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Current Products and Practice Section |
Introduction
The aim of internal fixation of traumatic and iatrogenic skeletal fracture is to achieve undisturbed fracture healing. The need for plates and screws for fixation is only temporary, until the fracture has united. Accordingly surgeons including those of AO-ASIF school recommend that all metallic implants used for fixation of fractures be removed in due course (Müller et al., 1979
). Reasons for removal of the implants include the possibility of bone atrophy due to stress shielding by rigid bone plates and screws (Pavolainen et al., 1978). Other disadvantages re hypothesized carcinogenic potential, the possibility of corrosion, disturbance in normal growth pattern, and implant migration in children (Simon et al., 1978). Internationally, the removal of the metallic hardware varies from routine removal from all patients in some countries to selective removal only from patients who have symptoms as is usual in the UK (Chapman and Woo, 1988). The use of biologically inert resorbable implants would eliminate the need for a second operation for their removal, and offers major clinical advantages for the fixation of facial bone fractures in trauma and orthognathic surgery. They would be enormously advantageous in paediatric craniofacial surgery. Clinical studies have shown that absorbable implants have been used successfully as a rigid fixation device in mandibular osteotomy and craniofacial surgery and that normal growth pattern is probably not disturbed by use of these implants (Simon et al., 1978; Suurohen et al., 1992). The resorbable plates and screws available from one company have been widely used in our combined trauma service (Leeds/Wakefield).
History of Absorbable Implants
The use of absorbable implants in the repair of bone fractures began in the late 1960s. Fabrication of implants was accomplished by melt moulding and extrusion of polymer into pins and rods. Subsequently more complex designs such as screws and small plates became possible in the late 1970s and early 1980s (Böstman, 1991).
Chemical composition of Absorbable Implants
Alpha compounds such as polyglycolic acid, polylactic acid, and polyesters polyparadioxanon are organic macromolecular compounds that are degradable and absorbable by the body. They also possess the chemical and physical properties necessary for internal fixation devices (Böstman, 1991).
Experimental Studies
Several investigations have shown that these polymers are completely absorbable within bony tissue and that new bone is deposited on and within the implants as degradation proceeds. The degradation procedure appears to be mainly by hydrolytic activity and to a lesser extent through non-specific enzymatic action. The rate of degradation is dependant on the molecular weight, crystallinity, thermal history, and geometry of the implant, as a porous thin sheet depolymerizes much more rapidly than a dense block.
The degradation process in itself does not imply immediate absorption of an implant, as experiments show that 70 per cent of the material from the implant remains in situ for 3 months. Studies also show that the principal route of ultimate elimination is respiration with excretion in the urine and faeces playing only a minor role (Böstman, 1991).
The main clinical complication reported associated with the use of polylactic acid and polyglycolic acid implants is the development of inflammatory foreign body reaction. Clinical reports show that these problems may be due to delayed resorption rate of the polymer (Brady et al., 1973; Böstman, 1991).
Products Available
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, with no complications reported in their usage to date (Investigational Products in the United States, 1995).
In the Oral and Maxillofacial Department at Pinderfields General Hospital we currently use Lactosorb resorbable craniomaxillofacial fixation system in facial trauma and are in process of using it in orthognathic surgery. Lactosorb is a patented copolymer of PLLA (82 per cent) and PGA (18 per cent), and offers a good balance between initial strength and resorption rate.
Features of this system include:
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Unit price for 1•5-mm plate in lactosorb system ranges from £52•50 to £94•50 depending on the shape and number of holes compared with the conventional metal A-O system, which ranges from £34•85 to £61•50. A 1•5-mm screw ranges in price from £27•30 to £30•75 for lactosorb system depending on length compared with A-O 1•5-mm screws at £9•40.
Unit price for a 2•00 mm. plate in lactosorb system ranges from £52•50 to £94•05 compared with the A-O system at £35•50 to £49•05 and for the matching screw prices range from £23•65 to £34•15 for lactosorb compared with the A-O system at £10•75.
Lactosorb resorbable craniomaxillofacial fixation system is provided by Poly-Medics and supplied by Walter Lorenz Surgical, Ins. The U.K. representative is Athrodax healthcare international Ltd, Great Western Court, Ross-on-Wye, Herefordshire HR9 7XP, U.K.
References
Bergsma, E. J., Rozema, F. R., Bos, R. R. and deBruign, E. C. (1993) Foreign body reactions to resorbable poly (L-lactide) bone plates and screws used for fixation of unstable zygomatic fractures, Journal of Oral and Maxillofacial Surgery, 51, 666–670.[Medline]
Böstman, O. M. (1991) Current concepts review, absorbable implants for the fixation of fractures, Journal of Bone and Joint Surgery, 73-A, 148–155.
Brady, J. M., Cutright, D. E., Miller, R. A. and Battistone, G. C. (1973) Resorption rate, route of elimination and ultra structure of implant site of polylactic acid in the abdominal wall of the rat, Journal of Biomedical Materials Research, 7, 155–166.[Medline]
Chapman, M. W. and Woo, S. L. Y. (1988)In: principles of Fracture Healing in Operative Orthopaedics,M. W. Chapman and Michael Madison (Eds)J. B. Lippincott, Lippincott, pp. 115–123.
Investigational Products in the United States: (1995)Compay data Polyamides Inc,Warsaw, Indiana 46580Biomet, Inc form No. Y-INT-022/051525/HP.
Müller, M. E., Allgöwer, M., Schneider, R. and Willenegger, H. (1979)Manual of internal fixation. Techniques recommended by the AO Group, 2nd edn,Springer, New York.
Pavolainen, P., Karajarju, E., Slatis, P., et al. (1978) Effect of rigid plate fixation on structure and mineral content of cortical bone.Clinical Orthopedics and Related Research, 136, 287.
Simon, B. R., Woo, S. L-Y., McChartly, M., et al. (1978)Parametric study of bone remodeling beneath internal fixation plates of varying stiffness,Journal of Bioengineering,. 2, 543.
Suurohen, R., Laine, P., Sarkiala, G., et al. (1992)Sagittal split osteotomy fixed with biodegradable, self reinforced poly-L-lactide screws,International Journal of Oral and Maxillofacial Surgery, 21, 303–308.
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