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Bone mineral density and fracture prevalence in long-term kidney graft recipients

Durieux, Stéphanie1; Mercadal, Lucille2; Orcel, Philippe1; Dao, Hahn1; Rioux, Christophe3; Bernard, Maguy4; Rozenberg, Sylvie1; Barrou, Benoit5; Bourgeois, Pierre1; Deray, Gilbert2; Bagnis, Corinne Isnard2 6

Clinical Transplantation

Background.  Renal transplantation triggers an early bone loss that increases the subsequent risk of osteoporosis and fractures. Little is known about the long-term outcome of bone status and fracture prevalence several years after transplantation. Therefore, we conducted a cross-sectional evaluation of bone status to find out the frequency and predictors of osteoporotic fractures in late kidney graft patients.

Methods.  Changes in spinal, hip, and total body bone mineral density were assessed using a DEXA Hologic QRD 1000 scanner, and fractures were quantified in all kidney graft patients presenting for routine evaluation with a minimal follow-up of 5 years after transplantation (with a mean follow-up 8.5±3.1 years). We measured biochemical markers of bone metabolism and collected clinical and dietary intake data.

Results.  Fifty-nine renal graft recipients were enrolled in the study within 9 months. Osteoporosis, according to the World Health Organization definition, was observed in 31 patients (53% of the total population) and fractures occurred in 26 patients (44% of the total population and 51.6% of patients with osteoporosis). Femoral neck bone mineral density was positively correlated with patient’s weight and cyclosporin current dosage. Steroid cumulative dosage correlated only to lumbar spine Z score. Dietary calcium, serum 25 hydroxyvitamin D, parathyroid hormone, and urinary N-telopeptides excretion were normal.

Conclusions.  These data emphasize the substantial prevalence of osteoporosis and fractures among very long-term kidney graft recipients. Therapeutic intervention in these patients is urgently needed.

1 Department of Rheumatology, Groupe Hospitalier Pitié-Salpétrière, Paris, France.

2 Department of Nephrology, Groupe Hospitalier Pitié-Salpétrière, Paris, France.

3 Department of Biostatistics, Groupe Hospitalier Pitié-Salpétrière, Paris, France.

4 Department of Biochemistry, Groupe Hospitalier Pitié-Salpétrière, Paris, France.

5 Department of Urology and Renal Transplant Unit, Groupe Hospitalier Pitié-Salpétrière, Paris, France.

6 Address correspondence to: Dr. Corinne Isnard Bagnis, Service de Néphrologie, Groupe Hospitalier Pitié Salpétrière, 83 Boulevard de l’Hôpital, 75013 Paris, France. E-mail: corinne.bagnis@psl.ap-hop-paris.fr.

Received 15 October 2001.

Revision Requested 30 January 2002.

Accepted 15 May 2002.

Transplant and patient survival improved drastically in the past two decades and long-term survival of renal graft patients is common. But kidney graft function is only one of the determinants of life quality in those patients. Although successful kidney transplantation corrects many of the metabolic abnormalities secondary to chronic renal failure, it is associated with relevant bone loss. The risk of osteoporosis (OP) is not sufficiently considered as part of the routine management of patients receiving a kidney graft. Mineral bone loss is known to occur within 6 months after kidney grafting mostly at the trabecular site (1) with an increased risk of fractures. Contrary to early bone loss, there is limited information about the prevalence of fractures among late renal graft recipients. Therefore, we conducted a cross-sectional evaluation of bone status and fracture prevalence in a population of patients with a posttransplantation time of at least 5 years.

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PATIENTS AND METHODS

Patients

From April 1998 to January 1999, all renal graft recipients followed for at least 5 years posttransplantation, presenting for routine examination in our institution, were included in this cross-sectional study. Informed consent was obtained orally from all subjects after the nature of the procedures was fully explained. Demographic data collected from hospital and physicians records included age, gender, race, interval since transplantation, cause of renal failure, and number of rejection episodes. Immunosuppressive treatment and potential risk factors for OP were collected from reviewing medical charts.

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Methods

Bone mass measurement.

Bone mineral density (BMD) was measured by dual-energy x-ray absorptiometry with a Hologic QRD 1000 scanner (Waltham, MA) at the lumbar spine and the femoral neck. Vertebral bone density values represented the average of three vertebrae (the second to the fourth lumbar vertebrae). Hip-bone density was measured at the femoral neck level. Measurements for bone-mineral content, in g/cm2, reflected values from the second to the fourth lumbar vertebrae. Results were also expressed as Z scores relative to mean normal values for subjects of the same age and gender and as T scores for sex-matched young adults. Pretransplant measurements of bone densitometry were not available. World Health Organization’s (WHO) diagnostic criteria were applied to define OP (T score below −2.5 at the femoral neck and/or the lumbar site) and osteopenia (T score between −1 and −2.5 at same sites).

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Radiological investigations.

Thoracic and lumbar spine lateral radiographs were obtained in all patients at the time of study. Vertebral fractures were assessed qualitatively (presence or absence) by two independent investigators, and the number of vertebral fractures between the fifth thoracic and the fourth lumbar vertebrae was counted.

Peripheral fracture incidence was retrospectively obtained by interviewing the patients and reviewing radiographic findings whenever possible. The site of the fracture (ribs, upper or lower limbs) was noted. Heavy traumatic fractures, resulting from accidental injuries, were excluded from the counting of the peripheral fractures.

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Laboratory investigations.

Samples of blood were obtained at time of the study for subsequent measurements in the serum. Biological parameters were measured using standardized procedures. Serum creatinine, calcium, and phosphorus, albumin, and urinary calcium were measured by automated techniques. Plasma parathyroid hormone (PTH) 1-84 levels were measured using a two-site immunoradiometric assay for the intact molecule. Serum 25 hydroxyvitamin D (25 OH-D3), thyroid hormones (thyroid-stimulating hormone, free thyroxine [FT4]), sex hormones (follicle-stimulating hormone, luteinizing hormone, or testosterone), and urinary type I collagen cross-linked N-Telopeptides (NTx) were measured using a commercial assay. Bone alkaline phosphatase was measured by an Alkphase assay (Dade Behring, Deerfield, IL).

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Dietary intake.

At the time of the study, a physician asked patients about daily eating habits. Calcium intake was then calculated using a validated food frequency questionnaire (2).

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Immunosuppressive therapy.

As part of induction therapy, all patients received monoclonal or polyclonal anti-T-cell medication. Twenty-six patients were given azathioprine as a third immunosuppressive agent together with cyclosporin A (5 mg/kg) and daily prednisone (20 mg); 28 patients were managed with daily prednisone 20 mg and azathioprine only. One patient received prednisone and cyclosporin. Daily prednisone dosage was gradually tapered to 10 mg per day by 4 to 6 months after transplantation. Acute rejection episodes were treated using a standardized protocol of intravenous methylprednisolone 10 mg/kg for 3, consecutive days. Prednisone cumulative dosage was calculated using the individual charts. No other immunosuppressive regimen was used in our study population. Patients did not receive any antiosteoporotic treatments. Calcium and vitamin D were not considered as antiosteoporotic treatments. Before transplantation, only one patient had been treated with steroids for lupus nephritis.

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Statistical Analysis

All results are expressed as mean ± standard deviation (SD). Student’s t test was used to compare continuous variables, and Fischer’s exact test was used to compare nominal variables between groups. Spearman’s correlation coefficient and logistic regression analysis were used as necessary. A P value of less than 0.05 was considered statistically significant.

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RESULTS

Demographic and Clinical Characteristics

All patients were first cadaveric renal allograft recipients. A total of 65 patients were evaluated. Full clinical data were not available for six patients. Therefore, the study analysis included 59 transplanted patients (32 males, 27 females), 71% white, 7% African American, and 22% from other ethnic origins. Minimum posttransplantation time was 60 months. Maximum posttransplantation time was 237 months. The mean interval since transplantation at entry into study was 102±37 months. One or more rejection episodes were noted in 30% of the patients. Causes of renal failure leading to treatment with regular dialysis included: glomerulopathies (n=24), genetic diseases (n=8), undetermined nephropathies (n=6), diabetic nephropathies (n=2), and miscellaneous diseases (n=19). All except five patients received a mean daily dose of 73.6±33.1 mg per day of azathioprine. Twenty-seven (46%) patients received a mean daily dose of 75.5±81.6 mg per day cyclosporin A (Table 1).

Table 1

Table 1

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Biochemical Parameters

Serum calcium and urinary calcium excretion were within the normal laboratory reference range for all patients. Serum 25 OH-D3, PTH 1-84, bone alkaline phosphatase, and urinary NTx excretion were normal (Table 1). Mean creatinine dosage was 131±64 μmol/L. Testosterone dosages were within the normal range in all male patients. Follicle-stimulating hormone or luteinizing hormone dosage, or both, was elevated in 12 of 32 women and correlated with their postmenopausal status. Thyroid hormone dosage was within the normal range, except in one patient but without clinical significance.

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Bone Mineral Density

Using the WHO criteria, OP was observed in 31 patients (53%). According to gender, 53% of males and 52% of females had a T score < −2.5 SD. Osteopenia at both or either site of measurement was observed in 24 patients (40%). Mean T score was −2±1.3 at the lumbar spine and −1.9±1.2 at the femoral neck in the total population. The proportion of patients with a lumbar Z score of less than −2 was 39% (23/59 patients). A femoral Z score of less than −2 was noted in 9/59 (15%) patients. At the lumbar spine site, 22 patients (37%) had a T score below −2.5 SD and 26 patients (44%) between −1 and −2.5 SD. At the femoral neck, values were 22 (37%) and 24 (40%), respectively. No significant differences were observed between characteristics of OP patients and non-OP patients, except for weight (Table 2). Only four (7%) patients had normal bone mineral density.

Table 2

Table 2

Femoral bone mineral density correlated with patient’s weight (r=0.39;P <5.10 3) and with cyclosporin current dosage (r=0.32;P <0.05). Lumbar spine BMD correlated with cyclosporin current dosage (r=0.28;P <0.05). Among patients with defined osteoporosis, 18/31 (58%) patients were not currently treated with cyclosporin. Among patients without osteoporosis, 14/28 (50%) patients received a mean dose of 86.1 mg per day of cyclosporin (Table 2). A large proportion of patients was not taking cyclosporin (28 patients). Among these patients without cyclosporin, 15 (53%) were osteoporotic. The mean dose of cyclosporin for patients with and without OP is given in Table 2. Steroids cumulative dosage correlated to lumbar spine Z score (r=−0.26;P <0.05).

Gender, ethnic origin, age, interval posttransplantation, serum creatinine, PTH 1-84, 25 OH-D3, urinary NTx, and daily azathioprine dosage could not be identified as independent risk factors for femoral neck or vertebral OP by logistic regression analysis. Data were stratified according to gender. No significant correlation was found between spinal BMD and vertebral fractures in women (r=−0,18, Spearman correlation test).

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Fractures

During the follow-up, 47 vertebral or nonvertebral fractures were documented in 26 (44%) of 59 patients (Table 3). Only partial data were available for eight patients, who were excluded from analysis. Seventeen eligible patients (32 fractures) had a defined osteoporosis (T score < −2.5 SD). Seven patients with significant osteopenia (−1 SD < T score ≤ −2.5 SD) had experienced 12 fractures. Of all these fractured patients, 14 patients had more than one fracture. Four patients had both vertebral and nonvertebral fractures. Incidence of fractures and comparison with low BMD (T score < −2.5 SD) is described in Table 3. Nonvertebral fractures were statistically more prevalent in patients with a low bone mass. In contrast, vertebral fractures were not statistically associated with a low bone mass (Table 3). Peripheral fractures were predominantly located at the wrist and the lower limb (ankle, foot, and femoral neck). Ribs fractures were reported in one case and probably underestimated. Mean age of fractured patients was 52.4 vs. 46.7 years in nonfractured patients.

Table 3

Table 3

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DISCUSSION

Our study shows that densitometric OP or osteopenia, defined using the WHO criteria, seems to be extremely common in nonselected long-term kidney graft recipients. Because only patients with a posttransplantation interval over 5 years (mean posttransplantation time: 8.5 years) were included, our study best reflects bone status in late renal transplanted patients.

Renal transplantation can be associated with dramatically reduced bone mass, the first 3 to 6 months after transplantation being the most critical period (1–3). The early bone loss profile is well known in kidney-transplanted patients as shown by Julian et al. (1), who described a 6.8% decrease in the lumbar spine BMD 6 months after transplantation and an 8.8% decrease after 18 months. Horber et al. (2) documented a similar loss in 34 recipients followed during 5 months after kidney transplantation, with a mean loss of 1.6% per month for the lumbar Z score.

Bone loss occurs at a high rate early (12 months) after transplantation but long-term changes in BMD are controversial. Bone loss has been found to be negligible after the second year posttransplantation (3,4). Grotz et al. (4) showed that the mean BMD values collected from 190 patients from 2 to 20 years after transplantation (mean: 55 months) were within the normal range, reflecting the classical age-dependent decrease in BMD. But an ongoing accelerated bone loss of 1.7±2.8 % per year was observed in 55 kidney graft recipients compared with age-matched controls (5).

Few studies evaluated the bone status of long-term renal graft recipients. A high prevalence of lumbar and femoral osteopenia was reported (28.6% and 10.5% of the patients, respectively) when patients are screened after a mean time posttransplantation of 97.5 months (5). In female patients, prevalence of densitometric lumbar OP was 33.3% and that of femoral OP was 10% (5).

Risk factors of transplantation OP have been widely studied in the literature. Well-established factors for OP contribute to defining an individual risk profile for the patients. Some of these factors were found to be correlated to bone loss, such as age (6), postmenopausal status, female gender (6,7), renal function, and duration of dialysis before transplantation (8). The major risk factor for most of the authors was steroid cumulative dosage (1, 5, 6, 7). In our study, steroid cumulative dosage was not well correlated to bone mass. This is not surprising because in studies conducted in the early posttransplantation period, the correlation reflects the high intensity of bone loss observed in the first posttransplantation years associated to the high daily dosage of steroids. Our results suggest that other factors than steroid cumulative dosage might enhance long-term bone loss.

Beside steroids, other immunosuppressive agents have a known impact on bone status. Azathioprine was not found to modify bone metabolism in animal models or in humans. The role of cyclosporin A is still controversial. When cyclosporin A was administered to rats in doses comparable with those used in transplantation, severe bone loss has been reported (9). In these studies, bone turnover was accelerated, with increased bone resorption and bone formation, mediated in part by gonadal dysfunction and by increased expression of interleukin-6 (10,11). In other studies, cyclosporin A has been found to inhibit bone resorption in rats (12) and to decrease osteoclast differentiation in vitro (13). In human organ transplantation, cyclosporin A is used together with steroids in most of immunosuppressive regimens. Thus, it is difficult to determine whether cyclosporin A has a specific effect on bone in humans. As suggested by increased serum osteocalcin in renal-, cardiac-, or liver-transplanted patients, cyclosporin A may stimulate bone formation in humans (10). The combined effect of both steroids and cyclosporin is unclear. In the first few months, the rapid bone loss explained by a predominant decrease in bone formation may mainly be attributed to steroids. In our study, bone mineral density was significantly correlated with cyclosporin but not steroid-cumulative dosage, suggesting a possible protective effect of cyclosporin on bone mass in long-term posttransplant patients. Aroldi et al. (8) found that cyclosporin alone did not decrease BMD by contrast to steroids in renal transplant recipients. It has indeed previously been suggested that patients treated with cyclosporin may be at lower risk of bone loss in the long-term after renal transplantation (3,14,15). In a 2-year prospective study in 52 kidney recipients, Ezaitouni et al. (3) reported that the Z score was decreased at 3 months at the vertebral (−1.40) and femoral neck sites (−1.34) with no difference thereafter in BMD measurements (6, 12, and 24 months). Bone remodeling markers were significantly increased in the first 6 months, then returned to baseline after 2 years, in parallel with decreased cyclosporin dosage. The authors described an increase in vertebral BMD measured by axial computed tomography after 1 year, correlated both with cyclosporin dosage at 1 year and with bone alkaline phosphatase level at 6 months. This suggests that cyclosporin has a beneficial effect by stimulating bone remodeling and counterbalancing the negative effect of steroids on bone mass (3).

Prevalence and characteristics of fractures in renal transplantation OP are scarcely evaluated in the literature. Our study shows an intermediate fracture rate. No specific risk factor was identified. In prospective longitudinal studies, occurrence of fractures was not mentioned during the early posttransplantation follow-up (1,2), but BMD measurements were below the fracture threshold in over half of the patients (1). In a cross-sectional study conducted in 100 graft recipients with a mean posttransplantation time of 63±53 months, the fracture rate was three times higher than in patients undergoing dialysis (15). Peripheral fractures occurred in 11/100 patients within a mean interval of 103 months after transplantation. Vertebral fractures were observed in 3/100 patients within a mean interval of 57 months after transplantation. Age and BMD were not correlated to fractures. In a retrospective analysis of 35 diabetic transplanted patients with a mean posttransplantation time of 49±28 months, 17 (48.5%) patients had peripheral fractures (16). Risk factors were the female gender and the steroid-cumulative dosage. In a cross-sectional study (31 patients with a mean posttransplantation time of 40 months), 14 patients (45%) had experienced both vertebral and nonvertebral fractures (17). The lack of correlation between fractures and bone mass can be attributed to the fact that steroids induce a special type of OP described in histomorphometric studies. The main effect of steroids is a depression of osteoblast function, which induces a reduced synthesis of bone matrix with greater thinning of trabeculae (18). Therefore, the quality of bone is altered, accounting for the fact that some patients with fractures may have normal bone mass measurements.

In our study, there was an overlap in the BMD measurements between patients with and without fractures. A possible reason for the occurrence of fractures in the nonosteoporotic group is altered trabecular bone architecture induced by corticosteroids. A recent study performed on 108 men (mean age 52.1 years) with osteopenia suggested that bone trabecular microarchitecture was a major determinant of vertebral fractures (19). Bone histomorphometry was not performed in this study, but we can assume that other factors such as pretransplantation dialysis or hyperparathyroidism can also modify bone microarchitecture. Further studies including bone biopsy should be performed to describe the trabecular architecture and its relationship to BMD and fractures. In our study, the prevalence of OP in long-term posttransplant patients is close to 50%. Osteoporotic fractures occurred before or after transplantation in 25% of the transplanted patients. Cyclosporin current dosage was associated to a higher bone mass, suggesting a possible protective effect of this drug in long-term posttransplantation recipients.

From our data, it seems useful that bone status should be evaluated routinely before and after kidney transplantation. Therapeutic strategies to prevent fractures in renal transplanted patients, based on risk factors analysis, are needed even years after transplantation to improve quality of life in late survivors.

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REFERENCES

1. Julian B, Laskow D, Dubovsky J, et al. Rapid loss of vertebral bone density after renal transplantation. N Engl J Med 1991; 325: 544–550.
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