Imaging performed in acetabular fracture is to evaluate for wall involvement, intra-articular entrapped bone fragment and associated femoral head fracture. On the AP view of the hip, six lines relating to the acetabulum and surrounding structures have been described: iliopubic or iliopectineal line, ilioischial line, radiographic teardrop, roof of the acetabulum, anterior rim of the acetabulum, and posterior rim of the acetabulum. Disruption of any of these normal lines indicates the possible presence of abnormality [3] (Fig. 21.3c).

On lateral view, the pelvis is divided into anterior and posterior columns. The anterior column (iliopubic component) consists of pubis and ilium, whereas the posterior column (ischial component) consists of ischium, ischial spine, and ischial tuberosity. The posterior wall fractures are the most common acetabular fractures and are often associated with multiple fragments as well as posterior hip dislocation.

Fractures of the femoral head are relatively infrequent in isolation given the protection of the femoral head by the acetabulum (Figs. 21.4b and 21.5a). When they occur, they are usually associated with hip dislocation and are caused by impaction or shearing by the acetabulum. Pipkin classification is the standard mean to classify femoral head fractures, with a main criterion of whether the fracture is above or below the fovea [5].

Pipkin Classification:

Type 3 and 4 fractures often require operative intervention, while the need for operative fixation of type 1 and 2 fractures depends on several factors including intra-articular fragment, irreducible dislocation, and involvement of the weight-bearing portion. The fractures of the femoral head are at an increased risk of avascular necrosis or nonunion because main arterial supply (medial femoral circumflex artery) may be injured.

Femoral neck fractures may result from a significant injury such as a fall but can also be seen in minor trauma among osteoporotic individuals. Garden classification is most commonly used to describe these fractures [6].

While displaced fractures are readily visualized on radiography, imaging features of nondisplaced fractures are often subtle. These features include:

The blood supply to the femoral head is through three routes, including the vessels in the ligamentum teres, capsular vessels, and branches of the nutrient vessels. There is an increased risk of avascular necrosis following femoral neck fractures because of nutrient and capsular vessel disruption. The frequency of avascular necrosis in displaced and nondisplaced fractures is 30–40 and 10 %, respectively [7]. If radiography is negative but there is still a high clinical suspicion, CT with multiplanar reconstruction or MRI should be considered.

Intertrochanteric fractures occur along the line between greater and lesser trochanter (Fig. 21.5c). Since the joint capsule attaches anteriorly to the intertrochanteric line and posteriorly to the medial half of the femoral neck, trochanteric fracture is theoretically extracapsular and heals well relative to the femoral neck fractures. It is important to distinguish stable from unstable trochanteric fractures. This can be determined by assessing the medial cortex and comminution of the posterior cortex (on lateral view). Comminuted fracture line at the medial cortex, displaced lesser trochanter fracture, and lesser trochanter fragment containing calcar femorale are generally considered unstable. Presence of a fracture gap medially and posteriorly will allow fracture displacement into varus or retroversion, causing instability. Unlike fractures of the femoral neck, trochanteric fractures are almost free from serious complications.

Subtrochanteric fractures are fractures located between the lesser trochanter and 5 cm below. It is usually a result of direct trauma and may occur in isolation or with intertrochanteric fractures. The fractures are usually displaced by muscle pulls, resulting in shortening and varus deformity.

Fractures of the distal femur usually occur with high-velocity trauma in young adults and low-energy trauma in osteoporotic bone of elderly persons. Therefore, it is commonly associated with several other injuries such as fractures of the patella, hip and tibial shaft, and ligamentous disruptions of the knee. Femoral shaft fractures occur at any age and are equally common in the uppermost, middle, and lower thirds of the shaft. Their pattern may be transverse, oblique, spiral, comminuted, or greenstick. Radiographic evaluation must include the hip and knee joints since joint dislocations may coexist.

The fractures of the supracondylar and condylar regions are divided into three types: extra-articular (transverse), ­partial articular, and complete articular. The key components in dividing distal femoral fractures are presence of (1) a fracture line extending into the intercondylar notch (intercondylar split), (2) osteochondral fragments in the area of or between the intercondylar split, and (3) coronal-plane (Hoffa) fracture. Partial articular fractures are an intra-­articular fracture that involves either the medial, lateral condyle, or in a coronal plane (best seen on a lateral radiograph). Complete articular fractures produce separate articular and metaphyseal segments, in which each can be simple or multifragmentary.

Osteochondritis dissecans (OCD) of the knee (Fig. 21.6) may present in a similar fashion to acute traumatic osteochondral fractures, although they differ. The condition is characterized by sequential degeneration or aseptic necrosis and recalcification. It can occur in persons of any age group but is most frequent between 10 and 20 years old, with male predominance. Most of the cases (85 %) involve the medial femoral condyle in the posterolateral aspect [8]. Radiography shows OCD as a well-circumscribed crescent-shaped area of radiolucency above the subchondral bone, separated from femoral condyle. MRI is helpful to help determining the vascularity of the lesion, bilaterality, occult lesions, and degree of loosening of the lesion.