Preoperative Radiography and Templating in Total Hip Arthroplasty

INTRODUCTION TO PREOPERATIVE PLANNING IN TOTAL HIP ARTHROPLASTY

Total Hip Arthroplasty (THA) is widely regarded as one of the most successful surgical interventions in modern medicine. However, the longevity of the implant and the functional outcome of the patient are inextricably linked to meticulous preoperative planning. Preoperative radiographic evaluation and templating remove intraoperative guesswork, mitigate the risk of neurovascular injury, and ensure the precise restoration of hip biomechanics.

The primary goals of preoperative templating are to determine the optimal size and position of the acetabular and femoral components, restore the anatomic center of rotation, optimize femoral offset to maximize abductor mechanics, and equalize limb lengths. Failure to execute a rigorous preoperative radiographic analysis can lead to catastrophic intraoperative complications, including periprosthetic fractures, excessive bone resection, instability, and severe limb-length discrepancies.

CLINICAL EVALUATION: THE MODIFIED HARRIS HIP SCORE

Before radiographic analysis begins, a comprehensive clinical evaluation must be documented. The Modified Harris Hip Score (HHS) remains a gold-standard, validated instrument for assessing hip function, pain, and deformity before and after arthroplasty. The maximum score is 100 points, heavily weighted toward pain relief and functional capacity.

Domains of the Modified Harris Hip Score

1. Pain (Maximum 44 Points)
* 44 points: None or ignores it.
* 40 points: Slight, occasional, no compromise in activities.
* 30 points: Mild pain, no effect on average activities; rarely moderate pain with unusual activity; may take simple analgesics.
* 20 points: Moderate pain, tolerable but makes concessions; some limitation of ordinary activity; requires stronger analgesics.
* 10 points: Marked pain, serious limitation of activities.
* 0 points: Totally disabled, crippled, pain in bed, bedridden.

2. Function and Daily Activities (Maximum 47 Points)
* Limp: None (11), Slight (8), Moderate (5), Severe (0).
* Support: None (11), Cane for long walks (7), Cane most of the time (5), One crutch (3), Two canes (2), Two crutches/Not able to walk (0).
* Distance Walked: Unlimited (11), Six blocks (8), Two or three blocks (5), Indoors only (2), Bed and chair (0).
* Stairs: Normally without railing (4), Normally using railing (2), In any manner (1), Unable (0).
* Shoes and Socks: With ease (4), With difficulty (2), Unable (0).
* Sitting: Comfortably in ordinary chair for 1 hour (5), On a high chair for 30 mins (3), Unable to sit comfortably (0).
* Public Transportation: Able to enter (1), Unable (0).

3. Absence of Deformity (Maximum 4 Points)
All of the following must be present to score 4 points (otherwise 0):
* Less than 30 degrees fixed flexion contracture.
* Less than 10 degrees fixed adduction.
* Less than 10 degrees fixed internal rotation in extension.
* Limb-length discrepancy less than 3.2 cm.

4. Range of Motion (Maximum 5 Points)
Calculated based on a composite index of flexion, abduction, adduction, and external/internal rotation.

💡 Clinical Pearl

Always correlate the patient's clinical deformity (e.g., fixed external rotation or flexion contractures) with the radiographic findings. A fixed external rotation deformity will artificially alter the appearance of femoral offset and neck length on a standard AP radiograph, rendering standard templating inaccurate.

STANDARD RADIOGRAPHIC PROTOCOL

A standardized radiographic series is mandatory for accurate templating. Poor-quality radiographs or improper patient positioning will lead to erroneous measurements and inappropriate implant selection.

Required Views

1. Anteroposterior (AP) Pelvis: The beam is centered on the pubic symphysis. The film must include the bilateral iliac crests, the entire pelvic ring, and the proximal one-third of both femurs.
2. AP and Lateral of the Affected Hip: The lateral view (cross-table or frog-leg) is critical for assessing the anterior/posterior bone stock and the sagittal bow of the proximal femur.

Patient Positioning and Magnification

• Internal Rotation: The hips must be positioned in 15 degrees of internal rotation. This neutralizes the natural anteversion of the femoral neck, placing it parallel to the coronal plane and allowing for an accurate assessment of true femoral offset and neck length.
• Magnification Markers: A radiopaque magnification marker (typically a sphere or a ruler with lead spheres exactly 100 mm apart) must be taped to the patient's thigh.
  • For the AP view: Place the marker on the upper medial thigh, at the level of the greater trochanter.
  • For the lateral view: Move the marker to the anterior thigh.
  • Rationale: The marker must be positioned at the exact coronal or sagittal depth of the bone being templated to calculate the precise magnification factor (typically 115% to 120% on standard analog films).

⚠️ Surgical Pitfall

Failure to internally rotate the hip by 15 degrees will foreshorten the femoral neck on the AP radiograph. This leads the surgeon to underestimate the patient's native offset, potentially resulting in the selection of an implant with insufficient offset, leading to postoperative abductor weakness and instability.

RADIOGRAPHIC ANALYSIS OF PATHOLOGIC STATES

Before overlaying templates, the surgeon must critically evaluate the radiographs for structural abnormalities that will dictate the surgical approach, implant type, and need for bone grafting.

Acetabular Evaluation

• Structural Integrity: Assess the medial wall (Kohler's line), the superior dome, and the teardrop.
• Protrusio Acetabuli: Medial migration of the femoral head past Kohler's line. This requires careful reaming and often necessitates medial bone grafting (impaction grafting) to lateralize the center of rotation.
• Developmental Dysplasia of the Hip (DDH): Evaluate the lateral center-edge angle and the amount of superior/lateral bone stock. Severe dysplasia may require a high hip center, structural autografting (e.g., femoral head autograft), or specialized trabecular metal augments.
• Post-Traumatic Deformity: In cases of old fracture-dislocations, obtain Judet views (obturator and iliac oblique) and a CT scan. These are essential to identify posterior wall defects or retained hardware that must be bypassed or removed.

Femoral Evaluation

• Medullary Canal Width: Assess the cortical thickness and canal diameter (Dorr classification). Narrow canals (often seen in young patients, dwarfs, or DDH) may require a straight stem, a conical modular stem, or a custom-milled implant.
• Deformity and Bowing: Paget disease, prior fractures, or congenital anomalies can cause severe anterior or lateral bowing. A lateral radiograph is paramount here. If the bow precludes the insertion of a standard straight stem, a corrective femoral osteotomy may be required concurrently with the arthroplasty.
• Retained Hardware: Ensure appropriate extraction instruments (e.g., broken screw extractors, trephines) are available if previous internal fixation devices are present.

STEP-BY-STEP PREOPERATIVE TEMPLATING TECHNIQUE

Templating can be performed using traditional acetate overlays on plain films or via digital templating software on a Picture Archiving and Communication System (PACS). The biomechanical principles remain identical.

Step 1: Establish Reference Lines and Assess Limb Length Discrepancy (LLD)

1. Draw a horizontal reference line connecting the inferior margins of the radiographic teardrops (the inter-teardrop line). If the teardrops are asymmetric or obscured, a line connecting the inferior margins of the ischial tuberosities (the bi-ischial line) can be used.
2. Draw a perpendicular line from the reference line down to the most prominent point of the lesser trochanter on both the operative and non-operative sides.
3. The difference in the perpendicular distance between the two sides represents the radiographic limb-length discrepancy.

Step 2: Acetabular Templating

1. Place the acetabular template over the affected hip. The goal is to restore the anatomic center of rotation without excessive removal of subchondral bone.
2. Positioning: The medial border of the template should rest adjacent to the radiographic teardrop (lateral to Kohler's line). The inferior margin should align with the inferior border of the teardrop or the obturator foramen.
3. Inclination: Orient the template at 40 to 45 degrees of abduction relative to the inter-teardrop line.
4. Mark the center of the templated acetabular component. This crosshair represents the new center of rotation of the reconstructed hip.

💡 Clinical Pearl

When templating the acetabulum, prioritize maximizing host bone contact. Do not upsize the cup excessively just to gain stability, as this removes critical anterior and posterior column bone stock. Aim for a line-to-line fit or a 1-2 mm under-ream for press-fit hemispherical components.

Step 3: Femoral Templating

1. Select a femoral template that matches the contour of the proximal medullary canal.
2. Fixation Strategy:
   • Cementless (Press-fit): The template should fill the metaphysis and achieve cortical contact at the medial calcar and lateral cortex.
   • Cemented: Allow for a 2 to 3 mm circumferential cement mantle between the template and the endosteal cortex. Measure the canal diameter below the tip of the stem to size the distal medullary cement restrictor plug.
3. Ensure the stem is aligned with the neutral axis of the femoral canal to avoid varus or valgus malpositioning.

Step 4: Restoring Offset and Limb Length

1. With the femoral stem template in optimal position, evaluate the various neck length and offset options (e.g., standard, high offset, +0, +4, +8 head lengths).
2. Align the center of the femoral head on the template with the previously marked new center of rotation of the acetabulum.
3. Adjusting for LLD: If the patient has a 10 mm short leg on the operative side, the center of the templated femoral head should be positioned 10 mm superior to the acetabular center of rotation. When the hip is reduced intraoperatively, this will lengthen the leg by 10 mm, equalizing the limbs.
4. Restoring Offset: The horizontal distance from the center of rotation to the central axis of the femoral shaft must match the contralateral normal hip (or the patient's native anatomy if bilateral disease is present). High-offset stems lateralize the femur without increasing limb length, effectively tensioning the abductors.

Step 5: Determining the Neck Resection Level

1. Once the optimal stem size, offset, and neck length are selected, mark the level of the anticipated femoral neck cut on the radiograph.
2. Measure the distance from the top of the lesser trochanter to the medial point of the neck resection.
3. This measurement (e.g., "15 mm above the lesser trochanter") serves as a critical intraoperative landmark to guide the oscillating saw during the femoral neck osteotomy.

DIGITAL TEMPLATING CONSIDERATIONS

Digital radiographs are now the standard of care in modern orthopaedic practice. Digital templating requires specialized software integrated with a comprehensive library of manufacturer-specific implant CAD models.

Advantages of Digital Templating

• Dynamic Scaling: The software automatically calculates the magnification factor based on the 100 mm calibration marker, eliminating manual mathematical errors.
• Modularity: Surgeons can rapidly cycle through different stem sizes, neck angles, and head lengths with a single click.
• 3D Templating: Advanced CT-based 3D templating allows for multi-planar reconstruction. This is invaluable in complex cases (e.g., severe DDH, post-traumatic deformity, or revision surgery) where 2D radiographs fail to capture rotational deformities or volumetric bone defects.

⚠️ Surgical Pitfall

Digital templating is only as accurate as the calibration. If the magnification marker is placed incorrectly (e.g., taped to the bed rather than the patient's thigh), the software will scale the image improperly, leading to gross overestimation or underestimation of the required implant size. Always perform a "sanity check" by measuring a known anatomic landmark, such as the diameter of the contralateral normal femoral head.

CONCLUSION

Preoperative templating is not merely a sizing exercise; it is a comprehensive biomechanical simulation of the surgical procedure. By rigorously evaluating the preoperative radiographs, calculating limb-length discrepancies, and meticulously templating both the acetabular and femoral components, the orthopaedic surgeon establishes a definitive surgical blueprint. This disciplined approach minimizes intraoperative delays, reduces the incidence of complications, and ensures the restoration of optimal hip kinematics, ultimately leading to superior functional outcomes and implant longevity.