffness values were compared among groups. The absolute stiffnesses of plated radii in axial and abaxial compression were 52% to 83% of the intact stiffnesses in all fixation groups. No difference was found in torsion. There was no difference in normalized stiffnesses between small-breed radii stabilized with CTLPs and large-breed radii stabilized with LCDCPs; however, small-breed radii stabilized with DCPs were less stiff than were any other group. Plated radii of small-breed dogs had normalized stiffnesses equal to or less than plated radii of large-breed dogs. The complications typically associated with plating of radial fractures in small-breed dogs cannot be ascribed to an overly stiff bone-plate construct.

Can Locking Screws Allow Smaller, Low-Profile Plates to Achieve Comparable Stability to Larger, Standard Plates?

The open reduction and internal fixation of radial shaft fractures and osteotomies with standard 3.5-mm plates can be complicated by tendon irritation, hardware prominence, and fracture through the screw holes. With the advent of locking plate technology, implant companies and some surgeons have recommended expanding the indications for these devices; for example, using smaller, low-profile locking plates to suffice where a standard, larger plate would traditionally be used. We analyzed whether there is merit to this strategy. We hypothesized that, in an established cadaveric fracture fixation model, a smaller, low-profile plate with multiple locking screws could maintain adequate fixation stiffness with the potential to minimize hardware-related complications. Seven matched pairs of fresh-frozen cadaver radii were used. A 5-mm osteotomy gap was created at the midpoint of each specimen and the simulated fracture in one radius from each pair was fixed with a 3.5-mm plate and six nonlocking, standard screws. The contralateral radius was fixed using an equivalent-length 2.7-mm plate with eight locking screws. The radii were subjected to controlled bending and torsional loads and the bending and torsional stiffnesses were documented. Cyclic dorsal-to-volar bending was then applied and resistance to fatigue bending assessed. The 2.7-mm locking plate was approximately one third as stiff as the 3.5-mm nonlocking plate (P < 0.02). Under physiological loading conditions, the 3.5-mm plate was superior to the 2.7-mm plate with respect to bending stiffness in all four directions, torsional stiffness in both directions, osteotomy gapping, and osteotomy angulation (P < 0.02 for all tests). The performance gap did not narrow with cyclic testing. The theoretical structural benefit from the locking screws did not make up for the smaller size of the 2.7-mm plate. This held true in all bending planes, torsion, and cyclic loading, and outweighed any biologic differences between the specimens, including the presence or absence of osteoporosis. This is the first study to rigorously compare these two constructs and we conclude that the mechanical properties of the standard 3.5-mm plate are superior to the locking 2.7-mm plate in all regimes tested.

Stability reconstruction and related complications after total spondylectomy for thoracolumbar tumors

Objective To investigate the spinal stability reconstruction and related complications after total spondylectomy for thoracolumbar tumors. Methods From January 1997 to December 2009, 34 cases with thoracolumbar tumors treated with total spondylectomy, including 20 males and 14 females with an average age of 43 years. The tumors were located in thoracic vertebra in 27 cases and lumbar vertebra in 7, including 6 spinal metastases and 28 primary tumors. The total spondylectomy was piecemeal in 23 cases and en bloc in 11. The reconstruction methods included posterior short-segment fixation in 19 cases, multi-segmental fixation in 13, anterior intervertebral fixation in 2, titanium mesh with auto-bone graft in 20 cases, titanium mesh with bone cement in 7, bone cement filling in 2, auto-bone strut graft in 2, titanium mesh with strengthened rings in 1, artificial vertebral body replacement in 2, posterior massive bone bridging graft in 8 and bone particles graft in 26. Results The mean follow-up time was 43 months. Seven patients died, 5 suffered recurrence. Poor grafted bone fusion was found in 5 cases. The intervertebral sagittal Cobb angle of adjacent vertebra was ??after operation, 9.3?at the end of follow-up or before revision operation. The average lost angle was 14.1?for cases with multi-segmental fixation, and 21.5?with short-segment fixation. Complications included internal fixation broken (5) and loosening (2), titanium mesh subsidence (6) and displacement (3). There were 5 revision surgeries. Conclusion There are relative more complications about spinal stability reconstruction after total spondylectomy. To achieve temporary stiff fixation and long-term fusion, the posterior multi-segment fixation and massive bone bridging graft combined with the anterior artificial vertebral body replacement are effective. Key words: Spine; Neoplasms; Internal fixators; Postoperaive complications

Differential fracture healing resulting from fixation stiffness variability: a mouse model

BackgroundThe mechanisms underlying the interaction between the local mechanical environment and fracture healing are not known. We developed a mouse femoral fracture model with implants of different stiffness, and hypothesized that differential fracture healing would result. MethodsFemoral shaft fractures were created in 70 mice, and were treated with an intramedullary nail made of either tungsten (Young's modulus = 410 GPa) or aluminium (Young's modulus = 70 GPa). Mice were then sacrificed at 2 or 5 weeks. Fracture calluses were analyzed using standard microCT, histological, and biomechanical methods. ResultsAt 2 weeks, callus volume was significantly greater in the aluminium group than in the tungsten group (61.2 vs. 40.5 mm3, p = 0.016), yet bone volume within the calluses was no different between the groups (13.2 vs. 12.3 mm3). Calluses from the tungsten group were stiffer on mechanical testing (18.7 vs. 9.7 N/mm, p = 0.01). The percent cartilage in the callus was 31.6% in the aluminium group and 22.9% in the tungsten group (p = 0.40). At 5 weeks, there were no differences between any of the healed femora. ConclusionsIn this study, fracture implants of different stiffness led to different fracture healing in this mouse fracture model. Fractures treated with a stiffer implant had more advanced healing at 2 weeks, but still healed by callus formation. Although this concept has been well documented previously, this particular model could be a valuable research tool to study the healing consequences of altered fixation stiffness, which may provide insight into the pathogenesis and ideal treatment of fractures and non-unions.

Polymethylmethacrylate augmentation of the pedicle screw: the cement distribution in the vertebral body

Many studies have proven that the polymethylmethacrylate (PMMA) augmentation of the pedicle screw can significantly increase stiffness and strength of spinal fixation. Some major complications have also been reported. However, there are no reports discussing cement distribution and its morphology in the osteoporotic vertebral body, which is critical in the analysis of the biomechanical strength of the pedicle screw and the risk of cement leakage after pedicle screw augmentation. In this study, we used computed tomography (CT) to evaluate the cement distribution in the osteoporotic vertebral body after PMMA augmentation of a pedicle screw and to analyze the factors leading to cement leakage. Two groups of patients were studied. Group A consisted 25 osteoporotic patients (mean age of 73years) with spinal instrumentation who had a total of 145 pedicle screws and cement augmentation with biopsy needles. Group B consisted of 23 osteoporotic patients (mean age of 74.6years) with spinal instrumentation who had a total of 125 cannulated pedicle screws with cement augmentation. All patients had CT evaluation of the cement distribution in the vertebral body after the surgery. The cement distribution in the vertebrae was divided into four zones in the axial CT view: anterior one-third, middle third, and posterior third of vertebral body, and the pedicle. The morphology of the cement distribution around the pedicle screw was defined as scattered type or concentrate type. The leakage pattern was divided to anterior-lateral, posterior-lateral, and canal leakage. The correlations among bone mineral density (BMD), the cement leakage rate, and cement distribution morphology were also analyzed. The results showed that most augmented pedicle screws had cement extension into three of the four zones of the vertebral body (66.3%), followed by two zones (20%), all four zones (11.5%), and only one zone (2.2%). Overall, 123 screws (84.8%) in Group A and 108 screws (86.4%) in Group B had cement concentrate type distribution. The cement leakage rate in Group A is 18.3% and 13.6% in Group B. Patients with a BMD <0.6g/cm(2) had significantly higher rates of cement leakage and tended toward a scattered cement distribution. There was only one patient who had a symptomatic leakage (sciatica) in Group B. We concluded that the cement distribution after pedicle screw augmentation with biopsy needle or cannulated screw technique was mostly localized in three zones of the vertebral body, and patients with lower BMD had a higher risk of cement leakage and scattered cement distribution.