Lower extremity

Lower extremity

Exam category: Procedures → Extremity
Why it matters: The knee has more eponym views than any other joint in the body — five, all doing roughly the same job (intercondylar fossa or patellofemoral joint) but each solving a different practical problem (patient can't kneel, can't prone, needs bilateral comparison).

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The big picture (plain English)

Work distal to proximal again: toes → foot → calcaneus → ankle → tib-fib → knee/patella → femur. The ankle has one classic naming trap (the "mortise" view), and the knee has a whole family of special views worth learning as a group rather than five isolated facts. Before any of that, the registry expects you to name the joints and the handful of conditions that send patients to you in the first place.

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1. The joints of the lower limb, and the one exception

The joints of the lower limb are all — with one exception — synovial joints, wrapped in a fibrous capsule containing synovial fluid, and all, with the same one exception, diarthrodial (freely movable).

The single exception is the distal tibiofibular joint. It is a fibrous joint, held by fibrous interconnections between the tibia and fibula rather than a synovial capsule. It is the syndesmosis type and is only slightly movable — amphiarthrodial. (Its most distal part is smooth and lined with a synovial membrane continuous with the ankle joint, which is why it is sometimes described loosely — but the joint itself is the fibrous one.)

Two movement facts get tested alongside it:

• The ankle (talocrural) joint is a saddle (sellar) joint. Its motion is limited to dorsiflexion (decreasing the angle between the dorsum of the foot and the front of the lower leg) and plantar flexion (pointing the foot downward).
• Inversion and eversion do not happen at the ankle joint. The subtalar (talocalcaneal) joint, in combination with other intertarsal joints, is a plane (gliding) joint that provides the gliding and rotation producing inversion (inward turning, varus) and eversion (outward turning, valgus). The lower leg does not rotate during either. Most sprained ankles come from a forced inversion or eversion.

One more named articulation: the Lisfranc joint is the tarsometatarsal articulation of the midfoot. The Lisfranc ligament spans the medial cuneiform to the base of the first and second metatarsals, and because no transverse ligament runs between those two metatarsal bases, the region is prone to stress injury. A moderate sprain shows as an abnormal separation between the first and second metatarsals; injuries range up to fracture-dislocations of those bases. They are easily missed if weight-bearing AP and lateral foot projections are not performed.

2. Pathology and terminology you'll be asked to name

Procedures questions test basic pathology and the medical terminology attached to each region, not just positioning.

3. Ankle — three frontal projections, and the true lateral

A true AP ankle doesn't fully show the ankle joint space, because the distal fibula sits more posterior than the tibia and gets partially overlapped by the talus. That single fact generates the whole frontal family, and the exam tests which rotation opens what:

The true lateral ankle is the mediolateral projection. The patient lies lateral recumbent, affected side down, knee flexed about 45°, so the lateral (outer) aspect of the ankle rests against the IR and the CR passes medial-to-lateral, directed to the medial malleolus. Dorsiflex the foot toward a right angle to help hold the true lateral. The lateromedial projection is a recognised alternative — more uncomfortable for the patient, but sometimes easier to get a true lateral from; it is not the routine.

How you know the lateral is true. This is where two different facts get mixed up, so keep them apart:

• The malleoli themselves are not superimposed on each other. In the anatomy, the lateral malleolus sits about ⅜ inch (1 cm) posterior to the medial malleolus (and about 1 cm more distal). Rotating the ankle until the two malleoli superimpose is a common positioning mistake — that produces a partially oblique ankle, not a lateral.
• What should be superimposed is the lateral malleolus over the tibia. The evaluation criterion for no rotation is the lateral malleolus superimposed over the posterior half of the tibia and talus, with the talocalcaneal joint open and the tarsal sinus and calcaneocuboid joint space open. On a lateral foot, the same criterion reads: the distal fibula superimposed over the posterior half of the tibia, with the tibiotalar joint open and uniform.

On a finished lateral image the calcaneus is in profile, with the talus and distal tibia-fibula above it and the navicular in front — and the tibia and fibula lie on top of one another rather than side by side. That last point is what separates a lateral from all three frontal projections at a glance.

4. Foot and toes — the sesamoid view and weight-bearing

Tangential (sesamoid) projection. This puts the sesamoid bones at the first MTP joint in profile to assess injury.

• Patient prone. Dorsiflex the foot so the plantar surface forms a 15–20° angle from vertical, then dorsiflex the great toe and rest it on the IR to hold the position. It is uncomfortable — don't hold the patient there longer than necessary.
• CR perpendicular to the IR, directed tangentially to the posterior aspect of the first MTP joint (angle slightly if the amount of dorsiflexion requires it for a true tangential).
• The supine reverse version exists but is not preferred. The patient lies supine and holds the toes back with a long gauze strip. It works, but the foot is now held up away from the receptor — and that increased object–image receptor distance (OID) brings magnification and loss of definition. That is the whole reason prone wins; it is not about comfort or a different projection.

Weight-bearing foot studies are performed to show the foot under the natural stress of body weight. On the weight-bearing lateral, the criterion is that the longitudinal arch is demonstrated in its entirety — which is exactly what a non-weight-bearing image cannot show, and why arch conditions are assessed this way.

5. The knee/patella eponym family

All five of these exist to show either the intercondylar fossa or the patellofemoral joint — group them by which structure they're for, not as five unrelated names.

Intercondylar fossa (PA axial), three ways to get the patient into position:

Patellofemoral joint (tangential/"sunrise"), two ways:

Whatever the method, the defining feature of a tangential patella projection is the same: the knee is flexed and the CR passes tangentially through the patellofemoral joint space, throwing the patella clear of the femur. The exact angle differs by method, so the angle is not the discriminator — the tangential path through a flexed joint is.

6. Patella and femur — the routine projections

The patella is routinely done PA, not AP. The PA is performed prone with the legs extended, CR perpendicular to the midpatella. Prone puts the patella against the receptor, and the OID principle from Image Production does the rest: less OID → less magnification → sharper detail. (Bontrager lists its clinical indication as evaluating patellar fractures before the knee is flexed for the tangential views — so the PA also comes first in the order of operations.)

The lateral patella is taken with the patient lateral recumbent, affected side down, the knee flexed only 5–10°, and the CR perpendicular to the mid-patellofemoral joint. The small flexion angle is deliberate: more flexion may separate fracture fragments if a fracture is present.

Femur. The routine is AP and lateral. Note the mid-and-distal femur projection must include the knee joint on the receptor, allowing for beam divergence — the lower receptor margin sits about 2 inches (5 cm) below the knee joint. If the site of interest is the proximal femur, a unilateral hip routine or pelvis is what's recommended instead. Rotate the leg internally about 5° for a true AP (15–20° for the proximal femur, as for an AP hip).

7. Weight-bearing views

Ankle and knee both have AP/lateral weight-bearing options in the current spec — used to assess joint alignment and space under normal physiologic load, which a non-weight-bearing image can't show.

8. The routine series — where to center, and how you know it's right

The named views above are the exceptions. This is what they're exceptions to — the routine series for each part and the centering point the beam goes to. The registry tests these directly, and it tests the evaluation criteria just as often: the specific finding on the finished image that proves the position was right.

Two extra facts about the knee carry real exam weight. First, the AP knee is rotated internally 3–5° for a true AP (until the interepicondylar line is parallel to the IR). Second, the AP knee CR angle is not fixed — it's chosen so the beam runs parallel to the tibial plateau, using the distance from the ASIS to the tabletop:

The lateral knee takes a 5–7° cephalad CR, and the AP weight-bearing foot study angles 15° posteriorly to a point midway between the feet.

How you know it's right

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Registry must-know facts

• All lower-limb joints are synovial and diarthrodial except one: the distal tibiofibular joint, a fibrous, syndesmosis-type, amphiarthrodial joint.
• The ankle joint is a saddle joint doing dorsiflexion/plantar flexion. Inversion and eversion come from the subtalar (talocalcaneal) joint with the other intertarsal joints — a plane (gliding) joint.
• Lisfranc joint = the tarsometatarsal articulation; the ligament runs medial cuneiform to the first and second metatarsal bases, and injury shows as separation between metatarsals 1 and 2.
• Gout first strikes the first MTP joint. Pott's fracture = complete fracture of the distal fibula with ankle-joint injury, often the medial malleolus/distal tibia. Osgood-Schlatter = the tibial tuberosity.
• Ankle rotation ladder: AP (0°) leaves the lateral malleolus over the talus; mortise (15–20° medial) opens the whole joint space; 45° medial oblique (special) shows the distal tibiofibular joint and distal fibula/lateral malleolus fractures.
• The routine lateral ankle is the mediolateral: lateral recumbent, affected side down, lateral aspect against the IR, CR to the medial malleolus. Lateromedial is the alternative, not the routine.
• True lateral ankle/foot: lateral malleolus (distal fibula) superimposed over the posterior half of the tibia, tibiotalar joint open. The malleoli are ~1 cm apart — superimposing them is the classic mistake and produces a partial oblique.
• Sesamoid tangential: prone, foot dorsiflexed 15–20° from vertical, great toe dorsiflexed onto the IR, CR tangential to the posterior aspect of the first MTP joint. The supine alternative costs OID → magnification and loss of definition.
• Weight-bearing lateral foot demonstrates the longitudinal arch in its entirety.
• Routine CR centering: toes → MTP joint of the digit; foot → base of the third metatarsal (AP angled 10° toward the heel); calcaneus axial → CR 40° cephalad from the long axis of the foot, lateral → 1 inch (2.5 cm) inferior to the medial malleolus; ankle → midway between the malleoli; tib-fib → midpoint of the lower leg (both joints on the image); knee → ½ inch (1.25 cm) distal to the patellar apex.
• AP toes: CR 10–15° toward the calcaneus with the foot flat, or perpendicular if a 15° wedge raises the forefoot parallel to the IR. Centre to the MTP joint either way.
• AP knee CR angle comes from the ASIS-to-tabletop measure: <19 cm → 5° caudad, 19–24 cm → 0°, >24 cm → 5° cephalad. The leg is rotated internally 3–5° for a true AP. The lateral knee uses 5–7° cephalad.
• AP knee is correctly positioned when the condyles look symmetric, the medial half of the fibular head is superimposed by the tibia, and the intercondylar eminence is centered in the fossa.
• Foot obliques rotate medially 30–40°; individual toe obliques rotate medially for digits 1–3 and laterally for digits 4–5.
• Camp Coventry (prone) / Holmblad (kneeling) / Béclère (supine, reversed AP) are three positioning options for the same PA axial intercondylar fossa view.
• Merchant (bilateral, supine, knees flexed 40°, CR 30° caudad) is preferred over Settegast (prone, 90° flexion, must rule out patellar fracture first, reduced diagnostic value from quad tightening) for the patellofemoral joint.
• Patella: PA not AP — prone puts the patella against the receptor, so less OID, less magnification, sharper detail. Lateral patella: lateral recumbent affected side down, knee flexed only 5–10°, CR perpendicular.
• Femur (mid and distal): routine AP and lateral, CR perpendicular to the midpoint of the receptor — not to a palpable landmark — with the knee joint included and the lower margin about 2 inches (5 cm) below it. AP is correct when the condyles look symmetric and the patella sits slightly medial; lateral when the femoral condyles superimpose with an open patellofemoral joint space. On both, the knee joint space itself will not look fully open — that is beam divergence, not rotation.

Common trap

Questions test whether you know which of the three fossa methods to pick for a patient who can't kneel or can't prone — picking Holmblad for a patient who can't kneel, or Camp Coventry for one who can't prone, is the exact wrong-tool-for-the-patient trap this family of views is built to test. A second trap lives in the lateral ankle: "superimposed" is correct for the lateral malleolus over the tibia and wrong for the two malleoli over each other, and questions are written to see whether you noticed which pair they named.

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Source: Bontrager, Textbook of Radiographic Positioning and Related Anatomy, 10th ed., Ch. 6 (lower-limb joint classification and the distal tibiofibular exception; the subtalar joint's gliding/rotation producing inversion and eversion; Lisfranc ligament and joint injury; gout and Osgood-Schlatter under Clinical Indications; the AP/mortise/45°-oblique ankle series and the 45° oblique's clinical indications; the mediolateral lateral ankle and its lateromedial alternative; lateral ankle and lateral foot Evaluation Criteria; the tangential sesamoid projection including the supine alternative's OID penalty; the weight-bearing lateral foot's longitudinal-arch criterion; AP toes CR angle and the 15° wedge alternative; PA patella, lateral patella and the Merchant bilateral method; routine CR centering points, the ASIS-to-tabletop angle guideline, and the evaluation criteria in section 8, taken from Ch. 6's per-projection "CR" and "Evaluation Criteria" entries rather than summarized from memory) and Ch. 7 (femur — mid and distal: the AP and lateral routine, internal rotation and knee-joint inclusion with the receptor margin 2 inches below the joint, the CR directed perpendicular to the midpoint of the receptor, and the section-8 evaluation criteria taken from that chapter's per-projection "CR" and "Evaluation Criteria" entries — condyle symmetry and the slightly medial patellar outline on the AP, condyle superimposition with an open patellofemoral joint space on the lateral, the 1-inch clearance of the knee joint space from the receptor margin, and its statement that beam divergence keeps the knee joint space from opening fully on either projection). Pott's fracture per Bontrager Ch. 15 (fracture terminology). The OID-versus-magnification principle applied to the PA patella is Bontrager's own general statement that placing a structure closer to the IR on the PA results in less magnification; Bontrager's listed indication for the PA patella is evaluating fractures before the knee is flexed for other projections. Weight-bearing views confirmed present in the current exam specifications' named-view list.