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How to Avoid Unintended Increase of Posterior Slope in Navigation-assisted Open-wedge High Tibial Osteotomy

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Abstract

Open-wedge high tibial osteotomy (HTO) with navigation provides accurate axial alignment correction in the frontal plane but can cause an unintended increase of the proximal tibial slope. This study examined the plausibility of preserving the original posterior slope of the proximal tibia during navigation-assisted open-wedge HTO. Three-dimensional virtual HTO preserving the original posterior slope was simulated and the opening gaps measured. Forty cases of navigation-assisted open HTO for unicompartmental osteoarthritis were performed maintaining an anterior opening gap measuring 67% of the posterior gap. The posterior slope of the proximal tibia was compared before and after surgery. In virtual surgery, a ratio of 0.67 between the anterior and posterior opening gap was needed to maintain the original slope. In navigation-assisted HTO, the mechanical axis was corrected to 2.9° valgus (range, 0.5°-6.2°) with fewer outliers (12.5%), and the posterior slope was maintained. The posterior slope was 7.9°±2.3° preoperatively and 8.3°±2.8° postoperatively (P>.05). The normal tibial posterior slope can be maintained if the anterior opening gap is approximately 67% of the posterior opening gap during the navigation-assisted HTO.

Unicompartmental osteoarthritis with malalignment can be resolved using high tibial osteotomy (HTO), which can result in changes in the slopes of both the coronal and the sagittal planes. Studies have reported that the posterior slope of the proximal tibia tends to increase after open-wedge HTO.^1,2 In addition, an undesired change in the tibial slope can influence knee kinematics and stability.^1,3,4

Long-term success of HTO depends on the quality of the correction and under- or overcorrection is a significant cause for HTO failure.^5-7 Under- or overcorrection is most commonly due to inadequate intraoperative visualization of the mechanical axis. The usefulness of a navigation system in open-wedge HTO with continuous visualization of the limb alignment intraoperatively has been reported previously.^8-10

This study examined methods for avoiding unintended increases in the posterior slope in navigation-assisted open-wedge HTO using computer-simulated 3-dimensional (3D) virtual surgery. In addition, radiologic results of navigation-assisted open-wedge HTO in which the anterior opening gap was approximately 67% of the posterior gap were analyzed.

Materials and Methods

Virtual Opening HTO Surgery Using a 3D Surface Model of the Leg

Between October 2005 and December 2006, 40 patients with unicompartmental osteoarthritis with a genu vara deformity were treated with navigation-assisted open-wedge HTO. A preoperative image using computed tomography (CT) was obtained in 1.25-mm sections in all patients, and the resulting data were converted to a 3D surface model using Mimics 9.1 (Materialise, Brussels, Belgium). The surface leg model included the femur head, the ankle joint, and the knee joint to set up the anatomic landmarks for defining the mechanical axis. Rapidform 3D scan modeling software (INUS Technology Inc, Seoul, South Korea) simulated virtual opening HTO, maintaining the original tibial slope using the same method as an actual procedure.

Two anatomic points were defined before opening the knee to measure the opening gap after valgus opening. Point M is the posteromedial cortex of the proximal tibia on the line of the osteotomy. Point A is the anteromedial cortex of the proximal tibia (posteromedial aspect of tibial tuberosity only) on the lines of the osteotomy. The distal part of the tibia was opened to the hinge on the lateral cortex along the anteroposterior axis. The length of the anterior and posterior opening gaps were measured using digitalized measurement tools for the Rapidform program (Figure1).

Surgical Technique and Radiologic Evaluation of Navigation-assisted Open-wedge HTO

The OrthoPilot navigation system (HTO version 1.3, B. Braun Aesculap; Tuttlingen, Germany) was used. The mechanical axis was visualized continuously, and the aim of the correction was to achieve a genu valgum of 3° (range, 2°-5°).

All procedures were performed in a standardized incomplete valgus open-wedge manner. The osteotomy began 3 cm distal to the medial joint line at the medial cortex of the proximal tibia. The osteotomy was directed to the proximal one-third of the fibular head, leaving 5 mm of the lateral tibial cortex intact. Careful valgization through stepwise insertion of the three coupled osteotomes was performed to avoid intraarticular fractures. At this time, the anterior opening gap was maintained at approximately 70% of the posterior opening gap.

When the required correction was achieved, the osteotomy was stabilized with two open-wedge plates (B. Braun Aesculap). The posterior plate was placed as posteriorly as possible at the posteromedial corner of the proximal tibia. The anterior plate was placed behind the oblique tuberosity osteotomy and was 2 to 4 mm shorter depending on the size of the posterior plate (Figure 2).

For the radiologic evaluation, the postoperative leg axis was examined by weight-bearing, full-leg radiography obtained 3 months’ postoperatively. To assess the accuracy of the correction, the mechanical tibiofemoral angle and intersection of the mechanical axis of the tibial plateau were examined. The posterior slope of the proximal tibia was measured using the proximal tibial anatomic axis method. A paired t-test was used for statistical analysis (SPSS 12.0; SPSS Inc, Chicago, Ill).

Results

Table 1 describes the relationship between the anterior and posterior opening gap, depending on the correction angle and the width of the proximal tibial plateau, after virtual open HTO to maintain the original posterior slope. The anterior gap was 67% of the posterior gap to preserve the original slope. One patient, an Asian woman, who had a proximal tibial plateau that measured 75 cm in width. If 10° of angular correction is planned, the posterior gap should be 9.54 mm and the anterior gap should be 6.42 mm for satisfactory valgus correction without changing the posterior slope (Figure 3).

After navigation-assisted open-wedge HTO, the mechanical axis was corrected adequately to a mean valgus of 2.9° (range, 0.5°-6.2°) and passed through 60% of the tibial plateau postoperatively. There were fewer outliers (12.5%) using this method. The posterior slope was well maintained, measuring 7.9°±2.3° preoperatively and 8.3°±2.8° postoperatively (P>.05). There were no specific complications related to the navigation (Tables 2 and 3).

Discussion

Open-wedge HTO is a reliable treatment for unicompartmental osteoarthritis with a varus deformity in relatively active and young patients and also can be performed to treat localized medial cartilage defects, osteochondritis dissecans, and condylar osteonecrosis.^4,5,11-13

During open-wedge HTO, both the sagittal and coronal alignment should be monitored. Study results show that the posterior slope of the proximal tibia tends to increase after open-wedge HTO.^1,2 In addition, an undesired change in the tibial slope can influence knee kinematics and stability.^1,3,4,14

The anatomic characteristics of the proximal tibia, a 3D structure with a triangular shape, may be most important in determining the tibial slope after open-wedge HTO.² Noyes et al suggested that the posterior slope would be increased inadvertently if the anterior gap around the tuberosity were equal to the posteromedial gap.¹²

Hernigou et al^5,15 and Koshino et al¹⁶ established tables using a mathematical model that provides a correction of varus malalignment in the coronal plane. However, there are few reports that show how much difference between the anterior and posterior gap is needed to maintain the original posterior slope and achieve the expected coronal limb alignment. Noyes et al¹² reported that to maintain a normal tibial slope, the opening gap at the tibial tubercle should be approximately half that of the opening gap at the posteromedial tibia.

This study attempted to establish the relationship between the anterior and posterior opening gap needed to maintain the original posterior slope during valgus opening through computerized 3D virtual open-wedge HTO. To accomplish this, a CT-based surface model of the leg was reconstructed from candidates for open-wedge HTO, and virtual surgery was performed in the same manner as in clinical practice. Two anatomic points, which simulated the opening gap in virtual surgery, could be measured precisely in the same manner as in an actual surgical field. In practice, these two anatomic points are where the two open-wedge plates are placed anteriorly and posteriorly. As a result, the virtual surgery demonstrated that the anterior opening gap should be 67% of the posterior gap to preserve the original posterior slope during open-wedge HTO.

Because some navigation systems provide intraoperative real-time alignment not only of the frontal but also of the sagittal plane, and because the newly released OrthoPilot software (HTO version 1.4) also provides information about the change in the posterior slope, it is believed that this wedge-shaped opening concept is still important. Surgeons who perform an incomplete osteotomy, leaving the lateral cortex intact, typically determine the opening shape during valgus opening. After opening, only a slight change in the posterior slope is possible. In this study, the most adequate ratio between the anterior and posterior opening gap was 0.67.

The long-term success of HTO strongly depends on the quality of the correction, and most investigators recommended a tibia femoral anatomic axis of 8° to 10° or a mechanical hip, knee, and ankle axis of 3° to 4° valgus.^5-7 Therefore, precise intraoperative assessment of the mechanical axis is important. Techniques such as the cable method, grids with lead-impregnated reference lines, and measurement of the joint orientation angle can be helpful but are not accurate in determining the mechanical femorotibial axis.^6,17,18 A navigation system has been used successfully in open-wedge HTO, with the advantage of continuous real-time visualization of the limb alignment.^8-10

In this study, navigated HTO was performed to maintain an anterior opening gap of approximately 67% of the posterior gap. Navigation significantly improves the accuracy, decreases the variability of the correction with fewer outliers, and maintains the posterior slope.

Complications such as pin track infections or fractures through the pin tracks were not encountered in this study, and the only disadvantages of navigated HTO were the additional time needed to complete the procedure and stab wound inflicted on the femur and tibia for fixing the navigation trackers.

Several factors affect the posterior slope after HTO. Marti⁴ suggested that an undesirable increase in the tibial slope could be avoided by making a complete posterior cut and ensuring appropriate release of posterior soft tissues. We believe that tight posteromedial soft tissue, including the superficial medial collateral ligament and the anterior location of the plate, is the main cause for a change in slope intraoperatively.

To avoid an increase of the posterior slope in open-wedge HTO 1) the osteotomy should be parallel to the joint line on the sagittal plane; 2) the posterior corticotomy should be complete and the posteromedial soft tissue of the proximal tibia should be released adequately; 3) the posterior plate should be placed as posteriorly as possible; 4) the postoperative full extension should be kept to the same degree as preoperative full extension using the navigation system; and 5) the anterior opening gap at the anteromedial cortex of the proximal tibia behind the tuberosity should be approximately 67% of the posterior opening gap at the most posteromedial corner of the proximal tibia.

Conclusion

Navigation-assisted HTO significantly improved the accuracy of the postoperative mechanical femorotibial axis and decreased the variability of the correction with fewer outliers. The normal tibial posterior slope can be maintained if the anterior opening gap is approximately 67% of the posterior opening gap.

References

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Authors

Drs Song, Seon, and Park are from the Center for Joint Diseases, Chonnam National University Hwasun Hospital, Jeonnam, Korea,

Correspondence should be addressed to Sang Jin Park, MD, Center for Joint Disease, Chonnam National University Hwasun Hospital, 160, Ilsim-ri, Hwasun-eup, Hwasun-gun, Jeonnam, 519-809, Korea.

Dr Song is a member of the B. Braun Aesculap speaker’s bureau. Drs Seon and Park have no financial relationships to disclose.