How is the screw length determined for a Dynamic Hip Screw (DHS) procedure in an elderly patient with potential osteoporosis?

DHS Screw Length Determination

The screw length in DHS fixation should be determined by measuring to within 5-10mm of the subchondral bone of the femoral head on both AP and lateral fluoroscopic views, prioritizing central positioning over maximal length to minimize tip-apex distance below 25mm.

Primary Technical Principle: Tip-Apex Distance (TAD)

The most critical factor in determining screw length is achieving a tip-apex distance (TAD) of less than 25mm, which is the strongest predictor of screw cut-out failure 1, 2. TAD is calculated as the sum of the distance from the tip of the lag screw to the apex of the femoral head on both AP and lateral radiographs, after correcting for magnification.

Screw Length Measurement Algorithm

Step 1: Intraoperative Fluoroscopic Assessment

• Measure the distance from the lateral cortex of the femur to the subchondral bone of the femoral head on both AP and lateral views 1
• The screw tip should be positioned 5-10mm from the subchondral bone to allow for controlled impaction while preventing penetration 3, 2

Step 2: Position Verification (More Important Than Length)

• Central-central positioning on both AP and lateral views produces the best outcomes and naturally determines appropriate length 3, 2
• If central positioning cannot be achieved, posterior-inferior placement is preferred over anterior-superior, as it supports the posteromedial cortex and reduces cut-out risk even with longer TAD 2
• Avoid anterior or superior screw positions, which significantly increase cut-out rates regardless of length 3, 4

Critical Technical Considerations

Osteoporotic Bone Modifications

In elderly patients with osteoporosis (Singh Index levels 2-4), screw length determination remains the same, as osteoporosis itself does not independently influence outcome when proper positioning and TAD are achieved 3, 5. However, these patients require:

• More careful attention to TAD measurement 1
• Acceptance that posterior-inferior positioning may require slightly longer screws but provides better mechanical support 2

Common Pitfalls to Avoid

• Excessive screw length attempting to maximize purchase increases TAD and cut-out risk 1, 2
• Peripheral placement (anterior or superior) increases TAD even with shorter screws 2
• Making the final length decision before achieving adequate fracture reduction under fluoroscopy 4
• Ignoring the lateral view measurement—both planes must be assessed 1, 3

Practical Measurement Technique

1. After achieving acceptable reduction, place guide wire in central-central position on fluoroscopy 3, 2
2. Measure wire length to 5-10mm from subchondral bone on both AP and lateral views 3
3. Calculate TAD: if >25mm, reposition wire more centrally rather than shortening excessively 1, 2
4. Select screw length that maintains central position with TAD <25mm 1, 2

The key principle is that screw position determines appropriate length, not the reverse—attempting to maximize screw length at the expense of central positioning increases failure rates 3, 2, 4.