Standardization Concept
I fully support the goals of standardization. In the past, I have had to convert my data into several different formats, which not only takes time, but allows for the possibility of calculation errors. As discussed by Van der Helm, standardization allows both the comparison of data and the development of an intuition for normal values. It also helps a newcomer to understand the issues involved. In some case, there is no objectively "right" way, but if we can agree to one way, it will simplify the decisions and data comparison for many people in the future.
History
Since I am aware of Frans Van der Helm's (VdH) more recent work (Van der Helm, 1997), I know that the documents that I was given to review are now outdated. I am not aware of how J. Dapena (JD) would change his proposal, but I will try to comment on both the presented proposals and VdH's more recent proposal, found on their web site:www.fbw.vu.nl/research/Lijn_A4/shoulder/isg/proposal/protocol.htm
I am also aware of the Elbow Joint Standardization proposed some time ago to the ISB Standardization Committee by Bo Peterson, and the Wrist and Hand proposal by F. Werner and B. Buchholz. Where they overlap, there should be an effort not to have conflicting standards.
Title
This is a very minor point, but since they are different between the proposals, I support "joints of the shoulder complex" to emphasize the inclusion of the scapula and clavicle; the lay person (and possibly somebody beginning a study) usually associates the "shoulder" with the glenohumeral joint alone. VdH's new title could be "A standardized protocol for motion recordings of the shoulder complex". As I said, though, it's a minor point.
Introduction
I find VdH's extra paragraph on humeral motions etc. important to include. The breakdown in both proposals of the three types of project areas is very useful. VdH provides further useful clarification. VdH's newer proposal discusses, in addition, different tracking systems. This may not be necessary to the ISB standard, but could be helpful to new researchers, and clarifies the further discussion.
Notation
I am in agreement with JD that the notation for the matrices does not need to be included in the standard, although Craig (1986) could still be recommended. If the rotations themselves are indeed standardized, then the matrices will not need to be presented in papers. This section does not appear in VdH's recent proposal, but rather is clarified where necessary.
Part 1: Definition of a Global Coordinate System (GCS)
In the original version of the standardization proposal, both JD and VdH support the use of Z vertical. In VdH's more recent proposal, Y is vertical. I continue to support the Z-vertical approach, for the reasons that they initially stated, and because the longitudinal axes of the humerus and thorax are then along the Z axes, which agrees with normal robotics terminology.
Part 2: Definition of Local Coordinate Systems of the Segments
In general, it would be immensely helpful to have figures showing the anatomical locations of the landmarks. This is in VdH's more recent work.
Thorax
The origin of the thorax is not particularly important, but since one must be chosen, I support JD's (IJ+C7)/2. Since GZt is supposed to approximate the longitudinal axis of the thorax (see VdH's notes), it is intuitively easier to picture if it passes through the middle of the thorax.
It may be difficult to track the specified thorax points when using video techniques due to the arm crossing between the landmarks and the camera(s). A paragraph should be added already here that, even if these specific landmarks cannot be followed, the relationship between these landmarks and those chosen must be recorded. (This is true for all of the local landmarks.)
Clavicle
A minor point, but since there appears to be a discrepancy, I would support VdH's subscripts corresponding to the project type. CB could precede CA, if it is the preferred method.
Has the relationship between these two coordinate systems been determined? If so, this should be included. If possible, the one should then be adapted to the other so that reported angles are consistent.
VdH communicated more recently that his group now prefers the use of AA (Angulus Acromialis, most latero-dorsal point of scapula) over AC to avoid gimbal lock.
Scapula
As with the clavicle, the landmark AA is now preferred by VdH over AC.
Humerus
It is important here to include the work of Meskers et al. (1998) to indicate that GH can be estimated from other bony landmarks. This is included in VdH's recent proposal. Other approaches also exist relating external markers to the glenohumeral joint centre, such as calibrated arm movements, or moving markers by a specified distance in a specified location. It would be helpful to give (possibly standardized) instructions to locate the GH.
The use of a cylindrical cuff, as suggested by VdH, to operationalize the longitudinal axis of the humerus, must be calibrated to assure the correct direction of the axes.
As with the thorax, the medial epicondyle may be obstructed from camera(s). If other points are used, their positions relative to the these standardized points should be recorded. This is described in detail in VdH's new proposal.
The medial and lateral epicondyles may not be the best landmarks, for two reasons:
a) Williams (1996) identified poor repeatability and reproducibility of marking the lateral epicondyle due to its shape.
b) Although it is not necessary to identify the elbow flexion axis to describe the humerus orientation, it could be intuitively better. The flexion axis lies 7-11 mm below the epicondyles, through the centre of the trochlea (e.g. Veeger et al., 1997); see also Peterson's proposal for the elbow.
This should be discussed further.
Part 3: Global and Local Linear Positions
There was apparently a disagreement over using the term "positions" or "displacements". Since displacements must be calculated from positions, the term positions might as well be used.
It is not necessary to specify that the distances should be in metres (as opposed to cm or mm), but it would be advantageous to recommend metric as the standard units.
This section does not appear in VdH's new proposal, although it should be addressed.
Part 4: Global Orientations
The thorax, clavicle and scapula rotation orders agree between the two proposals, but the humerus rotations differ. See comments re Relative Orientations. The order of rotations should agree between the two sections.
Part 5: Relative Orientations
VdH's proposal should more clearly state that option 2 (5B) is the standard option, as emphasized in JD's proposal. In VdH's new proposal there appears to still be no recommendation for one over the other.
As with the landmarks, it would be helpful to have figures to accompany the rotation orders, although this is difficult to present in 3D.
I much prefer the humerus rotations proposed by VdH. This is the issue in the standardization proposal about which I feel the strongest. In the past, people have tried to use "flexion-abduction-rotation", i.e. X-Y'-Z'' (JD), in an attempt to use clinical language, but in fact this only confuses the matter, since abduction must occur about the flexion axis. More recently, people have used the term "elevation" (i.e. the angle of the humerus from the thorax: flexion is elevation in the sagittal plane; abduction is elevation in the frontal plane). This is preceded by a rotation dictating the plane of elevation and followed by a rotation giving the axial rotation of the humerus. Using these angles, it is easy to conceptualize where the arm is in space; see also note 5.6. A figure from Romilly et al. (1994) compares the two rotation orders. Peterson's proposed elbow standard also supports longitude-latitude-rotation.
I have yet to come up with the ideal name for the first rotation of the humerus, but I now support "horizontal flexion-extension". The problem that I encountered with "plane of elevation", is that people believed that the arm was actively rotating in this plane, rather than it being a mathematical description to reach a given position.
I have not explained these issues particularly well here. If there is continued confusion about this issue, I will try to explain it better, because I believe that the flexion-abduction rotation order is misleading and confusing.
JD's suggestion to use a different calculation order when there is a danger of gimbal-lock is a good one, but the presentation of the results should remain as suggested by VdH.
Part 6: Joint Torques
Agreed.
Reference Section
This should be expanded, as has been done in VdH's new proposal. It may also be worth referencing appropriate web sites (e.g. the ISB standardization, and the International Shoulder Group).
References to these comments
- Meskers CGM, van der Helm FCT, Rozendaal LA and Rozing PM (1998a) In vivo estimation of the glenohumeral joint rotation center from scapular bony landmarks by linear regression. J. Biomech., 31:93-96.
- Romilly DP, Anglin C, Gosine RG, Hershler C and Raschke SU (1994) A functional task analysis and motion simulation for the development of a powered upper-limb orthosis. IEEE Trans. Rehabil. Eng., 2(3):119-129.
- Van der Helm FCT (1997) A standardized protocol for motion recordings of the shoulder. In Proc. 1st Conf. Int. Shoulder Group, Veeger HEJ, Van der Helm FCT and Rozing PM (eds.), Shaker Publishers, Maastricht, pp. 7-12.
- Veeger HEJ, Yu B, An KN and Rozendal RH (1997) Parameters for modelling the upper extremity. J. Biomech., 30(6):647-652.
- Williams JR (1996) Some aspects of the biomechancis of the elbow joint related to prosthetic design. DM thesis, Oxford Univ., Oxford.
Conclusion
A lot of work has clearly gone into the proposals. They should be updated by Van der Helm's revised proposal, currently on their web site, and presented at the upcoming ISB conference.
