Introduction
Surgical fixation of the acromioclavicular (AC) joint dislocation or unstable fracture of the distal clavicle involves a complicated procedure for orthopedic surgeons. The recently developed clavicular hook plate is designed to be fixed on the distal clavicle with its hook being inserted beneath the acromion, just posterior to the AC joint. The hook works as a lever for depressing the clavicle to the level of the acromion at the AC joint, and facilitate bony or ligamentous healing [
1,
2]. Several studies have described favorable surgical outcomes and fast bony union or ligamentous healing with few complications [
2-
7]. However, complications such as postoperative pain, acromial erosion (
Fig. 1), or fractures are known to occur [
8-
12]. To minimize these complications, the pressure between the hook tip and undersurface of the acromion needs to be dispersed [
2,
8]. Since the designing of the hook plate requires it to be placed across the distal clavicle and the acromion around the AC joint, a thorough knowledge of the structural relationship between these two bony structures is important for the safe positioning. Although one previous study described the surgical anatomy around the AC joint in relation to the hook plate implantation [
2], it mainly focused on the anatomy of the horizontal plane of the acromion and did not include the coronal plane structural relationship between these two bones. The current study therefore aims to investigate the in vivo coronal plane structural relationship between the distal clavicle and acromion around the AC joint using computed tomography (CT) of the shoulder joint. We believe that the results generated from the data would help in the selection and bending of the hook plate, and be valuable to facilitate in improving the future design of the hook plate from the perspective of surgical fixation of the AC joint using this technique.
Discussion
The current study analyzed the coronal plane structural anatomy around the AC joint and its variability between individuals, and evaluated the differences between genders. Furthermore, we propose several clinically relevant anatomical parameters in the perspective of surgical fixation of the AC joint using the hook plate.
Although several authors account complications related to the stress concentration on the undersurface of the acromion after hook plate fixation [
8,
9,
16], surgical anatomy of the AC joint in relation to the hook plate fixation is seldom reported in literature, except one previous study. ElMaraghy et al. [
2] analyzed the acromion anatomy of 15 cadaveric samples using a three-dimensional model. They analyzed the antero-posterior width at the lateral aspect of the acromion, medial to lateral length of the acromion, and its thickness. They concluded that these anatomical parameters are unique for each patient, and therefore ‘standard’ acromial dimensions could not be found. Another conclusion was that a substantial portion of the samples revealed the hook to be in contact with the supraspinatus tendon, subacromial bursa, and acromial bone, with stress concentrated at the hook tip. This study is relevant since it evidences the concerns of subacromial structural damage after hook plate fixation. However, the study did not include the clinically relevant coronal plane structural relationship between the distal clavice and the acromion during execution of the hook plate technique.
In the current study, the angle between the distal clavicle at the AC joint and the acromion just posterior to the AC joint was observed to be variable (AC angle, 16.8° ± 10.38°). Since the plate to be fixed on the distal clavicle and the hook to be inserted beneath the acromion are designed as a fixed angle in shape, the point of contact between the hook tip and the undersurface of the acromion seems inevitable in many cases (
Fig. 4). The hook plate is designed such that it maintains the normal biomechanics of the AC joint, and the hook portion is free to move during arm elevation [
17]. Many authors suggest that if the point contact between hook tip and undersurface of the acromion occurs, this movement of the hook under the acromion might consequently result in bony erosion [
2,
8,
10,
18,
19]. Hence, to avoid stress concentration between the hook tip and the undersurface of the acromion, bending of the hook portion or plate portion of the plate in accordance with the AC angle might be necessitated. We therefore suggest that the AC angle, which shows very high individual variability in our results, should be thoroughly examined for each patient (
Fig. 2,
5).
The depth of the acromion at the AC joint might be important when choosing the depth of the acromial hook plate. Inserting a plate with smaller depth of the hook portion than the depth of the acromion could result in over-reduction of the AC joint as well as occurrence of pin point contact between the hook tip and the acromion. Many studies recommend using an accurate depth of the hook of the plate to avoid excessive pressure on the undersurface of the acromion [
3,
8,
18].
In terms of AC height difference, the mean value in the current study was 3.5 mm, and almost all cases showed positive values (except 1 case with -0.7 mm). This indicates that the distal clavicle is usually located at a higher position than the acromion in the AC joint. It might be clinically important for determining the proper reduction of the AC joint. For example, in AC joints with large positive AC height difference before injury, neutralization of the AC height difference after surgical fixation could indicate excessive depression of the distal clavicle. Over-reduction of the AC joint causes excessive stress on the distal clavicle, and especially on the point contact area of the acromion. The large AC angle, inappropriate selection of the depth of the hook, and over-reduction of the AC joint result from the inconsequential to the normal positive AC height difference, and all these factors aggravate the stresses imposed on the undersurface of the acromion by the hook.
We observed significant differences between genders in the anatomy around the AC joint, which we believe to be a clinically relevant observation. Similarly, Wisanuyotin et al. [
20] also reported that the anatomy of the clavicle is significantly different between genders. In the present study, the female gender has a significantly larger AC angle (13.8° vs. 19.8°), and thinner distal clavicle (12.2 mm vs. 10.9 mm) and acromion (8.7 mm vs. 7.7 mm) as compared to the male gender. This might indicate that the female patients are at a higher risk of having complications related to the stresses on the undersurface of the acromion, although we do not have evidences to prove this assumption. ElMaraghy et al. [
2] similarly suggests that females might have more complications after surgical fixation of the AC joint using the hook plate. They describe that despite the hook being positioned posterior to the AC joint, the base of the hook still rests in the middle of supraspinatus fossa, which may subsequently lead to subacromial impingement between the hook and the supraspinatus tendon. Further studies analyzing the relationship between the occurrence of postoperative complications and the degree of the AC angle or the gender are required to validate this hypothesis.
The current study has many limitations. The main drawback of this study is that we could not conduct a statistical comparison between the groups with and without subacromial erosion after surgical fixation with the hook plate. After experiencing the first case with subacromial erosion subsequent to the use of the hook plate, we initiated bending the hook of the plate prior to final fixation in accordance with the AC angle assessed with intraoperative fluoroscope. Hence, there are insufficient cases with subacromial erosion for performing the statistical comparison to confirm that large AC angle is a prognostic factor for subacromial erosion. Second, since there are no prior literatures describing the AC angle that can be used as a reference value for sample size calculation in comparison between gender groups, we were unable to perform sample size calculation before collecting data. However, we performed a retrospective power analysis for the AC angle between the two gender groups. The result showed the beta error probability of the analysis was low enough for a clinical study (power=80.1%). Third, our measurements to estimate the structural relationship between the acromion and the distal clavicle were based only on the coronal plane, which may not be the correct representation of the three-dimensional relationship. Fourth, all the CT scans were performed on patients in the supine position; thus, our findings may differ slightly from scans performed in the standing or sitting position. Fifth, this study includes the shoulder joints with anterior instability, contusion, and neoplasm, which may cause subtle abnormalities in the structural relationship at the AC joint. However, we used strict criteria in selecting subjects in order to reflect the closest to normal anatomy of the AC joint. In spite of all these limitations, we believe that our observations could be helpful in improving the future design of the hook plate, and also be the corner stone for future studies investigating the relationship between the postoperative acromial complications and the anatomy around the AC joint.