ZZ Implant Template Description

ZZ.1 Implant Mating

In this section, the usage of mating features for assembly of implants is declared.

ZZ.1.1 Mating Features

These Attributes establish a Cartesian coordinate system relative to the Frame of Reference of the implant. When two implants are assembled using a pair of mating features, a rigid spatial registration can be established, that transforms one Frame of Reference so that the mating features align. The figure below gives a simple example in 2D how two implants (symbolized by two rectangles) are matched according to a mating feature pair. For each 2D and 3D template present, a set of coordinates is assigned to each Mating Feature Sequence Item.

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Figure ZZ.1-1 IMPLANT TEMPLATE MATING (EXAMPLE).

ZZ.1.2 Mating Feature ID

It is recommended to give Mating Features that are somehow related, the same Mating Feature ID (0068,63F0) in different implant templates. This may help applications to switch between components while keeping connections to other components. The Example in Figure ZZ.1-2 shows that the first and the last hole in the plates get the same Mating Feature ID in each Template.

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Figure ZZ.1-2 IMPLANT TEMPLATE MATING FEATURE IDS (EXAMPLE).

ZZ.1.3 Mating Feature Sets

The Mating Features are organized in sets of alternative features: Only one feature of any set shall be used for assembly with other components in one plan. This enables the definition of variants for one kind of contact a component can make while ensuring consistent plans.

An example for Mating Feature Sets is shown in Figure ZZ.1-2. A hip stem template shows a set of five mating features, drawn as circles on the tip of its cone. Different head components use different mating points, depending on the base radius of the conic intake on the head.

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Figure ZZ.1-2 2D MATING FEATURE COORDINATES SEQUENCE (EXAMPLE).

ZZ.1.4 Degrees of Freedom

For each Item of the Mating Feature Sequence (0068,63E0), degrees of freedom can be specified. A degree of freedom is defined by one axis, and can be either rotational or translational. For each 2D and 3D template present, the geometric specifications of the mating points can be provided.

ZZ.1.5 Implant Assembly Templates

Instances of the Implant Assembly Template IOD are utilized to define intended combinations of implant templates. An Implant Assembly Template consists of a sequence of component type definitions (Component Type Sequence (0076,0032)) which references Implant Template Instances and assigns roles to the referenced implants. In the example in Figure ZZ.1-3, the component types “Stems” and “Heads” are defined. Four different stems and two different heads are referenced. Both groups are flagged mandatory and exclusive, i.e. a valid assembly requires exactly one representative of each group.

The Component Assembly Sequence (0076,0060) declares possible connections between components referenced by the component groups. Each sequence item refers to exactly two implant templates which are part of at least one component group in the same Implant Assembly Template Instance. An Component Assembly Sequence Item references one mating feature in each of the templates according to which the assembly is geometrically constrained. The double-pointed dashed lines represent the Items of the Component Assembly Sequence in Figure ZZ.1-3.

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Figure ZZ.1-3 IMPLANT ASSEMBLY TEMPLATE (EXAMPLE).

ZZ.2 Planning Landmarks

Registration of implant templates with patient images according to anatomical landmarks is one of the major features of implantation planning. For that purpose, geometric features can be attached to Implant Template Instances. Three kinds of landmarks are defined: Points, lines, and planes. Each landmark consists of its geometric definition which is defined per template and a description.

When registering an Implant Template to a patient dataset like an Image or a Surface Segmentation, the planning software should establish a spatial transformation that matches to planning landmarks to corresponding geometric features in the patient dataset.

ZZ.3 Implant Registration and Mating Example

  1. In this section, an example is presented that shows the usage of Implant Templates together with an Implant Assembly Template to create an Implantation Plan with patient images. The example is in 2D but can easily be extended to 3D as well. The example looks at a simplified case of hip reconstruction planning, using a monoblock stem component and a monoblock cup component.

  2. Planning consists of 2 steps: Selection and placement of the best fitting cup from the cups referenced by the Assembly Template based on the dimension of the patient’s hip is the first step. With that done, a stem is selected that can be mated with the selected cup and has a neck configuration that leads to an optimal outcome with regard to leg length and other parameters. Therefore, the available stems are placed so that the features align. The femoral planning landmarks are used to calculate the displacement of the femur this configuration would result in. The workflow is shown in the following set of figures.

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Figure ZZ.3-1 IMPLANT TEMPLATES USED IN THE EXAMPLE.

  1. In the first step, the planning landmarks marked with the green arrows in Figure ZZ.3-2 are aligned with compliant positions in the patient’s x-ray.

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Figure ZZ.3-2 CUP IS ALIGNED WITH PATIENT’S ACETABULUM USING 2 LANDMARKS

  1. In the second step, the femoral length axis is detected from the patient’s x-ray and the stem template is aligned accordingly using the femoral axis landmark. The proximal and distal fixation boundary planes are used to determine the insertion depth of the stem along that axis.

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Figure ZZ.3-3 STEM IS ALIGNED WITH PATIENT’S FEMUR.

  1. In the third step, the image is split into a femoral and a pelvic part according to the proposed resection plane of the stem template. The mating features are used to calculate the spatial relation between the femoral and the pelvic component.

  2. [pic]

Figure ZZ.3-4 FEMORAL AND PELVIC SIDE ARE REGISTERED.

ZZ.3.1 Degrees Of Freedom

The hip joint has several degrees of freedom, of course. The Implant Template should contain this information in the Mating Features. In the given 2D projections, the rotational freedom of the joint is expressed by one single rotation around the axis of projection intersecting with the printing space at the 2D coordinate of the Mating Feature. Therefore, a Degree Of Freedom Sequence Item added to either the stem, the cup, or both.

In planning, this information could be used to visualize the rotational capacities of the joint after implantation.

Note: Technically, the degree of freedom could also have been added to the cup or even (each with half the range of freedom) to both. But since we are used to see femurs rotation with respect to pelvises and not the other way around, it seemed natural to do it that way.

[pic] Figure ZZ.3-5 ROTATIONAL DEGREE OF FREEDOM

ZZ.4 ENCODING EXAMPLE

  1. The Templates used in the example can be encoded as follows:

Table ZZ.4-1Attributes Used to Describe a Mono Stem Implant for Total Hip Replacement

Attribute Value Comment
SOP Common Module
SOP Class UID 1.2.840.10008.5.1.4.43.1
SOP Instance UID 1.2.3.4.5.6.7.0.1
Generic Implant Template Module
Manufacturer ACME
Implant Name MONO_STEM
Implant Size MEDIUM
Implant Part Number ACME_MST_M
Effective DateTime 26.06.2009 12:00
Implant Template Version 1
Implant Template Type ORIGINAL
Implant Target Anatomy Sequence
>Anatomic Region Sequence
>>Code Value T-12710
>>Coding Scheme Designator SRT
>>Code Meaning Femur
Frame of Reference UID 1.2.3.4.5.6.7.1.1
Overall Template Spatial Tolerance 1.0
HPGL Document Sequence
>HPGL Document ID 1
>View Orientation Code Sequence
>>Code Value R-10206
>>Coding Scheme Designator SRT
>>Code Meaning Antero-Posterior
>HPGL Document Scaling 1.0
>HPGL Document IN PA … HPGL commands
>HPGL Contour Pen Number 2
>HPGL Pen Sequence
>>HPGL Pen Number 2
>>HPGL Pen Label Contour
>>HPGL Pen Number 3
>>HPGL Pen Label Landmarks
>>HPGL Pen Number 4
>>HPGL Pen Label Mating Features
>Recommended Rotation Point 39.6/72.4
>Bounding Rectangle 14.2/5.7/46/78.8
Material Code Sequence
>Coding Scheme Designator SRT
>Code Value F-61207
>Code Meaning Stainless Steel Material
Implant Type Code Sequence
>Coding Scheme Designator DCM
>Code Value 112315
>Code Meaning Monoblock Stem
Fixation Method Code Sequence
>Coding Scheme Designator SRT
>Code Value R-42808
>Code Meaning Uncemented Component Fixation
Mating Feature Sets Sequence
>Mating Feature Set ID 1
>Mating Feature Set Label Head Rotation Point
>Mating Feature Sequence
>>Mating Feature ID 1
>>2D Mating Feature Coordinates Sequence
>>>Referenced HPGL Document ID 1
>>>2D Mating Point 39.6/72.4
>>>2D Mating Axes 1/0/0/1
>>Mating Feature Degree of Freedom Sequence
>>>Degree of Freedom ID 1
>>>Degree of Freedom Type ROTATION
>>>2D Degree of Freedom Sequence
>>>>Referenced HPGL Document ID 1
>>>>2D Degree Of Freedom Axis 0/0/1
>>>>Range of Freedom -15/15

Table ZZ.4-2 Attributes Used to Describe a Mono Cup Implant for Total Hip Replacement

Attribute Value Comment
SOP Common Module
SOP Class UID 1.2.840.10008.5.1.4.43.1
SOP Instance UID 1.2.3.4.5.6.7.0.2
Generic Implant Template Module
Manufacturer ACME
Implant Name MONO_CUP
Implant Size MEDIUM
Implant Part Number ACME_MCP_M
Effective DateTime 26.06.2009 12:00
Implant Template Version 1
Implant Template Type ORIGINAL
Implant Target Anatomy Sequence
>Anatomic Region Sequence
>>Code Value T-15710
>>Coding Scheme Designator SRT
>>Code Meaning Hip Joint
Frame of Reference UID 1.2.3.4.5.6.7.1.2
Overall Template Spatial Tolerance 1.0
HPGL Document Sequence
>HPGL Document ID 1
>View Orientation Code Sequence
>>Code Value G-5215
>>Coding Scheme Designator SRT
>>Code Meaning Anterior Projection
>HPGL Document Scaling 1
>HPGL Document IN PA … HPGL commands
>HPGL Contour Pen Number 2
>HPGL Pen Sequence
>>HPGL Pen Number 2
>>HPGL Pen Label Contour
>>HPGL Pen Number 3
>>HPGL Pen Label Landmarks
>>HPGL Pen Number 4
>>HPGL Pen Label Mating Features
>Recommended Rotation Point 12.9/0
>Bounding Rectangle 0/0/25.8/12.9
Material Code Sequence
>Coding Scheme Designator SRT
>Code Value F-61207
>Code Meaning Stainless Steel Material
Implant Type Code Sequence
>Coding Scheme Designator DCM
>Code Value 112307
>Code Meaning Acetabular Cup Monoblock
Fixation Method Code Sequence
>Coding Scheme Designator SRT
>Code Value R-42808
>Code Meaning Uncemented Component Fixation
Mating Feature Sets Sequence
>Mating Feature Set ID 1
>Mating Feature Set Label Hip Joint Mating Feature
>Mating Feature Sequence
>>Mating Feature ID 1
>>2D Mating Feature Coordinates Sequence
>>>Referenced HPGL Document ID 1
>>>2D Mating Point 12.9/0
>>>2D Mating Axes 0.707/0.707/-0.707/0.707

Table ZZ.4-3Attributes Used to Describe the Assembly of Cup and Stem

Attribute Value Comment
SOP Common Module
SOP Class UID 1.2.840.10008.5.1.4.44.1
SOP Instance UID 1.2.3.4.5.6.7.0.3
Implant Assembly Template Module
Implant Assembly Template Name Acme Hip Assembly
Implant Assembly Template Issuer ACME
Effective DateTime 26.06.2009 12:00
Implant Assembly Template Version 1
Implant Assembly Template Type ORIGINAL
Implant Assembly Template Target Anatomy Sequence
>Anatomic Region Sequence
>>Code Value T-15710
>>Coding Scheme Designator SRT
>>Code Meaning Hip Joint
Procedure Type Code Sequence
>Code Value P1-14810
>Coding Scheme Designator SRT
>Code Meaning Hip Joint Reconstruction
Component Types Sequence
>Component Type Code Sequence Sequence Item 1
> Coding Scheme Designator DCM
> Code Value 112310
> Code Meaning Femoral Stem
>Exclusive Component Type YES
>Mandatory Component Type YES
>Component Sequence
>>Referenced SOP Class UID 1.2.840.10008.5.1.4.43.1
>>Referenced SOP Instance UID 1.2.3.4.5.6.7.0.1
>>Component ID 1
>Component Type Code Sequence Sequence Item 2
> Coding Scheme Designator DCM
> Code Value 112305
> Code Meaning Acetabular Cup Shell
>Exclusive Component Type YES
>Mandatory Component Type YES
>Component Sequence
>>Referenced SOP Class UID 1.2.840.10008.5.1.4.43.1
>>Referenced SOP Instance UID 1.2.3.4.5.6.7.0.2
>>Component ID 2
Component Assembly Sequence
>Component 1 Referenced ID 1 The stem
>Component 1 Referenced Mating Feature Set ID 1
> Component 1 Referenced Mating Feature ID 1
>Component 2 Referenced ID 2 The cup
> Component 2 Referenced Mating Feature Set ID 1
> Component 2 Referenced Mating Feature ID 1

ZZ.5 Implant Template Versions and Derivation

The Generic Implant Module contains several Attributes to express the relations between different versions of implant templates. These Attributes are

(0022,1097) Implant Part Number Number (or alphanumerical code) assigned by the manufacturer of an implant to one particular release of one particular part. Whenever changes on the implant design are made, a new implant part number is assigned.
(0068,6226) Effective DateTime Date and time from which on an Implant Template Instance is valid.
(0068,6221) Implant Template Version Number assigned by the creator of an ORIGINAL Implant Template Instance. When an implant manufacturer issues a new version of an implant template without doing changes on the implant itself, it issues a new instance with the same part number but a different template version.
(0068,6222) Replaced Implant Template Sequence When a manufacturer issues a new version of an Implant Template, the instance contains a reference to it direct predecessor.
(0068,6223) Implant Type When a software vendor, user or other entity creates a “proprietary” version of an Implant Template by adding Attributes, the resulting Instance is labeled DERIVED.
(0068,6225) Original Implant Template When an Instance is DERIVED, it contains a reference to the ORIGINAL instance it was derived from (directly or with several derived versions in between).
(0068,6224) Derivation Implant Template Sequence When an Implant Template Instance is derived from another instance, it contains a reference to the Implant Template Instance it was directly derived from.

Different versions of Implant Templates reflect the changes a manufacturer is doing on the Implant Templates he issues. The Implant Templates which are issued by a manufacturer (or a third party who is acting on behalf of the manufacturer) are always ORIGINAL. Software vendors, PACS integrators, or other stakeholders will add information to such templates for different purposes. The Instances which are generated by this process is called derivation and the resulting instances are labeled DERIVED. Implantation Plans, i.e. electronic documents describing the result of implantation planning, are specified in an instance of the Implantation Plan SR Document. There, the implants which are relevant for one plan are included by reference. When such plans are exchanged between systems or organizations it is likely that the receiving party has access to other versions of templates as the sending party has. In order to maintain readability of exchanged plans, the following is required:

  1. All necessary information about an implant which is relevant to display and understand a plan is present in the ORIGINAL Implant Templates which were issued by a manufacturer. This is assured by these Attributes being Type 1 in the IOD.

  2. When deriving Instances, information may only be added but not removed from the ORIGINAL Instance. This information may be encoded in standard or private Tags.

  3. Derived Instances contain the information about the source Instances they were derived from. All Instances contain a reference to the ORIGINAL Instance they were derived from. If an application receives a plan which references an implant it does not have in its database, it will find the UID of the ORIGINAL Instance in the plan, too. It can query its database for an instance which was derived from that Instance and thereby find an Instance it can use to present the plan.

Figure ZZ.5-1 shows an example of the relationships between two versions of a manufacturer’s Implant Template and several different Implant Templates derived by software vendors from these versions.

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Figure ZZ.5-1 IMPLANT VERSIONS AND DERIVATION.