Denition, classication and morphometry of entrance of the bicipital groove and its associations with lesions of adjacent structures

The denition of the entrance of the bicipital groove (EBG) is still unclear, and the relationships between the EBG and lesions of the long head of the biceps brachii tendon (LBT) and biceps pulley complex (BPC) have long been controversial. The purpose of this anatomic and imaging study was to dene the EBG and to examine morphological parameters, classications, and their relationships with lesions of the LBT and BPC. correlated with type B and C EBGs by imaging analysis.


Introduction
Shoulder pain is one of the most common musculoskeletal complaints, and many diseases, including tendinitis and tenosynovitis of the long head of the biceps brachii tendon (LBT), rotator cuff disease and lesions of the biceps pulley complex (BPC), are origins of shoulder pain [1,2]. These origins usually involve a key anatomical structure-the entrance of the bicipital groove (EBG) [3]. The bicipital groove lies between the greater and lesser tuberosity of the humerus through which the LBT passes [4]. The transverse humeral ligament passing from the lesser tuberosity to the greater tuberosity of the humerus overlies the LBT as it emerges from the capsule of the shoulder joint [5]. The subscapularis tendon, coracohumeral ligament, and transverse humeral ligament are all believed to contribute to LBT stability within the bicipital groove [6]. Although many studies have examined the pathology of EBG-related lesions, the de nition of the EBG remains equivocal.
The relationship between variations in the anatomical structure of the EBG and local lesions or injury is controversial. Several articles have reported that the morphology of the bicipital groove affects LBT disorders [7][8][9]; some anecdotal studies have not supported any correlation between intra-articular biceps tendon pathology and bicipital groove morphology [10]. Some authors have argued that the transverse ligament does not exist as a separate entity [5,11] but is considered as an extension of the subscapularis tendon and only contributes slightly to the stabilization of the LBT in the bicipital groove [12].
In order to clarify the dispute, we developed a clear de nition of the EBG as an anatomic and functional segment along the LBT from the upper margin of the transverse humeral ligament to the intra-articular soft tissue (synovium, a hammock-like synovial sling or part of the BPC) surrounding the LBT adjacent to the entrance of the capsule (Fig. 1A). The EBG is a channel with variable length and a constant position and consists of two parts: an intra-articular segment and an extra-articular segment. The intra-articular segment of the EBG is a soft tissue channel wrapping the LBT near entrance of the capsule. The extraarticular segment is a semiosseous channel from the entrance of the capsule to the upper margin of the transverse humeral ligament. Usually, the level of the upper margin of the transverse humeral ligament is the turning point for a cliff of the lesser tuberosity (Fig. 1A, 1B). Additionally, we classi ed anatomical landmarks of the EBG to examine their correlations with lesions of the LBT and BPC in an anatomic and imaging study. shoulders from cadaveric specimens without visible evidence of pathologic processes (tumor, infection, or fracture). The BPC was checked for lesions after removing the deltoid muscle, and then the capsule was incised to assess the EBG, LBT and BPC.
Lesions of the BPC were assessed and evaluated by the Habermeyer classi cation [21,22]. Type was de ned as normal; type II was de ned as subluxation; and type III was considered dislocation of the LBT, de ned as when the LBT was positioned more medially within the EBG than usual [18][19][20].

Dry unpaired humeri bones
A total of 113 adult intact humeri were collected from skeletons preserved in hermetic boxes by the Department of Anatomy, School of Basic Medicine, Southern Medical University, China.. The age and sex of the donors were unknown. The width, depth, and angles of the EBG were measured ( Figure 1B and Figure 2A, 2B).

Images
We retrospectively reviewed 278 images of 3-dimensional (3-D) CT and MRI scans of the shoulder at the Hainan Province Hospital of Traditional Chinese Medicine and the Third A liated Hospital of Southern Medical University from January 2016 to January 2020. Criteria for selection were age from 18 to 90 years old; available 3-D CT and MRI scans of the shoulder; and absence of tumor, infection, surgery, or fracture of the shoulder. The mean age of the patients, including 176 males and 102 females, was 56.38±21.27  years. The width, depth, and angle of the EBG were measured as above on 3-D CT images ( Figure 6B). Lesions of the LBT and BPC were assessed and evaluated as above on MRI scans. Dislocation and subluxation of the LBT were also evaluated on MRI scans.

Classi cation of the EBG
We classi ed the intra-articular EBG into 3 types after anatomical observation and refer to the classi cation system of Welcker and Dierickx [23]. In particular, the intra-articular EBG was classi ed as either type : with or without a minor synovial sling ( Figure 3A); type II: part of the BPC or a hammock-like synovial sling wrapped the intra-articular LBT without an interval or gap ( Figure 3B); or type III: the BPC sling wraps the intra-articular LBT with gaps or intervals ( Figure 3C).
Consequently, the extra-articular EBG in the lateral view was classi ed as either type i: exhibiting a shape similar to a shaft and in the lateral view appearing as a shaft without narrowing or expanding at the level of turning over the cliff of the lesser tuberosity ( Figure 4A, 4B, 4C); type ii: having a "sphecidae" shape and in the lateral view girdled by the supratubercular ridge and basal part of greater tuberosity at the level of turning over the cliff of the lesser tuberosity ( Figure 4D, 4E, 4F); or type iii: with a are shape and expanded in the lateral view similar to a are at the level of turning over the cliff of the lesser tuberosity ( Figure 4G, 4H, 4I).
The EBG was also classi ed into 3 types. In particular, the EBG was classi ed into type A: with a U shape, a depth of more than 3-4 mm and a total opening angle that was smaller than 80-90° at the level of turning over the cliff of the lesser tuberosity ( Figure 5A, 5B, 5C, 5D); type B: with a shallow arc shape, a depth of less than 3-4 mm and a total opening angle that was larger than 80-90° at the level of turning over the cliff of the lesser tuberosity ( Figure 5E, 5F, 5G, 5H); or type C: with a shhook shape, a bony spur that arose from the medial and/or lateral wall of the EBG at the level of turning over the cliff of the lesser tuberosity, and the medial or lateral wall was almost vertical ( Figure 5I, 5J, 5K, 5L).
The shape of the articular surface of the humeral head adjacent to the EBG was classi ed into 2 types. In particular, type a was a smooth arc ( Figure 6A, 6B, 6C), and type b was a wavelike line ( Figure 6D, 6E, 6F).

Statistical analysis
All measurements and classi cations were independently evaluated by 3 researchers. Interrater agreement was assessed using kappa coe cients as slight, fair, moderate, substantial and almost perfect [24]. All data are presented as the mean±standard deviation. The means of the length, width, depth, and angles were compared among different types by one-way classi cation ANOVA with SNK q or Welch's test with Dunnett's T3 and Dunnett's C tests, as appropriate. Independent samples t tests were performed to compare the width, depth and angles between type and type II groups based on the articular surface of the humeral head adjacent to the EBG.. P values below 0.05 were considered signi cant. Statistical analyses were performed using the SPSS 17.0 (SPSS, Inc., Chicago, IL, USA).

Results
Interobserver reliability was substantial or almost perfect Interobserver kappa values were 0.88 to 0.92 and 0.62 to 0.74 for classi cation of the intra-articular EBG and extra-articular EBG in the lateral view. The kappa values were 0.79 to 0.93 for classi cation of the EBG in the superoinferior view into types A, B and C and 0.80 to 0.93 for classi cation of the shape of the articular surface of the humeral head adjacent to the EBG. These data indicated substantial or almost perfect reliability across the 3 observers.

Measurement and classi cation distribution of the EBG in cadaveric specimens
The length, width, depth, medial wall angle and total open angle of the EBG in cadaveric specimens are presented in Table 1. The classi cation distributions of the EBG are presented in Table 2. Lesions of the LBT were found in 2 specimens: one type lesion ( Figure 7A) and one type II lesion ( Figure 7B) based on the Curtis-Snyder classi cation [16], and one small tear ( Figure 7A) and 1 medium tear ( Figure 7B) based on the Post classi cation [17]. There were 2 subluxations among all 34 type B LBTs, and no dislocations of the LBT were found ( Figure 7B, 7C, 7D). Consequently, only 2 lesions of the BPC, one type ( Figure 8A) and one type II lesion ( Figure 8B, 8C), were detected based on the Habermeyer classi cation [21,22]. Both injured LBTs were subluxated and were type B and type C; meanwhile, both injured LBTs were type and type iii EBGs ( Figure 7A, 7B). No mesotendon of the LBT was found within the EBG.
Measurement of the EBG and its relationship with classi cation in the dry humeri The width, depth, medial wall angle and total open angle of the EBG in the dry humeri are presented in Table 4. The total open angle was the largest (F=63.52; P<0.001) and the depth and medial angle were the smallest (F=79.59 and 36.37, respectively; both P<0.001) in type B, and type C had the smallest width

Measurement of the EBG and the distribution based on the classi cation of images
Data on the width, depth, and angles in the images are presented in Table 6. The width and total open angle were the largest (F=280.75 and 538.56, respectively; both P<0.001) and the depth and medial angle were the smallest (F=18.63 and 85.25, respectively; both P<0.001) in type B. Additionally, the depth and medial angle were the smallest (F=30.10 and 18.74, respectively; both P<0.001) and the width and total open angle were the largest (F=14.93 and 27.14; both P<0.001) in type iii. The width, depth, medial wall angle and total open angle of the EBG in the images were not signi cantly different among the types based on classi cation of the articular surface adjacent to the EBG (t=0.14, 0.44, 0.68, and 0.63, respectively; all P≥0.50).

Discussion
Many studies have mentioned the EBG [17,[25][26][27][28][29][30][31], but they did not de ne the EBG by rule and line. Many studies on tenodesis of the LBT valued a location 10 mm distal to the proximal EBG that anatomically lies superior to the pectoralis major tendon [25,26,30]. Moreover, the point or segment of the EBG varied among studies based on the perspective from the schematic diagram [17,30,32]. As the segment of the EBG is very important for shoulder pathology, physical examinations and surgical operations, we believe that it is necessary to accurately de ne the EBG. We de ned the EBG as an anatomic and functional segment along the LBT from the upper margin of the transverse humeral ligament to the intra-articular soft tissue surrounding the LBT adjacent to the entrance of the capsule (Fig. 1A). The EBG is divided into two parts: an intra-articular segment and an extra-articular segment. Usually, the point of the upper margin of the transverse humeral ligament is the turning point over a cliff of the lesser tuberosity (Fig. 1A,  1B).
The dimensions and morphology of the intertubercular sulcus are involved in LBT lesions [14,33] [34], although there are still many controversies [10,21]. Lesions of the LBT create three types of bony abnormalities within the intertubercular sulcus: rotator cuff calci cations, medial and lateral spurs (small bony excrescences), and degenerative changes [14,33]; bony abnormalities are considered to be correlated with lesions of the LBT as a matter of course. We found that type C of the EBG classi cation in the superoinferior view correlated with LBT lesions. The groove was designated as narrow, normal, or shallow based on mean measured opening angles of less than 66°, 94°, and 118°, respectively [10,35], and the morphometry of the bicipital groove on MRI had no correlation with lesions of the intra-articular LBT [10]. However, we speculated that the EBG rather than the middle segment of the bicipital groove may be more clinically signi cant. Previous studies may have drawn different conclusions based on measurements of different segments of the bicipital groove. In our study, all subluxations or dislocations were distributed in type B based on the EBG classi cation in the superoinferior view, both in cadavers and images.
Lesions of the LBT and BPC are considered to be associated with shoulder pain. The supratubercular ridge, which increases mechanical wear and tear [33], could increase the instability of the LBT within the EBG and thus favor disease of the LBT [33]. The LBT may dislocate and slide over the lesser tuberosity if the anterior supporting structures are torn and the medial wall itself is de cient [13]. We classi ed the EBG into different types based on anatomical variations of the bony landmarks. The extra-articular segment of the EBG, which is a semiosseous channel, was classi ed into 3 types in the lateral view and 3 types in the superoinferior view. Additionally, the articular surface near the EBG was classi ed into 2 types. Most LBT and BPC lesions were found in type II and type III EBGs based on the classi cation in the lateral view. Some studies have considered that the supratubercular ridge may predispose patients to LBT dislocation [13,33,36], and our data supported the notion that the supratubercular ridge might correlate with pathology. To our surprise, bony spurs arising from the wall of the intertubercular sulcus (EBG) at the level of the upper margin of the transverse humeral ligament, which belonged to type iii, did not appear to be a protective sign but rather a sign of LBT lesions (Fig. 5). Osseous spurs seemed to correlate with lesions of the LBT and BPC.
The types of articular surface near the EBG did not seem to correlate with any lesion of the LBT or BPC. A type b articular surface near the EBG was most common, and this type may be a helpful sign indicating the direction of the LBT under shoulder arthroscopy. Classi cation of the intra-articular EBG may be practically useful to evaluate the type and lesion of the BPC under shoulder arthroscopy. Both lesions of the LBT were type I, which meant that the soft tissue wrapping of the LBT might protect the LBT rather than serving as the origin of biceps-related complaints [23]. No mesotendon of the LBT was found within the EBG, which meant that the LBT lacked nutrient vessels in this segment.
Importantly, the images for this retrospective analysis were collected from patients with shoulder disorders; therefore, the numbers of lesions and injuries of the LBT and BPC were much higher than those in the general population. Our hypothesis and speculative conclusion based on anatomical and imaging observations and measurements require further investigation. The greatest possible limitation was the lack of arthroscopic veri cation and study. By clarifying the concept, location and classi cation of the EBG, we hope to gain better insight into the relationships between variations in the EBG and peripheral tissue injury, which should be the subject of further anatomical and clinical research.

Conclusion
In this work, we de ned the EBG and examined the morphological parameters, classi cations, and their relationships with lesions of the LBT and BPC through an anatomic and imaging study. The EBG is an anatomical concept worthy of clinical attention. Anatomical variations in the EBG correlated with lesions of the LBT and BPC.

Funding
This work was funded by Guangdong Basic and Applied Basic Research Foundation (No.2020A151501998).

Disclaimer
No nancial biases exist for any author and their immediate families from any commercial entity related to the subject of this article.