Effect of prosthesis design on muscle length and moment arms in reverse total shoulder arthroplasty.
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.
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.
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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; firstname.lastname@example.org.
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 CoR BIORSA 36 mm, +10 mm 29 mm diameter CoR 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 Infraspinatus 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 Infraspinatus Normal +5% MGMH +4% LGMH +5% BIORSA +5% MGLH +3% Teres Minor Normal +13% MGMH +11% LGMH +11% BIORSA +11% MGLH +10%
Please note: Illustration(s) are not available due to copyright restrictions.
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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|
|Date:||Apr 15, 2013|
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