Digital Templating for Total Hip Arthroplasty (THA)

Digital Templating for Total Hip Arthroplasty (THA)

Prepared by Dr. Kayahan KARAYTUG

Overview

Preoperative digital templating is a cornerstone of modern total hip arthroplasty (THA). It enables accurate restoration of hip biomechanics, facilitates implant selection, and reduces intraoperative uncertainty. Despite advances in imaging, digital templating remains subject to technical and anatomical limitations.

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Objectives of THA Templating

Digital templating in THA aims to:

• Restore the center of rotation
• Address limb length discrepancy
• Optimize femoral offset
• Evaluate acetabular bone stock
• Assess proximal femoral geometry and canal morphology
• Guide femoral neck resection
• Anticipate implant size and instrumentation

Accurate templating contributes to reduced operative time and helps prevent dislocation, periprosthetic fracture, limb length inequality, and implant loosening.

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Analogue Versus Digital Templating

Traditional acetate templating assumes a fixed magnification (typically 120%). However, actual magnification varies based on patient habitus and radiographic geometry, with reported values ranging from 109% to 128%. This variability compromises implant size prediction and may lead to oversizing, which has been associated with femoral shaft fractures in 3–24% of cases.

Digital templating has largely replaced analogue methods by compensating for magnification error, improving efficiency, and providing a permanent planning record.

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Limitations of 2D Digital Templating

Despite its advantages, digital templating relies on 2D imaging of 3D anatomy, resulting in:

• Limited assessment of intramedullary femoral morphology
• Reduced accuracy in predicting stem fit and version
• Dependence on correct radiographic calibration

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Role of 3D (CT-Based) Templating

CT-based 3D templating enables detailed evaluation of:

• Femoral canal geometry
• Cortical thickness
• Acetabular orientation

Reported implant size prediction accuracy is higher compared to 2D methods (86–94% vs. 80–84%). However, increased radiation exposure, cost, and logistical burden limit routine CT use. Consequently, 2D digital templating remains the clinical standard.

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Calibration and External Markers

Accurate calibration is critical for reliable digital templating.

Calibration Methods

• External calibration markers (ECMs / scaling balls)
• Backboard-based systems (e.g., Kingmark™)

This approach relies on placing a marker at the same coronal plane as the hip center of rotation.

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Challenges with External Calibration Markers

Accurate marker placement is technically demanding. Common issues include:

• Anterior displacement in obese patients
• Lateral displacement beyond the imaging field
• Difficulty identifying the hip center externally
• Radiographer workload and variability
• Patient discomfort and modesty concerns

Incorrect placement results in mismatched magnification between the marker and the region of interest.

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Standardized Imaging Protocol

Following multidisciplinary collaboration, a standardized protocol was introduced:

• All THA patients operated on by a single surgeon
• Standardized AP pelvic radiographs
• Consistent use of an external calibration marker

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Study Objective (THA)

To assess whether increased institutional familiarity with standardized ECM use:

• Improves calibration marker positioning accuracy over time
• Enhances the precision of digital templating in THA

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Key Clinical Pearls

• Calibration accuracy is the primary determinant of reliable THA templating
• 2D digital templating remains the gold standard in routine practice
• CT-based 3D templating improves accuracy but is not universally practical

References

1. Knight JL, Atwater RD. Preoperative planning for total hip arthroplasty: Quantitating its utility and precision. J Arthroplasty. 1992;7(4):403–409.
2. González Della Valle A, et al. Preoperative planning accuracy for total hip arthroplasty. J Arthroplasty. 2008;23(4):517–523.
3. The B, et al. The accuracy of digital templating in total hip arthroplasty. J Bone Joint Surg Br. 2005;87(6):760–765.
4. Merle C, et al. Three-dimensional planning in total hip arthroplasty: Accuracy and reproducibility. Orthop Traumatol Surg Res. 2012;98(1):1–6.
5. Viceconti M, et al. Large-scale validation of a CT-based approach for pre-operative planning of total hip replacement. J Biomech. 2003;36(12):1899–1906.
6. Hananouchi T, et al. Accuracy of CT-based 3D preoperative planning for cementless total hip arthroplasty. J Orthop Sci. 2012;17(5):536–543.
7. King RJ, et al. The use of calibration markers in digital templating for total hip arthroplasty. Skeletal Radiol. 2009;38(6):583–589.