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Biomechanical investigation of an alternative concept to angular stable plating using conventional fixation hardware
© Windolf et al; licensee BioMed Central Ltd. 2010
Received: 22 January 2010
Accepted: 21 May 2010
Published: 21 May 2010
Angle-stable locking plates have improved the surgical management of fractures. However, locking implants are costly and removal can be difficult. The aim of this in vitro study was to evaluate the biomechanical performance of a newly proposed crossed-screw concept ("Fence") utilizing conventional (non-locked) implants in comparison to conventional LC-DCP (limited contact dynamic compression plate) and LCP (locking compression plate) stabilization, in a human cadaveric diaphyseal gap model.
In eight pairs of human cadaveric femora, one femur per pair was randomly assigned to receive a Fence construct with either elevated or non-elevated plate, while the contralateral femur received either an LCP or LC-DCP instrumentation. Fracture gap motion and fatigue performance under cyclic loading was evaluated successively in axial compression and in torsion. Results were statistically compared in a pairwise setting.
The elevated Fence constructs allowed significantly higher gap motion compared to the LCP instrumentations (axial compression: p ≤ 0.011, torsion p ≤ 0.015) but revealed similar performance under cyclic loading (p = 0.43). The Fence instrumentation with established bone-plate contact revealed larger fracture gap motion under axial compression compared to the conventional LC-DCP osteosynthesis (p ≤ 0.017). However, all contact Fence specimens survived the cyclic test, whereas all LC-DCP constructs failed early during torsion testing (p < 0.001). All failures occurred due to breakage of the screw heads.
Even though accentuated fracture gap motion became obvious, the "Fence" technique is considered an alternative to cost-intensive locking-head devices. The concept can be of interest in cases were angle-stable implants are unavailable and can lead to new strategies in implant design.
The devices known as angular stable internal fixators have enhanced the armamentarium for surgical fracture treatment [1–3]. The mechanical principle of these implants is the locking of the screw head into the plate, resulting in a load transfer via plate and screws [1–3]. This increases the stability of the construct and eliminates the risk of loss of reduction due to screw toggling. Furthermore, the periosteal blood supply of the bone under the device is preserved, since there is no need for contact or compression between plate and bone. Biomechanical studies [4–6] have shown the advantages of angle-stable plate fixation over conventional plating. However, several unique complications have been noted, such as difficulty with implant removal and implant cut out in osteoporotic bone . Furthermore, locking implants increase the cost of surgery, which is why many surgeons are restricted in the use of angle-stable fixation hardware. Developing countries and countries with a small budget health care system rarely use these techniques [8, 9].
The objective of this study was to compare the biomechanical performance of a newly proposed crossed screw technique ("Fence") utilizing a conventional LC-DCP (limited contact dynamic compression plate) to LCP (locking compression plate) and standard LC-DCP stabilization in a human cadaveric diaphyseal gap model. Fracture gap motion and fatigue properties under cyclic loading were evaluated under axial compression and torsion.
The null hypothesis was that the construct created with conventional screws in a crossed configuration would yield biomechanical results comparable to those achieved with the other instrumentations.
Specimens and study-groups
For the LCP (Series 1) and the conventional LC-DCP constructs (Series 2), standard plating techniques were used (Figure 3A). The LCP plates (10-hole, 4.5/5.0-mm broad LCP) were attached with 4.9-mm self-tapping head locking screws inserted through the threaded portion of the combination hole provided in the plate. Head locking screws were tightened using a torque limiter. 5-mm-thick spacers were used to offset the plates from the cortex (Figure 3B). The LC-DCP plates were placed directly on the cortex and attached using 4.5-mm self-tapping bicortical cortex screws, since 4.9-mm screws of this type are not available. All conventional screws and bolts were tightened by hand following the clinical practise.
The bones were cut proximally and distally at a distance of 60 mm from the ends of each plate, and potted in Polymethylmethacrylate (PMMA, Beracryl, W. Troller Kunststoffe AG, Jegenstorf, Switzerland). At either end of the plate, a 5-mm distance was ensured between the plate and the potting material (Figure 3B).
Data acquisition and analysis
For comparisons within test-series 1 and 2 (elevated Fence versus LCP; contact Fence versus LC-DCP), paired t-tests were employed on cycles to failure and range of gap motion at 1, 2000 and 4000 cycles. Furthermore, a Repeated Measures ANOVA (analysis of variance) was used to compare between gap motion at 1, 2000 and 4000 cycles within each group. A statistical software package (SPSS 18.0, SPSS Inc., Chicago, USA) was used. Level of significance was set to α = 0.05.
Cortical bone density was 625 ± 204 mgHA/cm3 (mean ± SD) for the elevated Fence specimens, 612 ± 202 mgHA/cm3 for the LCP samples, 608 ± 70 mgHA/cm3 for the contact Fence group and 585 ± 129 mgHA/cm3 for the LC-DCP specimens. The donor's mean age was 76 years (range 70 - 83 years, 7 male and 1 female).
92 ± 25
2.1 ± 0.6
171 ± 19
2.9 ± 0.1
148 ± 29
3.5 ± 0.4
299 ± 118
3.0 ± 1.0
We investigated the concept of a lower-cost plating technique that would confer the same benefits as those offered by locking plates. The hallmark feature of this technique was the criss-cross pattern of screw routing ("Fence"), using conventional (non-locking-head) locking bolts. An additional advantage of the Fence technique is the variable direction of angulation and screw insertion. Thus, it might be possible to fix additional fragments especially when treating multi-fragmentary fractures. Furthermore, periprosthetic fractures may be addressed using the Fence technique passing the stem of the prosthesis anterior or posterior and at the same time achieving angular stability. The technique with its advantages and opportunities might, however, be more demanding to apply compared to e.g. an LCP instrumentation. A certain experience and skill level of the surgeon is required to avoid complications like screw collisions during implant placement. For further commercialization of the technique an easy-to-use drill template might be an option for ease of the procedure.
In a first step, we compared the Fence technique with established bone contact to conventional, non-locked plating. The conventional constructs were most rigid under axial loading, but failed earliest during cyclic torsional testing, while none of the contact Fence specimens failed. This suggests that the contact Fence technique carries potential to enhance the construct's fatigue properties under cyclic loading conditions compared to conventional plating. However, it has to be taken into account that different screw types with slightly different core diameters were used (cortex screw vs. locking bolt). An influence of this factor can not be excluded. Several authors have investigated the biomechanical properties of locking plates versus conventional plates; findings have been mixed [5, 10–14]. Even though not tested in a direct comparison, we found that the LC-DCP constructs failed markedly earlier than did the LCP instrumentations, which would agree with the findings by Lill et al.  that flexible constructs are better able to withstand cyclic loading.
In a second test-series, we evaluated a non-contact Fence instrumentation and compared it to an LCP fixation. Both osteosyntheses reflected comparable fatigue properties. However, the Fence technique showed significantly higher fracture gap motion under axial and torsional loading. There is insecurity about the optimal amount of micro-motion in the fracture gap for enhanced bone healing. Hypothetically, a less rigid construct could be advantageous by potentially stimulating callus formation. On the other hand, extensive motion could lead to delayed unions or could cause pseudarthrosis. Although the senior author treated 12 patients successfully with this technique in his trauma centre, further studies-and, in particular, clinical trials-will be required for a definitive assessment of the utility of the technique described in this paper. However, such work would appear to be justified in light of the results of the present study.
Other aspects requiring further investigation might be the screw angulation and the distance of an elevated construct from the cortex of the bone. Ahmad et al.  compared LCPs applied at different distances (flush to bone; 2 mm, and 5 mm off the bone), with a DCP control, and found comparable biomechanical behaviour and similar results in the DCP and the LCP constructs in which the plate was applied at or less than 2 mm from the bone. LCP constructs 5 mm off the cortex showed increased plastic deformation and lower failure loads. Similarly, Fulkerson et al. , investigating locked-screw constructs, found that increasing the bone-plate distance significantly decreased construct stability. We believe that in our study 5-mm elevation of the plates produced a lever-arm effect at the unsupported free part of the screws which considerably affected the mechanical behaviour of the elevated constructs. Regarding angulation of the Fence pattern, a standardized screw angle of 60° was chosen. The potential effect of this angle on the construct stability was not subject to our investigation. With increasing screw angle the entry points of adjacent screws would approach each other at the near cortex, which could induce a potential weak point. We concluded that fatigue performance and rigidity of the Fence construct may be further optimized by adjusting the bone-plate distance and the screw pattern angulation. Another drawback of the method might be the interdependency within screw pairs. Given only one screw pair is used, failure of one screw would lead to simultaneous loss of stability of the second screw and hence, to failure of the construct.
Our experiment was subject to the limitations common to biomechanical studies. The in vivo loading environment could only be mimicked in a restricted way. We decided to test successively in axial compression and torsion considered as most relevant loading patterns. The sample size was small due to limited availability of bone specimens. We, therefore, decided to carry out only pairwise comparisons without considering the relations between unmatched study-groups. However, conclusions drawn from our findings, based on a low sample size still need to be viewed critically.
This study introduces a plating technique with crossed screw configuration ("Fence") as a potential alternative to cost-intensive locking-head devices. The fatigue performance was found comparable to angular stable plating, whereas the "Fence" construct allowed larger motion in the fracture gap. A potential influence on bone healing can not be evaluated here. The technique can be of interest in cases were angle-stable implants are unavailable or may lead to new strategies for implant development.
We thank B. Gueorguiev for his valuable support and expertise in optical motion tracking and data evaluation.
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