The lower limb

8 The lower limb




The bones of the lower limb



The femur (Fig. 8.1; see also Fig. 6.1)


The femur is the longest and the strongest bone of the body. It has a head, neck, shaft and an expanded lower end.



The head is more than half of a sphere and is directed upwards, medially and forwards. It is intra-articular and covered with cartilage apart from a central pit called the fovea, where the ligamentum teres is attached. The blood supply of the femoral head is derived from three sources as follows:






The neck of the femur is about 5 cm long and forms an angle of 125° in females to 130° in males with the shaft. It is also anteverted, that is, it is directed anteriorly at an angle of about 10° with the sagittal plane.



Its junction with the shaft is marked superiorly by the greater trochanter and inferiorly and slightly posteriorly by the lesser trochanter. Between these anteriorly is a ridge called the intertrochanteric line, and posteriorly a more prominent intertrochanteric crest. The capsule of the hip joint is attached to the intertrochanteric line anteriorly but at the junction of the medial two-thirds and the lateral third of the neck posteriorly.


The shaft of the femur is inclined medially so that whereas the heads of the femurs are separated by the pelvis, the lower ends at the knees almost touch. It also has a forward convexity. The shaft is cylindrical with a prominent ridge posteriorly, the linea aspera. This ridge splits inferiorly into medial and lateral supracondylar lines, with the popliteal surface between them. The medial supracondylar line ends in the adductor tubercle.


The lower end of the femur is expanded into two prominent condyles united anteriorly as the patellar surface, but separated posteriorly by a deep intercondylar notch. The most prominent parts of each condyle are called the medial and lateral epicondyles. Above the articular surface on the lateral side is a small depression that marks the origin of the popliteus muscle.



Radiological features of the femur (see Fig. 6.1)










Radiological features of the patella




Dislocation of the patella


Lateral dislocation of the patella is more common than medial dislocation and occurs following valgus injury with associated imposed bowstringing of the extensor mechanism over the knee joint. Anatomical structures have evolved to prevent dislocation, including relative hypertrophy of the vastus medialis muscle and overgrowth of the lateral femoral condyle.


Wiberg describes three shapes of patella:





Evaluation of patellofemoral alignment is routinely achieved using the skyline position with the beam centred on the patellofemoral joint from below. Patellofemoral tracking is best achieved using the Merchant views, with the beam directed from above down to a cassette held over the tibia. The Merchant views can therefore be acquired in weightbearing and in varying degrees of flexion and extension.



Transverse fracture of the patella is associated with separation of the fragments by the pull of the quadriceps muscle. Comminuted fractures as a result of direct trauma, on the other hand, usually leave the extensor expansion intact. In association with disruption of the articular cartilage, these fractures are treated by patellectomy rather than K-wire stabilization.







The bones of the foot (Figs 8.4, 8.5)


In addition to metatarsals and phalanges, there are seven tarsal bones in the foot. These are the talus, calcaneus, navicular, cuboid and three cuneiform bones. Of these, the talus and calcaneus are most important radiologically.








Radiological features of the bones of the foot (Fig. 8.6)



Plain radiographs


Boehler’s critical angle of the calcaneus (see Fig. 8.6A) is the angle between a line drawn from the posterior end to the anterior end of its superior articular facet and a second line from the latter point to the posterosuperior border of the calcaneus. It is normally 30–35°, with an angle less than 28° occurring when there is significant structural damage to the bone.



Heel-pad thickness (see Fig. 8.6A) is measured on a lateral radiograph of the calcaneus between the calcaneal tuberosity posteroinferiorly and the skin surface. Normal thicknesses are 21 mm in the female and 23 mm in the male. Thickening of the heal pad occurs in patients with gigantism or acromegaly.


On a lateral radiograph of the foot in children over 5 years old the long axis of the talus points along the shaft of the first metatarsal. In the younger child the talus is more vertical and its long axis points below the first metatarsal (see Fig. 8.6B, C).







The joints of the lower limb



The hip joint (Fig. 8.7)








Radiological features of the hip joint



Plain radiographs


On radiographs of the hip joint pads of fat, seen as linear lucencies, outline the capsule of the hip joint and closely applied muscle. Bulging of these is an early sign of joint effusion.


A small accessory ossicle, the os acetabuli, is sometimes seen at the superior margin of the acetabulum and should not be confused with a fracture here.


Similarly, irregularity of the superior margin of the acetabulum in children is a normal variant.


In assessment of radiographs of the hip in infants (Fig. 8.8), the following lines and angles are as described:








In assessment of radiographs of the adult hip, routine frontal radiographs allow the identification of six lines (see Fig. 6.3). The continuation of the inferior margin of the superior pubic ramus superiorly to the upper outer margin of the acetabular roof forms the outer margin of the anterior column or wall of the acetabulum (line 1). The continuation of the inferior margin of the inferior pubic ramus superiorly to the upper outer margin of the acetabular roof forms the outer margin of the posterior column or wall of the acetabulum (line 2). The continuation of the superior margin of the superior pubic ramus superiorly forms the iliopectineal line (line 3). The continuation of the superior margin of the inferior pubic ramus superiorly forms the ilioischial line (line 4). The roof of the acetabulum forms line 5. The ‘tear drop’ is formed by the reflections of the cotyloid fossa and quadrilateral plate and represents line 6. Disruption of any of the described lines is employed in the localization of disease processes on conventional radiographs.







Magnetic resonance imaging of the hip (Fig. 8.10)


MR imaging of the hip may be performed using the body coil and wide 25–30 cm field of view, allowing simultaneous visualization of both hips and comparison of the normal and abnormal sides. Dedicated imaging of a single hip, to identify the labrum, requires the use of a phased array quadrature surface coil.


Dec 19, 2015 | Posted by in PEDIATRIC IMAGING | Comments Off on The lower limb

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