Cellular reactions to biodegradable magnesium alloys on human growth plate chondrocytes and osteoblasts.

PURPOSE: In recent decades operative fracture treatment using elastic stable intramedullary nails (ESINs) has mainly taken precedence over conservative alternatives in children. The development of biodegradable materials that could be used for ESINs would be a further step towards treatment improvement. Due to its mechanical and elastic properties, magnesium seems to be an ideal material for biodegradable implant application. The aim of this study was therefore to investigate the cellular reaction to biodegradable magnesium implants in vitro. METHODS: Primary human growth plate chondrocytes and MG63 osteoblasts were used for this study. Viability and metabolic activity in response to the eluate of a rapidly and a slower degrading magnesium alloy were investigated. Furthermore, changes in gene expression were assessed and live cell imaging was performed. RESULTS: A superior performance of the slower degrading WZ21 alloy's eluate was detected regarding cell viability and metabolic activity, cell proliferation and morphology. However, the ZX50 alloy's eluate induced a favourable up-regulation of osteogenic markers in MG63 osteoblasts. CONCLUSIONS: This study showed that magnesium alloys for use in biodegradable implant application are well tolerated in both osteoblasts and growth plate chondrocytes respectively.

Development of a Dual Reporter System to Simultaneously Visualize Ca2+ Signals and AMPK Activity.

In this study, we introduce a new separation of phases-based activity reporter of kinase (SPARK) for AMP-activated kinase (AMPK), named AMPK-SPARK, which reports the AMPK activation by forming bright fluorescent clusters. Furthermore, we introduce a dual reporter system, named GCaMP-AMPK-SPARK, by incorporating a single-fluorescent protein (FP)-based Ca2+ biosensor, GCaMP6f, into our initial design, enabling simultaneous monitoring of Ca2+ levels and AMPK activity. This system offers the essential quality of information by single-channel fluorescence microscopy without the need for coexpression of different biosensors and elaborate filter layouts to overcome spectral limitations. We used AMPK-SPARK to map endogenous AMPK activity in different cell types and visualized the dynamics of AMPK activation in response to various stimuli. Using GCaMP-AMPK-SPARK, we revealed cell-to-cell heterogeneities in AMPK activation by Ca2+ mobilization. We anticipate that this dual reporter strategy can be employed to study the intricate interplays between different signaling networks and kinase activities.

Fracture of the ulnar styloid process negatively influences the outcome of paediatric fractures of the distal radius.

In paediatric patients with fractures of the distal radius, the consequences of associated ulnar styloid fractures are often underestimated. These may include persisting pain or functional deficits. The aim of the present study was to report the outcome of these fractures using a modified DASH-Score. All children with distal radius fractures treated in a two years period were analysed; only patients with a concomitant fracture of the ulnar styloid were included in the study.In addition, children with a non-union of the styloid at cast removal were asked to complete a postal questionnaire; the data were compared to those in a group of patients with isolated distal radius fractures. Patients reporting problems and those with a modified DASH score over 0.5 were invited for a long-term follow-up clinical and radiological examination. A concomitant fracture of the ulnar styloid was present in 11% of all distal radius fractures. At the time of cast removal 46 patients (89%) showed a delayed union of the ulnar styloid. The modified DASH Score of these patients at an average of 31 months (range: 24-40 months) was significantly worse (3.8; range: 0-24.2) compared to 0.7 (range 0-27.7) in the patients with isolated radius fractures after a mean of 27 months (range: 21-42 months). At follow-up, 7 patients showed a non-union of the ulnar styloid. Fractures of the base of the styloid process were more likely to develop non-union compared to fractures of its tip. The presence of an ulnar styloid fracture negatively influences the outcome of distal radius fractures. Patients with lesions of the ulnar styloid should be followed until union is observed and/or they are asymptomatic.

Fractures of the pelvis in children: a review of the literature.

Trauma is the leading cause of death in children. Pelvic ring injuries account for 0.3-4% of all paediatric injuries. The pattern of fractures differs to that seen in adults as it is more ductile. Pelvic ring injuries tend to be more stable as the relatively thick periosteum restricts bony displacement. Intrapelvic viscera are not well protected and can sustain injury in the absence of pelvic fractures. These injuries have traditionally been treated non-operatively. In this paper, we comprehensively review the literature and propose a protocol for treatment taking into consideration associated organ injuries, hemodynamic status of the patient, patient's age, type of fracture and the stability of the pelvic ring.

Does early post-operative exercise influence bone healing kinetics? Preclinical evaluation of non-critical sized femur defect healing.

Physical exercise is a well-known modality for maintaining healthy locomotor mechanism. A detailed preclinical research on physical exercise effect on bone healing kinetics could help to improve the rehabilitation process after fracture treatment and bone remodeling. Our aim was to evaluate the effect of early post-operative exercise effect on bone microstructural changes in a rat model. Twenty Sprague Dawley male rats underwent bi-cortical 1.6 mm hole drilling in both femur diaphysis, after which (n = 10) underwent continuous treadmill training (TR) over two weeks, while the other group of rats (n = 10) was assigned to non-training (NT) control group. New bone formation labeling was performed by subcutaneous fluorochrome injections at day 5, 14 and 31. In vivo micro-computed tomography (µCT) scans were performed once a week during the 6-week post-operative period. Ten animals (five from each group) were euthanized at 3rd week while remaining animals were euthanized at 6th week. Femur samples were extracted and underwent ex vivo µCT and histological evaluation, while serum was used for evaluating alkaline phosphatase (ALP). µCT data demonstrated increased volume and surface of newly formed bone in defect area of TR group. Bone volume/Tissue volume (BV/TV) ratio and number of osteocytes showed an increase in TR group after 3-week period. Fluorochrome distances were increased between day 5 and 14 within the training group. Serum ALP level increased in both groups over 3- and 6-weeks. Post-operative exercise increases the bone healing kinetics and stimulates the new bone formation during and after the training protocol has ended.

The combined effect of zinc and calcium on the biodegradation of ultrahigh-purity magnesium implants.

Magnesium (Mg)-based implants are promising candidates for orthopedic interventions, because of their biocompatibility, good mechanical features, and ability to degrade completely in the body, eliminating the need for an additional removal surgery. In the present study, we synthesized and investigated two Mg-based materials, ultrahigh-purity ZX00 (Mg-Zn-Ca; <0.5 wt% Zn and <0.5 wt% Ca, in wt%; Fe-content <1 ppm) and ultrahigh-purity Mg (XHP-Mg, >99.999 wt% Mg; Fe-content <1 ppm), in vitro and in vivo in juvenile healthy rats to clarify the effect of the alloying elements Zn and Ca on mechanical properties, microstructure, cytocompatibility and degradation rate. Potential differences in bone formation and bone in-growth were also assessed and compared with state-of-the-art non-degradable titanium (Ti)-implanted, sham-operated, and control (non-intervention) groups, using micro-computed tomography, histology and scanning electron microscopy. At 6 and 24 weeks after implantation, serum alkaline phosphatase (ALP), calcium (Ca), and Mg level were measured and bone marrow stromal cells (BMSCs) were isolated for real-time PCR analysis. Results show that ZX00 implants have smaller grain size and superior mechanical properties than XHP-Mg, and that both reveal good biocompatibility in cytocompatibilty tests. ZX00 homogenously degraded with an increased gas accumulation 12 and 24 weeks after implantation, whereas XHP-Mg exhibited higher gas accumulation already at 2 weeks. Serum ALP, Ca, and Mg levels were comparable among all groups and both Mg-based implants led to similar relative expression levels of Alp, Runx2, and Bmp-2 genes at weeks 6 and 24. Histologically, Mg-based implants are superior for new bone tissue formation and bone in-growth compared to Ti implants. Furthermore, by tracking the sequence of multicolor fluorochrome labels, we observed higher mineral apposition rate at week 2 in both Mg-based implants compared to the control groups. Our findings suggest that (i) ZX00 and XHP-Mg support bone formation and remodeling, (ii) both Mg-based implants are superior to Ti implants in terms of new bone tissue formation and osseointegration, and (iii) ZX00 is more favorable due to its lower degradation rate and moderate gas accumulation.

Behaviour of human physeal chondro-progenitorcells in early growth plate injury response in vitro.

PURPOSE: The aim of this study was to investigate the proliferation and differentiation behaviour of a defined cell population gained from the human growth plate, namely, chondro-progenitorcells (CPCs), in the initial inflammatory phase of growth plate injury response in vitro. METHODS: Growth plate cells were sorted via FACS and differentiated along adipogenic and osteogenic lineage to confirm their progenitor features. To mimic the inflammatory phase of injury response at the growth plate they were treated with IL-1ß and exposed to cyclic mechanical loading. A BrdU assay was used to investigate CPC proliferation. CPC differentiation behaviour was analysed by RT-PCR. RESULTS: CPCs (CD45-, CD34-, CD73+, CD90+, and CD105+) showed a successful differentiation along adipogenic and osteogenic lineage. Under conditions simulating the inflammatory phase of injury response at the growth plate in vitro CPCs differentiated towards hypertrophy while chondrogenesis and ossification were inhibited. Proliferation was not significantly altered. CONCLUSION: This study showed that CPCs can be isolated from the human growth plate and expanded in vitro. In the first phase of injury response at the growth plate these cells differentiate towards hypertrophy. As longitudinal growth is obtained by chondrocyte proliferation and volume increase during hypertrophy this maturation might be the first step towards post-traumatic growth disorders such as unwanted premature ossification of the growth plate.

Immunological reaction to magnesium-based implants for orthopedic applications. What do we know so far? A systematic review on in vivo studies.

Magnesium-based implants (Mg) became an attractive candidate in orthopedic surgery due to their valuable properties, such as osteoconductivity, biodegradability, elasticity and mechanical strength. However, previous studies on biodegradable and non-biodegradable metal implants showed that these materials are not inert when placed in vivo as they interact with host defensive mechanisms. The aim of this study was to systematically review available in vivo studies with Mg-based implants that investigated immunological reactions to these implants. The following questions were raised: Do different types of Mg-based implants in terms of shape, size and alloying system cause different extent of immune response? and; Are there missing links to properly understand immunological reactions upon implantation and degradation of Mg-based implants? The database used for the literature research was PubMed (U.S. National Library of Medicine) and it was undertaken in the end of 2021. The inclusion criteria comprised (i) in vivo studies with bony implantation of Mg-based implants and (ii) analysis of the presence of local immune cells or systemic inflammatory parameters. We further excluded any studies involving coated Mg-implants, in vitro studies, and studies in which the implants had no bone contact. The systematic search process was conducted according to PRISMA guidelines. Initially, the search yielded 225 original articles. After reading each article, and based on the inclusion and exclusion criteria, 16 articles were included in the systematic review. In the available studies, Mg-based implants were not found to cause any severe inflammatory reaction, and only a mild to moderate inflammatory potential was attributed to the material. The timeline of foreign body giant cell formation showed to be different between the reviewed studies. The variety of degradation kinetics of different tested implants and discrepancies in studies regarding the time points of immunological investigations impair the conclusion of immunological reactions. This may be induced by different physical properties of an implant such as size, shape and alloying system. Further research is essential to elucidate the underlying mechanisms by which implant degradation affects the immune system. Also, better understanding will facilitate the decision of patients whether to undergo surgery with new device implantation.

Implant degradation of low-alloyed Mg-Zn-Ca in osteoporotic, old and juvenile rats.

Implant removal is unnecessary for biodegradable magnesium (Mg)-based implants and, therefore, the related risk for implant-induced fractures is limited. Aging, on the other hand, is associated with low bone-turnover and decreased bone mass and density, and thus increased fracture risk. Osteoporosis is accompanied by Mg deficiency, therefore, we hypothesized that Mg-based implants may support bone formation by Mg2+ ion release in an ovariectomy-induced osteoporotic rat model. Hence, we investigated osseointegration and implant degradation of a low-alloyed, degrading Mg-Zn-Ca implant (ZX00) in ovariectomy-induced osteoporotic (Osteo), old healthy (OH), and juvenile healthy (JH) groups of female Sprague Dawley rats via in vivo micro-computed tomography (µCT). For the Osteo rats, we demonstrate diminished trabecular bone already after 8 weeks upon ovariectomy and significantly enhanced implant volume loss, with correspondingly pronounced gas formation, compared to the OH and JH groups. Sclerotic rim development was observed in about half of the osteoporotic rats, suggesting a prevention from foreign-body and osteonecrosis development. Synchrotron radiation-based µCT confirmed lower bone volume fractions in the Osteo group compared to the OH and JH groups. Qualitative histological analysis additionally visualized the enhanced implant degradation in the Osteo group. To date, ZX00 provides an interesting implant material for young and older healthy patients, but it may not be of advantage in pharmacologically untreated osteoporotic conditions. STATEMENT OF SIGNIFICANCE: Magnesium-based implants are promising candidates for treatment of osteoporotic fractures because of their biodegradable, biomechanical, anti-bacterial and bone regenerative properties. Here we investigate magnesium‒zinc‒calcium implant materials in a rat model with ovariectomy-induced osteoporosis (Osteo group) and compare the related osseointegration and implant degradation with the results obtained for old healthy (OH) and juvenile healthy (JH) rats. The work applied an appropriate disease model for osteoporosis and focused in particular on long-term implant degradation for different bone conditions. Enhanced implant degradation and sclerotic rim formation was observed in osteoporotic rats, which illustrates that the setting of different bone models generates significantly modified clinical outcome. It further illustrated that these differences must be taken into account in future biodegradable implant development.

Antimicrobial activity of gentamicin palmitate against high concentrations of Staphylococcus aureus.

The reduction of implant related infections plays a pivotal role in orthopaedic surgery as an increasing number of people require implants (up to 200,000 per year in the United States (source: Joint Implant Surgery & Research Foundation 2010)). The aim of the current study is to prevent and thus decrease the number of bacterial infections. Both pre and post operative systemic antibiotic treatment and gentamicin containing bone cements (polymethylmethacrylate, PMMA) are commonly used strategies to overcome infections. In this study, the antimicrobial efficacy of gentamicin sulfate loaded bone cement was compared with titan discs coated with a new form of gentamicin, gentamicin palmitate. Adherence prevention, killing rates and killing kinetics were compared in an in vitro model, using Staphylococcus aureus (S. aureus), which together with Staphylococcus epidermidis (S. epidermidis) represents 60% of bacteria found responsible for hip implant infections (An and Friedman, 1996, J Hosp Infect 33(2):93-108). In our experiments gentamicin, which was applied as gentamicin palmitate on the surface of the implants, showed a high efficacy in eliminating bacteria. In contrast to gentamicin sulfate containing bone cements, gentamicin palmitate is released over a shorter period of time thus not inducing antibiotic resistance. Another benefit for clinical application is that it achieves high local levels of active ingredient which fight early infections and minimize toxic side effects. Furthermore, the short term hydrophobic effect of gentamicin palmitate can successfully impede biofilm formation. Thus, the use of self-adhesive antibiotic fatty acid complexes like gentamicin palmitate represents a new option for the anti-infective coating of cementless titan implants.

Anatomical structures at risk with the proud retrograde femoral nail.

BACKGROUND: Post-operative knee pain is common following retrograde nailing, with its etiology often multifactorial although a well-established cause is nail protrusion from the intercondylar notch. The aim of this study was to assess the structures at risk if the nail is left proud of the femoral articular surface. METHODS: A retrograde femoral nail (Synthes Distal Femoral Nail) was inserted into the distal femur of 15 cadaveric lower limbs using the standard technique. The nail was left 10 mm proud of the articular surface and locked in this position. The knee was then put through a full range of movement while recording the intra-articular structures that came into contact with the distal end of the nail as well as the position of the knee when this occurred. This was repeated with the nail 5 mm proud. RESULTS: With the nail 10 mm proud, it impinged on the anterior horn of the medial meniscus in 14 cases and the anterior horn of the medial meniscus as well as the tibial insertion of the ACL in one case at 15° of flexion. At 70° of flexion the nail came into contact with the distal margin of the patellar articular surface in the midline in all 15 cases. With the nail 5 mm proud, it impinged on the anterior horn of the medial meniscus in seven cases and the tibial insertion of the ACL as well as the anterior horn of the medial meniscus in eight cases as the knee was brought into full extension. In flexion the distal margin of the patellar articular surface in the midline came into contact with the nail at 70° of flexion in all 15 cases. CONCLUSION: Correct positioning of a retrograde femoral nail is of paramount importance to avoid further iatrogenic injury to intra-articular structures.

The risk of nerve injury with minimally invasive plate osteosynthesis of distal fibula fractures: an anatomic study.

AIM: The aim of our study was to identify the structures which may be at risk of injury when using a minimally invasive technique for the osteosynthesis of the lateral malleolus and the influence of the size of the implant on the frequency of injury to these structures. METHOD: Forty plates were percutaneously inserted in 20 cadaveric legs. The region around the plate was then dissected to examine the relation of nerves and soft tissues to the plate. RESULTS: The superficial peroneal nerve was in direct contact with the plate in 11 of the 20 cases (55%) of the 10 hole plates. We encountered only one case of the superficial peroneal nerve skirting the proximal edge of a 6 hole plate (p = 0.0164). CONCLUSION: Consequently we recommend meticulous attention is paid to the dissection of soft tissues in both the proximal and distal incisions. The length of the plate may be checked with intraoperative imaging prior to its insertion, and the site of both proximal and distal incisions may be marked on the skin. After careful dissection down to the bone, preserving nerves and tendons, the periosteal elevator should be introduced both from the proximal as well as the distal incisions to prepare the extra-periosteal tunnel for the insertion of the plate, in order to avoid the entanglement of the superficial peroneal nerve with the metal work, particularly in plates of longer than six holes.

Anatomical landmarks in the paediatric distal radius: a new method for measuring epiphyseal height.

PURPOSE: Elastic stable intramedullary nailing for fixation of paediatric forearm fractures has become the preferred method of osteosynthesis. The aim of the study was to develop a method for finding the correct entry point in the distal radius of children of all ages as a distance from the radial styloid, a landmark which is readily palpable at the wrist. METHOD: MRI scans of the wrist in 19 children aged 2-7 years (mean 5) were reviewed. We noted that the proximal edges of the ossific nucleus of capitate and the tip of the radial styloid were at the same vertical height from the physis. This allowed us to review 519 wrist radiographs of children aged 2-14 years (mean 7.98) to measure the epiphyseal height, as the radial styloid does not ossify until the age of 14 years. RESULTS: The epiphyseal height had a mean of 16.67 mm (13-21 mm). The epiphyseal height in boys was greater than the girls by a mean of 1.2 mm (p < 0.0005). CONCLUSION: For the absolute safe distance, we recommend an insertion site 40 mm proximal to the tip of the radial styloid, which is safe in all children with an open distal radial physis. This simple and reproducible method allows the identification of the insertion site for the nail with only the minimal use of imaging.

The risk injury to the posterior interosseous nerve in standard approaches to the proximal radius: a cadaver study.

PURPOSE: The aim of this study was to provide guidance on the safe zones for the exposure of the proximal radius by measuring the distance from the PIN to various anatomical landmarks in the proximal forearm in pronation and supination. METHODS: Twenty cadaveric arms were used for this study. On the anterior aspect of the forearm, the distance between insertion of the biceps tendon and the arcade of Frohse as well as the shortest distance between the PIN and the ulnar aspect of the radial neck were measured. On the posterior aspect of the forearm, the shortest distance between the PIN and the ulnar border of the interosseous membrane was measured at 30 and 50 mm distal to the articular surface of the radial head. RESULTS: The distance between the PIN and ulnar aspect of the radial neck had a mean of 21.6 mm in supination and 13.3 mm in pronation. The distance between the radial tuberosity and the arcade of Frohse was 18.6 mm. The mean distance between the PIN and the radial border of ulna at 30 mm distal to the articular surface of the proximal radius was 12.3 mm in supination and 22.3 mm in pronation. At 50 mm distal to the articular surface of the proximal radius the mean distance was 8 mm in supination and 16.2 mm in pronation. CONCLUSIONS: The course of this nerve is variable as it winds around the radial neck within the belly of the supinator muscle. Safe distances for dissection have been presented in our study.

Iatrogenic ulnar nerve injury after pin fixation and after antegrade nailing of supracondylar humeral fractures in children.

BACKGROUND AND PURPOSE: Ulnar nerve injury may occur after pinning of supracondylar fractures in children. We describe the outcome and compare the rates of iatrogenic injuries to the ulnar nerve in a consecutive series of displaced supracondylar humeral fractures in children treated with either crossed pinning or antegrade nailing. METHODS: Medical charts of all children sustaining this fracture treated at our department between 1994 and 2009 were retrospectively reviewed regarding the mode of treatment, demographic data including age and sex, the time until implant removal, the outcome, and the rate of ulnar nerve injuries. RESULTS: 503 children (55% boys) with an average age of 6.5 years sustained a type-II, type-III, or type-IV supracondylar fracture. Of those, 440 children were included in the study. Antegrade nailing was performed in 264 (60%) of the children, and the others were treated with crossed pins. Iatrogenic ulnar nerve injury occurred in 0.4% of the children treated with antegrade nailing and in 15% of the children treated with crossed pinning. After median 3 (1.6-12) years of follow-up, the clinical outcome was good and similar between the 2 groups. INTERPRETATION: Intramedullary antegrade nailing of displaced supracondylar humeral fractures can be considered an adequate and safe alternative to the widely performed crossed K-wire fixation. The risk of iatrogenic nerve injury after antegrade nailing is small compared to that after crossed pinning.

Post-traumatic coxa vara in children following screw fixation of the femoral neck.

BACKGROUND AND PURPOSE: The rare displaced fractures of the femoral neck in children need accurate reduction and rigid fixation. The implants commonly used for internal fixation in children are pins or screws. We evaluated the long-term outcome in children who sustained fractures of the proximal femur that were treated by screw fixation. PATIENTS AND METHODS: All 22 children (mean age 12 (5-16) years) with fractures of the femoral neck that were treated with screw fixation (mean 2.4 (1-3) screws) at our department between 1990 and 2006 were evaluated. For measurement of outcome, the Harris hip score (HHS) was used and the development of post-traumatic coxa vara was assessed from the difference in the neck-shaft angle postoperatively and at the latest follow-up examination, after mean 4 (2-15) years. RESULTS: A loss of reduction was observed in 12 patients. There was a statistically significant correlation between the HHS and the changes in the neck-shaft angle. INTERPRETATION: Loss of reduction was found in more than half of the children. Screw fixation cannot be recommended for the treatment of femoral neck fractures in children due to a substantial number of post-traumatic coxa vara.

Increased Staphylococcus aureus Biofilm Formation on Biodegradable Poly(3-Hydroxybutyrate)-Implants Compared with Conventional Orthopedic Implants: An In Vitro Analysis.

OBJECTIVE: To compare the biofilm formation on a biodegradable material, poly(3-hydroxybutyrate) (PHB), with that on conventional titanium (Ti) and steel (St) implant material. METHODS: Pins made of the different materials were incubated in Müller-Hinton broth inoculated with 2 × 10 colony-forming units (CFU)·mL of Staphylococcus aureus for 2 and 7 days and then sonicated for the disruption of the biofilms. CFU were counted to quantify the number of bacteria in the biofilm, and the cell proliferation assay 2,3-Bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H- tetrazolium-5-carboxanilid salt was used to evaluate their metabolic activity. Scanning electron microscopy visualized the structure of the biofilm. RESULTS: We found a significantly higher metabolic activity and CFU count in the biofilm of PHB pins compared with St and Ti pins (analysis of variance, P < 0.0001). Scanning electron microscopy revealed structured biofilms on PHB pins already after 2 days of incubation, which was not observed on the other tested implants. CONCLUSION: PHB implants seem to provide an environment that advantages the formation of biofilms of S. aureus, a common pathogen in implant-related infections. The amount of biofilm is higher on PHB implant compared with conventionally used orthopedic titanium and steel implants. To overcome the potential risk of surgical site infections linked to the clinical use of PHB implants, possible modifications of the material, increasing its antibacterial properties, need to be further investigated.

The influence of biodegradable magnesium implants on the growth plate.

Mg-based biodegradable materials are considered promising candidates in the paediatric field due to their favourable mechanical and biological properties and their biodegrading potential that makes a second surgery for implant removal unnecessary. In many cases the surgical fixation technique requires a crossing of the growth plate by the implant in order to achieve an adequate fragment replacement or fracture stabilisation. This study investigates the kinetics of slowly and rapidly degrading Mg alloys in a transphyseal rat model, and also reports on their dynamics in the context of the physis and consecutive bone growth. Twenty-six male Sprague-Dawley rats received either a rapidly degrading (ZX50; n = 13) or a slowly degrading (WZ21; n = 13) Mg alloy, implanted transphyseal into the distal femur. The contralateral leg was drilled in the same manner and served as a direct sham specimen. Degradation behaviour, gas formation, and leg length were measured by continuous in vivo micro CT for up to 52 weeks, and additional high-resolution µCT (HRS) scans and histomorphological analyses of the growth plate were performed. The growth plate was locally destroyed and bone growth was significantly diminished by the fast degradation of ZX50 implants and the accompanying release of large amounts of hydrogen gas. In contrast, WZ21 implants showed homogenous and moderate degradation performance, and the effect on bone growth did not differ significantly from a single drill-hole defect. STATEMENT OF SIGNIFICANCE: This study is the first that reports on the effects of degrading magnesium implants on the growth plate in a living animal model. The results show that high evolution of hydrogen gas due to rapid Mg degradation can damage the growth plate substantially. Slow degradation, however, such as seen for WZ21 alloys, does not affect the growth plate more than drilling alone, thus meeting one important prerequisite for deployment in paediatric osteosynthesis.

The potential of isotopically enriched magnesium to study bone implant degradation in vivo.

This pilot study highlights the substantial potential of using isotopically enriched (non-radioactive) metals to study the fate of biodegradable metal implants. It was possible to show that magnesium (Mg) release can be observed by combining isotopic mass spectrometry and isotopic pattern deconvolution for data reduction, even at low amounts of Mg released a from slowly degrading 26Mg enriched (>99%) Mg metal. Following implantation into rats, structural in vivo changes were monitored by µCT. Results showed that the applied Mg had an average degradation rate of 16±5µmyear-1, which corresponds with the degradation rate of pure Mg. Bone and tissue extraction was performed 4, 24, and 52weeks after implantation. Bone cross sections were analyzed by laser ablation inductively coupled plasma mass spectrometry (ICP-MS) to determine the lateral 26Mg distribution. The 26Mg/24Mg ratios in digested tissue and excretion samples were analyzed by multi collector ICP-MS. Isotope pattern deconvolution in combination with ICP-MS enabled detection of Mg pin material in amounts as low as 200ppm in bone tissues and 20ppm in tissues up to two fold increased Mg levels with a contribution of pin-derived Mg of up to 75% (4weeks) and 30% (24weeks) were found adjacent to the implant. After complete degradation, no visual bone disturbance or residual pin-Mg could be detected in cortical bone. In organs, increased Δ26Mg/24Mg values up to 16‰ were determined compared to control samples. Increased Δ26Mg/24Mg values were detected in serum samples at a constant total Mg level. In contrast to urine, feces did not show a shift in the 26Mg/24Mg ratios. This investigation showed that the organism is capable of handling excess Mg well and that bones fully recover after degradation. STATEMENT OF SIGNIFICANCE: Magnesium alloys as bone implants have faced increasing attention over the past years. In vivo degradation and metabolism studies of these implant materials have shown the promising application in orthopaedic trauma surgery. With advance in Mg research it has become increasingly important to monitor the fate of the implant material in the organism. For the first time, the indispensible potential of isotopically enriched materials is documented by applying 26Mg enriched Mg implants in an animal model. Therefore, the spatial distribution of pin-Mg in bone and the pin-Mg migration and excretion in the organism could be monitored to better understand metal degradation as well as Mg turn over and excretion.

Time-Dependent Changes in T1 during Fracture Healing in Juvenile Rats: A Quantitative MR Approach.

Quantitative magnetic resonance imaging (qMRI) offers several advantages in imaging and determination of soft tissue alterations when compared to qualitative imaging techniques. Although applications in brain and muscle tissues are well studied, its suitability to quantify relaxation times of intact and injured bone tissue, especially in children, is widely unknown. The objective observation of a fracture including its age determination can become of legal interest in cases of child abuse or maltreatment. Therefore, the aim of this study is the determination of time dependent changes in intact and corresponding injured bones in immature rats via qMRI, to provide the basis for an objective and radiation-free approach for fracture dating. Thirty-five MR scans of 7 Sprague-Dawley rats (male, 4 weeks old, 100 ± 5 g) were acquired on a 3T MRI scanner (TimTrio, Siemens AG, Erlangen, Germany) after the surgical infliction of an epiphyseal fracture in the tibia. The images were taken at days 1, 3, 7, 14, 28, 42 and 82 post-surgery. A proton density-weighted and a T1-weighted 3D FLASH sequence were acquired to calculate the longitudinal relaxation time T1 of the fractured region and the surrounding tissues. The calculation of T1 in intact and injured bone resulted in a quantitative observation of bone development in intact juvenile tibiae as well as the bone healing process in the injured tibiae. In both areas, T1 decreased over time. To evaluate the differences in T1 behaviour between the intact and injured bone, the relative T1 values (bone-fracture) were calculated, showing clear detectable alterations of T1 after fracture occurrence. These results indicate that qMRI has a high potential not only for clinically relevant applications to detect growth defects or developmental alterations in juvenile bones, but also for forensically relevant applications such as the dating of fractures in cases of child abuse or maltreatment.