Three-Dimensional Footprint Mapping of the Deltoid and Trapezius: Anatomic Pearls for Acromioclavicular Joint Reconstruction

Acromioclavicular Joint Anatomy and Biomechanics: The Significance of Posterior Rotational and Translational Stability

The shoulder girdle extends from the sternoclavicular joint to the scapular stabilizing muscles posteriorly. It consists of 3 joints and 2 mobile regions. The shoulder girdle is statically stabilized by the acromioclavicular and coracoclavicular capsuloligamentous structures and dynamically stabilized by the trapezius, deltoid, and deltotrapezial fascia. During humerothoracic elevation, the clavicle elevates, protracts, and rotates posteriorly through the sternoclavicular joint while the scapula tilts posteriorly and rotates upward. The purpose of this article is to review the anatomy and biomechanics of the acromioclavicular joint and the shoulder girdle.

Deltotrapezial Stabilization of Acromioclavicular Joint Rotational Stability: A Biomechanical Evaluation

Background

Despite advances in surgical management of acromioclavicular (AC) joint reconstruction, many patients fail to maintain sustained anatomic reduction postoperatively.

Purpose

To determine the biomechanical support of the deltoid and trapezius on AC joint stability, focusing on the rotational stability provided by the muscles to posterior and anterior clavicular rotation. A novel technique was attempted to repair the deltoid and trapezius anatomically.

Study Design

Controlled laboratory study.

Methods

Twelve human cadaveric shoulders (mean ± SD age, 60.25 ± 10.25 years) underwent servohydraulic testing. Shoulders were randomly assigned to undergo serial defects to either the deltoid or trapezius surrounding the AC joint capsule, followed by a combined deltotrapezial muscle defect. Deltotrapezial defects were repaired with an all-suture anchor using an anatomic technique. The torque (N·m) required to rotate the clavicle 20° anterior and 20° posterior was recorded for the following conditions: intact (native), deltoid defect, trapezius defect, combined deltotrapezial defect, and repair.

Results

When compared with the native condition, the deltoid defect decreased the torque required to rotate the clavicle 20° posteriorly by 7.1% (P = .206) and 20° anteriorly by 6.1% (P = .002); the trapezial defect decreased the amount of rotational torque posteriorly by 5.3% (P = .079) and anteriorly by 4.9% (P = .032); and the combined deltotrapezial defect decreased the amount of rotational torque posteriorly by 9.9% (P = .002) and anteriorly by 9.4% (P < .001). Anatomic deltotrapezial repair increased posterior rotational torque by 5.3% posteriorly as compared with the combined deltotrapezial defect (P = .001) but failed to increase anterior rotational torque (P > .999). The rotational torque of the repair was significantly lower than the native joint in the posterior (P = .017) and anterior (P < .001) directions.

Conclusion

This study demonstrated that the deltoid and trapezius play a role in clavicular rotational stabilization. The proposed anatomic repair improved posterior rotational stability but did not improve anterior rotational stability as compared with the combined deltotrapezial defect; however, neither was restored to native stability.

Clinical Relevance

Traumatic or iatrogenic damage to the deltotrapezial fascia and the inability to restore anatomic deltotrapezial attachments to the acromioclavicular joint may contribute to rotational instability. Limiting damage and improving the repair of these muscles should be a consideration during AC reconstruction.

Arthroscopic Excision for Painful Acromial Fracture Nonunion After Reverse Total Shoulder Replacement

Acromial stress fracture is a well-known complication after reverse shoulder arthroplasty. Although some patients are asymptomatic, others may present with reproducible pain to palpation and signs of impingement. Generally, the initial mode of treatment is conservative; however, surgical intervention is considered in patients with painful nonunion and displaced fragments. Although open reduction techniques are most common, Levy type I fragments may be amenable to excision. This technical note provides a stepwise approach to arthroscopic excision for painful nonunited acromial stress fractures after reverse shoulder arthroplasty.

The Role of the Trapezius in Stabilization of the Acromioclavicular Joint: A Biomechanical Evaluation

Background

Acromioclavicular joint (ACJ) injuries are common, and many are adequately treated nonoperatively. Biomechanical studies have mainly focused on static ligamentous stabilizers. Few studies have quantified ACJ stabilization provided by the trapezius.

Purpose/Hypothesis

To elucidate the stabilization provided by the trapezius to the ACJ during scapular internal and external rotation (protraction and retraction). It was hypothesized that sequential trapezial resection would result in increasing ACJ instability.

Study Design

Controlled laboratory study.

Methods

A biomechanical approach was pursued, with 10 cadaveric shoulders with the trapezius anatomically force loaded to normal. The trapezius was then serially transected over 8 trials, which alternated between clavicular defects (CD) and scapular defects (SD); each sequential defect consisted of 25% of the clavicular or scapular trapezial attachment. After each defect, specimens were tested with angle-controlled scapular internal and external rotation (12°) with rotary torque measurements to evaluate ACJ stability.

Results

The mean resistance in rotary torque for 12° of scapular internal rotation (protraction) with native specimens was 7.0 ± 2.0 N·m. Overall, internal rotation demonstrated a significant decrease in ACJ stability with trapezial injury (P < .001). Eight sequential defects resulted in the following significant percentage decreases in rotary torque from native internal rotation: 1.5% (25% CD; 0% SD), 5.6% (25% CD; 25% SD), 5.1% (50% CD; 25% SD), 6.5% (50% CD; 50% SD), 3.8% (75% CD; 50% SD), 7.1% (75% CD; 75% SD), 6.7% (100% CD; 75% SD), and 12.3% (100% CD 100% SD) (P < .001). The mean resistance in rotary torque for 12° of scapular external rotation (retraction) with native specimens was 7.1 ± 1.7 N·m. External rotation did not demonstrate a significant decrease in ACJ stability with trapezial injury (P = .596). The 8 sequential defects resulted in decreases in rotary torque from native external rotation of 0%, 3.8%, 4.0%, 3.2%, 3.5%, 3.4%, 4.2%, and 0.7%.

Conclusion

Trapezial injury resulted in increased instability in the setting of scapular internal rotation (protraction) of the ACJ.

Clinical Relevance

These findings validate the inclusion of deltotrapezial fascial injury consideration in the modified Rockwood classification system. Repair of the trapezial insertion on the ACJ may provide improved outcomes in the setting of ACJ reconstruction.

Combined Vertical, Horizontal, and Rotational Acromioclavicular Joint Stabilization: "Closing the Circle" Technique

The biomechanical and anatomical complexity of the acromioclavicular joint makes its repair techniques particularly challenging. High rates of reduction subsidence and construction failures transversally affect both anatomic and nonanatomic repair techniques. The importance of addressing both vertical and horizontal instability has been highlighted in recent years. The authors aim to describe a surgical technique that combines vertical, horizontal, and rotational stabilization, in an attempt to restore the coracoacromioclavicular circle of stability.

Addressing Arthroscopic-Assisted Acromioclavicular Joint Reconstruction in the Beach Chair Position With Concomitant Labral Pathology in the Lateral Decubitus Position

High-grade Acromioclavicular (AC) injuries are complete dislocations, involving ruptures of the AC and coracoclavicular ligaments. They occur following trauma after a fall, direct-blow to an adducted arm, or indirectly by falling onto an abducted, outstretched extremity. Given this traumatic etiology, additional intra-articular pathologies can arise and may go unnoticed because of the painful and prominent AC joint (ACJ). Previous studies have evaluated patients with high-grade ACJ injuries with diagnostic arthroscopy at the time of an ACJ reconstruction. They found associated injuries to the labrum/biceps, rotator cuff, and articular cartilage. The arthroscopic-assisted ACJ reconstruction (AA-ACJR) technique has made it possible to identify the associated injuries and treat them concurrently. The previous studies have performed this reconstruction in the beach chair position (BCP) and have addressed the concomitant pathology in the same position. As opposed to the BCP, the lateral decubitus position (LDP) allows for easier application of traction to the arm and, thus, improves visualization of the glenoid, especially the inferior and posteroinferior portions. It is imperative to gain appropriate access to the inferior glenoid for anchor placement to address this component of traumatic instability. We present the technique for addressing high-grade ACJ injuries with AA-ACJR in the BCP preceded by labral repair in the LDP.