After the distraction, the biomechanical tests showed the concurrence of one intended (i.e., sagittal) and two undesired (i.e., coronal and horizontal) rotations (Fig. 6). The minimal change in the tibial slope and twist occurs in the WI (0), SP (0), and DS (mid) conditions. Without precise manipulation, this indicates the inherent effects of kinematic or kinetic changes in the cruciate knee ligaments and patellofemoral contact forces [23–25]. The reported change in the sagittal angle of the distracted plateau induces improper changes in the postoperative range of motion, tibial shear force, and anterior tibial translation of the knee. Based on the findings of this study, the optimal strategy for sawing procedures can be preoperatively planned to minimize or adjust the posterior tibial slope and patellofemoral contact force (Figs. 3(A), (B)). The focus of this study was to highlight the impact of two osteotomy procedures, WI and SP, on the knee biomechanics of the periarticular ligaments and joints.
The proposed PSI design comprises three characteristics: 1) anatomical fitness between the PSI and attached tibia, 2) precise control of the distraction angle and tibial slope, and 3) surgical indicator using an aligning rod. The first characteristic is the consequence of employing CT images to reconstruct the PSI configuration (Figs. 2(A), (B)). The inner PSI surfaces fit well with the patient’s targeted tibia region. The correlation of the cutting slot and the two sawing parameters can be illustrated in terms of the projected slot edge lines (Line C) onto the sagittal and horizontal planes (Fig. 7(A)). Before distraction, the WI can be defined by the slot edge and line normal to the coronal plane. The surgeon saws the medial tibia along the slot to achieve an accurate WI. Similarly, the slot edge can guide the sawing depth to precisely determine the SP. This constitutes the second characteristic of the ideal cutting guide for achieving a precise osteotomy.
The lateral hinge orientation can be finely controlled through the design of the first slot edge (Fig. 7(B)). The hinged motion of the proximal and distal tibiae deviated owing to non-middle distraction. At the DS (post) and DS (ant), the testing results illustrate the significant effect of the non-middle distraction on inducing the sagittal and horizontal rotations (Fig. 6). From the authors’ experiences, the middle site can be 3D printed as a mark on the cutting slot surface for the surgeon’s reference. If the tibial slope adjustment and patellofemoral contact is desired, the method used to control the WI and SP is determined to be more effective than that of the DS (p < 0.05).
A detailed comparison of the PSI functions, as discussed in studies published from 2016 until the present, is provided in Table 1. One of the six comparative indices that are easier to achieve is the indication of the osteotomy site, followed by the saw depth, distraction height, and plate position, while edge inclination and SP comprise the most challenging indices. On comparing the studies conducted by Yang, Jacquet, Sandro, and Jud, it is noted that the cutting slots were designed but not finely used postoperatively as the adjusting or controlling tibial slope and twist references [6, 7, 10, 13]. Even with a precisely designed cutting slot, the bone saw with a stop to avoid overcutting and ensure that the saw was orthogonal to the slot edge was necessary to achieve precise control of the target outcome. Except for studies conducted by Yang, Jacquet, and Jud [6, 7, 13], there is no detailed discussion concerning the precise PSI positioning on the tibial surface pins prior to the utilization of guiding pins.
During distraction, surgeons often face difficulty in accurately confirming whether the presumed distraction height or angle has been reached. In clinical practice, gradual distraction and the final WBL can be monitored and confirmed using common fluoroscopy methods [3]. However, the surgical accuracy of these methods is prone to technical errors owing to manual and visional methods. Consequently, computer-navigated techniques and novel instrumentation have been introduced to improve OWHTO accuracy [4–13]. This study provides a method to intraoperatively monitor the 3D rotation of the distracted tibia in real time (Fig. 8(A)). Furthermore, the aligning holes were not initially coaxial. Using computer-aided technology, PSI designers can simulate the angle change of the two PSI holes to be aligned when the distracted tibiae rotate. This allows surgeons to concentrate on the distraction procedure without having to excessively monitor the fluoroscopy system. Once the metal rod can smoothly pass through the two holes, it implies that the presumed distraction angle or WBL percentage has been reached (Fig. 8(B)). From the authors’ experiences, this requires sophisticated procedures to simulate the coronal, sagittal, and horizontal rotations of the distracted tibia, which is reconstructed using CT images.
It is worth noting that certain biomechanical and surgical limitations are inherent in this study. Improper WI and SP might induce hinge fractures. In the situation of excessive sawing depth—SP (5), the distraction potentially risks the fracture of the lateral cortex owing to the remaining deficient bone. Contrarily, the lack of sawing depth in the case of SP (-5) potentially exposes the proximal part of the osteotomized tibia to intercondylar fracture. Similarly, the inclined wedge of WI (-10) and WI (+ 10) should be cautiously created and distracted to avoid hinge fractures. Limited experiences concerning the higher inclination of WI and SP can be concluded as the PSI design guidance. The simulation of the aligning holes was based on the 3D model of the proximal tibia, excluding the distal tibia effect and the periarticular tissue constraints. These testing results do not support the mathematical formula for predicting the spread angle in the literature [26–28]. The same distraction height leads to different distraction angles, showing that WI, SP, and DS affect the distraction angle (Fig. 6(A)). The formulation to correlate the PSI parameters and three tibial rotations is still ongoing in the current authors’ laboratory.
In conclusion, the orientation of the lateral hinge was determined by the WI and SP. By controlling the two sawing parameters, the ideal PSI can be well-designed to serve as a target guide, thus precisely maintaining or adjusting both the distraction angle and tibial slope. Preoperative planning of the sawing procedure involves a trade-off between distraction efficiency and undesired effects (i.e., sagittal and horizontal rotations) on the knee biomechanics and potential lateral hinge risk. The most critical design area for OWHTO PSI is the cutting slot that guides the WI and SP.