Biomechanical Study of a Subscapularis Repair Technique for Total Shoulder Arthroplasty

Approach and Humeral Preparation

Total shoulder arthroplasty with a suture-incorporating prosthesis is performed with the patient in the beach chair position. A standard deltopectoral approach to the shoulder is used to provide exposure of the subscapularis and rotator interval. The surgeon may perform subscapularis tenotomy, subscapularis peel, or lesser tuberosity osteotomy to gain access to the glenohumeral joint. The authors' preferred technique is to perform a subscapularis peel that involves removal of the entire tendon from its footprint on the lesser tuberosity.

Once the subscapularis is released, the glenohumeral joint can be disarticulated to deliver the proximal humerus into the surgical field. Any osteophytes that distort the native anatomy are removed with a rongeur. Preparation of the humerus begins with an anatomic neck cut. A sound is used to identify the humeral canal, followed by reaming and broaching until a snug fit without toggle is achieved. The size of the final broach corresponds to the size of the final humeral implant.

Through-implant Subscapularis Suture Repair

After broaching, the final humeral prosthesis is ready to be inserted. On the back table, the prosthesis is loaded with 6 nonabsorbable sutures, for a total of 12 suture strands to be tied (Figure 1). The first 2 sutures are passed through the lateral fin of the implant; the superior suture is blue, and the inferior suture is tiger striped. The remaining 4 sutures are passed through specially designed suture holes along the medial edge of the implant. It is important to alternate the suture color to facilitate knot tying later in the case. Before implantation, 2 holes are drilled in the bicipital groove to receive the 2 sutures that were initially passed through the lateral fin on the prosthesis (Figure 2). The authors use the prosthesis itself as a guide for the location of these holes and pass the sutures with a suture passer. The implant is then impacted into the humeral canal, with care taken to maintain tension on the medial suture strands so that they retain their order (Figure 3). With the implant seated, the 8 medial suture strands are passed through the subscapularis tendon in a uniform fashion (Figure 4). The authors' preferred method is to pass the suture with an Arthrex FastPass Scorpion suture passer (Arthrex, Inc, Naples, Florida), but free needles also can be used.

Figure 1: The prosthesis is loaded with 6 nonabsorbable sutures, for a total of 12 suture strands to be tied.

Figure 2: Before implantation, 2 holes are drilled in the bicipital groove to receive the 2 sutures that were initially passed through the lateral fin on the prosthesis.

Figure 3: The implant is impacted into the humeral canal, with care taken to maintain tension on the medial suture strands so that they retain their order.

Figure 4: With the implant seated, the 8 medial suture strands are passed through the subscapularis tendon in a uniform fashion.

Suture Configuration

The knot tying sequence follows a specific pattern, as shown in Figure 5. No sutures are tied to themselves, and each strand is tied to a different strand in the configuration. Therefore, the suture-implant complex does not become taut until the final 2 knots are tied. Knot tying is facilitated by the placement of suture strands in the prosthesis so that strands of the same color are tied together, as shown in Figure 5A. The tying sequence begins by tying the superiormost strand of the medial 8 strands (blue) to 1 of the superior strands from the lateral fin (also blue), as shown in Figure 5B. Next the inferiormost strand of the medial 7 strands (tiger stripe) is tied to 1 of the inferior strands from the lateral fin (also tiger stripe), as shown in Figure 5C. Next the remaining inferior suture from the lateral fin (tiger stripe) is tied to the third strand from the top of the remaining 6 medial strands (also tiger stripe), as shown in Figure 5D. Then the final superior suture from the lateral fin (blue) is tied to the third strand from the bottom of the remaining 5 medial strands (also blue), as shown in Figure 5E. Finally, the top 2 and bottom 2 medial strands are tensioned to tighten the construct and then they are tied together. These final 2 knots break the rule of matching suture colors, as shown in Figure 5F. The final construct should have secure knots that are evenly spaced about the tendon footprint (Figure 6).

Figure 5: Initial layout of the 12 strings of sutures (A). Superior knot tying (B). Inferior knot tying (C). First lateral to medial knot tying (D). Second lateral to medial knot tying (E). Medial row knot tying and final knot configuration (F).

Figure 6: The final subscapularis repair should have secure knots that are evenly spaced about the tendon footprint.

Cadaveric Study

To test the strength of subscapularis repair performed with a through-implant technique, the authors obtained 20 matched pairs of fresh frozen cadaveric shoulders with overlying soft tissues removed, an intact rotator cuff, and a potted humerus. Cadavers were all male, with a mean±SD age of 58.1±10.6 years and no previous shoulder injury or surgery. An uncemented short-stemmed humeral prosthesis that facilitates through-implant repair (Univers Apex; Arthrex) was implanted in all specimens. Cadavers were prospectively randomized to side (right vs left) and surgical technique (lesser tuberosity osteotomy vs subscapularis peel). For each cadaver, the subscapularis was repaired with a lesser tuberosity osteotomy with standard suture technique¹⁰ on 1 side, whereas the contralateral subscapularis was repaired with a novel through-implant suture repair and subscapularis peel technique. Specimens that received lesser tuberosity osteotomy repair constituted the control group, and those that received the subscapularis peel technique and through-implant suture repair constituted the experimental group. Lesser tuberosity osteotomy repairs were made with 4 No. 5 FiberWire sutures (Arthrex) that were passed through bone tunnels in the lateral bicipital groove and around the humeral stem before implantation. The free suture tail was then passed through the tendon with a modified Mason-Allen stitch and tied over the lesser tuberosity osteotomy.¹¹ The same technique as described earlier was used for all through-implant subscapularis suture repairs in the experimental group. Using a hydraulic testing system (Instron, Canton, Massachusetts), each specimen was cycled at 10 to 100 N for 500 cycles and then pulled to failure at 33 mm/s. Displacement of the subscapularis footprint was measured from its original position after 10 cycles and again after 500 cycles. Ultimate load to failure of each construct was quantified, and the mechanism of failure was documented. The stiffness of each construct was calculated as force/displacement at the bone-tendon junction. Statistical analysis was performed with Student's t test to compare mean and SD, with significance set at alpha=0.05.

Limitations

Limitations of the study include its short-term nature. Biomechanical studies and short-term clinical outcome studies do not provide information on the ultimate healing potential of this construct. However, this study was conducted similarly to other studies of subscapularis repair.^4,7,10,13 Further clinical study of the suture-incorporating humeral prosthesis is needed to determine whether it offers a true benefit in terms of shorter operative time and improved patient outcomes.