====== Twining line ======

{{ ::twining_line.png?400|}}

Studies have used the [[Twining line]] (from the [[tuberculum sellae]] to the [[torcula]]), or the [[floor of the fourth ventricle]] as reference lines and defined different cutoff values of “steepness”. The other important factors to be considered in [[Pineal region approach]] are anatomic features of the 
[[quadrigeminal cistern]] and [[superior cerebellar cistern]], as both host important neurovascular structures and serve as anatomic corridors 
((Rhoton AL Jr. Tentorial incisura. Neurosurgery. 2000 Sep;47(3 Suppl):S131-53. 
PubMed PMID: 10983307.
))
((Hart MG, Santarius T, Kirollos RW. How I do it--pineal surgery:
supracerebellar infratentorial versus occipital transtentorial. Acta Neurochir
(Wien). 2013 Mar;155(3):463-7. doi: 10.1007/s00701-012-1589-5. Epub 2012 Dec 27. 
PubMed PMID: 23269352.))
((Syed HR, Jean WC. A Novel Method to Measure the Tentorial Angle and the
Implications on Surgeries of the Pineal Region. World Neurosurg. 2018
Mar;111:e213-e220. doi: 10.1016/j.wneu.2017.12.037. Epub 2017 Dec 16. PubMed
PMID: 29258947.
))
((Gu Y, Hu F, Zhang X. Purely endoscopic resection of pineal region tumors using
infratentorial supracerebellar approach: How I do it. Acta Neurochir (Wien). 2016
Nov;158(11):2155-2158. Epub 2016 Aug 9. PubMed PMID: 27506850.))
((Qi S, Fan J, Zhang XA, Zhang H, Qiu B, Fang L. Radical resection of
nongerminomatous pineal region tumors via the occipital transtentorial approach
based on arachnoidal consideration: experience on a series of 143 patients. Acta 
Neurochir (Wien). 2014 Dec;156(12):2253-62. doi: 10.1007/s00701-014-2224-4. Epub 
2014 Sep 23. Erratum in: Acta Neurochir (Wien). 2015 Feb;157(2):349. PubMed PMID:
25246142.
)).

The current trend to use the Twining line to define this angle has significant pitfalls. 

The goal of a study of Syed et al., was to provide a new and accurate way to measure the [[tentorial angle]] and demonstrate its impact on surgeries of the [[pineal region]].

n-angle to measure the tentorial angle was introduced using the [[floor of the fourth ventricle]] and the [[torcula]]. Comparisons with older techniques were made to illustrate reliability. Midline sagittal MR images were used to measure the tentorial angle in 240 individuals to obtain population-based data. A cohort of 8 patients who underwent either the infratentorial or the transtentorial approach to the pineal or upper vermian region were examined in search of correlations between tentorial angle and surgical approach.

The data in this study showed that the [[Twining line]] technique understates the tentorial angle in people with low-lying torcula. The n-angle is more reliable in reflecting the true steepness of the tentorium regardless of torcula position. On average, men have slightly steeper tentoriums. In the clinical cohort, all patients who underwent infratentorial surgery had tentorial angles <55°, whereas the majority of patients who underwent transtentorial surgeries had angles >67°.

The n-angle provides a reliable and accurate way to describe the slope of the tentorium. The population-based average of 60° may be a useful measurement to influence the choice of surgical approach, either under or through the tentorium, to the pineal region
((Syed HR, Jean WC. A Novel Method to Measure the Tentorial Angle and the
Implications on Surgeries of the Pineal Region. World Neurosurg. 2018
Mar;111:e213-e220. doi: 10.1016/j.wneu.2017.12.037. Epub 2017 Dec 16. PubMed
PMID: 29258947.
)).
----
The recognition of anterior or posterior displacement of the fourth ventricle on axial computed tomography (CT) has proven to be difficult because the apparent position of this structure is variable and dependent on scanning angle. In most cases direct visualization of a lesion and its relationship to normal anatomic structures allows for the correct assessment. However, in some instances it would be advantageous for lesion localization to be able to identify relatively subtle displacements of the fourth ventricle. This is possible on CT by determining the position of this structure relative to Twining's line (the line between the tuberculum sellae and torcula). The position of the fourth ventricle, tuberculum sellae, and torcula relative to an arbitrary fixed point can be established in virtually all cases, and thus the position of the fourth ventricle relative to Twining's line can be determined. In a control group of 100 patients with normal CT examinations, the ratio of the distance from the tuberculum sellae to the center of the fourth ventricle and the distance from the tuberculum sellae to torcula (Twining's line) was between 0.47 and 0.53. In 66% of the cases the ratio was 0.49-0.51. THere were 54 posterior fossa masses evaluated by this technique. Determination of fourth ventricular position by this method proved to be of particular value in recognizing brainstem glioma, and in determining the site of origin of laterally placed posterior fossa masses
((Zimmerman RD, Russell EJ, Leeds NE. Axial CT recognition of anteroposterior
displacement of fourth ventricle. AJNR Am J Neuroradiol. 1980 Jan-Feb;1(1):65-70.
PubMed PMID: 6779592.
)).