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4. Biomechanics of rTSA
4.1 Biomechanics of Rotator Cuff-Deficient Shoulder
The glenohumeral joint is the most mobile joint in the human body. The stability of the joint is provided by a combination of both static and dynamic factors. The bones, the ligaments, and the capsule represent the static stabilizers. The ligamento-capsular complex is crucial to the end range of motion when they are stretched [1]. The rotator cuff muscles are m. supraspinatus, m. infraspinatus, m. teres minor, and m. subscapularis and are the dynamic stabilizers. They provide “contraction-compression” model of stability [2]. The contraction both centers the head and compresses it against the glenoid fossa. They are most effective in the mid- and end-range of motion. Their action is best described as stability by balancing the force couples. Infraspinatus, teres minor, and subscapularis provide a net inferiorly directed force; deltoid muscle provides a net superiorly directed force resulting in net force balance in coronal plane. In the analogous manner, subscapularis is balancing infraspinatus and teres minor muscles in sagittal plane [3]. The rotator cuff actively stabilizes and opposes upward motion of the humeral head during contraction of the deltoid muscle.
Loss of the normal force couples about the shoulder with massive rotator cuff tears leads to an alteration of the compressive forces and consequently to deterioration of the concentric arc of motion. Without the force provided by the inferior rotator cuff, the humeral head starts to migrate superiorly resulting from the unopposed contraction of the deltoid muscle. So instead of an arm elevation with muscle contraction, it becomes more of a translational movement [4].
4.2 The History of the Concept of rTSA
The usage of reverse total shoulder arthroplasty (rTSA) has gained significant popularity, and instead of being a procedure of desperation, it is a procedure of choice for the management of the cuff-deficient arthritic shoulders.
However, attempts to compensate for rotator deficiency with the idea of fixed fulcrum with a ball in the proximal humerus and a socket in the glenoid date back to the 1970s when a number of reversed implants were designed. It was Neer that designed the reverse prosthesis with the hope that the given stability might compensate the rotator deficiency. He designed three consequent versions of fixed fulcrum rTSAs between 1970 and 1973: Mark I, Mark II, and Mark III. The first, Mark I, included a large ball for more motion and a glenoid implant fixed with acrylic cement, but because of the size of the ball, the rotator cuff cannot be attached. Mark II was different by smaller ball to allow rotator cuff repair, but this led to limited range of motion. Mark III design kept the smaller ball and added an axial rotation feature between the humeral stem and diaphysis to allow more motion. Unfortunately, this prosthesis was also unsuccessful due to dislodgement of the scapula in one patient. So there was a consensus that the scapula was not adequate to handle the forces transferred to it; with these constrained designs, the reversed arthroplasty was abandoned [5, 6].
It was Paul Grammont in 1985 that designed the first reverse shoulder prosthesis that worked. The first prosthesis designed in 1985 included two pieces, one metallic glenoid or “glenosphere,” which represented two-thirds of a sphere, which was cemented, and the other humeral, polyethylene, which represented one-third of a sphere and was also cemented. The results of eight cases were published in 1987 [7]. The center of this prosthesis was still too lateral, which led Grammont to evolve his prosthesis to a half-sphere.
The second prosthesis, designed in 1989, is the prosthesis “Delta™ III,” whose name recalls that it is only animated by the deltoid muscle alone. The first implantation dates back to 1991. The prosthesis, modular, contains four original pieces: glenoid baseplate (without cement), glenosphere (third of sphere and not two-thirds), humeral stem, and humeral cup. The glenoid baseplate is fixed using a central peg and four screws including two, upper and lower, divergent, which are essential to neutralize the shear forces. Osteointegration of glenoid baseplate is favored by a hydroxyapatite coating [8, 9].
His idea was to medialize the center of rotation by eliminating the neck of the glenoid implant. Thus, forces of the deltoid acting on the fixed fulcrum can convert the upward pull force of the deltoid into rotatory movement capable of elevating the arm. New center of rotation can recruit more of the deltoid fibers, minimize the shearing forces, and turn them into compressive. The glenoid component is one-third of a sphere with a large diameter allowing greater range of movement before impingement occurs.
When Paul Grammont presented his concept of rTSA, it has two main differences from previous designs: (1) a large glenoid hemisphere with no neck, fixed directly to the glenoid, and (2) a small humeral cup oriented with a nonanatomic inclination of 155 covering less than half of the glenosphere [10]. The result was stable prosthesis with medialized and lowered center of rotation, minimized shearing forces acting on the glenoid, increased deltoid lever arm, and lowered humerus [8, 10, 11].
4.2.1 Center of Rotation and Glenoid Positioning
Since the rTSA glenosphere is fixed to the native glenoid surface, the distance from the glenoid surface to the center of rotation is directly proportional to the mechanical torque about the component and the shear forces at the glenoid bone prosthesis interface.
The concept of the rTSA with a fixed fulcrum for rotation was originally adapted from the total hip arthroplasty designs. These original designs maintained the center of rotation of a normal shoulder joint. Unfortunately, fixation of the glenoid component could not withstand the shear forces created at the bone prosthesis interface and resulted in unacceptably high rates of early mechanical failures.
In order to address this issue, Grammont eliminated the neck of the previous implant designs by utilizing one-third of a sphere that was fixed directly onto the glenoid. This change medialized the center of rotation to the glenoid surface, effectively minimizing the shear forces across the glenoid bone prosthesis interface [10, 12].
Medialization of the center of rotation thus effectively converts the mechanical torque at the glenosphere into more compressive forces across the prosthesis–bone interface.
Also, another beneficial outcome is that medicalization of the center of rotation by 10 mm increased the deltoid moment by 20% and that distalization of the center of rotation by 10 mm increased the efficacy of the deltoid by another 30%.
Positioning of the glenoid component is currently an area of active research; the study of glenoid position may have important consequences for maximizing the impingement-free arc of motion.
In a cadaveric biomechanical study conducted by Nyffeler et al., four different positions of glenosphere were tested: glenosphere centered on the glenoid, leaving the inferior glenoid rim uncovered; glenosphere flush with the inferior glenoid rim; glenosphere extending beyond the inferior glenoid rim; and glenosphere tilted downward 15°. The relation between different glenoid component positions and glenohumeral range of motion was examined. They concluded that placing the glenosphere distally significantly improved adduction and abduction angles compared with all other test configurations [13].
Gutiérrez et al. in an in vitro study analyzed abduction/adduction motion and its dependence on five surgical (location of the glenosphere on the glenoid and tilt angle of the glenosphere on the glenoid) and implant-related factors (implant size, center-of-rotation offset, and humeral neck-shaft angle) on impingement-free abduction motion. They concluded that largest average increase in the range of impingement-free abduction motion resulted from a more lateral center-of-rotation offset. The position of the glenosphere on the glenoid was associated with the second largest average increase in abduction motion (when the glenosphere position was changed from superior to inferior). The largest effect in terms of avoiding an adduction deficit was provided by a humeral neck-shaft angle of 130, followed by an inferior glenosphere position on the glenoid, a 10-mm lateral offset of the center of rotation, inferior tilt of the glenosphere, and a 42-mm-diameter prosthetic size [14].
4.2.2 Stability
Although improving glenohumeral stability is the ultimate aim of RSA, subluxation and dislocation of RSA devices still occur. Dislocation rates have been shown in the range of 2.4, 6.3, 8.6, 16.7, and 31% [15–17].
Glenosphere-humerosocket stability is an important variable in selecting an appropriate RSA and is closely correlated to compressive force, socket depth, and to a lesser extent on implant size. Since the dynamic stabilization normally provided by the rotator cuff muscles is absent in the patient undergoing a rTSA, in order to maintain the relative position of the humerus against the glenoid, the rTSA design places the convex surface on the glenoid and the concave surface on the humerus. This effectively “constrains” the joint and prevents the humerus from translating superiorly against glenoid even during deltoid contraction.
The radii of curvature of the humerus and the glenoid are identical, imposing concentric motion.
Increased constraint secondary to the deeper and more conforming concavity of the humeral articular surface prevents glenohumeral translation while providing sufficient stability for functional range of motion. This high degree of intrinsic stability frees the reverse total shoulder prosthesis from dependence on active stabilization by concentric compression and provides a stable fulcrum for the remaining musculature.
The angle that the total joint force vector can subtend without risk of dislocation with the center line is thereby increased to ≥45°.
In a study evaluating the hierarchy of stability factors in the reverse shoulder, Gutierrez et al. found that the net compressive force acting on the glenohumeral articulation is the most significant element of stability [18].
Clinically, the compressive force is largely generated by active and passive structures of soft tissue together with the negative pressure within the glenohumeral joint. To date, techniques described to enhance RSA stability through soft tissue tension have focused on tensioning of the deltoid. This may be accomplished by lowering the humerus relative to the glenoid, by lengthening the humerus by inserting a thicker polyethylene humeral component and retaining as much proximal humerus as possible, or by lateralizing the humerus.
Stability also depends on glenoid component positioning. Glenosphere retroversion >20 has been shown to reduce anterior stability while the arm is in the resting position. In addition, placing the glenosphere in a position of inferior offset has been shown to increase stability by approximately 17%. Humeral component version has little effect on stability.
4.2.3 Role of Deltoid in rTSA
The rTSA was developed to optimize functional outcome by making better use of the patient’s remaining musculature. The system is designed both to re-tension and to reposition the deltoid in relation to the joint’s center of rotation. The lever arm of the deltoid muscle is almost doubled with a reverse TSA prosthesis; thus, the efficacy of the deltoid for abduction is also approximately doubled.
A medialized center of rotation increases the deltoid’s moment arm by 20–42% and recruits additional fibers of the anterior and posterior deltoid to serve as abductors.
The fibers that are medial to the center of rotation in a normal shoulder come to lie lateral to the center of rotation and thereby become abductors and/or elevators. Thus, it is presumed that the longer lever arm resulting from the reverse prosthesis allows the recruitment of more deltoid fibers for elevation and abduction. Conversely, the anterior and posterior deltoid fibers lose their external and internal rotator moment.
Thus, active external rotation in particular is often further compromised after rTSA [19, 20].
