Named after the German radiologist who first described it in the early twentieth century, Kohler’s disease refers to a foot bone disorder seen in young children 3-7 years of age.
It is a rare condition characterized by pain and tenderness on the top of the foot just in front of the ankle, caused by avascular necrosis of the navicular bone, a small bone of the ankle/hind foot.
Avascular necrosis means damage to the bone tissue as a result of loss of blood supply. Kohler’s disease mostly affects boys, the ratio between boys and girls being about 4:1.
The navicular is one of the tarsal bones of the foot. The tarsus is the hind part of the foot (like wrist of the hand) and is formed by seven small bones. The navicular bone is located in front of the ankle on the inner side.
The navicular bone is the last bone to ossify in the foot. Ossification refers to deposition of minerals in the bone, which makes it hard. An un-ossified bone is soft and therefore can be compressed easily.
The process of ossification starts at a point in or near the center of the navicular bone, called the ossification center. This center becomes mineralized first and from here the process of ossification spreads to the rest of the bone. In earlier stages, a single artery supplies this ossification center, however, the number of blood vessels increases to five or six later on.
Because of this late hardening (ossification) of the navicular bone, it is theorized that this soft un-ossified bone gets compressed between the already ossified bones located at its front and back. The anatomical location of the navicular bone predisposes it to strong compressive forces as one stands or walks; these forces increase with increase in body weight. Therefore, the more delayed the ossification, the more likely is the risk of bone damage due to compression. In boys the ossification of the navicular bone starts almost a year later than in girls and this might be the reason why the disease is seen more often in boys.
The compressive forces on the bone cause constriction of the single blood vessel supplying the ossification center of the bone. No blood supply means no nutrients and the result is death of the bone cells and collapse of the ossification center. This bone damage causes pain and inflammation in the area.
However, the lack of blood supply is only temporary, as other vessels grow forward, restoring the blood supply to the area. With time, new bone forms to replace the damaged bone tissue.
- Usually, only one foot is involved
- Pain on the top of the foot in front of the ankle
- Tenderness and swelling in the area above the navicular bone
- The child may limp due to pain and try to shift the weight to the outer side of the foot
The age of the patient, as well as the symptoms, point towards the diagnosis. An X-ray or bone scan helps to confirm the diagnosis. X-rays show a collapsed (flat) navicular bone, which appears more opaque than normal.
The disease is self-limiting and the only treatment required is eliminating pain and swelling, and supporting the affected foot.
In mild cases, use of an insole device may be sufficient, such as an arch support or a heel wedge. If the symptoms are severe, a below knee cast may be applied for about six to eight weeks. If pain persists after that, the cast needs to be reapplied for another six weeks. After the cast is removed, use of arch supports is recommended for about six months.
Painkillers are prescribed to reduce pain and swelling.
The condition heals even without treatment but takes much longer (about 15 months); even with treatment, symptoms may persist for about three months. However, the disease does not have any negative influence on the final shape or growth of the navicular bone and complications occur very rarely.