The term 'Metabolic Bone Disease' (or 'Disorder') is not an actual disease, but a catch-term for a wide variety of things that can soften or deform a tortoise's shell and skeleton. Some MBD's are due to disease processes, like 'Fibrous osteodystrophy', 'Hypertropic osteopathy', 'Paget's Disease', etc. Most MBD's are due to problems with diet and basic cares. The most common is 'Nutritional Secondary Hyperparathyroidism' or NSHP. Common names for this condition are 'rubber jaw' in lizards, or 'soft shell' in turtles and tortoises. The root cause is usually a combination of too little calcium, too much phosphorous, or too little vitamin D, but can also be aggravated by poor temps, poor hydration, etc. Pyramiding is usually a form or symptom of NSHP as well. [Mader]
Since NSHP is almost entirely due to problems in basic husbandry (mostly diet, but also cares, housing, etc.), it can be prevented by providing the tortoise with the right diet and husbandry, appropriate to the species. We may need to supplement key elements- especially calcium, vitamin D, fiber, iron, etc., or work to augment the temps, lighting, humidity, and so forth.
Tortoises are very flexible when hatched, and need a diet and cares designed to promote shell and skeleton development from the very beginning. If the shell does not 'firm up' in a timely fashion, various forms of NSHP and pyramiding (see right) will develop.
NSHP shows up in tortoises as a softened, leathery, or rubbery shell*, but by the time the shell shows obvious problems, there may be problems in the bones. Things like pyramided or raised scutes, deformed jaws, weak and/or deformed limbs, splayed walking, dragging limbs, paralysis, and cloacal prolapses are signs something is wrong. (* Note: Some turtles and tortoises are supposed to have soft, leathery, or rubbery shells such as the Pancake Tortoise and Soft-shell Turtles.)
IMPORTANT- Very young tortoises are a bit soft or flexible so they can fit in the egg and so the shell can move as they grow. This often worries keepers, but there is a difference between a young shell and a soft shell. Young tortoise shells are springy, like they are made of a stiff plastic- the plastron may feel a bit springy for some time. Soft shells, however, feel more 'leathery' and do not resist being compressed as much. If the shell just gets softer instead of firming up, you should take action.
As long as the tortoise is still eating, the shell is not too soft, and there are no other worrisome symptoms, the tortoise can probably be treated at home. Do not, however, suddenly start dumping lots of calcium in the poor animal or flooding it with UVB lighting. Try...
If the shell is very soft; the tortoise is not eating; is showing other symptoms; or is not responding to your cares, you should see a vet. The vet should do blood and other tests, an x-ray, and take a complete history to try to determine if the problem is NSHP or another bone problem. The vet would then determine a course of treatment based on the findings. NSHP is usually treated with vitamin D3 injections and supplemental calcium orally, then follow-ups over two weeks. More severe cases may also need dietary support and other treatment. It is important to follow the vets suggestions as carefully as possible for the best chance at recovery.
Treated properly, the tortoise should recover fully, although some shell or skeletal deformity will remain, and there may be residual muscle or nerve problems. The earlier it is caught, the better the chances for a complete recovery.
Untreated NSHP can be fatal, so make sure the tortoise gets the right cares.
When scutes grow in a conical or piled-up fashion with the bone deforming behind the scutes, we call it 'pyramiding'. Some tortoises, such as Star Tortoises, pyramid naturally, and some tortoises, such as some Testudo species, seem a little more resistant to the problem. As common as the condition is in captive tortoises like Sulcata, Leopards, and Red-foots, there is very little hard research on it. See also Mike Pingleton's article on pyramiding, reprinted in the Library.
Pyramiding has been blamed in the past on excess protein in the diet, but since the shell underneath is also pyramided, it is clearly a form of MBD, so the causes are the same as that. Experiments have shown that keeping a growing tortoise's shell misted throughout the day helps prevent pyramiding [Fife, Weisner, etc.], but a counter-argument is that this causes thinner scutes and does not address the shell issues beneath the scutes- pyramided shell and spongy bones with a lower than usual density. [Highfield 2010]
The most likely root cause is a disruption of the natural diet and cycles. Wild tortoises generally go through seasonal changes that affect what kind and how much food is available, and the shell grows in response which gives all the different kinds of tissues the chance to 'catch up'.
This would be the same as the prevention of MBD as well as trying to offer naturalistic climate and diets.
The first signals of pyramiding may be thick growth rings or a deepening grove between the scutes instead of the normal smooth growth. Additional new growth will add more height to the previous layers if things are not corrected and grooves or valleys will start to develop between scutes. While pyramiding usually happens early, it can start to develop later in some species (like Red-foots) if conditions allow.
There is no way to eliminate pyramids once they start. Treatment focuses on preventing further pyramiding by doing those things that prevent pyramiding. If things are corrected, new growth will be reasonably flat- although it may take years to see a significant difference.
Pyramiding by itself is mostly a cosmetic issue and should not, by itself, be considered a sign of failure. Pyramided tortoises live as long and successful a life as those without pyramids. However- because it represents a flaw in our cares or understandings, we want to continue to learn more about it and work to prevent it.