Mechanical Axis Alignment: LDFA, MPTA, HKA Explained
Lower limb alignment for orthopaedic surgeons — LDFA, MPTA, HKA, FTA measurement, normal values, and role in OA and surgical planning.
Why Alignment Matters in Knee OA
The mechanical axis of the lower limb — the line from the centre of the femoral head to the centre of the ankle — determines how load is distributed across the knee joint. In a normally aligned limb, this axis passes through or just medial to the centre of the knee, distributing approximately 60% of the load through the medial compartment and 40% through the lateral compartment.
When alignment deviates from normal, the consequences are predictable. Varus malalignment (bow-legged) shifts load medially, accelerating medial compartment cartilage loss. Valgus malalignment (knock-kneed) does the opposite. Understanding and quantifying this alignment is fundamental to OA management and surgical planning.
The Four Key Angles
HKA (Hip-Knee-Ankle angle) is the angle between the mechanical axis of the femur (centre of femoral head to centre of knee) and the mechanical axis of the tibia (centre of knee to centre of ankle). Normal value is 180 degrees (straight line) with an acceptable range of 177–183 degrees. Values below 177 degrees indicate varus; above 183 degrees indicate valgus. HKA is the single most important alignment measurement for osteotomy planning and TKA component positioning.
LDFA (Lateral Distal Femoral Angle) is the lateral angle between the femoral mechanical axis and the distal femoral joint line. Normal value is 87–88 degrees (range 85–90). An LDFA outside this range indicates that the malalignment originates in the femur — this distinction is critical because femoral-origin deformity requires distal femoral osteotomy (DFO), not proximal tibial osteotomy.
MPTA (Medial Proximal Tibial Angle) is the medial angle between the tibial mechanical axis and the proximal tibial joint line. Normal value is 87 degrees (range 85–90). An MPTA below 85 degrees in a varus knee indicates tibial-origin deformity, making the patient a candidate for high tibial osteotomy (HTO).
FTA (Femorotibial Angle) is the anatomical angle between the femoral and tibial shafts, measured on a standing AP long-leg radiograph. Normal value is 173–175 degrees (5–7 degrees of anatomical valgus). While less precise than mechanical axis measurements, FTA remains widely used because it can be measured on standard knee radiographs without requiring full-length leg films.
Measurement Technique
Accurate alignment measurement requires a standing full-length leg radiograph (hip to ankle on a single film). The patient stands with patellae facing forward, feet shoulder-width apart, and full weight on both limbs. The radiograph must capture the femoral head superiorly and the ankle mortise inferiorly.
On the radiograph, the femoral mechanical axis is drawn from the centre of the femoral head to the centre of the knee (midpoint of the intercondylar notch). The tibial mechanical axis runs from the centre of the tibial plateau to the centre of the ankle (midpoint of the talar dome). The intersection of these lines defines the HKA angle and the deviation of the mechanical axis from the knee centre.
Manual measurement using digital tools on a PACS workstation is the standard approach. The primary source of variability is identifying the centre of the femoral head (especially with hip OA or prosthesis) and the centre of the knee joint. AI-assisted tools can automate landmark detection and angle calculation, improving reproducibility. At Salnus, our analysis pipeline integrates automated LDFA, MPTA, and FTA calculation directly into the DICOM viewing workflow, providing instantaneous quantitative assessment.
Clinical Application: When Alignment Changes Treatment
In early OA (KL grade 1–2) with varus malalignment, alignment data influences the choice between conservative management and early surgical intervention. A patient with KL-2 OA and 5 degrees of varus has a significantly higher risk of progression than a patient with the same KL grade and neutral alignment — the MOST study demonstrated that varus malalignment increases the odds of medial OA progression by a factor of 3–4.
In moderate OA (KL grade 2–3), alignment determines whether the patient is a candidate for joint-preserving surgery. HTO is indicated for medial compartment OA with tibial-origin varus (MPTA below 85 degrees), while DFO is indicated for lateral compartment OA with femoral-origin valgus (LDFA above 90 degrees). Incorrect identification of the deformity origin leads to inappropriate surgical selection and suboptimal outcomes.
In advanced OA (KL grade 3–4) being considered for total knee arthroplasty, preoperative alignment assessment guides component positioning strategy. The target for conventional TKA is mechanical alignment (HKA 180 degrees), but kinematic alignment strategies that preserve the patient's constitutional alignment are gaining evidence. Accurate preoperative measurement of LDFA and MPTA is essential for both approaches.
Osteotomy Planning: Where Precision Matters Most
For HTO and DFO, the correction angle is calculated directly from the alignment measurements. The goal is to shift the mechanical axis from its current position (typically passing through the medial compartment) to a target position (usually 62% of the tibial plateau width from the medial edge — the Fujisawa point).
The correction angle is the difference between the current HKA and the target HKA. A 1-degree error in the preoperative measurement translates directly to a 1-degree error in the surgical correction — which over time can mean the difference between a successful outcome and early failure. Patient-specific 3D-printed guides designed from preoperative CT data can transfer the planned correction angle to the operating room with sub-degree accuracy.
If you work with alignment-critical cases and would like to evaluate AI-assisted measurement and planning tools, visit the Salnus Surgeon Portal or contact us for a demonstration.
Disclaimer: This article is for educational purposes only. All surgical planning decisions should be made by qualified orthopaedic surgeons based on comprehensive patient assessment. Salnus AI tools are designated for research use only (RUO).
References:
- Paley D. Principles of Deformity Correction. Springer, 2002.
- Sharma L, et al. The role of knee alignment in disease progression and functional decline in knee osteoarthritis. JAMA. 2001;286(2):188-195.
- Cooke TDV, et al. Radiographic assessment of bony contributions to knee deformity. Clin Orthop Relat Res. 2007;(454):28-31.
Reviewed by the Salnus biomedical engineering team.