Executive Summary

Reconstructive orthopedic surgery depends heavily on the stabilization of fractured, deformed, or resected bone using internal fixation devices such as pins, plates, rods, and screws. These devices hold bone fragments in anatomical alignment while healing occurs; however, modern practice increasingly recognizes that bone not only heals around these implants but can grow over, across, and sometimes encapsulating them. This bone growth is both a biological response to stability and a factor that shapes long-term outcomes—including strength, fatigue resistance, risk of implant removal, and the need for revision procedures.

This white paper examines (1) the biological pathways of bone growth in the presence of foreign fixation hardware, (2) the mechanical conditions that stimulate or suppress peri-implant bone formation, (3) the stages of healing unique to bones stabilized by plates and pins, (4) complications of excessive or deficient bone overgrowth, and (5) best practices for reconstructive surgical planning and follow-up.

1. Introduction

Internal fixation using metal implants—stainless steel, titanium, cobalt-chromium—has been a cornerstone of reconstructive orthopedics for nearly a century. Historically considered inert scaffolds, these implants are now understood to participate actively with the surrounding bone in a dynamic interplay of stability, strain, biological stress signaling, osteogenesis, and remodeling.

Unlike external fixation, which leaves the bone surface unobstructed, internal fixation introduces new surfaces onto which bone cells may attach, proliferate, and remodel. Bone growth over plates and pins is especially relevant in:

severe or complex fractures joint reconstruction limb-lengthening procedures pediatric corrections tumor resection and reconstruction trauma with high comminution osteoporotic fractures requiring broad surface fixation

Understanding how the body responds to these implants is critical for preventing malunion, nonunion, excessive callus formation, and biomechanical failure.

2. Biological Mechanisms of Bone Growth Over Implants

2.1 Mechanotransduction and Strain Signaling

Bone cells—osteocytes, osteoblasts, and osteoclasts—respond to mechanical strain through biochemical pathways involving:

Piezo1 mechanosensitive ion channels Wnt/β-catenin pathway RANK/RANKL/OPG remodeling balance Prostaglandin E2 signaling Integrin-mediated cytoskeletal responses

Plates reduce strain at the fracture site. If strain is optimal (approximately 2–10%), osteogenesis is stimulated; if strain is too low, bone may become stress shielded and resorb.

When mechanical load is transferred partly into the plate, the bone may thicken around the implant margins to reestablish load pathways—this is one driver of bone growth over the implant.

2.2 Foreign Body Response Without Rejection

Unlike soft tissues, bone does not produce fibrous encapsulation around implants unless motion or infection is present. Instead, osteoblasts may treat the implant as a stable substrate for mineralization. Titanium implants, in particular, encourage osteointegration due to:

oxide surface microtopography biocompatibility relative inertness low corrosion potential

Bone can bridge across the plate edges, cover screw heads, and even integrate along pin tracts if the device remains immobile.

2.3 Biological Stages Favoring Overgrowth

Bone typically grows over implants during:

Inflammation (0–1 week): hematoma formation and early progenitor recruitment Soft Callus Formation (1–3 weeks): fibrocartilage stabilizes the fracture Hard Callus Formation (3–10 weeks): mineralized osteoid envelops fracture and often creeps over hardware edges Remodeling (months to years): osteoclasts shape mature bone; implants may become partially buried

Overgrowth is most pronounced when wide plates, bridging plates, or buttress plates are used.

3. Mechanical and Surgical Determinants of Bone Growth Over Implants

3.1 Plate Design and Its Influence on Overgrowth

Different plate designs change the extent of peri-implant bone formation:

Compression plates: allow load sharing; moderate overgrowth Locking plates: create rigid constructs; higher risk of bone encapsulation due to reduced micromotion Buttress plates: guide callus growth in predictable directions Intramedullary rods with distal screws: encourage endosteal growth around screw tips or entry sites

Surface roughness heavily influences whether bone will attach; roughened titanium leads to greater overgrowth than polished stainless steel.

3.2 Pin and Screw Tract Behavior

Bone around pins and screws behaves differently:

Threads: increase local strain, stimulating bone ingrowth Cortical compression: leads to denser bone margins Thermal necrosis risk: overheating during drilling suppresses bone growth Micro-motion: leads to fibrous instead of osseous integration

Pins traversing multiple cortices may encourage bone bridges that later complicate hardware removal.

3.3 Stability, Load Sharing, and Stress Shielding

Bone overgrowth is influenced by the distribution of load between bone and implant:

High implant load > high stability > high overgrowth risk Balanced load sharing > optimal healing > limited but healthy overgrowth Excessive rigidity (especially with long locking plates) > stress shielding > resorption near plate edges and overgrowth on opposite sides

Strain gradients often determine callus direction, volume, and thickness.

4. The Healing Process with Plates and Pins

4.1 Primary vs. Secondary Bone Healing

Primary (direct) bone healing: occurs when plate fixation is rigid; bone remodels across fracture without significant callus Secondary (indirect) healing: involves callus formation and commonly produces bone overgrowth near implants

Locking plates tend to favor more primary healing; dynamic compression plates allow microstrain that leads to controlled secondary healing.

4.2 Callus Formation Over Implants

Callus may grow:

over the top surface of plates around the edges of screws filling recesses between plate holes bridging to soft tissues (rare but occurs in large callus formation)

Large callus burdens can complicate subsequent surgeries but often produce excellent biomechanical stability.

4.3 Endosteal vs. Periosteal Responses

Endosteal bone grows along implants penetrating the medullary canal, often forming dense collars Periosteal bone proliferates over plates especially when periosteum is preserved during surgery

Periosteal stripping reduces overgrowth but increases nonunion risks.

5. Potential Complications of Bone Growth Over Implants

5.1 Difficulty or Inability to Remove Hardware

Excessive overgrowth can bury plates or screws, making removal:

longer more invasive higher risk for neurovascular injury potentially impossible without bone resection

This is especially common with locking plates in young, healthy patients with high osteogenic capacity.

5.2 Stress Shielding and Remodeling Imbalance

If the plate carries too much of the load:

bone underneath may resorb bone beside may thicken excessively refracture risk increases after plate removal

The remodeled bone may have a “stress-transfer ridge” that mimics the contour of the plate.

5.3 Impingement and Pain Syndromes

Bone overgrowth may cause:

tendon or ligament impingement reduced joint mobility pain on motion protrusions noticeable under thin soft tissue envelopes

Clavicle plates and wrist plates are common sites.

5.4 Overgrowth at Pin Tracts

Pins left for long durations may be encased in bone, causing:

pin-tract infections difficulty extracting the pin cortical breakage during removal

6. Best Practices in Surgical Technique and Follow-Up

6.1 Optimizing Implant Choice

Consider:

patient age and osteogenic activity expected healing time need for later hardware removal anatomic region and soft tissue coverage desired balance between rigidity and micromotion

Titanium is ideal for permanent fixation; stainless steel may be preferred when future removal is expected.

6.2 Periosteal Preservation

Minimize periosteal stripping to promote healthy but controlled bone formation.

6.3 Encouraging Balanced Load Sharing

Techniques include:

dynamic compression shorter plates when appropriate avoiding unnecessarily rigid constructs staggering screws to allow controlled micromotion

6.4 Monitoring Overgrowth Postoperatively

Use:

radiographs CT when mechanical conflict is suspected palpation and clinical examination tracking callus symmetry and rate of consolidation

6.5 Decision-Making for Hardware Removal

Removal may be recommended if:

bone has sufficiently remodeled overgrowth is causing impingement or pain screws or pins are near tendons patient is young with long life expectancy

Avoid early removal before bone strength recovers.

7. Future Directions

Research trends include:

bioresorbable magnesium-based plates that avoid long-term overgrowth growth-factor-coated implants that direct bone growth away from plate surfaces ultra-low rigidity plates calibrated to stimulate optimal micromotion trabecular titanium surfaces engineered to guide rather than accidentally receive bone real-time strain-monitoring implants that detect healing progress

The long-term goal is predictable, guided bone remodeling that utilizes implants as temporary scaffolds rather than permanent stress-shielding obstacles.

Conclusion

Bone growth over pins and plates is not a complication but a predictable biological response to stability, strain, and surface characteristics. When harnessed properly, this overgrowth provides robust long-term healing and functional recovery. However, when excessive or misdirected, it can create mechanical challenges, limit mobility, and complicate hardware removal.

Modern reconstructive surgery increasingly focuses on guiding peri-implant bone growth rather than simply accommodating it. Understanding the interplay between biomechanics and biology is essential for optimizing outcomes in trauma, orthopedic reconstruction, and limb-saving procedures.