AP radiograph of both legs and enlarged detail of right knee
AP radiograph of both legs and enlarged detail of right knee
These radiograph of both legs demonstrates bowing deformities of both legs, and the detailed view of the right knee shows widening of the metaphyses with ragged, "frayed" margins.
What is rickets, anyway? Basically, it's osteomalacia in a child. If one of the many causes of osteomalacia becomes operant during childhood while the bones are still growing, the resulting abnormalities in the growing bones are called rickets.
The major problem in rickets is that something prevents the normal orderly conversion of cartilage into bone. This is best seen at the ends of the fastest growing bones in the body, i.e. the costochondral junctions of the middle ribs, then the distal femur, the proximal humerus, both ends of the tibia and finally, the distal ulna and radius. One may see widened, bulky physeal plates and irregularity ("fraying"), disorganization, and splaying ("cupping") of the bone at the junction of the metaphysis and physis. The widened growth plate is due to lack of mineralization of the cartilage matrix, and is weaker than a normal growth plate. This may predispose the patient to a "slipped" epiphysis (epiphysiolysis).
If rickets begins in infancy or early childhood, the long bones will show characteristic bowing deformities. Bowing of the long bones is allowed by the weaker rachitic bone slowly responding to stresses. Initially these stresses may be due to the child crossing its legs as it sits, and later are predominantly due to the stresses associated with weight bearing.
Rickets and osteomalacia 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 rickets and 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 bowing and widened, frayed metaphyses seen in this patient are hard to miss, though, and fairly characteristic for rickets.
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 is far more helpful for diagnosing osteomalacia than any radiographic test.