Influence of proximal cancellous bone void volume on the rigidity of tibial plateau fracture fixation: An observational study

Abstract

BACKGROUND

Depressed tibial plateau (TP) fractures frequently result in metaphyseal cancellous bone voids following fracture reduction. The influence of cancellous bone void volume (CBVV) and location on the rigidity of fracture fixation constructs remains poorly understood despite potential implications for surgical decision-making.

AIM

To assess the effect of CBVV on the rigidity of TP fracture fixation.

METHODS

TP fractures with different levels of CBVV (Scenario 1: 10% distal, Scenario 2: 50% distal, and Scenario 3: 50% proximal) were created in fourth-generation Sawbones models and fixed according to standard surgical practice. The samples were loaded under controlled conditions, and rigidity was assessed. Descriptive statistics were used to test for normality, and statistical significance was determined using one-way analysis of variance.

RESULTS

All samples completed all loading sequences without failure. Loading to the capacity of the load frame (3 kN) did not result in fixation failure; therefore, no compressive load at failure was recorded. As expected, vertical displacement increased with increasing compressive load. The average fracture gap opening at the maximum applied load (2900 N) was 0.057 ± 0.056 mm, 0.066 ± 0.044 mm, and 0.124 ± 0.077 mm for scenarios 1-3, respectively. The average construct stiffness was 1469 ± 57 N/mm, 1484 ± 73 N/mm, and 1467 ± 23 N/mm for scenarios 1-3, respectively. No statistically significant differences were observed in vertical displacement, gap opening, or stiffness (P > 0.05).

CONCLUSION

No statistically significant differences in rigidity were found between constructs with varying CBVV. However, a trend was observed, with the 50% proximal deficit demonstrating the least rigidity. Further research is warranted to investigate if proximal cancellous bone voids may warrant adjunctive support during TP fracture fixation.

Key Words: Tibial plateau fracture; Cancellous bone void; Bone defect; Fracture fixation; Locking plate; Biomechanics

Core Tip: The biomechanical significance of metaphyseal cancellous bone loss in tibial plateau (TP) fractures remains poorly defined. Using standardized synthetic fracture models, this study demonstrated that increasing cancellous bone void volume did not significantly reduce fixation rigidity. Nevertheless, proximal bone defects exhibited greater fracture gap opening and a tendency toward reduced construct stability. Further research is required to investigate whether selective use of bone grafting or augmentation in TP fracture fixation is advisable.

INTRODUCTION

Tibial plateau (TP) fractures, which commonly occur due to high-energy trauma in younger individuals or osteoporotic changes in older adults, present a significant clinical challenge due to their complex anatomy and critical role in load-bearing and joint stability[1]. These fractures typically result from axial compressive forces, often accompanied by varus or valgus stress at the knee joint. The fracture pattern is influenced by the magnitude and direction of the force, the degree of knee flexion at injury, and bone quality. Younger patients often sustain wedge-shaped or split fractures, whereas elderly individuals are more likely to develop depression-type fractures resulting from lower-energy trauma[2].

Effective management requires a sound understanding of fracture biomechanics to ensure stable fixation and optimal functional recovery[3]. The principal aims of treatment are to restore articular congruity, maintain alignment, and provide stable fixation that allows for early mobilization. While non-operative management may suffice for undisplaced fractures, surgical fixation is generally required for displaced or depressed fractures, or those associated with ligamentous injury[1]. Standard fixation constructs typically employ subchondral raft screws and lateral locking plates[4].

Metaphyseal bone defects commonly accompany TP fractures and can compromise fixation stability. Restoration of the articular surface and subchondral support often requires bone grafting or augmentation using autograft, allograft, or bone substitutes such as calcium phosphate cement[5-7]. The goal is to restore anatomy, support early motion, and reduce the risk of post-traumatic osteoarthritis[8]. However, autografts carry donor-site morbidity, while allografts have limited mechanical strength and potential integration issues[9].

A critical bone defect is one that prevents spontaneous healing under standard management[10]. While thresholds exist for diaphyseal fractures (> 25 mm for tibial shaft[11]), no such guidelines are established for TP fractures[12]. The volume and location of cancellous bone voids likely influence the biomechanical stability of fixation constructs, yet quantitative evidence remains limited.

The present study aimed to investigate how the volume and anatomical position of cancellous bone voids affect the rigidity of TP fracture fixation using a lateral locking plate in Sawbones models.

MATERIALS AND METHODS

For the purpose of this experimental TP fractures with different levels of cancellous bone voids were created in Sawbones. The fractures were fixed according to standard practice. The samples were loaded under controlled conditions and the rigidity of each sample was assessed.

Samples

Twelve fourth-generation Sawbone tibias were used for this study. These high-fidelity models replicate not only the morphology of a typical tibia but also its mechanical properties[13]. Specifically, Sawbones composite models comprise a porous foam core designed to replicate cancellous bone, enclosed within layers of epoxy resin that simulate the cortical component. The mechanical properties (stiffness and strength) of both materials are calibrated to correspond to those of healthy cancellous and cortical bone, respectively[14].

Proximal tibial Schatzker type II fractures were created in each sample. To ensure consistency and repeatability, fractures were cut with the help of bespoke cutting guides produced specifically for this purpose (Figure 1). Following a step-by-step process, the lateral fragment was first cut along a predefined plane using a handsaw. The depression of the articular surface was then simulated using a pulse bandsaw. All cuts were made using the bespoke cutting guides, which also enabled control of the amount of bone deficit and its relative position (Figure 1).

Figure 1 The 3D printed guides use for the reproducible generation of tibia plateau fractures. A: Left; B: Closed.

Deficit volumes and the bespoke cutting guides were defined with the help of the digital 3D model of the Sawbone samples and computer aided design (Solid Works). To this end, the initial total cancellous bone volume in the lateral tibial condyle was calculated first and the cutting planes splitting this into different ratios of bone-deficit were identified. Two different deficit volumes (low and high) and two different relative positions (proximal and distal) were considered to produce three scenarios. Deficit volume was defined as a percentage of the total cancellous bone in the lateral tibial condyle, with 10% and 50% representing low and high degrees of deficit, respectively. To produce these deficits a transverse plane splitting cancellous bone volume in half was first identified (mid-volume plane). To reproduce the two cases of high deficit the cancellous bone above (proximal 50% deficit) or below (distal 50% deficit) the mid-volume plane was removed. To reproduce the low deficit scenario a new cutting plane was defined distal to the mid-volume plane enclosing 10% of the total cancellous bone volume. Distal and proximal positions were defined relative to a transverse plane dividing the cancellous volume of the lateral condyle into equal halves. The specific scenarios included in the study were: 10% distal deficit (Figure 2A), 50% distal deficit (Figure 2B), 50% proximal deficit (Figure 2C).

Figure 2 The three scenarios of bone deficit tested. A: 10% distal deficit; B: 50% distal deficit; C: 50% proximal deficit.

Four samples were produced for each scenario. Considering that key supporting bone screws are placed in the proximal half of the condyle, among the three scenarios, the last one (50% proximal deficit) was considered to be the most detrimental to fixation stability.

The fractures were fixed with 3.5 mm LCP proximal lateral tibia locking plates (DePuy Synthes, Warsaw, IN, United States) using standard techniques. Following reduction of the articular surface with a temporary K-wire placed in subchondral bone, the plate was positioned on the lateral TP with locking screws placed proximally and distally, as shown in Figure 3.

Figure 3 Saw bone Lateral tibia plateau fracture fixed with locking plate. A: Lateral X-ray view; B: Anterior-posterior X-ray view.

Mechanical testing

The rigidity of the samples was assessed using a 3 kN INSTRON ElectroPuls™ E3000 Load frame, capable of performing quasi-static as well as dynamic loading scenarios.

Samples were fixed to the load frame using a bespoke support that positioned them to simulate a standing orientation (Figure 4). Compressive loading was simultaneously applied to both articular compartments. Following five initial preconditioning load-unload cycles at 500 N, the loading protocol included three load levels with peak forces of 500 N, 1000 N, and 1500 N. One final load sequence was applied, with the end conditions defined as fixation failure or reaching the load frame’s maximum capacity. Compressive displacement was applied at a rate of 20 mm/minute, while force and load-frame displacement were recorded at 100 Hz[3].

Figure 4 Mechanical testing arrangement. A: The testing set-up; B: Closeup showing the bespoke support, loading structures and the marker clusters for fracture gap tracking.

All samples were subjected to compressive loading distributed simultaneously across both articular compartments. A specially designed and manufactured dual-loading applicator was used to divide the applied load between the medial compartment (65% of the applied load) and the lateral compartment (35% of the applied load). The applicator was attached to the load frame’s actuator via a cradle equipped with a freely rotating ball bearing[13].

The relative movement of the lateral fragment with respect to the rest of the bone was captured using a motion capture system. Bespoke marker clusters were mounted on either side of the fracture (Figure 4B). Their displacement in 3D space was recorded using four motion capture cameras (Vicon, Oxford, United Kingdom), which captured marker trajectories at 100 Hz. Marker trajectory data were filtered using a 6 Hz low-pass Butterworth filter in Visual3D (HAS-Motion, Ontario, Canada) to assess the gap opening. Gap opening was defined as the change in the straight-line distance between the bases of the two clusters (Figure 4).

Outcome measures

Outcome measures and statistical analyses were performed for the following: Load frame vertical displacement at 500 N, 1000 N, 1500 N, and at the overall maximum applied load. Fracture gap opening at 500 N, 1000 N, 1500 N, and at the overall maximum applied load. Construct stiffness, assessed as the slope of the force–load frame displacement graph. Compressive load at fixation failure.

Statistical analysis

Descriptive statistics were used to assess the normality of the data. The statistical significance of differences between conditions was assessed using one-way analysis of variance (ANOVA) (α = 0.05).

RESULTS

All samples successfully completed all loading sequences without failure. Loading to the load frame’s capacity (3 kN) did not result in fixation failure. Consequently, no measurements were recorded for compressive load at failure.

As expected, load frame vertical displacement increased with increasing compressive load (Figure 5A). The average stiffness of the entire construct for scenarios 1 to 3 was 1469 ± 57 N/mm, 1484 ± 73 N/mm, and 1467 ± 23 N/mm, respectively (Figure 5B). The average opening of the fracture gap at the maximum applied force (2900 N) was 0.057 ± 0.056 mm, 0.066 ± 0.044 mm, and 0.124 ± 0.077 mm for scenarios 1 to 3, respectively (Figure 5C).

Figure 5 Biomechanical performance across the three cancellous bone defect scenarios. A: Average vertical displacement for increasing loading; B: Stiffness; C: Fracture gap opening under maximum compression for the three conditions considered.

ANOVA revealed no statistically significant differences in gap opening [F (2,9) = 1.44, P = 0.286], or stiffness [F (2,9) = 0.11, P = 0.894]. Post-hoc power analysis based on the observed ANOVA effect sizes demonstrated statistical power of 29.9% for fracture gap opening (Cohen’s f = 0.57, 95%CI: 0.00-1.03) and 6.7% for construct stiffness (Cohen’s f = 0.16, 95%CI: 0.00-0.51), at an α level of 0.05. These values are well below the conventionally targeted 80% power, reflecting the small sample size for this study.

DISCUSSION

This study aimed to investigate in an exploratory biomechanical study, the effect of cancellous bone void volume (CBVV) on the rigidity of TP fracture fixation. The results revealed no statistically significant differences in rigidity between fractures with varying CBVV (10% distal, 50% distal, and 50% proximal deficits). While no significant differences were observed in stiffness, vertical displacement, or fracture gap opening, a trend was identified in which fractures with a 50% proximal cancellous bone deficit exhibited slightly less rigidity, as reflected in the higher fracture gap opening (0.124 ± 0.077 mm compared with 0.057 ± 0.056 mm for the 10% distal deficit). These findings suggest that more proximal cancellous bone voids may impact the mechanical stability of fixation and may warrant the use of additional adjuncts in clinical practice. These results can support further testing on this topic, however as it stands, it cannot support clinical recommendations. This is due to the small sample size and the resultant low statistical power of the results obtained. As such, the results presented here should be interpreted within the context of hypothesis-setting for further research.

The lack of significant differences in stiffness and vertical displacement is consistent with previous studies evaluating the biomechanical properties of TP fractures and their fixation. These studies indicated that while cancellous bone defects can affect the mechanical properties of bone, the size of the defect alone does not always result in significant changes in fixation stability[12,15]. However, the observed trend in gap opening with the 50% proximal defect may indicate a greater biomechanical impact of proximally located cancellous bone voids. This aligns with prior findings suggesting that fractures with substantial cancellous bone defects-particularly in proximal regions-may require adjunctive support or alternative fixation strategies[16,17].

Although all samples completed the loading sequences without failure, it is important to note that no compressive load failure was reached during testing, as the maximum applied force of 3 kN was insufficient to induce fixation failure. This highlights a limitation of the testing protocol, as the load frame’s capacity was not exceeded, preventing a more comprehensive assessment of the ultimate strength of the fixation construct. Future studies could benefit from using a load frame with higher capacity to better evaluate fixation failure and load-to-failure characteristics.

Another notable observation was that gap opening increased progressively with higher applied loads, while stiffness remained relatively stable across the three fracture scenarios. This finding is consistent with studies that examined the effect of cancellous bone defects on TP fracture fixation, which reported that stiffness remained relatively unchanged across different defect volumes, although more proximal defects resulted in larger displacement[17].

The findings from this study are exploratory, and indicate that TP fractures with proximal cancellous bone voids (50% proximal deficit) may require additional support, but further studies are required to investigate this. Evidence supports the use of bone grafting and augmentation in fractures with significant cancellous bone loss, particularly in weight-bearing regions such as the proximal tibia[18]. The findings of this study reinforce the notion that fractures with substantial bone voids, especially in the proximal weight-bearing region, may benefit from adjunctive bone grafting or structural augmentation to enhance fixation rigidity and reduce the risk of collapse or implant migration.

There are limitations to the current study. The small sample size (n = 12) and the use of Sawbones models limit the generalizability of the findings to human tibial fractures. Although Sawbones models are widely used for biomechanical testing and have been validated in many studies[13,14], they do not fully replicate the biological complexities of human bone, including variations in bone quality and the healing response. Further research involving larger sample sizes and human cadaveric models could provide more definitive insights into the biomechanical effects of cancellous bone voids on TP fracture fixation.

CONCLUSION

This study demonstrates that the presence of cancellous bone voids, particularly in the proximal tibial region, may influence the rigidity of TP fracture fixation, with a trend toward reduced rigidity in fractures with a 50% proximal cancellous bone deficit. The data from this study show no statistically detectable group differences in this synthetic model and observed proximal-defect differences require confirmation. These findings emphasize the importance of considering both the volume and location of cancellous bone voids when planning fixation strategies for TP fractures.