Corrigendum: Consensus Recommendations for the Diagnosis and Management of X-Linked Hypophosphatemia in Belgium.

[This corrects the article DOI: 10.3389/fendo.2021.641543.].

Consensus Recommendations for the Diagnosis and Management of X-Linked Hypophosphatemia in Belgium.

X-linked hypophosphatemia (XLH) is the most common genetic form of hypophosphatemic rickets and osteomalacia. In this disease, mutations in the PHEX gene lead to elevated levels of the hormone fibroblast growth factor 23 (FGF23), resulting in renal phosphate wasting and impaired skeletal and dental mineralization. Recently, international guidelines for the diagnosis and treatment of this condition have been published. However, more specific recommendations are needed to provide guidance at the national level, considering resource availability and health economic aspects. A national multidisciplinary group of Belgian experts convened to discuss translation of international best available evidence into locally feasible consensus recommendations. Patients with XLH may present to a wide array of primary, secondary and tertiary care physicians, among whom awareness of the disease should be raised. XLH has a very broad differential-diagnosis for which clinical features, biochemical and genetic testing in centers of expertise are recommended. Optimal care requires a multidisciplinary approach, guided by an expert in metabolic bone diseases and involving (according to the individual patient's needs) pediatric and adult medical specialties and paramedical caregivers, including but not limited to general practitioners, dentists, radiologists and orthopedic surgeons. In children with severe or refractory symptoms, FGF23 inhibition using burosumab may provide superior outcomes compared to conventional medical therapy with phosphate supplements and active vitamin D analogues. Burosumab has also demonstrated promising results in adults on certain clinical outcomes such as pseudofractures. In summary, this work outlines recommendations for clinicians and policymakers, with a vision for improving the diagnostic and therapeutic landscape for XLH patients in Belgium.

A cell-based combination product for the repair of large bone defects.

Regenerative cell-based implants using periosteum-derived stem cells were developed for the treatment of large 3 cm fresh and 4.5 centimeter biological compromised bone gaps in a tibial sheep model and compared with an acellular ceramic-collagen void filler. It was hypothesized that the latter is insufficient to heal large skeletal defects due to reduced endogenous biological potency. To this purpose a comparison was made between the ceramic dicalciumphosphate scaffold (CopiOs®) as such, the same ceramic coated with clinical grade Bone Morphogenetic Protein 2 and 6 (BMP) only or a BMP coated cell-seeded combination product. These implants were evaluated in 2 sheep models, a fresh 3 cm critical size tibial defect and a 4.5 cm biologically exhausted tibial defect. For the groups in which growth factors were applied, BMP-6 was chosen at a dose of 344 µg for 3 cm and 1.500 µg or 3.800 µg for 4.5 cm defects. An additional group in the 4.5 cm defect was tested using BMP-2 in a dose of 1.500 µg. For all the cell based implants autologous periosteum-derived cells were used which were cultured in monolayer during 6 weeks. For the fresh defect 408 million cells and for the biologically exhausted tibial defect 612 million cells were drop-seeded on the BMP coated scaffolds. Bone healing was studied during 16 weeks postimplantation, using standard radiographs. While fresh defects responded to all treatments, regardless the use of cells, the biologically hampered defects responded in half of the cases and only if the BMP-cell combination product was used, supporting the concept that cell-based therapies may become attractive in treating defects with a compromised biological status.

The challenge of the infected pilon tibial non-union: treatment with radical resection, bone transport and ankle arthrodesis.

A significant number of patients with pilon tibial fractures develop complications, the most devastating being a combination of infection and non-union with bone loss. The results of the Ilizarov bone transport technique were retrospectively evaluated in ten patients. All underwent an extensive resection and reconstruction aiming at an ankle arthrodesis. The outcome was registered by clinical and radiographic examination as proposed by Paley's functional and bone results classification. A good healing at the level of the docking site could be obtained in all patients but with a re- intervention in 8 of the 10. In 5 of these patients, re-intervention with a transcalcaneal nailing leaded to the final healing. Other options are debridement of the docking site (2 patients) and a new Ilizarov procedure (1 patient). If patients are prepared to participate in a long-term treatment with the risk of multiple interventions a reconstruction can be performed, resulting in a limb with an acceptable function, allowing all activities of daily life and even a professional occupation. To obtain this final result with a definite union at the docking site a secondary retrograde intramedullary nailing is considered a valuable and safe procedure.

Neurofibromatosis type 1-related pseudarthrosis: Beyond the pseudarthrosis site.

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder affecting approximately 1 in 2,000 newborns. Up to 5% of NF1 patients suffer from pseudarthrosis of a long bone (NF1-PA). Current treatments are often unsatisfactory, potentially leading to amputation. To gain more insight into the pathogenesis we cultured cells from PA tissue and normal-appearing periosteum of the affected bone for NF1 mutation analysis. PA cells were available from 13 individuals with NF1. Biallelic NF1 inactivation was identified in all investigated PA cells obtained during the first surgery. Three of five cases sampled during a later intervention showed biallelic NF1 inactivation. Also, in three individuals, we examined periosteum-derived cells from normal-appearing periosteum proximal and distal to the PA. We identified the same biallelic NF1 inactivation in the periosteal cells outside the PA region. These results indicate that NF1 inactivation is required but not sufficient for the development of NF1-PA. We observed that late-onset NF1-PA occurs and is not always preceded by congenital bowing. Furthermore, the failure to identify biallelic inactivation in two of five later interventions and one reintervention with a known somatic mutation indicates that NF1-PA can persist after the removal of most NF1 negative cells.

Warning About the Use of Critical-Size Defects for the Translational Study of Bone Repair: Analysis of a Sheep Tibial Model.

The repair of large long bone defects requires complex surgical procedures as the bone loss cannot simply be replaced by autologous grafts due to an insufficient bone stock of the human body. Tissue engineering strategies and the use of Advanced Therapy Medicinal Products (ATMPs) for these reconstructions remain a considerable challenge, in particular since robust outcomes in well-defined large animal models are lacking. To be suitable as a model for treatment of human sized bone defects, we developed a large animal model in both skeletally immature and mature sheep and made close observations on the spontaneous healing of defects. We warn for the spontaneous repair of large defects in immature animals, which can mask the (in)effectiveness of ATMP therapies, and propose the use of large 4.5 cm defects that are pretreated with a polymethylmethacrylate (PMMA) spacer in skeletally mature animals.

Prevalence of neurofibromatosis type 1 in congenital pseudarthrosis of the tibia.

UNLABELLED: A strong relationship between congenital pseudarthrosis of the tibia (CPT) and neurofibromatosis type 1 (NF1) has been suggested, but prevalence varies widely throughout the literature and the criteria used for diagnosis are very heterogeneous. Literature focus is mainly on treatment and no specific review on the prevalence of NF1 has been published. Based on our own observations, we hypothesized the prevalence of NF1 in patients with CPT to be higher than what is previously accepted. We conducted a comprehensive literature review on this topic and compared results with our study population. Twenty-one out of twenty-five patients in the study population matched the NIH diagnostic criteria for NF1 (84.0 %, CI95 = 69.6-98.4 %). These results are higher than the prevalence reported in the literature (55.4 %, CI95 = 50.4-60.4 %). CONCLUSIONS: The prevalence of NF1 in patients with CPT might be higher than what is reported until now because the criteria of NF1 generally appear only after the diagnosis of CPT. We propose a repeat meticulous examination and a multidisciplinary approach with a clinical genetic counseling in all CPT patients. WHAT IS KNOWN: • Congenital pseudarthrosis of the tibia and neurofibromatosis type 1 are closely related. • Literature focus is mainly on treatment and little epidemiologic research is available. What is New: • Prevalence of neurofibromatosis type 1 in patient with congenital pseudarthrosis of the tibia might be higher than what is reported until now. • A multidisciplinary approach with meticulous clinical examination and genetic counseling might lead to an earlier diagnosis of neurofibromatosis type 1 in patients with congenital pseudarthrosis of the tibia.

Extraction of the Wichita Fusion Nail after Knee Arthrodesis.

To avoid a new exposition and partial damage of a knee arthrodesis site due to the removal of the Wichita fusion nail (WFN), a new extraction technique was developed, using a femoral osteotomy at the proximal end of the nail. Fixing the osteotomy with an Ilizarov frame offered the possibility to perform an additional correction of length and/or alignment if necessary.

Computational modeling of bone fracture non-unions: four clinically relevant case studies.

The human skeleton has a remarkable regeneration capacity. Nevertheless, 5-10 % of the bone fractures fails to heal and develops into a non-union which is a challenging orthopedic complication requiring complex and expensive treatment. This review paper will discuss four different computational models, each capturing a particular clinical case of non-union: non-union induced by reaming of the marrow canal and periosteal stripping, non-union due to a large interfragmentary gap, non-union due to a genetic disorder [i.e. NF1 related congenital pseudoarthrosis of the tibia (CPT)] and non-union due to mechanical overload. Together, the four computational models are able to capture the etiology of a wide range of fracture non-union types and design novel treatment strategies thereof. Further research is required to corroborate the computational models in both animal and human settings and translate them from bench to bed side.

Ilizarov bone transport as a treatment of congenital pseudarthrosis of the tibia: a long-term follow-up study.

PURPOSE: Most studies on congenital pseudarthrosis of the tibia (CPT) report on the short-term union rate and refracture rate but do not take into account the long-term outcome. This review includes patients treated with an Ilizarov bone transport, who all reached skeletal maturity. It describes long-term results and highlights any prognostic factors that could predict the final outcome. METHODS: The records of patients with CPT treated with an Ilizarov bone transport in our institution were retrospectively evaluated. RESULTS: A total of 12 consecutive patients were studied. The mean follow-up was 24.5 years (range 6-39 years). Primary consolidation was seen in ten patients (83 %). Half of these patients had a refracture. At final follow-up, eight patients experienced union and four remained un-united, of whom one had an amputation. CONCLUSIONS: The present data confirm a good primary healing rate. However, tibial union at final follow-up was only seen in 67 %, indicating that refracture is the main issue. United bone is often of inferior biological and mechanical quality, so lifetime protection with intramedullary devices, braces or a combination of both is recommended.

Bringing computational models of bone regeneration to the clinic.

Although the field of bone regeneration has experienced great advancements in the last decades, integrating all the relevant, patient-specific information into a personalized diagnosis and optimal treatment remains a challenging task due to the large number of variables that affect bone regeneration. Computational models have the potential to cope with this complexity and to improve the fundamental understanding of the bone regeneration processes as well as to predict and optimize the patient-specific treatment strategies. However, the current use of computational models in daily orthopedic practice is very limited or inexistent. We have identified three key hurdles that limit the translation of computational models of bone regeneration from bench to bed side. First, there exists a clear mismatch between the scope of the existing and the clinically required models. Second, most computational models are confronted with limited quantitative information of insufficient quality thereby hampering the determination of patient-specific parameter values. Third, current computational models are only corroborated with animal models, whereas a thorough (retrospective and prospective) assessment of the computational model will be crucial to convince the health care providers of the capabilities thereof. These challenges must be addressed so that computational models of bone regeneration can reach their true potential, resulting in the advancement of individualized care and reduction of the associated health care costs.

Validation of a finite element model of a unilateral external fixator in a rabbit tibia defect model.

In case of large segmental defects in load-bearing bones, an external fixator is used to provide mechanical stability to the defect site. The overall stiffness of the bone-fixator system is determined not only by the fixator design but also by the way the fixator is mounted to the bone. This stiffness is an important factor as it will influence the biomechanical environment to which tissue engineering scaffolds and regenerating tissues are exposed. A finite element (FE) model can be used to predict the system stiffness. The goal of this study is to develop and validate a 3D anatomical FE model of a bone-fixator system which includes a previously developed unilateral external fixator for a large segmental defect model in the rabbit tibia. It was hypothesized that the contact interfaces between bone and fixator screws play a major role for the prediction of the stiffness. In vitro mechanical testing was performed in order to measure the axial stiffness of cortical bone from mid-shaft rabbit tibiae and of the tibia-fixator system, as well as the bending stiffness of individual fixator screws, inserted in bone. µCT-based case-specific FE models of cortical bone and SCREW-BONE specimens were created to simulate the corresponding mechanical test set-ups. The Young's modulus of rabbit cortical bone as well as appropriate screw-bone contact settings were derived from those FE models. We then used the derived settings in an FE model of the tibia-fixator system. The difference between the FE predicted and measured axial stiffness of the tibia-fixator system was reduced from 117.93% to 7.85% by applying appropriate screw-bone contact settings. In conclusion, this study shows the importance of screw-bone contact settings for an accurate fixator stiffness prediction. The validated FE model can further be used as a tool for virtual mechanical testing in the design phase of new tissue engineering scaffolds and/or novel patient-specific external fixation devices.

Humeral lengthening by distraction osteogenesis: a safe procedure?

This study was conducted to assess the safety of humeral lengthening using an Ilizarov frame. We retrospectively reviewed 26 humeral segments in 17 patients that were lengthened at our department between 1993 and 2011. There were varying aetiologies including achondroplasia, epiphyseal dysplasia, Oilier disease, trauma or infection of the proximal humeral growth-plate, unicameral bone cyst and brachial plexus injury. Mean age at start of surgery was 17.05 years (range : 5-40). The mean lengthening achieved was 8.85 (3-13) cm. Mean lengthening percentage was 353% (range : 10-48). Average healing index was 30.56 days/cm (range : 17.46-4232). There was a significant difference in healing index between achondroplasia patients (28.79 days/cm) compared to others (33.41 days/cm). Minor problems included pin tract infection (14 segments). More important obstacles were temporary elbow flexion contracture (7 segments), premature consolidation (6 segments), radial nerve dysaesthesia (6 segments) and loosening of a Schanz screw (1 segment). Complications included one fracture and one progressive bowing after frame removal. One planned lengthening was not completely achieved. Despite a lot of obstacles, humeral lengthening using an Ilizarov frame provided a reliable method to treat the functional or cosmetic problems of upper limb shortening.

The Pentaconcept in skeletal tissue engineering. A combined approach for the repair of bone defects.

Tissue engineering has become a hot topic in modern medicine. Its application in a surgical setting, such as for the treatment of skeletal defects, still has to tackle some problems that might look simple at first sight, but need a well-structured handling combining surgery and science, with in a central position the patient, who is both cell donor and receptor of the tissue engineered end product. To achieve this goal in a clinical setting, a five steps pathway is described and designated as the Pentaconcept, integrating all ingredients for successful reconstructive procedures.

Reduction of treatment time in external ring fixation using the monofix device.

One of the disadvantages of the Ilizarov method is the long treatment time needed for extensive lengthening or complex deformity correction. To minimize the discomfort of the circular frame the authors have introduced a 'frame reduction' technique in which the ring frame is converted towards a unilateral fixator, the 'Monofix'. Both its efficacy and patient satisfaction encouraged the authors to apply this method on a routine basis since 1998 for lower and upper limb corrections. The technique is demonstrated, its indications and potential problems are discussed.

Ilizarov treatment for aseptic delayed union or non-union after reamed intramedullary nailing of the femur.

The authors reviewed eight patients treated with an Ilizarov frame for a non-infected delayed union or non-union after reamed intramedullary nailing of the femoral diaphysis. The hardware was completely removed in all cases but two, in which the distal fragment of a broken nail was left in situ. In five cases without shortening, progressive compression was applied. In the other three patients a simultaneous lengthening was performed using an additional percutaneous osteotomy. All patients achieved a good consolidation, with an average time to healing of 32 weeks and restoration of length and alignment if necessary. Bone grafting was never required, illustrating the sufficient biological potential for repair in non-infected femoral non-union.

Ilizarov treatment for extreme bilateral genu recurvatum in a pseudoachondroplasia patient: a case report.

The history of a 17-year-old female patient with pseudoachondroplasia, with an extreme bilateral genu recurvatum due to tibial growth disturbance, is presented. She was treated by the Ilizarov method over a nine- month period for consecutive bilateral correction of the lower legs, leading to an excellent functional result. The authors advocate the Ilizarov method as the treatment of choice for severe juxtaarticular osseous deformities, where the technical limits of a standard osteotomy with internal fixation are being exceeded.

Bone morphogenetic protein signaling in the murine distraction osteogenesis model.

A murine distraction osteogenesis model was standardized to allow analysis of the molecular pathways associated with postnatal de novo bone formation. The authors examined the presence and expression of Bone Morphogenetic Proteins (BMPs) -2, -3, -4, -6 and -7, and the BMP receptors Alk3 and Alk6 at different stages. Strong signals were detected for BMP-4 at the end of the distraction period and for BMP-6 during the entire experimental period. Signals for BMP-7 (Osteogenic Protein-1) were very low, suggesting a less important role during the normal process of distraction bone healing. Immunohistochemical staining revealed the presence of BMP-4 in the early chondroblasts, while BMP-6 was detected in the more mature cartilage cells. The data indicate a BMP molecular profile reminiscent of the embryonic maturation process in endochondral bone formation.