How is neck length calculated in total hip replacement (THR)?

Neck Length Calculation in Total Hip Replacement

In total hip replacement, neck length is determined by measuring the distance from the center of rotation of the femoral head to the lateral shoulder of the femoral stem, which directly affects femoral offset and leg length restoration—this is calculated during preoperative digital templating by measuring the vertical distance from the planned femoral head center to the resection level of the femoral neck.

Preoperative Templating Methodology

Digital templating is the standard method for calculating neck length and selecting appropriate components in THR. The process involves:

• Measure the distance from the center of the femoral head to the planned neck resection level on calibrated anteroposterior (AP) pelvis radiographs 1, 2
• Calculate the required neck length by determining the vertical offset needed to restore the anatomical center of rotation of the hip 3
• Account for both leg length discrepancy and femoral offset simultaneously when selecting neck length, as these parameters are interdependent 1, 3

Key Anatomical Reference Points

The calculation requires identifying specific landmarks:

• The rotational center of the hip serves as the primary reference point for measuring craniocaudal positioning 3
• The lateral shoulder of the greater trochanter provides the reference for femoral offset measurement 3
• The planned femoral neck osteotomy level determines the starting point for neck length calculation 2

Accuracy Considerations

Digital templating demonstrates high precision when properly executed:

• Computerized 3D planning restores the rotational center with mean accuracy of 0.73 mm craniocaudally and limb length with mean accuracy of 0.3 mm 3
• Modular neck systems allow fine-tuning of neck length to optimize both leg length and femoral offset restoration 3
• Templated sizes correspond to actual implanted components in 93% of cases for neck-shaft angle selection 3

Clinical Pitfalls to Avoid

Several factors can compromise neck length calculation accuracy:

• Failure to obtain properly positioned AP pelvis radiographs with 15-20 degrees internal rotation leads to inaccurate measurements of the femoral neck axis 4
• Not accounting for pelvic obliquity creates false impressions of required leg length correction 5
• Patient obesity and external hip rotation significantly affect templating accuracy and must be corrected before measurement 2
• Magnification errors occur when calibration markers are not used, though studies show minimal difference in accuracy with or without calibration devices like KingMark 2

Practical Algorithm for Neck Length Selection

Follow this systematic approach:

1. Obtain standardized AP pelvis radiograph with both hips internally rotated 15 degrees 4
2. Identify and mark the center of the femoral head on both operative and contralateral sides 3
3. Measure the vertical distance from head center to planned neck cut on the operative side 1
4. Compare to the contralateral normal hip to determine required restoration 1
5. Select modular neck length that restores femoral offset to within 1 mm of the contralateral side 3
6. Verify that the selected neck length achieves leg length equality (mean discrepancy should be <3 mm) 1, 3

Intraoperative Verification

Large discrepancies between trial components and templated neck length indicate potential surgical error and should prompt reassessment of component positioning 6. The surgeon should recognize when trial components differ by more than one size from the preoperative plan, as this suggests either templating error or intraoperative technical issues 2, 6.