Orthopedics - Oleksa A.P. 2006

Pathophysiology of joints
Developmental defects and congenital skeletal anomalies
Renal osteodystrophy

Rickets-like bone changes in children and adolescents were first described in 1883 by R.G. Lucas. The true nature of these alterations became clear following the studies by Marriott and Howland, who in 1916 discovered hypocalcemia, hyperphosphatemia, and associated acidosis in uremia. Later, dystrophic skeletal lesions were described in certain forms of enzymopathies with isolated renal tubular dysfunction.

In medical literature, renal osteopathy is described under various names, such as renal osteodystrophy (osteodystrophia renalis), renal rickets (rachitis renalis), generalized fibrous osteitis of the Kidney (osteitis fibrosa generalisata renalis), and others.

These terms indicate a destructive bone process resulting from chronic functional insufficiency of the renal glomeruli and tubules.

The causes of renal failure leading to pathological bone changes most frequently include Renal Hypoplasia, Nephrotic Syndrome (Glomerulonephritis), cystic degeneration, Butler-Albright transient tubular acidosis, and many other disorders accompanied by tubular dysfunction—specifically, impaired filtration, resorption, and back-diffusion. This results in acidosis, azotemia, and hyperphosphatemia. Serum calcium levels, along with decreased calciuria, drop due to impaired intestinal absorption (Lichtwitz A., De Seze S. et al., 1960).

Intestinal calcium absorption is hindered by its excessive precipitation with phosphates and a deficiency of calcium-binding protein in the intestinal mucosa, which is regulated by activated vitamin D. Vitamin D activation in renal failure is impaired due to the inability of damaged Kidneys to synthesize adequate amounts of 1,25-dihydroxyvitamin D. This leads to a negative calcium balance with a decrease in total and ionized Blood calcium.

A positive phosphorus balance plays a crucial role in the Pathogenesis of renal osteopathy. With reduced renal clearance, phosphates accumulate in the blood, causing hyperphosphatemia.

Hypocalcemia, hyperphosphatemia, and acidosis stimulate parathyroid hormone (PTH) secretion by the Parathyroid glands, as PTH acts to restore disrupted Homeostasis. In healthy kidneys, parathyroid hormone enhances tubular calcium reabsorption and inhibits phosphate reabsorption. Additionally, PTH helps normalize blood calcium by stimulating Bone tissue resorption.

The uncoupling of Calcium and phosphorus homeostasis causes persistent phosphatemia despite parathyroid hyperfunction, which ultimately drives systemic calcium METABOLISM disorders and structural bone changes.

The disruption of the calcium-phosphorus balance in renal osteopathy is independent of vitamin D action and dietary mineral composition.

It is known that calcium and phosphorus metabolism in kidney diseases is disrupted even before the onset of azotemia; it is transient in nature and initially compensated by heightened regulatory mechanisms, namely the parathyroid glands. Recurrent Renal Dysfunction and a drop in Glomerular Filtration below 30 mL/min become permanent and lead to established osteopathy.

Clinical manifestations of the disease can vary, but the leading features are polyuria and polydipsia, indicating impaired renal function. Due to reduced urinary concentrating ability, the urine has a low specific gravity. Albuminuria is typically detected. Blood tests reveal marked azotemia, hypocalcemia, and elevated phosphorus levels (pronounced phosphatemia). However, these values may vary somewhat depending on the disease course in individual patients.

Parents report that the child lacks appetite, occasionally vomits, suffers from headaches, and As a result is pale, underweight, and experiencing stunted growth. Objective Examination primarily reveals short stature and limb deformities, most commonly Genu Valgum accompanied by flat feet. The chest may present a "Pigeon chest" deformity with characteristic rachitic rosary along the Ribs. Intellectual development is normal. No cardiovascular pathology is detected. Biochemical blood tests indicate impaired renal function.

X-ray examinations reveal characteristic signs of renal osteodystrophy, which radiologists classify into two groups (Reinberg S.A., 1964).

In type A, bone changes affecting the enchondral apparatus are most pronounced. Growth plates at the site of osteoid tissue are significantly widened and wavy. The metaphyseal Regions of the bones are also expanded, acquiring a "wineglass" appearance, with irregular metaphyseal ends lacking the zone of provisional calcification. The diaphyses of long bones are not curved, and Osteoporosis is absent. Unlike infantile rickets, the epiphyseal ossification centers are nearly unchanged, with smooth contours.

This form of the disease has a favorable prognosis.

Type B radiologically presents signs of osteodystrophy with severe Mineral Metabolism disorders. The metaphyseal regions of the bones are markedly ballooned and widened. Generalized osteoporosis with structural bone changes is highly pronounced. Bones exhibit a coarse-meshed Structure with indistinct trabeculae (Fig. 72), resembling a honeycomb or mottled cotton-wool appearance with signs of bone remodeling (Looser's zones). The cortical layer of long bones is somewhat broadened, frayed, and laminated, with multiple small defects visible in its superficial subperiosteal areas, especially in the epiphyses and metaphyses.

Ossification is delayed, and children experience significant growth retardation. The bones, particularly the tibias and femurs, are shortened and S-shaped. Coxa vara and genu valgum are constant and characteristic diagnostic signs that develop as the child grows.

Treatment. A comprehensive therapeutic approach aims to correct acidosis, hyperphosphatemia, and Vitamin D deficiency. During dialysis, the dialysate must contain an adequate amount of calcium to ensure complete removal of excess blood phosphate and correction of acidosis.

The diet should exclude phosphorus-rich foods. Patients are prescribed phosphate binders (activated charcoal, aluminum hydroxide).

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Fig. 72. Resorptive Changes in the pubic bones in renal osteodystrophy (Lori K., 1993).

Vitamin D therapy for patients with predominant signs of Osteomalacia begins with low doses—6–10 thousand IU daily. After 3–4 months of treatment, general weakness and pain disappear, normal blood levels of calcium, parathyroid hormone, and alkaline phosphatase are restored, and bone density increases. Once these laboratory parameters normalize, the vitamin D dose is reduced. In cases of pronounced hyperparathyroidism and vitamin D resistance, the dose is increased to 150–200 thousand IU daily, with careful monitoring of blood calcium and phosphate levels.

Dihydrotachysterol at 0.25–0.375 mg daily is also effective. Good therapeutic results are achieved with 1-hydroxycholecalciferol, as it normalizes calcium absorption and parathyroid hormone secretion. It is initially prescribed at 0.5–2.5 mcg/day. Once treatment success is attained, the dose is tapered to 0.25–0.1 mcg/day.

If conservative treatment fails and extraskeletal ossifications develop, a subtotal parathyroidectomy is necessary, as an adenoma of these glands is very frequently detected.

Surgical correction of long bone deformities should be performed no earlier than two years after sustained clinical and biochemical remission. However, it should be borne in mind that the earlier treatment is initiated in children, the greater the likelihood of preventing severe bone deformities and, consequently, surgical interventions.