Acromioclavicular (AC) joint dislocations are common shoulder injuries.1,2 They account for approximately 12% of dislocations involving the shoulder.3 AC dislocations are diagnosed radiographically by the widening of the AC joint and vertical displacement of the lower border of the clavicle in relation to the lower acromion border.4 The normal AC joint is classically described as being no greater than 4 mm in width.5 However, different radiographic projections may demonstrate various AC joint spaces and lead to faulty interpretations.6 In addition, the anteroposterior views have difficulty in demonstrating the distal clavicle and acromion because the distal clavicle and acromion are superimposed on the spine of the scapula.7 Bilateral anteroposterior stress radiographs of the shoulders are reported as limited usefulness, painful, and not recommended in acute injury.8,9
The AC joint dislocations can be repaired with Kirschner wires (K-wire), Steinmann pins, screws, plates, or coracoclavicular ligament reconstruction.10-15 Central fixation of AC separation with K-wire is a commonly used method because of its simplicity.16 It can enable good results in the shoulder function, especially in the treatment of acute Rockwood grade III AC joint separations.17,18 But percutaneous fixation of the AC joint is technically demanding because of the anatomical relationship between the thin acromion and the curved distal clavicle.11 An easily identified and applied radiographic projection can provide clear demonstration of the AC joint and reduce the difficulty frequently encountered in the percutaneous transfixation of AC dislocations. The C-arm fluoroscopy is used as a routine intra-operative guidance to facilitate this procedure. The current study aims to introduce the new projections, the axial and tangential views of the AC joint, to help evaluate the severity of the injury and serve as intraoperative fluoroscopic guidance to facilitate percutaneous fixation of the AC dislocations with K-wires.
Radiographic anatomical correlations of the AC joint were studied in three fresh-frozen human cadaveric shoulders without evidence of fracture, tumor, or deformity. These shoulder specimens were obtained from one male and two female donors, one from each donor. The average age of the donors was 60 years (range, 53–69 years). Muscular and tendinous soft tissues were removed thoroughly to clearly and directly expose the AC joint.
The study subjects consisted of 20 adult Chinese volunteers (12 male and 8 female) having a mean age of 28 years (range, 23–46 years), mean height of 170 cm (range, 159–180 cm), and mean weight of 64.8 kg (range, 44–87 kg). Potential participants were excluded if they had history of fracture, tumor, deformity, or trauma in the shoulders. All the participants had given informed consents. The lead garment was used to protect the volunteers from unnecessary radiation during the study. All imaging processes were performed by one single technician.
The instruments included the digital radiograph (DR) system, SIEMENS AXIOM Aristos FX Plus (Munchen, Germany), the PACS workstation (Jinan, China), the C-arm fluoroscopy unit (Ziehm Exposcope, Nuremberg, Germany), and the measurement software (MB ruler, Markus Bader, Germany).
The shoulder specimens were used to identify the projection directions of the axial and tangential radiographs of the AC joints. The three specimens were positioned by hand. The DR unit was positioned for the lateral view of the shoulder specimen. The shoulder was rotated externally and the lateral side was tilted inferiorly. We observed the images of the AC joint while manually adjusting the specimen's position. When the clearest irregular quadrilateral track image appeared, the axial view of the AC joint was obtained (identified) and the radiographic features were recorded. A K-wire was positioned in the direction of the radiographic beam in the axial view, with the shoulder specimen kept still. The axial view of AC joint clearly showed the center of the quadrilateral track, the starting point for the K-wire insertion. Adjust the K-wire position to obtain an optimal intra-osseous trajectory into the quadrilateral track (Figure 1A). When the projection of the K-wire appeared as a dot inside of the quadrilateral tract, it was advanced into the AC joint. To get the tangential view of the AC joint, the shoulder specimen was then changed to the position where the K-wire was perpendicular to the radiographic beam (Figure 1B). During this procedure of taking axial and tangential views of the AC joint of the specimens, the DR tube should tilt caudally if the lareral side of shoulder does not tilt inferiorly. The DR tube should also be tilted caudally to obtain clear views in the volunteers since they stood upright.
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Figure 1. Identify the projection directions of the axial and tangential radiographs of the AC joints of shoulder specimens. 1A: In the ideal axial view of the AC joint, the Kirschenner wire appears as a round point in the quadrilateral track. 1B: In the tangential view of AC joint, the Kirschnner wire locates in the cavity of the distal clavicle and the acromion
Figure 2. When taking the axial radiograph (2A) and tangential radiograph (2B), the angles between the coronal plane of the body and the perpendicular plane of FPD are measured using the MB Ruler. (The horizontal green lines run parellel to the FPD and the red lines represent the coronal plan of the body. The angles appear automatically.)
Figure 3. Identify the quadrilateral track and the gap of the AC joint of volunteer. 3A: The axial radiograph of the AC joint; 3B: the tangential radiograph.
Routine anteroposterior views of both AC joints were obtained with the volunteers standing or sitting, with the back against the FPD and the arms hanging unsupported at the side.7 The minimal distances from the distal clavicle to acromion were measured on both anteroposterior and tangential radiographs on the PACS workstation, respectively.
The C-arm fluoroscopy was applied to obtain the axial and tangential views of the AC joints as the validation of this technique in clinical application. The volunteer lay supine on the operation table. The C-arm fluoroscopic unit was positioned for the lateral view of the shoulders with the X-ray image intensifier close to the target shoulder. The X-ray tube was moved to Angle A along the tract of the C-arm. To get the axial view of the AC joint, the C-arm was rotated to Angle B along the axis of the horizontal support arm. To get the tangential view of the AC joint, the C-arm was rotated to Angle C along the axis of the horizontal support arm. The axial and tangential views were observed. If the axial or tangential views were not satisfied, adjust the C-arm unit from the recommended angles to obtain the optimal views. The new angles were recorded and labeled as adjusted Angles A, B, and C, respectively.
Statistical analyses were performed using SPSS 13.0 for Windows (SPSS, Chicago, IL, USA). Values were expressed as mean±SD. Comparisons of Angles A, B, and C and the minimal distances of the AC gap between the left and right AC joints were made by paired Student's t test respectively. Comparisons of Angles A, B, and C between male and female volunteers were made by Student's t test respectively. Comparisons of Angles A, B, and C and the adjusted Angles A, B, and C were made by paired Student's t test respectively. Differences were regarded as statistically significant when P values were less than 0.05.
The axial and tangential views of the AC joints of the shoulder specimens were obtained. The anterior and posterior border of the quadrilateral track of the axial radiographs was the projections of the anterior and posterior cortex of the distal clavicle. The superior and inferior borders were overlapped projections of the superior and inferior cortex of the distal clavicle and acromion. On the axial views, the K-wire was inserted into the AC joint and appeared as a dot. If the specimen position deviated from the ideal position, the projection of the K-wire presented as a nearly round or oval point within the quadrilateral track with the projection of the superior cortex of distal clavicle partially superimposed on the upper edge of the acromion. The tangential radiographs provided clear views on the gap of AC joint, the cavity of the acromion, and the distal clavicle.
For each volunteer, the clearest quadrilateral track and the gap of the AC joint were successfully identified on the axial and tangential radiographs (Figure 3). The Angles A, B, and C were recorded (Table 1). Statistical analyses showed no significant differences in the Angles A, B, and C between left and right AC joints (P >0.05). The Angles A, B, and C obtained from the male were greater than the corresponding angles for the female (Table 2) (P <0.05). The Angles A, B, and C for male were (20.8±2.4)°, (23.3±3.2)°, and (117.5±3.7)° respectively. The corresponding angles for female were (18.3±2.3)°, (20.1±2.4)°, and (113.1±3.3)° respectively. On both tangential and anteroposterior views, there was no significant difference in the minimal distances of AC joint space between left and right joints (P >0.05). On the tangential views, the minimal distance was (6.1±1.2) mm, wider than the same measurement on the anteroposterior radiographs, which was (4.3±1.1) mm (P <0.05).
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Table 1. The angular measurement data of the left AC joint of the volunteers (°)
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Table 2. Comparison of the angles A, B, and C between male and female volunteers (°)
Palpation of the AC joint was applied to determine the direction of the radiographic beam during taking the axial and tangential views. Among the first six volunteers, 8–13 shots were taken to obtain an optimal axial view and 4–7 for a tangential view. Among the last six volunteers, only 2–5 shots were taken to get an axial view and 2–4 for a tangential view.
The clear axial and tangential views of the AC joints were obtained with the C-arm fluoroscopy unit. One to three shots of fluoroscopy were taken to get an axial view and one to two shots of fluoroscopy for a tangential view. Angle A was adjusted in one patient and Angles B and C were adjusted in four patients (Table 3). There was no statistical difference between the Angles A, B, and C and the corresponding adjusted Angles A, B, and C (P >0.05).
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Table 3. The angular measurement data of the AC joint of the volunteers (°)
In the present study, the axial and tangential radiographs of the AC joint of the shoulder specimens were successfully obtained. On the axial views, the AC joint appeared as a clear, irregular quadrilateral tract. On the tangential views, the AC joint gap and the cavity of the distal clavicle and the acromion were demonstrated to the utmost. The axial and tangential views of the AC joints were taken on the volunteers using the same method applied in the shoulder specimen, and the related angles were recorded and measured. Palpation of the AC joint facilitated a quick determination of the direction of the radiographic beam, therefore made the axial and tangential views an easy task to take. Applying the same angles obtained with the DR system, C-arm fluoroscopy unit can also produce the similar axial and tangential radiographs.
The shoulder radiographic evaluation can be completed by four positions: standard anteroposterior view, axillary axial view, profile of the scapula and anteroposterior view in arm abduction.19 Other recommended radiographs include the Zanca, lateral, and trauma views. An anteroposterior view with 10° to 15° of cephalic tilt of the beam allows better visualization of the distal clavicle in cases suspicious for clavicle injury.20 In the study, the axial and tangential views of the AC joint were obtained with caudal tilt of the radiographic beam and with the body rotating externally. The two views are different from the projections in the literatures. To the best of our knowledge, we are the first to reveal the radiographic features of the axial and tangential views.
The axial views can demonstrate the overlapped projection of the distal clavicle and the acromion. It can identify the perpendicular migration of the distal clavicle as the anteroposterior view, and the anterior or posterior displacement as the axillary view. The radiograph can be used to better demonstrate the anterior dislocation of the AC joint combined with or without superior or inferior displacement of the distal clavicle.21-24 The anterior AC dislocation is a rare lesion, which is almost indiscernible in X-rays taken with conventional projections.23 Nieminen et al23 confirmed such a lesion with oblique craniocaudal radiographs. The axial radiograph can demonstrate the AC joint as clear as, if not better than, the oblique craniocaudal radiographs.
The tangential views can clearly reveal the AC joint gap and the cavity of the acromion and distal clavicle. The AC joint is wider in male than in female25 but on average is no wider than 4 mm in the anteroposterior projection.5 Any joint wider than 6 mm is considered as abnormal.25 Shin et al11 reported that the preoperative horizontal distance between the distal clavicle and acromion averaged 3.1 mm in uninjured joints and 7.1 mm in dislocated joints. In our study, the average minimal distance between the distal clavicle and the acromion is 4.3 mm (range, 1.7–6.3 mm) on the anteroposterior views, while the distance is 6.1 mm (range, 3.9–8.3 mm) on the tangential views. Compared to the anteroposterior radiographs, the tangential radiographs can demonstrate the profile of the AC joint better and avoid the affection of the projection of the radiographic beam in demonstrating the width of the AC space.
There are over 100 techniques to manage AC joint dislocations including transarticular AC techniques, coracoclavicular fixation techniques, and ligament or muscle transfer procedures. Pinning across joints is a commonly used one, which is economic-friendly. For example, one of the most frequently performed surgical techniques to treat acute Rockwood grade III AC joint injuries is temporary articular transfixation of the AC joint with K-wires.26-32 Although some drawbacks regarding the technique have been mentioned in the literature such as migration, infection, and poor biomechanical quality, this technique enables good results in shoulder function and the functional outcome parameters reveal a high consistency in the long run over the years.18 However, this technique is a technically difficult endeavor because of the anatomical relationship between the thin acromion and the curved distal clavicle11 as well as the limitations of fluoroscopic visualization of radiographic landmarks. To enhance the safety of K-wire fixation, the orthopaedic surgeons have to stand much radiation exposure through the procedure and spend more time for the changing in different views. The axial and tangential radiographs of the AC joint can be used to guide the articular transfixation using K-wire or Steinmann pins. This technique can overcome the above-mentioned difficulties. The two views offer several potential advantages over routine imaging methods. These include rapid acquisition of axial view that demonstrates the path of K-wires. During operation, the K-wire is positioned along the radiographic beam of the axial view after satisfied reduction of the AC dislocation is achieved. The orientation of the K-wire is adjusted to allowing an optimal intra-osseous trajectory into the quadrilateral track. The K-wire is then inserted across the AC joint into the lateral clavicle. The tangential radiographs can show clearly whether the pin was in the cavity of the distal clavicle and the acromion. The length of the acromion and the distal clavicle on the tangential radiographs are a reliable reference for the selection of K-wire, guide pin or screws with appropriate size. Furthermore, once the axial and tangential views have been obtained, there is no need to change the C-arm angularly to get other projecting views. The two views in combination allow the surgeon to operate while receiving immediate or nearly immediate visual feedback regarding the path of the implants. Consequently, this will minimize the risk of potentially dangerous radiation exposure for the patient and surgeons. The operation time will also be shortened. With the axial and tangential views, the procedure only takes a few minutes. The two radiographs can be also applied in the assessment of the reduction and fixation of the AC joint.
These results hint at a potential application of useful radiographic examination for AC joint injuries; however, our study is limited by the small sample size. The limited data obtained in our study can only reflect the radiological features of the AC joint of a small portion of general population, accordingly cannot be generalized to the public as a whole. Future studies with a large number of randomly selected participants are needed to provide more precise information of the AC joints, which will make the result more reliable in referring to the general population.
Acknowledgment: The assistance of Dr. PENG A-qin, Dr. ZHAO Chang-ping, Dr. WANG Peng-cheng, Dr. WU Xi-rui, and Dr. ZHANG Guo-chuan is acknowledged.
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