Mechanism of Periscrew (Pedicle Screw) Loosening
Screw loosening occurs through a biomechanical process involving loss of preload force, micromotion at the bone-screw interface, and progressive bone resorption around the screw threads, ultimately leading to loss of fixation stability. 1
Primary Mechanical Mechanisms
The fundamental mechanism involves several interconnected processes:
- Loss of preload: The initial clamping force (preload) applied during screw insertion gradually decreases over time due to settling of the bone-screw interface and plastic deformation of the materials 2, 3
- Micromotion and cyclic loading: Repetitive mechanical stresses cause counterclockwise rotation of the screw, with studies documenting up to 7 degrees of rotation accompanied by screw elongation of up to 200 micrometers 3
- Joint separation forces: External forces from weight-bearing or occlusal loading create joint-separating forces that exceed the remaining preload, allowing progressive loosening 2
Bone Quality as Critical Factor
Low bone mineral density is the single most important risk factor for screw loosening, particularly in osteoporotic patients:
- Osteoporotic bone (T-score < -2.5) shows screw loosening rates of 32.3% compared to 12.7% in normal bone density patients (P < 0.029) 1
- The mean Hounsfield Units (HU) of the screw trajectory are significantly lower in osteoporotic patients (147 ± 94) versus non-osteoporotic patients (208 ± 91, P < 0.001) 1
- Women with low bone density are at particularly high risk and should receive additional augmentation with cement, hooks, or lamina taping to prevent loosening 1
Biomechanical Stress Distribution
The mechanism involves specific patterns of stress concentration:
- Screw elongation and thread distortion: Under cyclic loading, screws can elongate and threads become compressed, with visible distortion of the joint architecture on microscopic examination 3
- Interface compression: The bone-screw interface undergoes compression and distortion, creating gaps that allow micromotion 3
- Progressive bone resorption: Micromotion triggers an inflammatory response leading to osteolysis around the screw threads, further reducing fixation 1
Clinical Consequences
The loosening process has direct clinical implications:
- Reduced fusion rates: Patients with screw loosening show fusion rates of only 71.4% compared to 93.9% in those without loosening (P = 0.038) 1
- Progressive instability: Once initiated, the loosening process tends to accelerate as micromotion increases bone resorption 1
- Early detection critical: Most screw loosening occurs within the first 6 months after surgery, making early monitoring essential 4
Prevention Strategies Based on Mechanism
Understanding the mechanism guides prevention:
- Maximize initial preload: Using torque drivers to achieve recommended torque values (typically 35 Ncm) prevents initial loosening 5, 2
- Optimize screw-bone contact: Wider diameter screws (5.0-6.0 mm) show only 5.8% loosening compared to 14.5% for standard diameter (3.75-4.0 mm) due to increased surface area 5
- Address bone quality preoperatively: Patients with osteoporosis require augmentation strategies or alternative fixation methods 1
Common pitfall: Assuming screw loosening is purely a hardware problem when it fundamentally reflects inadequate bone-screw interface stability, particularly in osteoporotic bone. The solution requires addressing both mechanical factors (proper torque, screw design) and biological factors (bone quality, loading conditions) 1, 2.