Geometric Analysis of the Grammont Reverse Shoulder Prosthesis: An Evaluation of the Relationship between Prosthetic Design Parameters and Clinical Failure Modes

*Roche, C; **Flurin, PH; ***Wright, T; ****Crosby, L; *****Zuckerman, J
*Exactech, Gainesville, FL; ** Bordeaux-Merignac Sports Clinic, FR;
***Univ. of Florida Dept. of Ortho., Gainesville, FL; ****Wright State University, Dayton, OH;
*****NYU Hospital for Joint Diseases, NY, NY

Introduction

The Grammont reverse shoulder prosthesis has been demonstrated to alleviate pain and improve function in patients suffering from cuff tear arthropathy. Not surprisingly, these successes have led to an expansion of its indications, despite the well-documented complication rates. The incidence of scapular notching is reported to be high as 50%; the incidence of instability/dislocation is reported to be as high as 10%. Such rates have led surgeons to intentionally implant the prosthesis in a manner not intended by the manufacturers (e.g. fixing the glenosphere with a 15º inferior tilt or with a 4mm distal shift).¹ (Figure 1) The fact that surgeons are making these attempts begs the question: Do the specific design parameters of the Grammont make it susceptible to these failure modes? Therefore, the purpose of this study is to evaluate the relationship between the design parameters associated with the Grammont reverse shoulder and the commonly reported clinical failure modes.

Figure 1. Modified illustration from Nyfeller1 depicting 4 positions in which the glenosphere is commonly implanted.

Materials and Methods

The Grammont reverse shoulder was geometrically modeled using a 3-D computer-aided design software (Unigraphics; UGS, Inc.). An assembly analysis was conducted to quantify the effect of the design parameters on the functionally relevant measurements during simulated humeral abduction/adduction. The following design parameters were varied in equal increments across these ranges: humeral neck angle (130º to 165º), humeral liner constraint (0.250 to 0.3125), glenosphere thickness (17mm to 21mm), and glenosphere diameter (34mm to 44mm). The functionally relevant measurements assessed were: range of motion (ROM), jump distance, and offset. (Figure 2) By implication, the relationship between the aforementioned design parameters and functional measurements will elucidate the failure mechanisms associated with the commonly reported clinical complications for reverse shoulder arthroplasty (scapular notching, dislocation, improper deltoid tensioning, etc…).

Figure 2. Graphical definitions of the functionally relevant measurements assessed in this study. It is important to note that ROM is defined as the humeral motion occurring between inferior and superior impingement; scapular motion was not considered.

Results

The Grammont reverse shoulder (i.e. 155° neck angle, humeral constraint of 0.275, 36x19mm glenosphere – denoted by shading in tables) was observed to impinge inferiorly and superiorly at 35° and 95° abduction, respectively. (Figure 3) Increasing neck angle by 5º positively shifts ROM by 5º and increases offset by 0.25 to 0.5mm. (Figure 4; Table 1) Increasing humeral constraint by 0.0125 decreases ROM by 4º and increases jump distance by 0.5mm. (Table 2) Increasing glenosphere thickness by 1mm increases ROM by 5º; offset and jump distance were not affected. (Table 3) Increasing glenosphere diameter by 2mm increases jump distance by 0.5mm; ROM was not affected. (Table 4)

Table 1. Relationship between humeral neck angle and ROM in a Grammont-style reverse shoulder.

Table 2. Relationship between the degree of humeral liner constraint and ROM in a Grammont-style reverse shoulder.

*Assumes a constant humeral liner constraint between sizes

Table 3. Relationship between glenosphere thickness and ROM in a Grammont-style reverse shoulder.

*Assumes a constant humeral liner constraint between sizes

Table 4. Relationship between glenosphere diameter and jump distance (at varying positions
during humeral abduction) in a Grammont-style reverse shoulder.

Figure 3. Graphical depictions of the Grammont reverse shoulder at varying positions of humeral abduction.	Figure 4. Comparison of the relationship between jump distance and ROM for varying humeral neck angles in a Grammont-style reverse shoulder.

Discussion and Conclusions

The results of this study predict that the Grammont design impinges inferiorly on the scapula prior to the patient being able to adduct his/her arm to their side. Similar results have been verified both radiographically and clinically.² Based upon these observations, the authors conclude that the specific combination of humeral neck angle, glenosphere geometry, and humeral liner geometry are interrelated but not necessarily optimized in the Grammont design and thus could make it susceptible to scapular notching and dislocation via inferior impingement. The knowledge of these relationships can serve as the basis for optimizing a Grammont-style reverse shoulder prosthesis. To demonstrate the application of these relationships, we designed a novel reverse shoulder prosthesis with the objective to maximize ROM and minimize impingement while maintaining the center of rotation on the face of the glenoid. (Figure 5) Doing so predicted increases in ROM by as much as 50%. (Figure 6; Table 5) Future reverse shoulder designs should consider shifting the inferior impingement point to a location that permits a ROM better accommodating a patient’s activities of daily living.

Figure 5. Equinoxe reverse shoulder (Exactech Inc.; Gainesville, FL)	Figure 6. Comparison of the relationships between jump distance and ROM for the Equinoxe and Grammont reverse shoulder designs.

*Assumes 42mm Grammont has the same humeral liner constraint as the 36mm Grammont

Table 5. ROM comparison between the Equinoxe and Grammont reverse shoulder designs.

References

1. Nyffeler RW, Werner CM, Gerber C. Biomechanical relevance of glenoid component positioning in the reverse Delta III total shoulder prosthesis. J Shoulder Elbow Surg. 2005 Sep-Oct;14(5):524-8.

2. De Wilde LF, Audenaert EA, Berghs BM. Shoulder prostheses treating cuff tear arthropathy: a comparative biomechanical study. J Orthop Res. 2004 Nov;22(6):1222-30.

Production of this poster sponsored by:
Exactech, Inc., General Research Fund