Background
High tibial osteotomy (HTO) with a medial opening wedge is a surgical treatment for correcting knee osteoarthritis limited to the medial compartment [
1‐
3]. It is more favorable than lateral closing wedge osteotomy due to the complications such as compartment syndrome, neurological complications, lateral muscle detachment, proximal fibula osteotomy, and limb shortening [
4‐
6]. However, the opening wedge can potentially make the proximal tibia highly unstable and necessitates instrumentation by plate and screws to keep the graft in place and stabilize the osteotomized construct [
6‐
8]. Several plate systems have been used for medially opening HTO, such as TomoFix, Puddu, Position HTO plate, and traditional DCP plate [
4,
5,
7,
9,
10]. Among them, the TomoFix system is comparatively more commonly used to stabilize the opening wedge by using locking screws [
5,
9,
11].
From the biomechanical viewpoint, both implant stress and wedge stability are highly correlated with implant stiffness and placement site [
6,
12,
13]. For the T-shaped plate, the anteromedial and medioposterior regions have been selected as the sites of plate placement [
6,
14‐
16]. The placement-induced effects of HTO plates have been investigated in terms of the tibiofemoral loads and the tibial slope [
15,
17‐
19]. Blecha et al. proposed that the contact sites of the tibiofemoral joint occur at the more anterior region for up-right standing with a lower flexion angle [
6]. They suggested that the optimal site of the plate placement is at the anteromedial region to provide more stable support [
6,
14]. However, Rodner et al. revealed that the anteromedial placement of the HTO plate increases the tibial slope at that region and shifts the tibiofemoral contact to the posterior side. This increases the shearing force on the tibial plateau, which might lead to instability and injury of the soft tissues [
15]. Consequently, they recommend that surgeons place the plate at the medioposterior site for better maintenance of the tibial slope [
15,
18]. Up to the authors'knowledge, however, detailed comparison of the construct performance between two placements has still not been extensively investigated and the optimal placement remains a controversy. This constitutes the motive of this study.
This study aims to evaluate the placement-induced effects of the TomoFix plate on the construct performance. The hybrid use of the T- and I-shaped plates is applied as the control group to provide a wider base to support the tibiofemoral loads. This is done to eliminate the placement-selecting problem induced by the smaller TomoFix plate. The short- and long-term effects of plate placement are evaluated by biomechanical tests and fatigue simulation. The findings of the current study provide insight into the device- and surgery-related factors associated with better outcomes of HTO fixation.
Discussion
Compared with the TomoFix system, the TIP can provide two load-transferring legs to transmit the femorotibial loads across the medial wedge (Fig.
1). This study hypothesizes that the two-leg construct has two structural advantages over the one-leg counterparts. The TIP construct can form a wider base to support the opening wedge and avoid surgical hesitance about plate placement. The testing results demonstrate that both the wedge stiffness and failure load of the TIP construct are consistently the highest among the three variations (Fig.
5). Additionally, the two-leg base can behave as a force-couple mechanism to resist the bending loads of the osteotomized tibia [
24]. At the edges
aa and
cc, the wedges of both APT and MPT constructs are respectively compressed and distracted (Fig.
9). The transformation from compression to distraction shows the bending characteristics of the TomoFix constructs. However, the distraction at the wedge tip is unfavorable to callus formation and even induces wedge fracture (Fig.
4a) [
15,
27].
Comparatively, the three edges of the TIP wedge are consistently compressed and more protected, thus effectively suppressing the increase in tibial slope (Fig.
9). Some studies have claimed that increased tibial slope will redistribute tibiofemoral contact pressures posteriorly on the tibial plateau, open the osteotomy anteriorly, and increase anterior tibial translation and subluxation [
15,
28,
29]. At the edges
aa and
cc, the wedge micromotion of the APT construct is about 2.36 and 3.66 times that of the MPT construct. This indicates the potential risk of deterioration of tibial slope, wedge stiffness, and residual life in the APT tibia (Figs.
5b and
8a). The plateau fracture of the TIP construct can be attributed to the weakened strength of the resected plateau for the sake of spring placement (Fig.
2a).
The screws of the TIP construct are nonlocking fashion in this study. This induces the interfacial slippage at the plate-screw junctions and makes the contact stress highly concentrated [
24] (Fig.
7). In turn, the residual life of the TIP implant is significantly less than those of the TomoFix counterparts (Fig.
8). This can be overcome by the use of the plate-screw locking mechanism. However, there are two rigidity-induced concerns about the TIP construct. The minute micromotion of the TIP wedge might be unfavorable to the bony fusion of the opening wedge [
30,
31]. Moreover, the stress-shielding effects of the stiffer TIP system might lead to osteoporosis and delay the bony union [
32,
33]. These concerns should be further evaluated by the long-term examination of the clinical study.
For the three variations, the relationships between the failure load and wedge stiffness are evaluated by calculating their coefficients of correlation (R value). For the TIP, APT, and MPT, the R values of the failure load and wedge stiffness are 0.86, 0.91, and 0.94, respectively. It indicates that the higher stability of the construct wedge can effectively improve the construct resistance to bone failure. The highly negative correlation (R = −0.99) between the edge aa micromotion and wedge stiffness can serve as the well validation between the experimental and numerical results.
This study neglects the plate-bending effect on the biomechanical properties of the various constructs. In the simulative and experimental models, all plates are well pre-bended before fixing them to the tibial model. However, in the technique manual of the AO TomoFix system, the bicortical screw pulls the distally osteotomized segment towards the plate and forces the plate into suspension, creating an elastic preload, which imposes pressure upon the lateral hinge. From the biomechanical viewpoint, however, the current authors presume that the use of bicortical screw to bend the plate potentially alerts the initially distracted height of the medial wedge and exerts the additional shearing force onto the lateral hinge. This might make the lateral hinge at the higher risk of combining the tensile and shearing loads. The pre-bending effect should be confirmed by further studies.
Similar to any model that attempts to simulate femorotibial complexity, there are some limitations and assumptions inherent in this study. After osteotomy, intervention-induced loads originating from the distraction of the remaining intact cortex, medial collateral ligament, and patellar ligament exist. These loads constitute the stabilizing force on the graft [
3,
6]. However, the intervention loads are excluded due to the ease of experimental setup. Limited to the available data sources, the femorotibial loads of the degenerative knee are briefly defined in the tests and analyses. Using elastic springs, only compressive loads are assumed to exist in the experimental and numerical evaluations. The deformation of the rubber discs leads to the weaker stiffness of the tested specimen than that of the counterpart in the numerical analysis (Figs.
2a and
5b). Bone remolding is not simulated in this study; thus the rigidity-induced problem of the overstayed and stiffer plate cannot be evaluated
in vivo.
Conclusions
Both placement site and support base have significant effects on the construct behaviors. Even without using locking screws, the two-leg TIP forms an effective stabilizer to resist the bending and compressive loads and, thus, significantly enhancing construct stiffness and suppressing wedge fracture. The MPT construct shows performance more comparable to the TIP construct than the APT. Consequently, the TIP construct with locking screws is recommended for the patients with demands of heavy and/or dynamic loads. In the situation of using a single plate, the MPT construct can be an alternative for stabilizing the medial HTO wedge.
Competing interests
The authors declare that they have no competing interests.
Author’s contributions
CAL, SYH, and SCL conceived of the study, participated in the design of the study and performed the data analyses. CAL and SCL formulated the model and drafted the manuscript with the help of CMC and CST. All authors carried out the experiments, read, and approved the final manuscript.