How do deltoid muscle moment arms change after reverse total shoulder arthroplasty?

J Shoulder Elbow Surg (2015) -, 1-8

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How do deltoid muscle moment arms change after reverse total shoulder arthroplasty? David R. Walker, PhDa,*, Aimee M. Struk, MEd, ATCb, Keisuke Matsuki, MD, PhDc, Thomas W. Wright, MDd, Scott A. Banks, PhDa a

Department of Mechanical Engineering, University of Florida, Gainesville, FL, USA Department of Orthopaedic Surgery, University of Florida, Gainesville, FL, USA c Shoulder and Elbow Surgery Center, Funabashi Orthopedic Hospital, Chiba, Japan d Orthopaedics and Sports Medicine Institute, University of Florida, Gainesville, FL, USA b

Background: Although many advantages of reverse total shoulder arthroplasty (RTSA) have been demonstrated, a variety of complications indicate there is much to learn about how RTSA modifies normal shoulder function. This study used a subject-specific computational model driven by in vivo kinematic data to assess how RTSA affects deltoid muscle moment arms after surgery. Methods: A subject-specific 12 degree-of-freedom musculoskeletal model was used to analyze the shoulders of 26 individuals (14 RTSA and 12 normal). The model was modified from the work of Holzbaur to directly input 6 degree-of-freedom humeral and scapular kinematics obtained using fluoroscopy. Results: The moment arms of the anterior, lateral, and posterior aspects of the deltoid were significantly different when RTSA and normal cohorts were compared at different abduction angles. Anterior and lateral deltoid moment arms were significantly larger in the RTSA group at the initial elevation of the arm. The posterior deltoid was significantly larger at maximum elevation. There was large intersubject variability within the RTSA group. Conclusions: Placement of implant components during RTSA can directly affect the geometric relationship between the humerus and scapula and the muscle moment arms in the RTSA shoulder. RTSA shoulders maintain the same anterior and posterior deltoid muscle moment-arm patterns as healthy shoulders but show much greater intersubject variation and larger moment-arm magnitudes. These observations provide a basis for determining optimal implant configuration and surgical placement to maximize RTSA function in a patient-specific manner. Level of evidence: Basic Science Study, Kinesiology. Published by Elsevier Ltd. Keywords: Reverse; musculoskeletal; shoulder; arthroplasty; computational

The University of Florida Health Center Institutional Review Board approved this study (study number 463-2005). *Reprint requests: David R. Walker, PhD, Department of Mechanical Engineering, University of Florida, MAE-A Bldg 3rd Flr, Gainesville, FL 32611, USA. E-mail address: [email protected] (D.R. Walker). 1058-2746/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jse.2015.09.015

Reverse total shoulder arthroplasty (RTSA) has increasingly been used in the United States since its U.S. Food and Drug Administration approval in 2003.23 RTSA serves to restore some mobility in rotator cuff–deficient shoulders while relieving the pain of osteoarthritis. Although several advantages of reverse shoulder

2 arthroplasty have been reported (eg, treatment of rotator cuff arthropathy),5,36 there are still a variety of complications (eg, acromial fracture), making it apparent that much remains to be learned about how RTSA modifies normal shoulder function.7,24,28,31,36 A better understanding of shoulder motion and muscle function after RTSA will support improved design rationales and surgical implementation of this treatment. Fundamentally, the ability of a muscle to move the shoulder is a function of 2 factors: the muscle moment arm and the muscle force-generating capacity.2,14,15,32 Several cadaveric studies have reported how changes in RTSA geometry affect shoulder muscle moment arms.9,12,13,19,20,25-27,29 Although insightful, these cadaveric studies do not capture the full variation of morphology, size, and motion that would be encountered in a RTSA clinical cohort, and there are few reports of in vivo RTSA shoulder muscle moment arms.10 Subject-specific in vivo measures of shoulder muscle moment arms will be important inputs to further improve RTSA methods and outcomes. Computational modeling provides a powerful method to analyze human biomechanics, and advanced models of the upper extremity have been developed for a variety of applications.1,6,8,9,11-13,16,18-20,25-27,29,34 Detailed muscle properties input into to these musculoskeletal models are typically derived from cadaveric studies.2,14,15,32 For example, Holzbaur et al16 used a musculoskeletal model based on cadaveric measurements of muscle properties to determine muscle moment arms and joint moments and found their results compared well with experimental data. Ling et al21 used computational models to assess muscletendon transfers in the shoulder. Other investigators have used similar musculoskeletal models to study RTSA biomechanics during active abduction.19,27 Common to all of these upper extremity models is the use of generic muscle properties (origin and insertion geometry, tendon slack lengths, optimal fiber lengths, etc) and the assumption of a constant scapulohumeral rhythm fixing scapular upward rotation to humeral elevation.2,6,14-16,32,34 This generic approach is useful for broadly quantifying the geometric and moment-generating relationships of the shoulder muscles but would be difficult to use to prescribe a surgical intervention, for example, where the patient’s specific muscle configuration and shoulder rhythm could be very different from an average idealized individual. Recent studies of shoulders with RTSA have shown much greater scapular movement, or a smaller scapulohumeral rhythm, than is observed in healthy shoulders17,33 and a much wider range of joint center (JC) locations relative to the glenoid.30 These 2 factors suggest that subject-specific shoulder geometry and movement patterns may significantly alter the muscle moment arms after RTSA. The goal of this study was to use subjectspecific geometric and kinematic data for 12 healthy shoulders and 14 shoulders with RTSA to implement subject-specific musculoskeletal models and determine

D.R. Walker et al. deltoid muscle moment-arm variation for arm abduction. These models will establish how current RTSA shoulders affect muscle moment arms relative to healthy shoulders and provide objective relationships between surgical placement of the RTSA implant components and the resulting muscle moment arms.

Materials and methods Overview Previously collected kinematic data for 12 healthy shoulders and 14 shoulders with RTSA3 were used to scale, configure, and drive subject-specific computational models. The patient-specific models were used to simulate dynamic abduction, and muscle analyses were performed to determine the muscle moment arms of the anterior, lateral, and posterior aspects of the deltoid.10,16

Kinematic data Single-plane fluoroscopy and model-image registration were used to measure the scapula and humeral bone kinematics in 12 healthy shoulders (Figs. 1 and 2) and humeral and glenoid implant kinematics in 14 shoulders with RTSA (Fig. 3).22,35 These 3dimensional (3D) kinematics and subject-specific bone and implant geometry data were used to configure and drive each patient-specific model. For the RTSA shoulders, computed tomography–derived 3D models of the humerus and scapula were also used for 3D model-image registration. Because the RTSA implants are rigidly fixed to the bones, it is possible to determine the fixed 3D relationship between the implant and bone and to use the measured implant kinematics to drive the subject-specific bone models for each individual with RTSA. With explicit in vivo 3D kinematic data for each patient’s scapular and humeral motions, it was not necessary to assume a fixed scapulohumeral rhythm as is commonly done with computational models of the shoulder.10,16 The native glenoid (NG) center was defined as the geometric center of the glenoid on the 3D scapular models for all individuals (Fig. 4) created using Geomagic (Morrisville, NC, USA). The JC is known as the center of the glenosphere or the humeral head when referencing RTSA/normal individuals, respectively. A sphere was fit to the humeral articular surface, and the humeral offset (HO) was determined as the distance from the center of the sphere to the NG for all shoulders. In RTSA shoulders only, the JC was measured from the NG to the center of the glenosphere implant. The JC change after RTSA was measured as the distance (JC – HO).

Joint definition The OpenSim shoulder joint, defined by Holzbaur et al,10,16 was modified and defined as a 12 degree-of-freedom (DOF) system in which the scapula and humerus were modeled as 6 DOF universal joints with respect to the thorax.16 The thorax was defined as the fixed reference frame. A 2-dimensional (2D) translational correction was applied to the scapular and humeral kinematics if any rib cage motion occurred during imaging. Each moving body rotational order was defined as a 3-1-2 Cardan sequence. Each

Deltoid moment arms during abduction

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Figure 1 Computed tomography reconstruction of the bone (Top left: Transverse view; Top right: Coronal view; Bottom left: Three dimensional anatomical bone models; Bottom right: Sagittal view).

Figure 2 Bone coordinate definitions of the humeral and scapular bones.

model DOF was driven by the kinematic data collected for each patient.

Muscle definitions The shoulder model consisted of 16 muscle actuators. Muscle attachment points, via points and wrapping surfaces, were

Figure 3 Three-dimensional bone and implant meshes (orange and blue) registered to 2-dimensional fluoroscopy X-ray images.

consistent with a Holzbaur model10,16 and were modified to include only the muscles that span the shoulder girdle. The lateral deltoid wrapping surface was modified to be a cylinder instead of an ellipse to increase the robustness of the model when the JC is changed. All modifications of wrapping geometry remained consistent with the Holzbaur et al16 model.

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Figure 4

Implemented bone (white transparent) and implant (dark gray) configurations.

Table I Joint center and humeral offset ranges for reverse total shoulder arthroplasty and normal shoulders

and then scaled to represent an average-sized RTSA individual with a 33.1-cm humeral length.

Variable

Medial(-)/ lateral (mm)

Superior(-)/ inferior (mm)

Anterior(-)/ posterior (mm)

Statistical methods

RTSA–JC RTSA–HO Normal-HO

0–14 21–40 22–28

–3 to 14 11–35 –3 to 7

–6 to 6 –12 to 12 –12 to 7

Comparison of the RTSA cohort and normal cohort deltoid moment arm means and standard deviations were performed using 2-way repeated-measures analysis of variance with the level of significance at 0.05. The Tukey honestly significant difference was used to perform pairwise post hoc comparisons.

HO, humeral offset; JC, joint center; RTSA, reverse total shoulder arthroplasty.

Results Patient-specific model scaling The modified OpenSim model was scaled for each of the 14 RTSA individuals using their in vivo motion-capture marker data. Marker placement defined the humeral length, which was used to uniformly scale the upper extremity segments according to the long axis of the humerus.16 Humeral dimensions were determined from computed tomography scans for each of the normal individuals. All scaling was performed using the scaling tool in OpenSim.10

Model moment-arm analysis Arm abduction was simulated using the humeral and scapular 3D kinematic data collected in previous work.22,35 The joints in RTSA shoulder-specific models were configured to represent each individual’s shoulder joint configuration. A muscle analysis was performed on each arm pose during abduction for each participant, and muscle moment arms were calculated. The moment arms predicted for each RTSA participant and normal individual were normalized to their own humeral length

HO in RTSA shoulders varied over a much greater range than in native shoulders (Table I). HO in the lateral direction for RTSA shoulders was at least 6 mm smaller than the smallest humeral offset in the healthy shoulders. The center of rotation in RTSA shoulders was more inferior than in healthy shoulders. The range of anterior/posterior placement of the rotation center for RTSA shoulders was bounded by the range for normal shoulders. Similar patterns of muscle moment-arm changes were observed for normal and RTSA shoulders with abduction in the anterior and posterior heads of the deltoid but not the lateral head of the deltoid. The magnitude of the moment arms was statistically different between normal and RTSA shoulders for all 3 heads of the deltoid (Fig. 5). RTSA shoulders demonstrated a smaller range in average deltoid moment arms but a greater absolute magnitude over the abduction arc compared with the normal shoulders (Fig. 5). The moment arm of the anterior deltoid was positive with the arm at the side and decreased monotonically, but the RTSA shoulder decreased with a smaller slope (Fig. 5, A). At initial

Deltoid moment arms during abduction

Figure 5 Deltoid moment arms varied over the arc of shoulder abduction. (A) The anterior deltoid showed a decreased trend in both healthy and reverse total shoulder arthroplasty (RTSA) moment arms, but RTSA moment arms were significantly larger over the abduction arc. (B) The lateral deltoid was almost constant across the abduction arc in healthy shoulders but was decreasing and had larger initial magnitude in RTSA shoulders. (C) The posterior deltoid moment arm increased from the initial negative values; the magnitude became greater as glenohumeral abduction crossed 30 . The mean  1 standard deviation is shown for normal (orange) and RTSA (blue) shoulders, and pairwise differences are denoted by open circles.

angles of glenohumeral abduction (5 to 25 ) the average moment arm of the lateral deltoid was significantly greater in the RTSA group than in the normal group (Fig. 5, B). The lateral deltoid average moment arm was nearly constant and positive in normal shoulders but showed a slight decreasing trend with abduction in RTSA shoulders (Fig. 5, B). The posterior deltoid abduction moment arm was negative with the arm at the side (eg, it acted as an adductor and increased monotonically with increasing glenohumeral abduction) (Fig. 5, C). There were significant pairwise differences between normal and RTSA shoulder moment arms at 40 , 45 , and 50 of glenohumeral abduction for the anterior deltoid, 5 to 25 for the lateral deltoid, and 50 of glenohumeral abduction for the posterior deltoid.

Discussion The incidence of complications with RTSA has been reported to be between 19% and 68%.7,31,36 A better

5 understanding of how surgical placement of RTSA implants changes muscle moment arms and muscle performance may aid in refinement of implant design and surgical technique to improve clinical and functional outcomes.9,12,13,19,20,25-27,29 We found that deltoid muscle moment arms in shoulders with RTSA exhibited similar patterns of change over the arc of glenohumeral abduction compared with normal shoulder anatomy, except within the anterior and lateral deltoid. As with any computer modeling study, a number of simplifications and limitations are important to state. First, we had the scapular and humeral bone (or implant) geometry in each shoulder to determine that individual’s 3D shoulder kinematics. But we were not able to identify all of the muscle origins and insertions for each muscle on each bone. Thus, it was necessary to use a uniformly scaled version of the Holzbaur et al16 OpenSim shoulder model, animated with subject-specific humeral and scapular kinematics. The use of this scaled generic model may produce different muscle moment arms than a truly subject-specific set of shoulder geometric variables. Second, we found that the lateral deltoid muscle wrapping surface in the original model16 produced discontinuous muscle moment arms when the HO was moved according to the surgical placement of the RTSA implants and each individual’s 3D abduction kinematics. A custom cylindrical wrapping surface was developed for the lateral deltoid, providing a surface that was more robust to joint configuration changes. This change in wrapping surface geometry had a very limited effect on muscle moment arms for arm abduction movements. Exploring how best to define custom wrapping surfaces for joints with geometry that differs significantly from the nominal model is an important area for future studies, especially when simulating differing dynamic activities (eg, scaption and external rotation). Third, the current analysis only evaluated muscle moment arms for arm abduction. This is an important movement for shoulder function, but a more comprehensive set of movements should be studied before any claims about beneficial JC placement are made. We found that the center of rotation in RTSA shoulders varied over a much greater range than in natural shoulders and that most of the variation was in the coronal plane (Fig. 5). Subject-specific joint configurations were input to our computational model so direct relations between JC location and muscle moment arms could be determined. Muscle moment arms in the RTSA group were significantly different (P < .05) from the normal group for the anterior, lateral, and posterior deltoid, with some moment arms varying by more than 20 mm across all aspects of the deltoid for the RTSA group. In general, we found that a more medial, inferior, or anterior RTSA JC resulted in a larger muscle moment arm, whereas a more lateral, superior, or posterior RTSA JC resulted in a smaller muscle moment arm for the arm abduction activity. Comparing

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Figure 6 Fluoroscopic images of 0 , 45 , and 75 of humeral elevation with superimposed lateral deltoid (black lines) moment arm measurements to illustrate how the model calculates moment arm compared with a conventional surgical definition for a patient with a 38mm glenosphere diameter. The lateral deltoid moment arm (orange line) at (A) 0 , (B) 45 , and (C) 90 .

healthy shoulder geometry with RTSA shoulder geometry, Boileau et al4 stated that ‘‘the center of rotation (COR) of the glenohumeral joint is characteristically shifted in an inferior/medial direction and the change in COR varies depending on implant design.’’ Our measurements of RTSA JCs and HOs are consistent with these earlier findings. We also provide JC geometry in a normal shoulder cohort, which may provide useful reference values when considering how to reconstruct the joint geometry in diseased shoulders where cuff deficiency and osteoarthritis can alter the natural JC.30 The reported values for shoulder muscle moment arms vary widely. In normal shoulders, Holzbaur et al16 experimentally measured anterior deltoid moment arms of 15 to 20 mm, whereas our healthy subject-specific model analyses showed an average 30 mm anterior deltoid moment arm for abduction at 60 . In RTSA shoulders, Ackland1 and Kontaxis and Johnson11 found anterior deltoid moment arms of 13 mm, whereas our models showed an average 30  13 mm anterior deltoid moment arm with the arm at the side. The anterior deltoid magnitude was significantly larger in the RTSA group than in the normal group due to the more posterior and inferior placement of the JC compared with the normal shoulders (Table I). For the lateral deltoid in RTSA shoulders, previous studies have reported moment arms of 14,27 20,1 40,2 50,33 52,34 and 599 mm compared with 30  6 mm from our models. A simple line drawing on the fluoroscopic images shows good agreement between an intuitive graphical estimate of the lateral deltoid moment arm and the model-calculated values (Fig. 6). RTSA shoulders showed a decreasing lateral deltoid moment arm during early abduction (Fig. 5), opposite the trend seen in normal shoulders, which may be due to loss of deltoid wrapping with abduction of the more laterally offset humerus. The anterior and posterior deltoids showed smaller moment arm changes in RTSA than in normal shoulders during 0 to 50 of glenohumeral

abduction, perhaps due to the fixed JC in shoulders with RTSA. The net inferior and posterior placement of the humerus with RTSA is the likely explanation for larger anterior deltoid and smaller posterior deltoid moment arms for replaced shoulders at initial elevation. Differences in measurement methods likely account for differences in moment arm values between our work and previous reports. For example, we used subject-specific shoulder joint geometry and in vivo kinematics for each case, whereas previous work has used simplified models of shoulder motion (eg, an assumed scapulohumeral rhythm), or generic shoulder joint geometry.

Conclusion The goal of the study was to assess the effect of RTSA on deltoid muscle moment arms on a patientspecific basis. We observed significant variation in deltoid muscle moment arms as a function of HO changes and also that RTSA shoulders generally showed larger anterior and lateral deltoid muscle moment arms than healthy shoulders, especially in early glenohumeral abduction (0 to 30 ). These 2 observations support a conclusion that it may be possible to enhance deltoid muscle moment arms by placing the RTSA HO in a patient-optimized position to provide the best muscle moment-generating capacity. Further studies are required to determine the sensitivity of muscle moment-arm changes to changes in joint geometry, how moment arms vary over different functional motions, and how HO changes affect shoulder muscle force production potential. Our study findings provide some objective relationships between reconstructed joint geometry and deltoid muscle moment arms that may be used to improve

Deltoid moment arms during abduction

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shoulder implant design, surgical techniques, and rehabilitation strategies.

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Disclaimer

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The authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.

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