Consequently, alterations in tibial slope should be avoided in specific situations but planned in others. In opening wedge HTO, the above-mentioned pathologies can be adequately addressed with a single cut. Additionally, the proximal tibio-fibular joint, the peroneal nerve and anterior compartment of the leg are remote from the surgical site. Most problems associated with violating these are avoided. Also, achieving smaller corrections of 5 mm or less is technically easier.
Preoperative planning is based on a radiographic evaluation of the extent of arthrosis and lower extremity alignment on bilateral weight-bearing anteroposterior views in extension, bilateral weight-bearing posteroanterior tunnel views in 30 degrees of flexion, and lateral and skyline views. True lateral views, ie, with the medial and lateral femoral condyles overlying, are necessary for assessment of the wear pattern and for accurate measurement of the posterior tibial slope angle. We use the Noyes method on hip-to-ankle views to estimate the amount of correction required.6
Fixation is with four-hole Puddu tapered wedge plates (Arthrex Inc.; Naples, Fla.). The taper allows better control of the posterior tibial slope. Alternative fixation methods include other plate and screw systems, bone grafting alone and external fixators. Helpful to the procedure is a mobile low-dose ionizing radiation fluoroscope.
Optimal Guide Pin Placement. The pin is drilled from approximately 4 cm below the medial joint line and, using the tip of the fibular head as reference, advanced across the superior aspect of the tibial tubercle to 1 cm below the lateral joint line. This is repeated as often as necessary to attain the best possible pin placement. Note that the orientation of the guide pin will be oblique and, that while greater obliquity increases the risk of fracture into the lateral compartment, it will, on the other hand, give the osteotomy increased depth.
Creating the Osteotomy. Osteotomizing the tibia parallel to the posterior tibial slope and below the guide pin should avert intra-articular fracture. We use a small oscillating saw to perforate the medial tibial cortex, and complete the osteotomy with thin, flexible osteotomes. Continuous or frequent imaging verifies that the osteotome is not misdirected nor the lateral cortex violated. Stop 1 cm short of the lateral cortex.
Opening the Osteotomy. Confirming that the anterior and posterior cortices are osteotomized prior to opening the osteotomy is another precaution taken to avoid intra-articular fractures. Distract the osteotomy slowly to the pre-planned correction using a calibrated wedge. Evaluate alignment using an external guide aligning the centre of the hip with the centre of the ankle.
Assessment of Posterior Tibial Slope. To maintain the tibial slope, the osteotomy distraction anteriorly should be that at the posteromedial border. A single limb of the calibrated wedge inserted posteromedially is helpful.
Plate Fixation. The plate is secured with 6.5 cancellous screws proximally and 4.5 cortical screws distally. Fluoroscopic evaluation will prevent intra-articular screw placement.
Bone Grafting. Allograft, autograft or, occasionally, bone substitutes are used in corrections greater than 7.5 mm.
Medial Collateral Ligament. If this becomes too taut, it is fenestrated and allowed to slide.
Weight bearing. Depending on the amount of correction and the stability of the construct, weight-bearing progresses from partial to full protected (with crutches) over 12 weeks – at which time there is, in most cases, complete union.
(Note: Technical Points section based on Fowler PJ, Tan JL, Brown GA: Medial opening wedge high tibial osteotomy: How I do it. Operative Techniques in Sports Medicine. 8(1):32-38, 2000.)
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