Modified Dunn Osteotomy for Moderate and Severe Slipped Capital Femoral Epiphysis
Authors: Agashe Mandar , Shah Shalin , Kapoor Darshan , Vaidya Sandeep 
Published: May 2024

Background:
Moderate to severe slipped capital femoral epiphysis is a condition that benefits from treatment with modified Dunn realignment osteotomy and fixation
1,2
. The results are acceptable when the procedure is performed by experienced surgeons
3,4
.

Description:
The patient is placed in the lateral decubitus position. Surgical hip dislocation is performed after a digastric trochanteric osteotomy. An extended retinacular flap is carefully prepared, and the epiphysis separated from the metaphysis, exposing the entirety of the femoral neck. Following this, an open reduction is performed by excising the posteromedial callus and trimming the femoral neck in order to allow capital realignment while avoiding tension on the retinacular vessels. Finally, the reduced slip is fixed with 6.5 or 7-mm cancellous screws and Kirschner wires. The femoral head is then repositioned in the acetabulum, and the trochanter is fixed distal to its original position with use of 3.5-mm self-tapping cortical screws. Closure is performed in layers after achieving adequate hemostasis.

Alternatives:
Operative alternatives to this technique include cuneiform osteotomy, compensatory osteotomy of the base of the neck or intertrochanteric osteotomy and controlled repositioning, and osteoplasty
5–7
. Osteotomies performed distally and away from the deformity can still lead to cam impingement, thereby failing to reduce the risk of future joint degeneration. In situ pinning has also been described for use in moderate to severe cases of slipped capital epiphysis, but the residual deformity leads to substantial femoroacetabular impingement and thus affects the prognosis of such a hip
8
.

Rationale:
This technique accounts for the complex anatomy of the hip joint, especially in terms of its precarious blood supply. Long-term studies have shown that corrective osteotomy performed near the physis can correct the alignment at the head-neck junction, thereby avoiding future femoroacetabular impingement. In the hands of experienced surgeons, results have been shown to be favorable
3,4
.

Expected Outcomes:
In a series of 30 consecutive hips, including 20 moderate and 10 severe slips, the slip angle improved significantly following the modified Dunn procedure (by 43.63° ± 8.42°; p < 0.001). Osteonecrosis was observed in 2 hips (6.7%), and 1 hip had postoperative subluxation that required intervention
9
. In their series of 23 patients, Slongo et al. encountered 2 cases (9%) of osteoarthritis and osteonecrosis
10
. In their series of 40 patients from 2 different institutions, Ziebarth et al. reported no cases of chondrolysis or osteonecrosis
11
.

Important Tips:

The steps described need to be followed sequentially and meticulously in order to safely dislocate the hip while preserving the blood supply.The availability of an anterior pouch in draping, to deliver the leg during dislocation, will reduce the dependence on an assistant and maintain sterilityThe trochanteric osteotomy should be started with the help of a saw, completed with an osteotomy and must be about 1 to 1.5 cm thick in order to achieve good, stable fixation following relocation of the femoral head and to achieve good abductor function post-relocation.The use of curved, stout, thick scissors (“episiotomy scissors”) can help to ease the step of cutting the ligamentum teres during dislocation.The retinacular flap should be widely exposed along with removal of the posterior callus so that no tension remains on the blood supply.

Acronyms and Abbreviations:

SCFE = slipped capital femoral epiphysisFAI = femoroacetabular impingementMDP = modified Dunn procedureAVN = avascular necrosisTHR = total hip replacementFAER = flexion, abduction and internal rotationCT = computed tomographyMRI = magnetic resonance imagingAP = anteroposterior