Templating in Total Hip Arthroplasty
1. Why Template?
Preoperative templating helps to plan and restore:
- Hip biomechanics
- Hip center
- Femoral offset
- Leg length
- Acetabular cup position
- Femoral stem position
- Implant size
It is particularly useful in patients with unusual anatomy, such as:
- Developmental dysplasia
- Protrusio
- Abnormal femoral neck angles
- Proximal femoral deformity
Templating also helps anticipate implant sizes and ensure appropriate implants are available in the operating room.
2. First Step: Obtain Good-Quality Radiographs
The most important first step is good-quality radiographs.
AP pelvis
The AP pelvis should include:
- Both hip joints
- Both proximal femora
- At least the proximal one-third of each femur
Check pelvic positioning
The coccyx should be approximately 3 cm from the symphysis pubis and aligned with it.
This helps ensure that:
- The pelvis is not rotated
- Pelvic tilt is appropriate
- Acetabular and femoral measurements are reliable
Poor positioning
If the coccyx is too close to the symphysis pubis:
- The radiograph becomes an outlet-type view
If the coccyx is too far away:
- The radiograph becomes an inlet-type view
Do not template from poorly positioned radiographs.
3. Important Acetabular Landmarks
Two important landmarks are:
Teardrop
- Marks the inferior boundary for cup placement
Kohler’s line
- Represents the ilioischial line
- Marks the medial-most aspect of the acetabular bone
These landmarks are important when determining the position and depth of the acetabular component.
4. Evaluate Femoral Rotation
The femur should be appropriately rotated on the AP pelvis.
The femoral neck and head should be seen in an appropriate profile.
The lesser trochanter is particularly useful:
- Too much lesser trochanter visible suggests external rotation
- Little or no lesser trochanter visible suggests internal rotation
In severe osteoarthritis, obtaining an appropriate profile may be difficult because of restricted hip movement.
In such cases, templating the normal side can be useful.
5. Assess Femoral Neck Anatomy
Assess:
- Varus or valgus neck
- Femoral neck-shaft angle
- Femoral offset
A horizontal line from the greater trochanter toward the femoral head can help estimate the relationship of the hip center.
Femoral offset
Femoral neck offset is the horizontal distance between:
- The center of the femoral stem
- The hip center
Restoration of offset is important for appropriate hip biomechanics.
6. Assess the Proximal Femur
A good AP view of the hip including the proximal femur helps evaluate:
- Bone quality
- Femoral canal morphology
- Proximal femoral deformity
- Trochanteric anatomy
Dorr Classification
| Type | Characteristics |
|---|---|
| Dorr A | Thick cortex, narrow femoral canal |
| Dorr B | Good cortical bone and reasonable canal dimensions |
| Dorr C | Thin cortex, wide canal |
Most stems fit well in Dorr B femora.
In a Dorr C femur with a wide canal and thin cortex, a cemented stem may need to be considered.
7. Lateral Hip Radiographs
Useful views include:
- Frog-leg lateral
- Cross-table lateral
Frog-leg lateral
Useful for assessing:
- Proximal femoral anatomy
- Femoral bowing
- Proximal femoral deformity
Cross-table lateral
Provides additional information about:
- Acetabular anatomy
- Anteroposterior dimensions of the acetabulum
A technically poor cross-table view may need to be repeated.
8. Identify Case-Specific Challenges
Look carefully for:
- Large medial osteophytes
- Superior or lateral migration of the femoral head
- Protrusio
- Dysplasia
- Acetabular bone loss
- Proximal femoral deformity
For example, a large medial osteophyte may require medial reaming to reach the appropriate depth.
Superior migration and superior acetabular wear may result in significant superior cup uncoverage, which should be anticipated during templating.
9. Stepwise Approach to Templating
The lecture describes a stepwise approach:
Step 1
Determine magnification
Step 2
Determine leg-length difference
Step 3
Template the acetabular cup
Step 4
Template the femoral component
Step 5
Plan the final hip reduction
Usually, templating is performed on the affected side.
However, if the deformity is severe and makes templating difficult, the normal side can be used.
10. Digital Templating
Digital templating has become widely used because it makes the process easier.
Typical process:
- Upload radiographs
- Select procedure and side
- Select appropriate radiograph
- Determine magnification
- Determine leg length
- Select cup and stem
- Position the components
- Plan the final reduction
The lecture emphasizes that digital templating is easier, but not necessarily more accurate than conventional templating.
11. Determine Magnification
Ideally, the radiograph should contain a calibration marker.
The marker should be positioned:
- At the level of the bone
- In the coronal plane
The software can then use the known marker size to calculate magnification.
If a marker is not available, the lecture describes using approximately 20 to 22% magnification as a practical estimate.
12. Determine Leg Length
A horizontal reference line can be drawn using:
- The transischial line
- The line connecting the two teardrops
- The inferior aspects of the obturator prominences
The distance from this reference line to appropriate femoral landmarks can then be compared between the two sides.
Digital software can provide:
- Leg-length difference
- Femoral head diameter
- Femoral offset
- Proximal femoral canal diameter
13. Templating the Acetabular Cup
Important landmarks:
- Teardrop
- Kohler’s line
In a standard hip:
- The cup should generally be positioned around the lateral aspect of the teardrop
- The inferior aspect of the cup should be at or slightly below the level of the teardrop
The exact position depends on the patient’s anatomy.
Protrusio
Templating helps determine:
- Desired cup position
- Amount of medial bone that may need to be packed
- How to restore the anatomical hip center
Dysplasia
The femoral head may be superiorly migrated.
Templating helps determine how to bring the cup toward the anatomical hip center.
14. Templating the Femoral Component
Assess:
- Neck cut
- Stem size
- Stem position
- Femoral offset
- Varus or valgus alignment
- Canal fill
For a metaphyseal-filling or taper stem, the stem should appropriately fill the metaphysis.
The tip of the stem should be centered in the femoral canal, rather than positioned in varus or valgus.
Severe proximal femoral deformity may require a different stem design, such as a Wagner cone-type stem.
15. Plan the Final Reduction
After determining:
- Cup position
- Stem position
- Stem size
- Hip center
- Offset
the final reduction can be simulated.
This allows assessment of:
- Leg-length restoration
- Femoral offset
- Hip center
- Cup inclination
The example in the lecture demonstrates how the stem can be moved to fine-tune leg length and offset.
16. Postoperative Assessment
Good-quality postoperative radiographs are essential.
Assess the pelvis
Check:
- Pelvic rotation
- Cup position
Assess the acetabular component
Evaluate:
- Cup inclination
- Cup height
- Cup depth
- Relationship to the teardrop
- Relationship to Kohler’s line
A good cross-table lateral view can be used to assess cup and stem anteversion.
After hip replacement, the lecture specifically advises against routinely obtaining a frog-leg lateral view.
17. Assess the Femoral Stem
Evaluate:
- Leg length
- Femoral offset
- Neck-shaft angle
- Stem size
- Canal fill
- Periprosthetic fracture
- Varus or valgus alignment
The tip of the stem should be centered within the femoral canal.
18. Common Errors
One study discussed in the lecture found that a common error after THA was excessive leg lengthening.
The contributing factors included:
- Inferior acetabular component position
- Excessive femoral offset
- Inadequate medialization of the acetabular component
Good preoperative templating can help minimize these errors.
19. Digital vs Conventional Templating
Both methods can work well.
Conventional templating
- Uses physical templates
- Has been used successfully for many years
- Can be accurate when performed correctly
Digital templating
- Easier to perform
- Provides automated measurements and “wizards”
- Makes implant manipulation and planning more convenient
The key message from the lecture is that digital templating is not necessarily more accurate than conventional templating. Correct technique is more important than the particular method used.
20. Most Important Principle: Templating Is a Plan, Not a Command
The surgeon should cross-check the template intraoperatively.
The templated implant size may need to change based on:
- Actual broaching
- Bone quality
- Component fit
- Cup stability
- Stem stability
- Hip stability
Therefore:
Do not become completely dependent on the template.
The template tells you what you expect to use, but intraoperative findings determine what you actually use.
21. Role of Intraoperative X-rays
The lecture’s approach differs according to surgical position.
Posterior approach
Routine intraoperative X-rays are generally not considered helpful because the patient is lateral and obtaining a good AP pelvis is difficult.
Direct lateral approach
Similarly, routine intraoperative X-rays are not considered particularly useful.
Direct anterior approach
Because the patient is supine, obtaining an intraoperative radiograph is easier.
It can therefore be useful, particularly when beginning to perform the anterior approach.
High-Yield Points
- Good-quality AP pelvis is the starting point.
- Coccyx should be approximately 3 cm from the symphysis pubis and aligned with it.
- Teardrop is an important landmark for inferior cup position.
- Kohler’s line marks the medial-most acetabular boundary.
- Check femoral rotation using the lesser trochanter.
- Assess femoral neck-shaft angle and offset.
- Assess proximal femoral morphology using Dorr classification.
- Dorr C femur may require consideration of a cemented stem.
- Determine magnification before templating.
- Restore hip center, leg length and offset.
- Template the cup first, then the femoral component.
- The stem should have appropriate canal fill and be centered in the canal.
- Digital templating is easier, not necessarily more accurate.
- Always correlate the template with intraoperative findings.
- Hip stability remains the most important intraoperative check.
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