Bioabsorbable Materials

Bioabsorbable implants are chosen for low-load indications where avoiding a second removal operation, eliminating MRI artefact, or allowing easy revision through the implant outweighs the lower mechanical strength.

The topic lists "tunnel widening in ACL surgery" as a disadvantage, the controversies section asks whether it matters, and three of the evidence cards report more femoral tunnel widening with bioabsorbable screws - but the mechanism is never explained. Examiners ask why, and the answer separates a general ACL-graft problem from a biomaterial-specific contribution.

Tunnel widening is multifactorial - mechanical plus biological:

The first three operate with any graft fixation, but the fourth is the bioabsorbable-specific contribution and is why pure-polymer (especially fast-degrading or bulky) screws widen the femoral tunnel more than titanium in the comparative trials - the same acid-dump/foreign-body biology that causes sterile cysts elsewhere is acting on the tunnel wall. It is also the rationale for bio-composites: adding β-TCP/hydroxyapatite buffers the acid and converts the track to bone (osteoconduction), so the screw site ossifies rather than enlarges.

Clinically the debate (covered in Controversies) is whether the extra widening matters for stability or revision; mechanistically, the point to state is that bioabsorbable widening is biological osteolysis from degradation, layered on top of the mechanical graft-motion causes common to all reconstructions.

• Do bioabsorbable screws offer any real clinical advantage over metal? High-level evidence (Cochrane and overlapping meta-analyses) shows equivalent functional outcomes but higher treatment-failure and intraoperative-breakage rates for bioabsorbable interference screws. The argued benefits — easier revision, no MRI artefact, no removal surgery — are largely theoretical or device-specific rather than proven in outcome scores.
• "Tunnel widening" — does it matter? Bioabsorbable (especially pure-polymer) screws are associated with more femoral tunnel widening and prolonged effusion, but whether this translates into worse long-term stability or harder revision is unresolved.
• Are composites the answer? Bio-composites (polymer + β-TCP/HA) demonstrably degrade more completely and promote osteoconduction, but high-quality comparative RCTs showing they reduce clinically important complications versus pure polymers (and justify higher cost) are limited.
• PLLA's slow resorption — benefit or liability? Prolonged strength retention is useful, but bulk PLLA can persist for years and cause late foreign-body reactions, so "absorbable" does not mean "gone".
• Cost-effectiveness is now the key research question — efficacy is broadly settled, so future trials should weigh device cost against the avoided-removal-surgery and adverse-event profiles.

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.

Understanding the mechanism is the single most-tested concept in the basic-science viva. Degradation is non-enzymatic hydrolysis of ester bonds — water diffuses into the polymer and cleaves the backbone, progressively reducing molecular weight until fragments are small enough to be cleared by macrophages and metabolised via the Krebs cycle to CO₂ and water.

Bulk erosion vs surface erosion — alpha-hydroxy polyesters (PLA, PGA) undergo bulk erosion: water penetrates faster than bonds break, so the whole implant hydrolyses simultaneously. Strength is lost throughout the cross-section before significant mass is lost, and trapped acidic monomers cannot escape — this autocatalysis is what drives the rapid late "acid dump". Surface-eroding polymers (polyanhydrides, not standard ortho polyesters) degrade from the outside in and retain core strength longer.

Two distinct timelines matter clinically:

• Strength-retention time — how long the implant holds load (must outlast bone/graft healing).
• Mass-loss / resorption time — how long until the implant disappears (much longer than strength loss).

For PLLA the gap is large: mechanical strength may be lost over months while the bulk material persists radiographically for 3-5 years or more, which is why late PLLA foreign-body reactions can present years after surgery.

The topic states the implants are "weaker than metal," "cannot be used for load bearing in adults," and yet cause "no stress shielding" - but never gives the mechanical framework that ties these together. The single number that explains all three is the elastic (Young's) modulus, and understanding it is a classic basic-science viva line.

Two properties, two consequences:

• Elastic modulus (stiffness) governs stress shielding. A construct far stiffer than bone carries most of the load and unloads the surrounding bone, which then resorbs (Wolff's law). Metals are roughly an order of magnitude stiffer than cortical bone, so a rigid metal implant shields bone; a bioabsorbable polymer, whose modulus is much closer to bone (though still below it) and which progressively softens as it hydrolyses, transfers load gradually back to healing bone - hence "no stress shielding."
• Ultimate strength governs whether the implant survives the applied load. Polymers have a far lower strength than metal, so they fail under high diaphyseal/articular loads - hence "low-load indications only" and the brittle intra-operative breakage seen in the Cochrane data.

So the same low modulus that makes bioabsorbables attractive (load shared with bone, no stress shielding, no MRI artefact) is inseparable from their weakness (low-load indications only). They are not a weaker substitute for metal so much as a different point on the stiffness-strength spectrum - chosen where matching bone and disappearing matter more than peak strength. (Bioabsorbable metals such as magnesium alloys, which sit between polymers and permanent metal in strength, are covered in the dedicated magnesium-implant topic.)

1. Böstman OM, Pihlajamäki HK. Adverse tissue reactions to bioabsorbable fixation devices. Clin Orthop Relat Res. 2000;(371):216-27. PMID 10693569.
2. Debieux P, Franciozi CES, Lenza M, et al. Bioabsorbable versus metallic interference screws for graft fixation in anterior cruciate ligament reconstruction. Cochrane Database Syst Rev. 2016;7:CD009772. DOI
3. Mascarenhas R, Saltzman BM, Sayegh ET, et al. Bioabsorbable versus metallic interference screws in ACL reconstruction: a systematic review of overlapping meta-analyses. Arthroscopy. 2015;31(3):561-8. DOI
4. Myers P, Logan M, Stokes A, Boyd K, Watts M. Bioabsorbable versus titanium interference screws with hamstring autograft in ACL reconstruction: a prospective randomized trial with 2-year follow-up. Arthroscopy. 2008;24(7):817-23. DOI
5. Barber FA, Dockery WD, Hrnack SA. Long-term degradation of a poly-lactide co-glycolide/β-tricalcium phosphate biocomposite interference screw. Arthroscopy. 2011;27(5):637-43. DOI
6. Rokkanen PU, Böstman O, Hirvensalo E, et al. Bioabsorbable fixation in orthopaedic surgery and traumatology. Biomaterials. 2000;21(24):2607-13. DOI
7. Ambrose CG, Clanton TO. Bioabsorbable implants: review of clinical experience in orthopedic surgery. Ann Biomed Eng. 2004;32(1):171-7. DOI

Evidence verified against PubMed. Per PubMed attribution requirements, DOIs are linked above where available.