Bioabsorbable Implants in Orthopaedic Surgery

 

Definition

• Bioabsorbable implants are materials that gradually degrade within the body, are absorbed, and are ultimately excreted after fulfilling their intended function.

Introduction

• Internal fixation implants for fractures are generally required only temporarily, until fracture union occurs.
• Metallic implants often necessitate a second surgical procedure for removal, which involves:
  • Additional patient discomfort
  • Increased financial cost
  • Risk of operative complications
• To overcome these disadvantages, biodegradable and bioabsorbable implants were developed.
• These implants gradually degrade once their mechanical role is completed, eliminating the need for removal surgery.

Problems Associated with Metallic Implants

• Local soft tissue irritation
• Chronic pain
• Release of metallic ions
• Increased risk of infection
• Requirement of a second surgery for implant removal
• These limitations have driven the increasing use of biodegradable implants.

Historical Background

• In eighteen ninety-three, low molecular weight polyglycolic acid was synthesized by Bischoff and Walden.
• In nineteen sixty-two, the first synthetic absorbable suture was developed from polyglycolic acid by American Cyanamid Company.
• In nineteen seventy-five, a ninety to ten copolymer of glycolide and lactide known as polygalactin nine ten was introduced as a commercial absorbable suture.
• Polyglycolide and polylactide sutures have been used for decades with no reported carcinogenic, teratogenic, toxic, or allergic effects.
• In nineteen sixty-nine, the use of polyglycolic acid for reinforcing pins, screws, and plates in bone surgery was first proposed.

Materials Used for Bioabsorbable Implants

• Polyglycolic acid
• Polylactic acid
• Polydioxanone
• Self-reinforced polymers
• Polylactide co-glycolide
• Polycaprolactone

Characteristics of an Ideal Biomaterial

• Does not provoke inflammatory or toxic reactions
• Completely metabolized after fulfilling its function
• Leaves no residual trace in the body
• Easily manufactured into the required implant form
• Has an acceptable shelf life
• Can be sterilized effectively

Polyglycolic Acid

• First material used to produce a fully synthetic absorbable suture
• Highly crystalline and insoluble in most solvents
• Exhibits high tensile strength and stiffness
• Loses approximately fifty percent of strength within two weeks
• Loses full strength by four weeks
• Completely absorbed within four to six months

Polylactic Acid

• Exists in two optical isomers:
  • D-polylactide
  • L-polylactide
• L-polylactide is the naturally occurring form.
• A blend of D and L polylactide is known as DL-polylactide.

Mechanical Properties and Applications

• Polyglycolic acid and crystalline polylactic acid:
  • High tensile strength
  • Low elongation
  • Suitable for weight-bearing fixation devices and sutures
• DL-polylactide:
  • Low tensile strength
  • High elongation
  • Used mainly as a drug delivery system

Other Bioabsorbable Polymers

• Polycaprolactone:
  • Degradation time of approximately two years
  • Copolymers are used in monofilament sutures
• Polyanhydrides:
  • Excellent biocompatibility
  • Degradation time adjustable from days to years
  • Primarily used for drug delivery
• Polyorthoesters:
  • Used as drug delivery systems

Mechanism of Degradation

• After implantation, the material must remain intact until healing is achieved.
• Degradation occurs gradually through chemical hydrolysis of unstable molecular bonds.
• Degraded products are absorbed and excreted by the body.

Types of Degradation

Bulk Degradation in Semicrystalline Polymers

• Occurs in two phases:
  • Water penetrates the implant and attacks chemical bonds in the amorphous regions
  • Long polymer chains break into shorter fragments
  • Enzymatic degradation of fragments follows
• Occurs when water penetration exceeds the rate of polymer dissolution.

Surface Erosion

• Water penetrates the implant more slowly than polymer breakdown.
• Degradation occurs layer by layer at the surface.

Clinical Applications of Bioabsorbable Implants

Knee Surgery

• Widely used in anterior cruciate ligament reconstruction as:
  • Interference screws
  • Transfixation screws
• Osteochondral fractures can be fixed arthroscopically using biodegradable pins.
• Meniscal fixation devices and biodegradable suture anchors allow advanced soft tissue reconstruction in complex knee injuries.

Shoulder Surgery

• Used in repair and reconstruction of:
  • Rotator cuff tears
  • Shoulder instability
  • Biceps tendon lesions
• Suitable for labral repair and biceps tendon tenodesis.
• Clinical studies show comparable outcomes between polyglycolic acid and polylactic acid implants at two-year follow-up.
• Eliminates the need for bone tunnels in many procedures.

Spine Surgery

• Early clinical studies demonstrated use as interbody spacers in lumbar fusion.
• Implants persisted longer than the expected twelve to eighteen months.
• Clinical and radiographic results support use in transforaminal lumbar interbody fusion.
• Comparative animal studies showed superior distraction, stiffness, and fusion with bioabsorbable composite cages.
• Bioabsorbable anterior cervical plates demonstrated stability comparable or superior to resorbable mesh systems.

Paediatric Orthopaedics

• Suitable for fixation of growth plate fractures.
• Studies show comparable results to metallic implants in paediatric elbow fractures.
• Self-reinforced polylactic acid screws provide sufficient fixation in subtalar arthrodesis.
• Described applications include olecranon fracture fixation in children.

Foot and Ankle Surgery

• First use in ankle fracture fixation reported in nineteen eighty-five.
• Applications include:
  • Medial malleolar fractures
  • Talar fractures
  • Intra-articular osteochondral fractures
• Used in fixation of:
  • Hallux valgus osteotomies
  • Syndesmotic injuries
  • Lisfranc joint dislocations

Hand and Wrist Surgery

• Mini-plate systems available for:
  • Fracture fixation
  • Osteotomies
  • Arthrodesis
• Typically use self-reinforced polylactide plates with small-diameter screws.

Additional Applications

• Used in fixation of fractures involving:
  • Humeral condyle
  • Distal radius and ulna
  • Radial head
• Bioabsorbable meshes available for acetabular reconstruction.
• Widely used in:
  • Craniofacial surgery
  • Maxillofacial surgery
  • Dental surgery

Advantages of Bioabsorbable Implants

• No long-term soft tissue irritation
• No stress shielding or osteopenia
• No requirement for implant removal surgery
• Particularly useful in paediatric fracture fixation
• Do not interfere with callus formation or fracture healing
• Can act as carriers for local drug delivery, including antibiotics

Limitations and Drawbacks

• Lower stiffness compared to metallic implants
• Higher cost
• Risk of fixation failure
• Lower elastic modulus leading to screw back-out

Complications

• Delayed sterile inflammatory or foreign body reaction
• Painful, erythematous, fluctuant swelling over healed surgical sites
• Average onset approximately twelve weeks after implantation
• Failure of fixation
• Postoperative wound infection

Conclusion

• Bioabsorbable implants represent an important advancement in orthopaedic fixation.
• They reduce the need for secondary surgery and associated complications.
• Proper material selection and indication are critical for success.
• Continued improvements in material science may further expand their role in orthopaedic practice.