Solving the surgical bunion conundrum

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Please take a look at this great paper, from Dr. LaPorta and evaluate.

A new concept is proposed to evaluate bunion deformity and assist in surgical planning drawing on the work of Paley's principles of the mechanical axis. Application of the principle can assist the surgeon to know how much correction is required for a successful outcome, and likely also predict failure.

My question: is it a safe presumption to use Paley's principles, in this way?

Food for thought.

 

I don't think this line drawn on the radiograph represents the "mechanical axis" of the medial column or first ray.

 

Simon,

Thanks for your input. Your statement somewhat reflects my concerns. But, perhaps you can elaborate why you believe that to be the case?

 

Ignoring the 2D nature of the radiograph for now. Why should a line joining the base of the proximal phalanx and the talar head bisection represent the mechanical axis for the entire first ray? Just doesn't make sense. They've just picked some points and drawn a line through them! Let's face it, each of the joints along that segment should be capable of having it's own axis of rotation. Moreover, you can't find an axis in this way since biomechanical axes of rotations are artificial kinematic constructs, an axis (or instant centre of rotation) can only "exist" when there is motion occurring. So in the static radiograph, by definition there is no axis since no rotation is occurring.

Unlike the subtalar joint, I don't think the joints along this segment have enough constraint for us to deduce that the axial position will not be defined by the applied force- like the midtarsal joints. Indeed, look at the literature on the instant centre of rotation of the talo-navicular joint in isolation of the rest of the joints of the medial column...

 

Simon,

Your challenge, if I interpret correctly, is that the entire premise for mechanical axis construction, is wrong. However, this is based on the work of Paley. For that purpose it matters not the XR image is two dimensional, although CT scanning can provide a three dimensional perspective, but is not generally required for angular deformity appraisal. Paley's work is universally accepted, in the orthopedic realm of mechanical axis deviation and the pre-operative planning of deformity. LaPorta et al explain, in their article how this concept was used and then adapted for the foot.

The line, from the talus to the phalanx represents the medial column. Deviation away from this line represents deformity. The intervening segments are also represented. The CORA for first ray deformity is identified. The reference points allow the surgeon to know when the mechanical axis of the first metatarsal segments deviates away from that of the medial column. That is quantifiable entity, to know how much correction is required.

 

But it's not the mechanical axis, end of story. Rather, it's a line drawn on a radiograph. You can measure deviation away from this line, but this line is not likely to be the axis of rotation of the medial column. Perhaps it is a semantic issue.

The axis of rotation of the medial column is highly unlikely to be in a fixed position and will occur in varying spatial locations depending on the loading forces applied at any instant in time for a given locomotor task. There will also be inter subject variability in this.

Orthopedics by definition are obsessed with straightening things, the presumption being that straight lines are a good thing. You can draw a straight line connecting any two points you like on a radiograph and appraise angular deviations away from this line; this does not make the line you have drawn the "mechanical axis" it just makes it a line. Perhaps therein lies the difference between orthopedics and biomechanics.

I suspect that this really does run down to semantics. Your description above suggest the term "axis" is being employed in the mathematical sense as a boundary for a Cartesian coordinate system. However, the authors use of the term "mechanical axis" suggests something very different in a biomechanical sense.

 

Simon,

Thank you for your input. It is appreciated.

Semantics: perhaps so.

Currently the surgeon might (among other) employ a measurement of the angular displacement between the first and second metatarsal for pre-operative surgical assessment & planning. This practice has become entrenched in surgical decision making in the podiatric and orthopedic world. I have always found that to be a somewhat arbitrary exercise. Neither conventional method, nor Dr LaPorta's proposal, is an exact mathematical and/or biomechanical certainty. It is perhaps another practical tool (and possibly an upgrade to existing methods), for the surgeon, to hopefully produce a more predictable result.

Ultimately, practical application of the tool, and further applied analysis, together with long term outcomes measures can perhaps provide more clues, to know if this can offer validity.

Best wishes,