Percutaneous In Situ Cannulated Screw Fixation of the Slipped Capital Femoral Epiphysis S
Percutaneous In Situ Cannulated Screw Fixation of the Slipped Capital Femoral Epiphysis
DEFINITION
Slipped capital femoral epiphysis (SCFE) is a common hip disorder in adolescents in which the neck and femur displace anterolaterally (most commonly into varus and extension) with respect to the proximal femoral epiphysis.
SCFE can be classified as stable or unstable. A child with a stable SCFE has pain and a possible limp but is able to ambulate with or without walking aids, whereas a child with an unstable SCFE is unable to ambulate even with crutches. A stable slip has a nearly 0% risk of osteonecrosis, but an unstable slip has
risk of osteonecrosis somewhere between 10 and 50%.10
SCFEs have also been described by duration of symptoms as chronic (>3 weeks of symptoms), acute (<3 weeks of symptoms) or acute on chronic (long-standing mild symptoms with an increase in symptoms of >3 weeks' duration). This latter classification correlates less well with the risks of avascular necrosis (AVN) and chondrolysis.
ANATOMY
The proximal femoral physis and epiphysis are located within the hip capsule. Although the proximal physis provides length and shape to the femoral neck, most SCFEs occur in adolescence, when little growth remains at this growth plate.
The blood supply to the proximal femoral epiphysis comes from the medial femoral circumflex artery, which travels along the femoral neck. From the circumflex arise the lateral epiphyseal vessels, which enter the epiphysis posterosuperiorly. Small contributions come from the vessels of the round ligament and the posterior inferior epiphyseal vessels off the medial femoral circumflex artery. Injury to this tenuous capsular blood supply can result in osteonecrosis.
PATHOGENESIS
In SCFE, the epiphysis stays within the acetabulum while the neck and distal femur slip (most commonly into extension and varus).
SCFE occurs more commonly in boys than girls (60% of patients are boys). Most patients (up to 75%) are adolescents (boys 13.5 years, girls 12.0 years on average). Most patients are obese and in the 90th to 95th
weight percentile for age. SCFE occurs bilaterally in about 25% of patients.9
Biochemical factors likely play a role. Hormonal changes that occur during adolescent growth influence the strength of the physis.4
Biomechanical factors also likely play a role. The physis in SCFE is subjected to higher shear force. The physis is more oblique during adolescence and in obese children; both factors increase shear in normal activities. The proximal femur is relatively retroverted in many cases of SCFE, which also increases the shear force on the
physis. The reinforcing perichondral ring of the proximal physis also weakens with age until growth plate closure.
If the physis of an SCFE patient is studied histologically, it looks widened, with abnormal chondrocyte maturation and endochondral ossification. The slip occurs mainly through the hypertrophic zone of the physis.6 Patients younger than age 10 years should be evaluated for an underlying endocrine abnormality, including
hypothyroidism, renal osteodystrophy, and panhypopituitarism.11
NATURAL HISTORY
The natural history of untreated SCFE and the ultimate outcome are difficult to predict, although it is widely accepted in adult reconstructive circles that most cases of degenerative hip arthritis are secondary to an underlying structural cause, such as SCFE. The risk of progression exists while the physis remains open. The
slip severity increases with the duration of symptoms.7
The development of degenerative joint disease is related to the severity of the slip.3
PATIENT HISTORY AND PHYSICAL FINDINGS
Physical examination methods include the following:
The resting position of the knee and foot is observed with the patient lying supine and it is compared to the other side. Excessive external rotation is a result of the slip.
Hip range of motion (ROM) between affected and normal sides (for stable SCFE only) is compared. Because of the slip, the affected side has decreased flexion, abduction, and internal rotation of the hip. There may be guarding with ROM.
In SCFE that presents with knee pain, passive knee ROM is normal and effusion is absent.
In stable SCFE, the patient has an antalgic gait. The foot may be externally rotated. In unstable SCFE, the patient is unable to bear weight at all on the affected side.
Patients complain of hip or groin pain, thigh pain, or knee pain, which may be exertional and usually occurs without a history of trauma.
The patient may have a limp (stable slip) or frank inability to ambulate (unstable slip).
Examination of the hip can reveal an externally rotated foot and knee, guarding of the hip with ROM, and decreased flexion and internal rotation of the hip.
Findings on the knee examination are normal.
IMAGING AND OTHER DIAGNOSTIC STUDIES
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Plain radiographs of the pelvis, including anteroposterior (AP) and frog-leg lateral views, should be obtained in any pediatric patient with hip, thigh, or knee pain.
A widened physis on AP or lateral views can be an early sign of SCFE.
The Klein line, which can demonstrate SCFE, is drawn along the superior neck of the femur, on the AP view. In a normal hip, this line should touch at least some part of the epiphysis, but in SCFE, it will not cross the epiphysis (FIG 1).
The metaphyseal blanch sign of Steel is a crescent-shaped double density along the medial femoral neck where the slipped epiphysis overlaps the metaphysis on the radiograph.
Images of the contralateral hip should be scrutinized for evidence of bilateral SCFE. If present, both sides should be treated.
The severity of an SCFE can be described by displacement relative to the width of the metaphysis7:
Mild: less than one-third the width Moderate: one-third to half the width Severe: more than half the width
Another method of describing slip severity is measuring the difference between the epiphyseal shaft angle on each side16:
Mild: less than 30 degrees Moderate: 30 to 50 degrees Severe: 50 degrees or greater
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If the patient is younger than 10 years of age, underlying endocrine abnormalities should be investigated with laboratory studies, including thyroid function tests and basic chemistries.
DIFFERENTIAL DIAGNOSIS
SCFE
Legg-Perthes disease Hip labral tear
Femoral neck stress fracture Septic arthritis of the hip Knee derangement
Greater trochanteric bursitis
FIG 1 • An AP radiograph of the pelvis shows the Klein line drawn on the left hip. The line does not cross the epiphysis, indicating the SCFE. (Copyright Richard S. Davidson, MD.)
NONOPERATIVE MANAGEMENT
Immobilization in a spica cast was the historical treatment but is no longer recommended for SCFE.
Once SCFE is identified in any patient with an open physis, management is surgical to avoid further slippage and the possible development of femoral head AVN.
SURGICAL MANAGEMENT
When SCFE is identified in a patient with an open physis, surgical management with percutaneous in situ cannulated screw fixation should be undertaken on an urgent basis if the slip is stable or on an emergent basis
if unstable.2 There is a fair amount of evidence that the risk of AVN in unstable SCFE can be reduced if the hip is decompressed in some manner within 24 hours.5
Before surgery, the patient should remain strictly non-weight bearing on the affected leg to prevent conversion of a stable SCFE to an unstable SCFE.
In unstable SCFE, purposeful reduction of the displacement is controversial. Both open and closed reduction techniques have been associated with osteonecrosis, but the unstable slip itself may be the more likely cause of osteonecrosis.1, 13, 19
Because of the risk of contralateral slip, prophylactic pinning of the contralateral hip can be considered and discussed with the patient and family, especially if the patient is younger than age 10 years or has an endocrine abnormality.8, 14, 15
Preoperative Planning
All imaging studies are reviewed. The plain films of the contralateral hip should be scrutinized for evidence of early or clinically silent slip.
Laboratory studies should be reviewed in patients younger than 10 years of age.
Positioning
We place the patient supine on the radiolucent operating room (OR) table with the entire affected leg prepared and draped free to the umbilicus. The ipsilateral arm is padded and positioned across the chest.
The fluoroscopy monitor is placed at the patient's head and the C-arm unit is positioned for AP views of the hip from the contralateral side of the operating table. The hip is flexed 90 degrees and abducted 45 degrees to obtain lateral views. Not using a fracture table allows multiple views to be sure that the guide pin and the bone screws do not penetrate the femoral head.
Some surgeons prefer to perform in situ screw fixation on a fracture table.
Approach
In situ cannulated screw fixation of SCFE involves percutaneous placement of the guidewire into the central third of the epiphysis perpendicular to the physis. In most cases, this involves a small incision on the anterolateral thigh. This is followed by drilling and placement of the screw over the guidewire. Some screws are self-tapping and self-drilling.
Careful examination of spot fluoroscopy images ensures that the screw tip is within the femoral head, as detailed in the Techniques section.
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TECHNIQUES
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Guidewire Placement
The goal of guidewire placement is to place the tip of the wire perpendicular to the physis in the middle third of the epiphysis, about 3 mm from the subchondral bone. Spacing of the screw threads can be measured (usually 1 mm) and compared on the image intensification screen as a ruler. For the most common SCFE (varus slip), the starting point is on the anterior neck to place the wire properly in the middle of the epiphysis.
Determining the Course of the Guidewire
Under image guidance (AP view with the image machine placed vertical), the hip is internally and externally rotated until the neck appears to be its longest. At this position of rotation, the femoral neck is horizontal to the operating table and perpendicular to the image beam (TECH FIG 1A).
A guidewire is placed on the anterior hip and image intensification is used to align the point of the wire over the center of the femoral head. The guidewire is then aligned along the neck (TECH FIG 1B). The skin is marked at the tip to identify the position of the center of the femoral head.
The marker then follows the guidewire laterally to the lateral aspect of the femur. This line represents the course along which the guide pin for the bone screw will follow in the AP image (TECH FIG 1C). The marked line is drawn with a pen (TECH FIG 1D).
Determining the Entry Point along the Femoral Neck
Although we know that the femoral neck is following this marked line, the position of the femoral head in the sagittal plane is determined by flexing the hip 90 degrees and abducting the hip 45 degrees for the frog-leg lateral view. A line is drawn on the image as a diameter of the femoral head, and then a line is drawn perpendicular to the diameter at its center representing the desired path of the bone screw in the sagittal plane (TECH FIG 2A). The angle this line makes with the femoral neck tells the position of the femoral head with respect to the end of the femoral neck. The position at which this line crosses the femoral neck line also defines the entry point for the guide pin along the femoral neck (TECH FIG 2B,C). The degree of slip of the epiphysis posteriorly with respect to the neck is estimated.
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TECH FIG 1 • Patient is supine on the operating table. A. The image intensifier is set for an AP view and an image is obtained. The hip to be operated on is rotated internally and externally until the length of the neck appears longest. The femoral neck is now horizontal. This position is maintained. B. A long guidewire is placed on the patient at the hip with the point centered on the center of the femoral head and the guidewire over the center of the neck. C. This is confirmed with an image view. D. An OR pen is used to mark the skin with this line. The femoral shaft is palpated laterally on this line; this point is marked as 0 degree. The point over the femoral head is marked as 90 degrees. (A,C: Copyright Richard S. Davidson, MD.)
Having taken the steps earlier, the surgeon has now determined how to make the femoral neck horizontal to the operating table and perpendicular to the image beam and has determined the angle that the femoral head is with respect to the end of the femoral neck and the entry point on the anterolateral neck for the guide pin and screw.
Determining the Skin Entry Site
The final issue for the surgeon to determine is where to enter the skin. The skin is like a circle around the hip. The mark for the center of the femoral head is at 90 degrees and the lateral (palpable) femoral shaft is at 0 degree (TECH FIG 3A). If the head-neck angle measured is, for example, 30 degrees, the entry point
on the skin should be 30 degrees from the lateral palpable femoral shaft toward the femoral head.
Helpful hint: This position can be obtained by taking a length of suture that goes from the femoral head mark to the lateral femur mark (representing 90 degrees) and dividing it into thirds (30 degrees). The surgeon then measures from the lateral femoral shaft (0-degree mark)
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toward the head along the drawn line. A 1-cm incision is made through skin and spread with a hemostat down to bone along the drawn line but at an angle of 30 degrees toward the horizontal (ie, the head-neck angle) (TECH FIG 3B).
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TECH FIG 2 • The hip is flexed 90 degrees and abducted 45 degrees. A. An AP image is obtained. B. The image shows the femoral neck and the displaced femoral head. C. The neck to head angle is measured, and the point at which a line through the center of the head and perpendicular to the physis intersects the neck is noted. This is the entry point for the guide pin and bone screw. (A: Copyright Richard S. Davidson, MD.)
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TECH FIG 3 • A. The hip is then returned to the “neck horizontal” position. B. A 1-cm incision is made along the drawn line at the number of degrees from the lateral femur (0 degree). The fascia is spread with a clamp. C. The guide pin is inserted into this incision along the marked line but at the measured neck-head angle. The level of insertion at the neck is at the point observed in TECH FIG 2C. (C: Copyright Richard S. Davidson, MD.)
The guide pin should enter the incision in line with the drawn line and at an angle of 30 degrees to the horizontal (TECH FIG 3C). The point of the guide pin should be positioned on the anterolateral femoral neck where the entry was estimated earlier.
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Inserting the Guidewire
The guide pin is drilled part-way into the femoral neck and the hip is rotated to assess the guide pin
trajectory on the lateral view. In obese patients, it is easy to inadvertently bend the guidewire from the pressure of the soft tissues. This can be minimized by extending the incision a bit and by using the cannulated depth gauge to help protect the wire. When satisfactory, the guide pin is advanced to within 3 mm of the articular surface and measured, and a bone screw is chosen. Multiple image views should confirm the guide pin position before inserting the bone screw to avoid stress risers from too many holes in the bone.
Helpful hint: The surgeon can rotate the hip to prevent bending of the guide pin while flexing to get multiple views.
The chosen bone screw is inserted to within 3 mm of the articular surface.
Helpful hint: If the end of the guide pin is threaded, the bone screw may advance the guide pin through the articular surface. The hip must not be moved because of the risk of breaking off the end of the guide pin in the hip joint. Instead, the surgeon can retract the guide pin partially into the bone screw and then
continue to advance the bone screw to the appropriate position. Again, multiple views are obtained to ensure that the bone screw does not penetrate the articular surface.
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Drilling and Cannulated Screw Placement
The guidewire is measured with the cannulated depth gauge to determine desired screw length.
The cannulated drill is drilled over the guidewire. Care is taken to keep the drill colinear with the guidewire to avoid binding. The drill should stop 1 or 2 mm before the tip of the guidewire to keep the guidewire in place.
The bone is tapped with the cannulated tap over the guidewire. The 6.5- to 7.5-mm cannulated screw is placed over the guidewire. The guidewire is removed.
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Radiographic Evaluation
Radiographic evaluation (TECH FIG 4) ensures good screw position to minimize risk of complications.
Spot fluoroscopic AP and frog-leg lateral views are used to make sure that four or five threads of the screw are within the epiphysis to decrease the risk of slip progression.
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TECH FIG 4 • Radiographic evaluation of the threads across the physis on the AP (A) and frog-leg lateral
(B) views. (Copyright Richard S. Davidson, MD.)
The approach and withdraw technique allows evaluation of the screw tip to ensure that it remains within the femoral head. With live fluoroscopy, the hip is ranged from internal to external rotation at varying degrees of flexion and the screw tip is observed to approach and withdraw from the subchondral bone. The
closest point is observed and the screw should remain within the femoral head.12
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PEARLS AND PITFALLS |
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Guidewire ▪ In general, the larger the slip, the more proximal the starting point along the anterior placement neck.
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Guidewire ▪ When the hip is moved from resting position to frog-leg lateral position or back again, bending the pin can bend from pressure of the soft tissues. This can be minimized by using a 1- to 2-cm incision and by using the cannulated depth gauge to protect the pin.
Guidewire ▪ Drilling should stop 1-2 mm before the end of the guidewire to keep it seated in bone. pullout The surgeon should check spot fluoroscopy as the drill is backed out to make sure the guidewire remains in place.
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POSTOPERATIVE CARE
No immobilization is recommended. Patients should remain non-weight bearing or touch-down weight bearing on crutches on the affected leg for about 6 weeks to allow healing.
Patients are gradually allowed to return to activities after 6 weeks.
If the SCFE and fixation were unilateral, patients and families must be made aware of symptoms to watch for that could indicate slipping in the other hip, such as hip, thigh, or knee pain.
Patients should be followed at regular intervals until the proximal femoral physis closes completely on radiographs.
OUTCOMES
Outcomes of SCFE treatment are related to the severity of slip and the development of complications. The
best long-term results are seen with in situ screw fixation.3 In patients with mild and moderate SCFE treated with in situ screw fixation, results are good to excellent.
COMPLICATIONS
Chondrolysis is articular cartilage necrosis of the femoral head that causes hip pain and limited motion.
It has been associated with SCFE and its treatment, and the incidence has decreased with modern treatment techniques of in situ screw fixation.
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FIG 2 • Osteonecrosis and collapse of the femoral head in a severe case of SCFE. The in situ fixation had become prominent and has been removed. (Copyright Richard S. Davidson, MD.)
Risk factors include unrecognized pin or screw penetration, spica cast treatment, more severe slips, and female sex. The joint space appears narrowed to less than 3 mm on radiographs, and treatment involves revision of prominent hardware, limited weight bearing until symptoms improve, physical therapy, and nonsteroidal anti-inflammatories. Overall, patients may have cartilage recovery and good midterm
functional outcomes.18
Osteonecrosis is a difficult and often debilitating complication associated with SCFE and its treatment. It involves the death and possible collapse (FIG 2) of the epiphysis because of disruption of its blood supply. Patients present with groin, thigh, or knee pain and limited hip motion.
Risk factors for osteonecrosis are unstable SCFE, pins in the posterosuperior quadrant of the epiphysis, an increased number of pins, and severe displacement.17 A stable slip has a nearly 0% risk of osteonecrosis, but an unstable slip can have as high as a 50% risk of osteonecrosis.10
Treatment involves removal of prominent hardware if the physis has closed, limited weight bearing until healing, symptomatic management, and later reconstructive procedures.
Further slippage after in situ fixation occurs most frequently with improper screw placement outside of the desired middle third of the epiphysis and insufficient thread purchase into the epiphysis.
REFERENCES
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