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Author Topic: Reverse Planning Method: Maintaining & Correcting Mechanical Axis During LL  (Read 18634 times)

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maximize

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The correct alignment of the mechanical axis is a subject that should be important to all who are interested in leg lengthening. In this post, I will discuss how the mechanical axis of the legs can be maintained and even corrected during internal femur/tibial lengthening procedures with a technique called the Reverse Planning Method (RPM)  developed by Dr. Baumgart.

First, I will provide an overview of the mechanical and anatomic axis and the valgus (x legs) problem created by traditional internal femoral lengthening procedures. Then I will provide an overview of the RPM. I will explain in some detail how the RPM can be used during femoral and tibial lengthening procedures. Lastly, I will discuss the requirements for RPM and which surgeons do or don't seem to use this technique.

This is a long post, but it is a very important topic which I have not seen discussed on any LL forum. I hope you find it interesting and informative. I tried to keep the explanation as simple as possible, but it still might get a bit technical at times. If you have any questions or there are points I haven't been clear on, please let me know.

The Mechanical and Anatomic Axes

The two important axes of the leg are the anatomical axis and the mechanical axis. The anatomical axis runs along the pathway of the intramedullary canal of the femur. The mechanical axis is found by drawing a straight line through the femoral head and the midway point of the ankle. In a normally aligned limb, the mechanical axis will run straight through the middle of the knee, intersecting the medial tibial spine. If it does not, you either have varus (bow legs) or valgus (x legs) knees.



Problems with Traditional Internal Femur Lengthening

Internal femur lengthening devices will by their nature lengthen along the femur's anatomic axis. This can lead to shifts in the mechanical axis towards a valgus (x legs) position. Such shifts in the mechanical axis have been documented by Dr. Paley. Paley demonstrated that in normally aligned limbs, lengthening along the anatomical axis of the femur with internal Precise nails led to a lateral shift of the mechanical axis by 1 mm for each 1 cm of lengthening.

The following diagram from Paley's article demonstrates how this shift happens. Internal lengthening is compared in this picture with external lengthening, which can theoretically maintain the mechanical axis. In practice, however, we know that external monorails for femurs can lead to distortion and misalignments, as the monorail devices and pins can be subject to surgeon error and bending/warping with muscular forces and weight bearing.



Internal lengthening of the femurs is considered an ideal method of leg lengthening for most because it is fast, safe, predictable, and effective. However, this expected shift of the mechanical axis is a concern for many of us, because any degree of varus/valgus can predispose joints to arthritis. If internal femoral lengthening is then to be considered a good method of lengthening, a solution is needed to maintain the mechanical axis in normal alignment.

Dr. Baumgart has published such a solution, called the Reverse Planning Method (RPM). This method completely addresses the issue of axial shift during internal leg lengthening, allowing internal lengthening to be used to not only maintain ideal mechanical axis, but to correct abnormalities in the mechanical axis as well. Overall, I think it is a really smart and intuitive approach.

In brief, the method involves: (1) identifying abnormalities in the existing femoral/tibial anatomy if any are present, and then (2) predetermining based on existing abnormalities and what amount of lengthening is desired exactly how the two bone segments should be positioned and nailed together during surgery so that when lengthening is done, an ideal mechanical axis will result.

The Reverse Planning Method (RPM)

The RPM starts with standing full length xrays, ie. long standing radiographs (LSR). A determination is made from these radiographs whether any abnormality is present in the existing mechanical axis of the limb. The mechanical axis is drawn from the femoral head through the center of the ankle as usual. If the axis is perfect, with the line going through the medial spine of the tibia, it must simply be preserved by surgery. If a varus or valgus alignment is identified, this should be corrected during leg lengthening.

In cases of pre-existing misalignments (varus/valgus), the next step is to determine whether the problem originates from the femur, tibia, or both. To do so, the the medial proximal tibial angle (MPTA) is assessed. This is the angle created by the plane of the tibial plateau (knee joint) intersecting the path of the tibia's intermedullary canal. The MPTA should be 85-90°. If the MPTA is 85-90° (normal), it means the tibia is well formed. If the MPTA is abnormal (<85° or >90°), it implies a tibial deformity and thus a tibial correction is indicated.

To assess femoral abnormalities, the mechanical lateral distal femoral angle (mLDFA) is assessed. This angle is formed by the plane of the medial-lateral condyles of the femur intersecting with the plane of the mechanical axis. Again this should be 85-90°. If it is not, it suggests an abnormality to the femur which should be corrected.



The next steps depend on which bone segment you will be operating on and via what approach. For intramedullary nailing, femurs can be entered from the hip (anterograde) or from the knee (retrograde). Tibias can only be entered from the knee (anterograde). I will review each of the three approaches, though you will see they all follow the same basic principle. I will only cover varus/valgus corrections, although this technique allows for torsional and anteroposterior (front to back) deformity correction as well.

If the figures that follow are too big to load fully in your window, you can click them to load them in a new window.

Antegrade Femoral RPM:



To perform RPM limb lengthening in cases with an abnormal femur, one must next identify the center of rotation of angulation (CORA) for the deformity. This is the point at which the bone "bends". The CORA is marked for a femoral deformity in figure 1a. An osteotomy level (OL) is chosen to be as close to the CORA as is reasonably possible (1c). In addition, at this stage, the ideal position of the center of the hip (CH) is marked at where it would be expected to be if the mechanical axis was perfect after lengthening is done (1c).

Next, a tracing is performed of the upper femoral segment (from femoral head to osteotomy line) on a second sheet of paper (1d). The intramedullary nail is drawn to scale within this segment. This tracing is then superimposed on the original radiograph so that the center of the femoral head (CH) lines up with the ideal axis. The nail should creates a gap equivalent to the desired amount of lengthening (1f). At this point, it is important to make sure the nail enters the distal femoral segment at a workable position and angle. If for any reason it does not, the osteotomy level must be adjusted and the prior steps repeated. The mechanical lateral proximal femoral angle (mLPFA) as shown above is now assessed as well, and should be in the corrected limb around 90° (1f, 1h).

At this point, 1f shows the alignment of the bony fragments after limb lengthening is complete. We must then go backwards as shown in figure 1g, by sliding the proximal segment tracing down along the axis of the nail until the bone segments meet. This shows the alignment of the bones that must be attained at the time of surgery and nail implantation. If all planning has been correct, and the surgery is successful at replicating this alignment, when lengthening is done, the femoral head will be exactly where you intend it to be, and the knee/leg will be perfectly aligned.

The same process of RPM applies in cases where there is a normal pre-operative axis, except that the procedure is simpler as there is no deformity to correct for.

Retrograde Femoral RPM:

The retrograde approach (from the knee up) for femoral lengthening is much less common in CLL. However, it is indicated in cases where deformities of the upper end of the femur present obstructions to anterograde nailing. Because you are entering from the level of the knee, the precise entry point needs to be very carefully considered and controlled. Dr. Monegal has said he likes this approach when indicated as I believe he said he finds it gives him the most axial control. Recent controlled animal studies however have shown patellofemoral degeneration and arthritis as a result of retrograde femoral nailing, so I suspect anterograde approach should still be preferred for most of us.



Retrograde femoral lengthening is done according to RPM in the same way as retrograde. The level of deformity (CORA) is identified if a femoral deformity is present (1a). The osteotomy line is determined in proximity to any such deformity (1c). The ideal position of the femoral head after lengthening is marked (1c). The upper femoral segment is traced with the nail drawn to scale (1d), and then superimposed over the original radiograph in an ideal alignment (1e). The nail must be aligned at this stage with the femoral notch where it will enter at the level of the knee (1e). Again, in 1f, the segments are lastly brought together to illustrate the ideal alignment/angulation of the nail and bone segments at the time of surgery. If this alignment is attained intraoperatively, when lengthening is complete, the mechanical axis will again be perfect.

Anterograde Tibial RPM:



Tibial RPM follows the same principles as femoral RPM. First the level of the deformity (CORA) is identified within the tibia if any deformity is present, and along with it, the best osteotomy line (OL) is marked (fig 1a, 1c). Next the ideal position of the center of the ankle (CA) after lengthening is done is drawn (1c). The distal fragment of the tibia from the osteotomy line down is traced and a nail is drawn to scale within it (1d). This tracing is superimposed on the original radiograph so that the nail lines up with the entry point at the knee into the tibia and the ankle lines up with its ideal position along the mechanical axis (1e). Again, this segment is then slid back until the bone fragments meet (1f). This position indicates the ideal alignment of the bone fragments during nailing, so that if attained intraoperatively, lengthening will result upon completion in a perfect mechanical axis.

Requirements for RPM

Implementing the RPM during leg lengthening requires several factors. Perhaps the most important is that the reamers (devices used to clear a path through the medulla of the bones) used must be rigid so that the path cleared is very controled and intentionally made. This is particularly important when it comes to reaming the second piece of bone which, as shown above, is usually angled relative to the first for corrections. Dr. Monegal has suggested that while the Fitbone system is designed with rigid reamers expressly for this reason, non-Fitbone nail systems are generally implemented with flexible reamers. Such reamers will follow the path of least resistance along the curving path of the medulla through the bone, and they will not be suitable for RPM.

Furthermore, to maintain precision during reaming, Dr. Baumgart and the official Fitbone protocol suggest reaming under image enhancement fluoroscopy (real time xray) in two dimensions. This means front to back view, and lateral side view. To better interpret the results from this fluoroscopy, Dr. Baumgart suggests placing some small Schanz screws in the bone for markers at the start of surgery. These can then be used to measure and align the angles of the two bone fragments. Additionally, he suggests performing the surgery on a radiolucent table with a grid of radiopaque strips to better allow the surgeon to judge alignments from the fluoroscopy.

Lastly, implementing the RPM requires a predetermination of the amount of lengthening to be done. All of the alignments procedures discussed above require you to know exactly how much distraction will be performed. It is not ideal to "wait and see" how much you lengthen based on how you feel, as the angles of the bone fragments determined by RPM will change depending on the expected length of distraction.

As a safety point generalizable to all internal leg lengthening, Dr. Baumgart also suggests putting a small hole to vent the osteotomy site before reaming. This will allow any fat to escape at that point rather than build up with pressure inside the bone during reaming. This reduces risk of fat embolism.

Who Does RPM?

Dr. Monegal suggests that all Fitbone surgeons use this technique, and they meet yearly to discuss the technique and refinements they can make to it. Dr. Monegal again suggests this is possible because the Fitbone system ships with rigid reamers which allow controlled reaming along any axis. On the other hand, he suggests this is not the case for other systems, and this is partly why he prefers the Fitbone model.

I am guessing Dr. Paley does not use this technique with Precise, based on the 1 mm shift in axis for each 1 cm of internal femoral lengthening he has described.

Dr. Guichet is a peculiar case, as although there is no evidence he uses RPM, he has published study of 31 patients of his who had femoral lengthening with Albizzia nail lengthening average 3.4 to 6.3 cm, and although he states he was expecting to see mechanical axis deviation, none was actually seen in these cases. He does not offer an explanation for why this might be. I wonder if all the manual racheting combined with full weight bearing might have helped keep things straight somehow.

I am also not sure what Betz does. I understand he previously worked with Fitbone before developing his own Betz nail, so maybe he has carried the principles of RPM to his new approach.

I would like to inquire with these three surgeons on the subject of RPM and flexible vs. rigid reamers. I'm curious what their opinions are.

Conclusions

It seems apparent to me that the Reverse Planning Method should be considered standard of care for all internal CLL. It outlines how to do internal CLL safely while producing a normal mechanical axis. Hopefully, if more people are aware of it and inquiring about it with their surgeons, it will only become more widespread and CLL will become safer and more effective for everyone. I hope this summary has made sense on explaining the basic principles of this approach.

The primary remaining question I have about this methodology is what the expected margin of error is. No procedure is 100% perfect, and although fluoroscopy can allow good guidance of the surgery, it still relies on the surgeon's ability to judge on the spot whether the alignment intraoperatively matches the planned alignment that they are aiming for. Nailing and reaming additionally are not perfectly controllable processes, and I wonder how physically the bony fragments are aligned and immobilized to ensure the process is as accurate as possible.

Dr. Monegal has suggested he will be sending me some further information on the surgery. I am eagerly awaiting these materials and will update the thread with anything further of note I learn about this process.

Any questions or comments are welcome.

References:
http://www.ncbi.nlm.nih.gov/pubmed/22933497
http://www.ncbi.nlm.nih.gov/pubmed/19685230
http://www.ncbi.nlm.nih.gov/pubmed/12728034
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4302230/
http://fischermedical.dk/wp-content/uploads/FITBONE-TAA-Surgical-technique.pdf
http://www.ncbi.nlm.nih.gov/pubmed/15920419
« Last Edit: June 04, 2015, 07:09:19 AM by maximize »
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Metanoia

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Hi, very informative post. Thank you.
Just for your info Betz and Baumgart went separate ways long time ago. Betz definitely does not use Baumgart's method. In general he doesn't do much planning for the surgeries, he is more working like a traumatologist. And of course he doesn't give a s*it about his patient's deformities.
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Taller

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Maximize, I want to thank you for all of the insightful and informative posts you've shared on this forum in the last few days. They've all been of great quality; quite helpful and thought-provoking. Your presence is really valued here.
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just_me

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Very intereseting reading - Good work maximize.
About the second picture from the top under the headline "Problems with Traditional Internal Femur Lengthening": It is not possible to see the whole picture - It is too big. Is it possible for you
somehow to correct it so we can see the whole picture?
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maximize

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Thanks for the feedback, guys. Glad you liked it.

I can no longer modify my original post, so here's a direct link to that image: Problems with Traditional Internal Femur Lengthening.

Uppland

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Maximize, I want to thank you for all of the insightful and informative posts you've shared on this forum in the last few days. They've all been of great quality; quite helpful and thought-provoking. Your presence is really valued here.

I think this is what we all want to say, thanks maximize.

On another note, can anyone contact Dr Guichet and ask if he does RPM? I'm thinking about mailing him myself but I've got a lot of stuff to ask about so It'll take a few days.
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just_me

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Yes, Doctor Monegal seems like very knowledgeable about legalignments - I guess he must be doing a lot of legalignment and reconstructive surgeries. It is very good to know if you have legdeformity issues.
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theuprising

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I would be very annoyed if I were a Paley, Guichet or Betz patient and did femurs. Their method looks likely to cause problems especially those who did over 5cm. This is definitely the future of femur lengthening. Great topic.
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heightangel

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This method is good in theory, but in practice it doesn't work sometimes. Not all patients operated on by these doctors end up aligned. Most end up worse. You must have a lot of experience to do it correctly. I would only trust Baumgart himself but he is so expensive.
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KiloKAHN

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Would a 6 or 7 mm shift in mechanical axis be significant enough to cause osteoarthritis though? If it is, osteotomy can be done on each femur to correct it though, so it's not hopeless if you've done internals without the rpm.

Curious about that study Paley is releasing in September that is supposed to address this.
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Surgery performed by Dr Mangal Parihar at Mangal Anand Center for Limb Lengthening & Reconstruction. http://www.limblengtheningforum.com/index.php?topic=634.0
Pre-surgery height:  164 cm (Surgery on 6/25/2014)
Post-surgery height: 170 cm (Frames removed 6/29/2015)

theuprising

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The Paley study should be very interesting in that if he states that alignment issues are insignificant then it looks like someone developed a redundant method.

Can someone explain why a person would have rpm on their tibia considering all lengthening takes place along the mechanical axis anyway?
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Dr Monegal

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Dear Uprising

You are Right as the mechanical and anatomical axis are the same in normal tibiae.
It is not in femurs thought.

What is most important is to make allignment tests before Planning as you can see many different deformities as well in tibiae that should be considered and studied in both AP and Lateral views to prevent malalignment and knee ROM.

Metaphisis is full of cancellous bone and due to distraction It migth cause deviations during lengthening. Sometimes poller or guide screws are needed in order to keep allignment during the distraction/consolidation periods.

On the other hand, tibia Allignment becomes tricky When doing externals for 2 reasons:

1- distraction is applied parallel to the mechanical axis
2- progressive stifness of the callus might cause bending/losening of the pins

For that reason It is quite common to ser tibia valga or femur varus deformities after LL with Externals.

Excellent point

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maximize

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I would be very annoyed if I were a Paley, Guichet or Betz patient and did femurs. Their method looks likely to cause problems especially those who did over 5cm. This is definitely the future of femur lengthening. Great topic.

One of the reasons I think it's important to raise the profile of this issue within the LL community is that although the reverse planning method was developed by a Fitbone affiliated surgeon (Dr. Baumgart), it is not proprietary to the Fitbone process. Paley, Guichet, and Betz could all integrate this methodology into their surgical approaches. They would just have to work to revise the reaming systems they use, so that they can do both flexible and rigid reaming. No changes would be needed to the actual nails, and any competent LL orthopedist could make the necessary adjustments in technique with the right tools.

Would a 6 or 7 mm shift in mechanical axis be significant enough to cause osteoarthritis though? If it is, osteotomy can be done on each femur to correct it though, so it's not hopeless if you've done internals without the rpm.

Curious about that study Paley is releasing in September that is supposed to address this.

We know from observational studies that every single degree of valgus or varus your knees have increases your risk significantly of lateral (valgus) or medial (varus) compartment osteoarthritis. Whether each individual will develop osteoarthritis though is impossible to predict. Valgus deformity induced by lengthening along the femoral anatomic axis can be corrected by distal femoral opening wedge osteotomy as discussed in the previous thread, but it is a significant surgery requiring large incisions and the insertion of large plates along the distal femur. Most would probably live with a few degrees/mm of valgus rather than go through that.

I'm also curious what Paley has pending for publication. I highly suspect it will be a short term study on his patients showing normal knee/ankle ROM and function 1-5 years after internal Precise LL despite the expected valgus deviation.

From my perspective, a perfect mechanical axis should always be our goal at the end of LL. I don't think anything can change that.

Can someone explain why a person would have rpm on their tibia considering all lengthening takes place along the mechanical axis anyway?

RPM would be relevant for the tibias if you are starting with tibial deformities. For example, a lot of normal people have a bit of varus due to mild bowing of the tibia. If such a person wants to get CLL, RPM to the tibias could give you length while correcting this at the same time.

tibia Allignment becomes tricky When doing externals for 2 reasons:

1- distraction is applied parallel to the mechanical axis
2- progressive stifness of the callus might cause bending/losening of the pins

For that reason It is quite common to ser tibia valga or femur varus deformities after LL with Externals.

We've seen more than one of the femur varus deformities even on this site just like you describe with femoral monorails. That's why I don't think they're a good option. In theory though, external tibia LL should I think be very able to maintain the mechanical axis if done by a good surgeon and with a full circumferential frame. For example, using the Taylor Spatial Frame or Hexapod on tibias with a competent surgeon, a good alignment should absolutely be expected. The only problems with the external tibia approach as KiloKAHN can I think attest are it's painful, debilitating, and slow.

Thanks as always for posting, Dr. Monegal. As we discussed, if you can forward any materials regarding the practical aspects of alignment/immobilization during reaming/nailing with this approach, as well as margin of error, I'd very much appreciate it. Please feel free to e-mail me anything you think might help with understanding this process.

cherry

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Thanks to thz great info              ..cherry
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cherry

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Hi tall .ur senior .i hv doubts .i want to do internel ....which metjod is safe .and which method u did...cherry
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KiloKAHN

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@Maximize:

What do you think of Dr Parihar's response regarding that study by Paley/Herzenberg about mechanical axis shift?

Quote
LL Forum: In this article by Drs Paley and Herzenberg (http://www.ncbi.nlm.nih.gov/pubmed/22933497), it said that for internal femoral lengthening "there is a lateral shift of the mechanical axis by approximately 1 mm for every 1 cm of lengthening". Wouldn't this mean that internal femur lengthening has a side effect of causing valgus deformity and potentially osteoarthritis of the knee for putting increased pressure on one side of the knee joint? If so, can the nail be inserted in such a way so as to reduce this lateral shift of the mechanical axis?

Dr Parihar: Lot of potential problems with that article. It's a good hypothesis, but far from being proven yet. There are other articles that have shown the opposite (that there is no real change in the mechanical axis.)
        a. They did not correlate the length achieved with the amount of axis deviation. If there is a cause-effect relationship, one should be able to show a positive correlation.
        b. One patient actually moved in the opposite direction (medial axis deviation).
        c. They state in the article that "Because of the potential for errors in measurement or radiological magnification, the data were analysed by considering a total shift in mechanical axis deviation of ≤ 2 mm to be inconsequential. With this assumption, further analysis of these 26 limbs showed that 15 limbs had an insignificant total lateral change in mechanical axis deviation of ≤ 2 mm”. i.e. >50% of the limbs did not have a significant change in the mechanical axis.
        d. The correct comparison would be the immediate postoperative Axis, and the 6 month postoperative axis - because the surgery itself (osteotomy and insertion of the nail) may change the axis.

http://www.limblengtheningforum.com/index.php?topic=2783.msg43784#msg43784
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Surgery performed by Dr Mangal Parihar at Mangal Anand Center for Limb Lengthening & Reconstruction. http://www.limblengtheningforum.com/index.php?topic=634.0
Pre-surgery height:  164 cm (Surgery on 6/25/2014)
Post-surgery height: 170 cm (Frames removed 6/29/2015)

applesandoranges

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I believe that Dr. Paley has a new article coming out which puts the mechanical axis deviation issue to rest. Would like to check that out for sure. If there's no problem with the deviation then that's definitely another advantage to do femur LL over Tibia, especially if it's only like 3-4cm which is fine for me.
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maximize

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@Maximize:

What do you think of Dr Parihar's response regarding that study by Paley/Herzenberg about mechanical axis shift?

Those are all valid points. My thoughts are as follows.

The anatomic axis of the femur is 5-7 degrees deviated from the mechanical axis. Thus a shift in the mechanical axis is mathematically required and expected by lengthening along the anatomic axis (femur). However, I think I am coming to believe that current methods of internal LL are slightly inaccurate by nature, and so what you get will vary in practice. Furthermore, xrays are slightly inaccurate by nature as well, and if you are trying to measure down to the mm, even subtle variations in how close the patient is standing to the film or their exact foot position may throw it off.

I think it is a combination of those two factors that obscure the shift we expect to see from these surgeries.

Regarding surgical variations, you can view the Precise operative technique here:
http://ellipse-tech.com/wp-content/uploads/2014/06/preciceproductpage/p2femuroperativetechniquelc0083a.pdf

There are many moments where a slight variance in the expected pathway of lengthening could be introduced and those small variances could dwarf the expected small mechanical axis shift. A lot of what I'm about to say is speculation on my part as I've never talked to a leg lengthening surgeon about this or been present to watch one. However the principles are general to orthopedic surgery.

When you ream the canal, the exact point of entry is very important. Additionally, the ream suggested is flexible and will follow the "path of least resistance" through the bone. The exact shape and direction the canal that is created will follow will therefore not be 100% predictable down to the mm. Everyone's femurs will allow a slightly different pathway.

Next when you create the osteotomy, the distal segment now becomes "loose" and unfixed relative to the proximal segment. Even a slight shift or rotation in this segment as the nail is inserted through it could now throw things off in unexpected but small ways.

Then when you screw (fix) the lengthening nail into place, there is a small bit of shift that could be anticipated depending on how snug the lengthening nail is within the reamed canal, how weak the walls of the reamed canal are, and what forces are applied as the fixation screws go into the bone and add lateral compression forces.

Here is an example of a Precise internal lengthening xray:



You can see that the nail runs a little "lateral" at the top through the femur and then ends up more "medial" at the bottom where it is fixed. ie. It is not the perfect expected pathway you would predict if you were drawing the nail over the original preop xray. Other xrays of internal lengthening will often show other similar slight variations depending on the case.

Dr. Parihar is absolutely correct - the only way to truly prove that leg lengthening shifts the axis would be to do xrays immediately after surgical fixation and then again after lengthening. Doing so would take all the small variances I just described out of the equation as once fixed, we expect the nail should no longer shift. However, this is honestly unnecessary in my opinion as we know a small predictable shift is mathematically inevitable.

Dr. Guichet published a list of cases where he showed no statistically significant trend of deviations in the axis. Paley's cases show no obvious major trends either. But a deviation is an expected mathematical outcome. Errors in xray and surgical procedure may obscure this outcome. However, i can see no reason not to plan for it to ensure it is as minimal as possible, and that is what the reverse planning method is about. There is no downside for a surgeon to use it, and it applies to all devices and surgical approaches.

184dream

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Re: Reverse Planning Method: Maintaining & Correcting Mechanical Axis During LL
« Reply #18 on: December 12, 2020, 09:13:36 AM »

anyone why this method still not applicable ?
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