Quantitative analysis of lower limb and pelvic deformities in children with X-linked hypophosphatemic rickets

Lower limb maltorsion and acetabular deformity in children and adolescents with X-linked hypophosphatemia

Background

X-linked hypophosphatemia (XLH) is a rare monogenetic skeletal disorder. Lower limb deformities contribute substantially to impaired gait quality and burden of disease in patients with XLH. Standardized data regarding onset and severity of lower limb maltorsion are unavailable. This study aimed to evaluate lower limb maltorsion using rotational magnetic resonance imaging (MRI) and computed tomography (CT).

Methods

Rotational MRI and CT of children and adolescents with verified XLH were evaluated retrospectively. Femoral and tibial torsion, acetabular anteversion, and axial acetabular coverage were measured and compared with published age-adapted radiographic, clinical measurements and MRI data, respectively.

Results

Fifteen patients (mean age, 10.7 years) were included in the study. Decreased femoral torsion was observed in 47% (14/30 femora) and femoral retrotorsion in 17% (5/30 femora). Ten of 13 hips with coxa vara deformity presented with decreased femoral antetorsion. Reduced external tibial torsion manifested in 64% (18/28 tibiae). Abnormal axial femoral head coverage was present in 67% (20/30 hips), whereas 53% (16/30 hips) showed increased acetabular anteversion.

Conclusion

Femoral and tibial torsional pathologies were found in children and adolescents with XLH. The occurrence of severe femoral retrotorsion in a 2-year-old child prior to ambulation raises questions regarding the biomechanical impact of gait on the development of torsional deformities in XLH.

X-linked hypophosphataemia

X-linked hypophosphataemia is a genetic disease caused by defects in the phosphate regulating endopeptidase homolog X-linked (PHEX) gene and is characterised by X-linked dominant inheritance. The main consequence of PHEX deficiency is increased production of the phosphaturic hormone fibroblast growth factor 23 (FGF23) in osteoblasts and osteocytes. Chronic exposure to circulating FGF23 is responsible for renal phosphate wasting and decreased synthesis of calcitriol, which decreases intestinal phosphate absorption. These mechanisms result in lifelong hypophosphataemia, impaired growth plate and bone matrix mineralisation, and diverse manifestations in affected children and adults, including some debilitating morbidities and possibly increased mortality. Important progress has been made in disease knowledge and management over the past decade; in particular, targeting FGF23 is a therapeutic approach that has substantially improved outcomes. However, patients affected by this complex disease need lifelong care and innovative treatment strategies, such as gene repair of PHEX, are necessary to further limit the disease burden.

Impact of burosumab on lower limb alignment in children with X-linked hypophosphatemia

Background

Osteotomy and hemiepiphysiodesis are used to treat lower limb deformities in the rare musculoskeletal disease X-linked hypophosphatemia (XLH), but postsurgical complications and malalignment recurrence are possible. This retrospective analysis assessed whether treatment with burosumab, a fully human IgG1 monoclonal antibody to fibroblast growth factor 23 approved for treatment of rickets in XLH, improves lower limb malalignment toward age-specific normal values in children with XLH.

Methods

Children with confirmed XLH received burosumab for 160 weeks in the open-label phase 2 study CL205, or conventional therapy (Pi/D) or burosumab for 64 weeks in the randomized, open-label phase 3 study CL301, with crossover from Pi/D to burosumab through 88 weeks. Full-length, anteroposterior lower limb radiographs were reviewed. The mechanical femoral-tibial angle (MFTA) of lower limbs was measured at baseline and postbaseline. Each MFTA was classified as normal (within 1 standard deviation [SD] of age-specific normal range) or clinically normal (within 2 degrees of normal).

Results

Overall, 116 limbs were included (CL205, n = 26; CL301, n = 90). Varus or valgus limbs were observed at baseline in 21 (80.8%) limbs in CL205 and in 69 (76.7%) limbs in CL301. In CL205, mean (SD) MFTA decreased from 13.0° (6.7°) at baseline to 5.7° (6.0°) at week 64 and 1.0° (4.8°) at week 160. In CL301, mean (SD) MFTA decreased from 15.5° (13.6°) at baseline to 8.5° (10.0°) at week 64 in the burosumab arm, and in the crossover arm, from 9.2° (10.4°) at week 64 to 6.9° (9.4°) at week 88 (after 22 weeks of burosumab). The proportion of normal or clinically normal limbs increased with burosumab in CL205 (baseline to week 160, 19.2% to 58.3%) and in the CL301 burosumab arm (baseline to week 64, 19.6% to 37.0%) but not in the CL301 crossover arm (week 64-88, 34.1% to 33.3%).

Conclusions

In children with XLH, long-term treatment with burosumab is capable of correcting the MFTA of varus and valgus lower limbs to a neutral alignment without requiring surgical intervention.

Key Concepts

1.Treatment with burosumab led to the correction of lower limb angular deformity to neutral alignment in children with X-linked hypophosphatemia (XLH).2.Continued treatment with burosumab for at least 1 year appears to have further positive effects on the correction of lower limb angular deformity in children with XLH.3.Initial treatment with burosumab is indicated in young children with XLH for whom hemiepiphysiodesis is being considered.

Level of Evidence

III.

Temporary hemiepiphysiodesis using eight-plates for angular deformities of the lower extremities in children with X-linked hypophosphataemic rickets

PURPOSES

Temporary hemiepiphysiodesis (TH) using eight-plates is one of the most frequently performed surgeries for correcting angular deformities of the lower extremities in adolescents. Rarely have studies examined children with X-linked hypophosphataemic rickets (X-LHPR) treated with TH using eight-plates. This study was conducted to investigate the efficacy, the endpoint, and the complications of TH using eight-plates to correct angular deformities of the lower extremities in skeletally immature children.

METHODS

We reviewed a total of 26 children (86 physes, 52 knees) with X-LHPR (mean age of 6.2 years, range from 2 to 13 years) who underwent TH using eight-plate to correct angular deformities of the lower extremities. Radiographs and clinical records of these patients were evaluated for demographic data and related clinical factors.

RESULTS

The average correction of the mechanical lateral distal femoral angle (mLDFA) was 11.7 ± 8.7° (range from 1.0 to 29.7°), and the average correction of the mechanical medial proximal tibial angle (mMPTA) was 8.4 ± 5.0° (range from 0.3 to 16.7°). The mean deformity correction time was 22.7 months (range from 7 to 60 months), and the mean follow-up after eight-plate removal was 43.9 months (range from 24 to 101 months). Overall, 76.9% (20/26 patients) of the angular deformities of the knee were completely corrected and 15.4% (4/26) of the patients received osteotomy surgery. The femoral correction velocity (0.9° per month) was significantly higher than the proximal tibial (0.6° per month) (p = 0.02). The correction velocity of the mLDFA and mMPTA with the TH procedure was faster than that in the absence of intervention (0.9° vs. 0.2°, 0.7° vs. 0.4° per month, p < 0.05). The correction velocity of the mLDFA (1.2° vs. 0.5° per month, [Formula: see text]) and mMPTA (0.7° vs. 0.5° per month, p = 0.04) of patients whose age ≤ five years old was faster than that of patients whose age > five years old. A total of 69.2% (18/26) patients experienced one TH procedure using eight-plates only. Two patients had screw loosening (2/26, 7.7%). One patient (1/26, 3.8%) had a rebound phenomenon after the removal of eight-plate and had the TH procedure again. There was no breakage, infection, physis preclosure, or limited range of movement found in the follow-up.

CONCLUSION

TH using eight-plates is a safe and effective procedure with a relatively low incidence of complication and rebound, and it could be used as part of a streamlined treatment for younger X-LHPR patients with resistant or progressive lower limb deformity despite optimal medical treatment. Early intervention can achieve better results.

Changes in adipose bone marrow and bone morphology in X-linked hypophosphatemic rickets

INTRODUCTION

X-linked hypophosphatemic (XLH) rickets causes significant bone deformities in the lower limbs resulting from a bone mineralization defect. According to Frost's Mechanostat theory, compensatory modeling of the bones takes place during increased mechanical loads. In addition, mechanical stimuli modulate the differentiation of mesenchymal stem cells; common precursors to bone marrow adipocytes and osteoblasts.

HYPOTHESIS

Bone deformities of the lower limbs lead to increased femoral bone mass and decreased fatty infiltration of the bone marrow (FIBM) in children with XLH rickets compared to a control group.

PATIENTS AND METHODS

Eleven children (10.3years [6-17]) with XLH rickets and 22 healthy children (10.2years [5-15.5]) underwent lower limb Magnetic Resonance Imaging. A calculation of FIBM was performed at the mid-femur, as well as a calculation of the total bone cross-sectional area (CSA), the cortical CSA, the anteroposterior and mediolateral axes of the femur, bone marrow and the thickness of the femoral cortices.

RESULTS

Total bone CSA, total cortical CSA and bone marrow CSA were higher in the XLH group than in the control group (p<0.05). The mid-lateral diameters of the femur and bone marrow were more elongated than those of the control group (p<0.001). Only the anterior cortex was thinned in the XLH group (p=0.001), while there was no difference with the control group for the posterior, medial and lateral cortices. The total percentage of FIBM was 72.81% [±3.95] and 77.4% [±5.52] for the XLH and control groups respectively (p<0.001).

DISCUSSION

The increase in bone mass in the XLH population reflects an adaptation of bone tissue to the bone deformities present in this pathology. The decrease in FIBM indicates a lower risk of osteoporosis in the XLH population and may constitute a new monitoring parameter in this pathology.

LEVEL OF STUDY

III; Case-control study.

Growth pattern in children with X-linked hypophosphatemia treated with burosumab and growth hormone

BACKGROUND

X-linked hypophosphatemia (XLH) is characterized by increased serum concentrations of fibroblast growth factor 23 (FGF23), hypophosphatemia and insufficient endogenous synthesis of calcitriol. Beside rickets, odonto- and osteomalacia, disproportionate short stature is seen in most affected individuals. Vitamin D analogs and phosphate supplements, i.e., conventional therapy, can improve growth especially when started early in life. Recombinant human growth hormone (rhGH) therapy in XLH children with short stature has positive effects, although few reports are available. Newly available treatment (burosumab) targeting increased FGF23 signaling leads to minimal improvement of growth in XLH children. So far, we lack data on the growth of XLH children treated with concomitant rhGH and burosumab therapies.

RESULTS

Thirty-six patients received burosumab for at least 1 year after switching from conventional therapy. Of these, 23 received burosumab alone, while the others continued rhGH therapy after switching to burosumab. Children treated with burosumab alone showed a minimal change in height SDS after 1 year (mean ± SD 0.0 ± 0.3 prepubertal vs. 0.1 ± 0.3 pubertal participants). In contrast, rhGH clearly improved height during the first year of treatment before initiating burosumab (mean ± SD of height gain 1.0 ± 0.4); patients continued to gain height during the year of combined burosumab and rhGH therapies (mean ± SD height gain 0.2 ± 0.1). As expected, phosphate serum levels normalized upon burosumab therapy. No change in serum calcium levels, urinary calcium excretion, or 25-OHD levels was seen, though 1,25-(OH)₂D increased dramatically under burosumab therapy.

CONCLUSION

To our knowledge, this is the first study on growth under concomitant rhGH and burosumab treatments. We did not observe any safety issue in this cohort of patients which is one of the largest in Europe. Our data suggest that continuing treatment with rhGH after switching from conventional therapy to burosumab, if the height prognosis is compromised, might be beneficial for the final height.