Fractures in the hand are common and often amenable to non-operative treatment. Certain unstable fractures and those associated with a complex hand injury are best treated operatively. The appeal of plate and screw fixation of hand fractures has been limited by concern that implants will interfere with the function of the gliding structures in the hand. In addition, a second operation for removal of the implant is often necessary. On the other hand, stable fixation with a plate ensures maintenance of alignment while allowing immediate mobilization.

          Bioresorbable fixation has been suggested as a means of overcoming some of the drawbacks of plate fixation in the hand while retaining the advantages. Our recent work in the Orthopaedic Biomechanics Laboratory at Beth-Israel Deaconess Medical Center addresses the feasibility of using bioresorbable plates and screws for the treatment fractures.

Background

The Biomechanics of Internal Fixation for Hand Fractures
          Most biomechanical studies have shown that plate fixation on the dorsal surface of the bone provides stronger fixation than other techniques such as intramedullary or crossed Kirschner wires, interfragmentary compression screws alone, or plate fixation of the lateral surface of the bone.1-5 Positioning the plate on the dorsal aspect of the bone is particularly advantageous under an apex dorsal load such as that encountered by the fractured metacarpal.5 Furthermore, the addition of an interfragmentary compression screw across an oblique fracture provides greater rigidity than dorsal plating alone.

Physiologic Loading in the Hand
          The strength and moment arm of the wrist extensors and the extrinsic and intrinsic hand musculature have been studied both clinically and in the laboratory by Ketchum and colleagues.6 They found that the intrinsic muscles of the index finger contributed combined forces equivalent to approximately 80% of those generated by the extrinsic digital flexors. The intrinsic musculature was responsible for 70% of the moment needed to produce metacarpophalangeal flexion with simultaneous interphalageal joint extension (the "intrinsic plus" position).7 This information is useful for determining the boundary conditions for biomechanical testing of metacarpal fractures under physiologic loading conditions.

The Development of Bioresorbable Internal Fixation Devices
          The development and testing of the bioresorbable implants that we are investigating was performed by Gogolewski and colleagues at the AO Research Institute in Davos, Switzerland. They quantified the effect of thermal treatment8, injection molding9, and solid-state extrusion10 on the mechanical properties and molecular stability of poly (L-lactide), poly (L/D-lactide), and poly (L/DL-lactide) implants. They also analyzed the in vivo degradation and host response to biodegradable fixation devices in sheep.9

          Several problems have been identified which have hampered the widespread use of bioresorbable fixation in modern traumatology. Many devices made of polyglycolic acid (PGA) homopolymers demonstrate rapid loss of in vivo strength and refractures have been common.11, 12 At the other extreme, remnants of pure polylactic acid (PLA) implants have been identified up to eight years after implantation13, raising the question as to whether PLA is too "biostable" to be used as a bioresorbable material.14 Furthermore, degradation of bioresorbable implants can cause a non-infectious inflammatory tissue response (so-called sterile abscess) requiring operative drainage in up to 26% of patients, most commonly when PGA or its copolymers are used.2, 15, 16 Several theories have been proposed regarding the etiology of this inflammatory reaction. Many investigators feel that this reaction represents an inability of the local tissues to rapidly clear the breakdown products of PGA.15 This theory is supported by the fact that glycolic acid has been isolated from inflammatory tissue15, and these reactions are distinctly less common with implants derived primarily from the more slowly degrading PLA polymers.13 Gogolewski and colleagues have developed implants made of a copolymer of 70% PLA and 30% PGA which has a more desirable rate of degradation that allows the plate to share the load until the fracture has healed, without the risk of sterile abscess formation seen with pure PGA homopolymers.

Bioresorbable Implants for Hand Fractures

Previous Work
          A handful of studies have addressed the use of bioresorbable intramedullary implants for the fixation of hand fractures. Intramedullary fixation devices made of polylactic acid-carbon were tested in fresh frozen human phalanges and pig metacarpals and shown to provide more resistance to bending loads than crossed K-wires.1 In vivo testing of polyglycolide (PGA) intramedullary implants in unstable metacarpal and phalangeal fractures in rabbits demonstrated rapid reduction in strength of the implants (73% at 2 weeks), but the results of treatment were comparable to crossed Kirschner wires.17 Intramedullary PLA implants have been used in rabbit femora and human thumb metacarpophalangeal arthrodeses with healing in each case and fragmentation of the implant over a period of 3 years.16

           There is a need to build upon this initial work because intramedullary implants are mechanically inferior to other techniques for fixation of hand fractures. Moreover, implants made of pure PGA or PLA homopolymers have demonstrated less than ideal strength and degradation characteristics over time.

Work Underway in the OBL
          We are currently testing 2.0-millimeter plates made of the 70/30 PLA/PGA co-polymer developed by the AO/ASIF for the fixation of metacarpal fractures. (Figure 1) Our study employs the use of synthetic metacarpal bones with anatomical and mechanical properties comparable to human metacarpals (Pacific Research Labs, Vashon, WA). The advantages of using synthetic bones include minimal variation of structural properties and the ability to immerse the bones in saline for prolonged periods of time in order to simulate in vivo degradation of the implants without affecting the mechanical properties of the bone.

          In the first phase of the study we are using a transverse osteotomy to simulate an unstable fracture pattern that is not amenable to fixation with interfragmentary compression screws.5 Bones fixed with the bioresorbable implants (6-hole 2.0-mm plates and 8 x 2.0-mm resorbable screws) are compared to bones fixed with 6 hole 2.0-mm plates made of titanium. A standard gap is created at the osteotomy site in order to isolate the structural properties of the plate itself. Later in the study, we plan to test more clinically relevant fracture patterns such an oblique fracture that can be secured with an interfragmentary compression screw in addition to the plate. The ends of each bone are set in acrylic to facilitate mechanical testing. (Figure 2)

           Mechanical testing is performed on fresh implants fixed to synthetic metacarpals using the gap osteotomy model described above. (Figure 3) The static load to failure is measured under an eccentric axial load and a torsional load. These maximum loads will be used to set boundary conditions for 2 Hz cyclic fatigue tests in both testing modes. Fatigue testing will be repeated using a variety of cyclic loading conditions in order to determine the endurance limit of the plated bone construct. (Figure 4). The use of cyclical loading more accurately simulates the biomechanical conditions produced in the hand during early rehabilitation from metacarpal fractures.

          Additional plates will be fixed to composite metacarpals and submerged in phosphate buffered saline (pH 7.4) to simulate in vivo hydrolysis. Plates will be removed at weekly intervals for 12 weeks and subjected to the fatigue tests described above. This data will be used to determine the loss of bending and torsional strength over time.

Conclusions

           The potential advantages of bioresorbable implants include less stress shielding of the bone than would be expected with metallic implants, less interference with modern imaging techniques, and elimination of the need for subsequent operations to remove the implant. Recent improvements in the materials and design of bioresorbable plates and screws have addressed some of the problems with the first generation of resorbable implants. Our investigations will provide useful information regarding the mechanical performance of bioresorbable implants in the treatment of hand fractures as a critical first step towards their successful use in treating patients.

Kevin Bozic, MD is a Resident in the Harvard Combined Orthopaedic Residency Program

Jesse B. Jupiter, MD is Chief of the Hand Service at Massachusetts General Hospital, and Professor of Orthopaedic Surgery at Harvard Medical School

Address correspondence to:
Jesse B. Jupiter, MD; Massachusetts General Hospital;
ACC 527, 15 Parkman St.; Boston, MA 02114
email: jjupiter1@partners.org

This study funded in part by grants from AO North America (Paoli, PA) and the AO Development Institute (Davos, Switzerland)