[Processing properties and viscosities of PMMA bone cements]. / Verarbeitungseigenschaften und Viskositäten von PMMA-Knochenzementen.

Processing properties of PMMA bone cements can be divided into four phases: 1. mixing, 2. waiting, 3. processing and 4. curing. Each of these phases is subject to several external influencing factors, such as temperature and humidity, which must be considered during application. Instructions for use provided by the manufacturers of PMMA bone cements and mixing and application systems contain important information on correct application. The processing properties of PMMA bone cements and possible factors influencing the curing process are of great importance for safe procedures in the operating room. Knowledge of the viscosity and consistency of the PMMA bone cement from the dough phase to complete curing facilitates preparation and application, which in the long term significantly improves the requirements placed on PMMA cements regarding the function and service life of the implant.

[Erythema and swelling after im-plantation of a cardioverter defibrillator (ICD)]. / Erythem und Schwellung nach Implantation eines Kardioverter-Defibrillators (ICD).

HISTORY AND CLINICAL FINDINGS: We report the case of a 60-year-old man with localized erythema and edema, caused by an implanted cardioverter defibrillator (ICD). EXAMINATION: An infection could be excluded by clinical examination, para clinical parameters and clinical chemistry. The histologic examination was indicative for hypersensitivity reaction. Patch test was negative but nickel sensitization was seen by lymphocyte transformation test and nickel could be measured in the pacemaker eluate. TREATMENT AND COURSE: Based on the synopsis of negative patch testing, the hypersensitivity resembling histology and the corresponding in vitro analysis, the assumed cause for the dermatitis is a contact allergy to nickel as pacemaker component. After explantation, complete restitution of the skin could be observed. CONCLUSIONS: Unusual skin reactions in patients with cardiac pacemakers are mostly referred to as "Pacemaker dermatitis". There are various potential causes: first, an infection needs to be excluded. Then, the possibility of both "pressure damage" caused by the pacemaker and the mostly symptom free "pacemaker erythema with telangiectasia" - synonym "post implantation erythema" - needs to be considered. When suspecting an incompatibility of the implant, further allergologic testing is required. Often, the removal of the pacemaker is inevitable.

Laser engravings as reason for mechanical failure of titanium-alloyed total hip stems.

INTRODUCTION: Two revisions of broken ß-titanium total hip stems had to be performed in our hospital after 2 and 4 years in situ. Since both fractures were located at the level of a laser engraving, a failure analysis was conducted. MATERIALS AND METHODS: Both retrieved hip stems were disinfected and collected in our retrieval database after patient's signed agreement. Each fragment was macroscopically photographed. Fracture surfaces were analyzed using scanning electron microscopy (SEM). Quantification of element content was conducted using energy dispersive X-ray (EDX) analysis. RESULTS: Both stems show fatigue fracture, as displayed by the lines of rest on the fracture surface. The origin of fracture was identified directly at the laser engraving of the company logo at both stems by means of SEM. The EDX analysis showed an oxygen level beneath the laser engraving about twice as high as in the substrate, causing material embrittlement. CONCLUSIONS: Laser engravings need to be reduced to a minimum of necessary information, and should be placed at locations with minimum mechanical load. Biomechanical analyses are recommended to identify less loaded areas in implant components to avoid such implant failures.

Influence of cementless hip stems on femoral cortical strain pattern depending on their extent of porous coating.

The extent of porous coating of cementless total hip stems is held responsible for radiological periprosthetic changes, the rate of thigh pain, and even its long-term success. However, there is only sparse knowledge on how the biomechanical loading conditions of the femur are influenced by the extent of porous coating in the early phase after implantation of a cementless hip stem. Aiming to evaluate the effect of surface structuring on the strain pattern of the femur, we implanted three anatomic hip stems with different extents of porous coating (full, two-thirds proximal, and penguin type) in second-generation composite femora coated with a photoelastic layer. A cortical strain mapping was conducted before and after insertion of the implants under standardized loading conditions considering relevant muscle forces. The results of the statistical analysis of three different implantation sequences proved that composite femora are suitable for repeated measurements within the applied experimental setup. Cortical strain changes including stress-shielding effects medially (-60%) and laterally (-50%) were validated with a cadaver femur. The extent of porous coating had no significant influence on the surface strain pattern for an immediate postoperative situation.

How stiffness and distal interlocking of revision hip stems influence the femoral cortical strain pattern.

BACKGROUND: Stress shielding and nonphysiological load transfer after primary or revision total hip replacement (THR) prepare the ground for resorptive bone remodeling. The quality of the bone stock influences the risk of periprosthetic fractures and the severity of future revision surgeries. The question of whether or not bending stiffness and distal screw interlocking influence load transfer of a modular revision hip stem with a solid, hollow, and hollow-slotted stem extension led to the conception of this experimental study. The results were compared with a standard hip stem for primary THR. METHODS: Revision stems were implanted in photoelastically coated composite femora. Cortical strain mapping was conducted before and after insertion of the implants under standardized loading conditions, considering the relevant muscle forces. Statistical analysis was based on a 95% confidence interval and a variance analysis for repeated measurements. RESULTS: Significant stress shielding was observed after insertion of all types of hip stems compared with the intact femora. There was also a marked difference between strain alterations induced by standard and revision hip stems. With revision stems, the most distinct stress shielding effects were registered with the solid stem extension, particularly in the femoral diaphysis. Distal interlocking screws only had a local action on strain pattern and tended to enhance stress shielding at the midstem area when using the more flexible components. CONCLUSION: More flexible revision stems provide a cortical strain pattern of the femur closer to the preoperative status. This may reduce resorptive bone remodeling in the long term. However, any type of revision stem tested in this study caused higher stress shielding than the hip stem for primary THR, especially in the diaphyseal region medially and laterally. With sufficient proximal anchorage, the influence of distal interlocking screws on the femoral strain pattern was localized.

New method for determining in vitro structure stiffness of ceramic acetabular liners under different impact conditions.

Increasing both patient mobility and prosthesis life span requires improvements in the range of motion and wear behavior of the liner. With the use of new composite alumina-zirconia ceramic materials, the same stability of the liner can be achieved at lower wall thickness than it is possible with alumina-only materials. The aim of this study was developing a method for determining the in vitro structure stiffness of ceramic acetabular liners against impact stresses. The first trials were performed with a common alumina acetabular liner type (Ceramtec; Biolox forte; diameter 28 mm; thickness 7 mm) and a new type of alumina-zirconia (Ceramtec Biolox delta; same dimensions) liner. The clinically established alumina liner was reproducibly damaged using worst case Separation/subluxation equivalent to one-fourth or half of the head diameter, and an impact load of 15 J. The liners containing the new alumina-zirconia material were not damaged in any of the trials up to an impact load of 20 J and half head diameter offset.

Alloplastic reconstruction of the extensor mechanism after resection of tibial sarcoma.

Reconstruction of the extensor mechanism is essential for good extremity function after endoprosthetic knee replacement following tumor resection. Only a few biological methods have been able to reliably restore a functional extensor mechanism, but they are often associated with significant complication rates. Reattachment of the patellar tendon to the prosthesis using an alloplastic patellar ligament (Trevira cord) can be an appropriate alternative. In vivo and in vitro studies have already shown that complete fibrous ingrowth in polyethylene chords can be seen after a period of six months. However, until now, no biomechanical study has shown the efficacy of an alloplastic cord and its fixation device in providing sufficient stability and endurance in daily life-activity until newly formed scar tissue can take over this function. In a special test bench developed for this study, different loading regimes were applied to simulate loads during everyday life. Failure loads and failure modes were evaluated. The properties of the cord were compared before and after physiological conditioning. It was shown that rubbing was the mode of failure under dynamic loading. Tensile forces up to 2558 N did not result in material failure. Thus, using an artificial cord together with this fixation device, temporary sufficient stable fixation can be expected.

Tissue engineering of the anterior cruciate ligament-sodium dodecyl sulfate-acellularized and revitalized tendons are inferior to native tendons.

The acellularization of tendons using detergents (sodium dodecyl sulfate, Triton-X, tri-nitro-butyl-phosphate) is a new source of scaffolds for tissue engineering in anterior cruciate ligament (ACL) repair. In vitro testing demonstrated that acellular tendon scaffolds are biocompatible and show good biomechanical properties, but in vivo confirmation of these results is not yet available. Therefore, the aim of this study was to see in vivo if an acellular allogenic construct colonized with autologous fibroblasts improves the quality of ACL reconstruction. ACL replacement was performed in 31 New Zealand White rabbits using a standardized model. Fifteen animals received autologous semitendinosus tendon, whereas 16 animals were treated with a tissue-engineered construct. This construct was made by acellularization of allogenic semitendinosus tendons using sodium dodecyl sulfate and subsequent in vitro colonization with autologous fibroblasts. Eight weeks postoperatively, macroscopic, biomechanical (ultimate load to failure, elongation, stiffness; n = 8/9), and histological (n = 5) examinations were performed. Biomechanical testing showed decreasing strength of the constructs at 8 weeks after implantation compared with the direct postsurgical strength. However, tissue-engineered constructs (F = 19.7 +/- 20.3 N) were significantly weaker than autologous tendons (F = 61.2 +/- 31.2 N). Histologically, the autologous tendons showed signs of partial necrosis and tissue remodeling. The tissue-engineered constructs exhibited an inflammatory reaction and showed both repopulated and acellular regions. In conclusion, in vivo results were much more unfavorable than in vitro results had suggested. Further studies have to be performed to test if modifications of the acellularization process yield better results in vivo.

Mechanical properties of modern calcite- (Mergerlia truncata) and phosphate-shelled brachiopods (Discradisca stella and Lingula anatina) determined by nanoindentation.

We measured distribution patterns of hardness and elastic modulus by nanoindentation on shells of the rhynchonelliform brachiopod Mergerlia truncata and the linguliform brachiopods Discradisca stella and Lingula anatina. The rhynchonelliformea produce calcitic shells while the linguliformea produce chitinophosphatic shells. Dorsal and ventral valves, commissure and hinge of the calcitic shell of M. truncata show different nanohardness values (from 2.3 to 4.6 GPa) and E-modulus (from 52 to 76 GPa). The hardness of the biocalcite is always increased compared to inorganic calcite. We attribute the effects to different amounts of inter- and intracrystalline organic matrix. Profiles parallel to the radius of curvature of the valves cutting through the different layers of shell material surprisingly show quite uniform values of nanohardness and modulus of elasticity. Nanoindentation tests on the chitinophosphatic brachiopods D. stella and L. anatina reflect the hierarchical structure composed of laminae with varying degree of mineralization. As a result of the two-phase composite of biopolymer nanofibrils reinforced with Ca-phosphate nanoparticles, nanohardness, and E-modulus correlate almost linearly from (H=0.25 GPa, E=2.5 GPa) to (H=2.5 GPa, E=50 GPa). The mineral provides stiffness and hardness, the biopolymer provides flexibility; and the composite provides fracture toughness. Gradients in the degree of mineralization reduce potential stress concentrations at the interface between stiff mineralized and soft non-mineralized laminae. For the epibenthic chitinophosphatic D. stella the lamination is also present but less pronounced than for the infaunal L. anatina, and the overall distribution of material strength in the cross-sectional profile shows a maximum in the center and a decrease towards the inner and outer shell margins (modulus of elasticity from 30 to 12 GPa, hardness from 1.7 to 0.5 GPa). Accordingly, the two epibenthic forms, calcitic M. truncata and chitinophosphatic D. stella display fairly bulky (homogeneous) nanomechanical properties of their shell materials, while the burrowing infaunal L. anatina is distinctively laminated. The strongly mineralized laminae, which provide the strength to the shell, are also brittle, but keeping them as thin as possible, allows some bending flexibility. This flexibility is not required for the epibenthic life style.

Development of novel implant abutments using the shape memory alloy nitinol: preliminary results.

PURPOSE: The development of gap-free abutments is a challenging problem, because the gap between the implant and the abutment, which is a consequence of current manufacturing limitations, can serve as a reservoir for pathogens. This may lead to peri-implantitis, a major cause of implant failure. Therefore, the aim of this study was to design and fabricate a gap-free abutment using a shape memory alloy with improved ability to prevent microleakage at the implant-abutment gap. MATERIALS AND METHODS: The abutment was designed using the shape memory alloy nitinol and based on mathematical calculations considering the temperature-related, reversible changes to its crystalline alloy structure. The abutment prototypes were tested for their susceptibility to microbes in vitro, under static and dynamic conditions, by contaminating the abutments before assembly using a bacterial solution. Microbacterial tests were performed after cultivation of the implants for 1 week. The results were tested for statistically significant differences using the chi-square test. RESULTS: The mathematical calculations met the clinical requirements using a contact pressure of 2 3 108 Nm2 with a preload of 1.9 kN on cooled abutments. After recooling, the contact pressure was 1.3 Nm2, allowing for easy disassembly. Microbacterial analysis revealed no penetration of Escherichia coli under static conditions either in the control group or in the prototypes. Under dynamic conditions, however, the prototypes showed significantly reduced bacterial leakage compared to the controls. CONCLUSIONS: The data presented here demonstrate that dental implants fabricated with gap-free abutments using a shape memory alloy showed significantly reduced bacterial leakage versus conventional implants. This improvement could minimize clinical problems such as peri-implantitis and consequently enhance the long-term success of dental implants.

Biomechanical and immunohistochemical properties of meniscal cartilage after high hydrostatic pressure treatment.

Meniscal allograft processing procedures, in particular gamma irradiation, deteriorate the biomechanical and biological properties of the transplanted tissue. High hydrostatic pressure (HHP) treatment, widely used in food technology to inactivate microorganisms while preserving natural compounds, might serve as a gentle alternative to gamma irradiation in the processing of meniscal allografts. We therefore investigated the effects of HHP treatment on the biomechanical and immunohistochemical properties of meniscal cartilage. Specimens of bovine menisci were treated with HHP for 10 min (20 degrees C) at 300 MPa and 600 MPa. Untreated control samples were left at room temperature and ambient pressure. We performed repetitive cycling indentation-tests to assess the biomechanical properties-in particular the viscoelastic behavior-of HHP treated and untreated meniscal specimens. Immunohistochemical analysis for collagens type I, II, and III and for the proteoglycans versican, aggrecan and for link-protein was performed by immunolabeling cross-sections of untreated and at 600 MPa HHP treated specimens. Comparing untreated and HHP treated meniscal specimens there were no significant differences for all tested biomechanical parameters. All cross-sections of untreated and HHP treated specimens stained positive for the collagens and proteoglycans. We demonstrated that meniscal cartilage can be treated by HHP at levels as high as 600 MPa without affection of the biomechanical and immunochistochemical properties. Therefore, HHP treatment might serve as a gentle alternative to gamma irradiation in the processing of meniscal allografts. Further research is necessary to verificate the present results in vivo.

In vitro analysis of an allogenic scaffold for tissue-engineered meniscus replacement.

Scaffolds play a key role in the field of tissue engineering. Particularly for meniscus replacement, optimal scaffold properties are critical. The aim of our study was to develop a novel scaffold for replacement of meniscal tissue by means of tissue engineering. Emphasis was put on biomechanical properties comparable to native meniscus, nonimmunogenecity, and the possibility of seeding cells into and cultivating them within the scaffold (nontoxicity). For this purpose, native ovine menisci were treated in vitro in a self-developed enzymatic process. Complete cell removal was achieved and shown both histologically and electron microscopically (n = 15). Immunohistochemical reaction (MHC 1/MHC 2) was positive for native ovine meniscus and negative for the scaffold. Compared to native meniscus (25.8 N/mm) stiffness of the scaffold was significantly increased (30.2 N/mm, p < 0.05, n = 10). We determined the compression (%) of the native meniscus and the scaffold under a load of 7 N. The compression was 23% for native meniscus and 29% for the scaffold (p < 0.05, n = 10). Residual force of the scaffold was significantly lower (5.2 N vs. 4.9 N, p < 0.05, n = 10). Autologous fibrochondrocytes were needle injected and successfully cultivated within the scaffolds over a period of 4 weeks (n = 10). To our knowledge, this study is the first to remove cells and immunogenetic proteins (MHC 1/MHC 2) completely out of native meniscus and preserve important biomechanical properties. Also, injected cells could be successfully cultivated within the scaffold. Further in vitro and in vivo animal studies are necessary to establish optimal cell sources, sterilization, and seeding techniques. Cell differentiation, matrix production, in vivo remodeling of the construct, and possible immunological reactions after implantation are subject of further studies.

Tissue engineering of the anterior cruciate ligament: a new method using acellularized tendon allografts and autologous fibroblasts.

INTRODUCTION: The availability of autogenous tendons (middle part of patellar tendon, semitendinosus/gracilis, or quadriceps tendon) for cruciate ligament reconstructions is restricted and related to withdrawal morbidity. Allografts and synthetic ligament materials often show problems regarding long-term stability and immunological reactions. Therefore, the aim of this study was to develop and characterize a new scaffold based on acellular allografts seeded with autologous cells for tissue engineering of the anterior cruciate ligament (ACL). MATERIALS AND METHODS: Semitendinosus tendons of New Zealand White (NZW) rabbits were harvested and acellularized using the detergent sodium dodecyle sulfate (SDS) as the main ingredient. After that, cultured (37 degrees C, 5% CO(2), medium) dermal fibroblasts were injected into the tendons. These constructs were further cultivated for 4, 7, or 14 days under the same culture conditions. Native, acellular, and seeded tendons underwent biomechanical testing (ultimate load to failure [N], stiffness [N/mm], and elongation [%], each n = 9] and histological hematoxylin-eosin (H.E.) staining. Detailed immunohistochemical (collagen I, III, IV, VI, pro-collagen I, versican, and vimentin) analyses were conducted to detect changes in the composition and structure of the extracellular matrix (ECM) after acellularization. RESULTS: Histologically, a cell-free, crimped slack tendon structure after acellularization and a good integration of the cells after injection (4, 7, and 14 days) were seen. Metabolic activity of the seeded cells was demonstrated by positive immunohistochemical staining for pro-collagen I, which was negative in nonseeded constructs. Major differences in staining patterns of the various other ECM components were not observed. Biomechanically, the maximum load to failure of these tendons was comparable to native tendons (P = 0.429; native 134.5 +/- 12.9 N; acellular 118.5 +/- 7.3 N; seeded 132.3 +/- 5.6 N). Stiffness and elongation were comparable between native and acellular tendons, but differed significantly after seeding (P < 0.001). CONCLUSION: The described method is suitable to make tendons completely cell free without changing their major biomechanical properties. Preservation of the ECM and of the collagen fiber structure by this method should give an ideal environment for autologous cell integration and metabolic activity in contrast to other approaches for tissue acellularization. The cell disruption and extraction of cell detritus should minimize adverse immunogenic reactions.

Two column lesions in the thoracolumbar junction: anterior, posterior or combined approach? A comparative biomechanical in vitro investigation.

There are various surgical techniques for the treatment of spinal fractures in the thoracolumbar region. Several implants have been developed for anterior or posterior instrumentation. Optimal treatment of unstable thoracolumbar osseous and ligamentous injuries remains controversial. To compare the stabilizing effects of an antero-lateral, thoracoscopically implantable plate system (macsTL, Aesculap, Germany) with the stability provided by a fixateur interne (SOCON, Aesculap, Germany), this in vitro investigation examined six human bisegmental (T12-L2) spinal units. Specimens were tested intact, and with simulation of osseous lesions in the anterior and ligamentous lesions in the posterior column (combined A/B-fracture). While loaded in the main anatomical planes such as flexion/extension, left and right lateral bending and left and right axial rotation with a bending moment of 7.5 Nm in a special testing jigs, motion analysis was performed. Quantitative interpretation of the stabilizing effect was achieved using a contactless three-dimensional motion analysis system. Each specimen was tested in four different scenarios: the first step measured movements of intact spinal segments. For the second step, specimens underwent simulation of combined A/B-fracture provided with bisegmental (T12/L2) antero-lateral fixation and bone strut graft from the iliac crest. For the third step, segments were additionally stabilized by the fixateur interne. The last measurement (fourth step) was performed after removing the anterior instrumentation. Range of motion (ROM) values were compared and statistically evaluated. Compared to the intact specimens the anterior instrumentation of the combined lesion, simulated A/B-fracture, leads to a stabilizing effect in flexion/extension and lateral bending. In contrast to these findings the torsional instability increased for the upper segment and bisegmentally. A maximum rigidity, beyond intact values, was registered for each anatomical plane with the combined instrumentation: antero-lateral and fixateur interne. After removing the anterior screw plate system maximum movements, in all segments for flexion/extension and lateral bending, bisegmentally and for the upper segment in axial rotation, were less than ROM values measured with the anterior system only. With respect to these findings a combined ventro-dorsal stabilization procedure should be considered for ligamentous disruptions of the posterior column in combination with A-fractures in the thoracolumbar junction.

Biomechanical and immunohistochemical analysis of high hydrostatic pressure-treated Achilles tendons.

BACKGROUND: Reconstruction of bone defects caused by malignant tumors is carried out in different ways. At present, tumor-bearing bone segments are devitalized mainly by extracorporeal irradiation or autoclaving, but both methods have substantial disadvantages. In this regard, high hydrostatic pressure (HHP) treatment of the bone is a new, advancing technology that has been used in preclinical testing to inactivate normal cells and tumor cells without altering the biomechanical properties of the bone. The aim of this study was to examine the biomechanical and immunohistochemical properties of tendons after exposure to HHP and to evaluate whether preservation of the bony attachment of tendons and ligaments is possible. METHODS: For this, 19 paired Achilles tendons were harvested from both hindlimbs of 4-month-old pigs. After preparation, the cross-sectional area of each tendon was determined by magnetic resonance imaging (MRI). For each animal, one of the two tendons was taken at random and exposed to a pressure of 300 MPa (n = 9) or 600 MPa (n = 10). RESULTS: The contralateral tendon served as an untreated control. The biomechanical properties of the tendons remained unchanged with respect to the tested parameters: Young's modulus (MPa) and tensile strength (MPa). This finding is in line with immunohistochemical labeling results, as no difference in the labeling pattern of collagen I and versican was observed when comparing the HHP group (at 600 MPa) to the untreated control group. CONCLUSIONS: We anticipate that during orthopedic surgery HHP can serve as a novel, promising methodical approach to inactivate Achilles tendon and bone cells without altering the biomechanical properties of the tendons. This should allow one to preserve the attachment of tendon and ligaments to the devitalized bone and to facilitate functional reconstruction.

[Biomechanical properties of articular cartilage after high hydrostatic pressure treatment]. / Biomechanische Eigenschaften von Gelenkknorpel nach hydrostatischer Hochdruckbehandlung.

AIM: Reconstruction of bone defects due to malignant tumors can be realized by several methods. Up to now, two methods, irradiation and autoclaving, are available for extracorporeally devitalizing resected tumor-bearing osteochondral segments. Previous investigations have shown that human normal and tumor cells in culture were irreversibly impaired when subjected to extracorporeal high hydrostatic pressure (HHP) of 350 MPa. The aim of this study was to examine the biomechanical and immunohistochemical properties of cartilage after exposure to HHP MATERIALS AND METHODS: Osteochondral segments of bovine femoral condyles were exposed to pressure of 300 and 600 MPa (n=20 each). Biomechanical and biological properties of untreated and treated segments were evaluated by repetitive ball indention testing and immunohistochemical labelling aggrecan, link protein and collagen II. The contralateral segments served as untreated control. RESULTS: No significant alterations concerning stiffness and relaxation of osteochondral segments even after 600 MPa were observed. Immunohistochemically, staining was positive in all cases and no differences in the labeling pattern of proteoglycanes were observed between untreated and HHP-treated specimens. CONCLUSION: These findings give hope that HHP eventually will be used as a new gentle way of treating resected cartilage and bone without alteration of biomechanical properties to inactivate tumor cells in order to allow autologous reimplantation.

Fractures of cementless thin-walled cups.

Cementless threaded cups are commonly used in total hip arthroplasty. A method to match the elasticity of the cup to that of natural bone is reducing the wall thickness of the implant. Despite good results with this philosophy of implantation, we recently observed 3 cases of implant fractures after the implantation of such thin-walled cups called "Bicon." We performed a thorough analysis of 1 of these cases including a histological examination and a technical failure analysis (including scanning electron microscopy) to establish the chronology and cause of the failure. Multiple fractures of the thin-walled metal-back cup were determined to be caused by fatigue. We have concluded that due to the specific material and design parameters of this cup, under adverse circumstances such as the lack of primary or secondary osteointegration, there is a risk of failure due to fatigue.

Biomechanical investigation of the effect of high hydrostatic pressure treatment on the mechanical properties of human bone.

Several methods are available for reconstruction of bone defects due to malignant tumors. To extracorporally devitalize resected tumor-bearing bone segments two methods, that is, extracorporal irradiation or autoclaving, are available up to now. However, both methods have substantial disadvantages like decrease of bone's mechanical strength. To develop an alternative method for tumor inactivation in skeletal segments, high hydrostatic pressure (HHP) was applied. Previous investigations have shown that human normal and tumor cell lines as well as tumor-afflicted human bone specimens were irreversibly damaged at 350 MPa when subjected to HHP. This study was aimed to examine the alterations of biomechanical properties of human bone after exposure to HHP. Trabecular and cortical bone specimens were harvested from six pair of fresh-frozen human cadaveric femora. The bone specimens from one side were exposed to different pressure values of 300 or 600 MPa over 10 min. Bone samples from the contralateral sites were used as untreated controls. Biomechanical properties were investigated by a quasi-static compression test for trabecular specimens and by a quasi-static four-point bending test for cortical specimens, respectively. Biomechanical properties of the cortical and trabecular bone did not decrease after exposure to 300 MPa regarding the testing parameters Young's modulus and ultimate strength (200.7 +/- 38.7 MPa for HHP treated cortical bone versus 186.5 +/- 34.3 MPa for the untreated control group). After pressure treatment at 600 MPa Young's modulus and ultimate strength respectively remained almost unchanged in trabecular bone and were reduced about 15% in cortical bone (p < 0.001 and p =0.002, respectively). We anticipate that in orthopedic surgery HHP can serve as a novel, promising methodical approach for tumor cell inactivation, which occurs at pressure levels of about 300 MPa. Thereby immediate reimplantation of treated bone segments by preservation of the essential biomechanical properties of bone could become possible. Even after HHP treatment at 600 MPa the strength of bone only decreases up to 15%.

Pitfalls in the use of acetabular reinforcement rings in total hip revision.

INTRODUCTION: For the reconstruction of acetabular bone defects different types of acetabular reinforcement rings are being used. In clinical practice, these implants showed to some extent good long-term results. In the present work pitfalls and complications after the implantation of acetabular reinforcement rings as well as possible solutions are being discussed. MATERIAL AND METHODS: In the first case recurrent dislocation was caused by the malposition of the acetabular component with an impingement of the protruding bone cement and the anterior edge of the acetabular ring as well as muscle insufficiency as a result of the shortening of the leg length. The second case revealed an impingement of the iliopsoas tendon due to a protruding acetabular reinforcement ring. During revision, bone cement was used to smoothen the protruding anterior edge of the acetabular reconstruction ring in order to obtain a relieved sliding of the tendon. Furthermore, we report on the case of a delayed neuropathy of the sciatic nerve after reconstruction of the acetabulum with an acetabular reinforcement ring. RESULTS: Intraoperatively an impingement of the sciatic nerve at the protruding dorsal edge of the acetabular reinforcement ring and the surrounding scar tissue was found. In a further case an aseptic loosening of an acetabular reinforcement ring caused the formation of an excessive granuloma with a large intrapelvic portion. The granuloma led to persisting senso-motoric deficits of the femoral nerve. In summary, based on these clinical cases possible pitfalls, associated with the use of acetabular reinforcement rings, are shown. The mal-positioning and the intra-operative re-shaping of the implant by the surgeon are pointed out as the substantial factors for the occurrence of an impingement phenomenon and total hip instability. Furthermore, in case of an adequate orientation of the cemented polyethylene insert an improper position of the acetabular ring which results in protruding edges has to be considered as a cause of a prosthetic impingement. CONCLUSION: The cases presented emphasize the necessity of prevention of such pitfalls intra-operatively as well as accurate analysis of implant failures. Furthermore, they suggest explicit preoperative planning before deciding on the strategy of revision surgery of acetabular reinforcement rings.