Modular necks improve the range of hip motion in cases with excessively anteverted or retroverted femurs in THA

BACKGROUND

Anteversion of an acetabular component often is difficult to ascertain in patients with THA in whom excessively anteverted or retroverted femurs may result in limited ROM or risk of dislocation. Restriction of motion, however, is determined by the combination of version of both components.

QUESTIONS/PURPOSES

We therefore determined the combined anteversion values that provide adequate ROM. We varied acetabular version by differing implantations and varied femoral version with modular necks.

METHODS

ROM was tested by changing cup anteversion after setting the femoral version to 20° or 60° anteversion or 20° retroversion. The angle of the modular neck was adjusted in 11 increments of 5° each. Range of internal rotation (IR) at 90° flexion, external rotation (ER) at 0° extension, and flexion (Flex) were measured when any impingement occurred before dislocation. We defined a required ROM as having 40° IR, 30° ER, and 110° Flex.

RESULTS

At 60° anteversion, ER was less than 10° even when the acetabular component was set at 10° retroversion because of posterior impingement. When a modular neck with 25° retroversion was used, ER improved to greater than 30°. At 20° retroversion, IR was 31° even when the acetabular component was opened to 35° anteversion. IR improved to 34° and 40° with 20° and 25° anteverted modular necks, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

In cases with excessive femoral anteversion or retroversion, the required ROM could not be achieved by simply changing the version of acetabular components. The adjustment of femoral versions using the modular necks allowed additional improvement of ROM.

Decreased accuracy of acetabular cup placement for imageless navigation in obese patients

BACKGROUND

Optimal acetabular cup position is an important determinant of the success of total hip arthroplasty (THA), and navigation systems have been developed and applied to improve placement precision. However, the registration method touching bony landmarks through soft tissues may decrease the accuracy in obese cases and in patients with acetabular dysplasia.

METHODS

Placement accuracy in 69 joints in which THA was performed with imageless navigation was calculated by comparing the placement angle in the anterior pelvic plane and the placement angle in the x-ray table plane with the patient in the supine position. We defined the difference between the placement angle in the anterior pelvic plane and the angle displayed on the navigation screen as the error and the difference between the placement angle in the plane of the x-ray table and the target angle of acetabular component position as the difference in target angle. Patients were divided into obese (BMI ≥ 25) and nonobese (BMI < 25) groups and into acetabular dysplasia and nondysplasia groups.

RESULTS

The mean ± SD navigation errors for all cases were 2.4° ± 2.0° for inclination and 3.7° ± 2.3° for anteversion. The mean difference in target angle for all cases was 2.8° ± 2.5° for inclination and 4.2° ± 3.0° for anteversion. The error in anteversion was significantly higher in the obese group (4.8° ± 2.5°) than in the nonobese group (3.2° ± 2.6°) (P = 0.01). No significant difference was observed between the acetabular dysplasia and nondysplasia groups.

CONCLUSIONS

Imageless navigation can be performed with an error of approximately 3° at the anterior pelvic plane. However, accuracy was found to decrease in obese cases.

3D CT analysis of combined cup and stem anteversion in cases of cup navigation in hip arthroplasty

Navigation of the cup in total hip arthroplasty is well analyzed and shows accurate results, reducing cup outliers of Lewinnek's "safe zone." With regard to the combined anteversion of cup and stem, however, a "new" safe zone with a range of 25° to 50° has been published. The aim of this study was to analyze total anteversion (cup and stem) by postoperative 3D computed tomography in isolated cup navigation cases. In 46 patients, the mean combined anteversion was 34.4° (range, 16.3°-57.3°, SD ± 9.3°) with 10 outliers. The mean cup anteversion was 19.5° (range, 11°-27°, SD ± 3.7°). Regarding Lewinnek's "safe zone" (cup only), we observed 5 outliers. An improvement of technique of stem implantation or navigation may reduce outliers of combined anteversion.

The association between the sagittal femoral stem alignment and the resulting femoral head centre in total hip arthroplasty

Adequate stem alignment is essential for the success of Total Hip Arthroplasty (THA) to avoid dislocation and impingement. One factor that has not been sufficiently investigated so far is the stem tilting in the sagittal plane, which has an influence on the position of the centre of the femoral head and thus also on prosthesis torsion. We aimed to evaluate sagittal stem position using 3D-CTs in patients with THA and to develop a mathematical-geometrical model to simulate the functional correlation between sagittal stem tilting and the influence on functional anteversion. Thirty patients with THA underwent a CT-scan. By 3D-reconstruction of the CT-data, femoral-/prosthesis-axis, torsion and sagittal tilt were determined. In accordance with the position of the femoral and prosthesis axes, the rotatory (rAV) (surgically adjusted) and functional (depending on sagittal tilt) anteversion (fAV) was measured. A three dimentional-coordinate transformation was also performed using the Euler-angles to derive a mathematical-geometrical correlation between sagittal stem tilting and corresponding influence on anteversion. The mean rAV was 8° (-11.6-26°), the fAV 18° (6.2-37°), and the difference 10° (8.8-18°). The mean degree of stem tilting was 5.2° (0.7-9°) anterior towards the femoral axis. The individually measured parameters are reflected in the mathematical-geometrical model. Depending on the extent of the sagittal deviation, a clear influence on the torsion emerges. For example, a stem implanted at a 15° anteverted angle with a sagittal tilt by two degrees towards anterior results in a fAV of 20°. A clear association between the sagittal stem alignment and the impact on the fAV was demonstrated. Hence, the rotatory anteversion intended by the surgeon may be functionally significantly different. This might pose an increased risk of dislocation or impingement. The sagittal tilt of the prosthesis should therefore be considered in the context of impingement and dislocation diagnosis. In this respect, we recommend a 3D-analysis of stem alignment.

Factors affecting bony impingement in hip arthroplasty

Computer modeling of 10 patients' computed tomographic scans was used to study the variables affecting hip arthroplasty range of motion before bony impingement (ROMBI) including acetabular offset and height, femoral offset, height and anteversion, and osteophyte removal. The ROMBI was compared with the ROM before component impingement and the native hip ROM. The ROMBI decreased with decreased total offset and limb shortening. Acetabular offset and height had a greater effect on ROMBI than femoral offset and height. The ROMBI lost with decreased acetabular offset was not fully recoverable with an increase in femoral offset or osteophyte removal. Bony impingement increased and component impingement decreased with decreased acetabular offset and increased head diameter.

A three-dimensional parameterized and visually kinematic simulation module for the theoretical range of motion of total hip arthroplasty

BACKGROUND

Proper option and optimal alignment of implants are major factors in obtaining the desired range of motion of total hip arthroplasty. It would be very useful if a tool is available to assist clinicians to preoperatively determine proper implants and their optimal intraoperative alignment for individual patient.

METHODS

Based on the secondary development function of ADAMS/VIEW, a three-dimensional parameterized module was developed to simulate six motions of implants for total hip arthroplasty. Total eight parameters (stem abduction, acetabular size, head size, general head-neck ratio, stem-neck angle, acetabular anteversion, acetabular inclination and femoral antetorsion) were included and three definitions of acetabular orientations (operative, radiographic and anatomical) were available. Any initial position of implants and simulation precision could be defined on the demand.

FINDINGS

After entering these eight parameters, a special model of total hip arthroplasty would be constructed. Single or combined motion until the prosthetic impingement could be simulated visually and the corresponding maximal rotation angle would be quantified.

INTERPRETATION

Clinical factors, such as patients with severe deformity of hip joint or a large range of motion demand, should be considered during total hip arthroplasty. This module can provide references for clinicians on proper option and intraoperative implantation of components, and be suitable for postoperative evaluation. Furthermore, it is a tool to investigate the "safe-zones" of total hip arthroplasty and also can be employed for the design of new implant system.

Effective incorporating spatial information in a mutual information based 3D-2D registration of a CT volume to X-ray images

This paper addresses the problem of estimating the 3D rigid poses of a CT volume of an object from its 2D X-ray projection(s). We use maximization of mutual information, an accurate similarity measure for multi-modal and mono-modal image registration tasks. However, it is known that the standard mutual information measures only take intensity values into account without considering spatial information and their robustness is questionable. In this paper, instead of directly maximizing mutual information, we propose to use a variational approximation derived from the Kullback-Leibler bound. Spatial information is then incorporated into this variational approximation using a Markov random field model. The newly derived similarity measure has a least-squares form and can be effectively minimized by a multi-resolution Levenberg-Marquardt optimizer. Experiments were conducted on datasets from two applications: (a) intra-operative patient pose estimation from a limited number (e.g. 2) of calibrated fluoroscopic images, and (b) post-operative cup orientation estimation from a single standard X-ray radiograph with/without gonadal shielding. The experiment on intra-operative patient pose estimation showed a mean target registration accuracy of 0.8mm and a capture range of 11.5mm, while the experiment on estimating the post-operative cup orientation from a single X-ray radiograph showed a mean accuracy below 2 degrees for both anteversion and inclination. More importantly, results from both experiments demonstrated that the newly derived similarity measures were robust to occlusions in the X-ray image(s).

Hip flexion after Birmingham hip resurfacing: role of cup anteversion, anterior femoral head-neck offset, and head-neck ratio

We aimed to investigate the factors affecting range of flexion after hip resurfacing. A total of 82 cases, operated by a single surgeon, were assessed at a mean of 43 months. The Einzel-Bild-Roentgen-Analysis for the acetabular cup software was used to measure socket orientation. Range of flexion had a moderate positive correlation with cup anteversion (R = 0.26, P = .017), weak but significant negative correlation with neck diameter (R = -0.23, P = .042), and none with anterior femoral head-neck offset. Using multivariate analysis that adjusted for age, sex, cup anteversion and inclination, head-neck offset ratio, head-neck ratio, and neck diameter, the only significant correlate of flexion was cup anteversion (P = .017). Care should be taken during cup placement to allow adequate anteversion to be maintained in Birmingham hip resurfacing because this can affect flexion range of motion.

Tailor-made Surgical Guide Reduces Incidence of Outliers of Cup Placement

Malalignment of the cup in total hip arthroplasty (THA) increases the risks of postoperative complications such as neck cup impingement, dislocation, and wear. We asked whether a tailor-made surgical guide based on CT images would reduce the incidence of outliers beyond 10 degrees from preoperatively planned alignment of the cup compared with those without the surgical guide. We prospectively followed 38 patients (38 hips, Group 1) having primary THA with the conventional technique and 31 patients (31 hips, Group 2) using the surgical guide. We designed the guide for Group 2 based on CT images and fixed it to the acetabular edge with a Kirschner wire to indicate the planned cup direction. Postoperative CT images showed the guide reduced the number of outliers compared with the conventional method (Group 1, 23.7%; Group 2, 0%). The surgical guide provided more reliable cup insertion compared with conventional techniques.

LEVEL OF EVIDENCE

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

Do large femoral heads reduce the risks of impingement in total hip arthroplasty with optimal and non-optimal cup positioning?

The purpose of this study was to assess whether large femoral heads (36-38 mm) improve the range of motion in total hip arthroplasty compared to standard (28-32 mm) femoral heads in the presence of optimal and non-optimal cup positioning. A mathematical model of the hip joint was generated by using a laser scan of a dried cadaveric hip. The range of motion was assessed with a cup inclination and anteversion of reference and with non-optimal cup positions. Large femoral heads increased the range of motion, compared to the 28-mm femoral head, in the presence of a hip prosthesis correctly implanted and even more so in the presence of non-optimal cup positioning. However, with respect to the 32-mm femoral head, large femoral heads showed limited benefits both in the presence of optimal and non-optimal cup positioning.

Quantification of pelvic tilt in total hip arthroplasty

In THA, anterior or posterior tilt of the pelvis changes the position of the acetabular component on the coronal plane of the body as compared with its anatomic position in the pelvic bone. To understand the occurrence and clinical importance for patients with pelvic tilt on an operating room table in the lateral decubitus position, we studied 436 patients (477 hips) undergoing primary THA using an imageless computer navigation system that measured tilt. We determined the distribution and magnitude of pelvic tilt, especially tilt of 10 degrees or greater. The distribution of tilt had a range of 25 degrees posterior to 20 degrees anterior. Twenty-nine of 477 (6.1%) hips had zero tilt; 251 (52.6%) had tilt of 1 degrees to 5 degrees; 120 (25.2%) had tilt of 6 degrees to 9 degrees; and 77 (16.1%) had tilt of 10 degrees or greater. The conversion factor for acetabular anteversion has been determined by a mathematical formula by Lembeck et al. and was confirmed by us in practice. Measurement of pelvic tilt during the performance of THA will improve the accuracy of cup position, especially allowing anteversion to be measured on the coronal plane.

LEVEL OF EVIDENCE

Level II, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.

Femoral anteversion in THA and its lack of correlation with native acetabular anteversion

Several studies support the concept that, for optimum range of motion in THA, the combined femoral and acetabular anteversion should be some constant or fall within some "safe zone." When using a cementless femoral component, the surgeon has little control of the anteversion of the component since it is dictated by native femoral anteversion. Given this constraint, we asked whether the surgeon should use the native anteversion of the acetabulum as a target for implant position in THA. Forty-six patients scheduled for primary THA underwent CT scanning and preoperative planning using a computer workstation. The native acetabular anteversion and the native femoral anteversion were measured. Prosthetic femoral anteversion was measured on the workstation by three-dimensional templating of a straight-stemmed tapered implant. The mean of the sum of the native acetabular anteversion and native femoral anteversion was 28.9 degrees; however, 17% varied by 10 degrees to 15 degrees and 11% by more than 15 degrees. The mean of native femoral anteversion and prosthetic femoral anteversion was 13.8 degrees (range, -6.1 degrees-32.7 degrees) and 22.5 degrees (range, 1 degrees-39 degrees), respectively. Based on our data, we believe the surgeon should not use the native acetabular anteversion as a target for positioning the acetabular component.

A stature-specific concept for uncemented, primary total hip arthroplasty

BACKGROUND AND PURPOSE

Variations in hip anatomy limit the femoral canal fit of standard uncemented hip stems. In addition, there are still issues with leg length discrepancy and offset reconstruction, potentially resulting in impingement, dislocation, and wear. Modular stems with different shapes for femoral canal fit and multiple neck options may improve the outcome and reduce complications.

PATIENTS AND METHODS

173 patients (190 hips) received an uncemented THA with 1 of 2 different stem shapes for canal fit and a modular neck for stature-specific hip reconstruction. Median follow-up time was 9 (7-13) years. During the follow-up period, 20 patients died (22 hips) and 12 patients (13 hips) were lost to follow-up. 155 hips were available for evaluation, including clinical and radiological outcome.

RESULTS

1 stem was revised for a periprosthetic fracture following trauma; 10 cups and 2 modular necks were revised (1 for breakage and 1 during cup revision). At 10 years, stem survival was 100%, modular neck survival was 99% (CI: 95-100), and cup survival was 94% (CI: 87-97). No leg length discrepancies were measured in 96% of cases. Offset with anatomic lateralization was achieved in 98%. Median Harris hip score was 94 (47-100) and median Merle d'Aubigné score was 16 (10-18). Relevant radiolucent lines and osteolysis were not found.

INTERPRETATION

The uncemented modular neck, dual-stem system used in this series allows accurate reconstruction of the joint by adapting the implant to the needs of the patient. This may improve the outcome of primary THA, which is supported by the results of this medium-term follow-up evaluation.

Minimal radiation dose computed tomography for measurement of cup orientation in total hip arthroplasty

We studied a computed tomography (CT) protocol with minimal radiation dose for measurements of cup orientation after total hip arthroplasty. Inclination and version angles were measured directly on cadaver pelvis fixed with an acetabular component. We performed the CT scanning on the phantom made of this model in 6 protocols as follows: tube current of 300, 100, 50, 30, 20, and 10 mA. The 30 mA protocol maintained good image quality for measurements with the smallest dose (1.1 millisievert), which corresponds to less than 2 times that of an anteroposterior radiograph of the pelvis. The differences of measured angles between direct and CT measurements on the phantom were less than 1 degrees. Measurements of cup orientation were possible by this protocol in 26 hips in 22 patients after total hip arthroplasty.

Accuracy of acetabular component positioning with a fluoroscopically referenced CAOS system

OBJECTIVE

This study evaluated the accuracy, repeatability, and reproducibility of a fluoroscopic referenced system used for guiding acetabular component positioning.

METHODS

Calibration of the Medtronic StealthStation Treon Plus system was performed using a Weber gage block to assess linearity. Metrologic validation of repeatability and reproducibility was done using a cadaveric pelvis with an uncemented cup placed in the target position of 45 degrees inclination and 17.5 degrees anteversion. A baseline assessment was done with a National Institute of Standards and Technology (NIST) traceable coordinate measuring machine (CMM).

RESULTS

Weber gage block analysis revealed a mean bias of 0.69 mm. For the cadaveric pelvis, the anterior pelvic plane was determined using the bilateral anterior superior iliac spines with the symphysis pubis as the inferior landmark. The mean CMM measurement was inclination of 46.023 degrees (SD=1.075; range: 43.318-46.844 degrees) and anteversion of 15.787 degrees (SD=0.411; range: 15.068-16.384 degrees). One surgeon performed a repeatability assessment (n=8), finding mean inclination of 42.8 degrees (SD=1.5; range: 39.5-44.5 degrees) and anteversion of 17.5 degrees (SD=3.0; range: 14.5-22.5 degrees). Three surgeons performed a reproducibility assessment (n=24), finding mean overall inclination of 48.5 degrees (SD=0.9; range: 46-50 degrees) and anteversion of 17.8 degrees (SD=2.5; range: 13.5-23.5 degrees). All measurements were within a predefined acceptability range of+/-5 degrees.

DISCUSSION

The accuracy and reproducibility of the fluoroscopic referencing method was found to be suitable for determination of cup position in the surgical setting. Anteversion measurements were more variable for the fluoroscopic method and this may be related to the difficulty for the surgeon in predictably picking the anatomical points from the fluoroscopic image.

Femoral neck anteversion, acetabular anteversion and combined anteversion in the normal Indian adult population: A computed tomographic study

BACKGROUND

Abnormal femoral neck anteversion (FNA) and/or acetabulum anteversion (AA) have long been implicated in the etiogenesis of hip osteoarthritis (OA), developmental dysplasia of the hip (DDH), and impingement, instability and wear in total hip arthroplasty (THA). Since studies on the Indian population are sparse on this topic, the purpose of this study was to report the normal values of FNA, AA and the combined anteversion (CA= FNA+ AA) in Indian adults.

MATERIALS AND METHODS

FNA, AA and CA were prospectively measured in 172 normal hips in 86 Indian adults using standardized computed tomographic (CT) methods and this data was compared with the established Western values.

RESULTS

The median values and interquartile ranges were 8 degrees (6.5-10.0 degrees ) for FNA, 19 degrees (16.0-22.0 degrees ) for AA and 27 degrees (23.5-30.0 degrees ) for CA. AA and CA values were significantly (P<0.05) lower in males, and there was also a trend towards lower FNA in males. Although a negative correlation was observed between the FNA and AA, this was not strong and may not be clinically relevant.

CONCLUSION

When compared with the Western data, the FNA values were 3-12 degrees lower and the CA values were 3-5 degrees lower in Indian adults. The AA values were comparable, but were skewed towards the higher side. Further studies are needed to assess the clinical relevance of our basic science data in pathogenesis of OA, and to validate it in relation to hip surgeries like corrective osteotomies and THA.

Accuracy of computer navigation for acetabular component placement in THA

The accuracy and precision of any computer-aided surgical device is critical to its utility. We asked the following question: how accurate and precise are the values measured by an imageless computer navigation system as compared with those measured using postoperative CT scans? Twenty-five patients (26 hips) underwent primary THA using an imageless computer navigation system for placement of the acetabular component. Inclination and anteversion were measured in the operative coordinate system as defined by Murray. Accuracy, precision, and bias were computed, and Bland-Altman analysis was used to assess levels of agreement. The accuracy (mean +/- standard deviation of the absolute difference between computer-assisted navigation and CT) was 1.8 degrees +/- 1.2 degrees for inclination and 2.0 degrees +/- 2.0 degrees for anteversion. Precision was 3.4 degrees for inclination and 5.5 degrees for anteversion. Bias was 0.52 degrees for inclination and 0.35 degrees for anteversion. Limits of agreement were 4.26 degrees for inclination and 5.58 degrees for anteversion. An imageless computer navigation system can precisely determine acetabular cup position.

Computer-assisted total hip arthroplasty: coding the next generation of navigation systems for orthopedic surgery

This article outlines the scientific basis and a state-of-the-art application of computer-assisted orthopedic surgery in total hip arthroplasty (THA) and provides a future perspective on this technology. Computer-assisted orthopedic surgery in primary THA has the potential to couple 3D simulations with real-time evaluations of surgical performance, which has brought these developments from the research laboratory all the way to clinical use. Nonimage- or imageless-based navigation systems without the need for additional pre- or intra-operative image acquisition have stood the test to significantly reduce the variability in positioning the acetabular component and have shown precise measurement of leg length and offset changes during THA. More recently, computer-assisted orthopedic surgery systems have opened a new frontier for accurate surgical practice in minimally invasive, tissue-preserving THA. The future generation of imageless navigation systems will switch from simple measurement tasks to real navigation tools. These software algorithms will consider the cup and stem as components of a coupled biomechanical system, navigating the orthopedic surgeon to find an optimized complementary component orientation rather than target values intraoperatively, and are expected to have a high impact on clinical practice and postoperative functionality in modern THA.

Validation of a new method for determination of cup orientation in THA

Our goal was to validate accuracy, consistency, and reproducibility/reliability of a new method for determining cup orientation in total hip arthroplasty (THA). This method allows matching the 3D-model from CT images or slices with the projected pelvis on an anteroposterior pelvic radiograph using a fully automated registration procedure. Cup orientation (inclination and anteversion) is calculated relative to the anterior pelvic plane, corrected for individual malposition of the pelvis during radiograph acquisition. Measurements on blinded and randomized radiographs of 80 cadaver and 327 patient hips were investigated. The method showed a mean accuracy of 0.7 +/- 1.7 degrees (-3.7 degrees to 4.0 degrees) for inclination and 1.2 +/- 2.4 degrees (-5.3 degrees to 5.6 degrees) for anteversion in the cadaver trials and 1.7 +/- 1.7 degrees (-4.6 degrees to 5.5 degrees) for inclination and 0.9 +/- 2.8 degrees (-5.2 degrees to 5.7 degrees) for anteversion in the clinical data when compared to CT-based measurements. No systematic errors in accuracy were detected with the Bland-Altman analysis. The software consistency and the reproducibility/reliability were very good. This software is an accurate, consistent, reliable, and reproducible method to measure cup orientation in THA using a sophisticated 2D/3D-matching technique. Its robust and accurate matching algorithm can be expanded to statistical models.

Activities of daily living after total hip arthroplasty. Is a 32-mm femoral head superior to a 26-mm head for improving daily activities?

Range of motion (ROM) of the hip joint after total hip arthroplasty (THA) could be increased by using a larger prosthetic femoral head, but it is not known whether the activities of daily living (ADL) are influenced by THA with different head sizes. Our objective was to compare postoperative ADL in patients who underwent THA using a head diameter of 26 mm or 32 mm. We assessed the range of motion and the mode of ADL after cementless primary THA. Comparison was performed between 25 joints of 24 patients who underwent THA with a 26-mm femoral head (26-mm group) and 24 joints of 20 patients with a 32-mm head (32-mm group). The postoperative range of flexion and abduction was significantly larger in the 32-mm group than in the 26-mm group. With respect to the mode of performing selected ADL such as putting on and removing pants, socks, and cutting toenails, many patients adopted the compensatory position of lumbar flexion with hip flexion plus knee extension in the 26-mm group, while a majority of the patients from the 32-mm group employed the mode of hip flexion with knee flexion. Patients with the 32-mm head showed better postoperative ADL of the ipsilateral side compared with the 26-mm head.

A comparison of surgeon estimation and computed tomographic measurement of femoral component anteversion in cementless total hip arthroplasty

BACKGROUND

The intraoperative estimation of the anteversion of the femoral component of a total hip arthroplasty is generally made by the surgeon's visual assessment of the stem position relative to the condylar plane of the femur. Although the generally accepted range of intended anteversion is between 10 degrees and 20 degrees, we suspected that achieving this range of anteversion consistently during cementless implantation of the femoral component was more difficult than previously thought.

METHODS

We prospectively evaluated the accuracy of femoral component anteversion in 109 consecutive total hip arthroplasties (ninety-nine patients), in which we implanted the femoral component without cement. In all hips, we measured femoral stem anteversion postoperatively with three-dimensional computed tomography reconstruction of the femur, using both the distal femoral epicondyles and the posterior femoral condyles to determine the femoral diaphyseal plane. The bias and precision of the measurements were calculated.

RESULTS

The surgeon's estimate of femoral stem anteversion was a mean (and standard deviation) of 9.6 degrees +/- 7.2 degrees (range, -8 degrees to 28 degrees). The anteversion of the stem measured by computed tomography was a mean of 10.2 degrees +/- 7.5 degrees (range, -8.6 degrees to 27.1 degrees) (p = 0.324). The correlation coefficient between the surgeon's estimate and the computed tomographic measurement was 0.688; the intraclass coefficient was 0.801. Anteversion measured by computed tomography found that forty-nine stems (45%) were between 10 degrees and 20 degrees of anteversion; forty-three stems (39%) were between 0 degree and 9 degrees of femoral anteversion; eight stems (7%) were in anteversion of >20 degrees; and nine stems (8%) were in retroversion.

CONCLUSIONS

The surgeon's estimation of the anteversion of the cementless femoral stem has poor precision and is often not within the intended range of 10 degrees to 20 degrees of anteversion. The implications of this finding increase the importance of achieving a safe range of motion by evaluating the combined anteversion of the stem and the cup.

Stress tensor field visualization for implant planning in orthopedics

We demonstrate the application of advanced 3D visualization techniques to determine the optimal implant design and position in hip joint replacement planning. Our methods take as input the physiological stress distribution inside a patient's bone under load and the stress distribution inside this bone under the same load after a simulated replacement surgery. The visualization aims at showing principal stress directions and magnitudes, as well as differences in both distributions. By visualizing changes of normal and shear stresses with respect to the principal stress directions of the physiological state, a comparative analysis of the physiological stress distribution and the stress distribution with implant is provided, and the implant parameters that most closely replicate the physiological stress state in order to avoid stress shielding can be determined. Our method combines volume rendering for the visualization of stress magnitudes with the tracing of short line segments for the visualization of stress directions. To improve depth perception, transparent, shaded, and antialiased lines are rendered in correct visibility order, and they are attenuated by the volume rendering. We use a focus+context approach to visually guide the user to relevant regions in the data, and to support a detailed stress analysis in these regions while preserving spatial context information. Since all of our techniques have been realized on the GPU, they can immediately react to changes in the simulated stress tensor field and thus provide an effective means for optimal implant selection and positioning in a computational steering environment.

Navigated cup implantation in hip arthroplasty

BACKGROUND AND PURPOSE

Many studies have suggested that navigation-based implantation can improve cup positioning in total hip arthroplasty (THA). We conducted a systematic review and meta-analysis to compile the best available evidence, and to overcome potential shortcomings because of small sample sizes in individual studies.

METHODS

The search strategy covered the major medical databases from January 1976 through August 2007, as well as various publishers' databases. The internal validity of individual studies was evaluated independently by 3 reviewers. We used random-effects modeling to obtain mean differences in cup angulation and relative risk (RR) of cup positioning outside Lewinnek's safe zone.

RESULTS

Of 363 citations originally identified, 5 trials of moderate methodology enrolling a total of 400 patients were included in the analysis. Mean cup inclination and anteversion were not statistically significantly different between the conventional groups and the navigated groups. Navigation reduced the variability in cup positioning and the risk of placing the acetabular component beyond the safe zone (RR = 0.21, CI: 0.13-0.32).

INTERPRETATION

Based on the current literature, navigation is a reliable tool to optimize cup placement, and to minimize outliers. However, long-term outcomes and cost utility analyses are needed before conclusive statements can be drawn about the value of routine navigation in THA.

Cup positioning in primary total hip arthroplasty using an imageless navigation device: is there a learning curve?

In this study, the success of cup positioning in total hip arthroplasty (THA) using an imageless navigation system was analyzed (1) during the learning period and (2) after the learning period for using the navigation system. Sixty THAs were performed in which threaded cups were placed with use of a computer-assisted navigation device (B. Braun Aesculap, Tuttlingen, Germany). Half of the procedures (30), group A, were done by the same surgeons under the learning curve for using the navigation system; the other half (30), group B, were done by surgeons who were no longer considered under the learning curve. Intraoperative acetabular component parameters (inclination, anteversion) for both groups were compared with postoperative radiographic alignment values. In group A, significant differences were seen between intraoperative and postoperative cup orientation. In group B, no significant differences were seen between intraoperative and postoperative cup orientation. Additionally, the percentage of outliers decreased in group B. Operating and anesthesia times significantly decreased with the surgeon's experience in imageless cup navigation. There was an individual increase of precision during the learning curve for all surgeons. Imageless navigation is a dependable and accurate method of cup positioning during THA. However, accuracy of cup placement and length of operating room time were affected by surgeons' experience using the system. Imageless navigation may lead to a reduction in the length of the learning curve for surgeons beginning to perform THAs, improvement in the surgeon's ability to perform this procedure safely, and minimization of outliers.

Tailor-made surgical guide based on rapid prototyping technique for cup insertion in total hip arthroplasty

BACKGROUND

A surgical guide made by the rapid prototyping (RP) technique for cup insertion in total hip arthroplasty might be useful to avoid malalignment of the cup, which indicates postoperative complications.

METHODS

To address this research question, we applied a RP-based guide to 24 patients with their CT images. We designed it to fit onto the acetabular edge and to insert a Kirschner wire (K-wire) which indicated a planned cup direction. We intraoperatively placed it on the acetabular edge, inserted the K-wire through the guide on the superior acetabulum and implanted the cup while observing the alignment of the K-wire. We also recorded the additional time needed to use the guide.

RESULTS

The mean cup accuracy between planned and postoperative alignments was 2.8 degrees (SD = 2.1 degrees ) for abduction and 3.7 degrees (SD = 2.7 degrees ) for anteversion. The mean additional time was 3.5 (range 2-6) min.

CONCLUSION

We can use this guide with acceptable accuracy and without consuming an excessive amount of time.

Six sigma analysis of minimally invasive acetabular arthroplasty: a preliminary investigation

Minimally invasive techniques in THA may increase the difficulty of acetabular component insertion relative to the optimized position. We sought to determine the ability of eight surgeons to position an acetabular component placed using an anterior-lateral minimally invasive surgical (MIS) approach with conventional instruments or computer navigation using an optical imageless protocol compared with conventional true values determined by computed tomography (CT). We introduce a new approach, the Six Sigma process capability index, to assess outliers. Using the Six Sigma process capability index (Cp > 1.3) and the criteria of Lewinnek et al. of +/- 10 degrees for adequate precision, three-dimensional (3D) CT was capable for inclination and anteversion. Computer navigation and visual cues with conventional instrumentation were precise for anteversion but not for inclination. We conclude image-free computer navigation was not better than conventional instrumentation with the surgeons' visual cues for acetabular cup placement. Six Sigma analysis allows comparison of various methods of referencing with literature controls, and our data suggest CT referencing is the most precise method.

[Navigation in hip joint revision endoprosthetics]

The exact anatomical and biomechanical demands of correct positioning of prostheses can be critical for hip joint revision endoprosthetics. Preoperative planning cannot always be implemented due to the intraoperative situation. The reconstruction of the hip center, the correct alignment of the acetabulum and the shaft is difficult, especially in large defect situations. An exact positioning of the implant is possible for primary hip joint replacement by computer-assisted procedures. Although the potential benefits of revision endoprosthetics make sense, there are as yet only few publications which as a rule are limited to positioning of the acetabulum. It will be possible to further optimize revision endoprosthetics by analysis of the impingement, determination of alterations in leg length, measurement of shaft antetorsion and other parameters, by additionally incorporating shaft navigation and implant data. Navigation should, however, only be understood as an aid to orientation and does not supplant lack of knowledge and experience on the side of the operator. Information on positioning of the implant must therefore come from the operator.

Cup positioning in THA: current status and pitfalls. A systematic evaluation of the literature

The correct determination of cup orientation in THA regarding the intraoperative as well as the postoperative assessment due to the pelvic tilt and rotation with inexact incorporation of the pelvis is uncertain. The anterior pelvic plane (APP) seems to be the most reliable reference frame and computer-assisted navigation systems seem to provide the best tool for correct implantation to date. For the intraoperative assessment of the APP, the exact determination of the bony landmarks is mandatory. For the standard plain radiography, standardized positioning of the patient and approximation of pelvic tilt by a lateral view are mandatory. An additional CT must be carried out for certain indications. More emphasis has to be given to the individuality of pelvic tilt and range of motion.

Comparison of mini-incision total hip arthroplasty through an anterior approach and a posterior approach using navigation

This study reports on differences in the use of minimally invasive surgery for total hip arthroplasty related to the direction of cup insertion against the operating table, intraoperative hip range of motion, stability, and a choice of cup liners for both a mini-incision posterior approach (MPA) and a mini-incision anterior approach (MAA) using Stryker Navigation's CT-Hip system. The MPA group consisted of 39 consecutive patients and the MAA group consisted of 33 consecutive patients. Clinically, there was no significant difference in the average Japanese Orthopedic Association hip score or the Oxford hip score preoperatively and at 6 months and 2 years follow-up. The intraoperative joint stability measurements showed no large difference between the two groups when malpositioning of the cup was eliminated.

Upward and inward displacements of the acetabular component increase stress on femoral head in single endoprothesis models

The centre of rotation of the hip can be displaced in hip dysplasia and revision arthroplasty. This study examined the effect of artificial femoral head load after acetabular component displacement in total hip arthroplasty. Sixteen total hip arthroplasty models of human cadaver specimens were reconstructed, and under different acetabular component position, the load around the femoral head was evaluated by strain gages. The results showed that the load was higher in the same specimens when the cup was moved 2 mm inward or upward, especially after the cup was moved more than 6 mm, and the load had an increasing effect in the inward group. In the upward group, an increasing effect happened at 8 mm upward displacement, but the stress value decreased from 4 mm to 6 mm upward displacement. In the same moving distance, the stress of inward displacement is obviously higher than upward displacement. Altogether, the results suggested that for both inward displacement and upward displacement of the acetabular cup, the load around the femoral head increased gradually, while the distance of the inward displacement and the superior displacement was increased. The greater the displacement, the bigger the loading contact stress. The upward displacement caused less stress change on the femoral head. The stress of the 6 mm upward position was lower than nearby positions; perhaps this site represented a stress buffering zone.

The relationship of the orientation of the transverse acetabular ligament and acetabular labrum to the suggested safe zones of cup positioning in total hip arthroplasty

BACKGROUND

There is a need to develop and validate a method for establishing cup orientation that is patient specific and independent of the anterior pelvic plane. It is our hypothesis that the transverse acetabular ligament and acetabular labrum can be used to do this. The objective of this study is to define the orientation of the plane formed by the transverse acetabular ligament and acetabular labrum and to examine whether these local landmarks lie within the limits of acceptance for cup positioning.

METHODS

Twenty-five consecutive patients, who were being investigated for labral tears with a MRI arthrogram of the hip, were enrolled in this prospective study. The orientation of the transverse acetabular ligament-labrum plane was determined by manually selecting points on the transverse acetabular ligament and labrum. The best-fit plane through these points was determined and its operative orientation expressed with respect to a constructed pelvic coordinate system.

RESULTS

The operative anteversion of the transverse acetabular ligament-labrum plane ranged from 5.3-36.1 inverted exclamation mark (mean 23.0 inverted exclamation mark + or - 7.4 inverted exclamation mark standard deviation). The inclination ranged from 38.4-50.3 inverted exclamation mark (mean 45.6 inverted exclamation mark + or - 3.2 inverted exclamation mark standard deviation).

CONCLUSIONS

The transverse acetabular ligament and acetabular labrum offer a possible solution to the many difficulties involved in cup placement during total hip arthroplasty. This paper highlights the variation in the orientation of these local acetabular landmarks and questions the logic of a set target for cup positioning.

Impingement as a mechanism of dissociation of a metasul metal-on-metal liner

This case report is of a patient with disassociation of the acetabular cup liner caused by impingement. The cup inclination (39 degrees) and anteversion (24 degrees) were good as measured by computer navigation. Impingement occurred because the head-neck ratio was 2.0, and the hip length and offset were short by one head length. Successful revision without intraoperative impingement was accomplished with one size head larger (32 mm; head-neck ratio, 2.3) and one size longer to correct hip length and offset.

Evaluation of component positioning in primary total hip arthroplasty using an imageless navigation device compared with traditional methods

Acetabular orientation affects the success of total hip arthroplasty. Computer-assisted navigation systems may reduce positional errors. Total hip arthroplasty results were analyzed using an imageless navigation system. We hypothesized that reliability and accuracy would improve. One hundred forty-nine total hip arthroplasties were performed using minimally invasive surgical techniques in 3 cohorts: manual (n = 53), initial navigation cases (n = 49), navigation second series (n = 47). Manual patients' cup orientation variation from desired range was -19 degrees to +18 degrees anteversion (SD, 9.1 degrees ), -11 degrees to +25 degrees abduction (SD, 6.7 degrees ). Navigation variation from desired was -18 degrees to +15 degrees (SD, 7.3 degrees ) in group 1 and -15 degrees to +9 degrees (SD, 5.9 degrees ) in group 2 in anteversion and -15 degrees to +13 degrees (SD, 6.1 degrees ) in group 1 and -15 degrees to +11 degrees (SD, 4.7 degrees ) in group 2 in abduction. Results were statistically significant. There were significant differences for operating room time and estimated blood loss, but not incision size or body mass index. Navigation provided controlled, reproducible acetabular alignment; but a learning curve existed in terms of accuracy, estimated blood loss, and operating room time.

Imaging and navigation measurement of acetabular component position in THA

There are six different definitions of acetabular position based on observed inclination and anteversion made in either the (1) anterior pelvic plane or (2) coronal planes and based on whether each of the observations made in one of these two planes is (1) anatomic, (2) operative, or (3) radiographic. Anteroposterior pelvic tilt is the angle between the anterior pelvic plane and the coronal plane of the body. The coronal plane is a functional plane and the anterior pelvic plane is an anatomic pelvic plane. A cup may be in the "safe zone" by one definition but may be out of the "safe zone" by another definition. We reviewed published studies, analyzed the difference in varying definitions, evaluated the influence of the anterior pelvic tilt, and provided methods to convert from one definition to another. We recommend all inclination and anteversion measurements be converted to the radiographic inclination and anteversion based on the coronal plane, which is equivalent to the inclination and anteversion on the anteroposterior pelvic radiograph.

Computer-assisted versus manual alignment in THA: a probabilistic approach to range of motion

Dislocation remains a major complication after THA, and range of motion before impingement is important in joint stability. Variability in implant alignment affects resultant range of motion. We used a probabilistic modeling approach to assess the effects of implant alignment variability based on manual and computer-assisted surgical (CAS) techniques on resultant range of motion after THA. We implemented a contact detection algorithm within a probabilistic analysis framework. The normally distributed alignment variables (mean +/- 1 standard deviation) were cup abduction (manual = 45 degrees +/- 7.6 degrees , CAS = 45 degrees +/- 5.7 degrees ), cup anteversion (manual = 20 degrees +/- 9.6 degrees , CAS = 20 degrees +/- 4.5 degrees ), and stem anteversion (manual and CAS = 10 degrees +/- 1.5 degrees ). The outcomes of the probabilistic analysis were range of motion distributions with 1% and 99% bounds. The upper bounds of motion for manual and CAS alignment were similar because bony impingement was the limiting factor. The lower bounds of range of motion were substantially different depending on the type of surgical alignment; manual alignment produced a smaller range of motion in 3% to 5% of cases. CAS implant alignment produced range of motion values above minimum acceptable levels in all cases simulated.

Combined anteversion technique for total hip arthroplasty

Combined cup and stem anteversion in THA based on femoral anteversion has been suggested as a method to compensate for abnormal femoral anteversion. We investigated the combined anteversion technique using computer navigation. In 47 THAs, the surgeon first estimated the femoral broach anteversion and validated the position by computer navigation. The broach was then measured with navigation. The navigation screen was blocked while the surgeon estimated the anteversion of the broach. This provided two estimates of stem anteversion. The navigated stem anteversion was validated by postoperative CT scans. All cups were implanted using navigation alone. We determined precision (the reproducibility) and bias (how close the average test number is to the true value) of the stem position. Comparing the surgeon estimate to navigation anteversion, the precision of the surgeon was 16.8 degrees and bias was 0.2 degrees ; comparing the navigation of the stem to postoperative CT anteversion, the precision was 4.8 degrees and bias was 0.2 degrees , meaning navigation is accurate. Combined anteversion by postoperative CT scan was 37.6 degrees +/- 7 degrees (standard deviation) (range, 19 degrees -50 degrees ). The combined anteversion with computer navigation was within the safe zone of 25 degrees to 50 degrees for 45 of 47 (96%) hips. Femoral stem anteversion had a wide variability.

LEVEL OF EVIDENCE

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

Optimal orientation of implanted components in total hip arthroplasty with polyethylene on metal articulation

BACKGROUND

In many studies related to the total hip arthroplasty, it has been found that incorrect alignment of the total hip component is one of the major factors influencing mechanical failures. Although various recommendations for cup orientation have been presented, there were few studies that seek to determine a proper orientation of the implants based on the human motion data. The objective of this study is to determine an optimum orientation of the hip implant considering various daily activities.

METHODS

Firstly, the cup orientations free of impingement were calculated for a given set of implant geometric parameters and the required range of motion for daily activities measured in 10 subjects. Next, the optimum values for the cup orientation and stem anteversion avoiding impingement and minimizing cup wear were determined for the proposed motion criteria.

FINDINGS

Different cup orientation was obtained as optimum for each combination of the neck angles (40 degrees and 50 degrees) and oscillation angles (120 degrees and 135 degrees). The corresponding optimum stem anteversion was also different when different neck angle was used.

INTERPRETATION

As the margin for the impingement-free orientation of the cup was small, the optimum cup orientation and stem anteversion should be adopted specific for each combination of the neck angle and oscillation angle.

The combined anteversion technique for acetabular component anteversion

The combined anteversion technique for acetabular component placement of total hip arthroplasty is beneficial because of the surgeons' limited ability to control the anteversion of a cementless femoral stem. Our data show that the cementless stem anteversion can be 15 degrees different than anticipated. By determining femoral stem anteversion before positioning cup anteversion, the cup anteversion can be adjusted for the stem anteversion. The combined anteversion technique should provide a mean near 35 degrees with a safe zone of 25 degrees to 50 degrees.

An in vivo model for intraoperative assessment of impingement and dislocation in total hip arthroplasty

We have developed an intraoperative model to quantify total hip arthroplasty impingement and dislocation mechanics using fluoroscopy and shape-matching techniques. Two patient groups were investigated: group 1 consisted of 12 hips using 28- or 32-mm femoral heads and an anterolateral surgical approach, and group 2 consisted of 17 hips using 22- or 26-mm femoral heads and a posterolateral surgical approach. During intraoperative hip stability testing consisting of extension and external rotation motions, group 1 was more unstable, and prosthetic impingement was the major reason for dislocation. With flexion and internal rotation motions, group 2 was more unstable, and superior-lateral impingement or soft tissue traction was the major reason for dislocation. Intraoperative quantitative assessment of hip mechanics provides a safe and clinically relevant method to characterize potential complications and evolve techniques to prevent them.

Results of the Birmingham Hip Resurfacing dysplasia component in severe acetabular insufficiency

The dysplasia cup, which was devised as an adjunct to the Birmingham Hip Resurfacing system, has a hydroxyapatite-coated porous surface and two supplementary neutralisation screws to provide stable primary fixation, permit early weight-bearing, and allow incorporation of morcellised autograft without the need for structural bone grafting.

A total of 110 consecutive dysplasia resurfacing arthroplasties in 103 patients (55 men and 48 women) performed between 1997 and 2000 was reviewed with a minimum follow-up of six years. The mean age at operation was 47.2 years (21 to 62) and 104 hips (94%) were Crowe grade II or III.

During the mean follow-up of 7.8 years (6 to 9.6), three hips (2.7%) were converted to a total hip replacement at a mean of 3.9 years (2 months to 8.1 years), giving a cumulative survival of 95.2% at nine years (95% confidence interval 89 to 100). The revisions were due to a fracture of the femoral neck, a collapse of the femoral head and a deep infection. There was no aseptic loosening or osteolysis of the acetabular component associated with either of the revisions performed for failure of the femoral component. No patient is awaiting a revision.

The median Oxford hip score in 98 patients with surviving hips at the final review was 13 and the 10th and the 90th percentiles were 12 and 23, respectively.

THA using an anatomic stem in patients with femoral head osteonecrosis

Treating young patients with femoral head osteonecrosis (ON) remains challenging. Anatomic stems were introduced in the 1980s and 1990s to improve the proximal canal fit in an attempt to enhance long-term implant survival, an important aspect of treating young patients. We began using one design in 1993 and asked three questions to confirm whether the design criteria improved outcomes in patients with ON: (1) What is the long term survivorship of these implants?; (2) What is the amount and rate of wear?; and (3) What is the incidence of osteolysis? We retrospectively reviewed 56 patients (69 hips) who underwent THA for femoral head ON with a cementless anatomic stem proximally coated with hydroxyapatite. Four patients (four hips) were lost to followup and 16 patients (19 hips) died. In the remaining 36 patients (46 hips) the minimum followup was 10 years (mean, 11.2 years; range, 10-13 years). The mean age at operation was 48.6 years. The average Harris hip score at last followup was 87 points. Worst-case survivorship was 58.1% at 13 years and best-case was 93.3%. The average linear wear of the polyethylene liner was 2.02 mm and the average annual wear was 0.18 mm per year. Thirty-seven hips (80%) had femoral osteolysis and 14 (30%) had acetabular osteolysis. One patient who had extensive femoral osteolysis and stem loosening was revised at 11.2 years postoperatively. The high rates of polyethylene wear and osteolysis are of concern.

Bony impingement affects range of motion after total hip arthroplasty: A subject-specific approach

Hip range of motion after total hip arthroplasty has been shown to be dependent on prosthetic design and component placement. We hypothesized that bony anatomy would significantly affect range of motion. Computer models of a current generation hip arthroplasty design were virtually implanted in a model of pelvis and femur in various orientations ranging from 35 degrees to 55 degrees cup abduction, 0 degrees to 30 degrees cup anteversion, and 0 degrees to 30 degrees femoral anteversion. Four head sizes ranging from 22.2 to 32 mm and two neck sizes ranging from 10-mm and 12-mm diameter were tested. Range of motion was recorded as maximum flexion-extension, abduction-adduction, and axial rotation of the femur before any contact between prosthetic components or bone was detected. Bony impingement preceded component impingement in about 44% of all conditions tested, ranging from 66% in adduction to 22% in extension. Range of motion increased as head size increased. However, increasing head size also increased the propensity for bony impingement, which tended to reduce the beneficial effect of increased head size on range of motion. Reducing neck diameter had a greater effect on prosthetic impingement (mean, 3.5 degrees increase in range of motion) compared to bone impingement (mean, 1.9 degrees ). This model allowed for a clinically relevant assessment of range of motion after total hip arthroplasty and may also be used with patient-specific geometry [such as that obtained from preoperative computed tomography (CT) scans] for more accurate preoperative planning.

Resolving inconsistencies in defining the target orientation for the acetabular cup angles in total hip arthroplasty

BACKGROUND

Dislocation following total hip arthroplasty is a major complication and malorientation of the acetabular cup is one of the primary factors affecting dislocation. Different conventions used to describe the cup orientation produce significant variations in the recommendations for correct positioning, which in turn make it difficult for clinicians to properly interpret and apply previously reported studies.

METHODS

We examined nine articles presenting recommendations for the range of target orientations of the acetabular cup to minimize the risk of dislocation (referred to as the 'safe zone'). Those studies included five ways to define the cup orientation and two methods to define the reference frame. We converted those recommendations to a single representation based on the radiographic angles expressed in the pelvic frame reference.

FINDINGS

After conversion, the mean recommended anteversion angle was shifted downward by 5 degrees (P<0.01). Also, the target orientation recommendations became more consistent, especially for the anteversion angles where the standard errors of the upper and lower limits were reduced by 61% (P=0.02) and 23% (P=0.04), respectively.

INTERPRETATION

The choice of reference frame and the definition for acetabular cup orientation angles can have a significant effect on the target orientation for the acetabular cup. Recommendations for the target orientation should always explicitly state which reference frame and angle definition is being used. The averaged recommendation of the studies assessed here is 41 degrees inclination and 16 degrees anteversion in radiographic angles or 39 degrees inclination and 21 degrees anteversion in operative angles, both expressed in the pelvic reference frame.

Isolated fracture of the ceramic head after third-generation alumina-on-alumina total hip arthroplasty

BACKGROUND

While most reports of component fracture following alumina-on-alumina total hip arthroplasty have involved the acetabular liner, few have involved fracture of the alumina femoral head. In the present multicenter study, we investigated ceramic head fractures in a cohort of patients who underwent third-generation alumina-on-alumina total hip arthroplasty.

METHODS

We performed a retrospective study of 312 patients (367 hips) who underwent alumina-on-alumina total hip arthroplasty without cement at four participating centers with the use of a 28-mm BIOLOX forte femoral head and a BIOLOX forte liner from July 2001 to October 2003. Three hundred and five patients (359 hips) were evaluated at a mean of forty-five months postoperatively. Clinical follow-up with use of the Harris hip score and radiographic evaluation were performed at six weeks; at three, six, and twelve months; and every six months thereafter. Retrieved ceramic implants were examined by means of visual inspection.

RESULTS

Five hips (1.4%) in five patients were revised because of a ceramic head fracture during the follow-up period. The ceramic head fractures occurred during normal daily activities at a mean of 22.6 months postoperatively. A short neck had been used in all five hips in which a fracture occurred, compared with 121 (34.2%) of the 354 hips in which a fracture did not occur (p = 0.009). The fracture involved a circular crack along the circumference of the thinnest portion of the head component at the proximal edge of the bore. The fracture also involved multiple vertical cracks extending radially along the longitudinal axis from the circumference of the circular crack line to the lower edge of the head component.

CONCLUSIONS

In the present study, the rate of ceramic head fracture associated with one design of a short-neck modular alumina femoral head was 1.4% (five of 359). The extent to which these findings are generalizable to other designs that utilize this type of femoral head is unknown.

The influence of acetabular component position on wear in total hip arthroplasty

Our experience has implicated cup inclination as an important factor in wear, whereas others have suggested that the hip center of rotation (COR) must be closely reestablished to reduce wear. We conducted a retrospective study to determine the relative importance of these 2 factors. One hundred thirty-nine total hip arthroplasties were studied after a mean follow-up of 9.2 years (range, 6-3 years). Forty-nine of 139 operated hips had a contralateral normal hip, which allowed the most accurate measurement of the influence of change in the COR. Wear was related to the inclination of the cup but not to a change in the COR. Secondarily, wear was less with a ceramic-polyethylene polyarticular surface than with metal-polyethylene. The importance of this data is related to cup implantation techniques. The hip COR can be moved superiorly and/or medially to permit cup inclination below 45 degrees with correct cup coverage.