Image Interpretation Course
by Heidi Nunn DCR(R) PgCert
The Knee
Ottawa rules | Suprapatellar bursar | Patella | Tibial plateau | Segond fracture
Intercondylar eminence | Proximal fibula | Osteochondritis dissecans
Osgood-Schlatter's disease | Pellegrini Stieda | Synovial osteochondromatosis
Diaphyseal aclasia | Non-ossifying fibroma
(click on images to enlarge)
Ottawa rules
- State that a knee xray is only required for patients with knee injuries with any of the following:
Age 55 or over.
Isolated tenderness of the patella (no bone tenderness of the knee other than the patella).
Tenderness at the head of the fibula.
Inability to flex to 90 degrees.
Inability to weight bear both immediately and in the casualty department (ie, 4 steps – unable to transfer weight twice onto each lower limb regardless of limping).
Suprapatellar Bursa
- The fat pad posterior to the quadriceps tendon is divided into anterior and posterior compartments by a soft tissue density; the suprapatellar bursa. This should measure 5mm or less:
- This is located within the joint capsule, therefore it may expand to form a joint effusion (>5mm). This is indicative of soft tissue injury (meniscal or ligamentous). The majority of knee injuries affect the soft tissues, with no bony injury demonstrated:
- Intra-articular fractures will cause leakage of blood and marrow into the joint which will again distend the bursa to form a lipohaemarthrosis. Fat (lucent) lies on top of the fluid (blood), which is radiographically dense. Indicates a fracture even if one is not seen.
Patella
- Fractures may be comminuted, transverse or vertical. Sometimes a skyline projection may be required to demonstrate a vertical fracture, however, this is contraindicated for transverse fractures, as flexing the knee will cause the fracture to displace:
- Skyline views can also be useful to demonstrate subtle subluxations:
- Look for osteochondral fractures associated with patella dislocations, ie, medial patella or lateral femoral condyle:
- Ruptures of the infra patellar tendon may occur. Evidence of this is an increased distance between the inferior pole of the patella and the tibial tuberosity. This distance normally should be roughly the length of the patella, plus or minus 20%.
- Bipartite patella is often seen, which is an unfused secondary ossification centre. This is characteristically located at the supero-lateral aspect and should not be mistaken for a fracture. However, a bipartite patella may also fracture:
- undisplaced:
- displaced:
Tibial Plateau
- Associated with ruptures of the medial collateral ligament and anterior and posterior cruciate ligaments.
- Lateral tibial plateau is the most common site. Can be subtle and may be easily overlooked. However, this is a significant injury:
- An imaginary line drawn along the lateral femoral condyle and the medial cortex of the proximal fibular shaft should not have more than 5mm of tibial condyle located laterally. If more than this is seen laterally, a split/depression fracture should be suspected:
- Look closely for the lucent fracture line (split) and an area of increased bone density / sclerosis indicating impaction (depression):
- Because the tibial plateau slopes posteriorly 15 degrees, it can be difficult to determine the degree of depression. Displacement is therefore often underestimated.
Segond fracture
- Refers to a characteristic linear fragment arising from the lateral margin of the lateral tibial plateau. This fragment is attached to the lateral capsular ligament and is associated with disruption of the anterior cruciate ligament. It is a more significant injury than it initially appears:
Intercondylar Eminence (Tibial Spine)
- Avulsion fractures occur at the insertion of the anterior cruciate ligament (most common) and posterior cruciate ligament. The avulsion may be a partial avulsion or complete detachment. Most commonly seen in adolescents:
Proximal Fibula
- Fibular head and neck fractures are usually accompanied by lateral tibial plateau fractures or knee ligament injuries (cruciate or collateral ligaments). As with the segond fracture, an apparently small fracture in this region is often more significant than it appears:
- Fractures seen more distally at the proximal fibula shaft are often accompanied by ankle injuries (Maisonneuve):
Osteochondritis Dissecans
- Refers to a sub-articular lucency with sclerotic margins, within which is seen an oval segment of bone (area of avascular necrosis). This will often give symptoms similar to the presence of a loose body, as there subsequently becomes a gradual separation of this loose bone:
- Osteochondritis dissecans is characteristically seen at the lateral aspect of the medial femoral condyle. Other common sites may be within the talus, or the capitellum:
Osgood-Schlatter’s Disease
- Osteochondritis of the tibial tuberosity. Occurs in adolescents with repeated trauma to the knee. The normal ossification centre for the tibial tuberosity may demonstrate a wide variety of normal variants and is often fragmented. Diagnosis is made clinically, and radiography is not indicated.
Pellegrini Stieda
- Refers to post-traumatic calcification within the medial collateral ligament. Should not be mistaken for an acute injury:
Synovial Osteochondromatosis
- Refers to multiple cartilaginous loose bodies, which calcify within the synovium of the joint, causing symptoms of pain and locking:
- Most commonly seen in the knee, elbow and ankle:
Diaphyseal Aclasia
- An exostosis / osteochondroma refers to a thin cartilage-capped outgrowth. This may be pedunculated (on a stalk), or sessile (no stalk). Diaphyseal aclasia refers to multiple exostoses / osteochondromas which arise from the metaphyseal region and point away from the joint. The knees are usually involved. Usually asymptomatic, but can become large and may fracture. May become malignant, particularly if located axially rather than from a long bone.
Non-Ossifying Fibroma
- A benign lytic lesion often seen at the distal femur and proximal tibia. Refers to a large fibrous cortical defect (>2cm) located within the diametaphyseal region of a long bone. Has a bubbly appearance. Asymptomatic and is usually an incidental finding:
