Today, there are 2 radically different kinds of total shoulder arthroplasties or replacements for individuals who suffer from severe shoulder joint pain due to arthritis or injury. While traditional shoulder arthroplasty has concentrated on replacing normal joint anatomy with plastic or metal components, the reverse total shoulder arthroplasty (RTSA) involves not only replacing joint components, but also repositioning them and altering the normal biomechanics of the shoulder. In cases where patients have sustained major muscle or tendon damage, making conventional replacement constructs more likely to fail, RTSA has become a viable solution for shoulder pain and disability. To understand how an RTSA works, a patient should first understand basic shoulder anatomy.
The glenohumeral, or shoulder, joint is classified as a ball-and-socket joint. It is formed by the articulation of the head (ball) of the humerus, or upper arm bone, with the glenoid cavity (socket) of the scapula (shoulder blade) (Fig.1). Since the cavity is shallow, there is little contact between the bones, but the glenoid labrum, a ring of cartilaginous fiber that lines its circumference, deepens the cavity by about 50%, allowing for more surface contact and a better fit. Nevertheless, the joint capsule, or envelope of fibrous connective tissue that attaches to the bones of the joints to seal the joint space and to help provide stability, is very loose. This makes the glenohumeral joint the most mobile of the body, capable of flexion (bending), extension (straightening), adduction (pulling toward the body), abduction (pulling away from the body) medial and lateral rotation (turning toward or away from the midline of the body), and circumduction (moving in a circle).
Each of these muscles originates from the scapula and has a tendon that attaches to the head of the humerus. Together they form “a cuff” around the shoulder joint, called the rotator cuff, which provides stability. This cuff is also important to shoulder movement and is frequently injured from playing sports or doing other activities involving repetitive arm and shoulder motion.
The capsule consists of a number of tendons (tissue that connects muscle to bone) along with bursae, or small fluid- illed sacs, strategically located to aid movement and prevent friction. Because it lacks strong ligaments (tissue that connects bone to bone) to support it, the glenohumeral joint is also known as a muscle-dependent joint. It is primarily stabilized by the biceps brachii, or muscle on the anterior (front) side of the upper arm, and the tendons of what are called the rotator cuff muscles. These include the supraspinatus, subscapularis (Fig. 2), infraspinatus, and teres minor muscles (Fig. 3).
Variations on Shoulder Arthroplasty
In traditional shoulder replacements, a plastic cup is used for the glenoid cavity of the scapula and a metal ball for the head of the humerus (Fig. 4). Outcomes from this procedure are generally good, but when a patient with shoulder arthritis also has a massive rotator cuff tear, conventional anatomic total shoulder replacements have higher complication rates. This is because, without an intact rotator cuff to stabilize the glenohumeral joint, its centers of force shift upward, causing the arthroplasty to loosen and fail.1 The problem with patients who also have rotator cuff injuries was recognized decades ago when it led to the development of the RTSA. This new type of prosthesis or implant reversed the normal anatomic configuration of the shoulder by placing the ball component of the implant in the glenoid socket of the scapula and the socket component in the humerus. In the 1970s, early designs of this arthroplasty often failed because the implant would loosen, break, or otherwise become unstable. Over time, the prostheses were designed to use the deltoid muscle (the rounded muscle on top of the shoulder) as a lever to compensate for the deficient rotator cuff. However, the early versions of this new design also frequently failed as the replacement constructs proved to be too constraining.
The Grammont Arthroplasty
In 1985, French orthopaedist Paul Grammont advanced the design for RTSA when he came up with the idea of altering the normal biomechanics of the shoulder joint. Over the next 20 years, engineering of the Grammont RTSA evolved to provide a viable solution for patients with arthropathy (joint pain) and rotator cuff deficiency (Fig. 4). The construct functioned by converting the forces acting on the joint and, as in the previous designs, using the deltoid muscle in the absence of an intact rotator cuff as a lever so the patient could lift his or her arm. The larger contact surface area of the glenohumeral articulation in this type of shoulder replacement meant that the implant was more stable than previous models; it prevented the humerus from shifting upward in patients with rotator cuff deficiency.
Lastly, patients who were having pain from shoulder arthritis could get relief from the resurfacing of the arthritic joint with a stable implant. While various manufacturers have since produced prostheses that feature their own specific modifications, all of these are based on Grammont’s original design. Additionally, even though the procedure is still largely used in cases of arthropathy and rotator cuff deficiency or as revision surgery when traditional arthroplasty fails, indications for RTSA have now expanded to include some fractures of the humerus.
Outcomes and Complications
RTSA offers new hope to patients suffering with shoulder pain and disability. According to doctors at Rush University Medical Center in Chicago, having this type of surgery substantially reduced shoulder pain in 9 out of 10 patients and led to a stabilized shoulder in at least 75% of cases. Despite these statistics and advancements in prosthesis design, outcomes for RTSA can vary widely and the procedure is associated with high complication rates ranging from 19 to 68% of cases.(2) For example, scapular notching or erosion caused by the humerus or the humeral prosthesis impinging on the inferior or lower neck of the scapula, is unique to reverse total shoulder surgery. Other common problems include instability and dislocation of the prosthesis. These complications have been attributed to poor soft tissue tension, implant misplacement, an improperly-sized implant, mechanical impingement, bone loss, and neurologic dysfunction. For instance, acromial (pertaining to the protruding bone on the top of the shoulder) and scapular fractures have been reported after surgery and attributed to minor postoperative injuries. Fractures can also occur during surgery through a variety of mechanisms including bone preparation and placing instrumentation in bone that is already weak and porous. Moreover, nerve injury can result from over-lengthening the arm with the implants and overstretching nerves. Additionally, the unavoidable dead space (opening in the tissues created by the procedure itself, which allows the accumulation of blood or serum) can permit a hematoma, a solid swelling of clotted blood within the tissues, to develop. As with other joint arthroplasties, infection associated with hematomas, dead space, revision surgeries, a breach in sterile technique, or a compromised immune system, can occur. Given these possible complications, RTSA should be recommended only for select patients and performed only by experienced surgeons. Even after a successful reverse total shoulder procedure, some patients may continue to experience limitations in joint mechanics and function.
A good option for a bad shoulder
Historically, shoulder pain and arthritis have been difficult to treat in patients with a rotator cuff deficiency or following failed conventional shoulder replacement surgery. Although there can be complications, reverse total shoulder surgery addresses the problems involved directly and to-date offers the best treatment option available for many patients with damaged rotator cuffs.