Fractures of the clavicle are common, comprising up to 5% of all fractures in adults [1]. Most clavicle fractures are localized at the level of the mid-diaphyseal third [2]. Dislocation of the fracture elements in midshaft clavicle fractures (MSCF) occurs due to the actions of the sternocleidomastoid muscle, which displaces the medial fragment superiorly and posteriorly, and of the deltoid and great pectoral muscles, which shift the lateral fragment inferiorly and anteriorly. These shifts cause a malaligned fracture that may result in symptomatic malunion of the clavicle and increase the risk of a nonunion [3–6].

In the last decades, many studies have reported that a shortened clavicle can lead to worse functional outcomes, pain, loss of strength, rapid fatigue, hyperesthesia of the hand and arm, difficulty sleeping on the affected side, and esthetic complications [5–14]. Godfrey et al. [15] reported that the degree of symptomatology and occurrence of mal- and nonunion after MSCF is related to the extent of shortening and displacement of the fracture elements. Mean post-traumatic shortening of the fractured clavicle has been reported to be approximately 1.2 cm; however, shortening of up to 3 cm has been reported [16]. It has been described that there are poorer outcomes when shortening of the clavicle is more than 15–20 mm or 9.7–15% as compared to the original length [5, 7–14].

For this reason, lately, the tendency has been to surgically reduce and fixate MSCF if shortened more than 15–20 mm, or if displaced more than the diameter of the clavicle’s shaft. However, due to the unique shape of the clavicle, consisting of an S-shape in two planes, reliable and reproducible measurements of the displacement and shortening can be challenging.

Although there is a plethora of available modalities and techniques to measure shortening of the MSCF, it still remains unclear which method is most accurate, reproducible, and useful in daily practice.

Therefore, the objective of this systematic review was to evaluate the reliability and reproducibility of measurements of shortening in MSCF using any available imaging technique.

Electronic databases (PubMed, EMBASE, and Cochrane) were searched from their inception to November 2016. Keywords used to develop our search strategy were “clavicle,” “fractures,” “imaging,” “shortening,” “displacement,” and “reliability.” The detailed search strategy is described in Additional file 1. The inclusion criteria and method of analysis were specified in advance and documented in a protocol that was not registered in PROSPERO (Additional file 2).

Data extraction and quality assessment

An electronic data extraction form was created and used to record data. Data from all included studies were extracted with respect to specific characteristics, that is, number of clavicles reviewed, study design, imaging technique, method of measurements, statistical analysis, and the author’s conclusion. PH and GH extracted data independently. If disagreement persisted after discussion, consensus was met consulting AvK.

Methods and quality were independently assessed (PH and GH, any discrepancies were discussed to achieve consensus, using a third reviewer (AvK) for all included studies. The 4-point scale COSMIN checklist box B for assessment of reliability was used.

The “worst score counts” algorithm was used for the analysis [17]. Briefly, each item from COSMIN box B was rated individually as “excellent,” “good,” “fair,” or “poor,” and an overall score was given by taking the lowest score of any of the items.

The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, both the PRISMA flowchart and checklist, were followed during the preparation of this review (Fig. 1 and Additional file 3).

In this systematic review, we evaluated the reliability and reproducibility of measurements of shortening in MSCF. The results of this systematic review demonstrate that the literature on this topic did yield only three fair and one poor quality studies. Since shortening plays an increasingly important role in deciding on surgical intervention of MSCF, it is important to have a reliable and accurate method of measuring. Despite the lack of high-quality studies, the available knowledge and literature should not be discarded.

Smekal et al. [20] published a paper validating the accuracy/reliability of measurements of different imaging modalities and techniques. They found that the posterior-anterior (PA) thorax approximated the measurements on CT the best. Measurements on 15° tilted caudo-cranial radiograph of the clavicle and clinical measurements showed the smallest agreement with CT measurements. However, they did not state the reproducibility of measurements. The measurements were performed in healed malunited clavicle fractures and not in the acute phase. This was done to ensure static conditions in time. This is a strong feature of the study since Plocher et al. [22] described progressive shortening in acute MSCF in time.

The PA thorax means a higher dose for the patient of 0.1 mSv compared to 0.02 mSv for a clavicle AP [23]. It also relies on the symmetry of the clavicle using the unfractured side for comparison. A study by Cunningham et al. [24] reported asymmetry of the intact clavicle of more than 5 mm in almost 30% of patients. This may mean that measuring shortening of the MSCF compared to the unfractured side may be less reliable than assumed.

Archer et al. [21] also used the assumption of symmetry which may compromise reliability. They found a limit of agreement of 3.48 cm indicating that plain AP film of the fractured clavicle is not reliable in the prediction of the shortening measured on the CT scan. However, they found an ICC of 0.90. The statistical method for calculating intrarater variability using the paired t test may be debatable but they report no significant differences in measurements in five of six observers.

Jones et al. [18] reported weak to no agreement in inter- and intrarater agreement for radiological shortening using AP and 30° caudo-cranial views. They did not report a standardized method of measuring the shortening on these views. In addition, they also reported minimal to moderate interrater agreement for displacement and comminution. Intrarater agreement was strong for comminution, moderate for displacement, and minimal for shortening.

In contrast to current standard practice in which AP and 15° caudo-cranial views are made, papers have been published that support the use of a 15–30° cranio-caudal AP or PA or PA thorax view as being the most accurate in measuring the shortening of MSCF. [20, 25–27]. Although commenting on accuracy, these studies did not report the reproducibility of these views. Silva et al. [19] proposed a standardized mode of measuring shortening in MSCF. Their paper focused on adolescents, not adults, and also did not report the imaging modality or technique used. After contacting the corresponding author, it was verified that measurements were performed on standard AP and 15° caudo-cranial views. They reported no difference in a standardized measurement or method of choice concerning inter- and intraobserver variability. More recent studies find both a moderate and excellent interrater agreement using a standardized method of measuring [28, 29].

Two studies were not included in the review because these studies did not meet the inclusion criteria as only interrater agreement and not intrarater agreement was reported. However, we believe these studies are worth mentioning here. Stegeman et al. [29] found an intraclass correlation coefficient of 0.97 (CI 0.95–0.99) between two observers measuring shortening in a standardized way on 32 AP X-rays of the fractured clavicle. Interestingly, they found only a moderate agreement (0.45 CI 0.12–0.69) for measuring absolute shortening on the AP panoramic view after consolidation indicating that the imaging technique may be influential on the reliability of measurements as well. Malik et al. [28] report an ICC of 0.926 (CI 0.909–0.941) between four observers using a standardized method of measuring shortening of the fractured clavicle in 196 AP chest X-rays. These images were made with the patient varying between supine, semi-upright, and upright positions. The goal of this study was to evaluate differences in measured shortening between the different positions of the patients. No additional information on statistical analysis or interrater agreement per subgroup was reported.

Other factors reported to influence reliable and reproducible measurements are variation in magnification due to X-ray positioning and possibly positioning of the patient [18, 28, 30]. Backus et al. [30] reported a statistically significant difference between upright and supine patient positioning concerning shortening and displacement. Malik et al. [28] found a significant step-wise progression of measured shortening between supine, semi-upright, and upright positioning of the patient.

Some limitations of this study have to be discussed. First, there is only limited available literature on the topic of measuring the fractured clavicle. Since four studies were included and none of them were rated as good or excellent quality according to the COSMIN checklist, it was not possible to draw definite conclusions or make definite recommendations. Second, although the COSMIN checklist is considered the best available option to evaluate the methodological quality of studies on measurement properties, the “worst score counts” algorithm might underestimate the overall quality of a paper (e.g., one poor score out of a total of 11 items results in a poor overall score). For that reason, we provided the scores for all items using the 4-point scale. Other limitations of this study include the possibility of publication bias and language restrictions. Third, the inclusion criteria used might have been too strict. Two papers that did not meet the inclusion criteria were identified but yet could be of value on the topic. Including these papers [28, 29], however, does not influence the final conclusion pertaining the lack of evidence on the subject.

In order to optimize future studies and the realization of comparable results, a standardized method of imaging and measuring is of great importance. When considering the optimal method of imaging and measuring the fractured clavicle, one should consider the following: Imaging modality and technique, patient positioning, radiation exposure, costs and the method for measuring shortening, and/or displacement. To identify a standardized method, a compromise between these factors should be made based on further research.

CT scans and PA thorax seem more accurate, but the first is more expensive and both expose the patient to a much higher radiation dose. Supine positioning of the patient may underestimate the actual shortening and displacement, which in turn can negatively influence the decision to surgically reduce and fixate the MSCF. Calibrated views will prevent magnification errors while measuring. Although not proven better, it might be a consideration to optimize consistency by measuring shortening and displacement in a standardized and possibly proportional way as proposed by other authors. [9, 13, 19, 30, 31]

The objective of this systematic review was to evaluate the reliability and reproducibility of measurements of shortening in MSCF using any available imaging technique.

We identified four studies on reliability of measurement of MSCF. Since these studies were only of fair and poor quality, it was impossible to draw definite conclusions. Shortening is one of the reasons to surgically treat the fractured clavicle, so further research is needed to identify the most effective, reproducible, and reliable method of imaging and measuring. In order to optimize future studies and the realization of comparable results, a standardized method of imaging and measuring is of great importance.