Foot pain often leads owners to leap to navicular disease as a primary cause. But as veterinarians, we know a multitude of other structures and disease processes can cause foot pain. According to experts, foot pain is the most common source of lameness in horses. And while lesions of the navicular bone certainly occur frequently, injuries of one—or even more than one—structure in the foot can lead to lameness.
For example, other components of the podotrochlear apparatus can be injured besides the navicular bone itself. These can include the collateral sesamoidean ligaments or the distal sesamoidean impar ligament.
“Because of the close anatomic relationship between each of the structures confined within the hoof capsule, concomitant injury must always be considered,” said Sue Dyson, MA, VetMB, PhD, FRCVS.
Indeed, Dyson and colleagues previously reported that 17% of 199 horses that underwent magnetic resonance imaging (MRI) had injuries to multiple structures in the foot (Dyson et al. 2005).
In this article, we’ll identify and review some of the many “other” causes of foot pain, emphasizing the importance of MRI in diagnosing such lesions.
The Role of MRI
Diagnostic analgesia (nerve or joint blocks with a local anesthetic such as mepivacaine or lidocaine), together with clinical evaluation, radiology, and ultrasonography, used to serve as a foundation for diagnosing the underlying cause of foot pain. The advent of MRI and the introduction of this modality to the equine industry in the 1990s revolutionized lameness diagnosis, particularly for foot, pastern, and fetlock region pain.
MRI, which is now widely available (although still an economic barrier for some owners), permits a precise peek at the structures encased within the foot. Demonstrating the value of MRI, Dyson co-authored a study in which her team retrospectively reviewed data from 702 horses diagnosed with foot pain (Parkes et al. 2015). They found that clinical examination findings could not discriminate between diseases of the podotrochlear apparatus and other causes of foot pain.
Unusual Suspects of Foot Pain in Horses
Other structures outside the podotrochlear apparatus that have been associated with foot pain include:
- Deep digital flexor tendon (DDFT).
- Collateral ligaments of the distal interphalangeal (DIP) joint.
- Distal interphalangeal joint (DIP joint) osteoarthritis (OA).
- Bone trauma to or cystic lesions in the middle or distal phalanges.
- Injuries to the ossified ungular cartilages.
- Penetrating injuries of the foot.
In the Parkes et al. study, only 62 (8.8%) of the 702 horses had navicular bone injuries. Instead, 299 of the 702 horses had injuries unrelated to the podotrochlear apparatus and DDFT. Those included the collateral ligaments of the DIP joint, injuries of ungular cartilages and associated ligaments, primary bone trauma of distal phalanx, OA of the DIP joint, and more.
In Dyson’s earlier study reviewing MRI images from 199 horses diagnosed with foot-related lameness, the horses responded to perineural analgesia of the foot, but clinical exam as well as radiological or ultrasonographic abnormalities failed to identify lesions that would explain the degree of the animal’s lameness. The most commonly diagnosed issue was tendonitis of the DDFT (33% of the 199 horses), followed by desmitis of a collateral ligament of the DIP joint (31%), traumatic injury to the middle or distal phalanges, and one case of sesamoidean desmitis.
We’ll now focus on causes of foot pain not involving the structures that make up the podotrochlear apparatus, including soft tissue and bony abnormalities.
DDFT Injuries
Injuries of the DDFT can occur at any location from the fetlock to the tendon’s insertion on the distal phalanx. “There are a variety of different types of injuries, some of which are likely to be degenerative in nature, and they can be multifocal,” Dyson explained. “Central core lesions, similar to injuries of the superficial digital flexor tendon in the metacarpal (cannon bone) region, usually occur proximal to (above) the navicular bone but may also occur close to the insertion. If a core lesion is detected using ultrasonography in the pastern region, it is highly likely that there will be additional lesions lower down within the hoof capsule.”
She added that full-thickness sagittal plane (front to back) splits are often multifocal, and dorsal abrasions are most common proximal to the navicular bone, often within the navicular bursa.
Ungular Cartilages of the Foot
Also referred to as the collateral cartilages, these are C- to L-shaped cartilaginous structures attached to the distal phalanx. They extend proximally on the medial and lateral aspects of the foot to just proximal to the coronary band. When these cartilages become ossified, they are then referred to as “sidebones.” The cartilages attach to surrounding structures—including the digital cushion, the three phalanges, and the navicular bone—by tiny ligaments. They also have close anatomic relationships with the collateral ligaments of the DIP joint and connections with the DDFT via a fibrocartilaginous ligament.
“Often, ossification of these collateral cartilages is discounted and not considered an important cause of lameness,” said Dyson. “Extensive ossification of one or both ungular cartilages, however, changes their biomechanical function as shock absorbers. Secondary osseous modeling changes occur with increased thickness of the compact bone (the outer shell of bone) and increased density of the spongious bone (the inner bone). This can predispose the cartilages to internal trauma of the ossified cartilage or the distal phalanx below the ossified cartilage. The cartilages can also fracture, especially at the base of the ossified cartilage. The closely associated ligaments are also more vulnerable to injury because they are attached to a solid structure, rather than a structure with the capacity to bend.”
According to Dyson and Nagy (2011), pain can stem from those cartilages. This includes primary injury to the cartilage as well as injury to the chondrocoronal and chondrosesamoidean ligaments that connect the cartilages to the surrounding structures mentioned above.
“Radiographs can easily identify ossification of the cartilages and secondary modeling changes, but MRI is needed to diagnose bone trauma of the ossified cartilage or the closely related distal phalanx and their related ligaments, the chondrocoronal and chondrosesamoidean ligaments,” said Dyson.
In that study, she also reported concurrent injuries to the distal phalanx and collateral ligaments of the distal interphalangeal joint in cases of traumatic injury to collateral cartilages.
Dyson added that “injuries of the ungular cartilages can occur in the absence of ossification.”
Collateral Ligaments of the DIP Joint
The collateral ligaments (CL) of the DIP joint provide medial and lateral support and limit joint rotation. Injuries can occur anywhere along the length of each ligament and, because of their close anatomical relationship with the joint capsule, are often associated with concurrent joint inflammation.
In addition to injury of the ligaments themselves, the site of osseous insertion—either the origin on the middle phalanx or insertion on the distal phalanx—of the ligaments can serve as a source of pain causing lameness. According to Dakin et al. (2009), “It has been suggested that this site is susceptible to significant biomechanical loading, irrespective of the site of injury within the ligament.”
Those concurrent osseous abnormalities can be associated with a poorer prognosis, said Dyson, which is why it is important to identify all lesions contributing to a horse’s lameness.
Dakin et al. used MRI to evaluate 313 feet in 289 horses with foot pain diagnosed with collateral desmopathy of the DIP joint. They identified concomitant osseous abnormalities in almost half of all feet—most frequently enthesis new bone and endosteal irregularity at the origin and insertion of the collateral ligaments. Those bony irregularities commonly occurred on the medial aspect of the limb (76.4%) and at the ligament insertion (78.3%) rather than origin. MRI was superior to radiography, ultrasonography, and nuclear scintigraphy for diagnosing osseous lesions associated with the collateral ligaments.
“In addition, 86 (27.5%) of the feet had additional injuries recognized within the hoof capsule,” Dyson relayed. Those included:
- Osseous cystlike lesions affecting the distal phalanx.
- Mineralization of the palmar process of the distal phalanx.
- Bony fragments associated with ligament insertion or origin.
- Subchondral bone abnormalities of the middle or distal phalanges.
- Osteoarthritis of the DIP joint.
- Moderate to severe ossification of the cartilages of the foot and fracture of the base of the medial ossified cartilage of the foot.
Horses included in this study were also evaluated with radiography, ultrasonography, and nuclear scintigraphy. Radiographic abnormalities were identified in a mere 9.4% of the 143 feet that had osseous lesions related to the collateral ligaments detected by MRI. Ultrasound also underestimated the frequency of occurrence of collateral ligament injury.
“Ultrasound was limited because much of the CLs are not accessible for evaluation,” said Dyson.
With nuclear scintigraphy, the researchers noted a positive association between the presence of increased radiopharmaceutical uptake (IRU) and osseous pathology associated with collateral ligament injury.
“Thus, nuclear scintigraphy can provide complementary information to MRI but should not be relied on as a sole diagnostic imaging technique,” Dyson advised.
Take-Home Message
Multiple injuries not related directly to the navicular bone, bursa, or other components of the podotrochlear apparatus can cause foot pain in horses. MRI is a valuable tool to help diagnose these pathologies, which do not necessarily occur in isolation. Thus, complete examination of all structures within the foot using all available tools—including clinical examination, ultrasound, radiographs, and nuclear scintigraphy—will yield the most information.
Related Reading
- Equine Stifle Radiology Tricks and Surprises
- Understanding the Biomechanics of the Horse’s Fetlock Joint
- Horses With Uneven Feet: Causes, Assessment, and Management
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