Lower Extremities
Knee/Patella
Anatomy & Pathology
Key Anatomical Structures
- Femur (distal end):
- Medial and lateral condyles: Large rounded articular surfaces that rest on the tibial plateaus.
- Intercondylar fossa (notch): Deep depression separating the condyles posteriorly; site for cruciate ligament attachment.
- Epicondyles: Non-articular prominences superior to the condyles; serve as attachment sites for collateral ligaments.
- Tibia (proximal end):
- Medial and lateral tibial condyles / plateaus: Flat surfaces forming the weight-bearing portion of the knee joint.
- Intercondylar eminence (spines): Raised ridges separating the tibial plateaus; attachment for cruciate ligaments.
- Tibial tuberosity: Anterior prominence for the patellar tendon insertion.
- Patella:
Sesamoid bone within the quadriceps tendon that articulates with the patellar surface (trochlear groove) of the femur.- Base: Superior border.
- Apex: Inferior tip; directed toward the tibial tuberosity.
Functions to protect the knee joint and improve leverage of the quadriceps muscle during extension.
- Joints:
- Femorotibial joint: Diarthrodial, hinge-type joint between femoral and tibial condyles.
- Patellofemoral joint: Diarthrodial, gliding-type articulation between the patella and femur.
- Proximal tibiofibular joint: Plane (gliding) joint aiding ankle motion.
- Soft tissue & supportive structures:
- Menisci (medial and lateral): Crescent-shaped fibrocartilage pads that deepen tibial surfaces and act as shock absorbers.
- Ligaments:
- Cruciate ligaments (ACL, PCL): Provide anterior-posterior stability.
- Collateral ligaments (MCL, LCL): Provide medial-lateral stability.
- Quadriceps and patellar tendons: Maintain alignment and transmit extension force.
Common Pathologies
- Fractures: Include supracondylar femur, tibial plateau, patellar, or avulsion fractures at the tibial tuberosity.
- Osteoarthritis (degenerative joint disease): Narrowing of joint space, subchondral sclerosis, and osteophyte formation due to cartilage wear.
- Osgood-Schlatter disease: Inflammation and fragmentation of the tibial tuberosity in adolescents from repetitive strain of the quadriceps.
- Chondromalacia patellae (runner’s knee): Softening and erosion of the patellar cartilage, causing anterior knee pain; best demonstrated on tangential (sunrise/merchant) views.
- Joint effusion: Fluid accumulation within the joint capsule, often secondary to trauma, infection, or arthritis.
- Ligamentous injury (ACL, PCL, MCL, LCL): May cause joint instability, abnormal widening, or avulsion fragments visible radiographically.
- Meniscal tear: Fibrocartilage injury not directly visible radiographically but often inferred from joint-space narrowing or associated fractures; MRI used for confirmation.
- Patellar subluxation or dislocation: Lateral displacement of the patella from the trochlear groove, often due to ligament laxity or trauma.
Routine Projections (ARRT Required)
Supplemental Projections
Knee – Weight-Bearing AP
CR Location & Positioning
- SID: 40″ (may use 72″ to reduce magnification)
- Patient position: Standing, equal weight on both feet
- Adjustments: Knees fully extended, toes forward
- CR: Perpendicular, ½ inch below apices of patellae
- Pt. Instructions: Stand still, equal weight on both legs
- Exposure: 65–70 kVp
Evaluation Criteria
- Coverage: Both knees including joint spaces
- Rotation checks: Equal distance between femoral condyles and tibial plateaus
- Motion checks: Sharp trabeculae
- Technique checks: Joint spaces visible, density balanced
- Clinical aim: Detects joint space narrowing (arthritis, varus/valgus deformity)
Knee – Tunnel (PA Axial Holmblad Method)
CR Location & Positioning
- SID: 40″
- Patient position: Kneeling on table, standing against IR, or kneeling on stool
- Adjustments: Flex knee 70° from full extension (20° from CR); tibia touching IR
- CR: Perpendicular to lower leg, entering popliteal fossa, exiting patellar apex
- Pt. Instructions: Hold still
- Exposure: 65–70 kVp
Evaluation Criteria
- Coverage: Intercondylar fossa, tibial plateaus, intercondylar eminence
- Rotation checks: Symmetric condyles; centered intercondylar eminence
- Motion checks: Crisp bony detail
- Technique checks: Open fossa, posterior condyles visible
- Clinical aim: Loose bodies, cartilage defects, osteochondritis
Knee – Tunnel (PA Axial Camp-Coventry Method)
CR Location & Positioning
- SID: 40″
- Patient position: Prone
- Adjustments: Flex knee 40–50°; foot supported; femur flat
- CR: Perpendicular to tibia, angled 40–50° caudad to knee joint
- Pt. Instructions: Hold still
- Exposure: 65–70 kVp
Evaluation Criteria
- Coverage: Intercondylar fossa, tibial plateaus, intercondylar eminence
- Rotation checks: Minimal tibiofibular overlap; symmetric condyles
- Motion checks: Sharp trabeculae
- Technique checks: Clear joint space, no patellar superimposition
- Clinical aim: Same as Holmblad
Knee – Tunnel (AP Axial Béclère Method)
CR Location & Positioning
- SID: 40″
- Patient position: Supine
- Adjustments: Flex knee until femur forms 60° angle with tibia; condyles equidistant from IR
- CR: Perpendicular to tibia, entering ½ inch below patellar apex
- Pt. Instructions: Hold still
- Exposure: 65–70 kVp
Evaluation Criteria
- Coverage: Intercondylar fossa, eminence, joint space
- Rotation checks: Symmetric condyles, centered fossa
- Motion checks: No blur
- Technique checks: Patella not superimposing fossa, good contrast
- Clinical aim: Similar to other tunnels; used when patient cannot kneel
Patella – Tangential (Settegast / Sunrise)
CR Location & Positioning
- SID: 40″
- Patient position: Supine or prone
- Adjustments: Flex knee slowly until patella is perpendicular to IR; loop bandage may hold foot
- CR: 15–20° cephalad (depending on knee flexion), through patellofemoral joint
- Pt. Instructions: Hold still
- Exposure: 65–70 kVp
Evaluation Criteria
- Coverage: Patella in profile, patellofemoral joint open
- Rotation checks: Symmetric condyles, patella centered
- Motion checks: Sharp margins
- Technique checks: Good contrast of patella and femoral sulcus
- Clinical aim: Subluxation, fractures, joint space evaluation
Hows & Whys of Knee Radiography
Anatomy
- What are the menisci, and what is their function?
Crescent-shaped fibrocartilage that deepen the tibial plateaus and absorb shock. - What structures form the intercondylar fossa?
The posterior aspects of the femoral condyles. - Where is the tibial tuberosity located, and what attaches there?
On the anterior proximal tibia; the patellar tendon inserts here. - Which part of the patella is superior, and which is inferior?
The base is superior; the apex is inferior. - What is the relationship of the medial vs. lateral femoral condyles?
The medial condyle extends more distally and is larger.
Positioning
- Why is the CR angled 3–5° cephalad for patients with a thick pelvis (>24 cm)?
To project the tibia and femur joint surfaces open, compensating for natural slope of the tibia. - Why is the knee flexed 20–30° for the lateral projection?
To relax muscles/fat pads and open the joint space without displacing fractures. - Why is only 5–10° of flexion used for the lateral patella?
To avoid tightening the quadriceps tendon, which could displace a fracture fragment. - Why is the leg rotated internally 45° for the medial oblique knee?
To open the proximal tibiofibular joint and show lateral condyles in profile. - Why is the leg rotated externally 45° for the lateral oblique knee?
To demonstrate the medial condyles in profile and superimpose fibula over tibia. - Why must the intermalleolar line be parallel to the IR for the mortise ankle but not the knee?
The knee joint space requires CR angle correction instead of patient rotation due to tibial slope. - Why is the prone PA patella preferred over AP?
To reduce OID, giving sharper detail. - Why must the CR be angled 15–20° cephalad in the Settegast (Sunrise) projection?
To align the beam with the patellofemoral joint space. - Why are weight-bearing AP or Rosenberg knees performed?
To evaluate joint space narrowing under load, useful in arthritis.
Technique & Image Evaluation
- How do you know an AP knee was not rotated?
Femoral condyles symmetric; tibial eminence centered; slight fibular head overlap. - How do you know the CR angle was correct on an AP knee?
The femorotibial joint space is open and symmetric. - How can you tell the lateral knee is correctly positioned?
Condyles superimposed; open patellofemoral joint space; tibia and fibula slightly overlapped. - What error is indicated if the medial condyle appears anterior to the lateral on a lateral knee?
The knee was over-rotated toward the IR. - What error is indicated if the medial condyle appears posterior to the lateral on a lateral knee?
The knee was under-rotated away from the IR. - How can you tell the CR angle was insufficient on the lateral knee?
Medial condyle projects inferior to lateral condyle. - How can you tell the CR angle was excessive on the lateral knee?
Lateral condyle projects inferior to medial condyle. - How can you confirm correct positioning on a medial oblique knee?
Proximal tibiofibular joint open, lateral condyles in profile. - How can you confirm correct positioning on a lateral oblique knee?
Medial condyles visualized, fibula superimposed on tibia. - How do you know the intercondylar fossa (tunnel view) is open?
Fossa is unobstructed, intercondylar eminence centered, patella not superimposing fossa. - How can you confirm patient cooperation on knee images?
Sharp trabecular markings, no motion blur.
Clinical Applications
- Which projections best demonstrate the patella in profile?
Lateral and Sunrise (tangential). - Which projections demonstrate the patellofemoral joint space?
Lateral and tangential (Settegast, Hughston, Merchant). - Which projection best demonstrates the intercondylar fossa?
Tunnel views (Holmblad, Camp-Coventry, Béclère). - Why is the weight-bearing AP knee essential in arthritis studies?
Reveals joint space narrowing not visible on non–weight-bearing views. - Why are tunnel views helpful in evaluating knee trauma?
They can show “joint mice” (loose bodies), cartilage defects, or osteochondritis dissecans. - Why should flexion be limited in suspected patellar fracture?
Excess flexion could displace fracture fragments.
