The human shoulder takes on a special role in the human body due to of its specific requirements of stability and mobility . In Germany, approximately 12,000 shoulder prostheses are implanted each year, making them less common than artificial knees or hips . The main reason for lower numbers of shoulder prostheses is the high complexity of the human shoulder joint, whereby it is difficult to find adequate technical shoulder models for replacement. Therefore, the results of shoulder prostheses and other surgical interventions are often unsatisfactory, which motivates the endeavour to improve existing shoulder prostheses and optimize surgical interventions concerning the shoulder joint . For this reason, it is necessary to enhance the knowledge of biomechanical behaviour of the shoulder joint. This knowledge forms the basis for the development of modern concepts of reconstructive surgery and arthroplasty . One possibility to learn more about shoulder biomechanics is the use of an experimental shoulder testing apparatus e.g. for motion and sensitivity analysis. The majority of the simulators has focused on evaluating shoulder biomechanics while the joint is statically positioned, or when passive motions are externally applied with or without muscle loading . Kedgley et al. showed that simulators using continually variable muscle forces to drive shoulder movement produce motions with higher repeatability than passive systems because the approximation to the physiological case is closer . Only a few systems have investigated joint kinematics and kinetics using active muscle driven motion .
One limitation of most of the existing simulators is the fixed force generation for an active movement of the joint. Either an estimated fixed ratio of muscle forces is applied. Often the setpoint of the force of the deltoid muscle is set manually and the other muscles forces are scaled with a fixed ratio based on the physiological cross-sectional area (PCSA) of each muscle. The fixed ratio of muscle forces can result in an uncontrolled shoulder movement. Furthermore, alternatively an external rail is used to guide the humerus on a given path and tampers the forces by adding non physiological guidance forces.
Neither a fixed force ratio nor a guidance rail seems to generate physiological conditions to reproduce biomechanical shoulder motion behaviour. Apart from this electromyography (EMG) is used to determine muscle activation and estimate the muscle forces , . Due to cross-talk of the muscles, the reliability of the EMG-data is questionable.
To overcome the limitations of existing shoulder simulators a new and innovative shoulder testing apparatus was developed with two main objectives:
- A stable control algorithm for reproducible (<5°) free motion with a redundant muscle setup for cadaver specimen
- Qualitative and quantitative (<10° deviation) accordance between passive Teach-In and active free shoulder movement
In the context of a first pilot study the potentials of the new shoulder motion simulator are investigated.
2 Material and methods
2.1 Shoulder simulator – technical setup
One synthetic shoulder and one cadaver specimen were tested on the new biomechanical shoulder simulator (Figure 1). The new shoulder simulator contains six active pneumatic muscles (DMSP, Festo, Esslingen, Germany) which are connected via UHMWPE-ropes and ball bearing pulleys to the corresponding muscle tendons. Thus the three parts of the deltoid muscle and the rotator-cuff muscles [supraspinatus, infraspinatus + teres minor (combined), subscapularis] can be actively controlled. The advantages of the used pneumatic muscles are the high force density and the inherent compliance, which is needed to fit into the elastic properties of cadaver specimen. Furthermore, two passive muscles are realized with springs (pectoralis major muscle combined with latissimus dorsi muscle and biceps brachii muscle).
Adverse the pneumatic muscles have a highly nonlinear behaviour which makes control difficult. Therefore, a nonlinear adaptive controller for force and length was developed on the basis of Zeng and Wan . With an additional compensation for the nonlinear characteristics of the used pneumatic valves (VPWP Festo, Esslingen, Germany), a precise control over a wide range of muscle forces and lengths for a changing and unknown control path, namely the specimen, is expected.
An innovative and completely new approach which is realized in the new developed shoulder simulator and control algorithm, respectively, and which is contrary to the shoulder testing apparatus found in literature, is that motion of the shoulder can be controlled by muscle lengths rather than forces. This is necessary to create controlled and free shoulder-movements but depends on detailed information on movement and specimen specific muscle lengths over time. This information is acquired by a so called “Teach-In” process, where the operator moves the humerus on a desired trajectory, while the muscles are force controlled. During this movement the muscles follow the forced movement, and the control system records the required muscle lengths for the realisation of the specific trajectory. After this procedure the system can use the measured, over the trajectory varying muscle lengths, to replay the shoulder movement without the operator’s guidance.
The shoulder simulator setup is equipped with measurement devices for muscle lengths (WS10SG, ASM GmbH, Moosinning, Germany) and forces (KM30z, ME-Messsysteme, Henningsdorf, Germany), a 6D-force torque sensor (ATI, Apex, USA) for the joint reaction forces and moments, and an optical tracking system (Polaris Spectra, NDI, Ontario, Canada) to record the arm motion. A real-time control system (MicroAutoBoxII, dSPACE, Paderborn, Germany) was used for data recording, control and communication with all included devices.
2.2 Experimental validation
To evaluate the control concept and to analyse the achievable range of motion (ROM) of different movements, an experiment with a synthetic shoulder joint was initially realized. A simplified shoulder model, made from synthetic bones and elastic rubber bands representing the capsular ligament system (glenohumeral ligament & coracohumeral ligament), is attached to the shoulder simulator. This was done by fixating the inferior part of the scapula in polyurethane (PUR) foam and then mounting it to the motion simulator. Scapulae were mounted in the setup with the neutral plane of the glenoid tilted 10° superiorly, the scapula tilted 20° anteriorly, and the plane of the scapula parallel to the middle deltoid line of action . The muscle and tendon attachments and pulling directions are estimated based on anatomical landmarks and the joint surfaces of the synthetic shoulder are lubricated with petrolatum.
In a second step the experiments are repeated with a human cadaver specimen (Figure 2). One fresh-frozen, upper extremity was obtained from a female donor (age: 85 years). The cadaver was free of disease, and the joint capsule was not vented. The arm was thawed at approximately 20°C for 24 h and kept hydrated with normal saline during preparation. The scapula was exposed from the medial border to the suprascapular notch before being embedded with PUR foam in a rectangular block. Then the fixed shoulder was placed into the motion simulator. The distal end of the humerus is likewise attached to a cylinder where the rigid body of the optical tracking system and the spring representing the biceps brachii muscle is fixed. The artificial muscles are connected by chirurgical suture using fibre wire cords.
The desired movement of the arm was then performed manually by the operator while the artificial muscles are on force control and the variable muscle lengths are recorded. After this “Teach-In” procedure the movement of the arm was performed by the active length controlled muscles. That way both synthetic and cadaver specimen were articulated in different characteristic motions (abduction/adduction, internal/external rotation with adducted arm, anteversion/retroversion) through a physiologic ROM.
In the context of the experiment with the synthetic shoulder model the general usability of the simulator and its novel control concept were evaluated. This setup enabled a reproducible active movement (reproducibility of active <5°) with a good agreement (<10°) between the passive “Teach-In” movement and the replayed active movement (exemplarily an abduction is shown in Figure 3).
Optimal results were performed at abduction/adduction with a successful ROM from 0° to 90°. The range of internal- and external rotation was 80°–0°–25°.
With the use of a human cadaver shoulder movements could be successfully performed as well. Although, the successful ROM was smaller than with the synthetic model movements could be performed (abduction movement in Figure 4).
The difference of the passive “Teach-In” and active replay movement was in general greater than with the synthetic shoulder model, because the sutured attachments of the artificial muscles were more elastic, and therefore, the elongation of the muscles was adulterated. With the human cadaver a full ROM of 35°–85° (abduction/adduction), −30° to 0° (anteversion) and −25° to 0° to 20° (internal/external rotation) could be performed.
4 Discussion and conclusion
A new and innovative motion simulator for testing the biomechanical behaviour of the shoulder joint was presented. This first study shows that repeatable movements of the shoulder can be easily performed with the new developed shoulder simulator.
The pneumatic muscles in combination with the implemented adaptive force and length controller allowed the precise control of a compliant actor in combination with an unknown control path. In combination with the novel muscle control concept free but stable and controlled movements with a redundant muscle setup of individual specimen are possible, without the use of EMG data or external rails. This enables a major improvement towards free motion simulation compared to existing biomechanical shoulder simulators. Furthermore, the new shoulder simulator offers the opportunity to analyse the needed muscle forces, instead of estimating them a-priori. In this context our cadaver experiment confirmed the thesis that there is no approximately constant ratio of the muscle forces during an abduction movement (Figure 5).
The follow-up behaviour of the active replay movement compared to the passive “Teach-In” movement with the synthetic shoulder satisfied the qualitative and quantitative aspiration, with deviations <10°. Although the follow-up behaviour with cadaver specimen was not as good as with a synthetic model, a good qualitative accordance as well as a high reproducibility of active motion were achieved. This can be further optimized by enlarging the stiffness of the connection to the muscle tendon.
There exist some limitations coming along with the new shoulder simulator. For the pneumatic muscles, an optimization for a more physiological behaviour is one objective of our ongoing work. Because especially at turning points of the muscle length setpoints, e.g. at the changing point between abduction to adduction, the muscle tension was loose, so that no force was transmitted. This is not in accordance with the physiological ground tone of each muscle.
Our ongoing work will focus on the optimization of the length controller to solve the problem by always applying a force greater than a specified ground tone, even if it leads to deviations from the setpoint. Optimized tendon clamps and more suitable suture techniques are already under construction and evaluation. Further biomechanical studies with the shoulder simulator are planned to provide answers to specific clinical questions and to investigate different surgical strategies.
The authors would like to thank Professor A. Prescher, Institute for Anatomy and Björn Rath, Orthopaedic Clinic, RWTH Aachen University, for providing the cadaver specimen preparation. This project has been funded in parts in the framework of the START program of the Medical Faculty of the RWTH Aachen University.
Research funding: The author state no funding involved. Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent is not applicable. Ethical approval: The conducted research is not related to either human or animal use.
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