CYCLICAL INTRAVENOUS THERAPY with pamidronate is increasingly used to treat children and adolescents with osteogenesis imperfecta (OI) and other osteopenic disorders in children.(1–3) In OI patients, histomorphometric studies of iliac bone samples have shown that this treatment increases cortical thickness and the amount of trabecular bone, but suppresses bone turnover.(4) Typically pamidronate is given in intravenous infusion cycles that are repeated every month to every 4 months.(1–3) As long as growth continues, each of these infusion cycles leaves a dense transverse line in the metaphysis.(1,2,5) The histological nature of these metaphyseal lines has not been elucidated, but it has been speculated that they consist of calcified cartilage and might reduce biomechanical strength.(6)
For obvious reasons, it is difficult to obtain bone tissue from sites where these sclerotic lines are most conspicuous radiographically, i.e., in fast growing long bones. Nevertheless, sclerotic lines are also observed in the iliac wing, which has an apophyseal growth plate (Fig. 1A). This is close to the location where bone biopsy samples are typically obtained for histomorphometric analysis. Although our standard site for transiliac biopsy specimens is 2 cm distal from the apophyseal growth cartilage,(7) growth plate tissue is sometimes inadvertently included in a biopsy specimen. This reduces the value of the specimen for histomorphometric analysis, but offers the opportunity to study sclerotic lines associated with pamidronate treatment.
The present patient was a 12-year-old girl with OI type VII who had received cyclical intravenous pamidronate every 4 months for the past 6.9 years.(8) A biopsy of the left iliac bone was performed after dual tetracycline labeling as part of a follow-up program of long-term pamidronate therapy.(4) The sample was embedded in polymethylmetacrylate, sectioned, and stained with Goldner-Masson Trichrome, as described in detail previously.(7) The entire cancellous bone compartment excluding the endocortical surfaces was analyzed histomorphometrically in three sections that were separated by at least 150 μm, as described previously.(7) Results were compared with age-matched reference data.(7) Trabecular bone volume per tissue volume of the entire cancellous bone compartment was in the upper one-half of the reference range (30.9%; age-specific reference values: 24.4 ± 4.3%), with normal trabecular thickness (132 μm; reference, 148 ± 23 μm; Fig. 1B). An identically processed iliac bone sample obtained at the start of pamidronate therapy (6.9 years earlier) had revealed a trabecular bone volume of 12.2% (Fig. 1C). Thus, trabecular bone volume had increased about 2.5-fold during the treatment period.
The bone obtained after pamidronate treatment was arranged in bands parallel to the growth plate (Fig. 1B). The distance between the bands was 1.0-1.3 mm. This was similar to the distance between sclerotic lines in the corresponding radiograph (Fig. 1A). In the band closest to the growth plate, about one-quarter of the mineralized tissue consisted of calcified cartilage (Table 1). The relative amount of cartilage decreased continuously with increasing distance from the growth plate.
Averaged over the entire cancellous compartment, parameters of bone formation were within normal limits (e.g., bone formation rate per bone surface was 52 μm3/μm2/year; reference, 37 ± 17 μm3/μm2/year), whereas resorption parameters were elevated (osteoclast surface per bone surface was 2.3%; reference, 0.94 ± 0.38%; the number of osteoclasts per bone perimeter was 0.82/mm; reference, 0.29 ± 0.14/mm). As noted in our previous studies, many of the osteoclasts were unusually large, with a high number of nuclei (compare Figs. 1D and 1E).(4)
The increased number of osteoclasts probably reflects the fact that bone turnover is usually elevated close to the growth plate.(7) It is expected that bone formation parameters should be equally elevated at that location, but bone formation rate was within normal limits. This is in accordance with our earlier observation that pamidronate has a more pronounced suppressive effect on histomorphometric parameters of bone formation than on the number of osteoclasts.(4) This lack of treatment effect on osteoclast number may seem paradoxical with an antiresorptive therapy, but pamidronate may exert its effect through interference with osteoclast function rather than by causing osteoclast apoptosis.(9)
In this iliac bone sample, the percentage of calcified cartilage was high in the sclerotic line closest to the growth plate but decreased with increasing distance from the growth plate (Table 1). This suggests that the sclerotic lines seen on radiographs do not represent bars of growth plate cartilage that have been “frozen” by antiresorptive therapy. Rather, they seem to be horizontal trabeculae undergoing turnover. Therefore, new mineralized bone is gradually substituted for the old bone containing calcified cartilage, but the size and shape of these trabeculae are not changed. These are the typical features of the bone remodeling process.(10) The present biopsy specimen did not allow for analysis of sclerotic lines further away from the growth plate, but our previous observations are in accordance with the view that low remodeling activity may contribute to the persistence of the sclerotic lines during pamidronate treatment.(4)
One might argue that the arrangement of cancellous bone in horizontal trabeculae is not optimal from a biomechanical perspective. In addition, it must be acknowledged that pamidronate does not alter the underlying genetic defect, and therefore, the newly produced bone is likely to have impaired mechanical properties at the material level. Thus, pamidronate does not correct the structural and material deficiencies of OI bone but rather increases its amount.
We speculate that the sclerotic lines that are seen in children during pamidronate treatment arise through similar mechanisms as the transverse lines that occur in the metaphyseal bones of growing children after nonspecific events, such as fever or fractures.(11) These lines are thought to arise after temporary interruption of growth plate activity, when osteoblasts start to deposit bone matrix on the metaphyseal side of the growth plate. When growth resumes, the growth plate moves away from these newly created horizontal trabeculae, which then become visible as so-called “growth arrest lines,” or Harris lines, on radiographs.(11) Temporary interruption of growth plate cartilage resorption at the time of a pamidronate infusion may have a similar effect as growth arrest.