Articular Palmar Process Fracture

Figure 26: In this example of an articular palmar process fracture the fracture line is visible in the dorsopalmar view (white arrowheads). The fracture line extends toward the articular surface (black arrowhead) so articular involvement is highly suspected.

Notice that the fracture line is not visible in the lateral view. This is typical of an articular fracture - the fracture line is more dorsally located than a nonarticular fracture and is obscured by the superimposed bone of the distal phalanx.

Figure 27: As with the nonarticular fracture the oblique radiograph is needed to show the exact location of the fracture. The fracure line (arrowheads) is seen to extend to the articular surface. Discontinuity of the articular surface (a "step lesion") is seen - the black lines indicate the margins of the bone at the articular surface. The fracture is in the medial aspect of the distal phalanx. Which oblique radiographic projection is this?

Extensor Process Fracture

Figure 28: Fractures of the extensor process may be seen as incidental findings or may be a cause of lameness. Radiographic evidence of an extensor process fracture does not prove that it is the cause of lameness. The results of the radiographic study must be combined with the findings of the lameness examination and intra-articular anesthesia to determine the significance of the finding.

 A small fracture fragment is seen arising from the extensor process. The fracture fragment is relatively round and smooth and there is no radiographic evidence of degenerative joint disease. These findings suggest that the fracture is chronic and may, therefore, be an incidental finding.

Figure 29: This radiograph is from a horse with a chronic extensor process fracture. The original fracure line is faintly visible (arrowhead). The unusual, bulging shape of the extensor process is the result of remodeling. Despite the size of the fracture fragment and the extensive remodeling there is little evidence of degenerative joint disease (in fact, this fracture was an incidental finding - lameness was the result of navicular disease!)

Smooth periosteal response is present on the dorsal surface of the distal phalanx (arrow). This is usually an indicator of prior inflammation of the distal phalanx and may be related to the fracture.

Fracture of the Middle Phalanx

Figure 30: Fracture of the middle phalanx is most commonly seen in horses that perform activities that require sliding and turns on the hindquarters (the weight of the horse is on the hindlimbs only as the horse pivots). Polo ponies, Western performance horses (cutting and reining horses) and jumpers are the most likely to suffer a fracture of this bone during athletic activity. They may also occur during leisure activity (lunging, light riding, unrestrained paddock exercise). These fractures are typically comminuted and involve the articular surfaces of the proximal and distal interphalangeal joint.

The radiographs above are not of good quality but this is because a cast has been placed on the distal limb to stabilize the fracture for transport to the university. This is an appropriate level of care and can significantly improve the chance for successful repair of the fracture.

Figure 31: In the dorsopalmar view a fracture line is seen (arrows) extending from the proximal interphalangeal joint, obliquely through the middle phalanx to the distal interphalangeal joint. In the lateral view 2 fracture lines are seen (the arrowheads are at the proximal and distal aspects of each fracture line).

Because of the complexity of these fractures, radiographs tend to underestimate the number of fracture lines (and therefore, fracture fragments) that are present. Oblique views are also obtained in an effort to better define the fracture configuration. If available, computed tomography can be extremely useful in the evaluation of these fractures and allow for accurate surgical planning.

Fracture of the Proximal Phalanx

Figure 32: In this section we will show examples of proximal phalangeal fractures that involve the body of the bone - those fractures that involve only the proximal articular margin of the bone will be discussed with the fetlock joint. Fracture configurations of the body of the proximal phalanx are quite variable and range from incomplete sagittal fractures to comminuted fractures.

Incomplete fractures of the proximal phalanx begin at the proximal articular surface in the sagittal groove. They extend a variable distance into the proximal phalanx along a sagittal plane but do not exit the bone (this would be a complete fracture). They are common in Standardbred racehorses.

These fractures are usually only visible in the dorsopalmar (or dorsoplantar) radiograph. In the acute phase the fracture line may be difficult to impossible to visualize. Within 7-10 days bone resorption will occur along the margins of the fracture making the fracture line wider. Sclerosis of the surrounding bone may create increased opacity around the fracture. These changes allow the fracture line to be easily seen (red arrow).

Careful evaluation of the lateral view may show faint periosteal reaction along the dorsoproximal margin of the bone. This is not seen in the acute stage since periosteal new bone takes 2-3 weeks to be visible radiographically.

Although the diagnosis is obvious in this case, an acute incomplete fracture can be virtually impossible to detect. If an incomplete fracture is suspected from the clinical history, a conservative approach is indicated. This may consist of resting the horse and repeating the radiographs in 7-10 days. It is important to take several dorsopalmar projections at different angles to the joint and using different techniques when evaluating for a possible incomplete fracture. Slight overexposure of the dorsopalmar view will make a fracture line easier to see. On the other hand, slight underexposure of the lateral view will make subtle periosteal reponse easier to see. If the owner does not want to wait to retake films then nuclear scintigraphy can be performed to evaluate for the presence of bone activity. If a fracture is present a focal, intense area of isotope uptake will be present in the dorsal first phalanx (that is in fact how the above fracture was initially diagnosed).

Incomplete fractures of the proximal phalanx may progress and become complete fractures. Complete fractures may remain in the sagittal plane and exit the bone at the center of the distal articular surface or may exit along the lateral or medial aspect of the bone. Although this is only faintly visible in this view, oblique views demonstrated that the fracture in this case exited along the lateral aspect of the bone (arrowhead) proximal to the articular surface.

The red arrows indicate the fracture line within the bone. Although there is only one fracture two lines are visible. This is because the plane of the fracture is different in the dorsal and palmar cortices of the bone. The fracture line appears to cross over into the distal metacarpal bone (black arrow). The fracture is only in the proximal phalanx - this appearance is the result of superimposition of the articular surface of the proximal phalanx (dotted line indicates the palmar aspect of the articular surface) with the distal third metacarpus.

The prognosis of a fracture, particularly in an athlete, is significantly affected by articular involvement. In this case the fracture enters only the proximal interphalangeal joint. The prognosis is better than if it entered both the proximal and distal interphalangeal joints.

Figure 33: Lag screw fixation of this fracture configuration is the preferred treatment. This helps to stabilize the fracture and prevent further damage to the articular surface. If the alignment and compression is good very little secondary arthritic change should develop in the joint.

The distal screw enters the proximal phalanx along its lateral margin.T he 2 proximal screwheads appear to be placed within the bone but are actually on the dorsolateral bone surface. They were placed in this fashion to follow the slight "spiral" path of the fracture. The fracture line is still faintly visible but is much narrower indicating that good compression has been achieved.

Figure 34: In the "worst case" scenario, an incomplete fracture may progress to a highly comminuted fracture as in this example. If this occurs there is no surgical option. If a horse is economically valuable and has potential as a breeding animal an attempt may be made to treat the fracture with a cast or external fixator. The healing time is prolonged and the horse will often be significantly painful until the fracture heals. Even if fracture healing occurs the horse will generally be lame as a result of the severe arthritic changes that develop.

Horses with this type of fracture are often humanely destroyed. This is often the wisest choice both humanely and economically.

LAMINITIS

Laminitis is defined as inflammation of the laminae of the foot. Factors that may trigger the onset of laminitis include endotoxemia, overeating, local trauma and corticosteroid administration. Research suggests these and many other factors can trigger a peripheral vascular response within the feet. Vascular changes including decreased capillary perfusion and significant arteriovenous shunting lead to ischemic necrosis of the laminae.

Clinically the affected horse is lame and painful with the pain localized to the feet. There is increased heat in the feet and the palpable digital pulses are increased. Laminitis is most common in the forefeet but may occur in all 4 feet. It may also be seen in a single foot if the horse is non-weight bearing on the contralateral limb. Most horses with laminitis will stand with the forefeet stretched forward so that the majority of the weight is borne on the heels. They are generally quite reluctant to move and may spend a lot of time recumbent.

The radiographic changes of laminitis are the result of edema of the sensitive laminae and of loosening of the interconnections between the sensitive and insensitive laminae of the hoof.

Laymen often use the term "founder" as synonomous with laminitis. This is the "f" word of equine practice!

Radiographic Evaluation for Laminitis

Figure 35: Lateromedial views of the feet are the only views needed to evaluate for laminitis. The dorsal 65-degree proximal-palmarodistal oblique view may be used to evaluate the vasculature of the distal phalanx and to determine if bone resorption is present. However, this is not needed in most examinations.

It is important to be able to locate the dorsal surface of the hoof wall and the location of the coronary band when evaluating radiographs in laminitic horses.

This allows measurements to be made that help define the severity of the disease process and the prognosis for the horse. Placing a metallic marker (nail, horseshoe nail, etc) along the dorsal surface of the hoof wall with its proximal aspect at the coronary band allows easy identification of these structures.

In this case a horseshoe nail has been used to mark the hoof. The head of the nail is at the coronary band. Notice that although the radiographic technique used has overexposed the dorsal soft tissues of the hoof the dorsal margin can be identified by the marker.

Notice that in this normal horse the marker is parallel to the dorsal surface of the hoof wall. The hoof is excessively long in this horse but the skeletal structures are normal .

Laminar Edema

Some individuals with laminitis will have only laminar edema. This causes an increased thickness of the laminae that is seen as increased distance between the dorsal hoof wall and dorsal surface of the distal phalanx.

Figure 36: Radiographically, this appears as increased thickness of the dorsal soft tissues. The distance between the marker and the dorsal surface of the distal phalanx is measured perpendicular to the hoof wall, in three areas.

·Proximal - 2mm distal to the junction of the extensor            

 process and dorsal cortex of P3

·Distal - 6 mm proximal to the tip of P3

·Middle - halfway between proximal and distal

In normal horses the 3 measurements are the same. In a study evaluating Thoroughbred racehorses the dorsal soft tissue thickness was approximately 15 mm. A value of 18 mm or less is considered normal for light horses. The value may be slightly higher in Warmbloods and higher in Draft breeds.

The thickness of the dorsal soft tissues is affected by the size of the horse and also by radiographic magnification. In order to compensate for these factors a method of measurement has been used that compares the thickness of the dorsal soft tissues to the palmar cortical length of the distal phalanx. Use of a ratio removes the effect of horse size and magnification since both factors in the ratio are equally affected by these variables.

Figure 37: The soft tissue thickness in the middle (2) and distal (3) areas is compared to the length of the palmar cortex of the distal phalanx (1). The palmar cortex extends from the dorsal tip of the the distal phalanx to the articular margin(indicated by white line). In this example the dorsal soft tissues measured 11 mm and the palmar cortex measured 59 mm on the original radiographs. The ratio is therefore 19%.

In a study of Thoroughbred racehorses the normal soft tissue : palmar cortical length ratio was 23% in the middle area and 23.5% distally. It is suggested that a ratio of 28% or greater is consistent with laminar thickening.

Palmar Deviation of The Distal Phalanx

Figure 38: Laminar edema causes the interdigitations between the sensitive and insensitive laminae to loosen, especially those along the dorsal surface of the distal phalanx. As the horse bears weight, P3 moves downward in the hoof capsule causing separation of the lamina.

In addition, the deep digital flexor tendon pulls the tip of the distal phalanx in a palmar direction. The effect of these two actions is palmar deviation of the tip of P3. Because of this palmar movement of the tip of P3, the bone appears to "rotate" within the hoof capsule.

The common term for this palmar deviation is "rotation of P3."

Two methods may be used to determine the degree of palmar rotation of the distal phalanx.

Figure 39: Method 1 - Lines are drawn along the dorsal aspect of the hoof wall and distal phalanx (red lines). Notice how the metallic marker on the hoof wall helps in this process. A line is then drawn parallel to the ground surface of the hoof to intersect these two lines. The angles (1) and (2) are compared and in a normal horse should be approximately equal. If rotation is present angle (2) will be greater than angle (1). In the example used here angle (1) measured 58 degrees and angle (2) measured 60 degrees.

Figure 40: Method 2 - The distance between the dorsal surface of the hoof and the dorsal surface of the distal phalanx is measured in the three areas described above (proximal, middle and distal). The three measurements should be approximately equal. If rotation is present the distal and/or middle measurements will be greater than the proximal one. In the example used here the measurements are proximal = 25 mm, middle = 25 mm and distal = 28 mm.

Method 1 is the preferred method of evaluation since it determines the degree of rotation and the degree of rotation has been shown to be inversely related to the ability of the horse to return to athletic function.

Favorable prognosis - less than or equal to 5.5 degrees of rotation

Guarded prognosis - 6.8 to 11.5 degrees of rotation

Unfavorable prognosis - greater than or equal to 11.5 degrees of rotation.

Figure 41, 42: This is an example from a clinical case. Although rotation of the distal phalanx is clearly evident, placement of lines along the dorsal surface of the hoof and the distal phalanx allows the measurement of the degree of rotation. In this case there is approximately 10 degrees of rotation. The lucent area in the dorsal laminar tissue is gas. This is an indication of laminar separation.

"Sinking"

A variation of laminitis in which the entire distal phalanx sinks within the hoof capsule is commonly referred to as sinking (the horse is then referred to as a "sinker"). In these horses all of the laminae of the hoof (not just the dorsal laminae) loosen, and the weight of the horse drives P3 distally within the hoof capsule.

Clinically these horses tend to stand with the forefeet under the body (not out in front as in classic laminitis). They are extremely painful and reluctant to move. As the distal phalanx separates from the hoof and moves distally, an obvious palpable depression may develop at the coronary band.

Radiographically, sinkers have evidence of thickened dorsal soft tissues and an increase in the ratio of dorsal soft tissue thickness to palmar cortical length (some researchers consider an increase in this ratio to be an indicator of sinking). Additionally, the extensor process of P3 moves distally with respect to the coronary band. The coronary band is not usually visible as a distinct structure in a radiograph - this is why it is important to mark its position. Because the entire distal phalanx is moving distally, the dorsal surface of the hoof capsule and of P3 remain parallel.

The exact vertical distance between the coronary band and extensor process is quite variable between horses so it is difficult to determine if a horse is a sinker from one film series. Sequential film series may be compared for a change in the vertical distance between the coronary band and extensor process. An increase in this distance is considered evidence of sinking.

Preliminary work has been performed to establish the distance between these structures in normal horses but reference numbers for all horses are not yet available. Also, the method used to determine this distance is relatively complicated.

Figure 43: The vertical distance between the coronary band and extensor process is designated D. The true distance (corrected for magnification) can be calculated by using the formula

Actual Length of D = Length of D measured on the radiograph X Actual length of the marker

Length of marker measured on radiograph.

Chronic Laminitis

If a horse has had chronic (> 3-4 weeks) laminar inflammation, radiographically detectable remodeling of the distal phalanx will occur.

Figure 44: Flaring of the dorsal solar border of P3 is a characteristic change of chronic laminitis. The tip of P3 may have a distinct "ski-tip" appearance (see inset left) or may appear fuzzy and indistinct. Thickening of the dorsal cortex of P3 may occur (arrows right).

If the change is active the margins of the cortex may appear slightly fuzzy; if inactive the margins will be smooth. These radiographic changes do not usually regress if the laminitis resolves - therefore, they may be seen in animals that have no current clinical evidence of laminitis.

Figure 45: If severe and long-standing laminitis is present. resorption of much of the distal phalanx may occur. This radiograph is from a pony with severe, chronic laminitis (remember, there are two types of ponies - those that have laminitis and those that will have laminitis!!). The hoof is misshapen and the distal half of the distal phalanx is no longer visible. The proximal sesamoid bones are very lucent.

Figure 46: this change is consistent with disuse osteopenia. The pony is, for obvious reasons, bearing little weight on this limb.

MISCELLANEOUS

Osteomyelitis

Osteomyelitis may occur in any of the phalanges, usually as the result of a penetrating wound or surgery.

Osteomyelitis of the distal phalanx occurs relatively frequently following penetration of the sole by a sharp object (nail, sharp metal, etc). The radiographic appearance of osteomyelitis of the distal phalanx is somewhat different from that of other bones. Because the distal phalanx has a modified periosteum there is little evidence of periosteal proliferation. The dominant feature of osteomyelitis of the distal phalanx is bone lysis. Bone lysis may not be radiographically visible for 10-14 days following injury and in the early phase the lysis can be quite subtle. This is why it is important to re-radiograph the distal phalanx if the horse fails to respond to appropriate treatment following penetrating injury to the foot.

Figure 47: Dorsopalmar and dorsopalmar oblique views of the distal phalanx are needed to evaluate for osteomyelitis. In this radiograph, an area of bone resorption is evident along the solar margin of the distal phalanx (arrows). The areas of opacity in the tissue around the distal phalanx are material within the hoof.

Bone Cyst

Occasionally, bone cysts (syn. - subchondral bone cysts) occur in the phalanges as a result of osteochondrosis - a developmental orthopedic disease. The cysts may occur adjacent to any joint but are most typically seen in the distal articular surface of the proximal phalanx, proximal articular suface of the middle phalanx and at the articular surface of the distal phalanx.

Remember that osteochondrosis is the result of a failure of enchondral ossification. A cyst is formed by the retention of cartilage within the bone immediately adjacent to the articular surface. This thickened area of cartilage undergoes necrosis and is visible as a circular lucency in the subchondral bone. Initially, the articular cartilage over the cyst may be intact. If a defect develops in the articular cartilage the necrotic material within the cyst drains into the joint and causes synovial inflammation. This begins the cycle of degenerative joint disease.

Figure 48:This dorsopalmar view shows a very large cyst in the center of the distal phalanx (arrows). There is no obvious connection between the cyst and the distal interphalangeal joint in this radiograph. A dorsopalmar horizontal beam view would also demonstrate the lesion and possible connection to the joint (this is not always visible radiographically however).

Keratoma

Keratomas are benign tumors that arise from the keratin containing cells of the lamina of the hoof. They are relatively rare. The tumors grow as soft tissue masses within the hoof capsule. Because there is little room for expansion of the mass, with increasing size resorption of the distal phalanx occurs as a result of pressure necrosis. Clinically, the horses are chronically lame. In some cases the soft tissue mass may be palpable above the coronary band.

Radiographically, an area of bone resorption will be seen in the distal phalanx. The area of bone resorption tends to be relatively large by the time the horse is significantly lame and radiographs are obtained. The bone resorption may occur anywhere within the distal phalanges.

Figure 49: These radiographs are typical of a keratoma. The area of bone resorption is visible in the lateral view (black arrows) but is considerably more obvious in the dorsopalmar view (white arrows) . Although there is bone loss as in osteomyelitis the large size of the lesion and the distinct margination make a diagnosis of osteomyelitis unlikely. Another key differentiating factor in this case may be the history - gradual onset of lameness with no history of penetrating wound (keratoma) vs. acute onset of relatively severe lameness following a penetrating wound to the foot (osteomyelitis).

Very rarely, other types of soft tissue tumors arising from the laminar tissue will create this radiographic appearance. Tumor types that have been reported in the literature include hemangioma, squamous cell carcinoma and intraosseous mast cell tumor.

Ossification of the Accessory Cartilages (Sidebone)

Figure 50: Ossification of the accessory cartilages of the distal phalanx occurs to some extent in most horses. It is only when the ossification is extensive that a clinical problem may develop. Many horses with radiographic evidence of cartilage ossification have no lameness related to it.

Excessive ossification is thought to be related to trauma to the cartilages as a result of concussion to the quarters of the hoof. The concussive force to this area may be worse in horses with poor conformation, as a result of poor shoeing or as a result of work performed on hard surfaces. When draft horses worked on cobblestone streets sidebone was more often a cause of lameness.

Figure 51:In the dorsopalmar view the accessory cartilages are visible as mineralized structures extending proximally. The lateral cartilage (arrow) is large and well mineralized. The lucent line between the ossified cartilage and the remainder of the distal phalanx is an area of non-ossified cartilage between the bone and the ossified cartilage, not a fracture line. The medial accessory cartilage has less obvious mineralization (arrowhead).

In the lateral view the faint mineral opacity palmar to the middle phalanx (arrows) is the superimposed ossified lateral accessory cartilage

REFERENCES

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

Butler JA et al. Clinical Radiology of the Horse. Blackwell Scientific Publications. 1993

Stashak TS. Adams' Lameness in Horses 4th ed. Lea & Febiger. 1987

Thrall DE. Textbook of Diagnostic Veterinary Radiology 4th ed. Saunders. 2002

Linford Rl, O'Brien T, Trout DR. Qualitative and morphometric radiographic findings in the distal phalanx and digital soft tissues of sound Thoroughbred racehorses. AJVR 54(1),1993.

Stick JA et al. Pedal bone rotation as a prognostic sign in laminitis of horses. JAVMA 180(3),1982.

Cripps P, Eustace RA. Radiological measurements from the feet of normal horses with relevance to laminitis. Eq Vet J 31(5),1999.