Imaging Anatomy

For Medical Students

Structure and Location

On an anteroposterior radiograph the distal femur shows two condylar outlines, the intercondylar notch between them, and the epicondyles at the widest point. The joint line appears as the gap between femoral and tibial subchondral bone, because cartilage is not radio-opaque.

On a true lateral the condyles superimpose. The roof of the intercondylar notch appears as a dense line running obliquely, called Blumensaat's line, and it is the most useful landmark on the view. Behind the condyles the posterior condylar outlines should coincide if the view is truly lateral.

On magnetic resonance imaging the articular cartilage is visible as a layer over the subchondral bone, and the fat in the marrow gives the metaphysis a bright signal on the sequences that show fat.

Relations

Blumensaat's line is the notch roof, so everything above it is bone and everything below it is notch contents.

The physeal scar, a faint transverse line across the metaphysis, marks where the growth plate was and persists into adult life.

The suprapatellar pouch appears as a fluid plane above the trochlea when distended, lying between the quadriceps and the femur.

Function

Each view exists to show a particular relationship. The anteroposterior view shows coronal alignment and the joint line. The lateral shows the notch roof, the condylar profiles and the position of the patella. A notch or tunnel view opens the notch by flexing the knee, so the notch outline can be seen rather than superimposed.

Knowing which view shows what is the difference between requesting an image and requesting the right one.

Clinical Relevance

A lateral radiograph that is not truly lateral misrepresents everything measured on it, and the test is whether the posterior condylar outlines coincide.

Interpretation beyond normal anatomy, including every index and ratio used around the knee, belongs to Section 5. What belongs here is what the normal structures look like and which view shows them.

MRCS Level

Structure and Location

Radiographic anatomy worth naming: on the lateral view, Blumensaat's line, the posterior condylar outlines, the trochlear floor where it meets the anterior cortex, and the patella's position relative to the trochlea. On the anteroposterior view, the condylar outlines, the notch, the epicondyles and the physeal scar.

Magnetic resonance anatomy by plane. Sagittal images show the cruciates, the notch roof, the condylar profiles and the posterior horns. Coronal images show the collaterals, the notch walls and the meniscal bodies. Axial images show the trochlea and the patellofemoral relationship, which is why they are the plane for patellar problems.

Cartilage on magnetic resonance has a layered appearance reflecting collagen orientation, and the subchondral plate below it is a distinct low signal line.

Relations

On a sagittal magnetic resonance image the anterior cruciate runs parallel to the roof of the notch, so Blumensaat's line is also the reference for the ligament's normal orientation.

The posterior cruciate is seen on the sagittal images as a curved low signal band from the medial wall of the notch to the posterior tibia.

The popliteus tendon appears on coronal and axial images in its sulcus on the lateral condyle, and its sheath is continuous with the joint.

The trochlea on axial images is seen with its two facets and the central groove, and the level at which the axial slice is taken changes the apparent depth, because the groove is shallower proximally.

Blood Supply and Innervation

Vascular anatomy is not directly visible on plain radiographs. On magnetic resonance the popliteal vessels are seen behind the capsule on both sagittal and axial images, and their position relative to the posterior condyles is assessable.

Contrast enhanced studies and angiography show the genicular anastomosis, and the middle genicular artery entering the notch can sometimes be identified.

Marrow signal reflects perfusion and cellularity, so a change in marrow signal is a change in the bone's vascular or cellular state rather than in its shape.

Function

Each modality answers a different question about the same bone. Radiographs show alignment and bone outline. Computed tomography shows bone in three planes and is the modality for rotational anatomy. Magnetic resonance shows cartilage, ligament, marrow and the soft tissue relations.

Rotational anatomy of the distal femur is assessed on axial computed tomography, because the references are the epicondyles and the posterior condyles and both need a true axial plane.

Clinical Relevance

The level of an axial slice changes the apparent trochlear depth, so comparing axial images means comparing the same level.

The anterior cruciate lying parallel to Blumensaat's line is a normal relationship and the starting point for assessing the ligament on magnetic resonance.

The physeal scar is normal and is not a fracture. In a child the physis itself is open, and distinguishing physis from fracture is a Section 10 and Section 5 matter.

Every measured index on these views is deliberately absent here. Measurement method and normal ranges belong to Section 5, and decision 005 records why no figure appears on this page.

FRCS Level

Relations

Imaging is how the relations on the other pages of this chapter are verified in a particular patient before operating. The relations that are worth checking rather than assuming are the position of the popliteal vessels behind the condyles, the level of the physeal scar in a young adult, and the rotational relationship between the epicondylar axis and the posterior condyles.

All three vary, and all three are visible preoperatively.

Function

Planning imaging exists to answer a question that the examination cannot. A request that does not have such a question attached produces findings rather than answers, and incidental findings around the knee are common enough that this matters.

Three dimensional reconstruction and templating change what is knowable before surgery rather than what is true about the anatomy.

Clinical Relevance

Rotational assessment of the distal femur is the imaging question with the most direct surgical consequence, because the references disagree and imaging is how the disagreement is quantified in a given knee.

In revision planning, the position of existing tunnels and hardware relative to the native attachments is an imaging question, and it determines whether a procedure is single or two stage.

Where imaging shows a relation that differs from the textbook, the patient's anatomy wins. That is the practical reason this chapter is written in relative terms rather than fixed ones.