Bio degadable implants used in Orthopaedics by Dr.Vinay

CHAIR PERSON-Prof&HOD Dr.B.S.J.K.Reddy

Presenter-Dr.Vinay

1st Year PG.

Dated 30/09/2013.

SSMC,TUMKUR.

The implants which undergo gradual degradation by biological process and absorbed and excreted by the body are called Biodegradable Implants.

Implants for the internal fixation of fractures have a common property of being needed temporarily,until the fracture has united.

In-accordance with this limited requirement.,the trauma surgeons recommend that all metallic implants used for the fixation of fractures be removed in due-course.

There will be need for a second operation for removal of metallic implants,with considerable additional inconvenience,expense and at some risk of operative complication.

This led to the development of so called biodegradable/bioabsorbable/biologically inert implants which undergo gradual degradation after their purpose is being served.

The history of absorbable implants in the repair of bone tissue began in late 1960’s.

The first studies were performed in the field of maxillofacial and mandibular surgery.

Implants of more complex design,such as screws and small plates became possible in late 1970’s and early 1980’s.

In the light of present knowledge.,the best suited and commonly available organic components used in preparing bio-degradable implants are

Polyglycolic acid Polylactic acid Polyparadiaxonone Chemically these compounds are Alpha

poly-esters.

Polyglycolic acid and polylactic acid copolymers in a ratio of 90:10 and polydiaxonose are used as absorbable sutures world wide.

The most efficient method of producing high molecular weight polyglycolic acid or polylactic acid is a ring opening polymerization of the corresponding cyclic diester,glycolide or lactide by catalysis.

The most important characteristic which determines the behaviour of these polymers is the glass transition temperature at which the polymer becomes rigid and brittle.

This ranges from 58 degree celsius for polylactic acid and 16 degree celsius for polypara diaxonone.

The commercially available resorbable polymers include pure polyglycolic (PGA) acid in the form of PINS and SCREWS. Pure poly-l-lactic acid (PLLA) and a co-polymer of PLLA and PGA.

The last gathered the best physical and chemical properties of both PLLA and PGA, and experimental studies have shown that the fixation devices made from this copolymer maintain most of their strength for 8 weeks and will completely resorb in the body in 12–15 months.

Tensile and flex strength are comparable to titanium plating system Plates are easy to adapt with aid of heat pack;A wide selection of implant sizes and shapes are available A convenient hex-drive breakaway delivery system simplifies screw placement;

Eliminates growth restriction and implant migration for paediatric craniofacial reconstruction;

Resorbs completely and may eliminate the need for second operation;

Does not induce late stage inflammatory reaction

A headed bioabsorbable tissue anchor has a continuous thread spiraling around a tapering central core.

At the distal end, the headed bioabsorbable tissue anchor terminates in a flat point. At the proximal end, the bioabsorbable tissue anchor has a flat, disk-shaped head for engaging tissue, and slots formed in the head for engaging a driver.

The anchor is cannula shaped to engage the driver.

The headed bioabsorbable tissue anchor has a large thread surface per turn of thread. As the anchor is turned into bone for engaging cancellous bone, the disk-shaped head engages and anchors the tissue to the bone.

The implants of polyglycolic acid,polylactic acid or polydiaxonone are completely absorbable within the bone tissue and the new bone is deposited on or within the implant as the degradation proceeds.

The degradation of these polymers occurs

1.Mainly by hydrolysis 2.And to lesser extent through non

specific enzymatic action.

Factors which influence the degradation process are

1.molecular weight 2.crystallinity 3.Thermal history 4.Geometry Poly-lactic acid copolymers have the slowest

rate of degradation(half-life 6months) The co-polymers of polyglycolyic acid and

poly paradiaxonone are much more rapid. The principal route of elimination is

respiration and to lesser exent in urine and faeces.

The fact that the interfragmentary compression cannot be achieved with polyester pins or rods should be recognized when the indications for the use of these implants are being considered.

Polyester screws do provide compression of the fracture but are more difficult to insert than the comparable metallic screws.

Only fractures affecting the cancellous bone can be managed effectively with the array of implants currently available.

At the moment the INDICATIONS for the biodegradable implants are

1.Radial head Fracture 2.Wedge fractures of the patella 3.Fractures of the proximal and distal ends

of the metatarsals and metacarpals 4.osteochondrosis Dessicans or

Osteochondral fractures of the femur condyle.

5.Ankle Fracture 6.Adult capitellum Fracture 7.Displaced elbow fractures in children 8.Distal radius fracture 9.Pediatric fracture 10.Small fractures or osteotomies.

1.No irritation of soft tissue 2.No Osteopenia 3.No need of secondary operation to

remove the implants. 4.Useful in pediatric fracture fixation. 5.No interference with the callus

formation and fracture healing. 6.Anti-biotic releasing Bioabsorbable

screws to reduce implant related infecion.

The unique complication of these implants is the delayed inflammatory reaction or sterile inflammatory foreign body reaction.

The other complications include 1.Failure of fixation 2.Postoperative wound infection

The clinical presentation of the delayed sterile inflammatory reaction is……

The patient has no local or systemic signs of the problems with the wound in the immediate post-op period.,then a painful erythematous,fluctuant swelling suddenly develops about the healed wound.

The mean interval between the fixation of fracture and clinical manifesation of reaction is twelve weeks.

A sinus draining he liquid remnants of the implant material often form

Bacterial cultures of the drainage from the sinuses are negative.

1.Fixation achieved with these type of implants is often neither rigid not stable enough to hold the fracture with motion or weight bearing force before union.

2.Cast support and use of cruthces for lower extremity fractures has been recommended,which limits their further use.

3.Bioabsorbable Implants have excessively low moduli resulting in backing out of screws

4.Poor handling characteristics when compared to metals.

5.The price of 45/50 mm fibre reinforced rod is approximately 15 times that of a metallic cancellous screw.

6.The treatment of diaphyseal fractures of long bones however would necessitate a larger device such as a long plate and intramedullary nail,that has a slow rate of degradation.

Nevertheless the unsolved problem of irritation of soft tissue and of osteopenia beneath metallic plates will continue to simulate research on absorbable implants.