MRI is a Reliable Method for Measurement of Critical Shoulder Angle and Acromial Index

Abstract

Objective The objectives of this study are to compare absolute values of acromial index (AI) and critical shoulder angle (CSA) obtained in both radiographs and magnetic resonance image (MRI) of the shoulder; and to compare the interobserver and intra-observer agreement for AI and CSA values measured in these image modalities. Methods Patients who had medical indication of investigating shoulders conditions through radiographs and MRI were included. Images were taken to two fellowship-trained shoulder surgeons, which conducted measurements of AI and CSA in radiographs and in MRI. Twelve weeks after the first evaluation, a second evaluation was conducted. Inter- and intra-observer reliability was presented as an Intraclass Correlation Coefficient (ICC) and agreement was classified according to Landis & Koch criteria. The differences between two measurements were evaluated using Bland-Altman plots. Results 134 shoulders in 124 subjects were included. Mean intra-observer ICC for CSA in X-rays and in MRI were 0.936 and 0.940, respectively; for AI, 0.908 and 0.022. Mean inter-observer ICC for CSA were 0.892 and 0.752 in X-rays and MRI respectively; for AI, ICC values were 0.849 and 0.685. All individual analysis reached statistical power ( p < 0.001). Mean difference for AI values measured in X-rays and in MRI was 0.01 and 0.03 for observers 1 and 2, respectively. Mean difference for CSA values obtained in X-rays and MRI was 0.16 and 0.58 for observers 1 and 2, respectively. Conclusion Both MRI and X-rays provided high intra- and interobserver agreement for measurement of AI and CSA. Absolute values found for AI and CSA were highly correlated in both image modalities. These findings suggest that MRI is a suitable method to measure AI and CSA. Level of Evidence II , Diagnostic Study.

Keywords: acromion; magnetic resonance imaging; radiography; rotator cuff tear; shoulder impingement syndrome.

Fig. 1

True AP view radiograph of the shoulder, showing CSA measurement. The angle is formed between two lines: one from the superior to the inferior pole of the glenoid, and other from the latter to the lateral edge of the acromion.

Fig. 2

True AP view radiograph of the shoulder, showing anatomical parameters required to measurement of the AI. This index is obtained dividing the distance from the glenoid surface to the most lateral edge of the acromion (GA) by the distance from the glenoid surface to the lateral cortex of the proximal humerus (GH). AI = GA/GH.

Fig. 3

Frequently, in MRI the most lateral edge of the acromion is not in the same plane of the glenoid midline. Thus, we used the cursor to mark the lateral acromion (3A) and then scroll the images until the glenoid midline (3B), where the measurements are made.

Fig. 4

Bland-Altman plots showing difference vs . average distribution of AI ( A, B ) and CSA ( C, D ) indexes measured in X-rays and in MRI for observers 1 and 2, respectively.

Fig. 1

Radiografia em incidência anteroposterior verdadeira do ombro, mostrando a medida do ângulo crítico do ombro (ACO). O ângulo é formado por duas linhas: uma do polo superior para o inferior da glenoide e outra deste até a borda lateral do acrômio.

Fig. 2

Radiografia anteroposterior verdadeira do ombro, mostrando os parâmetros anatômicos necessários para medida do índice acromial (IA). Este índice é obtido dividindo a distância da superfície glenoidal à borda mais lateral do acrômio (GA) pela distância da superfície glenoidal à cortical lateral do úmero proximal (GH). IA = GA/GH.

Fig. 3

Com frequência, na ressonância magnética, a borda mais lateral do acrômio não está no mesmo plano da linha média da glenoide. Assim, utilizamos o cursor para marcar o acrômio lateral (3A) e depois rolar as imagens até a linha média da glenoide (3B), onde são feitas as medidas.

Fig. 4

Gráficos de Bland-Altman mostrando a diferença em comparação à distribuição média do índice acromial (IA) ( A, B ) e ângulo crítico do ombro (ACO) ( C, D ) medidos em radiografias e ressonâncias magnéticas (RM) para os observadores 1 e 2, respectivamente.