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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 7  |  Issue : 1  |  Page : 13-18

Femur deformity correction planning in sagittal plane using mechanical axis


1 Department of Orthopaedic, Saint Petersburg State University Hospital; Department for Bone Pathology, H. Turner National Research Center for Children's Orthopedics and Trauma Surgery, Saint Petersburg, Russia
2 Department injuries and their consequences treatment, Ministry of Health of Russia, Vreden Russian Research Institute of Traumatology and Orthopedics; Department of General Surgery, St. Petersburg State University, Saint Petersburg, Russia
3 Department of Technical, “Ortho-SUV” Ltd, Russia

Date of Submission18-Apr-2021
Date of Decision13-Jun-2021
Date of Acceptance17-Jun-2021
Date of Web Publication30-Jun-2021

Correspondence Address:
Dr. Victor A Vilenskiy
Department of Orthopedic, #3, Saint-Petersburg State University Hospital, Saint-Petersburg
Russia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jllr.jllr_14_21

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  Abstract 


Recently, deformity correction planning using reference lines and angles is a standard procedure. At the same time, both anatomical and mechanical axes are used for the frontal plane. However, until nowadays, there was no algorithm for planning femoral deformity correction in the sagittal plane based on the mechanical axes of the bone fragments. The aim of this study was to develop a method of femur deformity correction planning in the sagittal plane based on mechanical axes. Materials and Methods: On the basis of computer tomography data of 23 adults with nondeformed femurs, we measured the angular relationships between the anatomical axis of the proximal femur in sagittal plane and mechanical axis (∠maj) and the angular relationships between the femoral neck axis and mechanical axis in sagittal plane (∠laj). Results: It was found that ∠maj was 16.0° ±7.6°, and ∠laj was 10.2° ±2.4°. Based on the data obtained, we developed a method for planning the correction of femoral deformities in the sagittal plane. According to this method, the mechanical axis of the proximal bone fragment can be determined using any of three options: (1) “Joint line based.” In this method, the proximal joint line of the femur is drawn, then from the center of the femur head with the angle 85° (mean value of mPPFA) to this line is drawn a line that is mechanical axis; (2) “Femoral neck based.” The neck axis is drawn first, then from the center of the femur head with the angle 16° (mean value ∠maj) to this line is drawn a line that is mechanical axis; and (3) “Anatomical axis based.” First, the anatomical axis of the proximal femur is drawn, then a line parallel to the anatomical axis from the center of the femoral head is drawn, then a line at an angle of 10° (mean value ∠laj) to it is drawn, that is the mechanical axis. Determination of the mechanical axis of the distal fragment in sagittal plane is made by the following: the distal joint line is drawn and divided into five equal segments. Then, the point located on the border of the front 2/5 and the rear 3/5 of the segment is found. From that point, at an angle of 81° (mean mPDFA), a mechanical axis of the distal femur fragment is drawn. The intersection of the mechanical axes of the proximal and distal fragments defines the apex of the deformity. Conclusion: The proposed method for planning deformity correction based on mechanical axes for the sagittal plane complements the existing planning methods for the frontal plane and improves the quality, namely the accuracy of preoperative planning.

Keywords: Deformity correction planning, mechanical axis-based planning, sagittal plane femur deformities, three dimensional planning


How to cite this article:
Vilenskiy VA, Solomin LN, Utekhin AI. Femur deformity correction planning in sagittal plane using mechanical axis. J Limb Lengthen Reconstr 2021;7:13-8

How to cite this URL:
Vilenskiy VA, Solomin LN, Utekhin AI. Femur deformity correction planning in sagittal plane using mechanical axis. J Limb Lengthen Reconstr [serial online] 2021 [cited 2021 Dec 8];7:13-8. Available from: https://www.jlimblengthrecon.org/text.asp?2021/7/1/13/320028




  Introduction Top


Recently, deformity correction planning using reference lines and angles (RLA) is a standard procedure.[1],[2],[3] At the same time, both anatomical and mechanical axes are used for the frontal plane.[1],[4] Most surgeons prefer planning using mechanical axes that is proved by the goal of deformity correction – to restore the common mechanical axis of the limb and the angles formed by its intersection with the joint lines.[4]

The algorithm for finding the apex of deformity (fulcrum) in the frontal plane is known. It is necessary, on the basis of RLA, to determine the axes of the proximal and distal fragments. The intersection of these lines is the apex of the deformity. At the same time, at least three methods for identification of the mechanical axis of the proximal femur fragment in the frontal plane are known: on the basis of the proximal femoral joint line, on the basis of the femoral neck axis, and on the basis of the anatomical axis of the femur.[1],[4] The use of each of the three methods has its positive aspects and indications.

Recently, it is known at what point and at what angle the mechanical axis intersects the joints lines of the femur and tibia in the sagittal plane[5] [Figure 1]. However, until now, there is no algorithm for planning femoral deformity correction in the sagittal plane based on the mechanical axes of the bone fragments. And for this, first of all, it is necessary to develop methods for determining the mechanical axes of the proximal and distal bone fragments in the sagittal plane.
Figure 1: Mechanical reference lines and angles for the sagittal plane: (a) General scheme, where mPPFA is the mechanical posterior proximal femoral angle, mPDFA is the mechanical posterior distal femoral angle, mPPTA is the mechanical posterior proximal tibial angle, mADTA is the mechanical anterior distal tibial angle; (b) points of intersection of the mechanical axis with the distal joint line of the femur and the proximal joint line of the tibia

Click here to view


Thus, the AIM of our study is to develop a method of femur deformity correction planning in the sagittal plane based on mechanical axes.


  Materials and Methods Top


The study included data from 23 volunteers (23 limbs) aged 18 to 65 years (36.4 + 15.5), for whom the study was done simultaneously for two legs, to assess the deformity of one of them. Fourteen out of 23 volunteers (61%) were female. This group of patients was previously analyzed to determine mechanical RLA for the sagittal plane[5] and the methodology of this study was generally similar. As in the previous study, the criteria for inclusion in the study were:

  1. Age over 18 years
  2. Absence of deformity of the femur in the frontal and sagittal planes, confirmed by radiographs by RLA assessment
  3. Absence of the torsion component of the deformity according to the computed tomography data.


The absence of deformity was confirmed according to the standard procedure.[1] In this case, the following RLA were measured: mechanical lateral distal femoral angle; mechanical medial proximal tibial angle; mechanical lateral distal tibial angle; anatomical medial proximal femoral angle; anatomical distal lateral femoral angle; anatomical medial neck–shaft angle; and anatomical posterior distal femoral angle (aPDFA).

Tomograms in these patients were obtained using a Toshiba Aquilion 64 Computer Tomography (Toshiba Medical Systems, Japan). The step of the slices was 1 mm. The absence of flexion or hyperextension in the knee joint (0/0/0) was assessed by three-dimensional (3D) reconstructions of tomography data by drawing lines along the anterior cortical layer of the femur in the lower third and the tibia in the upper third.[3] To confirm the absence of torsion, the technique proposed by Strecker et al. was used.[6]

The correspondence of all the listed parameters to the range of known normal values indicated the absence of deformity in the frontal and sagittal planes.

To analyze the computer tomography data, the Radiant Dicom Viewer 2020.1 software (Poznan, Poland) was used. A 3D reconstruction of the lower limb was viewed on the monitor screen from its internal surface.

This study consisted of two parts. In the first, we developed various options for the identification of the mechanical axes of the femur in the sagittal plane. In the second part of the study, on the basis of the data obtained, we developed a method for planning the correction of femoral deformities in the sagittal plane using mechanical axes.

Thus, in the first part of the study, we measured:

  1. The angle between the axis of the femoral neck and the mechanical axis;
  2. The angle between the anatomical axis of the proximal femur fragment and the mechanical axis.


For this, the following reference points were set:

  1. (a) Center of the femoral head [Figure 2]a. Using the ellipse tool, a circle was created with a diameter equal to that of the femoral head and its center was determined;
  2. (b) Anterior point of the distal joint line of the femur in lateral view. It is assumed that it corresponds to the place where the articular cartilage ends along the anterior surface of the femur [Figure 2]b;
  3. (c) Posterior point of the distal joint line of the femur. It is assumed that it corresponds to the place where the articular cartilage ends along the posterior surface of the femur [Figure 2]b;
  4. (j) – The point of intersection of the mechanical axis and the distal joint line of the femur [determined according to the scheme shown in [Figure 1]b;
  5. (m) – The midpoint of the femoral neck [Figure 2]c.
Figure 2: Mechanical axis of the proximal femur: (a-c) Reference points; (d-e and g) drawing of the lines; (f and h) angles identification (explanations are in the text)

Click here to view


Then, the following lines were drawn:

Line am – the axis of the femoral neck for lateral view [Figure 2]d;

Line aj – mechanical axis of the femur for lateral view [Figure 2]e;

Line al – a line parallel to the anatomical axis of the proximal femur fragment for lateral view [Figure 2]g;

After that, the following angles were measured [Figure 2]:

  1. ∠maj [Figure 2]f – The angle between the axis of the femoral neck and the mechanical axis;
  2. ∠laj [Figure 2]h – The angle between the line al and mechanical axis.


The results of investigation were processed using parametric analysis methods. To determine compliance with the normal distribution, the Shapiro–Wilk test was used. In the obtained variation series, the arithmetic mean values (M) and standard deviations were calculated.

The study was approved by the Institutional Local Ethics Committee.


  Results Top


The following results were obtained [Figure 3]. ∠maj was 16.0° ±7.6°, and ∠laj was 10.2° ±2.4°. It should be noted that the determination of ∠maj was impossible in two cases due to poor visualization of the femoral neck.

Based on the data obtained, we developed a method for planning the correction of femoral deformities in the sagittal plane. According to this method, the mechanical axis of the proximal bone fragment can be determined using any of three options:
Figure 3: Results of the study: (a) Obtained angular relationships of the mechanical axis of the femur for the sagittal plane and a line parallel to the anatomical axis of the proximal femur fragment (∠laj); (b) obtained angular relationships of the mechanical axis of the femur for the sagittal plane and the axis of the femoral neck (∠maj); (description in the text)

Click here to view


  1. “Joint line based.” Point (b) is placed in the projection of the apex of the greater trochanter then the line ab is drawn, that is the proximal joint line of the femur for the sagittal plane [Figure 4]a. From point (a) at an angle of 85° (mean value of mPPFA), a line is drawn, which is the mechanical axis of the proximal femur fragment for the sagittal plane;
  2. “Femoral neck based.” Draw a line am connecting the center of the femoral head and the middle of the neck [Figure 4]b. From point (a) at an angle of 16° (mean value ∠maj), the mechanical axis of the proximal femur fragment is drawn for the sagittal plane;
  3. “Anatomical axis based.” Draw the anatomical axis of the proximal femur, and then draw a line al parallel to the anatomical axis from the center of the femoral head [Figure 4]c. Then, from point (a) at an angle of 10° (mean value ∠laj), a mechanical axis of the proximal femur fragment is drawn for the sagittal plane.
Figure 4: The scheme for determining the mechanical axis of the proximal femur, three options: (a) “joint line based;” (b) “femoral neck based;” and (c) “anatomical axis based” (explanations in the text)

Click here to view


The mechanical axis of the distal femur in the sagittal plane is determined by the following way. Measure the length of the distal joint line of the femur cd. Segment cd is divided into five equal segments. Then, find the point (j) located on the border of the front 2/5 and the rear 3/5 of the segment cd. From point (j), at an angle of 81° (mean mPDFA), a mechanical axis of the distal femur fragment for the sagittal plane is drawn [Figure 5]. The intersection of the mechanical axes of the proximal and distal fragments defines the apex of the deformity [Figure 6].
Figure 5: The scheme of the distal femur mechanical axis step-by-step construction (explanations in the text)

Click here to view
Figure 6: The scheme for the femur deformity correction planning in sagittal plane: (a) Mechanical axes of the proximal and distal fragments of the femur are drawn, their intersection is the apex of the deformity; (b) the femur is cut at the apex of the deformity, and the axes are aligned

Click here to view


As shown Fig. 6. The scheme for the femur deformity correction planning in sagittal plane: a - mechanical axes of the proximal and distal fragments of the femur are drawn, their intersection is the apex of the deformity; b - the femur is cut at the apex of the deformity, and the axes are aligned

As shown in [Figure 7], a case of planning and femur deformity correction in the sagittal plane based on mechanical axes is presented.
Figure 7: Clinical case, 8-year-old girl, with meta-epiphyseal dysplasia, procurvature of the left femur, shortening 6 cm: (a) On the radiographs, the axis of the proximal femur fragment was determined by the femoral neck based method, the axis of the distal femur fragment was determined, the apex of the deformity is found; (b) deformity correction was planned according to the skiagrams, taking into consideration the shortening of 6 cm; (c) the X-ray result of deformity correction after the frame removal

Click here to view



  Discussion Top


This study is based on knowledge about the mechanical axis of the femur in the sagittal plane and the angles formed by its intersection with the proximal and distal joint lines.[5] The proposed method of deformity analysis and planning of its correction is similar to that used for planning correction of femoral deformities in the frontal plane,[1],[2] which has become routine over the past 20 years.

Until now, for the sagittal plane, there was only one planning algorithm – based on anatomical axes.[7] Thus, there was an internal contradiction: In cases when the planning for the frontal plane was made using mechanical axis and for the sagittal plane using anatomical, this led to the fact that the apex of deformity in the AP and lateral views might not coincide. The surgeon was faced with a difficult choice: at the level of which of the determined apexes is it preferable to perform the osteotomy? Moreover, when planning only along the anatomical axes in the frontal and sagittal planes, the indicated mismatching was also noted, which is quite explainable by the fact that the anatomical axis of the femur in the frontal plane is a straight line, in the sagittal plane it is an arch.

Why had the orthopedic surgeons neglected the obvious necessity of using mechanical axes for planning in the sagittal plane? There are several answers to this question. First, it was argued that deformities in the sagittal plane are better tolerated by the patient, since the deformity is in the plane of motion in the knee and ankle joint, with the exception of distal femur deformities.[2],[8]

Second, until recently, knowledge of the mechanical axis for the sagittal plane was insufficient for use in planning. Hence, it was argued that with full extension in the knee joint, the mechanical axis of the lower limb normally passes anteriorly from the knee joint,[1],[2] and with flexion of 5°, the mechanical axis crosses the knee joint “slightly in front” from the point of its rotation (intersection of the posterior cortex line and Blumensaat line). Moreover, Standard et al.[3] suggested a “modified” mechanical axis, which is the line connecting the center of the femoral head and a point on the border of the anterior one-third and mid one-third of the distal joint line of the femur. It was assumed that the angle formed by the intersection of the mechanical axis and the distal joint line of the femur would be equal to aPDFA. Thus, it was argued that the anatomical and mechanical axes of the femur intersect the distal joint line of the femur at one point with the same angle.

The concept of the mechanical axis of the femur for the sagittal plane used in navigation systems for knee arthroplasty is interesting. Thus, in a number of systems, the mechanical axis is represented by a line connecting the center of the femoral head and a point located 1 cm anteriorly to the Blumensaat line, while in other navigation systems, the mechanical axis is a line connecting the center of the femoral head and a point that separates the distal joint line of the femur is 65% in front and 35% behind.[9]

The earlier study by our group[5] contradicts the above information. As a result of this study, the points of intersection of the mechanical axis for the sagittal plane and the joint lines of the femur and tibia were identified, as well as the reference values of the angles formed by these intersections [Figure 1]. This study is a continuation of the previous work and, thanks to the newly identified relationships, allows the drawing of the mechanical axis of the proximal femur fragment for the sagittal plane in three different ways: (1) joint line based; (2) femoral neck based; and (3) anatomical axis based.

For what reason are three methods of determining the mechanical axis of the proximal fragment necessary? Let us give an analogy with planning in the frontal plane, which also uses three methods of drawing the mechanical axis of the proximal fragment. However, no guideline answers the question of what are the indications for using a particular method. In the original sources,[2],[10] and in the monograph by Paley,[1] it is argued that the first method that the orthopedic surgeon should use is the method “from the anatomical axis of the proximal femur fragment” (anatomical axis based method). In case of proximal femur deformity, which does not allow drawing the anatomical axis of the proximal fragment, the author recommends to draw the mechanical axis using the “joint line based” method. However, it is known that in the case of minimal torsional deformities, the values of the reference angles are distorted.[11] Torsion exerts the greatest influence on the method of determining the mechanical axis of the proximal fragment using “joint line based” method.[4] In such cases, it seems logical to use the neck axis based method. However, our study also does not provide a clear answer to this question, only providing options for the surgeon.

The study has the following limitations. The proposed method for femur deformity correction planning in the sagittal plane involves working with radiographs made in the lateral projection. The disadvantage of this method is its “two-dimensionality,” i.e. deformities in the transverse plane (torsion) are not taken into account, which is critical. Therefore, it is recommended to use this method only in cases of deformities that do not include a torsion component. However, the newly obtained data, together with the knowledge of the mechanical axis and the planning method for the frontal plane, are likely to serve as the basis for 3D femur deformity correction planning.


  Conclusion Top


The proposed method for planning deformity correction based on mechanical axes for the sagittal plane complements the existing planning methods for the frontal plane and improves the quality, namely the accuracy of preoperative planning. In the nearest future, the newly obtained data are likely to serve as the basis for 3D planning, which will eliminate planning errors associated with pathological bone torsion.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient (s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Paley D. Principles of Deformity Correction. New York, Springer: 2002. p. 102-8.  Back to cited text no. 1
    
2.
Paley D, Herzenberg JE, Tetsworth K, McKie J, Bhave A. Deformity planning for frontal and sagittal plane corrective osteotomies. Orthop Clin North Am 1994;25:425-65.  Back to cited text no. 2
    
3.
Standard SC, Herzenberg JE, Conway JD, Siddiqui NA, McClure PK. The Art of Limb Alignment. 8th ed. Baltimore: Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore; 2019.  Back to cited text no. 3
    
4.
Solomin LN. The Basic Principles of External Skeletal Fixation Using the Ilizarov and Other Devices. 2nd ed. Milan: Springer-Verlag Italia; 2012. p. 1593.  Back to cited text no. 4
    
5.
Solomin LN, Utekhin AI, Vilenskiy VA. Reference values of the femur and tibia mechanical axes and angles in the sagittal plane, determined on the basis of three-dimensional modeling. J Limb Lengthen Reconstr 2020;6:116-20.  Back to cited text no. 5
  [Full text]  
6.
Strecker W, Keppler P, Gebhard F, Kinzl L. Length and torsion of the lower limb. J Bone Joint Surg 1997;79:1019-23.  Back to cited text no. 6
    
7.
Paley D, Pfeil J. Prinzipien der kniegelenknahen Deformitätenkorrektur. Orthopade 2000;29:18-38.  Back to cited text no. 7
    
8.
Goodier WD, Calder PR. External fixation for the correction of adult post-traumatic deformities. Injury 2019;50 Suppl 1:S36-44.  Back to cited text no. 8
    
9.
Chung BJ, Kang YG, Chang CB, Kim SJ, Kim TK. Differences between sagittal femoral mechanical and distal reference axes should be considered in navigated TKA. Clin Orthop Relat Res 2009;467:2403-13.  Back to cited text no. 9
    
10.
Paley D, Tetsworth K. Mechanical axis deviation of the lower limbs. Preoperative planning of uniapical angular deformities of the tibia or femur. Clin Orthop Relat Res. 1992;:48-64. PMID: 1611764.  Back to cited text no. 10
    
11.
Jamali AA, Meehan JP, Moroski NM, Anderson MJ, Lamba R, Parise C. Do small changes in rotation affect measurements of lower extremity limb alignment? J Orthop Surg Res 2017;12:77.  Back to cited text no. 11
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]



 

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