Naturally we tend to take things for granted unless something is amiss, and so it is with our bones. Sure, people take note when they have arthritis or osteoporosis, and a fracture commands the bone owner’s complete attention. For the moment, however, appreciate how close to normal your bones are. A good way to do so is to understand that normality is not universal and that there are some amazing and strange ways that bone can go bad—so strange and rare that most orthopedists, myself included, have never encountered a patient so afflicted. However, we do read about these diseases in journal articles, and the topics may appear on certification exams. Here is a brief overview of three rare conditions that will definitely heighten your appreciation of normal bones.
The first is the genetic deficiency of an enzyme, alkaline phosphatase, which bone-building cells need to function. Bone growth and turnover are normally a balance between bone production and degradation; but in this condition, hypophosphatasia, bone is slowly resorbed and not replaced. In the severe early-childhood form of this disease, the ribs melt away, and the patient requires mechanical assistance for breathing. Early death ensues, at least until recently. Now the disease can be thwarted by periodic, lifelong injections of a synthetic form of the missing enzyme.
Although not fatal, the second condition, vanishing bone disease, is even more vexatious because nobody knows its cause. It can occur at any age but shows up most commonly in young adults where large segments of bone in a given region, typically face or upper arm, just gradually disappear. When it attacks the ribs, it can be fatal, otherwise it is just disabling, since the affected part of the skeleton lacks stability. For instance, contracting the muscles in a transformed arm just telescope the limb rather than bend and straighten the joints. Sometimes the process stops spontaneously, but it is not known to reverse. Radiation may provide some benefit but risks later malignant transformation. At times grafted bone can span the defect and improve stability and function.
In the third condition, an overabundance of bone rather than a dearth causes problems. It’s called fibrodysplasia ossificans progressive (FOP). A chemical messenger, bone morphogenetic protein, normally stimulates bone cells to make bone, which is a process integral to bone growth and fracture healing. In FOP, bone morphogenetic protein does not get switched off, and it stimulates bone formation in extra-skeletal locations, starting in the neck and shoulder muscles and gradually working its way down the spine and into the limbs. Any muscle irritation, an injection or bruise for instance, leads to the local formation of an extra-skeletal lump of bone. Surgical excision of the knot leads to more bone formation. The condition begins in childhood, gradually stiffens all joints in the spine and limbs, and makes even self-care activities impossible. Sadly, chewing and chest expansion become restricted, so either malnutrition or pneumonia causes early death.
The first malady discussed, hypophosphatasia, is far more common than the other two and is treatable. The others are exceedingly rare. Total worldwide cases number no more than a few hundred. Awareness of the three allows a glimpse into the amazing and complex life of normal bone, which is worthy of our utmost appreciation.