Computer Image-Guided Precise Acromioplasty for Reducing the Critical Shoulder Angle

Abstract

The shoulders with critical shoulder angle (CSA) of greater than 33-35° are associated with rotator cuff tears, whereas a CSA of less than 30° is likely to be osteoarthritic. However, anterior acromioplasty or lateral acromioplasty could not reduce high CSAs to the desired range (30-33°), with satisfactory accuracy and efficacy. Thus, we introduce a computer image-guided precise acromioplasty (CIG-PAP) technique, an individualized treatment based on three-dimensional planning. We believe that the introduction of this technique will provide an alternative approach to reduce a large CSA to the desired range (30-33°).

Technique Video

Video 1

This video demonstrates the preoperative three-dimensional (3-D) planning and the intraoperative procedures of the computer image-guided precise acromioplasty (CIG-PAP) for reducing the critical shoulder angle.

Introduction

The critical shoulder angle (CSA) describes the lateral extension of the acromion in relation to the inclination of the glenoid and has been demonstrated to be an effective predictor for the development of shoulder pathology.1, 2, 3, 4 Numerous studies have documented that shoulders with a CSA of less than 30° were likely to be osteoarthritic, whereas a CSA of greater than 33-35° was associated with rotator cuff tears (RCTs).5, 6, 7, 8, 9, 10 Gerber et al. demonstrated that patients with a CSA reduced to 33° or less had superior abduction strength and lower retear rate after arthroscopic rotator cuff repair (RCR).¹¹ Therefore, accurately reducing the large CSA to the desired range (30-33°) seems to become one of the important goals in the clinical treatment of RCTs.

Several studies have shown that anterior acromioplasty reduced the CSA by a mean of 1-3.2°.12, 13, 14, 15 In comparison, lateral acromioplasty could significantly decrease the CSA by a mean of 3.6-6.5°.^11,16,17 However, whether anterior or lateral, 20-52% of patients still had postoperative CSA > 35° after acromioplasty, while CSA were over-reduced to below 30° in some cases.11, 12, 13, 14^,16, 17, 18, 19 Currently, avoiding insufficient or over-reduction of the CSA is still a challenging problem.

In this article, we present a computer image-guided precise acromioplasty (CIG-PAP) technique, which is an individualized treatment based on three-dimensional (3-D) planning to reduce a large CSA to the desired range (30-33°). CIG-PAP is especially indicated for patients with RCTs combined with preoperative CSA greater than 35°. We hope that using this technique concomitant to arthroscopic RCR would provide clinical benefits to patients. Conversely, contraindication for CIG-PAP would include the following: irreparable RCTs, inflammatory arthritis, and concomitant fracture or dislocation of the shoulder.

Surgical Technique (With Video Illustration)

3-D Reconstruction and Coordinate System Establishment

Digital Imaging and Communications in Medicine (DICOM) data of shoulder are obtained. Three-dimensional reconstruction is made using Mimics 20.0 (Materialise, Leuven, Belgium). Subsequently, 3-D shoulder model is imported into Blender 2.81 (Amsterdam, Netherlands) to establish a Cartesian coordinate system for position alignment, according to previous studies.^20,21

Preoperative Planning and Measurement

Preoperative planning is performed on 3-D shoulder model using Blender 2.81. First, preoperative CSA is measured on the standard anterior view to confirm the value of CSA greater than 35° (38° in this article). The CSA is measured between a line connecting the superior and inferior borders of the glenoid and another line from the inferior borders of the glenoid to the most inferolateral point of the acromion.¹ And then, the postoperative target CSA is set at 31°, which is in the middle of the desired range of 30-33° (Fig 1A). Next, a cutting plane is established and positioned at the measuring line of 31° (Fig 1B). Hence, the external acromion of the cutting plane (green region) is the part of the bone to be removed (Fig 1C). Lastly, the resected bone borders are identified and measured on the computer, including the anterior acromial edge of acromioclavicular joint (Point A), anteromedial resection margin (Point B), posterolateral resection margin (Point C), and the distance of AB (approximately 13 mm) and BC (approximately 27 mm) (Fig 1D).

Fig 1.

Demonstration of preoperative planning and measurement (right shoulder). (A) Preoperative CSA (yellow line, 38°) is measured on the standard anterior view of 3-D shoulder. And the postoperative target CSA is set at 31° (white line). (B) The cutting plane is created and positioned at the measuring line of 31°. (C) The green region outside the cutting plane is the part of the acromion to be resected. (D) The measurement of resected bone borders, including the distance of AB (approximately 13 mm) and BC (approximately 27 mm). “A” denotes anterior acromial edge of acromioclavicular joint, “B” denotes anteromedial resection margin, and “C” denotes posterolateral resection margin; CSA, critical shoulder angle.

Arthroscopic CIG-PAP

After general anesthesia combined with brachial plexus block, the patient is placed in the lateral decubitus position with the arm in 30° abduction and with 10 pounds of traction. The bony landmarks are identified and marked, together with anterior, posterior, and lateral portals (Fig 2).

Fig 2.

The patient is placed in the lateral decubitus position with continuous extremity traction (right shoulder). The bony landmarks are outlined, and the anterior, posterior, and lateral portals are marked.

After the posterior and anterior portals are created, a routine diagnostic arthroscopy is performed in the articular cavity. Any intra-articular pathology is assessed and addressed appropriately if necessary, including articular cartilage, glenoid labrum, glenohumeral ligament, biceps, and subscapularis.

The 30° arthroscope is then inserted into the subacromial space, and a lateral portal is established to assess the reparability of the RCT based on tear location, size, degree of retraction, tension, tissue quality, and other variables. Once the RCT is determined to be reparable, the following procedures will be performed.

First, all soft tissue undersurface of the acromion and acromioclavicular joint is cleaned using radiofrequency electrocautery (Arthrex, Naples, FL). After fully revealing the acromioclavicular joint, anterior and lateral borders of the acromion, the anterior acromial edge of acromioclavicular joint (point A) is identified and marked with a spinal needle (Fig 3; Video 1).

Fig 3.

Identifying and marking the point A with a spinal needle (right shoulder in lateral decubitus position). (A) Gross view after inserting the spinal needle. (B) Arthroscopic view using 30° arthroscope from the posterior portal. A denotes anterior acromial edge of acromioclavicular joint. The asterisk (∗) denotes the clavicle.

Second, the posterior portal is used as the viewing portal, and the lateral portal is used as the working portal for measurement. According to the preoperative planning, the anteromedial resection margin (point B) is identified by measuring the distance of AB (approximately 13 mm in this article), and marked with a spinal needle (Fig 4; Video 1).

Fig 4.

Marking the point B with a spinal needle by measuring the distance of AB on the undersurface of the acromion (right shoulder in lateral decubitus position). (A) Gross view of the measurement and marking. (B) Arthroscopic view using 30° arthroscope from the posterior portal, and measuring the distance of AB from the lateral portal (scale from 10 mm to 23 mm, ∼13 mm). A denotes the anterior acromial edge of acromioclavicular joint; B denotes the anteromedial resection margin.

Third, the lateral portal is used as the viewing portal, and the anterior portal is used as the working portal for measurement. The posterolateral resection margin (point C) is identified by measuring the distance of BC (approximately 27 mm in this article), and also marked with a spinal needle (Fig 5; Video 1).

Fig 5.

Marking the point C with a spinal needle by measuring the distance of BC on the undersurface of the acromion (right shoulder in lateral decubitus position). (A) Gross view of the measurement and marking. (B) Arthroscopic view using 30° arthroscope from the lateral portal, and measuring the distance of BC from anterior portal (scale from 10 mm to 37 mm, ∼27 mm). B denotes the anteromedial resection margin; C denotes the posterolateral resection margin.

Then, a 0# PDS® (polydioxanone) suture (Ethicon, Somerville, NJ) is introduced under arthroscopy from point B to C through the inner core of the straight needles, and the blue resection line (BC) is made (Video 1).

Next, the region of the acromion outside the blue resection line is gradually resected by a 5.5-mm shaver burr (Arthrex) via the lateral portal (Fig 6; Video 1). Finally, the cutting surface of the acromion is deactivated using radiofrequency electrocautery (Arthrex).

Fig 6.

Removing the acromial bone outside the resection line BC (right shoulder in lateral decubitus position). (A) Gross view after marking the resection line BC with a 0# PDS® (polydioxanone) suture (Ethicon, Somerville, NJ). (B) Arthroscopic view using 30° arthroscope from the posterior portal, and working from the lateral portal. B denotes the anteromedial resection margin; C denotes the posterolateral resection margin; ※ symbol denotes the deltoid fascia.

Arthroscopic RCR is then performed depending on the tear location and size. Standard anteroposterior (AP) radiograph (type A1 or C1 according to the Suter-Henninger classification²⁰) (Fig 7) or CT scans of the shoulder is taken 3-5 days postoperation to assess the post-operative CSA.

Fig 7.

The pre-operative three-dimensional planning and postoperative measurement for CSA (right shoulder). (A) Target CSA of preoperative planning is set at 31°. (B) Postoperative CSA measured on the standard anteroposterior radiograph is 30.5°. CSA, critical shoulder angle.

Postoperative Management

As the CIG-PAP is concomitant with arthroscopic RCR, the patient should follow the rehabilitation protocol of RCR. The upper limb is placed into a shoulder and elbow brace for immobilization immediately after the operation. Active activity of the elbow and wrist should be started on the second day postoperatively. Passive or active-assisted shoulder range-of-motion exercises should begin 1-2 weeks after the operation. Active shoulder motion should be started after 8 weeks and gradually strengthened and reintegrated into normal activities at 12 weeks postoperatively.

Discussion

Since the reduction of CSA is difficult to predict intraoperatively, the favorable range of postoperative CSA is not always achieved after acromioplasty. Olmos et al. suggested using the approximation of “1 degree of CSA reduction for each 1 mm of resection” to quantify the amount of acromion to be resected by lateral acromioplasty; however, they found that 46.3% of patients still had a postoperative CSA > 35°.¹⁷ Similarly, to calculate the amount of acromion resection for reducing the CSA, Franceschetti et al. developed a formula by measuring the parameters on the AP radiographs.²² Although they reported using the complex formula could reduce the large CSAs to the range of 30-35°, how to accurately apply the preoperative planning to arthroscopic acromioplasty needs to be further clarified.²²

The CIG-PAP provides a new potential strategy for accurate reduction of CSA without insufficient or over-resection of the acromion (Table 1). This all-arthroscopic personalized acromioplasty is concomitant with arthroscopic RCR. And it is recommended to perform this technique first to increase space for visualization and suture passage during RCR (Table 2). The most difficult manipulation of this technique is how to accurately locate the area of osteotomy during arthroscopy, according to the 3-D planning.

Table 1.

Advantages and Disadvantages

Advantages	Disadvantages
•All-arthroscopic visual operation technique	•New learning curves for operators
•No extra incisions and specialty instruments	•Preoperative CT scans of the shoulder and 3-D planning are needed.
•Individualized acromioplasty	•Lack of clinical follow-up data
•Avoiding insufficient or overreduction of the CSA
•Greater reduction of the CSA with the lower volume of resected bone.

CSA, critical shoulder angle; CT, computed tomography; 3-D, three dimensional.

Table 2.

Tips and Tricks

•Clean all soft tissue undersurface of the acromion and acromioclavicular joint to get a better operation field.

•To increase space for visualization and suture passage during RCR, it is recommended to perform the CIG-PAP first.

•The acromion should be gradually resected from the undersurface to the upper until the deltoid fascia is exposed.

•The spinal needles should be vertically inserted close to the acromial edge as far as possible.

•After inserting the spinal needles, the assistant should keep the needle tails from swinging greatly.

•The cutting surface of acromion should be deactivated using radiofrequency electrocautery.

CIG-PAP, computer image-guided precise acromioplasty; RCR, rotator cuff repair.

In the early stage, we measured the resected bone borders on the patient’s skin after the bony landmarks were marked.²³ This method was satisfactory for some thin patients. However, it was inaccurate for the patients with unclear bony landmarks. Moreover, the body surface marker points would shift after the shoulder was filled with water during the operation.

This article modified the procedure to locate the area of osteotomy with greater accuracy. We selected the anterior edge of the acromioclavicular joint (point A) as an initial marker point, because it was relatively stable and easy to identify under arthroscopy. And we directly measured and marked the resected bone border under the arthroscopy instead of marking it on the body skin. We hope these improvements would reduce the CSA with greater consistency between preoperative 3-D planning and intraoperative osteotomy. Our preliminary cohort results are promising.

The main disadvantage of this technique is that preoperative 3-D planning is necessary. Therefore, there are inevitable learning curves for the 3-D planning tools described in this article. However, any other similar application software can be chosen depending on the operator’s proficiency and preference. In fact, many other 3-D planning tools described in the literature can also realize the process.24, 25, 26 Maybe an automatic 3-D planning program will simplify the procedure in the future.

In general, the CIG-PAP provides a new individualized treatment for accurate correction of CSA. Further research and clinical trials need to be carried out to evaluate the clinical benefits.

Supplementary Data

Video 1

This video demonstrates the preoperative three-dimensional (3-D) planning and the intraoperative procedures of the computer image-guided precise acromioplasty (CIG-PAP) for reducing the critical shoulder angle.