Osteodensitometry in uncemented total hip arthroplasty using computertomography.

The aim of this investigation was to evaluate a new method developed for the measurement of bone mineral density and bone remodelling phenomena after total hip arthroplasty using computer tomography. Computertomography is a radiological technique to examine bone structures in high resolution. Using an extended scale it is possible to investigate bone scans and implants with fewer metal artifacts. For osteodensitometry measurement a special software (IMPact HIP) for the analysis of the data was used. The measured parameters were the overall bone mineral density (mg Calcium-Hydroxyapatite/ml) and the cortical bone structure. A standard scan mode enable to compare the computertomography scans at follow-up. Nineteen total hip arthroplasty patients (20 hips) with a mean age of 58 years (31-70) were operated on using an uncemented titanium alloy stem with a tapered design. The periprosthetic bone was assessed using computertomography-assisted osteodensitometry two weeks and one year after surgery. We observed a decrease of the overall bone mineral density (15%) and of the cortical bone structure (20%) one year after insertion of the stem in the proximal part of the femur. The area corresponds to the Gruen zones 1 and 7. On the other hand, a decrease of mineral density of 5% for the overall bone and of 3% for the cortical bone was found at the level of the tip of the stem, which corresponds to the Gruen zones 3, 4 and 5. Computertomography-assisted osteodensitometry allows to investigate the bone remodelling after total hip arthroplasty by separating the analysis of the overall bone mineral density and of the cortical structure. The present method is a reliable tool for quality-control in total hip arthroplasty.     

A long-lasting bisphosphonate partially protects periprosthetic bone,but does not enhance initial stability of uncemented femoral stems: A randomized placebo-controlled trial of women undergoing total hip arthroplasty

Low bone quality may compromise the success of cementless total hip arthroplasty in high-risk patients such as elderly women. Zoledronic acid is a long-lasting antiresorptive agent, which is known to reduce short-term periprosthetic bone loss. However, its effect on femoral stem stability is not well known. Forty-nine female patients with a mean age of 68 years (range, 51–85 years) scheduled to undergo cementless total hip arthroplasty due to osteoarthritis were randomized in this double-blind, placebo-controlled trial to receive a single postoperative infusion of zoledronic acid or placebo. Patients were evaluated for up to four years postoperatively for femoral stem migration measured by radiostereometric analysis, bone mineral density (BMD) measured by dual X-ray absorptiometry, functional recovery, and patient-reported outcome scores. Implant survival was determined at nine years postoperatively. Zoledronic acid did not reduce the femoral stem migration that occurred predominantly during the settling period of the first 3–6 months. Subsequently, all femoral stems were radiographically osseointegrated. Zoledronic acid maintained periprosthetic BMD, while the expected loss of periprosthetic bone during the first 12 months was found in controls. Thereafter, periprosthetic BMD of Gruen zone 7 decreased even in the zoledronic acid group but remained 14.6% higher than that in the placebo group at four years postoperatively. Functional recovery was comparable across the groups. At nine years postoperatively, no revision arthroplasty had been performed. In conclusion, in women at high-risk for low BMD, zoledronic acid had a long-lasting, partially protective effect on periprosthetic bone loss, but the treatment did not enhance the initial femoral stem stability.     

Quality of intertrochanteric cancellous bone as predictor of femoral stem RSA migration in cementless total hip arthroplasty

In cementless total hip arthroplasty, osteoporosis may jeopardize the achievement of immediate stability and lead to migration of anatomically shaped femoral stems. Poor quality of proximal cancellous bone per se may also affect the rate of osseointegration. In a selected group of female total hip arthroplasty patients (mean age 64 years) with unremarkable medical history, intertrochanteric cancellous bone biopsy was taken from the site of stem implantation. Local bone quality, determined by structural μCT imaging and destructive compression testing of the biopsy tissue, was used as the predictor of three-dimensional stem migration determined by radiostereometric analysis (RSA) up to 24 months. The patients exhibited major differences in mechanical properties of the intertrochanteric cancellous bone, which were closely related to the structural parameters calculated from μCT data. Unexpectedly, the major differences observed in the quality of trochanteric cancellous bone had only minor reflections in the RSA migration of the femoral stems. In statistical analysis, the μCT-based bone mineral density quartile (low, middle, high) was the only significant predictor for stem translation at 24 months (p=0.022) but only a small portion (R(2)=0.16) of the difference in translation could be explained by changes in bone mineral density quartile. None of the other parameters investigated predicted stem migration in translation or rotation. In conclusion, poor quality of intertrochanteric cancellous bone seems to contribute to the risk of implant migration less than expected. Probably also the importance of surgical preservation of intertrochanteric cancellous bone has been over-emphasized for osseointegration of cementless stem.     

The influence of acetabular cup material on pelvis cortex surface strains, measured using digital image correlation

Acetabular cup loosening is a late failure mode of total hip replacements, and peri-prosthetic bone deterioration may promote earlier failure. Preservation of supporting bone quality is a goal for implant design and materials selection, to avoid stress shielding and bone resorption. Advanced polymer composite materials have closer stiffness to bone than metals, ceramics or polymers, and have been hypothesised to promote less adverse bone adaptation. Computer simulations have supported this hypothesis, and the present study aimed to verify this experimentally.A composite hemi-pelvis was implanted with Cobalt Chromium (CoCr), polyethylene (UHMWPE) and MOTIS^®carbon-fibre-reinforced polyether etherketone (CFR-PEEK) acetabular cups. In each case, load was applied to the implanted pelvis and Digital Image Correlation (DIC) was used for surface strain measurement. The test was repeated for an intact hemi-pelvis. Trends in implanted vs. intact bone principal strains were inspected to assess the average principal strain magnitude change, allowing comparison of the potential bone responses to implantation with the three cups.The CFR-PEEK cup was observed to produce the closest bone strain to the intact hip in the main load path, the superior peri-acetabular cortex (+12% on average, R²=0.84), in comparison to CoCr (+40%, R²=0.91) and UHWMPE cups (?26%, R²=0.94). Clinical observations have indicated that increased periacetabular cortex loading may result in reduced polar cancellous bone loading, leading to longer term losses in periprosthetic bone mineral density. This study provides experimental evidence to verify previous computational studies, indicating that cups produced using materials with stiffness closer to cortical bone recreate physiological cortical bone strains more closely and could, therefore, potentially promote less adverse bone adaptation than stiffer press-fitted implants in current use.     

Cancellous and cortical morselized allograft in revision total hip replacement: A biomechanical study of implant stability

To restore femoral intramedullary bone stock loss in revision surgery of failed total hip arthroplasties, impacted morselized cancellous allograft is recommended. This study investigated the mechanical properties of both impacted cortical (group A) and cancellous (group B) morselized bone graft for reconstruction of femoral bones. Ten matched pairs of fresh frozen human femora were prepared by over-reaming to create a smooth-walled cortical shell. Each pair had one cortical and one cancellous impacted morselized allograft and cement. Stem subsidence was evaluated by a cyclic axial load, which was applied by a servohydraulic test. The stem subsidence was measured for initial subsidence (subsidence at the first 1000 cycles), the total axial subsidence (subsidence at the end of cycles under load) and the final axial subsidence (subsidence after the unloading phase). Torque test was measured by torsional loads through the prosthetic femoral heads. Total axial subsidence was significantly higher in group B (mean: 1.32+/-0.32 mm) compared to group A (mean: 0.94+/-0.26 mm) (P<0.01).There was no significant difference between the two groups in terms of initial subsidence (P=0.09) and final axial subsidence.The mean maximum torque before failure was 39.5+/-22.2 N-m for the cortical morselized allograft and 32.5+/-18.1N-m for cancellous.We concluded that impacted morselized cortical bone graft used for reconstruction of contained femoral bone loss in revision hip arthroplasty, may reduce the stem subsidence. Further animal experimentation for mechanical and histological evaluation of in vivo application is warranted.     

Activity intensity,assistive devices and joint replacement influence predicted remodelling in the proximal femur

Bone morphology and density changes are commonly observed following joint replacement, may contribute to the risks of implant loosening and periprosthetic fracture and reduce the available bone stock for revision surgery. This study was presented in the ‘Bone and Cartilage Mechanobiology across the scales’ WCCM symposium to review the development of remodelling prediction methods and to demonstrate simulation of adaptive bone remodelling around hip replacement femoral components, incorporating intrinsic (prosthesis) and extrinsic (activity and loading) factors. An iterative bone remodelling process was applied to finite element models of a femur implanted with a cementless total hip replacement (THR) and a hip resurfacing implant. Previously developed for a cemented THR implant, this modified process enabled the influence of pre- to post-operative changes in patient activity and joint loading to be evaluated. A control algorithm used identical pre- and post-operative conditions, and the predicted extents and temporal trends of remodelling were measured by generating virtual X-rays and DXA scans. The modified process improved qualitative and quantitative remodelling predictions for both the cementless THR and resurfacing implants, but demonstrated the sensitivity to DXA scan region definition and appropriate implant–bone position and sizing. Predicted remodelling in the intact femur in response to changed activity and loading demonstrated that in this simplified model, although the influence of the extrinsic effects were important, the mechanics of implantation were dominant. This study supports the application of predictive bone remodelling as one element in the range of physical and computational studies, which should be conducted in the preclinical evaluation of new prostheses.     

Finite element analysis of retroacetabular osteolytic defects following total hip replacement

Periprosthetic osteolysis in the retroacetabular region with cancellous bone loss is a recognized phenomenon in the long-term follow-up of total hip replacement. The effects on load transfer in the presence of defects are less well known. A finite element model incorporating a retroacetabular defect behind a cementless component was validated against a 4th generation sawbone pelvis. Computational predictions of surface strain and von Mises stresses were closely correlated to experimental findings. The presence of a cancellous defect increased von Mises stress in the cortical bone of the medial wall of the pelvis. At a load of 600 N this was under the predicted failure stress for cortical bone. Increases in the cup size relative to the acetabulum caused increased stress in the cortical bone of the lateral wall of the pelvis, adjacent to the acetabulum. We are confident that our modeling approach can be applied to patient specific defects to predict pelvis stress with large loads and a range of activities.     

Disuse in adult male rats attenuates the bone anabolic response to a therapeutic dose of parathyroid hormone.

Intermittent treatment with parathyroid hormone (PTH) increases bone formation and prevents bone loss in hindlimb-unloaded (HLU) rats. However, the mechanisms of action of PTH are incompletely known. To explore possible interactions between weight bearing and PTH, we treated 6-mo-old weight-bearing and HLU rats with a human therapeutic dose (1 microg.kg(-1).day(-1)) of human PTH(1-34) (hPTH). Cortical and cancellous bone formation was measured in tibia at the diaphysis proximal to the tibia-fibula synostosis and at the proximal metaphysis, respectively. Two weeks of hindlimb unloading resulted in a dramatic decrease in the rate of bone formation at both skeletal sites, which was prevented by PTH treatment at the cancellous site only. In contrast, PTH treatment increased cortical as well as cancellous bone formation in weight-bearing rats. Two-way ANOVA revealed that hPTH and HLU had independent and opposite effects on all histomorphometric indexes of bone formation [mineral apposition rate (MAR), double-labeled perimeter (dLPm), and bone formation rate (BFR)] at both skeletal sites. The bone anabolic effects of weight bearing and hPTH on dLPm and BFR at the cortical site were additive, as were the effects on MAR at the cancellous site. In contrast, weight bearing and hPTH resulted in synergistic increases in cortical bone MAR and cancellous bone dLPm and BFR. We conclude that weight bearing and PTH act cooperatively to increase bone formation by resulting in site-specific additive and synergistic increases in indexes of osteoblast number and activity, suggesting that weight-bearing exercise targeted to osteopenic skeletal sites may improve the efficacy of PTH therapy for osteoporosis.     

Comparative effects of alendronate and alfacalcidol on cancellous and cortical bone mass and bone mechanical properties in ovariectomized rats.

The purpose of the present study was to compare the effects of alendronate and alfacalcidol on cancellous and cortical bone mass and bone mechanical properties in ovariectomized rats. Twenty-six female Sprague-Dawley rats, 7 months of age, were randomized by the stratified weight method into four groups: the sham-operated control (Sham) group and the three ovariectomy (OVX) groups, namely, OVX + vehicle, OVX + alendronate (2.5 mg/kg, p.o., daily), and OVX + alfacalcidol (0.5 mug/kg, p.o., daily). At the end of the 8-week experimental period, bone histomorphometric analyses of cancellous bone at the proximal tibial metaphysis and cortical bone at the tibial diaphysis were performed, and the mechanical properties of the femoral distal metaphysis and femoral diaphysis were evaluated. OVX decreased cancellous bone volume per total tissue volume (BV/TV), and the maximum load of the femoral distal metaphysis, as a result of increases in serum osteocalcin (OC) levels, and also the number of osteoclasts (N.Oc), osteoclast surface (OcS) and bone formation rate (BFR) per bone surface (BS), and BFR/BV, without any effect on cortical area (Ct Ar), or maximum load of the femoral diaphysis. Alendronate prevented this decrease in cancellous BV/TV by suppressing increases in N.Oc/BS, OcS/BS, BFR/BS, and BFR/BV, without any apparent effect on Ct Ar, or maximum load of the femoral distal metaphysis and femoral diaphysis. On the other hand, alfacalcidol increased cancellous BV/TV, Ct Ar, and the maximum load of the femoral distal metaphysis and femoral diaphysis, by mildly decreasing trabecular BFR/BV, maintaining trabecular mineral apposition rate and osteoblast surface per BS, increasing periosteal and endocortical BFR/BS, and preventing an increase in endocortical eroded surface per BS. The present study clearly showed the differential skeletal effects of alendronate and alfacalcidol in ovariectomized rats. Alendronate prevented OVX-induced cancellous bone loss by suppressing bone turnover, while alfacalcidol improved cancellous and cortical bone mass and bone strength by suppressing bone resorption and maintaining or even increasing bone formation.     

Improvement of femoral bone-cement adhesion in cemented revision hip arthroplasty by application of an amphiphilic bonder in a dynamic femur expulsion testing in vitro ]

Cemented femoral stems have shown decreased longevity compared to cementless implants in hip revision arthroplasty. The aim of this study was to evaluate the effect of an amphiphilic bonder on bone cement stability in a biomechanical femur expulsion test. A simplified hip simulator test setup with idealised femur stem specimens was carried out. The stems were implanted into bovine femurs (group 1: no bonder, n=10; group 2: bonder including glutaraldehyde, n=10; group 3: bonder without glutaraldehyde, n=10). A dynamic loading (maximum load: 800 N; minimum load: 100 N; frequency: 3 Hz; 105 cycles) was performed. Subsequently, the stem specimens were expulsed axially out of their implant beds and maximum load at failure was recorded. The static controls showed a mean maximum load to failure of 4123 N in group 1, 8357.5 N in group 2 and 5830.8 N in group 3. After dynamic loading, the specimens of group 2 reached the highest load to failure (8191.5 N), followed by group 3 (5649.5 N) and group 1 (3462 N), respectively. In group 2, we observed nine periprosthetic fractures at a load of 8400 N without signs of interface loosening. Application of an amphiphilic bonder led to a significant improvement of bonding stability, especially when glutaraldehyde was added to the bonder. This technique might offer an increased longevity of cemented femur revision stems in total hip replacement.     

The role of estrogen receptor-beta, in the early age-related bone gain and later age-related bone loss in female mice

The molecular and cellular mechanism of estrogen action in skeletal tissue remains unclear. The purpose of this study was to understand the role of estrogen receptor-beta, (ERbeta) on cortical and cancellous bone during growth and aging by comparing the bone phenotype of 6- and 13-month-old female mice with or without ERbeta. Groups of 11-14 wild-type (WT) controls and ERbeta knockout (BERKO) female mice were necropsied at 6 and 13 months of age. At both ages, BERKO mice did not differ significantly from WT controls in uterine weight and uterine epithelial thickness, indicating that ERbeta does not regulate the growth of uterine tissue. Femoral length increased significantly by 5.5% at 6 months of age in BERKO mice compared with WT controls. At 6 months of age, peripheral quantitative computerized tomography (pQCT) analysis of the distal femoral metaphysis (DFM) and femoral shafts showed that BERKO mice had significantly higher cortical bone content and periosteal circumference as compared with WT controls at both sites. In contrast to the findings in cortical bone, at 6 months of age, there was no difference between BERKO and WT mice in trabecular density, trabecular bone volume (TBV), or formation and resorption indices at the DFM. In 13-month-old WT mice, TBV (-41%), trabecular density (-27%) and cortical thickness decreased significantly. while marrow cavity and endocortical circumference increased significantly compared with 6-month-old WT mice. These age-related decreases in cancellous and endocortical bone did not occur in BERKO mice. At 13 months of age, BERKO mice had significantly higher total, trabecular and cortical bone, while having significantly lower bone resorption, bone formation and bone turnover in DFM compared with WT mice. These results indicate that deleting ERbeta protected against age-related bone loss in both the cancellous and endocortical compartments by decreasing bone resorption and bone turnover in aged female mice. These data demonstrate that in female mice, ERbeta plays a role in inhibiting periosteal bone formation, longitudinal and radial bone growth during the growth period, while it plays a role in stimulating bone resorption, bone turnover and bone loss on cancellous and endocortical bone surfaces during the aging process.     

Improving stress shielding following total hip arthroplasty by using a femoral stem made of β type Ti-33.6Nb-4Sn with a Young’s modulus gradation

Stress shielding-related bone loss occurs after total hip arthroplasty because the stiffness of metallic implants differs from that of the host femur. Although reducing stem stiffness can ameliorate the bone resorption, it increases stress at the bone–implant interface and can inhibit fixation. To overcome this complication, a novel cementless stem with a gradient in Young’s modulus was developed using Ti-33.6Nb-4Sn (TNS) alloy. Local heat treatment applied at the neck region for increasing its strength resulted in a gradual decrease in Young’s modulus from the proximal to the distal end, from 82.1 to 51.0 GPa as calculated by a heat transfer simulation. The Young’s modulus gradient did not induce the excessive interface stress which may cause the surface debonding. The main purpose of this study was to evaluate bone remodeling with the TNS stem using a strain-adaptive bone remodeling simulation based on finite element analysis. Our predictions showed that, for the TNS stem, bone reduction in the calcar region (Gruen zone 7) would be 13.6% at 2 years, 29.0% at 5 years, and 45.8% at 10 years postoperatively. At 10 years, the bone mineral density for the TNS stem would be 42.6% higher than that for the similar Ti-6Al-4V alloy stem. The stress–strength ratio would be lower for the TNS stem than for the Ti-6Al-4V stem. These results suggest that although proximal bone loss cannot be eliminated completely, the TNS stem with a Young’s modulus gradient may have bone-preserving effects and sufficient stem strength, without the excessive interface stress.     

Simvastatin did not prevent nor restore ovariectomy-induced bone loss in adult rats

Current published results on whether statins have beneficial effects on bone metabolism have been conflicting so far. In order to further investigate if statins were promising candidates for the treatment for osteoporosis, we conducted a study in which rats were ovariectomized (OVX) at 6 months of age, allowed to lose bone for 60 days and followed by oral administration of simvastatin at the dose levels of 0.3-10 mg/kg/d for 60 days. PGE2 (6 mg/kg) was used as a positive control. Study endpoints included bone histomorphometry on the proximal tibial metaphysis (PTM) and the tibial diaphysis (TX), dual-energy X-ray absorptiometry on the right femur and micro computed tomography (ICT) on the 5th lumbar vertebra (LV). After 120 days of OVX, cancellous bone lost by 80% in the PTM and 18% in the LV accompanied by increased bone formation and resorption. Simvastatin at all dose levels did not affect bone volume, bone formation rate and bone erosion surface when compared to 120 day ovariectomized animals at all bone sites studied. By contrast, PGE2 restored cancellous and cortical bone area to sham control levels. In conclusion, this study demonstrated that unlike PGE2, oral administration of simvastatin did not have effects on cancellous or cortical bone formation and resorption; and consequently was not able to prevent further bone loss or restore bone mass in the osteopenic, OVX rats.     

A losing battle: weight regain does not restore weight loss-induced bone loss in postmenopausal women

Previously, we reported significant bone mineral density (BMD) loss in postmenopausal women after modest weight loss. It remains unclear whether the magnitude of BMD change in response to weight loss is appropriate (i.e., proportional to weight loss) and whether BMD is recovered with weight regain. We now report changes in BMD after a 1‐year follow‐up. Subjects (n = 23) in this secondary analysis were postmenopausal women randomized to placebo as part of a larger trial. They completed a 6‐month exercise‐based weight loss program and returned for follow‐up at 18 months. Dual‐energy X‐ray absorptiometry (DXA) was performed at baseline, 6, and 18 months. At baseline, subjects were aged 56.8 ± 5.4 years (mean ± s.d.), 10.0 ± 9.2 years postmenopausal, and BMI was 29.6 ± 4.0 kg/m². They lost 3.9 ± 3.5 kg during the weight loss intervention. During follow‐up, they regained 2.9 ± 3.9 kg. Six months of weight loss resulted in a significant decrease in lumbar spine (LS) (?1.7 ± 3.5%; P = 0.002) and hip (?0.04 ± 3.5%; P = 0.03) BMD that was accompanied by an increase in a biomarker of bone resorption (serum C‐terminal telopeptide of type I collagen, CTX: 34 ± 54%; P = 0.08). However, weight regain was not associated with LS (0.05 ± 3.8%; P = 0.15) or hip (?0.6 ± 3.0%; P = 0.81) bone regain or decreased bone resorption (CTX: ?3 ± 37%; P = 0.73). The findings suggest that BMD lost during weight reduction may not be fully recovered with weight regain in hormone‐deficient, postmenopausal women. Future studies are needed to identify effective strategies to prevent bone loss during periods of weight loss.     

Cementless Vektor-titan stem in total hip arthroplasty.

INTRODUCTION: Cementless THR is a well established, and a widely accepted optimal procedure for younger patients. The cementless Vektor-Titan stem is made of Ti6AI7Nb, has got the shape of a three-dimensional cone, and an optimal proximal anchoring property. MATERIALS AND METHODS: The aim of this prospective study was to scrutinise the outcome of 250 Vektor-Titan stems in cementless THRs with an average follow-up time of 3.0 years (Min: 1, Max: 6). The average age of the patients including 148 women and 102 men was calculated with 54.6 years (Min: 22.5, Max: 77.7). RESULTS: The score according to Merle d'Aubigné improved from preoperative 9.3 (Min: 7, Max: 13) to postoperative 17.0 (Min: 14, Max: 18). Distal cortical hypertrophy and proximal atrophy was detected in 4 cases. Single atrophy of the proximal femur was found in additional 3 cases. Progressive radiolucent lines in zone 1 and 7 according to Gruen were observed in one case. Postoperative local and general complications were seen as two subfascial hematomas, two single dislocations, two recurrent dislocations of the hip prosthesis, 6 lesions of the sciatic nerve (one persisting), two deep venous thrombosis, two pneumonias, and one lethal pulmonary embolism. A stable proximal fixation was achieved in 242 of 250 cases (96.8%). CONCLUSION: The results of this study using the Vektor-Titan stem in cementless total hip arthroplasty showed that the principle of proximal fixation was optimized. Long term follow-up studies are needed to confirm these good results.     

Estrogen-induced osteogenesis in intact female mice lacking ERbeta

We recently found that estrogen receptor (ER) antagonists prevent high-dose estrogen from inducing the formation of new cancellous bone within the medullary cavity of mouse long bones. In the present investigation, we studied the role of specific ER subtypes in this response by examining whether this is impaired in female ERbeta(-/-) mice previously generated by targeted gene deletion. Vehicle or 17beta-estradiol (E(2)) (range 4-4,000 microg. kg(-1). day(-1)) was administered to intact female ERbeta(-/-) mice and wild-type littermates by subcutaneous injection for 28 days. The osteogenic response was subsequently assessed by histomorphometry performed on longitudinal and cross sections of the tibia. E(2) was found to cause an equivalent increase in cancellous bone formation in ERbeta(-/-) mice and littermate controls, as assessed at the proximal and distal regions of the proximal tibial metaphysis. E(2) also resulted in a similar increase in endosteal mineral apposition rate in these two genotypes, as assessed at the tibial diaphysis. In contrast, cortical area in ERbeta(-/-) mice was found to be greater than that in wild types irrespective of E(2) treatment, as was tibial bone mineral density as measured by dual-energy X-ray absorptiometry, consistent with previous reports of increased cortical bone mass in these animals. We conclude that, although ERbeta acts as a negative modulator of cortical modeling, this isoform does not appear to contribute to high-dose estrogen's ability to induce new cancellous bone formation in mouse long bones.     

Effects of disrupted beta1-integrin function on the skeletal response to short-term hindlimb unloading in mice.

The study was designed to determine whether beta1-integrin plays a role in mediating the acute skeletal response to mechanical unloading. Transgenic (TG) mice were generated to express a dominant negative form of beta1-integrin under the control of the osteocalcin promoter, which targets expression of the transgene to mature osteoblasts. At 63 days of age, wild-type (WT) and TG mice were subjected to hindlimb unloading by tail suspension for 1 wk. Pair-fed, normally loaded WT and TG mice served as age-matched controls. Bone samples from each mouse were processed for quantitative bone histomorphometry and biomechanical testing. The skeletal phenotype of TG mice was characterized by lower cancellous bone mass in the distal femoral metaphysis (-52%) and lumbar vertebral body (-20%), reduced curvature of the proximal tibia (-20%), and decreased bone strength (-20%) and stiffness (-23%) of the femoral diaphysis with relatively normal indexes of cancellous bone turnover. Hindlimb unloading for only 1 wk induced a 10% decline in tibial curvature and a 30% loss of cancellous bone in the distal femur due to a combination of increased bone resorption and decreased bone formation in both WT and TG mice. However, the strength and stiffness of the femoral diaphysis were unaffected by short-term hindlimb unloading in both genotypes. The observed increase in osteoclast surface was greater in unloaded TG mice (92%) than in unloaded WT mice (52%). Cancellous bone formation rate was decreased in unloaded WT (-29%) and TG (-15%) mice, but, in contrast to osteoclast surface, the genotype by loading interaction was not statistically significant. The results indicate that altered integrin function in mature osteoblasts may enhance the osteoclastic response to mechanical unloading but that it does not have a major effect on the development of cancellous osteopenia in mice during the early stages of hindlimb unloading.     

Effects of Materials of Cementless Femoral Stem on the Functional Adaptation of Bone

The objective of this paper is to identify the effects of materials of cementless femoral stem on the functional adaptive behaviors of bone.The remodeling behaviors of a two-dimensional simplified model of cementless hip prosthesis with stiff stem,flexible 'iso-elastic' stem,one-dimensional Functionally Graded Material (FGM) stem and two-dimensional FGM stem for the period of four years after prosthesis replacement were quantified by incorporating the bone remodeling algorithm with finite element analysis.The distributions of bone density,von Mises stress,and interface shear stress were obtained.The results show that two-dimensional FGM stem may produce more mechanical stimuli and more uniform interface shear stress compared with the stems made of other materials,thus the host bone is well preserved.Accordingly,the two-dimensional FGM stem is an appropriate femoral implant from a biomechanical point of view.The numerical simulation in this paper can provide a quantitative computational paradigm for the changes of bone morphology caused by implants,which can help to improve the design of implant to reduce stress shielding and the risk of bone-prosthesis interface failure.     

Response of cortical and cancellous bones to mild calcium deficiency in young growing female rats: a bone histomorphometry study.

The purpose of the present study was to clarify the differences in the alterations of cellular activities of osteoblasts and osteoclasts, mineralization, and bone mass in cortical and cancellous bones of young growing rats with mild calcium deficiency. Twenty female Sprague-Dawley rats, 6 weeks of age, were randomized by the stratified method into two groups with 10 rats in each group: 0.5% (normal) calcium diet group and 0.1% (low) calcium diet group. After 10 weeks of feeding, bone histomorphometric analysis was performed on cancellous bone of the proximal tibia as well as cortical bone of the tibial shaft. Calcium deficiency increased eroded surface (ES/bone surface [BS]) and the number of osteoclast (N.Oc/BS) with an increase in osteoblast surface (ObS/BS), but decreased bone formation rate (BFR/BS) in cancellous bone. However, cancellous bone volume was preserved, while cortical bone area was decreased as a result of decreased periosteal bone gain and enlargement of the marrow cavity. These results suggest that short-term mild calcium deficiency in young growing female rats increased bone resorption by increasing osteoclastic recruitment, and suppressed mineralization followed by increased osteoblastic recruitment in cancellous bone, but cancellous bone loss was counteracted through redistribution of calcium from cortical bone to cancellous bone.     

Long-term study of bone remodelling after femoral stem: a comparison between dexa and finite element simulation

A hip replacement with a cemented or cementless femoral stem produces an effect on the bone called adaptive remodelling, attributable to mechanical and biological factors. The objective of all of cementless prostheses designs has been to achieve a perfect transfer of loads in order to avoid stress-shielding, which produces an osteopenia. In order to quantify this, the long term and mass-produced study with dual energy X-ray absorptiometry (DEXA) is necessary. Finite element (FE) simulation makes possible the explanation of the biomechanical changes which are produced in the femur after stem implantation. The good correlation obtained between the results of the FE simulation and the densitometric study allow, on one hand, to explain from the point of view of biomechanical performance the changes observed in bone density in the long-term, where it is clear that these are due to a different transfer of load in the implanted model compared to the healthy femur; on the other hand, it validates the simulation model, in a way that it can be used in different conditions and at different time periods, to carry out a sufficiently precise prediction of the evolution of the bone density from the biomechanical behaviour in the interaction between the prosthesis and femur.