## 1 Introduction
In short, to date, surgical procedures for subtrochanterior nonunion rely on methods that achieve bone union based on secondary bone healing through static fixation, either intramedullary nails or extramedullary devices. Non-dynamic surgical procedures in the subtrochanterior area, the most stressed zone of the human skeleton, entail specific issues. The stability of the nonunion site is primarily achieved through the device (not the bone). Because of this, in addition to the intrinsic characteristics of nonunion (bone defects and previous surgical procedures that can reduce the contact area between fragments), only a minimal amount of beneficial loading forces (compression forces) are directly conducted to the bone. These forces are essential for increasing stability at the nonunion site (decreasing the strain at the fracture site) to achieve bone union and reduce biomechanical stress on the device [18]. They are especially relevant in this area because high bending varus shear forces can cause significant strain in the interfragmentary tissue, promoting nonunion. In addition, the device is under high stress and can break or fail if the bone union is not achieved in a timely manner. Although the overall union rate is reported to be between 71% and 98% [1,8,17], recent studies indicate a failure rate of up to 29% [10]. Furthermore, these procedures are challenging. Full weight bearing is not immediately allowed postoperatively (a devastating circumstance for elderly patients) [7], and bone grafting is often necessary to promote bone union [6,7,9,13]. Dynamic fixation outperforms nondynamic fixation and will overcome these problems in this highly stressed zone. Dynamic fixation progressively results in greater compression forces, reinforcing the stabilization of the nonunion directly through the bone contact areas. A dynamic device exerts continuous compression forces, enhancing the stability of the bone fragments at the nonunion site and promoting quicker healing without the need for bone grafting. Additionally, it decreases stress on the device (increasing the time to achieve bone union) and allows for full weight-bearing from the first postoperative day. The DHS is the only device that can achieve dynamic compression at the nonunion site. However, due to its design, it cannot properly conduct the vertical physiological vector of the loading forces predominant in this area and produces excessive medialization of the shaft (especially if there is no intact lateral cortex), increasing the possibility of loss of reduction and, consequently, nonunion [19]. For the sliding hip screw to function according to its biomechanical principle, an intact lateral cortex is mandatory [19]. The lateral cortex is always damaged in subtrochanteric nonunions, so the use of the DHS in subtrochanteric fractures has reported a high rate of complications, making it unsuitable for these injuries. Furthermore, in subtrochanteric fractures or nonunions, the screw insertion site is mostly at or just near the fracture site. It cannot maintain a proper hold in the lateral cortex. Due to this, this principle has not been successfully applied to subtrochanteric injuries. We propose that achieving some degree of dynamic fixation in the subtrochanteric zone using a DHS would be feasible, avoiding excessive medialization of the shaft. This may be accomplished through a valgus overcorrection of the hip angle applied to the proximal fragment. In addition, it can be performed without the need for a lateral closed osteotomy by simply decompressing the nonunion site. This paper aims to describe a new surgical technique for addressing dynamic fixation in subtrochanteric nonunion in varus through over-valgus correction fixed with a DHS and to evaluate its radiological and clinical outcomes in actual patients.