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Bicondylar Tibial Plateau Fractures: A Biomechanical Study

Mueller, Kelly, L.; Karunakar, Madhav, A.; Frankenburg, Elizabeth, P.; Scott, Derek, S.

Clinical Orthopaedics and Related Research: July 2003 - Volume 412 - Issue - p 189-195
doi: 10.1097/01.blo.0000071754.41516.e9
SECTION II ORIGINAL ARTICLES: Trauma
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The optimal treatment of bicondylar tibial plateau fractures remains controversial. The current study was designed to answer the following questions: (1) can a lateral fixed angle plate provide similar construct stability to dual plating techniques and (2) does the size of the medial buttress plate used in dual plating techniques have an effect on construct stability? Bicondylar tibial plateau fractures were created, reduced, and instrumented in a matched pair design using a cadaveric simulated bicondylar tibial plateau fracture model. Tibias were instrumented with one of three constructs: a lateral periarticular plate and posteromedial small fragment dynamic compression plate, a lateral periarticular plate and posteromedial ⅓-tubular plate, or a lateral fixed angle plate. Biomechanical testing was done to determine construct stiffness, maximum load to failure, and medial condylar displacement for each of the three constructs. There was no significant difference measured between the two dual plating constructs and the lateral fixed angle plate for overall construct stiffness or with respect to medial condylar fragment displacement. A lateral fixed angle plate may have clinical applications in the treatment of bicondylar tibial plateau fractures.

From the University of Michigan, Department of Orthopaedic Surgery, Ann Arbor, MI.

Reprint requests to Madhav A. Karunakar, MD, Department of Orthopaedic Surgery, University of Michigan Medical Center, 1500 East Medical Center Drive, Ann Arbor, MI 48109–0328. Phone: 734–615-3100; Fax: 734–764-9159; E-mail: mkarun@umich.edu.

Received: July 23, 2002.

Revised: September 27, 2002.

Accepted: October 30, 2002.

During the past 2 decades, advances in surgical technique and implants have improved the outcome for patients with tibial plateau fractures. In 1979, Schatzker et al 14 introduced a classification for tibial plateau fractures that distinguished low-energy split depression fractures from higher energy bicondylar (Schatzker V and VI) fractures. The goals of operative treatment of these fractures include anatomic reduction with restoration of articular congruity, and rigid fixation to allow early motion. Classic dual plating accomplished these goals, but was fraught with soft tissue complications. 3,9,13

The treatment of these complex fractures has continued to evolve and considerable controversy still exists regarding the optimal fixation construct. The classic dual plating techniques involved large fragment lateral and medial plates. This has evolved to lower profile small fragment plates and extraperiosteal medial buttress plates. 7,8 Although these techniques have become more widely accepted, limited data are available on the biomechanical properties of these fixation constructs.

Although fixed angle plates have been implemented with good success for intraarticular distal femur fractures, 10,12 this technique only recently has been suggested for tibial plateau fractures. 4 During the past few years, new implants have been developed to allow minimally invasive surgery. The Less Invasive Stabilization System (LISS, Synthes, Paoli, PA) is a plating system in which threaded screws lock into the plate to form a fixed angle construct. This implant may allow fixation of bicondylar tibial plateau fractures without requiring extensive medial dissection or hardware insertion.

The current study was designed to answer the following questions: (1) can a lateral fixed angle plate provide similar construct stability to dual plating techniques and (2) does the size of the medial buttress plate used in dual plating techniques have an effect on construct stability?

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MATERIALS AND METHODS

Twenty-four (12 matched pairs) fresh frozen human cadaveric tibias and mating femurs were obtained and stored at −20° C. The average age of the donor was 79 years (range, 54–96 years). The tibias were stripped of all soft tissues. Radiographs of the proximal tibia and distal femur of each specimen were obtained and bone mineral density testing was done using dual energy xray absorptiometry (DEXA). Each tibia was cut transversely 20 cm distal to the plateau and secured with cement in a 5 × 7.6-cm (2” × 3”) aluminum channel leaving 17 cm of tibia exposed.

The whole bone stiffness of each specimen was established using the Material Testing System (MTS model number 810, Minneapolis, MN). Mechanical testing was done as follows: the bone constructs were mounted in 10° flexion and preyield compression loads to 311.4 N (70 lbs) were applied to the tibial plateau at a constant rate of 44.5 N/ second using the mating distal femur. Continuous load and displacement data were collected using LabView data acquisition system (National Instruments, Austin, TX) and the slopes of the loaddisplacement curve were calculated as a measure of whole bone stiffness.

After initial stiffness was determined, a reproducible bicondylar tibial plateau fracture with metadiaphyseal dissociation was created on each tibia using a band saw based on a previously described model. 6 The fracture model is illustrated in Figure 1. The first cut was directed from the intercondylar eminence to a point on the lateral cortex 4 cm distal to the plateau. The medial condylar fracture was directed from the same starting point to a point on the medial cortex 6 cm distal to the plateau. The metadiaphyseal dissociation was made by connecting the distal points of the simulated condylar fractures.

Fig 1.

Fig 1.

The specimens were assigned systematically into one of three treatment groups, such that mean bone mineral density of the groups was not significantly different. The fractures were anatomically reduced and instrumented with one of three constructs: Group (A) double plating with eight-hole periarticular plate placed laterally and five-hole small fragment dynamic compression plate placed posteromedial; Group (B) double plating with eight-hole periarticular plate placed laterally and five-hole ⅓ tubular plate placed posteromedial, and Group (C) five-hole Less Invasive Stabilization System (Synthes) plate placed laterally. Four small fragment (3.5 mm) cortical screws were placed through the periarticular plate to create a subchondral raft. 8 Two 3.5-mm cortical subchondral screws with washers were placed in specimens instrumented with the lateral fixed angle plate to provide compression and maintain the articular surface reduction. This technique is recommended as the fixed angle plate does not allow for compression across the fracture site.

The three constructs were selected based on a review of recent literature and on the senior author’s clinical practice. The periarticular plate and ⅓-tubular plate construct was selected because it is the standard construct used in the treatment of bicondylar tibial plateau fractures by the senior author. The dynamic compression plate was selected to provide a more rigid medial buttress to address the question of whether the size of the medial buttress plate has an effect on construct stability. The LISS was selected because it is a commercially available fixed angle plate designed for use in the proximal tibia. Because bicondylar tibial plateau fractures often involve a displaced posteromedial fragment, 1,6 the medial fixation in the current study was placed posteromedial. Each specimen was visually inspected by the author, senior author, and an unbiased orthopaedic surgeon and all reductions were deemed anatomic. Postfixation radiographs confirmed anatomic reduction in all specimens. The fixation constructs are illustrated in Figure 2.

Fig 2A–C.

Fig 2A–C.

Mechanical testing was repeated for the instrumented specimens in the same manner described above. The specimens were first axially loaded nondestructively to determine postfixation stiffness of the construct, and then loaded to failure. Failure testing was accomplished at a constant displacement rate of 0.20 mm/second to an ultimate construct displacement of 1.00 cm. Because of limitations of the testing system, the failure test had to be displacement controlled. This rate of testing was chosen to closely mimic the preyield test rate. Continuous MTS load and displacement values were collected with the LabView data acquisition system at a rate of 200 Hz. Displacement between the condylar fragments was measured using clip gauges custom manufactured in the authors’ laboratory. Each clip gauge consisted of a linear strain gauge mounted to a phosphorus bronze strip with copper pins. The clip gauges are designed to measure only linear motions and are not sensitive to twisting or bending motions. Before mounting on the specimens, each gauge was calibrated over a 1 cm range. Mounting was done by attaching the acrylic blocks to the bony surfaces with glue. The clip gauges then were positioned securely into the blocks across the intercondylar, lateral, and medial condylar fracture lines perpendicular to the axes of motion. Figure 3 illustrates the testing setup and position of the clip gauges. Radiographs of the proximal tibia were repeated after failure testing.

Fig 3.

Fig 3.

Data were analyzed using SPSS statistical software (SPSS Inc, Chicago, IL). Univariate descriptive statistics were used to determine ranges, means, and standard deviations. A multivariate general linear model was used to test for differences in age, bone mineral density, and gender across the three fixation constructs. Nonparametric paired comparisons (Wilcoxon signed rank tests) were used to test for differences in the biomechanical properties of the constructs. Results of a sample size analysis run on pilot data indicated that for a paired analysis, four specimens per group would have 80% power to detect a probability of .02 per group (p < .05, two-sided).

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RESULTS

The group tested included 24 tibia and femur pairs from 12 cadavers. Eight specimens were instrumented with the dynamic compression plate (Group A), eight with a ⅓-tubular plate (Group B), and eight with the LISS plate (Group C). Age, gender, and bone mineral density were not significantly different among the three groups (p = 0.10). The preyield stiffness of the intact (whole bone) specimens before fracture and instrumentation was not significantly different among the three groups (p ≥ 0.30). The postfixation construct stiffness was not significantly different among the three groups (p ≥ 0.27). The maximum load to failure was 1260 N (range, 753–1703 N) for Group A, 1839 N (range, 749–2738 N) for Group B, and 1760 N (range, 858–2294 N) for Group C. The difference was not significant among the three groups (p ≥ 0.27). The testing data are summarized in Table 1.

TABLE 1

TABLE 1

Although there were no catastrophic failures, the load-displacement curves showed failure in each specimen tested. Comparison of the postfixation and postfailure radiographs revealed the following modes of failure for the three constructs. For both periarticular plate constructs, screw loosening or deformation occurred at the plate-screw interface. There was no evidence of loosening or deformation at the plate-screw interface with the fixed angle plate. The ⅓-tubular plates had more cases of proximal screw loosening and deformation of the medial plate than the dynamic compression construct.

Mean absolute displacement at the intercondylar fracture site was 0.58 mm for Group A, 1.1 mm for Group B, and 1.0 mm for Group C. Displacement of the lateral condylar fragment was 0.64 mm for Group A, 0.86 mm for Group B, and 0.85 mm Group C. Maximum displacement of the medial fragment averaged 1.17 mm (range, 0.70–1.99 mm) for Group A, 1.35 mm (range, 0.63–2.5 mm) for Group B, and 1.78 mm for Group C (range, 0.24–4.5 mm). There was no significant difference among the three fixation constructs with respect to intercondylar, lateral, or medial fragment displacement (p ≥ 0.14).

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DISCUSSION

Complex fractures of the tibial plateau present a treatment challenge for orthopaedic surgeons. The general principles of surgical treatment include anatomic restoration of the articular surface and rigid fixation to allow early mobilization. Initial attempts at treating such fractures closed consistently resulted in a poor outcome. 11,14 Classic descriptions of open reduction and internal fixation of bicondylar tibial plateau fractures recommended medial and lateral plating with large fragment buttress plates. 13 This technique, historically referred to as the dead bone sandwich, fell into disfavor largely because of soft tissue complications resulting from injury and surgical trauma. 3,9,11,15

To reduce the incidence of complications, treatment of these complex fractures evolved toward lower profile internal fixation, indirect reduction techniques, and improved soft tissue handling. 2,4,9 Periarticular small fragment implants on the lateral side and extraperiosteal small fragment plates on the medial side have largely replaced the classic dual large fragment plating technique. 2,7,8 The reported advantages included less prominent hardware, less surgical dissection, and fewer soft tissue complications.

Horwitz et al 7 presented a biomechanical analysis comparing three different large fragment fixation constructs for bicondylar tibial plateau fractures. Using an optical tracking system, they recorded vertical subsidence of the medial condylar fragment. They found that a lateral buttress plate alone provided significantly less stability when compared with either a traditional large fragment dual plating construct or a lateral buttress plate with small fragment anteromedial antiglide plate. They also found no significant difference between a large fragment medial buttress plate and an antiglide reconstruction plate in controlling medial fragment displacement. They concluded that a lateral buttress plate with a small fragment anteromedial plate was equally effective at controlling medial vertical subsidence when compared with traditional dual large fragment plates.

The current study was designed to answer the question: does the size of the medial buttress plate used in dual plating techniques have an effect on construct stability? The results of this study did not show any statistically significant difference between the two medial buttress plate constructs. However, visual inspection of the postfailure radiographs had a higher rate of screw loosening and deformation of the medial buttress plate in the ⅓-tubular group. The clinical significance of this observation is unclear as most patients with bicondylar tibial plateau fractures are maintained with nonweightbearing restrictions and are not subject to the failure loads tested in this biomechanical model.

Fixed angle plates have been used successfully for fractures of the distal femur. 10,12 This technique eliminated the need for extensive medial dissection, thereby reducing the extent of soft tissue stripping, bone devascularization, and associated complications seen with these fractures. 10,12 Use of a lateral fixed angle plate for the distal femur reportedly has resulted in improved rates of bony union and fewer wound complications. 12 This concept only recently has been applied to the proximal tibia.

The Less Invasive Stabilization System (Synthes) is a plating system in which threaded screws lock into the plate forming a fixed angle. The surgical technique requires anatomic fixation of the articular surface followed by closed reduction of the metadiaphysis and percutaneous submuscular placement of the plate. This device is clinically appealing as it may obviate the need for extensive medial dissection and internal fixation in the operative treatment of bicondylar tibial plateau fractures. Because no biomechanical data exist for this plate in the proximal tibia, this study was designed to determine whether a fixed angle plate could provide equally effective fixation for complex tibial plateau fractures as more standard dual plating techniques.

The current data suggest that under axial loading conditions, the overall construct stiffness is similar for a lateral fixed angle plate when compared with dual plating techniques. Each of the three constructs was equally effective at controlling medial fragment displacement when an anatomic reduction was achieved. These findings suggest that a fixed angle plate may offer an alternative means of fixation for bicondylar tibial plateau fractures. This may be particularly useful in cases when a medial incision is prohibited because of soft tissue injury.

The authors recognized that the use of a biomechanical model is a limitation of this study. Soft tissue tension, which contributes to alignment and maintenance of reduction, is lost in a laboratory model. The testing apparatus applied only an axial load without rotation at the fracture site. This testing mode was selected because clinically, axial load is the largest component of failure. Although cyclic failure occurs clinically, it is difficult to test biomechanically because interval healing that occurs is not reproducible in this model. Clip gauges were used in this study to measure displacement at the fracture lines. Although fracture displacement is three-dimensional, only the primary direction of displacement (linear motion) can be detected with this method.

The average age of the donor for cadaveric specimens used in this study was 79 years (range, 54–96 years), which is substantially older than the population of patients who typically sustain these high-energy injuries. Ding et al 5 showed that trabecular bone is tougher in young patients, and that the mechanical properties of trabecular bone decrease with age. Osteopenia and degenerative changes found in these specimens may have influenced the response to loading. However, a fixation construct that maintains fracture reduction in these specimens is likely to also be sufficient in younger patients with better bone quality.

Despite these limitations, in this biomechanical model, the overall construct stiffness of a lateral fixed angle device was similar to more conventional techniques. In addition, the lateral fixed angle device was effective at controlling medial condylar displacement. This device may have a clinical application in the operative management of bicondylar tibial plateau fractures.

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Acknowledgements

The authors thank Jan Hall, MS, for assistance with the statistics, Jason Kaiser, Sean Matuszak, and Dennis Kayner for contributions to the current study, and Synthes for donating the implants.

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References

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