In this pilot study 7 patients between nine and eighteen years of age were treated by FHRO with the new technique between May 2017 and October 2019. Aim of this study was to evaluate feasibility, safety and reconstruction outcome. All except one patient, the parents or the legal guardian provided informed consent to this study, who was excluded. Three of the patients were male and three were female. Ethical approval was obtained by the ethical committee of the Canton Zurich. Inclusion criteria were pain and restricted hip motion, severe deformity of the femoral head and an intact peripheral cartilage with a central necrosis. Preoperative computer simulation of the surgery was performed for each patient and patient-specific instruments (PSI) to precisely execute the bony cuts were designed and manufactured. Surgeries were performed in different centers by four senior hip surgeons who had experience in FHRO surgeries.
Preoperative computer simulation
CT scans of the patients᾽ hips were obtained in supine position and anterior-posterior hip orientation according to a specifically designed protocol (MyOsteotomy CT protocols, Medacta International, Castel San Pietro, Switzerland). The CTs 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 were imported in 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 femur and pelvis were generated from the segmented images using the Marching Cube algorithm.(22) The 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 remodelling templates in the simulation. In case of a pathological contralateral side, a geometric sphere was used as a remodeling template. 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], because the 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 medial neck pilar [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]. Afterwards, the reduction of the mobile fragment was simulated by applying 3D rotations and translations such that sphericity, articular step off and medial neck pilar size are optimized. The intersecting volume between the mobile fragment in its reduced position and the stable part was then used to calculate the second osteotomy plane which completes the 3D wedge to be resected. Fine-tuning through iterative refinement of osteotomy planes and reduction was required in each case until the optimal strategy was determined [Fig. 4]. After each FHRO simulation, the congruency and fit of the reshaped head into the acetabulum was assessed in order to reveal necessity and extent of the additional PAO [Fig. 5].
PSI are a surgical navigation concept in which cutting, drilling, and reduction instruments are computer-designed and matched to the preoperative simulation of the surgery. The undersurfaces of the instruments are shaped as negatives of the bone anatomy such that the tools can be later placed exactly at the planned position 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 surgery. A 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 femur 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 in the base block of the PSI to improve its stability.For the portion covering the cartilageneous cover of the head an offset of 4 mm was integrated. Two drill sleeves with Ø2.6 mm were designed to allow 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 will be inserted and precisely aligned according to the planned osteotomy planes. The PSI were additively manufactured as CE-conform 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 application in the surgery, autoclave sterilization was performed in the surgical centers.
The patient was positioned in lateral decubitus position. The pathologic hip was accessed via the surgical hip dislocation approach(26). The fascia lata was split anterior to the gluteus maximus muscle, until the greater trochanter could be visualized and the head could be accessed. The trochanter was then osteotomized and flipped anteriorly together with the attached gluteus medius and minimus muscles on one side and vastus lateralison muscle on the other side. The joint capsule was presented through the gap between piriformis and gluteus minimus and incised in a z-shaped fashion, whereby special attention was paid to protect the retinacular vessels. (27) After sectioning of the ligamentum teres, the hip could be dislocated with traction, adduction and external rotation. (26) The medial femoral circumflex artery was secured in form of a pediculated periosteal flap. (1) For dissection of the retinacular flap, the stable part of the trochanter was piecemeal resected 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 positioning of the PSI. Finding the correct position of the PSI is not straight forward in general and could be only achieved by comparison with a manufactured replica of the patient bone [Fig.8-A]. After fixation of the PSI with 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)s 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 free hand determined, 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 a cephalad or 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 Ø3.5 mm cortical screws. Articular step offs between the contact area of the fragments were smoothed out using a scalpel to restore a transition-free joint surface. Retinaculum and capsular flap were loosely adapted before the trochanter was reattached at an advanced position.
The radiological outcome was measured by two independant readers on pre- and postoperative pelvic AP radiographs.(19) For the evaluation of the head shape the femur head diameter in ratio 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. For the evaluation of the hip containment, the extrusion index (ratio of head extrusion distance and containment)(28), the LCEA (AP), the Tönnis angle(29) and the Shenton line(30) were measured. Additionally, the CCD-angle was obtained to track if the surgery affects varus or valgus alignment and the preoperative Waldenstroem classification(31) for disease state definition. For effort 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 using Wilcoxon signed ranks test.