A new peridural needle-the 'LOLO' needle.LK Delima, DF Delima
Peridural needles in present day use have a compromised function because of their circular cross section. An ideal peridural needle must be sharp to cut the spinal ligaments and at the same time blunt enough to ensure prevention of dural perforation. This ideal function is only possible if the bevel and tip of the needle are not inversely related as is the case in circular cross section needles. The 'LOLO' peridural needle developed by us is free of this compromise. It has a square cross section with a long, sharp eccentric tip. The bevel is flat and rectangular. The side opening is 4 mms from the tip of the needle through which the catheter is deflected out. Such a design is sharp to traverse the spinal ligaments and also effectively ensures dural tenting and prevent perforation. These properties have been proved by a myelographic study and compared to other conventional needles.
Keywords: Anesthesia, Epidural, instrumentation,Equipment Design, Human, Needles, Support, Non-U.S. Gov′t,
An ideal peridural needle must have properties, sharpness which will cut the spinal ligaments and bluntness which will ensure dural 'tenting' and prevent dural perforation. However, it is not possible to obtain these two extreme properties in the same needle using needles of conventional design. Because the present peridural needles have a circular cross section. The bevel and tip of circular cross section needles are inversely related so that shorter the bevel, the blunter is the needle tip and vice versa. The bevel of such a needle will always be oval with a sharp, pointed tip. This it is impossible to provide in conventional needles a tip and bevel that is both sharp on the spinal ligaments and blunt on the dura mater. Recognition of this problem lead to certain modification like filing down the point of the Tuohy's needle, and the Crawley needle. But all these needles were handicapped by the circular cross section. The problem remained unsolved, because an increase in the bluntness of the tip made it difficult to insert the needle as seen in the Cheng needle.On the other hand, increasing the sharpness of the needle made the changes of dural perforation extremely high as seen with the Tuohy's needle.
The aim of this study was to produce a needle wherein the bevel and tip would not be interrelated. It would then be possible to have a needle that would be sharp on the spinal ligaments and at the same time blunt to ensure dural tenting and prevent dural perforation.
All needles were made by the authors using hand or electrical tools. Recognising the fact that to produce a needle that is both sharp on the spinal ligaments and blunt on the dura, is impossible with a needle with a circular cross section, the 'LOLO' peridural needle was made to have a square cross section.
An 18 gauge or 20 gauge spinal needle was selected and its tip worked to a solid, square cross section. One side of the tip was ground at an angle resulting in a flat, rectangular bevel with a sharp and long tip that is eccentrically placed [Figure - 1] and [Figure - 2]. The side opening of the 'LOLO' peridural needle is situated 4-5 mm from the tip of the needle. This conforms to the dimensions of the peridural space in the lumber region. Thus when the tin of the needle touches the dural sheath, the side opening lies within the peridural space.
The 'LOLO' peridural needle for continuous use remains the same except for the inclusion of the baffle plate in the tip of the needle [Figure - 1]. This deflects the catheter through the side opening in a cephalic direction. The hub of the needle was winged to facilitate tactile localisation of the space. The shaft of the needle was marked at regular intervals.
Thus the 'LOLO' peridural needle is designed to have a long and sharp tip that negotiates the spinal ligaments and a blunt fiat rectangular bevel which ensures dural tenting and prevents dural perforation.
To prove the superiority of the 'LOLO' needle in ensuring dural tenting and preventing dural perforation a myelographic study was carried out. The dural sac was outlined with contrast medium. The 'LOLO' needle was introduced under image intensifier control to touch the dural sheath and then further advanced to 'tent' the dura. The extent of dural 'tenting' was visualised and recorded on a radiograph. The needle was then withdrawn and a radiograph taken to determine whether the dural sheath was perforated or not. This procedure was carried out in 20 patients using the 'LOLO' needle and the Tuohy's needle each.
The degree of dural 'tenting' possible was extreme using the 'LOLO' peridural needle [Figure - 3]. On withdrawal of the needle the dural sheath was always found to be intact [Figure - 4]. In all the 20 cases, it was impossible to perforate the dura unless the dura was impinged against the vertebral body or intervertebral disc.
With the Tuohy's needle although localisation of the peridural space was not difficult, there was very little dural tenting possible and attempts to create the same tenting effect as seen with the 'LOLO' needle resulted in a perforation and was indicated by an extradural leak in the contrast medium [Figure - 5]. In all the 20 cases using the Tuohy's needle, although localisation of the peridural space was possible without dural perforation, the latter invariably occurred when the dura was tented to create the same effect as seen with the 'LOLO' needle.
The 'LOLO' peridural needle is based on sound physical principles that do not compromise function. The square cross section of the needle offers many advantages:
(1) The rule of shorter the bevel, blunter the tip holds good only in circular cross section needles where the tip and bevel are interrelated. In square cross section needles the tip and the bevel are independent of each other and hence it is possible to have a sharp tip and a blunt bevel.
(2) the square bevel with an eccentric tip hits the dura with a flat surface lessening the chances of a dural perforation.
(3) in circular cross section needles, for a catheter to emerge from its side opening, atleast half the diameter of the needle must be cut off, whilst in square cross section needles just one face of the square has to be filed off to enable the catheter to emerge. This results in a stronger needle.
The long and sharp tip of the 'LOLO' peridural needle actually separates the spinal ligaments during its insertion thus lessening the chances of backache. This is because the spinal ligaments run in a longitudinal direction. This is in contrast to the short and blunt tips of conventional needles which tear the spinal ligaments whilst traversing it.
Having traversed the spinal ligaments the tip of the needle comes in contact with the dural sheath which is a hollow, deformable cylinder. Initially only the tip of the needle is in contact with the dural sheath. In conventional needles all the force of insertion is concentrated at the needle point resulting in a greater force per unit area with an eventual perforation as seen in the myelographic study. In the 'LOLO' peridural needle the force of insertion is spread out over the long and sharp tip resulting in less force per unit area of the dura with the chance of a dural perforation being much less. This principle is similar to an analogy with a knife. It is easier to divide the skin with the tip of the knife than when the entire cutting edge is merely pressed against the skin.
Having initially 'tented' the dura with its long sharp tip, on further insertion, the rectangular blunt bevel now comes in contact with the dura and 'tent' it further. When this occurs, the force of insertion is now spread out, not linearly as when the tip is in contact with the dura, but over the larger two dimensional area of the flat blunt rectangular bevel making the chances of dura perforation even more remote as the 'LOLO' peridural needle is advanced further. This is in contrast to other conventional needle where the chances of dural perforation are increased as the needle is further inserted. Thus dural 'tenting' with the 'LOLO' peridural needle occurs sequentially, initially with the tip of the needle upto a certain extent when the flat, rectangular bevel takes over. Thus the needle in effect gets 'blunter' as it is introduced further. This explains the extreme degree of dural 'tenting' that occurs with needle as shown in the myelographic study.
The myelographic study conclusively proves the extent of dural tenting that occurs with the 'LOLO' peridural needle. We do not say that it is difficult to locate the space with conventional needles. However, the degree of dural 'tenting' that occurs with these needles is minimal and when it occurs it is almost immediately followed by a dural perforation. This leaves very little room for error. The 'LOLO' peridural needle not only seeks out the peridural space but due to its marked 'tenting' effect, it causes a local widening of the peridural space making catheter insertion easier.
The side opening which is situated 4-5 mm from the tip of the needle conforms to the anatomy of the epidural space. When the tip of the needle is in contact with the dural sheath, the side opening lies just within the peridural space. This has many advantages:
(1) There is better transmission of negative pressure.
(2) Directional spread of anaesthetic is possible.
(3) There is less chance of the opening getting blocked.
(4) Therefore insertion is a continuous operation with no need for frequent insertion and removal of the stylet.
(5) Dural perforation by the catheter does not occur because the catheter emerges some distance from the tip and takes a smooth curve before it passes upwards into the peridural space., A catheter emerging from the tip of the needle as with the Tuohy's needle, will immediately impinge on the dura and may perforate it. Further catheter kinking is much less in side opening needles.
(6) The objection to the side opening that may be raised is that if a perforation by the tip occurs, it may be difficult to detect the same. This might be true in conventional needles. However, in the 'LOLO' peridural needle, the degree of 'tenting' is so great that in the event of a perforation, the elastic dura snaps back to its original position with the side opening invatiably lying in the subarachnoid space.
The inclusion of the baffle plate in the tip of the 'LOLO' peridural needle for continuous use gives the needle distinct advantages:
(1) depending on the curve of the baffle plate, the curve that the catheter takes i.e. catheter bias, as it emerges can be increased.
(2) as the catheter curves smoothly up into the peridural space, there is no need to depress the needle hub to direct the catheter upwards and dural puncture by the catheter does not occur.,
(3) as both the inner edge of the needle and the baffle plate are not part of the cutting system, they can be machined smooth. No catheter shearing occurs and the catheter can be withdrawn if stuck and reintroduced without the danger of catheter shearing.
We conclude that the 'LOLO' peridural needle has the properties of an ideal peridural needle with no compromise in any of its functions it is intended for. It is sharp on the spinal ligaments which it separates and is blunt on the dura which it sequentially 'tents'.
1. The authors gratefully acknowledge the Dean, B. Y. L. Nair Hospital for use of hospital data. Dr. D. D. Tanna, Professor, B. Y. L. Nair Hospital, the Departments of Cardiology and Radiology, B. Y. L. Nair Hospital for the myelographic study.
2. This work was sponsored by a grant from the Nair Golden Jubillee Research Foundation, Bombay.
3. Paper read at the Annual Conference, Indian Society of Anaesthetists, National Conference, Visakhapatnam, December 1986. Annual Conference, Indian Society for the study of Pain, February, 1987, Bombay.
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5]