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Biomechanical Properties of Volar Hybrid and Locked Plate Fixation in Distal Radius Fractures
SCIENTIFIC ARTICLE
Biomechanical Properties of Volar Hybrid and Locked Plate Fixation in Distal Radius Fractures Shima C. Sokol, MD, Derek F. Amanatullah, MD, PhD, Shane Curtiss, AS, Robert M. Szabo, MD, MPH Purpose We compare the biomechanical properties of a volar hybrid construct to an alllocking construct in an osteoporotic and normal comminuted distal radius fracture model. Methods Groups of 28 normal, 28 osteoporotic, and 28 over-drilled osteoporotic left distal radius synthetic bones were used. The normal group consisted of synthetic bone with a standard foam core. The osteoporotic group consisted of synthetic bone with decreased foam core density. The over-drilled osteoporotic group consisted of synthetic bone with decreased foam core density and holes drilled with a 2.3 mm drill, instead of the standard 2.0 mm drill, to simulate the lack of purchase in osteoporotic bone. Within each group, 14 synthetic bones were plated with a volar locking plate using an all-locking screw construct, and 14 synthetic bones were plated with a volar locking plate using a hybrid screw construct (ie, both locking and nonlocking screws). A 1-cm dorsal wedge osteotomy was created with the apex 2 cm from the volar surface of the lunate facet. Each specimen was mounted to a materials testing machine, using a custom-built, standardized axial compression jig. Axial compression was delivered at 1 N/s over 3 cycles from 20 N to 100 N to establish stiffness. Each sample was stressed to failure at 1 mm/s until 5 mm of permanent deformation occurred. Results Our results show no difference in construct stiffness and load at failure between the all-locking and hybrid constructs in the normal, osteoporotic, or over-drilled osteoporotic synthetic bone models. All specimens failed by plate bending at the osteotomy site with loss of height. Clinical Relevance Although volar locking plates are commonly used for the treatment of distal radius fractures, the ideal screw configuration has not been determined. Hybrid fixation has comparable biomechanical properties to all locking constructs in the fixation of metaphyseal fractures about the knee and shoulder and might also have a role in the fixation of distal radius fractures. (J Hand Surg 2011;36A:591–597. Copyright © 2011 by the American Society for Surgery of the Hand. All rights reserved.) Key words Biomechanical, distal radius fracture, hybrid fixation, internal fixation, locking, volar plate. HE DISTAL RADIUS fracture is one of the most common fractures. The incidence of distal radius fractures is expected to increase by 50% by the year 2030 as the population in industrial countries continues to age and life expectancies increase.1 Osteoporosis contributed to an estimated 397,000 wrist frac-
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From the Department of Orthopaedic Surgery, University of California, Davis, Sacramento, CA. Received for publication August 17, 2010; accepted in revised form December 16, 2010. No benefits in any form have been received or will be received related directly or indirectly to the subject of this article. Medartis supplied all testing materials. Corresponding author: Robert M. Szabo, MD, MPH, University of California, Davis, School of Medicine, 4860 Y Street, Suite 3800, Sacramento, CA. 95817; e-mail: [email protected]. 0363-5023/11/36A04-0004$36.00/0 doi:10.1016/j.jhsa.2010.12.032
tures in 2005, according to the National Osteoporosis Foundation.2 The advent of locking plate technology has facilitated the treatment of complex fractures, particularly in cases involving severe comminution and poor bone quality. Locked volar plating for the treatment of dorsally angulated, comminuted distal radius fractures has the advantage of an easy surgical approach, less tendon irritation, and a decreased need for subsequent hardware removal.3–7 Willis and coworkers found that volar locking plates had greater stiffness under axial compression and dorsal bending than the same volar plate with nonlocking screws in a dorsally comminuted synthetic radius fracture model.8 Locking plates have been shown to exhibit significantly greater stiffness (p ⬍ .05) and strength
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(p ⬍ .001) against non-cyclic loads compared to nonlocking plates in comminuted fractures.9 Locking plates allow for hybrid fixation of the fracture meaning that both nonlocking and locking screws may be used. In hybrid plate fixation, nonlocking screws provide plate– bone compression and increase friction at the plate– bone interface. Locking screws are then used to stabilize the construct, particularly in comminuted fragments and osteoporotic bone.10 Hybrid constructs have shown similar biomechanical properties to all-locked constructs in the fixation of proximal tibia and osteoporotic humeral shaft fractures.10,11 Although combining screw types is common in surgical practice, to our knowledge, no studies have addressed hybrid fixation of distal radius fractures. The purpose of this study was to compare the biomechanical strength of a hybrid construct to an all-locking screw construct for the fixation of distal radius fractures in both a normal and osteoporotic model. METHODS For each model system (normal, osteoporotic, and overdrilled osteoporotic), 28 synthetic large, left, fourthgeneration radius synthetic bones (Sawbones, Pacific Research Laboratories, Vashon, WA) were randomized to either all-locking or hybrid fixation, with 14 synthetic bones in each group. We used a custom drill guide to place at least 2 drill holes proximal and distal to the site of the planned osteotomy to ensure that the left, narrow, short (23 ⫻ 43 mm), volar, 1.6-mm thick, titanium (ASTM F67) locking fracture plate (Medartis, Switzerland) was positioned in a standard fashion on the volar surface of each distal radius, just proximal to the volar lip. We used a 2.0-mm drill, according to the manufacturer’s recommendations, before inserting 2.5-mm, selftapping, cortical or locking screws. None of the locking screws were locked into place more than 3 times, per the manufacturer’s protocol. We applied the volar plates to the bone with screws placed in the predrilled holes, and then we drilled the remaining screw holes with a standard drill guide. All specimens were fixed proximally with 1 compression screw in the center of the oblong hole. We then placed a locking screw proximal and distal to this screw to obtain secure proximal fixation. The proximal locking screws were not tightened until all the distal screws were placed. Distally, we fixed each synthetic bone with either an all-locking screw or a hybrid construct (Fig. 1). For the hybrid construct, nonlocking screws were placed in the outer holes of the distal and proximal row of the distal fragment portion of the plate. The screws were tightened to compress the plate to the volar
FIGURE 1: Completed specimens. A All-locking construct with locking screws placed in all holes. B Hybrid construct with 4 nonlocking screws placed in the outer holes to compress the plate to bone, then 3 locking screws placed centrally in the distal fragment. Nonlocking screws are gold in color. Locking screws are blue.
FIGURE 2: Lateral view of plated specimens. A All-locking construct elevated off bone. B Hybrid construct compressed to bone.
surface of the distal radius (Fig 2). Next, we placed locking screws in the center hole in the proximal row and in the central 2 holes in the distal row. All screws were placed bicortically, attempting to keep all screws flush with the distal cortex. After the volar plate was applied, we created a 1-cm dorsal wedge osteotomy based on previously used fracture models.12 The osteotomy was created using a customized cutting jig, centered 2 cm proximal to the volar articular margin of the
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a concave plate that was mounted to the base of the material testing machine, thus centering the applied load and minimizing off-axis loading. A concave attachment was mounted to the distal radius, 1 cm dorsal to the volar lip, in line with the radial shaft to allow for articulation with the convex load cell attachment. The proximal radius was mounted on a sphere to allow for articulation with the concave mounting plate. Specimens were loaded in axial compression similar to previously published studies.12,16,17 We applied a 10 N preload before testing. Axial compression was delivered at 1 N/s over 3 cycles from 20 N to 100 N to determine axial stiffness. We loaded each specimen to failure in axial compression at a rate of 1 mm/min. Load versus displacement curves were generated from the cyclic ramp and failure load data. Stiffness was calculated as the slope of the linear portion of the load versus displacement curve between 20 N and 100 N or prior to the yield point during failure. The load at failure was the highest force value attained in the final load versus displacement curve. Failure was defined as 5 mm permanent deformation with axial compression. We visually inspected each specimen to determine mode of failure. FIGURE 3: Test specimen loaded in material testing machine.
lunate fossa. We then fractured the volar cortex manually. The all-locking and hybrid constructs were made using 3 different model systems: (1) a normal synthetic bone model, using the standard 17 pcf (0.27 g/cm3) polyurethane foam core; (2) an osteoporotic synthetic bone model, with a custom polyurethane foam core 10 pcf (0.16 g/cm3) to simulate the strength of an osteoporotic distal radius; and (3) an over-drilled osteoporotic synthetic bone model in which a 2.3-mm drill was used instead of 2.0-mm drill to decrease screw purchase in the osteoporotic bone model. Decreased foam core density and over-drilling of the cortices have been used in other studies focusing on the fixation of osteoporotic diaphyseal long bones.10,13–15 The cortical dimensions of the distal radius remained the same in all the model systems. All the specimens were prepared in an identical and systematic manner to the normal model except for the variation of foam core density and drill diameter. Each synthetic bone was transected at the distal third of the radial shaft. We mounted the specimens to a material testing machine (Instron, Norwood, MA) using a custom-built, standardized axial compression jig with 3 degrees of freedom at each end (Fig. 3). A convex attachment was secured to the load cell and matched to
Statistical analysis The mean stiffness and maximal load at failure were calculated for each group of 14 samples. Errors are reported as standard deviation for each mean. All comparisons between groups and model systems were made using a 2-tailed, unpaired t-test. A power analysis was used to determine that 14 specimens in each group would be sufficient to detect a difference of 300 N/mm between groups, assuming a standard deviation of 300 N/mm with a power of 80% and an alpha error of 0.05. RESULTS In the normal synthetic bone model, there was no statistically significant difference in the mean initial, second round, third round, or failure stiffness between the all-locking and hybrid constructs (Table 1). The failure load did not differ between the 2 constructs (Fig. 4). There was no statistically significant difference in any testing parameter between the hybrid and all-locking constructs when tested in the osteoporotic synthetic bone model (Table 1). In the over-drilled osteoporotic synthetic bone construct, there was no difference in the mean initial, second round, and third round stiffness between the 2 constructs (Table 1). There was no statistically significant difference in the failure stiffness and failure loads between the all-locking and hybrid constructs in this testing group (Table 1, Fig. 4).
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TABLE 1.
Average Stiffness of Each Fixation Method
Group
Initial (N/mm)
Round 2 (N/mm)
Round 3 (N/mm)
Failure (N/mm)
Normal 1089 ⫾ 284
1495 ⫾ 332
1526 ⫾ 343
826 ⫾ 343
935 ⫾ 172
1358 ⫾ 254
1397 ⫾ 264
755 ⫾ 204
Hybrid
958 ⫾ 192*
1340 ⫾ 250
1362 ⫾ 264
756 ⫾ 244
All-locked
911 ⫾ 261†
1282 ⫾ 225
1310 ⫾ 228
666 ⫾ 146
Hybrid All-locked Osteoporotic
Over-drilled osteoporotic Hybrid
708 ⫾ 160*‡
1171 ⫾ 190*
1196 ⫾ 201*
428 ⫾ 109*‡
All-locked
756 ⫾ 204†
1175 ⫾ 226
1202 ⫾ 232†
472 ⫾ 111†§
*Significant decrease compared to normal hybrid group (p ⬍ .05). †Significant decrease compared to normal all–locked group (p ⬍ .05). ‡Significant decrease compared to osteoporotic hybrid group (p ⬍ .05). §Significant decrease compared to osteoporotic all-locked group (p ⬍ .05).
FIGURE 4: Average load to failure for each fixation group. *, Significant difference in average load to failure between groups (p ⬍ .05).
There was a statistically significant difference in initial stiffness and failure stiffness between normal and over-drilled osteoporotic synthetic bone models when hybrid fixation was applied (p ⬍ .001, in both cases; Table 1, Fig. 5). The over-drilled osteoporotic synthetic bone model with hybrid fixation was also significantly less stiff after rounds 2 and 3 of testing when compared to the hybrid fixation in the normal synthetic bone model (p ⫽ .004 and .005, respectively; Table 1). Initial stiffness and failure stiffness were significantly decreased in the over-drilled osteoporotic synthetic bone model compared to the osteoporotic synthetic bone model (p ⬍ .001 in both cases; Table 1, Fig. 5). Other differences did not reach statistical significance. Failure loads for hybrid fixation in the osteoporotic (p ⫽ .043) and over-drilled osteoporotic synthetic bone models
(p ⬍ .001) were significantly less than hybrid fixation in the normal synthetic bone model (Fig. 4). Stiffness also decreased among the all-locking constructs depending on bone quality. Initial stiffness for the all-locking construct in the over-drilled osteoporotic synthetic bone model was significantly less than the all-locking construct in the normal synthetic bone model (p ⫽ .019, Fig. 5). Round 3 stiffness (p ⫽ .05) and failure stiffness (p ⬍ .001) were also significantly less for the over-drilled osteoporotic synthetic bone model than for the normal synthetic bone model when all-locking fixation was used (Table 1). Compared to the osteoporotic synthetic bone model, failure stiffness in the over-drilled osteoporotic synthetic bone model was also significantly less with an all-locking construct (p ⫽ .001; Table 1). Failure loads, however, did not
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FIGURE 5: Average initial stiffness of each fixation group. *, Significant difference in average initial stiffness between groups (p ⬍ .05).
vary significantly among the groups (Fig. 4). All specimens failed by plate bending and loss of height at the osteotomy site. No specimens failed by screw pullout or breakage. DISCUSSION Volar locking plates allow for the placement of either locking or nonlocking screws. Although volar locking plates are commonly used, studies detailing the ideal screw configuration of the distal segment are lacking. Weninger and coauthors reported that a locking screw inserted immediately proximal to the fracture site improved mechanical stability in a comminuted distal radius synthetic bone fracture model.18 Recently, Mehling and co-authors reported that filling all the distal holes with locking screws produced a stronger construct than filling only one row, either proximal or distal.19 The authors recommended inserting at least 4 locking screws in the distal fragment, with at least 2 in the most distal row. Our study found that a hybrid construct with only 3 locking screws in combination with nonlocking screws provided equal fixation to an all-locked construct for an extra-articular, comminuted distal radius fracture model, regardless of bone quality or screw purchase. A hybrid configuration allows for compression fixation and fixed angle stabilization. Using nonlocking screws before adding locking screws might strengthen the construct by first bringing the plate closer to bone. A locking plate that lies elevated off the bone has reduced torsional strength and stiffness.20 –22 Studies have shown that hybrid fixation has similar biomechanical properties as all-locking constructs. Hybrid fixation of cadaver proximal tibia fractures showed no statistically significant difference in deformation compared to an all-locking construct. This study was performed in
specimens with normal bone density, tested in cyclic axial loading.11 Gardner and colleagues demonstrated that hybrid constructs had similar biomechanical properties to all-locking constructs, using an osteoporotic humeral shaft synthetic bone model.10 Our results support the findings of Gardner and associates, that switching nonlocking screws for more expensive locking screws, after the plate is compressed to bone, is not necessary.10 We did note a trend toward improved initial and failure stiffness in the all-locking construct compared to the hybrid construct in the over-drilled osteoporotic synthetic bone model, although this difference was not statistically significant. Alternatively, the amount of compression obtained with the hybrid configuration might not be substantial enough to improve stability. The volar cortex of the distal radius does not precisely match the contour of the volar locking plate, making tight compression of the plate onto bone impossible. As a result, although the hybrid-fixed plate lies closer to the bone than an alllocking construct, a tight friction fit might be lacking to improve stability of a hybrid construct (Fig. 2). To standardize the study groups, we inserted all locking screws bicortically. In practice, we routinely use unicortical screws in distal radius fractures to avoid extensor tendon complications.23–27 Furthermore, the dorsal cortex, which is thin and comminuted in these injuries, is believed to provide little additional stability. A recent presentation (Greenberg and Izadi) at the 2010 American Society for Surgery of the Hand meeting demonstrated that locking screws placed 75% of the distance to the dorsal cortex did not compromise stability when compared to screws placed to the distal cortex or bicortically. These results were not available before developing our study design. However, this study did not evaluate fixation with nonlocking screws,
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and adequate plate– bone compression requires that nonlocking screws engage the far cortex. This is a potential disadvantage of hybrid fixation instead of an all-locking construct with unicortical fixation. Adding a limited number of bicortical, nonlocking screws placed flush with the far cortex might be a reasonable alternative for stabilizing comminuted distal radius fracture without causing extensor tendon irritation. Flush screw placement can be obtained by drilling the proximal cortex just up to the distal cortex, measuring this distance, and then drilling the distal cortex. A screw 1 to 2 mm longer than the measured distance is selected. Given the complex geometry of the dorsal cortex of the distal radius, the relationship of the screw tip to the dorsal cortex might be difficult to determine intraoperatively. In addition to fluoroscopy with multiple views, a limited dorsal incision can be used to evaluate screw penetration. Unlike the femur, the distal radius has no validated model of osteoporosis.13 Our osteoporotic synthetic bone model showed decreased failure loads compared to our normal synthetic bone model with hybrid fixation. Decreases in all other parameters were not statistically significant in the osteoporotic synthetic bone model. When alllocking fixation was used, only failure stiffness reduction was statistically significant in the osteoporotic synthetic bone model when compared to the normal synthetic bone model. The lack of difference between the osteoporotic and normal synthetic bone models with either fixation method suggests that our initial model for osteoporosis in the distal radius was not adequate. Although the foam core had decreased density in our osteoporotic synthetic bone model, the cortices of the 2 different models did not differ. Osteoporotic bone can have substantial cortical thinning, which limits fixation of unlocked screws compared to locking screws.10,28 The distal radius is composed mostly of trabecular bone, but studies have suggested that cortical area contributes significantly to its mechanical strength.29 Therefore, we created another test group in which both cortices of the osteoporotic synthetic bone were drilled with a 2.3-mm drill instead of the conventional 2.0-mm drill. Over-drilling of the cortices has been used in other studies focusing on fixation of osteoporotic bone.10,14,15 Gardner and coauthors over-drilled to 0.3 mm less than the diameter of the screw to simulate purchase in an osteoporotic humeral shaft fracture model.10 Spivak and coauthors showed 4.1-mm transpedicular screws predrilled with a
3.9-mm drill had up to 60% decrease in initial pull-out strength when compared to the same screws predrilled with a 3.1-mm drill.14 In our study, we drilled the cortices using a 2.3-mm drill instead of the recommended 2.0-mm drill. This led to a noticeable decrease in screw purchase, particularly in the nonlocking screws. Our over-drilled osteoporotic synthetic bone model showed significantly decreased stiffness and failure loads when compared to our normal synthetic bone model when hybrid or all-locking fixation was applied (Fig. 5). Further investigation is necessary to determine whether over-drilling a synthetic bone model with decreased foam core density is an adequate model for testing fixation in osteoporotic bone. There were several weaknesses of our study. Because of the wide variability in bone quality of cadaver specimens, we used synthetic bone to allow for a more consistent model. The fourthgeneration synthetic bone model replicates the biomechanical properties of natural bone and has been validated for use in the femur and tibia but not in the radius.30,31 The exact biomechanical properties of the fourth-generation synthetic bone radius model and the correlation of a radius synthetic bone model to normal or osteoporotic bone have not been conducted. Our study tests only the initial stiffness of fixation and does not take into consideration the biological aspects of bone healing. We used a dorsal wedge osteotomy to simulate a comminuted distal radius fracture model. This fracture pattern is unstable dorsally, leading to loss of volar tilt if not treated with stable fixation. This fracture model is also similar to a dorsal opening wedge osteotomy used to restore volar tilt in distal radius malunions.17 The results of our study, however, cannot be applied directly to intra-articular fractures, which might benefit more from fixation with angular stability. We also did not test cyclic loading, which might have demonstrated different properties of locking and nonlocking fixation. Specimens were loaded to failure in axial compression similar to previously published studies.12,16,17 We believe that hybrid fixation with only 3 locking screws in the distal fragment might provide equivalent biomechanical strength and stiffness under axial load to an all-locking construct for the treatment of extraarticular, comminuted distal radius fractures, regardless of bone quality and screw purchase. Our results suggest that nonlocking screws used to compress the plate to bone need not necessarily be changed for more expen-
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sive locking screws. Using bicortical, nonlocking screws to compress the plate to bone, however, does not appear to provide any biomechanical advantage. This might increase the risk of extensor tendon injury by using longer nonlocking screws to achieve adequate fixation and compression in poor-quality bone.
16.
17.
REFERENCES 1. Figl M, Weninger P, Jurkowitsch J, Hofbauer M, Schauer J, Leixnering M. Unstable distal radius fractures in the elderly patient— volar fixed-angle plate osteosynthesis prevents secondary loss of reduction. J Trauma 2010;68:992–998. 2. National Osteoporosis Foundation. Fast Facts: Fractures. Available at: http://www.nof.org/node/40. Accessed July 10, 2010. 3. Jupiter JB, Fernandez DL, Toh CL, Fellman T Ring D. Operative treatment of volar intra-articular fractures of the distal end of the radius. J Bone Joint Surg 1996;78A:1817–1828. 4. Drobetz H Kutscha-Lissberg E. Osteosynthesis of distal radial fractures with a volar locking screw plate system. Int Orthop 2003;27:1– 6. 5. Orbay JL, Fernandez DL. Volar fixed-angle plate fixation for unstable distal radius fractures in the elderly patient. J Hand Surg 2004; 29A:96 –102. 6. Orbay JL, Fernandez DL. Volar fixation for dorsally displaced fractures of the distal radius: a preliminary report. J Hand Surg 2002;27A:205–215. 7. Kamano M, Honda Y, Kazuki K, Yasuda M. Palmar plating for dorsally displaced fractures of the distal radius. Clin Orthop Relat Res 2002;397:403– 408. 8. Willis AA, Kutsumi K, Zobitz ME, Cooney WP III. Internal fixation of dorsally displaced fractures of the distal part of the radius. A biomechanical analysis of volar plate fracture stability. J Bone Joint Surg 2006;88A:2411–2417. 9. Boswell S, McIff TE, Trease CA, Toby EB. Mechanical characteristics of locking and compression plate constructs applied dorsally to distal radius fractures. J Hand Surg 2007;32A:623– 629. 10. Gardner MJ, Griffith MH, Demetrakopoulos D, Brophy RH, Grose A, Helfet DL, Lorich DG. Hybrid locked plating of osteoporotic fractures of the humerus. J Bone Joint Surg 2006;88A:1962–1967. 11. Estes C, Rhee P, Shrader MW, Csavina K, Jacofsky MC, Jacofsky DJ. Biomechanical strength of the Peri-Loc proximal tibial plate: a comparison of all-locked versus hybrid locked/nonlocked screw configurations. J Orthop Trauma 2008;22:312–316. 12. Koh S, Morris RP, Patterson RM, Kearney JP, Buford WL Jr, Viegas SF. Volar fixation for dorsally angulated extra-articular fractures of the distal radius: a biomechanical study. J Hand Surg 2006;31A: 771–779. 13. Sommers MB, Fitzpatrick DC, Madey SM, Vande Zanderschulp C, Bottlang M. A surrogate long-bone model with osteoporotic material properties for biomechanical testing of fracture implants. J Biomech 2007;40:3297–3304. 14. Spivak JM, Neuwirth MG, Labiak JJ, Kummer FJ, Ricci JL. Hydroxyapatite enhancement of posterior spinal instrumentation fixation. Spine (Phila PA 1976) 1994;19:955–964. 15. Jazrawi LM, Bai B, Simon JA, Kummer FJ, Birdzell LT, Koval KJ.
18.
19.
20.
21.
22.
23. 24.
25.
26.
27. 28.
29.
30.
31.
597
A biomechanical comparison of Schuhli nuts or cement augmented screws for plating of humeral fractures. Clin Orthop Relat Res 2000;377:235–240. Leung F, Zhu L, Ho H, Lu WW, Chow SP. Palmar plate fixation of AO type C2 fracture of distal radius using a locking compression plate—a biomechanical study in a cadaveric model. J Hand Surg 2003;28B:263–266. McCall TA, Conrad B, Badman B, Wright T. Volar versus dorsal fixed-angle fixation of dorsally unstable extra-articular distal radius fractures: a biomechanic study. J Hand Surg 2007;32A:806 – 812. Weninger P, Schueller M, Drobetz H, Jamek M, Redl H, Tschegg E. Influence of an additional locking screw on fracture reduction after volar fixed-angle plating-introduction of the “protection screw” in an extra-articular distal radius fracture model. J Trauma 2009;67:746 – 751. Mehling I, Muller LP, Delinsky K, Mehler D, Burkhart KJ, Rommens PM. Number and locations of screw fixation for volar fixedangle plating of distal radius fractures: biomechanical study. J Hand Surg 2010;35A:885– 891. Kowalski MJ, Schemitsch EH, Harrington RM, Chapman JR, Swiontkowski MF. A comparative biomechanical evaluation of a noncontacting plate and currently used devices for tibial fixation. J Trauma 1996;40:5–9. Stoffel K, Dieter U, Stachowiak G, Gachter A, Kuster MS. Biomechanical testing of the LCP— how can stability in locked internal fixators be controlled? Injury 2003;34 Suppl 2:B11–19. Fitzpatrick DC, Doornink J, Madey SM Bottlang M. Relative stability of conventional and locked plating fixation in a model of the osteoporotic femoral diaphysis. Clin Biomech (Bristol, Avon) 2009; 24:203–209. Rampoldi M, Marsico S. Complications of volar plating of distal radius fractures. Acta Orthop Belg 2007;73:714 –719. Arora R, Lutz M, Hennerbichler A, Krappinger D, Espen D, Gabl M. Complications following internal fixation of unstable distal radius fracture with a palmar locking-plate. J Orthop Trauma 2007;21:316 – 322. Benson EC, DeCarvalho A, Mikola EA, Veitch JM, Moneim MS. Two potential causes of EPL rupture after distal radius volar plate fixation. Clin Orthop Relat Res 2006;451:218 –222. Hattori Y, Doi K, Sakamoto S, Yukata K. Delayed rupture of extensor digitorum communis tendon following volar plating of distal radius fracture. Hand Surg 2008;13:183–185. Protopsaltis TS, Ruch DS. Volar approach to distal radius fractures. J Hand Surg 2008;33A:958 –965. Power J, Loveridge N, Lyon A, Rushton N, Parker M, Reeve J. Bone remodeling at the endocortical surface of the human femoral neck: a mechanism for regional cortical thinning in cases of hip fracture. J Bone Miner Res 2003;18:1775–1780. Spadaro JA, Werner FW, Brenner RA, Fortino MD, Fay LA, Edwards WT. Cortical and trabecular bone contribute strength to the osteopenic distal radius. J Orthop Res 1994;12:211–218. Cristofolini L, Viceconti M, Cappello A, Toni A. Mechanical validation of whole bone composite femur models. J Biomech 1996;29: 525–535. Heiner AD, Brown TD. Structural properties of a new design of composite replicate femurs and tibias. J Biomech 2001;34:773–781.
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Biomechanical Properties of Volar Hybrid and Locked Plate Fixation in Distal Radius Fractures Shima C. Sokol, MD, Derek F. Amanatullah, MD, PhD, Shane Curtiss, AS, Robert M. Szabo, MD, MPH Purpose We compare the biomechanical properties of a volar hybrid construct to an alllocking construct in an osteoporotic and normal comminuted distal radius fracture model. Methods Groups of 28 normal, 28 osteoporotic, and 28 over-drilled osteoporotic left distal radius synthetic bones were used. The normal group consisted of synthetic bone with a standard foam core. The osteoporotic group consisted of synthetic bone with decreased foam core density. The over-drilled osteoporotic group consisted of synthetic bone with decreased foam core density and holes drilled with a 2.3 mm drill, instead of the standard 2.0 mm drill, to simulate the lack of purchase in osteoporotic bone. Within each group, 14 synthetic bones were plated with a volar locking plate using an all-locking screw construct, and 14 synthetic bones were plated with a volar locking plate using a hybrid screw construct (ie, both locking and nonlocking screws). A 1-cm dorsal wedge osteotomy was created with the apex 2 cm from the volar surface of the lunate facet. Each specimen was mounted to a materials testing machine, using a custom-built, standardized axial compression jig. Axial compression was delivered at 1 N/s over 3 cycles from 20 N to 100 N to establish stiffness. Each sample was stressed to failure at 1 mm/s until 5 mm of permanent deformation occurred. Results Our results show no difference in construct stiffness and load at failure between the all-locking and hybrid constructs in the normal, osteoporotic, or over-drilled osteoporotic synthetic bone models. All specimens failed by plate bending at the osteotomy site with loss of height. Clinical Relevance Although volar locking plates are commonly used for the treatment of distal radius fractures, the ideal screw configuration has not been determined. Hybrid fixation has comparable biomechanical properties to all locking constructs in the fixation of metaphyseal fractures about the knee and shoulder and might also have a role in the fixation of distal radius fractures. (J Hand Surg 2011;36A:591–597. Copyright © 2011 by the American Society for Surgery of the Hand. All rights reserved.) Key words Biomechanical, distal radius fracture, hybrid fixation, internal fixation, locking, volar plate. HE DISTAL RADIUS fracture is one of the most common fractures. The incidence of distal radius fractures is expected to increase by 50% by the year 2030 as the population in industrial countries continues to age and life expectancies increase.1 Osteoporosis contributed to an estimated 397,000 wrist frac-
T
From the Department of Orthopaedic Surgery, University of California, Davis, Sacramento, CA. Received for publication August 17, 2010; accepted in revised form December 16, 2010. No benefits in any form have been received or will be received related directly or indirectly to the subject of this article. Medartis supplied all testing materials. Corresponding author: Robert M. Szabo, MD, MPH, University of California, Davis, School of Medicine, 4860 Y Street, Suite 3800, Sacramento, CA. 95817; e-mail: [email protected]. 0363-5023/11/36A04-0004$36.00/0 doi:10.1016/j.jhsa.2010.12.032
tures in 2005, according to the National Osteoporosis Foundation.2 The advent of locking plate technology has facilitated the treatment of complex fractures, particularly in cases involving severe comminution and poor bone quality. Locked volar plating for the treatment of dorsally angulated, comminuted distal radius fractures has the advantage of an easy surgical approach, less tendon irritation, and a decreased need for subsequent hardware removal.3–7 Willis and coworkers found that volar locking plates had greater stiffness under axial compression and dorsal bending than the same volar plate with nonlocking screws in a dorsally comminuted synthetic radius fracture model.8 Locking plates have been shown to exhibit significantly greater stiffness (p ⬍ .05) and strength
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(p ⬍ .001) against non-cyclic loads compared to nonlocking plates in comminuted fractures.9 Locking plates allow for hybrid fixation of the fracture meaning that both nonlocking and locking screws may be used. In hybrid plate fixation, nonlocking screws provide plate– bone compression and increase friction at the plate– bone interface. Locking screws are then used to stabilize the construct, particularly in comminuted fragments and osteoporotic bone.10 Hybrid constructs have shown similar biomechanical properties to all-locked constructs in the fixation of proximal tibia and osteoporotic humeral shaft fractures.10,11 Although combining screw types is common in surgical practice, to our knowledge, no studies have addressed hybrid fixation of distal radius fractures. The purpose of this study was to compare the biomechanical strength of a hybrid construct to an all-locking screw construct for the fixation of distal radius fractures in both a normal and osteoporotic model. METHODS For each model system (normal, osteoporotic, and overdrilled osteoporotic), 28 synthetic large, left, fourthgeneration radius synthetic bones (Sawbones, Pacific Research Laboratories, Vashon, WA) were randomized to either all-locking or hybrid fixation, with 14 synthetic bones in each group. We used a custom drill guide to place at least 2 drill holes proximal and distal to the site of the planned osteotomy to ensure that the left, narrow, short (23 ⫻ 43 mm), volar, 1.6-mm thick, titanium (ASTM F67) locking fracture plate (Medartis, Switzerland) was positioned in a standard fashion on the volar surface of each distal radius, just proximal to the volar lip. We used a 2.0-mm drill, according to the manufacturer’s recommendations, before inserting 2.5-mm, selftapping, cortical or locking screws. None of the locking screws were locked into place more than 3 times, per the manufacturer’s protocol. We applied the volar plates to the bone with screws placed in the predrilled holes, and then we drilled the remaining screw holes with a standard drill guide. All specimens were fixed proximally with 1 compression screw in the center of the oblong hole. We then placed a locking screw proximal and distal to this screw to obtain secure proximal fixation. The proximal locking screws were not tightened until all the distal screws were placed. Distally, we fixed each synthetic bone with either an all-locking screw or a hybrid construct (Fig. 1). For the hybrid construct, nonlocking screws were placed in the outer holes of the distal and proximal row of the distal fragment portion of the plate. The screws were tightened to compress the plate to the volar
FIGURE 1: Completed specimens. A All-locking construct with locking screws placed in all holes. B Hybrid construct with 4 nonlocking screws placed in the outer holes to compress the plate to bone, then 3 locking screws placed centrally in the distal fragment. Nonlocking screws are gold in color. Locking screws are blue.
FIGURE 2: Lateral view of plated specimens. A All-locking construct elevated off bone. B Hybrid construct compressed to bone.
surface of the distal radius (Fig 2). Next, we placed locking screws in the center hole in the proximal row and in the central 2 holes in the distal row. All screws were placed bicortically, attempting to keep all screws flush with the distal cortex. After the volar plate was applied, we created a 1-cm dorsal wedge osteotomy based on previously used fracture models.12 The osteotomy was created using a customized cutting jig, centered 2 cm proximal to the volar articular margin of the
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a concave plate that was mounted to the base of the material testing machine, thus centering the applied load and minimizing off-axis loading. A concave attachment was mounted to the distal radius, 1 cm dorsal to the volar lip, in line with the radial shaft to allow for articulation with the convex load cell attachment. The proximal radius was mounted on a sphere to allow for articulation with the concave mounting plate. Specimens were loaded in axial compression similar to previously published studies.12,16,17 We applied a 10 N preload before testing. Axial compression was delivered at 1 N/s over 3 cycles from 20 N to 100 N to determine axial stiffness. We loaded each specimen to failure in axial compression at a rate of 1 mm/min. Load versus displacement curves were generated from the cyclic ramp and failure load data. Stiffness was calculated as the slope of the linear portion of the load versus displacement curve between 20 N and 100 N or prior to the yield point during failure. The load at failure was the highest force value attained in the final load versus displacement curve. Failure was defined as 5 mm permanent deformation with axial compression. We visually inspected each specimen to determine mode of failure. FIGURE 3: Test specimen loaded in material testing machine.
lunate fossa. We then fractured the volar cortex manually. The all-locking and hybrid constructs were made using 3 different model systems: (1) a normal synthetic bone model, using the standard 17 pcf (0.27 g/cm3) polyurethane foam core; (2) an osteoporotic synthetic bone model, with a custom polyurethane foam core 10 pcf (0.16 g/cm3) to simulate the strength of an osteoporotic distal radius; and (3) an over-drilled osteoporotic synthetic bone model in which a 2.3-mm drill was used instead of 2.0-mm drill to decrease screw purchase in the osteoporotic bone model. Decreased foam core density and over-drilling of the cortices have been used in other studies focusing on the fixation of osteoporotic diaphyseal long bones.10,13–15 The cortical dimensions of the distal radius remained the same in all the model systems. All the specimens were prepared in an identical and systematic manner to the normal model except for the variation of foam core density and drill diameter. Each synthetic bone was transected at the distal third of the radial shaft. We mounted the specimens to a material testing machine (Instron, Norwood, MA) using a custom-built, standardized axial compression jig with 3 degrees of freedom at each end (Fig. 3). A convex attachment was secured to the load cell and matched to
Statistical analysis The mean stiffness and maximal load at failure were calculated for each group of 14 samples. Errors are reported as standard deviation for each mean. All comparisons between groups and model systems were made using a 2-tailed, unpaired t-test. A power analysis was used to determine that 14 specimens in each group would be sufficient to detect a difference of 300 N/mm between groups, assuming a standard deviation of 300 N/mm with a power of 80% and an alpha error of 0.05. RESULTS In the normal synthetic bone model, there was no statistically significant difference in the mean initial, second round, third round, or failure stiffness between the all-locking and hybrid constructs (Table 1). The failure load did not differ between the 2 constructs (Fig. 4). There was no statistically significant difference in any testing parameter between the hybrid and all-locking constructs when tested in the osteoporotic synthetic bone model (Table 1). In the over-drilled osteoporotic synthetic bone construct, there was no difference in the mean initial, second round, and third round stiffness between the 2 constructs (Table 1). There was no statistically significant difference in the failure stiffness and failure loads between the all-locking and hybrid constructs in this testing group (Table 1, Fig. 4).
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TABLE 1.
Average Stiffness of Each Fixation Method
Group
Initial (N/mm)
Round 2 (N/mm)
Round 3 (N/mm)
Failure (N/mm)
Normal 1089 ⫾ 284
1495 ⫾ 332
1526 ⫾ 343
826 ⫾ 343
935 ⫾ 172
1358 ⫾ 254
1397 ⫾ 264
755 ⫾ 204
Hybrid
958 ⫾ 192*
1340 ⫾ 250
1362 ⫾ 264
756 ⫾ 244
All-locked
911 ⫾ 261†
1282 ⫾ 225
1310 ⫾ 228
666 ⫾ 146
Hybrid All-locked Osteoporotic
Over-drilled osteoporotic Hybrid
708 ⫾ 160*‡
1171 ⫾ 190*
1196 ⫾ 201*
428 ⫾ 109*‡
All-locked
756 ⫾ 204†
1175 ⫾ 226
1202 ⫾ 232†
472 ⫾ 111†§
*Significant decrease compared to normal hybrid group (p ⬍ .05). †Significant decrease compared to normal all–locked group (p ⬍ .05). ‡Significant decrease compared to osteoporotic hybrid group (p ⬍ .05). §Significant decrease compared to osteoporotic all-locked group (p ⬍ .05).
FIGURE 4: Average load to failure for each fixation group. *, Significant difference in average load to failure between groups (p ⬍ .05).
There was a statistically significant difference in initial stiffness and failure stiffness between normal and over-drilled osteoporotic synthetic bone models when hybrid fixation was applied (p ⬍ .001, in both cases; Table 1, Fig. 5). The over-drilled osteoporotic synthetic bone model with hybrid fixation was also significantly less stiff after rounds 2 and 3 of testing when compared to the hybrid fixation in the normal synthetic bone model (p ⫽ .004 and .005, respectively; Table 1). Initial stiffness and failure stiffness were significantly decreased in the over-drilled osteoporotic synthetic bone model compared to the osteoporotic synthetic bone model (p ⬍ .001 in both cases; Table 1, Fig. 5). Other differences did not reach statistical significance. Failure loads for hybrid fixation in the osteoporotic (p ⫽ .043) and over-drilled osteoporotic synthetic bone models
(p ⬍ .001) were significantly less than hybrid fixation in the normal synthetic bone model (Fig. 4). Stiffness also decreased among the all-locking constructs depending on bone quality. Initial stiffness for the all-locking construct in the over-drilled osteoporotic synthetic bone model was significantly less than the all-locking construct in the normal synthetic bone model (p ⫽ .019, Fig. 5). Round 3 stiffness (p ⫽ .05) and failure stiffness (p ⬍ .001) were also significantly less for the over-drilled osteoporotic synthetic bone model than for the normal synthetic bone model when all-locking fixation was used (Table 1). Compared to the osteoporotic synthetic bone model, failure stiffness in the over-drilled osteoporotic synthetic bone model was also significantly less with an all-locking construct (p ⫽ .001; Table 1). Failure loads, however, did not
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FIGURE 5: Average initial stiffness of each fixation group. *, Significant difference in average initial stiffness between groups (p ⬍ .05).
vary significantly among the groups (Fig. 4). All specimens failed by plate bending and loss of height at the osteotomy site. No specimens failed by screw pullout or breakage. DISCUSSION Volar locking plates allow for the placement of either locking or nonlocking screws. Although volar locking plates are commonly used, studies detailing the ideal screw configuration of the distal segment are lacking. Weninger and coauthors reported that a locking screw inserted immediately proximal to the fracture site improved mechanical stability in a comminuted distal radius synthetic bone fracture model.18 Recently, Mehling and co-authors reported that filling all the distal holes with locking screws produced a stronger construct than filling only one row, either proximal or distal.19 The authors recommended inserting at least 4 locking screws in the distal fragment, with at least 2 in the most distal row. Our study found that a hybrid construct with only 3 locking screws in combination with nonlocking screws provided equal fixation to an all-locked construct for an extra-articular, comminuted distal radius fracture model, regardless of bone quality or screw purchase. A hybrid configuration allows for compression fixation and fixed angle stabilization. Using nonlocking screws before adding locking screws might strengthen the construct by first bringing the plate closer to bone. A locking plate that lies elevated off the bone has reduced torsional strength and stiffness.20 –22 Studies have shown that hybrid fixation has similar biomechanical properties as all-locking constructs. Hybrid fixation of cadaver proximal tibia fractures showed no statistically significant difference in deformation compared to an all-locking construct. This study was performed in
specimens with normal bone density, tested in cyclic axial loading.11 Gardner and colleagues demonstrated that hybrid constructs had similar biomechanical properties to all-locking constructs, using an osteoporotic humeral shaft synthetic bone model.10 Our results support the findings of Gardner and associates, that switching nonlocking screws for more expensive locking screws, after the plate is compressed to bone, is not necessary.10 We did note a trend toward improved initial and failure stiffness in the all-locking construct compared to the hybrid construct in the over-drilled osteoporotic synthetic bone model, although this difference was not statistically significant. Alternatively, the amount of compression obtained with the hybrid configuration might not be substantial enough to improve stability. The volar cortex of the distal radius does not precisely match the contour of the volar locking plate, making tight compression of the plate onto bone impossible. As a result, although the hybrid-fixed plate lies closer to the bone than an alllocking construct, a tight friction fit might be lacking to improve stability of a hybrid construct (Fig. 2). To standardize the study groups, we inserted all locking screws bicortically. In practice, we routinely use unicortical screws in distal radius fractures to avoid extensor tendon complications.23–27 Furthermore, the dorsal cortex, which is thin and comminuted in these injuries, is believed to provide little additional stability. A recent presentation (Greenberg and Izadi) at the 2010 American Society for Surgery of the Hand meeting demonstrated that locking screws placed 75% of the distance to the dorsal cortex did not compromise stability when compared to screws placed to the distal cortex or bicortically. These results were not available before developing our study design. However, this study did not evaluate fixation with nonlocking screws,
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and adequate plate– bone compression requires that nonlocking screws engage the far cortex. This is a potential disadvantage of hybrid fixation instead of an all-locking construct with unicortical fixation. Adding a limited number of bicortical, nonlocking screws placed flush with the far cortex might be a reasonable alternative for stabilizing comminuted distal radius fracture without causing extensor tendon irritation. Flush screw placement can be obtained by drilling the proximal cortex just up to the distal cortex, measuring this distance, and then drilling the distal cortex. A screw 1 to 2 mm longer than the measured distance is selected. Given the complex geometry of the dorsal cortex of the distal radius, the relationship of the screw tip to the dorsal cortex might be difficult to determine intraoperatively. In addition to fluoroscopy with multiple views, a limited dorsal incision can be used to evaluate screw penetration. Unlike the femur, the distal radius has no validated model of osteoporosis.13 Our osteoporotic synthetic bone model showed decreased failure loads compared to our normal synthetic bone model with hybrid fixation. Decreases in all other parameters were not statistically significant in the osteoporotic synthetic bone model. When alllocking fixation was used, only failure stiffness reduction was statistically significant in the osteoporotic synthetic bone model when compared to the normal synthetic bone model. The lack of difference between the osteoporotic and normal synthetic bone models with either fixation method suggests that our initial model for osteoporosis in the distal radius was not adequate. Although the foam core had decreased density in our osteoporotic synthetic bone model, the cortices of the 2 different models did not differ. Osteoporotic bone can have substantial cortical thinning, which limits fixation of unlocked screws compared to locking screws.10,28 The distal radius is composed mostly of trabecular bone, but studies have suggested that cortical area contributes significantly to its mechanical strength.29 Therefore, we created another test group in which both cortices of the osteoporotic synthetic bone were drilled with a 2.3-mm drill instead of the conventional 2.0-mm drill. Over-drilling of the cortices has been used in other studies focusing on fixation of osteoporotic bone.10,14,15 Gardner and coauthors over-drilled to 0.3 mm less than the diameter of the screw to simulate purchase in an osteoporotic humeral shaft fracture model.10 Spivak and coauthors showed 4.1-mm transpedicular screws predrilled with a
3.9-mm drill had up to 60% decrease in initial pull-out strength when compared to the same screws predrilled with a 3.1-mm drill.14 In our study, we drilled the cortices using a 2.3-mm drill instead of the recommended 2.0-mm drill. This led to a noticeable decrease in screw purchase, particularly in the nonlocking screws. Our over-drilled osteoporotic synthetic bone model showed significantly decreased stiffness and failure loads when compared to our normal synthetic bone model when hybrid or all-locking fixation was applied (Fig. 5). Further investigation is necessary to determine whether over-drilling a synthetic bone model with decreased foam core density is an adequate model for testing fixation in osteoporotic bone. There were several weaknesses of our study. Because of the wide variability in bone quality of cadaver specimens, we used synthetic bone to allow for a more consistent model. The fourthgeneration synthetic bone model replicates the biomechanical properties of natural bone and has been validated for use in the femur and tibia but not in the radius.30,31 The exact biomechanical properties of the fourth-generation synthetic bone radius model and the correlation of a radius synthetic bone model to normal or osteoporotic bone have not been conducted. Our study tests only the initial stiffness of fixation and does not take into consideration the biological aspects of bone healing. We used a dorsal wedge osteotomy to simulate a comminuted distal radius fracture model. This fracture pattern is unstable dorsally, leading to loss of volar tilt if not treated with stable fixation. This fracture model is also similar to a dorsal opening wedge osteotomy used to restore volar tilt in distal radius malunions.17 The results of our study, however, cannot be applied directly to intra-articular fractures, which might benefit more from fixation with angular stability. We also did not test cyclic loading, which might have demonstrated different properties of locking and nonlocking fixation. Specimens were loaded to failure in axial compression similar to previously published studies.12,16,17 We believe that hybrid fixation with only 3 locking screws in the distal fragment might provide equivalent biomechanical strength and stiffness under axial load to an all-locking construct for the treatment of extraarticular, comminuted distal radius fractures, regardless of bone quality and screw purchase. Our results suggest that nonlocking screws used to compress the plate to bone need not necessarily be changed for more expen-
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sive locking screws. Using bicortical, nonlocking screws to compress the plate to bone, however, does not appear to provide any biomechanical advantage. This might increase the risk of extensor tendon injury by using longer nonlocking screws to achieve adequate fixation and compression in poor-quality bone.
16.
17.
REFERENCES 1. Figl M, Weninger P, Jurkowitsch J, Hofbauer M, Schauer J, Leixnering M. Unstable distal radius fractures in the elderly patient— volar fixed-angle plate osteosynthesis prevents secondary loss of reduction. J Trauma 2010;68:992–998. 2. National Osteoporosis Foundation. Fast Facts: Fractures. Available at: http://www.nof.org/node/40. Accessed July 10, 2010. 3. Jupiter JB, Fernandez DL, Toh CL, Fellman T Ring D. Operative treatment of volar intra-articular fractures of the distal end of the radius. J Bone Joint Surg 1996;78A:1817–1828. 4. Drobetz H Kutscha-Lissberg E. Osteosynthesis of distal radial fractures with a volar locking screw plate system. Int Orthop 2003;27:1– 6. 5. Orbay JL, Fernandez DL. Volar fixed-angle plate fixation for unstable distal radius fractures in the elderly patient. J Hand Surg 2004; 29A:96 –102. 6. Orbay JL, Fernandez DL. Volar fixation for dorsally displaced fractures of the distal radius: a preliminary report. J Hand Surg 2002;27A:205–215. 7. Kamano M, Honda Y, Kazuki K, Yasuda M. Palmar plating for dorsally displaced fractures of the distal radius. Clin Orthop Relat Res 2002;397:403– 408. 8. Willis AA, Kutsumi K, Zobitz ME, Cooney WP III. Internal fixation of dorsally displaced fractures of the distal part of the radius. A biomechanical analysis of volar plate fracture stability. J Bone Joint Surg 2006;88A:2411–2417. 9. Boswell S, McIff TE, Trease CA, Toby EB. Mechanical characteristics of locking and compression plate constructs applied dorsally to distal radius fractures. J Hand Surg 2007;32A:623– 629. 10. Gardner MJ, Griffith MH, Demetrakopoulos D, Brophy RH, Grose A, Helfet DL, Lorich DG. Hybrid locked plating of osteoporotic fractures of the humerus. J Bone Joint Surg 2006;88A:1962–1967. 11. Estes C, Rhee P, Shrader MW, Csavina K, Jacofsky MC, Jacofsky DJ. Biomechanical strength of the Peri-Loc proximal tibial plate: a comparison of all-locked versus hybrid locked/nonlocked screw configurations. J Orthop Trauma 2008;22:312–316. 12. Koh S, Morris RP, Patterson RM, Kearney JP, Buford WL Jr, Viegas SF. Volar fixation for dorsally angulated extra-articular fractures of the distal radius: a biomechanical study. J Hand Surg 2006;31A: 771–779. 13. Sommers MB, Fitzpatrick DC, Madey SM, Vande Zanderschulp C, Bottlang M. A surrogate long-bone model with osteoporotic material properties for biomechanical testing of fracture implants. J Biomech 2007;40:3297–3304. 14. Spivak JM, Neuwirth MG, Labiak JJ, Kummer FJ, Ricci JL. Hydroxyapatite enhancement of posterior spinal instrumentation fixation. Spine (Phila PA 1976) 1994;19:955–964. 15. Jazrawi LM, Bai B, Simon JA, Kummer FJ, Birdzell LT, Koval KJ.
18.
19.
20.
21.
22.
23. 24.
25.
26.
27. 28.
29.
30.
31.
597
A biomechanical comparison of Schuhli nuts or cement augmented screws for plating of humeral fractures. Clin Orthop Relat Res 2000;377:235–240. Leung F, Zhu L, Ho H, Lu WW, Chow SP. Palmar plate fixation of AO type C2 fracture of distal radius using a locking compression plate—a biomechanical study in a cadaveric model. J Hand Surg 2003;28B:263–266. McCall TA, Conrad B, Badman B, Wright T. Volar versus dorsal fixed-angle fixation of dorsally unstable extra-articular distal radius fractures: a biomechanic study. J Hand Surg 2007;32A:806 – 812. Weninger P, Schueller M, Drobetz H, Jamek M, Redl H, Tschegg E. Influence of an additional locking screw on fracture reduction after volar fixed-angle plating-introduction of the “protection screw” in an extra-articular distal radius fracture model. J Trauma 2009;67:746 – 751. Mehling I, Muller LP, Delinsky K, Mehler D, Burkhart KJ, Rommens PM. Number and locations of screw fixation for volar fixedangle plating of distal radius fractures: biomechanical study. J Hand Surg 2010;35A:885– 891. Kowalski MJ, Schemitsch EH, Harrington RM, Chapman JR, Swiontkowski MF. A comparative biomechanical evaluation of a noncontacting plate and currently used devices for tibial fixation. J Trauma 1996;40:5–9. Stoffel K, Dieter U, Stachowiak G, Gachter A, Kuster MS. Biomechanical testing of the LCP— how can stability in locked internal fixators be controlled? Injury 2003;34 Suppl 2:B11–19. Fitzpatrick DC, Doornink J, Madey SM Bottlang M. Relative stability of conventional and locked plating fixation in a model of the osteoporotic femoral diaphysis. Clin Biomech (Bristol, Avon) 2009; 24:203–209. Rampoldi M, Marsico S. Complications of volar plating of distal radius fractures. Acta Orthop Belg 2007;73:714 –719. Arora R, Lutz M, Hennerbichler A, Krappinger D, Espen D, Gabl M. Complications following internal fixation of unstable distal radius fracture with a palmar locking-plate. J Orthop Trauma 2007;21:316 – 322. Benson EC, DeCarvalho A, Mikola EA, Veitch JM, Moneim MS. Two potential causes of EPL rupture after distal radius volar plate fixation. Clin Orthop Relat Res 2006;451:218 –222. Hattori Y, Doi K, Sakamoto S, Yukata K. Delayed rupture of extensor digitorum communis tendon following volar plating of distal radius fracture. Hand Surg 2008;13:183–185. Protopsaltis TS, Ruch DS. Volar approach to distal radius fractures. J Hand Surg 2008;33A:958 –965. Power J, Loveridge N, Lyon A, Rushton N, Parker M, Reeve J. Bone remodeling at the endocortical surface of the human femoral neck: a mechanism for regional cortical thinning in cases of hip fracture. J Bone Miner Res 2003;18:1775–1780. Spadaro JA, Werner FW, Brenner RA, Fortino MD, Fay LA, Edwards WT. Cortical and trabecular bone contribute strength to the osteopenic distal radius. J Orthop Res 1994;12:211–218. Cristofolini L, Viceconti M, Cappello A, Toni A. Mechanical validation of whole bone composite femur models. J Biomech 1996;29: 525–535. Heiner AD, Brown TD. Structural properties of a new design of composite replicate femurs and tibias. J Biomech 2001;34:773–781.
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