Coexistence of the suprascapular notch and the suprascapular foramen—a rare anatomical variation and a new hypothesis on its formation based on anatomical and radiological studies

The patient from our first radiological study (case 1) suffered from deep and diffuse poorly localized pain of the left shoulder and weakness of abduction and external rotation of the left upper extremity in the glenohumeral joint. Atrophy of the left supraspinatus muscle was also observed. These symptoms are the main clinical manifestations of suprascapular nerve entrapment (Gosk et al. 2007; Post and Mayer 1987; Zehetgruber et al. 2002).

Although suprascapular nerve entrapment is uncommon, it should be considered in the differential diagnosis of patients with shoulder pain and weakness, which includes brachial plexopathy, cervical spine disease, cervical discopathy, glenohumeral joint diseases, tendonitis or bursitis and rotator cuff tear (Gosk et al. 2007; Zehetgruber et al. 2002). The diagnosis of suprascapular nerve entrapment is based on physical examination, medical history and the results of additional investigations (Cummins et al. 2000). The most important of the latter are the results from electrodiagnostic studies, including studies of nerve conduction velocity from Erb’s point to the supra- and infraspinatus muscles and electromyography. The normal latency ranges for the supra- and infraspinatus muscles are 1.7–3.7 and 3.3–4.2 ms, respectively (Kraft 1972; Kullmer et al. 1998).

The patient from our radiological study (case 1) refused further diagnostic evaluation of his shoulder pain. However, based on his clinical presentation and the radiographical findings, we consider that he presented with suprascapular nerve entrapment.

Although many anatomical variants of the suprascapular notch have been reported in the literature, coexistence of the suprascapular notch and the suprascapular foramen has been described only three times (Hrdicka 1942; Natsis et al. 2007; Sinkeet et al. 2010). The first report was that of Hrdlicka who in 1942 found one such case among 2792 dried scapulae (0.036 %). The second report was that of Natsis et al. (2007), who discovered this rare type of anatomical variants in three of 423 scapulae from the German general population (0.7 %). Scientists have classified scapulae with both a notch and a foramen at the site of the SSN as a distinct type (Type V). More recently, there have also been descriptions from Africa; Sinkeet et al. (2010) found one such a case in 138 investigated Kenyan scapulae (0.72 %).

In our study, the frequency of the coexistence of a suprascapular notch and a suprascapular foramen was 0.33 %, which was higher than that reported by Hrdlicka (1942) (0.036 %) and lower than that described by Natsis et al. (2007) in German scapulae (0.7 %) and by Sinkeet et al. (2010) (0.72 %) in the Kenyan population. It would be reasonable to suppose that these frequencies depend on the population, similar to the frequency of complete ossified STSL.

To our knowledge, no hypotheses have been published to explain the coexistence of a suprascapular notch and foramen. Here, we present four hypotheses on the formation of this phenomenon which are based on the latest anatomical findings (Avery et al. 2002; Bayramoğlu et al^. 2003) (Fig. 4).

In our opinion, the first hypothesis (Fig. 4a) is the most probable as it assumes that the ossification of the single bundle ACSL would create the bony bridge above the suprascapular foramen. The osseous transformation of the STSL would not occur, and so it would be absent on a dry scapula, and a suprascapular notch would be formed. The ACSL was first described by Avery et al. (2002) as a fibrous band extending along the anterior aspect of the SSN, just below the STSL. Its proximal and distal attachments insert separately onto the borders of the SSN and run parallel or obliquely to the STSL. The topography of the bony bridge observed in our anatomical and radiological study was similar to that observed by Avery et al. (2002) on the anterior coracoscapular ligament. Therefore, the first hypothesis is the most probable. In our opinion, the ossification of the ACSL can increase the risk of suprascapular neuropathy because of the higher potentiality for nerve irritation by the bony margins of the foramen and the lack of elasticity that the ACSL normally demonstrates. The etiopathology of the suprascapular nerve entrapment proposed by Rengachary et al. (1979), called the “sling effect”, assumes that during motions of the arm, the nerve makes minimal transitional movements. Therefore, an angulated nerve can be pressed against the sharp bony margin when travelling through the suprascapular foramen during the action of the upper limb. The repeated kinking irritates the nerve and induces microtrauma that might result in this neuropathy. Although in the case presented here it is impossible to determine the course of the SN, the area of the suprascapular region was highly reduced in comparison to the potential area of the normal notch without a bony bridge. The frequency of the ACSL has been found to be 18.8 % (Bayramoglu et al. 2003—Turkish population), 28 % (Piyawinijwong and Tantipoon 2012—Thai population) or 60 % (Avery et al. 2002—American population). All scientists confirm its presence as an additional etiological factor of the condition.

The second potential mechanism (Fig. 4b) described in this article which explains the variation is calcification of the inferior part of a bifid STSL. In such a case, the ligament has two bands (superior and inferior) that are separately fixed to the one border of the suprascapular notch. Both parts of this bifid STSL travel independently one below the other, but they have a common opposing attachment. An ossified inferior band of the bifid STSL might create a bony bridge (Fig. 4b). Despite the fact that frequency of the bifid STSL varies from 3.3 to 15.6 % (Bayramoglu et al. 2003; Duparc et al. 2010), partial ossification (not whole) of the ligament seems unlikely.

The third hypothesis (Fig. 4c) explaining the coexistence of a suprascapular notch and foramen depends on incomplete ossification of a trifid STSL. This type of superior transverse scapular ligament has three parts: the superior, middle and inferior band. When only the middle band ossifies, the bony bridge is formed, and the suprascapular notch and foramen are present on dry scapulae (Fig. 4c). A survey of recent literature revealed only two cases of trifid STSL (Polguj et al. 2012; Ticker et al. 1998). In the study of Ticker et al. (1998), the ligament had the middle band partially ossified, possibly confirming this hypothesis. However, Polguj et al. (2012) did not find any signs of ossification in the ligament in their study.

The fourth potential mechanism explaining the coexistence of a suprascapular notch and foramen is the calcification of the superior band of a bifid ACSL (Fig. 4d). However, this explanation seems rather unlikely because, to our knowledge, no case of partially or completely ossified bifid ACSL has ever been reported.

The SSF and the SSN are the most probable passages for the SN. Data obtained from our photographic documentation analysis indicate that the areas of both spaces were considerably decreased by the presence of the bony bridge in comparison to the reconstructed potential area of the suprascapular nerve passage. The space available for the passage was decreased by about 37.4–38.6 (radiological study) and 36.5 % (anatomical study). In our opinion, the coexistence of the suprascapular notch and suprascapular foramen narrows the space for passage of the nerve and, therefore, might also increase the risk of suprascapular neuropathy.