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SECTION II ORIGINAL ARTICLES: Trauma

Bioabsorbable Miniplating Versus Metallic Fixation for Metacarpal Fractures

Waris, Eero MD*,**; Ashammakhi, Nureddin MD, PhD; Happonen, Harri MSc; Raatikainen, Timo MD, PhD§; Kaarela, Outi MD, PhD; Törmälä, Pertti PhD, BMS; Santavirta, Seppo MD, PhD; Konttinen, Yrjö T. MD, PhD*,**,¶

Author Information
Clinical Orthopaedics and Related Research: May 2003 - Volume 410 - Issue - p 310-319
doi: 10.1097/01.blo.0000063789.32430.c6
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Abstract

List of Abbreviations Used: PGA polyglycolide, polyglycolic acid, PDS polydioxanone, PLGA copolymer of lactide and glycolide, PLGA 80/20 copolymer of 80% lactide and 20% glycolide, P(L/DL)LA stereocopolymer of LL- and DL-lactides, P(L/DL)LA 70/30 stereocopolymer of 70% LL-lactide and 30% DL-lactide, PLA polylactide, polylactic acid, PLLA poly-L-lactide, poly-L-lactic acid

Metacarpal and phalangeal fractures are common injuries. Unstable fracture patterns require operative reduction and fixation. Metallic osteofixation devices provide rigid fixation, allowing early functional use. Their use, however, is associated with certain disadvantages and may lead to unsatisfactory results. In the hand, metallic implants may interfere with tendon gliding, and may disturb joint movement and function. Prominent or protruding metallic hardware can be associated with continuous discomfort, pain, and infections. The stress shielding caused by a rigid metallic device can result in osteopenia. 16 The complication rate has decreased through the use of Ti rather than stainless steel and through the use of smaller, lower-profile plating systems. 23 However, a secondary procedure to remove the device often is required. Stern et al 27 and Berman et al 2 have reported a need for plate removal in 25% of cases of metacarpal and proximal phalangeal fractures.

Reliable, biocompatible bioabsorbable devices have been developed for osteosynthesis and their clinical applications have been studied. 26 These devices most commonly are made of PLA, PGA, and their copolymers. Ultrahigh strength implants can be manufactured from these polymers by using self-reinforcing techniques, which involve reinforcement of the polymeric matrix with fibers, fibrils, or molecular chains of the same material. 28 In the hand, self-reinforced bioabsorbable pins and screws are being used for fixation of metacarpal and phalangeal fractures 14,18,19 and to stabilize fusions. 13 Self-reinforced PLLA tacks are being applied in reinsertion of the ulnar collateral ligament of the metacarpophalangeal joint of the thumb. 31

Bioabsorbable self-reinforced miniplates and screws also have been developed and now are preferred to metallic miniplates and screws in craniofacial surgery. 1 The techniques and implants have been adapted for use in the hand, with the development of specific miniplates and screws. The current study was done to examine the fixation properties of self-reinforced PLGA 80/20 and self-reinforced P(L/DL)LA 70/30 miniplating systems in transverse metacarpal osteotomies. The results were compared with those of Ti plating and the use of Kirschner (K) wires.

MATERIALS AND METHODS

Metacarpal Bones from Cadavers

The current study was approved by the National Authority for Medicolegal Affairs in Finland. Fifty-six pairs of second metacarpal bones were collected from cadavers (44 men and 12 women). None of the donors had a known history of musculoskeletal illness. The donors were 25 to 90 years at death (mean, 64 years; standard deviation, 14 years). Harvesting was done 1 to 7 days (mean, 4 days; standard deviation, 1 day) postmortem. The bones were dissected, wrapped in normal saline-soaked gauze, and frozen to −70° C to maintain mechanical properties as close to fresh living bones as possible. This collection and preservation protocol was validated previously. 30

The frozen specimens were allowed to thaw at room temperature for 12 to 24 hours before the laboratory studies. The bones were immersed in normal saline and kept moist throughout the experiments. Each bone was cleaned of remaining soft tissues and its dimensions were measured. The mean length of the bones was 69.9 mm (standard deviation, 4.7 mm), the midshaft cross-sectional axes were 9.7 mm (standard deviation, 1.1 mm) and 9.1 mm (standard deviation, 0.8 mm), and the cortical thickness was 2.4 mm (standard deviation, 0.5 mm). There was no difference in the covariates between the test groups (p < 0.05).

Implants and Fixation Methods

A transverse osteotomy was done with a circular diamond saw (diameter, 30 mm; thickness, 0.30 mm; Giflex-TR diamond disc, Bredent, Senden, Germany) at the middiaphyseal region of each metacarpal bone. The osteotomized bones were randomized and then fixed using one of the seven fixation methods (Fig 1): (1) dorsal or (2) dorsolateral self-reinforced PLGA 80/20 plate (BioSorb™ PDX, Bionx Implants Ltd, Tampere, Finland) with 2-mm screws, (3) dorsal or (4) dorsolateral self-reinforced P(L/DL)LA 70/30 plate (BioSorb™ FX) with 2-mm screws, (5) dorsal Ti plate (S plate, Howmedica Leibinger, Freiburg, Germany) with 1.7-mm screws, (6) dorsal Ti plate (M plate, Howmedica Leibinger) with 2.3-mm screws, or (7) crossed 1.25-mm K wires (Synthes, Mathys Medical, Bettlach, Switzerland). The mechanical integrity of intact metacarpal bones also was studied.

Fig 1A–E.
Fig 1A–E.:
Fixation configurations of the (A) dorsal self-reinforced PLGA 80/20 or self-reinforced P(L/DL)LA 70/30 plate and 2-mm screws, (B) dorsolateral self-reinforced PLGA 80/20 or self-reinforced P(L/DL)LA 70/30 plate and 2-mm screws, (C) dorsal Ti plate and 1.7-mm screws, (D) dorsal Ti plate and 2.3-mm screws, and (E) crossed 1.25-mm K wires are shown.

The bioabsorbable devices used in the current study all were made by using a self-reinforcing technique and they were sterilized with gamma radiation at a dose of 2.5 Mrads. The plates were prebent slightly at room temperature to contour them to the bone (Fig 2) and then placed on the dorsal or dorsoradial side of the shaft of the metacarpal bone, after which the osteotomy was closed, screw holes were drilled and tapped, and the plate was tightened with 2-mm diameter miniscrews of the same material as the plates. The control groups were stabilized with Ti miniplates and screws (1.7 and 2.3 mm hand plating systems) or crossed K wires. Titanium plates and screws were applied similarly to bioabsorbable platings except that the tapping step was not needed. The geometries of the plating systems are shown in Table 1. Kirschner wire fixation was accomplished with two 1.25-mm stainless steel K wires, fixed by the retrograde method across the osteotomy site at an angle approximately 40° to the long axis of the bone in the midlateral plane.

TABLE 1
TABLE 1:
Geometry of Plates and Screws Used in the Current Study
Fig 2.
Fig 2.:
The bioabsorbable plate and screw used in the current study are shown. The plate has been prebent slightly at room temperature to contour it to the bone.

Biomechanical Testing

Biomechanical testing was done with Lloyd LR 30K testing equipment (Lloyd Instruments Limited, Fareham, England). For each study group, eight specimens were subjected to bending tests and eight were subjected to torsion tests.

Bending Test

Palmar and dorsal apex loadings were applied, using the three-point bending method. This method has been used to study the bending stabilities of metacarpal and phalangeal bones. 3,21,22 The loading crosshead was centered over the osteotomy site in the middle of a span of 30 mm. A constant crosshead speed of 1 mm per minute was used.

The specimens were tested in apex palmar bending by applying crosshead displacement of 1 mm. The specimens then were rotated 180° along their longitudinal axes and loaded until failure occurred in apex dorsal bending. A load-displacement curve was plotted for each specimen.

Flexural rigidity (extrinsic stiffness) of the bone is shown by the slope of the load-deformation curve in its elastic region where deformation increases linearly with increasing load before the yield point. It is equal to EI, where E is Young’s modulus and I the cross-sectional moment of inertia. It was calculated according to the equation EQUATION

where F/x = the slope of the load-displacement curve in the initial linear section (N/m), and L = length of the span (m). 30

The specimens were allowed to break in apex dorsal bending. The breaking point was defined as a decrease in load with increasing displacement. The maximum bending moment (M, in N-m) resulting in failure was calculated using the equation EQUATION

where F = the force recorded at the failure point (N) and L = the support span (m). 3 Failure modes were observed visually.

Torsion Test

The specimens for torsional tests were placed in bone cement (Palacos® R, Schering-Plough, Brussels, Belgium) in cylindrical plastic containers (diameter, 30 mm; depth, 15 mm) at both ends. The specimen then was fastened horizontally into the testing machine at the cemented ends and subjected to torsional force. A constant strain rate of 0.24° per second was used. The torque-twist relationship was determined. The torsional rigidity, (GJ) (extrinsic stiffness, N-m2/rad) was calculated according to the equation EQUATION

where T/θ = the slope of the torque-twist curve in the initial linear section (N-m/rad) and L = length of the unembedded portion of the bone specimen (m). 30 Failure torque was recorded at the breaking point.

Statistical Methods

Mean values and standard deviations were calculated for each test group and for intact bone. Differences among the groups were compared using analysis of variance (ANOVA) and if differences were found to be relevant, pairwise comparisons were done with the t test for normally distributed variables and the nonparametric Mann-Whitney U test was done for skewed distributions. The level of statistical significance was set at a probability less than 0.05.

RESULTS

Apex Palmar Loading

The rigidity of dorsal self-reinforced P(L/DL) LA plating was higher than that of dorsal self-reinforced PLGA (p = 0.011) and 1.7-mm Ti plating (p = 0.032). There was no significant difference in rigidity between dorsal self-reinforced PLGA and 1.7-mm Ti plating (p = 0.340). Both types of dorsolateral bioabsorbable plating were more rigid than the corresponding dorsal platings (p < 0.001). Of the plated bones, dorsal 2.3-mm Ti plating had the highest rigidity (p = 0.020, versus the second highest), whereas crossed K wires provided the highest stability when analyzing all fixation groups (p < 0.001). Intact control bone had a mean rigidity of 0.4269 N-m2 (standard deviation, 0.0630) (Fig 3).

Fig 3.
Fig 3.:
The flexural rigidity in apex palmar loading with the values presented as means ± standard deviation is shown. SR = self-reinforced.

Apex Dorsal Loading

Apex dorsal loading is considered the most important test for metacarpal stability, because transverse metacarpal fractures typically are angulated, creating an apex dorsal deformity secondary to the pull of the interosseous muscles.

In rigidity, both dorsal bioabsorbable platings (self-reinforced PLGA, p = 0.005; self-reinforced P(L/DL)LA, p < 0.001) were weaker than dorsal 2.3-mm Ti plating. Of the two, self-reinforced PLGA plating was more rigid than self-reinforced P(L/DL)LA plating (p = 0.005) and it was equal in rigidity to the dorsal 1.7-mm Ti plate (p = 0.491). Both types of dorsal bioabsorbable plating were more rigid than crossed K wires (self-reinforced PLGA, p < 0.001; self-reinforced P(L/DL)LA, p = 0.001). When applying the bioabsorbable plates dorsolaterally, the rigidity decreased (self-reinforced PLGA, p < 0.001; self-reinforced P(L/DL)LA, p = 0.006) compared with the corresponding dorsal plating. There was no difference in rigidity between the dorsolateral bioabsorbable platings and crossed K wires (p = 0.284) (Fig 4A).

Fig 4A–B.
Fig 4A–B.:
(A) The flexural rigidity and (B) maximum bending moment in apex dorsal loading with the values presented as means ± standard deviation are shown. SR = self-reinforced.

Analysis of the maximum bending moment (Fig 4B) revealed that both types of dorsal bioabsorbable plating had higher values than crossed K wires (p < 0.001), but there was no difference when compared with 1.7-mm Ti plating (p = 0.331). The values measured in the dorsal bioabsorbable plate groups did not differ (p = 0.567). When intact bone was excluded, dorsal 2.3-mm Ti plating had the highest maximum bending moment (p = 0.034, versus the second highest) among the fixation methods. A tendency toward lower values was seen when dorsolateral bioabsorbable plating was compared with corresponding dorsal plating (self-reinforced PLGA, p = 0.084; selfreinforced P(L/DL)LA, p = 0.002). The mean value for crossed K wires was the lowest, but the difference compared with dorsolateral self-reinforced P(L/DL)LA was not significant (p = 0.090). Intact control bone had a rigidity of 0.477 N-m2 (standard deviation, 0.099) and a maximum bending moment of 9.04 N-m (standard deviation, 2.07).

Visual evaluation of failure mechanisms revealed that osteotomies plated dorsally with self-reinforced PLGA were broken either at the plate (five of eight specimens) or the bone cortex (three of eight specimens). Dorsal self-reinforced P(L/DL)LA plates were stretched slightly at the screw hole site, after which the fixation failed as above (three of eight specimens at the plate, and three of eight specimens at the cortex). In two specimens, the dorsal self-reinforced P(L/DL)LA plate stretched without breaking. Dorsolateral bioabsorbable plates broke (self-reinforced PLGA, eight of eight specimens; self-reinforced P(L/DL)LA, four of eight specimens) or were stretched without breaking (self-reinforced P(L/DL)LA, four of eight specimens) at the screw hole. Bones plated dorsally with 1.7-mm Ti failed because of plate breakage (five of eight specimens) or because the Ti screws came out of the bone (three of eight specimens), whereas all bones plated dorsally with 2.3-mm Ti could not resist the force created around the screws, leading to broken cortex (four of eight specimens) or longitudinal splitting of the diaphysis (four of eight specimens). Kirschner wires pulled through the cortex (eight of eight specimens) when angulation at the osteotomy site was increased. The areas subjected to the highest stress in plating systems were on the bone adjacent to the screws and at the screw holes of the plate proximal to the osteotomy. The screws and their heads did not break in any instance.

Torsional Loading

In torsional loading (Fig 5A) both types of dorsal bioabsorbable plating were more rigid than dorsal 1.7-mm Ti plating (self-reinforced PLGA, p = 0.036; self-reinforced P(L/DL)LA, p = 0.011) but weaker than 2.3-mm Ti plating (self-reinforced PLGA, p = 0.046; self-reinforced P(L/DL)LA, p = 0.022). The bioabsorbable platings were equivalent statistically (p = 0.721) and they did not differ from K wire fixation (p = 0.178). Torsional rigidity of intact control bone was 1.045 N-m2/rad (standard deviation, 0.253).

Fig 5A–B.
Fig 5A–B.:
(A) The torsional rigidity and (B) failure torque with the values presented as means ± standard deviation are shown. SR = self-reinforced.

In terms of failure torque (Fig 5B), the bioabsorbable and 2.3-mm Ti platings were statistically equal (p = 0.510). Furthermore, the bioabsorbable platings had higher failure torque than 1.7-mm Ti plating (p = 0.008, versus dorsal self-reinforced P(L/DL)LA plating) or crossed K wires (p = 0.002, versus dorsal self-reinforced P(L/DL)LA plating). Failure torque of intact control bone was 5.89 N-m (standard deviation, 1.35 N-m).

DISCUSSION

After metallic fixation of hand fractures, satisfactory 4,9,11 and less impressive results, and postoperative recovery of function 2,17,24,25,27 have been reported. The most common adverse effects are joint stiffness, contractures, malunion, and nonunion. The major disadvantages of metallic plates are long-term interference with tendon excursion and tendon adhesion. 17,27

The consolidation of metacarpal and phalangeal fractures typically occurs in 4 to 7 weeks, 32 after which the osteosynthesis devices become unnecessary or even harmful. 17,27 Because bioabsorbable fixation materials are eliminated from the body through natural metabolic pathways, 1 the presence of permanent foreign material in the body can be avoided. The use of bioabsorbable devices offers obvious clinical advantages provided that they are available in forms offering secure and stable osteofixation.

In the current study, the 2-mm self-reinforced PLGA 80/20 and self-reinforced P(L/DL)LA 70/30 plating systems provided fixation stabilities comparable with the 1.7-mm Ti plating system in bending tests. In apex dorsal loading, the rigidity and maximum bending moment of dorsal bioabsorbable plating were higher than those of K wires. However, in apex palmar loading, K wire fixation was superior to any of the bioabsorbable or Ti plate fixation methods. The reason for this is the direction of force, which isolates and leaves the plate alone to carry the load. An interfragmentary screw considerably increases the rigidity of a plated configuration. 3 In torsion, the bioabsorbable platings were more rigid than 1.7-mm Ti plating, and no significant difference was found when comparing them with K wire fixation. Regarding failure torque, the bioabsorbable platings had values statistically comparable with that of the 2.3-mm Ti plating system.

By placing bioabsorbable miniplates laterally, interference with extensor apparatus can be avoided. Dorsolateral placement of the bioabsorbable plates increased rigidity in apex palmar loading but decreased stability in apex dorsal loading.

There has been much interest in the development of bioabsorbable fixation systems for hand surgery. Prevel et al 21 studied the biomechanics of nonreinforced PLGA (80% PLLA, 20% PGA, Lactosorb®) plates and screws used for fixation of transverse metacarpal osteotomies and compared the results with those of various Ti platings. This study can be criticized, because the investigators analyzed the biomechanical results of the Ti and bioabsorbable groups using relative values of different intact control bones. Because there were significant differences in the biomechanical properties of the control bones used in the bioabsorbable and Ti groups, comparison between these groups cannot be considered reliable. Bozic et al 6 evaluated the biomechanical properties of nonreinforced six-hole P(L/DL)LA 70/30 plates and 2.5-mm screws in a synthetic metacarpal bone model. Compared with Ti plate with 2-mm screws, these P(L/DL)LA plates were found to provide significantly lower stiffness (105 N/mm versus 273 N/mm) and failure load (314 N versus 925 N) values in apex dorsal bending.

Prevel et al 21 and Bozic et al 6 did not report the dimensions of the nonreinforced bioabsorbable plates used in their studies; however, Prevel and Bozic reported that the plate profiles were significantly higher than that of the self-reinforced plates used in the current study (written communication, CD Prevel, MD, KJ Bozic, MD, 2001). In loaded bones, the strength properties of nonreinforced devices are limited and they need to be manufactured as large thick implants to compensate for their brittleness and low strength. Comparison of the current data with those of Bozic et al 6 suggests that the self-reinforced manufacturing technique is crucial to produce small yet strong bioabsorbable fixation devices for use in hand fracture fixation.

Self-reinforced bioabsorbable osteosynthesis devices possess the highest strength and ductility of all bioabsorbable implants available for clinical use. The bioabsorbable plate fixations used in the current study failed through plate or bone cortex breaking, in approximately equal proportions. This is because the elasticity modulus of self-reinforced bioabsorbable devices is close to that of cortical bone. Therefore, the stress shielding associated with rigid metallic devices is avoided. Furthermore, the strength of bioabsorbable devices decreases while the healing process and rebuilding of normal bone structure takes place. The strength retention of self-reinforced PLGA 80/20 is from 6 to 8 weeks 20 and that of selfreinforced P(L/DL)LA 70/30 is from 18 to 36 weeks. 29 Complete absorption takes 1 to 3 years for self-reinforced PLGA 80/20 and 2 to 3 years for self-reinforced P(L/DL)LA 70/30.

Self-reinforced pins and screws are being used increasingly for stabilization of small fragment fractures in the upper limbs, 18,19,26 in the feet, 19,26 and around the ankles. 26 In most patients the postoperative clinical course is uneventful. Bioabsorbable fixation devices rarely are associated with a clinically manifested inflammatory foreign-body reaction during the degradation process. During the past few years, this complication has been observed in 5.3% of patients with self-reinforced PGA devices and rarely observed (0.2%) in patients with self-reinforced PLLA devices. 5 Such a complication is remote with the substantially amorphous, self-reinforced PLGA 80/20 and self-reinforced P(L/DL)LA 70/30, provided that the surgical technique is adequate, the area of implantation is well-vascularized, and a minimal amount of the material is used. The suitability of a PLGA sheet as a separating agent between extensor tendon and metacarpal bone after controlled trauma in monkeys has been studied. 8 The sheet degraded in 50 days, and the tissue reaction was minimal. Extensor tendon mobility was attained quickly and was better than in control hands without implants. Fibrosis and adhesions developed in the latter between the traumatized tendon and subjacent bone.

The use of bioabsorbable threads 7 and pins has been reported in the treatment of hand fractures. In a biomechanical study with proximal phalangeal osteotomies, self-reinforced pins provided comparable rigidity to K wires in bending and axial loading, but failed in torsion. 10 Uneventful functional recovery has been achieved using self-reinforced PLLA pins in four metacarpal and three phalangeal fractures. 19 Self-reinforced PGA and self-reinforced PLLA pins have been used in the fixation of eight metacarpal and five phalangeal fractures. 18 With a mean followup of more than 4 years, normal capacity for work was achieved in 12 patients and no complications were reported except for one minor redisplacement in a patient with a comminuted phalangeal fracture. In the same study, the outcome in patients with Bennett’s fractures was less favorable in two of five patients. Secure stabilization was achieved using self-reinforced PLLA and self-reinforced PGA pins in osteochondral and intraarticular metacarpal and phalangeal fractures in 46 patients. 14 Failure of union was seen in four patients. These failures were related to technical problems, not to the implants. No differences in functional outcome among 30 patients were observed when extraarticular hand fractures were stabilized either with intramedullary self-reinforced PGA pin or K wire, both fixations having a metallic wire loop. 15 A comparative study of nonreinforced PDS pins and K wires for fixation of hand fractures, arthrodeses, and osteotomies showed no differences in union time or in complications in 23 patients. 12 Additional operative procedures were needed in the K wire group more often than in the bioabsorbable pin group.

The advent of new self-reinforced PLGA 80/20 and self-reinforced P(L/DL)LA 70/30 miniplating systems makes it possible to expand the applications of bioabsorbable osteofixation devices in hand surgery. Self-reinforced miniplates have been used successfully for more than 10 years in craniomaxillofacial surgery and their strength retention properties and biocompatibility have been extensively validated. 1 In preliminary experiments (written communication, M Ninkovic MD, PhD, 2001), self-reinforced P(L/DL)LA 70/30 plating systems have met mechanical demands in clinical settings and provided reliable stability for metacarpal and phalangeal fractures and have resulted in uneventful healing during followups as many as 18 months. The use of self-reinforced bioabsorbable miniplate hand systems obviates the long-term interference of the plate with tendon gliding. Also, there is no need for a secondary removal operation, which carries a risk of damaging neurovascular structures.

Internal fixation with new low-profile self-reinforced PLGA 80/20 and self-reinforced P(L/DL)LA 70/30 plate and screw systems provides sufficient stability for metacarpal fixation. Early clinical experience of self-reinforced bioabsorbable plates suggests that they are useful. Prospective randomized clinical trials should be done to prove their clinical usefulness.

Acknowledgements

The authors thank Pasi Ohtonen, MSc, for doing the statistical analysis.

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