To understand the deformities of lower limb, it is important to grasp and establish the parameters defining normal alignment.
The alignment of the lower limb is governed by arrangement of femur, tibia, hip, knee, and ankle.
To understand it netter the complex three-dimensional shapes of bones and joints can be simplified to basic line drawings in frontal, sagittal, or transverse anatomic planes.
Anatomical and Mechanical Axes of Lower Limb
Each long bone has a mechanical and an anatomic axis. The mechanical axis of a bone is the straight line connecting the joint center points of the proximal and distal joints in frontal or sagittal plane. Most of the mechanical axes are considered in the frontal plane.
The anatomic axis of a bone It is the line passing through the mid point of diaphysis.
Thus mechanical axis is always a straight line in both frontal or sagittal plane but the anatomic axis line may be straight in the frontal plane but curved in the sagittal plane, as in the femur.
As mechanical axis is considered mostly in the frontal plane, we need to define only the frontal plane joint center points of the hip, knee, and ankle.
Midpoint of Joints
For the hip, the joint center point is the center of the circular femoral head [ Can be identified with Mose circle or goniometer]
Center of the knee joint can be found by [All methods almost approximate to the same point.]
- Marking a point at the top of the femoral notch
- Midpoint of the femoral condyles
- Center of the tibial spines
- Midpoint of the tibial
Using the top of the femoral notch or tibial spines is the quickest method.
The ankle joint center point can be marked by
- Mid-width of the talus
- Mid-width of the tibia and fibula at the level of the plafond
- Mid-width of the soft tissue outline
The mid-width of the talus or the plafond is the easiest to use.
Orientation of Joints and Line Denoting Them
A line that represents the orientation of a joint in a plane is called joint orientation line. Different joints have different orientations.
In the frontal plane
- Across the flat subchondral line of the tibial plafond in either the distal tibial subchondral line
- The subchondral line of the dome of the talus.
In the sagittal plane
The frontal plane
- Across the flat or concave aspect of the subchondral line of the two tibial plateaus.
- Tangential line to the most distal points on the convexity of the two femoral condyles.
In the sagittal plane
- The proximal joint line of the tibia is drawn along the fiat subchondral line of the plateaus
- The distal femoral articular shape is circular
- .The distal femoral joint orientation can be drawn as a straight line connecting the two points where the femoral condyles meet the metaphysis of the femur.
- In children, this can be drawn where the growth plate exits anteriorly and posteriorly.
- Blumensaat’s line [represents the intercondylar notch, can be used as the joint orientation line]
There are two lines and both show orientation in different directions
- A line from the proximal tip of the greater trochanter to the center of the femoral head represents the hip joint orientation line of the hip joint in the frontal plane.
- Alternatively, the mid-diaphyseal line of the femoral neck can represent the orientation of the hip joint. It uses the center of the femoral head as one point and the mid-diaphyseal width of the neck as the second point.
Angles are formed because the joint lines in the frontal and sagittal planes have a characteristic orientation to the mechanical and anatomic axes.
Joint Orientation Angles
The angle formed between the joint line and either the mechanical or anatomic axis is called the joint orientation angle. Prefix m or a signifies if the axis used is mechanical or anatomical.
F stands for femur and T for Tibia.
aLDFA is the lateral angle formed between the anatomic axis of the femur and the knee joint line of the femur.
aPPTA is is the posterior angle between the anatomic axis of the tibia and the joint line of the tibia in the sagittal plane.
The prefixes a or m may be omitted in following situations
- Sagittal plane orientation angles usually refer to the anatomic axis because mechanical axis lines are rarely used in the sagittal plane
- Because the mechanical and anatomic axes of the tibia are parallel and the angles would have same value. Prefix m or a may not be
- By convention lateral proximal femoral angle [LPFA] uses mechanical axis and medial proximal femoral angle uses the anatomic axis, the prefixes may not be applied.
Thus prefixes are most important in distal femoral angles.
Neck shaft angle (NSA). The normal NSA is 125°-131°.
A line from the tip of the greater trochanter to the center of the femoral head. It is also called the horizontal orientation angle for the proximal femur
Normal value 89.9± 5.2°.
normal mLDFA to be 87.5±2.5°
In saggital plane, posterior slope of the proximal tibia in the sagittal plane is 10.7± 1.8°.
Posterior distal femoral angle – 83.1±3.6°
Ankle joint is in slight valgus [up to 8° of valgus can be seen].
In saggital plane there is anterior tilt of the distal tibia.
Joint Convergence Angles
The angle formed between joint orientation lines on opposite sides of the same joint is called the joint line convergence angle. In the knee and ankle joints, these lines are normally parallel.
Anatomic Axis – Joint Line Intersection Points
Anatomic Axis to Joint Center Distance or aAJCD
This denotes the distance between mechanical and anatomical axes at any given joint. This is calculated in coronal or frontal plane.
The distance from point of intersection of anatomic axis lines with the joint line can be described relative to the center of the joint line or to one of its edges.
In the frontal plane, the distance on the joint line between the intersection with the anatomic axis line and the joint center point is called the anatomic axis to joint center distance (aJCD).
For hip the, the anatomic axis passes through piriformis fossa and the distance between that point and center of head of femur gives us aJCD.
In knee, both antomic axes of femur and tibia pass appx through medial tibial spine and aJCD becomes the distance between this point and center of joint. For all practical purpose medial tibial spine is the reference point.
Normal aJCD for knee is about 10± 5 mm.
For ankle, the value is 4±4 mm.
Anatomic Axis to Joint Edge distance (aJED)
This value is calculated in sagittal plane.
aJED is the distance between the point of intersection of the anatomic axis line with the joint line and the anterior edge of the joint.
In distal end of femur, the anatomical axis devides the joint in anterior one third and posterior two thirds.
Similarly, tibia is divided in anterior one fifth and posterior four fifths.
Ankle is divided into two equal halves.
The anatomic axis:joint edge ratio (aJER) is the ratio between the aJED and the total width of the joint.
aJER = aJED/total width of joint
The normal value is 1/3 for femur and 1/5 for tibia. The value for ankle is 1/2 [ Correlate with where axes intersect in lateral view]
Similarly, anatomic axis: joint center ratio (aJCR) is the ratio of the aJCD and the total width of the joint.
Mechanical Axis Deviation
Joint alignment and joint orientation are two considerations in evaluation of the frontal plane of the lower extremity.
Alignment refers to the collinearity of the hip, knee, and ankle and orientation refers to the position of each articular surface in relation to to the axes tibia and femur.
Normally anatomic tibiofemoral angle is 6° valgus.
For alignment and orientation standing AP radiographs of the entire lower extremity on a single film is done.
Mechanical axis passes from center of the femoral head to the center of the ankle and passes almost through center of knee [Although normal alignment is often depicted with the mechanical axis passing through the center of the knee, a line drawn from the center of the femoral head to the center of the ankle typically passes immediately medial to the center of the knee].
When the mechanical axis does not lie close to center of knee, it is called mechanical axis deviation.
Malalignment occurs when the center of the knee does not lie close to this line.
The distance between the mechanical axis line and the center of the knee in the frontal plane is the mechanical axis deviation.
9.7±6.8 mm medial deviation is considered normal variation.
- HSS Newsletter [includes credit for mentioned images too]
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