Treatment of a Fractured Bavicular Bone

How to Use Accurate Diagnostic Tools for Evidence Based Treatment of a Fractured Navicular Bone Kristina Grewal, DVM; Monique Craig, BSCE

With the use of calibrated digital radiographs and photographs the healing process can be accurately monitored. This evidence-based approach provides the practitioner with a thorough database, enabling an individualized therapeutic protocol.

Introduction “Beau”, a 5yo, 17.1h QH/TB, presented with a fracture of the navicular bone of the left front foot on June 2, 2010 to Alamo Pintado Equine Medical Center, Los Olivos, California. Digital radiographic and Magnetic Resonance Imaging studies were performed to visualize and delineate the extent of the fracture. Surgical fixation was offered to repair the oblique fracture1 but was declined by the client due to the young age of the horse. Less aggressive treatment was pursued with casting and immobilization of the foot with heel elevation shoe treatment2 . After a year of treatment “Beau” was not showing any response; no improvement in lameness score or level of discomfort. The horse came to us with a 4/5 Obel grade lameness on July 29, 2010. After 3 months of an evidence-based treatment protocol, “Beau” was started back into training and was no longer exhibiting any signs of lameness. After 4 months of a strict graduated exercise program he was back in full training and is currently back in competition. It was advantageous to our protocol to have access to a quality set of previous diagnostic studies.

Materials and Methods Measurements A comprehensive series of digital skyline navicular views were taken to monitor the fracture. Calibrated lateral radiographs were used to make accurate measurements 3 including palmar angle and sole thickness. Radiographs (Lat, DP) and digital photographs (Lat, Solar, DP) were taken at each stage of the trimming and shoeing treatments.

Some care needs to be taken to be able to make an accurate measurement of any physical object from photographs or radiographs. Accurate measurements require some form of calibration – optimally, a scale marker is placed in the image at the time it is taken.

Calibration of Photographs For lateral and frontal photos of the hoof, we used a specialized hoof block which has a built in scaling system on its sides (figure 1). Two calibration points are on the outer surface of the block and two more calibration points are in wells that go into the block

and under the foot. All measurements are computed so they are accurate along the centerline of the block.

Figure 1: The specialized block is used for lateral and frontal photos of the hoof. Special calibration points on the sides and front of the block aid in calibration.

By using this “four point calibration” scheme, the block allows for the camera to be held at any distance from the block. Using the four points, and the known geometry of the block (spacing between points, the depths of the wells, etc) the Metron softwarea can compensate for the focal length of the camera and always provide accurate measurements in the plane corresponding to the centerline of the block.

Calibration of Radiographs In radiographs there is always magnification in the resulting images due to divergence of the generator beam. Without careful consideration of calibration, it is not possible to measure things accurately in radiographs. Because of the magnification effect, it is important when taking a radiograph to place a metal marker of known size in the same plane in which measurements are to be made, called the “plane of interest”. When taking a lateral radiograph of a horse’s lower digit, it is useful to consider the “plane of interest” to be the centerline of the hoof capsule. Accurate measurements can then be made anywhere in the imaginary plane which is aligned with the centerline of the hoof capsule.

Our calibration block has metal parts within it that will become apparent in a radiograph (figure 2.) The Metron software automatically locates this metal scale marker, and then uses it to calibrate the image.

Figure 2: In the original radiograph (A), the metal scale marker built into the block can be seen. In (B) it has been automatically located by the Metron software, which uses it to scale the image so that measures (C) can be made accurately in the image.

A difference of about 20% is a typical error margin that can be made if calibration is not taken into account by the imaging software.

Figure 3: There can be as much as 20% error in measurements made in radiographs that have not been calibrated. Image B shows the true length of 1.44”, whereas image A has the magnified value of 1.73”.

Therapeutic Protocol Initial treatment consisted of semi-confinement in a 20’x30’ turnout with unlimited hand walking and therapeutic trimming and shoeing every 4 weeks. As lameness signs diminished, exercise was exponentially increased. Composite shoesb, gluec and packingd were used. Trimming and shoe placement was guided through evaluation of radiographs and based on a three dimensional trimming approach4 . Using external landmarks proximal to the hoof associated with the bony column allows visualization of the distal phalanx within the hoof capsule. The primary landmarks, coronary gaps, are located by palpation of the dorsal aspect of the coronary band (figure 4.) Secondary landmarks were also used to reference to the bony column on the distal aspect of the first phalanx. The bony eminences of the medial and lateral aspects of the distal end of the first phalanx coincide with the insertion of the collateral sesamoidean ligaments.

Figure 4: Projecting the coronary gaps to the sole to aid in aligning the trim and shoe placement to the bony column.

The coronary gaps are projected orthogonally to points on the sole (figure 4) and serve as a guide for trimming and shoe placement that references the bony column, rather than

following deviations in the growth of the hoof. Caudal support of the arch and sole was achieved by evenly packing the collateral grooves and sole from the frog apex to heel with an FDA-approved RTV silicon packing material. The composite shoe was then set to the sole with a small ribbon of methacrylate glue applied on the periphery of the shoe.

The shoe was beveled at the toe to ease breakover.

Results A second MRI was performed on February 28, 2011 at Alamo Pintado Equine Medical Center to confirm our clinical observation and radiographic conclusion of fibrous union healing of the fracture (figure 5.) There is high signal material present within the oblique sagittal fracture line of the medial aspect of the navicular bone. Mild displacement of the distal solar aspect of the fracture is present but the proximal portion maintains alignment.

A low signal osteophyte and bony callus is present along the proximal aspect of the fracture but it has not yet bridged the fracture line. There is irregularity to the distal articular margin of the navicular bone as well as regions of low to moderate signal. Mild effusion is present within the distal inter-phalangeal joint. Marked hypertrophy and mild signal increase of the suspensory ligament of the navicular bone is present. The deep digital flexor tendon and ligament of the foot are normal.

Figure 5: Top two MRI views are from June 2009; Lower two MRI views are from February 2011.

The MRI results are consistent with healing of the navicular bone via fibrous union.

There is some mild displacement of the two halves of the fracture along with sclerosis of the medullary cavity of the bone. There is some mild to moderate bone edema still present as well as moderate desmopathy of the suspensory ligament of the navicular bone.

Figure 6: Skyline navicular views from June 2009 (A); April 2010 (B); July 2010 (C); September 2010 (D). . .

Discussion In recent years, many advances have been made in understanding the kinematics and biomechanics of the equine distal limb. This has given way for the opportunity to move from the research bench to evidence-based clinical application and give the veterinarian a clearer path to guide individualized treatment. Evidence-based medicine allows for more rational clinical decision making and the ability to economically and more efficiently practice high quality medicine5 . This directly benefits our patients and clients In our case, we were fortunate to have pre-existing quality data. We were able to assess any underlying hoof abnormalities from the initial injury through previous treatments, not

just at the start of our treatment. This allowed for expedited clinical observations, such as an abnormally high palmar angle for this horse. Earlier radiographs demonstrated that the foot was stable and the horse was capable of growing sole (figure 7A.) Based on this clinical evidence, the palmar angle could more quickly be adjusted back to normal range without risk of injury to the patient.

Figure 7: In April 2010 (A), the palmar angle was within normal limits (3.1 deg.) and there was adequate sole depth. Upon initial presentation to us in July 2010 (B) there was a high palmar angle and minimal sole depth.

In this case, the excessive palmar angle was restored by reduction of the heels (figure 8.)

The heels were trimmed so the most caudal aspect of the heels was level with the exfoliated frog. The sole being thin was left untouched until adequate sole depth was measured. The sole depth and palmar angle were measured during each shoeing session.

The sole depth improved from 0.18 inches to 0.42 inches, and the palmar angle was reduced from 9.3 degrees to 4.0 degrees. Traditional shoeing therapy includes shoeing with a regular flat shoe with four three degree wedge pads6 .

Figure 8: Images (A) were taken at the time the horse came into our care. Images (B) were taken 3 months later.

We chose the use of composite shoes and glue because of the ability to allow the entire hoof capsule and collateral cartilages to flex, thus increasing blood flow. This is obviously important for tissue healing. The wide webbed and egg / heart bar styling of the shoe allows for full digital support due to load sharing. Force plate analysis has shown that egg bar shoes increase the compressive force on the heels in a manner similar to a wedge pad7 . The final component is the packing, which is specially designed for

shock absorption and to support the frog and caudal area of the hoof. This is accomplished by supporting the internal arch of the hoof resulting in a beneficial change to both the orientation of the pedal bone and navicular bone, and also the stance of the lower limb (figure 9.) The use of packing appears to help stabilize motions of the internal structures of the hoof (figure 10) and may have an effect on reducing the stress in the DDFT and on the collateral sesamoidean ligaments. The use of packing perhaps emulates the effect of packed dirt and mud in the natural barefoot horse. The combination of the packing with the shoe allows for loading of the frog which is integral for helping to increase blood flow8 .

Figure 9: The use of packing: the angulation of the navicular bone (2.3 degree increase) and pedal bone (2.8 degree increase) are beneficially changed. This type of support may replace or diminish the need for a wedge pad. (note: these images for illustrative purposes, these are not of the horse of our report)

Figure 10: From our study utilizing a Cadaver leg in a press: packing affects the descent of the sole and the bony column.

In a case where traditional therapy was not effective using an evidence-based approach allowed us to find a path that was effective for this patient.

References and Footnotes 1. Colles, CM, How to Repair Navicular Bone Fractures in the Horse, in Proceedings, The Annual Convention of the AAEP, Vol 47, page 270, 2001.

2. Turner, TA, Malone, E, How To Treat Navicular Bone Fractures, in Proceedings, The Annual Convention of the AAEP, Vol 42, page 370, 1997.

3. Vargas Rocha, J, Lischer, C, Kummer, M, Haessig, M, and Auer JA. Evaluating the Measuring Software Package Metron-PX for Morphometric Description of Equine Hoof Radiographs. Journal of Equine Veterinary Science, Vol. 24, No. 8, August, 2004.

4. Craig, M, unpublished data 1996 - 2011.

5. Holmes, N, Ramey, D, An Introduction to Evidence-Based Veterinary Medicine. Vet Clin Equine 23 (2007) 191-200.

6. Auer, J, Stick, J, Equine Surgery 2nd ed. Philadelphia: W. B. Saunders, 1999; 789790.

7. Reilly, P, In-Shoe Force Measurements and Hoof Balance, Journal of Equine Veterinary Science, Vol. 30, No. 9, September, 2010.

8. Bowker, R, Van Wulfen, K., Springer S., Linderm K., Functional anatomy of the cartilage of the distal phalanx and digital cushion in the equine foot and a hemodynamic flow hypothesis of energy dissipation, Amer. J. of Vet. Research, Vol 59, No. 8, 1998.

a

Metron software, www.Metron-Imaging.com EponaShoe, www.EponaShoe.com c Equibond, East Coast Farrier Supply, Kirkwood, PA 17536 d EponaShoe HoofPack, www.EponaShoe.com b