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Effect of prosthesis design on muscle length and moment arms in reverse total shoulder arthroplasty.

The first widespread use of a reverse shoulder concept appeared in the 1970s and provided a viable solution for patients with decompensated cuff tear arthropathy (CTA). Previously, these patients had to live with a non-functional arm due to insufficient muscles surrounding the shoulder joint. (1) However, early ball-in-socket designs that had a lateralized center of rotation (e.g., Bailey-Walker and Kessel designs) suffered from a high incidence of glenoid fixation failure due to large stresses at the bone-implant interface. (2,3) Despite these issues, the lateralized ball-in-socket concept has continued to be used by some surgeons. In 1991, Dr. Paul Grammont offered the first major evolution in reverse shoulder design. Grammont's modification moved the center of rotation (CoR) closer to the glenoid face and reversed the anatomic geometry, thus medializing the entire construct. Moving the CoR to the glenoid face reduced stresses at the bone-implant interface and decreased the incidence of glenoid side failures. (4) As one complication was reduced, other failure modes emerged, namely dislocation and scapular notching. (5) Notching occurs when the cup or humeral bone impinges against the scapula and erodes the scapular neck creating a notch-like defect in the bone. This contact can also lead to dislocation acting as a shoe horn to displace the humeral component off the glenosphere. Sirveaux and coworkers introduced a scale for grading scapular notching as a result of its prevalence in reverse shoulders. (6) The reported rates of scapular notching in the late 1990s and early 2000s led to the emergence of next generation designs aimed at reducing notching. In general, scapular notching can be minimized by inferiorly shifting the humerus, lateralizing the humerus, or a combination of the two to avoid impingement. Inferiorly shifting the humerus is most commonly achieved by inferiorly shifting the glenosphere or inferiorly tilting the glenosphere. Regarding humeral lateralization, there are two general philosophies: the first is to lateralize the CoR away from the glenoid far enough to prevent impingement while keeping the humeral component embedded in the proximal humerus (lateral glenoid, medial humerus--LGMH). The second is to move the humeral component laterally by keeping the CoR close to the glenoid face and using a combination of larger glenosphere size and a lateral offset between the center of the humeral cup and the center of the humeral stem (medial glenoid, lateral humerus). Grammont's original concept of medializing the center of rotation and keeping the humeral component inside the proximal humerus is termed (medial glenoid, medial humerus--MGMH). Recently, another solution for lateralizing the construct has emerged through the use of bone graft behind the glenoid plate (BIORSA). It should be noted that regardless of the reverse shoulder design, all concepts shift the CoR medially and inferiorly. These designs have effectively reduced notching rates (reported rates as low as 13% with MGLH design). (7) As the incidence of instability and notching are reduced, other outcomes become the focus of design evolution. One outcome commonly reported with reverse shoulders is poor external rotation strength and range of motion. An indicator of this outcome is a patient whose hand internally rotates as their arm is raised in front of their face resulting in a hornblower's sign. (8) While this is not a wholly debilitating outcome, patient satisfaction and outcomes are generally lower since many activities of daily living require external rotation, such as combing the hair and brushing of teeth, even feeding oneself. This study focuses on the effects that next generation reverse shoulder designs have on the lengths and moment arms of the muscles responsible for external rotation: posterior deltoid (PD), infraspinatus (IS), and teres minor (TM).


Three dimensional bone models of the scapula, humerus, clavicle, and ribcage were imported into a solid modeling software (Unigraphics NX 7.5, Siemens, Inc., Plano, TX) along with 3D representations of four different commercially available reverse shoulder assemblies (Table 1). The first design is based on the Depuy Delta[TM] prosthesis, which has a CoR on the glenoid face and a humeral component that is placed into the proximal humeral bone. Countersinking the humeral component inside the bone results in a small medial offset between the CoR and the axis of the stem in the intramedullary canal. This results in a medial glenoid CoR with a medialized humerus (MGMH) meaning that the location of the humerus is the closest to the scapula of all three designs. The second design is a modification of the first and is based on the Tornier BIO-RSATM concept. This implant is similar to the MGMH, but the glenoid plate is lateralized by a 10 mm graft placed between the glenoid face and the implant (BIORSA). The third design is based on the DJO RSP prosthesis with a glenoid CoR that is 10 mm lateral of the glenoid face and a humeral component that rests inside the proximal humerus. This results in a lateralized glenoid CoR and a medialized humerus (LGMH), so the position of the humerus is more lateral than the Grammont-style design. The fourth design is based on the Exactech Equinoxe[R] and has a medialized glenoid CoR with a humeral component that rests on top of the resected humerus, as opposed to inside like the other two designs. The result of resting atop the humeral cut is the liner ends up much more medial relative to the IM axis of the humerus. This concept has a medialized CoR of the glenosphere and a lateralized humerus (MGLH); as a result, the humerus is positioned further lateral than the previous two designs. For each design, the most commonly utilized commercially available implant was modeled based on published specifications (i.e., 36 mm x 18 mm glenosphere for MGMH and BIORSA, 32 mm x 26 mm glenosphere for LGMH, and 38 mm x 21 mm glenosphere for MGLH).

The normal humeral anatomy is represented by the uncut humerus and scapula separated by a 4 mm gap meant to account for the thickness of articular cartilage on both the humerus and glenoid face as well as the labrum surrounding the joint. The reverse shoulder assemblies were implanted into the bones following the manufacturers' recommended surgical techniques with regard to humeral preparation and glenoid plate placement. All humeral assemblies were placed in 20[degrees] of retroversion.

Once the bone and implant geometries were prepared, the PD, IS, and TM muscles were added to the models using origins and insertions defined by Gray's anatomy and confirmed by the senior investigator (HDR). (9) Each muscle was split into three segments in order to capture the breadth of the muscle (either superior, middle, inferior or anterior, middle, posterior depending on muscle orientation). With the arm at the side (0[degrees] of abduction), the arm was taken through a range of motion (internal and external rotation) to identify points of muscle wrapping around the bony geometry. In the portions of the range of motion where the muscle line of action intersected the bone, wrapping points were added to allow the muscles to better conform to the bony geometry and prevent non-physiological results.

Through the range of motion, the muscle length and moment arms were recorded and compared with the normal anatomy. The length of the muscle in the normal shoulder with the arm in the neutral position was assumed to represent the resting length for all muscles surrounding the shoulder. This provides a basis to indicate the amount of tension/ laxity there is after the reverse shoulder is implanted and how much extension/contraction is required to achieve the motions analyzed. The resting length was used to normalize the values and eliminate the effect of the bony geometry on the results (i.e. larger bones yield larger moment arms and longer muscle lengths). Using the axis of the intramedullary canal, the center of rotation of the glenosphere, and the line of action of the muscle segment, moment arm values for external rotation were calculated directly rather than using the motion versus change in length approximation.


As the arm is internally rotated from neutral to 40[degrees] internal rotation, the IS and TM are lengthened by 5% and 13%, respectively, in the native shoulder. Similarly, from neutral to 40[degrees] external rotation, the IS and TM must contract by 9% and 15%, respectively. The change in length is an estimation of the contraction the muscle will have to achieve to move the arm. The resting length of the muscle relative to the normal anatomy affects the amount of tension the muscle can generate. The neutral lengths of the muscles in all assemblies relative to normal as well as the change in length over the range of motion are listed in Table 2.

The moment arms for the external rotators were calculated for all the reverse shoulder designs as well as the normal anatomy. For each of the assemblies analyzed, the moment arms are plotted along with the normal shoulder. During external rotation, the IS and TM moment arms are increased relative to normal anatomy for all reverse shoulder designs (Figs. 1 and 2). The increase in moment arms for the MGMH, LGMH, and BIORSA is roughly 20% over the external rotation range analyzed. The MGLH design doubles the increase in moment arms for both muscles to 40% through external rotation relative to the other designs. Unlike the IS and TM, the posterior-deltoid moment arms for the reverse designs are similar to or below anatomic levels for all design except the lateralized humerus (MGLH). However, it should be noted the moment arm for the posterior-deltoid is roughly 20% of that for the IS and TM (Fig. 3). This indicates it may be difficult for this muscle to externally rotate the arm despite doubling the moment arm. The trends for these can be seen in Figure 1.


As the indications for reverse shoulders continue to expand and surgical techniques are adjusted to avoid particular complications, it is important to understand what happens to the soft-tissues surrounding the joint. It is clear that implanting a reverse shoulder medializes the humerus relative to normal anatomy resulting in shortening of the external rotators. While the detailed mechanics of muscle contraction are beyond the scope of this document, the concept of a length-tension curve first presented by Blix in 1894 is still widely used to describe muscle behavior. (10) Based on the theory that optimal muscle length yields maximum force output, shortening a muscle will decrease the force capacity of the muscle over a given range of motion. This is one possible explanation for the poor external rotation reported by some patients receiving a reverse shoulder. The decrease in muscle force output can prevent activities of daily living, and as patient expectations continue to increase, the focus on continually improving patient outcomes is also increased. One way to help improve patient strength in external rotation is to optimize the moment arms of the external rotators in the range of motion where they are required. The plot of external rotator moment arms in the results section demonstrates that throughout external rotation, the lateralized humerus design improves the moment arm for the external rotators more than the other design options. This is meant to improve the function of the external rotators the way medializing the CoR helped the deltoid during abduction to combat the hornblower's sign.


The shortening of external rotators after reverse shoulder arthroplasty has the potential to decrease the functional strength of those muscles. Previously, surgical technique modifications, such as adjusting retroversion of the humeral stem, have been suggested to improve impingement free motion, but the effect on tension or moment arm were not mentioned. (11,12) However, surgical technique modifications are unable to improve muscle moment arms. The lateral offset of the stem relative to the center of rotation dictates the moment arm of the muscle. Changing the lateral offset of the humerus both improves cuff tension and moment arm of the external rotators. By improving both tension and moment arm of these muscles, a lateralized humerus design has the potential to improve the rotator function of any remaining posterior rotator cuff muscles and posterior deltoid relative to the other design options that are commercially available.

This study has multiple shortcomings; it is purely theoretical and deals with only one bone model. Future work will include identifying how size of the anatomy influences the relationships analyzed here and reviewing clinical outcomes of patients reported in the literature for each of the design philosophies to look for statistically significant differences that can validate the theories put forth in this article.

Caption: Figure 1 Teres Minor external rotation moment arm as a function of external rotation at 0[degrees] abduction. The plot indicates that external rotator moment arm is greater than anatomic in all reverse shoulder designs, but the lateralized humerus increases the moment arm more than others at higher external rotation angles.

Caption: Figure 2 Infraspinatus external rotation moment arm as a function of external rotation at 0[degrees] abduction. The plot indicates that external rotator moment arm is greater than anatomic in all reverse shoulder designs when externally rotated beyond neutral, but the lateralized humerus increases the moment arm more than others at higher external rotation angles.

Caption: Figure 3 Posterior Deltoid external rotation moment arm as a function of external rotation at 0[degrees] abduction. Unlike the TM and IS muscles, only the lateralized humerus design increases the moment arm of the PD at higher external rotation angles.

Disclosure Statement

Funding for this study was provided by Exactech, Inc., Gainesville, Florida. Matthew A. Hamilton, Christopher P. Roche, and Phong Diep, are employed by Exactech, Inc. Pierre-Henri Flurin, M.D., and Howard D. Routman, D.O., are consultants for Exactech, Inc., and receive royalties on products related to this article.


(1.) Neer CS 2nd, Watson KC, Stanton FJ. Recent experience in total shoulder replacement. J Bone Joint Surg Am. 1982 Mar; 64(3):319-37.

(2.) Ahir SP, Walker PS, Squire-Taylor CJ, et al. Analysis of glenoid fixation for a reversed anatomy fixed-fulcrum shoulder replacement. J Biomech. 2004 Nov; 37(11):1699-708.

(3.) Bayley JI, Kessel L. Shoulder Surgery. Springer-Verlag, 1982.

(4.) Boileau P, Watkinson DJ, Hatzidakis AM, Balg F. Grammont reverse prosthesis: design, rationale, and biomechanics. J Shoulder Elbow Surg. 2005 Jan-Feb;14(1 Suppl S):147S-61S.

(5.) Wall B, Nove-Josserand L, O'Connor DP, et al. Reverse total shoulder arthroplasty: A review of results according to etiology. J Bone Joint Surg Am. 2007 Jul;89(7):1476-85.

(6.) Sirveaux F, Favard L, Oudet D, et al. Grammont inverted total shoulder arthroplasty in the treatment of glenohumeral osteoarthritis with massive rupture of the cuff. Results of a multicentre study of 80 shoulders. J Bone Joint Surg Br. 2004 Apr; 86(3)388-95.

(7.) Roche C, Marczuk Y, Wright TW, et al. Scapular notching in reverse shoulder arthroplasty: Radiographic analysis of implant position in male and female patients. Bone Joint J. 2013 Apr; 95-B(4):530-5.

(8.) Walch G, Boulahia A, Calderone S, Robinson AH. The 'dropping' and 'hornblower's' signs in evaluation of rotator-cuff tears, J Bone Joint Surg Br. 1998 Jul; 80(4): 624-8.

(9.) Gray H, Lewis W. Anatomy of the Human Body (20th ed). Philadelphia: Lea & Febiger, 1918.

(10.) Oatis CA. Kinesiology: The Mechanics & Pathomechanics of Human Movement. Lippincott Williams & Wilkins, 2004.

(11.) Stephenson DR, Oh JH, McGarry MH, et al. Effect of humeral component version on impingement in reverse total shoulder arthroplasty. J Shoulder Elbow Surg. 2011 Jun;20(4):652-8.

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Matthew A. Hamilton, Ph.D., Christopher P. Roche, M.S., M.B.A., Phong Diep, B.S., Pierre-Henri Flurin, M.D., and Howard D. Routman, D.O.

Matthew A. Hamilton, Ph.D., Christopher P. Roche, M.S., M.B.A., and Phong Diep, B.S., are employed by Exactech, Inc., Gainesville, Florida. Pierre-Henri Flurin, M.D., is at the Bordeaux-Merignac Clinique du Sport, Merignac, France. Howard D. Routman, D.O., is with Atlantis Orthopaedics, Palm Beach Gardens, Florida. Correspondence: Matthew A. Hamilton, Ph.D., Exactech, Inc., 2320 NW 66th Court, Gainesville, Florida 32653;

Table 1 Description of Components Used in Each of the
Reverse Assemblies

Description   Glenosphere     Glenoid Baseplate
              Diameter        Size

LGMH          32 mm, +10 mm   26 mm diameter
                Lateral CoR
MGMH          36 mm, +0 mm    29 mm diameter
BIORSA        36 mm, +10 mm   29 mm diameter

MGLH          38 mm, +2 mm    Oval 34 mm height
                Lateral CoR     x 25 mm width

Description   Glenoid Plate Location

LGMH          Aligned with inferior rim of
                glenoid. No inferior tilt.
MGMH          Aligned with inferior rim of
                the glenoid. No inferior tilt.
BIORSA        Aligned with inferior rim of the
                glenoid. Lateralization achieved
                with a 10 mm bone graft between
                glenoid plate and glenoid face.
                No inferior tilt.
MGLH          Aligned with inferior rim of the
                glenoid. No inferior tilt.

Table 2  Comparison of Resting Length and Extension/Contraction
of the IS and TMI Muscles Through 40[degrees] of Internal and
External Rotation

Description     Resting Length Relative   Difference in Length
                to Normal Shoulder (+)    0[degrees] to 40[degrees]
                tension; (-) laxity       ER (+) extension;
                                          (-) contraction


  Normal            0%                       -9%
  MGMH            -20%                       -9%
  LGMH            -13%                       -9%
  BIORSA          -14%                       -9%
  MGLH            -11%                      -11%

Teres Minor

  Normal            0%                      -15%
  MGMH            -33%                      -17%
  LGMH            -22%                      -18%
  BIORSA          -23%                      -18%
  MGLH            -19%                      -19%

Description     Difference in Length
                0[degrees] to 40[degrees]
                IR (+) extension;
                (-) contraction


  Normal           +5%
  MGMH             +4%
  LGMH             +5%
  BIORSA           +5%
  MGLH             +3%

Teres Minor

  Normal          +13%
  MGMH            +11%
  LGMH            +11%
  BIORSA          +11%
  MGLH            +10%


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Article Details
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Author:Hamilton, Matthew A.; Roche, Christopher P.; Diep, Phong; Flurin, Pierre-Henri; Routman, Howard D.
Publication:Bulletin of the NYU Hospital for Joint Diseases
Article Type:Report
Geographic Code:1USA
Date:Apr 15, 2013
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