The “coracoid tunnel view”: a simulation study for finding the optimal screw trajectory in coracoid base fracture fixation

Scapula fractures occur at a rate of 0.7% of all fractures, with an approximate of 3–5% accounted for fractures of the shoulder girdle [5, 6, 17]. It is estimated that fractures of the coracoid process represent 3–13% of all scapular fractures [12]. Fractures of the coracoid are often categorized into two sub-types according to the Ogawa classification, based on their location relative to the coracoclavicular ligament attachment [14]. Type I fractures are located proximal to the coracoclavicular ligament attachment, and Type II fractures distal to the coracoclavicular ligament attachment [14]. Good results are reported by managing Type I fractures with open reduction and internal fixation [1, 8, 13, 14]. However, because of the difficulty in radiographic visualization of the coracoid anatomy [10], placement of a screw down the coracoid pillar through its base into the scapular neck is challenging even for the experienced surgeon. Spatial awareness of the scapula and its surrounding structures is crucial. Misjudgment in drilling angle, position or depth may result in intra-articular, hardware, and/or neurovascular damage [8, 11, 21]. During surgery, fluoroscopy is used as guidance for screw fixation of a Type I coracoid fracture. Although fluoroscopy in anterior–posterior view, axillary scapular view and lateral scapular view may be useful in guiding the surgeon, it does not render the desired geographical insight due to the scapula’s complex propeller blade-like shape.

Previous studies attempted to increase anatomical insight of the coracoid by analyzing the morphometric data [4, 7, 16]. Another study developed a new radiographic technique to visualize the individual pillars of the coracoid by standardizing the standing position of the patient and the radiographic beam angulation [2]. However, these reported morphometric data of the coracoid might not be applicable to every patient undergoing surgery as there was substantial heterogeneity regarding the way of measurement. In addition, the developed radiographic techniques might be susceptible to imprecise imaging depending on the position of the patient on the operating table. Therefore, a new radiographic approach solely based on anatomical landmarks (i.e., without considering the morphometric data of the coracoid as guidance) might be more practical and universally applicable.

The primary aim of this study is to establish a new fluoroscopic view based on anatomical landmarks that facilitate the surgeon in screw fixation of Type I coracoid fractures. And the secondary aim of this study is to validate the view by evaluating the accuracy and the inter-rater reproducibility of using or not using the configuration of anatomical landmarks of the optimal view for coracoid screw fixation. We hypothesize that this new configuration of landmarks is a useful addendum for the orthopaedic surgeon in fixation of coracoid base fractures.

In the current study, we presented a protocol for fixation of coracoid base fractures using a newly developed fluoroscopic view: the CTV. We determined four radiographic landmarks to easily replicate this view: the coracoid, glenoid fossa, scapular notch and superior scapular border. The proposed imaging protocol increased the rate of correctly placed virtual screws according to our predefined criteria from 59% in the “before CTV instruction” session to 89% in the “after CTV instruction” session. Moreover, six out of eight surgeons improved their screw placement skills after making use of the CTV. One of these two surgeons had a perfect score in both sessions; therefore, an improvement was not observed.

During the development of the CTV imaging protocol, we noticed that all coracoid tunnels were similar, although, unique for each individual scapula. We, therefore, chose to standardize the drilling angle in our predefined criteria by following the surgical strategy of Ogawa and colleagues, which is drilling parallel to the glenoid fossa, for the reduction and stabilization of the coracoid base fracture [13]. More importantly, by drilling parallel to the glenoid fossa, the danger zone (topographic distribution of suprascapular nerve and the ascending part of the circumflex scapular artery) as described by Wijdicks and colleagues is most likely avoided, consequently reducing neurovascular risks [21].

Several studies analyzed the morphometry of the coracoid process to determine ideal internal screw fixation of the coracoid [4, 9, 19]. However, during an intra-operative intervention, the surgeon relies primarily on fluoroscopy to correctly place a screw. For this last step, we designed and investigated a useful method in addition to this work.

Few articles address the surgical treatment of coracoid fractures specifically. Bhatia and colleagues gave a detailed account on fixating coracoid fractures percutaneously using two clearly defined fluoroscopic views as a guide [2]. Ogawa and colleagues described an open surgical technique where the direction of the Kirschner wire is placed under the guidance of fluoroscopic images, and the position of the Kirschner wire was confirmed by tactual exploration with the fingertip at the coracoid base and penetrating sensations of the drill [13]. In a case report by Kawasaki and colleagues, a technique was described for reducing coracoid base fractures using only a fluoroscopic view in the axial plane of the coracoid base [9]. Their finding of tilting the C-arm until an axial circle of the coracoid base is visible is somewhat in agreement with the results of our study and support the idea of a CTV; which is, the oval-shaped structure of the coracoid base is only visible in a certain fluoroscopic viewing angle.

The simulation design of our study prevents unnecessary radiation exposure to patients and surgeons. However, our experimental study may lead to an underestimation of the difficulty in reproducing the CTV in an actual intra-operative setting. The use of macerated scapulae was very helpful in defining the optimal CTV; however, the CTV in patients will show over-projection of surrounding bones, which may render finding the CTV more difficult in patients than in our study [2, 3, 9, 13]. Finally, the virtual procedures were only performed in intact scapulae. Thus, the surgeons were placing virtual screws in perfect samples. In spite of these limitations, we believe that the presented protocol can be of aid to the surgeon, because only in case of correct alignment of the anatomical landmarks (i.e., coracoid tip, glenoid fossa, scapular notch and superior border), the coracoid tunnel is visible and correct coracoid base fixation is possible (Fig. 6).

In conclusion, our study demonstrates that the CTV is a useful radiographic technique for the orthopaedic surgeon to fixate fractures of the coracoid base. The anatomical landmarks are comprehensive and easy to find. In addition, it showed an improvement in correct (virtual) placement of screws according to our predefined criteria. Future clinical studies should validate the protocol and also determine the accuracy of the CTV in a clinical setting.