Head-locking durability of fixed and variable angle locking screws under repetitive loading.

Polyaxial locking screws are increasingly applied in fracture fixation. To investigate the durability of the head-locking mechanism, the removal torque of variable angle (VA) and fixed angle (FA) stainless steel and titanium locking screws was investigated without and after a cyclic loading test. Stainless steel (St) and titanium (Ti) 2.4 mm orthogonally inserted FA screws and 2.4 mm VA screws inserted in different inclinations (0°-15°) (n = 6 per group) were locked at 0.8 Nm. Removal torque was determined without (W) and after (A) cyclic loading (sinusoidal load, 5 Hz, constant amplitude of 25 N, up to 10'000 cycles, or failure). Significant differences in-between the groups were detected by Student's t-test (p < 0.05). Except VA Ti in 0deg and FA, all groups exhibited a drop in removal torque below the insertion torque without and after cyclic testing. The removal torque was (St: FA W:0.81 Nm ± 0.04 A:0.72Nm ± 0.04; VA0deg W:0.73 Nm ± 0.04 A:0.65 Nm ± 0.05; VA15deg W:0.51 Nm ± 0.05 A:0.50 Nm ± 0.08; Ti: FA W:0.82 Nm ± 0.03 A:0.70 Nm ± 0.04; VA0deg W:0.80 Nm ± 0.02 A:0.72 Nm ± 0.05; VA15deg W:0.55 Nm ± 0.03 A:0.54 Nm ± 0.06). In all groups, the removal torque after cyclic testing did not drop below 16% of the removal torque without cyclic testing. No head loosening was observed after cyclic testing. Stainless steel and titanium 2.4 mm fixed and variable angle locking screws provide a stable and lasting head-locking mechanism. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:949-952, 2016.

Concept of variable angle locking--evolution and mechanical evaluation of a recent technology.

Applications for fracture-adapted screw positioning offered by variable angle locking screws are increasing. The locking strength of the variable angle locking mechanism at different insertion angles was compared to conventional fixed angle locking screws. Stainless steel (S) and titanium (Ti) variable and fixed angle 2.4 mm locking screws, inserted at different inclinations (0°-15°), and locked at 0.8 Nm were subjected to a load-to-failure test. Ultimate failure moment at the screw-head interface and failure mode of the screws were determined. Significant differences were detected by one-way ANOVA (p < 0.05). Stainless steel and titanium variable angle locking screws inserted at 2°-10° inclination exhibited a failure moment comparable to the 0° position (S 1.67± 0.04 Nm; Ti 1.67 ± 0.14 Nm) and failed predominantly at the screw neck, with the head remaining fixed to the plate. Inserted at 15° inclination, screws revealed a lower failure moment (S 1.33 ± 0.06 Nm, Ti 1.58 ± 0.05Nm), and failed predominantly by breakout of the head thread. Fixed angle locking screws inserted at an inclination >2° did not lock properly in the plate hole, providing insufficient locking strength. Variable angle locking screws offer a stable head-locking mechanism at different inclinations, comparable to the locking strength of orthogonal inserted fixed angle locking screws. Marginal inclinations >15° should be used with care.

A biomechanical study on proximal plate fixation techniques in periprosthetic femur fractures.

INTRODUCTION: Proximal plate fixation is a crucial factor in osteosynthesis of periprosthetic femur fractures. Stability and strength of different fixation concepts for proximal plate fixation were compared. MATERIALS AND METHODS: Twelve fresh frozen, bone mineral density matched human femora, instrumented with cemented hip endoprosthesis were osteotomized simulating a Vancouver B1 fracture. Specimens were instrumented with locking compression plates, fixed proximally with either locking attachment plate (LAP), monocortical screws, cerclage plus monocortical screws (1cerclage) or cerclages only (4cerclages). Cyclic testing was performed with monotonically increasing load until failure. Relative movements at the proximal plate-femur interface were registered by motion tracking. RESULTS: The LAP construct exhibited a significantly longer cumulative survival (failure criterion 1mm separation at the proximal plate fixation) compared to the monocortical (p=0.048) and 4cerclages constructs (p=0.012) but not to 1cerclage constructs. CONCLUSION: Bicortical screw anchorage improves proximal plate fixation in periprosthetic fractures. The cerclage-screw combination is a valuable alternative especially in osteoporotic bone.

Mechanical behavior of fixation components for periprosthetic fracture surgery.

BACKGROUND: Reliable periprosthetic fracture treatment needs detailed knowledge on the mechanical behavior of the fixation components used. The holding capacity of three conventional fixation components for periprosthetic fracture treatment was systematically investigated under different loading directions. METHODS: Locking compression plates were fixed to a 7 cm long part of diaphyseal fresh frozen human femur with either a single 1.7 mm cerclage cable, a 5.0mm monocortical or a bicortical locking screw (n=5 per group). Constructs were loaded in lateral, torsional and axial direction with respect to the bone axis in a load-to-failure test. Corresponding stress distribution around the screw holes was analyzed by finite element modeling. FINDINGS: Both screw fixations revealed significantly higher stiffness and ultimate strength in axial compression and torsion compared to the cerclage (all P<0.01). Ultimate strength in lateral loading and torsion was significantly higher for bicortical screws (mean 3968 N SD 657; mean 28.8 Nm SD 5.9) compared to monocortical screws (mean 2748 N SD 585; mean 14.4 Nm SD 5.7 Nm) and cerclages (mean 3001N SD 252; mean 3.2 Nm SD 2.0) (P≤0.04). Stress distribution around the screw hole varied according to the screw type and load direction. INTERPRETATION: Fixation components may be combined according to their individual advantages to achieve an optimal periprosthetic fracture fixation.

Biomechanical performance of different cable and wire cerclage configurations.

PURPOSE: Cerclage technology is regaining interest due to the increasing number of periprosthetic fractures. Different wiring techniques have been formerly proposed and have hibernated over years. Hereby, they are compared to current cerclage technology. METHODS: Seven groups (n = 6) of different cable cerclage (Ø1.7 mm, crimp closure) configurations (one single cerclage looped once around the shells, one single cerclage looped twice, two cerclages each looped once) and solid wire cerclages (Ø1.5 mm, twist closure) (same configurations as cable cerclages, and two braided wires, twisted around each other looped once) fixed two cortical half shells of human femoral shaft mounted on a testing jig. Sinusoidal cyclic loading with constantly increasing force (0.1 N/cycle) was applied starting at 50 N peak load. Cerclage pretension (P), load leading to onset of plastic deformation (D) and load at total failure (T) were identified. Statistical differences between the groups were detected by univariate ANOVA. RESULTS: Double looped cables (P442N ± 129; D1334N ± 319; T2734N ± 330) performed significantly better (p < 0.05) than single looped cables (P292N ± 56; D646N ± 108; T1622N ± 171) and were comparable to two single cables (P392N ± 154; D1191N ± 334; T2675N ± 361). Double looped wires (P335N ± 49; D752N ± 119; T1359N ± 80) were significantly better (p < 0.05) than single looped wires (P181N ± 16; D343N ± 33; T606N ± 109) and performed similarly to single looped cables. Braided wires (P119N ± 26; D225N ± 55; T919N ± 197) exhibited early loss of pretension and plastic deformation. CONCLUSION: Double looped cerclages provided a better fixation stability compared to a single looped cerclage. Double looped wires were comparable to a single looped cable. The use of braided wires could not be recommended mechanically.

Underneath the cerclage: an ex vivo study on the cerclage-bone interface mechanics.

INTRODUCTION: Cerclages regain interest due to a rising number of periprosthetic fractures. The contact distribution at the circumferential cerclage-bone interface is still unknown. Local interface pressure depends on the amount of contact area. Cortical damage at the interface would provoke cerclage loosening. Therefore, the contact area, the bone pressure along the interface and the cortical resistance underneath loaded cerclages were determined in an ex vivo model. MATERIALS AND METHODS: Human diaphyseal femoral bone was used with differing cross-sectional geometry. Bone contact points of fixed 1.5 mm wire and 1.7 mm cable cerclages were identified from axial radiographs. Pressure distribution at the cerclage-bone interface was recorded with a pressure-measuring film using a distraction setup with two cortical half shells. Bone shells with installed cerclages were separated with up to 400 N force and were subsequently analyzed histologically to detect cortical damage. RESULTS: Both cerclage types exhibited a point contact fixation with non-loaded spanned zones in-between. Cables cover larger contact areas. Both cerclages exhibited an inhomogeneous interface pressure distribution depending on the bone surface geometry. Histology revealed intact cortical bone without cortical affection after loading of both cerclage types. CONCLUSION: Point contact fixation of the cerclages installs non-loaded, spanned zones where the periosteum is not compressed, rendering a strangulation of the blood supply unlikely. Cortical bone withstands static concentric pressure produced by the cerclage. Cortical groove formation is attributed to instability under functional load and not to weakness of the cortex itself.

Effect of fracture gap on stability of compression plate fixation: a finite element study.

In compression plating, anatomical reduction and compression across the fracture site are the basic principles necessary to achieve primary bone healing. However, varying amounts of gap at the fracture site frequently occur due to technical pitfalls, such as overbending of the plate and inaccurate reduction, and due to the fracture configuration itself. Little is known as to how fracture gap affects stability of the bone-plate construct. We analyzed the effects of fracture gap size (1 and 4 mm) and bone defect (25%, 50%, 75%, 100%) on the biomechanical stability of the compression plate-bone construct through validated finite element analysis. The stiffnesses of eight different models were compared with the stiffness of an ideally compressed model (0 mm/0%). Stress concentration in form of peak von Mises stress (PVMS) was also evaluated. The decrease in stiffness depended mainly on the depth of bone defect. The decrease in stiffness was similar in models with the same defect and different gap size. Considerably more stress was concentrated around the central hole of the plate in gap models with the depth of bone defects of 75% and 100% than with smaller defects. We concluded that even a thin fracture gap (1 mm) with no contact between the fracture after plating decreases stiffness exponentially; contact at the fracture surfaces of > or =50% was necessary to avoid undue stress concentration in the plate.

Internal fixator for use in the mouse.

Knockout techniques enable us to expand our knowledge about bonerepair processes. Since they require the use of mice, such studies necessitate the development of special technologies. Mechano-biological reactions play a determining role in fracture healing, and therefore controlled conditions of stability are essential. Achieving fixation with a low-mass implant avoids uncontrolled inertial loading and enables free ambulation. A miniaturised version of conventional internal fixation as used in humans has been developed. The method consists of a plate-like internal fixator with locked screws. It permits compression fixation or splinting fixation at selectable levels of flexibility. A guide is used for standardised application of the fixator, the screws and the osteotomy. Locking the screws to the plate enables elevation of the fixator from the bone surface; therefore, minimal contact damage to the periosteal blood supply can be expected. The technology is now in use in several laboratories.

Bone regeneration in long-bone defects: tissue compartmentalisation? In vivo study on bone defects in sheep.

Regeneration of living tissue varies with species, age and type of tissue, and undoubtedly with the biological and mechanical environment of the precise tissue. Autologous cancellous bone grafting is a well-known technique that provides bony regeneration. We investigated the efficiency of autologous bone grafting in a well-vascularised muscle environment, and additionally when isolated from the muscle and connected only to the bony environment. We designed a reproducible animal model producing a stable 3cm middiaphyseal bone and periosteal defect on sheep femurs and created a foreign-body membrane with a temporary poly-methylmethacrylate spacer. The foreign-body membrane had the outer dimension of the removed bone segment. We then ascertained the bony regeneration potential within the bone defect using autologous cancellous bone graft. Regeneration of bone is enhanced considerably by an autologous foreign-body membrane that separates the interfragmentary space from the muscular environment. This effect is independent of the autologous bone graft. The results suggest that bone behaves like a compartment that protects its specific humoral or its cellular environment, or both. Regeneration of bone can be enhanced by compartmentalisation of the bone defect.

Fixation compliance in a mouse osteotomy model induces two different processes of bone healing but does not lead to delayed union.

Delayed unions are a problematic complication of fracture healing whose pathophysiology is not well understood. Advanced molecular biology methods available with mice would be advantageous for investigation. In humans, decreased fixation rigidity and poor reduction are generally associated with delayed unions. In this study, these two factors were combined to observe their effect on bone healing in mice. Two plates with locking screws, one with 14 the bending stiffness of the other, were used to stabilize a 0.45mm gap osteotomy. muCT, radiographs, 4pt-bending tests and histological analysis demonstrated that the different plate types led to two different healing pathways. The less flexible bridging plate induced only intramembranous ossification whereas the more flexible bridging plate induced a mixture of endochondral and intramembranous ossification. However, the different plates led to a delay in healing of only 3-5 days in the period between 14 and 21 post-operative days. In mice, considerable fixation flexibility is necessary to induce secondary bone healing similar to that which occurs in humans, but this was not sufficient to induce a substantial delay in bone healing as would be expected in humans.

Fracture healing. The evolution of our understanding.

Our understanding of fracture healing has undergone an evolution over many decades with continuous improvement of fracture treatment. Solid union is a precondition of restoring the function of a fractured bone. The goal of the early treatment of the fracture was focussed upon enabling solid union in acceptable alignment of the fracture. This was achieved with reduction followed by application of external splints. The function of the articulations was often troubled by long lasting and extensive external immobilization, which required physiotherapy that lasted longer than bone union. The surgical reduction and stabilization aimed at early recovery of movement of the articulations and maintenance of the function of the soft tissues and blood supply. The AO group initiated 1958 by Maurice E. Müller and his colleagues prioritized the recovery of limb function and propagated precise reduction and fixation using mainly compression. Absolute stability of fixation, achieved using implants, allowed to move the articulations very early without pain, while the fracture united solidly. After such treatment the implants could not be removed before 1 1/2 to 2 years without risking increased incidence of re-fracture. This was in sharp contrast to the fact that after conservative treatment the bone was solidly united after 2 to 3 months. The analysis of this situation revealed that internal remodelling after absolutely stable fixation did not recognize the presence of the fracture. Primary healing, therefore, is not a healing in the strict sense of the word but a side effect of internal removal of necrotic bone. To maintain early function of the limb and stimulate the healing process the so called biological internal fixation was developed. It combines minimal surgical trauma, acceptable rather than precise reduction and flexible fixation usually achieved with so called internal fixateurs. Flexibility of mind and of tools aims at safe and early healing with full recovery of function and minimal risk of biological complications.

Surgical aspects of infection involving osteosynthesis implants: implant design and resistance to local infection.

Infection is still a major complication associated with surgery in orthopedics and traumatology. The implant design, the implant material, and the implantation technique may play essential roles in preventing infection. The goal of this paper is to discuss a series of experiments evaluating different aspects of both surgical procedures and implants. For this purpose, standardized local application of the human pathogen Staphylococcus aureus was performed in an animal model. Although these experimental results may be clinically relevant, direct extrapolation to clinical infection rates should be avoided.

In vivo measurement of bending stiffness in fracture healing.

BACKGROUND: Measurement of the bending stiffness a healing fracture represents a valid variable in the assessment of fracture healing. However, currently available methods typically have high measurement errors, even for mild pin loosening. Furthermore, these methods cannot provide actual values of bending stiffness, which precludes comparisons among individual fractures. Thus, even today, little information is available with regards to the fracture healing pattern with respect to actual values of bending stiffness. Our goals were, therefore: to develop a measurement device that would allow accurate and sensitive measurement of bending stiffness, even in the presence of mild pin loosening; to describe the course of healing in individual fractures; and help to evaluate whether the individual pattern of bending stiffness can be predicted at an early stage of healing. METHODS: A new measurement device has been developed to precisely measure the bending stiffness of the healing fracture by simulating four-point-bending. The system was calibrated on aluminum models and intact tibiae. The influence of pin loosening on measurement error was evaluated. The system was tested at weekly intervals in an animal experiment to determine the actual bending stiffness of the fracture. Transverse fractures were created in the right tibia of twelve sheep, and then stabilized with an external fixator. At ten weeks, bending stiffness of the tibiae were determined in a four-point-bending test device to validate the in-vivo-measurement data. RESULTS: In-vivo bending stiffness can be measured accurately and sensitive, even in the early phase of callus healing. Up to a bending stiffness of 10 Nm/degree, measurement error was below 3.4% for one pin loose, and below 29.3% for four pins loose, respectively. Measurement of stiffness data over time revealed a significant logarithmic increase between the third and seventh weeks, whereby the logarithmic rate of change among sheep was similar, but started from different levels. Comparative measurements showed that early individual changes between the third and fourth weeks can be used as a predictor of bending stiffness at seven weeks (r = 0.928) and at ten weeks (r = 0.710). CONCLUSION: Bending stiffness can be measured precisely, with less error in the case of pin loosening. Prediction of the future healing course of the individual fracture can be assessed by changes from the third to the fourth week, with differences in stiffness levels. Therefore, the initial status of the fracture seems to have a high impact on the individual healing course.

The effect of delayed autogenous bone grafting on the healing of diaphyseal defects in a canine model.

Bone grafting of osseous defects is often delayed to minimize the risk of infection, however, the effect of this delay on defect healing is not clear. Unilateral oblong unicortical diaphyseal femoral defects (30 x 4.5 mm) were created in 3 groups of 18 adult canines that were grafted with autogenous bone immediately, at 2 weeks, and at 6 weeks. After an 8-week healing period, paired femurs, h consisted of defect versus intact specimens, were assessed by biomechanical testing and microradiography. No differences were demonstrated biomechanically or radiographically between the healing defects with immediate versus delayed bone grafting. This study suggests that within the first 6 weeks postoperatively osseous defects can be grafted with autologous bone without adversely affecting bone graft incorporation.

Evolution of the internal fixation of long bone fractures. The scientific basis of biological internal fixation: choosing a new balance between stability and biology.

The advent of 'biological internal fixation' is an important development in the surgical management of fractures. Locked nailing has demonstrated that flexible fixation without precise reduction results in reliable healing. While external fixators are mainly used today to provide temporary fixation in fractures after severe injury, the internal fixator offers flexible fixation, maintaining the advantages of the external fixator but allowing long-term treatment. The internal fixator resembles a plate but functions differently. It is based on pure splinting rather than compression. The resulting flexible stabilisation induces the formation of callus. With the use of locked threaded bolts, the application of the internal fixator foregoes the need of adaptation of the shape of the splint to that of the bone during surgery. Thus, it is possible to apply the internal fixator as a minimally invasive percutaneous osteosynthesis (MIPO). Minimal surgical trauma and flexible fixation allow prompt healing when the blood supply to bone is maintained or can be restored early. The scientific basis of the fixation and function of these new implants has been reviewed. The biomechanical aspects principally address the degree of instability which may be tolerated by fracture healing under different biological conditions. Fractures may heal spontaneously in spite of gross instability while minimal, even non-visible, instability may be deleterious for rigidly fixed small fracture gaps. The theory of strain offers an explanation for the maximum instability which will be tolerated and the minimal degree required for induction of callus formation. The biological aspects of damage to the blood supply, necrosis and temporary porosity explain the importance of avoiding extensive contact of the implant with bone. The phenomenon of bone loss and stress protection has a biological rather than a mechanical explanation. The same mechanism of necrosis-induced internal remodelling may explain the basic process of direct healing.