CHAPTER 128 Ossification of the Cartilages of the Foot (Sidebone)
The cartilages of the foot, otherwise known as the ungular or collateral cartilages, are C-shaped structures that are attached to the distal phalanx and extend proximally on the medial and lateral aspect of the foot to just proximal to the coronary band. Sidebone refers to ossification of the cartilages of the foot that usually starts at the cartilage base, at its attachment to the distal phalanx, and progresses proximally. Less commonly, separate centers of ossification may develop that extend proximally or distally and may fuse with the palmar process of the distal phalanx. If ossification originating from a separate center occurs concurrently with ossification from the base of the cartilage, a radiolucent line may exist between the two sites and may persist throughout life. This line may be difficult to differentiate from a fracture. Heterogeneous areas of ossification may also mimic fracture healing. In some horses, there are discrete separate centers of ossification proximally, well separated from ossification further distally. Hypothesized mechanisms of ossification include hereditary disposition, hoof concussion, improper shoeing, hoof imbalance, and poor conformation.
A system for grading ossification of the cartilages of the foot on the basis of a weight-bearing dorsopalmar radiographic view uses the following criteria: grade 0, no ossification; grade 1, minimal ossification at the base of the cartilage; grade 2, mild ossification at the base of the cartilage to the palmar level of the distal interphalangeal joint; grade 3, moderate ossification to the level of the proximal edge of the navicular bone; grade 4, advanced ossification extending clearly above the navicular bone but remaining in the distal half of the middle phalanx; and grade 5, extensive ossification to the level of the proximal half of the middle phalanx (Figure 128-1).
Figure 128-1 A, Dorsopalmar radiographic view of a foot (medial is to the left in A–D) with grade 2 ossification of the medial cartilage of the foot (white line). B, Dorsopalmar radiographic view of a foot with grade 3 ossification of the lateral cartilage of the foot (white line). C, Dorsopalmar radiographic view of a foot with grade 4 ossification of the medial and lateral cartilages of the foot (white line). D, Dorsopalmar radiographic view of a foot with grade 5 ossification of the lateral cartilage of the foot (white line). M, Medial; L, lateral.
Mild ossification (grades 1 and 2) of the cartilages of the foot is widespread. Moderate or severe ossification of the cartilages of the foot is common in the front feet of heavy horses, but it is rare in ponies, Warmbloods, and Thoroughbreds. About 80% of Finnhorses in Finland and draft horses in Belgium have radiographic evidence of ossification of the cartilages of the foot. In a referral population of horses at the Animal Health Trust in Great Britain, high ossification grades are overrepresented in large native ponies (e.g., Dales, Highland, Fell, and Connemara breeds) and cob-types of horses compared with other breeds (Irish Draught, Crossbred, Thoroughbred, Thoroughbred cross, Warmblood, and pony). Heavy horses and horses with low height-to-body weight ratios have a greater degree of ossification than others. In horses examined bilaterally, left-right symmetry of ossification was present.
The effect of sex on ossification is in dispute, but there is no association with age. Extensive ossification is more commonly found in the lateral than the medial cartilages of the foot in Finnhorses and in Warmbloods. In the British study, the medial and lateral cartilages of the foot were similar but not symmetric in grade, with the lateral ossified cartilage usually larger than the medial.
The cartilages of the foot are hypothesized to reduce concussion to structures within the foot and to assist blood flow by compression of the venous plexuses of the foot during loading. Both functions are assisted by multiple vascular foramina that contain large central veins joined to a network of microvessels, termed venovenous anastomoses, which may be critical to energy dissipation during impact of the hoof with the ground. As the foot contacts the ground, blood is forced through the microvasculature, which supplies resistance to flow, allowing energy to be dissipated through the cartilages of the foot and fluid rather than through the ligamentous and bony structures within the feet. A negative pressure within the palmar aspect of the foot aids rapid refilling of the microvasculature for the next loading phase.
Horses with a low height-to-body weight ratio often move with a pronounced upward and downward action, landing heavily on their feet with each loading phase of the stride. This produces large concussive forces on the feet, from which the energy must be dissipated. Continuous energy dissipation through the cartilages of the foot may cause a change in the structure and subsequent ossification of the cartilages of the foot. In turn, ossification of the cartilages of the feet may be an evolutionary adaptation in these types of horses in response to their gait.
The distal aspect of the cartilages of the foot contains more venovenous anastomoses than the proximal parts. The greater abundance of venovenous anastomoses at the base of the cartilages may explain why ossification starts distally more often than from a separate center of ossification because this is where most of the energy is first concentrated at the start of dissipation. A recent study has indicated that there is greater radiopharmaceutical uptake (RU) at the base of the cartilages of the foot than further proximally, indicating greater bone modeling. The general symmetry of ossification between left and right feet suggests a hereditary predisposition, which is supported by the estimated high heritability in Finnhorses.
Although extensive ossification of the cartilages of the foot is said to be associated with shortening of stride and, in narrow feet with little room for expansion, is suggested to cause lameness, there is little evidence to support these claims. Nonetheless, there is growing evidence that horses with either marked asymmetry of ossification of the cartilages of the foot within a foot or extensive symmetric ossification may be at more risk of injury of the ossified cartilage or the ipsilateral aspect of the distal phalanx, compared with horses with no or mild ossification.