Saber shin is a malformation of the tibia

Tibia

The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....

. It presents as a sharp anterior bowing of the tibia.

Saber shin is a sharp-edged anteriorly convex tibia.

Causes

It can result from syphilis

Syphilis

Syphilis is a sexually transmitted infection caused by the spirochete bacterium Treponema pallidum subspecies pallidum. The primary route of transmission is through sexual contact; however, it may also be transmitted from mother to fetus during pregnancy or at birth, resulting in congenital syphilis...

 and yaws.

Saber shin is also one of the deformities that can result from Paget's disease of bone

Paget's disease of bone

Paget's disease is a chronic disorder that can result in enlarged and misshapen bones. The excessive breakdown and formation of bone tissue causes affected bone to weaken, resulting in pain, misshapen bones, fractures, and arthritis in the joints near the affected bones...

.

It can also result from Vitamin D deficiency.
and in Weismann-Netter syndrome.