What causes periscrew loosening?

Mechanisms of Periscrew Loosening

Periscrew loosening occurs primarily through three mechanisms: inadequate initial fixation, mechanical stress-induced bone loss, and particle-induced osteolysis, with bone resorption triggered by high radial stress being the dominant biomechanical pathway.

Primary Mechanical Mechanisms

Radial Stress-Induced Bone Resorption

• High radial stress from screws causes more bone damage than axial stress, representing the primary mechanism of screw loosening in orthopedic fixation 1
• Bone resorption begins at screws closest to stress concentration points (such as fracture lines or joint interfaces), then progressively affects adjacent screws 1
• The radial stress creates a zone of damaged bone around the screw that undergoes resorption, leading to periscrew lucency and eventual loosening 1

Inadequate Primary Fixation

• Inadequate initial fixation at the time of surgery is a fundamental cause of early loosening, particularly in patients with poor bone stock 2, 3
• Poor bone quality compromises the initial mechanical interlock between screw threads and bone, predisposing to micromotion 3

Mechanical Stress and Micromotion

• Mechanical stresses create micromotion at the implant-bone or cement-bone interface, which initiates the loosening cascade 3
• Malalignment during surgery progresses to instability and eventually loosening over time 3
• In cement-augmented screws, single-level cement fixation creates a fixed pivot point that causes screw toggling during flexion-extension loading, resulting in loosening 4

Particle-Induced Osteolysis

Inflammatory Cascade

• Polyethylene wear debris is the most common trigger for particle-induced osteolysis, though cement and metal particles also contribute 2, 3
• Macrophages phagocytose particle debris, creating a cell-mediated inflammatory response with granulation tissue formation 2, 3
• This inflammatory process leads to progressive bone loss around the implant, particularly in cementless constructs 3

Anatomical Distribution

• Osteolysis preferentially occurs at specific high-stress locations: femoral condyles near collateral ligament attachments, component peripheries, and access channels to cancellous bone including screw holes 2, 3
• Cementless implants have higher incidence of osteolysis compared to cemented constructs 3

Biomechanical Factors

Load Distribution Patterns

• Screws nearest to the primary load point (such as fracture lines or joint interfaces) experience the highest stress and are most vulnerable to loosening 1
• The volume of bone damaged by stress gradually decreases from central high-stress screws toward end screws 1

Implant Angulation Effects

• Buccolingual angulation of implants significantly influences screw loosening risk, while mesiodistal angulation shows less impact 5
• Unfavorable implant inclination increases off-axis loading, contributing to mechanical failure 6

Clinical Risk Factors

Patient-Related Factors

• Parafunctional habits (bruxism, clenching) increase mechanical stress on fixation 6, 5
• Generalized attrition patterns correlate strongly with increased screw loosening incidence 5
• Poor bone stock compromises initial fixation and accelerates loosening 2, 3

Technical Factors

• Single-unit restorations have higher loosening risk than multiunit constructs due to concentrated loading 5
• Cantilever designs increase off-axis forces that promote loosening 6
• Surgeon experience significantly impacts loosening rates, with less experienced operators having higher complication rates 5

Progressive Failure Pattern

Temporal Sequence

• Loosening typically manifests as a progressive process: initial micromotion → periscrew lucency → halo formation around screw body → eventual pullout 4
• The halo sign on imaging represents bone loss and/or fibrous tissue interface formation, indicating advanced loosening 4
• Small areas of osteolysis may remain stable with monitoring, but large areas indicate impending component failure requiring surgical intervention 3

Critical Diagnostic Considerations

Imaging Findings

• Periscrew lucency >2mm on radiographs indicates significant loosening 7
• Bone scintigraphy shows increased uptake at loose screws, but interpretation is complicated by 20% false-positive rate at 1 year and 12.5% at 2 years post-surgery in asymptomatic patients 2, 3
• A false-negative bone scan can occur if loosening happens at the cement-prosthetic interface without inciting new bone formation 2

Distinguishing Infection from Aseptic Loosening

• Both infection and aseptic loosening show increased periprosthetic uptake on bone scans, requiring WBC/marrow imaging for differentiation 2
• Aseptic loosening results from either inadequate initial fixation, mechanical loss of fixation over time, or biologic loss from particle-induced osteolysis 2