(HP-65) Ball Pelvimetry Technique

[SlideRule]

A prerecorded program for the Ball pelvimetry technique
American Journal of Obstetrics & Gynecology, vol 126, issue 4, 15 OCT 1976, pgs. 477-478, EMANUEL A. FRIEDMAN, M.D., MED.SC.D,

Pelvimetry can be simplified by a recently devised computer program. The program can be used with any of several advanced programmable pocket or desk-top calcul1ator. This approach is more accurate than either the nomogram or the slide rule and demand much less time and attention.

؜؜؜؜؜؜؜؜ ؜؜ ؜؜ ؜؜ ؜؜ The X-RAY cephalopelvimetric technique first described in 1935 by Ball and Marchbanks has several distinct advantages over other methods. It is capable of measuring the fetal head in any presenting position, providing a specific head volume in each case against which to judge the capacity of the pelvis based on measured dimensions. Moreover, it has essentially none of the technical drawbacks of other methods that require accurate positioning of the patient, precise placement of grids or rulings, or use of specially designed equipment. The geometric triangulation principles of the Ball technique have been previously outlined. Detailed expositions of how the measurements are made from the x-ray films are also available and need not be repeated here.
؜؜ ؜؜ ؜؜ ؜؜ ؜؜ Various aids have been introduced to simplify the mathematical manipulations needed to correct the film measurements for expected divergence error, to calculate the corresponding head volume and pelvic capacity and to determine volume deficits where they exist. It is likely this useful clinical tool has not found greater acceptance among obstetricians and radiologists by
virtue of the arithmetic involved. Nomograms and a special slide rule have apparently not entirely resolved this problem. A recently devised computer program has generated utilitarian tables that are proving to be most helpful in clinical practice at this institution. The sheer bulk of paper and the need to reduplicate the tables periodically as they deteriorate with use and time
are the only obvious disadvantages we have encountered to date.
؜؜ ؜؜ ؜؜ ؜؜ ؜؜ A computer program has now been evolved for use with any of several advanced programmable pocket or desk-top calculators. The 100-step sequence shown in Table I was written specifically for the HP-65* unit; it can be readily modified for other equipment. When the prerecorded magnetic card containing this program is inserted into the calculator, the operator can proceed to compute volumes, capacities. and deficits merely by entering the paired arrays of measured dimensions and object-film distances. As each pair is entered, the corrected volume is derived and stored for subsequent use in determining the deficit or for later reference purposes. In this program, subroutine A is used for entering the three important pelvic dimensions (widest transverse and anteroposterior diameters of the inlet and interspinous diameter at the midplane) and subroutine B for the two head &circumference measurements (obtained from the anteroposterior and lateral films, respectively). All necessary corrections and conversions are automatically accomplished and the head volumes are averaged. After the five dimensions have been duly processed, subroutine C computes the deficits seriatim. To enter a new case, only the storage registers need to be cleared.
؜؜ ؜؜ ؜؜ ؜؜ ؜؜ The flow of data is as follows: The first measured pelvic dimension (the widest transverse diameter of the inlet) is entered (key A) along with its' object-film distance (key R/S). The data are automatically sub-programmed (subroutine E) for purposes of correcting the measured dimension for magnification distortion and beginning the computation of the corresponding volume.
؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ V = piD² / 6
The program returns to the point from which it had branched, there to complete the volume calculation and to seek an appropriate storage register. It tests each of the first three registers in turn and enters the derived volume into the first one it locates that contains no previously entered volume. Since this is the first entry, the capacity corresponding to the widest transverse inlet diameter would be stored in the first register. Next, a second dimension (the anteroposterior diameter of the inlet, key A) and its object-film distance (key R/S) are entered; the volume is computed and stored in the second register (subprogram 1). Entering the third pelvic dimension (interspinous diameter, key A) and its object-film distance (key R/S) complete the calculations of pelvic capacities as this volume is stored in the third register.
One of the measured head circumferences (from the anteroposterior film, key B) with its corresponding object-film distance (key R/S) is then entered, branched (subprogram E) for divergence correction, and further corrected for fetal scalp volume (by adding 2 cm. to the corrected cranial circumference). Initial computation of the head volume from the corrected circumference
 ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ ؜؜ V = C³ / 6pi²
is undertaken, utilizing the steps shared in common with the calculation of volume from diameters. Returning to the point from which it branched, the program completes the volume calculation and proceeds to seek an appropriate storage register; the first head volume is stored in the fourth register. The remaining head circumference (from the lateral film, key B) and its object-film distance (key R/S) are handled in the same way, and the calculated volumes stored in the fifth register (subprogram 2). When both head volume storage registers are occupied, they are automatically averaged and the mean head volume stored in the sixth register. Finally, subprogram C determines the deficits between the mean head volume and the pelvic capacity as derived first from the widest transverse inlet diameter (key C), then from the anteroposterior inlet diameter (key R/S), and last from the interspinous diameter (key R/S again).
This approach is much more accurate than either the nomogram or the slide rule in clinical use. It is much less demanding in terms of time and attention than the aforementioned generated tables. The expense of a programmable calculator is offset by the advantages of the information it provides through the intermedium of this program, and of course by the many other uses to which it can be put.

Table I. Programmed steps for Ball pelvimetry
LBL A
STO 8
E
×
STO 3
RCL 1
1
+
STO 8
g DSZ
GTO 1
RCL 3
STO 1
RTN

LBL 1
RCL 2
1
+
STO 8
g DSZ
RCL 3
RTN
RCL 3
STO 2
RTN

LBL B
STO 8
2
STO 7
E
ENTER↓
×
÷
STO 5
RCL 4
1
+
STO 8
g DSZ
GTO 2
RCL 5
STO 4
RTN

LBL 2
RCL 5
RCL 4
+
2
÷
STO 6
RCL 5
RTN

LBL C
RCL 1
RCL 6
-
R/S
RCL 2
RCL 6
-
R/S
RCL 3
RCL 6
-
RTN

LBL E
RCL 8
R/S
STO 6
g R↓
100
ENTER↓
ENTER↓
RCL 6
-
÷
÷
RCL 7
+
3
g y^x
6
÷
g π
RTN

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