Metabolic Bone Disease

Metabolic Bone Disease (MBD) is a condition (or conditions) resulting in abnormal bone growth and/or repair. It encompasses the following conditions and syndromes; osteoporosis, osteomalacia, osteopenia, rickets, fibrous osteodystrophy, hypocalcemia and nutritional secondary hyperparathyroidism.

Causes:

MBD is generally the long term result of deficient dietary calcium and/or vitamin D3. This results in a negative calcium:phosphorous ratio and is caused by the following:

Too little calcium or too much phosphorus in the diet.
The presence of substances in the diet that impair the absorption of calcium (e.g. oxalates, phytates etc)
A deficiency of vitamin D3 from the lack of exposure to proper amounts of UVB either from unfiltered sunlight or high quality UVB lighting.
Inadequate protein or excessive protein.
kidney or liver disease (which impair conversion of vitamin D to it's active from), small intestinal disease and/or parasites(disrupts absorption), and disease of the thyroid or parathyroid glands (produce hormones which affect calcium metabolism). These are minor contributors - most cases are nutritionally based.
Improper (too cool) basking temperatures temperatures impair digestion and therefore calcium absorption . It also inhibits the production of D3 by UVB light.

Diagnosis:

The diagnosis is usually made from a dietary and environmental history as well as clinical signs.

Dietary and Environmental History

A proper diet is critical to the health of the tortoise. In general you want a diet that's high in calcium, high in fiber, low in phosphorous and adequate in protein. This is discussed in more detail in the diet sections and in the pyramiding section.

The tortoise also needs a proper environment. Without this, even a perfect diet can result in MBD.

Some critical factors are UVB and moisture levels. However too much is just as harmful as too little. To strike a proper balance, gradients are needed. The tort needs a warm basking area with UVB, but it also requires a cooler section without UVB. This allows it to not only regulate its body temperature, but to also control its UVB exposure. The same is true for moisture. It needs a warm moist area as well as a cool dry spot.

Clinical Signs:

Weak limbs resulting in an unsteady gait and the inability to raise the plastron off the ground when walking.
A depressed posterior carapace giving rise to the appearance of being humped back.
The whole carapace may be flattened rather than domed.
Pyramiding
The shell appearing too small for its body.
Soft shell
Abnormal beak, often appearing like a parrot's beak or a duck's bill.
Bowed or deformed legs.
Anorexia
Lethargy
Constipation

Treatment and Prevention:

MBD is far easier to prevent than to treat. Some of the important issues are:

Dietary calcium. The best source of calcium is a varied diet. Grocery store greens are often lower in calcium than the weeds and grasses available in the wild. To make up for this it is often recommended that all food be dusted with phosphorous free calcium. However, too much calcium can be as bad as too little. Another problem that's often overlooked is that bone is made of a variety of minerals including magnesium and boron. Calcium powders don't provide these. Instead I prefer to keep cuttlebone with them at all times. Cuttlebone is high in calcium and the necessary trace minerals.
Low dietary phosphorous. If you follow the diet guidelines on this site, this will not be an issue.
Adequate Protein. Protein is essential for muscle growth. Young and active animals require more than older inactive tortoises.
High Fiber. This is important for intestinal health.
Exercise.
Correct Lighting. A quality UVB bulb or daily access to unfiltered sun is essential.
Proper Temperature

The above is not only important for prevention, but is also a critical part of treatment.

The following quote is from Mader in "Reptile Medicine and Surgery" page 390: