Anatomy of the foot
The horse’s foot is completely enclosed by the hard hoof capsule, which is composed of two regions: a wall and a solar region. The central area of the solar region is known as the frog. The pedal bone is the lowest bone in the horse’s skeleton and is situated within the hoof capsule. The pedal bone is connected to the hoof wall by sensitive finger-like projections called laminae (Figure 1).
The joint between the pedal bone and the short pastern bone or second phalanx is known as the coffin or distal interphalangeal (DIP) joint and consists of cartilage and synovial fluid contained within a joint capsule. The deep digital flexor tendon is situated at the back of the foot and attaches to the pedal bone. A sheath, the digital flexor tendon sheath, surrounds the deep digital flexor tendon and extends to the mid to bottom of the pastern region.
Lastly, the navicular bone is situated behind the third phalanx and coffin joint and is surrounded by the navicular bursa. This bursa is a small sac filled with synovial fluid that prevents friction when the deep digital flexor tendon moves over the back of the navicular bone.
Life-threatening infections (synovial sepsis) can occur when penetrating wounds communicate with one of the three synovial structures. It is therefore very important that prompt assessment of penetrating foot wounds is performed to assess the site, depth and angle of penetration.
Which anatomical sites are affected by injuries at the different penetration sites?
There are a number of different sites in the horse’s foot that can be penetrated by sharp objects (Figure 2) and the affected site dictates the treatment that will be required and the eventual prognosis of the injury.[Subhead] Penetration of the toe at site A
A nail that penetrates the sole and extends into the toe region at site A can penetrate the sensitive laminae, which will be severely painful for the horse. The sharp object will cause inflammation of the laminae, a form of laminitis, which will get worse the longer the object stays in the foot.
As the solar region of a horse’s foot is not clean in the first place and sharp objects that penetrate the sole are also generally dirty, bacteria will also inevitably enter the laminae and solar tissue after a penetration. The sharp object may damage the blood supply to the injured area of the foot and devitalise the surrounding tissue, which can then provide an anaerobic environment favouring the growth of these bacteria. If a penetrating injury is not treated appropriately, infection will then become established and without drainage will become an abscess.
Another major issue at this location is penetration and damage to the underlying pedal bone. The pedal bone can become infected, which is known as septic osteitis, or the bone can become fractured by the penetrating object, although this is unusual. Infections of the pedal bone can be very difficult to treat as antibiotics cannot normally penetrate the pedal bone effectively.
The treatment of these forms of injuries normally involves debridement of the puncture wound and tract and resection of the infected area of pedal bone. This is normally performed under general anaesthesia and horses with this type of injury normally have a long period of rehabilitation as the surgical opening must heal from within.
Text: Dr Luke Poore
The full article appears in the February issue (130) of HQ > Shop now