This study assessed the variations and measurements of the cubital tunnel and the UN and found that the roof of the tunnel can be composed of two different structures: the AE or, more prevalently, the CuTR. The structure forming the roof was longer in males than in females, while the right elbows had wider roof structures. In cases where the AE was present, the AE was thicker than the CuTR. Interestingly, the diameter and CSA of the UN were larger when the AE was present. The AE muscle was present in 5% of this sample, which is consistent with previous studies in which the prevalence ranged from 5.4 to 13.3% [10, 21, 23, 27, 29] (Table 8). The prevalence of the AE in males ranged from 1.8 to 8.7% [21, 27], and therefore the prevalence found in the present study (3.3%) falls within this range. The prevalence of AE was lower in females (1.7%) than in males and was lower than that reported in other studies (4.5–10%) [10, 27]. The lower prevalence of AE in the present study could be attributed to the lower sample size of females (n = 13) compared to males (n = 17). When compared to the dissection study of Suwannakhan et al. [27], the prevalence of AE in the right and left elbows in the current study was similar. Suwannakhan et al. [27] reported a 3.6% prevalence of AE in left elbows and 2.7% in right elbows, similarly, the current study found a higher prevalence of AE in left elbows than right with a 3.3% and 1.7% prevalence in left and right elbows respectively. In contrast to the present study in which no bilateral presence of the AE was found, Suwannakhan et al. [27] reported one case of the bilateral presence of the AE in one female.

The thickness of the AE (0.70 ± 0.58 mm) was greater than the thickness of the CuTR (0.18 ± 0.08 mm) in this study, which supports the suggestion by Maslow et al. [17] that AE occupies more space than the CuTR. In contrast, the AE was reported by Grewal et al. [10] as 7 mm (range 2–13 mm) thick. The difference in measurement could be due to differences in study designs as Grewal’s study included patients with CuTS and were measured from magnetic resonance imaging (MRI) images. The present study found no significant differences in the thickness of the roof structure between sexes and sides of the body, but to the best of the authors’ knowledge, no other study has compared the thickness of the roof structure between sexes and sides. Males had a longer structure forming the roof of the cubital tunnel than females in the present study, which may be attributed to sexual dimorphism. Few previous studies have investigated the length of the CuTR. However, Grewal et al. [10] reported the mean length of the AE as 22.0 mm (range 15.0 – 32.0 mm) from MRI images and 35.0 mm (range 30.0 – 43.0 mm) using a microcaliper in their dissection study, which is similar to the length found in the current study (31.2 ± 1.7 mm). There was no significant difference between the UN diameter and CSA between sex and sides. Letissier et al. [14], reported that males had a larger UN CSA than females, although no significant differences were found between the left and right sides. Interestingly, the diameter, and therefore, the CSA of the ulnar nerve was significantly larger in elbows with an AE present in this study.

There were no significant differences in the diameter of the cubital tunnel between the sexes and sides of the body. In addition, no significant correlation between the UN diameter and the cubital tunnel diameter was found in this study, although James and Sutton [11] reported a linear relationship between the diameter of the tunnel and the CSA of the UN. However, these measurements were taken when the elbow was flexed at a 90-degree angle compared to the present study where the elbows were extended at an angle larger than 90 degrees [11], which may explain the difference in findings.

The AE is suggested to be a variant muscle in humans, with the function of the muscle not clearly defined [21]. There is no consensus on the potential role that AE plays in the development of CuTS. One study suggests that the presence of the AE may reduce the risk of the development of CuTS due to its decreased rigidity at the entry point of the UN into the tunnel [29]. Wilson et al. [29] reported that the AE was present significantly less frequently in patients with CuTS than in those who did not have CuTS, and thus concluded that AE is a “protective factor” in ulnar neuropathy. In contrast, due to the muscle’s hypertrophic ability, the AE may be a contributing factor in the development of CuTS [29]. In patients in which the AE was present, CuTS developed in the dominant arm more frequently (88.9%) than in those who had CuTS with a CuTR forming the roof of the tunnel [29]. Wilson et al. [29] therefore, proposed that hypertrophy of the AE muscle, secondary to extensive use of the dominant arm, contributes towards CuTS. Another study found four cases of UN neuropathy that were secondary to AE hypertrophy as a result of increased elbow flexion [18]. This statement is supported by a third study that reported the presence of hypertrophied AE in three baseball players who developed CuTS [14].

The length of the cubital tunnel roof increases with an increasing degree of elbow flexion, suggesting the roof structure becomes more taut when the elbow is increasingly flexed [11]. This is an important clinical consideration for UN decompression surgery as along with the length increase with flexion, the area of the tunnel also increases. An increased cubital tunnel area provides a larger, safer, surgical field for instruments [11]. The width of the cubital tunnel is clinically relevant as it ultimately describes the length of the portion of the UN that courses through the tunnel. In UN decompression surgery, the structure overlying the UN is cut to prohibit the compression of the nerve [26]. The present study found that the tunnel was wider on the right side of individuals than on the left side. The majority of the population has a right-handed dominance (88–90%) compared to left-handed (10–12%) [12]. The larger width of the tunnel roof structure in the right elbows may be further linked to increased use of the dominant arm.

There were some limitations of the study. Firstly, the number of males and females was not equal due to the limited sample size. The body donors were pre-dissected by medical students, and thus, in some of the individuals, several structures of interest were damaged and therefore the full range of measurements could not be taken. As the cadavers were formalin-fixed, the range of motion of the elbow joints was limited and, although care was taken to ensure consistency, the arm may not have been in the same position in all of the bodies while measurements were taken. The effect of variation of the elbow flexion angle on the dimensions of the tunnel could not be investigated due to the formalin fixation. No medical history was available for the body donors and it was therefore unknown whether any suffered from CuTS. Care must also be taken when interpreting the results of the CSA of the UN as the formula used assumed the nerve was round, however, the nerve may not have necessarily been round in every elbow. Lastly, the difference in reported measurements of the cubital tunnel could be attributed to the effect of ageing on both the AE [20] and the retinaculum [8], however, the age distribution in the sample was unknown and thus could not be taken into consideration.

The presence of ossification in the cubital tunnel was reported as a cause of UN entrapment in a recent case report by Vojtêch et al. [28]. The presence of an accessory ossicle in the cubital tunnel may be congenital or result from pathology and trauma. As the present study focused on the soft tissue of the cubital tunnel, the bony features were not observed. However, no obvious accessory ossicles were detected. Future anatomical studies should document the prevalence of an accessory ossicle in the cubital tunnel and this rare aetiology of cubital tunnel syndrome should be further investigated in clinical studies.