Once the wedge size has been determined, the correction can be obtained via a medial opening wedge (MOW) or a lateral closing wedge (LCW) HTO. Both methods have been used for osteotomy with ACL deficiency [
10‐
12,
14‐
16,
18,
37,
60]. Each technique has unique advantages and disadvantages.
The LCW HTO was the first method described [
11], and provides immediate cortical contact which has traditionally allowed patients to fully weight bear earlier without the need for bone grafting. The LCW HTO has a tendency to decrease posterior tibial slope, which is advantageous in the ACL deficient knee [
61,
62]. However, this method decreases proximal tibial bone stock, which may make a subsequent total knee arthroplasty technically more demanding [
63]. The approach is also in close proximity to the common peroneal nerve, anterior compartment and proximal tibiofibular joint [
9]. Injuries to these structures could result in iatrogenic nerve injury, compartment syndrome, and proximal tibiofibular joint instability.
The MOW HTO allows easier correction of coronal and sagittal alignment since one cut is easier to titrate than two parallel cuts [
59]. This method preserves bone stock and may help “tighten” the capsuloligamentous structures around the knee [
59], while avoiding the neurovascular risks of the LCW HTO. Disadvantages of the MOW include the possible need for bone graft and the risk of delayed or non union. Additionally, the potential to increase posterior tibial slope exists with an inexperienced surgeon or with lack of attention to detail. The absolute correction of tibial slope may be limited with MOW HTO by the triangular geometry of the tibia [
64]. Nonetheless, the advantages of MOW make it the current preferred technique to address varus gonarthrosis in the ACL-deficient patient [
8].
MOW HTO surgical technique
A general anesthetic is preferred as the use of a regional block could mask potential post-operative compartment syndrome. The patient is positioned supine on a radiolucent operating table with a tourniquet applied to the upper thigh. A bolster may be placed under the ipsilateral hip to prevent excessive external hip rotation. Pre-operative antibiotics are administered. A sterile positioning bundle can be used underneath the knee and leg during the osteotomy to optimize positioning during fluoroscopy. The positioning bundle can also be used to place under the heel to extend the knee prior to final fixation. If knee arthroscopy is indicated, this is performed first.
Appropriately draped C-arm fluoroscopy should be brought in from the same side as the operative leg while the surgeon stands on the opposite side. This position allows easier access to the surgical wound on the medial side of the leg.
A medial longitudinal incision is made midway between the tibial tubercle and the posteromedial border of the tibia. Sartorial fascia is incised above the gracilis tendon. Fascial attachments posterior to the MCL at the metaphyseal flare of the posteromedial aspect of the tibia are elevated. Blunt dissection across the posterior cortex of the tibia elevates soft tissue and aids in placing a blunt tipped Hohmann retractor to protect neurovascular structures. This instrument will be in direct line with the proposed osteotomy. Anteriorly, the tibial tubercle and the medial border of the patellar tendon are identified and the fascia along the medial aspect is opened with cautery. This facilitates insertion of a blunt retractor (bent Lane or Hohmann) beneath the tendon.
A break-away guide pin is drilled in at the superior margin of the planned osteotomy site under fluoroscopic guidance. The tibial width can be measured using the calibrated pin. The pin should be directed towards a point at least 1 cm distal to the joint line on the lateral cortex. Ensure that the proposed osteotomy passes superior to the tibial tubercle. If the patellar tendon insertion is too close to the proposed osteotomy, a proximally directed biplane osteotomy that will allow the level of the tibial osteotomy to be lowered without compromising the extensor mechanism can be made posterior to the tubercle in the tibial tubercle. A biplanar osteotomy should also be performed for corrections greater than 12.5 mm to avoid significantly decreasing the patellar height [
9,
65]. The biplane cut has the added advantage of increasing rotational stability of the osteotomy. In the sagittal plane the cut should be parallel to the tibial slope. In the case of an abnormally flat slope, the cut can angle back approximately 10°, which will provide more proximal bone for subsequent screw fixation. The planned osteotomy plane is marked with cautery. The superficial medial collateral ligament can be released with cautery in line with the proposed osteotomy. Alternatively, the superficial MCL may be elevated distal to the osteotomy and allowed to cover over the osteotomy and plate at the end of the procedure. If the superficial MCL is not released in some way, a medial tension band is created across the medial compartment, increasing compartment pressures [
32].
Blunt retractors should remain in place both posteriorly and anteriorly when making the bony cut. A small oscillating saw blade 45 mm in length is chosen and the cut is made below and in line with the guide-pin. The direction of the saw blade should be confirmed with fluoroscopy to ensure it does not drift towards the knee joint. Thin osteotomes are used to complete the cut. It is advised that a bone bridge of approximately 10 mm be maintained on the lateral side and that the lateral extent of the osteotomy be closer to the lateral cortex than to the lateral joint line. This will help avoid a lateral cortical breach as well as intra-articular fracture propagation. Once it appears that the osteotomy has been completed anteriorly and posteriorly and is far enough lateral, a wide, firm osteotome can be inserted to assess its mobility. The osteotomy is gradually opened to the desired wedge correction as measured by the gap at the posteromedial aspect of the osteotomy site. This can be done with stacked osteotomes or wedges, and a laminar spreader may be required to maintain the wedge correction.
The sagittal alignment and tibial slope are assessed with a lateral image and compared to pre-operative imaging. In the absence of cruciate deficiency, the slope is kept neutral by maintaining a gap in the osteotomy site posteriorly that is twice as big as the gap anteriorly [
64]. In the case of ACL deficiency, closing the gap anteriorly flattens the tibial slope. A 2° change in slope can be expected for every 1 mm of closure anteriorly [
64]. This anteroposterior relationship of the osteotomy gap is due to the axial triangular geometry of the proximal tibia. As mentioned, closure of the anterior aspect of the gap can be achieved by placing a positioning bundle under the heel with the knee in extension during plate fixation.
With the osteotomy complete and the size of the gap and angle of the slope confirmed, the desired hardware can be applied. A locking plate system is preferred to maintain axial and rotational stability during the consolidation period. Screws are inserted under fluoroscopic guidance to ensure safe positioning. Once fixation is complete and hardware position is confirmed with fluoroscopy, the defect (i.e. gap >10 mm) can be filled with bone graft or a bone graft substitute. The wound is closed over a drain followed by the application of dressings and a hinged knee brace.
For patients who are candidates for a concurrent ACL reconstruction, it must be ensured that the HTO hardware does not interfere with the tibial tunnel or fixation. The procedure should begin with arthroscopic assessment and treatment of concurrent meniscal and articular cartilage injuries. The osteotomy is then completed with slope either maintained or flattened at the discretion of the surgeon. Proximal screw fixation of the plate will have to be manipulated to avoid interference with the tibial tunnel. This can be accomplished by placing the plate as posterior as possible, with or without leaving the most anterior screw hole of the proximal aspect of the plate empty or short. If a screw hole is left empty, the surgeon must ensure the plating system allows for adequate fixation in the proximal fragment. Once the plate is secured to the bone, the ACL reconstruction is resumed including graft preparation and the drilling of both the femoral and tibial tunnels. The tunnel should exit the anterior tibial cortex at the proximal aspect of the osteotomy site. Tibial fixation of the ligament is at the surgeon’s discretion.