Bioabsorbable Materials
BIOABSORBABLE MATERIALS
PGA, PLA, and PLLA Implants
Material Classes
Critical Must-Knows
- Definition: Synthetic polymers (Polyesters) that degrade in the body via Hydrolysis, eliminating the need for removal surgery
- Definition: Commonly used for interference screws, suture anchors, and pediatric fractures
- Mechanism: Degradation via Hydrolysis (uptake of water breaks ester bonds)
- Management: Used where metal removal would be obligatory or difficult
Examiner's Pearls
- "Biocompatibility (Foreign body reaction is a risk)
- "Strength retention profile (Must last long enough for bone healing)
- "Risk of Foreign Body Reaction (Sterile abscess/cystic change) especially with PGA
- "Osteoconductive composites (added TCP/HA) help bone replacement
Clinical Imaging
Imaging Gallery
Exam Warning
Degradation Mechanism
Hydrolysis: It is NOT enzymatic. Water breaks ester bonds.
The 'Acid Dump' Risk
PGA (Rapid): Fast degradation releases acid spike → overwhems buffer → Sterile Abscess.
PLLA Profile
Hydrophobic: Very slow degradation (years). Less reaction, but foreign body remains.
Composition & Types
Common Polymers
1. Polyglycolic Acid (PGA):
- Hydrophilic (loves water).
- Fast degradation: Loses strength in ~6 weeks. Absorbed by ~6 months.
- Problem: High rate of foreign body reaction (sterile abscess) due to "acid dump".
- Use: Rapidly healing tissues / Sutures (Vicryl is Polyglactin).
2. Poly-L-Lactic Acid (PLLA):
- Hydrophobic.
- Slow degradation: Retains strength for over 12 months. Absorbed over 3-5 years.
- Problem: Crystalline debris can cause late reactions.
- Use: ACL screws, Meniscal arrows.
3. Co-polymers (PLGA/PLDLA):
- Mixing L-Lactic and D-Lactic acids creates an Amorphous (non-crystalline) structure.
- Tunable degradation rate (intermediate).
4. Bio-Composites:
- Polymer + Bioceramic (TCP or Hydroxyapatite).
- Benefit: The ceramic buffers the acidic byproducts (alkaline) and promotes bone ingrowth.
At a Glance
Bioabsorbable materials are synthetic polyester polymers that degrade in vivo via hydrolysis, eliminating the need for removal surgery. PGA (polyglycolic acid) degrades rapidly (weeks-months) with 5-10% risk of sterile abscess from acid byproduct accumulation, while PLLA (poly-L-lactic acid) is hydrophobic with slow degradation (3-5 years). Common applications include ACL interference screws, meniscal arrows, and paediatric transphyseal pins. Bio-composites (polymer + TCP/hydroxyapatite) buffer acidic byproducts and promote osteoconduction. Key advantages include no stress shielding and MRI compatibility, but they are weaker than metal and carry risks of tunnel widening and brittle screw breakage during insertion.
G-L-Very SlowDegradation Speed
Memory Hook:Glycolic Goes (Fast), Lactic Lasts (Slow)
Clinical Relevance
Advantages and Disadvantages
Advantages:
- No removal surgery needed.
- No stress shielding (load transfers gradually as implant weakens).
- No MRI artifact.
- Radiolucent (can see fracture healing).
Disadvantages:
- Weaker than metal (cannot use for load bearing in adults).
- Sterile Abscess / Osteolysis: 5-10% rate.
- Screw breakage during insertion (Brittle).
- "Tunnel widening" in ACL surgery.
Evidence Base
Reaction Rates
- Study of 2500 patients with PGA implants
- Sterile inflammatory foreign-body reaction occurred in 8%
- Presented as painful fluctuant swelling 2-3 months post-op
- Required aspiration or drainage
- Led to shift towards slower degrading polymers (PLLA)
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Complications
Bioabsorbable vs Metal Interference Screws
MCQ Practice Points
Exam Pearl
Q: What are the main types of bioabsorbable polymers used in orthopaedics?
A: (1) PLA (Poly-lactic acid): Most common, slow degradation (2-4 years), good strength. (2) PGA (Poly-glycolic acid): Fast degradation (6-12 months), high crystallinity, historic first-generation. (3) PLLA: L-isomer of PLA, slower degradation than racemic PDLLA. (4) Copolymers (PLGA): Tunable degradation by varying LA:GA ratio. All degrade via hydrolysis to lactic/glycolic acid.
Exam Pearl
Q: What is the mechanism of bioabsorbable polymer degradation?
A: Bulk hydrolysis of ester bonds by water. Polymers absorb water, autocatalytic degradation occurs (acidic byproducts accelerate breakdown), molecular weight decreases, mechanical strength lost, fragments phagocytosed by macrophages. Final products (lactic acid, glycolic acid) enter Krebs cycle and are excreted as CO₂ and H₂O. Not enzymatic degradation.
Exam Pearl
Q: What are the clinical applications of bioabsorbable implants in orthopaedics?
A: (1) Suture anchors: Shoulder, hip labral repairs. (2) Interference screws: ACL reconstruction (tibial fixation). (3) Meniscal arrows/darts: Meniscal repair. (4) Pediatric fractures: Avoid growth plate crossing with metal. Advantages: No second surgery for removal, no stress shielding. Not suitable for load-bearing cortical fixation.
Exam Pearl
Q: What are the complications specific to bioabsorbable implants?
A: (1) Sterile osteolysis: Acidic degradation products cause foreign body reaction, cyst formation. (2) Inflammatory reaction: Crystalline degradation particles. (3) Premature mechanical failure: Loss of strength before healing complete. (4) Incomplete resorption: Residual material detectable years later. (5) MRI artifact: Some products create signal void for extended period.
Exam Pearl
Q: Why did early bioabsorbable implants (PGA) cause high rates of sterile abscess formation?
A: Pure PGA degrades rapidly (3-6 months), releasing high concentrations of glycolic acid in a short period. This acidic environment causes: (1) pH drop, (2) Intense foreign body reaction, (3) Sterile fluid collection/sinus formation. Modern implants use slower-degrading PLA or copolymers (PLGA 85:15) with more gradual acid release, reducing inflammatory response.
Australian Context
FRACS Examination Relevance
Basic Science Viva:
- Bioabsorbable materials is a core biomaterials topic
- Expect questions on hydrolysis mechanism (NOT enzymatic)
- Know PGA vs PLLA degradation rates and clinical implications
- Understand byproduct metabolism (Krebs cycle)
Key Examination Points:
- Differentiate bulk erosion vs surface erosion
- Explain why PGA causes sterile abscess (acid dump)
- Discuss bio-composite advantages (buffering, osteoconduction)
- Compare with metal implants for ACL fixation
Common Questions:
- What screw would you use for ACL reconstruction?
- Why did early PGA implants fail?
- What are advantages of bio-composites?
Management Algorithm
References
- Bostman OM. Absorbable implants for the fixation of fractures. JBJS Br. 1991.
- Ambrose CG, Clanton TO. Bioabsorbable implants: review of clinical experience in orthopedic surgery. Ann Biomed Eng. 2004.
Bioabsorbable Quick Facts
High-Yield Exam Summary
Science
- •Polyesters (PLLA, PGA)
- •Degradation: Hydrolysis
- •Byproducts: Acid + Water + CO2
Comparison
- •PGA: Hydrophilic, Fast, Reactive
- •PLLA: Hydrophobic, Slow, Inert
- •PLGA: Tunable