How do you differentiate between Dynamic Hip Screw (DHS) failure due to lateral wall fracture versus barrel crossing the fracture line?

Differentiating Between DHS Failure Due to Lateral Wall Fracture vs. Barrel Crossing Fracture Line

Dynamic Hip Screw (DHS) failure due to lateral wall fracture can be differentiated from barrel crossing fracture line by measuring lateral wall thickness (<21mm indicates high risk for lateral wall fracture) and evaluating the position of the barrel relative to the fracture line on radiographs.

Lateral Wall Fracture as Cause of DHS Failure

Radiographic Findings

• Lateral wall thickness measurement:
  • Lateral wall thickness <21mm is the most reliable predictor of lateral wall fracture with 95% sensitivity and 88.2% specificity 1
  • Threshold value of 20.5mm has been established as critical - below this value, DHS alone should not be used 2
  • Visible disruption of the lateral cortex on post-operative radiographs

Biomechanical Factors

• Higher von Mises stress distribution around the blade entry point in thinner lateral walls (89.26% higher in 10mm walls compared to 30mm walls) 3
• Medialization of the femoral shaft is commonly seen (64.7% of cases with lateral wall fractures) 1
• Increased displacement of the implant components visible on follow-up radiographs

Clinical Outcomes

• Poor Harris hip score (mean 65.5 vs 73.1 in intact lateral wall) 1
• Only 32.3% of patients with lateral wall fractures achieve pre-injury mobility status 1
• Higher rates of reoperation and complications

Barrel Crossing Fracture Line as Cause of DHS Failure

Radiographic Findings

• Barrel of the DHS visibly crossing the fracture line on AP and lateral radiographs
• Inadequate fracture reduction with persistent fracture gap
• Progressive varus collapse on sequential radiographs
• Screw cut-out or implant migration

Biomechanical Factors

• Lack of sliding mechanism function when barrel crosses fracture line
• Increased stress at the barrel-plate junction
• Single-point fixation (without anti-rotation screw) leading to rotational instability
• Excessive collapse and medialization of the shaft

Clinical Outcomes

• Delayed or non-union of fracture
• Progressive varus deformity
• Implant failure (screw cut-out or plate breakage)

Key Differentiating Features

1. Pre-operative Assessment:

   • Measure lateral wall thickness on pre-operative AP radiographs
   • If <21mm, high risk for lateral wall fracture during or after DHS fixation 1, 2

2. Intra-operative Assessment:

   • Evaluate integrity of lateral wall during surgery
   • Check position of barrel relative to fracture line
   • Consider lateral wall reconstruction with trochanteric stabilizing plate if lateral wall is thin or comminuted 4

3. Post-operative Assessment:

   • Look for visible lateral wall disruption on post-operative radiographs
   • Evaluate barrel position relative to fracture line
   • Measure tip-apex distance (TAD) - excessive TAD increases risk of screw cut-out regardless of lateral wall status

Prevention Strategies

1. For Lateral Wall Fracture Prevention:

   • Use intramedullary nail instead of DHS for fractures with lateral wall thickness <21mm 1, 2
   • Consider adding trochanteric stabilizing plate (TSP) to DHS when lateral wall is thin 4
   • Take extreme precaution during reaming and insertion to prevent iatrogenic lateral wall damage

2. For Barrel Crossing Fracture Line Prevention:

   • Ensure anatomic reduction before DHS insertion
   • Position barrel completely within the proximal fragment
   • Add anti-rotation screw for two-point fixation to enhance stability 4
   • Ensure proper tip-apex distance (<25mm)

Pitfalls and Caveats

• Lateral wall thickness is more reliable than AO/OTA classification in predicting lateral wall fracture 2
• Osteoporosis has no significant bearing on lateral wall fracture occurrence 1
• Post-operative lateral wall fracture can occur even when the lateral wall appears intact during surgery
• Both complications can lead to poor functional outcomes, but lateral wall fracture has been specifically associated with medialization and treatment failure 1, 5
• AO/OTA 31-A2 classification, postoperative lateral wall fracture, posteriorly inserted lag screw, and varus reduction pattern are significant risk predictors for DHS failure 5

By systematically evaluating these factors, surgeons can differentiate between DHS failure due to lateral wall fracture versus barrel crossing the fracture line, and take appropriate preventive measures based on pre-operative planning.