Stabilisation of the Foot and Ankle Complex

Dissertation / Doktorarbeit von Gaspar Maximilian Gabriel Morey-Klapsing

Abstract:

Probably one of the main contributions of this thesis has been the use of a three-dimensional kinematic model accounting not only for ankle motion but also for the motion of the lateral and medial columns of the forefoot with regard to the rearfoot (Arampatzis et al., 2002), in a joint stability context. The obtained values may serve as reference for the planning of further studies and provide a base for building up new hypotheses. However this thesis did not aim to merely describe the kinematics but rather to provide more knowledge regarding the stabilisation of the foot and the ankle. Therefore another important contribution is surely the simultaneous study of the kinematics, the EMG and the ground reaction forces, which allows a better understanding of the whole stabilisation process.

The presented results have shown that forefoot motion is fundamental in foot and ankle stabilisation. The flexibility of the forefoot, especially in the frontal plane, permits a fast and appropriate adaptation to the ground. Furthermore the high mobility of the forefoot, allows the ankle to rotate slower and to a lesser extent. Possibly this reduction in required ankle motion can contribute considerably to injury prevention, since the forces acting at the ankle are high and a misalignment with regard to the ground reaction forces could rapidly lead to moments overwhelming the stabilising potential of the involved structures. In addition, the rapid adaptation of the forefoot to the ground can potentially provide more precise and earlier feedback regarding the ground characteristics than the structures surrounding the ankle joint. This way the corresponding adjustments in an immediate feedback could happen earlier, and the consequences of future interactions could be predicted more accurately.

The results from the presented studies support the notion that joint stabilisation does not rely primarily on proprioception. Prolonged peroneal latencies might in fact be due to deafferentiation consequent to the recurrent sprains. However prolonged latencies do not seem to be responsible for a functional instability. On one hand the differences in latency times between healthy and unstable ankles are relatively low and not consistently observed. Those studies identifying prolonged latencies in functionally unstable joints, report differences close to 15 ms (Konradsen and Ravn, 1990; Löfvenberg et al., 1995). Fifteen ms is a short time to have a high functional impact on joint stabilisation. Furthermore the presented results strongly suggest that the temporal characteristics of the EMG response play a less important role than those related to EMG amplitude.

Finally, healthy or not, the latencies are in general too long to provide reactive protection against strong sudden perturbations. During the inverting tilt plate tests the EMG activity during the first 50 ms is almost negligible, but both forefoot joints are almost maximally inverted. In addition the highest mediolateral ground reaction forces occur as soon as 25 ms after plate release. So during these first milliseconds, the stabilisation has to rely onto passive mechanisms. At the sudden everting tilts, the EMG activity shows the same temporal pattern as inverting tilts, but lower amplitudes. At the same time the kinematics are in general lower in amplitude and velocity. These findings together with the higher passive constraints to eversion than to inversion (Siegler et al., 1994) underpin the importance of passive constraints during the early phase of recovery from a sudden disturbance.

Awareness has shown to improve stabilisation even when no differences in activation prior to plate release were identified. On the other hand it has been shown that the early reactive response to a joint perturbation is unspecific in nature (similar EMG patterns despite of opposite disturbance directions). Accordingly Grüneberg et al. (2003) found specific responses to appear relatively late in time (~90 ms). Finally specific adaptive responses to an expected perturbation are given in a feedforward manner (specific changes in joint positioning and activation according to landing surface). All these findings together suggest that experience may play a crucial role in joint stabilisation. The more experience we have the better we can anticipate the effects of the next interaction with the environment. Consequently the anticipative adjustments would be more adequate. Nevertheless it has also been shown that it is possible to stabilise the foot and ankle joints despite of not being aware of the instant of tilt. The consequences are higher ground reaction forces and higher needed muscular activation levels.

As discussed somewhat earlier, these higher activation levels would probably be too late in absence of other stabilising mechanisms. Moritz and Farley (2004) found adaptive stabilising responses to surprising changes in surface stiffness prior to identifiable changes in the EMG. These changes were due to the passive dynamics inherent to the anatomy and physiology of our musculoskeletal system, which seems to include self-stabilising mechanisms in its design (Wagner and Blickhan, 2003). When observing all the presented results together, it can be suggested with reasonable evidence, that experience and awareness can improve the usage of the self-stabilising mechanisms. It is suggested that the observed proactive adjustments aim to produce the best suited conditions (segmental orientation and muscle tension) to cope with the perturbation to come.

The proved effectiveness of proprioceptive training in the rehabilitation of functionally unstable joints (Verhagen et al., 2000) can also be explained by the presented theoretical frame, without the need of assuming that stabilisation strongly relies on proprioception. The repeated experience would provide the information necessary for a better and even earlier prediction of the consequences of the next interaction. It could further help to generate more suited default strategies for the case of being surprised.

Although not experimentally proven, there is a general consensus on that severe ankle sprains usually happen because the foot is brought into a position where even fully activated muscles cannot resist the injuring moment. The only way for avoiding such extreme positions is to prevent them in advance. Accordingly Eils and Rosenbaum (2003) suggested, that the main function of braces is not the restriction of motion in acute situations, but holding the foot in a position where it is unlikely to elicit such extreme situations. This way sprains are mainly prevented rather than counteracted.

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