Do monocortical distal locking screws impair mechanical properties in opening wedge high tibial osteotomy with bone graft?—a sawbone biomechanical study

Introduction

High tibial osteotomy (HTO) is a surgical procedure generally considered for younger, active individuals who have unicompartmental knee osteoarthritis and are not ideal candidates for total knee replacement surgery. Medial opening wedge high tibial osteotomy (MOWHTO) has been increasingly recognized an effective re-alignment procedure for medial osteoarthritis of the knee with extra-articular varus deformity (1), and has shown to delay arthroplasty (2). As compared to Lateral Closing Wedge High Tibial Osteotomies (LCWHTO), MOWHTO has gained relative popularity with advantages of bone preservation, enhanced ligamentous stability, avoidance of fibular osteotomy and potential for technically simpler revision surgery.

In MOWHTO, a trapezoidal gap is created on the medial aspect of the proximal tibia with a biplanar osteotomy leaving an intact lateral hinge; the gap can either be left to heal secondarily or filled with a bone graft or an equivalent substitute. This inherently destabilizes the proximal tibia, therefore necessitating further stabilization with a fixation device consisting of an angular-stable plate with locking screws spanning the osteotomy site. The mechanical strength of the MOWHTO construct is critical for stability and bony union. A strong and rigid construct would minimize the risk of complications that can arise from mechanical instability from hinge fractures, such as loss of correction, loss of fixation, malunion, delayed union and non-union (3).

With the move to use smaller adaptive locking plates, stabilizing the MOWHTO construct has taken even greater priority. Thus, bicortical fixation of all distal screws for MOWHTO locking plate has conventionally been preferred (4,5) to engage both cortices of the tibia. Bicortical fixation of the distal screws of the MOWHTO locking plate has been perceived to confer better resistance to shear and rotational forces, and minimize the risks of implant failure from physiological loading. However, such differences may not be clinically or biomechanically significant in the setting of MOWHTO under physiological loads. Conversely, monocortical fixation may provide additional postulated benefits of decreased stress shielding, decreased incidence of symptomatic hardware from screw prominence and soft tissue injury, and therefore may theoretically lower the peri-operative complication and revision rates (6). Current technique guides do not yet provide specific direction on whether bicortical or monocortical fixation of distal screws is more optimal in MOWHTO, despite acknowledging the possibility of both fixation techniques (7).

To the best of our knowledge, few studies in prevailing literature directly compare mechanical stability between monocortical and bicortical fixation of distal screws in MOWHTO. A recent study conducted on three-dimensional (3D) bone models found that monocortical fixation of the three distal screws in MOWHTO did not worsen the stability of fixation (8). Another cadaveric study also supported the use of monocortical fixation; however, the comparison had been done on proximal screw holes instead (9). A rigorous mechanical evaluation in this regard would add much value to advancing surgical best practices in MOWHTO fixation.

This study therefore aimed to evaluate the differences in compressive loads and fatigue strength between monocortical and bicortical distal screw fixation in the setting of a MOWHTO construct in a controlled laboratory setting using artificial sawbones, in order to ensure consistency in simulating the qualities of the average human tibia and minimize inter-specimen variability in geometrical and structural properties (10). We hypothesized that there will be no significant differences in the mechanical properties of monocortical versus bicortical distal screw fixation in MOWHTO.

Methods

Specimen preparation

Twenty human-sized fourth-generation composite analogue tibia (Synbone AG, Zizers, Switzerland) sawbone models were used. These were divided into two groups of 10 specimens each: “bicortical group” and “monocortical group”. A priori power analysis was conducted using G*Power 3.1.9.7 to determine the required sample size for comparing two independent means. Based on similar biomechanical studies such as Lind-Hansen et al. (11), we assumed an effect size (Cohen’s d) of 1.3, a Type I error probability (α) of 0.05, and 80% power (1−β). Using a two-tailed Mann-Whitney U test, the analysis indicated that 10 specimens per group (total 20) would be sufficient to detect significant differences.

A biplanar MOWHTO with an 8 mm osteotomy gap was performed on each specimen by a single fellowship-trained orthopaedic surgeon, using consistent technique and positioning guides to limit variability in plate placement across specimens. The surgical technique utilized has been described in a prior publication (12). A 3D-printed 8 mm wedge of composite analogue tibia bone graft was then inserted into the osteotomy gap prior to plate fixation, to simulate filling of the osteotomy gap. Each MOWHTO sample was stabilized using a NewClip Technics Activmotion HTO plate (Haute-Goulaine, France) made of titanium alloy (Ti6Al4V), fixed with four proximal and four distal locking screws made of the same titanium alloy, each 4.5 mm in diameter (Figure 1). The four proximal screws were uniformly 60 mm in length. In the “bicortical group”, all four distal screws measured 42 mm in length and engaged both cortices. In the “monocortical group”, two proximal distal screws (4.5 mm diameter, 40 mm length) were bicortical, while the two distal-most screws (4.5 mm diameter, 35 mm length) were fixed monocortically.

Figure 1 Setup of artificial tibia post-8 mm biplanar MOWHTO and locking plate insertion. MOWHTO, medial opening wedge high tibial osteotomy.

Mechanical testing

Static and dynamic tests were performed on the prepared bone-implant constructs. Half of the specimens prepared were randomly allocated to quasi-static compression testing; the remaining half was allocated to dynamic testing in the form of cyclic compressive loading to failure. Compression tests were selected as the mode of load application for both static and dynamic tests, in order to optimally simulate the load that would be applied on the knee post-operatively.

For both types of mechanical testing, all specimens were individually mounted onto hydraulic testing machines, an Instron 5566 Universal Testing Machine (Instron, Massachusetts, USA) for the static tests and Zwick LTM3 Electrodynamic Testing Machine (ZwickRoell Pte Ltd., Ulm, Germany) for the dynamic tests respectively. A 3D-printed distal tibial support was used to mount the specimens onto the machines, minimizing any movement of the specimens during testing. Additional support was provided to the specimens via a proximal tibial loading fixture to ensure that force could be evenly spread from the machine through the tip of the bone. Throughout mechanical testing, the specimens were carefully observed to exclude the presence of hinge fractures.

Quasi-static compression testing

Specimens underwent quasi-static compression testing to simulate physiological weight bearing post-MOWHTO during standing. Each specimen was subjected to unilateral quasi-static compression testing with displacement-controlled single loading applied at a speed of 0.1 mm/s until failure (Figure 2). We determined failure at the point where a visible collapse of the lateral cortex of the tibia could be observed, along with an audible cracking sound. Load-displacement curves were acquired for each specimen within the Instron software and their ultimate compressive strengths were recorded.

Figure 2 Setup of specimen for quasi-static compressive testing.

Dynamic fatigue testing

Dynamic testing was also incorporated to simulate repetitive loading on the knee post-MOWHTO during physical activity. This consisted of load-controlled cyclic fatigue testing, with stepwise compression sinusoidal loading to failure at a frequency of 2 Hz. The force amplitude of each step was kept constant with feed-back control of the force signal within the machine. The lower compressive force limit of each load step was kept constant at 160 N; the upper compressive force limit of each load step was set at 800 N for the first step, and subsequently increased stepwise by 80 N every 20,000 cycles until failure (Figure 3).

Figure 3 Experimental setup and protocol for dynamic fatigue testing. (A) Setup of specimen for dynamic fatigue testing. (B) Schematic of testing protocol for cyclic load-controlled sinusoidal force loading.

Like the static testing phase, the load-displacement curves of each specimen were also acquired, and the maximal load and number of cycles to failure for each specimen. The maximal load and number of cycles to failure for each specimen were recorded and the load-displacement curves were also acquired.

Data analysis

The primary outcomes of this study are the median ultimate compressive strength of the monocortical and bicortical specimens for static testing; as well as median maximal load and number of cycles to failure for dynamic testing. All statistical analyses were conducted using Microsoft Excel (Microsoft Corp., USA). Box and whisker plots were constructed to visualize the distribution of the data points and facilitate appropriate choices of hypothesis testing. Owing to the small sample size, nonparametric tests were used to compare outcomes between the monocortical and bicortical groups, with statistical significance defined as P<0.05.

Results

For both static and dynamic mechanical testing, hinge fractures were absent in all 10 specimens at the point of mechanical failure. Instead, all 10 specimens failed at the diaphysis of the tibia beyond the distal tip of the locking plate (Figure 4).

Figure 4 Point of failure of all MOWHTO specimens after quasi-static and dynamic compressive testing, occurring beyond the distal tip of the locking plate underscoring their mechanical stability. MOWHTO, medial opening wedge high tibial osteotomy.

Quasi-static compression testing

Specimens from the monocortical group had a median ultimate load of 2.60 kN [mean ± standard deviation (SD) =2.59±0.15 kN], comparable to that of 2.64 kN (mean ± SD =2.56±0.33 kN) in the bicortical group (Table S1). Figure 5 depicts the distribution of the data points obtained. A 1-tailed Mann-Whitney U test confirmed that the median ultimate load of monocortical specimens was not significantly lower than in bicortical specimens, returning P>0.05.

Figure 5 Boxplot of ultimate load (kN) of monocortical and bicortical specimens upon quasi-static compression testing.

Dynamic fatigue testing

For cyclical loading to failure, the median maximum load attained before collapse of the specimens were similarly comparable between monocortical and bicortical specimens (Table S2). Again, Mann-Whitney-U testing showed no statistically significant difference in median maximum load (P=0.654), as with the actual number of cycles (P=0.249) withstood by both the monocortical and bicortical groups of specimens (Table S3). The distribution of these data points is summarized as box-and-whisker plots in Figure 6.

Figure 6 Boxplots of (A) maximal load (kN) attained and (B) number of cycles tolerated by monocortical and bicortical specimens upon dynamic fatigue testing.

Discussion

Key findings and explanations

Our findings demonstrate that the mechanical stability of MOWHTO with monocortical fixation of the distal two screws is noninferior to bicortical fixation. This applies whether for static or dynamic testing, where the median ultimate compressive strength and fatigue strength respectively were consistently shown to have no statistically significant difference between the two arms. The ultimate compressive load was also found to be sufficient, corresponding to about three times the body weight of a typical patient, and was consistently observed even for the monocortical specimens.

Furthermore, the absence of lateral hinge fractures in both monocortical and bicortical specimens underscored the safety of the hinge in all construct loading patterns. All samples had consistently failed at the diaphysis, which we note is scarcely observed physiologically in complications arising from MOWHTO (13). This could be a property arising from the purported stability of Activmotion locking plates used in this study, as it features a self-locking system (14) without including a supplemental screw (15) to protect against lateral hinge fractures.

Comparison with similar research

This effectively corroborates insights from literature thus far (9)—Itou et al. (8) also found no significant differences in stress distribution between monocortical fixation at the lateral hinge. More importantly, our study provides a valuable extension of the utility of monocortical fixation in the setting of controlled testing on artificial bone. With the assurance of no additional risk of lateral hinge fractures associated with monocortical fixation, our findings present monocortical fixation of the distal screws as an appealing option in vivo due to established reduction in risk of injuring the deep peroneal nerve (16).

Taken into the wider context of orthopedic applications, the noninferiority of monocortical fixation have been demonstrated in existing studies. A recent computational modelling study showed that monocortical fixation may provide comparable construct protection and stability to bicortical fixation in tibia diaphyseal fracture models during the early healing phase (17). Multiple reviews (18,19) on volar locking plate osteosynthesis for distal radius fractures corroborated that unicortical locking fixation with 75% of bone depth had equivalent stability with bicortical fixation, with similar findings in the clinical setting (20). Monocortical fixation has also been shown to have equivalent effectiveness without compromising construct stability in other types of fracture fixation, ranging from clavicular (21,22) to mandibular fractures (23). Purported benefits of monocortical fixation discussed in these contexts include protection of soft tissue and neurovascular structures and a potential reduction in irritation and rupture of adjacent tendons associated with penetrating the distal cortex (24). These can be harder to mitigate with a bicortical fixation approach given that neurovascular structures can occur as close as 5 mm from bicortical screws, such as in the case of clavicular fracture fixation (25) where serious limb or life-threatening complications may rarely occur. Symptomatic hardware associated with complicated fixation tends to present in a delayed rather than immediate manner (21), and minimizing screw prominence through monocortical fixation may help reduce such risks, potentially lowering morbidity and rates of revision surgery.

Strengths and limitations, implications, and future actions

The authors acknowledge some limitations in this study. Although an a priori power analysis was performed, the relatively small sample size of 10 specimens per group limits statistical power, and we plan to address this with larger cohorts in future studies. The use of synthetic composite tibiae allowed for consistent, repeatable biomechanical testing but lacks biological factors such as healing and remodeling present in vivo, which may affect clinical outcomes; accordingly, future cadaveric studies and clinical trials are planned to validate these biomechanical findings and better account for biological variability. Additionally, cyclic loading was applied only vertically, and the absence of multi-directional forces such as torsional and shear stresses limits the simulation of real-world physiological loads; therefore, future work will incorporate multi-directional loading to more closely replicate physiological conditions. Slight variations in locking plate positioning may also have influenced measurements. Nonetheless, standardized surgical techniques and consistent protocols were employed to minimize variability.

Nonetheless, this study has significant strengths. The controlled setting with artificial bone allowed for precise, repeatable testing of both monocortical and bicortical fixation under static and dynamic loads, which is a key strength in understanding their mechanical properties without the confounding factors of biological variability. We also rigorously demonstrated that monocortical fixation can provide comparable stability to bicortical fixation without additional risks of lateral hinge fractures, which could directly inform surgical practice. We envision future studies to be performed on a larger scale with greater sample size and for our findings to be verified clinically via the gold standard of a randomized controlled trial.

Conclusions

In summary, our study found that monocortical fixation of distal screws in MOWHTO is non-inferior to bicortical fixation, in terms of compressive load-bearing till failure and fatigue strength. Monocortical fixation of the two distal-most screws may be considered a biomechanically non-inferior modification to the standard fixation technique in MOWHTO.

Acknowledgments

We would like to acknowledge Mdm Heng Chee Hoon from School of Mechanical & Aerospace Engineering, Nanyang Technological University, Singapore, for her guidance towards our mechanical testing protocol implementation.

Footnote

Peer Review File: Available at https://atm.amegroups.com/article/view/10.21037/atm-24-223/prf

Funding: This work was supported by the SingHealth Duke-NUS Academic Medicine Research Grant (No. AM/SU062/2021).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://atm.amegroups.com/article/view/10.21037/atm-24-223/coif). H.R.B.A.R. serves as an unpaid editorial board member of Annals of Translational Medicine from July 2024 to June 2026. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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