An alternative definition of the scapular coordinate system for use with RSA

Correspondence between scapular anatomical coordinate systems and the 3D axis of motion: A new perspective on an old challenge

Several scapular anatomical coordinate systems have been reported in the literature to describe shoulder kinematics. Unfortunately, the use of different conventions hinders comparison across studies. Further, inconsistencies between a coordinate system and the scapula's 3D axis of motion means that scapular motion will be incorrectly attributed to axes about which it did not rotate. The objectives of this study were to: 1) determine the extent to which the axes of four common scapular coordinate system conventions correspond to the 3D axis of scapular motion (i.e., instantaneous helical axis, IHA), and 2) report the prevalence of scapulothoracic gimbal lock for each convention. Shoulder kinematics were tracked during scapular plane abduction in 45 participants using biplane videoradiography. Scapulothoracic kinematics were described using the original convention proposed by van der Helm, the convention recommended by the International Society of Biomechanics (ISB), a glenoid-based coordinate system, and a glenoid-oriented coordinate system. The 3D angle was calculated between the IHA and each axis of the four conventions (IHA-axis angular deviations). A repeated measures ANOVA was used to compare IHA-axis angular deviations between conventions. The glenoid-oriented and ISB conventions resulted in the smallest and largest IHA-axis angular deviations, respectively (21.7°±3.6° vs. 30.5°±5.2°, p < 0.01). Gimbal lock was approached in 17.8% of participants when using the original convention, 2.2% when using the ISB convention, and 0% when using the glenoid-based or -oriented conventions. These findings suggest the glenoid-oriented coordinate system may be worthy of further consideration when investigating shoulder kinematics during scapular plane abduction.

A survey of human shoulder functional kinematic representations

In this survey, we review the field of human shoulder functional kinematic representations. The central question of this review is to evaluate whether the current approaches in shoulder kinematics can meet the high-reliability computational challenge. This challenge is posed by applications such as robot-assisted rehabilitation. Currently, the role of kinematic representations in such applications has been mostly overlooked. Therefore, we have systematically searched and summarised the existing literature on shoulder kinematics. The shoulder is an important functional joint, and its large range of motion (ROM) poses several mathematical and practical challenges. Frequently, in kinematic analysis, the role of the shoulder articulation is approximated to a ball-and-socket joint. Following the high-reliability computational challenge, our review challenges this inappropriate use of reductionism. Therefore, we propose that this challenge could be met by kinematic representations, that are redundant, that use an active interpretation and that emphasise on functional understanding.

The effect of coordinate system variation on in vivo patellofemoral kinematic measures

BACKGROUND

The use of different coordinate system definitions for the patella leads to difficulties in comparing kinematic results between studies. The purpose of this work was to establish the effect of using a range of coordinate system definitions to quantify patellar kinematics. Additionally, intra- and inter-investigator repeatabilities of the digitization of anatomic landmarks on the patella were determined.

METHODS

Four different patellar coordinate system definitions were applied using digitisations in two and three dimensions and a single femoral coordinate system was used for comparison. Intra-investigator variability was established by having one investigator digitize the patellar landmarks of three subjects on five separate occasions. Inter-investigator variability was quantified by having five participants digitize the same landmarks on the same three subjects. Patellofemoral kinematics were quantified for ten subjects, at six angles of tibiofemoral flexion, using MRI.

RESULTS

As a result of changes in the patellar coordinate system, differences of up to 11.5° in flexion, 5.0° in spin, and 27.3° in tilt were observed in the resultant rotations for the same motion, illustrating the importance of standardizing the coordinate system definition.

CONCLUSIONS

To minimize errors due to variability while still maintaining physiologically sensible kinematic angles, a coordinate system based upon an intermediate flexion axis between the most medial and lateral points on the patella, and a superiorly-directed long axis located between the most proximal and distal points on the patella, with an origin at the centre of the most proximal, distal, medial, and lateral points on the patella is recommended.

CLINICAL RELEVANCE

The recommended anatomic coordinate frame may be employed in the calculation of dynamic in vivo patellar kinematics when used in combination with any method that reliably quantifies patellar motion.

The accuracy of an external frame using ISB recommended rotation sequence to define shoulder joint angle

When investigating shoulder kinematics, it may be necessary to limit shoulder joint angles at a specific level. Previous studies used external frames or external surfaces to assist the participant to reach the shoulder joint angles of interest. The accuracy of these methods, however, has not yet been investigated. In the current study, an external frame was designed to assist in maintaining specific shoulder postures in a wide range. The three degrees of freedom of rotation of the proposed frame were designed to be consistent with the description of shoulder joint angles recommended by the International Society of Biomechanics. Six participants used this frame to perform 118 different shoulder postures. The reference joint angles measured by a motion tracking system were compared with the frame-defined angles. The angle differences among all the participants ranged from 12.7° to 85.6°, with an average of 32.2° (SD 15.1°) across all postures. For the postures with elevation angles on or below horizontal, the average angle difference was 23.7° (SD 8.5°). Findings suggest that errors exist when using an external frame to assist in reaching specific shoulder postures. Error is minimized at elevation angles close to -30°, and the performance is poor for extreme shoulder postures.

Anatomically reproducible assessment of volumetric bone mineral density--based on clinical computed tomography

Local distribution and quality of bone mineral density (BMD) in the proximal humerus are important for rotator cuff repair, internal fixations, and arthroplasty. The current study aimed to develop a volumetric BMD (vBMD) measurement technique that can specify the volumetric region of interest in an anatomically reproducible manner and ensures fair comparison of vBMD among intrasubjective or intersubjective measurements. Computed toed into three-dimensional (3D) polygon models. A standardized volumetric block was asmographic images of 10 intact shoulders (from six women and four men, age range 46-69 years) were reconstruct signed, and it included whole insertion areas of the supraspinatus and infraspinatus tendons on the greater tuberosity. For standardizing a bone block, a mathematical method for reproducible landmark assignment and reproducible local coordinate system was developed. After converting the polygonal standardized volumetric block into 3D tetrahedron finite element models, its vBMD was calculated. The repeatability and reproducibility standard deviations of the vBMD measurement method were found to be less than 2%. The current study developed a vBMD measurement technique that can specify the vROI in an anatomically reproducible manner and ensures fair comparison of BMD among intrasubjective or intersubjective measurements, with less than 2% reproducibility error.

Transformation between different local coordinate systems of the scapula

The existence of multiple local coordinate systems (LCSs) for the scapula makes it difficult to compare the kinematics of the scapula across various studies and reports. This study aimed to build transformation matrices between different LCSs for the scapula and to provide the coordinates of previously measured muscles and ligaments around the scapula with respect to the International Society of Biomechanics (ISB) recommended LCS. The bony landmarks necessary for building various local coordinate systems were digitized on 13 CT scanned scapulae. The LCSs were built based on the digitized bony landmarks and then used for calculating the transformation equations. The approximate coordinates of 28 muscles and ligaments of the scapula were expressed with respect to the ISB-recommended LCS using the derived transformation equations. The results of this study may be used for the comparison of scapula kinematics data with respect to various LCSs and for building a scapula biomechanical model with respect to ISB-recommended LCS.

The effect of decreasing computed tomography dosage on radiostereometric analysis (RSA) accuracy at the glenohumeral joint

Standard, beaded radiostereometric analysis (RSA) and markerless RSA often use computed tomography (CT) scans to create three-dimensional (3D) bone models. However, ethical concerns exist due to risks associated with CT radiation exposure. Therefore, the aim of this study was to investigate the effect of decreasing CT dosage on RSA accuracy. Four cadaveric shoulder specimens were scanned using a normal-dose CT protocol and two low-dose protocols, where the dosage was decreased by 89% and 98%. 3D computer models of the humerus and scapula were created using each CT protocol. Bi-planar fluoroscopy was used to image five different static glenohumeral positions and two dynamic glenohumeral movements, of which a total of five static and four dynamic poses were selected for analysis. For standard RSA, negligible differences were found in bead (0.21±0.31mm) and bony landmark (2.31±1.90mm) locations when the CT dosage was decreased by 98% (p-values>0.167). For markerless RSA kinematic results, excellent agreement was found between the normal-dose and lowest-dose protocol, with all Spearman rank correlation coefficients greater than 0.95. Average root mean squared errors of 1.04±0.68mm and 2.42±0.81° were also found at this reduced dosage for static positions. In summary, CT dosage can be markedly reduced when performing shoulder RSA to minimize the risks of radiation exposure. Standard RSA accuracy was negligibly affected by the 98% CT dose reduction and for markerless RSA, the benefits of decreasing CT dosage to the subject outweigh the introduced errors.