Fixation in slipped capital femoral epiphysis avoiding femoral-acetabular impingement

Surgery was carried out under fluoroscopic control. Spontaneous reduction occurred in the unstable slips when positioning the patient. A modified cannulated screw 90–120-mm long was used. The modified AO cannulated screw (HIT-MEDICA, Rimini, Italy) had a distal segment, with the original six threads reduced to three and were 9-mm long and 6.4 mm in diameter. The screw was inserted over a guide wire after pre-drilling. The threaded portion of the screw was placed entirely within the epiphysis with the smooth shank crossing the physis, femoral neck, and lateral femoral cortex. The screw head was allowed to remain about 1.5–2 cm to the side of the lateral femoral cortex (always under the iliotibial band to avoid disturbance to the patients) to provide for neck growth. This distance was based on the predicted growth of the proximal femur, plus a few additional millimeters, to facilitate removal using a guide wire if necessary. The position of the screw in the central quadrant of the femoral epiphysis was confirmed by multiple fluoroscopic views. The screw tip was positioned more than 2.5 mm from the epiphyseal subchondral bone. After insertion of the screw, the hip was taken through a full range of movement with multiple fluoroscopic views to check the final hardware position. Post-operation, patients with a stable SCFE were allowed to weight-bear as tolerated. Those with an unstable SCFE used crutches with partial weight-bearing for 4 weeks.

Pre-operative assessment included clinical and radiological classification of the SCFE in terms of severity and type (stable/unstable). The pre- and post-operative head-neck slip angles were measured on anteroposterior (AP) and lateral radiographs (with the leg held in a device at 90° of flexion and 45° of abduction. This is a modified abduction of the 90° Dunn view position [22]) taken at a standard 100-cm beam distance [23]. Any change of 5° or more was considered significant. A negative value was considered a decrease and a positive value an increase in slip angle. Radiographic measurements were made digitally using the CARESTREAM DRX-Evolution system. Slip severity was measured on pre-operative lateral radiographs with the posterior sloping angle (PSA) [23].

PSA is the angle between the plane of the physis and the line perpendicular to the femoral neck axis. To assess cam deformity, two measurements were performed: alpha (α) angle in AP and lateral view radiographs in the affected and unaffected hips (Fig. 1).

Measurements were performed immediately after pinning, at 6 months and at 12 months post-operatively. The α angle was measured on AP and lateral view radiograms following the methodology of Clohisy et al. (2008) [22]. It is calculated by measuring the angle between two lines: (1) a line drawn connecting the center of the femoral head to the point on the anterolateral and lateral view aspect of the head-neck junction, where the radius of the femoral head is found in the acetabulum (i.e., where a prominence starts), and (2) a line drawn through the center of the femoral neck, connecting to the center of the femoral head. An α angle greater than 50° is suggestive of cam deformity [24]. Concurrently, neck length was measured on a line drawn from the lateral femoral cortex to the physis on the AP view, as shown in Fig. 2.

Post-operation and at the 12-month follow-up, limb length discrepancy was clinically measured (from anterior iliac superior spine to tibial malleolus) and differences between the affected and unaffected sides noted.

All measurements were independently determined by two observers (FF and AGA), and inter-observer correlations were evaluated by chi-square test.

Clinical follow-up assessment of the hip was made using the modification by Zahrawi et al. (1983) [25] of the classification of Heyman and Herndon (1954) [26] in which an excellent result has a normal range of hip movement, no limp, no pain, and leg shortening of < 1 cm. A good result had slight limitation of internal rotation, occasional pain, and leg shortening of < 1 cm. A fair result had persistent mild pain, loss of internal rotation, abduction, and leg shortening of > 1 cm. A poor result did not meet any of the above criteria. (We preferred this classification to the Harris Hip Score because it included leg shortening. We believe this is one parameter that must be considered and compared with traditional screw fixation in other papers.)

Descriptive statistics were used for the variables of our cohort. The paired Student’s t test was used to analyze the differences between a normal hip and a pathologic hip when the data was parametric, and the Pearson correlation was used to correlate the results with the collected data. Outcome data for each group are presented as the mean and standard deviation. The mean difference between groups and a 95% confidence interval are provided. A p value of < 0.05 was considered significant. Statistical analysis was performed using GraphPad Prism 5 computer software.

The clinical results in all patients were rated as excellent and all had resumed full activity at the 6th- and 12th-month post-operation. Limb length discrepancy of ≤ 1 cm (the affected limb was shorter) was recorded post operation and confirmed at follow-up in nine cases (numbers 1, 2, 4, 6, 7, 9, 10, 13, 15), and in two cases, the affected limb was longer (numbers 19, 22). There were no statistically significant differences between groups (p = 0.08) (Table 2).

No statistically significant differences were observed between affected and unaffected hips in flexion (p = 0.3), extension (p = 0.3), internal rotation (p = 0.3), and external rotation (p = 0.4) and no clinical signs of impingement were detected (Table 2).