Phalanges

The radiographic views needed to evaluate the phalanges will depend on the area of interest. The views needed to evaluate the third phalanx are different than those needed to evaluate the pastern.

Proper labeling of all phalangeal radiographs is important. The structures of the fetlock joint and distal to it are symmetrical and provide no anatomic landmarks for orientation. As with all limb radiographs the markers will be placed along either the lateral or cranial aspect of the limb.  

THIRD PHALANX

Routine evaluation of the third phalanx consists of 2 views

·Lateromedial (Lateral)

·Dorsal 65-degree Proximal-Palmarodistal Oblique (D65Pr-PaDiO)*

Optional views that may be used to evaluate the third phalanx include

· Dorso 65-degree Proximal 45-degree Lateral-Palmarodistal Medial Oblique (D65Pr45L-PaDiMO)*

· Dorso 65-degree Proximal 45-degree Medial-Palmarodistal Lateral Oblique (D65Pr45M-PaDiLO)*

· Dorsopalmar (Horizontal Beam)*

* If the hind foot is radiographed substitute plantar for palmar.

LATEROMEDIAL VIEW

Figure 1: The lateromedial view (commonly referred to as a lateral view) is obtained with the horse standing on a block. The x-ray beam is centered on the foot, at the level of the coronary band.
Figure 2: The lateral radiographic projection allows evaluation of most of the 1st phalanx and the entire 2nd and 3rd phalanges. This is the same view that is used to evaluate the navicular bone.

DORSAL 65-degree PROXIMAL -PALMARODISTAL OBLIQUE VIEW

Figure 3: This view is obtained with the horse standing on the cassette (contained within a holder for protection). As with other oblique views the name of the view describes the direction of the x-ray beam. The beam is aimed from dorsoproximal to palmarodistal at a 65 degree angle to the supporting surface. The beam is centered on the foot and directed at the level of the coronary band. The name listed above is the proper designation for the radiographic projection. However, in the real world this view is generally referred to as the dorsopalmar view.

Figure 4: This is the same dorsopalmar view that is used to evaluate the navicular bone. However, much less penetration of the x-ray beam is required to produce adequate radiographs of the third phalanx. The exposure factors used are decreased to avoid overexposure of the margin of the third phalanx.

In this radiograph the technique used allows the margins of the third phalanx to be visualized but the area of the distal interphalangeal joint is underexposed. A second view of the area using higher exposure factors would be needed to evaluate the joint.

DORSO 65-DEGREE PROXIMAL 45-DEGREE LATERAL-PALMARODISTAL MEDIAL OBLIQUE (D65Pr45L-PaDiMO)

As with all radiographic projections this view is named by the direction of the x-ray beam. As with the dorsopalmar view the horse is standing on the cassette (within a protective holder). The x-ray beam is aimed from dorsoproximal to palmarodistal at a 65-degree angle to the supporting surface AND from lateral to medial at a 45 degree angle to the dorsal surface. The beam is directed at the level of the coronary band.

Figure 5: This view allows evaluation of the lateral aspect and palmar process of the third phalanx. Notice how well the lateral aspect of the coffin joint is visualized.

This view is most often used to evaluate for the presence of a fracture of P3 - there is a fracture visible in this radiograph - can you find it?

The dorso 65-degree proximal 45-degree medial-palmarodistal lateral oblique (D65Pr45M-PaDiLO) is the same view of the medial aspect of the third phalanx. In this case the x-ray beam is aimed from medial to lateral at a 65-degree angle to the dorsal surface.

DORSOPALMAR (HORIZONTAL BEAM)

Figure 6: As indicated by the name the x-ray beam travels from dorsal to palmar in a horizontal direction. The horse is made to stand on a block in order to place the coronary band at the level of the x-ray beam. The cassette is behind the limb, perpendicular to the x-ray beam.

Although it is relatively easy to obtain this view in the forelimbs it may be difficult to get a horse to place the hind feet on blocks.

Figure 7: This view provides good evaluation of the proximal and distal interphalangeal and metacarpophalanageal joint spaces (the metacarpophalangeal joint has been "cropped" from the radiograph).

This view also allows evaluation of the symmetry of the third phalanx. Poor foot care can result in a "hoof imbalance" over time. This is visible in the foot itself but may also be seen as asymmetry of the distal phalanx.

The projections of bone along each side of the distal interphalangeal joint are ossified accessory cartilages. The ossification along the right side is extreme. This is commonly referred to as "sidebone." The linear lucency that separates the ossified cartilage from the bone on each side is not a fracture, it is an area of cartilage that has not yet ossified.

PROXIMAL INTERPHALANGEAL JOINT

Routine evaluation of the second and third phalanges and proximal interphalangeal joint consists of 2 views :

• Lateromedial (Lateral)

• Dorsopalmar (Horizontal Beam)*

These views are the same as those described above for the third phalanx.

Optional views that may be used to evaluate the pastern joint include

• Dorsolateral-Palmaromedial Oblique*

• Dorsomedial-Palmarolateral Oblique*

* If the hind foot is radiographed substitute plantar for palmar.

DORSOLATERAL-PALMAROMEDIAL OBLIQUE

Figure 8: In this view the x-ray beam passes from the dorsolateral aspect of the limb (approximately 45 degrees lateral to the dorsal surface) to the cassette at the palmaromedial aspect of the limb. Notice that the cassette is parallel to the angle of the pastern and that the x-ray beam is perpendicular to the cassette. The x-ray beam is centered at the level of the proximal interphalangeal joint.

To include the distal interphalangeal joint and third phalanx in the image the horse is positioned with the foot elevated on a block. In this case the block is designed to hold the cassette - this limits radiation exposure to personnel.
Figure 9: This view allows the dorsomedial and palmarolateral margins of the phalanges and the interphalangeal joints to be evaluated. This may be helpful in cases of arthritic change and in defining fracture lines in the first and second phalanges.

The opposite oblique view (dorsomedial - palmarolateral) is very similar in appearance. Again, correct markers and marker placement are needed for correct interpretation of the radiograph.

REFERENCES

• Morgan JP. Techniques of Veterinary Radiography 5th ed. Iowa State University Press. 1993

• Smallwood JE et al. A standardized nomenclature for radiographic projections used in veterinary medicine. Veterinary Radiology 26(1), 1985;pp 2-9.

• Shively MJ. Synonym equivalence among names used for oblique radiographic views of distal limbs. Veterinary Radiology 29(6), 1988;pp 282-284.

RADIOGRAPHIC ANATOMY OF THE PHALANGES

A brief review of the normal radiographic appearance and important anatomic structures of the phalanges is indicated prior to any discussion of radiographic abnormalities.

Let us begin our discussion of the normal radiographic appearance with the dorsal 65-degree proximal - palmarodistal oblique view (referred to as the dorsopalmar view for purposes of simplicity). Below are a labeled diagram

Figure 10 :and a radiograph of this view.

A = Proximal phalanx (P1), B = Middle phalanx (P2), C = Navicular bone, D = Distal phalanx (P3), 1 = Angle of the heel and the frog, 2 = Palmar margin of the distal interphalangeal joint, 3 = Dorsal margin of the distal interphalangeal joint, 4 = Solar margin, 5 = Vascular channels, 6 = Distal interphalangeal joint, 7 = Palmar process, 8 = Ossifying accessory cartilage of P3, 9 = Solar canal.

Notice that the structures of the distal interphalangeal joint and the navicular bone are not as clear in the radiograph as they are in the diagram. As noted above an exposure adequate to evaluate the solar margin of P3 results in underexposure of the thicker areas such as the joint and the navicular bone. Proper exposure of the distal interphalangeal joint would create significant overexposure of the margin of the third phalanx (burn-out!).

The soft tissue structures are normal in the radiograph. The thin band of mineral opacity along the left side of the third phalanx is opaque material on the solar surface of the foot. This is quite common in radiographs of the feet and should not be mistaken for pathology. Packing material (usually Playdough) is often placed into the sulci of the frog - if this is not done the air within the frog shows up as linear lucent bands. The packing material is usually of soft tissue opacity and may be seen superimposed over the third phalanx. This should not be mistaken for pathology.

The accessory cartilages of the third phalanx have varying degrees of ossification. This radiograph is from the same horse as the horizontal beam dorsopalmar view above. Notice the assymmetry between the palmar process regions (7). The palmar process on the left side is normal; on the right the palmar process is surrounded by a large area of bone proliferation which extends abaxially and palmar to the process. This is the ossified accessory cartilage.

The vascular channels (5) arise from the solar canal (9) and extend to the solar margin of P3 (4). These structures actually are vascular channels (unlike those in the navicular bone) and provide the blood supply to the bone. The vascular channels can be mistaken for fracture lines. In general, fracture lines are more straight and distinct and do not tend to travel toward the center of the bone. The vascular channels may appear widened and irregular if inflammation of the third phalanx is present. However, this change can be quite subtle. Causes of inflammation include laminitis, pedal osteitis and osteomyelitis.

Notice that both the palmar (2) and dorsal (3) margins of the distal interphalangeal joint are visible in the radiograph. This is the result of the angle at which the x-ray beam intersects the joint.

Below are a labeled diagram and radiograph of the lateromedial view. This is typically referred to as the lateral view.

Figure 11, 12:

A = Proximal phalanx (P1)

B = Middle phalanx (P2)

C = Distal phalanx (P3)

D = Navicular bone

1 = Extensor Process

2 = Dorsal surface

3 = Solar canal

4 = Solar margin

5 = Palmar processes (superimposed)

6 = Distal interphalangeal joint (coffin joint)

7 = Proximal interphalangeal joint (pastern joint)

The lateral radiographic projection allows good evaluation of the margins of the proximal interphalangeal joint (7) and distal interphalangeal joint (6). These joints may be referred to by horsemen as the pastern joint and coffin joint, respectively. The width of the joint spaces can be evaluated in the lateral view - however, the appearance of the joint spaces can be greatly affected by the way the animal is standing and the angle of the x-ray beam, so should not be over interpreted.

Evaluation of the lateral view is more straight-forward than some of the other views used to evaluate the phalanges. One area of confusion is the superimposition of the ossifying accessory cartilages and the navicular bone. This may create an impression of bony proliferation along the flexor surface of the navicular bone (red arrows). This degree of bony proliferation would be highly unusual on the navicular bone and if this were actually bony proliferation it should be visible in the palmaroproximal- palmarodistal view (aka flexor skyline view) of the navicular bone.

The soft tissue structures of the heel are prominent and are often confused with soft tissue swelling. Examination of the foot of the horse should help to confirm that there is normally a large amount of soft tissue in this region.

The oblique views of the third phalanx (D65Pr45L-PaDiMO and D65PrM-PaDiLO)are used to evaluate the palmar processes of the third phalanx and the margins of the distal interphalangeal joint. These views are included in a full radiographic series of the third phalanx, usually when a fracture is suspected.

Figure 13: In this radiograph the palmar process is clearly visible (black arrowhead). The lucent area adjacent to the palmar process is gas within the sulcus of the foot. The articular surface of the distal phalanx is indicated by the black arrow.

Many vascular channels are visible radiating from the articular surface of the bone to its solar margin (white arrows). With experience it becomes easier to differentiate vascular channels from fracture lines.

Proper exposure of the margin of the third phalanx causes the central area (including most of the distal interphalangeal joint) to be under-exposed.

Figure 14, 15: Horizontal beam dorsopalmar views are not considered a part of the routine radiographic evaluation of the phalanges. However, this view may be used when complete evaluation of the distal interphalangeal joint is needed.

A = Proximal phalanx (P1), B = Middle phalanx (P2), C = Distal phalanx (P3), D = Navicular bone, 1 = Proximal interphalangeal joint (pastern   joint), 2 = Nutrient foramen of the middle phalanx, 3 = Distal interphalangeal joint (coffin joint), 4 = Solar canal, 5 = Parietal sulcus of the distal phalanx, a and b = the height between the distal border of the distal phalanx and the ground surface

The distal interphalangeal, proximal interphalangeal and metacarpophalangeal joints are visible in a dorsopalmar horizontal beam view (the metacarpophalangeal joint has been "cropped" from the image above). The joint spaces decrease in width from distal to proximal - i.e. the distal interphalangeal joint is the widest and the metacarpophalangeal joint the narrowest.

Each joint space should be of equal width across its' entire surface. Remember that the appearance of a joint space is created by the articular cartilage and fluid within the joint, it is not actually a space. Symmetrical widening of a joint space suggests an increase in synovial fluid; symmetrical narrowing suggests loss of cartilage.

Asymmetry of the joint space may be the result of positioning or pathology. If it is the result of positioning all 3 of the joint spaces will demonstrate similar asymmetry. If it is the result of pathology only the affected joint space will be asymmetrical.

If the dorsopalmar horizontal beam is well-positioned it can be used to evaluate balance of the hoof. If a hoof is properly balanced the distances between the distal border of the distal phalanx and the ground surface will be symmetrical across the bone ("a" and "b" above will be equal). A diagnosis of hoof imbalance is generally made based on the appearance of the foot but can be substantiated with radiographs.

A routine series of the proximal interphalangeal joint consists of dorsopalmar and lateral views. Oblique views (DLPMO / DMPLO) may be added to provide additional information about the margins of the joint and the bone surfaces.

Correct labeling of these radiographs is imperative as there is no anatomic landmark to help differentiate the lateral and medial surfaces of the bones.

Figure 16: Small areas of roughened bone are present on the dorsomedial and dorsolateral margins of the middle phalanx. These are the areas of attachment of the collateral ligaments of the navicular bone (syn. suspensory ligaments of the navicular bone). This close-up view of the dorsolateral aspect of P2 shows this area (red arrow). This normal appearance may be mis-diagnosed as an area of proliferative periosteal response.

RADIOGRAPHIC ABNORMALITIES OF THE PHALANGES

This section will discuss some common radiographic abnormalities of the phalanges.

DEGENERATIVE JOINT DISEASE

Degenerative joint disease (DJD) is one of the most common causes of lameness in the horse. Degenerative joint disease may be primary (the result of "wear and tear") or secondary (due to an identifiable etiology such as joint instability, presence of a fracture fragment etc). The radiographic appearance of degenerative joint disease is the same no matter what the cause.

Radiographic changes of early and/or mild DJD include the following

Increase in intracapsular soft tissue (effusion and/or synovial thickening)

Osteophyte production (proliferation of bone at the junction of articular cartilage and bone)

Enthesiophyte production (proliferation of bone at the insertions of joint capsules, tendons and ligaments). With late and/or severe DJD the following radiographic changes may also be present.

Narrowing of the joint space.

Cystic areas of subchondral demineralization.

Ankylosis.

Degenerative joint disease of the interphalangeal joints

The layman's term for degenerative joint disease of the interphalangeal joints is "ringbone" - low ringbone occurs in the distal interphalangeal joint and high ringbone in the proximal interphalangeal joint.

Figure 17: In the lateral radiograph it is difficult to see any abnormality of the distal interphalangeal joint. However, close inspection of the dorsal aspect of the joint (inset) shows small, sharp osteophytes on the extensor process of the distal phalanx and at the margin of the articular surface of the middle phalanx (arrows). Notice that the osteophyte on the extensor process is more lucent than the adjacent bone - this is typical of osteophytes as they are forming. The radiographic changes seem fairly minor. However,the distal interphalangeal joint does not tolerate DJD well and relatively little arthritic change may be present for the degree of lameness.

Figure 18: In this radiograph there is more obvious osteophyte formation on the extensor process of the distal phalanx. Significant periosteal proliferation is also present on the dorsodistal aspect of the middle phalanx. These changes are evidence of more advanced degenerative joint disease.

The radiographic changes of DJD at the proximal interphalangeal joint may be as subtle as those shown in the distal interphalangeal joint above or may be much more obvious.

The radiographs below are from a 13-year old Appaloosa with lameness of the right fore limb. The radiographic changes are evidence of severe degenerative joint disease of the proximal interphalangeal joint.

DORSOPALMAR VIEW

Figure 19: Narrowing of the proximal interphalangeal joint space (red arrows) is present. The narrowing is severe and symmetric.

With careful evaluation subchondral lucencies can be seen in the distal surface of the proximal phalanx.
LATERAL VIEW

Figure 20: Significant periosteal response is present on the dorsal margins of the proximal interphalangeal joint (white arrows). Notice that the periosteal response extends well away from the joint margins. This is often termed "extra-articular" ringbone. This term is somewhat misleading as it implies that there is no involvement of the joint in the process.

Narrowing of the proximal interphalangeal joint space is also visible in this view but is more difficult to appreciate than in the dorsopalmar radiograph.
DMPLO VIEW

Figure 21: The oblique views are useful to show extension of the periosteal response to the dorsolateral and dorsomedial margins of the proximal interphalangeal joint (white arrow). The periosteal response often encompasses the entire dorsal surface of the joint, thus the term "ringbone."

Figure 22: Narrowing of the proximal interphalangeal joint space is also apparent (arrowheads).

FRACTURES

Fractures of the phalangeal bones are relatively common, usually occurring during athletic activity. Fractures of the distal phalanx occasionally occur from the horse kicking a stationary object (i.e. the wall).

Fractures of the distal phalanx are classified based on their location. The diagrams below show the common types of fractures

Figure 23.

I = Nonarticular oblique palmar/plantar process (wing) fracture

II = Articular oblique palmar/plantar process (wing) fracture

III = Sagittal articular fracture

IV = Comminuted fracture - articular or nonarticular

V = Solar margin fracture

VI = Extensor process fracture (variable size)

This classification scheme is from Adams' Lameness in Horses but other authors use a different classification system. For example Thrall's Veterinary Diagnostic Radiology uses the following system I = Nonarticular oblique palmar process (wing) fracture

II = Articular oblique palmar process (wing) fracture

III = Sagittal articular fracture

IV = Extensor process fractures (variable size)

V = Comminuted fracture of body or fracture owing to foreign body penetration or osteomyelitis

VI = Solar margin fracture

Because of this variability it may be better to describe the fracture configuration than to use a numbering system.

Fractures of the palmar process are the most common types - articular fractures are more common than nonarticular fractures. These fractures may be visible in the dorsopalmar view but oblique views are almost always needed to determine if articular involvement is present. Articular involvement has a significant effect on the prognosis and outcome of distal phalangeal fractures so is a key fact to be determined by the radiologic examination.

Nonarticular Palmar Process Fracture

Figure 24: In the radiographs above a fracture line (arrowheads) is visible in one of the palmar processes of the distal phalanx (lateral based on the labeling of the dorsopalmar view that has been omitted from the image). This case is somewhat unusual in that the fracture line is seen very well in the lateral view. However, with only these views it is difficult to determine if the fracture involves the articular surface.

Figure 25: The DLPMO view is used to provide better visualization of the lateral palmar process. The fracture line is much wider and easier to see (white arrows and arrowheads). The fracture line extends to the surface of the palmar process immediately adjacent to the articular surface (denoted by the red line). This is a nonarticular fracture but just barely!