Interference Screw in Orthopedic Fixation: A Thorough Overview

Orthopedic procedures require secure fixation to promote healing and restore function. Interference screws play a key role in stabilizing soft tissue grafts, particularly in ligament reconstruction surgeries such as anterior cruciate ligament (ACL) repair. They provide immediate fixation while integrating with surrounding tissues, making them essential in modern orthopedic surgery.

Understanding their function, materials, mechanical properties, and interaction with biological tissues is crucial for optimizing surgical outcomes.

Role In Orthopedic Fixation

Interference screws secure soft tissue grafts within bone tunnels, particularly in ligament reconstruction procedures. Their design compresses the graft against tunnel walls, creating a stable mechanical hold. This fixation method is widely used in ACL and posterior cruciate ligament (PCL) reconstructions, where maintaining graft position during early healing is critical. Unlike cortical buttons or suspensory devices, interference screws provide direct graft-to-bone contact, improving biological incorporation and reducing micromotion.

Screw diameter, length, and thread design influence effectiveness. Larger-diameter screws generate higher compression forces, enhancing stability but increasing the risk of graft damage. Thread geometry affects fixation strength, with deeper threads improving purchase within the bone tunnel. Tapered screws provide gradual compression, reducing graft slippage compared to cylindrical designs.

Insertion technique also impacts stability. Surgeons must position the screw to ensure uniform compression without excessive tension that could damage the graft. Studies suggest a parallel orientation to the graft enhances fixation strength. Pre-tensioning the graft before screw insertion further improves biomechanical outcomes by reducing laxity.

In clinical practice, interference screws demonstrate high success rates in maintaining graft stability during early recovery. A systematic review in The American Journal of Sports Medicine found interference screw fixation in ACL reconstruction yielded comparable or superior outcomes to alternative methods in graft incorporation and functional recovery. Biomechanical studies confirm interference screws provide sufficient strength for early rehabilitation, promoting faster recovery while minimizing graft failure risk.

Materials Used In These Devices

The composition of interference screws affects mechanical strength, biocompatibility, and long-term integration with bone. Traditional designs relied on metal alloys like titanium and stainless steel due to their durability and resistance to deformation. Titanium remains widely used for its balance of strength and biocompatibility, maintaining fixation stability without excessive wear or corrosion. However, metal screws are non-resorbable, sometimes requiring removal and interfering with imaging.

To address these limitations, bioabsorbable polymers have emerged as an alternative, offering sufficient fixation strength while gradually degrading to allow for bone remodeling. Polylactic acid (PLA) and polyglycolic acid (PGA) are commonly used, often combined to adjust degradation rates. PLA-based screws maintain integrity for months before hydrolytic breakdown, ensuring stability during critical healing phases. Newer formulations, such as poly-L-lactic acid (PLLA) and poly-D,L-lactic acid (PDLLA), enhance mechanical properties and control degradation rates.

Composite materials incorporating calcium-based additives improve bioactivity. Calcium phosphate and hydroxyapatite reinforcements promote osteoconductivity, facilitating bone ingrowth. A study in The Journal of Bone and Joint Surgery found calcium phosphate–reinforced screws improved graft incorporation compared to standard PLA designs. Magnesium-based biodegradable screws are also being explored for their mechanical strength and gradual resorption, with clinical trials showing promising outcomes in bone integration and reduced inflammation.

Mechanics And Strength In Fixation

The mechanical performance of interference screws depends on material properties, geometric design, and insertion technique. Fixation relies on friction between the graft and bone tunnel, with compressive forces influenced by screw diameter and thread depth. Larger screws with deeper threads enhance engagement but also increase the risk of graft deformation. Engineers must balance stability with pressure to avoid compromising graft integrity.

Thread geometry affects insertion torque and resistance to pullout forces. Deeper threads improve mechanical hold by increasing bone contact. Cylindrical screws distribute pressure uniformly, while tapered designs gradually increase compression, reducing graft slippage. The choice between these designs depends on surgical approach, with some studies suggesting tapered screws enhance early graft incorporation.

Insertion torque is critical; excessive force can cause microfractures, particularly in osteoporotic patients. Surgeons must calibrate torque to ensure fixation without compromising bone integrity. Research indicates excessive torque can lead to localized necrosis, while insufficient torque results in inadequate fixation and early graft loosening. Bioabsorbable screws often require lower insertion forces than metal screws due to differences in mechanical stiffness.

Biodegradation Processes

Bioabsorbable interference screws degrade through hydrolysis, where water molecules break down polymer chains, gradually reducing structural integrity. The rate of degradation depends on polymer type, molecular weight, and crystallinity. PLA screws degrade more slowly than PGA due to higher crystallinity and lower hydrophilicity, maintaining fixation strength during early graft healing.

As degradation progresses, screws fragment into smaller particles cleared through metabolic pathways. Local pH changes can accelerate this process, as polymer breakdown produces acidic byproducts. PLLA screws can take years to fully degrade, while PGA-based screws typically resorb within 12 to 24 months. Magnesium-based screws degrade through corrosion, releasing magnesium ions that are absorbed and excreted. Alloy composition can adjust the dissolution rate to align with graft incorporation timelines.

Tissue Response

The biological response to interference screws depends on material composition, degradation rate, and mechanical stability. Titanium screws elicit a mild reaction, with osteoblasts forming new bone around the implant. While they do not degrade, they provide stable long-term fixation. However, permanent implants can sometimes cause stress shielding, leading to bone resorption due to altered biomechanical loading.

Bioabsorbable screws interact dynamically with surrounding tissues as they degrade. The gradual loss of structural integrity is accompanied by cellular activity that replaces the resorbed material with new bone. PLA screws degrade into lactic acid byproducts, which are metabolized and cleared. This process stimulates bone remodeling but can sometimes cause localized inflammation or cyst formation if breakdown products accumulate too quickly.

Magnesium-based screws offer an alternative, releasing magnesium ions that promote bone regeneration while minimizing inflammation. Studies indicate controlled degradation of magnesium implants enhances bone remodeling, making them a promising option in orthopedic fixation.