Fatigue - Introduction

Human spines are exposed to dynamic loads during everyday activities. Especially at the workplace, these dynamic loads might be rather high due to the increased usage of machines along with increased specialisation, which can lead to longer periods of similar physical stress within the daily working routine. Therefore, the duration of the exposure to these loads might last for a reasonable portion of each day, and consequently, the exposure adds up over one’s lifetime and exerts a cumulative effect. In particular, vibrations combined with constrained sitting postures are regarded as a risk factor for disc degeneration and endplate failure. Workers who operate large construction machinery, forest harvesters and forklifts are prominent examples for workers especially exposed to these potentially harmful postures.

Another consequence of the increased usage of machines is that the physical constitution of workers in western societies is continually worsening due to the lack of regular exercise. One indication of this is that the mean body mass index has increased, which is of course not solely caused by using machines. This decreased constitution might cause reduced muscle protection and stiffening of the trunk during exposure to dynamic loads.

To prevent extensive loading and diminish the associated risks, the cumulative amount of exposure to vibrations should be limited according to regulations and recommendations (2002/44/EG; ISO 2631-1, 1997; ISO 2631-5, 2004). The scientific basis for these recommendations is scant, however. For example, little is known about the effect of external vibration loads transmitted from the seat to the body on the internal relative loads within spinal structures. Even less is known about different postures and intra-individual differences of human bodies in general and specifically differences in spinal structures. The main reason for this is that in vivo studies in humans are rarely possible. The experiments would either be too harmful for the potential subjects or they can only be performed on injured persons, such as e.g.

experiments with instrumented cages or fixators (Rohlmann et al., 2001; Wilke et al., 2001b).

However, it is important to become aware of this external-internal transfer mechanism in order to understand the mechanism of disc degeneration and other kinds of spinal injuries. This could improve the appraisal of occupational accidents, enable the determination of implant duty cycles and spur the development of future treatments or implants as well as improve prophylaxis in the workplace and in private life.

To close this gap, numerical whole-body models have been developed to examine the behaviour of spinal columns exposed to different types of vibration while sitting in different postures (VIBRISK, FIOSH). However, capturing the anatomical complexity and including all relevant active and passive elements within the load chain is still

challenging and even the improved models will on principle be unsuitable for determining the fatigue characteristics of spinal segments. Fatigue behaviour of functional spinal units (FSUs) has been determined in vitro (Brinckmann et al., 1988), but mainly for donors in the upper range of working age or after retirement (n = 70, 51±18 yrs). Furthermore, the numbers of cycles to failure were small (N < 5000), partly due to the high load used. In a recent study (Huber et al., 2006), the FSUs of younger donors (n = 30, 33±6 yrs) were shown to fail during 100,000 load cycles only if this loading with a high physiological load (peak-to-peak: 0-2 kN) coincided with pure bone mineral density (BMD).

The aim of this in vitro study was to investigate how specimen posture and vertebral characteristics like age, BMD and endplate size might be used to determine the fatigue strength of spinal specimens.

The hypothesis for this study is based on a recent publication (Seidel et al., 2008a).

The number of cycles to failure (NK) is thought to be related to the maximal failure load (Fmax) within a single overload experiment in the following way:

6

The given equation is a simplified version of the original one. It is only valid for cyclic loads with a constant mean load (Fmean) and a constant peak-to-peak load (Fpeak-to-peak). For more complex loading conditions, the contributions of different consecutive load cycles are summed up (Figure VI-1).

F _mean

F peak-to-peak

F _mean

F peak-to-peak

Figure VI-1 Parameters used for the equation to determine the fatigue strength Obviously, it is not possible to measure both the cycles to failure in a fatigue experiment and the single failure overload (or ultimate strength, respectively) with the same specimens. Therefore, an established equation for the maximal failure force (Brinckmann et al., 1989) was used. This equation is based on the endplate area (AREA) and the bone mineral density (BMD):

cm

Combining these two equations leads to a simple relation with one combined input (BMD · AREA) and one output value (NK). The values ci represents constants adding the parameters of the equation for the ultimate strength and the cyclic loading:

(

)

k c c BMD AREA

N = + ⋅ ⋅ . (VI-3)

As an alternative non-invasive method, the ultimate strength might also be predicted based on the age of the donors (AGE). The use of X-rays could therefore be omitted.

This is not of interest for in vitro studies, but important with regard to in vivo investigations. Several regressions were published in a meta-study (Seidel et al., 1998) and one of them (STEEP) is similar to that incorporated into the Annex of ISO 2631-5 (2004). In the original publications stresses are given, but in the context of this study forces are more appropriate. Therefore, the equations are given with an additional product (AREA) representing the endplate area. First, one with a low gradient (LOW):

and second, one with a steep gradient (STEEP):

AREA

Individual ultimate force can be calculated by the donors age and the corresponding endplate area. For an age of 56, the predicted ultimate strength is similar for both equations. The ultimate strength at this age will be about 3 MPa for both equations.

Because of the different slopes, the strength of a specimen from a 20 year old donor will be 4.4 MPa for the LOW equation and 5.4 MPa for the STEEP one. With the given hypothesis for the prediction of cycles to failures, the following equation arises (the variable ki represents constants):

(

)

k k k AREA k AGE AREA

N = + ⋅ + ⋅ ⋅ . (VI-6)

For further group wise analyses, the mean AREA is calculated out of the average of the superior L5 endplate and the inferior L4 endplate of all 18 specimens in this part (16.7±3.1 cm²). The ultimate force at the age of 56 for a mean AREA will be about 5 kN for both equations. Because of the different slopes, the strength of a specimen from a 20 year old donor will be 7.3 kN for the LOW equation and 9 kN for the STEEP one.