The Subscapularis-Sparing Windowed Anterior Technique (SWAT) for Anatomic Total Shoulder Arthroplasty.

Abstract

Background: Anatomic total shoulder arthroplasty (aTSA) has historically been performed via the standard deltopectoral approach, requiring violation of the subscapularis to access the glenohumeral joint. Subscapularis dysfunction has been documented in as many as 67% of cases and may lead to instability, weakness, and lower patient-satisfaction scores^11-16. However, the rate of subscapularis failure is much lower, at 1.6% to 3.0%, with a reoperation rate for a failed subscapularis of 0.9% to 3.0%^31,32. To preserve the subscapularis tendon, muscle-preserving techniques have been developed that allow for early postoperative motion and activity without prolonged immobilization. The subscapularis-sparing windowed anterior technique (SWAT) is a method for aTSA that preserves the integrity of the subscapularis as well as the deltoid. As a result of the complete preservation of the subscapularis, this technique offers the advantage of early postoperative motion with no restrictions. This technique also avoids the deltoid split that is required in some other subscapularis-preserving techniques.

Description: SWAT utilizes the standard deltopectoral incision. A window is created inferior to the subscapularis tendon and is utilized for the removal of inferior osteophytes, inferior translation of the humeral shaft, and capsular release. The rotator interval is also developed and is utilized to complete the humeral head cut, obtain glenoid exposure, and implant the components. Additionally, the use of preoperative planning allows accurate sizing of the humeral head component³⁰. Prior studies have shown that this technique can be utilized to reliably attain acceptable radiographic and clinical outcomes³⁰.

Alternatives: Alternatives include aTSA performed via the standard deltopectoral approach with a lesser tuberosity osteotomy, aTSA with a subscapularis peel or tenotomy, hemiarthroplasty, and other subscapularis-sparing aTSA techniques.

Rationale: The SWAT approach preserves the deltoid and the subscapularis by utilizing a deltopectoral approach and creating an inferior window to remove inferior osteophytes. This technique allows for adequate glenohumeral joint access, bone preparation, and implant selection and implantation. SWAT for aTSA is substantially different from other techniques described in the literature. Other subscapularis-sparing techniques require splitting of the deltoid^16-19,22 and substantial release of the inferior subscapularis^20,21 and have limitations related to difficult visualization of the humeral head for an accurate cut, difficult inferior osteophyte resection, and potential malalignment of the humeral components¹⁷. The SWAT has several benefits, including preservation of the deltoid, preservation of the subscapularis, the use of an inferior window to allow for complete removal of humeral osteophytes, adequate bone preparation, and accurate implant sizing and implantation.

Expected outcomes: Because of the complete preservation of the subscapularis, this technique offers the advantage of early postoperative motion with no restrictions. This technique also avoids the deltoid split that is required in some of the other subscapularis-sparing techniques. The SWAT aTSA is ideal for patients who would benefit from early mobilization and increased independence. The use of a sling can be discontinued early, and patients typically are satisfied with the level of function achieved in the first 2 weeks postoperatively. One prior study showed that this technique can be utilized to reliably attain acceptable radiographic and clinical outcomes³⁰. No revisions or cases of mechanical failure were noted during the early postoperative period³⁰. Therefore, the SWAT aTSA is a good option for patients without help at home and patients who will not tolerate, or whose quality of life will be substantially altered by, the use of a sling.

Important tips: Keep in mind that the SWAT can always be extended by takedown of the subscapularis at any point if there is concern regarding adequate access, especially if the surgeon is still learning the technique.The prevention of subscapularis rupture is based primarily on patient selection and intraoperative evaluation to confirm the integrity of the subscapularis. The subscapularis will not experience excessive stretching if the humerus is displaced inferiorly.Use of the inferior window to access and remove inferior osteophytes requires special care to protect the soft tissues by both directing the osteotome toward the glenoid during osteophyte removal as well as positioning the shoulder in adduction and external rotation. Place the elbow of the patient's arm toward their navel.Removal of osteophytes and release of the humeral attachments of the capsule through the inferior window are crucial, even in cases with a small osteophyte; the releases and osteophyte removal that occur with use of the osteotome act to release the inferior joint capsule, which is important for mobilizing the humerus inferiorly when accessing the glenoid.An intramedullary guide is utilized to assist in obtaining a reproducible 135° humeral head cut at the anatomic neck plane to match the neck-shaft angle of the humeral implant.It is also important to utilize a saw blade with a limited excursion width. When performing the head cut through the rotator interval, having a blade with a limited excursion and having the arm in adduction will protect the soft tissues (i.e., the axillary nerve and subscapularis tendon) and will be less likely to damage the glenoid.The trial broach positioning will allow a suboptimal head cut to be identified and corrected. If unsatisfied with the size of the head cut following removal of the guide, the surgeon can utilize a calcar planer as necessary to remove additional bone.Several techniques can be utilized to match the humeral components with the patient's premorbid anatomy and to avoid overstuffing. Preoperative templating with use of computed tomography scans and planning software helps to assess the appropriate head size. The use of a stemmed implant is preferred because it allows for the use of an intramedullary cutting guide, and having the stem helps to ensure appropriate implant positioning. Assessing the head cut and final stem position on fluoroscopy is also helpful.The final stem and head choices are impacted together on the back table and are implanted as an assembled humeral component. A tag suture is placed on the edge of the subscapularis and superior cuff in order to help facilitate implantation of the assembled humeral component.

Acronyms and abbreviations: SWAT = subscapularis-sparing windowed anterior techniqueaTSA = anatomic total shoulder arthroplastyLTO = lesser tuberosity osteotomyMRI = magnetic resonance imagingCT = computed tomography.