Unravelling the Mysteries of Navicular Disease
First let’s begin by locating the navicular bone in the horse. Each of your horse’s hooves contains two bones: the distal phalanx (coffin bone or P3) and the distal sesamoid bone (navicular bone). The navicular bone is a small, boat-shaped bone that is bordered by the coffin bone, middle phalanx (P2), and deep digital flexor tendon (DDFT). It is approximately six centimetres in length and two centimetres in width in the average 1200 pound horse.
The main purpose of the navicular bone is to act as a fulcrum for the DDFT as it changes direction and attaches to the bottom of the coffin bone. It is at this level in the foot where the DDFT naturally exerts major compressive forces against the distal one third of the navicular bone. The navicular bone also acts as a shock absorber for the coffin joint. Because of its two purposes, the tiny navicular bone is required to withstand constant concussive and compressive forces. Between the navicular bone and the DDFT is a small space called the navicular bursa. This small space contains synovial fluid, which provides frictionless movement for the DDFT as it passes over the navicular bone. Beneath the DDFT lies the digital cushion, which acts to provide further shock absorption for the foot. The collateral ligaments suspend the navicular bone proximally, while distally the bone is suspended by the impar ligament.
Navicular disease is the chronic degeneration of the navicular bone and its surrounding structures. The reason for this degeneration is not yet fully understood. There are two theories. The vascular theory suggests a profound decrease in blood supply to the navicular bone causes thrombosis (formation of blood clots) and ischemia (deficient blood supply) and subsequently pain and degeneration. However, this theory has largely been rejected due to researchers’ inability to identify ischemic bone or thrombosis. Researchers have also been unable to reproduce the clinical signs of navicular disease by occluding the blood supply to the bone.
The second and more widely accepted biomechanical theory suggests that pain and degeneration are due to an imbalance between the horse’s anatomical conformation and the mechanical load on the foot. Because of this imbalance, stress is placed on the navicular bone and its surrounding structures including the DDFT, collateral ligaments, impar ligament, and navicular bursa. Concussion, compression, friction, and tension from the ligaments are forces every foot is required to withstand. The biomechanical theory suggests the size, shape, and balance of the foot along with the size and shape of the navicular bone can drastically affect the surrounding tissues when these daily forces are applied.
Contact studies have found that the maximum amount of force is applied to the navicular region during the propulsion phase of the stride, which is when the coffin joint is extended. Several things happen at this particular moment during the stride:
