Nesrin Sarigul-Klijn

The effect of bone quality on pedicle screw loading in axial instability. A synthetic model.

Study Design. In this biomechanical analysis of pedicle screw bending moments, custom-fabricated vertebral analogues were loaded in axial compression to produce sagittal bending forces. Moments were measured directly from internally instrumented pedicle screws. Objectives. To establish the role of cancellous vertebral modulus on pedicle screw bending moments within the vertebral body and the vertebral pedicle. Summary of Background Data. Pedicle screws are often used to manage axial instability of the spine. Clinical studies report a high incidence of screw bending failure, resulting in kyphosis and pain in some patients. Factors predisposing to bending failure are not well understood, although recent studies have shown that vertebral morphometry is important. Methods. Axially canullated 7.0-mm pedicle screws, internally instrumented with paired strain gauges, were inserted into analogue vertebrae of uniform dimension. Cancellous modulus was varied from 25-100 MPa. Screws were rigidly mounted to a vertical testing frame, and axial loads were applied to the superior vertebral endplate, producing sagittal bending moments. Moments were recorded from gauges applied in the intrapedicular and intravertebral portions of the screw. Mean moments were compared using a Students t test, with significance defined as P < 0.05. Results. Cancellous modulus did not affect bending moments experienced in either the intrapedicular or intravertebral portions of the pedicle screws. Gauge accuracy was excellent, and with no gauge drift. Conclusions. Although small changes in pedicle morphometry can alter screw bending moments significantly, changes in cancellous modulus had no measurable impact on bending moments at these same loads. Bone density is likely to play a limited role in screw bending failure.

The effect of pedicle morphometry on pedicle screw loading : A synthetic model

STUDY DESIGN Static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw bending load was studied as a function of pedicle morphometry. OBJECTIVES To determine how sagittal bending moment in pedicle screws is affected by changes in pedicle height, length, and width. BACKGROUND DATA An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for axially unstable fractures. The majority of screws fail in sagittal bending within the pedicle. To date, little is known of the exogenous factors that affect in situ loads incurred by pedicle screws. METHODS Synthetic vertebral analogues were fabricated, varying pedicle height, length, or width independently. Pedicle screws internally instrumented with strain gages were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture. RESULTS Screw bending moments within the pedicle increased incrementally with increasing pedicle length, rising 30% as length increased from 8.0 mm to 12.0 mm. Screw moment increased 20% when pedicle height dropped below 15.0 mm, consistent with a threshold effect. Changes in pedicle width did not affect screw loads within the pedicle. CONCLUSIONS In situ pedicle screw loads increased significantly as pedicle length increased and as pedicle height decreased. Pedicle screws instrumented internally with strain gages are an effective research instrument allowing measurement of in situ loading along the axis of the screw.

Characteristics of pedicle screw loading. Effect of surgical technique on intravertebral and intrapedicular bending moments.

STUDY DESIGN A static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw load was studied as a function of variables in insertion technique. OBJECTIVES To determine how the sagittal bending moment in pedicle screws is affected by changes in pedicle screw length, insertional depth, and sagittal placement. BACKGROUND DATA An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for unstable burst fractures. The majority of screws fail in sagittal bending within the pedicle. Little is known of the insertion technical factors that affect in situ loads incurred by pedicle screws. METHODS Synthetic vertebral analogues were fabricated. Pedicle screws internally instrumented with strain gauges were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture. RESULTS Screw bending moments within the pedicle increased 33% and 52% when screws were left 3 mm and 5 mm short of full insertion. Intrapedicular moments increased 20% to 29% in screws inserted superiorly or inferiorly within the pedicle. Thirty-five-millimeter screws developed intrapedicular moments 16% higher than 40-mm and 45-mm screws. CONCLUSIONS In situ pedicle screw loads increased significantly as a direct result of variations in surgical technique. Screws left short of full insertion, placed off center in the sagittal plane of the pedicle, or less than 40 mm long developed increased intrapedicular bending moments.

Air-Launching Earth to Orbit: Effects of Launch Conditions and Vehicle Aerodynamics

Introduction T HE purpose of this study is to determine the benefits of airlaunching expendable or reusable launch vehicles (LV) by using quantitative methods. Air-launch vehicles consist of at least two stages, a carrier aircraft and a rocket-powered LV. The carrier aircraft can be either subsonic or supersonic capable and can even include balloons. Air launch is one of the leading concepts that can meet today’s launch requirements of both responsive and low cost. Previous work in this area has identified nonquantitative benefits and drawbacks of air-launch methods.1 In this Note, many different air-launch scenarios associated with different release, launch conditions, and vehicle aerodynamics are modeled and simulated using trajectory optimizations. The trajectory optimization is conducted using POST, a numerical integration program based on the three-degree-of-freedom equations of motion of a flight vehicle.2 More than 160 simulations were conducted in which launch altitude, speed, and flight-path angle were varied, and the effect of adding a wing was also modeled.

Intelligent Flight Trajectory Generation to Maximize Safe Outcome Probability after a Distress Event

A flight trajectory generation method called the distressed-aircraft-recovery technique for maximum safe-outcome probability (DART_MSOP), based on integration of three new algorithms, is developed that maximizes safe-outcome probability after a distress event by incorporating an abort airport together with a model of current aircraft dynamics. Several abort-probability models are studied under various constraints. The first new algorithm, a statistical-based initial-turn-determination algorithm, is developed to advise pilots to a reachable best landing site immediately after the distress event and before using the second new algorithm, a high-fidelity flight trajectory generation algorithm. A third new algorithm determines the flight maneuver for guidance of a perpetual-turning-attitude aircraft to fly the trajectory generated by the second algorithm. The third algorithm is only used if the aircraft has stuck controls or a similar malfunction that generates a nonzero amount of bank angle and causes the aircraft to turn only in one direction. As a three-dimensional high-fidelity algorithm, the second algorithm considers the probability of an abort to increase overall survivability by minimizing expected flight-path length as it shapes the trajectory. The performance of this new intelligent flight trajectory determination method DART_MSOP is evaluated using a case study based on a hypothetical in-flight distressed transport aircraft in northern California. Numerical simulations include variable failure rates to simulate different in-flight distress conditions, and multiple fixes along the path to accommodate realistic trajectories. DART_MSOP intelligent flight trajectory determination method should increase aviation safety if these algorithms are implemented in aircraft avionics systems.

A Novel Approach to Extract Power From Free-Flowing Water and High Altitude Jet Streams

It is well known to aeronautical engineers that oscillating wings may extract power from an air stream. The bending and torsion flexure of airplane wings exposed to an air flow may extract sufficient energy to result in structural damage or even catastrophic failure. Fortunately, this phenomenon can also be used to extract energy from a wind or water stream and convert it into electric power.Copyright

A Study of Air Launch Methods for RLVs

Many organizations have proposed air launch Reusable Launch Vehicles (RLVs) due to a renewed interest generated by NASA’s 2 Generation Space Launch Initiative. Air launched RLVs are categorized as captive on top, captive on bottom, towed, aerial refueled, and internally carried. The critical design aspects of various proposed air launch RLVs concepts are evaluated. It is found that many concepts are not possible with today’s technology. The authors introduce a new air launch concept that is possible with today’s technology called SwiftLaunch RLV.

Flight Mechanics of Manned Sub-Orbital Reusable Launch Vehicles with Recommendations for Launch and Recovery

An overview of every significant method of launch and recovery for manned sub-orbital Reusable Launch Vehicles (RLV) is presented here. We have categorized launch methods as vertical takeoff, horizontal takeoff, and air launch. Recovery methods are categorized as wings, aerodynamic decelerators, rockets, and rotors. We conclude that both vertical takeoff and some air launch methods are viable means of attaining sub-orbital altitudes and wings and aerodynamic decelerators are viable methods for recovery. These conclusions are based on statistical methods using historical data coupled with time-stepped integration of the trajectory equations of motion. Based on the additional factors of safety, customer acceptance, and affordability, we also conclude that the preferred architecture for a commercially successful manned sub-orbital RLV is Vertical Takeoff using hybrid rocket motor propulsion and winged un-powered Horizontal Landing onto a runway (VTHL).

Air Launching Earth-to-Orbit Vehicles: Delta V gains from Launch Conditions and Vehicle Aerodynamics

The advantages and disadvantages of the various methods for air launching expendable or reusable space launch vehicles are described. Many different air launch scenarios are modeled and simulated using trajectory optimizations. The trajectory optimization is conducted using POST, a numerical integration program based on the three-degree-of-freedom equations of motion of a flight vehicle. Results in terms of change in velocity gains are reported as a function of launch conditions and launch vehicle aerodynamics. Air launch benefits are presented for a range of air speeds, altitudes, and flight path angles. The most beneficial launch vehicle parameters are in the following order; launch velocity, launch flight path angle, and launch altitude. Increasing launch vehicle size had the largest effect on payload size. There is an optimum launch flight path angle that maximizes the benefit from air launching. The results show that once above about 15000 meters (49,200 feet), added launch altitude has little additional benefit. Nomenclature Ae = Nozzle exit area = Angle of attack β =

Flight testing of a new earth-to-orbit air-launch method

This paper describes the development and flight testing of a new air-launch method for safely launching personnel and cargo into low Earth orbit (LEO). A new rocket is also being designed that will be carried by and launched using the new air-launch method from a modified 747 airliner. This new air-launch method, called trapeze-lanyard air drop (t/LAD) launch, will greatly improve simplicity, safety, cost, and reliability of launching personnel into LEO. A t/LAD launch eliminates the need for wings or fins on the launch vehicle; greatly reduces ascent dynamic pressure, sideways accelerations and bending forces, and rocket engine thrust vectoring control; and allows the use of a simple and very safe vapor pressurization (Vapak) engine cycle for the launch vehicle. This paper reports on the flight-test results of dropping three 23%-scale drop test articles using the t/LAD launch method.