Claus Hilger
Determining the Condition of Rigid Endoscopes 

Methods and Instruments

Zustandsbeurteilung Starrer Endoskope

Methoden und Instrumente für das opt. System

After a certain period of use rigid endoscopes become dull which means that the former brilliant, clear and precise picture impression has deteriorated. The image appears dull and milky. Various parameters which determine the picture quality apparently change within a certain period of time.

The manufacturers see the cause for such dullness usually in mechanical damages of the lenses of the optical system. This evaluation is also often confirmed by the damage of the outer tubing. Such damages often lead to damages of the optical system i.e. cracked lenses and therefore resulting in a change of relevant optical qualities (focus, contrast) as well as the reduction of picture brightness (decline of transmission). This evaluation is surely correct and comprehensible.

But besides mechanical influences which can damage an optical system there are also wear and tear factors which can influence the picture quality just like a broken lens. These coherences are not named very often in case of repair.

Since the amount of the repair is substantially influenced by whether a lens is cracked or not, there is a demand for a method to test the condition of optical systems of rigid endoscopes at hospitals.

Continue....(English text)

Starre Endoskope trüben nach einiger
 Betriebszeit ein, d.h. der ehemals brillante,
 klare und scharfe Bildeindruck geht verloren.
 Bilder werden als trübe oder milchig
 eingeschätzt. Verschiedene, die Bildqualität
 einer Optik bestimmende Parameter,
 verändern sich offenbar im Laufe der Zeit. 

 Als Ursache für das Auftreten von
 Bildeintrübungen werden von den
 Herstellern gewöhnlich mechanische
 Beschädigungen der Linsen des optischen
 Systems (Linsenbrüche) genannt. Diese
 Einschätzung läßt sich auch häufig durch
 vorliegende Beschädigungen am äußeren
 Hüllrohr belegen. Solche Schäden führen
 vielfach zu Beschädigungen des optischen
 Systems, z.B. in Form von Linsenbrüchen
 und damit zu einer Veränderung relevanter
 optischer Eigenschaften (Bildschärfe,
 Kontrast) sowie zu einer Verringerung der
 Bildhelligkeit (Nachlassen der Transmission).
 Diese Einschätzung ist, sofern tatsächlich
 Linsenbrüche vorliegen, sicherlich korrekt
 und nachvollziehbar. 

 Neben mechanischen Einflüssen, die eine
 Optik schädigen können, existieren jedoch
 auch Verschleiß- und Alterungseinflüsse, die
 den Bildeindruck des sichtbaren Bildes
 ähnlich beeinflussen wie ein gering
 ausgeprägter Linsenbruch. Diese
 Zusammenhänge werden im Reparaturfall
 weniger häufig genannt. 

 Da es von nennenswerter Bedeutung für die
 Höhe der Reparaturkosten ist, ob ein
 Linsenbruch vorliegt oder nicht, besteht in
 den Krankenhäusern ein Bedarf an
 Testverfahren für das optische System
 Starrer Endoskope. 

 Fortsetzung.... (German text)


1. Introduction
2.  Basic Construction of Rigid Optics
3.  Behaviour with Bending Stress
4.  Testing Methods of the Optical system
5.  Summary and Evaluation

1. Introduction

    According to experience, there is in the course of time mechanical damage with most endoscopes , especially concerning the jacket tube. One reason is tilting the endoscope within the sheath while inserting. According to the degree of tilting this causes elastic or plastic deformations of the jacket tube. Plastic deformations can be noticed by naked eye inspection, whereby elastic ones cannot.

    Depending on the location of damage inside the inner endoscope – within or outside the area of arranged rod lens made of optical glass - the optical system can be effected or not by mechanical influence. So damage with the jacket tube might cause optical impairment. It might not, if damage is within an area of relative distance to an arranged rod lens. In this case there might be no loss in optical quality.

    The absence of obvious outer visible damage of the jacket tube does not exclude breakage of the inside lenses, as mechanical stress of the jacket tube could always take place in the elastic region, whereby an optical glass located within a visibly damaged area will be effected.

    In conclusion, the visual test of the jacket tube of the rigid endoscope is no adequate method of evaluation, concerning the condition of the optical system. Although different causes, both appear as a foggy image. In order to prove costs of repair it is necessary to know and name the exact reason of damage.

2. Basic Construction of Rigid Endoscopes
    The basic construction of a rigid endoscope is shown in fig. 1 (Laparascope 10 mm).

    The lenses of the optical system are arranged in the inner tube, and held in place with distance pieces. The distal region of the inner tube is closed by the deflector prism (that determines the direction of view) and with the eyepiece at the proximal part. The inner tube is enclosed by the jacket tube containing optical fibre cable and it`s connector.

    The jacket tube has a covering lens at the distal part and an ocular at the proximal part. One of its significant attributes of the jacket tube is the ability to seal against invading substances.

    Fig. 1: Parts of a rigid endoscope
    Top: Jacket tube (enclosing fibre cable), connector for fibre cable and outer eyepiece
    Middle: Inner tube with eyepiece
    Bottom: Deflector prism with (some broken) optical lenses and distance pieces

3. Behaviour with Bending Stress 4. Test Methods of the Optic System
I would like to introduce the methods of evaluation conditions of the optical system. Beyond that, there are no other methods used within this field, or externally by third persons, as far as I know.
5. Summary and Evaluation
This script introduces different internal and external methods of determining quality of rigid endoscopes. The transmission measurement method, as described in 4.2 is appropriate to the technician who wants to evaluate the ageing process concerning loss of transmission. The following results can be achieved: after x sterilisation cycles with method y, the endoscope 4711 lost z% in transmission. As shown above, these statements can also be achieved with other optical instruments. An indication of the endoscopes` life service (usually defined as the number of sterilisation cycles before becoming foggy) can be given.

The Monocular Tube being the developed magnifying glass is no appropriate method of measurable criteria of quality, but it allows a quick check concerning the grade of fogginess and proving or excluding a lens breakage. This method is practicable in hospital and whenever doubts occur concerning classification of costs. Being able to differentiate between ageing influences and improper handling, possible guarantee claims towards the manufacturer may be made. This is a direct economic advantage. With increasing experience, quality of repair, especially done by a third party can be proved.

The construction of the Monocular Tube is small, handy, can be used anywhere and is not dependant on an energy supply: after surgery, in the room of disposal, or before sealing with sterilisation foil. It is a useful method for the premium check of condition, before sending in for repair, and it also is an aid for the representative. Lens breakage and ageing influences can be proved and determined. A differentiation between proper and improper handling can be made.

Regarding the sum of $ 2500 or with a stock of more than 5 endoscopes, the purchase of at least one of both measuring instruments should be considered, or in very rare cases, training for the magnifying glass measurement method.

About the Autor:

Dipl. Ing. Claus Hilger
Keuloser Str. 75
D-36093 Kuenzell

Claus Hilger is an consultant in medical devices and is working for jurisdication and insurances (Additional information in German).

Aditional Links:

German Homepage of Rigid Endoscopes

Tools, Tips and Tricks (cunning techniques)