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Patient-specific instrumentation for high tibial osteotomy provides a personalized approach
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Medial opening-wedge high tibial osteotomy is a well-established procedure to correct varus malalignment of the lower limb in isolated medial compartment osteoarthritis of the knee. A shift of the weight-bearing line toward lateral, unloading the affected medial compartment, is the key biomechanical principle of this surgical procedure. Patient selection, preoperative planning and stability of the fixation are critical factors in performing a successful opening-wedge high tibial osteotomy (HTO). Despite modern imaging possibilities, such as low-dose CT and MRI which can generate 3-D bony reconstructions, most osteotomies are still traditionally planned based on 2-D, full-leg, bipodal standing radiographs. Furthermore, obtaining the planned correction is assumed to be the most important factor because long-term clinical results depend on the accuracy of the lower limb alignment. However, it remains difficult to accurately achieve the preoperatively planned correction as conventional techniques and traditional planning methods are falling short. Computer navigation used to perform knee osteotomies has gained popularity due to its real-time visualization of the lower limb during surgery, however its cost and steep learning curve has kept this technique from being a tool that is widely used by orthopedic surgeons. As patient-specific instrumentation (PSI) using 3-D medical software has proven its value in maxillofacial surgery and osteotomies around the forearm, there is growing interest in PSI for lower limb realignment procedures. For the handful of studies published about PSI in opening-wedge HTO, accuracy rates are promising and are hypothesized to be better than those associated with conventional techniques. Therefore, we present a novel technique using PSI in opening-wedge HTO that aims to improve the overall accuracy of its outcome and overcome the technical difficulties and potential complications surgeons may face during opening-wedge HTO surgery.
Preoperative planning and printing
Prior to surgery, the patient undergoes a full-leg bipodal standing radiograph and CT scan of the affected leg according to the DePuy Trumatch Scanning Protocol for the knee. This protocol is designed to create 3-D models of the lower limb and consists of slices with 5-mm spacing and 5-mm thickness in the hip and ankle, 0.5 mm spacing and thickness in the knee and with a 150-mm range center on the knee joint. The resultant Digital Imaging and Communications files of the CT scan are transferred to the Mimics segmentation software (Materialise) to separate the bony structures from the surrounding soft tissue. The virtual model of the limb is loaded into the 3-matic 3-D planning software (Materialise), in which the desired osteotomy direction and wedge opening are simulated (Figure 1). At the end of the planning process, a personalized fitting wedge and cast are 3-D-printed in biocompatible resin that is CE-marked and approved for sterilization in an autoclave or by gamma-ray sterilization (Figure 2). For safety reasons, the printed cast is labeled with the patient’s initials, side of surgery and degree of correction. The wedge contains two grooves that indicate the depth to which the printed wedge should be inserted. These grooves should match with the medial cortex of the proximal and distal tibial fragment for proper positioning. Although this planning method may seem time-consuming, the actual printing process is accelerated because of in-hospital availability of a small-type 3-D printer.
Source: Peter Verdonk, MD, PhD
Surgical technique
To start, the most noticeable landmarks, including the tibial tubercle, joint line and fibular head, are outlined on the patient’s skin. A 10-cm vertical medial skin incision is made on the tibia. Under fluoroscopic control, two parallel K-wires are introduced horizontally at 3 cm below the medial tibial joint line on the medial cortex and the wires are aimed laterally, proximally of the tibiofibular joint and 1 cm below to the lateral joint line. First, a horizontal osteotomy is made with a sawblade distal, in contact with the two K-wires on the medial side. Then, an oblique step osteotomy is made at the level of the tibial tubercle, as planned, on the 3-D virtual model. Next, the lateral cortex is perforated about three times using a 3.2-mm drill to reduce the risk of lateral hinge fractures. The horizontal osteotomy is gently opened by inserting five chisels in a progressive manner posteriorly, but without full engagement. The personalized, 3-D-printed wedge can now be introduced in the gap while applying valgus stress. To indicate appropriate placement of the wedge, the two grooves on the wedge should match with the medial tibial cortex. The electrocautery cord that represents the weight-bearing line is then used to check the limb alignment under fluoroscopy (Figure 3).
The bone graft preparations are started while the customized wedge remains in the osteotomy gap. The printed negative cast of the patient is used as a box in which the bone allograft (half femoral head) is prepared. The bone allograft is trimmed triangularly by a sawblade until the size matches the original printed wedge (Figure 4). Subsequently, the printed wedge is exchanged for the wedge-shaped bone graft which ultimately provides an identical alignment correction.
The osteotomy is fixed with a TomoFix locking plate (DePuy Synthes) to guarantee sufficient initial stability (Figure 5). Concerning fixation from proximal to distal, we start with three angle-stable screws and one cortical screw at the most proximal side followed by one cortical and three angle-stable screws at the distal side. A fourth angle-stable screw is then placed at the proximal side, exchanging the cortical screw. The osteotomy reconstruction also includes the use of press-fit bone allograft (Figure 6).
Future directions
This PSI technique provides an individualized, safe and potentially more precise approach for performing opening-wedge HTO. For example, even the thickness of the sawblade should be considered while planning the osteotomy using this medical software. The preoperative 3-D simulation allows the surgeon to plan the osteotomy strictly at the bony level by using the medial proximal tibial angle instead of traditionally used planning angles, such as the medial femorotibial angle and weight bearing line percentage that span the entire leg and eventually can be misinterpreted due to soft tissue laxity. Further, this technique can be of special interest to orthopedic surgeons who are young or have limited experience with HTO because the expected learning curve is short. Finally, we think that in more challenging knee osteotomies, including multiplanar deformities and double-level corrections (tibia and femur), PSI can guide the surgeon through the operation leading to improved accuracy outcomes, as this remains the most important factor in the endurance of lower-limb realignment procedures.
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- For more information:
- Wouter Van Genechten, MD, can be reached at department of orthopaedic surgery, Antwerp University Hospital, Wilrijkstraat 10, Edegem, Belgium; email: wouter.vangenechten22@gmail.com.
- Annemieke Van Haver, PhD, can be reached at department of orthopaedic surgery, More Institute, AZ Monica, Stevenslei 20, Antwerp, Belgium; email: mvhaver@hotmail.com.
- Peter Verdonk, MD, PhD, can be reached at department of orthopaedic surgery, More Institute, AZ Monica/Antwerp University Hospital, Stevenslei 20, Antwerp, Belgium; email: pverdonk@yahoo.com.
Disclosures: Van Genechten, Van Haver and Verdonk report no relevant financial disclosures.