3.2.2.1. AP size measuring apparatus
The method for CSA measurements uses a profile of the hand thickness in the plane transverse to the AP and FD l muscles. The CSA measuring apparatus used in the current work (figure 17) was the same as that developed by previous investigators: the “calliper technique” validated against MRI and CT measurements, which gave a good correlation [Bruee et al, 1989].
Measurements are dependent on three potentiometers (two horizontal (palmar and dorsal) and one vertical), that send output signals to an analogue to digital (A/D) board and into a TESTPOINT program which we wrote especially for CSA data capture and analysis (see Appendix V, B).
Hand rest
Palmar
potentiometer Dorsal
potentiometer
Signals to computer
Figure 17. The CSA measuring apparatus. This method uses callipers and 3 potentiometers (two verticals and one horizontal).
Chapter 3; H R T long-term muscle effect study
Because o f the high value o f the correlation coefficient with MRI/CAT (r =0.94), CSA measurements made using the calliper technique can therefore be used as reliable estimation o f AP muscle size. It should be noted that the calliper technique underestimates by a factor o f approximately 42%, the actual CSA o f the A P muscle. It was suggested that the underestimation might primarily be due to the fact that some o f the A P “hidden” under the bases o f the metacarpal bones. It was also suggested that a second source o f error would be owing to the faet that the springs on which the potentiometers lays (dorsal and ventral) compresses the A P during measurements.
3.2.2.2. Procedure for AP size measurement
The measurement method was standardised within and between subjects. For this purpose, the subject hand was marked as follows. A dot was placed on the dorsum o f the hand at the point o f convergence between first and second metacarpals (figure 19), found by palpation with the thumb fully abducted (figure 18) and partially opposed to straighten the skin web between thumb and first finger. A second point was marked on the palmar surface o f the hand in the position obtained by extending a line through the first point perpendicular to the dorsal surface between the first and second metacarpals. As an aid in finding this point, a simple apparatus consisting o f two pencils facing each other on a rigid metal frame was used. Finally a line was drawn from one point to the other over the centre o f the web, this line is followed by the eallipers. Adductor D o llic is (AP) First dorsal interosseus (FDl) Phalanges Proximal Distal Middle 2"" Metacarpal (AP is attach ed to 3'" m etacarpal on palm ar face)
Figure 18. Thumb position for CSA measurement. Subjects were required to hold thumb extended maximally (but comfortably) to expose the two muscles as much as possible.
Chapter 3: H R Tlon^-term muscle effect study (/) Q) m CD Distal Middle Proximal CSA m easurem ent trajectory Saddle-joint of the thumb M etacarpal R eference point Carpal
Figure 19. First reference point for CSA measurement. The number above each digit indicates the numbering system for the phalanges and metacarpals. The reference point is between the first and second metacarpals.
Subjects were seated comfortably with the elbow resting (on a foam-filled pad if requested by the subject) on the workbench. The experimenter placed the subject’s hand so that the dorsal side (o f the area between the thumb and the second metacarpal touched the perspex frame and the rest o f the hand was completely relaxed. The dorsal potentiometer was placed on the first point and the palmar potentiometer was placed on the second point (Figure 20).
The experimenter then manually straightened the subject’s skin web, and simultaneously moved the calliper upward over the markings on the hand, while the potentiometers outputs are recorded.
C h a p t e r H R T l o n f i - t e n n m u s c l e e f f e c t s t u d y Q. < 0 Q>
ÎI
<D Q)1
Q. < 0 0)| s
Q. a> Q) EII
| i1
I
Figure 20. The calliper technique for measuring A P CSA. Note that the first Dorsal in tero sseo u s is included in the CSA m easu rem en t (in the dorsal fa c e of the hand).
Calibration o f the CSA apparatus
To calibrate this apparatus, a triangular perspex object(Figure 21) o f known dimensions was placed so that the two horizontal potentiometers rested in the small grooves at the base o f the triangle.
70mm
34 mm
Figure. Calibration of CSA measuring apparatus. T he d im en sion s of the triangular object w ere known (Area = 1190mm^), and u sed to calibrate the CSA apparatus.
Chapter 3: H RTlott^-term muscle effect study
The calliper was then moved upward across the object. The difference o f outputs from the two facing potentiometers (the width of the object) was then plotted against the output from the vertical potentiometer (the height o f the object). The area of the triangle was calculated as area = (width x height) I 2. This voltage output was then related to the known area of the object.
3.2.2.3. A P size data analysis
The difference in the output signals of the horizontal potentiometers was plotted against the output of the vertical potentiometer, thus giving a profile of the hand between our reference point (the joint between the metacarpal bones o f the thumb and that of the index finger) and the middle of the skin web (Figure 22).
Calliper off metacarpal
Calliper on skin web only
Calliper off hand
Length
Figure 22. Example of a typical adductor pollicis muscle profile The green lines show the limits of the calliper profile and the black arrows represent the direction of scanning. Length is the output of the vertical potentiometer. Width is the difference between the outputs of the two horizontal potentiometers. The positions of the horizontal and vertical blue lines are controlled by the experimenter.
The record is measured by positioning two lines shown in blue such that:
a) The horizontal line (blue dashed) is placed at the thickness of the skin-fold at the web, after the calliper has left the muscle and
Chapter 3: HRT/on}>-term muscle effect study
b) The vertical line is placed immediately at the point where the calliper has left the metacarpal surface, generally indicated by a dip in the thickness record.
A complete set of records for one subject is shown in figure 23. Six profiles were obtained on each occasion and the areas of at least four were averaged depending on consistency. The mean coefficient of variation with these sets o f four measurements was 1.7%. 14 E E 5 ■o % -10 0 10 20 30 40 50 60 Height (mm)
Figure 23. Example of the superimposed CSA measures of one subject.
The dashed area represents the area of the muscle after bone and skin web have been excluded