In 2000, Ganz et al. described the practical anatomy of the medial femoral circumflex artery [10] and a surgical approach that could be used to safely dislocate the hip without the risk of iatrogenic osteonecrosis [9]. This technique facilitated an improved understanding of hip mechanics, the role of the acetabular labrum, and the association between specific femoral morphologies and damage to the acetabular rim and labral-chondral complex [11]. Utilization of the surgical dislocation technique in the setting of chronic SCFE substantiated damage to the anterior acetabular articular cartilage and labrum induced by impingement between the upper femoral metaphysis and the anterior acetabulum [12]. Further application of the surgical dislocation approach enabled modification of the Dunn technique of subcapital reorientation [6, 8]. The concept of a safe and reproducible technique to reorient a chronically or acutely displaced upper femoral epiphysis is attractive for surgeons because it has the potential to favorably influence the natural history of the severe SCFE deformity and to prevent complications associated with treatment of high-grade slips including chondrolysis, osteonecrosis, slip progression, hip pain, functional disability, and osteoarthritis. The literature supporting routine use of the modified Dunn technique is generally supportive. It seems that in experienced centers, the technique is associated with an acceptable complication rate when compared to complications that are associated with more traditional management of acute unstable SCFE [1]. Ziebarth et al., Slongo et al., and Huber et al. collectively reported 83 combined stable and unstable SCFE treated using the modified Dunn technique with two cases of osteonecrosis [13, 14, 8]. In contrast, Sankar et al., reporting results from several US center, reported that 7 out of 27 patients developed osteonecrosis [15]. This disparity suggests that careful training and supervised practice may be necessary to prevent an unacceptable rate of complications especially during implementation of this technically demanding procedure.

There are no clear surgical indications for the modified Dunn approach. When one considers that the majority of patients treated by in situ fixation function reasonably well for many decades following initial treatment [2, 16], the surgeon must justify the additional risk and higher magnitude of this procedure upon a high likelihood of a poor result following in situ fixation or persistence of a severe deformity. As such, SCFE that are at higher risk for failure are those that may benefit from this procedure. These may include:

A patient who presents with a severe slipped epiphysis must be thoroughly evaluated prior to surgical treatment. A thorough history should be used to elucidate factors that may be associated with non-idiopathic SCFE. These include age, weight, past medical history, and family history. If there is a reason to suspect a non-idiopathic SCFE, proper management will increase the likelihood of successful healing and minimize the probability of complication following treatment. Complete radiographic evaluation is necessary to fully evaluate the degree of deformity, chronicity of the SCFE, and state of the contralateral hip. In certain cases of chronic SCFE, a CT scan will permit assessment of physeal healing, enable 3-dimensional visualization of the deformity, and facilitate preoperative planning. The acetabular morphology is becoming increasingly appreciated as a factor in symptomatic SCFE mechanics and can be analyzed preoperatively [17].

The technique of the modified Dunn was first used by Leunig et al. to describe acetabular damage associated with SCFE [12] and was first described in detail by Leunig and Ganz [18, 19] and most recently by Ziebarth et al. [8]. It is recommended that surgeons using this technique have familiarity with the vascular anatomy of the upper femur and experience using the surgical dislocation approach.

Since the patient is placed in a lateral decubitus position, an operating table that will permit intraoperative radiographic evaluation of the operative hip is helpful. While it is not necessary to use a radiolucent operating table, the table must permit the surgeon to obtain fluoroscopic images which enable assessment of epiphyseal position, placement of fixation devices, and trochanteric positioning and refixation.

Since most patients with SCFE are obese, surgical visualization can be challenging. It is necessary to have proper retractors that enable both acetabular and femoral exposures. A complement of Langenbeck and Hohmann retractors designed for retraction of the hip joint facilitates difficult exposures. In addition, sharp periosteal elevators, pointed bone-holding forceps, and thin osteotomes are required during various steps of the procedure.

Fixation can be accomplished using cannulated screws, long 4.5 or 6.5 mm fully or partially threaded screws, or Steinmann pins either alone or in combination. There is no uniform method of reliable fixation, and each published series has reports of hardware failure. Surgeons have reported using threaded and smooth wires, solid screws, and cannulated screws. It is best to have various fixation options depending upon the size of the patient, the size of the epiphysis, and the surgeon’s ability to visualize the epiphysis intraoperatively.