Abstract
Symns 1 has described the first method for the quantitative determination of vibration sense. In his fork 2 pieces of steel are attached to the prongs so that when the fork is vibrating strongly a small window is seen between them and this window disappears at a certain amplitude of vibration. On the disappearance of the window the fork is applied to a bony point of the patient and a stop watch started. When the patient no longer feels the vibration the watch is stopped. It is assumed then that vibration sense is relatively less acute, the shorter the time interval during which it is perceived. This work has been continued by Wood 2 and Ahrens. 3
In observing a fork vibrating under a microscope it is evident that when the amplitude is small, the vibration recedes very slowly, so that even with the eye observing the fork directly and aided with an eyepiece micrometer, it is difficult to determine within several seconds of time when the fork is vibrating exactly at a given amplitude. Then, too, from the physiologic standpoint it has often been stated that in testing sensation one should approach the threshold value from below rather than from above as is done with Symns' fork. Further, there is no easy method of keeping the pressure of the fork constant on the bony point of the patient.
With these difficulties of the former method in mind, the technique here described was developed.
The materials necessary are shown in Fig. 1. The tuning fork is 10½ inches in length and has a rate of 128 vibrations per second. The base of the fork has been weighted so that altogether it weighs 501 gm. Around the shank of the base is a metal collar which slides freely up and down. Thus the fork can be supported in a vertical position by holding the collar so that it will not tip over and it will then rest with approximately 500 gm. pressure on the portion of the patient to be tested. The contact phlange of the bottom of the fork which rests against the patient is 1.0 cm. in diameter. The small box shown in Fig. 1 contains 15 rods made from piano wire which is 1.50 mm. in diameter. These rods range in length from 13.90 mm. to 19.00 mm. ± 0.01 mm. The rods can be pushed through a small cylindrical piece of rubber which is attached to the end of a silk thread about 20 cm. in length. On the other end of this silk thread is a small metal rod for convenience in handling. The piano wire rods are pushed through the rubber on the tip of the thread and are placed between the prongs of the fork thus spreading the prongs at will from 13.90 mm. to 19.00 mm. Then by quick traction on the thread the rod is jerked from between the prongs of the fork and the initial vibration of the fork will correspond closely to the amount of spread produced by the piano wire rod. Kielhauser 4 states that rods have been used by Müller, Pouillet and Pfaundler to start forks vibrating.
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