Influence of methodological errors on reliability

It is not possible to completely avoid methodological errors when using measurement tools. Recognition of that fact places the onus on the user to control for errors as much as possible. Various methodological errors in 3D-ULMA have been recognised in the literature (Butler et al., 2010,

Jaspers et al., 2011b, Jaspers et al., 2011c, Vanezis et al., 2015). Their contributions to the poor reliability of the 3D-ULMA model used in this research are outlined in the following paragraphs.

4.7.3.4.1 Anatomical coordinate system definition

Firstly, definition of the anatomical coordinate system (ACS) which

describes the angular position of axes, planes and rigid bodies, may have influenced reliability of the model. ACS definition is dependent on

identification of bony landmarks through palpation, placement of technical clusters, reliability of pointer acquisition and postural alignment of the upper limb during a static calibration. Altered biomechanical alignment of children with OBPP, especially of the shoulder complex (Nath et al., 2007, Hale et al., 2010), made accurate palpation of bony landmarks more challenging. Furthermore, due to the inherent variability and age of participants consistent implementation of the standardised position for static calibration proved difficult.

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Slight variation in definition of ACS influences axis definitions. This has been examined in the lower limb (Brennan et al., 2011) but exploration of its influence on upper limb kinematics is limited. It has been found that definition of the ST P/R axis was dependent on repositioning of the AC while ST M/L rotation and A/P tilt were less sensitive (van Andel et al., 2009). The ST P/R axis was the most difficult to measure reliably using the AM in children with OBPP suggesting that replacement of either the scapular markers or the cluster itself were subject to variation. The meeting point between the acromion and the scapular spine has been identified as the most accurate location for the AC (Shaheen et al., 2011). While this position was found to be least affected by skin deformation, replacement error was not assessed meaning the ability to reliably

replicate this position has not been determined. Caution has already been advised when interpreting the scapular segment due to sensitivity to

marker placement. This was identified by the difference seen between intra and inter-session errors (van Andel et al., 2009, Jaspers et al., 2011b, Jaspers et al., 2011c, Vanezis et al., 2015). Therefore, it was decided to use the position of the acromial angle for placement of the AC as it has been used in a paediatric population of TDC and children with HCP with its repeatability examined (Jaspers et al., 2011b, Jaspers et al., 2011c). Should the meeting point of the acromion and scapular spine be determined repeatable then its use in future studies may improve reliability of the AC, thereby improving reliability of ST axis definition.

SC with the arm in a position of rest, palm down on ipsilateral knee was used in this study. A recent study, albeit on adult cadaver subjects, found greater errors in scapular orientation when only SC was performed as opposed to DC (Cereatti et al., 2015). DC decreased error to -1.0⁰ to 14.2⁰ from an error of 6.2⁰ to 44⁰ in SC. DC at rest position and at a second angle close to end range was concluded to allow for greater compensation of soft tissue artefact than SC in adults (Brochard et al., 2011). However, reliability of DC while still within acceptable limits was less than SC. Therefore, DC was not adopted for this study based on the rationale that due to varied abilities in children with OBPP a standardised

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second calibration position would be difficult to determine prospectively. For future research, performing a second calibration at either 60⁰ GH joint abduction or, as close as possible, may improve reliability (Shaheen et al., 2011). To our knowledge, the impact of marker placement and palpation on other joints has not been reported in the literature but it is reasonable to assume that inaccurate palpation and marker placement would affect reliability of the model. This needs to be further examined in the literature to accurately inform interpretation of 3D-ULMA model’s findings.

4.7.3.4.2 Gimbal lock

Secondly, the well-recognised mathematical problem of gimbal lock and its presence in this research has been discussed (Chapter 3 Methods:

Section 3.7.2.1). While it is recommended that rotation sequences used in the calculation of joint kinematics should avoid singular positions (e.g. 180⁰ elevation) no one rotation sequence allows for this in the GH joint (Šenk and Chèze, 2006). Gimbal lock may have contributed to the overall poor reliability seen in both the Internal and External Rotation Tasks. Both were performed close to one of the identified gimbal lock positions i.e. 0⁰ of arm elevation (Anglin and Wyss, 2000, Šenk and Chèze, 2006, Phadke et al., 2011). When the humerus is parallel to the trunk POE cannot be distinguished from AR leading to illogical angles being determined by the mathematical model (Phadke et al., 2011). The start position of hand on ipsilateral knee, similar to that used by Jaspers et al. (2011b) and Jaspers et al. (2011c), was adopted to ensure a degree of shoulder elevation at all times. From the graphs it can be seen that the lowest degree of elevation was ~5⁰ with the majority of children with OBPP being elevated about 20⁰ at start and end of these tasks (Chapter 5 Kinematic Results: Section 5.4, Figure 5.3 and Section 5.5, Figure 5.4). This is close to the recommended 30⁰ of elevation recommended by Šenk and Chèze (2006) as a good starting point to avoid gimbal lock when using the ISB recommended sequence of rotation (YXY). The incidence of gimbal lock was not very high for these tasks (Appendix 3.7) suggesting that other factors, as discussed in the preceding sections, contributed to their poor reliability. The poor reliability of the ST joint in the Hand-to-Spine Task may be a

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consequence of the chosen rotation sequence as recommended by the ISB (Wu et al., 2005) as it was not found to be the best sequence for backward elevation of the GH joint (Šenk and Chèze, 2006).

4.7.3.4.3 Marker view

Thirdly, marker view was a challenge during the performance of some tasks. This is likely to have contributed to poor reliability due to critical loss of view at certain points. There are two main reasons for this problem. The orientation of arm segments for certain movements

compromised marker view e.g. forearm cluster during the Hand-to-Spine Task due to poor active supination and internal rotation posture of the arm or anterior trunk markers in the Hand-to-Mouth/Abduction Tasks. In addition, the motion analysis system used could only support four cameras. It is evident from the findings of this research that this is

insufficient to capture all potential orientations of the upper limb throughout all tasks. This has not been identified as a significant problem in the

literature, however most systems used more cameras (6-12) than were available for this research (Mackey et al., 2005, Fitoussi et al., 2009, Jaspers et al., 2011b, Jaspers et al., 2011c, Vanezis et al., 2015). Increasing the number of cameras may improve reliability of the current model in particular for the tasks identified above.

4.7.3.4.4 Standardised positions for task performance

Fourthly, direction was provided to standardise both start position and task performance through a consistent resting posture, verbal instruction and task demonstration. This reduces the amount of intrinsic variability within the measurement. Yet the difficulty in adopting standardised positions due to inherent variability of the upper limb, participant age and the desire to permit compensatory strategies, if required, rendered achieving

consistency more challenging. This may have resulted in larger error due to both intrinsic variability and extrinsic error and highlights the difficulties in measuring functional task performance.

Goniometry is the most objective measure of upper limb passive and active ROM in children with OBPP used in clinical practice (Chang et al.,

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2013). While its reliability has not been specifically explored in this population both inter-session and inter-observer reliability has been

explored in children (Riddle et al., 1987, de Winter et al., 2004, Wilk et al., 2009, Kolber et al., 2012). Assessment with goniometry is systematic with standard start positions, alignment of the upper limb segments, the

goniometer itself and performance of the movement, both active and passive. This means that these reliability results are not comparable to goniometry. However, by combining the information gained from both assessments, actual available ROM through goniometry and kinematic patterns via 3D-ULMA, an improved understanding of upper limb function can be achieved.

4.7.3.4.5 Sample size

Finally, sample size may have impacted on reliability results especially of joints and tasks that only required small movements for successful

completion. The sample size was calculated based on detecting a

difference in External Rotation ROM. This ROM is larger than the average excursions of the ST joint which may have impacted on the reliability findings of this joint in particular. A larger sample size of each of the NC is recommended for future research.

In summary, various methodological errors had an impact on the reliability findings of this research. The degree of each is difficult to quantify. The influence of intra-session reliability was not statistically explored therefore its contribution to the reliability findings cannot be quantified. The main issues identified were palpation error especially for the scapula, definition of ACS, difficulty for model to track specific tasks or rotation axes, marker view and finally sample size may be too small for certain rotation axes due to naturally smaller movement ROM.