Compressive forces of the Shute forceps

Compressive forces of the Shute forceps WARREN

H.

TERRENCE Omaha,

PEARSE,

J.

M.D.

KOLBECK,

M.D.

Nebraska

Strain-gauge instrumentation of the Shute parallel force&, together with use of the similarly instrumented Bill axis traction handle, permitted measurements of the forces of compression and traction exerted during delivery. Traction was identical with other reports for varying tyQes of forceps. Compression was about half that observed with the Simpson forceps when the Shute forceps were tightened 2 mm. However, it was often felt necessary to tighten the forceps to an average of 4 mm., increasing the average maximum compression to 4.1 pounds.

A PRIOR REP o RT a detailed the historical background of measurement of the forces of forceps delivery and described the strain-gauge instrumentation of a Simpson forceps for measurement of head compression and of a Bill axis traction handle for measurement of traction forces associated with obstetric delivery. The average maximum traction in a random series of 156 deliveries was 42.3 pounds, while an average maximum compressive force of 4.95 pounds was exerted against the fetal head by the instrumented left blade of the Simpson forceps. With the advent of the Shute forceps4 (Fig. 1) , an instrument was available which had the presumed advantage of lesscompressionof the fetal head. Following application, the recommended tightening of the Shute forceps approximated the two blades by 2 mm., at which point traction could be made. Because of this minimal compression, it was believed not necessary to relax or separate the blades between tractive efforts. All authors have measured traction forces From the Department Gynecology, University Medical Center. y;;;ived for publication iq;;;Qted

for

publication

of Obstetrics of Nebraska December ]anuary

of the same order of magnitude. Questions have arisen, however, about compression forces.l* 2 We believed that measurementsof the forces exerted by the Shute forceps would be of value in studying compression of the fetal head during delivery and in defining the safety and utility of the Shute forceps. Instrumsntation

and

recording

The strain gauge instrumentation of the forceps and of the Bill axis traction handle has been described previously. A strain gauge was mounted on the shank of the left blade of the Shute forceps with wires leading down through the handle and through a connecting cable. The sameBill axis traction handle was used and was attached to a transverse metal bar mounted at the end of the handle of the right blade of the Shute forceps. Wiring of this traction handle was as previously described. Force vector diagrams indicated that the axis of traction with the Bill handle was the same as with its more conventional use on the Simpson forceps. Recording was through a Sanborn twin-channel strain gauge recorder.

and

Results

Table I reports the maximum and “total” forces of traction and compression observed with the Shute forceps as compared with our previously reported results with the

8, 10,

44

Volume Number

113 1

Compressive

Table I. Average

forces

forces (41

of Shute

forceps

for forceps

Shute deliveries) (pounds)

(156

45

delivery

Simpson deliveries) (founds)

Traction

Maximum Total

44.7 1,056

42.3 796

4.1 392

5.0 152

Compression

Maximum Total

Fig. 1. Shute

Forceps.

The “total” figure is a Simpson forceps. time-force measurement of the area beneath the recorded curve. This allows consideration of the length of time over which varying forces are applied rather than’ the single measurement of the maximum force exerted at one point in time. As one might anticipate, the forces of traction are essentially the same. Almost identical results were reported by Laufe2 in his investigation of divergent forceps. In 83 deliveries, his observed average maximum traction was 42.3 pounds, with a “total” figure of 1,200. For compression with the divergent forceps, the average maximum was 6.34 pounds with a “total” of 241. Laufe’s measurements for the Simpson forceps were also somewhat higher than ours, probably relating to the location of the strain gauges at the heel of the blade rather than midway on the shank of the forceps. The “total” figure for the Shute forceps is deceptive since a longer time was usually used in application and in tightening the lock on the Shute forceps, together with recordings of the effect of various degrees of approximation of the two blades. The maximum compression was indeed less than with the Simpson forceps. It is Shute’s4 belief that, in general, the forceps need to be tightened only three serrations of the knurled knob, which is equivalent to 2 ml. of tightening of the blades on the head. At this level, the average maximum compression observed was 2.3 pounds. However, the several operators who used the instrument felt that the forceps often slipped on

Table II. Forceps rotation Compression

Maximum Total

Rotation 5.1 370

(6 patients) Delivery 4.5 357

the head with this degree of tightening, and, on the average, the operating obstetrician believed it necessary to tighten to six serrations, equivalent to 4 mm. Shute believes this slippage is more apparent than real, and, with the use of fundal pressure plus traction, no more than 2 mm. of compression should be necessary. Over all, the amount of tightening (the number of turns of the knob) was not directly proportional to the measured compression. For example, in two instances in which the knob was tightened ten turns (7 mm.), the average compression was 4.9 pounds, while in one instance in which it was tightened 14 turns (10 mm.) the maximum compression was 3.3 pounds. In the individual case, however, compression was directly proportional to tightening of the forceps and could readily be observed to increase in a stepwise fashion with each additional millimeter with which the forceps were brought together. The fetal head compression observed in 6 patients who had rotation of an occipitoposterior position to an occipito-anterior position with the Shute forceps followed by forceps delivery is noted in Table II. There was essentially no difference in the compression exerted on the head during rotation as opposed to that subsequently noted during delivery, and the “total” (time-force) figures were almost identical.

46

Pearse

and

May 1, 1972

Kolbeck

.41x1. J. Obstet.

Table III.

Forceps

compression

and

infant

weight Infant weight (grams)

Auerage maximum

Average total

2,000-2,500 2,500-3,000 3,000-3,500 3,500-4,000 4,000+

2.2 3.9 3.8 4.6 10.0

203 320 355 506 1,076

Table III indicates the effect of baby weight. With infants larger than 3,500 grams, there is a definite increase in the compression forces recorded. Comment “controlled compresShute emphasizes sion,” the fact that the blades cannot be changed in compressive force by manipulation or by traction once they have been tightened upon the head. Our graphs con-

REFERENCES

1. 2.

Kelly, J. V., and Sines, G.: GYNEC.~~: 521,1966. Laufe, L. E.: Obstet. Gynecol.

firmed this point, although in general the same was true of the Simpson forceps in the recording system we used. When the Shute forceps were tightened 2 mm. on the head, the average maximum compression exerted was about half that observed with the Simpson forceps. The several obstetricians participating in this study felt that it was often necessary to exert more compression on the head so that the forceps did not give the feeling of slipping. This led to a higher over-all maximum compression but one that was still less than that observed with the Simpson forceps. Perhaps more extensive experience with the parallel forceps would allow its use with only the 2 mm. tightening emphasized by Shute. The forceps were easy to use with minimal experience. It was a universal observation that they appeared to fit the fetal head better than any of the forceps in common use in this institution,

3. Ax

J.

OBSTET. 4.

34:

853,

1969.

Cynecol.

Pearse, W. H.: AM. J. OBSTET. 43, 1963. Shute, W. B.: AM. J. OBSTET. 442, 1959.

GYNECOL.

86:

GYNECOL.

77: