Validation of Computer-assisted Open-wedge High Tibial Osteotomy Using Three-dimensional Navigation

Abstract

An unintended increase in the posterior tibial slope after open-wedge high tibial osteotomy (HTO) can influence knee kinematics and stability. The objective for this study was to validate the change of the tibial slope obtained from three-dimensional (3D) navigation in open-wedge HTO by comparing it with that evaluated with computed tomography. Human cadaver knees were used. The open-wedge HTO was performed to maintain the anatomic tibial slope according to the navigation system. 3D navigation could provide surgeons with reliable information not only to determine appropriate coronal alignment but also to maintain the anatomic tibial slope in open-wedge HTO.

High tibial osteotomy (HTO) is an established operative procedure for correction of varus deformity in patients with unicompartmental osteoarthritis.^1,2 Medial open-wedge HTO with interposition of bone grafts or hydroxyapatite wedges has been reported.^3,4 It has become popular recently because it does not introduce peroneal nerve problems, as well as the simplicity of the procedure, shorter surgical time, more precise correction, enhancement of bone stock, and avoidance of changes in the proximal morphologic characteristics of the tibia.^3,5,6 In addition, concomitant anterior cruciate ligament (ACL) reconstruction may be easier with the medial open-wedge osteotomy than with the lateral closed-wedge osteotomy.⁷ However, studies have shown that the posterior tibial slope of the proximal tibia tends to increase when performing a medial open-wedge HTO.^5,8 Furthermore, an unintended increase in posterior tibial slope can influence knee kinematics, stability,⁹ and tibiofemoral contact pressure10 and potentially cause knee pain and loss of normal knee extension.¹¹ Therefore, in open-wedge HTO, both coronal alignment and sagittal alignment should be given close attention. Three-dimensional (3D) navigation is available for open-wedge HTO and can monitor simultaneously both the coronal and sagittal alignment, such as the change in the posterior tibial slope.

The objective of this study was to validate the change of the tibial slope obtained from 3D navigation in open-wedge HTO by comparing it with that evaluated with computed tomography (CT). In addition, the open-wedge angle along the anteromedial tibial cortex and the length of the anterior and posterior opening gaps of the open-wedge HTO, which maintain the anatomic tibial slope with 3D navigation, were measured with postoperative CT.

Materials and Methods

Specimen Preparation

Six fresh frozen human cadaver knees (62.5 ± 11.4 years; range, 50 – 82 years) were used in this study. The femur and tibia were cut approximately 20 cm from the joint line, and all soft tissue except ligaments, menisci, and joint capsule was removed. A preoperative 3D image was obtained in all specimens using a 16- detector CT scanner (Light Speed Ultra 16, GE Healthcare, Milwaukee, Wisconsin). The cadaveric knee was then connected with a skeleton model that included the pelvic, hip, and ankle joints by using an orthodontic resin so that navigation-assisted open-wedge HTO was simulated.

Procedures of Open-wedge High Tibial Osteotomy Using 3D Navigation

A kinematics-based image-free navigation system (OrthoPilot; B. Braun Aesculap, Tuttlingen, Germany) with HTO software version 1.4 (3D Open-wedge; B. Braun Aesculap) was used for all experiments (Figure 1). The transmitter was fixed on the distal femur and the distal tibia (tibial shaft) with a bicortical screw. To determine the mechanical leg axis, kinematic data including hip, ankle, and knee joints were registered. Anatomic landmarks, such as the medial epicondyle, lateral epicondyle, medial malleolus, lateral malleolus, central point of the ankle, and medial point of the tibial plateau were registered with a pointer. For 3D HTO navigation, an additional transmitter was fixed on the tibia part proximal from the cutting site with 2.5-mm k-wire (Figure 2) so that the distal and proximal tibia portions could be navigated directly against each other. The initial position of the proximal tibia was also registered. Once the registration was done, the mechanical leg axis was visualized continuously. The osteotomy began approximately 3 cm distal to the medial joint line at the medial cortex of the proximal tibia and was just proximal to the tibial tubercle, leaving 5- to 10 mm of the lateral tibial cortex intact. With monitoring of the mechanical leg axis and change of the tibial slope provided by navigation, the osteotomy was stabilized using a plate (POSITION HTO Plate; B. Braun Aesculap) with a 7-mm rectangular spacer block, maintaining the anatomic tibial slope (Figure 3). The plate was placed at the center of the medial cortex of the proximal tibia.

Figure 1: Experimental set up of the navigation-assisted open-wedge high tibial osteotomy.	Figure 2: A, Human cadaver knee with transmitters. B, Screen shot of the preoperative leg alignment.

Evaluation of Tibial Slope, Open-wedge Angle, and Anterior and Posterior Opening Gaps

The correction angle in the coronal plane and change of the tibial slope were determined in the preoperative and postoperative 3D-CT (Figure 4). The data obtained from CT was compared with those obtained from the navigation system. In addition, the open-wedge angle along the anteromedial tibial cortex and the length of the anterior and posterior opening gaps was measured in the postoperative 3D-CT (Figure 5). The anatomic points for measuring the opening gaps were determined using the methods described by Song et al.¹⁶ The point for the anterior opening gap was the anteromedial cortex of the proximal tibia (posteromedial aspect of tibial tuberosity only) on the lines of the osteotomy. Meanwhile, the point for the posterior opening gap was the posteromedial cortex of the proximal tibia on the lines of the osteotomy. A Wilcoxon signed-rank test was used for statistical analysis (SPSS 16.0; SPSS Science Inc, Chicago, Illinois)

Figure 3: A, Human cadaver knee after open-wedge high tibial osteotomy. B, Screen shot of the postoperative leg alignment and the change in the tibial slope.

Results

After open-wedge HTO, the femorotibial mechanical axis obtained from the navigation system was corrected from varus 2.3° ± 1.2° (range, varus 1° – 4°) to valgus 5.0° ± 2.8° (range, valgus 2°– 8°). Therefore, the change in the femorotibial mechanical axis was a mean valgus of 7.3° ± 1.6° (range, 5°– 9°). Meanwhile, the change in femorotibial angle on the coronal plane on CT was a mean valgus of 6.8° ± 1.1° (range, 6°– 8°). There was no statistically significant difference between the correction angle of the coronal plane in navigation and CT data (P >.05).

With regard to the posterior tibial slope, data obtained from navigation and CT are presented in Table 1. Because the osteotomy was fixed to maintain the anatomic tibial slope with a viewing navigation monitor, the increase of the posterior tibial slope on the navigation system was only 0.2° ± 0.4° (range, 0°– 1°) (Table 1). Based on CT measurement, the posterior tibial slope was maintained, measuring 10.1° ± 1.7° preoperatively and 10.6° ± 2.2° postoperatively. There was no statistically significant difference in change in posterior tibial slope between the navigation and CT data (P > .05).

The length of the anterior and posterior opening gaps, and the open-wedge angle along the anteromedial tibial cortex, are presented in Table 2. The anterior opening gap was 61% (range, 53% – 69%) of the posterior opening gap, and the open-wedge angle was 4.6° ± 0.4° (range, 4.0°– 5.1°) when the anatomic tibial slope was preserved using 3D navigation.

Figure 4: Measurement of the posterior tibial slope on 3D-CT. Figure 5: Measurement of the anterior and posterior opening gaps and open-wedge angle on postoperative 3D-CT.

Discussion

In this study, the changes of the tibial slope on 3D navigation after open-wedge HTO were compared with those on CT. In addition, the open-wedge angle along the anteromedial tibial cortex and the length of the anterior and posterior opening gaps in the open-wedge HTO that preserve the anatomic tibial slope were determined on postoperative CT.

Although HTO for patients with unicompartmental osteoarthritis is performed to correct varus deformity in the coronal plane, open-wedge HTO tends to increase posterior tibial slope in the sagittal plane simultaneously. An undesired increase in the posterior tibial slope can limit full extension of the knee joint and cause anterior knee pain. Watanabe et al¹¹ reported a revision HTO correcting sagittal alignment with a dynamic external ring fixator to treat anterior knee pain and a fixed flexional deformity associated with a previous failed medial open-wedge HTO.

An unintended increase in tibial slope also leads to excessive anterior translation and subluxation in patients with ACL deficiency.¹² Giffin et al⁹ evaluated the effects of altering tibial slope on the biomechanics of the knee. They observed that increasing the slope causes an anterior shift in tibial resting position, which is accentuated under axial loads. This finding suggests that increasing the tibial slope may be beneficial in reducing tibial sag in a PCL- deficient knee, whereas decreasing the slope may be protective in an ACL-deficient knee. Therefore, during open-wedge HTO, both coronal alignment and sagittal leg alignment should be monitored.

A navigation system has been used successfully in open-wedge HTO, with the advantage of continuous real-time visualization of the limb alignment.^13-16 However, previous navigation systems could only monitor coronal leg alignment. The new version 3D navigation systems are available for open-wedge HTO and can provide intraoperative real-time alignment not only of the coronal plane but also of the sagittal plane, such as the change in the posterior slope.

Noyes et al¹⁷ reported 3D analysis of the proximal tibia to show how the angle of the opening wedge along the anteromedial tibial cortex influences the tibial slope (sagittal plane) and valgus correction (coronal plane) during medial open-wedge osteotomy. They determined that the anterior osteotomy gap at the tibial tubercle must be half the posteromedial gap to maintain the normal sagittal tibial slope. Every millimeter of gap error at the tibial tubercle resulted in approximately 2° of change in the tibial slope. Their specific measurements and calculations provide the surgeon with the ability to determine the appropriate anteromedial tibial opening wedge to maintain or correct the tibial slope and to obtain the desired coronal axial alignment. Song et al¹⁶ examined methods for avoiding unintended increases in posterior tibial slope in open-wedge HTO using computer-simulated 3D virtual surgery. The virtual surgery demonstrated that the anterior opening gap should be 67% of the posterior opening gap to preserve the original posterior slope. In addition, they performed navigated open-wedge HTO with two different plate sizes to maintain an anterior opening gap of approximately 67% of the posterior opening gap. Only a slight increase (0.4°) was observed in the posterior tibial slope after open-wedge HTO.

This study showed that the anatomic tibial slope could be maintained after open-wedge HTO with intraoperative monitoring of the change in the tibial slope using 3D navigation. The anterior opening gap was 61% of the posterior opening gap and open-wedge angle was 4.6° when the anatomic tibial slope was preserved using 3D navigation. A plate with a wedge-shaped spacer block in place of a rectangular spacer block may be suitable for stabilizing the osteotomy while preserving the anatomic tibial slope in open-wedge HTO. Results suggested that 3D navigation could provide surgeons with reliable information to maintain the anatomic tibial slope in open-wedge HTO.

References

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Authors

Drs Yamamoto, Ishibashi, Tsuda, Tsukada, Kimura, and Toh are from the department of Orthopaedic Surgery, Hirosaki University Graduate School of Medicine, Hirosaki, Aomori, Japan.

Drs Yamamoto, Ishibashi, Tsukada, Kimura, and Toh have no relevant financial relationships to disclose.

Correspondence should be addressed to: Yuji Yamamoto, MD, Department of Orthopaedic Surgery, Hirosaki University Graduate School of Medicine, 5 Zaifu-cho, Hirosaki, Aomori, 036-8562, Japan.