Varus-producing distal femoral osteotomy

Introduction

Lower extremity alignment of the hip, knee, and ankle directly impacts which surfaces bear the highest load during axial weight-bearing. Proper lower extremity alignment is critical for effective knee function. Deviations from the optimal mechanical axis contribute to both earlier onset and accelerated progression of osteoarthritis (OA). In the case of genu valgum, this is caused by overloading the lateral compartment while off-loading the medial compartment of the knee. To slow progression of knee pathology secondary to a malaligned lower extremity mechanical axis, varus-producing distal femoral osteotomy (DFO) may be utilized to restore alignment within physiologic ranges and unload the lateral compartment in the valgus deformed knee.

Both genu valgum and varum have well-documented adverse impacts on patients’ quality of life. While genu varum is more common overall, genu valgum typically affects women more than men. Patients may sustain valgus deformity through a variety of mechanisms including trauma, lateral meniscus deficiency, or medial collateral ligament laxity as well as a variety of metabolic disorders affecting bone formation and growth. Most commonly, genu valgum results from a hypoplastic lateral femoral condyle.

Limb alignment can be assessed by use of the mechanical axis, also known as the weight-bearing axis, on radiographs. For optimal weight-bearing biomechanics, the axis should run from the center of the femoral head, pass through the center of the knee between the medial and lateral tibial spines, and end at the center of the talar dome. In cases of genu valgum, the mechanical axis may pass through the lateral femoral condyle or lateral to the tibiofemoral articulation entirely ( Fig. 79.1 ). The degree of valgus deformity is directly related to the risk of lateral compartment degenerative OA.

Except for post-traumatic malalignment, genu valgum most commonly occurs due to femoral malalignment and is best corrected with DFO. Because the femur is anatomically in valgus and the tibia in slight varus, DFO provides the advantage of maintaining the patient’s native tibial alignment while relieving the overloaded lateral compartment Consequently, DFO is the preferred method for correcting genu valgum. Several landmark studies have demonstrated superior results with osteotomies at the distal femur rather than proximal tibia to correct valgus knees. ^,

DFO correction of genu valgum can be performed via an opening-wedge technique at the lateral femur or a closing-wedge technique at the medial femur. Opening-wedge DFO is less technically challenging and offers the advantage of a more accessible surgical exposure. Furthermore, opening-wedge DFO allows for intraoperative adjustment and requires simpler fixation as compared to closing-wedge DFO. Closing-wedge DFO has a higher potential risk for debilitating injuries given the medial femoral cortex’s proximity to neurovascular structures. However, closing wedge DFO has the advantage of primary bone healing with direct bone apposition and potentially faster healing rates. In regard to both short- and long-term outcomes, results have been equivalent with either approach. Recent systematic reviews found no difference in patient-reported outcomes and highlighted comparable survival rates of the native knee. ^, For these reasons, we prefer opening-wedge DFO over closing-wedge osteotomy.

Preoperative considerations

Imaging

Radiographic evaluation for patients with valgus malalignment and suspected OA includes standard anterior and lateral views as well as bilateral long-limb views (also known as mechanical axis or long-leg views), bilateral Rosenberg views (PA weight-bearing with 45° flexion), and bilateral merchant or sunrise views. The bilateral long-limb view is necessary to establish the mechanical axis, while the Rosenberg and merchant views are utilized for evaluation of tibiofemoral and patellofemoral OA, respectively. For assessing tibiofemoral OA, the Rosenberg view is preferred to a standard anterior to posterior view, as it has greater sensitivity and specificity. An appropriate patient for a DFO will have isolated lateral compartment OA. If the patient demonstrates a valgus thrust or indeterminate limb alignment, a single leg standing view should be included. Magnetic resonance imaging (MRI) is not necessary, unless the surgeon has a suspicion of concurrent meniscal or cartilage pathology.