Duplication of Clavicle and triplication of Coracoid process

B. G. Sharma

Abstract A very rare case of duplication of the clavicle and triplication of the coracoid process of the scapula is presented. Duplication of the clavicle has been described in only six reports based on a search of the world literature. There is no mention of duplication or triplication of the coracoid process of the scapula in the world literature and this would appear to be the first report of this kind. A combination of bifurcation and triplication of the clavicle and coracoid process, respectively, in the present case illustrates that even anomalous bones search for their counterpart to form a joint. The process of duplication or triplication is explained with a new hypothesis.

Keywords Clavicle - Coracoid process - Duplication - Triplication - Supernumerary - Radiograph

Introduction

Anatomic anomalies of the skeletal system have long fascinated radiologists. Some of the skeletal anomalies such as block vertebrae, hemivertebrae, and cervical ribs may be of clinical importance. While anomalies like fusion or bifurcation of the ribs, coalition of the carpal or tarsal bones and development of the accessory sesamoid bones are largely of academic interest. Knowledge of the existence of such anomalies is important. A case of triplication of the coracoid process and duplication of the clavicle discovered incidentally, producing little or no clinical symptoms, is presented. Trifurcation (triplication) of the coracoid process is a very peculiar anomaly not previously described. A hypothesis is presented for the development of these anomalies.

Case reports

Case study 1

A healthy, well-built 34-year-old Nigerian farm worker was referred for a routine chest radiograph. The chest radiograph did not include the shoulder joints completely because of the patient's broad shoulder span. However, a bone abnormality was detected in the partially displayed left shoulder region. The patient could not recollect any injury to the left shoulder joint. On examination, all shoulder movements were normal to the full extent and no abnormality could be palpated.

Radiographs of the left shoulder joint in different positions (Fig. 1) revealed a small extra clavicle inferior to the lateral third of the left clavicle. The two clavicles were fused at the lateral ends and articulated with the acromion process of the scapula (Fig. 1A). The coracoid process was trifurcated with a common broad base (Fig. 1B, C). The supernumerary clavicle was 3.7 cm long with two cortices and a marrow cavity. The medial end of the supernumerary clavicle was broad with an articular surface. Three processes of the coracoid were clearly visible (Fig. 1B). The anterior process had the shape of a normal coracoid process with no articulation to any neighbouring bone. The middle process of the coracoid articulated with the broad medial end of the supernumerary clavicle through a joint (Fig. 1B, C). The third, posterior process of the coracoid was broad and long, and curved towards the middle of the main clavicle, where it articulated with a small accessory process arising from the posteroinferior aspect of the main clavicle (Fig. 1A–C). Few small ossicles were also present close to this joint. The main clavicle was slightly broadened in its mid portion at the site of the small accessory clavicular process and was relatively thinner in the lateral third portion. A diagnosis of duplication of the left clavicle with triplication of the coracoid process of the left scapula was made radiographically. The patient was completely symptom free on his three subsequent visits to the hospital and did not require any kind of surgical intervention for these asymptomatic anomalies.

Fig. 1A–C. The left shoulder joint. A Postero-anterior view. A small supernumerary clavicle is present below the lateral third of the main clavicle. The two clavicles are fused at the lateral ends. The main clavicle is slightly broadened at the site of the midclavicular accessory process and thinned in the lateral third portion. B Oblique frontal view. Note the trifurcation of the coracoid process. The anterior process (1) of the coracoid is non-articulating, the middle process (2) is articulating with the medial end of the duplicated clavicle, and the posterior process (3) is forming a joint with the midclavicular process. C Frontal view. The middle (2) and posterior (3) processes of the coracoid are demonstrated in their full extent. The anterior coracoid process (1) is seen as an end-on oval shadow

Case study 2

An 80-year-old male Libyan was referred for a chest radiograph to exclude lung infection. A 1.2-cm long stubble accessory process was detected on the inferior aspect of the left clavicle at the junction of the medial two thirds and lateral third (Fig. 2). The accessory process of the clavicle pointed inferolaterally and articulated with the coracoid process of the scapula. The distal end of the accessory clavicular process was broad with marginal spur formation. The bone trabeculae were clearly visible in the accessory clavicular process. A diagnosis of midclavicular accessory process with osteoarthrosis of the coracoclavicular articulation was given to this incidental finding. Degenerative changes were present in the shoulder joints. The patient died 6 months later due to the complications of chronic haemodialysis.

Fig. 2. The left shoulder joint. Note the accessory clavicular process articulating with the coracoid process. Early degenerative changes are present in this accessory joint

Discussion

Duplication of the clavicle is an asymptomatic condition and is usually detected by serendipity when a chest or a shoulder radiograph is taken. The first case of bifurcation of the clavicle was reported in 1921 in a 16-year-old boy [1]. Since then, five further cases of duplication (bifurcation) of the clavicle have been reported in the world literature. Bilateral clavicular anomalies had been reported in one case, where the lateral third of the right clavicle was duplicated and articulated with the coracoid process, and a spur-like projection was present in the left clavicle. Besides these anomalies, accessory bone elements were also present close to the right coracoclavicular joint, as well as between the spur-like projection and coracoid process on the left side [2]. In all of the reported cases, duplication involved the lateral part of the clavicle. The supernumerary clavicle ("Os Subclaviculare") was completely separated from the main clavicle in one case [3], and in two others the duplicated clavicle remained fused with the main clavicle on the medial aspect, giving rise to a fork-like bifurcation of the clavicle [1, 4]. In three previous cases [2, 5, 6], and in Case study 1 of this report, the two clavicles were partially united at the lateral ends like a half-headed arrow (

).

The supernumerary clavicle is always small, thin and situated below the lateral third of the main clavicle. Calcification of the coracoclavicular ligament is a relatively common condition that may mimic duplication of the clavicle and requires differentiation. Like any other bone, a supernumerary or accessory clavicle must have two cortices, bone trabeculae, a medullary cavity, and articulation. All these features were present in the patients discussed here.

Two theories have been proposed to explain the duplication of the clavicle: (a) displacement of one of the ossification centres in utero, and (b) presence of more than two ossification centres, where one of them is displaced to form a supernumerary clavicle [3]. Embryologically the clavicle develops from the clavicular blastema after about 5 postovulatory weeks. Very soon the clavicular blastema is mineralised and represents a membranous bone. Ossification of the clavicle begins in the mid part of the membranous bone from 5.5 postovulatory weeks onward by two separate centres, a medial and a lateral. The lateral centre is more advanced than the medial. The two centres unite to form a long S-shaped solid nontrabeculated clavicle [7]. Failure of any of these two centres to develop results in hypoplasia or aplasia of that segment of the clavicle. There is no mention of more than two ossification centres for the clavicle in the previous studies. If, hypothetically, a third ossification centre exists to form a supernumerary clavicle, then only the extra ossification centre will form an accessory clavicle and that should be half the length of the main clavicle. But this has not been ever reported. A supernumerary clavicle is always small and thin. The theory of displacement of an ossification centre also does not explain many points. If either of the two ossification centres is displaced, then that side of the main clavicle will be either absent or markedly hypoplastic. Thus, the supernumerary clavicle formed from the displaced accessory ossification centre would develop better on that side in comparison with the main clavicle.

After reviewing all of the cases reported in the literature, a 'fragmentation theory' of the ossification centre is proposed to explain the duplication of the clavicle. A small fragment probably separates out from the main ossification centre during early clavicular development and gives rise to a duplicate clavicle. A supernumerary clavicle thus formed will be small and thin. The time of fragmentation will determine the type of duplication. If the ossification centre divides completely in the later period of clavicular development, then it will have an appearance of a fork-like bifurcation of the clavicle. This is particularly so because the single centre had already formed some part of the shaft prior to splitting into two. The two cases of bifurcation of the clavicle reported by Rutherford [1] and Twigg and Rosenbaum [4] could be explained by this theory. If a small fragment of the ossification centre detaches completely from the main centre in the early period of development, then a small but completely separated supernumerary clavicle will be formed. The case presented by Goldthamer could be an example of this situation [3]. If, however, the small fragment of the ossification centre retains a little attachment to the major portion of the centre, then the supernumerary and the main clavicle will be fused at the lateral ends as shown in the cases of Reinhardt, Keats, and Schubert [2, 5, 6], as well as in the present Case study 1. The presence of small accessory processes in the mid main clavicles are peculiar abnormalities, which had articulated with the long curved posterior process of the coracoid in Case study 1 and with the coracoid process itself in Case study 2. Similar to Case study 2, Cockshott [8] had shown five examples of midclavicular accessory processes and coracoclavicular articulation. These coracoclavicular joints are believed to be rare among Caucasians and people of African descent, but occur more frequently in Asia, particularly in the Japanese population and those with Southern Chinese ancestry [8].

Duplication or triplication of the coracoid process has not been previously reported. Of interest with these peculiar anomalies of the coracoid and clavicle in Case study 1 is that all of the accessory elements found a counterpart to form a joint, suggesting that even extra bones are governed by the same regulatory factors of development as are normal bones to form articulations. Three earlier reports [2, 3, 8] and Case study 2 of this report, where the duplicated clavicles and the midclavicular processes articulated with the normal coracoid process, further support this observation.

The etiology of triplication of the coracoid process can also be explained by the same theory of fragmentation as proposed for the clavicle, i.e. the ossification centre for the coracoid process was single in the early period of development that formed a broad base, and soon the centre split in three parts. One fragment formed a normal coracoid process and the other two formed the accessory processes.

Unusual anomalies of the clavicle and coracoid process have been presented with a hypothesis that attempts to explain their developmental evolution.