Statistical shape analysis and principal component analysis of the clavicle
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"The chief use of the clavicle is to hold the shoulder blade at the proper distance from the breastbone, since motion of the shoulder would be hindered if the two came close together, as seen in four-footed animals that can use their forefeet only for walking, and not in the way that men need their hands."
- Therselben, 1790
Although anthropometric work on the clavicle by Broca dates as far back as 1869 according to Voisin1, the role of the clavicle has been described more than 200 years ago by Therselben. While one study stated that the clavicle is the most frequently fractured bone of the human skeleton2, data collected in Malmo, Sweden between 1952 and 1987 showed that fractures of the clavicle only accounted for 4% of all fractures, but that this represented 35% of all fractures in the shoulder region3. Other studies have shown similar results, estimating that clavicular fractures account for 5 to 10% of all fractures4,5, and 44% of all injuries to the shoulder girdle6.
According to Wheeless' Online Textbook of Orthopaedics, fractures to the middle third are the most common in adults and children, accounting for account for 80% while fractures to the lateral third are seen in 15% with the remaining 5% affecting the medial third. Numerous studies have verified this, statingthat 80% of clavicular fractures occur in the middle third, 10 to 18% in the lateral third and 2 to 10% in the medial third 7,8,9,10,11. This can be explained by a number of facts. Firstly, that the narrowest part of the clavicle is at the meeting point of the sternal convexity and the acromial concavity, which is the location of the majority of clavicle fractures2. Second of all, there is a significant decrease in bone density at the transition from middle to lateral thirds of the clavicle, explaining the increased frequency of fractures occurring in the middle and lateral thirds2. It has also been previously shown that the mid-portion of the clavicle is the thinnest and narrowest part of the bone and represents a transitional region of the bone, both in curvature and cross-sectional anatomy, making it a mechanically weak area that is most likely to fracture12.
In terms of location, the sternoclavicular and acromioclavicular joints hold the clavicle in its anatomical position with the latter having been described as a 'keystone' link between the scapula and clavicle13. Its integrity plays an important role in the movement of the shoulder girdle. The construct of the ACJ makes it a very strong joint able to tolerate a significant amount of force before disruption. This explains that in comparison to clavicular fractures, injuries to the ACJ account for approximately only 12% of those to the shoulder girdle seen in clinical practice14. Much higher incidences are seen in contact sports. In rugby, ACJ injuries have been shown to account for 32% of shoulder injuries15 and in American football they are the most common injury to the shoulder16. Even in non-contact sports such as recreational skiing, about 20% of injuries to the shoulder girdle involve the ACJ17. With the majority of injuries to the ACJ seen in young males, the same applies to fractures of the clavicle18.
The main aim in treating fractures of the clavicle is to provide comfort and pain relief. Management options for both ACJ injuries as well as fractures of the clavicle vary widely, spanning from conservative treatment to fixation requiring surgical intervention with the latter including a variety of options from minimally invasive techniques19 to the application of a fixation plate. Whenever a fixation implant is to be selected, several factors drive the decision. The fit on bone of any fixation plate has a direct impact on the strength of the construct. However, current clavicular implants overlook the variations in geometry of the bone. In 2007, a paper describing the anatomy of the clavicle stated that gender specific anatomical features should be taken into consideration when performing intramedullary fixation of the clavicle2. While the length, diameters and curvature of the clavicles measured were based on 196 embalmed specimens, cortical bone thickness and the medullary canal diameter were determined using only 10 fresh specimens. In a more recent article, it has been reported that cortical bone thickness and bone shape of the clavicle have large effects on bone responses until failure and on fracture location20. This study emphasized the need for geometrical personalization of clavicle models in order to consider various age, gender and shape discrepancies. However, the radiological aspect of this study was based only on six clavicles with the biomechanical testing based on 18 clavicles from nine subjects with a mean age of 78. From a clinical standpoint, a larger number and much younger population should be studied, considering that fractures to the clavicle are most commonly seen in the twenties although comminuted fractures may be more common in the third and forth decades.
The relationship between clavicular length restoration post-fracture and functional outcome remains controversial. Some studies have concluded no observable reduction in shoulder function 21,22,23, while others have stressed the importance of restoration of clavicular length post-fracture 24,25. Previous literature has also stated that up to 30% of patients treated non-operatively develop unsatisfactory outcomes clinically, radiologically, and subjectively because of bony shortening, malunion, poor alignment or deforrnity 24,25,26,27. In another study of outcome after closed treatment of the fractured clavicle, the majority of patients were not satisfied with the cosmetic deformity and 40% complained of impaired function28. Although most fractures of the clavicle have a good prognosis and can be managed conservatively, the role of surgical intervention and its importance must not be forgotten.
In the literature, some studies focused on clavicle tests 20,29,30,31, while others focused on geometry 2,12,20,32. However, these studies had very small sample sizes as previously stated. As plate and intramedullary fixation are accepted and widely used methods of treatment, a study analyzing the shape and looking into the necessity for modifying modern implants in order to optimize fixation based on personalized bony geometry has much clinical relevance with regard to the management of clavicular fractures. One recent study set out to characterize variations in clavicular anatomy and determine the clinical applicability of an anatomic precontoured clavicular plate designed for fracture fixation33. However, the authors themselves stated that their main limitation was that it was a two-dimensional analysis of plate fit rather than a three-dimensional analysis. By noting the shortcomings of the abovementioned studies, this thesis focuses on the anatomy of the clavicle and the analysis and application of it to the design of currently available clavicle fixation plates in both a systematic and structured manner.