|Year : 2020 | Volume
| Issue : 2 | Page : 121-125
Defining the anatomic axis joint center distance and anatomic axis joint center ratio of the distal femur in the coronal plane
Nando Ferreira, Andries Johannes Cornelissen, Marilize Burger, Aaron Kumar Saini
Department of Surgical Sciences, Division of Orthopaedic Surgery, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town, South Africa
|Date of Submission||29-Sep-2020|
|Date of Decision||18-Dec-2020|
|Date of Acceptance||19-Dec-2020|
|Date of Web Publication||31-Dec-2020|
Prof. Nando Ferreira
Department of Surgical Sciences, Division of Orthopaedic Surgery, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town, 7505
Source of Support: None, Conflict of Interest: None
Background: The aim of this radiographic study was to define the anatomic axis joint center distance (aJCD) and anatomic axis joint center ratio (aJCR) of the distal femur in the coronal plane for skeletally mature individuals. Methods: A cross-sectional radiographic study was conducted to calculate the horizontal distances between the anatomical axis and the center of the knee at the level of the intercondylar notch and the joint line. Ratios relating these points to the width of the femur were then calculated. Results: A total of 164 radiographs were included: 91 male (55.5%) and 73 female patients (44.5%) with a mean age of 44.9 ± 18.1 years, with 79 right (48.2%) and 85 left (51.8%). The mean intercondylar width was 75.6 ± 6.6 mm, the mean aJCD at the notch was 3.6 ± 1.8 mm, the mean aJCD at the joint line was 4.9 ± 1.8 mm, the mean aJCR at the notch was 45.2 ± 2.4, and the mean aJCR at the joint line was 43.5 ± 2.4. The intercondylar width was significantly different (P < 0.001) between males (79.5 ± 4.8 mm) and females (70.6 ± 5.0 mm). A significant difference between the aJCR at the notch (P = 0.004) and the aJCR at the joint line (P = 0.003) was observed in males and females. No differences between the aJCD at the notch and/or aJCD at the joint line were observed between males versus females, left versus right, and those younger versus those older than 65 years. Conclusion: This is the first objective description of the aJCR of the distal femur in the coronal plane. This ratio can be used to aid the planning and execution of distal femoral deformity correction, retrograde femoral nailing, and total knee arthroplasty. Level of Evidence: IV
Keywords: Anatomic axis, anatomic axis joint center distance, anatomic axis joint center ratio, distal femur, lower limb alignment
|How to cite this article:|
Ferreira N, Cornelissen AJ, Burger M, Saini AK. Defining the anatomic axis joint center distance and anatomic axis joint center ratio of the distal femur in the coronal plane. J Limb Lengthen Reconstr 2020;6:121-5
|How to cite this URL:|
Ferreira N, Cornelissen AJ, Burger M, Saini AK. Defining the anatomic axis joint center distance and anatomic axis joint center ratio of the distal femur in the coronal plane. J Limb Lengthen Reconstr [serial online] 2020 [cited 2021 Apr 13];6:121-5. Available from: https://www.jlimblengthrecon.org/text.asp?2020/6/2/121/305862
| Introduction|| |
Accurate analysis and correction of skeletal deformities rely on the ability to precisely define the location and magnitude of each deformity. This, in part, is dependent on the ability to accurately draw proximal and distal mechanical or anatomic axes, where the intersection of the axes defines the apex of the deformity for each limb segment.
Per definition, the mechanical axis is drawn between the centers of two ipsilateral joints, while the anatomic axis is represented by a mid-diaphyseal line. As the center of the joint is easily identified, drawing a proximal or distal segment mechanical axis only requires the adjacent joint–mechanical axis joint orientation angle (which can be obtained from the contralateral limb or population averages). For this reason, many reconstruction surgeons would default to mechanical axis deformity correction planning methods.
In certain scenarios, the anatomic axis becomes important for deformity analysis and correction. These include situations where acute correction is stabilized by intramedullary fixation or where limb length discrepancy is to be addressed simultaneously with deformity correction through the use of intramedullary lengthening devices and Baumgart's reverse planning method. In these instances, an exact nail entry point is paramount to avoid creating a translational deformity and mechanical axis deviation. To avoid producing this osteotomy rule 3 error, the intramedullary nail should be inserted along the anatomic axis of the proximal and distal segments.
The anatomic axis joint center distance (aJCD) is defined as the interval, measured in millimeters, between the center of the joint at the joint orientation line and the location where the anatomic axis intersects this line. Because of differences in the physical size of joints between individuals, the aJCD may vary significantly in the population. The anatomic axis joint center ratio (aJCR) is defined as the ratio between the location where the anatomic axis intersects the joint orientation line and the total width of the joint. As ratios have no units, they are independent of actual joint width and less variable than the aJCD. Although the aJCR for the distal femur and proximal tibia in the sagittal plane is known, being 1/3–2/3 anterior and 1/5–4/5 anterior, respectively, the same ratio for the distal femur in the coronal plane is not known.
The aim of this cross-sectional radiographic study was to accurately define the aJCD and aJCR of the distal femur in the coronal plane for skeletally mature individuals.
| Methods|| |
A retrospective cross-sectional radiographic study of anteroposterior femur radiographs of skeletally mature individuals was conducted. Radiographs that precluded accurate measurements were excluded. These included rotated radiographic views and femurs that showed deformity or previous fractures or surgery. Images without rotation were ensured by confirming the patella shadow directly overlying the center of the femoral condyles. Institutional ethics committee approval as well as hospital board approval was obtained before data collection.
Analyses were performed using the annotative analysis/measurement tools of the Phillips IntelliSpace (Phillips, Amsterdam, Netherlands) Picture Archiving and Communication System software. Measurements were independently performed by two experienced limb reconstruction surgeons (NF and AS). The two sets of measurements were averaged to produce the final dataset. Analysis consisted of drawing the distal femur joint orientation line followed by a second line, parallel to the joint orientation line at the deepest point of the intercondylar notch [line AB and CD on [Figure 1]]. The center line of the knee [line M1–M2 on [Figure 1] was drawn at the midpoint of the most medial and lateral bony projections of the femoral condyles in the plane of the deepest point of the intercondylar notch. A distal femoral mid-diaphyseal line (anatomic axis) as described by Moreland et al. was then drawn to intersect both lines AB and CD. The distance between M1 and X and M2 and Y was then measured to produce the aJCD at the notch and joint line, respectively. The aJCR at the notch and joint line was calculated as CX/CD and AY/AB, respectively [Figure 1].
|Figure 1: Example of measurements used to calculate the aJCD and aJCR. Red line: Mid-diaphyseal line (anatomical axis). AB: Joint orientation line. CD: Line parallel to joint orientation line at the deepest point of the intercondylar notch. M1: Midpoint of most medial and lateral bony projections of the femoral condyles, aJCD: Anatomic axis joint center distance, aJCR: Anatomic axis joint center ratio|
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Statistical analysis was performed using Stata 14.2 (StataCorp LP, Texas, USA). Parametric data are reported as mean and standard deviation with 95% confidence intervals (CIs). Categorical data are described as frequencies and/or counts. Differences between groups were investigated with an independent t-test, and statistical significance was accepted at the α = 0.05 level.
| Results|| |
A total of 164 radiographs were included. The cohort consisted of 91 male (55.5%) and 73 female patients (44.5%), with a mean age of 44.9 ± 18.1 years (95% CI 42.1–47.6) [Table 1].
The intercondylar width was significantly different (P < 0.001) between males (79.6 ± 4.8 mm) and females (70.6 ± 5.0 mm) [Supplementary Table 1]. A significant difference between the aJCR at the notch (P = 0.004) and the aJCR at the joint line (P = 0.003) was also observed between males and females [Figure 2]a and [Figure 2]b. No differences between the aJCD at the notch or aJCD at the joint line were observed between males versus females, left versus right, and those younger and older than 65 years [Figure 3]a and [Figure 3]b.
|Figure 2: (a) aJCR at the notch and (b) aJCR at the joint line for female versus male, left versus right, and <65 year versus >65 years groups, respectively. aJCR: Anatomic axis joint center ratio|
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|Figure 3: (a) aJCD at the notch and (b) aJCD at the joint line for female versus male, left versus right, and <65 years versus >65 years groups, respectively. aJCD: Anatomic axis joint center distance|
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| Discussion|| |
Understanding the spatial relations of the femoral anatomical axis is crucial when performing corrective osteotomies of the distal femur, retrograde femoral intramedullary nailing, and during total knee arthroplasty. The aim of this cross-sectional radiographic study was to accurately define the aJCD, and for the first time, the aJCR of the distal femur in the coronal plane for skeletally mature individuals. The aJCR was found to occur at a consistent ratio of 45:55 of the total width from medial to lateral at the level of the femoral notch in both males and females.
The aJCD for the distal femur in the coronal plane has been quoted several times to varying degrees of accuracy.,,, However, the rationale for, or size of, the sample from which the conclusions were drawn is not always apparent which influences the confidence with which these measurements can be used in clinical practice. A summary of quoted values reported in the literature and sample size of the cohorts is shown in [Table 2].
|Table 2: Published descriptions of the distal femur anatomical joint center distance|
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The aJCD is patently related to the joint width and patient size and using population averages for all patients could inadvertently result in mechanical axis translations or iatrogenic deformities. One way to overcome this problem is to use the aJCD of the contralateral limb, although this may be difficult or inaccurate in cases where a deformity is present. In contrast, the aJCR can be utilized in cases where neither the aJCD of the affected, nor the contralateral limb, is feasible to obtain. In addition, the aJCR can be applied to a patient of any size. The present study reports a consistent aJCR in males, females, different sides, and across younger and older patients.
During femoral deformity correction, mechanical axis planning is primarily aimed at realigning joints in the coronal plane. The advantage of mechanical access planning is the use of the mid-points of joints to achieve their overall realignment. Advances in intramedullary lengthening device technology have resulted in a move toward internal devices for deformity correction and limb lengthening. Unlike the tibia, where the mechanical and anatomical axes are co-linear, they are distinctly different in the femur. Using an intramedullary correction device in the femur by definition requires anatomical axis planning methods. Unlike the mechanical axis, the anatomical axis of the femur does not exist at the midpoint of the knee joint in the coronal plane. Accurate quantification of the anatomical axis is essential for accurate deformity correction. This study has shown aJCD which has a narrower range than previously described as well as a consistent ratio (aJCR) of 45:55 from medial to lateral at the level of the intercondylar notch. This allows more accurate deformity planning and correction across patients of different sizes.
Quantification of the aJCR has applications outside of femoral deformity correction since the optimal entry point for retrograde nailing of the femur also corresponds to this point. A number of authors have investigated this with cadaveric studies of femurs nailed in vitro [Table 3].,, However, the recommended sites are described as distances from a reference point and therefore do not consider the variations in femoral condylar width as does a ratio. Similarly, wide variation exists in the recommended entry point, as suggested by the surgical techniques of a range of retrograde femur nail manufacturers [Table 3].,,,, However, none of the manufacturer recommendations provided information on the evidence base or rationale behind their recommendations.
|Table 3: Retrograde femoral nail entry points from cadaveric studies and manufacturer's recommendations|
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Regarding total knee arthroplasty, one needs to be vigilant of variations in femoral anatomy and its effects on component alignment post resection., However, there is a paucity of literature based on the anatomical or radiological data to definitively and reproducibly guide the entry point for the intramedullary alignment rod on which the resection guides are placed. The recommendations from the operative techniques from a range of systems and manufacturers [Table 4] reveal notable variation. The present study therefore highlights another application of the aJCR related to the identification of the correct entry point for the intramedullary jig in total knee arthroplasty.
|Table 4: Manufacturers' recommendations for the entry point of the femoral intramedullary alignment rod on which resection guides are placed|
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Although we endeavored to only identify radiographs that were virtually perfect, subtle axial plane (rotation) and sagittal plane angulations could perceivably cause measurement errors. The only way to negate these effects would be to perform these measurements on computed tomography scan reconstructions. In clinical practice, normal radiographs would be used for preoperative planning and fluoroscopic images to execute intraoperatively; we feel that the current study represents the “real-world” scenarios where these measurements and ratios would be applied. This study represents the only series to evaluate the aJCR of the distal femur in the coronal plane and its generalizability might be limited by the fact of this was a single center cohort which might not be representative of other geographic regions. Additional studies across multiple international sites might assist in supporting and further refining the findings from this study.
| Conclusion|| |
This study reports the first objective description of the aJCR of the distal femur in the coronal plane. This ratio can be used to aid the planning and execution of distal femoral deformity correction, retrograde femoral nailing, and total knee arthroplasty.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4]