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Tips and Pearls

The Oxford Spherical Mill for Metaphyseal Debridement in Open Epiphyseal Reduction and Internal Fixation for Slipped Capital Femoral Epiphysis

Lloyd, Thomas MBChB, MRCS*; Duncan, James BMedSci (Hons), BMBS, MRCS*; Kothari, Alpesh MSc, DPhil, FRCS (Tr & Ortho); Elkhouly, Amr MBBCh, MCh, MSc, FRCSEd (Tr & Ortho)*; Davies, Nev FRCS (Tr & Ortho)*

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doi: 10.1097/BTO.0000000000000376
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Abstract

Slipped Capital Femoral Epiphysis (SCFE) is an adolescent hip disorder that affects up to 25/100,000 patients aged 8 to 15 years.1 It is characterized by displacement of the proximal femoral epiphysis relative to the metaphysis. A spectrum of severity has been described and classifications suggested to help predict prognosis and guide treatment.2,3 Complete epiphyseal-metaphyseal dissociation, in the pattern of a Salter Harris type 1 fracture,4 causes physeal instability, a greater risk of displacement and, consequently, avascular necrosis (AVN). The aim of treatment of SCFE is to stabilize the proximal femoral epiphysis, avoid further displacement, and prevent femoral head osteonecrosis.

The chronicity and severity of the epiphyseal slip typically determines the approach to management. In-situ, percutaneous stabilization with cannulated screws is the method of choice for stable, acute or minimally displaced SCFE. Chronic and severe slips are associated with increased rates of AVN due to compromise of the retinacular vasculature as it is stretched over a posteroinferior beak of callus. Percutaneous, in-situ stabilization does not account for this and so more invasive techniques are required. Open epiphyseal reduction and internal fixation using a Modified Dunn Procedure is one method for treating severe and chronic cases of SCFE.5

The Modified Dunn Procedure provides anterior access to the proximal femur to expose the SCFE, allowing epiphyseal mobilization and metaphyseal debridement of the extensive callus found at the posteroinferior aspect of the neck. The Ganz procedure for dislocating the hip protects the retinacular vasculature within the posterior capsular flap.6 Initial debridement can be performed using conventional instrumentation however debriding the cervical callus concentrically to allow relocation of the metaphysis on the epiphysis can be challenging. We describe the benefits of using the Oxford Spherical Mill from the Oxford Partial Knee Replacement (OPKR) system (Zimmer Biomet, Warsaw, IN) for metaphyseal debridement during open epiphyseal reduction and internal fixation for SCFE.

SURGICAL TECHNIQUE

Exposure and Epiphyseal Mobilization

The patient is positioned in the lateral decubitus position to allow a direct lateral approach to the hip and the Ganz procedure of dislocation. A trochanteric osteotomy is performed using the trochanteric branch of the medial circumflex femoral artery as a reference point for the osteotomy site. The piriformis tendon remains attached posteriorly whereas the greater trochanter (GT) and remaining attachments are flipped anteriorly, exposing the joint capsule. A z-shaped anterior capsulotomy is performed, revealing the slipped epiphysis and metaphyseal bump. The hip is dislocated anteriorly. The periosteum is incised in line with its fibers and the anterior flap peeled back using a periosteal elevator to expose the femoral neck. Dislocating the hip anteriorly and mobilizing only the anterior periosteal flap helps to protect the retinacular vasculature running posteriorly.6

The epiphysis is carefully mobilized from the metaphysis using a Cobb elevator and bone rongeurs. A high-speed burr is used in-line with the physis to help breakdown stubborn callus in chronic SCFE (Fig. 1). Once fully mobilized, the epiphysis remains attached to the posterior flap of capsule and periosteum that contains its blood supply. The physeal surface of the epiphysis is removed using a curette.

FIGURE 1
FIGURE 1:
Mobilisation of metaphysis from epiphysis.

Metaphyseal Debridement Using The Oxford Spherical Mill

SCFE is classically associated with anterosuperior translation of the metaphysis in relation to the epiphysis. This orientation is responsible for excess bone formation at the anterosuperior physis in addition to the posteroinferior aspect where the epiphysis is overhanging. A “beak” of prominent callus typically forms posteroinferiorly and extends along the femoral neck. The posterior position of the retinacular vessels predisposes to their impingement if there is incomplete debridement of this posterior “beak.”5,7 The aim is therefore to create a hemispherical surface on the femoral neck, ensuring that no posterior callus remains and accurate reduction of the epiphyseal-metaphyseal junction can be restored. The Oxford Spherical Mill (OPKR System, Zimmer Biomet, Warsaw, IN) has proved useful in achieving adequate debridement and hemispherical recontouring of the femoral neck (Fig. 2).

FIGURE 2
FIGURE 2:
Oxford Spherical Mill recontouring proximal femoral metaphysis.

The Oxford Spherical Mill is a rotating reamer that is used in combination with a milling post and flanged spigots for femoral preparation in unicondylar knee replacement procedures. It achieves spherical recontouring of the distal femur for articulation with the spherically concave inner surface of the femoral component of the OPKR system.

Acting as a femoral neck reamer, the Oxford Spherical Mill, can achieve uniform hemispherical recontouring of the femoral neck to allow accurate relocation to the metaphysis. Anterior dislocation allows direct visualization along the long axis of the femoral neck. Reaming is performed perpendicular to this axis. Debridement is performed freehand without the milling post or spigots. This allows direct visualization of the “nubbin” of bone formed within the aperture of the mill, helping gauge the depth of bone removed (Fig. 3). This “nubbin” of bone, typically 2-3 mm, is subsequently removed using bone rongeurs, leaving the recontoured spherical femoral neck that can be accurately reduced to the proximal femoral epiphysis (Fig. 4). Spigots can be used to help guide concentric debridement while also limiting the depth of bone removed to 7 mm. Care is taken to ensure concentric reaming and not to over-shorten the femoral neck, avoiding postoperative iatrogenic instability.8,9

FIGURE 3
FIGURE 3:
“Nubbin” of bone formed within window of Oxford Spherical Mill reamer that indicates depth of bone removed during recontouring.
FIGURE 4
FIGURE 4:
Recontoured proximal femoral neck following reaming with Oxford Spherical Mill. The proximal femoral neck is subsequently reduced onto the femoral head and internal fixation is performed using cannulated screws.

Two retrograde guidewires are passed along the exposed metaphysis of the femoral neck and through the lateral femoral cortex. Both are overdrilled before the guidewires are removed. The epiphysis is then reduced onto the femoral neck. A k-wire inserted in the epiphysis is used as a joystick for assistance. Two antegrade guidewires are passed through the drill holes in the femoral neck. Their passage is extended into the epiphysis. Each is measured and Asnis III 6.5 mm cannulated screws (Stryker, Michigan) are inserted to hold the femoral epiphysis in place. The GT osteotomy is then reduced and secured in place using cannulated lag screws. A derotation boot is applied to the ipsilateral leg to prevent postoperative dislocation.

DISCUSSION

The principle of the Modified Dunn procedure is to restore alignment of the proximal femoral epiphysis to the femoral neck while maintaining its blood supply, which is principally provided by retinacular vessels in the posterior capsule.10–14 Debridement of posterior callus from the femoral metaphysis mitigates the risk of impingement of retinacular vessels supplying the epiphysis and, consequently, progression to avascular necrosis of the femoral head.

Although several updates to the original Dunn procedure (1978) have been incorporated and resulted in a modified version, there have been no previous reports suggesting the use of a microplasty instrument to assist in this critical metaphyseal debridement. The Oxford Spherical Mill has proven an ideal instrument to achieve femoral neck recontouring necessary for accurate reduction.

REFERENCES

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Keywords:

slipped capital femoral epiphysis; open reduction internal fixation; modified Dunn procedure; slipped upper femoral epiphysis

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