A spinal cannula is a special hollow needle ( cannula ) with which nerve water ( liquor cerebrospinalis ) is taken from the spinal canal or through which drugs are injected into the liquor (see also lumbar puncture , spinal anesthesia ).
A number of different models are used in medicine, which are named after the inventors (e.g. Quincke cannula, Sprotte cannula).
Spinal cannulas differ in some design features from the "usual" cannulas, as they have to meet several special requirements for CSF puncture:
- Avoidance of post-puncture headache,
- Sufficient inside diameter for good liquor return flow,
- Stability to penetrate the resilient spinal ligaments,
- no skin particles should be carried into the puncture canal.
Avoidance of post-puncture headache
The membranes of the brain and spinal cord are injured by the lumbar puncture. These so-called meninges line the spinal canal and delimit the liquor space. As a result of the prick through the meninges, cerebrospinal fluid (nerve fluid ) drains into the environment, which can lead to the dreaded CSF negative pressure headache (also called post- puncture or post-spinal headache ).
In order to keep the loss of CSF low, the spinal cannula should leave the smallest possible “wound” in the membranes of the spinal cord. Therefore, very thin cannulas are used (for spinal anesthesia 25 to 27 gauge, corresponding to an outer diameter of 0.5 to 0.4 mm).
Different tip shapes have been designed to minimize trauma (atraumatic tips). The names of the different cannula types refer to the inventor or the appearance of the respective tip.
Sufficient inside diameter versus dimensional and fracture stability
If the tip of the cannula is correctly positioned in the liquor space, the nerve water drips out of the cannula. In order for the cannula to be easily passable for liquor and possibly medication, it must have a relatively large lumen (inner diameter) despite the small outer diameter . This is at the expense of the wall thickness and thus the stability.
So that the cannula does not bend too easily or even break off when it is inserted through the rigid ligaments that are often ossified by calcium deposits in old age, it is used with a mandrin (guide needle) that fills the lumen and thus improves stability.
In particular, the softer pencil and ball point cannulas are usually pushed through an insertion cannula that is first pierced through the skin in the direction of the spinal ligaments. This stabilizes the shape and direction of the spinal cannula.
Avoidance of punching cylinders
In addition to the necessary stabilization, the stylet closes the cannula lumen and thus ensures that no cylinder of tissue is pierced - especially through the Quincke cannula, which is open at the front. In addition to the possible blockage of the cannula, there is a risk of bacterially contaminated skin being carried into the depths of the puncture canal (risk of infection : intraspinal abscess , meningitis ).
Different types of spinal cannulas
Heinrich Irenaeus Quincke introduced CSF puncture as a routine examination in medicine. The Quincke cut comes from him, in which the tip of the cannula is beveled, as is typical for "conventional cannulas". The Quincke cannula is the most stable of the spinal cannulas, but has the disadvantage that the beveled bevel “cuts” through the tissue.
Pencil point cannulas according to Whitacre or Sprat
These cannulas are - like pins or pencil points - ground to a point (point ground) so that they do not cut the fibers of the meninges but instead force the tissue apart. The opening of the needle is ground in on the side.
Ball point cannula
Ball point cannulas or ball pen cannulas look like greatly reduced ballpoint pens; the lead corresponds to the rounded stylet, which is withdrawn after penetrating the liquor space. The “sleeve of the ballpoint pen” would then correspond to the actual cannula. This shape should further reduce the trauma.
- T. Standl et al .: Spinal anesthesia performance conditions and side effects are comparable between the newly designed Ballpen and the Sprotte needle: results of a prospective comparative randomized multicenter study. In: Anesth Analg. 98 (2), Feb 2004, pp. 512-517. PMID 14742396