Failure of femoral head fixation: a cadaveric analysis of lag screw cut-out with the gamma locking nail and AO dynamic hip screw

The most commonly reported failure mode of sliding hip screws in published literature is cut-out of the lag screw. This study investigates the resistance to failure of the femoral head, with lag screws used in two types of sliding hip screws, the gamma locking nail (Howmedica) and the dynamic hip screw (DHS) (Synthes). The investigation consisted of biomechanical tests under static loading conditions on 12 pairs of cadaveric femoral heads, to establish the failure loads due to screw cut-out for the two implant lag screws. The gamma nail appeared to reduce the tendency to cut-out in the osteoporotic bone (soft) associated with elderly patients in whom these devices are commonly used (p < 0.05). In high density bone (hard) the gamma lag screw also appeared to be stronger, because the DHS showed a tendency to bend. The larger diameter of the gamma nail lag screw resists bending and appears to reduce the risk of cut-out compared with the DHS.     

Forces required to initiate sliding in second-generation intramedullary nails

Second-generation intramedullary nails, which allow the fixation screw that is placed in the femoral head to slide distally and thus allow compression of the fracture of the femoral neck, have become a popular option for the treatment of ipsilateral fractures of the femoral neck and shaft. However, the sliding characteristics of the screw within the barrel of the nail or the side-plate have not been assessed biomechanically, to our knowledge. The goal of the current study was to investigate the forces required to initiate sliding of the proximal screw in intramedullary devices and to compare these forces with those required to initiate sliding of hip screws. The loading configuration simulated the typical angle of 135 degrees between the intramedullary nail and the proximal screw. The forces required to initiate sliding of the proximal screw, with the screw extended fifty-one, seventy-six, eighty-six, and 102 millimeters beyond the proximal end of the barrel, were measured for three different types of second-generation intramedullary nails (Recon, ZMS, and Gamma), a sliding compression hip screw, and an intramedullary hip screw, and these forces were then compared. With each amount of extension of the screw, the hip screws required lower forces to initiate sliding than did the second-generation intramedullary devices. Of the second-generation devices, the Gamma nail required the highest forces to initiate sliding; the Recon and ZMS nails required 20 to 40 percent lower forces compared with the Gamma nail. None of the devices jammed in any of the loading configurations that were tested. When the extension of the screw was increased, higher forces were required to initiate sliding.     

Field Investigation of a Sandwich Plate System Bridge Deck

This paper presents the results of a live-load test of the Shenley Bridge, the first bridge application of the sandwich plate system technology in North America. The investigation focused on the evaluation of in-service performance including lateral load distribution behavior and dynamic load allowance. Real-time midspan deflections and strain values were measured under both static and dynamic conditions and under various loading configurations to assess the in-service performance. Distribution factors were determined for interior and exterior girders subjected to single and paired truck loadings. In addition, dynamic load allowance was determined from a comparison of the bridge’s response under static conditions to the response under dynamic conditions. From a comparison of measured results to AASHTO LRFD, AASHTO standard, and CHBDC provisions, it was determined that the current provisions tend to produce conservative predictions for lateral load distribution, but can be unconservative for dynamic load allowance. As a result of the testing program containing a single field test, a finite-element model was also used for determination of lateral load distribution and yielded predictions similar to measured results. The results from the finite-element models were often less conservative than the code provisions.     

Use of the Gamma nail in the treatment of fractures of the proximal femur

Fractures of the proximal femur are, more than ever, an important challenge in the field of traumatology. The Gamma nail, a combination of advantages of the sliding screw with the intramedullary nail, represents an efficient technique in the management of these fractures. A series of 224 fractures of the proximal femur in which this nail was used is reported. The average age of the patients was 79.2 years. The mean healing time was 68.2 days in 99.4% of the cases. The incidence of perioperative complications was 10.3% showing that, in most of the cases, the complications occurred because of poor technique. Postoperative complications occurred with an incidence of 14.1%. Seven cases of migration of a proximal screw, six shaft fractures, and one broken nail were the most important complications. The device allowed for early mobilization and full weightbearing of the affected hip regardless of the type of fracture. With adequate surgical technique and experience, the advantages of the Gamma nail increases as the complication rate diminishes.     

A method for studying the biomechanical load response of the (in vitro) lumbar spine under dynamic flexion-shear loads

A method was developed to study the biomechanical response of the lumbar motion segment (Functional Spinal Unit, FSU) under a dynamic (transient) load in flexion. In order to inflict flexion-distraction types of injuries (lap seat-belt injuries) different load pulses were transferred to the specimen by means of a padded pendulum. The load response of the specimen was measured with a force and moment transducer. The flexion angulation and displacements were determined by means of high-speed photography. Two series of tests were made with ten specimens in each and with two different load pulses: one moderate load pulse (peak acceleration 5 g, rise time 30 ms, duration 150 ms) and one severe load pulse (peak acceleration 12 g, rise time 15 ms, duration 250 ms). The results showed that the moderate load pulse caused residual permanent deformations at a mean bending moment of 140 Nm and a mean shear force of 430 N at a mean flexion angulation of 14 degrees. The severe load pulse caused evident signs of failure of the segments at a mean bending moment of 185 Nm and a mean shear force of 600 N at a mean flexion angulation of 19 degrees. Significant correlations were found between the load response and the size of the specimen, as well as between the load response and the bone mineral content (BMC) in the two adjacent vertebrae. Comparisons with lumbar spine response to static flexion-shear loading indicated that the specimens could withstand higher bending moments before injury occurred during dynamic loading, but the deformations at injury tended to be smaller for dynamic loading.     

Creep induced by load cycling in a C-Mn steel

Tensile creep tests were made on a C-Mn steel (C0.21. Mn 0.81) at room temperature in which, after initial creep under static conditions, the load was periodically removed and replaced. It was observed that load cycling produced additional creep which did not occur in static loading, the magnitude of the effect depending on the applied stress at the start of cycling. The experimental data are interpreted as showing that the processes responsible for creep under these conditions are initiated by the reverse movement of mobile dislocations during unloading.     

Field Static Load Test on Kao-Ping-Hsi Cable-Stayed Bridge

Field load testing is an effective method for understanding the behavior and fundamental characteristics of a cable-stayed bridge. This paper presents the results of field static load tests on the Kao-Ping-Hsi cable-stayed bridge, the longest cable-stayed bridge in Taiwan, before it was open to traffic. A total of 40 loading cases, including the unit and distributed bending and torsion loading effects, were conducted to investigate the bridge behavior. The atmospheric temperature effect on the variations of the main girder deflections was also monitored. The results of static load testing include the main girder deflections, the flexural strains of the prestressed concrete girder, and the variations of the cable forces. A three-dimensional finite-element model was developed. The results show that the bridge under the planned load test conditions has linear superposition characteristics and the analytical model shows a very good agreement with the bridge responses. Further discussion of deflection and cable forces of the design specifications for a cable-stayed bridge is also presented.     

Rapid Pullout Test of Soil Nail

This technical paper describes the rapid pullout response of soil nail embedded in dry clean sand. In the rapid pullout test, soil nail is pullout by a tensile impulse load with loading duration that is long enough to eliminate the influence of the stress wave propagation phenomenon. The results of these experiments showed the influence of loading rate on pullout response is highly dependent on the roughness condition of the nail surface. For rough nail, the prepeak rapid pullout response was significantly stiffer in the load-displacement characteristic and higher in peak pullout strength when compared to the corresponding quasi-static pullout response. While for a smooth nail, a negligible difference between rapid and quasi-static pullout response was noticed. In light of these limited experimental results, the radiation damping effect appears to be the dominant contributor to the enhancements in prepeak rapid pullout response of rough nail. “Actual” damping coefficient that quantifies the damping resistance mobilized in a rapid pullout test was found not to be constant but to decrease with the increase in pullout displacement.     

Behavior of Concrete Beams Strengthened with Fiber-Reinforced Polymer Laminates under Impact Loading

Most of the research on application of composite materials in civil engineering during the past decade has concentrated on the behavior of structural elements under static loads. In engineering practice, there are many situations in which structures undergo impact or dynamic loading. In particular, the impact response of concrete beams strengthened with composite materials is of interest. This paper presents the results of an experimental investigation conducted to study the impact effects on concrete beams strengthened with fiber-reinforced polymer laminates. Two types of composite laminates, carbon and Kevlar, were bonded to the top and bottom faces of concrete beams with epoxy. Five beams were tested: two strengthened with Kevlar laminates, two strengthened with carbon laminates, and one unretrofitted beam as the control specimen. The impact load was applied by dropping a steel cylinder from a specified height onto the top face of the beam. The test results revealed that composite laminates significantly increased the capacity of the concrete beams to resist impact load. In addition, the laminates reduced the deflection and crack width. Comparing the test results of the beams strengthened with Kevlar and carbon laminates indicated that the gain in strength depends on the type, thickness, weight, and material properties of the composite laminate.     

Time-Dependent Behavior of RC Beams Retrofitted with CFRP Straps

A retrofitting technique that uses prestressed unbonded carbon-fiber-reinforced polymer (CFRP) straps to provide additional shear capacity has previously been shown to be successful under short-term static loading conditions. The current study explores the longer-term behavior of this retrofitting technique through two experiments (a sustained load and a cyclic load experiment) and the development of a model based on the modified compression field theory. The experiments indicated that the strain in the CFRP straps changes with time due to changes in the load sharing with the concrete (caused by creep) and the steel stirrups (caused by yield of these elements). The predictive model was initially validated against static experimental results before being applied to the longer-term experiments. The model predicts the trends in behavior well although it is conservative in its estimates of strap strain. The model was then used to determine the influence of stirrup yielding, the load level before and after retrofitting, and the duration of loading on the CFRP strap strains. The initial results suggest that the largest increases in long-term strap strain will occur when the straps are installed early in the structure’s service life although further experimental validation is required.     

A comparison of the modified gamma nail and the ender nail for treatment of intertrochanteric fractures

We made a comparison between the modified Gamma nailing and the Ender nailing in the treatment of 102 patients with intertrochanteric fracture. Sixty patients were treated with the modified Gamma nailing and forty-two patients with the Ender method. The preoperative conditions of the patients in the two groups were similar as showed by statistical analysis. More intraoperative bleeding was recorded in the modified Gamma nail group. However, there were a earlier full weight -bearing, a better hip function and a lower rate of operative complication in the patients treated with the modified Gamma nailing. We conclude that with the modified technique and the modified femoral component the Gamma nail is advanced in the treatment of intertrochanteric fracture. However, it is danger to apply in elderly patients accompanied with certain vital problems. For these patients the Ender nail may be considered.     

Grain size distribution effects in superplasticity

The influence of grain size distribution on the stress-strain rate behavior of superplastic metals has been investigated for steady-state as well as transient loading situations. The model, which considers a distribution of internal stresses based on grain size distribution, is a detailed development of one presented elsewhere [1]. Deformation is assumed to occur by a combination of grain- boundary creep (sliding accommodated by diffusion) and power law creep, constrained by equal strain rate in all grains during steady state. The analytical results of the model based on realistic grain size distributions simulate steady-state behavior, as well as loading and load relaxation behaviors of superplastic aluminum and titanium alloys. Several other features of material response are also explained.     

Nonlinear Finite-Element Modeling of Batter Piles under Lateral Load

In this paper, a finite-element model is developed in which the nonlinear soil behavior is represented by a hyperbolic relation for static load condition and modified hyperbolic relation, which includes both degradation and gap for a cyclic load condition. Although batter piles are subjected to lateral load, the soil resistance is also governed by axial load, which is incorporated by considering the P-Δ moment and geometric stiffness matrix. By adopting the developed numerical model, static and cyclic load analyses are performed adopting an incremental-iterative procedure where the pile is idealized as beam elements and the soil as elastoplastic spring elements. The proposed numerical model is validated with published laboratory and field pile test results under both static and cyclic load conditions. This paper highlights the importance of the degradation factor and its influence on the soil resistance-displacement (p-y) curve, number of cycles of loading, and cyclic load response.     

In-Situ Load Testing of Corrugated Steel Pipe-Arch Culverts

A large number of corrugated metal pipe-arch culverts are located under highways. This study investigates the field performance of four existing pipe-arch culverts under static and dynamic loads. Effects of various parameters were considered in selection of the culverts, including backfill height, loading conditions, age of placement, and culvert geometry. Static loads were applied at ten different locations above each culvert using heavily loaded test trucks. Six dynamic tests were conducted at speeds varying from 8 to 64?km/h. A portable instrumentation frame was installed inside each test culvert to monitor the deflections at five critical locations. During each test, strains were also measured using 14 strain gauges. Test results indicated that culvert response was influenced significantly by the backfill height. Nearly symmetrical deflection patterns were recorded for symmetrical loading about the longitudinal vertical plane through the crown. The maximum static deflections were larger than the maximum dynamic deflections for each culvert.     

Free Out-of-Plane Vibrations of Masonry Walls Strengthened with Composite Materials

The natural frequencies and the out-of-plane vibration modes of one-way masonry walls strengthened with composite materials are studied. Due to the inherent nonlinear behavior of the masonry wall, the dynamic characteristics depend on the level of out-of-plane load (mechanical load or forced out-of-plane deflections) and the resulting cracking, nonlinear behavior of the mortar material, and debonding of the composite system. In order to account for the nonlinearity and the accumulation of damage, a general nonlinear dynamic model of the strengthened wall is developed. The model is mathematically decomposed into a nonlinear static analysis phase, in which the static response and the corresponding residual mechanical properties are determined, and a free vibration analysis phase, in which the dynamic characteristics are determined. The governing nonlinear differential equations of the first phase, the linear differential eigenvalue problem corresponding to the second phase, and the solution strategies are derived. Two numerical examples that examine the capabilities of the model and study the dynamic properties of the strengthened wall are presented. The model is supported and verified through comparison with a step-by-step time integration analysis, and comparison with experimental results of a full-scale strengthened wall under impulse loading. The results show that the strengthening system significantly affects the natural frequencies of the wall, modifies its modes of vibration, and restrains the deterioration of the dynamic properties with the increase of load. The quantification of these effects contributes to the understanding of the performance of damaged strengthened walls under dynamic and seismic loads.     

Nonlinear Dynamics of a Harmonically-Excited Inelastic Inverted Pendulum

Issues of dynamic stability for a single-degree-of-freedom system subjected to a time-varying axial load are presented. The linearized differential equation of motion for the model structure is given by the well-known Mathieu equation. Parametric resonance leading to dynamic instability is known to occur for such a system. This paper examines the response of the geometrically exact model for two inelastic constitutive models—an elastic-perfectly plastic model and a cyclic Ramberg-Osgood model. Damage evolution, represented by degradation of the elastic stiffness, is also considered. Analysis results demonstrate behavior that is counterintuitive to what would be expected under static or monotonic loading conditions. Though simple, this structural model helps illustrate the complex features in the response of an inelastic dynamical system.     

Human patellar-tendon rupture

The first biomechanical analysis of a human patellar-tendon rupture during actual sports competition is reported. Cinematographic data for analysis were collected at a national weight-lifting championship. Dynamic equations to mathematically model the lifter were developed to compute time course and magnitudes of hip, knee and ankle-joint moments of force and of tensile loading of the patellar tendon before and during tendon trauma. Results provided evidence that the range of maximum tensile stress of the tendon may be considerably greater during rapid dynamic loading conditions, as in many sports situations, than maximum tensile stress obtained during static test conditions.     

Mechanical failure of the intramedullary hip screw in a subtrochanteric femoral fracture

The Gamma nail, an implant specifically designed for intertrochanteric and subtrochanteric femoral fractures, has been criticized for its high risk of secondary shaft fractures. A modified design, the intramedullary hip screw has recently been introduced to correct this complication. We present a case of mechanical failure of this new implant that occurred in a pathological subtrochanteric fracture. The centering sleeve of the implant became loose and migrated while the head screw penetrated the acetabulum. Refixation was successful.     

Load mechanics of the wrist

During the last decade, we have been studying load mechanics under a variety of conditions using pressure-sensitive film and, more recently, three-dimensional reconstructions and motion analysis. In all of these studies using pressure-sensitive film, the simulated pathologic or traumatic conditions that were tested showed that all areas in which an increase in contact area or pressure occurred, localized to one area of one joint, coincided with areas in which degenerative changes occurred in the simulated clinical situation. The areas in which there was a decrease or no change in contact area or pressure coincided with areas that were spared from degenerative changes in simulated clinical situations. More recent work has demonstrated that normal carpal kinematics during wrist flexion and extension do not have an instantaneous screw axis that is fixed in or limited to the capitate. These findings change the understanding previously based on studies suggesting that the center of rotation was fixed in the capitate. It was noted that during global wrist motion the radiolunate joint contributes more motion and flexion than the capitolunate joint, but the capitolunate joint contributes more motion and extension than the radiolunate joint. It also was demonstrated that translational motion is a normal component of carpal kinematics.     

Laboratory and Field Performance of Cellular Fiber-Reinforced Polymer Composite Bridge Deck Systems

This paper addresses the laboratory and field performance of multicellular fiber-reinforced polymer (FRP) composite bridge deck systems produced from adhesively bonded pultrusions. Two methods of deck contact loading were examined: a steel patch dimensioned according to the AASHTO Bridge Design Specifications, and a simulated tire patch constructed from an actual truck tire reinforced with silicon rubber. Under these conditions, deck stiffness, strength, and failure characteristics of the cellular FRP decks were examined. The simulated tire loading was shown to develop greater global deflections given the same static load. The failure mode is localized and dominated by transverse bending failure of the composites under the simulated tire loading as opposed to punching shear for the AASHTO recommended patch load. A field testing facility was designed and constructed in which FRP decks were installed, tested, and monitored to study the decks’ in-service field performance. No significant loss of deck capacity was observed after more than one year of field service. However, it was shown that unsupported edges (or free edges) are undesirable due to transitional stiffness from approach to the unsupported deck edge.