Carsten Rendenbach

Retinol deprivation partially rescues the skeletal mineralization defects of Phex-deficient Hyp mice.

X-linked hypophosphatemic rickets (XLH) is a genetic disorder caused by mutational inactivation of the PHEX gene, encoding a transmembrane endopeptidase expressed in osteoblasts. Since several experiments involving Phex-deficient Hyp mice have demonstrated that an increased expression of Fgf23 in osteoblasts is causative for the renal phosphate loss characteristic of XLH, we performed genome-wide expression analysis to compare differentiated osteoblasts from wildtype and Hyp mice. Here we did not only observe the expected increase of Fgf23 expression in the latter ones, but also a differential expression of genes that are either induced by or involved in retinoic acid signaling, which led us to analyze whether dietary retinol deprivation would influence the phenotype of Hyp mice. Unexpectedly, feeding a retinol-free diet resulted in a partial rescue of the growth plate and bone mineralization defects in 6 weeks old Hyp mice. When we fed the same diet for 24 weeks the amount of non-mineralized bone matrix (osteoid) was reduced by more than 70%, although phosphate homeostasis was unaffected. In contrast, a dietary normalization of serum phosphate levels in Hyp mice reduced the osteoid amount by less than 30%, thereby demonstrating a previously unknown impact of retinol on the cell-autonomous mineralization defect of Phex-deficient osteoblasts.

Long-term biomechanical analysis of donor site morbidity after radial forearm free flap.

BACKGROUND Although the radial forearm free flap (RFF) is a commonly used microvascular graft for head and neck reconstruction, long-term biomechanical results regarding donor site morbidity are rare. PATIENTS AND METHODS In a prospective panel study, 32 patients were included. Biomechanical assessment was performed preoperatively, three months postoperatively and two years postoperatively. The primary endpoint of the study was grip strength. In addition, the Mayo wrist score, DASH score (disabilities of the arm, shoulder and hand score), fine motor skill strengths (tip pinch, key pinch, palmar pinch) and the range of motion were analysed. Primary defects were closed with local full-thickness skin grafts (FTSG) from the donor site forearm avoiding a secondary defect site. RESULTS In the long-term analysis, grip strength was reduced in both arms. A significant improvement over time was found only for the donor arm. A persistent deficit of tip pinch strength and dorsal extension was recorded. Persistent sensory limitations occurred in four cases. Patient contentment after two years of follow-up was high and daily life routine was not restricted. CONCLUSION Gross and fine motor skill limitations are reversible short-term effects after RFF harvesting and do not restrict daily routine in the long term. These findings substantiate the value of the RFF as a workhorse in reconstructive surgery.

Long-Term Donor-Site Morbidity After Microsurgical Fibular Graft: Is There a Difference Between the Medial Approach and the Lateral Approach?

PURPOSE Microvascular fibula harvesting for osseous reconstruction is a valuable aid in maxillofacial surgery. We assessed whether the lateral and the medial harvesting techniques differed with respect to long-term donor-site morbidity. MATERIALS AND METHODS We conducted a retrospective cohort study of patients who had undergone free fibula harvesting at the University Medical Center Hamburg-Eppendorf, Hamburg, Germany, between 1987 and 2008. The primary predictor variable was the surgical approach. The primary and secondary outcome variables were the American Orthopaedic Foot & Ankle Society score and the result of the Short Form 36 Health Survey on quality of life, respectively. Other variables were age, gender, harvest length, operation time, and specific impairments and surgical complications. Statistical analysis was performed with SPSS, version 14.0 (SPSS, Chicago, IL); P < .05 was considered significant. RESULTS The 42 patients had a mean age of 55.5 years. The mean follow-up period was 81 months (range, 7-174 months). In the medial group, the mean American Orthopaedic Foot & Ankle Society score was 94.4 points and the total number of impairments was 34, as compared with 85.6 points and 46 impairments, respectively, in the lateral group. This tendency for less morbidity with the medial approach was only found in female patients. The Short Form 36 scores were not statistically different between the 2 surgical approaches. CONCLUSIONS The medial approach led to less functional impairment of the foot and ankle. Our results support the medial approach as a viable alternative, especially in women, but only in cases when a skin paddle is not necessary.

Prospective biomechanical evaluation of donor site morbidity after radial forearm free flap.

Although the radial forearm free flap (RFF) is a commonly-used microvascular flap for orofacial reconstruction, we are aware of few prospective biomechanical studies of the donor site. We have therefore evaluated the donor site morbidity biomechanically of 30 consecutive RFF for orofacial reconstruction preoperatively and three months postoperatively. This included the Mayo wrist score, the Disabilities of the Arm, Shoulder and Hand (DASH) score, grip strength, followed by tip pinch, key pinch, palmar pinch, and range of movement of the wrist. Primary defects were all closed with local full-thickness skin grafts from the donor site forearm, thereby circumventing the need for a second defect. Postoperative functional results showed that there was a reduction in hand strength measured by (grip strength: -24.1%, in tip pinch: -23.3%, in key pinch: -16.5, and in palmar pinch: -19.3%); and wrist movement measured by extension (active=14.3% / passive= -11.5%) and flexion = -14.8% / -8.9%), and radial (-9.8% / -9.8%) and ulnar (-11.0% / -9.3%) abduction. The Mayo wrist score was reduced by 9.4 points (-12.9%) and the DASH score increased by 16.1 points (+35.5%) compared with the same forearm preoperatively. The local skin graft resulted in a robust wound cover with a good functional result. Our results show that the reduction in hand strength and wrist movement after harvest of a RFF is objectively evaluable, and did not reflect the subjectively noticed extent and restrictions in activities of daily living. Use of a local skin graft avoids a second donor site and the disadvantages of a split-thickness skin graft.

Immediate effects of retinoic acid on gene expression in primary murine osteoblasts

Consistent with clinical observations demonstrating that hypervitaminosis A is associated with increased skeletal fracture risk, we have previously found that dietary retinol deprivation partially corrects the bone mineralization defects in a mouse model of X-linked hypophosphatemic rickets. That retinol-dependent signaling pathways impact the skeleton is further supported by various findings demonstrating a negative influence of retinoic acid (RA) on bone-forming osteoblasts. We hypothesized that RA would directly regulate the expression of specific target genes in osteoblasts, and we aimed to identify these by genome-wide expression analyses. Here we show that high dietary retinol intake in mice causes low bone mass associated with increased osteoclastogenesis and decreased osteoblastogenesis, but intact bone matrix mineralization. We additionally found that short-term treatment of primary osteoblasts with RA causes a rapid induction of specific genes involved in either retinol-dependent signaling (i.e. Rara, Crabp2) or skeletal remodeling (i.e. Twist2, Tnfsf11). In contrast, neither expression of established osteoblast differentiation markers nor the proliferation rate was immediately affected by RA administration. Collectively, our data suggest that the negative effects of vitamin A on skeletal integrity are explainable by an immediate influence of RA signaling on specific genes in osteoblasts that in turn influence bone remodeling.

Increased Col10a1 expression is not causative for the phenotype of Phex-deficient Hyp mice

X-linked hypophosphatemic rickets (XLHR) is a severe disorder of phosphate homeostasis and skeletal mineralization caused by mutations of PHEX, encoding a bone-specific endopeptidase. Phex-deficient Hyp mice have been extensively studied to understand the molecular bases of XLHR, and here it was found that Fgf23, encoding a major phosphaturic hormone, was transcriptionally activated in bone-forming osteoblasts. We and others could additionally show that Col10a1 expression is increased in Hyp osteoblasts and bones, thereby raising the possibility that ectopic production of type X collagen could contribute to the impaired mineralization of the Hyp bone matrix. Here we show that an additional deficiency of the Col10a1 gene does not overtly affect the skeletal phenotype of Hyp mice. More specifically, Col10a1-deficient Hyp mice displayed severe disturbances of skeletal growth, bone mass acquisition and bone matrix mineralization, and they were essentially indistinguishable from Hyp littermates. This was confirmed by non-decalcified histology and bone-specific histomorphometry quantifying all relevant parameters of growth plate maturation, trabecular bone architecture and osteoid accumulation. Taken together, our results show that increased Col10a1 expression in Phex-deficient osteoblasts is not a major cause of the XLHR phenotype, which was an important issue to address based on the previous findings.