By: 1 August 2008

Adequate attachment of soft tissues to bone has been one of the most difficult aspects in orthopaedic surgery and was initially left to natural soft tissue healing, with adequate scar tissue formation. For this to occur, the tissue had to be maintained in a position of contact by immobilising the limb for a long period of time.

Early methods of soft tissue attachment were developed to try and improve the ease of fixation strength and placement. Techniques involved the drilling of bone tunnels, with or without pull through sutures, or may have used fixation of soft tissues to bone using screws, washers, staples and even tapered plugs. A further development of bone fasteners and toggle anchors required a larger access to the underlying bone with a higher potential for damage during the implant insertion.

These initial developments were used in conjunction with the reconstruction of knee ligament injuries, where appropriate ligament tension was needed. Bone spikes, washers, staples and also interference screws were therefore developed to achieve this. The disadvantages, with the exception of the interference screws, were that they caused a lot of noticeable discomfort for the patient.

Over the last decade dramatic improvements in arthroscopic surgery has led to smaller scars and an early return to a normal function. As the needs arose, so did the ingenuity of man to develop an innovative material for anchoring soft tissue to bone. Despite the improvement of stability achieved with these earlier devices, problems still arose due to the size of these fixation implants that had to be passed through smaller arthroscopic wounds.

There was also a recognised difficulty in the fixation of these implants around the joints because they could become loose and cause further problems within the joint itself. Modern bone anchors have been developed to give an adequate anchorage and an appropriate tissue tension without any discomfort.

Goble and Somers developed the first “screw-in” bone anchor, that provided a combined technology of a versatile suture material with an easy insertion, in August 1985 and the U.S patent was awarded in December 1986.

So what are the successful attributes of a bone anchor?

1. They should be easy to insert:
The current designs of bone anchors allow them to be inserted arthroscopically or through a very small per-cutaneous incision.

2. Provide a firm anchor to the bone:
The strength of the current designs of anchors is superior to the more historical trans-osseous suturing techniques, which were used in shoulder surgery. However, the commonest point of failure with these anchors is the suture or at the knot/tissue interface, especially if it is not well tied! With the development of stronger suture cord materials, the suture does not fail unless it is damaged in some way during the surgery.

3. Must be simple and reliable to use:
Current anchors are disposable implants and may come with a drill bit or bone punch. These are easy to use and can be rapidly inserted into bone without much effort, especially through a percutaneous incision.

4. May be bio-absorbable or metallic depending:
Metal anchors were first produced for the soft tissue fixation of shoulder injuries. Unfortunately, due to the reported complications of articular damage from migrating anchors and the image distortion seen on postoperative magnetic resonance films, they became quickly unfashionable. Bio-absorbable anchors were then developed to avoid these problems. These new anchors have an equal pullout strength to the metal anchors, with a lower reported complication rate.

Biodegradable polymers which are currently found in modern suture anchors include poly-L-lactic acid, poly-D, L lactic acid, polydioxanone, polyglycolic acid and their copolymers.

5 Sutures must have strong mechanical properties:
Suture anchors are now available preloaded with a choice of conventional braided polyester sutures or a version of an ultrahigh molecular weight polyethylene. Most suture anchors have two associated suture strands. The manner in which these sutures are attached to the anchor can vary, with separate eyelets or slots. These can be parallel or orientated at different angles to one another. Some anchors have a very large single eyelet that can accommodate more than two suture strands.

What are the different types of anchoring systems?

There are many types of anchoring systems for most of the modern anchors. Most of them are barbed, a screw or a rivet type of bone fastener. Current anchors have a leading edge which may be either inserted into a pre drilled hole or directly screwed into bone. These bone anchors also have a fixed to or fully mobile suture mount (suture eyelet) within the anchor system itself .

Barbed anchors have fins orientated in a longitudinal direction. They are elastically deformed on compression (insertion) and plastically deformed on distraction (pulling) and so hold on to the bone well once inserted.

Screw fit anchors are similar to a conventional screw but instead of a head there is an eyelet (suture anchor interface) to accommodate the suture strands. Screw anchors have a higher load to failure value than non-screw designs. The biodegradable anchors have a lower load to failure compared with metal anchors.

Relevant issues for selection of suture anchors are summarised in Table 1 (Shoulder Concepts, 2008 Boileau.)

Table 1

Anchor designAnchor sizeThreaded/Barbed/expandable/tackSpecific thread designPartially or fully threaded Anchor materialNon absorbable (steel vs. titanium)AbsorbableInitial propertiesAbsorption characteristics
EyeletRecessed or prominentHard eyelet or suture eyeletEdge design Mechanical performanceStraight pull-outCyclic loadCortical vs. cancellous bone
Suture selectionAbsorbable or non-absorbableStandard or ultra high strength Revision issuesRetrievable or permanentBone loss/ osteolysisImpact on imaging
Ergonomic issues: ease of insertion Cost

How can bone anchors fail?

Barber et al. have published extensively on the fixation strengths of various suture anchors.

1. Soft tissue strength:
The tissue they are used to repair may be degenerate and weak and therefore fail at this interface.

2. Suture strength:
Although most modern suture materials are strong they may be damaged in the suture eyelets or when manipulating them through tissue. This can occur when using suture passing devices.

3. Bone anchor strength (purchase):
Caution most be exercised when dealing with very soft osteoporotic bone, as the bone may not hold the anchor very well.

Under cyclic-loading conditions, anchor fixation strength is decreased compared with single-pullout tests. The suture/anchor interface is known to be the weakest link. In most cases the suture can wear at the eyelet and so the use of larger suture materials with larger anchors can provide a better strength before failure. Suture breakage at the knot is the predominant failure mode for biodegradable anchors inserted into cortical bone.

New anchor developments

The knotless tissue anchor: This anchor has been developed to cater for the increased volume of arthroscopic procedures. Surgeons can find knot tying difficult and also knots are known to be the weak point in tissue repairs. Although knotless anchors have a similar pullout strength to knot-tying anchors, they have an increased gap formation if there is an immediate loading. The design of knotless tissue anchors involves the following:

1. Anchor:
Expandable anchors which after fastened to the bone may hold the suture material next to the anchor and so avoid using knots .

2. Loop locking mechanism:
Where the loop of the thread that forms a knot can be jammed into the body of the anchor

3. New suture material:
Here ultrasonic energy is used to weld/melt the suture material so that after fixation a knot can be avoided.


  1. The development of suture anchors for use in soft tissue fixation to bone. Goble E. M, Somers W. K, Clark, R, Olsen R. E : AJSM. Vol 22 (2),1994: 236-239.
  2. Suture anchor failure strength an in vivo study. Barber F.A, Cawley P, Prudich J.F. Arthroscopy 9(6).1993: 647-652.
  3. Internal fixation strength of suture anchors-update. Barber F.A, Herbert M.A , Click J.N. Athroscopy 13(3).1997:355-362.
  4. Shoulder Concepts 2008.Edt, Boileau P:119-210. (Nice Shoulder Course 2008).