Evaluation of percutaneous pinning of slipped capital femoral epiphysis
with a single cannulated screw

J.N. ANASTASOPOULOS, D.B. PETRATOS, K.P. TSAMBAZIS, G.S. MATSINOS, E.D. KARANIKAS
Athens Children’s Hospital "Agia Sophia"
2nd Orthopaedic Department

ABSTRACT
Objective: The evaluation of percutaneous pinning with a single cannulated screw as a method of treatment of S.C.F.E.
Materials and Methods: In a period of 4,5 years 31 children (35 hips - 21 boys and 10 girls) who had a slipped capital femoral epiphysis were treated by percutaneous pinning with a single cannulated screw. The mean age was 13 years for the boys and 10,7 years for the girls. There were 31 cases of chronic S.C.F.E., 2 acute and 2 acute-on-chronic. The degree of the slip was calculated according to the method of Southwick (24 mild – 8 moderate – 3 severe ). The follow up average was 32 months. The clinical criteria of Heyman – Herndon and the radiographic parameters by Boyer were used to grade the results.
Results: 31 hips were rated as either excellent or good, 2 as fair, 1 as poor and 1 as failure (Avascular necrosis of the femoral head). It seems that a relation exists between the post-operative clinical situation and the pre-operative degree of the slip.
Conclusion: Percutaneous central pinning of S.C.F.E. with a single cannulated screw provides reliable fixation. This method reduces significantly surgical time, blood loss, post-surgical scar and hospitalization cost while minimizing pin related complications.
Key words: S.C.F.E., percutaneous in situ pinning, cannulated screw.

INTRODUCTION
Slipped Capital Femoral Epiphysis (S.C.F.E.) is a common problem of the late childhood. It seems to be strongly associated with the hormonal changes of this period[30] and also with mechanical factors[24] of the proximal femur such as femoral retroversion[11], growth plate obliquity and the head-shaft angle[8]. The significance of the in time diagnosis and urgent operative treatment have been emphasized by the corresponding articles by Mayer[18] and Birch[3]. Authors, have reported several modalities of treatment according to the degree of slip, such as application of a hip spica cast[2,19], epiphysiodesis[12,14,15,20], osteotomy[10,22] and fixation with one[5,27] or multiple pins[23,28] to prevent further slippage of the epiphysis and to stimulate closure of the physis. The purpose of this retrospective study is the evaluation of percutaneous pinning of S.C.F.E. with single self tapping cannulated screw 6,5mm thread diameter, estimating the surgical technique and analyzing the results.

Figure 1. Diagram of the landmarks that are used to determine the skin point of entry of the guide-wire.

Figure 2. Drawing showing the positions of the cannulated screw in the proximal part of the femur related with its distance from the center of the epiphyseal plate.

MATERIALS AND METHODS
In a period of 4,5 years, 35 cases of S.C.F.E. in 31 children (21 boys and 10 girls) were treated by percutaneous (p.c.) pinning with a single self tapping 6,5 thread diameter cannulated screw in our department. Four cases were bilateral. The mean age was 13 years for the boys and 10,7 years for the girls. According to the data concerning the duration of symptoms (less or more than three weeks) and the history of trauma there were 31 cases of chronic, 2 acute and 2 acute-on-chronic. According to the ability of the child to walk even with crutches17 we had 32 stable and 3 unstable cases. Anteroposterior and frog-leg lateral radiographs were essential for the diagnosis. Children who had the diagnosis of S.C.F.E. were immediately admitted to the hospital and kept in bed rest until the operation. The degree of the slip was calculated according to the method that was described by Southwick[25]. The value of the head-shaft angle of the normal hip was subtracted from the head-shaft angle on the affected side as measured on the frog-leg lateral radiograph of the pelvis. A mild slip is defined as a difference in the head-shaft angle of less than 30 degrees; a moderate slip, 30 to 50 degrees; and a severe slip, more than 50 degrees[6]. There were 24 mild, 8 moderate and 3 severe cases of S.C.F.E. in the series. For the entire group the average slip was 27.8 degrees (range 6 to 80 degrees). For the bilateral slips 12 degrees was subtracted from the head-shaft angle to calculate the degree of the slip. 12 degrees was selected because it was the average head-shaft angle as measured on the frog-leg lateral radiographs of fifty asymptomatic adolescents.
Operative technique: The patient is positioned supine (figure 1) on ordinary table and a guide-pin is placed on the anterior surface of the proximal thigh so that the anteroposterior image (C-arm) shows in the desired varus-valgus position. The position of the guide-pin is marked with a marking pen. The guide-pin is then placed along the lateral aspect of the thigh (the limb is gently brought at frog-leg position with the shaft being parallel to the neck) so that it is in the correct anteroposterior position on lateral fluoroscopic image. The position of the pin is also marked on the skin. The guide-pin can then be placed through a simple stab (puncture) wound at the intersection of the two skin lines. Proper alignment, position and depth of insertion in the proximal femoral epiphysis are monitored on anteroposterior and lateral fluoroscopic images. The next steps are insertion and careful removal of the cannulated drill, insertion of the self tapping cannulated screw, removal of the guide-pin and finally a stitch is used to close the small incision which was made for the insertion of the drill and screw. The tip of the screw passes the physis proximally at a distance equal to the diameter of the thread (6.5mm).
Post-operative treatment: None of the children received antibiotics and only six patients were given painkillers because they were complaining of pain. The patients started physical therapy at the first post-operative day and they were usually discharged on the second post-op day. They followed a 3-week non-weight-bearing and a 3-week partial weight-bearing program before starting free walking. The mean Hospital stay was 4 days.
Evaluation of operative technique: The quality of operative technique was evaluated according to the distance D of the central axis of the screw from the center line of the femoral head on the anteroposterior and frog-leg lateral radiographs[21]. The screw was in position 1 when D‹½ d (d= diameter of the screw), in position 2 when ½ d<D‹d and in position 3 when D>d (figure 2). Therefore for every operation there was a pair of numbers that evaluated the technique of pinning. The first is for the position of the screw as seen on the anteroposterior radiograph and the second for the position as seen on the lateral radiograph. The screw was in the ideal 1.1 position in ten hips (figure 3), in the position 1.2 or 2.1 in fifteen hips, in the 2.2 position in six hips (figure 4), in the position 1.3 or 3.1 in three hips (figure 5) and in the poor 2.3 position in one hip.
Follow – up: The clinical criteria of Heyman – Herndon[13] and the radiographic parameters cited by Boyer et al[6]. were used to grade the results. The average duration of follow-up was 32 months and the patients were evaluated at 3-month intervals clinically and radiollogically.
Clinical criteria. Excellent: no limp or pain and normal range of motion of the hip
Good: no limp or pain but slight limitation of internal rotation
Fair: no limp or pain but slight limitation of internal rotation and abduction
Poor: mild limp, slight pain after strenuous exercise and slight limitation of internal rotation, abduction and flexion
Failure: limp, pain with activity and marked limitation of motion that led to reconstructive procedure.
Radiographic parameters6: Grade 0: no degenerative changes;
Grade I: one subchondral cyst and one osteophyte, slight subchondral sclerosis and a cartilage space of normal width;
Grade II: a few subchondral cysts and osteophytes, slight subchondral sclerosis and slight narrowing of the cartilage space;
Grade III: multiple subchondral cysts and osteophytes, marked subchondral sclerosis and moderate narrowing or obliteration of the cartilage space.

RESULTS
At the latest follow-up evaluation 13 hips were graded as excellent, 18 hips as good, 2 hips as fair, 1 hip as poor and 1 as failure. We did find an association between the severity of the slip and the clinical result but we did not find such an association between the surgical technique and the postoperative clinical situation (Mild slips: 46% excellent and 46% good results – Moderate slips: 25% excellent and 75% good results – Severe slips: 33,3% good, 33,3% poor and 33,3% failure results)
The most recent follow up radiographs showed radiographic changes of avascular necrosis (AVN) of femoral head in one hip. (Grade III according to the parameters cited by Boyer et al[6]). In this case (acute on chronic slip – severe – unstable) the epiphysis was reduced after application of skin traction on the affected leg for two days. The following p.c. pinning was perfect (grade 1,1) . Chondrolysis did not occur in any patient although there was intraoperative cartilage penetration with the guide-pin in three cases and with the cannulated screw in one case that was recognized the next day and managed with p.c. retrogression of the screw.
There was a deep vein thrombosis of the operated limb at the calf region one week post-op which was treated successfully with anticoagulative agents.

3A 3B
Figure 3.A,B. Percutaneous pinning of a mild S.C.F.E. (left) in a 13.2-year-old boy in the
1.1 position (ideal fixation).

4A 4B
Figure 4.A,B. Percutaneous pinning of a mild S.C.F.E. (right) in a 12.7-year-old boy in the 2.2 position.

5A 5B
Figure 5.A,B. Percutaneous pinning of a severe S.C.F.E. (left) in a 10.5-year-old girl in the 1.3 position.

DISCUSSION
The most popular modality of operative treatment of S.C.F.E. in the last years is the pinning of femoral epiphysis with one or more pins. Pinning with two or more pins provides better stability but the big number of the pins relates to chondrolysis and AVN of the femoral head (Increased possibility for injury of epiphyseal vessels and cartilage penetration). Blanco et al.5 reported a series of 114 hips that underwent pinning for treatment of S.C.F.E. and they were differentiated based on the number of the pins used. In the first group (1 pin) they had 4.6% complications and 2.3% re-operation. The corresponding results for the second group (2 pins) were 19.6% and 17.4% and for the third group (3 pins) were 36% and 12%. Stevens et al.[27] reported in their series of 130 hips that the percentage of AVN and chondrolysis were 16.6% and 13.8% when fixation was performed from the lateral portal using 3 or more pins while the corresponding percentages for pinning from the anterior portal with 1 or 2 pins were 5.3% and 0%. Walters and Simon[29], using an extension of Pythagorean theorem, reported that there is a possibility for pin penetration to exist although radiographs do not reveal it. The area of unrecognized pin penetration was named "radiological blind spot". This possibility increases as the pins are placed away from the center of the femoral head. They also compared the apparent depth of the pin (obtained by measurement on each radiograph of a biplane set) with the actual depth of the pin as measured with a caliper in an in vitro study. They found a 20% discrepancy if the pin was in the 1.1 position, a 100% discrepancy if it was in the 2.2 position and a 180% discrepancy if the pin was in the 3.3 position. Their conclusion was that the ideal position for the pin (position with the slightest possibility for penetration of cartilage) was only one; that with the pin perpendicular within the center of the femoral head at least 5mm in the subchondral bone.
The association between pin penetration and chondrolysis has been reported by many authors like Bishop et al.[4] who has referred that a great relation exists between the two situations, findings that were supported by Walters and Simon[29]. In our series cartilage penetration with guide pin in 3 cases and with cannulated screw in one case, although the last was not "transient", didn’t cause any chondrolysis.
Stambough et al.[26] referred that there must be a relation between AVN-chondrolysis and multiple pins placed in the superior-anterior quadrant of the proximal femoral epiphysis. Brodetti[7] based on angiography studies suggested the avoidance of the superior-posterior quadrant of the femoral head because this is the site where the supplying vessels for the weight-bearing surface pass through (lateral epiphyseal artery). In our series there wasn’t a case of placing the screw in the superior-posterior quadrant and only in one case the screw was close in the superior-anterior quadrant without causing AVN. Benett et al.[1] reported that the use of only a single screw reduced the risk of protrusion by a factor of three (without causing chondrolysis) and Crawford[9] asserted that insertion of the second screw increased the risk of complications a hundredfold.
According to the above elements it is clear that single screw fixation of S.C.F.E. with the screw perpendicular within the center of the physis causes the smallest percentage of complications. The question is if this technique decreases the stability of fixation. Karol et al.16 proved that double fixation yielded only a 33% increase in stiffness as compared with single pin fixation. The stiffness of neither double nor single screw fixation approximated that of the intact physis. In our series there was not a single case with postoperative slip progression.
The open pinning method via lateral approach of the proximal femur some times leads to impasse because the surgeon could not place the pins at the proper position. Following p.c. procedure there is the ability for appropriate placement of the pin through the anterior portal (as needed for the usual posterior slipping) and the ability to try many times to insert the guide pin at the correct position. This method like all the p.c. procedures provides decreased surgical time (in our series average surgical time 20min) and blood loss, the slightest post-operative pain (only six children needed pain relief), no need for post-operative antibiotics, decreased hospitalization time (average hospitalization time four days in our series) and consequently the smallest hospitalization cost.

CONCLUSION
Percutaneous pinning with a single cannulated screw is recommended because it provides stable fixation of slipped femoral epiphysis. It is associated with the smallest surgical time and blood loss, the slightest post-operative pain, the lowest hospitalization cost (few days in hospital – no need for antibiotics) and the best cosmetic results. We noticed an association between the degree of slip and the clinical results but the post-operative clinical situation is not influenced by slight divergences of the screw from the ideal central position.

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