What is the complete principle of Dynamic Hip Screw (DHS) in treating hip fractures, particularly in elderly patients with osteoporotic bone?

Dynamic Hip Screw (DHS) Principles for Hip Fractures

The Dynamic Hip Screw operates on the principle of controlled fracture impaction through a sliding lag screw mechanism that allows the fracture to compress along the axis of the femoral neck while maintaining stable fixation to the femoral shaft, but should only be used for stable intertrochanteric fractures—unstable patterns require cephalomedullary nail fixation instead. 1, 2

Core Biomechanical Principles

The DHS system consists of three fundamental components that work synergistically:

• Lag screw component: A large-diameter screw inserted into the femoral head that provides rotational stability and serves as the primary load-bearing element 1
• Sliding barrel mechanism: Allows controlled telescoping of the lag screw, permitting fracture impaction and compression as the patient bears weight, which promotes bone healing through dynamic compression 2, 3
• Side plate fixation: Secured to the lateral femoral cortex with cortical screws (typically 4-6 screws with bicortical purchase) to resist bending and torsional forces 1

Critical Technical Requirements

Achieving anatomic reduction is the single most important predictor of success—poor blade positioning increases cut-out risk by a factor of 7, regardless of implant type. 4

Reduction Goals

• Restore medial cortical continuity under fluoroscopic guidance in both AP and lateral views 1
• Achieve normal neck-shaft angle of approximately 130-135 degrees 1
• Maintain reduction throughout the fixation process to prevent malunion and hardware failure 1

Lag Screw Positioning

• Position the lag screw to reach within 5-10 mm of subchondral bone in the femoral head 1
• Optimal placement in the center-center or inferior-central position of the femoral head reduces cut-out risk 4
• Use tip-apex distance as a guide—keeping it minimal reduces failure rates 4

Specific Indications and Contraindications

DHS is appropriate ONLY for stable intertrochanteric fractures—this is a critical decision point that directly impacts mortality and morbidity. 1, 2

Appropriate Use (Stable Patterns)

• AO/OTA type 31-A1 and stable 31-A2 fractures without significant comminution 2, 3
• Fractures with intact posteromedial cortex and lesser trochanter 2
• Patients with adequate bone quality (Singh Index level 3-4 or higher) 5

Mandatory Contraindications (Use Cephalomedullary Nail Instead)

• Unstable fractures with lesser trochanter involvement or avulsion—failure rates exceed 50% with DHS in these patterns 1, 2
• Reverse obliquity fracture patterns 1, 2
• Subtrochanteric extension 1, 2
• Fractures with posteromedial comminution 1
• Pathologic fractures—DHS is not effective due to lack of bone healing, particularly with planned subsequent radiation 1

Special Considerations for Osteoporotic Bone

In elderly patients with severe osteoporosis, standard DHS fixation faces higher complication rates due to inadequate bone anchorage. 3

Enhanced Fixation Strategies

• Cement augmentation: PMMA injection into the femoral head through the lag screw improves fixation stability and prevents superior screw cut-out in osteoporotic bone, with fracture union achieved in 100% of cases at an average of 13.8 weeks 3
• Locking side plates: Fixed-angle locking screws provide 2.6 times greater construct strength compared to standard screws in osteoporotic bone, with improved maintenance of neck-shaft angle and reduced cut-out rates 6, 7

Technical Pitfall to Avoid

• Over-compression during fixation can cause fracture comminution in osteoporotic bone—controlled compression is essential 1

Perioperative Protocol Integration

The DHS principle must be integrated within a comprehensive perioperative care pathway to optimize outcomes:

• Surgery should be performed within 24-48 hours of hospital admission 8
• Do NOT use preoperative traction—it provides no benefit and is specifically contraindicated 1, 2
• Administer prophylactic antibiotics within one hour of skin incision 1
• Either spinal or general anesthesia is appropriate with no preference 2

Postoperative Management Principles

• Immediate weight-bearing as tolerated—the sliding mechanism is designed to function with early mobilization 1, 2
• Administer fondaparinux or low molecular weight heparin for DVT prophylaxis 1
• Continue regular paracetamol throughout the perioperative period for pain management 1
• Implement early mobilization protocols to reduce complications and improve outcomes 1

Fracture Healing Expectations

• Average time to union: 13.8-17.1 weeks depending on fracture stability and bone quality 3, 6
• Expected lag screw slippage: 3-4 mm as the fracture compresses—this is normal and therapeutic 6
• Monitor serial radiographs for fracture healing progression, maintenance of reduction, and hardware complications 2

Secondary Fracture Prevention

Every patient aged 50 years and over requires systematic evaluation for subsequent fracture risk using the Fracture Liaison Service (FLS) model. 2

• Order outpatient DEXA scan for bone density assessment 1, 2
• Initiate anti-osteoporotic treatment—can be started even without DXA scan in patients with hip fractures 8
• Provide patient education on fall prevention and bone health 8