Patients were recruited from a study on 3 treatment options for impingement. When asked to participate the patients could enter the clinical part of the study only or they could also undergo evaluation of shoulder motions using dynamic RSA (ethical committee consent R 475-1995-12-20). Controls were recruited from the working staff at orthopedic departments of Sahlgrenska hospital and Uddevalla hospital (ethics committee consent R 520-1998-08-20).
In the studies on the absolute active abduction 30 patients (mean age 50 (29–63) years, 20 men) who had been suffering from impingement (Neer stage 2) participated. They had had symptoms for at least 18 months. All patients were examined with ultrasonography. Exclusion criteria were presence of rotator cuff tear osteoarthritis and generalized joint disease such as rheumatoid arthritis. The corresponding control group consisted of 4 men and 7 women (mean age 38 (22–58) years) without shoulder symptoms.
In the studies on absolute passive abduction 21 patients (mean age 51 (29–63) years, 13 men) participated all of whom had had symptoms from impingement (Neer stage 2) for more than 18 months. Exclusion criteria were the same as for the previous group. The corresponding control group consisted of 4 men and 5 women (mean age 35 (22–58) years) without shoulder symptoms.
The sex distribution between the control groups and patient groups were not statistically significantly different in the studies on active and passive abduction (p = 0.09 and p = 0.28, respectively; Fisher's exact test) but the individuals in the control groups were younger (p = 0.008 and p = 0.004; Mann-Whitney test).
4–6 spherical tantalum markers (0.8 or 1.0 mm in diameter) were inserted under local anesthesia into the scapula (acromion) and the humeral head. A set-up involving 2 film exchangers placed side by side () and designed for simultaneous exposure was used.
Reference position. The global coordinate system is fixed to the cage (illustrated here at floor level). At the reference examination the two body-fixed coordinate systems (one scapular one humeral) are aligned to the cage system.
The vertical position of the film exchangers could be adjusted depending on the height of the patient. In front of the film exchangers a uniplanar calibration cage designed to suit the 2 film switchers was constructed and fixed. The exposure rate was set at 2 per second. 2–6 weeks after insertion of the one markers and using radiostereometric analysis (RSA Biomedical, Umeå, Sweden) the patients were studied standing during continuous active abduction and passive abduction () with the arm internally rotated. Because this recording technique limits the number of shoulder positions that can be studied, we chose to evaluate passive and active abduction used at an ordinary examination of the shoulder joint.
During motion the two body-fixed coordinate systems follow the motion of the bones which is illustrated here with changed position of the humeral coordinate system.
Together with one of the authors (EH) each subject (both patients and controls) performed several exercises of active abduction and passive abduction in order to feel as comfortable as possible before the radiographic examinations were started. The purpose was also to obtain as constant a speed as possible and to maintain the glenohumeral joint within the limits of the aperture (film size 35 × 35 cm).
When performing these exercises, the examiner stabilized the scapula with his hand until the patients themselves could maintain the scapula at as fixed a position as possible without any interference from the examiner. In this way we reduced the risk of additional body movements that could position the shoulder out of the field of view and we also avoided inclusion of any part of the examiner in the radiographic field of view during the recordings. Despite this, some of the examinations failed because the shoulder joint was not adequately visualized or the arch of motions was too small due to poor synchronization between the film exchangers and the patient.
The radiographic examination was initiated with a starting or reference position. A pair of stereo radiographs was taken corresponding to a well-defined anatomical position with the arm aligned to the longitudinal axis of the body and the forearm in external rotation with the palm facing forward. All subsequent recordings were related to this position of the arm.
The dynamic recordings lasted for 5–6 seconds (10–12 exposures). In the active abduction group only an average of 7 (5–10) representative pairs of stereographs (films) could be included in the final analysis in each patient due to difficulty in obtaining exact synchronization between the speed of the film exchanger and the motion of the arm. In the control group an average of 8 (7–9) film pairs were obtained. During passive abduction the corresponding values of representative pairs of stereographs (films) were mean 8 (6–10) and mean 8 (6–10), respectively.
A fictive point corresponding to the center of the humeral head was constructed to enable measurements of humeral head translations in a reproducible way. Circular templates were used to find the center of the head, but only on the 2 images of the reference position.
The radiographic films were scanned at 300 dpi using a flat-bed scanner (Sharp JX610, Osaka, Japan) and measured using dedicated software (Hallström and Kärrholm 2006
Using the RSA digital software the positions of the centers in each of the shoulders were measured on the 2 images and their 3-dimensional coordinates were computed in the same way as for a tantalum marker. Thus this plotting was done once for each shoulder. Thereafter the position of this point was transferred to all other subsequent examinations of the same shoulder, using its computed position relative to the humeral head markers. The presence of documented stable and sufficiently well-scattered tantalum markers in the humeral head is a prerequisite for these computations (Nilsson et al. 1990
We measured the relative rotations and translations of the humeral head by using the scapula as a fixed reference segment. In RSA, this is done by computation of the absolute motions of the individual bones (scapula and humerus) in the global coordinate system defined by the cage. Thereafter a reversed matrix calculation is used to “replace” the scapula to its original position. The humeral segment defined by its markers is subjected mathematically to the same inverse rotation matrix (). This enables computation of the relative difference between the two bones (segments) thus making it possible to evaluate motion occurring solely in one specific joint (here, the glenohumeral joint). In this study we also accounted for the computed humeral motions when related to the fixed cage coordinate systems. When these global motions (in RSA terminology, absolute motions) are computed the relative distribution of movement between the different parts of the body is disregarded. Thus the absolute motions of the humerus are the sum of any bending of the vertebral column movements of the chest, the scapula, and the humerus (Selvik 1974
). The absolute motions are an objective recording of what the examiner actually can observe whereas the relative motions may be more or less accurately estimated by the clinical examiner based on his or her observations of the position of visual or palpable anatomical landmarks.
Simplified sketch to illustrate absolute or global motions (top) and relative humeral motions (bottom). Absolute humeral motions include changes of position caused by scapular and trunk motions whereas relative motions do not.
In RSA rotations are calculated in a specific order: first, around the transverse; thereafter, around the longitudinal, and finally around the anterior-posterior axis. When the rotations recorded are pronounced as in our study this order of calculation will have an influence on the interpretation of the results. The reason for this is related to the fact that the coordinate system follows the moving segment (here, the humerus). Since the patient and the examiner abducted the arm corresponding to rotations around the anterior-posterior (AP) axis we decided to adjust the position of the cage coordinate system 90° by rotation around the longitudinal axis. This means that in this study, rotations were calculated in the order abduction/adduction (AP axis) internal/external rotation (longitudinal axis), and flexion/extension (transverse axis).
To estimate the contribution of humeral abduction that did not occur in the glenohumeral joint we also recorded the absolute abduction of the humerus. These data were extracted from the same recording as that used to obtain information about the relative motions. The absolute abduction is the rotation of the humerus around the anterior-posterior axis in relation to the cage coordinate system. It is the sum of the relative abduction in the glenohumeral joint, the rotation of the scapula, and of the trunk.
In terms of mean error of rigid body fitting, marker stability measurements in the active abduction group were concerning the reference segment (scapulae), 0.099 (0.012–0.350) mm (SD 0.070), and 0.084 (0.016–0.334) mm (SD 0.052) in the moving segment (humerus). Qualitative analysis of marker scatter (mean condition numbers) in the scapula were 175 (72–455) (SD 82), and 129 (46–313) (SD 65) in the humerus. In the evaluation of the passive abduction the corresponding marker stability parameters were 0.094 (0.015–0.303) (SD 0.06) and 0.083 (0.01–0.334) mm (SD 0.051). The corresponding qualitative analysis of marker scatter gave mean condition numbers of 182 (73–592) (SD 97) and 130 (47–356) (SD 70). In this study we accepted high condition numbers provided there was high marker stability (a mean error of less than 0.050) in a series of examinations.
The reproducibility of the active abduction movement (6 patients) and passive abduction movement (2 patients) was tested by repetitions of the mobility of the arm after an interval of 15 min. Data for each type of motion analyzed were interpolated linearly at 5-degree intervals of active abduction/passive abduction.
Of the 52 patients selected for active abduction and 33 patients selected for passive abduction, 2 chose not to take part in the RSA evaluation after the randomization. 3 patients had a late diagnosis of cuff rupture and 7 patients with too poor a marker scatter in either of the bones were also excluded.
Statistical analyses using SPSS version 13.0 were based on recordings between 20° and 55° of relative active and passive abduction in the glenohumeral joint using scapulae as a fixed reference segment. This interval was chosen to maximize the number of observations available from each group.
Non-parametric tests were used in evaluation where each patient contributed with one observation. Repeated measures ANOVA (MANOVA) was used when each subject contributed when a series of dependent observations was made on the same subject. Non-parametric correlation was used. The significance level was set at p < 0.05. The test of repeatability pooled standard deviations was reported (a way of averaging) standard deviations are presented as a simplification to account for changes of variations during the arc of motion in each individual.