FLUORIDE	International Society for Fluoride Research	Table of Contents 32(1) 1999

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Research Report

FLUORIDE CONCENTRATION IN BONE INFLUENCES PERIPROSTHETIC BONE MINERAL LOSS AFTER UNCEMENTED TOTAL HIP ARTHROPLASTY

A Bohatyrewicz,^a A Gusta,^a P Bialecki,^a T Ogoński,^b
E Dabkowska,^b A Spoz^a and Z Machoy^b
Sczecin, Poland

SUMMARY: Factors influencing bone remodelling after cementless total hip arthroplasty (THA) with Parhofer prosthesis were evaluated in a longitudinal study of 18 hips in 18 patients who had undergone uncemented THA due to osteoarthritis. Bone mineral density (BMD) in the femoral neck was determined by dual-energy X-ray absorptiometry (DXA) measurement performed preoperatively, in periprosthetic femoral regions and prospectively 2 weeks, 3, 6, 12 and 24 months after operative treatment. The concentrations of calcium, magnesium, zinc, and fluoride were measured in cortical and trabecular bone samples taken from the resected femoral head and neck. After the operation, the regional BMD decreased, but after 12 months it appeared to be stabilised. Preoperative femoral neck BMD and the fluoride content in trabecular bone indicated that osteopenia and lower fluoride concentrations correlated with bone density reduction after THA. No other factors (age, weight, sex, calcium, magnesium, and zinc concentrations in bone and fluoride concentration in cortical bone) showed significant associations.

Keywords: Bone fluoride concentration, Bone mineral density, Perioprosthetic bone density, Total hip arthroplasty.

1. Dept. of Orthopaedics and Traumatology, Pomeranian Medical Academy,
   Sokolowskiego 11, 70-891 Szczecin, Poland.
   
2. Dept. of Biochemistry and Chemistry, Pomeranian Medical Academy,
   Al. Powstańców Wlkp. 72, 70-111 Szczecin, Poland.

INTRODUCTION

Adaptive changes in periprosthetic bone occur following total hip arthroplasties (THA), and bone resorption and remodelling are known to be important factors in the long-term stability of a femoral prosthesis.¹ There is no evidence that severe resorptive changes cause clinical symptoms or prosthesis loosening. However, there is a concern that continuous femoral bone resorption may reduce the stability of the prosthesis stem. Consequently, evaluation and prevention of periprosthetic resorptive changes may be important for the longevity of the arthroplasty.²

The purpose of the present study was to determine the changes in bone mineral content around the prosthesis resulting from bone remodelling and to assess the importance of the following variables on the remodelling process: patient age, patient weight, duration of implantation, bone mineral density in contralateral femur, and fluoride, calcium, magnesium, and zinc concentrations in cortical and trabecular bone.

MATERIALS AND METHODS

During the follow-up period, each patient underwent clinical and radiographic examinations coupled with dual energy x-ray absorptiometry (DXA) measurement of the periprosthetic bone mineral density at 2 weeks, and 3, 6, 12 and 24 months after operative treatment.

Patients, supine on the table, were scanned using the software that was designed to measure the periprosthetic bone mineral content and density (DPX-L, Lunar, Madison, Wisconsin, USA). Seven periprosthetic regions, resembling the zones described by Gruen⁴ were defined according to the length of the inserted stem, thus permitting an accurate comparison of identical regions of interest during the longitudinal study. Medial and lateral sides of the femur from the level of medial neck resection to the stem tip were divided into three equal regions: Zones 1 to 3 for lateral side, and Zones 5 to 7 for medial side, and Zone 4 with equal height to each of the medial and lateral zones just distal to the stem tip (Figure 1). Scan resolution (size per pixel) was 0.6 x 1.2 mm, average scan dose was 4.8 mRem, and average scan time 5 minutes.

To evaluate the precision of DXA measurements, 8 patients received three consecutive scans after dismounting and remounting the table during each scan, and the coefficient of variation was calculated at each zone. The average precision at each zone was as follows: Zone 1, 5.4%; Zone 2, 3.4%; Zone 3, 2.3%; Zone 4, 1.8%; Zone 5, 3.7%; Zone 6, 4.1%; Zone 7, 6.3%.

The mineral density values from the different time periods and each subregion were calculated and referenced to the values 2 weeks after surgery. The percentage change in density in the operated hip was calculated as follows:

All values were expressed as the mean percentage BMD change ± the standard error. The DXA measurement of the contralateral femoral neck was performed during the hospitalization before operative treatment. The precision error was 2.3%.Linear regression analysis of the data was done to determine whether a correlation existed between the BMD change and clinical factors, including patient age, patient weight, duration of implantation, and BMD in contralateral femoral neck before arthroplasty.

Small samples of cortical bone from femoral neck and trabecular bone from femoral head were taken from the resected bone intraoperatively and stored frozen. The fluoride concentration was measured with an Orion fluoride ion-selective electrode after dissolving the defatted bone pieces in perchloric acid; calcium, magnesium, and zinc concentrations were measured by mass absorption spectrometry.⁵ The coefficient of variation was 2.7% for fluoride, 3.7% for calcium, 4.1% for magnesium, and 3.9% for zinc. Linear regression analysis was done to determine a correlation between BMD change and biochemical factors, including fluoride, calcium, magnesium, and zinc concentrations in cortical and trabecular bone.

By April 1995, 18 hip patients had been enrolled in this study. There were 4 men and 14 women with an average age of 63 years (range, 42-76 years).

RESULTS

Table 1. Bone loss after hip replacement (%)

Time	Zone 1	Zone 2	Zone 3	Zone 4	Zone 5	Zone 6	Zone 7
3 months	9.4 ± 2.9	9.1 ± 3.7	7.7 ± 3.1	7.8 ± 2.6	8.3 ± 2.6	10.1 ± 3.3	12.4 ± 2.9
6 months	13.1 ±4.0	12.2 ± 3.7	9.9 ± 3.6	9.3 ± 3.0	10.2 ± 3.6	13.4 ± 4.0	17.5 ± 3.2
12 months	16.8 ± 3.9	14.3 ± 4.0	13.2 ± 3.3	12.7 ± 3.1	14.5 ± 4.1	16.1 ± 4.3	21.2 ± 3.7
24 months	17.3 ± 4.2	14.4 ± 5.1	13.9 ± 4.2	12.6 ± 4.1	14.7 ± 4.3	16.9 ± 4.1	20.8 ± 4.8

At 12 and 24 months after the operation, the regional BMD in all seven zones showed a maximal decrease ranging from 7.3 to 38.8% of that density present at two weeks post-operatively, but after 12 months the bone density appeared to be stabilized. The most significant postoperative bone loss ranging from 12.1 to 38.8% was found in the zone 7 calcar area. The cortical zone 4 below the prosthesis showed the lowest, but still substantial decreases, ranging from 7.3 to 18.1%. The percent decrease in all Gruen zones at 24 months was compared with patient age and weight, and with fluoride, calcium, and magnesium concentrations in cortical and trabecular bone. The linear regression showed only two variables predictive of the bone loss: BMD in contralateral femoral neck for Gruen zones 1, 2, and 7, and fluoride content in trabecular bone for zone 1 and 2 (Figures 2 and 3).

Other clinical and biochemical variables displayed no relationship to bone loss. Correlation of patient age with bone fluoride, calcium, magnesium, and zinc concentration did not show any statistical significance.

DISCUSSION

It is acknowledged that there are several deficiencies in this study. First, the bone samples taken from degenerated femoral neck head were not the same as the bone surrounding the implanted prosthesis.⁶ Second, using the contralateral femur as an indicator of bone mineral content does not account for this inequality in the same patient.⁷ Finally, the sampling method including only patients with unilateral hip disease and PM Plasmapore hip prosthesis reduced the number of cases to only 18 and therefore did not necessarily produce a random sample of patients.

The decrease in periprosthetic BMD in the proximal Gruen zones 1 and 7 reflects altered stress distribution and typical increase in bone remodelling after prosthesis implantation, which may also be demonstrated by scintigraphic examination.⁸

Three factors adversely affect maintenance of bone mass after THA: adaptive bone remodelling and stress shielding secondary to size, material properties, and surface characteristics of contemporary prostheses; bone loss secondary to particulate debris; and bone loss as a consequence of natural aging.⁹ Engh et al.¹⁰ demonstrated that stem size and extent of porous coating influences femoral bone resorption after primary cementless hip arthroplasty. Bobyn et al.¹¹ suggested the optimal bending stiffness ratio between bone and implant should be between approximately 2:1 and 3:1, thereby leading to the lowest periprosthetic bone loss. The inverse relationship between bone mineral density in contralateral femur and periprosthetic bone loss might be explained by the decreasing of all mechanical properties in surrounding osteoporotic bone.^12,13

Grynpas¹⁴ suggested that bones with higher fluoride content show more resistance to acid dissolution and reduced rate of bone resorption. Whether the incorporation of fluoride into bone will reduce the BMD loss after THA remains to be determined.

CONCLUSIONS

• Total hip arthroplasty leads to significant decrease of bone mineral density measured around the prosthesis stem. Periprosthetic bone loss is greater in patients with low bone mineral content and low fluoride concentration in trabecular bone.
• Weight, age, calcium, magnesium and zinc concentrations in bone and fluoride concentration in cortical bone do not correlate with bone loss.

This paper was presented and discussed at the XXIInd Conference of the International Society for Fluoride Research in Bellingham, Washington USA (24-27 August, 1998).

Figure 1.  The seven regions of interest defined according to the length of the inserted stem.

BMD femoral neck (g/cm2)
Figure 2.  Correlation between percent changes of bone mineral density in Gruen 1, 2 and 7 zone at 24 months after arthroplasty and bone mineral density in contralateral femoral neck.

Fluoride content in trabecular bone (mmol/kg)
Figure 3.  Correlation between percent changes of bone mineral density in Gruen 1 and 2 zone at 24 months after arthroplasty and fluoride content in trabecular bone.

REFERENCES

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14. Grynpas MD. Fluoride effects on bone crystals. Journal of Bone and Mineral Research 5 Suppl 1 S169-S175 1990.

FLUORIDE 32 (1) 1999 pp 7-13	International Society for Fluoride Research
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