The literature shows that the quality of the head-neck taper junction is influenced by many different factors. Firstly, contamination of the taper by blood or fat decreases the strength of the connection and should therefore be avoided [13]. Furthermore, damage to the taper, e.g. during revision surgery with head replacement, has a negative effect on the junction and can lead to head breakage in metal-ceramic bearings [17]. Therefore an revision head with integrated metal sleeve should always be used in such cases [18]. Another decisive factor is the assembly force. If the assembly force is too low, there is an increased risk of corrosion at the interface due to fretting [19]. Too high assembly forces, on the other hand, can lead to proximal fractures of the femur, which then require extensive surgical treatment [20]. In contrast to the other factors described, there is no clinical standard for eliminating this potential risk. Therefore, the aim of the study was to determine the individual in situ assembly forces of different surgeons and to compare them with the forces recommended in the literature in order to derive a recommendation for action.
In a first step, experimental impactions were carried out on human cadavers. Since the sample size with N = 5 trials was very small due to the limited availability of body donors, an in vitro model was developed which reproduced the damping situation of the experiments on the cadavers. This in vitro model was intended to enable a larger number of trials. With the introduced parameter NPF it could be shown that the ratio between the introduced impulse and peak force on the stem was almost identical in the trials on cadavers and on the in vitro model. Therefore it can be assumed that the damping situation was almost identical in both experiments. Reproducing physiological damping behaviour of human soft tissues in vitro not only enables the comparison of the assembly forces of different surgeons, but can also be used to determine the forces actually acting on the patient. For example, Scholl et al [21] have carried out a similar investigation on a very rigid structure and have measured assembly forces up to approximately 25 kN. They explained the very high forces in comparison to other studies [14] by the point where the force was measured (striking pad of the hammer). In this context it should be noted that the participating surgeons considered the striking pad of the hammer used to be too small, which can be seen as a limitation of the experiments. Nevertheless, this study showed that such high forces do not occur neither on the hammer nor on the stem under physiological damping behaviour on the patient. The maximum forces measured on the hammer were 2881.6 N on cadavers and 4058.3 N on the model. The measured forces on the hammer were slightly higher than those measured on the stem. This is caused by different bearing situations of impactor tool and stem, friction and also differences between impact direction of the hammer and the neck axis.
However, the difference accounted for only a few percent. Krull et al [22] also carried out experiments on an in vitro model in the laboratory. They showed that both the stiffness of the tip of impactor tool and the damping of the taper bearing have major influence on the resulting forces. Thus, the damping is a much more significant influencing factor than the point of measurement. In this study, the focus was on the forces measured on the neck of the stem. The force acting on the neck of the stem is the force that counteracts the impact of the hammer and thus the force that is decisive for the Morse taper junction.
As already shown in similar studies, there are considerable differences between the assembly forces of individual surgeons [16, 21]. The tests on cadavers yielded forces between 1063.9 N and 2453.4 N. Even larger differences occurred testing on the in vitro model due to the higher number of cases. The forces ranged from a minimum of 822.5 N to a maximum of 3835.2 N. As the findings show, the range does not depend on sex, age or experience of the surgeons.
Based on the results of other studies, which examined the effects of assembly forces on parameters as pull-out force, turn-off moments and fretting, it appears that the majority of the surgeons involved applied too little force. Based on the resulting pull-out forces from different assembly forces, Ramoutar et al. recommend a minimum assembly force of 2.5 kN [15]. Consequently, 71.0% of the surgeons would not have applied a sufficiently high assembly force. Based on their findings regarding the relation between assembly force and resulting turn-off moments, Rehmer et al. recommend an assembly force of 4 kN [14]. From their investigations on fretting, Haschke et al. also recommend an assembly force of 4 kN [23]. According to this demand, 100% of the surgeons showed insufficient assembly forces in this study. However, Rehmer et al. and Haschke et al. also recommend avoiding assembly forces of more than 4 kN, as the risk of periprosthetic proximal fractures of the femur increases.
In conclusion, the assembly forces determined show a high variance between surgeons and are low compared to the data given in the literature. As the influences of different head sizes, material combinations or different soft tissue conditions wefre neglected in this study, it can be assumed that the actual variance of the assembly forces is even higher. In our opinion, it is therefore highly recommended to standardize the impaction of the femoral head by means of a new type of surgical instrument.