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DOI:10.2214/AJR.04.1433
AJR 2005; 185:1126-1131
© American Roentgen Ray Society

Original Research

Visualization of Intraarticular Structures of the Acromioclavicular Joint in an Ex Vivo Model Using a Dedicated MRI Protocol

Christian Fialka1, Christian Robert Krestan2, Paul Stampfl1, Klemens Trieb3, Seyedhossein Aharinejad4 and Vilmos Vécsei1

1 Department of Traumatology, Medical University of Vienna and Vienna General Hospital, Waehringer Guertel 18–20, Vienna A-1090, Austria.
2 Department of Radiology, Medical University of Vienna, Vienna, Austria.
3 Department of Orthopedics, Medical University of Vienna, Vienna, Austria.
4 Department of Anatomy, Medical University of Vienna, Vienna, Austria.

Received September 9, 2004; revised November 29, 2004;

 
Address correspondence to C. Fialka.


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of this study was to develop an MRI protocol that could visualize the intraarticular structures of the acromioclavicular (AC) joint.

MATERIALS AND METHODS. Using six fresh specimens from cadaveric shoulders, several MRI sequences were performed on 1.0-T scanners with a superficial coil (the temporomandibular joint coil). After the radiologic examination, the specimens were prepared for histology and 300-µm-thick, toluidine blue–stained sections were prepared that corresponded to the MR images. In each series of sections, immunohistochemistry using a type II collagen antibody was performed to further characterize the intraarticular structures.

RESULTS. The coronal 3D T1-weighted fast-field echo water-selective sequence allowed the identification of the intraarticular disk in all cases. Determination on MRI of other intraarticular structures—adipose tissue, synovial fluid, and the borders between neighboring tissues of different types—that corresponded to the histologic sections was possible. The use of a second plane in the 1.0-T sequences did not reveal additional information.

CONCLUSION. The described MRI protocol allows the visualization of the intraarticular fibrocartilaginous disk and the border between articular cartilage and the disk. Future clinical studies will indicate the diagnostic value of this protocol. We assume that this MRI protocol could help us to better understand AC joint disorders, in particular those located intraarticularly, and dislocations.


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Traumatic lesions of the acromioclavicular (AC) joint often result in clear symptoms and are assessed by clinical examination and radiography [1]. In patients with milder symptoms, stress radiography may be helpful [2]. Further diagnostic examinations, such as sonography or MRI, do not add additional information in these cases [3, 4]. Current treatment options in the acute phase of AC joint trauma depend exclusively on the degree of instability and the individual patient's activity profile [5–8].

Chronic, painful AC joint disorders, however, may not be related to instability [9–11]. Intraarticular disorders, such as chronic disruptions of the disk or degenerative changes of the surrounding osseous structures, have been considered causal in these cases. In line with this hypothesis, arthroscopic débridement of the joint or lateral clavicula resection can provide significant relief of symptoms in these patients [12–14]. One key diagnostic problem, according to the current literature, is the failure to visualize the intraarticular structures of the AC joint on MRI [3, 4, 9, 15, 16].

In clinical practice, therapeutic algorithms are strictly based on clinical symptoms and the measurable degree of instability. However, the lack of a radiologic protocol for the examination of the intraarticular compartment means that some intracapsular disorders are missed, which, if not addressed during surgical treatment, may contribute to poor outcomes. Therefore, a diagnostic procedure that would allow visualization of intraarticular structures could help to characterize intraarticular disorders and develop a more specific therapeutic concept for AC joint lesions in both acute and chronic cases.

Based on this reasoning, our study sought to establish a procedure that would allow direct visualization of the intraarticular disk. To accomplish this goal, different MRI protocols were compared in cadavers. The MRI findings were compared with corresponding histologic sections. We describe a superficial coil combined with 1.0-T MRI that allows differentiation between cartilage and the intraarticular disk in the AC joint in an ex vivo model that could serve as a basic protocol for further clinical examinations.



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  		Fig. 1A —MR images using different protocols. Images from cadaveric specimens 3 (A) and 5 (B). Presence of intraarticular fibrocartilaginous disk is confirmed by corresponding toluidine blue–stained sections. Arrows mark border between intraarticular disk and articular cartilage on acromial side. Images were acquired using dual T2- and proton density–weighted (upper left), dual T2-weighted turbo spin-echo (upper right), T2-weighted fast-field echo (lower left), and 3D water-selective (lower right) sequences. Center images show correlating histology sections.

 



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  		Fig. 1B —MR images using different protocols. Images from cadaveric specimens 3 (A) and 5 (B). Presence of intraarticular fibrocartilaginous disk is confirmed by corresponding toluidine blue–stained sections. Arrows mark border between intraarticular disk and articular cartilage on acromial side. Images were acquired using dual T2- and proton density–weighted (upper left), dual T2-weighted turbo spin-echo (upper right), T2-weighted fast-field echo (lower left), and 3D water-selective (lower right) sequences. Center images show correlating histology sections.

 

Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Six fresh cadavers were used for this study. The age of the cadavers included in this study was strictly ative alterations of the intraarticular structures [17]. MRI was performed using 1.0-T sequences. The specimens were examined histologically using the serial sections that corresponded to the MR images.



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  		Fig. 2A —Representative toluidine blue–stained histologic images of intraarticular structures in acromioclavicular joint from cadaveric specimens. Arrows indicate intraarticular fibrocartilaginous disk.

 



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  		Fig. 2B —Representative toluidine blue–stained histologic images of intraarticular structures in acromioclavicular joint from cadaveric specimens. and C, Representative immunohistochemistry of type II collagen-stained specimens indicates appearance of different cartilaginous structures. (Magnification, x1 [B] and x4 [C])

 



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  		Fig. 2C —Representative toluidine blue–stained histologic images of intraarticular structures in acromioclavicular joint from cadaveric specimens. Representative immunohistochemistry of type II collagen-stained specimens indicates appearance of different cartilaginous structures. (Magnification, x1 [B] and x4 [C])

 
Specimen Preparation
The shoulder specimens were isolated from the surrounding soft tissue, except for the AC joint capsule, before MRI. The clavicle was cut 3 cm medial to the joint, and the acromion was cut 2 cm lateral to the joint, leaving the capsule–ligament complex unaffected. The specimens were placed in 0.9% sodium chloride solution during MRI.

MRI Protocol
All specimens were examined on a 1.0-T MR system (T10-NT, Philips Medical Systems) using a surface coil (the temporomandibular joint coil). In the 1.0-T unit, coronal 3D T1-weighted fast-field echo and water-selective sequences were obtained using 2 excitations, a TR/TE of 24/11.95, and a flip angle of 50°. The field of view was 150 mm, and the reconstructed imaging matrix was 512 x 512. The measured voxel size was 0.59 x 0.61 x 2.00 mm, and the reconstructed voxel size was 0.29 x 0.29 x 1.00 mm. The total scanning duration was 3 min 56 sec.

Coronal dual T2-weighted sequences were obtained using 4 excitations, a TR/first-echo (proton-density-weighted) and a TE/second-echo (T2-weighted) TE of 2,400/11/120, a flip angle of 90°, and a turbo factor of 12. The field of view was 160 mm, and the reconstructed imaging matrix was 512 x 512. The measured voxel size was 0.31 x 0.4 x 2.0 mm, and the reconstructed voxel size was 0.31 x 0.31 x 2.0 mm. Total scanning duration was 9 min 46 sec.

Coronal T2-weighted fast-field echo sequences were obtained using 8 excitations, 264/14, and a flip angle of 35°. The field of view was 120 mm, and the reconstructed imaging matrix was 512 x 512. The measured voxel size was 0.62 x 0.62 x 1.5 mm, and the reconstructed voxel size was 0.23 x 0.23 x 1.5 mm. Total scanning duration was 6 min 46 sec.

Histology
After MRI was performed, the specimens were immersion-fixed in 7.5% formaldehyde and dehydrated in a series of alcohol solutions of increasing concentrations. Subsequently, the specimens were embedded in a mixture of methacrylic acid and polymerization agent (500 mL of methacrylic acid methyl ester, 25 g of benzoyl peroxide, and 199 mL of non-ylphenol). The polymerization was initiated at room temperature and continued in an incubator at 30°C.



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  		Fig. 3 —24-year-old asymptomatic male volunteer. Protocol was used in a manner identical to that used in ex vivo study. Superficial coil was fixed directly anterior to acromioclavicular joint. No evidence of major displacement due to respiration was observed. Arrows mark border between intraarticular disk and articular cartilage on lateral end of clavicle and acromion, respectively.

 
The specimens were cut into 2-mm sections in the coronal plane to achieve slices that corresponded to the MR images using a macro band saw. The specimens were rinsed with alcohol, transferred onto specimen holders, and attached to the latter using Technovit 7210 VLC (Hereus Kulzer Co.) with ultraviolet irradiation for 30 min. Then 300-µm-thick sections were prepared, polished (Planopol V grinding machine, Struers Tech), and stained with toluidine blue.

Five-micron-thick sections were prepared using a grinding machine, polished, rinsed with distilled water, and immersed in 30% H2O2 for 10 min to block endogenous peroxidase activity. The specimens were stained with a type II collagen antibody (dilution, 1:100; Southern Biotechnology), followed by rinsing and counterstaining with hemalaun.

Coronal MRI was begun at the anterior border of the lateral end of the clavicle, providing images of 2-mm thickness with a gap of 0.2 mm. The fixed specimen was then oriented in the same way at the beginning of the cutting, also providing the first slice at the anterolateral end of the clavicle, producing initial slices of 2-mm thickness that were reduced to 300 µm for initial staining. Beginning with the most anterior, the slices were numbered from anterior to posterior in the same way as performed for the MR images.

The examination was performed in consensus by two experienced clinicians (a musculoskeletal radiologist and an orthopedic trauma surgeon). The clinicians were blinded to the histology during the primary interpretation. The idea was to perform a descriptive examination that would clarify whether the intraarticular disk was visible and whether a clear border existed between the fibrocartilaginous and the articular cartilage. During a second interpretation, the findings were compared with the structures in the corresponding histologic sections using 4-fold magnification. Confirmation of the type of tissue was then performed by immunohistochemistry.


Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
With the use of the superficial coil and the previously described MRI protocols, the intraarticular structures could be clearly identified. The border zone between the subchondral bone and the articular cartilage, the border between the articular cartilage and the fibrous cartilage, the joint capsule (including its superior reinforcement), and other intraarticular soft-tissue structures, which were identified histologically as either synovia or fat, could be visualized on MRI (Fig. 1A, 1B).

A comparison of the applied MRI protocols revealed that the 3D water-selective sequence, followed by the dual T2- and proton density–weighted sequence, was the most reliable sequence for visualizing the border between the articular and the fibrous cartilage. The use of the T2 fast-field echo sequence did not allow reproducible identification of the intraarticular disk in most cases. A second MRI plane did not provide any additional information. Particularly in cases in which the disk was not reliably recognizable in the coronal plane, as was the case in most of the T2 fast-field echo sequences, the disk could not be detected in the horizontal plane either.

The cartilaginous structures, as seen on the MR images, were compared with the histologic findings. In the toluidine blue–stained specimens, a disklike structure comprising collagen was found in all cases. The location, form, and size of this structure were consistent with macroscopic findings in anatomic specimens and matched the appearance of the intraarticular disk. To confirm the type of tissue in this disklike structure, immunohistochemistry with type II collagen antibody was performed. The presence of parallel type II collagen fibers in the anterosuperior part of the joint confirmed the MRI and routine histology findings (Fig. 2A, 2B, 2C).


Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The AC joint, consisting of the lateral end of the clavicle and the medial end of the acromion, is surrounded by a fibrous capsule that is reinforced at the superior aspect by a quadrilateral ligament—that is, the superior AC ligament [18]. In the intraarticular compartment, a fibrocartilaginous disk is often present that, in some instances, has a meniscuslike structure [11, 19, 20]. This disk is located in the anterosuperior aspect of the AC joint and often reveals degenerative changes that are believed to occur at the beginning of the third decade of life [17]. The existing evidence suggests that the coracoclavicular ligaments act as anteroposterior stabilizers of the AC joint, whereas the superior ligament counteracts up to 90% of vertical forces of the acromion [21].

Acute lesions of the AC joint, depending on the acting force, may lead to disruption of the articular capsule, the superior ligament, and the coracoclavicular ligament [1]. Treatment strategies mainly depend on the degree of ligament instability [2]. Nevertheless, painful conditions are observed in daily practice that cannot be related to instability. According to the recent literature, extraarticular, but not intraarticular, structures can be detected on MRI [3, 4]. Only degenerative changes of the surrounding osseous structures, intraarticular effusions, or bone bruises at the lateral end of the clavicle, as indirect signs of intraarticular disorders, can be seen on MRI [3, 4, 22, 23].

Therapeutic options that are directed toward ligament instability risk missing potential accompanying disorders, such as disk disruptions. This is true for therapeutic algorithms that favor primary stabilization in all acute cases of Tossy III or Rockwood III lesions. On the other hand, the so-called skillful neglect principle for the same injuries may also result in chronic cases with painful impairment [24–26]. The surgical techniques in chronic cases mainly rely on the resection of the intraarticular structures or resection of the lateral end of the clavicle by arthroscopic or open techniques [12]. Reconstruction of the ligaments is not possible in delayed treatment. Only a nonanatomic reconstruction—that is, the Weaver-Dunn or the Bunnell procedure—is possible if the primary disorder has not been addressed [14, 27, 28]. Clinically, it is impossible to detect lesions of the intraarticular disk, especially in low-grade dislocations such as a Rockwood II. Sonographic and standard MRI protocols are sensitive enough to detect only periarticular disorders such as extraarticular ligament ruptures or degenerative changes in the osseous structures [3, 4, 9, 29–31]. Therefore, an MRI protocol that could assess the fibrocartilaginous disk would be helpful to initiate more sensitive and more specific therapeutic algorithms.

The main limitation of our study is the use of an ex vivo model for the development of the MRI protocol. In fact, the protocol will need further modification in clinical practice. In addition, the use of a superficial coil precludes the examination of the glenohumeral joint. This examination must be performed in addition to routine shoulder MRI. Thus, the information about possible intraarticular disorders may result in a better diagnosis in patients with symptomatic AC joints.

We have already applied this protocol in vivo. Using the asymptomatic shoulders of a volunteer, we showed that it is possible to obtain images comparable to those in the ex vivo model (Fig. 3). The surface coil routinely is placed directly anterior to the AC joint. On the basis of limited clinical experience, no evidence was seen of any major coil displacement due to respiration.

We are already performing a clinical examination for a consecutive group of acute AC joint injuries to compare the MRI findings of the affected joint with those of the contralateral unaffected side. These data, although they were not available at the time of this writing, will likely confirm the clinical value of this procedure.

In this article we have described a superficial coil that, used with 1.0-T high-resolution MRI, especially with 3D water-selective and dual T2- and proton density–weighted sequences, allows the differentiation between cartilage and the intraarticular disk in the AC joint. The visualization of intraarticular soft tissue may help in the understanding of AC joint disorders. Thus, the MRI protocol described in this study could serve as a basis for decision making in the treatment of symptomatic patients independently of the degree of joint instability. Future clinical studies, including high-resolution MRI, will definitely add to our knowledge of AC joint pathomorphology.


Acknowledgments
 
We thank Guenter Brand for outstanding technical assistance.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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