Fig. 2.1
Left clavicle, boney anatomy, and muscle attachments viewed from superiorly and inferiorly
The larger medial curvature allows more space for passage of neurovascular structures from the neck into the upper extremity through the costoclavicular interval. The transition from medial to lateral curvature occurs at approximately two-thirds the length of the bone as measured from its sternal end, a site that roughly corresponds to both the medial limit of attachment of the coracoclavicular ligaments and the entrance point of the main nutrient artery of the clavicle.
Males have significantly greater medial curvatures than females, most likely due to their greater size. Similarly, left-sided clavicles tend to be longer and have a greater medial curvature. On the other hand, it appears that the lateral curvature of the clavicle is not affected by either gender or anatomical side. Longer clavicles do not necessarily confer larger lateral curvatures (Table 2.1). The curvature of the IM canal represents the curvature an IM device should be shaped to in order to fit inside properly [5].
Table 2.1
Radius of curvature and standard deviation of the clavicle when grouped for gender and anatomical side from a 104 sample size clavicle morphometry study
Radius of curvature (mm) ± SD | ||||
Females—Right | Females—Left | Females | Overall | |
Medial | 83.51 ± 11.82 | 91.13 ± 15.90 | 87.54 ± 14.51 | 91.21 ± 14.4 |
Lateral | 31.86 ± 12.08 | 34.68 ± 12.57 | 33.35 ± 12.31 | 32.51 ± 11.1 |
Males—Right | Males—Left | Males | Overall | |
Medial | 93.63 ± 15.03 | 96.65 ± 11.06 | 94.99 ± 13.34 | 91.21 ± 14.4 |
Lateral | 31.51 ± 11.23 | 31.90 ± 7.77 | 31.69 ± 9.73 | 32.51 ± 11.1 |
The clavicle is made up of dense trabecular bone lacking a well-defined medullary canal. In cross-section, the clavicle changes gradually between an expanded prismatic medial end, a tubular midportion, and a flat lateral aspect. In addition to the variation in cross-sectional shape, the diameter of the clavicle and IM canal experience significant change in size. A recent clavicle morphometry study has found that the medial and lateral ends of the bone are the widest regions with an average diameter of 28 mm, while the midportion is significantly narrower, at 13.7 mm. The IM canal also shares this ‘hourglass’ shape, with a smallest diameter of 4.5 mm; however, it is important to note that the location of the narrowest diameter for both clavicle and IM canal occurs at different locations along the midportion (5% clavicle length difference). This offset was observed across all groups (male/female, right/left) and suggests that one may not estimate the location of narrowest region of the IM canal based on external visualization of the clavicle alone (Fig. 2.2). The size and location of the smallest diameter of the IM canal is of special interest as it is the limiting region for IM device design and must be understood for proper implant fit.
Fig. 2.2
Average clavicle and IM canal diameter as a function of percent clavicle length starting from the medial end from a 104 sample size clavicle morphometry study
Furthermore, clavicle and IM canal are not perfectly congruent. There exists an eccentricity of the IM canal center with respect to the clavicle because cortical thickness and shape are not consistent throughout any given cross-section. This finding also allows us to understand proper IM device fitting inside the canal, as the eccentricity tells us how close the device is to the surface of the bone and how thin the cortex is at any point around it [5] (Fig. 2.3).
Fig. 2.3
Clavicle cross-section illustrating IM canal eccentricity . The purple circle is the largest circular space that could potentially fit an IM device
The above-mentioned peculiarities of the clavicle, namely its curvature, narrowing offset between clavicle and IM canal, and IM canal eccentricity become important when intramedullary fixation of the clavicle is being considered. Since a 3-D morphometric analysis is a requisite for noting these features, a pre-operative CT scan and canal analysis would be warranted in pre-operative planning and implant selection for fracture treatment with an IM device.
As viewed in Fig. 2.1, several muscles attach to the clavicle. The muscles that contribute to shoulder motion are the deltoid, trapezius, and pectoralis. The subclavius muscle serves mainly as a protective layer between the clavicle and the neurovascular structures.
In view of the intimate relationship of the clavicle to the brachial plexus, the subclavian artery and vein, and the apex of the lung, it is surprising that injury to these structures in association with fracture of the clavicle is so uncommon. Brachial plexus palsy may develop weeks or years after injury as a result of compromise of the costoclavicular space by hypertrophic callus, with or without malalignment of the fracture fragments. Narrowing of the costoclavicular space because of malunion or nonunion can also lead to dynamic narrowing of the thoracic outlet. Prolonged compression of vascular structures can likewise be problematic. The course of the neurovascular structures makes it such that medially it is safer to drill superior to inferior and laterally anterior to posterior (Fig. 2.4) [6].
Fig. 2.4
Relationship of neurovascular structures to clavicle and safe drilling trajectories. From Sinha et al.
The scapula is a broad thin bone that serves as the origin or insertion for at least 18 different muscles. It also supports the upper extremity through its articulation with the humerus at the glenoid. The glenoid is a narrow, shallow concavity that relies on the surrounding labrum, ligaments, and muscle-tendon units for stability. The acromial and coracoid processes have enlarged during the evolutionary process, presumably to provide an origin for more powerful and mechanically efficient upper extremity musculature as well as to give the inherently unstable glenohumeral joint a measure of stability.
Clavicle Contribution to Motion of the Shoulder Girdle
The clavicle articulates on the medial end with the sternum and laterally with the acromion. The sternoclavicular (SC) and acromioclavicular (AC) joints are both diarthrodial articulations (hyaline articular cartilage-covered and synovium-lined mobile joints) with intervening fibrocartilage disks. Both joints lack inherent osseous articular stability and are maintained by strong ligaments. The acromioclavicular joint is unusual in that negligible motion occurs through the joint and yet degenerative arthritis is common, particularly after trauma. The function of the acromioclavicular articulation is unclear. Patients with acromioclavicular or coracoclavicular fusion or coracoclavicular screw fixation have full or nearly full shoulder motion. Most of motion between the shoulder girdle and the axial skeleton must therefore occur at the (SC) joint; a recent study in healthy asymptomatic volunteers documents some motion in the AC joint (19° of scapular tilting maximally) [7]. Motion at the SC joint is commonly thought to be 35° of elevation-depression, 35° of protraction-retraction, and 50° of rotation, as reported by Inman [8].
Ligament-cutting studies in cadavers suggest that the coracoclavicular ligaments limit superior displacement of the clavicle and the acromioclavicular ligaments limit posterior displacement [9]. The capsuloligamentous covering of the acromioclavicular ligament is most stout superiorly. Because one of the complications of distal clavicular excision is posterior displacement of the clavicle with impingement on the scapular spine, many surgeons emphasize preservation of the acromioclavicular ligaments, particularly superiorly.
Forces Seen by the Clavicle and Relation to Fracture
Clavicle fractures are common injuries (2.6–5% of all fractures), with 80% of the fractures occurring in the diaphysis [10]. The chapter will concentrate on the mechanics pertinent to mid-shaft injuries as they are the most common. It is not surprising that the middle third is the most common site of clavicular fracture, since the midportion is the thinnest and narrowest portion of the bone; it represents a transitional region of the bone, both in curvature and in cross-sectional anatomy, which makes it a mechanically weak area, and it is the only area of the clavicle that is not supported by ligamentous or muscular attachments. It is possible that this anatomy was selected during evolution because clavicular fracture protects the brachial plexus during difficult births (shoulder dystocia).
Mechanics of Clavicle Malunion
Clavicular malunion (Fig. 2.5) [11] is emerging as a clinical syndrome identified by shortening, deformity of the shoulder girdle (including the scapula), pain, and fatigue with overhead activity and weakness in strength testing [3, 11, 12]. The forces contributing to persistence or worsening of deformity after fracture include the weight of the shoulder as transmitted to the distal fragment of the clavicle, primarily through the coracoclavicular ligaments, and the deforming forces of the attached muscles and ligaments. The medial fragment is elevated by the clavicular head of the sternocleidomastoid muscle, which inserts onto the posterior aspect of the medial portion of the clavicle. The pectoralis major contributes to adduction and inward rotation of the shoulder.
Fig. 2.5
Anterior (a) and posterior (b) view of clavicle malunion. Shortening, drooping of the shoulder girdle and scapular winging of inferior-medial border of clavicle is evident. From Ristevski et al.
A study simulating the effect of clavicle shortening on upper extremity muscles found that shortening of the clavicle decreases the moment-generating capacity as well as the total force-generating capacity of the shoulder girdle muscles, especially elevation moments of the upper extremity muscles during abduction and internal rotation [13]. Flexion moments were affected less through physiologic range of motion. Additionally, shortening of the clavicle increases coronal angulation of the clavicle at the sternoclavicular joint thus providing a basic science support to the clinical syndrome of malunion.
A cadaveric study found that shortening of the clavicle produces change to the resting position of the shoulder girdle quite similar to the clinically observed malunions. Moreover, the motion of the shoulder girdle is progressively affected with increasing amounts of shortening [14].
Complications from Clavicle Fractures, Nonoperative Treatment
Traditionally, treatment has been conservative, due to influential historic literature with high rates of healing [15, 16]. More recent literature suggests that the results of nonoperative treatment are worse than what was previously thought. Inferior outcomes and higher risks of nonunion are documented when there is significant displacement (i.e., >2 cm or no cortical contact) and other risk factors such as comminution, z deformity, increasing age, female sex, and smoking [2, 3, 12, 16–18].
In a systematic review of 2144 clavicle fractures, a nonunion rate of 15% with nonoperative management was found [19]. Displacement and comminution were the primary risk factors contributing to nonunion. Operative treatment resulted in a relative risk reduction of 86% for nonunion. It is possible that a displacement of 2 cm noted on X-ray may be underestimating fracture severity. Because of the clavicle’s signature S-shape, the dimensions of the bone are often misrepresented on two-dimensional film. A recent three-dimensional clavicle morphometry study investigated the difference between total clavicle length and true clavicle length [5]. Total clavicle length is a straight end-to-end measurement in the frontal plane, while true length is the length of the bone if it were straight (Fig. 2.6). The study found an average of 1.5 cm difference between these two types of measurements. True length of the clavicle is a superior estimation of the bone’s length as it takes into account the curvatures in 3-D space, and suggests a longer region of bone overlap not appreciable on X-ray.
