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Fixation of soft tissue to bone is a basic technique in orthopaedic surgery. Historically, this involved the use of transosseous tunnels through which soft tissue structures were fixed using sutures. This time honoured technique is now increasingly being replaced by use of 'suture anchors', and 'interference' screws which provide faster fixation with high strength, often without the need for extensive surgical exposure.

Shoulder surgery is highly reliant on suture anchors. Typically an anchor has three parts

  1. The body of the anchor - this incorporates the fixation modality (threads for screw in, barbs, interference fit etc.)
  2. The 'eye' - a mechanism to hold the suture. While an 'eye' in the anchor material is still common, other methods use a loop of suture, a bridge of material and so on.
  3. The suture itself - modern high strength sutures are resistant to failure

These implants are made from metals, polymers, and composites

Metal anchors

These are either stainless steel or titanium. Alloys are often used to enhance properties. They have properties that have been successfully used for decades for orthopaedic fixations. However, they interfere with further imaging (especially MRI scans) and can limit options for revision surgery

Polymers

These are types of plastics, composed of a small repeating unit that can be utilised in orthopaedics for their properties. Polymers will usually not interfere with postoperative imaging. Revision surgery is easier as these either resorbs or can be treated like surrounding bone, i.e. drilled through. An example is PLLA, a homopolymer of poly-L-lactide.

Polymers can be biostable e.g. PEEK (Polyetheretherketone) or bioabsorbable (PLLA, PLGA)

They can cause an inflammatory response and produce cystic changes around them as they are absorbed. The complete process of absorption takes over 5 years, possibly upto 10 1.This does not form bone but a type of  fibrous tissue that is partially calcified is visible.

Biocomposites

These are combinations of a bioactive ceramics (calcium salts, either HA or beta-TCP is used at present) and a polymer. While retaining the properties of polymers, they have an added capacity to osseointegrate due to the osteoconductive properties. Biocomposites show increased bone formation and contact area compared with polymers2. They are less reactive than the polymers3

 

References

  1. Barber FA, Dockery WD. Long-term absorption of poly-Llactic acid interference screws. Arthroscopy 2006;22:820-826
  2. Hunt J, Callaghan J. Polymer-hydroxyapatite composite versus polymer interference screws in anterior cruciate ligament reconstruction in a large animal model. Knee surgery, sports traumatology. Arthroscopy 2008;16:655-660.
  3. Ruhe PQ, Hedberg EL, Padron NT, Spauwen PH, Jansen JA, Mikos AG. Biocompatibility and degradation of poly(DLlactic- co-glycolic acid)/calcium phosphate cement composites.
    J Biomed Mater Res A 2005;74:533-544.