Einsatz der Navigation beim Oberfl�chenersatz des H�ftgelenks

Computer-assisted navigation in Birmingham hip resurfacing: A case report

Component malpositioning during Birmingham hip resurfacing increases the risk for component wear, metallosis, component loosening, and the likelihood of dislocation and revision surgery. Computer-assisted navigation can increase the accuracy to which components are placed, and the utilization of this technology in Birmingham hip resurfacing is increasing. The present report summarizes the accuracy of acetabular component positioning in a Birmingham hip resurfacing case utilizing navigation. Intraoperative C-arm fluoroscopy following the use of the navigation tool confirmed excellent seating, positioning, and stability of the acetabular component. In addition, post-operative antero-posterior radiographs confirmed device accuracy and revealed a stable joint with no evidence of acetabular loosening or femoral fracture. Computer-assisted navigation may therefore be an effective tool to improve the accuracy of component positioning during Birmingham hip resurfacing.

Biomechanical optimization of subject-specific implant positioning for femoral head resurfacing to reduce fracture risk

Peri-prosthetic femoral neck fracture after femoral head resurfacing can be either patient-related or surgical technique-related. The study aimed to develop a patient-specific finite element modelling technique that can reliably predict an optimal implant position and give minimal strain in the peri-prosthetic bone tissue, thereby reducing the risk of peri-prosthetic femoral neck fracture. The subject-specific finite element modelling was integrated with optimization techniques including design of experiments to best possibly position the implant for achieving minimal strain for femoral head resurfacing. Sample space was defined by varying the floating point to find the extremes at which the cylindrical reaming operation actually cuts into the femoral neck causing a notch during hip resurfacing surgery. The study showed that the location of the maximum strain, for all non-notching positions, was on the superior femoral neck, in the peri-prosthetic bone tissue. It demonstrated that varus positioning resulted in a higher strain, while valgus positioning reduced the strain, and further that neutral version had a lower strain.

Computer-assisted hip resurfacing planning using Lie group shape models

INTRODUCTION

Hip resurfacing is a surgical option for osteoarthritis young and active patients. Early failures has been reported due to improper implant placement. Computer-assisted surgery is a promising avenue for more successful procedures.

PURPOSE

This paper presents a novel automatic surgical planning for computer-assisted hip resurfacing procedures. The plan defined the femoral head axis that was used to place the implant. The automatic planning was based on a Lie group statistical shape model.

METHODS

A statistical shape model was constructed using 50 femurs from osteoarthritis patients who underwent computer-assisted hip resurfacing. The model was constructed using product Lie groups representation of shapes and nonlinear analysis on the manifold of shapes. A surgical plan was drawn for the derived base shape. The base shape was transformed to 14 femurs with known manual plans. The transformed base plan was used as the computed plan for each femur. Both actual and computed plans were compared.

RESULTS

The method showed a success by computing plans that differ from the actual plans within the surgical admissible ranges. The minimum crossing distance between the two plans had a mean of 0.75 mm with a standard deviation of 0.54 mm. The angular difference between the two plans had the mean of 5.94° with a standard deviation of 2.145.94°.

CONCLUSION

Product Lie groups shape models were proved to be successful in automatic planning for hip resurfacing computer-assisted surgeries. The method can be extended to other orthopedic and general surgeries.

Our midterm results of the Birmingham hip resurfacing with and without navigation

We reviewed 148 consecutive hip resurfacings in order to assess the clinical outcomes of the BHR at midterm follow-up and to compare the accuracy of the navigation in the positioning of femoral component. We retrospectively analyzed 85 hips using the conventional jig to implant the femoral component and we prospectively followed 63 hips operated on by navigation. At a mean follow-up of 50.54 months, the Harris hip score improved significantly from 44.66 preoperatively to 98.45 postoperatively without any differences between the groups. Radiologically, we classic navigated group. Our clinical outcomes are excellent at midterm follow-up and the navigation definitely improves the implant position in both planes.

[Imageless computer navigation of hip resurfacing arthroplasty]

OBJECTIVE

Precise implantation of hip resurfacing arthroplasty by imageless computer navigation. Hence a malalignment of the femoral component, leading to early loss of the implant, can safely be avoided.

INDICATIONS

Coxarthrosis in patients with normal bone mineral density; only minor deformity of the femoral head that enables milling around the femoral neck without notching.

CONTRAINDICATIONS

Osteoporosis; large necrosis of the femoral head; metal allergy; small acetabular seat and corresponding wide femoral neck, leading to needless acetabular bone loss; pregnancy, lactation.

SURGICAL TECHNIQUE

Hip joint exposure by a standard surgical approach, bicortical placement of a Schanz screw for the navigation array in the lesser trochanter. Referencing of the epicondyles, the four planes around the femoral neck and head by use of the navigation pointer. Planning of the desired implant position on the touchscreen of the navigation device; a guide wire is inserted into the femoral head and neck using the navigated drill guide; navigated depth drilling is performed. The femoral head is milled using the standard instruments. The acetabular bone stock is prepared with the conventional instrumentation; high-viscosity cement is finger-packed on the reamed head and the femoral component is inserted. Hammer blows should be avoided to prevent microfractures. Verification of the implant position by the navigation device; displacement of the Schanz screw; joint reposition and closure of the wound.

POSTOPERATIVE MANAGEMENT

Standard postoperative management after hip arthroplasty.

RESULTS

The comparison of 40 navigated and 32 conventionally implanted ASR prostheses resulted in a significant reduction of outliers by use of computer navigation (navigated procedures: one outlier, conventional procedure: nine outliers; p<0.001). Accuracy of the navigation device was tested by analysis of planned and verified implant position: CCD angle accuracy was 1 degrees , antetorsion accuracy was 1 degrees , and offset accuracy was 1.5 mm. An ongoing computed tomography-based anatomic study proved a varus-valgus accuracy of the navigation device of 1 degrees .

Computer-assisted hip resurfacing using individualized drill templates

The goal of this study was to investigate whether individualized templates can provide an accurate and reliable computer-assisted system for femoral component placement during hip resurfacing. A consecutive series of 45 patients were examined. Using a 3-dimensional computer model of the femur, the drill trajectory for the central pin of the stem was planned. A surface-matched plastic drilling template was created using a rapid prototyping machine. This patient-specific drill guide was intraoperatively positioned on the patient anatomy, the central pin was drilled into the femoral neck, and the accuracy of the placement with respect to the planned central pin alignment was measured. With mean deviation between planned and actual central pin alignment of 1.14 degrees in varus and 4.49 degrees in retroversion, individualized templates were as accurate as conventional computer-assisted hip resurfacing.

Fluoroscopic navigation system for hip surface replacement

Metal-on-metal hip resurfacing arthroplasties represent an alternative to total hip arthroplasties for young and active patients, enabling the preservation of intact femoral bone and therefore improving the prognosis for future hip joint replacements. Follow-up studies have shown that the main reasons for early implant failure are mal-orientation of the implant stem in relation to the femoral neck axis, and notching of the femoral neck during femoral head preparation, as well as by exposed cancellous bone after implantation. A computer-assisted planning and navigation system for the implantation of femoral hip resurfacing implants has been developed which supports the surgeon during intraoperative fluoroscopy-based planning and navigation of implant positioning. This paper presents the results of a cadaver study performed to evaluate the system's functionality and accuracy.

Evaluation of a new fluoroscopy-based navigation system in the placement of the femoral component in hip resurfacing

Prosthesis-specific mechanical alignment instruments for the precise and reproducible positioning of the femoral component constitute one of the major improvements in modern hip resurfacing prostheses. However, mechanical failure of the femoral component is mostly attributable to the surgical technique, and in particular to notching of the femoral neck. In order to evaluate a novel computer-assisted fluoroscopy-based planning and navigation system, six DUROMTM hip resurfacing prostheses were implanted into artificial femurs by means of computer-assisted fluoroscopy-based navigation and prosthesis-specific mechanical alignment instruments. Subsequently, the planning and navigation system was tested within the scope of a cadaver study on three fixed whole-body preparations (six femurs). The average difference between planned and actual angle of the prosthesis was 0±0.7° for fluoroscopy-based navigation versus 6.5±7.8° for the in-vitro use of the prosthesis-specific mechanical alignment instruments, and 1±1.4° for fluoroscopic navigation in the cadaver study. The average discrepancy between planned and actual anterior offset was −1.2±1.2 mm versus 0.8±4 mm, and 0.3±2.2 mm in the cadaver study, and the time required for the total of five planning and navigation steps was 17.2±1.5 min versus 14±0.8 min and 20.2±2.5 min respectively. No notching of the femoral neck occurred under fluoroscopy nor under conventional treatment. During in-vitro studies, use of the computer-assisted fluoroscopy-based planning and navigation system resulted in enhanced accuracy compared with conventional prosthesis-specific mechanical alignment instruments. The system has yielded initial promising results within the scope of the cadaver study.

Accuracy of computer-assisted navigation for femoral head resurfacing decreases in hips with abnormal anatomy

Computer-assisted navigation systems for hip resurfacing arthroplasty are designed to minimize the chance of implant malposition. However, there is little evidence computer navigation is useful in the presence of anatomical deformity. We therefore determined the accuracy of an image-free resurfacing hip arthroplasty navigation system in the presence of a pistol grip deformity of the head and femoral neck junction and of a slipped upper femoral epiphysis deformity. We constructed an artificial phantom leg from machined aluminum with a simulated hip and knee. The frontal and lateral plane implant-shaft angles for the guide wire of the femoral component reamer were calculated with the computer navigation system and with an electronic caliper combined with micro-CT. There was a consistent disagreement between the navigation system and our measurement system in both the frontal plane and lateral plane with the pistol grip deformity. We found close agreement only for the frontal plane angle calculation in the presence of the slipped upper femoral epiphysis deformity, but calculation of femoral head size was inaccurate. The use of image-free navigation for the positioning of the femoral component appears questionable in these settings.

Femoral component positioning in hip resurfacing with and without navigation

Early failures after hip resurfacing often are the result of technical errors in placing the femoral component. We asked whether image-free computer navigation decreased the number of outliers compared with the conventional nonnavigated technique. We retrospectively compared 51 consecutive hip resurfacings performed using image-free computer navigation with 88 consecutive hip resurfacings performed without navigation. Patient demographics were similar. There were no differences in the average native femoral neck-shaft angles, planned stem-shaft angles, or postoperative stem-shaft angles. However, when the postoperative stem-shaft angle was compared with the planned stem-shaft angle, there were 33 patients (38%) in the nonnavigated group with a deviation greater than 5 degrees in contrast to none in the navigated group. Notching was present in four patients in the nonnavigated group and none in the navigated group. The average operative time was 111 minutes for the navigated group and 105 minutes for the nonnavigated group. Image-free navigation decreased the number of patients with potentially undesirable implant placements.

LEVEL OF EVIDENCE

Level III, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Is a valgus position of the femoral component in hip resurfacing protective against spontaneous fracture of the femoral neck?: a biomechanical study

The aim of our study was to investigate whether placing of the femoral component of a hip resurfacing in valgus protected against spontaneous fracture of the femoral neck. We performed a hip resurfacing in 20 pairs of embalmed femora. The femoral component was implanted at the natural neck-shaft angle in the left femur and with a 10 degrees valgus angle on the right. The bone mineral density of each femur was measured and CT was performed. Each femur was evaluated in a materials testing machine using increasing cyclical loads. In specimens with good bone quality, the 10 degrees valgus placement of the femoral component had a protective effect against fractures of the femoral neck. An adverse effect was detected in osteoporotic specimens. When resurfacing the hip a valgus position of the femoral component should be achieved in order to prevent fracture of the femoral neck. Patient selection remains absolutely imperative. In borderline cases, measurement of bone mineral density may be indicated.

Accuracy of a computer-assisted navigation system in resurfacing hip arthroplasty

We developed a method to assess the accuracy of an image-free resurfacing hip arthroplasty navigation system in a proximal femur with normal and abnormal anatomy. A phantom lower limb allowed deformation in varus/valgus and ante-retroversion. At specific points during the simulated surgical procedure, information was compared between a digital caliper and the computer navigation system angular measurements. Repeated serial tests were undertaken. In the setting of normal anatomical alignment of the proximal femur, the mean error of the system characterised as the difference between the measured computer navigation and caliper angles was 0.6 degrees in the frontal plane and 3.4 degrees in the lateral plane. In the setting of abnormal anatomical alignment, the mean error was 0.4 degrees in the frontal plane and 2.1 degrees in the lateral plane. This is the first study designed to assess the accuracy of a femoral navigation system for resurfacing hip arthroplasty in normal settings and in the presence of angular deformity. The study demonstrates satisfactory in-vitro accuracy.

Imageless computer navigation for placement of the femoral component in resurfacing arthroplasty of the hip

We have investigated the accuracy of placement of the femoral component using imageless navigation in 100 consecutive Birmingham Hip Resurfacings. Pre-operative templating determined the native neck-shaft angle and planned stem-shaft angle of the implant. The latter were verified post-operatively using digital anteroposterior unilateral radiographs of the hip. The mean neck-shaft angle determined before operation was 132.7 degrees (118 degrees to 160 degrees ). The mean planned stem-shaft angle was a relative valgus alignment of 9.7 degrees (SD 2.6). The stem-shaft angle after operation differed from that planned by a mean of 2.8 degrees (SD 2.0) and in 86% of cases the final angle measured within +/- 5 degrees of that planned. We had no instances of notching of the neck or varus alignment of the implant in our series. A learning curve was observed in the time taken for navigation, but not for accurate placement of the implant. Navigation in hip resurfacing may afford the surgeon a reliable and accurate method of placement of the femoral component.

Does computer-assisted surgery improve accuracy and decrease the learning curve in hip resurfacing? A radiographic analysis

BACKGROUND

Hip resurfacing is a technically demanding procedure in which accurate positioning of the femoral component is critical to the avoidance of early implant failures. The purpose of this study was to assess the accuracy of computer-assisted placement of the femoral component and to evaluate the impact of computer-assisted surgery on the learning curve associated with this procedure.

METHODS

The accuracy of positioning the femoral component was analyzed radiographically in hips undergoing resurfacing procedures performed by surgeons assigned to four different study groups: Group 1, in which the operations were performed with use of computer-assisted surgery by a fellowship-trained surgeon who was experienced in performing resurfacing arthroplasty (surgical experience, more than 250 hip resurfacings); Group 2, in which the operations were performed with use of computer-assisted surgery by senior residents who were inexperienced in performing resurfacing arthroplasty and who were closely supervised by faculty; Group 3, in which the operations were performed with use of conventional instruments by fellowship-trained faculty members; and Group 4, in which the operations were performed with use of computer-assisted surgery by a lesser experienced fellowship-trained faculty member (surgical experience, more than forty but less than seventy-five hip resurfacings) from Group 3.

RESULTS

The range of error in varus or valgus angulation that was observed for navigated procedures was 6 degrees in Group 1, 7 degrees in Group 2, and 5 degrees in Group 4. Compared with the preoperative neck-shaft angle value, the mean postoperative stem-shaft angle value increased by a mean of 4.7 degrees in Group 1, 7.2 degrees in Group 2, 6.5 degrees in Group 3, and 11.6 degrees in Group 4. When compared with the use of standard instrumentation, the use of computer-assisted surgery reduced the number of outliers and facilitated valgus insertion.

CONCLUSIONS

In the present study, computer-assisted surgery resulted in improved accuracy and precision in positioning the femoral component. In addition, computer-assisted surgery led to a reduction in the length of the learning curve for beginners in hip resurfacing and improved the surgeon's ability to perform this procedure safely.

[Fluoroscopic navigation versus conventional manual positioning of the femoral component for hip resurfacing: first experimental trial]

BACKGROUND

The most essential improvement of modern hip resurfacing arthroplasty is the metal-on-metal bearing as well as the integration of a procedure for the exact and repeatable positioning of the femoral component through a specific mechanical alignment instrument. Nevertheless, the main reasons for early implant failure are mal-positioning of the femoral component and notching of the femoral neck during femoral head preparation.

MATERIALS AND METHODS

In the context of an in vitro study, in each case six DUROM-Hip resurfacing prostheses were implanted in artificial femora with the prosthesis-specific mechanical alignment instrument, as well as under navigation control. The aim of the study was to evaluate the functionality and accuracy of a computer-assisted planning and navigation system on the basis of a navigation module library from Surgitaix AG (Aachen, Germany), as well as a comparison with the prosthesis-specific mechanical alignment instrument.

RESULTS

The main angulation error between planning and navigation of the stem-shaft angle was 0.2+/-1.2 degrees for the navigation system and 6.5+/-4.1 degrees for the mechanical alignment instrument, the main anterior offset error was 1.2+/-1.2 mm vs. -0.83+/-4.1 mm. The mean time for all five planning and navigation steps was 17+/-1.2 min vs. 14+/-0.8 min. The main distance error between planning and navigation was 1.9+/-0.6 mm for the navigation system, and 5.3+/-2.4 mm for the mechanical alignment instrument. Femoral notching was not observed for navigational or conventional positioning.

CONCLUSION

The computer-assisted fluoroscopic planning and navigation system for hip resurfacing showed, within the scope of this in vitro study, first promising experiences. The system approves a practicable planning with a high accuracy in implementation. Nevertheless, the potential benefit has to be evaluated in further clinical studies, especially from the perspective of a possible integration of this navigation system into the clinical workflow. Further studies should consider a fluoroscopy-assisted range of motion assessment under consideration of an additional cup-module to enhance the postoperative range of motion after hip resurfacing procedures.

Intraoperative navigation in hip surface arthroplasty: a radiographic comparative analysis study

BACKGROUND

The goal of the current prospective randomised radiological study was to determine the accuracy of conventional and computer-assisted femoral component implantation in surface arthroplasty (SRA).

METHODS

We analysed on standard radiographs the femoral component positions after 30 conventional instrumented (Group 1) and 30 navigated (Group 2) SRA femoral components. We evaluated: varus or valgus orientation, horizontal femoral offset and translation of the component.

RESULTS

The tendency to implant the femoral component in mild valgus position (2.8 degrees in Group 1 compared to 2.1 degrees in Group 2), more distally and ventrally in the femoral neck (in Group 1) and with femoral off-set increase (4.8 mm in Group 1 compared to 3.4 mm in Group 2) was found.

CONCLUSIONS

The navigation system enables a more accurate insertion of the femoral component.

Imageless navigation of hip resurfacing arthroplasty increases the implant accuracy

Surface arthroplasty of the hip is increasingly popular. Optimising the position of the femoral component is essential to avoid early implant failures such as femoral neck fractures. Sixty hip surface replacements were retrospectively analysed. In 30 patients imageless navigation was used, and 30 patients were operated upon using conventional jigs. Accuracy, implant position, operating time, and complications have been recorded. The navigation device improved the implant position with high accuracy. Implant-shaft angles <130 degrees and uncovered cancellous bone of the superior femoral neck could be safely avoided. After a significant learning curve, navigation took 15 minutes longer than conventional implantation. No complications were found in either group. Computer-assisted navigation allowed accurate implantation of the femoral component avoiding pitfalls of hip surface replacement. From our point of view the optimal placement of the femoral component outweighs the disadvantage of a longer operating time.

Hip resurfacing femoral neck fracture influenced by valgus placement

Femoral neck fracture is the most common short-term concern after hip resurfacing arthroplasty. Currently, there is little basis to decide between neutral and valgus placement. We loaded 10 notched cadaveric femur pairs to failure; one side was implanted at 0 degrees relative to the femoral neck and the other at 10 degrees valgus. All 20 were dual-energy X-ray absorptiometry-scanned. Failure load correlated with bone mineral density. Valgus placement increased the fracture load by an average of 28% over neutral for specimens with normal bone mineral density but had no effect on fracture load in specimens with low bone mineral density. For specimens with normal bone mineral density (typical of patients undergoing resurfacing arthroplasty), neutral-valgus placement had a greater effect than bone mineral density, explaining 54% of the fracture load variance. Component placement greater than 10 degrees valgus is likely undesirable because this can lead to an increase in component size and a greater likelihood of notching. To reduce fracture risk, we recommend placing the femoral component in valgus and selecting patients with higher bone mineral density.

Comparative repeatability of guide-pin axis positioning in computer-assisted and manual femoral head resurfacing arthroplasty

The orientation of the femoral component in hip resurfacing arthroplasty affects the likelihood of loosening and fracture. Computer-assisted surgery has been shown to improve significantly the surgeon's ability to achieve a desired position and orientation; nevertheless, both bias and variability in positioning remain and can potentially be improved. The authors recently developed a computer-assisted surgical (CAS) technique to guide the placement of the pin used in femoral head resurfacing arthroplasty and showed that it produced significantly less variation than a typical manual technique in varus/valgus placement relative to a pre-operatively determined surgical plan while taking a comparable amount of time. In the present study, the repeatability of both the CAS and manual techniques is evaluated in order to estimate the relative contributions to overall variability of surgical technique (CAS versus manual), surgeon experience (novice versus experienced), and other sources of variability (e.g. across specimens and across surgeons). This will enable further improvements in the accuracy of CAS techniques.

Three residents/fellows new to femoral head resurfacing and three experienced hip arthroplasty surgeons performed 20-30 repetitions of each of the CAS and manual techniques on at least one of four cadaveric femur specimens. The CAS system had markedly better repeatability (1.2°) in varus/valgus placement relative to the manual technique (2.8°), slightly worse repeatability in version (4.4° versus 3.2°), markedly better repeatability in mid-neck placement (0.7 mm versus 2.5 mm), no significant dependence on surgeon skill level (in contrast to the manual technique), and took significantly less time (50 s versus 123 s). Proposed improvements to the version measurement process showed potential for reducing the standard deviation by almost two thirds.

This study supports the use of CAS for femoral head resurfacing as it is quicker than the manual technique, independent of surgeon experience, and demonstrates improved repeatability.

Navigation reduces the learning curve in resurfacing total hip arthroplasty

Hip resurfacing is a novel technique with a substantial learning curve resulting in poor outcomes for many patients. We asked whether navigation would influence this learning curve and accuracy of implantation. Twenty medical students earning their degree in surgical technology participated in a randomized trial. We provided instruction about the surgical technique, including the use of conventional instrumentation, the use of a computed tomography-based planner for hip resurfacing, and a navigation system. The 20 students were then split into three groups undertaking these tasks in three different orders. Synthetic femurs replicated normal, osteoarthritis, slipped capital femoral epiphysis, and coxa valga. The mean error using the conventional method to insert a guidewire was 23 degrees; using the computed tomography plan method it was 22 degrees; and using navigation was 7 degrees. Students produced similar accuracy, even in their first attempt, on difficult anatomy when provided navigation. Motivated students rapidly achieved an expert level of accuracy when provided with navigation. Learning a conventional method first did not improve performance, even in difficult cases. Our data suggest navigation may play an important role in reducing the learning curve in hip resurfacing arthroplasty and other tasks in arthroplasty in which a high degree of accuracy is clinically important.

The accuracy of image-free computer navigation in the placement of the femoral component of the Birmingham Hip Resurfacing

A cadaver study using six pairs of lower limbs was conducted to investigate the accuracy of computer navigation and standard instrumentation for the placement of the Birmingham Hip Resurfacing femoral component. The aim was to place all the femoral components with a stem-shaft angle of 135°.

The mean stem-shaft angle obtained in the standard instrumentation group was 127.7° (120° to 132°), compared with 133.3° (131° to 139°) in the computer navigation group (p = 0.03). The scatter obtained with computer-assisted navigation was approximately half that found using the conventional jig.

Computer navigation was more accurate and more consistent in its placement of the femoral component than standard instrumentation. We suggest that image-free computer-assisted navigation may have an application in aligning the femoral component during hip resurfacing.

Computer-assisted femoral head resurfacing

Femoral head resurfacing is re-emerging as a surgical option for younger patients who are not yet candidates for total hip replacement. However, this procedure is more difficult than total hip replacement, and the mechanical jigs typically used to align the implant produce significant variability in implant placement and take a significant amount of time to position properly. We propose that a computer-assisted surgical (CAS) technique could reduce implant variability with little or no increase in operative time. We describe a new CAS technique for this procedure and demonstrate in a cadaver study of five paired femurs that the CAS technique in the hands of a novice surgeon markedly reduced the varus/valgus variability of the implant relative to the pre-operative plan (2 degrees standard deviation for CAS versus 5 degrees for a mechanical jig operated by an expert surgeon). We also show that the mechanical jig resulted in significantly retroverted implant placement. There was no significant difference in operative time between the two techniques.

Oberfl�chenersatz am H�ftgelenk

Currently, an increase in resurfacing arthroplasty in the treatment of hip osteoarthritis--especially in young adults--can be observed. New bearing technologies (mainly metal-on-metal surfaces) show better tribologic results than historical designs (e.g. the Wagner cup). At present, it is unclear whether these modifications and a definitively low dislocation rate--due to the large head diameter--can be supported by further good clinical results. The quantity as well as the quality of the available investigations prevents a definite opinion at the moment. Appropriate clinical studies with documented radiographic follow-up are necessary to compare the outcome of these new implants with standard techniques.