Equinoxe rTSA Platform Shoulder System Design Rationale

Christopher P. Roche, MS, MBA
Exactech, Inc.

Pierre-Henri Flurin, MD
Clinique de Sport Bordeaux-Mérignac

Lynn Crosby, MD
Georgia Regents Medical Center

Thomas W. Wright, MD
University of Florida College of Medicine

Joseph Zuckerman, MD
NYU Hospital for Joint Diseases

The Equinoxe reverse shoulder was first implanted in March 2007 (Figure 1). The primary development goal was to significantly reduce the complication rates reported with all previous reverse shoulder arthroplasty (rTSA) prosthesis designs, including: scapular notching, instability/dislocation, aseptic glenoid and humeral loosening, lack of active internal/external rotation, acromial/scapular fractures, and deltoid-fatigue.1-3 Additional concerns included difficulty in revisions, bone conservation, and lack of compatibility between primary and revision components. Many of these complications, failure modes, and concerns may be inter-related and a function of non-optimized prosthesis design. The following explains the specific design rationale and presents some validation tests that demonstrate successful mitigation of these complications.

Figure 1. Equinoxe Reverse Shoulder



Scapular notching is initiated by repetitive mechanical humeral liner impingement on the scapular neck and inferior glenoid.3 Relative to the Sirveaux grading scale,3 the geometric limit of impingement with most prosthesis designs is grade 2 (i.e., to the inferior glenoid baseplate screw). However, scapular notching has been documented to be progressive4-5 beyond these limits of impingement (e.g. Sirveaux grades 3 or 4) due to the biologic response. Scapular notching has recently been demonstrated to negatively impact clinical outcomes3,6-8 and negatively impact glenoid fixation,9 which is in stark contrast to initial reports.10-11

By minimizing humeral liner impingement, scapular notching can be minimized. To this end, a 3-D computer impingement model was developed prior to design of the Equinoxe shoulder to analyze 32 different geometric permutations of the Grammont reverse shoulder by independently varying humeral neck angle (7 angles: 135-165°), humeral liner constraint (5 constraints: 0.25-0.3), glenosphere thickness (7 thicknesses: 18-24mm), glenosphere diameter (6 diameters: 34-44mm), and inferior glenoid offset (7 offsets: 0-6mm inferiorly).12-15 Each design parameter was independently evaluated to identify its role in minimizing scapular impingement, maximizing overall range of motion (ROM), and maximizing “jump” distance. Specifically, three glenosphere sizes (38×21, 42×23, and 46x25mm) with a humeral neck angle of 145°, a curved-back glenoid baseplate with a 4mm superiorly offset cage peg, and different combinations of humeral liner were designed.

 The clinical results of these designs have been encouraging. A recent radiographic study demonstrated the Equinoxe to have a scapular notching rate of 13.2 percent in 151 patients at a mean follow-up of 28.3 } 5.7 months, with only 2.6 percent grade 2 notches and no grade 3 or 4 notches.16 Another large scale radiographic study demonstrated that 26 of 256 patients (10.2 percent) had a scapular notch, where 38, 42, and 46mm glenospheres had a notching rate of 14.2 percent, 4.4 percent, and 0 percent, respectively at an average follow-up of 22.2 } 8.7 months.15 These results represent a significant reduction in the reported scapular notching rate of the Grammont design, which has an average reported scapular notching rate of 68.2 percent.1-4,7,11,17-18


Aseptic glenoid loosening was the historical failure mode of pre-Grammont reverse shoulder designs that did not utilize a hemispherical glenosphere to minimize torque on the glenoid fixation surface.1,19-20 The Equinoxe RTSA leveraged the Grammont clinical history by maintaining the center of rotation (CoR) near the face of the native glenoid and incorporating the optimization analysis recommendations for ideal CoR placement 2mm lateral to the native glenoid. To neutralize this slightly increased torque on the fixation interface, the baseplate surface contact area with the native glenoid was increased by changing the shape, size, and backside curvature from the circular 29mm diameter flat-back Grammont design. Additionally, by minimizing humeral liner impingement, polyethylene wear was reduced; thereby reducing particles that are the primary osteolytic agents in aseptic loosening.21

The Equinoxe baseplate is an oval 25x34mm curve-back design (Figure 2). The design evolution of the glenoid prosthesis with anatomic shoulder arthroplasty (aTSA) started with circular profile devices that ultimately converged to an anatomic pear-shaped profile device. For the same reasons, the Equinoxe baseplate is superiorly elongated in the primary loading direction to neutralize any destabilizing action of the deltoid. By superiorly elongating the baseplate, the surface contact area is increased, affording the possibility to reduce the anterior/ posterior width from 29mm to 25mm and facilitate a more anatomic fit in smaller glenoids.22 Recent work evaluated eight baseplate designs and demonstrated that the Equinoxe had the largest surface contact area on the backside of the plate, 20 percent larger than the next largest baseplate.23 The screw hole pattern is maximized by positioning the screws to the edge of this enlarged periphery and increasing the number of screw options from four to six to provide surgeons with additional intra-operative flexibility. To maximize the length of screws used to achieve fixation, poly-axial compression screws that provide 20° of angular variability are utilized and each screw is locked with a cap to prevent backing out; the importance of angular screw variability was emphasized in a recent study with a competitive rTSA design.24

Figure 2. Equinoxe Glenoid Baseplate


The Equinoxe baseplate is 2 inch diameter curved backside geometry closely matches the native glenoid curvature preserving cortical and cancellous bone, and increases cortical bone contact to maximize baseplate support.22 Recent work quantified the cortical and cancellous glenoid bone removed by three different commercially-available rTSA prosthesis designs and demonstrated the Equinoxe removed the least glenoid bone and had the most cortical, cancellous, and overall glenoid bone surface contact area relative to the Depuy Delta III and DJO RSP.25

While preserving glenoid bone, achieving optimal screw length/placement, and maximizing baseplate surface contact are all important contributors to fixation, these are but a few of the variables that establish initial fixation. ASTM Standard F2028-1427 was developed to objectively evaluate and quantify the fixation of rTSA prostheses before and after a clinically relevant cyclic loading pattern. The ASTM rTSA glenoid loosening test has previously demonstrated differences in fixation between screw configurations, 27 medialized/lateralized CoR,28-30 glenoid baseplate designs,29-30 scapular defects and wear patterns,9, 32 and different densities of substrates.27,30-31 Two recent studies quantified the fixation of six different commercially available rTSA prostheses in both low and high density polyurethane blocks.30-31 These studies demonstrate that the Equinoxe and Delta III devices had significantly better glenoid fixation than each of the Zimmer, DJO, and BIO-RSA devices. Additionally, catastrophic failure was observed in at least one of each of the Zimmer, DJO, and BIO-RSA test components during cyclic loading; no failure occurred in either of the Equinoxe or Delta III devices. 30-31 While designs with a more lateralized CoR generally performed poorly, other factors may impact fixation. The Equinoxe design performed significantly better than the Zimmer device, despite each having identical 2mm lateralized CoRs. Thus, subtle differences in baseplate design can significantly impact fixation.

Aseptic glenoid loosening is more likely in eroded scapular morphologies. Generally, surgeons eccentrically ream an eroded glenoid to correct the defect. Eccentric reaming medializes the joint line and removes good, non-worn glenoid bone to correct the defect, which may compromise fixation. 32-33 To conserve glenoid bone, increase prosthesis surface contact area with cortical bone, and to better restore the native joint line when performing rTSA in eroded scapular morphologies, 25,32,34 the Equinoxe system provides unique augmented glenoid baseplates (Figure 3).

Figure 3. Equinoxe rTSA Baseplates; from left to right: Standard, 8° Posterior Augment, 10° Superior Augment, +10mm Extended Cage Peg, 10° Superior/8° Posterior Augment Baseplates



By minimizing humeral liner impingement with the scapula, the lever-out mechanism can be eliminated and the instability rate can be potentially reduced. Restoring the lateral position of the humerus may reduce the instability rate and also improve active internal/external rotation.1,10,21,34-42 The Grammont reverse shoulder medializes the humerus to such a degree that the rotator cuff is under-tensioned and deltoid wrapping around the greater tuberosity is reduced.34,36-42 By lateralizing the greater tuberosity to a more anatomic location, the Equinoxe can better restore rotator cuff muscle tension and deltoid wrapping (Figure 4).34,36-42

Figure 4. Impact of Humeral Medial/Lateral Positioning on Rotator Cuff Tensioning with TSA; from left to right: Anatomic Shoulder, Grammont, DJO RSP, Exactech Equinoxe reverse shoulders


A virtual shoulder model was developed to quantify muscle lengths,33-34,36,41-43 moment arms,36-39,43 and deltoid wrapping of different rTSA prosthesis designs, 34,39,41-44 implanted using a variety of implantation techniques41, and in a variety of different glenoid morphologies. 33-34 One recent study compared the muscle lengths and deltoid wrapping associated with the Depuy Delta III, DJO RSP, and Equinoxe designs along with the BIO-RSA technique. These results objectively demonstrated that designs and surgical techniques which resulted in more lateral humeral positioning were associated with more deltoid wrapping and better tensioning of the rotator cuff.41 The Delta III positioned the humerus most medially and shortened the rotator cuff by as much as 45.3 percent. We theorize that this magnitude of muscle shortening may be the primary mechanism for the limited improvements in active internal and external rotation reported with that prosthesis. The Equinoxe had the most lateral humeral position, most deltoid wrapping, and best restored the anatomic rotator cuff tension relative to the other rTSA prostheses evaluated. 41 These observations related to more anatomic rotator cuff tensioning and deltoid wrapping are likely responsible for the favorable ROM and clinical outcomes reported with the Equinoxe in a recent multi-center clinical study44 and compared favorably to that reported 1,3,5,8,11,35,45 for other rTSA designs.


There are other less common complications of rTSA. Aseptic humeral loosening is rare with aTSA.46 Given that loading of rTSA is generally of less magnitude and similar direction47-48 aseptic humeral loosening should also be rare with rTSA. We theorize that aseptic humeral loosening is reported more commonly with rTSA21,46 due to non-optimal humeral stem design and humeral implantation techniques which resect and/or spherically ream too much of the proximal humerus and create less rotationally-stable constructs than occurs with aTSA. Aseptic humeral loosening is further exacerbated when polyethylene particles illicit a biologic response.21 In the Equinoxe reverse shoulder design, aseptic humeral loosening was mitigated by utilizing the same rotationally-stable humeral stem and implantation technique of the clinically successful Equinoxe aTSA system. Additionally, by minimizing scapular notching, polyethylene particles are reduced. A recent computer analysis quantified the humeral bone removed by three different commercially-available designs and demonstrated the Equinoxe removed the least overall humeral bone relative to the Depuy Delta III and DJO RSP.25 Acromial/scapula fractures and the phenomenon of deltoid fatigue are other less common complications that are not well understood.

Acromial/scapula fractures have been reported to propagate from superior baseplate screws.49 We theorite the majority of these fractures are bone fatigue injuries due to repetitive overloading by the deltoid. The Equinoxe reverse shoulder CoR is maintained near the native glenoid to increase the deltoid abductor moment arm and minimize the necessary force generated by the deltoid to elevate the arm. By maintaining an efficient deltoid, the risk of stress-fractures and deltoid over-loading are minimized (Figure 5). It should be noted that there are a variety of rTSA design philosophies; with one philosophy lateralizing the CoR by up to 1cm to minimize scapular notching. While increasing glenosphere thickness does reduce impingement,12-13 it also increases the torque on the glenoid fixation surface and reduces the deltoid abductor moment arm, which increases the force required by the deltoid.36,38,39,50-51 A recent clinical study using a lateralized CoR rTSA design reported a 10.2 percent acromial/scapular fracture rate.52 This rate is significantly higher than that reported with the Grammont and other medialized CoR rTSA prostheses 46,49 and suggests that subtle design changes can have adverse clinical consequences.

Figure 5. Impact of Position of the Center of Rotation and Humerus on Deltoid Abductor Moment Arm & Deltoid Wrapping; from left to right: Anatomic Shoulder, Grammont, DJO RSP, Exactech Equinoxe reverse shoulders



Additional opportunities were identified in the Equinoxe rTSA to improve efficacy in revisions and conserve bone through better design. On the humeral side, the procedure was simplified by utilizing the same ream-broach humeral stem technique used with the Equinoxe aTSA system (rather than a non-standard 155° humeral neck cut utilized by the Grammont which also spherically reams the proximal humerus). Using the same humeral component for aTSA and rTSA allows standardizing many humeral instruments, reduces the number of trays needed and permits surgeons to leverage their existing training and surgical experiences. Crosby et al. reported numerus benefits of retaining the same humeral stem during revisions.53 By comparing revisions of platform humeral stems that were retained vs. non-platform humeral stems that were removed, revisions of platform humeral stems resulted in significantly less operating room time, less blood loss, and less overall procedure cost.53 On the glenoid side, many patients receiving rTSA in revision may have an implanted pegged or keeled glenoid. The size and position of the superiorly offset cage peg on the Equinoxe baseplate was designed to fill the central bone defect left by the explanted glenoid implant. The six screw-hole base plate allows appropiate positioning relative to the explanted glenoids to ensure that multiple options are available to achieve fixation. •


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