An osteological study of the clavicular facet of the coracoclavicular joint

Introduction

The coracoclavicular joint (CCJ) is a diarthrotic synovial joint present between the conoid tubercle of the clavicle and the superior surface of the horizontal part of the coracoid process of the scapula. 1 In humans, its occurrence is rare, whereas it is evident in primates. The joint has a capsule, synovial lined space, and hyaline articular cartilage that covers the facets on the coracoid and the clavicle close to the subclavius groove.2 Though the coracoclavicular joint is often disregarded as a structure without any significance, its presence has been established as an etiological factor in shoulder pain, and it could be associated with upper limb paraesthesia.3, 4, 5

This joint was first described by Gruber in 1861.6 During development, the cartilaginous procoracoid connects the coracoid process to the clavicle. The procoracoid ultimately forms the coracoclavicular ligament, which usually contains chondrocytes and cartilaginous nodules. The conoid tubercle of the clavicle and coracoid process occasionally grow toward each other, forming a joint covered by a capsule.7 This joint was found to be more prevalent in Asians than in the other races.8 In osteological studies, the prevalence ranged between 0.7% and 10%, and in the radiological studies it was between 0.6% and 21%.9 A higher prevalence of 1.7% to 30% was observed in cadaveric dissections due to the presence of articular cartilage and capsule.9, 10, 11 Thus, the incidence of CCJ shows wide variation according to the type of study. The present study was undertaken to determine the prevalence of the articular facet on the conoid tubercle of the clavicles in the South Indian population.

Materials and Methods

The study was conducted on 277 adult human dry clavicles (118 right and 159 left) of undetermined sex that were available in the Department of Anatomy, ESI Medical College, after excluding the damaged bones. These bones were accessible for undergraduate teaching during the study period of 2018-2022. The occurrence of a smooth articular facet present on the conoid tubercle determined the presence of CCJ. The maximum transverse diameter and anteroposterior diameter of the articular facets were measured using a digital Vernier caliper. The parameters were analyzed using SPSS software version 20 (IBM Corp. Armonk. NY).

Results

The prevalence of CCJ was 4.69%. On the left side, it was observed in 10 bones, and on the right side, in three bones. The shape of the facet was horizontally oval, and the transverse diameter was longer than the anteroposterior diameter (Figure 1). The mean transverse diameter was 8.81±1.86 mm, and the anteroposterior diameter was 6.16±2.39 mm. On the right, it was 10.82±1.6 mm and 8.06±1.09 mm, respectively, and on the left, it was 8.21±1.51 mm and 5.60±2.41 mm, respectively. The differences between the right and left sides were not statistically significant.

Figure 1

Shows the clavicular facet on the conoid tubercle of the clavicle (inferior view)

Discussion

The coracoclavicular joint is a rare synovial joint with articulation between the conoid tubercle of the clavicle and the superior surface of the horizontal part of the coracoid process of the scapula. In radiological studies, the CCJ was determined by the presence of triangular bony outgrowth near the conoid tubercle. However, slight axial rotation of the clavicle while taking an X-ray will exaggerate the conoid tubercle, which could be mistaken for CCJ. A cadaveric study is more reliable because the presence of the articular capsule and cartilage confirms the CCJ, although the sample size might be small. Nevertheless, the dry bone study provides a large sample size to identify the facet on the clavicle or the coracoid process of the scapula.11

Gruber studied 350 cadavers and observed CCJ in eight specimens. 6 Gumina et al. observed CCJ in eight dry bones (0.78%) among 1020 clavicles.8 In a radiological study on 1040 individuals from Northern India, the reported prevalence was 3.37%.17 An osteological study from India demonstrated an increased prevalence of 5.6%,16 whereas, in the adult Nigerian population, the prevalence was 0.55%.15 In a radiological study, it was observed in 0.82% (18 individuals) of the 2192 individuals examined, and in their osteological study on 392 skeletons, the incidence was 1.78%.10 Thus, CCJ has been studied using radiological, cadaveric, and osteological methods with differences in rate of prevalence. Many studies found the clavicular facet to be oval with a long horizontal axis followed by a circular shape.10, 12 The size of the facet in the study by Kaur and Jit ranged between 8 x 6 mm and 17 x 9 mm.12 In the present study, the size ranged between 4 x 6 mm and 11 x 6 mm.

The geographical distribution and prevalence of CCJ in various studies in the literature are shown in Table 1. The occurrence of CCJ is observed to be higher in Asia than in Western Europe, particularly in people of Chinese ancestry, which confirms geographical variations in its existence.13 Several studies reported that CCJ was more prevalent in males.17, 12, 14 The incidence of clavicular facet was 10.1% in males and 8.3% in females in Northwest India.12 Similarly, Nalla and Asvat also found the CCJ to be more prevalent in males (56.5%) than females (43.5%) in the South African population.14 CCJ was found to be more common on the left side,17 and the same was observed in the present study.

The incidence is also found to be higher in people above 30 years of age.19 Paparoidamis et al. observed a higher incidence of CCJ (13.9%) in individuals more than 76 years of age than in those between 61-75 years of age (3.7%).18 Likewise, in South Koreans, the joint was observed in 9.5% of the age group between 40-49 years and 11.4% of those aged 60 years.2 In individuals below 40 years of age, the CCJ was not found, suggesting that the occurrence could be due to ageing. 2 However, CCJ has also been reported in children. Nehme et al. observed CCJ in a 5-year-old and a 7-year-old child.10 Similarly, Kaur and Jit observed CCJ in a 13-year-old child but none in neonates or fetuses.12 Thus, they concluded that environmental factors such as activity could not play a role in its occurrence, and it could not be a congenital anomaly as well. Although it was assumed to be an autosomal dominant trait, there was no genetic evidence.20 According to Saunders, the CCJ does not follow the Mendelian pattern of inheritance. Despite sharing the same genetic makeup, individuals may or may not exhibit the trait. A physiological threshold determines whether the phenotype will be expressed or discontinued, and individuals will manifest the trait only when the threshold is crossed.21 Furthermore, the genetic cause could not be ruled out because of the variations observed in the prevalence of CCJ between different races.13

Although some authors say that there is no relation between the presence of the CCJ and the size of the clavicle and scapula, Nalla et al. observed an increase in the size of neighbouring bones. They observed longer first ribs and lengthened superior, medial, and lateral borders of the scapula with a significant increase in superior angle and proposed that CCJ develops to compensate for the space restrictions caused by the long and narrow scapula with normal thoracic inlet size, which could cause restriction of scapular movements.14 The presence of a CCJ in patients may present as thoracic outlet syndrome.22

Degenerative changes in the CCJ could be one of the causes of painful shoulders.4 CCJ could also predispose to the development of degenerative changes in other neighbouring joints, such as the acromioclavicular joint.9 Cheung et al. studied the microscopic structure of a resected CCJ and observed that the facet on the coracoid process had fibrocartilage lining, resembling the repair tissue following cartilage injury. In contrast, the clavicular facet was lined by mature hyaline cartilage.3 Thus, the presence of CCJ has been established as an etiological factor in shoulder pain and could be associated with upper limb paraesthesia as well.3, 4 Shoulder pain due to the confirmed presence of CCJ by imaging could be managed conservatively or by surgical resection depending on the severity of symptoms.3

Conclusion

In the South Indian population, with a prevalence of 4.69% in this study, the presence of CCJ could not be considered a rare anomaly. Awareness of the CCJ is essential in diagnosing and managing shoulder pain. Even though it is usually asymptomatic and incidentally diagnosed, the possibility of CCJ occurrence should be considered in the differential diagnosis of unexplained shoulder pathology.

Sources of Funding

None.

Conflict of Interest

None.