From 1942 to 2011, several classifications of suprascapular notch variations have been proposed in scientific literature [11, 12, 16–22]. However, little use has yet been made of new imaging techniques such as computer tomography or ultrasonography. This study is the first to classify the morphology of SSN based on computer scans of scapulae in a large population.
The two oldest classifications of the type of suprascapular notch were introduced by Hrdricka et al. (1942)  and Olivier (1960) . In the first, Hrdricka et al.  divide the SSN into five types based on visual observation: shallow (type II), medium (type III) and deep (type IV). In type I, the SSN was absent, and in type V, a complete foramen was formed. In the second classification, Olivier  also describes five types of suprascapular notch. In type I, the notch is very small and forms a shallow depression on the superior border of the scapula. Type II is also shallow but more visible. The notches in types III and IV are deep. In type V, a completely ossified STSL forms a bony foramen.
One of the most cited is Rengachary’s classification from 1979 , in which the suprascascapular notches are divided into six types. The first type lacks a discrete notch. Their second type is a wide, blunted, V-shaped notch, with its maximum width along the superior border of the scapula. The third type is symmetrical with nearly parallel lateral margins (U-shaped). The fourth type presents a very small V-shaped notch. In the fifth type, the medial part of the STSL is partially ossified, with a U-shaped notch. The sixth type has a completely ossified STSL which forms a bony foramen.
In 1998, Ticker et al.  distinguished two main types of suprascapular notches: U-shaped and V-shaped. Additionally, the degree of ossification of the superior transverse scapular ligament was evaluated separately.
In 2003, Bayramoglu et al. , based on the classification of Rengachary et al. , also described two main types: U- and V-shaped notches.
In the next classification from 2007, Natsis et al.  distinguished five types of SSN. The first type lacks a discrete notch (8.3 %). The second type presents a notch that is longest in its transverse diameter (41.85 %). The third type presents a notch which is longest in its vertical diameter (41.85 %). In the fourth type, the STSL is calcified and forms a bony foramen (7.3 %). In the fifth type, the scapula has a notch and a bony foramen (0.7 %).
In 2010, Duparc et al.  reported that a V-shaped SSN was seen in 36.7 % of shoulders and a U-shaped in 63.3 % of shoulders. In both types, the notch could be more or less open, narrower or wider.
Also in 2010, Iqbal et al. , based on the shape, described three types of SSN: U-shaped (13.2 %), V-shaped (20 %) and J-shaped (22 %).
The newest classification was from 2011. Polguj et al.  describe a quantitative classification of the SSN based on specific geometrical parameters that clearly distinguish five structural types. It is described in greater detail above in "Materials and methods".
The classifications of Hrdricka et al. , Olivier , Ticker et al. , Bayramoglu et al.  and Iqbal et al.  are qualitative and not based on specific geometrical parameters. The main reason for using the classification of Polguj  in our study is that it is a quantitative classification. It is a simple, clearly-described procedure which requires measurements to be taken. The classification of Polguj  is also easy to use and is based on specific geometrical parameters which clearly distinguish each type and subtype of suprascapular notch.
The frequency of type I (“narrow and deep”) was found to be higher in males (28.45 %) than in females (18.66 %). On the other hand type III (“wide and shallow”) was higher in females (63.06 %) than males (50.87 %). This aspect is especially important because males are approximately three to four times more likely to suffer from a suprascapular neuropathy than females [2, 5–8]. It may be partially explained by the “sling effect” proposed in 1979 by Rangrery et al. , which assumes that during movements of the arm, the nerve makes only minimal transitional movements and can be pressed against the sharp bony margin of the suprascapular notch by the action of the upper limb. This repeated kinking irritates the nerve and induces microtrauma that can result in suprascapular nerve entrapment . Therefore, hypothetically, when the suprascapular nerve passes by a “narrow and deep” suprascapular notch (type I), it could be more predisposed to injury by the sharp bony walls of this structure.
The second anatomical factor which might explain the higher frequency of suprascapular neuropathy in males than females may be the significantly higher frequency of scapulae with a bony foramen, corresponding to a completely ossified STSL–type IV, in males than females (6.03 % vs. 2.98 %). This appears to be especially important, because such morphological features are estimated to be one of the most important factors of suprascapular neuropathy [3, 11, 12, 16].
In our study, the frequency of scapulae with a bony foramen (type IV) was 4.72 % of all cases, which was higher than that seen by Sinkeet et al.  (3 %), Tubbs et al.  (3.7 %), Rengachary et al.  (4 %) or Wang et al.  (4.08 %). However it was lower then that described by Vallois  (6.5 %), Natsis et al.  (7.3 %) or Bayramoglu et al.  (12.5 %). In some populations, scapulae with a bony foramen was found to be very rare, while in others they were found to be common: Alaskan Eskimos (0.3 %)  vs. Egyptians 13.6 % . It would be reasonable to suppose that complete ossification of STSL frequency depends on the population. However, in our opinion, an important factor is the mean age of investigated specimens; in our study, the mean age of all patients was 62.5 years, and we assume this to be lower than in the cadaveric study.
Another factor of suprascapular nerve entrapment can be anatomical variations of structures in the suprascapular region, such as a bifid STSL [20, 21] or trifid STSL , hypertrophied subscapular muscle , a double suprascapular foramen  or the co-existence of a notch with a suprascapular foramen .
In 2001, Pecina et al.  discovered that another factor of suprascapular nerve entrapment can be a connective tissue band, the ligamentum spinoglenoidale, which may exist in up to 50 % of people. It formed a second fibro-osseous tunnel (spinoglenoid notch) for three or four motor branches of the nerve that supply the infraspinatus muscle.
Our study uses a new quantitative classification of SSN morphology, which, contrary to existing methods, is simple, reproducible, and based on specific geometrical parameters that clearly distinguish each type. As suprascapular nerve entrapment is a rare condition, these features are especially important in clinical practise because it has confirmed its practical viability in multicentre investigations focused on this pathology. To our knowledge, the literature reveals no similar study which describes SSN variation in such a large population.