In this retrospective multicenter pilot study, 7 patients [age range: 9–18 years] underwent our proposed approach for surgical FHRO treatment between May 2017 and October 2019. The surgeries were performed at different centers by four senior hip surgeons who had previous experience in FHRO surgeries. One out of 7 patients was excluded from this study because the parents/legal guardian did not provide informed consent. Ethical approval was obtained by the ethical committee of the Canton of Zurich. Three patients were male and the other three were female. Inclusion criteria were pain and restricted hip motion, severe deformity of the femoral head and intact peripheral cartilage with central necrosis. Preoperative computer simulation of the surgery was performed for each patient and patient-specific instruments (PSI) for surgical navigation were designed and manufactured.
Preoperative computer simulation: CT scans of the patients᾽ hips were obtained in a supine position and anterior-posterior [AP] hip orientation according to a specifically designed protocol [MyOsteotomy CT protocols, Medacta International, Castel San Pietro, Switzerland]. The CT scans were acquired with an axial resolution of 1 mm slice thickness using a Philips Brilliance 40 CT device [Philips Healthcare, Best, the Netherlands]. The data was imported into a commercial image processing software [Mimics Medical, Version 19; Materialise, Leuven, Belgium] and the bone anatomy was segmented from the surrounding soft tissues by applying global intensity-based thresholding and region-growing. 3D triangular surface models of the femur and the pelvis were generated from the segmented images using the Marching Cube algorithm (22). These models were imported into the in-house developed preoperative planning software CASPA [Computer-Assisted Surgical Planning Application, Version 5.29] to simulate the FHRO surgeries [Fig. 3]. The mirrored models of the healthy contralateral sides [Fig. 3, shadow contour] were used to approximate the pre-morbid femoral heads and served as remodeling templates in the simulation. In the case of a pathological contralateral side, a geometric sphere was used as a template instead. The sphere was manually centered in the mechanical joint center of the hip and resized until it covered the healthy portion of the femoral head.
The definition of the femoral head osteotomy planes is the most important step in the preoperative planning [Fig.3-A, grey planes]. These planes implicitly define the resection of the necrotic part [Fig.3-A, red wedge], the degree of head sphericity, the residual articular step-off between the contact surfaces of the fragments [Fig.3-B, red square], and the size of the remaining neck pillar [Fig.3., yellow line]. The locations of the osteotomies are constrained by the medial and lateral retinacular blood vessels feeding the femoral head.
The first osteotomy was defined along the lateral end of the necrotic area to create the mobile fragment [Fig.3, blue]. The reduction of the mobile fragment was simulated by applying 3D rotations and translations such that the sphericity, the articular step-off, and the neck pillar size are optimized. The intersectional volume of the mobile fragment in its reduced position and the stable part was then used to determine the orientation of the second osteotomy plane, which provides the definition for the 3D wedge that is to be resected. Iterative refinement of the orientation of the osteotomy plane was required in each case until the optimal strategy was determined [Fig. 4]. After each FHRO simulation, the congruency and fitting of the reshaped head into the acetabulum was assessed in order to reveal the necessity and extent of the additional PAO [Fig.5].
PSI design: PSI refers to a surgical navigation concept in which the cutting, drilling, and reduction instruments are computer-designed and matched with the preoperative simulation of the surgery. The undersurfaces of the instruments are shaped as the negatives of the bone anatomy such that the tools can later be placed exactly in the planned positions on the bone [Fig. 6]. PSI as navigation tools for corrective osteotomies were first introduced for the treatment of complex malunions of the forearm bones (23-25). We have adopted the PSI approach by designing new instruments tailored to the anatomy of the proximal femur and the FHRO. The main challenge was to design a PSI that can be placed on the proximal femur without compromising the vascular supply at the infero-medial curve of the femoral neck [Fig. 7]. The remaining footprint of the anterior bone surface on which the PSI can be placed is small and the surface relief of the bone is insufficiently pronounced to provide sufficient guide stability. For this reason, medial and lateral hooks were integrated into the base block of the PSI in order to improve its stability. An offset of 4 mm was integrated into the portion of the cartilaginous part of the head. Two drill sleeves of Ø 2.6 mm were designed to allow the temporary fixation of the PSI on the bone with surgical pins. The PSI also consisted of two cutting slits into which the blade of the surgical saw could be inserted and aligned according to the planned osteotomy planes. The PSI were manufactured as CE-conformed medical products by an industrial partner [Medacta International, Castel San Pietro, Switzerland] using biocompatible polyamide [P2200; EOS GmbH, Germany] and a selective laser sintering device [Formiga P395/ P396/ P100, EOS GmbH, Krailling, Germany]. Before surgery, autoclave sterilization was performed in the surgical centers.
Surgical technique: The patient was positioned in the lateral decubitus position. The pathologic hip was accessed via the surgical hip dislocation approach (26). The medial femoral circumflex artery was secured in the form of a pediculated periosteal flap (1, 7). For the dissection of the retinacular flap, the stable part of the trochanter was resected piecemeal down to the level of the neck and the periosteum was carefully dissected, allowing free access to the lateral and posterior neck bone. For the FHRO, the medial retinaculum was left attached to the calcar area (1).
The femoral neck was thereby accessible in its anterior, lateral, and posterior circumference and allowed the positioning of the PSI. Finding the correct position of the PSI is not straight forward and could only be achieved by comparison with a manufactured replica of the patient bone [Fig. 8-A]. After the fixation of the PSI using two surgical pins of Ø 2.5 mm, the sawing blade [thickness/width/length 1.00/25.00/90.00mm; Ref. Gomina 265.256.100] was introduced into each of the two cutting slits to perform the medial and lateral head osteotomies under continuous visual control. The level of the subsequent transverse osteotomy at the neck was determined freehand, allowing the necrotic central part and the pedicled lateral fragment to be liberated while the medial part of the head remained stable on the calcar bone [Fig. 8-B]. After resection of the necrotic part, the mobile fragment was reduced in a freehand fashion, but following the position obtained by the preoperative computer simulation. Under continuous control of the retinacular flap, the fragment could be moved in the cephalad or the caudad direction. It could be shifted posteriorly or anteriorly and could be rotated to finally obtain an optimal surface congruency. The reduced fragment was stabilized with two Ø 3.5 mm cortical screws. The articular step-offs between the contact areas of the fragments were smoothed out using a scalpel in order to restore a transition-free joint surface. The retinaculum and the capsular flap were loosely adapted before the trochanter was reattached.
Evaluation: The radiological outcome was measured by two independent readers (a fellowship trained radiologist and an orthopedic surgeon) on pre- and postoperative pelvic AP radiographs (19). For the evaluation of the head shape the ratio of the femur head diameter to the healthy contralateral side (10), the sphericity index [ratio of the minor and major axis of the ellipsoid femoral head] (19), and the Stulberg classification (10) were assessed. The Stulberg classification was measured to evaluate the chances of developing coxarthrosis in patients based on the severity of femoral head deformities, ranging from 1 (normal joint) to 5 [prognosis: severe early arthritis] (10). For the evaluation of hip containment, the extrusion index (ratio of head extrusion distance and containment) (27), the lateral center-edge angle (LCE), the Tönnis angle (28), and the Shenton line (29) were measured. Additionally, the centrum-collum-diaphyseal (CCD) angle was obtained to assess whether the surgery affected varus or valgus alignments and the preoperative Waldenströem classification (30) for the definition of the disease state. For effort, the evaluation time and costs associated with the new technique were recorded. Radiologic values were tested for normal distribution using the Shapiro-Wilk test and for statistical relevance (p ≤ 0.05) using Wilcoxon signed ranks test.