A Simple Modified Technique for In-Situ Screw Fixation in Slipped Capital Femoral Epiphysis

Abstract

Background

In-situ pinning has a definite role in the management of slipped capital femoral epiphysis (SCFE). We describe a modified technique for in-situ screw fixation on a regular radiolucent operating table which avoids certain complications innate with the existing techniques.

Materials

Sixty consecutive hips which underwent either in-situ fixation for SCFE (28 hips) or prophylactic fixation of the contralateral hip (32 hips) by the modified technique were analysed. The femoral head was divided into three zones (A-central, B-middle, C-peripheral) of equal circles. The zone of the screw was noted in both AP and lateral views. The angle between the physeal line and the screw in AP(S_AP) and lateral (S_lat) view, and the distance from screw tip to articular surface in both views were measured.

Results

In AP view, 55/60 (91.6%) screws were in zone-A, and five were in zone-B. In the lateral view, 56/60 (93.3%) screws were in zone-A, and four were in zone-B. There was no screw placed in zone-C in either of the views. The average deviation was < 15° in AP view and < 7° in lateral view from the ideal placement. The mean distance from the screw tip to the articular margin in AP was 5.15 mm and that in lateral was 6.15 mm. The interobserver agreement rate was found to be 0.8. No patient had intraoperative breakage of a drill bit or joint penetration, avascular necrosis, chondrolysis or screw-related complications at a minimum follow-up of one year.

Conclusion

In-situ pinning on the radiolucent table is safe and has distinct advantages. The modified technique of in-situ screw fixation adds to the safety and accuracy of the procedure.

Level of Evidence

Level IV.

Introduction

In-situ screw fixation is one of the preferred techniques for the treatment of SCFE. There are enough reports from the literature which has shown good long-term outcomes with in-situ pinning [1–4]. This is a time-tested method and has a definite role in the management of SCFE even today. In-situ screw fixation is essentially a technique performed under image intensifier guidance. The criteria for successful in-situ screw fixation are that the screw should be placed in the center of the epiphysis, preferably perpendicular to the physis without penetrating the joint. To achieve this perfectly, good biplanar fluoroscopic imaging is mandatory during the procedure.

Traditionally and in most studies, in-situ pinning has been described with the patient positioned on a fracture table [5–8]. The distinct advantages are ease of biplanar fluoroscopic imaging, need for fewer assistants, and lowered risk for further displacement of capital epiphysis as there is no need for repeated maneuvering of a limb. The major disadvantage is the inability to examine the hip at the end of the procedure for a range of movements and impingement (to assess the need for osteoplasty) in a sterile manner. The other disadvantages of fracture table are the need for position change and repeat draping in bilateral cases or for prophylactic pinning, which significantly increases the surgical time. Another potential problem with positioning on the fracture table is that in unstable slips, the weight of the limb hanging unsupported could increase the stress on the retinacular flap and potentially increase the chances of osteonecrosis. To overcome these limitations, in-situ pinning in the supine position on the radiolucent table has been described by some authors [9–12].

The practical difficulty in using a radiolucent table is bending or breakage of regular thin (2.5 or 3 mm) guide pin used for standard 6.5 mm/7 mm cannulated cancellous screws [9]. This is especially true in obese patients with bulky thighs, were the soft tissues (especially the iliotibial band) will bend the guide pin during maneuvering of a limb into flexion, abduction and external rotation to get the frog-leg lateral view. To overcome this technical difficulty, the author has developed a simple modification of in-situ screw fixation technique with the patient supine on a radiolucent table which has several advantages over the conventional technique. The technique utilizes the solid 4.5 mm drill bit, protection sleeve (custom-made 150 mm ex-fix rod), and cannulated 6.5 mm cancellous screws. The technique is described in detail below and the accuracy of placement of the screw with this technique was studied.

Methodology

Institutional review board approval was obtained for reviewing clinical and radiological data of all children with SCFE, operated between 2012 and 2015. The AP and frog-leg lateral radiographs of 60 consecutive hips which underwent either in-situ fixation for the slip (28hips) or prophylactic screw fixation of the contralateral hip (32hips) were taken for this study. Of 28 hips in in-situ pinning group, 3 were acute slips, five were acute on chronic, and 20 were chronic slips. We analyzed the six weeks post-operative radiographs of 60 hips for the accuracy of placement of screws by this technique. We did not take the immediate post-operative radiograph because it was not possible to take frog-leg lateral views for some of our patients who had modified Dunn’s procedure or primary osteoplasty. All procedures were performed by a single surgeon with specialised training in paediatric hip disorders.

Technique of In-Situ Screw Fixation

Pre-operative Planning

Anteroposterior (AP) and frog-leg lateral views are analyzed for the severity of slip. The starting point and trajectory of the screw in the AP and frog-leg lateral views are determined based on the severity of slip. Any of the standard described techniques can be used to determine the location of the entry point from the preoperative images [11, 12].

Patient Positioning

The patient was positioned supine on the radiolucent table. Sterile draping includes both the hips with entire lower limbs draped free to move intraoperatively. The image intensifier is placed on the opposite side in a static position to get an AP image of the hip joint. The affected limb is moved to flexed, abducted, and external rotation position to get the frog-leg lateral view, without moving the image intensifier.

Technique

Step 1 Mark the skin entry by placing the guidewire on the thigh. The preferred entry point is 0.5 to 1 cm below the trochanteric apophysis on the AP projection. The entry on the lateral projection will be decided according to the severity of the slip.

Step 2 Make the 1 to 2 cm skin incision and clear the soft tissue up to the bone with blunt artery forceps. Make sure that the iliotibial band was adequately dissected, for easy passage of custom-made protection sleeve (Fig. 1a).

Fig. 1.

In-situ screw fixation technique on a radiolucent table. a The sleeve (ex-fix rod) slide over the 4.5 mm drill bit up to the bone. b C-arm image showing the sleeve over drill bit up to the bone level; the guidewire seen is placed on the surface of the skin to guide the trajectory. c Flexion, abduction, and external rotation of limb with the sleeve and drill bit in-situ. d C-arm image of c. e Measuring the screw length using another guidewire of the same length. f, g AP and lateral C-arm images after placement of the screw

Step 3 Make the entry with 4.5 mm solid drill bit in the desired trajectory in the AP plane under image intensifier (Fig. 1b). The 4.5 mm solid drill bit is thicker (resists bending and breakage) and allows for better control of the direction compared to regular thin and flimsy guide wire of 2 or 2.5 mm thickness.

Step 4 Disconnect the power drive from the drill bit, leaving it in the bone. Drive the protection sleeve (custom-made 150 mm ex-fix rod) over the drill bit up to the bone level and gently flex, abduct and externally rotate the hip to take the frog-leg lateral view. Ex-fix rod as a sleeve protects the drill bit from bending or breaking (Fig. 1a–d). Confirm the trajectory of the drill bit in the lateral view and make the appropriate changes if needed.

Step 5 Once the desired trajectory is obtained in both the views, drive the drill bit further inside just short of the physis. At this stage, check the position of the drill bit using a protection sleeve with due adjustments in both planes to get in the center of the epiphysis. Then advance the drill bit into the epiphysis, just short of the subchondral bone.

Step 6 Remove the drill bit and place the standard instrument set guide wire into the track made by it. Screw length is measured by passing the identical length guidewire along with the one which is in the bone and measuring the difference of lengths (Fig. 1e).

Step 7 Use the cannulated tap to pre-cut the drilled track.

Step 8 The correct length 6.5 mm cannulated cancellous screw is advanced over the guidewire very carefully taking care that the guidewire does not penetrate the articular cartilage (Fig. 1f, g).

Step 9 Confirm the position of screw in multiple image intensifier views. Wounds closed in a standard fashion.

Step 10 Assess the hip range of movement and especially look for flexion-internal rotation (FIR) to evaluate for femoro-acetabular impingement (FAI). The author's practice is to do primary osteoplasty for FAI if FIR is less then 15º, using the standard anterior hip approach. Supine on radiolucent table facilitates this distinct advantage of dynamic hip assessment, without a change in position or need for repeat draping.

Accuracy Study of Technique

The femoral head was divided into three zones (A—central, B—middle, C—peripheral) of equal circles, as shown in Fig. 2. The zone of the screw was noted in both AP and lateral projections. The centrally placed screw was labelled as zone-A, and when the shaft of the screw was touching the first circle, it was considered to be in zone-B. When the screw shaft was touching the second circle or lying beyond that circle, it was labelled as zone-C. The following measurements were taken in both the AP and lateral views:

1. S_AP—Angle between the physeal line and the screw in the AP view

2. S_lat—Angle between the physeal line and the screw in the lateral view

3. D1 – Distance from screw tip to articular surface in the AP view

4. D2—Distance from screw tip to articular surface in the lateral view

Fig. 2.

a AP radiograph showing the marking of zones and that the screw is in zone A. b lateral radiograph showing the zones and that the screw is in zone A. c lateral radiograph showing sub-chondral distance measurement (4.68 mm in this case), d lateral radiograph showing measurement of S_lat. (here the deviation is 0º from the perpendicular)

Results

In the AP projection, 55/60 (91.6%) screws were in zone-A, and five were in zone-B. In the lateral projection, 56/60 (93.3%) screws were in zone-A, and four were in zone-B. There was no screw placed in zone-C in either of the views. Ideally, the screw needs to be placed perpendicular to the physis. The mean S_AP was 104.7° (range 70°–120.1°) and the mean S_lat was 84.8° (range 65.8°–106°). This essentially means that the average deviation was less than 15° in AP projection and less than 7° in lateral projection from the ideal placement. We also analyzed whether the screw was placed in the subchondral bone by measuring the distance from the screw tip to the articular margin in both AP and Lateral projections. The mean distance in AP was 5.15 mm (range 1–10.5 mm) and that in lateral view was 6.15 mm (range 2.1–15.5 mm). Two different observers took all the above measurements, and the interobserver agreement rate was found to be 0.8 by the intraclass correlation coefficient.

The mean preoperative slip angle in the in-situ group was 16°. Eight (28.5%) of the 28 patients in the in-situ group underwent primary osteoplasty for FAI (FIR < 15°), without a change in position. None of the patients had intraoperative breakage of the drill bit or joint penetration. No patients in this series had AVN, chondrolysis or screw-related complications in a minimum follow-up period of one year.

Discussion

The primary goal when treating an SCFE is to restore good long-term hip function using a surgical technique that is as safe and simple as possible. In-situ screw fixation is a time-tested, safe and simple surgical technique with good long-term outcomes. Even in today’s era of FAI, in-situ screw fixation definitely has its own role in the management of SCFE. However, technique-related complications like screw penetration, chondrolysis, slip progression, and guidewire breakage have been reported in the literature [12]. The practical difficulty of the fracture table for in-situ screw fixation for bilateral cases and the inability to assess the hip at the end of the procedure have made some surgeons shift to the regular radiolucent table.

We are sure that the radiolucent table has been used commonly for in-situ pinning among experienced paediatric orthopaedic surgeons. Therefore, our description may not be new. Lee et al. described a technique to determine the skin entry point for percutaneous pinning on a regular radiolucent table. However, the complexity of their planning method limits its usage [11]. Obtaining frog-leg lateral radiographs in patients with a stable SCFE was not associated with avascular necrosis or chondrolysis. Pring et al. described a preoperative planning technique to determine the pin-entry point for in-situ pinning on a radiolucent table. They found that the accuracy and complications are comparable to in situ screw fixation on a fracture table [12]. Blasier et al. evaluated slips over 10-years treated by single screw fixation on either a radiolucent (29 hips) or fracture table (36hips). They found that the screw deviation from the mid-point of the epiphysis was not significantly different on either the AP or lateral views [13]. All the above authors have consistently shown that the surgical time was significantly longer on the fracture table as compared to the radiolucent table. Further, in-situ pinning on a radiolucent table has an additional advantage that the radiation dose during the procedure is significantly less than that is performed on a fracture table [14].

All the previous authors used the regular guide pin provided with the standard instrument set. They cautioned about the bending or breakage of the guide pin while positioning the limb for lateral projection. Few recommended the use of a cannulated screwdriver to slide over the guide pin up to bone level while positioning for lateral projection to prevent guidewire deformation from bulky soft tissues and tight fascia. We recommend the use of 4.5 mm drill bit with a protection sleeve (ex-fix rod) instead of the regular guide pin. This thick drill bit helps for easy adjustment of trajectory in both the planes and the sleeve prevents the deformation of a drill bit. The addition of custom-made 150 mm ex-fix rod as a protective sleeve to the armamentarium came from the author's experience of drill bit breakage (while positioning for lateral projection). This greatly facilitated the author to get good lateral images, the key for successful in-situ screw fixation. From the accuracy analysis of our technique, we had very high accuracy in placing the screws in the ideal position, and none of them was in a non-acceptable position requiring revision. Also, none of the patients had any complications related to the placement of screws.

The theoretical disadvantage of the radiolucent table is the need for manipulation of a limb during the procedure to get a lateral image, which may be a concern in cases of potentially unstable slips. However, studies have shown that gentle manipulation of a limb on the radiolucent table does not increase the risk of AVN [12]. There are several advantages to our technique. First, it is simple and straightforward with readily available instruments. Second, both the sides can be done in one positioning (supine), which saves time. Third, the protection sleeve ensures that there would be no drill bit bending or breakage and safely allows acquiring a good lateral view. Fourth, the risk secondary to the usage of cannulated instruments can be avoided entirely (jamming of a guidewire in the cannulated drill bit and intraarticular penetration of guidewire). Fifth, allows dynamic examination of the hip after in-situ fixation, which helps in deciding the need for additional procedures like primary osteoplasty.

Conclusion

In summary, in-situ pinning on the radiolucent table is safe and have distinct advantages. Our modification of the technique is simple, readily available, and adaptable. This technique of in-situ screw fixation adds to the safety and accuracy of the procedure, has several advantages and avoids the complications usually faced while using the standard cannulated cancellous screw instrumentation.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Compliance with ethical standards

Conflict of Interest

All the authors declare that there is no conflict of interest.

Research Ethics and Patient Consent

Written consent for publication of patient details was obtained from the parent/guardian.