How to Evaluate the Palmar Compact Bone (Flexor CorteX) on Navicular Radiographs: Without Missing Subtle Lesions

You’re looking at a horse with a short, choppy stride in front. The lameness is low-grade, inconsistent, maybe bilateral. You nerve block the palmar digital nerves and see some improvement.

Radiographs look within normal limits. No cysts. No major sclerosis. No obvious fracture lines. You’re tempted to call it “inconclusive navicular changes” and move on.

But here’s the question:

Did you actually evaluate the palmar compact bone? Or did you just glance at a latermedial view and hope for the best?

The palmar compact bone of the navicular bone is one of the most common sites for early pathology in horses with heel pain. It’s also one of the easiest areas to overlook.

Not because the lesion isn’t there, but because the image is of poor quality or the interpretation is too quick.

In this article, we’ll walk through how to properly assess the palmar compact bone, what subtle lesions actually look like, and why standard views might not be enough to rule out navicular pathology – especially in field settings.

Why the Palmar Compact Bone Matters

When a horse presents with palmar foot pain, pathology of the navicular bone is always part of the differential diagnosis—often at the centre of it. Yet too often, the palmar surface of the navicular bone is either poorly imaged or under-interpreted.

The palmar compact bone is the interface with the navicular bursa and the deep digital flexor tendon . It is a region under high compressive load in performance and pleasure horses.

Subtle pathology here can cause pain and be the difference between a horse takes intermittent irregular steps on turns and one that’s consistent and confident in its stride. Changes in thickness and opacity of the palmar compact bone may in some circumstances reflect disease.

What makes it clinically tricky is this:

• These lesions may not be visible on a standard lateromedial view.
• They don’t always correlate with other osseous changes.
• And in early stages, they may not trigger dramatic radiographic signs—yet the horse is still lame.

If the palmar compact bone isn’t clearly visible, the diagnosis is incomplete. And that’s exactly where technique becomes critical.

Radiographic Views That Reveal or Conceal

If you can’t see the palmar compact bone clearly, you can’t assess it properly. And that’s the trap. Many practitioners rely on standard lateromedial (LM) and dorsoproximal-palmarodistal oblique (DPr-PaDiO) view and assume they’ve “imaged the navicular bone.”

But neither of those views shows the palmar compact bone with enough clarity to rule anything in, with the exception of advanced pathology, – or out.

Here’s what actually works:

1. Lateromedial (LM) View

Useful for overall navicular size, shape, and opacity of the trabecular bone (spongiosa). But the palmar surface? With a perfect LM view the thickness of the palmar compact bone can be assessed but alterations of contour or erosions may be missed .

2. Dorsoproximal-palmarodistal oblique (DPr-PaDiO_) View

Helps evaluate the distal border and the number, size and location of synovial invaginations. Gives some sense of bone outline but still doesn’t isolate the palmar surface.

3. Skyline (Palmaroproximal-Palmarodistal Oblique) View

This is the view that matters, but it needs to be interpreted in light of what is seen in the other images as well. The skyline projection isolates the palmar compact bone and displays its surface in profile. If you want to assess:

• Alterations in thickness or shape of the palmar compact bone
• Changes in opacity of the palmar compact bone
• Demarcation between the compact bone and the spongiosa

…this is the only view that reliably shows them – assuming that the image is of good enough quality. Poor quality images can be misleading.

“The skyline projection is the most reliable view to assess thickness and irregularity of the palmar compact bone and to detect erosions.” – Clinical Radiology of the Horse (Butler, Colles, Dyson et al.)

Positioning Errors That Lead to Missed Lesions

Even if you acquire a skyline image, you can still miss a lesion if:

• The foot isn’t positioned sufficiently far caudally during image acquisition.
• The x-ray beam isn’t angled correctly (usually 35° to 65°, depending on conformation of the hoof capsule).
• The exposure isn’t set to show fine contrast.
• The lesion is not at a site which is tangential to the x-ray beam.

Poor technique means poor definition – and that’s often the only difference between catching a lesion and missing it entirely.

Not every case of navicular disease presents with a textbook cyst or obvious fracture line. In fact, the lesions that matter most clinically are often the easiest to miss – because they’re small, asymmetric, or mistaken for normal variation.

How can I tell if the image is of diagnostic quality?

1. The palmar margin of the palmar compact bone should be a single line.
2. The thickness of the palmar compact bone should be compatible with what is seen in a lateromedial image.
3. The ergot or palmar aspect of the fetlock should not be superimposed over the bone.
4. It should be possible to assess the dorsal compact bone.

Top tips for interpretation

• There may be a crescent shaped radiolucent area within the palmar compact bone at the sagittal ridge; this is a normal finding
• The contour of the sagittal ridge varies among horses
• Poor positioning may result in apparent increase in opacity of the spongiosa which is an artefact
• The thickness of the palmar compact bone varies according to hoof capsule morphology and can change with disease
• If the hoof capsules are asymmetrical the navicular bones are likely to be asymmetrical: bone models according to Wolff’s law, so left -right comparisons can be misleading
• There is more than one type of navicular disease

Radiographic Interpretation in Context

Radiographs can show you the lesion, but they won’t tell you if it matters. A small cortical defect might be clinically silent but could be progressive. A perfectly normal-looking navicular bone might still be the cause of persistent lameness. This is where context becomes essential.

In one study, up to 35% of horses with no detectable radiographic abnormality had moderate to severe lesions of the palmar compact bone of the navicular bone or the deep digital flexor tendon (DDFT) on magnetic resonance images. These findings may reflect poor radiographic quality and/or interpretation or may reinforce that radiographic normality doesn’t exclude pathology. Radiographs are 2-dimensional images of 3-dimensional structures. Lesions may be focal and masked by superimposition of normal bone or ‘hidden’ because the x-ray beam was not tangential to the lesion.

Start with the Horse, Not the X-Ray

If you’re working up a horse with:

• Gradual-onset, bilateral forelimb lameness
• A shortened step, r toe-first landing or a propensity to stumble
• Lameness worse on firm ground especially on turns
• Positive response to a palmar digital or navicular bursa block

…then the navicular region deserves close scrutiny—even if the radiographs look “normal.”

Don’t Over-Rely on Lateromedialal Views Alone

An apparently normal lateromedial image of the navicular bone doesn’t rule out disease. You may still miss:

• Palmar compact bone irregularity
• Focal radiolucent areas in the palmar compact bone
• Alteration in thickness of the palmar compact bone
• Increased opacity of the spongiosa

The skyline view often tells a different story – especially in horses that block out to the bursa or show imaging changes in the DDFT on ultrasound or MRI.

Understanding Diagnostic Analgesia: Interpreting Blocks in Context

A positive response to a navicular bursa block can help localise pain to the navicular region – but remember the close anatomical relationships with the collateral sesamoidean ligaments, the distal sesamoidean impar ligament and the deep digital flexor tendon. Primary bursitis may also occur. . It’s important to remember that local anaesthetic solution placed in the distal interphalangeal (DIP) joint can diffuse rapidly into the navicular bursa, leading to a false-positive result. However, bear in mind that the navicular bone is an integral part of the DIP joint.

Magnetic resonance imaging (MRI) may be the only modality that can differentiate between true navicular bone pathology, deep digital flexor tendon (DDFT) lesions, or inflammation of the bursa and it is common to find lesions of several structures coexisting.

If a horse blocks sound to the bursa but shows only minimal radiographic changes, it may be because the horse has a primary soft tissue lesion, not a form of navicular disease.

Without this context, treatment decisions may be misguided—and prognosis inaccurately judged.

Don’t Ignore the Hoof Capsule

Hoof balance and conformation play a major role the biomechanical loads placed on the podotrochlear apparatus and the DDFT.

• Long toe, low heel conformation in particular must be addressed in the management strategy

You’re Not Looking for “Navicular Syndrome”

Navicular syndrome is not a diagnosis. You’re looking for a structural or biomechanical explanation for clinical pain.

Moreover, there are a variety of different types of navicular bone pathology which may have different aetiologies and prognosis.

..The skyline image of the navicular bone often gives information not available from other views, but all views should always be interpreted in association with each other. The PaPr-PaDiO images also potentially gives additional information about the palmar processes of the distal phalanx not available from other views. Always read the entire image. Do not ‘lesion spot’.

Case Example:
8-year-old Warmblood gelding, eventing. Presented with mild, persistent right forelimb lameness. Positive to navicular bursa block, but initial LM and DPr-PaDiO radiographs were unremarkable.

A well-positioned skyline projection revealed subtle focal palmar margin irregularity of the palmar compact bone and mild increased opacity of the trabecular bone . MRI later confirmed a partial-thickness tear of the deep digital flexor tendon and adjacent resorption of the palmar aspect of the palmar compact bone .

Without the skyline view, the possible true extent of pathology would have been missed.

When to Be Suspicious—and What to Do Next

You’ve acquired the skyline image. You think it is of diagnostic quality. The palmar margin of the palmar compact bone is focally rather ill-defined, , or maybe it’s just the angle with which the image was obtained. The horse blocks sound to the navicular bursa, but the radiographic changes are subtle.

This is where clinical judgment counts.

What Should Raise Your Suspicion?

Even if the images aren’t definitive, don’t ignore:

• A positive DP block with lameness returning as the block wears off
• A persistent forelimb lameness that improves with shoeing changes or wedging
• Abnormal hoof balance (for example, long toe and , underrun heels), toe-first landing or stumbling
• Subtle but consistent focal irregularity of the palmar aspect of the palmar compact bone,—especially if asymmetric

What You Can Do Without Advanced Imaging

Not every practice has CT or MRI. But that doesn’t mean you’re stuck.

Try this:

• Repeat the radiograph. Alter the angle of projection by assessing the angulation of the navicular bone in the LM image. Better positioning often reveals more.
• Image both forelimbs and compare the regularity of the palmar compact bone.
• Get a second set of eyes. Send all the radiographs to a colleague or radiology consultant.
• Consider ultrasonography of the palmar aspect of the pastern and via the heel bulbs. Some horses with DDFT lesions at the level of the navicular bone have multifocal lesions that extend into the pastern region.
• Trial treatment. Improve the trimming and shoeing and check the regularity with which the horse is trimmed and whether or not any pattern of the equine lameness severity has been seen in association with the shoeing cycle.
• Discuss modification of the type and intensity of work and the surfaces on which the horse is worked.
• Consider medication of the navicular bursa.
• Document progression. Re-image in 6–12 weeks if the horse is not improving or if the clinical picture worsens.

Sometimes, knowing when to wait – and what to monitor – is just as important as knowing when to act.

When to Refer for Advanced Imaging

Consider referral if:

• No significant radiographic abnormality is detected.
• The horse has failed multiple treatment attempts.
• The owner wants a definitive diagnosis, treatment and management plan and prognosis and can afford to proceed further. .
• A definitive diagnosis will change the management or prognosis significantly.

Skyline radiographs do not replace MRI, but good quality images interpreted correctly may give you the clue you need—before the case becomes chronic or career-limiting.

Final Takeaways

If you’re assessing a horse with palmar foot pain and you haven’t properly evaluated the palmar compact bone of the navicular bone, you’re working with incomplete information. Subtle lesions here are easy to miss—but they’re often the most important.

Here’s what to keep in mind:

• Don’t skip the skyline view. It potentially gives the most information about the palmar compact bone of the navicular bone, but it must be interpreted together with the other radiographic projections.
• Don’t overinterpret the significance of lateromedial image of the navicular bone in which no abnormality is seen. Early lesions of the palmar compact bone are usually not seen on a lateromedial image. .
• Subtle doesn’t mean insignificant. Mild irregularities of the palmar margin of the palmar compact bone of the navicular bone or radiolucent regions may be associated with pain and lameness.
• Positioning matters. A poorly angled view can hide an obvious lesion.
• Always interpret in context. Radiographs are just one piece of the puzzle. Look at blocks, balance, and the whole horse. The navicular bone is part of the podotrochlear apparatus and the DIP joint and there are other soft tissue injuries in the palmar aspect of the foot that can cause pain and lameness.
• If in doubt, re-image or refer. Early lesions are easy to overlook. Second opinions and repeat views are part of good medicine.

And remember—radiographic findings should guide, not end, the discussion.

While mild irregularity of the palmar compact bone of the navicular bone may respond to changes in trimming and shoeing and medication of the navicular bursa, concurrent DDFT damage – particularly in older sport horses- can significantly affect prognosis and may limit return to previous workload. A horse with primary navicular bone pathology may be a good candidate for palmar digital neurectomy, but a core lesion in the DDFT is often a contraindication for surgery.

Identifying the right cases for conservative management versus advanced imaging, surgery, or retirement is key to responsible decision-making.