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 AxesThe 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 LengtheningInternal 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.
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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 RPMImplementing 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.
ConclusionsIt 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/22933497http://www.ncbi.nlm.nih.gov/pubmed/19685230http://www.ncbi.nlm.nih.gov/pubmed/12728034http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4302230/http://fischermedical.dk/wp-content/uploads/FITBONE-TAA-Surgical-technique.pdfhttp://www.ncbi.nlm.nih.gov/pubmed/15920419
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Topic: Reverse Planning Method: Maintaining & Correcting Mechanical Axis During LL (Read 25116 times)