**6.1 Role of mechanical axis of lower extremity**

The mechanical axis of the lower extremity (straight line drawn from center of hip to center of ankle) normally lies just medial to the medial tibial spine. Varus malalignment brings this axis further inside the medial compartment or even medial to the joint. With axial loading, varus malalignment causes increased pressure in the medial compartment cartilage(Loening, James et al. 2000). Increased stress may negatively impact cartilage repair and restoration procedures. Without correction of the alignment to at neutral, the outcomes of cartilage procedures have been less successful in the presence of varus malalignment. This has led to increased performance of valgus-producing high tibial osteotomy (HTO) either via an opening- (OW-HTO) or closing-wedge (CW-HTO) technique. Mechanical axis correction to neutral or slight valgus is adequate in conjunction with cartilage repair or restoration procedures(Mina, Garrett et al. 2008). For medial compartment osteoarthritis, overcorrection to up to 62% of the width of the tibial plateau from the medial tibial border is warranted(Miller, Cole et al. 2008 ). A similar technique is used for lateral compartment chondral pathology in the setting of valgus malalignment via a laterally-based opening wedge technique.

### **6.1.1 Outcomes**

Outcomes after combined HTO and cartilage surgery for medial compartment cartilage pathology and varus malalignment have demonstrated significant improvements in subjective and objective clinical measures. Both CW-HTO and OW-HTO techniques have seen similar success concurrent with microfracture(Sterett and Steadman 2004; Sterett, Steadman et al. 2010; Pascale, Luraghi et al. 2011), abrasion arthroplasty(Matsunaga, Akizuki et al. 2007), and ACI(Franceschi, Longo et al. 2008; Gomoll, Kang et al. 2009; Minas, Gomoll et al. 2009).
