The arrows in this X ray point to sidebone, ossification of the ungular cartilages. | Courtesy Dr. Elizabeth AcuttSidebone refers to ossification of the collateral cartilages of the coffin bonealso known as the distal phalanx (P3) or pedal boneappearing as upward-extending bony growths. This ossification happens during a process in which the cartilage mineralizes, explains Brian Beasley, DVM, CJF, Dipl. ACVSMR, podiatrist at Grand Prix Equine, a veterinary practice in Newtown, Connecticut.Essentially, a flexible structure becomes progressively rigid, adds Carlos Carvajal de la Cerda, MVZ, CF, an instructor of equine podiatry at Colorado State Universitys Veterinary Teaching Hospital, in Fort Collins. Beasley and Carvajal de la Cerda share their insights into sidebone, from potential causes to best-practice solutions.Form Follows FunctionTo better understand sidebone its important to review the anatomy and function of the collateral (or ungular) cartilages, says Carvajal de la Cerda, highlighting key points:Collateral cartilages can be found above the palmar and plantar processes of the coffin bone (the winglike extensions at the rear of the coffin bone in the forelimbs and hind limbs, respectively). From there, the collateral cartilages extend dorsally to just above the coronary band.The collateral cartilages can be seen and/or palpated in the caudal (toward the rear) region of the hoof.These cartilaginous structures are highly vascularized (rich in blood supply) and, again, flexible under normal conditions.The importance of the ungular cartilages lies in their nature, explains Carvajal de la Cerda. They are designed to aid in shock absorption, along with other structures such as the digital cushion, frog, corium, sole, and hoof wall. When we examine a venogram or dissect a foot, we observe that the back part of the hoof is richly vascularized, forming a perfect hydraulic system to absorb and dissipate impact forces.The balance of these structures within the hoof ensures optimal function. The equine foot possesses a great mechanism to absorb the significant impacts that occur with every stride, Carvajal de la Cerda says. The faster a horse moves, the greater the forces on its limbs. Ground reaction forces (transmitted from the ground through the hoof and up through the bony column) also play a role and vary depending on the footing or surface the horse is working on.Disruption of this balance could explain why collateral cartilages ossify. The exact cause of sidebone is not fully understood, but several contributing factors have been proposed, says Carvajal de la Cerda, including:Repeated concussion or trauma to the collateral cartilageConformational imbalances that lead to increased stress on one side of the footAs for which horses the condition affects most, sidebone is relatively common in larger breeds such as draft horses, Friesians, and Warmbloods, says Carvajal de la Cerda. It may also occur in horses with angular limb deformities. While it is more frequently seen in older horses, young horses are not excluded.Sidebone is often discovered during radiographic examination for another reason. | Courtesy Dr. Sue DysonDiagnosis and Next StepsIn my practice, sidebone is most often an incidental finding discovered during radiographic examinations performed for other reasons, says Carvajal de la Cerda. He further explains:The extent of ossification can usually be observed in radiographs on the lateromedial (taken from the side, toward the horses midline) projection and evaluated more precisely on the dorsopalmar/dorsoplantar (front to back) view.Occasionally sidebone can also be seen or felt above the coronary band.In more complex or severe cases advanced imaging such as MRI or CT can help determine the lesions severity and assess if there are active changes.From there, the possible origin of the condition guides treatment, says Carvajal de la Cerda. Therapeutic shoeing that enhances shock absorption and provides omnidirectional breakover helps minimize further stress and improves comfort, he explains. Using radiographs to guide our shoeing allows us to set up the foot in a position where the horse can move more easily from a mechanical perspective, so we reduce the forces acting on it. If sidebone is determined to be the main reason for lameness, a short course of NSAIDs combined with stall rest may help alleviate inflammation and pain.More Complex ScenariosBeasley says sidebone is a common and generally inconsequential finding, particularly in large-breed horses. When it does lead to lameness, it usually reflects secondary effects rather than the ossification itself.Ossification of collateral cartilage compromises the ability to dissipate concussion during loading, explains Beasley. However, he notes lameness as a direct result of this ossification usually only occurs when ossified cartilage impinges on sensitive tissues, such as one of the stabilizing ligaments located within the hoof capsule, or when fractures of the ossified cartilage occur.In my own practice I will often block the medial (closer to the horses midline) and lateral (away from it) side of the foot independently to determine if an ossified collateral cartilage has the potential to be blamed for the lameness, he says. To confidently diagnose impingement of soft-tissue structures, advanced imaging techniques such as MRI are often necessary, as ultrasound is unable to penetrate the hoof wall.Beasley further explains that fracture of an ossified collateral cartilage likely results from internal trauma during the stance phase of the stride. Once ossified, they are no longer able to flex with the hoof capsule and around the coronary band as they extend proximally (toward the horses body), he says. As the bony column presses down on the digital cushion and frog, and ground forces press upward, the proximal part of the ossified collateral cartilages may collide with the inside of the proximal hoof wall and coronary band.When ossified collateral cartilage fractures, basically a local inflammatory cycle causes lameness, Beasley adds. These generally heal well, although occasionally theyll heal with a fibrous union, so there isnt a complete resolution of the fracture line. This can make the recheck radiographs a little tricky, but generally radiographic signs of bone healing are visible.A fracture misdiagnosis can occur, says Beasley, when ossification develops in both the distal (further away from the body) and proximal parts of the collateral cartilage. This will present as ossification adjacent to the coffin bone, a space that is actually still cartilage, and then ossification above, he says. This is often misdiagnosed as a fracture but is, in fact, the ossification process at two different locations.Researchers looking at more serious cases found:In a study1 of 462 horses in Britain, researchers used nerve blocks, radiographs, and MRI imaging to examine associations between the degree of collateral cartilage ossification and injuries in other hoof structures (see sidebar on page 45). The team concluded: Extensively ossified cartilages of the foot are significantly associated with collateral ligament or distal phalanx injury.In a 2015 retrospective study,2 also conducted in Britain, scientists examined radiographs of 271 cases of ossified collateral cartilages from 2005 to 2012, determining the following:They most commonly found fractures in cartilages with ossification grades of 4 or 5, best seen in flexed oblique (an angled X ray view with the limb slightly flexed) images.Cases with cartilage ossification grades of 4 or 5 were more likely than those with lower ossification grades to have modeling/adaptive changes.Abnormally shaped cartilages were more likely to have modeling/adaptive changes than the normally shaped cartilages.Likewise, in a 2020 British study3 of 929 images from 589 horses, authors said the following in their video abstract: New bone formation on the proximal and distal phalanges (long pastern, short pastern, and coffin bones) was seen more commonly in horses with Grades 4 or 5 ossification of the ungular cartilages compared to those with Grades 3 and below. In addition, there was a higher risk of new bone formation associated with lateral ungular cartilage compared to the medial. They further concluded: New bone formation on the proximal and distal phalanges may contribute to pain and lameness in horses with Grades 4 or 5 ossification of the ungular cartilages.In a 2011 U.S. study,4 researchers considered the difficulty of identifying abnormalities resulting from ossified collateral cartilage on radiographs and, thus, utilized MRI in 22 horses to characterize and grade ossified collateral cartilage presenting with a fractured coffin bone. From these cases the authors concluded: Ossified ungular cartilages may lead to fracture of the palmar process of the distal phalanx and injury of the ungular cartilage ligaments.Potentially Unrelated FindingsBeasley cautions that in cases in which a radiograph shows ossified collateral cartilage in a clinically lame horse, but a nerve block has not been performed, dont assume the ossification is causing the lameness, he says. It could be a red herring for something completely unrelated.Breed Type vs. PathologyTo better understand a possible farrier perspective, Beasley explains that in horseshoe forging competitions, farriers produced a handmade shoe with a medial heel wedge, rolled from the medial toe to the heel, for horses with sidebone. Historically, farriers crafted such shoes for draftier breeds that may be wide-chested and toed-in, putting more weight on the outside of the foot, he says. Farriers may have also felt and seen the ossification of the collateral cartilage. If somebody made that shoe, they were basically shoeing for the horses conformation.Because of this, Beasley speculates that there could be some disconnect between the farrier and veterinary communities (about) whether sidebone is pathologic or not.Along these lines, researchers on a 2014 Scandinavian study5 evaluated radiographs combined with sports data from 649 Swedish-Norwegian cold-blooded trotting horses. The authors acknowledge sidebone as a highly heritable condition within this breed but, based on their findings, ossification of the ungular cartilages does not cause decreased performance in cold-blooded trotters and is therefore most likely not a cause of clinical or subclinical lameness in this breed.SafeguardsBecause the exact cause of sidebone remains unclear, true prevention remains difficult, says Carvajal de la Cerda. Still, practical management aimed at reducing concussion and trauma to the hoof could help protect the collateral cartilages and limit ossification. He notes that providing appropriate footing and managing workload can help minimize repeated impact on the foot.Early intervention can also help prevent problems. Evaluating foal conformation from birth and applying corrective trimming (and shoeing when appropriate) can also reduce angular deformities, he says. This will result in better conformation of the adult horse, so the impacts can be more evenly distributed in the foot.Take-Home MessageOverall, the prognosis for horses with ossified collateral cartilages remains favorable, says Carvajal de la Cerda, and depends on factors such as the extent of ossification, the horses age and intended use, and whether lameness is associated with the finding.In my experience it is rare for sidebone to be the sole cause of lameness, he explains. Typically, it coexists with other pathologies that can be treated, while the sidebone itself remains an incidental finding. Overall, the long-term prognosis is good, especially if we apply the shoeing goals that I mentioned before. Long-term success also depends on collaboration. I would like to add that sidebone, as any other podiatry issue, needs efficient teamwork between your veterinarian and farrier, says Carvajal de la Cerda. It is only when we bring the two professions together that we can have real success.References1. Dyson S, Brown V, Collins S, Murray R. Is there an association between ossification of the cartilages of the foot and collateral desmopathy of the distal interphalangeal joint or distal phalanx injury? Equine Vet J. 2010;42(6):50411.2. Jones LE, Dyson SJ. Radiographic characterization of ossification of the ungular cartilages in horses: 271 cases (20052012). J Am Vet Med Assoc. 2015;247(7):80111.3. Tivey M-EL, Van Dijk J, Dyson S. Extensive ossification of the ungular cartilages and other osseous abnormalities of the proximal and distal phalanges. Equine Vet Educ. 2020;32:2530.4. Selberg K, Werpy N. Fractures of the distal phalanx and associated soft tissue and osseous abnormalities in 22 horses with ossified sclerotic ungual cartilages diagnosed with magnetic resonance imaging. Vet Radiol Ultrasound. 2011;52(4):394401.5. Hedenstrm UO, Wattle OS. Significance of ossificated ungular cartilages regarding the performance of cold-blooded trotters. Acta Vet Scand. 2014;56(1):74.6. Ruohoniemi M, Tulamo RM, Hackzell M. Radiographic evaluation of ossification of the collateral cartilages of the third phalanx in Finnhorses. Equine Vet J. 1993;25(5):453455.This article is from the Spring 2026 issue ofThe Horse: Your Guide to Equine Health Care. We at The Horse work to provide you with the latest and most reliable news and information on equine health, care, management, and welfare through our magazine and TheHorse.com. Your subscription helps The Horse continue to offer this vital resource to horse owners of all breeds, disciplines, and experience levels. 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