The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 88, Issue 9

Scientific Articles | September 01, 2006

Hybrid Locked Plating of Osteoporotic Fractures of the Humerus

Michael J. Gardner, MD¹; Matthew H. Griffith, MD¹; Demetris Demetrakopoulos, BS¹; Robert H. Brophy, MD, MS¹; Andrew Grose, MD¹; David L. Helfet, MD¹; Dean G. Lorich, MD¹

¹ The Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. E-mail address for M.J. Gardner: gardnerm@hss.edu

View Disclosures and Other Information

In support of their research for or preparation of this manuscript, one or more of the authors received grants or outside funding from Synthes, Inc. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at The Hospital for Special Surgery, New York, NY

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2006 Sep 01;88(9):1962-1967. doi: 10.2106/JBJS.E.00893

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Abstract

Background: Locked plating techniques recently have gained popularity and offer a different biomechanical approach for fracture fixation compared with traditional compression plating. In certain clinical situations, it may be preferable to employ a "hybrid" construct, in which an unlocked screw is used to assist with reduction and locked screws are subsequently used to protect the initial reduction. In the present study, we used an unstable osteoporotic fracture model of the humerus to determine (1) whether a hybrid construct behaved more like a locked construct or a conventional unlocked construct and (2) whether there was a difference between locked and unlocked constructs.

Methods: Thirty third-generation Sawbones humeri were divided into three groups of ten humeri each. A locking plate with combination holes was applied to each bone with use of either a locked construct, an unlocked construct, or a hybrid construct. To simulate purchase in osteoporotic bone, all screw-holes were drilled to 0.3 mm less than the diameter of the screw used. Each specimen was then osteotomized in the middle part of the shaft, and a 5-mm segment was removed. Oscillating cyclic torsion testing was performed to ±10 N-m for 1000 cycles, torsional stiffness was determined at periodic cyclic intervals, and the groups were compared.

Results: The locked and hybrid constructs demonstrated similar behavior. The initial stiffness was similar in these two groups. At ten cycles, the locked and hybrid constructs retained 96.3% and 95.4% of their initial stiffness, respectively. During the remainder of cycling the stiffness of the locked and hybrid constructs decreased in a linear fashion (R² = 0.89 and 0.88, respectively), and at 1000 cycles the stiffness of the locked and hybrid constructs averaged 80.0% and 79.2% of the initial values, respectively (p = 1.0). In contrast, the unlocked constructs initially were significantly less stiff than both the locked and hybrid constructs (p < 0.001). At ten cycles the unlocked constructs retained 80.4% of their initial stiffness, and at 1000 cycles they retained only 22.3% of their initial stiffness.

Conclusions: Hybrid constructs are mechanically similar to locked constructs, and both are significantly more stable than unlocked constructs under torsional cyclic loading.

Clinical Relevance: Combining screws in the hybrid configuration used in the present study did not compromise the mechanical performance of the construct. Hybrid constructs may decrease cost and may provide additional clinical value when treating fractures in osteoporotic bone.

Figures in this Article

Loss of screw purchase is an important factor related to the failure of fixation in osteoporotic bone^1-4. Fracture plating technology recently has evolved to include locked plating, in which screws with threads on the undersurface of the head are threaded into the plate, creating a fixed-angle device that does not allow screw toggle. Several plate designs feature screw-holes in which either a locking screw or a traditional compression screw may be used. During the placement of unlocked compression screws, the plate is pressed to the bone as the screw is tightened. Establishment of this friction fit is essential for secure fixation of the plate to the bone. Conversely, locking screw-plate systems act as single-beam constructs in the bone, and the plate does not require compression and friction generation to achieve mechanical stability.

Because of the different biomechanical principles associated with each type of fixation, it generally has been recommended to avoid applying both types of fixation within the same fracture fragment^5-7. Nevertheless, there are several specific situations in which it may be preferable first to establish a strong friction fit with an unlocked screw and then to secure this fixation with additional locking screws. Sometimes, in the case of a nonunion, the fracture site is fully stripped and débrided as part of the exposure, and the main goal is to obtain the most rigid fixation possible. In the setting of an osteoporotic fracture, loss of purchase in the poor quality bone is high, and it may be preferable to obtain an initial friction fit and protect this fixation with subsequent locking screws^5,7. Additionally, a compression screw may be used initially to appose the plate to the bone in order to optimize reduction, and it is unclear whether it is beneficial then to replace this screw with a locking screw.

The ability of hybrid constructs to maintain adequate purchase under repetitive loading has not been previously studied, to our knowledge. The primary objective of the present study was to determine if a hybrid construct behaved more like a locked construct or a conventional unlocked construct in an unstable osteoporotic fracture model of the humerus. A secondary objective was to determine if there was a difference between a locked construct and an unlocked construct in the same model. The null hypothesis was that there would be not differences in mechanical behavior between the three contructs during cyclic loading.

Materials and Methods

Materials and Methods | Results | Discussion | Appendix | References

Thirty third-generation Sawbones humeri (model 3304; Pacific Research Laboratories, Vashon, Washington) were divided into three groups (with ten humeri per group). Each specimen was potted in automobile body compound (Bondo, Atlanta, Georgia) at both ends. A 4.5-mm narrow Locking Compression Plate (LCP; Synthes, West Chester, Pennsylvania) was applied to each specimen with either all locked screws, all unlocked screws, or a hybrid combination of screws (one standard screw in the hole nearest the osteotomy site, and two locked screws distally in each fragment) (Fig. 1). Plates were placed in the appropriate position on the bone and were held manually in place without compressing the plates to the bone. All screw-holes were drilled to 0.3 mm less than the diameter of the screw used (4.7 mm for the 5.0-mm locked screws and 4.2 mm for the 4.5-mm cortical screws) to simulate the purchase in osteoporotic bone. This simulation method has been used previously^8,9 and also was verified by a previous pilot study in our laboratory. Locking screws were tightened to 4 N-m with use of a torque wrench to fully engage the threaded screw-heads with the plate threads, as suggested by the manufacturer. Unlocked screws were placed in the center of the compression hole and were tightened to 3.4 N-m, which has been used in previous protocols involving similar models^8,10. Each specimen was then osteotomized in the middle part of the shaft, and a 5-mm segment was removed to avoid contact of the two fragments during cyclic loading.

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Fig. 1: Schematic of the screw configurations of the three treatment groups: unlocked, locked, and hybrid.

Oscillating cyclic torsion testing was performed on a servohydraulic biaxial testing machine (Bionix; MTS Systems, Eden Prairie, Minnesota). The specimens were placed in the load frame vertically and were pinned into the grips. Torque was applied to a peak of ±10 N-m for 1000 cycles at 0.3 Hz. Rotational displacement was measured using a rotary variable differential transformer. Data were collected and recorded initially (after the first cycle), after ten cycles, and at each 100-cycle interval up to 1000 cycles with use of a personal computer and Labview data-acquisition software (National Instruments, Austin, Texas). Stiffness was determined from the slope of the load-deflection curve as N-m/deg.

The mean stiffness (and standard deviation) and the percentage of initial stiffness were calculated for each group initially and at each 100 cycles up to 1000 cycles. Analysis of variance tests were performed to compare the three treatment groups. Bonferroni post hoc tests were used to determine which groups were significantly different from each other, with the level of significance set at p < 0.05. Finally, from the curves generated for the percentage of initial stiffness cycling data, curve-fitting was performed and correlation coefficients (R²) were determined.

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Fig. 2: Graph illustrating the change in stiffness relative to the initial stiffness during cyclic torsion testing in the three groups. The unlocked group was significantly less stiff initially than the locked and hybrid groups and retained significantly less stiffness compared with the initial stiffness at each cyclic interval. There were no differences in the measurements between the locked and hybrid constructs at any cyclic interval. The progressive decline in stiffness of the locked and hybrid constructs was best fit to linear curves (R² = 0.89 and 0.88, respectively), whereas the unlocked construct curve was best fit to a logarithmic curve (R² = 0.98).

Results

Materials and Methods | Results | Discussion | Appendix | References

The fully locked and hybrid constructs demonstrated remarkably similar behavior to each other. The mean initial stiffness (and standard deviation) was 0.52 ± 0.068 N-m/deg for the locked constructs and 0.46 ± 0.071 N-m/deg for the hybrid constructs (p = 0.3). After ten cycles, the two constructs retained 96.3% ± 5.4% and 95.4% ± 4.6% of their initial stiffness, respectively (p = 1.0). After 1000 cycles, the locked and hybrid constructs retained 80.0% ± 10.2% and 79.2% ± 9.5% of their initial stiffness, respectively (p = 1.0). The stiffness curves generated for the locked and hybrid groups were best fit to a linear equation (R² = 0.89 and 0.88, respectively) (Fig. 2).

The unlocked constructs demonstrated a decline in stiffness with cycling, with a decrease to 80.4% ± 6.7% of the initial stiffness after ten cycles (Fig. 2). By the end of the first 100 cycles, the stiffness had declined even further, to 57.7% ± 6.6% of the initial stiffness. Stiffness continued to decline through 1000 cycles, appearing to asymptotically approach approximately 20% of the initial stiffness (final measured value, 22.3% ± 12.6%). The stiffness curve for the unlocked group was best fit to a logarithmic equation (R² = 0.98). In all specimens in the unlocked group, visual inspection at the peak torque of each cycle revealed that the plate became unstable as pullout and gross toggle of the screws occurred (Figs. 3-A and 3-B), which may explain the progressive decrease in stiffness. This toggling did not occur in any of the specimens in the locked or hybrid group.

Analysis of variance confirmed an overall significant difference between the three constructs in terms of both initial stiffness and stiffness at all cyclic intervals (p < 0.001). Bonferroni post hoc tests revealed that the initial stiffness in the unlocked group was significantly less than those in both the locked and hybrid groups (p < 0.001). At all cyclic intervals, the percentage of initial stiffness in the unlocked group was less than those in both the locked and hybrid groups (p < 0.001). No differences between the locked and hybrid specimens were found at any cyclic interval (p = 1.0).

Discussion

Materials and Methods | Results | Discussion | Appendix | References

Patients with osteoporosis may be at a greater risk for delayed union or nonunion following plate fixation of a humeral diaphyseal or metadiaphyseal fracture because of poor screw purchase or loosening^1,3,4,11,12. Inadequate screw purchase in the cortical bone may result in mechanically unstable fracture stabilization, fixation failure, and nonunion³. Locking plate systems with screw heads that thread into the plate and act as fixed-angle devices may facilitate improved stability in such cases^7,13-15. In the present study, we used a model that simulated screw purchase in osteoporotic bone to compare the mechanical characteristics of plating with all unlocked screws, all locked screws, or a hybrid combination of screw types. The first important finding was that after cyclic loading, the unlocked screw constructs had significantly lower stability compared with both the locked and hybrid constructs, as judged by the loss of stiffness over the course of loading. The locked and hybrid constructs both decreased in stiffness by about 7% between ten and 100 cycles, whereas stiffness in the unlocked group decreased by 23% during this interval.

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Fig. 3-A and Fig. 3-B: During cyclic testing, gross toggle of the screws within the plate and bone occurred in all specimens in the unlocked group. The plate toggled in one direction as the peak torque was reached in one direction (Fig. 3-A) and rocked the other way as peak load was approached in the opposite direction (Fig. 3-B). This toggling did not occur in any of the specimens in the locked or hybrid group.

The second important finding relates to the use of both screw types within a single fracture fragment, which potentially mixes different biomechanical fixation principles. To our knowledge, this concept has not been previously evaluated. Obtaining an initial compression fit between the plate and the bone with an unlocked compression screw, followed by the use of locked screws in the remaining holes, has been recommended for the treatment of fractures in osteoporotic bone^5,7. In addition, an initial compression screw may be used to draw the bone to the plate as an adjunctive reduction tool. We found that the locked and hybrid constructs were not significantly different under cyclic loading in terms of torsional stiffness. Thus, if the final two screws placed in each fragment are locked screws, the mechanical properties of the construct do not appear to depend on the type of screw placed initially. This also implies that if an unlocked screw is used for reduction, it may be unnecessary to replace it with a more expensive locked screw.

Several previous studies involving testing of a variety of different locking devices have indicated that locking constructs have potential advantages over unlocked screws and plates^{8,10,13,16,17}. The plate device tested in the present study has gained widespread popularity recently despite the general paucity of available biomechanical and clinical data^13,18-23. Although this model did not account for the biological effects of the different fixation types and precluded compression across the fracture site, the overwhelming difference in the mechanical behavior between the constructs suggests that more stable initial fixation may be obtainable in difficult clinical situations with use of locked screws.

In a study of twenty-four patients with delayed union or nonunion of an osteoporotic humeral diaphysis who were managed with locking plate fixation and bone graft, Ring et al. found that all patients had healing after a mean duration of follow-up of twenty months¹⁹. In a retrospective study of thirty-three patients with delayed union or nonunion of the humeral diaphysis, Wenzl et al. found that although patients who were managed with a locked plate were older, had longer-lasting nonunions, and had had more previous operations and more severe initial injuries, only in the unlocked group was there a hardware failure due to osteoporosis that required revision²⁴.

Many locked plating systems contain combination holes in which either locked or unlocked screws may be placed. The use of unlocked screws allows for compression across the fracture site, and, if micromotion is eliminated and absolute rigidity is achieved, primary healing and direct fracture remodeling may occur. However, this requires friction between the plate and the bone, and preloading of the bone may be difficult to achieve in osteoporotic patients. Locking screws, on the other hand, act as internal splints in which stable and controlled micromotion is preferable, which leads to secondary healing through callus formation. Although the different biomechanical principles of the two screw types warrant careful preoperative consideration regarding the sequence of placement, a combination of screw types may be preferable in situations in which osteoporosis is present or in which a compression screw is used as a reduction aid^5,7.

Hybrid locked plating may have several disadvantages that were not accounted for in this study. Biomechanically, an area of high stress concentration may have been produced at the site of the single compression screw without the neutralizing effects of additional compression screws²⁵. The neutralizing ability of locking screws as compared with unlocked screws is unknown, and we did not quantify local stresses at the sites of the unlocked screws in the hybrid constructs in this study. Additionally, plate length and working length have been shown to contribute substantially to construct stability^22,26,27, and these effects were not evaluated in a hybrid construct. Biologically, the use of a single compression screw creates more plate-bone compression than a pure locked construct does²⁸, potentially leading to disruption of the periosteal blood supply. One of the major theoretical advantages of locking plate technology is the preclusion of compressing the plate to the bone, thereby avoiding subsequent disruption of the periosteal cortical vascular supply^5,14,28. When a cortical screw is used as a reduction aid in a hybrid construct, a strong friction preload may not be necessary to achieve the desired reduction. These issues clearly warrant additional investigation.

We are aware of several limitations of the present study. First, Sawbones instead of cadaveric specimens were used for mechanical testing. We chose to use a synthetic bone model with osteoporotic screw purchase on the basis of a previously described protocol^8,9 to allow testing in a more standardized, homogeneous material. The third-generation Sawbones are fiberglass and epoxy composites and simulate the structural properties of both cortical and cancellous bone²⁹. Second, this model replicated a specific fracture type, a diaphyseal osteoporotic fracture of the humerus without the ability to compress the fracture ends, and specific screw configurations. Other combinations of screw types or empty holes may behave differently. Also, substantial cortical thinning may occur in osteoporotic bone³⁰, which may be more detrimental to the holding power of unlocked screws, with a coarser thread, compared to locking screws, with a finer thread. The altered drill-hole sizes that we used did not account for this cortical thinning and thus may overestimate the performance of an unlocked screw as part of a hybrid construct. Finally, the biological effects of fracture plating were not accounted for and clearly play a major role in bone-healing following fracture fixation^28,31-34.

In summary, we found that locking plate constructs were superior to unlocked plates and screws when tested under torsion in an unstable osteoporotic diaphyseal fracture model. The use of a single compression screw to obtain an initial friction fit did not add stability to the hybrid construct, but, more importantly, it did not significantly change torsional stiffness under cyclic loading compared with an all-locked construct. Locking screws are substantially more expensive than traditional unlocked screws, and it appears that at least some hybrid constructs are mechanically similar to all-locked screw constructs. Additional biomechanical and comparative clinical studies are necessary to validate these concepts, but the use of a locking plate along with a hybrid combination of locked and unlocked screws may provide an additional clinical tool for the treatment of fractures in patients with poor bone quality.

Appendix

Materials and Methods | Results | Discussion | Appendix | References

An appendix describing the pilot study that was performed to evaluate the osteoporotic synthetic bone model that was used in the present study is available with the electronic versions of this article, on our web site at (go to the article citation and click on "Supplementary Material") and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). ?

References

Materials and Methods | Results | Discussion | Appendix | References

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Topics

osteoporosis ; fracture ; bone plates ; bone screws ; humerus ; osteoporotic fractures ; compression ; locking screw

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Michael J. Gardner, Matthew H. Griffith, Demetris Demetrakopoulos, Robert H. Brophy, Andrew Grose, David L. Helfet, Dean G. Lorich; Hybrid Locked Plating of Osteoporotic Fractures of the Humerus. The Journal of Bone & Joint Surgery. 2006 Sep;88(9):1962-1967.

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