A new anatomical insight into the aetiology of lateral trunk of suprascapular nerve neuropathy: isolated infraspinatus atrophy

Abstract

Introduction: Although the pathomechanism of isolated infraspinatus atrophy (ISA) in throwing sports is known to be traction, it is unclear why only some players are affected. One likely explanation is that the infraspinatus pulling force exerted by its contracture generate the compressive resultant component force (Fn) compressing the lateral trunk of the suprascapular nerve (LTSN) against the edge of scapular spine. This paper makes two key assumptions (1) the course of LTSN in relation to the scapular spine, defined as the suprascapular-scapular spine angle (SSSA) is the key individual anatomical feature influencing the Fn magnitude, and thus potentially ISA development (2) SSSA is correlated with scapular notch type.

Materials and methods: The bone landmarks of the LTSN course were identified in 18 formalin-fixed cadaveric shoulders, and the SSSA was measured in 101 dry scapulae. The correlation between the SSSA and suprascapular notch type was evaluated. The Fn value was simulated mathematically based on the values of the SSSA of 101 dry scapulae and the prevalence of ISA in chosen throwing sports, as given in the literature: i.e., beach volleyball - 34% (group A1 - 34%; group A2-remaining 66% of scapulae) and tennis - 52% (group B1 - 52%; group B2-remaining 48% of scapulae).

Results: The mean SSSA value was 44.57° (± 7.9) and Fn 79 N (± 13.1). No statistically significant correlation was revealed between suprascapular notch type and SSSA. Groups A1 and B1 possessed significantly lower SSSA values (p < 0.000) and significantly higher Fn magnitude (p < 0.000) than groups A2 and B2 respectively. The average difference of Fn was 28.1% between group A1 and A2 and 31% between group B1 and B2.

Conclusions: The SSSA has a wide range of values depending on the individual: the angle influencing the magnitude of the compressive resultant force Fn on the LTSN at the lateral edge of the scapular spine via contraction of the infraspinatus muscle. The prevalence of ISA in throwing sports may be correlated with the SSSA of the LTSN. However, further combined clinical, MRI or/and CT studies are needed to confirm this.

Keywords: Isolated infraspinatus atrophy; Suprascapular nerve anatomy; Suprascapular neuropathy; Throwing sports.

Fig. 1

a, b The course of the lateral trunk of the suprascapular nerve, confirmed by anatomical examination. a Permanent soft preparation of the upper limbs, posterior-superior view of the suprascapular fossa. b Dry preparation of the scapula indicating the course of the lateral trunk of the suprascapular nerve. Large arrow—lateral trunk of the suprascapular nerve, small arrow—superior transverse scapular ligament, arrowhead—lateral edge of the scapular spine

Fig. 2

The tool used for measuring the suprascapular spine angle (SSSA), and the means by which the angle was determined with regard to the reference points on the bone (bottom of scapular notch, lateral point of scapular spine edge, medial and lateral points of upper aspect of scapular spine), and the method of preserving the position of the measuring device using plastic materials

Fig. 3

Suprascapular notch dimension measurements of maximal depth a; superior transverse b; middle transverse c according to classification proposed by Polguj et al. [31]

Fig. 4

The schematic drawing of resultant compressive force (Fn) of the pulling force of infraspinatus contraction (F), acting on the lateral trunk of the suprascapular nerve (LTSN) against the scapular spine lateral edge (SSLE). SSSA suprascapular spine angle

Fig. 5

The graph of the distribution of force Fn in the studied group of 101 scapulae

Fig. 6

The global biomechanical concept of the development of lateral trunk of suprascapular nerve neuropathy (LTSNN)