Tilted implants for the rehabilitation of edentulous jaws: a systematic review.

PURPOSE: The aim of this review was to evaluate the survival rate of upright and tilted implants supporting fixed prosthetic reconstructions for the immediate rehabilitation of partially and fully edentulous jaws, after at least 1 year of function. MATERIALS AND METHODS: An electronic search of databases plus a hand search on the most relevant journals up to December 2009 was performed. The articles were selected using specific inclusion criteria, independent of the study design. RESULTS: The literature search yielded 347 articles. A first screening based on the title and abstract identified 25 eligible studies. After full-text review of these studies, 10 articles were selected for analysis. Seven were prospective single-cohort studies and three had a retrospective design. A total of 462 patients have been rehabilitated with 470 immediately loaded prostheses (257 in the maxilla, 213 in the mandible), supported by a total of 1,992 implants (1,026 upright and 966 tilted). Twenty-five implants (1.25%) failed in 20 patients within the first year. All failures except one occurred in the maxilla. No significant difference in failure rate was found between tilted and upright implants, nor between maxillary and mandibular implants. No prosthesis failure was reported. Limited peri-implant bone loss was reported with no difference between upright and tilted implants. Full patients' satisfaction for function, phonetics, and esthetics was reported in three studies, based on questionnaires. CONCLUSIONS: The use of tilted implants to support immediately loaded fixed prostheses for the rehabilitation of edentulous jaws can be considered a predictable technique, with an excellent prognosis in the short-medium term. However, randomized long-term trials are needed to determine the efficacy of this surgical approach.

In vivo preclinical evaluation of the influence of osteoporosis on the anchorage of different pedicle screw designs.

We investigate the anchorage of pedicle screws with different surface treatments in osteoporotic bone. Eight ewes were divided into two groups of four animals each: four sheep underwent bilateral ovariectomy (OVX Group), whereas the operation was simulated in the remaining group (SHAM Group). Eighteen months after the first operation, the Dynesys(®) System was fitted to the sheep using pedicle screws with three different surface treatments: untreated, rough blasted (uncoated) and bioactive coated (bioactive). Uncoated screws showed a significantly higher bone ingrowth value compared with the untreated screws in the OVX group (9.3%, p < 0.005) and a significantly lower bone ingrowth value in the SHAM group (-11.0%, p < 0.05). Furthermore, the bioactive pedicle screws had a significant lower bone ingrowth value than the untreated screws in the SHAM group (-12.1%, p < 0.05). These results suggest that both tested surface treatments of pedicular screws may provide an advantage in terms of bone quality and osseointegration, when implanted in osteoporotic vertebrae.

Spine stability after implantation of an interspinous device: an in vitro and finite element biomechanical study.

OBJECT: Interspinous devices are widely used for the treatment of lumbar stenosis. The DIAM spinal stabilization system (Medtronic, Ltd.) is an interspinous implant made of silicone and secured in place with 2 laces. The device can be implanted via posterior access with the sacrifice of the supraspinous ligament (SSL) or via lateral access with preservation of the ligament. The aim of the present work was to evaluate the role of the laces, the SSL, and the device size and positioning to determine the device's ability in reducing segmental lordosis and in stabilizing motion. METHODS: Biomechanical tests were performed in flexion and extension on 8 porcine spines implanted with the DIAM either with or without the laces and the SSL. A finite element model of the human L4-5 spine segments was also created and used to test 2 sizes of the device implanted in 2 different positions in the anteroposterior direction. RESULTS: Implantation of the DIAM induced a shift toward kyphosis in the neutral position. Laces, the SSL, and device size and placement had a significant influence on the neutral position, the stiffness of the implanted spine, and the positions of the instantaneous centers of rotation. CONCLUSIONS: The shift of the neutral position toward kyphosis may be beneficial in reducing symptoms of spinal stenosis such as radicular pain, sensation disturbance, and loss of strength in the legs. The authors recommend preservation of the SSL and the use of the fixation laces, given their relevant mechanical role. Choosing the proper device size and placement should be achieved by using a correct surgical technique.

Lumbar total disc replacement: a numerical study.

AIM: The aim of this study was to evaluate the biomechanical effects of the Maverick(R) disc prosthesis at the implanted and adjacent level by the finite element (FE) method. MATERIALS AND METHODS: A 3D FE model of the L3-L5 segment was built. To simulate the different physiological movements (flexion, extension, lateral bending, axial rotation) pure moments of 10 Nm were applied. To evaluate the effect of the prosthesis, a 3D model of the device was built and inserted in the L3-L5 model. The ROMs obtained with the intact model were imposed as maximal rotations to the instrumented model, therefore implementing the Panjabi hybrid protocol. RESULTS: Increased ROMs at the implanted level and reduced ROMs at the adjacent level were predicted. A similar moment-rotation behavior was calculated after simulation of prosthesis insertion. No significant effect was predicted in terms of von Mises stress at the adjacent level after implantation of the prosthesis. CONCLUSIONS: Within the limitations of the models, the numerical results of this study predicted a preserved kinematics and stress at the adjacent segment, after insertion of the prosthesis.

Comparison of tilted versus nontilted implant-supported prosthetic designs for the restoration of the edentuous mandible: a biomechanical study.

PURPOSE: The purpose of the study was to evaluate, using finite element analysis, the stress patterns induced in cortical bone by three distinct implant-supported prosthetic designs. MATERIALS AND METHODS: The first two models consisted of a prosthesis supported by four implants, the distal two of which were tilted, with different cantilever lengths (5 mm and 15 mm). The third design consisted of a prosthesis supported by five conventionally placed implants and a 15-mm cantilever. RESULTS: In the tilted model with 5-mm cantilever and in the nontilted model, the maximum value of compressive stress (-18 MPa) was found near the cervical area of the distal implant. Higher values for compressive stress were predicted near the cervical area of the distal implant in the tilted model with a 15-mm cantilever, as compared to the tilted model with the 5-mm cantilever. For the tilted model with the 5-mm cantilever, peak values of tensile stress were predicted near the cervical area of both the distal (1.25 MPa) and the mesial implants (2.5 MPa). For the nontilted model, the peak value was found near the cervical area of the in-between implant (5 MPa). For the tilted model with 15-mm cantilever, tensile stress values were higher than in the tilted model with 5-mm cantilever. CONCLUSIONS: No significant difference in stress patterns between the tilted 5-mm and the nontilted 15-mm configuration was predicted. The tilted configuration with a 15-mm cantilever was found to induce higher stress values than the tilted configuration with a 5-mm cantilever.

A finite element analysis of tilted versus nontilted implant configurations in the edentulous maxilla.

The aim of this study was to evaluate stress patterns at the bone-implant interface of tilted versus nontilted implant configurations in edentulous maxillae using finite element models of two tilted and one nontilted configuration. Analysis predicted the maximum absolute value of principal compressive stress near the cervical area of the distal implant for all models. The tilted configurations showed a lower absolute value of compressive stress compared with the nontilted, indicating a possible biomechanical advantage in reducing stresses at the bone-implant interface.

Anterior cervical fusion: a biomechanical comparison of 4 techniques. Laboratory investigation.

OBJECT: Cervical instrumented fusion is currently performed using several fixation methods. In the present paper, the authors compare the following 4 implantation methods: a stand-alone cage, a cage supplemented by an anterior locking plate, a cage supplemented by an anterior dynamic plate, and a dynamic combined plate-cage device. METHODS Four finite element models of the C4-7 segments were built, each including a different instrumented fixation type at the C5-6 level. A compressive preload of 100 N combined with a pure moment of 2.5 Nm in flexion, extension, right lateral bending, and right axial rotation was applied to the 4 models. The segmental principal ranges of motion and the load shared by the interbody cage were obtained for each simulation. RESULTS: The stand-alone cage showed the lowest stabilization capability among the 4 configurations investigated, but it was still significant. The cage supplemented by the locking plate was very stiff in all directions. The 2 dynamic plate configurations reduced flexibility in all directions compared with the intact case, but they left significant mobility in the implanted segment. These configurations were able to share a significant part of the load (up to 40% for the combined plate-cage) through the posterior cage. The highest risk of subsidence was obtained with the model of the stand-alone cage. CONCLUSIONS: Noticeable differences in the results were detected for the 4 configurations. The actual clinical relevance of these differences, currently considered not of critical importance, should be investigated by randomized clinical trials.

Design concepts in lumbar total disc arthroplasty.

The implantation of lumbar disc prostheses based on different design concepts is widely accepted. This paper reviews currently available literature studies on the biomechanics of TDA in the lumbar spine, and is targeted at the evaluation of possible relationships between the aims of TDA and the geometrical, mechanical and material properties of the various available disc prostheses. Both theoretical and experimental studies were analyzed, by a PUBMED search (performed in February 2007, revised in January 2008), focusing on single level TDA. Both semi-constrained and unconstrained lumbar discs seem to be able to restore nearly physiological IAR locations and ROM values. However, both increased and decreased ROM was stated in some papers, unrelated to the clinical outcome. Segmental lordosis alterations after TDA were reported in most cases, for both constrained and unconstrained disc prostheses. An increase in the load through the facet joints was documented, for both semi-constrained and unconstrained artificial discs, but with some contrasting results. Semi-constrained devices may be able to share a greater part of the load, thus protecting the surrounding biological structure from overloading and possible early degeneration, but may be more susceptible to wear. The next level of development will be the biomechanical integration of compression across the motion segment. All these findings need to be supported by long-term clinical outcome studies.

Biomechanical studies on cervical total disc arthroplasty: a literature review.

BACKGROUND: Many models of cervical disc prostheses are currently commercially available or under clinical trial, and are based on several design concepts and built employing different materials. This paper is targeted to the understanding of the possible relationships between the geometrical, mechanical and material properties of the various cervical disc prostheses and the restoration of a correct biomechanics of the implanted spine. METHODS: Papers about cervical disc arthroplasty, based on ex vivo testing, mathematical models, and radiographic measurements, were included in the present review. FINDINGS: Although disc arthroplasty was found to be generally able to preserve a nearly physiological motion in the cervical spine, several alterations in the spine biomechanics due to disc arthroplasty were reported in the literature. An increase of the range of motion at the implanted level was observed in some ex vivo studies. Loss of mobility and heterotopic ossification were reported in radiographic investigations. Loss of lordosis at the implanted level was detected as well. Wear debris was usually found very limited and device stability seemed not to be an actual problem. INTERPRETATION: The possible relationships between the observed alterations in the spine biomechanics after disc arthroplasty and the properties of the various cervical disc prostheses have been reviewed. Clinical studies are needed to assess the validity of the considerations inferred from the biomechanical papers.

Cervical spine biomechanics following implantation of a disc prosthesis.

This study presents a finite element model of the C4-C7 segment in healthy conditions and after implantation of a disc prosthesis at a single level, in order to investigate of the influence of disc arthroplasty on the biomechanics of the cervical spine. A nonlinear finite element model of the C4-C7 segment in intact conditions was developed and run in flexion and extension. A detailed model of the Bryan disc prosthesis, including contacts between the different components of the device, was built and positioned at C5-C6. The calculated segmental motion resulted preserved after disc arthroplasty, with respect to the model of the intact spine, in both flexion and extension. A general preservation of the forces transmitted through the facet joints was obtained; a minor force increase at the implanted level was detected. The analysis of the instantaneous centers of rotation (ICR) in flexion-extension showed the preservation of a physiological kinematics. The mechanical behaviour showed an asymmetry between flexion and extension, probably due to the removal of the anterior longitudinal ligament and the anterior part of the annulus fibrosus, and the preservation of the posterior structures. In general, the disc prosthesis showed to be able to reproduce a nearly physiological motion. However, other important mechanical aspects, such as the possible micromotion at the bone-implant interface and the possible degenerative conditions of the spine, need to be evaluated before drawing a conclusion about total disc arthroplasty from an engineering point of view.

Bi-directional distraction in the treatment of micro-orbitism: a case report.

INTRODUCTION: Correction of micro-orbitism, resulting from clinical or congenital anophthalmia, has traditionally been performed by multiple segmentation of the orbital rim, orbital expanders and orbital conformers. Although distraction osteogenesis is a widely employed surgical approach in the treatment of patients with bony malformations, it has not been employed to enlarge micro-orbits. MATERIAL AND METHODS: The present article describes the development of a new bi-directional orbital distractor to treat a 17-year-old patient affected by micro-orbitism, caused by clinical anophthalmia. The deformity required an internal device to expand and to pull the orbit laterally. Surgical planning and device design were performed by means of patient-specific finite element analysis and a stereolithography model. The surgery consisted of a uni-lateral orbito-malar osteotomy performed via coronal and intraoral access. A 7-day-latency period was observed. The consolidation phase was chosen as six months. RESULTS: At the end of the distraction process, symmetry of the malar bones and orbital roofs was achieved. During removal of the device, newly formed bone was found at the original osteotomy and distraction gaps. CONCLUSION: The reported clinical case suggests that distraction osteogenesis can be a useful procedure for enlargement of micro-orbits. Despite this, a number of questions need to be addressed by long-term follow-up and careful study of future cases.