A technique to prescribe a vertical acceleration-time load on the human head–neck complex

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Abstract

An experimental technique is established to induce a prescribed upward acceleration-time load on the human head–neck complex. The primary objective is to replicate experimentally the initial loading history experienced at the base of the cervical spine during pilot ejection from a cockpit. A drop tower is specially modified to achieve this and seven head–neck complexes from male human cadavers were tested. The experimental acceleration-time results follow the desired profiles closely. High-speed photography is used to capture visual images of the ejection process to examine the dynamic response of specimens and to derive kinematic data on vertebrae motion during ejection.

Introduction

Modern day fighter pilots are equipped with an array of protective headgear and head mounted devices, which increase the loading on the neck, leading to possible neck injury. Moreover, ejection seats are installed to enable the pilot to escape from the aircraft in the event of an emergency. Pilot ejection occurs over a very short span of time, and thus generates sudden, high accelerations that may result in excessive loading of the cervical spine, leading to injury. Earlier study [1] has shown that 26–30% of successful ejections result in spinal injuries and of these, cervical injuries constitute a significant portion. The incidence rate for acute neck injuries among fighter pilots during ejection from a cockpit is therefore a major concern [2], [3]. To ensure a pilot’s safety, helmet assemblies are usually tested using a rocket sled test, such as the one using an Advanced Concept Ejection Seat (ACES) II ejection seat and different helmet configurations mounted on anthropometric mannequins [4]. However, rocket sled tests are extremely expensive to conduct and there are limited facilities worldwide. Other experimental studies of pilot ejection or sudden flight manoeuvres have been carried out through direct measurement of variables such as forces during actual flight, to ascertain the susceptibility of neck injury [5]. However, the load limits in these experiments involving human volunteers were extremely low, which is not similar to that in pilot ejection.

Recent studies have proposed the use of computational tools as an efficient approach for evaluating ejection safety [6], [7]. Nevertheless, the reliability of computational models can only be validated by experiments. Experimental techniques to replicate the acceleration-time history corresponding to actual pilot ejection must be established. Dynamic tests [8] on head–neck complexes are relatively scarce, compared to quasi-static testing [9], [10], [11], [12], [13] of the entire spine or vertebral segment in flexion or compression/bending. Research on spinal response under different loads is relevant to various situations; e.g. to study cervical spine whiplash injuries arising from traffic accidents, head–neck complexes from human cadavers have been subjected to transverse impact [14], [15], [16]. However, reports on experiments that simulate vertical ejection on cadaveric specimens are very limited due to practical difficulties. Gaynor et al. [17] conducted vertical acceleration tests on three embalmed cadavers. The vertical accelerator was an extensive set-up consisting of an aircraft ejection seat mounted on rails, capable of travelling a distance of 36.58 m (120 feet), the height of an eight-storey high elevator shaft. Stemper et al. [8] attempted to simulate vertical ejection on a cadaveric head–neck complex. The specimen was mounted in a supine position on a mini sled seated on horizontal rails. An acceleration of 14.8 G 100 ms duration was applied to the sled using an impact pendulum. However, this set-up introduced a 1 G acceleration on the supine oriented specimen due to gravity, which does not exist in actual pilot ejection. Nevertheless, Stemper et al. [8] clearly stated the advantages of performing a head–neck complex test versus whole body cadaver or manikin tests. Precise initial positioning of the body, the head–neck structure, and the application of an accurate acceleration pulse are difficult to achieve in whole body cadaver tests. Finally, manikins are not able to reproduce spinal injury due to hard and soft tissue damage.

This study explores an experimental method to induce a vertical acceleration to simulate actual pilot ejection in order to replicate the initial load/acceleration history (up to 0.1–0.15 s) to the head–neck structure. A low-cost and compact Vertical Ejection Simulation Tower (VEST) is designed and constructed to facilitate dynamic testing of this nature. Unlike a rocket sled test, the set-up focuses on the critical head–neck structure instead of the whole body.

Section snippets

Experimental techniques for high-G ejection

The initial phase of ejection involves the most abrupt change in acceleration, from 0 to 0.05 s; it is highly likely that any injury sustained occurs during this time. Fig. 1 illustrates the typical force history of an ejection sequence for an ACES II ejection seat with a 50th percentile Hybrid III mannequin [6] in a typical rocket sled test. This force history approximates that at the base of a pilot’s cervical spine during an actual ejection. Fig. 7 shows the corresponding acceleration

Ejection system and acceleration profiles

The VEST is driven by a guided drop mass within a 5 m tall drop tower (Fig. 4); there is a translating frame (Fig. 5), consisting of a horizontal upper catch bar and a horizontal lower support bar separated by two vertical columns. The frame is guided to move upwards by two vertical shafts through linear bearings at the ends of the two horizontal bars. The specimen holder (potting cup) is mounted on the lower horizontal support bar and the vertical shafts are fixed onto a base plate. The upper

Biomechanical response of cervical spine during ejection

During high-G upward ejection, the response of the vertebrae in a cervical spine is complex. To examine the kinematic response, a high-speed camera (Fastcam APX-C) is used to capture visual data at a rate of 2000 FPS (frames per second). As the cervical spine is covered by soft tissue, marker pins are inserted into each vertebra to facilitate tracking of its angular movement (Fig. 5). The soft tissue surrounding the cervical spine is left intact and undamaged, except at the areas where marker

Conclusion

A Vertical Ejection Simulation Tower (VEST) system based on a multiple pulley mechanism, was conceived, designed and constructed. This enabled simulation of the loading imposed on the human head–neck corresponding to the initial phase of pilot ejection (up to 0.1–0.15 s). Results obtained from the limited number of cadaveric specimens available within the duration of this research project, show that variation of the overall cervical spine curvature during ejection is within physiological limits.

Acknowledgement

This work was funded by the Defence Medical and Environmental Research Institute, DSO National Laboratories, Singapore, through a research grant (R-265-000-093-422).

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