Influence of the Stage of Massive Rotator Cuff Tear and Adductor Co-activation on Shoulder. Function during Activities of Daily Living: A Numerical Study.
Influence of the Stage of Massive Rotator Cuff Tear and Adductor Co-activation on Shoulder Function during Activities of Daily Living: A Numerical Study 1
Lemieux P.O.1, Gielo-Perczak K.2, Nuño N.1, Hagemeister N.1 Laboratoire de recherche en imagerie et orthopédie, École de technologie supérieure, Montréal, Canada 2 Biomedical Engineering Program, University of Connecticut, Storrs, CT, USA
Introduction The rotator cuff musculature has two known actions, which are to axially rotate the humerus (internally or externally) and to stabilize the humeral head into the glenoid socket. When a massive tear of the rotator cuff occurs, these two actions partially or totally disappear. This leads to poor shoulder function, although some patients remain asymptomatic [1]. Shoulder function in case of massive rotator cuff tear (MRCT) partially depends on the stage of the tear, which begins with a complete tear of the supraspinatus and progresses in the posterior portion (infraspinatus and teres-minor), in the anterior portion (subscapularis) or both [2]. Shoulder function may also be influenced by co-activation of adductors muscles, which is believed to replace the actions of the torn rotator cuff [3]. Active abduction and flexion with full elbow extension have previously been considered in both numerical and experimental investigations of MRCT [4, 5]. However, since humerus axial rotation may not be essential for these movements, the rotator cuff may be less active compared to other movements. A recent study has shown that ADL may involve high variations of humerus axial rotation and humerus elevation [6]. These ADL are regularly performed by elderlies who are more prone to suffer from MRCT. To date, numerical studies evaluating the sensitivity of various ADL to different stages of MRCT are lacking in the literature. Thus, the goal of the present numerical study was to assess the influence of two stages of MRCT on shoulder function while simulating four different ADL without carrying load at the hand. Methods The present study used the AnyBody Modeling System (AnyBody A/S, Aalborg, Denmark), from which the shoulder model is based on anthropometric data of the Delft shoulder group [7, 8]. The height and weight of the body are 1.80 m and 75 kg. A total of 118 musculotendinous units are simulated with a Hill-type muscle model adapted for inverse dynamics analysis [9]. Each muscle is simulated with multiple fibres to cover its large origin and/or insertion (Fig. 1). The completion of the movements prescribed to the model depends on two conditions. First, the
musculoskeletal system has to perform the movement by recruiting the available musculature. Meanwhile, the model must respect a glenohumeral stability constraint that ensures that the resultant force remains inside the glenoid cavity. If the musculoskeletal system cannot fulfil these conditions, the solicited fibres become overloaded (activity > 100%). In the present study, the completion ratio of each task was computed by dividing the simulation step at which muscle overload occurred by the total number of steps needed to perform the task. Adductor coactivation was also studied by verifying which adductor muscles (latissimus dorsi (LD), pectoralis major (PM), teres-major (TMa)) were activated during the simulation. For each of these muscles, an activity greater than 5% in one of the fibres was considered to be the co-activation threshold.
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Fig. 1. Front (top) and rear view (bottom) of the AnyBody shoulder model showing the pectoralis major (yellow), subscapularis (blue), supraspinatus (red), infraspinatus (green), teres-minor (purple), teres-major (cyan)). The upper halves (3 upper fibres) of the subscapularis and infraspinatus are numbered. The other muscles are hidden to improve visibility.
Four ADL involving a humerus axial rotation were simulated: hand to contralateral shoulder (H2CS); hand to mouth (H2M); combing hair (CH) and; hand to back pocket (H2BP). These ADL were simulated using the joint angles derived from motion capture [6]. A MRCT involving the upper half of the cuff was first simulated by removing the whole supraspinatus and the upper halves of the infraspinatus and subscapularis (Fig. 1). Thereafter, a complete MRCT was simulated by removing the whole rotator cuff (supraspinatus, infraspinatus, subscapularis, teres-minor). Results Table 1 shows the completion ratio and adductor activity for each ADL and for both types of MRCT. Table 1. Completion ratio and co-activated adductors for each ADL and for both stages of MRCT.
MRCT - Upper half ADL Completion (%) H2CS 100% H2M 100% CH 100% H2BP 48%
H2CS H2M CH H2BP
Adductors No adductor activity
MRCT - Whole cuff 68% LD, TMa 100% PM, LD, TM 100% PM, LD, TM 32% PM, LD, TM
Discussion The present numerical study assessed the influence of two stages of MRCT and adductor co-activation on shoulder function while simulating four ADL. The results suggest that the completion of ADL such as hand to contralateral shoulder (H2CS), hand to mouth (H2M), combing hair (CH) and hand to back pocket (H2BP) depends on the extent of the MRCT and the presence of adductor co-activation. All simulated ADL except H2BP were completed with the first stage of MRCT (upper portion only). This ADL is known to require a high humerus internal rotation near its maximal achievable value (i.e. range of motion) [6]. This suggests that MRCT that does not involve the inferior portion of the rotator cuff may still allow for ADL with reasonable amount of humerus axial rotation, but may become ineffective when humerus axial rotations is close to the end of active range of motion. Moreover, the tasks of H2M and CH were
completed with the second stage of MRCT (whole cuff). For these movements, a strong adductor coactivation strategy was employed by the muscular recruitment algorithm of the model. This strategy replaced the deficient rotator cuff in stabilizing and rotating the humerus. However, this strategy was not sufficient to ensure the completion of the H2CS and H2BP tasks. These tasks both involve a higher internal rotation compared to H2M and CH [6]. Moreover, the CH task required high humerus elevation (nearly 100°), while the other tasks required elevation below 70° [6]. Such high humerus elevation requires a stabilizing downward force which, in case of MRCT, is believed to be provided by adductor co-activation [3]. This suggests that the efficiency of adductor co-activation may depend on the level of humerus elevation and axial rotation involved in the task. Conclusion The present AnyBody shoulder model allows studying some aspects of MRCT such as adductor co-activation and type of tear. The findings may allow a more advised choice of the movement when studying the pathomechanics of MRCT in a musculoskeletal shoulder model. Moreover, simulation of humeral translation along with more challenging ADL (e.g. carrying load) may enhance these findings. Acknowledgements The AnyBody support team and the source of ADL data. References [1] Bedi, A., et al., J Bone Joint Surg Am, 2010. 92(9): p. 1894-908. [2] Rockwood, C.A., et al., Rotator cuff, in The Shoulder,2009, Elsevier Health Sc., p. 1552. [3] Steenbrink, F., et al., J Biomech, 2009. 42(11): p. 1740-5. [4] Hansen, M.L., et al., J Bone Joint Surg Am, 2008. 90(2): p. 316-325. [5] Magermans, D.J., et al., Clin Biomech (Bristol, Avon), 2004. 19(4): p. 350-7. [6] van Andel, C.J., et al., Gait Posture, 2008. 27(1): p. 120-7. [7] van der Helm, F.C., et al., J Biomech, 1992. 25(2): p. 129-44. [8] Veeger, H.E.J., et al., J Biomech, 1991. 24(7): p. 615-629. [9] Damsgaard, M., et al., Simul Model Pract Th, 2006. 14(8): p. 1100-1111.
