• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
bone and spine logo

Bone and Spine

Your Trusted Resource for Orthopedic Health Information

  • Home
  • About
  • Contact Us
  • Policies
  • Show Search
Hide Search
You are here: Home / Basics and Biomechanics / Normal Alignment of Lower Limb – Axes and Orientation

Normal Alignment of Lower Limb – Axes and Orientation

Dr Arun Pal Singh ·

Last Updated on October 29, 2023

To understand the deformities of the lower limb, it is important to grasp and establish the parameters defining normal alignment. The normal alignment of lower limb is governed by the arrangement of the femur, tibia, hip, knee, and ankle.

To understand it better, the complex three-dimensional shapes of bones and joints can be simplified to basic line drawings in frontal, sagittal, or transverse anatomic planes.

axes of lower lib
Image credit: Current Revisions in Musculoskeletal Medicine
Contents hide
1 Anatomical and Mechanical Axes of Lower Limb
2 Midpoint of Joints
2.1 Hip Joint
2.2 Knee Joint
2.3 Ankle Joint
3 Orientation of Joints and Line Denoting Them
3.1 Ankle
3.2 Knee
4 Angles
4.1 Joint Orientation Angles
4.2 Hip
4.3 Knee
4.4 Ankle Joint
4.5 Joint Convergence Angles
5 Anatomic Axis – Joint Line Intersection Points
5.1 Anatomic Axis to Joint Center Distance or aAJCD
5.2 Anatomic Axis to Joint Edge distance (aJED)
5.3 Ratios
6 Mechanical Axis Deviation
7 References

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 the 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 midpoint of the 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 the 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

Hip Joint

For the hip, the joint center point is the center of the circular femoral head [ Can be identified with Mose circle or goniometer]

Knee Joint

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 condyles

Using the top of the femoral notch or tibial spines is the quickest method.

Ankle Joint

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

Joint Orientation Lines
Joint Orientation Lines

A line that represents the orientation of a joint in a plane is called a joint orientation line. Different joints have different orientations.

Ankle

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

  • From the distal tip of the posterior lip to the distal tip of the anterior lip of the tibia

Knee

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]

Hip

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

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

Hip, knee, ankle Joint orientation angles
Joint Orientation Angles, Image credit: HSS.edu

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.

The angle may be measured medial (M), lateral (L), anterior  (A), or posterior  (P) to the axis line.

The angle may refer to the proximal   (P) or distal  (D) joint orientation angle  of a bone.

F stands for femur and T for Tibia.

Thus the mechanical lateral distal femoral angle or mLDFA is the lateral angle formed between the mechanical axis line of the femur and the knee joint line of the femur in the frontal plane.

aLDFA is the lateral angle formed between the anatomic axis of the femur and the knee joint line of the femur.

aPPTA is the posterior angle between the anatomic axis of the tibia and the joint line of the tibia in the sagittal plane.

[Frontal plane angles would be medial or lateral and sagittal plane angles would be anterior or posterior]

The prefixes a or m may be omitted in the 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 the same value. Prefix m or a may not be
  • By convention lateral proximal femoral angle [LPFA] uses a 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.

Hip

Neck-shaft angle (NSA). The normal  NSA is 125°-131°.

Lateral Proximal Femoral Angle

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°.

Knee

The distal femoral joint line is in slight valgus and the tibia is in slight varus to the proximal tibial joint line. [about 3 degrees]

Anatomical Lateral distal femoral Angle – 79-83 degrees

normal mLDFA to be 87.5±2.5°

Medial proximal tibial angle is 85-90 degrees.

In the saggital plane, the posterior slope of the proximal tibia in the sagittal plane is 10.7± 1.8°.

Posterior proximal tibial angle – 80.4± 1.6° [slight difference is seen the angle when measured from medial or lateral tibial plateau].

Posterior distal femoral angle –  83.1±3.6°

Ankle Joint

Ankle joint is in slight valgus [up to 8° of valgus can be seen].

Lateral distal tibial angle is 88.6±3.8 degrees.

In saggital plane there is the 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

aJCD
aJCD, Image Credit: HSS.edu

This denotes the distance between mechanical and anatomical axes at any given joint. This is calculated in the 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 anatomic axis passes through piriformis fossa and the distance between that point and center of the head of the femur gives us aJCD.

In the knee, both anatomic 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, the medial tibial spine is the reference point.

Normal aJCD for knee is about 10± 5 mm.

For the ankle, the value is 4±4 mm.

Anatomic  Axis  to Joint Edge distance (aJED)

This value is calculated in the 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 the femur, the anatomical axis divides the joint in anterior one third and posterior two thirds.

Similarly, the tibia is divided into anterior one fifth and posterior four-fifths.

The ankle is divided into two equal halves.

Ratios

The anatomic axis: joint edge ratio (aJER) is the ratio between the aJED and the total width of the joint.

Thus

aJER = aJED/total width of joint

The normal value is 1/3 for the femur and 1/5 for the tibia. The value for the 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 the evaluation of the frontal plane of the lower extremity.

Alignment refers to the collinearity of the hip,  knee, and ankle and orientation refer to the position of each articular surface in relation to he 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].

Mechanical Axis Deviation
Mechanical Axis Deviation,
Image Credit: HSS.edu

When the mechanical axis does not lie close to the center of the 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.

It is described as either medial or lateral [also called varus or valgus respectively] malalignment respectively.

9.7±6.8  mm medial deviation is considered a normal variation.

References

  • https://www.hss.edu/images/newsletters/1uiowety.pdf [includes credit for mentioned images too]
  • Chao EY, Neluheni EV, Hsu RW, Paley D. Biomechanics of    Orthop  Clin North  Am 1994, 25:379-386.
  • Hsu RW, Himeno S, Coventry  MB, Chao EY. Normal axial alignment of the lower extremity and load-bearing distribution at the knee.  Clin Orthop 1990, 255:215-227.
  • Moreland JR, Bassett LW, Hanker  GJ Radiographic analysis of the axial alignment of the lower J  Bone Joint Surg Am 1987,69:745-749.
  • Paley D, Tetsworth K Mechanical axis deviation of the lower limbs: Preoperative planning of uniapical angular deformities of the tibia or Clin Orthop 1992, 280:48-64.
  • Paley D, Chaudray M, Pirone AM, Lentz P, Kautz D Treatment of malunions and mal-nonunions of the femur and tibia by detailed preoperative planning and the Ilizarov techniques.  .  Clin North Am 1990, 21:667-691
  • Paley D, Herzenberg JE, Tetsworth  K, McKie J, Bhave A. Deformity planning for frontal and sagittal plane corrective osteotomies.  Orthop  Clin North Am 1994, 25:425-465.

Basics and Biomechanics This article has been medically reviewed by Dr. Arun Pal Singh, MBBS, MS (Orthopedics)

About Dr Arun Pal Singh

Dr. Arun Pal Singh is a practicing orthopedic surgeon with over 20 years of clinical experience in orthopedic surgery, specializing in trauma care, fracture management, and spine disorders.

BoneAndSpine.com is dedicated to providing structured, detailed, and clinically grounded orthopedic knowledge for medical students, healthcare professionals, patients and serious learners.
All the content is well researched, written by medical expert and regularly updated.

Read more....

Primary Sidebar

Know Your Author

Dr. Arun Pal Singh is an orthopedic surgeon with over 20 years of experience in trauma and spine care. He founded Bone & Spine to simplify medical knowledge for patients and professionals alike. Read More…

Explore Articles

Anatomy Anatomy Fractures Fractures Diseases Diseases Spine Disorders Spine Disorders Patient Guides Patient Guides Procedures Procedures
featured image of gower sign for segmenatal instability of lumbar spine

Clinical Tests for Lumbar Segmental Instability

Lumbar segmental instability may not always be visible on standard …

mesurement of scoliosis for braces

Braces for Scoliosis- Types, Uses and Results

Braces for scoliosis are recommended to prevent the scoliotic curve …

discogenic back pain

Discogenic Back Pain Causes, Diagnosis and Treatment

Discogenic back pain is a common cause of axial low back pain [the …

Elbow arthrodesis using internal fixation

Elbow Arthrodesis- Indications, Methods and Complications

Elbow arthrodesis refers to the surgical fusion of the elbow joint. It …

skeletal traction in upper tibial pin

Skeletal Traction – Indication, Uses and Complications

Skeletal traction is a type of traction where the force is applied …

Popular articles

kyphosis in tubercular spine

Kyphosis in Spinal Tuberculosis

Kyphosis in spinal tuberculosis is one …

complex regional pain syndrome lower limb

Complex Regional Pain Syndrome or CRPS- Causes, Symptoms and Treatment

Complex regional pain syndrome or CRPS …

freiberg disease

Freiberg Disease or Osteochondrosis of Metatarsal Head

Freiberg disease or Freiberg infraction …

salter pelvic osteotomy

Pelvic osteotomies – Indications and Types

Pelvic osteotomies are done to stabilize …

Bone and Spine

© 2025 BoneAndSpine.com · All Rights Reserved
The content provided on BoneAndSpine.com is intended for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Read Disclaimer in detail.