[Electromagnetic navigation of transpedicular punctures: more precise than the standard?]. / Elektromagnetische Navigation transpedikulärer Punktionen: Präziser als der Standard?

AIM: Before clinical implementation of an approved electromagnetic tracking system (CAPPA IRAD EMT) an experimental trial was performed to investigate the accuracy of the system and its safety in application for transpedicular vertebral punctures in comparison to the classical fluoroscopic method. MATERIAL AND METHODS: A total of 110 transpedicular punctures were performed bilaterally using 11 vertebrae of 5 realistic artificial phantoms and 1 pedicle was punctured with the conventional technique using c-arm fluoroscopy and the other with the electromagnetic tracking system. As a target a radiopaque non-ferromagnetic marker was implanted bilaterally in the anterior wall of the vertebrae. For evaluation of the precision the distance from the end of the puncture to the target and the gradual deviation of the actual channel from the ideal trajectory were assessed in three-dimensional computer tomography. Calculations and statistical analysis were performed according to the Wilcoxon test by means of SPSS 16.0.1 for Windows. RESULTS: The mean distance from the target was 6.6 mm (± 3.9 mm standard deviation SD) with electromagnetic navigation compared to 3.2 mm (± 2.8 mm SD) with fluoroscopic assistance and the mean aberration from the ideal trajectory was 18.4° (± 4.6° SD) compared to 6.5° (± 3.5° SD), respectively. The difference of accuracy was highly significant regarding both parameters (p < 0.001). CONCLUSIONS: The minimum requirement for accuracy of transpedicular punctures could not be achieved with electromagnetic navigation. Unless proven otherwise, the lack of accuracy is attributed to unstable referencing. Despite evidence of successful employment for soft tissue punctures the system cannot currently be recommended for osseous applications of the spine.

[Dihydrotestosterone improves the osseointegration of cobalt-chromium implants]. / Dihydrotestosteron verbessert die knöcherne Einheilung von Kobalt-Chrom-Implantaten.

BACKGROUND: The osseointegration of actually rarely implanted cobalt-chromium implants can be critical in an elderly population. The aim of our study was to evaluate the effect of preoperative testosterone treatment on the osseointegration of cobalt-chromium implants. MATERIALS AND METHODS: Press-fit implantation of 1.6 mm-diameter cobalt-chromium-molybdenum (CoCrMo) implants was performed in rats without pre-treatment in one group (n = 10) and after pre-treatment with 1 mg dihydrotestosterone (DHT) 2 days before surgery in the other group (n = 10). After 14 days, the specimens were examined by a pull-out test, histology and histomorphometry. RESULTS: The biomechanical testing delivered inconsistent data leading to no significant difference (6.45 ± 6.94 N vs. 4.66 ± 3.77 N). Histology showed closed contact between surrounding tissue and the implants in both groups. The bone/implant contact area was significantly enhanced after treatment with DHT (42.23 % ± 9.25 vs. 57.57 % ± 16.71, p < 0.05), while the ratio of osteoid was reduced (38.68 % ± 16.7 vs. 27.38 % ± 13.02, not significant). CONCLUSIONS: Pre-treatment with DHT enhances osseointegration of cobalt-chromium implants through enhanced mineralisation of peri-implant tissue. The treatment might additionally shorten postoperative rehabilitation due to its positive effects on musculature.

[Diagnostic value of CT discography in unclear radiculopathy]. / Stellenwert der CT-Diskografie bei unklarer Radikulopathie.

A clinically evident radiculopathy without correlation in the imaging studies represents a serious problem regarding the indication, planning and execution of an operative procedure for its treatment. Both the diagnosis and treatment of such cases are deemed to be difficult without clear morphological correlation. Moreover, the surgeon lacks an important basis for the adequate planning and above all the justification of surgical treatment. Although discography with post-discographic computer tomography (CT discography) is still controversially discussed as an invasive diagnostic measure, the literature shows that this method is not only useful but also indispensable in certain cases. Based on these findings and our own empirical data, we recommend CT discography to be considered for imaging in patients suffering from lumbar radiculopathy with equivocal or insufficient MRI findings. The technique allows an accurate diagnosis and precise planning of a targeted surgical intervention such as endoscopic sequestrotomy or decompression.

[Influence of disc prosthesis position on segmental motion in the lumbar spine]. / Einfluss der Position einer lumbalen Bandscheibenprothese auf das segmentale Bewegungsverhalten.

AIM OF THE STUDY: Total disc arthroplasty is reported to maintain segmental motion. From finite element studies a rather posterior and central implantation of the prosthesis is recommended. However, there is yet no in vitro study with cadaveric specimens investigating the topic of implant positioning. METHODS: Ten human lumbar spines were subjected to biomechanical testing. Flexion/extension and side-bending moments were applied from 2.5-7.5 Nm on a spine load simulator. First, the intact specimens were tested in 3 load cycles while motion was monitored with regard to the facet joints under different loads by an ultrasound-based system. An unconstrained total disc prosthesis was then implanted in a central position and the different load cycles were repeated. Finally the implant was positioned in a decentral position with an average offset of 6.2 mm for repetitive data acquisition. RESULTS: Comparison of the facet joint motion in central and eccentric prosthesis positions resulted in the following averaged differences. During flexion of the lumbar spine an average difference of the reference point excursions of 0.38 mm was recorded on the ipsilateral facet joint with reference to the decentral position. For extension, the difference was 0.33 mm on average, for right side bending a difference of 0.63 mm was recorded while left side bending resulted in an average difference of 0.24 mm. The deviation of the reference markers on the contralateral facet joint showed the following average differences: for flexion 0.23 mm and for extension 0.54 mm, respectively. For side bending right/left the differences amounted to 0.18 mm and 0.39 mm. With regard to segmental motion there was no statistically significant difference for both the ipsilateral (p = 0.0564) and the contralateral (p = 0.2593) reference marker. CONCLUSIONS: The comparison of the segmental motion after central and decentral implantation of a lumbar total disc prosthesis reveals differences that have, nevertheless, no statistical significance. However, for clinical use it is recommended to strive for a central position of the implant.

[Successful surgical intervention for lumbar tuberculous spondylodiscitis in spite of delayed indication - 5 year follow-up]. / Erfolgreiche operative Sanierung einer tuberkulösen Spondylodiszitis der Lendenwirbelsäule trotz verzögerter Indikation - 5-Jahres-Ergebnis.

Tuberculosis constitutes an important clinical entity with 90 million new infections worldwide during the last decade. 10 % of these infections affect osseous structures based on haematogenous spread. Therefore, tuberculous spondylodiscitis remains a major illness in orthopaedics which needs to be considered in cases of unspecific back pain. Pathognomonic characteristics are absent and clinical, chemical and radiological parameters are not reliable. The consequences of a delayed surgical intervention are discussed based on a case report with 5 year follow-up. Both adequate early CT and MRI scans and identification of the pathogen are essential in order to plan the therapy. Final re-evaluation of a conservative treatment regime needs to be done no later than six to eight weeks depending on the clinical constellation. Persistent or progressive spondylodiscitis with osseous destructions or neurological deficits demands a radical surgical intervention.

[Perineural cysts causing severe back pain and pathological fracture of the massa lateralis of the sacrum]. / Perineurale sakrale Zysten als Ursache von Ischialgie und pathologischer Massa-lateralis-Fraktur des Os sacrum.

We report on the case of an 81-year-old woman who was hospitalised because of severe pain at the sacro-iliacal joint radiating into the left leg without any accompanying neurological defect. X-rays demonstrated a step in the left massa lateralis of the sacrum, thus CT scans and MRI were performed, and multiple perineural cysts (Tarlov's cysts) were found. The patient underwent microsurgical treatment by fenestration of the cyst wall and evacuation of the fluid content in order to avoid further expansion of the cysts. Under consequent treatment with pain killers, complete mobilisation of the patient could be achieved. Ambulant follow-up by clinical controls and X-rays demonstrated a substantial resolution of the patient's preoperative symptoms. Symptomatic perineural cysts should be included into differential diagnosis of severe low back pain, thus CT or MRI scans should be performed in case of long-lasting or intrackable pain.

[Procalcitonin (PCT) as diagnostic tool for the monitoring of spondylodiscitis]. / Procalcitonin (PCT) als Verlaufsparameter der Spondylodiszitis.

AIM: The diagnosis of spondylodiscitis is often prolonged, but it is an important differential diagnosis of backache. The discrimination between a bacterial infection and an aseptic inflammation with laboratory examinations like ESG, CRP or leukocytes is not possible. The aim of the present study was to determine the value of procalcitonin (PCT) as a diagnostic tool and monitoring parameter for spondylodiscitis and for the discrimination between bacterial infection and aseptic inflammation of the spine. METHOD: A total of 17 patients with spondylodiscitis and 18 patients with disc herniation as control were included in this study and ESG, CRP, leukocytes, fibrinogen, PNM elastase und PCT were examined for 50 days. The median age was 65 (17-78) years and the ratio of males to females was 8 : 9 in patients with spondylodiscitis and 62 (32-87) years and 7 : 11 in patients with disc herniation. For microbiological examination, CT-guided punctures were performed in patients with spondylodiscitis. RESULTS: In 64 % of the 17 patients with spondylodiscitis a microbiological agent was detected, in 73 % of these cases staphylococcus aureus was isolated. The laboratory parameters indicating an infection were increased except for two cases in patients with spondylodiscitis, the mean value of CRP was 115 mg/dL. Influenced by the therapy these parameters decreased during the observation period. Except for one patient with an infection of a cardiac pacemaker, the PCT concentration was not elevated in both groups (< 0.5 ng/mL). In the group with disc herniation there were no elevated laboratory parameters during the entire observation period. CONCLUSION: PCT is not useful as diagnostic tool or monitoring parameter for spondylodiscitis. Furthermore, it is not useful for the discrimination between a bacterial infection and an aseptic inflammation of the spine.

[The orthoMIT project. Gentle surgery using innovative technology]. / Das Projekt orthoMIT. Schonendes Operieren mit innovativer Technik.

The main objective of orthoMIT is to develop an integrated platform for "gentle" surgery in orthopedics and traumatology with particular emphasis on hip, knee and spinal column surgery. The goal of the 24 partners of the orthoMIT consortium and further associated industrial partners is an integrated approach to reduce the cost of orthopedic interventions: full workflow integration in the operating room, full treatment integration extending through to rehabilitation, and full integration of the typical value chain of medical product development, from the laboratory to production and commercialization by the firms and industrial partners involved in the project including education and training. Development, therefore, focuses on new strategies in minimally invasive surgery and improved, knowledge-based planning and navigation systems. Other topics include the development of improved methods of interventional imaging using ultrasound and computed tomography (flat panel detectors) and new, miniaturized instruments with integrated sensors and a modular integration of various applications and components into an integrated surgical workstation.

[Influence of inlay height on motion characteristics of lumbar segments in total disc replacement]. / Einfluss der Inlaygrösse einer lumbalen Bandscheibenprothese auf das Bewegungsverhalten.

AIM OF THE STUDY: Maintaining segmental motion is one of the most reported theoretical advantages of total disc replacement (TDR). Several inlay sizes are available for reconstruction of the physiological disc height. The influence of the implant height on the range of motion (ROM) was investigated in a biomechanical study. METHODS: A total of 10 human lumbar cadaver spines were subjected to biomechanical testing. Flexion/extension and side-bending moments were applied from 2.5-7.5 Nm on a spine load simulator allowing for all 6 degrees of freedom. Motion under different loads was monitored by the Zebris system in 3 dimensions. Initially intact specimens were tested in 3 load cycles. Then a total disc prothesis was implanted with an 8.5 mm inlay and the cycles were repeated. Finally in 5 cases a 1-mm larger inlay was inserted while in the remaining 5 cases the inlay was exchanged with a 2-mm larger implant. Neutral zone (NZ) and ROM were recorded under the different loads. RESULTS: The average motion for the various loads showed no significant difference when the intact motion segment was compared to the specimen containing the 8.5-mm inlay. After the larger inlay had been mounted the average reduction of the ROM in flexion/extension was 25% under the load of 7.5 Nm, 26% under a torque of 5.0 Nm and 30% when 2.5 Nm were applied. The NZ was reduced by 37%. For side-bending the ROM was reduced by 21% under a load of 7.5 Nm, by 26% under 5.0 Nm and by 35% under a torque of 2.5 Nm. The NZ was decreased by 27%. The reduction of the ROM was significant (p=0.0057). CONCLUSION: Segmental lumbar motion is maintained after TDR. The size of the inlay can significantly change the ROM in lumbar spine segments treated by TDR.

[Intraspinal echinococcosis within the lumbar spine of an 18-year-old male patient]. / Intraspinale Echinokokkose der Lendenwirbelsäule eines 18-jährigen männlichen Patienten.

Echinococcosis of human pathogenetic relevance is a very rare disease in Europe. The vertebral manifestation of the latter zoonosis constitutes one percent of all cases. Intraspinal echinococcosis appears worldwide just sporadically, whereas extradurally located cysts are identified more often than intradurally located ones. The diagnostic and specific therapeutic consequences of this very seldom entity are discussed based on a case report.

[Comparison of the treatment of slipped capital femoral epiphysis with K-wires and cannulated titanium screws]. / Vergleich der Therapie der Epiphyseolysis capitis femoris mit Kirschner-Drähten und kanülierten Titanschrauben.

AIM: The treatment of slipped capital femoral epiphysis (SCFE) is usually treated operatively, but there is still no consensus about the method to be used. Up to a 30 degrees degree of slipping, the epiphysis is normally fixed in situ. The aim of our study was to compare the intermediate results after fixation in situ by K-wires versus cannulated titanium screws (Königsee-Implantate, Königsee-Aschau, Germany). METHODS: In this study 46 patients with SCFE grade I and II and mostly chronic slipping of the epiphysis were included. After fixation in situ and, if necessary, careful, closed reposition, the patients were clinically and radiologically followed-up for one year. The clinical results were documented by the score adapted from Heyman and Herndon. Furthermore, MRI scans were done to evaluate the vitality of the epiphysis pre- and postoperatively, when titanium screws were used. RESULTS: Clinical follow-up showed comparable results in the clinical scores after fixation by K-wires or cannulated titanium screws (3.13 +/- 1.02 vs. 3.10 +/- 1.01). After the treatment with titanium screws we saw a higher rate of abnormal gait (33.3 % vs. 19 %), a decreased rate of the positive Drehmann sign (10 % vs. 38 %) and a lower rate of revisions (16 % vs. 50 %) in comparison to K-wire fixation. After displacement of the K-wires we saw chondrolysis and prearthrosis in one case. Removal of the K-wires was done without any complications, while the removal of the cannulated titanium screws failed in 4 of 10 cases. CONCLUSION: The treatment of SCFE with K-wires and cannulated titanium screws showed comparable results in the clinical follow-up. The treatment with cannulated titanium screws reduces the number of necessary revisions, but the removal of the material is hindered. Because of the lower rate of complications we prefer in the meantime the use of cannulated steel screws.

[Therapy effects of testosterone on the recovery of bone defects]. / Therapieeffekte von Testosteron auf die Heilung von Knochendefekten.

AIM: Androgens have proliferative effects on osteoblasts and increase fracture healing by systemic and local stimulation of bone formation. The aim of the present study was to evaluate if the systemic stimulation by androgens leads to increased bone-defect healing. METHODS: 1.5-mm trepanation defects were created in the femoral diaphysis of 30 Sprague-Dawley rats. 10 animals were used as untreated controls and 10 animals per group were treated by intramuscular injection of 1 or 10 mg dihydrotestosterone two days prior to surgery. After 14 days the samples were explanted and examined by macroscopy, histology and histomorphometry. RESULTS: All animals were included into the study and were analysed. Clinical observation showed no complications. Macroscopic examination and histology showed no significant differences. All defects were filled with trabecular bone in direct contact to the surrounding bone. Histomorphometry showed a significantly decreased bone content in the controls in comparison to both therapy groups, while the therapy groups showed no significant differences between each other. CONCLUSION: The stimulation of healing of bone defects with androgens leads to a significantly higher bone content inside the defects. In clinical application, androgens may be a possibility to increase bone formation, especially in elderly patients. Furthermore, it may be possible to shorten postoperative rehabilitation because of the effects of androgens on muscles.

[Chronic recurrent multifocal osteomyelitis and tumoral calcinosis--is there an association?]. / Die chronisch-rezidivierende multifokale Osteomyelitis und die tumoröse Kalzinose--assoziierte Erkrankungen?

BACKGROUND: Idiopathic tumorous calcinosis is a rare benign disease of the periarticular tissue near large joints. Deposits of hydroxyapatite in single or multiple pseudocysts lead to consecutive pain or complaints by attaching the surrounding tissues. The etiology of this disease is not definitively clear. CASE REPORT: We describe the case of an 11-year-old turkish girl with a well known chronic recurrent multifocal osteomyelitis (CRMO) and hyperphosphataemia. Furthermore, she developed a tumorous calcinosis around the left hip, which recurred after surgery, and around the ankle joint. CONCLUSIONS: CRMO and tumorous calcinosis can be associated diseases. The development of tumorous calcinosis in patients with CRMO and hyperphosphataemia should be excluded.

[Reduction of spondylolisthesis by temporary adjacent segment distraction]. / Reposition der Spondylolisthese durch temporäre Distraktion im angrenzenden Segment.

AIM OF THE STUDY: Multiple instrument systems are currently available for the reduction of spondylolisthesis, where posteriorly oriented tensile forces are directly acting on the slipped vertebra. The aim of this clinical study was to evaluate the clinical efficacy of a new indirect reduction manoeuvre applied to the lumbar spine. METHOD: A total number of 32 patients (14 female, 18 male) suffering of spondylolisthesis were reduced by transpedicular instrumentation during June 2001 until October 2003. The cranial adjacent vertebra was temporarily instrumented and the reduction of the slipped vertebra was facilitated by the application of traction on the instrumentation leading to tension of the longitudinal ligaments. Posterior transforaminal lumbar interbody fusion (TLIF) or anterior lumbar interbody fusion (ALIF) was then carried out according to the degree of degenerative shortening of the anterior long. spinal ligament. On the last follow up (average 32 month postoperatively; min.: 22 month) both the reduction of the spondylolisthesis and the ossification of the interbody fusion was evaluated radiologically. Physical function and patients satisfaction was measured by means of the SF 36 questionnaire. RESULTS: The dislocation was reduced in all cases (81% on average). At the time of the last follow up bony fusion was depicted on the radiographs in all 32 patients. A distinct improvement in all categories of the SF 36 (in 5 out of 8 categories statistic significant) could be demonstrated. CONCLUSION: Temporary intraoperative instrumentation of the cranial adjacent segment proofs to be a simple an effective method for the sufficient reduction of spondylolisthesis.

[Special surgical technique of minimally invasive total knee replacement]. / Spezielle chirurgische Technik der minimalinvasiven Totalendoprothetik des Kniegelenkes.

AIM: Due to the initial disappointing experience with minimally invasive knee arthroplasty, many surgeons still prefer the conventional technique. Sharing technical details, personal insights and results may help abbreviate the individual learning curve towards successful MIS. METHOD: Based on the experience of over 250 minimally invasive knee arthroplasties an operative algorithm was developed and evaluated to assure optimal realization of a gentle joint replacement using a mini-mid-vastus incision. Short-term results were compared to those of the conventional technique with particular regard to pain, use of analgesic medication, flexion of the knee and achievements at physiotherapy. RESULTS: In comparison to the conventional procedure, MIS produced significantly better results regarding pain and function during early rehabilitation and did not affect the perfect positioning of the implants. The morphine-equivalent dose was less than a half on day 2 after operation. Flexion on days 1 and 3 was 70 degrees and 75 degrees respectively. 80 degrees were obtained after 4 days and after 6 weeks flexion was at an average of 115 degrees . In contrast, results of the conventional control group were notably worse, with 50 degrees and 65 degrees on days 1 and 3, 80 degrees being obtained on day 6 only and a mean flexion of 100 degrees after 6 weeks. CONCLUSION: The special surgical technique of minimally invasive knee arthroplasty accelerates and facilitates the rehabilitation of the patient and thereby defines a new quality standard.

[epiDRB--a new minimally invasive concept for referencing in the field of computer-assisted orthopaedic surgery]. / epiDRB -- ein neues minimal-invasives Konzept für die Referenzierung im Rahmen der Computer-assistierten orthopädischen Chirurgie.

AIM: Optoelectronic navigation for computer-assisted orthopaedic surgery (CAOS) is based on a firm connection of bone with passive reflectors or active light-emitting diodes in a specific three-dimensional pattern. Even a so-called "minimally-invasive" dynamic reference base (DRB) requires fixation with screws or clamps via incision of the skin. Consequently an originally percutaneous intervention would unnecessarily be extended to an open procedure. Thus, computer-assisted navigation is rarely applied. Due to their tree-like design most DRB's interfere with the surgeon's actions and therefore are at permanent risk to be accidentally dislocated. Accordingly, the optic communication between the camera and the operative site may repeatedly be interrupted. The aim of the research was the development of a less bulky, more comfortable, stable and safely trackable device that can be fixed truly percutaneously. METHOD: With engineering support of the industrial partner the radiolucent epiDRB was developed. It can be fixed with two or more pins and gains additional stability from its epicutaneous position. The intraoperative applicability and reliability was experimentally tested. RESULTS: Its low centre of gravity and its flat design allow the device to be located directly in the area of interest. Thanks to its epicutaneous position and its particular shape the epiDRB may perpetually be tracked by the navigation system without hindering the surgeon's actions. Hence, the risk of being displaced by accident is minimised and the line of sight remains unaffected. CONCLUSION: With the newly developed epiDRB computer-assisted navigation becomes easier and safer to handle even in punctures and other percutaneous procedures at the spine as much as at the extremities without an unproportionate amount of additional trauma. Due to the special design referencing of more than one vertebral body is possible at one time, thus decreasing radiation exposure and increasing efficiency.

[Minimally-invasive computer-assisted fluoroscopic navigation for kyphoplasty]. / Minimal-invasive Computer-assistierte fluoroskopische Navigation der Kyphoplastie.

AIM: The transpedicular placement of a hollow needle into vertebral bodies for kyphoplasty requires utmost accuracy and thereby permanent multiplanar X-ray control. Facing the increasing number of vertebral compression fractures, the aim of this work was the implementation of computer-assistance to optimise the issue. Prior to clinical implementation, experimental trials were undertaken to analyse the quality-improving options of the technique. METHOD: The virtual image of the planning and the puncture were correlated with the postoperative X-ray image of the needle. The real canal in the bone was then correlated with the preoperative planning in a CT-based 3D model and differences were calculated. As a measure of accuracy the deviation of the needle from the ideal intruding vector and the distance between its top and the centre of a predefined target were scrutinised and related to the indications of the navigation system. Operating time, radiation exposure and general applicability were additionally assessed. All data were compared with those of a conventional control group. RESULTS: Planning and navigation could be executed with high accuracy. With an exactly transpedicular approach, neural structures were safely circumnavigated without once missing the target. In the control group the distance fault was up to 9 mm. The navigated drilling differed from the ideal trajectory by 1 degrees to max. 4 degrees. Conventional C-arm control led to a divergence of 4 degrees to 8 degrees . Radiation exposure could be reduced through computer assistance by 76 % to a fourth of the conventionally resulting amount and the pure operating time thereby decreased by 40 %. The inconvenient course of repeated positioning of the C-arm was overcome. CONCLUSION: In challenging cases of deteriorated anatomy and difficult radiomorphologic orientation, especially of the lower thoracic spine, the CAOS-procedure succeeds in finding the optimal pedicular approach to the vertebral body, helps to avoid collateral damage and minimises the overall risk of the procedure. High accuracy and reduced radiation exposure justify the clinical use of fluoroscopic navigation for transpedicular instrumentation.

[Intraoperative 3D reconstruction of the PMMA plug for computer-assisted revision of total hip arthroplasty based on 2D X-ray images]. / Intraoperative 3D-Rekonstruktion des PMMA-Köchers zur computerassistierten Revisionshüftendoprothetik auf Basis von 2D-Röntgen-Bildgebung.

AIM: Computer-assisted cement removal out of the femoral medullary canal requires the exact definition of the plug's shape within a 3D coordination system. Aiming at a sufficiently precise reconstruction based on segmentation of just a few 2D X-ray images a special mathematic procedure is needed for automatic surface interpolation. The geometric specification of the resulting virtual model should take into account the characteristic geometry of the navigated tools in order to achieve the best possible removal. METHOD: Studies were performed on anatomic specimens of entire human femora, that underwent cemented THA before being cut every 5 mm. The cross-sections were scanned at the high resolution of 600 dpi. Segmentation of the cement was performed with the help of a virtual deformable template and was both used for simulation of X-ray projections from various points of view and for validation of the reconstructed 3D model. By this means systematic errors such as those possible during X-ray acquisition, tracking or segmentation could be avoided and the precision of the procedure could be measured exclusively. RESULTS: With increasing number of X-rays the distance from the reconstructed 3D model to the original could continuously be reduced. Using only two x-rays a maximum error was measured with 6.5 mm, whereas 5 pictures taken from different angles showed to be enough to ensure an error below 1 mm in the distal part of the femur. By the use of 6 or more pictures no significant improvement could be attained. CONCLUSION: The innovative procedure is essential for future 2D image-based fluoroscopic navigation of PMMA removal and bears the options of computer-controlled and robotic material working, respectively.