Drawing the Mechanical Axis of the Leg
A leg whose weight-bearing line misses the center of the knee overloads one side of the joint, and in a knee that already has arthritis that side wears out faster.
Key terminology
- Mechanical axis
- This line runs from the center of the femoral head to the center of the ankle. It is the weight-bearing line of the leg.
- Genu varum
- This bow-legged alignment leaves the knee outside the line from hip to ankle. It loads the medial side of the knee.
- Genu valgum
- This knock-kneed alignment leaves the knee inside the line from hip to ankle. It loads the lateral side of the knee.
- Mechanical axis deviation (MAD)
- This distance runs from the mechanical axis to the center of the knee, in millimeters. It records how far and in which direction the line misses the knee.
- Medial compartment
- This inner half of the knee is where the medial femoral condyle sits on the medial tibial plateau. It takes most of the load in a varus knee.
- Anatomic axis
- This line runs down the middle of a bone's shaft. In the femur it angles away from the mechanical axis.
- Hip-knee-ankle (HKA) angle
- This angle forms where the femoral and tibial mechanical axes meet at the knee. A value of 180° is neutral.
- Joint orientation angles
- These angles measure the knee joint line against each bone's mechanical axis. The femoral one is the mLDFA and the tibial one is the MPTA.
Takeaways
- The mechanical axis runs from the center of the femoral head to the center of the ankle. It is the line body weight travels down the leg, drawn on a full-length standing AP radiograph. In a normal limb it passes just medial to the center of the knee. In 25 normal men the femoral and tibial mechanical axes met in a mean of 1.1° to 1.5° of varus (Moreland et al, J Bone Joint Surg Am 1987;69:745-749, PMID 3597474).
- A mechanical axis medial to the knee center is varus, and one lateral to it is valgus. Varus is genu varum (bow legs) and valgus is genu valgum (knock knees). The distance from the line to the knee center is the mechanical axis deviation (MAD), in millimeters. A frontal plane deformity in the femur or tibia shifts the MAD and tilts the joints above and below it (Paley and Tetsworth, Clin Orthop Relat Res 1992;(280):48-64, PMID 1611764).
- Varus alignment overloads the medial compartment, and valgus overloads the lateral compartment. In 237 people with knee OA followed for 18 months, varus was linked to higher odds of medial progression (adjusted OR 4.09). Valgus was linked to higher odds of lateral progression (adjusted OR 4.89). Worse malalignment went with more joint space loss on that side (Sharma et al, JAMA 2001;286:188-195, PMID 11448282).
- The femoral shaft and the femoral mechanical axis point in different directions, and the tibial ones nearly match. The femoral anatomic axis runs down the shaft, and the neck sets the head medially, so the 2 lines meet at an angle (about 6° in most adults). The anatomic axis of the femur does not pass through the center of the knee. The tibial shaft line runs close to the tibial mechanical axis (Moreland et al, J Bone Joint Surg Am 1987;69:745-749, PMID 3597474).
- Alignment is read on a full-length standing AP radiograph with the patella straight forward and the knee straight. In a 3D model, internal rotation moved the MAD medially and external rotation moved it laterally. Adding knee flexion up to 30° pushed the MAD error to about 25 mm and the HKA error to about 8.5°. Standing showed 2.62° more deformity than non-weight-bearing (Brunner et al, Knee Surg Sports Traumatol Arthrosc 2023;31:1483-1490, PMID 36595052; Sun et al, Biomed Eng Online 2021;20:119, PMID 34838009).
- Neutral alignment is an average, and many healthy adults stand in a few degrees of varus. The hip-knee-ankle (HKA) angle is where the femoral and tibial mechanical axes meet, and 180° is neutral. Among 250 healthy adults aged 20 to 27, 32% of men and 17% of women had constitutional varus of 3° or more. Mild varus on a young, painless film can be that person's normal (Bellemans et al, Clin Orthop Relat Res 2012;470:45-53, PMID 21656315).
- Joint orientation angles show whether a deformity sits in the femur or the tibia. The mLDFA measures the knee joint line against the femoral mechanical axis, and the MPTA measures it against the tibial mechanical axis. Both sit near 87° in a normal limb, so the abnormal one locates the deformity. The method draws a joint line at the hip, knee and ankle and a mechanical axis for each bone (Paley and Tetsworth, Clin Orthop Relat Res 1992;(280):48-64, PMID 1611764).
- Malalignment clearly predicts progression of knee OA and less clearly predicts new OA. In knees without OA (MOST cohort), varus was linked to incident OA (adjusted OR 1.49) and valgus was not. Rotterdam linked varus to new OA (OR 2.06), mainly in overweight people. Framingham found no link using knee films (Sharma et al, Ann Rheum Dis 2010;69:1940-1945, PMID 20511608; Brouwer et al, Arthritis Rheum 2007;56:1204-1211, PMID 17393449; Hunter et al, Arthritis Rheum 2007;56:1212-1218, PMID 17393450).