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Torquing the tibia

A small group of us are currently trying to tidy up certain aspects of the Conventional Gait Model (Newington, Helen Hayes, Davis, Kadaba, PiG, VCM) in such a way as to address a number of known problems but also preserves its strengths. We’ve had quite a bit of discussion about tibial torsion recently which flags a number of different issues. Looking back at my book, I see I spent quite a bit of space discussing how different degrees of femoral anteversion affect gait data but not quite so long on tibial torsion.

Tibial torsion is the twist along the shaft of the tibia. If you were to look along the long axis of the bone and imagine where the knee joint axis is in relation to the proximal tibia (in black in the picture below) and the ankle joint axis (trans-malleolar axis) in relation to the distal tibia (grey) then the angle between them is the tibial torsion (20° in this example).

Tibial torsion

At birth tibial torsion, thus defined, is small (<5° on average) and normally increases with age to about 15-20° at adolescence after which it remains constant. (Note that this is different to femoral anteversion which starts off large and reduces over time). There is, however, considerable variability between individuals.

The importance of tibial torsion clinically and to biomechanical modelling is fundamentally that it means that the distal tibia is pointing in a different direction to the proximal tibia. In other words your knee points in a different direction to your ankle. This is particularly important for understanding gait problems in the transverse plane – if for example you want to know why someone is walking with internal foot progression (pigeon-toed in old language).

PiG, the current Vicon implementation of the Conventional Gait Model, deals with the issue by defining both a proximal and distal tibial segment differing only in a rotation about the long axis of the tibia. The proximal tibial segment is used for most measures of knee kinematics and kinetics and the distal tibial segment is used for the ankle kinematics and kinetics (and knee kinematics when generating outputs from the static trial, for some reason!).

At one level this is quite logical but it has several disadvantages:

  • There are two segments but only one bone!
  • The way tibial torsion is incorporated in the model is quite different to the way femoral anteversion is incorporated and this leads to confusion about both. (This is a particular issue as one of the principal advantages of the CGM is that it is inherently quite easy to understand).
  • Tibial torsion becomes an implicit feature of the gait graphs rather than an explicit feature. Thus if you want to consider what factors are affecting transverse plane alignment  from the gait graphs you cannot do so without also knowing the value of tibial torsion that has been used. (This is particularly important if the value of tibial torsion has been calculated with the use of a medial malleolar marker placed for the static trial but not reported along with the gait data). There are a number of ways around this but it would be better if the information was explicit in the graphs.

I much prefer the way we do things when using Visual3D which is to use only the distal tibia. Knee rotation (transverse plane) is then defined pretty much as in the diagram above except the knee axis is taken as the transepicondylar axis in the femur. The measurement is thus the combination of tibial torsion and any true rotation in the knee joint (subject to soft-tissue artefact of course, but that’s another story). What makes things even nicer is that we plot joint angles from the static trial along with our consistency plots (see graph below).

Knee rotation.png

Thus the data in black is knee rotation from 5 walking trials showing a range  between about 12° and 22° internal rotation which indicates that the ankle joint axis (in the tibia) is internally rotated with respect to the the knee joint axis (in the femur). The solid red line is knee rotation measured during the static trial which is by definition equal to the torsion that the system measures in the tibia. It is clear that the (very) abnormal knee rotation is almost entirely explained by torsion within the tibia (and you can then sit around and debate whether the remaining signal is real or a consequence of soft tissue artefact – yes rigid clusters are vulnerable to STA as skin markers it is just that the artefact is different!).

Another nice feature about this approach is that if you have measured tibial torsion clinically then you can compare that measurement with that which the system has made (the static trial measurement) and very easily think through the clinical consequences of any difference, by thinking how your interpretation might change if the solid red line were higher or lower.

Final paragraph for the advanced reader!

Just when we thought we’d considered all the issues and agreed a sensible way forwards someone mentioned kinetics. PiG expresses joint moments in the coordinate system of the distal segment by default and it really doesn’t make any sense to report the knee joint moment about a coordinate system defined by the ankle joint axis! Perhaps this is the reason that the two tibial reference frames were defined in the first place. A much simpler solution, however, is to express the joint moments in the coordinate system of the proximal segment (until now I’ve generally considered this an arbitrary choice but, as result of reflecting on this issue, I’ve now convinced myself that if you are going to define rotation of segments about their long axis by distal landmarks or functional axes and want to use an orthogonal axis system then you have no real choice but to use the proximal segment). The other solution which I actually prefer is to express the joint moments as projections of the total moment vector onto the axes of the joint coordinate system (as recommended by Anthony Schache and myself in this paper). At first glance reporting “components” of a vector about a non-orthogonal axis system appears quite offensive to any self-respecting engineer but this is actually more appropriate if you want to interpret those moments clinically in the context of the requirement of different muscle groups to exert moments about the axes that we regard movement as occurring.

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Coping with maternity leave

How do you ensure that staff going on maternity or paternity leave do not get deskilled during their period away from gait analysis?

Here’s an idea to provide a regular knowledge update. The Verne mobile consists of  6 fully articulated Verne‘s allowing the user to set them in any desired pose. They are arranged in a circle to reinforce the importance of cyclic movement patterns. Comes complete with a customised worksheet* of cyclic gait patterns for the user to re-create. Choose three from the following list to suit any laboratory or clinic:

Made in attractive colours to blend in with the decor of any nursery.

Congratulations Julie on the birth of William

* it doesn’t really – this bit’s a joke – but I was fascinated at the wide variety of gait patterns that we now have some form of kinematic data for (the mobile is real though!).

Clearing the air

Every so often I’m asked about why we tend to do clinical gait analysis barefoot and in AFOs (and shoes). One answer is that the barefoot condition tends to give a better indication of the full extent of a patient’s problems whereas walking in AFOs may be a better indication of how they function in everyday life. Another, however, is that sometimes walking in AFOs can help in identifying which particular impairments are having the most effect on gait. This was certainly the case when, a couple of weeks ago, I was reviewing one of the case studies we often use for teaching purposes but which exhibited features that I had not previously understood.

The analysis is of a seven year old girl with diplegic cerebral palsy (GMFCS III). She can take a few steps unaided but normally walks with a K-walker. We actually tested her in and out of the K-walker barefoot and in shoes and AFOs. the K-walker didn’t make that much difference to the kinematics with either condition so we’ll focus on the two unassisted walking conditions.

toe-walking-graphs

Perhaps the most obvious feature of the barefoot data is that she walks right up on her toes in considerable plantarflexion (feature c). The physical examination data shows that plantarflexor contractures (no passive dorsiflexion with knees extended beyond 10° plantarflexion ) can account for some of this but there are also signs of spasticity (from modified Tardieu and Ashworth tests). There is also, however, some suggestion of late (feature b) and reduced (feature a) knee flexion in swing. There is no clear explanation of this from the physical exam although there is a response to the Duncan-Ely test when performed quickly which might indicate some rectus femoris spasticity. Along with these specific findings the assessment indicates generalised weakness, persistent bilateral femoral neck anteversion and some mild tightness of the hip flexors.

The gait analysis with AFOs is quite different. The solid AFOs cast in a neutral position (which might have been assumed to be too aggressive given the physical examination) do appear to be holding the ankle in neutral  and substantially limit movement at the ankle (feature h).  The pelvis is a little more anteriorly tilted (feature d), possibly to move the centre of mass anteriorly as the new sagittal plane foot alignment will move the centre of pressure anteriorly (the steps were too short to get reliable kinetics). This would also exert a greater external extending moment at the knee which accounts for the hyperextension in late stance (feature g). The increased pelvic tilt leads to increased maximum hip flexion whereas the hyperextension pushes the knee back and maintains maximum peak hip extension. The overall effect is an increased range of movement at the hip (feature e). Perhaps most interestingly though, given that there is a question as to whether the rectus is spastic or not, is that peak knee flexion in swing is essentially normal (feature f). The slope of the knee graph through toe off is if anything a little steeper than normal. Such free flexion of the knee suggests that rectus spasticity is not a problem. Peak knee flexion is still delayed but this is clearly seen to be a consequence of the knee being too extended as it starts to flex in middle single support rather than of any stiffness. In summary, the data from the barefoot condition is inconclusive as to whether rectus femoris spasticity is contributing to the gait pattern but the data from the AFO condition provides quite strong evidence that it is not.

I hope that this has answered the question I posed at the beginning of this post but it does prompt another question – if there is no rectus spasticity then why is peak knee flexion so reduced in the barefoot condition?

I think the answer to this may lie in the observation that if a person is walking on their toes (and in plantarflexion) then it actually requires considerably less knee flexion for clearance in swing than in normal walking. In other words this girl may be showing reduced knee flexion in swing simply because she doesn’t need it when walking barefoot not because there is anything wrong with her knee function.In AFOs the ankle is held in neutral which makes clearance much more difficult and she has no option but to flex the knee more. It is interesting to note that when walking with shoes and AFOs she walks 20% slower than in bare feet and looks considerably less stable and fluent in her movements.

Rather than waste a lot of text in trying to explain why this occurs I’ve recorded a short video using Verne to illustrate that this is the case.

I go into the underlying concepts in relation to normal gait in this screen cast and have explored some of the other consequences of this for those walking in a more crouched gait pattern in this video blog.

First master’s students complete studies

Our school Progression Board met on Wednesday and formally approved the award of degrees for the first cohort of students to complete our new masters in clinical movement analysis. I’m sure there is a strong sense of achievement and satisfaction among the students. They’ve worked hard for three years, all of them balancing the requirements of studying alongside their day jobs working in gait analysis services in widely different locations. ~It would be great to post a picture of them all working together but of course they’ve all been studying by distance learning from their own location and the whole group has only ever met in cyberspace (although individuals have visited each other or met at conferences) so instead I’ll insert an advert for anyone who might want to apply for next year!

enrol 2016

During the first two years they worked through a programme of learning exercises drawing them into deeper understanding of clinical gait analysis and this year they have focussed on a research project of their own devising. The five projects have been:

  • How does arm swing change during walking in children with unilateral CP when an orthosis successfully alters foot strike pattern?
  • Effect of rounded bottom profile shoes on foot clearance in children with stiff knee gait.
  • A comparison of knee adductor moments from the Plug-in Gait model and a 6 Degrees of freedom model at  self-selected and slow walking speeds.
  • A cross sectional exploratory study to evaluate the validity of the Salford foot model in chronic stroke survivors.
  • A comparison of the repeatability of two different foot models at self-selected speed in healthy adults.

As well as the obvious success of the students, there has also been a strong sense of achievement for me and the other teaching staff. The programme has grown out of the CMAster project sponsored by the EU Lifelong Learning Programme and in partnership with KU Leuven and VU Amsterdam. We spent two years planning the programme and then three years delivering it so this week really marks the completion of a five year project. We’re all very proud of what we’ve achieved and Adam Shortland, our external examiner, is equally enthusiastic.

There is still time to apply to enrol for this year programme (to start in late September). Further details including how to apply can be found at this link. There is more information, including some material to support those considering enrolling at this site. The application period will end on 31st July and it can take a little time to get yourself sorted so if you are interested now is the time to take action.

CAN THE GROUND REACTION MOVE FOR YOU? UPDATE

This isn’t really a proper post, its just to remind you of the competition I started last week (which you can read about at this link if you missed it) and give you an update. I’ve had a number of entries so far from across three continents.

The answer to my original question – “is it possible to walk in such a way that the second peak of the ground reaction is substantially higher than the first?”– is clearly “yes, it is possible“. I’ll thus definitely be sending a copy of my book to someone.

I also asked for a description of how this has been achieved and I think there is still room for improvement. I’ve had some quite lengthy explanations but what I was really hoping for is a very simple explanation that pin-points what is going on in a small number of words. Ideally the explanation would have two fairly short sentence. The first would describe how the pattern was achieved, for example, “I allowed my knee to flex more than usual after initial contact and then tried to push forcefully into hyper-extension at the end of stance” (I don’t suppose this works – it’s just an example of the sort of sentence I’m after). Ideally it should be specific enough for me to replicate your results without having to look at your video.

I’m not giving an example of the second sentence because even an example would give some of the game away but I’d be looking for a biomechanical explanation of why doing this affects the ground reaction. Given that the ground reaction is a force, your explanation will almost certainly refer to how you have modified the movement of your centre of mass.

If you’ve entered but would like to modify your entry in the light of these comments then just send me another e-mail. Closing date still this Monday.

If engaging in this exercise (even just thinking about it) has made you think more seriously about enrolling yourself, or one of your staff, on the Masters in Clinical Gait Analysis by Distance Learning then you can find out more at this link. The programme is part-time and work-based and thus designed to be taken alongside your normal job in the laboratory where you normally work. There is no requirement to come to Salford at all.

You might also like to know that we are running a three day gait analysis course in Salford from 11th to 13th May this year. You can find out more at this link.