Tantalum Markers Research Articles

Validation of a low-dose hybrid RSA and fluoroscopy technique: Determination of accuracy, bias and precision

Analyzing skeletal kinematics with radiostereometric analysis (RSA) following corrective orthopedic surgery allows the quantitative comparison of different implant designs. The purpose of this study was to validate a technique for dynamically estimating the relative position and orientation of skeletal segments using RSA and single plane X-ray fluoroscopy. Two micrometer-based in vitro phantom models of the skeletal segments in the hip and knee joints were used. The spatial positions of tantalum markers that were implanted into each skeletal segment were reconstructed using RSA. The position and orientation of each segment were determined in fluoroscopy images by minimizing the difference between the markers measured and projected in the image plane. Accuracy was determined in terms of bias and precision by analyzing the deviation between the applied displacement protocol and measured pose estimates. Measured translational accuracy was less than 100 μ m parallel to the image plane and less than 700 μ m in the direction orthogonal to the image plane. The measured rotational error was less than 1°. Measured translational and rotational bias was not statistically significant at the 95% level of confidence. The technique allows real-time kinematic skeletal measurements to be performed on human subjects implanted with tantalum markers for quantitatively measuring the motion of normal joints and different implant designs.

3D MRI‐based tumor delineation of ocular melanoma and its comparison with conventional techniques

The aim of this study is to (1) compare the delineation of the tumor volume for ocular melanoma on high-resolution three-dimensional (3D) T2-weighted fast spin echo magnetic resonance imaging (MRI) images with conventional techniques of A- and B-scan ultrasound, transcleral illumination, and placement of tantalum markers around tumor base and (2) to evaluate whether the surgically placed marker ring tumor delineation can be replaced by 3D MRI based tumor delineation. High-resolution 3D T2-weighted fast spin echo (3D FSE) MRI scans were obtained for 60 consecutive ocular melanoma patients using a 1.5 T MRI (GE Medical Systems, Milwaukee, WI), in a standard head coil. These patients were subsequently treated with proton beam therapy at the UC Davis Cyclotron, Davis, CA. The tumor was delineated by placement of tantalum rings (radio-opaque markers) around the tumor periphery as defined by pupillary transillumination during surgery. A point light source, placed against the sclera, was also used to confirm ring agreement with indirect ophthalmoscopy. When necessary, intraoperative ultrasound was also performed. The patients were planned using EYEPLAN software and the tumor volumes were obtained. For analysis, the tumors were divided into four categories based on tumor height and basal diameter. In order to assess the impact of high-resolution 3D T2 FSE MRI, the tumor volumes were outlined on the MRI scans by two independent observers and the tumor volumes calculated for each patient. Six (10%) of 60 patients had tumors, which were not visible on 3D MRI images. These six patients had tumors with tumor heights < or = 3 mm. A small intraobserver variation with a mean of (-0.22 +/- 4)% was seen in tumor volumes delineated by 3D T2 FSE MR images. The ratio of tumor volumes measured on MRI to EYEPLAN for the largest to the smallest tumor volumes varied between 0.993 and 1.02 for 54 patients. The tumor volumes measured directly on 3D T2 FSE MRI ranged from 4.03 to 0.075 cm3. with a mean of 0.87 +/- 0.84 cm3. The tumor shapes obtained from 3D T2 FSE MR images were comparable to the tumor shapes obtained using EYEPLAN software. The demonstration of intraocular tumor volumes with the high-resolution 3D fast spin echo T2 weighted MRI is excellent and provides additional information on tumor shape. We found a high degree of accuracy for tumor volumes with direct MRI volumetric measurements in uveal melanoma patients. In some patients with extra large tumors, the tumor base and shape was modified, because of the additional information obtained from 3D T2 FSE MR images.

SU-FF-T-332: 3D MRI-Based Tumor Delineation of Ocular Melanoma and Its Comparison with Conventional Techniques

Purpose: The aim of this study was to compare the delineation of the tumor volume for ocular melanoma on high-resolution 3D T2-weighted fast spin echo MRI images with conventional techniques of A- and B-scan ultrasound, transcleral-illumination and placement of tantalum markers around tumor base. Method and Materials: High-resolution 3D T2-weighted fast spin echo (3D FSE) MRI scans were obtained for 50 consecutive ocular melanoma patients using a 1.5 T MRI in a standard head coil. These patients were subsequently treated with proton beam therapy at the UC Davis Cyclotron. The tumor was delineated by placement of tantalum rings around the tumor periphery during surgery. The patients were planned using EYEPLAN software and the tumor volumes were obtained. For analysis, the tumors were divided on tumor height and basal diameter. In order to assess the impact of high-resolution 3D T2 FSE MRI, the tumor volumes were outlined on the MRI scans by two independent observers and the tumor volumes calculated for each patient. Results: 12% of 50 patients with tumor heights ⩽3 mm were not visible on 3D MRI images. A small intra-observer variation with a mean of (−0.5 ±4)% was seen in tumor volumes delineated by 3D T2 FSE MR images. The mean variation of tumor volume measurements between MRI scan to EYEPLAN was (0.1 ± 2.5)%. The tumor shapes obtained from 3D MRI images were comparable to the tumor shapes obtained using EYEPLAN software. Conclusion: The demonstration of intraocular tumor volumes with the high-resolution 3D FSE MRI is excellent and provides additional information on tumor shape. We found a high degree of accuracy for tumor volumes with direct MRI volumetric measurements in uveal melanoma patients. The MRI scan provided shape information on the tumor, which was comparable with the shape data obtained from EYEPLAN software.

The effect of standard lumbar discectomy on segmental motion: 5-year follow-up using radiostereometry

We measured the effects of lumbar discectomy on segmental motion over a period of 5 years. Twenty-four patients with lumbar disc herniation were treated by standard lumbar discectomy at the L4-L5 or L5-S1 level. Peroperatively, tantalum markers were inserted into L4, L5, and the sacrum. Radiostereometric analysis was performed at discharge from hospital and 5 years postoperatively. The treated level was compared with the corresponding untreated level. Thus, patients who had discectomy at the L4-L5 level served as controls for patients with L5-S1 lesions and vice versa. The relative rotation and translation in relation to the three cardinal axes were calculated. Inducible displacements over the two discs were calculated between the supine and standing positions. At the L4-L5 level, there were no differences in inducible displacements between the operated and control levels at discharge or 5 years postoperatively. At the L5-S1 level we found decreasing inducible movement in the sagittal plane over time for discectomy patients. The reason for decreasing mobility over time after discectomy at the L5-S1 but not at the L4-L5 level is unknown. Mechanical factors caused by the more vertical orientation of the L5-S1 disc in combination with degenerative changes could be one explanation.

Aortic Stentgraft Movement Detection Using Digital Roentgen Stereophotogrammetric Analysis on Plane Film Radiographs - Initial Results of a Phantom Study

To evaluate the feasibility of aortic stentgraft micromovement detection using digital roentgen stereophotogrammetric analysis on plane film radiographs. An aortic stentgraft used for demonstration purposes was marked with 10 tantalum markers of 0.8 mm in diameter. The stentgraft was placed on a Plexiglas phantom with 5 tantalum markers of 1 mm in diameter simulating a fixed segment needed for mathematical analysis. In a subsequent step, the stentgraft was placed onto an orthopaedic spine model to simulate in vivo conditions.in a next step. Two radiographs taken simultaneously from different angles were used for simulating different stentgraft movement, e. g. translation, angulation, aortic pulsation and migration in the spine model. Movement of the stentgraft markers was analysed using a commercially available digital RSA setup (UmRSA(R) 4.1, RSA Biomedical, Umea, Sweden). Our study shows the feasibility of measuring aortic stentgraft movement and changes in stentgraft shape in the submillimeter range using digital roentgen stereophotogrammetric analysis. Translation along the 3 cardinal axes, change in stentgraft shape, simulation of aortic pulsation and simulation of in vivo conditions could be described precisely. Aortic stentgraft movement detection using digital roentgen stereophotogrammetric analysis on plane film radiographs is a very promising, precise method.

A new tibial coordinate system improves the precision of anterior–posterior knee laxity measurements: a cadaveric study using Roentgen stereophotogrammetric analysis

Roentgen stereophotogrammetric analysis (RSA) can be used to measure changes in anterior–posterior (A–P) knee laxity after anterior cruciate ligament (ACL) reconstruction. Previous measurements of A–P knee laxity using RSA have employed a tibial coordinate system with the origin placed midway between the tips of the tibial-eminences. However, the precision in measuring A–P knee laxity might be improved if the origin was placed on the flexion–extension axis of rotation of the knee. The purpose of this study was to determine whether a center-of-rotation tibial coordinate system with the origin placed midway between the centers of the posterior femoral condyles, which closely approximates the flexion–extension center-of-rotation of the knee, improves the precision in measuring A–P knee laxity compared to the tibial-eminence-based coordinate system. A–P knee laxity was measured using each coordinate system six times in three human cadaveric knees implanted with 0.8-mm diameter tantalum markers. For each laxity measurement, the knee was placed in a custom loading apparatus and biplanar radiographs were obtained while the knee resisted a 44 N posterior shear force and 136 N anterior shear force. A–P knee laxity was determined from the change in position of the tibia, with respect to the femur, resulting from the posterior and anterior shear forces. The precision for each coordinate system was calculated as the pooled standard deviation of A–P knee laxity measurements. The precision of the center-of-rotation coordinate system was 0.33 mm, which was about a factor of 2 better than the 0.62 mm precision of the tibial-eminence coordinate system ( p = 0.006). The 0.33 mm precision with the center-of-rotation coordinate system suggests that an observed change of either 0.56 mm (i.e. 1.7 standard deviations) or greater in A–P knee laxity over time is a real change and not due to measurement error when the new tibial coordinate system is used and other factors contributing to variability are controlled as was done in this study. Accordingly, clinicians and researchers should consider the use of this alternate tibial coordinate system when making serial measurements of A–P knee laxity using RSA because the improved precision allows for the observation of smaller differences.

Stability of cemented all-polyethylene keeled glenoid components. A radiostereometric study with a two-year follow-up.

We studied the stability of cemented all-polyethylene keeled glenoid components by radiostereometric analysis (RSA) in 16 shoulders which had received a total shoulder replacement. There were 14 women (one bilateral) and one man with a mean age of 64 years. The diagnosis was osteoarthritis in eight and rheumatoid arthritis in seven. Two of the shoulders were excluded from the RSA study because of loosening of the tantalum markers. Three tantalum markers were inserted in the glenoid socket, two in the coracoid process and two in the acromion. The polyethylene keeled glenoid component was marked with three to five tantalum markers. Conventional radiological and RSA examinations were carried out at five to seven days, at four months and at one and two years after operation. Radiolucent lines were found in all except three shoulders. Migration was most pronounced in the distal direction and exceeded 1 mm in four shoulders. In ten shoulders rotation exceeded 2 degrees in one or more axes with retroversion/anteversion being most common. No correlation was found between migration and the presence of radiolucencies on conventional radiographs.

Marker Configuration Model-Based Roentgen Fluoroscopic Analysis

It remains unknown if and how the polyethylene bearing in mobile bearing knees moves during dynamic activities with respect to the tibial base plate. Marker Configuration Model-Based Roentgen Fluoroscopic Analysis (MCM-based RFA) uses a marker configuration model of inserted tantalum markers in order to accurately estimate the pose of an implant or bone using single plane Roentgen images or fluoroscopic images. The goal of this study is to assess the accuracy of (MCM-Based RFA) in a standard fluoroscopic set-up using phantom experiments and to determine the error propagation with computer simulations. The experimental set-up of the phantom study was calibrated using a calibration box equipped with 600 tantalum markers, which corrected for image distortion and determined the focus position. In the computer simulation study the influence of image distortion, MC-model accuracy, focus position, the relative distance between MC-models and MC-model configuration on the accuracy of MCM-Based RFA were assessed. The phantom study established that the in-plane accuracy of MCM-Based RFA is 0.1 mm and the out-of-plane accuracy is 0.9 mm. The rotational accuracy is 0.1°. A ninth-order polynomial model was used to correct for image distortion. Marker-Based RFA was estimated to have, in a worst case scenario, an in vivo translational accuracy of 0.14 mm ( x-axis), 0.17 mm ( y-axis), 1.9 mm ( z-axis), respectively, and a rotational accuracy of 0.3°. When using fluoroscopy to study kinematics, image distortion and the accuracy of models are important factors, which influence the accuracy of the measurements. MCM-Based RFA has the potential to be an accurate, clinically useful tool for studying kinematics after total joint replacement using standard equipment.

CT and MR Imaging of Nitinol Stents with Radiopaque Distal Markers

To evaluate imaging characteristics and artifacts of a nitinol stent with distal tantalum markers with computed tomography (CT) angiography and magnetic resonance (MR) angiography. A vascular phantom was built to simulate in-stent restenosis. A nitinol stent with tantalum markers (Luminexx stent) was evaluated with CT angiography in different orientations relative to the z-axis and with MR angiography in different positions relative to both B0 and the readout gradient. Stenosis measurements were compared with conventional digital subtraction angiography for both modalities. In-stent signal intensity obtained with different flip angles was assessed in two nitinol stents with distal markers (Luminexx stent and SMART stent) and one without markers (Memotherm-FLEXX stent). Stenosis detection was not possible with CT angiography when the stent was perpendicular to the z-axis because of streak-like artifacts induced by tantalum markers. Stenosis evaluation with multiplanar reformation was accurate when the stent was in parallel and oblique orientations relative to the table axis. With MR angiography, metallic artifacts were mostly related to the stent orientation with B0, whereas orientation of the readout gradient had little influence. The mean error (overestimation) for stenosis measurements varied between 0.1% and 7.4% for CT imaging in parallel and oblique positions and 3.6% and 9.5% for MR imaging. Higher flip angles did not improve signal intensity inside the three stents tested. CT and MR angiography can be used for evaluating the patency of stents with distal markers that are parallel or oblique relative to the table axis (iliac, carotid, or femoral stents). MR angiography is preferred if the stent is perpendicular to the table axis (renal stent).

Comparison of two digital radiostereometric analysis methods in the determination of femoral head penetration in a total hip replacement phantom

Radiostereometric analysis (RSA) has been used extensively to evaluate the magnitude and direction of penetration of the femoral head into the acetabular component of a total hip replacement as a result of polyethylene wear and creep. The accuracy and precision of an RSA study depends on several factors, including the radiographic technique, the analytical software, and the positioning of the tantalum markers. This study had three sequential purposes. First, an in vitro phantom model was used to quantify the accuracy and precision of digital images versus conventional radiography in RSA measurements of penetration of the femoral head into the acetabular shell in a total hip replacement. The Umeå RSA software package was used for analysis of both the conventional films, which were digitized at a resolution of 300 DPI, and digital radiographs, which were converted from a DICOM format at a resolution of 218 DPI. Digital radiography was found to be superior. Next, two methods of RSA analysis currently in use for determining femoral head penetration into polyethylene of total joint replacements were compared. Using the phantom model, we compared the Umeå RSA system (Biomedical Innovations AB) to the RSA-CMS (RSA Clinical Measurement Solution) and in both cases used the digital radiographs. The Umeå RSA system was found to be superior. Finally, two methods of marking the position of the acetabular component with tantalum beads were compared: one in which beads were inserted into previously described towers protruding from the back of the acetabular shell and another in which beads were placed into the peripheral flange of the polyethylene liner using the Umeå RSA analysis system of the digital radiographs. The results using the two marker configurations were similar.

Model study of blood flow through a stent in the aorto-iliac bifurcation.

Atherosclerotic ailments play a major role in industrial societies. Atherosclerotic disease causes stenotic narrowing of the arterial vessel system. These narrow passages can be widened with balloon angioplasty and stents are introduced to keep the passage open. In this study, stents designed for the aorto-iliac bifurcation are considered. Most of these stents used in peripheral arteries are self-expanding stents made from Nitinol, a thermo-reactive alloy. The insertion is done under radiographic control. However, the delicate metallic structure of the stent usually casts only a weak radiographic shadow and thus is difficult to detect. A stent with an innovative radio opaque tantalum marker (Luminexx, C.R. BARD--Angiomed, Karlsruhe), overcomes this problem. However, the markers extend into the blood flow and the question arises whether the markers may cause the generation of thrombi. In a model study in an enlarged model of the aorto-iliac bifurcation with an inserted stent the flow was visualized. The enlarged scale permits the observation and video recording of the flow in great detail. The Reynolds similarity is kept. A subsequent analysis of the flow in the vicinity of the markers shows a short residence time. These results indicate that the additional markers do not increase the risk of thrombus generation.

Length of the patellar tendon after anterior cruciate ligament reconstruction with patellar tendon autograft: A prospective clinical study using roentgen stereometric analysis

Purpose: This prospective study was designed to assess postoperative changes in patellar tendon length after removal of its central one third for anterior cruciate ligament (ACL) reconstruction using precise Roengen stereometric analysis (RSA). Type of Study: Prospective clinical study. Methods: Ten consecutive patients underwent arthroscopically assisted ACL reconstruction using the mid-third bone–patellar tendon–bone (BPTB) autograft. The tendon defect was left open and the paratenon was approximated with sutures. Tantalum beads were inserted in the distal part of the patella and in the tibial tuberosity through the bone defects. The distance between the tantalum markers in the patella and the tibial tuberosity was measured with RSA at the first day and at 1, 2, 3, 6, and 12 months after surgery. Results: A decrease of patellar tendon length was observed in all cases. The amount of shortening averaged 1.6 mm (range, 0.5 to 3.2 mm), 3.2% of the initial length. In 4 patients, tendon shortening was detected during the first 4 weeks after surgery. In 6 cases, decrease of patellar tendon length started between the fourth and the eighth week. The shortening process continued in 8 knees until the 12th week. No further changes in patellar tendon length were observed after the 12th postoperative week. In 2 patients, shortening of the patella tendon exceeded 2 mm. The amount of patellar tendon shortening was not related to anterior knee pain. Conclusions: After removing the BPTB autograft, irreversible shortening of the remaining two thirds of the patellar tendon occurred during the first 12 postoperative weeks.Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 18, No 8 (October), 2002: pp 859–864

EVALUATION OF A FEMORAL STEM WITH REDUCED STIFFNESS

Femoral stems with reduced stiffness were introduced in total hip arthroplasty to facilitate proximal load transfer and thereby reduce periprosthetic bone loss. Poor fixation and unacceptably high revision rates turned out to be a major problem with these prostheses. The purpose of the present study was to evaluate the early performance of a low-modulus stem (one that is less stiff) with the potential to address the problem of fixation by the use of a surface coating. The coating consisted of a titanium porous mesh proximally covered with a mixture of hydroxyapatite and tricalcium phosphate. Thirty-nine men and twenty-six women (sixty-eight hips) with noninflammatory arthritis were studied. The patients were randomized to receive either a porous-coated composite stem with reduced stiffness (Epoch) or a stiff stem with a porous coating (Anatomic). Both stems were inserted without cement, had a ceramic coating on the proximal two-thirds, and were supplied with tantalum beads. At the time of the operation, tantalum markers were also placed into the proximal part of the femur. The patients were followed for up to two years after the operation and were evaluated with use of repeated radiostereometric measurements, measurements of bone-mineral density, conventional radiography, and Harris hip scores. Both stems showed optimum fixation with median subsidence and stem rotations that were close to zero. Repeated measurements of bone-mineral density revealed early loss of bone mineral in all Gruen regions in both treatment groups. The loss of bone mineral around the Epoch stems was significantly reduced at two years in Gruen regions 1, 2, 6, and 7 (p < 0.0005 to 0.04). Measurements on postoperative radiographs showed no difference in stem positioning or fill, but the Epoch stems had significantly more endocortical contact on both the anteroposterior (p < 0.0005) and the lateral radiograph (p = 0.02). At two years postoperatively, the Epoch stems had fewer sclerotic lines surrounding the stem (p = 0.002) and less sclerosis at the tip of the prosthesis (p = 0.001) compared with the Anatomic stems. The clinical results in terms of the Harris hip score, which was determined in all hips, and pain or discomfort, which was evaluated in thirty-seven hips treated at the same hospital, were not found to be different, with the numbers available. Contrary to previous studies of other designs with reduced stiffness, the Epoch stem achieved excellent primary fixation. Despite this rigid fixation, the proximal loss of bone-mineral density was less than that associated with the stem with a stiffer design. These results should encourage additional long-term studies with a larger patient population.

Evaluation of an external device measuring knee joint rotation: an in vivo study with simultaneous Roentgen stereometric analysis

An external device (“rottometer”) specially designed to measure knee joint rotation was developed and evaluated with respect to its validity. Simultaneous measurements were made with the rottometer and Roentgen stereometric analysis (RSA) in five patients with implanted tantalum markers in the tibia and femur. Measurements of internal and external rotation were made at 90° and 60° of knee flexion using 3, 6 and 9 N m torques. The coefficients of determination ( r 2) between the results obtained with the rottometer and RSA were around 0.9 for the total rotation. The rottometer consistently overestimated the rotation by about 100% and this systematic error was most constant at 90° flexion for the different torques. The magnitude of this error from soft tissue deformation as well as the rotatory movements in the hip, foot and ankle joints must be considered when using external devices to measure knee rotation in clinical studies. The most accurate registrations were found in 90° flexion with 9 N m force ( r 2=0.94).

Evaluation of accuracy and precision of bone markers for the measurement of migration of hip prostheses. A comparison of conventional measurements.

Our aim was to determine whether tantalum m arkers improved the accuracy and/or precision of methods for the measurement of migration in total hip replacement based on conventional measurements without mathematical correction of the data, and with Ein Bild Roentgen Analyse - Femoral Component Analysis (EBRA-FCA) which allows a computerised correction. Three observers independently analysed 13 series of roentgen-stereophotogrammetric-analysis (RSA)-compatible radiographs (88). Data were obtained from conventional measurements, EBRA-FCA and the RSA method and all the results were compared with the RSA data. Radiological evaluation was also used to quantify in how many radiographs the intraosseous position of the bone markers had been simulated. The results showed that tantalum markers improve reliability whereas they do not affect accuracy for conventional measurements and for EBRA-FCA. Because of the danger of third-body wear their implantation should be avoided unless they are an integral part of the method.

Evaluation of accuracy and precision of bone markers for the measurement of migration of hip prostheses

Our aim was to determine whether tantalum markers improved the accuracy and/or precision of methods for the measurement of migration in total hip replacement based on conventional measurements without mathematical correction of the data, and with Ein Bild Roentgen Analyse – Femoral Component Analysis (EBRA-FCA) which allows a computerised correction. Three observers independently analysed 13 series of roentgen-stereophotogrammetric-analysis (RSA)-compatible radiographs (88). Data were obtained from conventional measurements, EBRA-FCA and the RSA method and all the results were compared with the RSA data. Radiological evaluation was also used to quantify in how many radiographs the intraosseous position of the bone markers had been simulated. The results showed that tantalum markers improve reliability whereas they do not affect accuracy for conventional measurements and for EBRA-FCA. Because of the danger of third-body wear their implantation should be avoided unless they are an integral part of the method.

Model-based Roentgen stereophotogrammetry of orthopaedic implants

Attaching tantalum markers to prostheses for Roentgen stereophotogrammetry (RSA) may be difficult and is sometimes even impossible. In this study, a model-based RSA method that avoids the attachment of markers to prostheses is presented and validated. This model-based RSA method uses a triangulated surface model of the implant. A projected contour of this model is calculated and this calculated model contour is matched onto the detected contour of the actual implant in the RSA radiograph. The difference between the two contours is minimized by variation of the position and orientation of the model. When a minimal difference between the contours is found, an optimal position and orientation of the model has been obtained. The method was validated by means of a phantom experiment. Three prosthesis components were used in this experiment: the femoral and tibial component of an Interax total knee prosthesis (Stryker Howmedica Osteonics Corp., Rutherfort, USA) and the femoral component of a Profix total knee prosthesis (Smith & Nephew, Memphis, USA). For the prosthesis components used in this study, the accuracy of the model-based method is lower than the accuracy of traditional RSA. For the Interax femoral and tibial components, significant dimensional tolerances were found that were probably caused by the casting process and manual polishing of the components surfaces. The largest standard deviation for any translation was 0.19 mm and for any rotation it was 0.52°. For the Profix femoral component that had no large dimensional tolerances, the largest standard deviation for any translation was 0.22 mm and for any rotation it was 0.22°. From this study we may conclude that the accuracy of the current model-based RSA method is sensitive to dimensional tolerances of the implant. Research is now being conducted to make model-based RSA less sensitive to dimensional tolerances and thereby improving its accuracy.

Reply to “Comment on ‘Dominant diffusing species during cobalt silicide formation’ ” [J. Appl. Phys. 87, 8216 (2000)

Ideally, when a marker is used to determine the dominant diffusing species during phase formation, the marker should be chemically inert with respect to its surroundings, should not interfere significantly with the overall development of the reaction, and should not selectively couple to the diffusing motion of either element involved in the reaction. Finding a single marker that satisfies all these requirements is unfortunately extremely difficult and, in general, compromises have to be made. d’Heurle and Zang point out certain deficiencies tantalum has when used as a marker during CoSi2 formation. We acknowledge these deficiencies, but contend that the additional experiments carried out using tantalum markers in different systems (Ta marker in NiSi2) and different markers for the same system (Xe marker for CoSi2) suggest that these deficiencies are unlikely to make results of the original investigation completely invalid.

Early Subsidence of the Exeter Femoral Stem within the Cement Mantle in Primary Arthroplasties and in Revisions Using Impacted Allografts and Cement: A Roentgen Stereophotogrammetric Analysis

In five primary hip arthroplasties and in six revisions with impacted, morselized allografts and cement, roentgen stereophotogrammetric analysis (RSA) was planned to be used to measure the subsidence of the polished and double-tapered Exeter femoral stem, relative both to the cement and the femur. Only in two primary arthroplasties and in two revisions could the tantalum markers be adequately imaged for RSA evaluation. The stems in the primary arthroplasties subsided 1.4 to 2.0 mm and all the subsidence occurred within the cement mantle. The two revisions subsided 1.9 to 3.1 mm within the cement mantle, but there was also a slight subsidence of the cement in relation to the femur, 0.5 to 0.4 mm, respectively. No radiographic signs of stem loosening were found in any of the hips. Our findings of subsidence of the Exeter femoral stem within the cement mantle with no radiographic signs of loosening are in accordance with those reported by the Exeter group. The present technique using tantalum markers in the medioproximal aspect of the cement mantle must be considered as difficult.

The validity of two methods of mandibular superimposition: A comparison with tantalum implants

The aim of this investigation was to examine and compare the validity of Björk's and Ricketts' methods for the superimposition of serial cephalometric radiographs of the mandible for the analysis of changes over the duration of routine orthodontic treatment in growing subjects (approximately 2 years). Pre- and posttreatment lateral cephalometric radiographs of 23 children, with tantalum markers implanted in the mandible, were studied. The differences in position of six dental and skeletal landmarks between superimposition on Björk's structures and on Ricketts' corpus axis were compared with those on the basis of the implants. A rotational effect was found for corpus axis resulting from differential movement of Xi point with growth, whereas Björk's method yielded results essentially similar to those of the implant-based superimposition. This resulted in statistically significant median differences between the two methods for all landmarks except pogonion and menton. The magnitude of the differences increased with distance from the central core of the mandible and were generally greater horizontally than vertically. Although most differences were less than 2 mm, approximately 10% of the subjects showed differences greater than 4 mm for molar and incisor landmarks. These findings suggest that, for growing subjects, Björk's method should be preferred Ricketts', which cannot be relied on to indicate the true (intramandibular) changes during orthodontic treatment in growing subjects. (Am J Orthod Dentofacial Orthop 1998;113:263-70.)