Clin. Biomech. 1992; 7: 187- 190

Letters

Mechanical effects of sudium fluoride on bovine cortical bone In our paper titled ‘Effect of fluoride dosage on bone density, sonic velocity and longitudinal modulus of rabbit femurs’ (Culcif Tissue Znt 1992; 50: 88-92) we addressed the same problems as Rahn et al. (Crin Biomech 1991; 6: 185-189) but with very different results. Weanling rabbits, about 1 kg weight (about 8 weeks old) were intoxicated with NaF solution of specified concentration in accordance with the dose schedule. The diets were maintained for 13 weeks to sexual maturity. Dosage, specified as mg F- per kg body weight per day (a unit) varied from 4 to 100 units.

Fluoride concentration in the bone at death in- creased linearly with dose leve1 up to 90 units. Density of compact bone was determined by use of Archimedes principle. Ultrasonic velocity was measured in ground sections of the femur in the radial direction to the long bone axis.

Compact bone density peaked at 23 units (2.04 g cmw3), 2% above controls and then decreased markedly to a low value of 1.64 g cme for the 85 unit dose. The ultrasonic velocity, at 10 MHz, peaked at the 17 unit dose leve1 at 3.28 km s-‘, about 5% above the control level, and fel1 to a low of 2.6 km s-l at about 80 unit dosage. Assuming that the longitudinal modulus is the density times the square of the sonic velocity, a peak in this parameter was found at the 18 unit dosage, 14% greater than the control value at 22 GPa and fel1 to a minimum of 11 GPa at the 82 unit dosage level.

Body weight at death divided by the total midshaft cross-sectional area of the femur has the dimensions of stress and is defined for this application as the static load stress. The average load stress of the controls was used as reference to calculate the relative load stress. A peak was found in the relative load stress at the 10 unit dosage leve1 which falls to 30% of the reference value at the 87 unit dosage.

These dosages are very high but al1 the rabbits survived the full 13 weeks.

The maximum fluoride concentration in the bone cited by Rahn et al. (4500 ppm) is in the lowest part of our data, corresponding to a dosage of 30 units. There is evidente of biologieal variation in our data that masks the peaks we found, particularly for density. It is only because of the large dosage range we employed that the density peak can be detected. The peak is most obvious for the longitudinal modulus. In our experi- ments the bone fluoride concentration ranged up to 13000 ppm.

S Lees Bioengineering Department, Forsyth Dental Centre, 140 Fenway, Boston, Massachusetts 02115, USA

Authors’ response

Dr Lees in his letter reports results from a paper he has recently published (Calcif Tissue Int 1992; 50: 88-92). Although his paper and ours both address the effects of fluoride on cortical bone, the experimental designs and protocols are so disparate that the results are difficult to compare. Dr Lees used dosages of 4 to 100 units (mg per kg) per day, while we used dosages equivalent to 0.5 to 1.8 units per day.

It should be noted that we did not use a dosage of 30 units as suggested by Dr Lees’ letter. Because high fluoride intakes have been shown to result in toxicity and reduced mechanica1 strength, we restricted our dosages to those acceptable and common for human treatment.

Other differences include treatment time (6 years in ours; 13 weeks in theirs), animal model (cows in ours; rabbits in theirs), mechanica1 parameter (mechanically determined, Young’s modulus in ours; acoustically determined elastic coefficient in theirs), and density parameter (dual energy X-ray absorptiometry to deter- mirre ‘area density’ in ours; Archimedes’ principle to measure density in theirs). Therefore, any discrepan- cies in studies are likely due to these many differences.

In our study, which followed 17 cows for six years, we found no significant changes in material properties and a non-significant trend towards reduced bone mineral density with increased dietary fluoride.

In our paper, we stated that it is possible that the smal1 range of dosage levels and relatively smal1 sample size in our study may have obscured observa- tion of significant responses. Dr Lees finds also that the biological variation interfered with clear interpretation of data, ‘particularly for density’.

In this regard, our studies are consistent although Dr Lees is showing no effect or an obscured effect at even higher dosages than we used.

Overall, we fee1 that the dissimilarity in dosage levels is an important differente between the studies and thus distinguishes the findings.

R Vanderby PhD Orthopedie Surgery and Engineering Mechanics, University of Wisconsin, 600 Highland Avenue, Madison, Wisconsin 53792, USA

Measurement of grip strength

After reading the paper by Clarke et al. (Effect of body posture and grip strength in patients with cervical spondylosis. Clin Biomech 1991; 6: 123-6), 1 felt disappointed that the authors had, like so many before them, chosen to use an instrument inappropriate for the job.

Grip strength is usually understood to be the force

188 Clin. Biomech. 1992; 7: NO 3

that the hand can exert and yet the authors chose to use an instrument that measures pressure (Mediscus Grip Strength Monitor, IME, Wareham, UK). Pressure is force divided by area, which in such instruments is highly variable. Readings of pressure can vary enormously for the same grip strength (force) because of the different contact areas of the hand with the inflatable rubber bag.

Variability in pressure wil1 occur due to the defor- mation of the inflatable rubber bag as the grip force is applied. This causes the instrument to be non-linear and so adds to the inappropriateness of using such inaccurate devices. Why then was the data given in millimeters of mercury to the nearest decimal point? 1 realise that most people have used millimetres of mercury in the past to measure pressure, but the SI units are pascals (newtons/square metre).

Papers are so much more credible when the authors use instruments that actually measure the correct parameters in question and provide the correct scien- tific units. However, in this particular case, the authors can stil1 claim that there is a differente in grip strength between standing and sitting even if we do not know what the real values are.

Malcolm Ellis BSc PhD MBES The University of Leeds, Rheumatology and Rehabilitation Research Unit, 36 Clarendon Road, Leeds LS2 9PJ, UK

Although from my own experimental work, 1 appre- ciate the validity of the observation that grip strength is higher when standing than when sitting (Clarke et al., Clin Biomech 1991; 6: 123-6), there are a few points of concern 1 would like to raise.

Firstly, grip strength is normally measured as a force rather than as pressure1-4, and the criticisms levelled by Kondraske’ at a device similar to that used here6 hold true, that is that force can only be determined directly from pressure if the contact area is constant, and the contact area of the hand with the rubber bag described wil1 vary from one individual to the next according to the physical characteristics of the hand. Comparison between subjects would thus be invalid. Additional errors, admittedly less significant, wil1 be present because of the use of air, a compressible medium, in the bag. Linear pressure readings could be obtained if a non-compressible medium such as mineral oil was used instead. The equipment we use in Leeds to obtain reliable measurement of grip strength is essen- tially a strain gauge torsion dynamometer in the form of a pair of rigid bars, which are adjustable to suit the size of the hand, linked to a personal computeri.

The timing of the experiments is not clear from the text. Knowing it to be impossible to maintain a maxima1 grip for anything near 10 min, one would presume that the patient was allowed to relax between grips, but no indication of the grip test duration and recovery time are given. By testing repeatedly like this,

other variables such as fatigue are introduced and should be addressed in the analysis. Sequentia1 analysis of grip strength (grip duration of around 5 sI,~) would show characteristic patterns in parameters other than maxima1 grip over the 10 min period.

Alison Cutts PhD The University of Leeds, Rheumatology and Rehabilitation Research Unit, 36 Clarendon Road, Leeds LS2 9PJ, UK

References

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Helliwell PS, Howe A, Wright V. An evaluation of the dynamic qualities of isometrie grip strength. Ann Rheum Dis 1988; 41: 934-9 Helliwell P, Howe A, Wright V. Functional assessment of the hand-reproducibility, acceptability and utility of a new system for measuring strength. Ann Rheum Dis 1987; 46: 203-8 Mathiowetz V, Kashman N, Volland G et al. Grip and pinch strength: Normative data for adults. Arch Phys Med Rehabil1985; 66: 69-74 Jones AR, Unsworth A, Haslock 1. A microcomputer controlled hand assessment system used for clinical measurement. Eng Med 1985; 14: 191-8 Kondraske GV. Measurement of the quality of hand contractions. Med Bio1 Eng Comput 1985; 23: 399 Duym BW, Pfurtscheller G. Measurement of the quality of hand contractions. Med Bio1 Eng Comput 1991; 22: 245-50

The letters from Dr Malcolm Ellis and Dr Alison Cutts are welcomed. The main points are:

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That for our group of out-patients grip strength was stronger when standing and Dr Cutts reports that her experimental work confirms this, although she does not say whether her work is with healthy volunteers or patients. That Drs Ellis and Cutts both allow our claim to have established this finding, and That both of them criticize strongly the use of the grip strength bag technique for a variety of reasons. Perhaps the chief reason is the use of the bag which Dr Ellis says wil1 make the area of contact between the hand and the bag ‘highly variable’. Al1 that can be said of this criticism is that initial work in Salisbury does not show correlation between the size of the hand and strength, and therefore one bag size is considered adequate for adults. However, further work is being undertaken to investigate the correla- tion between hand size, grip strength, and other hand assessment parameters. Dr Ellis must produce evidente for his assertion before apparently con- demning the grip strength bag technique. Neither he nor Dr Cutts should despise its popularity, men- tioned by Dr Ellis. We fee1 that the coupling between the hand and the measuring machine is important and that rigid contacts such as metal bars and limited padded areas are unsuitalbe. Perhaps others fee1 the same, and this is why the bag remains 60 popular .

Authors’ response