AP and frogleg lateral views of the left hip
AP and frogleg lateral views of the left hip
Click either image for a more detailed view.
Osteomalacia and rickets are essentially the same process. Rickets is a term that is usually applied when this process occurs in skeletally immature patients (children). The abnormalities of osteomalacia/rickets are magnified in the rapidly growing bone found at the ends of bones -- especially around ossification centers (epiphyses and apophyses).
Osteomalacia and rickets are both characterized by an abnormally high ratio of osteoid (inadequately mineralized bone matrix) to mineralized bone, and over 30 causes or associated diseases have been identified. However, for our purposes, the Cliff Notes version of osteomalacia will do. Basically, we will consider only two main causes of osteomalacia: you have your problems with vitamin D metabolism and you have your problems with renal tubular phosphate loss. That's about it. There are, of course, syndromes that fall outside of this extremely simplified approach. However, these other causes are rare enough that I have not yet encountered them in my practice.
When considering renal tubular disorders, think of just two things: X-linked hypophosphatemia and Fanconi's syndrome. X-linked hypophosphatemia (a.k.a. familial vitamin D-resistant rickets) is the most common form of renal tubular rickets and osteomalacia. This has also been called X-linked hypophosphatemia, primary renal hypophosphatemic rickets or familial vitamin D-resistant rickets. As one of these names implies, it is due to a hereditary defect of the renal tubules, leading to decreased reabsorption of phosphate and therefore reduced serum phosphate levels. As another name implies, this decreased reabsorption does not respond to usual amounts of vitamin D. This defect is passed on with an X-linked dominant mode of inheritance. In general, this disorder exhibits rachitic epiphyseal and metaphyseal abnormalities predominantly in the lower limbs. This is best seen when comparing knee and wrist radiographs in the same patient. These patients also may demonstrate a generalized bone modeling error resulting in short, squat bones.
With a renal tubular cause of rickets, the radiographic picture can be very confusing, since one often has overlapping findings of osteomalacia and secondary hyperparathyroidism. This overlap is sometimes referred to as renal osteodystrophy.
Vitamin D metabolism is a slightly more complex topic. Humans produce Vitamin D in their skin following exposure to ultraviolet light. We also obtain it from dietary sources or vitamin supplements. It is then hydroxylated by the liver to form 25 OH D, which circulates, bound to a specific binding protein. This circulating pool of 25 OH D is then further hydroxylated by the kidney as needed, to form the physiologically active form of the hormone, 1,25 (OH)2 D. One can block this process at almost any of its stages, and thereby lead to osteomalacia or rickets. For example, if we were to make it impossible for someone to get out in the sunlight (make them a radiology resident) and feed them a diet of Twinkies, we could give them osteomalacia. Or, we could give them liver or kidney disease, and interrupt the process a bit further down the chain.
Now, how does one go about diagnosing osteomalacia or rickets? Well, it can be tricky. Some findings, such as osteopenia or coarsening of the trabeculae, are very nonspecific, and not helpful for diagnosis. The findings of osteomalacia are also often very subtle. The Looser zone seen in this patient is hard to miss, though. This finding is not pathognomonic for rickets, and a stress fracture could possibly yield a similar radiographic appearance. However, a wide, linear lucency such as this in cortex of a long bone should always make one consider the possibility of osteomalacia.
The findings of osteomalacia are generally much less specific than those of rickets. Generally, all that one sees is a diffuse osteopenia, which looks just like that seen in osteoporotic patients. In some cases of osteomalacia (rare in my experience), collections of osteoid may build up to the point that these "seams" of osteoid may be seen on plain radiographs as linear lucencies oriented perpendicular to the cortical margin. If large enough, these "Looser's zones" or pseudofractures may help lead one to the diagnosis of osteomalacia. Occasionally, one may see bowing of the long bones in an adult. In general, though, bone biopsy and serum chemistries are far more helpful for diagnosing osteomalacia than any radiographic test.