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.

Validation of a simple radiographic method to determine variations in pelvic and acetabular cup sagittal plane alignment after total hip arthroplasty

BACKGROUND AND PURPOSE

Orientation of acetabular component, influenced by pelvic tilt, body position, and individual variations affects the outcome following total hip arthroplasty (THA). Currently available methods of evaluation are either imprecise or require advanced image processing. We analyzed intersubject and intrasubject variability of pelvic tilt, measured by sagittal sacral tilt (ST) and its relationship with acetabular component tilt (AT) by using a simple method based on standard radiographs.

MATERIALS AND METHODS

ST was measured on lateral radiographs of pelvis including lumbosacral spine obtained in supine, sitting, standing, and lateral decubitus position for 40 asymptomatic THA patients and compared to computed tomography (CT) data obtained in supine position. AT was measured on lateral radiographs (measured acetabular tilt: MAT) in each position and compared to measurement of AT on CT and an indirectly calculated AT (CAT).

RESULTS

Mean ST changed from supine to sitting, standing, and lateral decubitus positions as follows: 26.5 +/- 15.5 degrees (range 4.6-73.4 degrees ), 8.4 +/- 6.2 degrees (range 0.6-24.5 degrees ), and 13.4 +/- 8.4 degrees (range 0.1-24.2 degrees ; p < 0.0001, p = 0.002, p = 0.006). The MAT on radiographs was not significantly different from the MAT measured on CT (p = 0.002) and the CAT (p = 0.06). There is a good correlation between change in ST and MAT in sagittal plane (r = 0.93).

CONCLUSION

Measurement of ST on radiographs is a simple and reliable method to track changes in pelvic tilt in different body positions. There is significant intersubject and intrasubject variation of ST and MAT with postural changes and it may explain causes of impingement or instability following THA, which could not be previously explained.

Primary total hip arthroplasty with a flattened press-fit acetabular component in osteoarthritis and inflammatory arthritis: a prospective study on 416 hips with 6-10 years follow-up

INTRODUCTION

A flattened cup was designed to create a more physiological load transfer to the pelvic bone compared to hemispherical cups, and to allow more bone contact compared to low-profile' spherical cups. To investigate these theoretical advantages and the potential influence of the quality of the acetabular bone, a clinical study was performed in patients with osteoarthritis (OA) and inflammatory arthritis (IA). The aims of the study were (1) to evaluate the fixation of the cup, postoperatively and later when osseous integration should have taken place, (2) to assess perioperative complications such as acetabular fractures and (3) to monitor the polar gap, a potential risk factor for osteolysis.

PATIENTS AND METHODS

A prospective study was performed on all consecutive OA and IA patients with an indication for primary total hip arthroplasty (THA). Three hundred and nine OA patients (340 hips) and 65 IA patients (76 hips) were included. The acetabular component was the flattened press-fit EPF-PLUS cup, the femoral component the tapered cementless Zweymueller SL-PLUS stem. All revisions and complications were recorded. Clinical and radiographical evaluation was performed on regular basis during 6-10 years.

RESULTS

The incidence of early loosening of the cup was 0 out of 340 in the OA group and 1 out of 76 in the IA group. The incidence of acetabular fractures was 7 out of 340 in the OA group and 3 out of 76 in the IA group. Failure rate for the acetabular component due to aseptic loosening or osteolysis after 6-10 years was 0% in the OA group and 4.8% in the IA group. In all cases available for follow-up the polar gap had disappeared and full osseous integration had taken place in both the groups.

INTERPRETATION

This study shows that the flattened press-fit acetabular component creates adequate initial mechanical stability to allow osseous integration and that the cup can be safely used in both OA and IA patients. However, after 6-10 years, in the IA group failure of the cup due to aseptic loosening occurred once and failure due to osteolysis occurred three times, while these type of failures did not occur in the OA group.

Biomechanical comparison of the primary stability of suturing Achilles tendon rupture: a cadaver study of Bunnell and Kessler techniques under cyclic loading conditions

INTRODUCTION

Biomechanical studies investigating suture techniques for Achilles tendon repair used single load to failure tests in order to evaluate the maximal load capacity of the repaired construct. During early rehabilitation the repair is repetitively loaded such as exercise or daily living activities like walking. Cyclic loading seems to duplicate the physiological loading conditions more closely than single cycle failure tests. Aim of this study was to test the most commonly used Achilles tendon repair techniques (Bunnell and Kessler repair) under cyclic loading conditions.

MATERIALS AND METHODS

Following tenotomy fresh human cadaveric tendons were sutured either with the Bunnell or Kessler technique. After repair, cyclic loading tests were performed with a uniaxial biomechanical testing machine Lloyd LR-5K Plus. Both groups were sutured with 0.7 mm PDS.

RESULTS

Except at maximum load we could not find significant differences between tendons sutured by Bunnell and Kessler techniques. During the cyclic testing there were no differences between both groups with respect to displacement. This applies also to the stiffness of the constructs, which we defined from the load to failure measurements. The failure modes in both groups differed; the tendons repaired by Kessler technique were cut by the tendons and in the Bunnell group the suture material tore in each specimen tested.

CONCLUSION

In our study Bunnell and Kessler techniques showed similar biomechanical properties using the same suture material. The typical failure mode of the Bunnell technique shows potential to optimise biomechanical behavior by using stronger suture material.

Revision of metal-on-metal resurfacing arthroplasty of the hip: the influence of malpositioning of the components

We have reviewed 42 patients who had revision of metal-on-metal resurfacing procedures, mostly because of problems with the acetabular component. The revisions were carried out a mean of 26.2 months (1 to 76) after the initial operation and most of the patients (30) were female. Malpositioning of the acetabular component resulted in 27 revisions, mostly because of excessive abduction (mean 69.9 degrees ; 56 degrees to 98 degrees ) or insufficient or excessive anteversion. Seven patients had more than one reason for revision. The mean increase in the diameter of the component was 1.8 mm (0 to 4) when exchange was needed. Malpositioning of the components was associated with metallosis and a high level of serum ions. The results of revision of the femoral component to a component with a modular head were excellent, but four patients had dislocation after revision and four required a further revision.

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.

Incidence and effect of calcifications after open-augmented Achilles tendon repair

BACKGROUND AND AIMS

The objective of this study was to analyse the incidence and effect on clinical outcome of post-operative Achilles tendon calcifications after open-augmented repair according to the Silfverskjöld technique.

PATIENTS/METHODS

This retrospective study presents the results of follow-up examinations on 104 patients with Achilles tendon rupture who were treated according to the Silfverskjöld technique. Post-operative calcifications were identified by means of sonography, and clinical outcome was evaluated using the 100 points Thermann score. The average follow-up examination period was 3.7 years with an average patient age of 42.9 years. Two groups were identified and included those with (Group I) and those without (Group II) calcifications. Clinical outcome was evaluated using the Thermann score for both groups and was compared statistically by means of the two random sample t-test.

RESULTS

Fifteen patients (Group I) developed tendon calcifications (14.4%) and 89 none (Group II). Group I scored 88.0 points and Group II 88.1 (good to very good outcome). There were no negative effects on clinical outcome (t = 0.98).

CONCLUSIONS

The incidence of tendon calcification after open-augmented repair of Achilles tendon rupture was 14.4% with no negative effects on clinical outcome as measured by the Thermann score.

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.

Validation of a computer navigation system and a CT method for determination of the orientation of implanted acetabular cup in total hip arthroplasty: a cadaver study

BACKGROUND

Successful hip reconstruction to restore the normal hip biomechanics requires precise placement of implants. Computer assisted navigation in total hip arthroplasty has been proposed to have the potential to help achieve a high accuracy in implant placement. The goal of the study was to evaluate the accuracy of an imageless computer navigation system on cadavers and to validate a non-invasive computed tomography method for post-operative determination of acetabular cup orientation.

METHODS

Total hip arthroplasty was performed on seven cadaver hips with the aid of an imageless computer navigation system. The achieved cup orientation were recorded using three methods, (1) intra-operatively using the imageless computer navigation system, (2) post-operatively with direct bone digitization and (3) with a computed tomography based three dimensional model interpreted by three raters. Measurement from the direct bone digitization was taken as the gold standard to evaluate the other two methods. The intra-rater and inter-rater consistency of the computer tomography-model method were assessed by Cronbach's alpha determination.

FINDINGS

Compared with the cup orientation obtained from the direct bone digitization, the average difference for anteversion and abduction were 3.3 (3.5) degrees (P=0.045) and 0.6 (3.7) degrees , respectively, for navigation reading. The average differences for computer tomography-model for three raters were 0.5 (2.1) degrees , 0.8 (1.5) degrees and 3.2 (3.3) degrees (P=0.043) for anteversion and 0.4 (1.6) degrees , 0.3 (1.6) degrees and 2.1 (2.7) degrees for abduction. The intra-rater consistency ranged from 0.626 for a novice rater to over 0.97 for experience raters. The inter-rater consistency (including novice and experienced raters) was over 0.90.

INTERPRETATION

While the values for cup orientation determined with imageless computer navigation were comparable to those from direct bone and implant digitization, the measurement for anteversion obtained was not as accurate as that for abduction. The proposed computer tomography-model method has an excellent intra-rater consistency for experienced raters, as well as an excellent overall inter-rater consistency. The study confirms that a non-invasive computed tomography based model analysis can be used in clinical practice as a valid method for post-operatively evaluating the orientation of the acetabular component.

MR imaging of ankle inversion injuries

Ankle inversions are common in the general population and in athletes. Multiple concurrent injuries are a common result of an ankle-inversion injury. Syndesmosis injury, lateral ankle ligament tears, peroneal retinaculum or tendon injury, osteochondral lesion, or fracture may occur. Chronic pain or instability may result from one or more of these injuries. MR imaging provides superior soft tissue resolution, high sensitivity for occult fractures, and the ability to image the articular cartilage and ankle ligaments directly. This article discusses the MR imaging evaluation of acute and chronic ankle inversion injuries.

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.

Component position in 2-incision minimally invasive total hip arthroplasty compared to standard total hip arthroplasty

Minimally invasive surgery (MIS) for total hip arthroplasty (THA) has sparked controversy in the orthopedic community, including debate regarding the reliability and reproducibility of component placement. We reviewed a single surgeon's 1-year experience by comparing postoperative radiographs of 67 MIS 2-incision THA and 28 standard THA for acetabular inclination, acetabular version, and femoral stem angulation. Acetabular inclination/version averaged 42.2 degrees/16.5 degrees and 38.7 degrees/15.5 degrees for MIS and THA, respectively. Femoral angulation averaged 0.007 degrees varus and 0.411 degrees varus for MIS and standard THA approaches, respectively. Radiographic assessment of component position of THA in 2-incision MIS vs a standard direct lateral approach reveals no significant differences. Components are placed in acceptable positions with both techniques.

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.

Modified posterior approach to total hip arthroplasty to enhance joint stability

We modified the posterior approach by preserving the external rotator muscles to enhance joint stability after primary THA. We asked whether this modified posterior approach would have a lower dislocation rate than the conventional posterior approach, with and without a repair of external rotator muscles. We retrospectively divided 557 patients (670 hips) who had undergone primary THA into three groups based on how the external rotator muscles had been treated during surgery: (1) not repaired after sectioning, (2) repaired after sectioning, or (3) not sectioned and preserved. The minimum followup was 1 year. In the group with preserved external rotator muscles, we observed no dislocations; in comparison, the dislocation rates for the repaired rotator group and the no-repair group were 3.9% and 5.3%, respectively. This modified posterior approach, which preserves the short external rotator muscles, seemed effective in preventing early dislocation after primary THA.

LEVEL OF EVIDENCE

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

Precise estimation of postoperative cup alignment from single standard X-ray radiograph with gonadal shielding

This paper addresses the problem of estimating postoperative cup alignment from single standard X-ray radiograph with gonadal shielding. The widely used procedure of evaluation of cup orientation following total hip arthroplasty using single standard anteroposterior radiograph is known inaccurate, largely due to the wide variability in individual pelvic position relative to X-ray plate. 2D-3D image registration methods have been introduced to estimate the rigid transformation between a preoperative CT volume and postoperative radiograph(s) for an accurate estimation of the postoperative cup alignment relative to an anatomical reference extracted from the CT data. However, these methods require either multiple radiographs or a radiograph-specific calibration, both of which are not avaiable for most retrospective studies. Furthermore, these methods were only evaluated on X-ray radiograph(s) without gonadal shielding. In this paper, we propose to use a hybrid 2D-3D registration scheme combining an iterative landmark-to-ray registration with a 2D-3D intensity-based registration to estimate the rigid transfromation for a precise estimation of cup alignment. Quantitative and qualitative results evaluated on clinical and cadaveric datasets are given which indicate the validity of our approach.

How do alternative bearing surfaces influence wear behavior?

Metal, ceramic, and polyethylene liners represent contemporary bearing choices for total joint replacement. Each has limitations in terms of design, sensitivity to manufacturing, and surgical placement. With polyethylene, larger femoral heads represent a design restriction and a potential wear issue. One side benefit is that polyethylene does not click, squeak, or create stripe wear. The attraction of hard-on-hard bearings (metal-on-metal, ceramic-on-ceramic) is that their typically ultra-low wear alleviates concerns with large femoral head designs. However, hard-on-hard bearings produce stripe wear due to the effects of the rigid liner edge. Slight subluxation (microseparation) during swing phase of gait can result in stripe wear on the ball and liner rim. In addition, high levels of implant wear with vertically placed cups can be anticipated. Currently, only alumina-on-alumina bearings can claim virtually no biologic risk. Thus, the role of laboratory studies is to isolate relevant aspects of performance by cup design and to predict the risk-benefit ratios in patients requiring total hip replacement.

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.

Precision and bias of imageless computer navigation and surgeon estimates for acetabular component position

Computer navigation has the potential to permit accurate placement of components. We first hypothesized acetabular inclination and anteversion using navigation would be within 5 degrees of postoperative computed tomography scans, then secondly, computer precision would be better than that of surgeons. In the first phase, we obtained postoperative CT scans in 30 hips to ascertain the computer navigation values for inclination and anteversion of the cup. In the second phase, in 99 patients with 101 hips, we determined the surgeon's precision by comparing surgeons' blind estimates for trial cup position with computer navigation values. The navigation precision for inclination was 4.4 degrees with a bias of 0.03 degrees and for anteversion was 4.1 degrees with a bias of 0.73 degrees. The experienced surgeons' precision was 11.5 degrees for inclination and 12.3 degrees for anteversion, whereas the less experienced surgeons' precision was 13.1 degrees for inclination and 13.9 degrees for anteversion. The data supported the first hypothesis as computer navigation had a bias for inclination and anteversion of less than 1 degrees with precision less than 5 degrees. The precision of computer navigation was better than that of surgeons. This imageless computer navigation system allows more accurate acetabular component placement.

Computed tomography-based surgical navigation for hip arthroplasty

Component malpositioning and postoperative leg length discrepancy are the most common technical problems associated with total hip arthroplasty (THA). Surgical navigation offers the potential to reduce the incidence of these problems. We reviewed 317 patients (344 hips) that underwent THA using computed tomography-based surgical navigation, including 112 THAs using a simplified method of measuring leg length. Guided by the navigation system, cups were placed in 40.8 degrees +/- 2 degrees of operative abduction (range, 35 degrees -50 degrees) and 30.8 degrees +/- 3.2 degrees (range, 19 degrees -43 degrees) of operative anteversion. We subsequently measured radiographic abduction on plain anteroposterior pelvic radiographs and calculated abduction and anteversion. Radiographically, 97.1 % of the cups were in the safe zone for abduction and 92.4% for anteversion. The mean incision length was less than 8 cm for 327 of the 344 hips. Leg length change measured intraoperatively was 6.6 +/- 4.1 mm (range, -2-22), similar to measurements from the pre- and postoperative magnification-corrected radiographs. Computer assistance during THA increased the consistency of component positioning and allowed reliable measurement of leg length change during surgery.

Enhanced acetabular component positioning through computer-assisted navigation

Optimal positioning of the acetabular component improves the long-term success of total hip arthroplasty by reducing the rate of adverse outcomes, such as component wear and dislocation. Mechanical guides designed to facilitate proper component orientation are inadequate, as they do not account for variations in patient position and pelvic motion during surgery. Pioneering image-guided surgical navigation systems were developed to provide surgeons with improved methods for intraoperatively measuring orientation and alignment. Although enhanced orientation has been reported with such systems, they require preoperative CT scans and are therefore limited by the need for preplanning, the necessity of matching CT data with the actual patient position, and the additional costs associated with CT. The recent development of CT-free navigational tools addresses these disadvantages and offers real-time surgical feedback regarding the actual position of the acetabular component and instruments relative to the pelvis. Proper training and enhanced identification of bony landmarks will improve upon the success of these systems.

Navigated non-image-based positioning of the acetabulum during total hip replacement

We tested the hypothesis that the non-image-based navigation system used in our department was able to measure accurately the 3D positioning of the acetabular cup of a total hip replacement (THR) and to increase the accuracy of its implantation during THR. We studied 50 consecutive navigated implantations of a THR and compared the intra-operative measurement of the cup by the navigation system to the post-operative measurement by computed tomography (CT) scan. The mean difference between the navigated and CT scan measurements for cup inclination was 2 degrees . The mean difference between the navigated and CT-scan measurements for cup flexion was 4 degrees . These differences were significant but considered to be clinically irrelevant in most cases. A total of 73% of the cases were within the safe zone defined prior to the study. The non-image-based system used allows a precise orientation of the cup during THR.

Factors affecting hip range of motion in surface replacement arthroplasty

BACKGROUND

Surface replacement arthroplasty aims to re-create normal hip biomechanics; however the pathoanathomy of the hip, prosthetic component design, surgical technique and other factors may have a significant impact on the range of motion restoration attained following surface replacement arthroplasty. However, there is paucity of information on the effect of such factors.

METHODS

A computerized three-dimensional hip model was created from preoperative computerized tomography images of a patient who was scheduled for a surface replacement arthroplasty. The effects of the femoral component size, translation and orientation on the range of motion were analysed as was the effect of increasing the seating depth and modification of the version of the acetabular component.

FINDINGS

Increasing the femoral component size led to global improvement in range of motion while translation increased range of motion in one direction but reduced it in the opposite direction. Change in the femoral component orientation had minimal effects on range of motion in comparison to the effect of changes in the version of the acetabular component. Increasing the seating depth of the acetabulum only caused reduced range of motion in internal rotation in 90 degrees flexion.

INTERPRETATION

To restore hip range of motion, surgeons performing surface replacement arthroplasty should aim to reproduce the natural femoral head-neck offset. Although increasing the femoral component size may achieve this, more acetabular bone will be resected. Knowing the specific zones of impingement of each arc of movement, selective translation of the femoral component or femoral neck osteoplasty can restore femoral neck offset in more critical areas without affecting acetabular bone stock. Over deepening of the acetabulum or leaving rim osteophytes should also be avoided to prevent impingement.

Acetabular augmentation ring for recurrent dislocation of total hip arthroplasty: 60% stability rate after an average follow-up of 74 months

Between 1988 and 2002, 47 patients (50 hips) were treated with acetabular shell augmentation arthroplasty for recurrent idiopathic dislocation of their total hip arthroplasty. Apparent causes for dislocation such as deep infection, component malposition, or polyethylene wear were excluded. Follow-up averaged 74 months (range, 12-178 months), and clinically, 30 hips (60%) did not present a subsequent dislocation at most recent follow-up. In five hips (10%), deep infection after the augmentation procedure necessitated removal of the entire prosthesis. In our opinion, this technique cannot be recommended as it has an unacceptable failure and high infection rate.

Accuracy considerations in navigated cup placement for total hip arthroplasty

Computer assisted orthopaedic surgery (CAOS) technology has recently been introduced to overcome problems resulting from acetabular component malpositioning in total hip arthroplasty. Available navigation modules can conceptually be categorized as computer tomography (CT) based, fluoroscopy based, or image-free. The current study presents a comprehensive accuracy analysis on the computer assisted placement accuracy of acetabular cups. It combines analyses using mathematical approaches, in vitro testing environments, and an in vivo clinical trial. A hybrid navigation approach combining image-free with fluoroscopic technology was chosen as the best compromise to CT-based systems. It introduces pointer-based digitization for easily assessable points and bi-planar fluoroscopy for deep-seated landmarks. From the in vitro data maximum deviations were found to be 3.6° for inclination and 3.8° for anteversion relative to a pre-defined test position. The maximum difference between intraoperatively calculated cup inclination and anteversion with the postoperatively measured position was 4° and 5°, respectively. These data coincide with worst cases scenario predictions applying a statistical simulation model. The proper use of navigation technology can reduce variability of cup placement well within the surgical safe zone. Surgeons have to concentrate on a variety of error sources during the procedure, which may explain the reported strong learning curves for CAOS technologies.

Computed tomography-based navigation to determine the socket location in total hip arthroplasty of an osteoarthritis hip with a large leg length discrepancy due to severe acetabular dysplasia

For osteoarthritis hips due to severe acetabular dysplasia such as Crowe type 3 or 4, placement of the socket is a difficult procedure in total hip arthroplasty. Because the acetabular bone stock is poor, suitable location for the socket is very limited with respect to achieving good coverage with the host bone. A 51-year-old woman who had an osteoarthritis hip with a large leg length discrepancy due to severe acetabular dysplasia required total hip arthroplasty. The purpose of the total hip arthroplasty was to improve the hip disorder as well as to reduce the leg length discrepancy to achieve good gait function. We present technical solutions to aid the surgeons in placing the acetabular socket at the proper location by using computed tomography-based navigation system.

Anatomic hip range of motion after implantation during total hip arthroplasty as measured by a navigation system

Simulation of prosthetic impingement is important for preventing complications after total hip arthroplasty (THA). Although the anatomical hip range of motion (ROM) in patients after THA is an essential parameter for these simulations, previous simulation studies substituted various clinical hip ROMs for the anatomical hip ROM. Using a navigation system, anatomical hip ROM was accurately assessed after implantation during primary THA in 30 patients. We found that the hip could be passively moved to 113 degrees of flexion, 34 degrees of extension, 46 degrees of abduction, 75 degrees of internal rotation, and 36 degrees of external rotation. Almost all reference hip ROMs used in previous simulations were smaller than these values. Therefore, wider hip ROM values should be used as parameters for such simulations.

Range of motion measurement of an artificial hip joint using CT images

Total hip arthroplasty (THA) is one of the most effective treatments for osteoarthritis and rheumatoid arthritis. Dislocation of the femoral head from the acetabular socket is a major problem of THA. To prevent dislocation, it is important to know the range of motion (ROM) after THA. Although various studies on the ROM were carried out, there exist only a few reports on ROM evaluation in individual patients. This is because in clinical cases, bone-to-bone and bone-to-component contacts must be considered besides the impingement of components. In this study, a new method for evaluating ROM of internal/external rotation, which takes into account all combinations of contacts between the bones and components, was proposed. A computer simulation demonstrated that the RMS error of the proposed method was approximately 3 degrees . The method was applied to 33 THAs under various conditions of flexion and adduction angles. The method was able to detect any type of impingement. The evaluated ROM was in good agreement with that measured during the THA operation (correlation coefficient = 0.91).

Mid-term results of cementless total hip replacement using a ceramic-on-ceramic bearing with and without computer navigation

We have developed a CT-based navigation system using infrared light-emitting diode markers and an optical camera. We used this system to perform cementless total hip replacement using a ceramic-on-ceramic bearing couple in 53 patients (60 hips) between 1998 and 2001. We reviewed 52 patients (59 hips) at a mean of six years (5 to 8) postoperatively. The mid-term results of total hip replacement using navigation were compared with those of 91 patients (111 hips) who underwent this procedure using the same implants, during the same period, without navigation. There were no significant differences in age, gender, diagnosis, height, weight, body mass index, or pre-operative clinical score between the two groups. The operation time was significantly longer where navigation was used, but there was no significant difference in blood loss or navigation-related complications. With navigation, the acetabular components were placed within the safe zone defined by Lewinnek, while without, 31 of the 111 components were placed outside this zone. There was no significant difference in the Merle d'Aubigne and Postel hip score at the final follow-up. However, hips treated without navigation had a higher rate of dislocation. Revision was performed in two cases undertaken without navigation, one for aseptic acetabular loosening and one for fracture of a ceramic liner, both of which showed evidence of neck impingement on the liner. A further five cases undertaken without navigation showed erosion of the posterior aspect of the neck of the femoral component on the lateral radiographs. These seven impingement-related mechanical problems correlated with malorientation of the acetabular component. There were no such mechanical problems in the navigated group. We conclude that CT-based navigation increased the precision of orientation of the acetabular component and control of limb length in total hip replacement, without navigation-related complications. It also reduced the rate of dislocation and mechanical problems related to impingement.

Comparison of fluoroscopic and imageless registration in surgical navigation of the acetabular component

OBJECTIVE

This study compared the repeatability and reproducibility of acetabular component positioning using imageless and fluoroscopic-referenced navigation methods.

METHODS

A single cadaveric pelvis had a modular acetabular component securely fixed. Cup position was evaluated using imageless and fluoroscopic registration techniques. These were compared to measurements of a coordinate measuring machine (CMM) and a validated CT scan protocol.

RESULTS

The CMM-determined anatomical acetabular inclination measurement was 46.02 degrees (SD = 1.07), while the CMM-determined anatomical anteversion (pubic symphysis) was 15.79 degrees (SD = 0.41). Computed tomography revealed inclination of 42.2 degrees (SD = 0.65); anteversion with pubic tubercle referencing of 12.1 degrees (SD = 0.14); and anteversion with pubic symphysis referencing of 14.3 degrees (SD = 0.89). Evaluation of repeatability (one surgeon; n = 8) with the imageless system (pubic tubercle) revealed inclination of 41.8 degrees (SD = 0.46) and anteversion of 11.2 degrees (SD = 0.8). For the fluoroscopic system (pubic symphysis), inclination was 42.8 degrees (SD = 1.6) and anteversion was 17.6 degrees (SD = 3.1). Evaluation of reproducibility (three surgeons; n = 24) with the imageless system revealed inclination of 41.8 degrees (SD = 0.82) and anteversion of 15.2 degrees (SD = 1.06). For the fluoroscopic system, inclination was 48.5 degrees (SD = 0.9) and anteversion was 17.8 degrees (SD = 2.5). Imageless referencing of cup inclination and anteversion were found to be process capable using the Six Sigma Cp and Cpk capability indices. Fluoroscopic referencing was process capable for cup inclination but not for cup anteversion (Cp - 1.1; Cpk - 1.0). An F-test revealed significantly greater variance with fluoroscopic referenced anteversion (p < 0.002).

CONCLUSIONS

Imageless referencing was process capable for computer navigation of cup placement in the ex-vivo setting. Fluoroscopic referencing for pelvic landmarks is problematic as locating points from radiographic images is difficult, especially for cup anteversion.

Accuracy of navigation-assisted acetabular component positioning studied by computed tomography measurements: methods and results

The postoperative position of the acetabular component is key for the outcome of total hip arthroplasty. Various aids have been developed to support the surgeon during implant placement. In a prospective study involving 4 centers, the computer-recorded cup alignment of 37 hip systems at the end of navigation-assisted surgery was compared with the cup angles measured on postoperative computerized tomograms. This comparison showed an average difference of 3.5 degrees (SD, 4.4 degrees ) for inclination and 6.5 degrees (SD, 7.3 degrees ) for anteversion angles. The differences in inclination correlated with the thickness of the soft tissue overlying the anterior superior iliac spine (r = 0.44; P = .007), whereas the differences in anteversion showed a correlation with the thickness of the soft tissue overlying the pubic tubercles (r = 0.52; P = .001). In centers experienced in the use of navigational tools, deviations were smaller than in units with little experience in their use.

Fiabilité limitée du plan pelvien antérieur pour l’implantation assistée par informatique de la cupule d’une prothèse totale de hanche

PURPOSE OF THE STUDY

The anterior pelvic plane, also called the Lewinnek plane, is commonly used as the reference plane to guide imageless computer assisted surgery for total hip arthroplasty (THA) because this plane is considered to be globally vertical in the standing position. To our knowledge, no study has evaluated this hypothesis or the potential variations in orientation as a function of gender, position of the subject, or THA insertion. The purpose of this work was to examine these different hypotheses in a radio-clinical study.

MATERIAL AND METHODS

The orientation of the anterior pelvic plane was measured in relation to the vertical plane on plain lateral x-rays of the pelvis in the standing position. X-rays were studied for 106 patients: 1) 82 patients with a THA (40 with at least one dislocation, 42 with a stable hip selected randomly, 19 with a standing lateral x-ray before and after arthroplasty) and 24 control subjects for whom lateral images were obtained in the supine and standing positions to assess potential position-related changes in orientation.

RESULTS

The orientation of the anterior pelvic plane was not affected by gender or age. The anterior pelvic plane formed an angle greater than 5 degrees with the vertical plane in 38% of patients and more than 10 degrees in 13%. The orientation of the anterior pelvic plane was not significantly different between the study groups (control versus THA) nor between the THA groups (stable versus dislocated). The supine position modified significantly the orientation of the anterior pelvic plane which changed on average from 1.20 degrees to -2.25 degrees ; the change was greater than 7 degrees in twelve subjects. Implantation of a THA did not modify signi-ficantly the orientation of the anterior pelvic plane in the standing position for the 19 subjects [the variations were small (-1 degrees to 7 degrees on average, range -21 degrees to 8 degrees ) but greater than 5 degrees for 7 of 19 subjects].

DISCUSSION

Most teams use the anterior pelvic plane to guide computer-assisted navigation, considering that this plane is vertical in the standing position. Our findings show however that this is not true for 38% of subjects with a margin of error of 10 degrees , i.e. about half of the anatomic anteversion of the acetabulum. Moving to the standing position would produce a significant variation in the orientation of the anterior plane of the pelvis. This is a source of error which has not been integrated into most imageless navigation systems. Similarly variations in the position of the pelvis from the standing to sitting and supine positions which can produce impingement or dislocation have not been taken into consideration.

CONCLUSION

Variations in the orientation of the anterior pelvic plane in relation to the vertical would suggest that this plane is not a reliable reference. To our knowledge, there is no reliable reference which can be easily identified during the operation which would take into account variations in the position of the pelvis. We thus believe it would be preferable to attempt to operate without a reference plane, relying on a more kinematic approach to guide computer-assisted implantation of the THA cup.

Nonmodular flexible press-fit cup in primary total hip arthroplasty: 15-year followup

The key rationale for a nonmodular flexible press-fit cup was to maximize long-term stability with a cementless, porous metal-coated cup that is low in stiffness (no metal shell) and to achieve fixation solely through biradial eccentricity between the cup and the acetabular cavity. We asked whether the promising results achieved at 5 and 10 years would be maintained at 15 years. We reviewed 261 patients who received the first 280 Morscher Press-Fit Cups. One hundred twenty patients (126 hips) died. One hundred one patients (112 hips) had a clinical and radiographic followup after a minimum of 13.5 years (mean, 14.7 years; range, 13.5-17.0 years). Twenty-four patients with 26 hips were clinically examined or interviewed by telephone. Three of the 101 patients were lost to followup. We judged the results excellent or good in 96% of the hips. The 15-year overall survivorship was 95.3% and with the end point of aseptic loosening, the survivorship was 97.5%. Wear was greater in cups with an inclination greater than 45 degrees and in metal-polyethylene pairings compared with ceramic-polyethylene pairings. This cup design performs well over the long term.

Containment versus impingement: finding a compromise for cup placement in total hip arthroplasty

Recommendations for cup containment and impingement may provide conflicting directions for component orientation in total hip arthroplasty. For optimal containment, the cup is positioned with respect to the acetabular bone, resulting in coincidence of the rim of the cup and the acetabulum. This results in good coverage and symmetric load transfer, leading to good long-term stability, but occasionally necessitates more abduction of the cup than that recommended by the safe zone. On the other hand, placement of the cup for an optimal range of motion would lead to only partial containment, with a higher risk of component loosening and revision. The most effective compromise is to use a prosthesis that has a large safe zone, realised by a high head-to-neck ratio, and orienting the cup such that a good containment is achieved and the safe zone is respected. Computer navigation or smart aiming devices may help to find the best relative orientation.

Difference between 2 measurement methods of version angles of the acetabular component

We evaluated the relationship and the difference between measurements of version angles (VAs) of the acetabular components in total hip arthroplasty taken using 2 different methods. One VA was measured on an anteroposterior radiograph of the hip joint (VAP) and the other on a cross-table lateral radiograph (VCL) in 97 hips after surgery (clinical data) and 6 sawbone pelvic models (model data). There was a positive correlation between VAP and VCL for both data. Mean and standard deviation of the differences (VCL - VAP) between the 2 measurements were 5 degrees +/- 4.2 degrees in clinical data and -0.01 degrees +/- 0.32 degrees in model data. These differences on clinical data should be taken into consideration when comparing VAs in the literature using different measuring methods.

Impingement with total hip replacement

Impingement is a cause of poor outcomes of prosthetic hip arthroplasty; it can lead to instability, accelerated wear, and unexplained pain. Impingement is influenced by prosthetic design, component position, biomechanical factors, and patient variables. Evidence linking impingement to dislocation and accelerated wear comes from implant retrieval studies. Operative principles that maximize an impingement-free range of motion include correct combined acetabular and femoral anteversion and an optimal head-neck ratio. Operative techniques for preventing impingement include medialization of the cup to avoid component impingement and restoration of hip offset and length to avoid osseous impingement.

Postoperative Calculation of Acetabular Cup Position Using 2-D–3-D Registration

A method to accurately measure the position and orientation of an acetabular cup implant from postoperative X-rays has been designed and validated. The method uses 2-D-3-D registration to align both the prosthesis and the preoperative computed tomography (CT) volume to the X-ray image. This allows the position of the implant to be calculated with respect to a CT-based surgical plan. Experiments have been carried out using ten sets of patient data. A conventional plain-film measurement technique was also investigated. A gold standard implant position and orientation was calculated using postoperative CT. Results show our method to be significantly more accurate than the plain-film method for calculating cup anteversion. Cup orientation and position could be measured to within a mean absolute error of 1.4 mm or degrees.

Poor accuracy of freehand cup positioning during total hip arthroplasty

Several studies have demonstrated a correlation between the acetabular cup position and the risk of dislocation, wear and range of motion after total hip arthroplasty. The present study was designed to evaluate the accuracy of the surgeon's estimated position of the cup after freehand placement in total hip replacement. Peroperative estimated abduction and anteversion of 200 acetabular components (placed by three orthopaedic surgeons and nine residents) were compared with measured outcomes (according to Pradhan) on postoperative radiographs. Cups were placed in 49.7 degrees (SD 6.7) of abduction and 16.0 degrees (SD 8.1) of anteversion. Estimation of placement was 46.3 degrees (SD 4.3) of abduction and 14.6 degrees (SD 5.9) of anteversion. Of more interest is the fact that for the orthopaedic surgeons the mean inaccuracy of estimation was 4.1 degrees (SD 3.9) for abduction and 5.2 degrees (SD 4.5) for anteversion and for their residents this was respectively, 6.3 degrees (SD 4.6) and 5.7 degrees (SD 5.0). Significant differences were found between orthopaedic surgeons and residents for inaccuracy of estimation for abduction, not for anteversion. Body mass index, sex, (un)cemented fixation and surgical approach (anterolateral or posterolateral) were not significant factors. Based upon the inaccuracy of estimation, the group's chance on future cup placement within Lewinnek's safe zone (5-25 degrees anteversion and 30-50 degrees abduction) is 82.7 and 85.2% for anteversion and abduction separately. When both parameters are combined, the chance of accurate placement is only 70.5%. The chance of placement of the acetabular component within 5 degrees of an intended position, for both abduction and anteversion is 21.5% this percentage decreases to just 2.9% when the tolerated error is 1 degrees . There is a tendency to underestimate both abduction and anteversion. Orthopaedic surgeons are superior to their residents in estimating abduction of the acetabular component. The results of this study indicate that freehand placement of the acetabular component is not a reliable method.

A computer model of the position of the combined component in the prevention of impingement in total hip replacement

Dislocation remains a major concern after total hip replacement, and is often attributed to malposition of the components. The optimum position for placement of the components remains uncertain. We have attempted to identify a relatively safe zone in which movement of the hip will occur without impingement, even if one component is positioned incorrectly. A three-dimensional computer model was designed to simulate impingement and used to examine 125 combinations of positioning of the components in order to allow maximum movement without impingement. Increase in acetabular and/or femoral anteversion allowed greater internal rotation before impingement occurred, but decreases the amount of external rotation. A decrease in abduction of the acetabular components increased internal rotation while decreasing external rotation. Although some correction for malposition was allowable on the opposite side of the joint, extreme degrees could not be corrected because of bony impingement.

We introduce the concept of combined component position, in which anteversion and abduction of the acetabular component, along with femoral anteversion, are all defined as critical elements for stability.

Ceramic liner fracture after cementless alumina-on-alumina total hip arthroplasty

Advances in technology have reduced the risk of fracture of ceramic total hip arthroplasty implants, but concerns remain about fracture of both components. We retrospectively reviewed 133 patients (157 hips) who had cementless alumina-on-alumina total hip arthroplasties with a sandwich-type acetabular component. Six patients (seven hips) died and five patients (six hips) were interviewed by telephone (95% followup). The 122 patients (144 hips) examined had a minimum followup of 36 months (average, 45 months; range, 36-68 months). All acetabular cups and femoral stems were radiographically stable at the last followup. Five hips in five patients (3.5%) were revised because of ceramic liner fractures. Ceramic liner fractures occurred at a mean of 35 months (range, 24-48 months) postoperatively. Acetabular cups in the fracture group (n = 5) were more anteverted than those in the nonfracture group (n = 139). In three patients the fracture apparently occurred during squatting, resulting in hyperflexion and wide hip abduction. Early ceramic liner fracture was associated with impingement associated with excessive anteversion of the acetabular cup in Korean patients who habitually squat.

Positionnement acétabulaire assisté par ordinateur dans les prothèses totales de hanche

PURPOSE OF THE STUDY

Actetabular component malpositioning during total hip arthroplasty (THA) increases the risk of dislocation, reduces the range of motion, and can be the cause of early wear and loosening. There have been numerous reports on the optimal orientation of the acebaular component in THA. Lewinnek et al recommended an abduction angle of 40+/-10 degrees and an anteversion of 15+/-10 degrees for cup alignment in THA. In order to prevent malpostioned hip implants and improve the reproducibility of implant alignment in THA, numerous computer-assisted orthopedic systems have been described, using computed tomography (CT)-base or imageless navigation. Among the imageless systems available, one is based on Bone Morphing technology initially described by Stindel for computer-assisted knee arthroplasty and adapted for THA. The purpose of this study was to compare computer-assisted acetabular component insertion versus free hand placement.

MATERIAL AND METHODS

A controlled randomized matched prospective study was performed in two groups of 30 patients. The study was approved by the French Ethics Committee. In the first group, cup positioning was assisted by an imageless computer-assisted orthopedics system based on Bone Morphing(R) (CAOS+ group). In the control group, cup placement was free hand (CAOS- group). The same cementless cup was used in both groups. The same surgeon performed all procedures using an anterolateral approach. Cup anteversion and abduction angles were measured on 3D CT scan reconstructions obtained postoperatively for each patient by an independent observer using a special cup evaluation software.

RESULTS

There were 16 males and 14 females in each group, mean age was 62 years (range 24-80) years, and mean body mass index was 25 in each group. Mean additional time of the CAOS procedure was 12 minutes (range 8-20). Intraoperative subjective agreement of the surgeon with the computer guidance system demonstrated a high correlation in 23 cases, a weak correlation in six cases and poor correlation in one case. There were no statistical differences between the CAOS+ and the CAOS- group regarding means of the abduction and anteversion angles, but a significant range of variance, the lowest variations being observed in the CAOS+ group.

DISCUSSION

This study has shown the accuracy of cup positioning using a CT-free navigation system in a prospective randomized controlled protocol.