CHAPTER 5 The Pelvic Limb

PELVIS

When radiographing the pelvis, it is typical to obtain ventrodorsal and lateral views. For the ventrodorsal view, the pelvic limbs can be either extended or flexed. The projection obtained with the pelvic limbs flexed is referred to as the frog-leg projection. For the lateral view, it is typical for the pelvic limb closest to the imaging plate to be pushed cranially and for the other, nondependent, limb to be pulled caudally to avoid overlapping the femurs.

The pelvis is composed of four paired bones: ilium, ischium, pubic bone, and acetabular bone. The acetabular bone is a small triangular bone that lies between the caudoventral aspect of the ilium and the cranioventral aspect of the ischium (Figure 5-1). It fuses with adjacent bones at approximately 12 weeks of age to form the ventral portion of the acetabulum.¹ Before fusion of the acetabular bone, there are many radiolucent junctions: between the cranial aspect of the acetabular bone and the ilium; between the caudal aspect of the acetabular bone and the ischium; and between the ilium and the ischium, dorsal to the acetabular bone (this junction is contiguous with the margins of the acetabular bone). Thus, superimposition of all of these radiolucent junctions makes it difficult to identify the acetabular bone in its entirety, although portions of it can be seen (Figure 5-2). Care should be taken to avoid misinterpreting the numerous cartilaginous junctions in the acetabular region of young animals as fractures (Figure 5-3). Superimposition of fecal material with the pelvis can also lead to an incorrect diagnosis of a pelvic fracture, especially in lateral radiographs (Figure 5-4).

Figure 5-1 Transverse computed tomography image through the acetabular region of a 3-month-old golden retriever. The acetabular bone is visible at the central region of the acetabulum (arrows).

Figure 5-2 Ventrodorsal radiograph of the left coxofemoral joint of a 4-month-old Labrador retriever (A) and corresponding labeled radiograph (A1). The acetabular bone is the triangular opacity that makes up the central portion of the acetabulum.

Figure 5-3 Lateral radiograph of the pelvis of a 10-week-old mixed-breed dog (A) and corresponding labeled radiograph (A1). The cartilaginous junction between the ilium and the ischium, and at the cranial margin of the acetabular bone, is quite conspicuous and can be misdiagnosed as a fracture. In A1, the margin of the acetabular bone that is identified is the cranial margin; the caudal margin of the acetabular bone cannot be seen in this radiograph.

Figure 5-4 Lateral radiograph of the pelvis of a 4-month-old Labrador retriever. There is a large amount of feces in the rectum. A vertical linear gas collection in the fecal matter superimposed on the ilium and the acetabulum (white arrow) could be misinterpreted as a fracture. However, upon careful inspection, the radiolucent line extends peripheral to the pelvis (black arrow), indicating that it cannot be part of the pelvis.

The ilium and the sacrum join at the sacroiliac joint, a combined synovial and cartilaginous joint that is united by a thin joint capsule. The union between the ilium and the sacrum is a synchondrosis.² The cartilage between the ilium and the sacrum creates a radiolucent line separating the two structures. This radiolucency, which persists throughout life, can be confused with a sacroiliac fracture or subluxation in patients of all ages (Figures 5-5 and 5-6). In a ventrodorsal radiograph, the sacroiliac junction appears to be composed of multiple longitudinal radiolucent lines rather than a single line because the articulation is not perfectly opposing; interdigitations create alternating lines of opacity and lucency (Figure 5-7). The caudal margin of the normal sacroiliac joint is characterized by a smooth transition at the junction of the ilium and the sacrum (see Figure 5-6); the character of this transition is the key to detecting sacroiliac subluxation. If there is displacement of the ilium with respect to the sacrum, a malalignment, or step, will be formed at this transition.

Figure 5-5 Ventrodorsal radiograph of the sacroiliac region of a 4-month-old Labrador retriever. The cartilaginous junction between the ilium and the sacrum is radiolucent (arrows), which can be confused with a fracture or subluxation. Fecal material is superimposed on the right sacroiliac joint, making a portion of the joint indistinguishable.

Figure 5-6 Ventrodorsal radiograph of the sacroiliac region of a 5-year-old Australian shepherd. The cartilaginous junction between the ilium and the sacrum persists throughout life; meaning there will always be a radiolucent region between these two bones (black arrows). This should not be confused with an iliosacral subluxation or fracture. Note the smooth transition between the ilium and the sacrum (white arrows); this transition will have a slight malalignment, or a step, when there is a subluxation of the sacroiliac joint.

Figure 5-7 Transverse computed tomography image through the sacroiliac joint in a 12-year-old Labrador retriever; the ventral aspect of the dog is at the top of the image. Note the interdigitations and undulating nature of the sacroiliac junction (arrows). The x-ray beam striking the ventral aspect of the subject cannot project a structure with this configuration as a single radiolucent line; rather, a series of adjacent radiolucencies are projected.

The ilium is divided subjectively into a cranially located wing, the most cranial portion of which is the iliac crest, and a caudally located body (Figure 5-8). The iliac crest is fused with the wing of the ilium in most dogs by 2 years of age, but a permanently incomplete fusion is common, occurring in more than 10% of dogs³ (Figure 5-9). This incomplete fusion can be misinterpreted easily as a fracture. Usually the nonfused iliac crest is seen in the lateral view; in the ventrodorsal view, the iliac crest is superimposed on the wing of the ilium and any incomplete union is typically not visible. However, if there is slight obliquity to the pelvis in the ventrodorsal view, the radiolucent junction between the iliac crest and wing of the ilium can occasionally be seen in that view as well (Figure 5-10).

Figure 5-8 Ventrodorsal (A) and lateral (B) radiographs of the pelvis of a 7-year-old Australian shepherd and corresponding labeled radiographs (A1, B1).

Figure 5-9 Lateral radiograph of the caudal lumbar/cranial pelvic region of a 6-year-old Labrador retriever. The crest of the ilium has not fused to the wing (arrows). This is commonly encountered in dogs and should not be misinterpreted as a fracture.

Figure 5-10 Lateral radiograph of the iliac crest region (A) and ventrodorsal radiograph of the right iliac crest (B) of a 10-year-old German shepherd. The iliac crest has not fused to the wing of the ilium, creating the expected radiolucency in the lateral view (black arrows in A). In this dog, the pelvis was slightly oblique when the ventrodorsal view was made; the separation between the iliac crest and the iliac wing can also be seen in this view (black arrows in B).

The acetabulum is formed at the confluence of the ilium, the ischium, and the pubis. It serves as a major articular component of the coxofemoral joint. The acetabulum is characterized by a depression, the acetabular fossa, that occupies the central portion of the articular surface in the ventrodorsal view (see Figure 5-8, A1, and 5-11). Congruency of the coxofemoral joint to characterize joint laxity is commonly assessed radiographically in the extended hip ventrodorsal view. Congruency should be evaluated cranial to the acetabular fossa (see the region of the white arrowheads in Figure 5-11). The acetabular fossa disrupts the smooth articulation between the femoral head and the acetabulum; thus congruency cannot be assessed in this area. In some young dogs, there is a small separate center of ossification located at the craniolateral margin of the acetabulum; this will eventually fuse with the ilium and should not be confused with a fracture fragment (Figure 5-12).

Figure 5-11 Radiograph of the left acetabulum of a 7-year-old Australian shepherd. The acetabular fossa (black arrows) occupies the central portion of the acetabulum. Congruency of the coxofemoral joint should be assessed in the region of the articulation cranial to the acetabular fossa (white arrowheads) but not in the region of the acetabular fossa. This joint is very mildly incongruent, based on the joint space being slightly wider just cranial to the acetabular fossa than at the most craniolateral aspect of the joint.

Figure 5-12 Ventrodorsal radiograph of the right acetabular region of a 4-month-old Weimaraner. There is a small opacity adjacent to the craniolateral aspect of the acetabulum due to a separate ossification center (arrow). This should not be confused with a fracture fragment.

When the acetabulum is assessed, two additional views are sometimes used. One is the dorsal acetabular rim (DAR) view, which is a special view designed to assess the configuration of the dorsal acetabular rim. For the DAR view to be acquired, the anesthetized patient is placed in sternal recumbency and the pelvic limbs are pulled cranially. If correctly positioned with the x-ray beam centered on the pelvis, the x-ray beam will pass through the long axis of the ilial shaft and result in the superimposition of the wings of the ilium, the body of the ilium, the acetabulum, and the tuber ischii. This will result in an unobstructed projection of the dorsal rim of the acetabulum.⁴ In the DAR view of a normal dog, the lateral aspect of the coxofemoral joint space should be oriented horizontally, and the rim of the acetabulum should be relatively pointed (Figure 5-13). The second additional acetabular view is an open-leg lateral view of the acetabulum, which provides an oblique, unobstructed view of the acetabulum and the femoral head. To acquire this view, the subject is placed in lateral recumbency with the coxofemoral joint of interest positioned dependently, against the x-ray table. The nondependent leg is abducted laterally to the extent that its final position is actually dorsal to the dorsal aspect of the spine. The x-ray beam is then centered on the dependent hip joint of interest. The open-leg lateral view provides an unobstructed, slightly oblique view of the acetabulum and proximal femur (Figure 5-14).

Figure 5-13 Dorsal acetabular rim views of a 7-month-old Vizsla (A) and an 11-month-old Mastiff (B). In A, the dorsal acetabular rim is indicated by the black arrow. Note how the coxofemoral joint space just beneath the acetabular rim is oriented horizontally and that the acetabular rim is relatively pointed. In this young dog, the capital physis is also open (white arrow). The large protuberance (white arrowhead) is the tuber ischii; its physis is also visible. The dog in B is slightly older and was provided for comparison. The acetabular rim is indicated by the black arrow.

Figure 5-14 Open-leg lateral radiograph (A) and close-up view (B) of the right coxofemoral joint of a 4-year-old mixed breed dog. Note the unobstructed view of the coxofemoral joint. In A, note the position of the left femur (arrow), having been abducted laterally and dorsally.

The ischium, which is composed of a body, ramus, and tuberosity, contributes to the formation of the acetabulum, the obturator foramen, and the pubic symphysis (see Figure 5-8).¹ The pubic symphysis is characterized by a fibrous union in most dogs younger than 5 years of age; after that time there is usually complete bony fusion.² Before fusion, the relative radiolucency of the fibrous union of the pubic symphysis can be misinterpreted as a fracture (Figure 5-15). In male dogs, superimposition of fecal material, the caudal vertebrae, and the prepuce decreases the conspicuity of the pubic symphysis in many ventrodorsal radiographs by creating confusing summation opacities. The pubic symphysis is not visualized in lateral radiographs because the primary x-ray beam does not strike the junction in a parallel fashion in this projection.

Figure 5-15 Ventrodorsal radiographs of the pubic symphysis region of an 11-week-old golden retriever (A), an 18-month-old Welsh Corgi (B), and a 1-year-old golden retriever (C). Various stages of pubic symphysis (black arrows in each illustration) fusion can be seen in these radiographs. In A and C, the caudal vertebrae are superimposed on the pubic symphysis. In B, the os penis (white arrows) is superimposed on the pubic symphysis. In B, there is a triangular segment of bone at the caudal aspect of the pubic symphysis (white arrowheads); this is a separate center of ossification of the medial portion of the ischium at the ischiatic arch.

The caudal portion of the ischium, the ischiatic tuberosity, develops from a separate ossification center (Figure 5-16). Typically, ossification of the ischiatic tuberosity begins at the most lateral aspect of the ischium (see Figure 5-16, A). With age, the lateral aspect of the ossification center fuses with the ischium, and ossification of the cartilaginous tuber ischii continues medially (see Figure 5-16 B, C, and E). As ossification of the tuber ischii proceeds, some dogs will also develop a secondary center of ossification at the caudal aspect of the pubic symphysis, associated with the ischiatic arch (see Figures 5-15, C, and 5-16, C and E). In lateral radiographs of the pelvis, the secondary ossification centers of the caudal aspect of the pelvis are easily confused with fractures (see Figure 5-16, D, F, and G).

Figure 5-16 A, Ventrodorsal radiograph of the left tuber ischii of a 14-week-old golden retriever. B, Ventrodorsal radiograph of the left tuber ischii of a 7-month-old Labrador retriever. Ventrodorsal radiograph of the left tuber ischii (C) and lateral radiograph of the tuber ischii (D) of an 8-month-old Labrador retriever. Ventrodorsal radiograph of the left tuber ischii (E), lateral radiograph of the tuber ischii (F), and open-leg lateral (G) radiographs of the left tuber ischii of a 1-year-old Labrador retriever. In the 14-week-old dog (A), the secondary ossification center of the tuber ischii appears as a focal opacity at the lateral margin of the ischium (arrows). In the 7-month-old dog (B), the development of the ossification center of the lateral aspect of the tuber ischii has progressed and is nearly fused, but there is now ossification of the more medial aspect of the ossification center (arrow). In the 8-month-old dog (C-D), the lateral aspect of the ossification center of the tuber ischii has fused and ossification of the medial portion of the secondary center has progressed (black arrows in C). Also in this dog is a triangular opacity (the patient’s left side marked with white arrowheads in C) that represents a secondary center of ossification associated with the ischial arch. The incompletely fused tuber ischii (black arrows in D) and incompletely developed ischiatic arch (white arrowheads in D) could be confused with fractures in the lateral view. The 1-year-old dog (E-G) has tuber ischii ossification similar to the 8-month-old, but the separate center of ossification associated with the ischiatic arch is more conspicuous (arrows in E) as there is not overlap of feces or the caudal vertebrae. As in the 8-month-old dog, the incompletely developed tuber ischii (black arrows in F) and ischiatic arch (white arrowheads in F) could be mistaken for fractures. In the open-leg lateral view of the pelvis, the x-ray beam strikes the cleavage plane between the ossification center of the tuber ischii directly, leading to a very conspicuous radiolucent region (arrow in G) that could even more likely be confused with a fracture.

The pubic bone is composed of two rami and a body (see Figure 5-8). The pubic bone and the ischium bound the obturator foramen (see Figure 5-8). Assessing the symmetry of the obturator foramina in ventrodorsal radiographs is a good method to determine the degree of rotation of the pelvis that may have been present during radiography. Assessing the degree of rotation is important because rotation influences the apparent radiographic depth of the acetabulum and apparent radiographic congruity of the coxofemoral joint. With the dog in dorsal recumbency, the obturator foramina are not normally parallel with the dorsal plane; the plane of each foramen is angled from ventromedially to dorsolaterally. As a result, when the long axis of the body is rotated, the obturator foramen on the side opposite of the direction of rotation appears larger, and the coxofemoral joint on that side appears to have greater depth and congruency than the opposite coxofemoral joint (Figures 5-17 and 5-18). In addition, the iliac crest on the side of the direction of rotation appears wider because it is struck more en face by the primary x-ray beam (see Figure 5-18).