Secondary Logo

Bioabsorbable Implants in the Treatment of Hand Fractures: An Update

Hughes, Thomas, B

Section Editor(s): Meals, Roy A MD, Guest Editor; Harness, Neil G MD, Guest Editor

Clinical Orthopaedics and Related Research: April 2006 - Volume 445 - Issue - p 169-174
doi: 10.1097/01.blo.0000205884.81328.cc
SECTION I: SYMPOSIUM: Problem Fractures of the Hand and Wrist
Free
SDC

Bioabsorbable implants have been used extensively in medicine. Many have suggested interest in these materials for hand fractures since they may lead to less implant morbidity and subsequent stiffness, and they have additional advantages: they are radiolucent, they eliminate hardware removal procedures, they limit stress shielding, and they incrementally transfer load to healing fractures. Disadvantages include weakness relative to metal implants, rapid loss of initial implant strength, higher refracture rates, and foreign body reactions. Newer polymers have led to a reduction in these complications and more stable fixation. While not yet incorporated into the armamentarium of most surgeons, there is promise that in the near future they may assist in the management of some hand fractures.

Level of Evidence: Level V (expert opinion). See the Guidelines for Authors for a complete description of levels of evidence.

From the Allegheny General Hospital, Pittsburgh, Pennsylvania.

The author certifies that he has no commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) that might pose a conflict of interest in connection with the submitted article.

Correspondence to: Thomas B. Hughes, MD, Allegheny General Hospital, 320 East North Avenue, Pittsburgh, PA 15212. Phone: 412-359-6754; Fax: 412-359-8055; E-mail: Thughes424@aol.com.

The treatment of unstable fractures or complex injuries of the hand frequently is facilitated by the implementation of rigid internal fixation. The stability imparted allows anatomic restoration with early mobilization and return of function. This stability is typically provided with metallic implants that do not come without some compromises.

Complication rates can be high, with major complications reported in 36% of patients in one series.13 These included nonunion, stiffness, plate prominence, infection and tendon rupture. Wires are frequently left out of the skin to facilitate removal and this can lead to pin tract infections. Pin infection rates between 7% and 15% been associated with pin loosening and migration, osteomyelitis, tendon rupture, and nerve injury.1,20 Rigid fixation may facilitate early motion, but less stiff implants may lead to more callous formation and better union rates.21 Stress shielding is also a well know complication of rigid plate fixation that may be lessened with less rigid plating techniques.4

The use of bioabsorbable implants has been proposed to reduce many of these problems. Pins that do not need to be removed need no longer be left out of the skin. Plates that slowly resorb transfer stress to the bone gradually, preventing bony weakness over time. This transfer also lessens the stress risers that normally exist at the ends of a plate and at the screw holes after plate removal. Although these are theoretical advantages, there is little scientific data to support this hypothesis.

Bioabsorbable implants, however, do not come without a price of their own. The rigidity of these implants compares favorably with, but does not equal, the strength of metal implants.2,15,23,24 Biochemical breakdown of older implants leads to a rapid loss of what initial strength was present leading to higher implant failure rates and refracture.15,23,24 Additionally, these biochemical processes lead to synovial reactions, sterile fluid and sinus formation, and fibrous encapsulation.17,18

In searching for information for this review, we determined that there are there are few studies in which authors investigate the use of bioabsorbable implants in the hand. Most of the available literature had been incorporated into this review. Although some of the biomechanical data are encouraging,2,6,10,11,15,16,19,24 the clinical series have small cohorts and frequently are retrospective or case series reviews.9,23,25 There are large cohorts of patients who have had treatment of fractures with bioabsorbable devices, but they are a retrospective collection of fractures from all anatomic areas.17,18 The lack of controlled studies that investigate the benefits of bioabsorbable fixation in hand trauma over traditional treatment methods is likely the greatest impediment to their widespread acceptance in the hand surgeon's armamentarium.

We present an update on the current state of bioabsorbable implants and their use in fracture fixation. In particular, it will discuss the information primarily as it relates to the treatment of hand fractures. The high complication rates noted with rigid internal fixation of hand fractures suggest a need for better fixation methods. The proposed benefits of bioabsorbable fixation would seem to be well suited to preventing some of these complications. In the last five years, clinical and biomechanical studies looking specifically at the use of these implants in phalangeal, metacarpal, and carpal models have added to our knowledge base.2,9,10,19,23-25 This knowledge base, however, requires additional information on clinical and functional outcomes in randomized controlled series with adequate patient cohorts to convincingly support their use over more traditional methods of fracture fixation.

Back to Top | Article Outline

Bioabsorbable Materials

While the use of absorbable gut sutures is an ancient practice, the more modern absorbable materials have been primarily polymers of poly-alpha-hydroxy acids in the polyester family.8,14 These polymer chains have properties that are specific to the independent monomers that comprise them and to the bonds that exist between the monomers.

Polymers are composed of building blocks termed monomers.5,8,22 These monomers form covalent bonds that bind them strongly together. The nature and arrangement of these bonds determines the structure and properties of the polymer. The polymerization process is controlled by temperature, pressure, chemical composition, and timing of the chemical reaction involved. Different monomers may be combined to obtain a material with a fusion of the characteristics of the individual components.

Polymerization is started when the initiator molecule is cleaved in the presence of the base monomer, forming a free radical. This free radical gains an electron by bonding to the monomer, forming a new covalent bond and, as a byproduct, a monomer with a free radical. This monomer free radical then binds to another monomer forming a bond and another free radical end. This continues in a process known as propagation. The propagation phase lasts a fraction of a second and is followed by the termination reaction, which occurs when two growing chains' free radicals combine, forming a stable polymer. This may be a homopolymer (comprised of a single type of monomer molecule) or a copolymer (formed from two or more different monomer molecules).

For each chiral molecule involved in the polymer there may be two enantiomers. A chiral molecule has a mirror image that is of identical chemical composition but opposite orientation. This opposite orientation reflects polarized light in opposite directions, which is termed the dextrorotatory (D) and a levorotatory (L) configuration, and is based on the chirality of the molecule. These different enantiomers are important in bioabsorbable polymers in that they affect the rate of absorption, and therefore the strength, of the implants.

The materials primarily used in orthopaedics are polyglycolic acid (PGA) and polylactic acid (PLA). Polyglycolic acid was introduced in 1970 as the suture material Dexon (Davis and Geck, Danbury, CT). It is more susceptible than PLA to hydrolysis and early breakdown, typically being absorbed in several months. It is frequently used in combination with PLA to lead to earlier hydrolysis of an implant. Polyglycolic acid is also more susceptible to gamma radiation and ethylene gas oxide sterilization.

Many variables control final implant mechanical properties. The polymerization reaction can be modified to create cross-links that increase fiber rigidity. The chemical reaction can be controlled by altering the temperature and the rate of heating and cooling. Once the polymers are formed they are additionally influenced by the surrounding temperature. The glass transition temperature is the temperature above which the substance is brittle and below which it is more ductile. The glass transition temperature for bioabsorbable materials is usually well above body temperature. For PGA it is near 40°C whereas for PLA it is more than 60°C. It is a useful property because heating the material allows it to be contoured easily. In clinical applications the plates are warmed in a bath until they are pliable, contoured, and then allowed to stiffen at room temperature before insertion (Fig 1).

Fig 1A

Fig 1A

The L isomer of PLA (PLLA) is the enantiomer found in largest amounts in orthopaedic implants.8,12,22 This isomer has a high degree of crystallinity and is more resistant to hydrolysis. A pure PLLA remains detectable for between 18 months and four years in vivo. The D isomer (PDLLA) is amorphous and provides less tensile strength. It promotes resorption of the implants over time.

Biodegradation occurs through two phases. In the first phase, hydrolysis of the bonds linking the polymer's monomers occurs. Hydrolysis occurs first in the amorphous portions of the implant (the PDLLA regions), but does not alter the mechanical properties of the implant because of the crystalline portions that remain provide most of the implant's strength. After hydrolysis of the amorphous portions, the crystalline regions undergo hydrolysis and fragmentation occurs. Fragmentation leads to a reduction in the mechanical properties of the implant and eventually implants fail. The second phase of degradation is the enzymatic breakdown of the monomers themselves into lactic and glycolic acids. These are converted into pyruvate and then processed through the citric acid cycle into carbon dioxide and water.

The rapid breakdown of these implants is thought to be the underlying cause for the clinical scenario of sterile sinus formation, synovitis, and other foreign-body reactions. This is supported by the observation of a higher incidence of these reactions with the more rapidly absorbed PGA than with PLA. The rates of degradation are controlled by copolymer ratio (the ratios of PLLA, PDLLA, and PGA) and by configuration structure, crystallinity, molecular weight, morphology, stresses, residual monomer, porosity, and site of implantation.

Back to Top | Article Outline

Mechanical Properties

The mechanical properties of bioabsorbable implants are obviously different from metallic fixation devices. They are less brittle with a lower modulus of elasticity, which causes less stress shielding. They undergo more creep and stress relaxation, causing concerns of greater fracture motion and higher rates of nonunion. These mechanical properties can be improved by reinforcing the materials. Second generation bioabsorbable materials are reinforced with materials that have the same molecular composition as the original material. These “self-reinforced” materials are formed by extrusion and are composed of polymeric chains oriented in parallel to provide support and increase structural rigidity. They have been found to greatly improve sheer and bending strength (Fig 2).3

Fig 2A

Fig 2A

Much of the research regarding bioabsorbable implants in hand fracture fixation has come from Finland.9,17,18,23-25 Waris et al18 used a porcine fresh frozen metacarpal model to compare 1.5 mm PLA pins and a 2.0 mm PLA plate with 1.5-mm and 1.25-mm stainless steel Kirschner wires (K-wires). In apex palmar and dorsal bending the 1.5 mm PLA pins were equivalent to 1.5 mm K-wires. However, in apex lateral bending and torsional rigidity they compared more closely to 1.25 mm K-wires. The 2.0 mm PLA plate was equivalent in strength in all directions with the 1.5 mm K-wires and equivalent to titanium plating in lateral apex bending and torsional rigidity. The titanium plates, however, were stronger than all other devices in apex palmar and dorsal bending.24

Waris et al23 then compared PLA/PGA plates with titanium plates and K-wires in a human cadaver model. They investigated three-point bending and torsional rigidity after fixation of an oblique osteotomy. Dorsal and dorsolateral 2.0 mm PLA/PGA plates of differing chemical characteristics were compared with dorsal 1.7 mm and 2.3 mm titanium plate fixation. In apex palmar and dorsal three-point bending the rigidity of all PLA/PGA plates was comparable to that of the 1.7 mm titanium plate, and their torsional rigidity was higher and equivalent to the 2.3 mm titanium plates.

Bozic et al2 examined the mechanical properties of a 2.5 mm bioabsorbable plate and screw system (AO Development Institute, Davos, Switzerland) compared with a 2.0 mm titanium construct (Synthes, Inc., Paoli, PA). Over a 12-week period these plates, placed on cadaveric metacarpals, were maintained in a saline bath at 37.3°C to simulate degradation via hydrolysis. Intermittently throughout that period mechanical testing measured strength and stiffness of the constructs. They determined titanium plates were 2.5 times stiffer than the PLA plates. The load to failure was greater for titanium. Additionally, the load to failure of the PLA plates was higher initially than when measured at Week 8. The failure of the titanium and the PLA was initially through the plate at the osteotomy site. As hydrolysis progressed, however, the failure occurred at the screw heads. Finally, they found the PLA construct lost 53% of the bending strength by week 12.

These studies, along with others in the literature,6,10,11,15,16,19,24 indicate what might have already been evident: bioabsorbable implants are strong, but not as strong as metal. Especially in torsion, and when there is comminuted bone and greater strength is required of the implant, failure is a concern in the PLA devices.

The real question therefore is how strong is strong enough? In fractures of the hand, joint contractures and tendon adhesions provide secondary morbidity, and therefore stable fixation of fractures is directed at allowing early motion. It is unclear if bioabsorbable plates are adequate to allow this rapid mobilization (Fig 2).10 It is also unclear how the complication rate of bioabsorbable plate fixation compares with metal implants. Although the theoretical advantages are many, there still needs to be verification that the secondary complications of refracture, sterile abscess formation, and fibrous encapsulation are not more troublesome than the plate irritation, stress shielding, and tendon adhesion formation seen with metal implants.

Back to Top | Article Outline

Clinical Applications

The initial clinical use of bioabsorbable materials centered on suture materials and the repair of soft tissues. More recently, bioabsorbable materials have gained widespread acceptance in applications involving the repair of soft tissues to bone (Fig 3).7 Rotator cuff anchors composed of PLA/PGA composites have been shown to have comparable strength to metal anchors, and absorbable interference screws are commonly used in anterior cruciate ligament reconstruction. The potential uses for fracture fixation are varied and have yet to be fully explored. Several clinical series are available that can provide some insight into potential advantages and pitfalls in the treatment of fractures with bioabsorbable implants.

Fig 3

Fig 3

Rokkanen et al reported in 1996,18 and again in 2000,17 on the use of bioabsorbable fixation for the treatment of fractures and osteotomies. These studies were aimed at determining the rates of complications with the use of absorbable rods, screws, tacks, plugs, arrows, and wires in 3200 patients. The overall complication rate was 10%. Bacterial wound infection occurred in 4% and failure of fixation occurred in 4%. Foreign body reactions occurred in 2% of patients, but this was with PGA implants. None of these reactions occurred with PLA implants. The authors estimated, however, that in 2500 operations until 1996 they had avoided nearly 1000 hardware removal operations.

Complicated injuries of the hand frequently require operative fixation. Combined skeletal and soft-tissue injuries require stability to pursue soft-tissue rehabilitation. Bioabsorbable implants lend themselves to this situation nicely by degrading and preventing long-term problems with tendon glide and adhesions. Waris et al used a self-reinforced PLA 70/30 plate (BioSorb, Bionx Implants Ltd, Tampere, Finland) in the treatment of three patients with complicated combined injuries of the hand.25 In one case, 2.0-mm plates were used to treat an open third metacarpal fracture with multiple extensor tendon lacerations and a dorsal soft-tissue defect requiring flap coverage. The fracture went on to union, but more interestingly the middle finger had a 40° arc of motion while the index and ring fingers only had a 30° arc. All three cases presented went on to union and good function.

Scaphoid fractures and nonunions have also been treated successfully with PLA screws.9 Three acute fractures and three nonunions were treated operatively with an absorbable screw. Five went onto union, the only nonunion being in a previously operated patient who went on to have a complete wrist fusion for radiocarpal arthrosis. There was one excellent, four good, and one poor result, the last being in the persistent nonunion. The authors believed the treatment was limited because of the screw designs available rather than the material properties. There were a limited number of screw sizes, and more options may have lead to better fixation. They also believed the procedures might have been better if the screws were cannulated. This would allow for more precise placement of the screw and the guide wire would be visible on an image intensifier. The devices they used also had no compressive component, such as variable pitch threads, so any compression achieved was because of positioning at the time of implant placement rather than through the implant itself. One advantage to this type of implant is better postoperative imaging to confirm union.

Back to Top | Article Outline

DISCUSSION

The use of bioabsorbable materials in orthopaedics is gaining acceptance.17,18

Their use in hand surgery has been limited in scale to this date, however there is some promising information that suggests they could be used more extensively in the future. The high complication rates of hand fracture plating13 show a need to investigate alternative methods of fixation. The biomechanical data available clearly show an ability to fix fractures in a hand model. The fixation is rigid, but not as rigid as metallic fixation. Biomechanical models will need to be developed to look at fracture stability with early finger motion. These models can then be compared to metallic fixation to prove or disprove the notion of “sufficient” stability.

Incorporating these materials into fracture fixation in the hand certainly has many theoretical advantages. The opportunity to avoid implant complications such as adhesions, pain, stress shielding, and secondary operative removal is certainly desirable. It is unclear at this time that bioabsorbable plates will prevent implant complications as suggested. The use of longer lasting and stronger materials may negate some of their theoretical advantages. Studies investigating the length of time to resorption may be of some help. More importantly, comparative studies will be required to demonstrate real advantages in implant complication rates. Studies determining final range of motion, rates of required tenolysis and implant failure, and patient functional outcomes are necessary to lead to widespread acceptance of bioabsorbable implants.

The complication rate observed with plate fixation of the hand has many factors, implant selection being only one. The most important factor in the rates of complication is likely the complexity of the original injury. Although Page and Stern13 showed a high complication rate, they concluded that their worst results were in phalangeal and open fractures. There is little evidence to suggest that in this complicated group of patients, bioabsorbable fixation could have lead to better results. Waris et al25 showed promising results in three cases of complex trauma; but this is insufficient to prove effectiveness, let alone superiority to metallic fixation. Larger patient cohorts are needed to detect complication rates, even in retrospective studies. Prospective matched cohort studies with sufficient patient numbers are required to suggest that bioabsorbable plates offer a better alternative to metallic fixation.

Although current data suggest these implants may provide sufficient mechanical properties to stabilize hand fractures, it is unclear if the stability is sufficient for early motion. The proposed benefits of bioabsorbable fixation have not yet been demonstrated in the treatment of hand injuries in large series. Early clinical studies show some level of success in treating hand injuries with bioabsorbable implants. Future studies are necessary to better assess rates of healing, functional outcomes, and comparative complication rates when compared with metal implants.

Back to Top | Article Outline

References

1. Botte MJ, Davis JL, Rose BA, von Schroeder HP, Gellman H, Zinberg EM, Abrams RA. Complications of smooth pin fixation of fractures and dislocations in the hand and wrist. Clin Orthop Relat Res. 1992;276:194-201.
2. Bozic KJ, Perez LE, Wilson DR, Fitzgibbons PG, Jupiter JB. Mechanical testing of bioresorbable implants for use in metacarpal fracture fixation. J Hand Surg. 2001;26:755-761.
3. Ciccone WJ II, Motz C, Bentley C, Tasto JP. Bioabsorbable implants in orthopaedics: new developments and clinical applications. J Am Acad Orthop Surg. 2001;9:280-288.
4. Claes L. The mechanical and morphological properties of bone beneath internal fixation plates of differing rigidity. J Orthop Res. 1989;7:170-177.
5. Daniels AU, Chang MK, Andriano KP. Mechanical properties of biodegradable polymers and composites proposed for internal fixation of bone. J Appl Biomater. 1990;1:57-78.
6. Fitoussi F, Lu W, Ip WY, Chow SP. Biomechanical properties of absorbable implants in finger fractures. J Hand Surg [Br]. 1998;23:79-83.
7. Goradia VK, Mullen DJ, Boucher HR, Parks BG, O'Donnell JB. Cyclic loading of rotator cuff repairs: A comparison of bioabsorbable tacks with metal suture anchors and transosseous sutures. Arthroscopy. 2001;17:360-364.
8. Gunatillake P, Adhikari R. Biodegradable Synthetic Polymers for Tissue Engineering. Eur Cell Mater. 2003;20:1-16.
9. Kujala S, Raatikainen T, Kaarela O, Ashammakhi N, Ryhanen J. Successful treatment of scaphoid fractures and nonunions using bioabsorbable screws: report of six cases. J Hand Surg. 2004;29: 68-73.
10. Lionelli GT, Korentager RA. Biomechanical failure of metacarpal fracture resorbable plate fixation. Ann Plast Surg. 2002;49:202-206.
11. Maruyama T, Saha S, Mongiano DO, Mudge K. Metacarpal fracture fixation with absorbable polyglycolide rods and stainless steel K-wires: a biomechanical comparison. J Biomed Mater Res. 1996;33: 9-12.
12. Miller RA, Brady JM, Cutright DE. Degradation rates of oral resorbable implants (polylactates and polyglycolates): rate modification with changes in PLA/PGA copolymer ratios. J Biomed Mater Res. 1977;11:711-719.
13. Page SM, Stern PJ. Complications and range of motion following plate fixation of metacarpal and phalangeal fractures. J Hand Surg. 1998;28:827-832.
14. Pietrzak WS, Sarver DR, Verstynen ML. Bioabsorbable polymer science for the practicing surgeon. J Craniofac Surg. 1997;8:87-91.
15. Prevel CD, Eppley BL, Ge J, Winkler MM, Katona TR, D'Alessio K, Sarver D. A comparative biomechanical analysis of resorbable rigid fixation versus titanium rigid fixation of metacarpal fractures. Ann Plast Surg. 1996;37:377-385.
16. Prevel CD, Katona T, Eppley BL, Moore K, McCarty M, Ge J. A biomechanical analysis of the stability of titanium bone fixation systems in proximal phalangeal fractures. Ann Plast Surg. 1996;37: 473-481.
17. Rokkanen PU, Bostman O, Hirvensalo E, Makela EA, Partio EK, Patiala H, Vainionpaa SI, Vihtonen K, Tormala P. Bioabsorbable fixation in orthopaedic surgery and traumatology. Biomaterials. 2000;21:2607-2613.
18. Rokkanen P, Bostman O, Vainionpaa S, Makela EA, Hirvensalo E, Partio EK, Vihtonen K, Patiala H, Tormala P. Absorbable devices in the fixation of fractures. J Trauma. 1996;40 (Suppl):S123-S127.
19. Roure P, Ip WY, Lu W, Chow SP, Gogolewski S. Intramedullary fixation by resorbable rods in a comminuted phalangeal fracture model. A biomechanical study. J Hand Surg [Br]. 1999;24:476-481.
20. Stahl S, Schwartz O. Complications of K-wire fixation of fractures and dislocations in the hand and wrist. Arch Orthop Trauma Surg. 2001;121:527-530.
21. Terjesen T, Apalset K. The influence of different degrees of stiffness of fixation plates on experimental bone healing. J Orthop Res. 1988;6:293-299.
22. Tormala P. Biodegradable self-reinforced composite materials; manufacturing structure and mechanical properties. Clin Mater. 1992;10:29-34.
23. Waris E, Ashammakhi N, Happonen H, Raatikainen T, Kaarela O, Tormala P, Santavirta S, Konttinen YT. Bioabsorbable miniplating versus metallic fixation for metacarpal fractures. Clin Orthop Relat Res. 2003;410:310-319.
24. Waris E, Ashammakhi N, Raatikainen T, Tormala P, Santavirta S, Konttinen YT. Self-reinforced bioabsorbable versus metallic fixation systems for metacarpal and phalangeal fractures: a biomechanical study. J Hand Surg. 2002;27:902-909.
25. Waris E, Ninkovic M, Harpf C, Ashammakhi N. Self-reinforced bioabsorbable miniplates for skeletal fixation in complex hand injury: three case reports. J Hand Surg. 2004;29:452-457.
© 2006 Lippincott Williams & Wilkins, Inc.