Three-dimensional recording and description of motions of the shoulder mechanism.

A measurement technique is presented for recording positions of the bones of the shoulder mechanism, i.e., thorax, clavicula, scapula and humerus, in 3-D space, based on palpating and recording positions of bony landmarks. The palpation technique implies that only static positions can be measured. Accuracy of retrieving bony landmarks is checked on-line using rigid body assumptions. The measurement error is calculated afterwards and is comparable with cinegraphic methods. Axial rotation of the clavicula is estimated by minimizing rotations in the acromioclavicular joint. A number of motion definitions is compared by means of interindividual variation and subjective interpretability. Two useful definitions are proposed for describing motions of the shoulder mechanism. Four conditions have been recorded: abduction and anteflexion of the humerus both with and without additional weight in the hand. Abduction and anteflexion result in large differences in scapular and clavicular motions. The effect of additional weight in the hand on the position of the shoulder girdle is negligible.

Loading of shoulder girdle muscles in consequence of a glenohumeral arthrodesis.

Sometimes a glenohumeral arthrodesis operation is performed at patients with a brachial plexus lesion. In the glenohumeral arthrodesis the humerus is fused to the scapula. The fusion angles between scapula and humerus determine the restoration of function after the operation. In this study the effect of fusion angles on the hand position and maximal force exertion has been simulated using a musculoskeletal model of the shoulder girdle. Mean scapular and clavicular positions (10 subjects) are used as input variables. Mean fusion angles (18 patients with a glenohumeral arthrodesis) are used as reference position. Output variables are the maximal force which can be exerted with the hand and muscle forces of the thoracoscapular muscles. In order to achieve a mobility area in the mid-sagittal plane, the humerus should be internally rotated 60° with a little abduction and forward flexion. Maximal force can be exerted when the scapula is laterally rotated and protracted beside the thorax. In that position m. serratus anterior has good moment arm to counterbalance the external moment. It is recommended to use an external fixator for the glenohumeral arthrodesis in order to be able to adjust the fusion angles outside the operation theatre.

Interaction between the joints in the shoulder mechanism: the function of the costoclavicular, conoid and trapezoid ligaments.

By developing a measurement method based on the palpation of bony landmarks, the three-dimensional positions of the scapula and clavicle can be measured at several angles of humerus elevation. An analysis of these measurements shows the interaction between all joints of the shoulder mechanism. With the help of a biomechanical shoulder model the role of some of the extracapsular ligaments in the motion pattern of scapula and clavicle can be derived. In addition, the interaction between the rotations in the acromioclavicular and sternoclavicular joints is shown, and the possible implications for the treatment of joint problems in the shoulder are discussed.

Geometry parameters for musculoskeletal modelling of the shoulder system.

A dynamical finite-element model of the shoulder mechanism consisting of thorax, clavicula, scapula and humerus is outlined. The parameters needed for the model are obtained in a cadaver experiment consisting of both shoulders of seven cadavers. In this paper, in particular, the derivation of geometry parameters from the measurement data is described. The results for one cadaver are presented as a typical example. Morphological structures are modelled as geometrical forms. Parameters describing this form are estimated from 3-D position coordinates of a large number of datapoints on the morphological structure, using a least-squares criterion. Muscle and ligament attachments are represented as a plane or as a (curved) line. Muscle paths are determined by a geometrical form of the bony contour around which the muscle is wrapped. Muscle architecture is determined by the distribution of muscle bundles over the attachment area, mapping the distribution of the origin to the insertion. Joint rotation centers are derived from articular surfaces. Hence, muscle moment arms can be calculated. The result of this study is a set of parameters for each cadaver, describing very precisely the geometry of the shoulder mechanism. This set allows positioning of muscle force vectors a posteriori, and recalculation of position coordinates and moment arms for any position of the shoulder.

Inertia and muscle contraction parameters for musculoskeletal modelling of the shoulder mechanism.

To develop a musculoskeletal model of the shoulder mechanism, both shoulders of seven cadavers were measured to obtain a complete set of parameters. Using antropometric measurements, the mass and rotational inertia of segments were estimated, followed by three-dimensional measurements of all morphological structures relevant for modelling, i.e. muscle origins and insertions, muscle bundle directions, ligament attachments and articular surfaces; all in relation to selected bony landmarks. Subsequently, muscle contraction parameters as muscle mass and physiological cross-sectional area were measured. The method of data collection and the results for inertia and muscle contraction parameters as prerequisities for modelling are described.

The palpator: an instrument for measuring the positions of bones in three dimensions.

In order to measure the three-dimensional (3D) position of the shoulder girdle bones, as well as the 3D positions of muscle attachments and joint surfaces in cadaver experiments, a measurement instrument, called the palpator, was developed. The palpator is composed of an open chain of four links connected by four hinges. By recording the rotation of the hinges using potentiometers, the position of the end-point of the palpator can be calculated. After the identification of 23 parameters of the palpator, a measurement accuracy of 1.43 mm is obtained.