Implant Options: Glenoid and Humerus



Implant Options: Glenoid and Humerus


Georges Haidamous, MD

Mark A. Frankle, MD



INTRODUCTION

Reverse total shoulder arthroplasty (RTSA) is the most commonly performed shoulder replacement procedure in the United States.1 New generation RTSA designs have revolutionized the treatment of various shoulder conditions beyond cuff tear arthropathy.2,3,4 A variety of implant designs are available5; however, the clinical superiority of one over the other has not been established.6,7,8 In the absence of well-defined criteria guiding implant choice, this process is still primarily surgeon dependent. Therefore, it is important to understand how different implant options affect the spatial positioning of the humerus with respect to the scapula and, in turn, shoulder biomechanics and postoperative outcomes.


ORIGINAL RTSA DESIGN CONCEPTS

In the early 1970s, Neer proposed the concept of reversing the ball and socket in cuff-deficient patients using a fixed-fulcrum design.9 Three versions of this highly constrained system were developed, but the concept was abandoned because of early glenoid component failure.10 Consequently, other design attempts were equally unsuccessful.11,12,13,14,15,16 In 1985, Paul Grammont reintroduced Neer’s concept, using (1) a two-thirds of a sphere ceramic glenoid component called the “glenosphere,” and (2) a cemented 155° polyethylene (PE) stem.17 The glenosphere shape was then modified by Baulot into a press-fit hemisphere, fixed by divergent superior and inferior screws,18 also known as the Delta III glenosphere. The hemispheric design further medialized the center of rotation (COR) to the glenosphere/glenoid interface. Medialization of the COR, in addition to the 155° stem, placed the humerus in a more medial and distal position. This implant combination was thought to result in a more efficient deltoid lever arm compensating for the dysfunctional rotator cuff, as well as increased deltoid muscle tension that augmented joint compression forces and stabilized the construct.19 Nevertheless, early outcome reports on the Delta III design have propagated an unsubstantiated causality between COR lateralization and increased glenoid loosening caused by increased torque on the fixation interface observed with previous (ie, more lateralized) designs.20,21


LIMITATIONS OF ORIGINAL RTSA DESIGN AND PROPOSED MODIFICATIONS

The original design, however, had its shortcomings, which included limited rotational range of motion (ROM) due to the reduced tension of the rotator cuff muscles as well as notching due to increased contact between the humeral component PE liner and the inferior scapular neck.2,22,23,24 As such, changes were proposed with the goal of repositioning the COR closer to that of a normal shoulder and increasing the clearance distance between the humerus and the scapular neck.25,26,27,28,29 This was achieved by (1) lateralizing the glenosphere’s COR with metal30 and bone graft augmented (BIO-RSA) constructs31 and (2) lateralizing the humerus using an onlay stem design.32 In addition, the latter was also achieved by decreasing the neck-shaft angle (NSA) from 155° to 135°.28,32 This allowed for different implant combinations and has resulted in more than 29 implant designs that are available in the marketplace.5 This multitude of designs have led researchers to attempt to understand which construct most closely replicates the normal shoulder and leads to optimal outcomes using classification systems based on glenosphere and stem lateralization.24,33 However, the majority of the research comparing these configurations has been based on biomechanical and computer simulation studies,24,25,26,27,28,29 whereas patient outcomes remain less well understood across designs6,7,8 as they are influenced by several factors not assessed in biomechanical models, such as surgical technique, amount of bone resection and loss, and soft-tissue tension.


IS THERE AN IDEAL RTSA DESIGN?

Major advancements have been made in RTSA over the past 35 years after Grammont introduced his concept, allowing patients undergoing this procedure to experience improved outcomes.2,34,35 Nevertheless, several challenges still exist, such as managing severe bone loss, improving implant stability and bone fixation, and surpassing the RTSA’s expected “70% of a normal shoulder” functionality.36 The ideal RTSA design would theoretically optimize muscular balance around the shoulder, reestablish anatomic force vectors across
the COR, and maximize impingement-free ROM by restoring the native scapulohumeral positions, while simultaneously preserving the stability of the construct. In addition, components would have excellent bony fixation.

It is still uncertain whether the RTSA concept we know today will be capable of reproducing the normal shoulder function, especially with evidence of important kinematic differences.37 Nevertheless, literature suggests that a particular component combination might be optimal for specific pathologies and outcomes.6,7,8,38 For example, a computer modeling study investigated factors leading to the greatest impingement-free ROM in 126 RTSA implant combinations and showed that the hierarchy of implant and technique-related factors leading to an improvement in abduction ROM are different from those leading to an improvement in rotational ROM.39 TABLE 22.1 demonstrates how different component parameters can have different effects on outcomes.


GLENOID FIXATION

Achieving secure glenoid fixation is essential to minimize glenosphere failure, which has been attributed to the inability of the baseplate to overcome the forces exerted on the implant/bone interface before adequate bony ingrowth occurs.49 To achieve bony ingrowth, it is important to minimize micromotion to less than 150 µm50 between the baseplate and screws.51 Increasing the length, diameter, and inclination of the peripheral screws can decrease micromotion,51 which does not appear to be affected by hybrid screw use (ie, locking and nonlocking screws) as long as at least one locking screw is utilized.52 The use of longer screws may compensate for the use of fewer screws53,54 when bone quality or bone loss interferes with screw placement. In addition, fixation of the baseplate also depends greatly on the quality of the bone,49,55 which becomes more challenging in the setting of severe glenoid bone loss.56 Bone loss often results in reduced backside contact between the baseplate and glenoid bone, which further decreases the component’s threshold for withstanding joint forces.57

Another essential requirement for achieving baseplate fixation is obtaining adequate baseplate compression via both the baseplate’s central axis and peripheral screws.58 Central axis fixation is usually in the form of a screw or a post (FIGURE 22.1). Biomechanical evidence suggests that baseplates with central screws achieve a 10-fold increase in compression forces compared to posts (2000 vs 200 N), as well as a 2.3-fold increase in load to failure.58 Adequate baseplate compression coupled with decreased micromotion at the peripheral screws is crucial to decreasing glenosphere failure. Earlier reports showed that the use of 3.5-mm nonlocking screws with a central screw baseplate resulted in a glenosphere failure rate of 10%.31 Electron microscopy assessment of the retrieved components revealed that implant failure was due to decreased bone ingrowth.59 A subsequent biomechanical evaluation comparing the use of peripheral 3.5-mm nonlocking screws to 5.0-mm locking ones showed that the latter resulted in 29% less micromotion.51 These benefits were also validated in a clinical study demonstrating a glenoid failure rate of 0.4% at 5-year follow-up.60


GLENOSPHERE LATERALIZATION

Regardless of implant design, the glenohumeral COR shifts medially and inferiorly compared to that of a normal shoulder following RTSA; however, the degree of this differs between designs.61 Currently, there are two methods to achieve lateralization on the glenoid side. The first is through prosthetic lateralization of either the baseplate or the glenosphere. For the glenosphere, lateralization is dependent on the degree of sphericity rather than the size of the component (FIGURE 22.2). The second method is by using bone graft augmentation (BIO-RSA). The two methods appear to have different biomechanical properties. A finite element analysis study comparing prosthetic to bony lateralization revealed that with BIO-RSA, only 5 mm of lateralization is mechanically acceptable as opposed to 10 mm with prosthetic lateralization and that baseplate stresses and displacement are increased with the former.62 To decrease bony impingement, a glenosphere lateral offset of 5 mm or more has been shown to be ideal.63,64 Gutierrez et al used computer simulation to examine the effects of different implant factors on abduction ROM.27 They observed that impingement occurred at the acromion superiorly, at the glenoid and scapular neck inferiorly, and that the most important factor for maximizing ROM is the lateral COR offset of the glenosphere (FIGURE 22.3C and D). A linear correlation was also detected between COR lateralization and abduction (ie, as COR lateralization increases, abduction increases).

Glenosphere lateralization also significantly increases anterior stability of the prosthetic construct65 and restores the length of the internal and external rotators of the shoulder7 (FIGURE 22.3B). A biomechanical analysis evaluating changes in muscle lengths and moment arms from pre- to post-RTSA implantation using 8 mm of glenosphere lateral offset showed that the subscapularis and teres minor maintained their lengths and rotational moment arms and exhibited an increase in their flexion forces.66 Likewise, findings from a computer modeling study demonstrated that an 8 mm increase in lateral glenosphere offset increased the length of the infraspinatus by 5%, the teres minor by 11%, and the teres major by 7%.25 Clinical studies support these findings, particularly with respect to the improvement in external rotation (ER).7,39 In a prospective randomized study, Grenier et al compared patients with 10-mm bone graft augmentation versus those who


did not.7 They found a statistically significant improvement in ER in bone graft patients with an intact teres minor. In addition, there is evidence that patients with a severe ER deficit (<0°), who received glenospheres with 6 and 10 mm lateralization, experienced significant improvement in ER (>49°) without additional latissimus dorsi transfer.67







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Jun 23, 2022 | Posted by in ORTHOPEDIC | Comments Off on Implant Options: Glenoid and Humerus
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