The initial stability and contact mechanics of a press-fit resurfacing arthroplasty of the hip

Finite element analysis was used to examine the initial stability after hip resurfacing and the effect of the procedure on the contact mechanics at the articulating surfaces. Models were created with the components positioned anatomically and loaded physiologically through major muscle forces. Total micromovement of less than 10 μm was predicted for the press-fit acetabular components models, much below the 50 μm limit required to encourage osseointegration. Relatively high compressive acetabular and contact stresses were observed in these models. The press-fit procedure showed a moderate influence on the contact mechanics at the bearing surfaces, but produced marked deformation of the acetabular components. No edge contact was predicted for the acetabular components studied.

It is concluded that the frictional compressive stresses generated by the 1 mm to 2 mm interference-fit acetabular components, together with the minimal micromovement, would provide adequate stability for the implant, at least in the immediate post-operative situation.

Clinical outcome of total hip arthroplasty using the normalized and proportionalized femoral stem with a minimum 20-year follow-up

Currently, there are several femoral stem designs available for use, but few have an extended track record. We have previously reported on 10- and 15-year outcome studies of total hip arthroplasty (THA) using a cemented normalized and proportionalized femoral stem from a single surgeon series. This is a follow-up study reporting the minimum 20-year outcome of this femoral stem design. The study began with THA performed in a consecutive series of 184 patients; stem fixation was achieved using first-generation cementing techniques. The overall early complication rate was 10%. There were 23 patients (31 hips) who had been followed-up for a minimum 20-year period (average 21.3 years). Mean d'Aubigne and Postel scores improved from 5.9 to 11.3; mean Harris hip scores improved from 43.8 to 92.8. Kaplan-Meier survivorship was 93% at 20 years (95% confidence interval); there were no stem failures. The use of a cemented normalized and proportionalized femoral stem in primary THA provides satisfactory long-term clinical and radiological outcomes in patients.

Loss in mechanical contact of cementless acetabular prostheses due to post-operative weight bearing: A biomechanical model

The primary stability of cementless acetabular components is a prerequisite for their clinical success. The target of the present study was to analyse possible effects of post-operative joint loading on the initial mechanical stability of a press-fitted acetabular prosthesis. For this purpose, a three-dimensional finite element model of the pelvic bone with acetabular reconstruction was set-up. The analysis included two steps: (1) simulation of the prosthesis press-fit implantation and (2) simulation of the instant of peak resultant hip loading during the one-legged stance. The difference between the contact pressures at the bone/implant interface, at the end of the second step and those at the end of the first step was calculated and assumed as an index of variation in mechanical contact due to post-operative weight bearing. The results show that, due to hip loading, contact pressures given by press-fit increase in the postero-superior acetabular region but decrease in the antero-inferior acetabular region. The calculated area in which the contact pressures decrease extend to about 30% of the total contact surface. These results imply that post-operative joint loading significantly reduces the mechanical stability given by press-fit. The decrease in contact pressures at the bone/implant interface may result in a lack of osteointegration, possibly hindering the implant secondary stability. It may also create a route for wear debris, possibly favouring periprosthetic osteolysis, which may lead to further loss in contact and clinical failure of the implant due to loosening.

Experimental validation of noninvasive referencing in navigated procedures on long bones

Navigation procedures in orthopedic surgery require fixation of reference markers to the anatomic region of interest. Inadequate fixation might lead to micromotion or loosening of the reference marker, consequently causing registration failures or errors in navigation. Osseous rigid fixation is usually achieved by minimally invasive Schanz screws or pins. The goal of this study was to evaluate a non invasive external fixation device, a headband so far used in cranial navigation, as an alternative invasive fixation technique to reference markers in the femur. A common navigation system with an adapted trauma software application was used to track the positions of the soft tissue-attached headband relative to an invasive reference marker on the femur during manipulations of the thigh. Relative translative and rotational movements of the headband were measured during defined movements of the hip and knee and manipulations of the headband itself. The results revealed high translative and rotational movements, up to 6 mm and 3 degrees , respectively, due to minor manipulations of the affected lower extremity. Noninvasive soft tissue fixation with a headband does not allow rigid fixation for accurate navigated registration or operative procedures at the femur. Necessary intraoperative movements or manipulations would cause substantial registration failures. Invasive fixation techniques with screws or pins are still the method of choice.

Assessment of cup position from plain radiographs: impact of pelvic tilting

The acetabular cup position after total hip arthroplasty (THA) regarding its inclination and version angles are influential parameters concerning the postoperative range of motion and dislocation stability. Standard anterior-posterior X-rays remain an important diagnostic instrument to observe the postoperative outcome and to secure quality control after THA, where an optimal positioning of the patient is recommended when taking these X-rays. The purpose of this preliminary study was to determine the effect of pelvic tilting regarding the positioning calculation of the acetabular cup from standard radiographs using a modified method according to Pettersson et al. (Acta Radiol Diagn, 23:259-263, 1982). In our model experiment, we were able to show that pelvic tilting to either side causes a considerable difference between the radiographic and calculated version angles following approximately linear functions. However, pelvic tilting to either side, leads, regarding the calculation of the inclination, to an average deviation between radiographic and calculated inclination angles less than 2 degrees .

Alteration of the hip joint centre during acetabular reaming

Change of the hip joint centre location during preparation of the acetabular cavity for the acetabular component can affect the outcome of total hip arthroplasty. Deviations from the preoperative geometry can compromise an otherwise successful operation with regard to hip dislocations, leg length inequality and range of motion of the hip joint. Eighteen acetabula from pelvic specimens were measured before and after acetabular reaming to determine the change of hip centre location. Two different acetabular reamers were applied to the acetabular cavity: a chamfered reamer intended for minimal invasive hip surgery (MIS) and a conventional hemispherical reamer. An optical 3D scanning system created 3D models of the cavities prior to and after the reaming procedure. The two 3D models were merged into a single 3D model and displacements in all three dimensions were calculated The results showed no significant difference between MIS and conventional reaming with regard to transition vector length (p=0.9). The mean length of the transition vector was 3.6 mm (SD. 2.4 mm). Our findings suggest that the alteration of the hip centre location is not influenced by the changes made to the MIS reamers in comparison with conventional reamers. In comparison with previous studies the drift of the hip centre caused by the acetabular reaming is reduced due to new reaming techniques and prosthesis designs.

Resektion der infizierten Achillessehne

BACKGROUND

Structural full-thickness defects of the Achilles tendon represent a severely disabling injury which should be treated by reconstruction. This study presents functional outcomes from standardised follow-up of non-reconstructed Achilles tendons with soft tissue coverage alone.

PATIENTS AND METHODS

Seven patients were treated with various techniques of soft tissue reconstruction without restoration of the Achilles tendon. After a mean of 11 months postoperatively, the lower extremity was evaluated generally with regard to function, AOFAS score (ankle and hindfoot), and isokinetic torque testing by the Biodex system.

RESULTS

All patients showed high mobility, muscle strength, and range of motion at follow-up. The average AOFAS score was 84.7 (maximum 100), and the torque loss in plantar flexion was 44.5% on average compared to the uninvolved side and thus comparable with results after secondary tendon reconstruction.

CONCLUSION

After complete loss of the Achilles tendon, compensatory techniques can hinder significant loss of torque and endurance, compared with secondary tendon reconstruction, allowing unsupported mobility and even top athletic performance.

The potential for bone loss in acetabular structures following THA

Attempts to preserve periacetabular bone stock following total hip replacement have largely ignored the potential for stress shielding in the acetabulum. We sought to quantify the change in stress distribution in acetabular bone with components of varying material stiffness by developing a high-resolution 3-D finite element model from CT scans of a young female donor. Periprosthetic bone stresses and strains on the left pelvis were compared with hemispherical cups of various material properties and with a horseshoe shaped polymeric design described in the recent literature. We observed unphysiologic periacetabular bone stress and strain fields for all designs tested. For hemispherical components, reduction of the acetabular shell material modulus caused modest changes in bone stress compared to the changes in implant geometry. The horseshoe shaped cup more effectively loaded the acetabular structures than the hemispherical design. Our results suggest stress and strain fields in pelvic structures after introduction of hemispherical acetabular components predict inevitable bone adaptation that can not be resolved by changes in implant material properties alone. Radical changes in implant design may be necessary for long-term maintenance of supporting structures in the reconstructed acetabulum.

Präzision in der orthopädischen Computernavigation

Navigation has become increasingly integrated into orthopaedic surgery, especially in the area of endoprosthetic procedures. Simplification of the instrumentation along with the use of imageless systems has increased the ease of use for the orthopaedic surgeon. Principle navigation systems enable an accuracy of corrections and alignments within intervals of 1 mm or 1 degrees . Consequently, potential intra- and interobserver failures during the registration procedure typically range within a few millimetres or degrees. Analysis of the actual algorithms used for the registration process of the lower extremity mechanical axis and the articular surfaces reveal valid and reproducible results. With the help of navigation, it is possible to achieve a higher degree of precision in total hip and knee implant placement, including a distinct reduction in variance as compared to conventional techniques. Similarly, application of navigation during a high tibial osteotomy or at the osteotomy of the distal radius also enables a more precise correction of the axis of the affected extremity, in addition to improved reproducibility. Despite these promising early results, large prospective clinical studies comparing conventional techniques versus computer assisted navigation are thus far only available for total knee arthroplasty. Whether navigated prosthesis placement can truly extend the longevity of an implant will require continued observation in the years to come. In addition, further prospective studies are required to determine the benefit of navigation in other orthopaedic procedures.

Orientation of the acetabular component

We compared the orientation of the acetabular component obtained by a conventional manual technique with that using five different navigation systems.

Three surgeons carried out five implantations of an acetabular component with each navigation system, as well as manually, using an anatomical model. The orientation of the acetabular component, including inclination and anteversion, and its position was determined using a co-ordinate measuring machine.

The variation of the orientation of the acetabular component was higher in the conventional group compared with the navigated group. One experienced surgeon took significantly less time for the procedure. However, his placement of the component was no better than that of the less experienced surgeons. Significantly better inclination and anteversion (p < 0.001 for both) were obtained using navigation. These parameters were not significantly different between the surgeons when using the conventional technique (p = 0.966).

The use of computer navigation helps a surgeon to orientate the acetabular component with less variation regarding inclination and anteversion.

A new tool for measuring cup orientation in total hip arthroplasties from plain radiographs

Orientation of the hip cup is important in total hip arthroplasties. Orientation includes abduction (inclination) and anteversion. Anteversion can be considered as true (anatomic) and planar (radiographic) anteversion. Some measurement methods either are too complicated or are less precise. We developed a new protractor to measure cup orientation using postoperative anteroposterior radiographs centered at the hip. The new protractor measures true and planar anteversion and abduction easily and precisely. We verified its accuracy using a software simulator and simulated 45 radio- graphs of total hip arthroplasties with 15 different anteversions ranging from 15 degrees -29 degrees and 45 actual radiographs of total hip arthroplasties. We then measured the planar ante- version with our method and the method of Lewinnek et al. Maximal errors were 3 degrees and 2.61 degrees , respectively, and mean errors were 0.96 degrees and 1.2 degrees , respectively. The standard deviations were 0.74 degrees with our method and 0.57 degrees with the method of Lewinnek et al. For the real radiographs, the mean of absolute difference between the two methods was 1.34 degrees , and the standard deviation was 1.13 degrees . We found no difference between the two methods and no difference in our findings compared with those of Pradhan.

Biomechanics of the Birmingham hip resurfacing arthroplasty

The effects of the method of fixation and interface conditions on the biomechanics of the femoral component of the Birmingham hip resurfacing arthroplasty were examined using a highly detailed three-dimensional computer model of the hip. Stresses and strains in the proximal femur were compared for the natural femur and for the femur resurfaced with the Birmingham hip resurfacing. A comparison of cemented versus uncemented fixation showed no advantage of either with regard to bone loading. When the Birmingham hip resurfacing femoral component was fixed to bone, proximal femoral stresses and strains were non-physiological. Bone resorption was predicted in the inferomedial and superolateral bone within the Birmingham hip resurfacing shell. Resorption was limited to the superolateral region when the stem was not fixed. The increased bone strain observed adjacent to the distal stem should stimulate an increase in bone density at that location. The remodelling of bone seen during revision of failed Birmingham hip resurfacing implants appears to be consistent with the predictions of our finite element analysis.

Misinterpretation of cup anteversion in total hip arthroplasty using planar radiography

INTRODUCTION

Anteroposterior pelvic radiographs are routinely used to monitor cup orientation in total hip arthroplasty (THA). Analysis of planar radiographs leads to a certain degree of measurement error for the cup anteversion (AV). With the current study, we wanted to clarify whether planar radiography can be used for accurate evaluation of the THA position.

MATERIALS AND METHODS

The postoperative orientation of pelvic implants in 42 patients was analyzed according to five documented mathematical algorithms using planar radiographs. Postoperative computed tomography (CT) pelvis scans were available for all patients. A CT-based navigation system was used to determine AV.

RESULTS

The comparison showed that all five formulas presented substantial variations for the AV angle. Of these, Widmer's algorithm presented the smallest difference compared to the CT. Misinterpretation of postoperative planar radiographs is a common problem in THA.

CONCLUSION

Planar radiographs are too imprecise for exact evaluation of the correct cup AV after THA. CT-based analysis may be necessary if exact values are required.

Early clinical and radiographic analysis of the Fitmore cup

The Fitmore titanium mesh cementless acetabular component in 115 hip arthroplasties was reviewed at an average of 33 months of follow-up. None were revised nor had infection. One hip dislocated 4 years postoperatively. Two femoral components were revised. The average Harris Hip Score at the last follow-up was 90 points. In the 96 sets of radiographs available, there was no loosening or new radiolucency. One hip had nonprogressive osteolysis adjacent to a screw. This press-fit cup has its polar region flattened and is rim loading. Noncontact (gaps) at the acetabular floor is expected and is not critical for fixation. In all but 6 hips, these gaps filled. In 5 hips, a minimal gap (<or=0.5 mm) remained in part of the equatorial region. The Fitmore cup osseointegrates successfully.

Using intraoperative pelvic landmarks for acetabular component placement in total hip arthroplasty

Dislocation after total hip arthroplasty is frequently due to acetabular malpositioning. Positioning of the acetabular component using anatomical landmarks may reduce the incidence of dislocation from improper acetabular orientation. The pelvis provides 3 bony landmarks (ilium, superior pubic ramus, and superior acetabulum), which, when used to define a plane, allows cup orientation in abduction and version. Landmarks evaluated in 24 cadaveric acetabuli allowed slightly increased abduction and anteversion of the cup, compared with native acetabuli. Six hundred seventeen primary total hip arthroplasties were performed between 1996 and 2003 using this technique. Mean cup abduction was 44.4 degrees with 13.2 degrees of anteversion. This technique allows satisfactory reproducible cup orientation based on individual pelvic morphology. Review of patient outcome data suggest high patient satisfaction and lower dislocation rate without additional equipment, time, or cost.

The transverse acetabular ligament: an aid to orientation of the acetabular component during primary total hip replacement

Ensuring the accuracy of the intra-operative orientation of the acetabular component during a total hip replacement can be difficult. In this paper we introduce a reproducible technique using the transverse acetabular ligament to determine the anteversion of the acetabular component. We have found that this ligament can be identified in virtually every hip undergoing primary surgery. We describe an intra-operative grading system for the appearance of the ligament. This technique has been used in 1000 consecutive cases. During a minimum follow-up of eight months the dislocation rate was 0.6%. This confirms our hypothesis that the transverse acetabular ligament can be used to determine the position of the acetabular component. The method has been used in both conventional and minimally-invasive approaches.

Positionnement de la cupule d’une prothèse totale de hanche par navigation sans image basée sur la cinématique articulaire

PURPOSE OF THE STUDY

Most navigation systems for computer-assisted total hip arthroplasty (THA) require prior computed tomography (CT) or acquisition of multiple bone landmarks on the pelvis. In order to avoid these problems, we developed a computer-assisted navigation system without CT based on a kinematic approach to the hip joint. The principle is to orient the cup in relation to the cone describing the hip joint range of motion. The purpose of this work was to analyze preliminary results.

MATERIAL AND METHODS

Eighteen primary THA were implanted with the system (16 women, two men, mean age 68 +/- 7.8 years, age range 54-83 years, 18 degenerative hip disease). Two optoelectronic captors were fixed percutaneously on the pelvis and the distal femur. The acetabulum was prepared first followed by the femur using reamers and broaches of increasing size. The last broach placed in the femur was equipped with a large head adapted to the newly prepared acetabulum. The range of hip motion was recorded to determine the maximal range of motion cone. The acetabular cup was thus positioned in order the prosthesis range of motion included entirely the maximal range of motion of the hip joint.

RESULTS

One patient fell three weeks after implantation causing posterior dislocation; there was no recurrence. The Postel-Merle-d'Aubligné score improved from 8 +/- 2.9 (range 3-12) preoperatively to 17 +/- 0.8 (range 16-18) at last follow-up. None of the patients complained about the captor insertion and there were no cases of hematoma or fracture. Operative time was 35-40 minutes longer for the first four cases and was progressively reduced 15-20 minutes for the last four cases. Mean leg length discrepancy was 5.6 +/- 7.5 mm (range 0-25) before implantation and 0.6 +/- 3 mm (range -5 to 10 mm) at last follow-up. CT-scan measurements revealed a mean anteversion of the femoral implant of 18.2 +/- 8.5 degrees (range 0-31). Anatomic anteversion of the cup (measured from the pelvis landmark and thus independently of the position of the pelvis) was 24.7 +/- 8.8 degrees (range 12-40). The sum of the femoral and anatomic acetabular anteversions was 43 +/- 13.1 degrees (range 22-71). Anteversions were 16 degrees for the cup and 16 degrees for the stem for the one case of dislocation.

CONCLUSION

This method can be used in routine without lengthening operative time significantly. It safely controls leg length and helps position the cup. This study demonstrated that there is no ideal position for the cup which can be used for all patients. Because of the wide range of inclination and anteversion figures, half of the cases were outside the safety zone recommended by Lewinnek.

Imageless navigation for insertion of the acetabular component in total hip arthroplasty: is it as accurate as CT-based navigation?

In a prospective randomised clinical study acetabular components were implanted either freehand (n = 30) or using CT-based (n = 30) or imageless navigation (n = 30). The position of the component was determined post-operatively on CT scans of the pelvis. Following conventional freehand placement of the acetabular component, only 14 of the 30 were within the safe zone as defined by Lewinnek et al (40 degrees inclination sd 10 degrees ; 15 degrees anteversion sd 10 degrees ). After computer-assisted navigation 25 of 30 acetabular components (CT-based) and 28 of 30 components (imageless) were positioned within this limit (overall p < 0.001). No significant differences were observed between CT-based and imageless navigation (p = 0.23); both showed a significant reduction in variation of the position of the acetabular component compared with conventional freehand arthroplasty (p < 0.001). The duration of the operation was increased by eight minutes with imageless and by 17 minutes with CT-based navigation. Imageless navigation proved as reliable as that using CT in positioning the acetabular component.

The effects of implant surface roughness and surgical technique on implant fixation in an in vitro model

OBJECTIVES

The aim of the present study was to determine the relationship between implant surface parameters, surgical approach and initial implant fixation.

MATERIAL AND METHODS

Sixty tapered, conical, screw-shaped implants with machined or etched surface topography were implanted into the explanted femoral condyle of goats. The implant sites were prepared either by a conventional technique, by undersized preparation, or by the osteotome technique. Peak insertion & removal torque, bone-to-implant contacts (BIC) and morphological bone appearance were assessed by scanning electron microscope (SEM) and micro-computer tomography. (micro-CT).

RESULTS

Insertion and removal torque values were significantly higher for etched implants inserted with the undersized technique (115.2 +/- 31.1, 102.9 +/- 36.4 N cm) respectively. Also, the average BIC value was higher for the etched implants placed with the undersized technique (87.5 +/- 5.6), which was statistically significant compared with machined and etched implants inserted by conventional technique.

CONCLUSION

In conclusion, this study shows that the surgical technique has a decisive effect on implant fixation (represented in this study by installation torque value/removal torque value and histomorphometric evaluation) in trabecular bone. Nevertheless, additional in vivo studies have to be done to prove the importance of surgical protocol for the final implant-bone response.

Method to reduce variations of inclination angle of the acetabular component during mini-incision hip arthroplasty

BACKGROUND

To reduce variations of cup inclination after total hip arthroplasty using the mini-incision posterior approach, we introduced two techniques, one at a time. The first technique is measuring a pelvic tilt angle in the frontal plane in the initial lateral position in the operating room. The second technique is using a tilt-meter to adjust the direction of a cup holder. The purpose of this study was to evaluate the usefulness of these techniques.

METHODS

For 106 hips operated on, the cementless acetabular component was impacted using a cup holder targeting 45 degrees in inclination and 20 degrees in anteversion. These hips were divided into three consecutive groups. For the first group (30 hips), no radiograph in the lateral position was obtained, and the alignment frame of the cup holder was aligned parallel to the floor by eye measurement. For the second group (56 hips), we measured the pelvic tilt angle, and tilted the alignment frame by eye measurement. For the third group (20 hips), we measured the pelvic tilt angle and tilted the alignment frame using the tilt meter. Inclination and anteversion angles were measured on postoperative radiographs. The absolute value of the difference between the measured angle and the target angle was defined as the inclination error or anteversion error, respectively.

RESULTS

The inclination error was more than 5 degrees for 33% in the first group, 20% in the second group, and 0% in the third group (P = 0.015, chi-squared test). There was a significant difference between the first group and the third group (P = 0.0039). For the anteversion error, there were no significant differences among the three groups.

CONCLUSIONS

Adjustment using a tilt-meter after measuring a pelvic tilt angle is a useful method to reduce the rate of large inclination error.

Avoiding sural nerve injuries during percutaneous Achilles tendon repair

BACKGROUND

Sural nerve injury is a reported risk during percutaneous repair of the Achilles tendon.

HYPOTHESIS

Exposure of the sural nerve during percutaneous repair can minimize the risk of nerve injury.

STUDY DESIGN

Case control study; Level of evidence, 3.

METHODS

The authors retrospectively examined the results of 84 patients who were treated for acute Achilles tendon rupture at 2 different hospitals. Both hospitals used the same percutaneous repair technique, except that the sural nerve was exposed in the 38 patients (mean age, 38 years; range, 23-68 years) of one hospital; the nerve was not exposed in the 46 patients (mean age, 42 years; range, 24-71 years) of the other hospital (the nonexposure group).

RESULTS

All patients recovered and returned to work after 44 days (range, 5-202 days). All patients returned to their previous sports levels within 1 year. On the 100-point Hannover Achilles Tendon Score, the mean score was 81 points (range, 44-100 points). The overall incidence of sural nerve related complications was 18%. All sural nerve lesions occurred in the nonexposure group. In the total study population, there were 3 cases of deep vein thrombosis, 1 rerupture, and 1 case of infection.

CONCLUSION

Sural nerve injuries can be minimized during surgery by carefully placing the stab incisions to expose the nerve so as to avoid it during repair. If the sural nerve is exposed, percutaneous repair of the ruptured Achilles tendon is a safe and reliable method of treating Achilles tendon ruptures.

Deformation of press-fitted metallic resurfacing cups. Part 1: Experimental simulation

The interference press fit of a metallic one-piece acetabular cup employed for metal-on-metal hip resurfacing procedures was investigated experimentally under laboratory conditions in the present study, in particular regarding the cup deformation. Tests were carried out in cadavers as well as polyurethane foams of various grades with different elastic moduli to represent different cancellous bone qualities. The cadaver test was used to establish the most suitable configuration of the foam model representing realistic support and geometrical conditions at the pelvis. It was found that a spherical cavity, with two identical areas relieved on opposite sides, was capable of creating a two-point pinching action of the ischeal and ilial columns on the cup as the worst-case scenario. Furthermore, the cup deformation produced from such a two-point loading model with a grade 30 foam was similar to that measured from the cadaver test. Therefore, such a protocol was employed in subsequent experimental tests. For a given size of the outside diameter of the cup of 60 mm, the cup deflection was shown to be dependent largely on the cup wall thickness and the diametral interference between cup and prepared cavity at implantation. For a relatively thin cup with a wall thickness between 2.3 mm (equator) and 4 mm (pole) and with a modest nominal diametral interference of 1 mm, which corresponds to an actual interference of approximately 0.5 mm, the maximum diametral cup deflection (at the rim) was around 60 μm, compared with a diametral clearance of 80-120 μm between the femoral head and the acetabular cup, generally required for fluid-film lubrication and tribological performances. Stiffening of the cup, by both thickening and lateralizing by 1 mm, reduced the cup deformation to between 30 and 50 μm with actual diametral interferences between 0.5 and 1 mm.

Estimation of pelvic tilt on anteroposterior X-rays--a comparison of six parameters

OBJECTIVE

To compare six different parameters described in literature for estimation of pelvic tilt on an anteroposterior pelvic radiograph and to create a simple nomogram for tilt correction of prosthetic cup version in total hip arthroplasty.

DESIGN

Simultaneous anteroposterior and lateral pelvic radiographs are taken routinely in our institution and were analyzed prospectively. The different parameters (including three distances and three ratios) were measured and compared to the actual pelvic tilt on the lateral radiograph using simple linear regression analysis.

PATIENTS

One hundred and four consecutive patients (41 men, 63 women with a mean age of 31.7 years, SD 9.2 years, range 15.7-59.1 years) were studied.

RESULTS

The strongest correlation between pelvic tilt and one of the six parameters for both men and women was the distance between the upper border of the symphysis and the sacrococcygeal joint. The correlation coefficient was 0.68 for men (P<0.001) and 0.61 for women (P<0.001). Based on this linear correlation, a nomogram was created that enables fast, tilt-corrected cup version measurements in clinical routine use.

CONCLUSION

This simple method for correcting variations in pelvic tilt on plain radiographs can potentially improve the radiologist's ability to diagnose and interpret malformations of the acetabulum (particularly acetabular retroversion and excessive acetabular overcoverage) and post-operative orientation of the prosthetic acetabulum.

Self-calibrating 3D-ultrasound-based bone registration for minimally invasive orthopedic surgery

Intraoperative freehand three-dimensional (3-D) ultrasound (3D-US) has been proposed as a noninvasive method for registering bones to a preoperative computed tomography image or computer-generated bone model during computer-aided orthopedic surgery (CAOS). In this technique, an US probe is tracked by a 3-D position sensor and acts as a percutaneous device for localizing the bone surface. However, variations in the acoustic properties of soft tissue, such as the average speed of sound, can introduce significant errors in the bone depth estimated from US images, which limits registration accuracy. We describe a new self-calibrating approach to US-based bone registration that addresses this problem, and demonstrate its application within a standard registration scheme. Using realistic US image data acquired from 6 femurs and 3 pelves of intact human cadavers, and accurate Gold Standard registration transformations calculated using bone-implanted fiducial markers, we show that self-calibrating registration is significantly more accurate than a standard method, yielding an average root mean squared target registration error of 1.6 mm. We conclude that self-calibrating registration results in significant improvements in registration accuracy for CAOS applications over conventional approaches where calibration parameters of the 3D-US system remain fixed to values determined using a preoperative phantom-based calibration.

[Periprosthetic bone loss after total hip endoprosthesis. Dependence on the type of prosthesis and preoperative bone configuration]

The changes of the periprosthetic bone density were examined with DEXA in 81 patients over a period of 1 year after implantation of cementless total hip endoprosthesis. Four types of endoprostheses (Vision 2000/Duraloc, ALPHA-Fit/ALPHA-Lock Plus, CLS/Allofit, Mayo/Trilogy) were implanted. Information on the changes of the periprosthetic bone density depending on the type of the prosthesis and the bony situation at the femur before operation was expected from these measurements. In all types of stems the strongest reduction of the bone density was found in the region of the calcar femoris, and the smallest changes were found distally and medially of the tip of the prostheses. In the prosthesis with shorter stem the change of the bone density was altogether clearly lower than in prostheses with longer stem. With increasing size of the prosthesis with proximally porous coating made from cobalt-chrome alloy, proximal atrophy was observed more frequently, whilst in the prosthesis made from titanium alloy with completely rough-blasted surface the distal hypertrophy increased. A low preoperative corticalis-bone marrow index strengthened the proximal atrophy in proximally porously coated prosthesis made from cobalt-chrome alloy and led in the prosthesis with completely rough-blasted surface more often to distal hypertrophy of the bone.

The safe-zones for combined cup and neck anteversions that fulfill the essential range of motion and their optimum combination in total hip replacements

Reduction of the range of motion (ROM) until prosthetic impingement of a total hip replacement may lead to frequent impingement, subluxation and dislocation especially for patients with good hip movement. The ROM until prosthetic impingement can be calculated using the technical ROM (theta) and the cup and neck positions by a previously created mathematical formula. A larger (theta) with proper cup and neck positions results in a larger ROM. However there was only one paper written in English, which revealed the optimum theoretical combination of cup and neck anteversions. ROM of more than 110 degrees flexion, 30 degrees internal-rotation at 90 degrees flexion, 30 degrees extension and 40 degrees external-rotation were defined as the criteria for essential ROM for ADL. The safe-zones for combined cup anteversion (betaanat) and neck anteversion (b) were defined as the areas that fulfill all the criteria of ROM without prosthetic impingement. The safe-zones were created for 35 degrees , 45 degrees and 55 degrees cup abductions (alpha) and for 120 degrees and 135 degrees (theta). The safe-zones for combined (betaanat) and (b) were much larger for a 135 degrees (theta) than a 120 degrees (theta). Their safe-zones showed that (b) should be reduced if (betaanat) is increased and choosing a lower (alpha) requires that the sum of (betaanat) and (b) should be higher and vice versa. A (theta) of more than 135 degrees is recommended as it further increases the size of the safe-zone and provides a larger ROM, and the optimum values of combined cup and neck anteversions can be estimated by the formula: (alpha) + (betaanat) + 0.77(b) = 84.3.

Two-incision total hip replacement: Intra-operative fluoroscopy versus imageless navigation for cup placement

The purpose of this study was to describe the surgical technique and to investigate results of a modified two-incision total hip replacement using either intraoperative fluoroscopy or imageless navigation. Twenty-nine patients (30 hips) with a minimum follow-up of one year were enrolled in this study. The patients were evaluated at 3, 6, 12 weeks, 6 months, and 1-year. The functional recovery as represented by the Harris hip score and WOMAC scale were better in the fluoroscopy group of patients at the early postoperative stage (3 wks). Thereafter, both groups showed rapid recovery with no difference in scores. Injury to the lateral femoral cutaneous nerve was the most commonly seen complication and it occurred in 6 hips (fluoroscopy 2; imageless 4). The symptoms were transient and resolved in 6 months in all 6 cases. This study demonstrated that the role of intraoperative fluoroscopy could safely be replaced by an imageless navigation system for the MIS-2 THA.

Total hip arthroplasty in osteopetrosis using computer-assisted fluoroscopic navigation

Hybrid total hip arthroplasty with computer-assisted fluoroscopic navigation was performed on a patient with osteoarthritis due to an autosomal dominant form of osteopetrosis. The surgical procedures were difficult because the bone was extremely hard and brittle with obliteration of the medullary cavity. Especially, preparation of a femoral canal for the stem was technically challenging and required changes from a conventional surgical procedure. Therefore, we used a computer-assisted fluoroscopic navigation system to create the femoral cavity for the stem, and an accurate placement of the prosthesis was achieved. Navigation guidance can be a useful tool when performing arthroplasty in patients with hip osteoarthritis associated with osteopetrosis.

Position of the acetabular cup -- accuracy of radiographic calculation compared to CT-based measurement

OBJECTIVE

A variety of mathematical and trigonometric methods has been described for determining the position of the acetabular cups from conventional radiographs. However, these formulae are subject to unduly large inaccuracies. The aim of the study was to compare the reliability and the accuracy of radiological and CT-based determination of acetabular cup position.

MATERIALS AND METHODS

The positions of acetabular cups of 31 patients were calculated in conventional plain radiographs of the hip using the method described by Widmer. Further, in all patients computed tomograms of the pelvis were performed and the cup position was measured with the aid of a CT-based computer-assisted navigation software. As reference values inclination and anteversion of the cups were calculated in 3D reconstructions of the pelvis with the aid of an image processing software.

RESULTS

The radiological measurement as well as the CT-based method showed good intra- and inter-observer reliability and no significant difference in the calculation of the inclination (p=0.409). However, CT-based determination of anteversion was significantly more exact than radiological measurement (p<0.001). The calculation of the cup anteversion from the X-rays showed serious deviations from the reference method and a substantial error range (X-ray: mean deviation +1.74 degrees, range -16.6 degrees to +29.8 degrees , S.D. +/-9.32 degrees; CT-based: mean deviation -0.74 degrees, range -6.6 degrees to +5.3 degrees, S.D. +/-2.87).

CONCLUSION

For any clinical problem or for clinical studies in which acetabular positions of acetabular cups have to be exactly determined, CT-based measuring methods are obviously the method of choice. Evaluations based only on conventional plain X-rays and calculation of the acetabular cup position using the formula described by Widmer must be regarded as unreliable, particularly, because of problems in measuring the anteversion.

Pfannenposition und Orientierung im Vergleich

BACKGROUND

Aim of this in-vitro study was to compare the hip cup placement for total hip replacement when using different navigation systems compared with the traditional, non-navigated technique.

METHODS

Five different navigation systems were used: the CT-less systems Navitrack, Orthopilot and Surgetics Station, as well as the CT-based Navitrack and VectorVision. Three different surgeons carried out five cup implantations using all navigation systems and the manual approach on a surgery dummy. Cup orientation (inclination and anteversion) and the cup position (achieved cup center) were determined with a coordinate measuring machine.

RESULTS

In the manual group the variability of the cup orientation was higher in comparison and hardly influenced by the surgeon. Navigation was identified as a significant factor for smaller deviations from planned inclination and anteversion angles (p<0,001 for both). Cup position was not affected by surgeon in the manual group (p=0,966). Compared with manual technique, the cup misplacement vector was significantly smaller in the CT-Navitrack group (p<0,001) but higher in the Navitrack (CT-less) and VectorVision group (p<0,001).

CONCLUSIONS

The use of computer navigation will help the surgeon to orientate the acetabular component more accurately but not necessarily with regard to cup positioning.

Total hip arthroplasty through a minimal posterior approach using imageless computer-assisted hip navigation

With decreased exposure in a minimal posterior hip incision, navigation with computer assistance provides an alternative method to accurately place the components. This study compares the results of a series of 82 navigated total hips to a retrospective cohort of 50 hips done with conventional instruments. The surgical incision split the gluteus maximus but did not extend distally into the fascia. The goal of cup placement was 40 degrees to 45 degrees of abduction (ABD) and 17 degrees to 23 degrees of flexion (FLX). Postoperative radiographs were digitized and analyzed. In the study group, 82 hips were done with computer assistance and compared with 50 done with conventional methods (manual) through the same incision. Radiographic analysis showed that there were significantly fewer cases inside the desired range of ABD and FLX in the manual group (6%) compared with the navigation group (30%), P = .001, with significant differences in the variances of ABD and FLX (P = .011 and .028). Improved accuracy of cup placement was found with increased experience in the use of navigation by the surgeon over the time of the series. The use of a computer-assisted surgery navigation system with a minimal posterior incision for a total hip arthroplasty results in significantly more reproducible acetabular component placement.

Long-term results with cementless Fitek (or Fitmore) cups

Fitek cementless cups have been adopted in our department in 1989. The first 100 consecutive Fitek implants were analyzed clinically (Harris hip score) and radiographically (anteroposterior and lateral x-rays) with a mean follow-up of 9.7 years. We did not have any case of cup loosening or any other problem requiring cup revision. In this series, we had 86 excellent, 10 good, 2 fair, and 2 poor results. The 2 poor results were because of 2 cases of aseptic loosening of the stem (1 cemented and 1 cementless). The x-rays showed an average angle of cup inclination of 36.5 degrees (range 16 degrees -54 degrees ) after surgery and no variations at the last follow-up. Bidimensional linear wear of the acetabular component showed 6 cases of measurable wear with an average wear rate per year of 0.265 mm. The overall wear rate per year was 0.02 mm. At the time of the last follow-up examination, we had 3 femoral osteolysis and no case of acetabular osteolysis. In our series, we observed "lack of contact" zones above the polar depression in 71 cases immediately after surgery. The average thickness of these lines was 1 (range 0.5-3.5) mm. Of these, at the last follow-up, 61 cases (86%) showed a complete "filling" of the "lack of contact," whereas in 10 (24%), the "filling" was incomplete (4 cases still showing a radiolucent line [<or=0.5 mm] in zone II). In the first group with "complete filling," we found 23 (37%) cases with bone ingrowth and no migration of the cup, whereas 38 (63%) cases showed bone ingrowth with evidence of cup migration. The Mann-Whitney nonparametric U test and the Kruskal-Wallis test showed that the survival rate of the 100 analyzed cups, after a follow-up time of 9.7 years, was 100% (end point: revision for any cause). Fitek cup showed good clinicoradiographic results.

Morphologic features of the acetabulum

INTRODUCTION

The embryology and development of the hip joint are complex. The acetabulum is not always of the same shape, width, or depth. Minor anatomical abnormalities in the acetabular shape, joint congruences are frequent. Controversies still exist on the importance of these variations and help to prevent problems following in surgical procedures such as acetabular reconstruction and femoracetabular impingement.

MATERIAL AND METHODS

The aim of this study is to provide the location of the unusual facets, the acetabular point, and the anterior ridge of the acetabulum based on a morphological study of human pelvic bones. Morphologic features of the acetabulum, particularly determination of unusual facets, were studied in 226 human coxal bones.

RESULTS

In adult coxal bones the acetabular fossa has an irregular clover-leaf shape, the superior lobe being smaller than the anterior and the posterior lobes. Measured lunate surface area varied between 14.5 and 30.5 cm2. A smooth unusual facet was found anteroinferior to the lunate surface in 62 acetabulums. Measured along the long axis, its size varied between 11 and 17 mm. Three different shapes of the unusual facet were as follows: oval (32.26%), piriform (45.16%), and elongated (22.58%). The prevalence of the piriform facet shape was higher in males. In 59.68% of the bones it extended to the superior ramus of the pubis, and in the remaining 40.32% it was limited within the acetabular margin. It is postulated that this facet could be a consequence of a particular posture, which results in traction of the ligaments attached to this area. Four distinct configurations were identified relative to the anterior acetabular ridge. The majority 98 (43.36%) were curved; 64 (28.33%) were angular; 37 (16.37%) were irregular; and 27 (11.94%) were straight.

CONCLUSION

There have been no reports on details such as unusual facets, acetabular point, and anterior ridge of the acetabulum in a single research. These findings will be of help in planning reorientation procedures, using spikes, screws, and press-fitting for fixation.

Bone-implant interface shear modulus and ultimate stress in a transcortical rabbit model of open-pore Ti6Al4V implants

This experimental study on laser-textured implants aimed to evaluate periimplant bone elasticity and ultimate stress of the bone-implant interface in a rabbit femur model. After randomization, two cylindrical Ti6Al4V samples (3.5 mm wide, 5.5 mm long) were transcortically implanted in each femur of 15 female New Zealand White Rabbits. Polished implants had been laser-textured with 100, 200, and 300 microm diameter pores, and another corundum blasted implant was additionally textured with 200 microm pores. Twelve weeks into the experiment, a modified push-out test was performed. The median shear modulus indicating the elasticity of the periimplant bone was 41.12 MPa for the proximal implant location and 25.38 MPa for the distal, without evidence for significant differences between implant types. Taking into account the median ultimate shear stress for 200 microm implants with and without corundum blasting, no significant difference could be demonstrated. However, for blasted 200 microm implants a statistically significant (p<0.025) relative gain in ultimate shear stress of 41% and 17% was proven in comparison with 100 and 300 microm implants, respectively. Non-blasted 200 microm implants reached 48% relative gain in respect of 100 microm samples.

Greater accuracy in positioning of the acetabular cup by using an image-free navigation system

In a prospective and randomised clinical study, acetabular cups were implanted free-hand (control group n=22) or with computer assistance using an image-free navigation system (study group n=23). The cup position was determined postoperatively on pelvic CT. An average inclination of 42.3 degrees (range: 30 degrees -53 degrees ; SD+/-7.0 degrees ) and an average anteversion of 24.0 degrees (range: -3 degrees to 51 degrees ; SD+/-15.0 degrees ) were found in the control group, and an average inclination of 45.0 degrees (range: 40 degrees -50 degrees ; SD+/-2.8 degrees ) and an average anteversion of 14.4 degrees (range: 5 degrees -25 degrees ; SS+/-5.0 degrees ) in the computer-assisted study group. The deviations from the desired cup position (45 degrees inclination, 15 degrees anteversion) were significantly lower in the computer-assisted study group (p<0.001 each). While only 11/22 of the cups in the control group were within the Lewinnek safe zone, 21/23 of the cups in the study group were placed in this target region (p=0.003).

The impact of the CCD-angle on range of motion and cup positioning in total hip arthroplasty

BACKGROUND

Biomechanical analysis and clinical experience reveal that offset total hip stems increase soft tissue tension and reduce the risk for dislocation in total hip arthroplasty. Most of these stems have a smaller neck-shaft-angle to increase the offset. This study investigates if changing the neck-shaft-angle has an impact on how cup and stem should be positioned with regard to range of motion.

METHODS

A mathematical model of a total hip arthroplasty was developed to analyze range of motion until impingement between cup and neck. Range of motion was determined for each combination of neck-shaft-angles and additional parameters like cup inclination, cup anteversion, stem antetorsion, head/neck ratio and design of the cup opening.

RESULTS

A maximized range of motion is achieved for neck-shaft-angles between 125 degrees and 131 degrees . Reducing the neck-shaft-angle by one degree requires reducing the cup anteversion by about 2 degrees and increasing the cup inclination by 0.45 degrees . Stems with neck-shaft-angles more than 135 degrees are not recommended when the head/neck ratio is 2.3 or less.

INTERPRETATION

Stems with a reduced neck-shaft-angle for an increased offset should be coupled with cups that are inclined slightly higher and less anteverted as compared to a standard stem. Precise recommendations for optimal component positioning can only be given for a specific prosthesis system regarding all parameters.

New technology in cemented total hip arthroplasty

The long-term results of total hip arthroplasty (THA) are predicated by excellent surgical techniques. New technology offers the hope of improving outcomes by providing to surgeons tools that make surgical procedures predictable. Techniques that improve the bone-cement-prosthesis composite should enhance long-term fixation. Less invasive surgical techniques that allow rapid recovery from THA have been recently described. Image-guided surgery may enable surgeons to accurately reconstruct the arthritic hip and improve outcomes.

ULTRASONOGRAPHIC ANATOMY OF THE NORMAL CANINE CALCANEAL TENDON

To provide a series of detailed ultrasonographic images of the canine calcaneal tendon, greyhound cadaver pelvic limbs were scanned with a high-resolution transducer, and images compared with dissected specimens. The three components of the calcaneal tendon are the tendons of insertion of the gastrocnemius, superficial digital flexor, and a conjoined tendon formed by tendons from the biceps femoris, semitendinosus, and gracilis. Each of these three tendons was visible in transverse ultrasound images, each measuring 2.4-3.2-mm thick at mid-calcaneal tendon. Improved understanding of the anatomy of the calcaneal tendon will support clinical ultrasonography of this region.

Anatomic referencing of cup orientation in total hip arthroplasty

Pelvic tilt and rotation can drastically affect the apparent cup orientation on conventional anteroposterior pelvic radiographs. It was hypothesized that nonstandardized radiographic cup version and abduction can differ significantly from the corresponding anatomic angles if not measured to the anterior pelvic plane, defined by the pubic tubercles and the anterior superior iliac spine. Differences in preoperative and postoperative pelvic orientation and their influence on radiographic measurements of the two angles were analyzed. Conventional radiographs and preoperative and postoperative computed tomography scans of 37 total hip arthroplasties were compared. Calculations were made with the preoperative planning station of a computer navigation system. Significantly smaller values of cup version were seen on nonstandardized radiographs, whereas abduction could be measured reliably when referenced to horizontal pelvic landmarks seen on radiographs. The underlying cause for this difference was the variation of pelvic tilt that ranged 27 degrees (range, -7 degrees -20 degrees). Influence of pelvic tilt on the apparent cup orientation can be seen with simple nomograms. The orientation of the anterior pelvic plane before and after surgery did not differ. We think that version measurement on nonstandardized radiographs without anatomic referencing is highly inaccurate.

LEVEL OF EVIDENCE

Diagnostic study, Level II-1 (development of diagnostic criteria on basis of consecutive patients--with universally applied reference "gold" standard). See Guidelines for Authors for a complete description of levels of evidence.

Untersuchung unterschiedlicher Biomaterialien in einer Knochenzellkultur des menschlichen Steigbügels

BACKGROUND

Cell culture studies may provide information on the behavior of biomaterials in the intended implant environment. Cell cultures from such an environment could be used for the development of middle ear implants.

MATERIAL AND METHODS

Secondary bone-like cell cultures derived from human stapes were exposed to different materials [Al(2)O(3) ceramic, glass ceramic (Ceravital), gold and titanium]. Proliferation was studied for up to 40 days.

RESULTS

The proliferation of cultured stapes bone-like cells did not differ significantly between the four tested biomaterials. The well known cytotoxic effect of copper, which was used as a control, was evident.

CONCLUSIONS

Four biomaterials [Al(2)O(3) ceramic, glass ceramic (Ceravital), gold and titanium] have similar biocompatibility and no toxicity when tested in human stapes cell cultures. This in vitro model may be of considerable value for the further development of middle ear implants, e.g., when coated with bone morphogenetic proteins.

Metal-on-metal bearings in cementless primary total hip arthroplasty

One hundred cementless titanium primary total hip arthroplasties with 28 mm Metasul bearings were prospectively studied (osteoarthritis in 76% of hips, mean age 59.6 years). Ninety-eight were reviewed after a 6-year average follow-up (range, 17-126 months) with clinical results graded excellent and good in 97%. One femoral component was revised for aseptic loosening at 7.8 years. Postoperative cobalt level was higher than the upper "normal" value (5 microg/L in whole blood) for 16 cases. No significant relationship could be established between cobalt concentration increase and any demographic or surgical data, including activity level, except anteversion of the cup >25 degrees. In this early experience, impingement due to a head sleeve has been the main cause of dislocation and failure, and systemic cobalt survey appeared to be a good indicator of metal-on-metal bearing mechanical behavior.

Radiographic evaluation of the Duraloc cup after 4 years

We implanted 71 metal-backed, porous-coated, hemispheric, press-fit Duraloc-100 cups in 68 consecutive patients. In 61 patients, the femoral stem was a cementless Spotorno and in ten a cemented Lubinus SP II. A 28-mm Biolox ceramic head was used with both stems. After an average follow-up of 4 (3.7-5.9) years, we examined 67 hips. Radiolucencies were described in three zones according to DeLee and Charnley, and migration was measured on serial radiographs using the computer-assisted EBRA method. Total migration of more than 1 mm within the first 2 years occurred in 22/62 cups. Nine cups showed more than 1.5 mm total migration within the first 2 years and more than 2 mm within the whole period. The presence of post-operative radiolucencies correlated significantly with a total migration value of more than 2 mm within the first 2 years (p=0.02). Post-operative radiolucencies in zone 1 correlated with a total migration value of more than 2 mm within the first 2 years (p=0.027) and more than 2.5 mm within the whole period (p=0.051). These correlation values might reflect the quality of operative technique, particularly reaming, and implant selection.

Compliant positioning of total hip components for optimal range of motion

Impingement between femoral neck and endoprosthetic cup is one of the causes for dislocation in total hip arthroplasty (THA). Choosing a correct combined orientation of both components, the acetabular cup and femoral stem, in manual or computer-assisted implantation will yield a maximized, stable range of motion (ROM) and will reduce the risk for dislocation. A mathematical model of a THA was developed to determine the optimal combination of cup inclination, cup anteversion, and stem antetorsion for maximizing ROM and minimizing the risk for cup-neck impingement. Single and combined hip joint motions were tested. A radiographic definition was used for component orientation. Additional parameters, such as stem-neck (CCD) angle, head-neck ratio, and the design of the acetabular opening, were also considered. The model showed that a maximized and safe ROM requires compliant, well-defined combinations of cup inclination, cup anteversion, and stem antetorsion depending on the intended ROM. Radiographic cup anteversion and stem antetorsion were linearly correlated. Additional internal rotation reduced flexion, and additional external rotation reduced extension, abduction and adduction. The articulating hemispheric surface of acetabular cups should be oriented between 40 degrees and 45 degrees of radiographic inclination, between 20 degrees and 28 degrees of radiographic cup anteversion, and should be combined with stem antetorsion so that the sum of cup anteversion plus 0.7 times the stem antetorsion equals 37 degrees. Final component orientation must also consider cup containment, implant impingement with bone and soft tissue, and preoperative skeletal contractures or deformities to achieve the optimal compromise for each patient.

Effect of the initial implant fitting on the predicted secondary stability of a cementless stem

A numerical model able to investigate the influence of biomechanical factors on the long-term secondary stability of implants would be extremely useful for the design of new cementless prosthetic devices. A purely biomechanical model of osseo-integration has been developed, formulated as a rule-based adaptation scheme. Due to its complexity, the problem was divided into three steps: preliminary implementation of the model (proof of concept); implementation of the complete model and investigation of the model solution; and model validation. The paper describes the first of these three steps. The model was implemented as a discrete-states machine, and the few parameters required were derived from the literature. It was then applied to a real clinical case. The study was conducted using the frictional contact finite element model of a human femur implanted with a cementless anatomical stem. A stable solution was achieved after between three and 15 iterations for all initial positions considered. Similar initial conditions yielded similar final configurations. The model predicted all initial configurations, with the exception of a partial osseo-integration, ranging between 62% (distal fit) and 78% (proximal fit) of the viable interface. This is in good agreement with the values reported in the literature that never exceed 75%, even in the best conditions, and report better clinical results for proximal fit. For the varus configuration, which lacks cortical support, the algorithm predicted a completed loosening.

Genauigkeit eines bildfreien Navigationssystemes für die Hüftpfannenimplantation – eine anatomische Studie / Accuracy of an Image-free Cup Navigation System – an Anatomical Study

The position of the acetabular cup is of decisive importance for the function of a total hip replacement (THR). Using the conventional surgical technique, correct placement of the cup often fails due to a lack of information about pelvic tilt. With CT-based and fluoroscopically-assisted navigation procedures the accuracy of implantation has been significantly improved. However, additional radiation exposure, high cost and the increased time requirement have hampered the acceptance of these techniques. The present anatomical study evaluates the accuracy of an alternative procedure--image-free navigation. This method requires little extra effort, does not substantially delay surgery, and needs no additional imaging. Press-fit cups were implanted in 10 human cadaveric hips with the help of the image-free navigation system, and the position of the cups was checked intraoperatively with a CT-based navigation system and postoperatively by computed tomography. All cups were implanted within the targeted safe zone with an average inclination of 44 degrees (range 40 degrees-48 degrees, SABW 2.7 degrees) and an average anteversion of 18 degrees (range 12-24 degrees, SABW 4.1 degrees). Analysis of accuracy of the image-free navigation software revealed only a small, clinically tolerable deviation in cup anteversion and cup inclination in comparison with the CT-based navigation system and the post operative CT scans. The evaluated image-free navigation system appears to be a practicable and reliable alternative to the computer-assisted implantation of acetabular cups in total hip arthroplasty.

Failures and successes in total hip replacement--why good ideas may not work

The course of development of total hip replacement (THR) is neither harmonious nor linear. Progress and set-backs alternate. Progress in THR manifests itself through reductions in the number and the severity of complications (infection, aseptic loosening, prematurely worn components, etc.). Innovation is the motor of progress. However, today's innovation may well be tomorrow's revision! Progress has been achieved in part through new implant materials and designs that provide improvements in such things as stress distribution in surrounding bone, tissue compatibility and osseointegration, and resistance to both wear and cyclic fatigue. Of at least equal importance, however, are improvements achieved in operative procedures (e.g., cementing technique) and finally, in clinical quality control: more complete documentation of implant and patient variables, establishment of implant registers, and utilization of outcome studies to guide the course of further development. Causes of failure in THR are numerous. However, unexpected side effects of innovations are the most frequent cause. An innovation may solve one problem, but also creates new ones. Problems in innovation which can lead to failures include: over-generalization of expected patient responses, ignoring past experience or assigning wrong causes to encountered problems, and finally, ignoring the dynamic nature of the living system (which can be described as using "necro-" instead of biomechanical thinking). Quality control in both manufacturing and clinical practice has to be improved. The pioneer times have come to an end. Today's patients should have the right to be operated on by a well trained surgeon and to be provided with well tested, well-understood implant materials and devices. Progress in endoprosthetics has led to the present high level of clinical success. Paradoxically, however, success is the greatest obstacle to further progress. This is because the curve of progress as a result of effort expended has turned asymptotic in endoprosthetics, as it does in many endeavors. In such situations the more a product (e.g., surgical implant and procedure, automobile design, computer program) becomes successful, the more efforts (and finances) are needed for further progress. On the other hand, the "scissors"--created by crossing what might be feasible with what resources are available--open widely and can cut sharply. In fact, financial restrictions may force orthopaedic surgeons and the medical device and technology industry to turn to lower technologies in the future. However, whatever new developments in endoprosthetics may bring (be they sophistications or simplifications), we must remain open-minded and not assume things to be facts until there is evidence to support them.

Load transfer and fixation mode of press-fit acetabular sockets

Adequate initial fixation is a prerequisite for osseointegration and secondary stability of noncemented cups. Physiologic force transmission between the cup and acetabulum guarantees the best long-term fixation. To study load transfer within the natural hip joint and in the bone-implant interface of 2 different hemispherical noncemented press-fit cups, 10 hips were investigated in an experimental setup simulating single-leg stance. Load distribution and contact area were measured using prescale pressure-sensitive films and digital image analysis. Three dominant locations near the periphery of the acetabulum could be identified. Main load transfer occurs in the cranial region of the acetabulum, where it is buttressed by the iliac bone; the second location is at the posterior-inferior region at the ischial facet, and the third location is at the anterior region, where support is provided by the pubic bone. Peripheral rim contact was present in both cups but not completely circumferential. It showed marked loading at the same 3 locations similar to the natural hip joint. The ilioischial diagonal axis produced the highest press-fit. Peak local forces were found at the ischial and iliac facets. Local forces can be grouped into an iliac, an ischial, and a pubic group contributing 55%, 25%, and 20% to the total hip joint force. Pole contact was not present in the natural hip and with the biradial press-fit cup with flattened pole area but was observed with the pure hemispherical cup. Hence, stable fixation of an acetabular cup is achieved best by a 3-point-like bony support at the iliac, ischial, and pubic bone. The acetabular fovea does not provide functional support of the femoral head or endoprosthetic socket. In revision surgery, remaining peripheral bone stock at the iliac, ischial, and pubic locations allows stable implantation of primary cups.