Slipped capital femoral epiphysis occurs with an incidence of up to ten per
100,000 population, depending on the race of the patient and the geographic
region studied1. The
treatment for slipped capital femoral epiphysis has evolved with time;
currently, orthopaedists in both general and subspecialty practice most
commonly use percutaneous in situ screw stabilization to treat the
disorder2,3.
Because of the posterior displacement of the epiphysis and retroversion of the
femoral neck, the classic "laterally based" starting point for
insertion of screws (as would be used for operative treatment of femoral neck
fractures in adults) needs to be located more anteriorly and superiorly on the
lateral-proximal aspect of the
femur3,4.
The distortion in femoral head-neck anatomy with slipped capital femoral
epiphysis can make radiographic visualization of the articular surface (to
determine joint encroachment or penetration by the pin or screw) difficult.
Unrecognized joint penetration with a screw can lead to
chondrolysis5-7.
Over the last twenty-five years, knowledge of the risk for joint penetration,
as well as the use of high-quality image intensifiers, has reduced but not
eliminated this
risk8-12.
We present the case of a patient who was referred to our facility as a
result of hip pain after screw placement for slipped capital femoral
epiphysis, and we demonstrate the use of an easily obtainable radiograph that
can help to identify the position of the screw tip in a patient who has
unexplained hip pain following screw stabilization for slipped capital femoral
epiphysis. Our patient and her family were informed that data concerning the
case would be submitted for publication.
Aten-year-old girl with pain in the left hip and decreased hip motion was
evaluated five months after she had been treated elsewhere for a stable left
slipped capital femoral epiphysis with in situ screw fixation. The procedure
had been performed one month following the onset of symptoms, and two screws
had been used to stabilize the slipped capital femoral epiphysis (Figs.
1,
2-A, and 2-B).
The condition of the patient failed to improve after the initial operation;
instead, she had increasing hip pain, loss of hip motion, and an externally
rotated hip posture. She had no pain at rest and was still an active swimmer.
Physical examination demonstrated limited left hip flexion to 45° and pain
with resisted straight-leg raising. The rotational arc of motion was 30°
(30° external to 60° external) and painful. Anteroposterior and
frog-leg lateral radiographs demonstrated that the screw tips were near the
joint surface. Only when a so-called false-profile oblique radiograph of the
hip was obtained was it possible to clearly confirm penetration of the screw
tips into the hip joint (Fig.
3).
The superior screw, which was 85 mm long and 6.5 mm in diameter, was
removed surgically and replaced with a shorter, cannulated screw that was 75
mm long and 7.3 mm in diameter. The inferior screw, which was 6.5-mm in
diameter, was withdrawn slightly (Figs. 4-A
and 4-B).
Postoperatively, the condition of the patient improved markedly. Range of
motion measurements at one year after surgery demonstrated left hip internal
rotation to —5°, external rotation to 80°, flexion to 90°,
and no pain during the examination, and radiographs demonstrated appropriate
screw length with no encroachment into the hip joint.
In the late 1970s, Walters and Simon demonstrated that pin penetration into
the hip can easily go unrecognized on both anteroposterior and lateral
radiographs and that there is a high correlation between chondrolysis and
persistent penetration of a
pin7. Their classic
illustrations have helped orthopaedic surgeons understand the great risk of
pin penetration, particularly if the pin tip is not centered
(Fig. 5). To identify pin
penetration with radiographs, they recommended the use of their template and a
comparison of the location of the pin on the anteroposterior, true lateral,
and frog-leg lateral radiographs. If the pin is in a danger area on two
different views or is in a danger area on one and a caution area on another,
the pins should be withdrawn.
Greenough et al. also reviewed complications from slipped capital femoral
epiphysis pinning5
and reported a 34% complication rate. Pin penetration occurred in >13% of
the patients and accounted for 64% of their complications. No chondrolysis was
reported. Stambough et
al.12 had four pin
penetrations in their eighty patients. Two patients had chondrolysis, and two
patients had femoral head osteonecrosis.
Riley et al. showed a 40% complication rate with pin-and-screw
fixation6, with
forty-one of the eighty hips demonstrating pin penetration. Four of nine hips
with chondrolysis clearly had pin penetration, as did five of fourteen hips
with osteonecrosis. Sepsis developed in one hip joint after pin
penetration.
An increasing awareness of the potential complications of pin penetration
has resulted in improved techniques to aid in preventing the problem. The
development of improved image intensifiers has allowed better radiographic
visualization intraoperatively. Learning that stable slips (95% of all slipped
capital femoral epiphyses) can be treated with a single, large-diameter
cannulated screw was a critical further step for safer
surgery3,11.
The cannulated screw design allows placement of a preliminary guide pin, which
can be carefully positioned (and studied with image-intensifier views) prior
to placement of the permanent screw. The screw tip can then be placed
centrally to minimize the chance of penetrating the outer edge of the femoral
head7,10.
A further aid in preventing penetration of the screw was the description by
Moseley of the "approach-withdraw phenomenon," a technique used to
study the hip joint with the image intensifier at the time of pin
placement13. With
this technique, a pin is introduced in the standard fashion. When the pin is
near the subchondral bone, the extremity is rotated from maximal internal
rotation to maximal external rotation, during which the pin is seen to
"approach" and then "withdraw" or move away from the
subchondral bone. The point of transition between these two motions indicates
the true position of the screw tip. With movement in multiple planes, to be in
a safe location the screw tip must be radiographically documented as never
crossing the subchondral bone plate.
Other techniques have been proposed to aid in determining joint
penetration. Bassett described a technique in which the joint is inspected
endoscopically through the channel drilled for the
screw14. Lehman et
al. described a technique in which the joint is inspected arthrographically
after water-soluble dye is injected through the screw
cannula15. This
method may not be completely reliable, however, because if the guide pin has
penetrated the joint but the cannulated screw has not, the arthrogram may
still indicate joint penetration.
Morrissy stated that, in comparison with the radiographs obtained after
pinning a femoral neck fracture, the radiographs obtained after pinning a
slipped capital femoral epiphysis can be deceptive due to the relative
anterior-to-posterior direction of the
screws3. Nguyen and
Morrissy highlighted the importance of the anatomy and the corresponding
starting point by showing radiographs of a slipped capital femoral epiphysis
model in various positions with various severities of
slip11. In the case
of our patient, the starting point of the screw was too inferior and too
lateral, resulting in the need for two screws. The use of two screws can lead
to an increased risk of a subtrochanteric femoral fracture as well as screw
injury to the vasculature of the superior portion of the femoral
head6,16,17.
Ideally, pin penetration is avoided in the operating room through use of
the methods discussed above. If penetration is noted on any radiographic view,
the screw must be adjusted (or replaced). Unfortunately, recognition at the
time of the operation does not always occur (as in the case of our patient),
and postoperative documentation of the problem can be difficult to achieve.
Limited motion due to postoperative pain often results in a less-than-ideal
radiographic study. A fine-cut computed tomographic study can be diagnostic
but may delay the time of diagnosis by days.
The false-profile oblique radiograph of the hip can be performed easily in
the office setting and provides a unique view of the surface of the hip
joint18. Lequesne
and de Seze described "le faux profil du bassin" (the
false-profile view of the pelvis), a radiographic view that they believed
represented a true lateral view of the acetabulum and hip and that could be
useful in the identification of dysplasia and other types of hip
pathology19. The
image is obtained with the patient standing with the affected hip against the
radiographic film. The ipsilateral foot is placed pointing forward, parallel
with the film plate. The opposite hip is rotated 25° posteriorly,
maintaining the ipsilateral foot in its original position
(Fig. 6). This positioning
allows the hip in question to be more anterior when viewed on the final
radiograph.
In the case of our patient, the false-profile view nicely demonstrated
screw penetration. If it had not, a fluoroscopic examination with the patient
under anesthesia or, more commonly, a computed tomographic scan could have
been performed to establish the diagnosis.
In summary, slipped capital femoral epiphysis is a common condition that
requires surgical stabilization. Knowledge of how the hip anatomy varies from
that of a normal hip is essential for accurate surgical pinning.
Intraoperative techniques have been reported to aid in determining both
placement of the screw and position of the screw tip in relation to the joint.
Use of a high-quality image intensifier makes the procedure more predictable.
The procedure can be performed with the patient positioned on either a
fracture table or a fluoroscopy table. While the patient is still in the
operating room, every effort should be made to confirm that screw-tip
penetration has not occurred. In our opinion, the best method for ascertaining
this is through the use of Moseley's "approach-withdraw
phenomenon" technique. Standard postoperative radiographs confirm screw
placement but can be misleading. If a patient has unexplained symptoms, a
false-profile view of the hip can help identify joint encroachment by the
screw tip and thus allow early correction of the problem. ?
Note: The authors thank Richard F. Santore, MD, for his advice
on use of the false-profile view and his help in interpreting false-profile
radiographs.