Last Updated on June 4, 2025
The Blumensaat line is a radiographic landmark seen on lateral views of the knee joint. It represents the roof of the intercondylar notch of the femur and appears as a dense white line on X-rays due to its cortical bone interface.
Named after Otto Blumensaat, a German surgeon who first described it in 1938, the line plays a key role in evaluating knee pathologies.
The visibility and orientation of the Blumensaat line depend on proper lateral radiographic technique, typically taken with the knee flexed between 30 and 45 degrees.
When visualized correctly, this line serves as a reference point for assessing patellar height, identifying ACL injuries, and guiding femoral tunnel placement during ACL reconstruction.
Morphological Variations of Blumensaat Line
The shape of the Blumensaat line is not always straight and can vary between individuals. These variations are typically categorized as [1]
- Straight Line: It is seen in 37% of the people.
- Small Hill: It shows a protrusion spanning less than half of the line at the proximal part of Blumensaat’s. It is seen in 19% of the people.
- Large Hill: This represents a protrusion spanning more than half of the line in the proximal part of the line. It is seen in 44% of the people.
These morphological differences generally do not alter the clinical utility of the line, but being aware of them is important when interpreting subtle knee pathologies or planning surgical procedures.
The importance of morphology is stressed in anterior cruciate ligament surgery where close attention must be paid to femoral tunnel evaluation and placement.
Relevant Anatomy
The intercondylar notch is the open space that lies between the medial and lateral femoral condyles.
It provides attachment to both the anterior and posterior cruciate ligaments. The anterior medial wall serves as the insertion point of the posterior cruciate ligament, and the posterior lateral wall serves as the insertion point of the anterior cruciate ligament.
Blumensaat’s line represents the roof of the intercondylar notch as seen in lateral view. In the following x-ray, it is the sclerotic line between two arrows.
Significance and Use of Blumensaat Line
Position of the Patella
In an x-ray taken at 30 degrees of knee flexion a line drawn through the roof [dome] of the intercondylar notch, which should intersect the lower pole of the patella. If the patella is above this line, it is called patella alta or high patella.
Blumensaat ACL (BlA) Angle
The ACL Blumensaat line angle is normally <15º. It is calculated by drawing Blumensaat’s line and one line parallel to the distal portion of the ACL. The angle is calculated at the intersection of these two lines. An angle of >15º indicates an abnormal course and probable anterior cruciate ligament tear.
Radiographic Evaluation of ACL Reconstruction [2]
Femoral component:
- A line is drawn along the posterior cortex of the femur. It should intersect line (Blumensaat line) at the inferior portion of the femoral component of the graft
- The angle between a line drawn along the femur diaphysis and the femoral tunnel angle must be appx 39°. Angles of ≤17° are associated with rotational instability tibial component.
Tibial Component
The anterior wall of the tibial tunnel should be entirely posterior to and parallel Blumensaat line. An anterior component will lead to graft impingement and failure. A too posterior graft will lead to translational laxity.
Lateral Femoral Notch Sign
The terminal sulcus is a shallow groove located on the lateral femoral condyle, which marks the junction between the weight-bearing surface of the femur (which articulates with the tibia), and the non-weight-bearing surface (which articulates with the patella).
The deep lateral femoral notch sign (LFNS) is a radiographic finding that appears as an abnormally deep groove or depression in the terminal sulcus of the lateral femoral condyle.
A notch depth > 1.5–2 mm is often considered abnormal and suggestive of ACL disruption.
A normal terminal sulcus does not extend more than 10 mm posterior to the Blumensaat line.
Role of Blumensaat Line in Retrograde Femoral Nailing
In retrograde femoral nailing, the entry point for the nail is typically made at or near the intercondylar notch.
On the AP view, the guide wire should be centered exactly in the middle of the intercondylar notch. On the lateral view it should be located in the extension of Blumensaat’s line. The entry point of the nail is in line with the axis of the medullary canal, just below the crest of the intercondylar notch.
Surgeons often use intraoperative fluoroscopy to confirm their entry point.
MRI Correlation of Blumensaat Line
On sagittal MRI slices through the intercondylar notch, the ACL normally runs nearly parallel to the Blumensaat line.
Normal ACL appears taut and closely follows the orientation of the Blumensaat line. However, a torn ACL deviates from the expected path or shows a discontinuity, often with an increased angle to the Blumensaat line.
A deviation greater than 15° from the line is strongly suggestive of a complete ACL rupture.
After ACL reconstruction, MRI can also be used to assess graft orientation relative to the Blumensaat line as discussed above.
References
- Iriuchishima T, Ryu K, Aizawa S, Fu FH. Blumensaat’s line is not always straight: morphological variations of the lateral wall of the femoral intercondylar notch. Knee Surg Sports Traumatol Arthrosc. 2016 Sep;24(9):2752-2757. [PubMed]
- Radiographic evaluation of anterior cruciate ligament reconstruction. Radiopaedia. https://radiopaedia.org/articles/radiographic-evaluation-of-anterior-cruciate-ligament-reconstruction?lang=us
- AO Foundation. (n.d.). Retrograde nailing approach – Distal femur. AO Surgery Reference. https://surgeryreference.aofoundation.org/orthopedic-trauma/periprosthetic-fractures/knee/approach/retrograde-nailing-approach-distal-femur
- Gopinathan P. Imaging the Anterior Cruciate Ligament-Points to Ponder. J Orthop. 2018 Feb 28;15(1):A1-A2. [PubMed]
