New quantitative radiographic parameters for vertical and horizontal instability in acromioclavicular joint dislocations

Abstract

Purpose: The aim of this study was to identify the most accurate and reliable quantitative radiographic parameters for assessing vertical and horizontal instability in different Rockwood grades of acromioclavicular joint (ACJ) separations. Furthermore, the effect of projectional variation on these parameters was investigated in obtaining lateral Alexander view radiographs.

Methods: A Sawbone model of a scapula with clavicle was mounted on a holding device, and acromioclavicular dislocations as per the Rockwood classification system were simulated with the addition of horizontal posterior displacement. Projectional variations for each injury type were performed by tilting/rotating the Sawbone construct in the coronal, sagittal or axial plane. Radiographic imaging in the form of an anterior-posterior Zanca view and a lateral Alexander view were taken for each injury type and each projectional variation. Five newly defined radiographic parameters for assessing horizontal and vertical displacement as well as commonly used coracoclavicular distance view were measured. Reliability, validity and the effect of projectional variation were investigated for these radiographic measurements.

Results: All radiographic parameters showed excellent intra- and interobserver reliability. The validity was excellent for the acromial centre line to dorsal clavicle (AC-DC) in vertical displacement and for the glenoid centre line to posterior clavicle (GC-PC) in horizontal displacement, whilst the remaining measurements showed moderate validity. For AC-DC and GC-PC, convergent validity expressed strong correlation to the effective distance and discriminant validity demonstrated its ability to differentiate between various grades of ACJ dislocations. The effect of projectional variation increased with the degree of deviation and was maximal (3 mm) for AC-DC in 20° anteverted malpositioning and for GC-PC in 20° retroverted malpositioning.

Conclusions: AC-DC and the GC-PC are two novel quantitative radiographic parameters of vertical and horizontal instability in ACJ dislocations that demonstrate excellent reliability and validity with reasonable inertness to malpositioning. The use of AC-DC for assessing vertical displacement and GC-PC for assessing horizontal displacement in a single Alexander view is recommended to guide the appropriate management of ACJ dislocations. A better appreciation of the degree of horizontal instability, especially in lower Rockwood grades (II, III) of ACJ dislocations, may improve management of these controversial injuries.

Keywords: AC joint; AC joint separation; AC–DC; Acromioclavicular joint; Dislocation; GC–PC; Horizontal instability; Instability; Intra- and interobserver reliability; Radiographic parameters; Rockwood classification; Validity; Vertical instability.

Fig. 1

Sawbone modelling. Specially designed holding device where a Sawbone Model was mounted. This model allows to simulate different ACJ dislocation as well as navigation in axial, sagittal and coronal direction according to measurement protocol

Fig. 2

Overview of the radiographic imaging. First, an anterior–posterior Zanca view radiograph was taken with the standardized technique and second, a lateral Alexander view radiograph with an angle of 10° cranial tilt of the beam

Fig. 3

Illustration of the six performed radiographic measurements. On the anterior–posterior Zanca view, coracoclavicular distance (CC) was measured for vertical displacement. On the Alexander view, acromial centre line to dorsal clavicle (AC–DC) and centre cranialization (CCran) were measured for vertical displacement and Maximal Overlap (OL), Lateral Extension (LE) and glenoid centre line to posterior clavicle (GC–PC) for horizontal displacement

Fig. 4

Effect of projectional variation for each type of injury. Mean and 95%CI are presented for each ACJ dislocation group depending of the amount of projectional variations included (neutral n = 7, 10 degrees n = 49 and 20 degrees n = 91). AC–DC is represented in a left, GC–PC in b middle and CC in c left

Fig. 5

Effect of projectional variation relating to the malposition. Mean deviations of the projectional variations (±10°, ±20°) are shown for AC–DC (a) and GC–PC (b) compared to neutral. Mean values are calculated including the different injuries. Lower values indicate underestimation, higher values overestimation. Axial variations are shown in dotted, sagittal variations dot dashed and coronal variation dashed