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

Intraoperative radiographs for placing acetabular components in hip resurfacing arthroplasty

BACKGROUND

Various clinical and biomechanical studies suggest certain acetabular positions may be associated with higher wear and failure rates in modern metal-on-metal hip resurfacing arthroplasties. However, there are no widely available, reliable, and cost-effective surgical techniques that ensure surgeons are able to place an acetabular component within the safe range of inclination angles after hip resurfacing surgeries.

QUESTIONS/PURPOSES

We investigated the accuracy of intraoperative radiographs to determine the acetabular inclination angle in resurfacing arthroplasty procedures.

PATIENTS AND METHODS

The study group included the first 100 resurfacing arthroplasties performed after we started routinely checking the intraoperative acetabular inclination angles. The acetabular component was repositioned if the intraoperative acetabular inclination angle was out of the target range of 30° to 50°. The control group included the previous 100 resurfacing arthroplasties performed without the benefit of intraoperative radiographs. A posterior minimally invasive surgical approach was used in both groups. Demographics and diagnoses were similar in both groups.

RESULTS

The average (± SD) difference between the intraoperative and 6-week radiographs was 2.7° ± 2.5°. The acetabular inclination angles at 6-week followup were within the targeted range more frequently in the study group than in the control group (outliers: 4% versus 29%).

CONCLUSIONS

These data suggest a single intraoperative radiograph is a quick, reliable, and cost-effective method for ensuring the acetabular inclination angle is within the targeted range.

Validation of navigation assisted cup placement in total hip arthroplasty

PURPOSE

Computer navigation has the potential to provide precise intraoperative knowledge to the surgeon. Previous studies with navigation have confirmed its function for improved component position but few studies have reported the accuracy and precision of navigation system in clinical use. With this study we propose to evaluate the efficacy of navigation in guiding cup placement.

METHODS

Fifty-six patients undergoing primary total hip arthroplasty were prospectively included in this study. Stryker imageless navigation system which is accurate to 0.5° was used in all cases. Intraoperative data was collected for the acetabular component position using navigation for the freehand cup placement and the final cup placement done using navigation. Postoperative evaluation of component position was done with computed tomography (CT) and the deviation from intraoperative freehand and navigation values were calculated.

RESULTS

The mean inclination of the freehand reading was 39.5° (range, 20°-58°), mean version of freehand reading was 10.7° (-6°- 27°), and the mean navigation reading was 43.2° (37°-49°) for inclination and 13.0° (-8° - 24°) for version. On postoperative CT scan analysis the mean inclination was 45.3° (34°-56°) and mean version was 15.1° (4°-25°). The deviation of the freehand inclination from the post operative CT scan reading was 11.4° (1°-30°) and the version deviated by a mean of 10.8° (2°-26°). The deviation of the navigation reading from the CT scan reading had a mean of 5.3° (1°-13°) for inclination and 5.6° (1°-17°) for version.

CONCLUSION

The accuracy of the navigation system over conventional freehand cup placement is validated by this study.

Use of image-free navigation in determination of acetabular cup orientation: analysis of factors affecting precision

We have been using an image-free total hip arthroplasty (THA) navigation system (OrthoPilot; Aesculap, Tuttlingen, Germany) to ensure accurate and reproducible acetabular cup orientation. In this study, the accuracy of this system in the assessment of acetabular cup orientation was evaluated by comparing the intraoperative inclination and anteversion angles presented by the navigation system and the corresponding postoperative values obtained by computed tomography measurement. In the intraoperative accuracy analysis, we additionally examined the influence of factors such as body mass index and soft tissue thickness on assessment error. Intraoperative and postoperative results obtained from 115 consecutive navigated THAs were compared and analyzed. In both inclination and anteversion angles, good agreement was observed; a discrepancy of more than 5° was observed in 1 and 3 cases, respectively. In the analysis of factors potentially affecting the accuracy of the intraoperative assessment, no correlation between each parameter and the intraoperative and postoperative discrepancy was demonstrated.

Minimally invasive and computer-navigated total hip arthroplasty: a qualitative and systematic review of the literature

Background

Both minimally invasive surgery (MIS) and computer-assisted surgery (CAS) for total hip arthroplasty (THA) have gained popularity in recent years. We conducted a qualitative and systematic review to assess the effectiveness of MIS, CAS and computer-assisted MIS for THA.

Methods

An extensive computerised literature search of PubMed, Medline, Embase and OVIDSP was conducted. Both randomised clinical trials and controlled clinical trials on the effectiveness of MIS, CAS and computer-assisted MIS for THA were included. Methodological quality was independently assessed by two reviewers. Effect estimates were calculated and a best-evidence synthesis was performed.

Results

Four high-quality and 14 medium-quality studies with MIS THA as study contrast, and three high-quality and four medium-quality studies with CAS THA as study contrast were included. No studies with computer-assisted MIS for THA as study contrast were identified. Strong evidence was found for a decrease in operative time and intraoperative blood loss for MIS THA, with no difference in complication rates and risk for acetabular outliers. Strong evidence exists that there is no difference in physical functioning, measured either by questionnaires or by gait analysis. Moderate evidence was found for a shorter length of hospital stay after MIS THA. Conflicting evidence was found for a positive effect of MIS THA on pain in the early postoperative period, but that effect diminished after three months postoperatively. Strong evidence was found for an increase in operative time for CAS THA, and limited evidence was found for a decrease in intraoperative blood loss. Furthermore, strong evidence was found for no difference in complication rates, as well as for a significantly lower risk for acetabular outliers.

Conclusions

The results indicate that MIS THA is a safe surgical procedure, without increases in operative time, blood loss, operative complication rates and component malposition rates. However, the beneficial effect of MIS THA on functional recovery has to be proven. The results also indicate that CAS THA, though resulting in an increase in operative time, may have a positive effect on operative blood loss and operative complication rates. More importantly, the use of CAS results in better positioning of acetabular component of the prosthesis.

Accurate acetabular component orientation after total hip arthroplasty using an acetabular alignment guide

Between February 2005 and August 2006, we recorded acetabular component orientation in 90 patients (100 hips) who underwent primary total hip arthroplasty (THA), to determine whether using an alignment guide ensures accurate acetabular positioning. In the alignment-guide group (46 patients; 48 hips), a guide was placed on the pelvis, a Kirschner wire (K-wire) was attached to the guide, and orientation of the acetabular component was confirmed by both the surgeon and an assistant. In the control group (44 patients; 52 hips), a K-wire was not used and the angle was confirmed by the surgeon alone. Radiographic acetabular component inclination and anteversion and computed tomography anteversion were determined. There was no significant difference in mean component orientation between the 2 groups. However, the SD was significantly smaller in the alignment-guide group, showing that consistent acetabular component orientation in primary THA is highly reproducible when an acetabular alignment guide with an attached K-wire is used.

Accuracy of computer navigation for acetabular component placement in THA

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

Does computer-assisted surgery benefit leg length restoration in total hip replacement? Navigation versus conventional freehand

Leg length discrepancy following total hip replacement (THR) can contribute to poor hip function. Abnormal gait, pain, neurological disturbance and patient dissatisfaction have all been described as a result of leg length inequality after THR. The purpose of this study was to determine whether the use of computer navigation in THR can improve limb length restoration and early clinical outcomes. We performed a matched-pair study comparing 48 computer-assisted THR with 48 THRs performed using a traditional freehand alignment method. The same implant with a straight non-modular femoral stem was used in all cases. The navigation system used allowed the surgeon to monitor both acetabular cup placement and all the phases of femoral stem implantation including rasping. Patients were matched for age, sex, arthritis level, pre-operative diagnosis and pre-operative leg length discrepancy. At a minimum follow-up of six months, limb length discrepancy was measured using digital radiographs and a standardised protocol. The number of patients with a residual discrepancy of 10 mm or more and/or a post-operative over-lengthening were measured. The clinical outcome was evaluated using both the Harris Hip Score and the normalised Western Ontario and McMaster Universities (WOMAC) Arthritis Index. Restoration of limb length was significantly better in the computer-assisted THR group. The number of patients with a residual limb length discrepancy greater than 10 mm and/or a post-operative over-lengthening was significantly lower. No significant difference in the Harris Hip Score or normalised WOMAC Arthritis Index was seen between the two groups. The surgical time was significantly longer in the computer-assisted THR group. No post-operative dislocations were seen.

Navigated cup implantation in hip arthroplasty

BACKGROUND AND PURPOSE

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

METHODS

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

RESULTS

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

INTERPRETATION

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

Femoral neck cut level affects positioning of modular short-stem implant

A trend in total hip arthroplasty surgery has been to design more bone-preserving procedures, especially for younger patients. This study investigated the final implant positioning of a short metaphyseal femoral neck type of implant to determine whether leg length, caput collum diaphysis (CCD) angle, and offset could be re-created with different levels of femoral neck resection. Ten cadaveric hips in 6 whole-body specimens were used, with 3 fiducial markers to allow registration of computer navigation points to computed tomography scan data. Three femoral neck resection levels were investigated: 0 mm, +5 mm (the recommended level of resection), and +10 mm from the base of the femoral neck. Results showed that the CCD angle was significantly higher with 0-mm neck cut and the offset was lower, whereas the highest neck cut had longer leg-length results. Surgeons who use a short metaphyseal stem need to realize the importance of a proper femoral neck cut to restore anatomic parameters as well as the possible benefit of computer-assisted surgery to restore these anatomic parameters during surgery.

Intra- and intersurgeon variability in image-free navigation system for THA

In image-free navigation systems, cup orientation is determined in the pelvic coordinate by registration of bony landmarks. While the value of navigation relates primarily to the reliability and accuracy of cup placement, the reliability of registration plays a role in cup placement. We therefore examined intra- and intersurgeon variability in registration and the distance between registration points in each bony landmark. Thirty-seven THAs were performed in the lateral position and 15 THAs in the supine position. The cup was fixed using a navigation system. The registration was repeated two more times by operator and assistant, and the intra- and intersurgeon variability of cup abduction angle and anteversion was analyzed by ICC (intraclass correlation coefficients). In 25 hips, the distance between intrasurgeon registration points and between intersurgeon registration points in each landmark were calculated. The ICC in the lateral position ranged between 0.59 and 0.81, and between 0.85 and 0.95 in the supine position. The ICCs of cup abduction angle for the intra- and intersurgeon variability were 0.92 and 0.95 for the supine position and 0.65 and 0.59 for the lateral position. Those of anteversion were 0.93, 0.85, and 0.81, 0.72, respectively. The variability in locating the ASIS in the lateral position was greater than that in the supine position. The variability of registration points depended on bony landmarks and patient position but the range of variability we found would not likely result in a large variability in cup placement.

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

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

Role of navigation in total hip arthroplasty

Current uses of computed tomography-based and imageless navigation systems for total hip arthroplasty include proper placement of the acetabular component, measurement of limb-length changes, enablement of minimally invasive surgery, and proper placement of components for hip resurfacing procedures. This article provides an overview of the rationale for computer-assisted surgery in total hip arthroplasty and hip resurfacing. The experience of the senior author (M.L.S.) with computer-assisted surgery for hip arthroplasty has demonstrated improved position of the acetabular component as compared with the position attainable with use of mechanical instruments, maintenance of appropriate position of the acetabular component during minimally invasive surgery, and appropriate positioning of the femoral and acetabular components during the learning curve for hip resurfacing procedures.

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.

Current state and future of joint replacements in the hip and knee

Joint replacements of the hip and knee are among the most clinically successful operations. According to figures compiled by the American Academy of Orthopaedic Surgeons, the number of primary total hip replacements performed in the USA was 220,000 in 2003. This was 38% more than in 1996 and this number is expected to rise to 572,000 (plus another 97,000 revisions) by 2030. The number of primary total knee replacements performed in 2003 was approximately 418,000 and is expected to rise exponentially with the increasing numbers of baby boomers and the aging population. Current research focuses not only on extending implant longevity, but also on improving function to meet the increased demands of today's patients, who are likely to be younger and more active than their predecessors two decades ago. Potential advancements in arthroplasty surgery include new, more wear-resistant bearing surfaces, porous metals to enhance osseointegration and replace lost bone stock, a clearer understanding of the biological processes associated with periprosthetic osteolysis, minimally invasive surgery and computer assisted surgery. Long-term studies are needed to establish the efficacy of these new technologies.

Error in acetabular socket alignment due to the thick anterior pelvic soft tissues

The aim of this study was to assess how much the digitizing error attributable to the thick soft tissues in the anterior pelvic area can affect the targeted acetabular socket alignment. The acetabular socket orientation angle was measured by using a synthetic pelvic model and an OrthoPilot total hip arthroplasty (Aesculap AG & Co KG, Tuttlingen, Germany) navigation system. The anterior pelvic plane was defined using 3 bony landmarks: the ipsilateral and contralateral anterior superior iliac spines (ASIS) and the center of the 2 pubic tubercles. The digitization error of 1 cm at the ipsilateral ASIS, contralateral ASIS, and the center of the two pubic tubercles resulted in 1.8 degrees , 4.4 degrees , and -6.8 degrees in anteversion, and 1.4 degrees , 0.2 degrees , and -0.2 degrees in abduction, respectively. The current navigation system based on the anterior pelvic plane may produce considerable error in guiding the acetabular socket anteversion for patients with thick soft tissue in the anterior pelvic area.

Navigated non-image-based registration of the position of the pelvis during THR. An accuracy and reproducibility study

The precise recording of the position of the pelvis is a prerequisite for total hip replacement (THR). The anterior pelvic plane is an accepted reference for determining the 3D pelvic orientation. We hypothesized that cutaneous palpation of this plane was accurate and reproducible. Ten consecutive navigated implantations of THR prostheses were studied. In each case, four palpations of both anterior iliac spines and the pubic symphysis were performed with a navigated stylus. The first palpation was made on actual bone contours through a skin puncture and was considered as the reference. The other three palpations were made over the intact skin as a normal intra-operative procedure. There was no significant difference between the pelvic orientations measured by the three cutaneous palpations, or between the orientations measured by transcutaneous palpation and the mean result with cutaneous palpation. Cutaneous palpation can be considered as a reliable technique for the definition of pelvic orientation during THR with the non-image-based system employed.

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.

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.

Navigating the fluoroscope's C-arm back into position: an accurate and practicable solution to cut radiation and optimize intraoperative workflow

OBJECTIVES

During complex image-guided orthopedic trauma procedures, repetitive fluoroscopic scout imaging is performed. A number of preparatory positioning images often must be taken to reproduce a comparable projection. These scout images have no intrinsic clinical relevance but nevertheless expose the patient and the surgical team to considerable radiation, which could be avoided. This study presents and validates a method to decrease intraoperative radiation.

METHODS

Precision, time requirements, and number of scout images for repositioning the fluoroscope, with and without navigation aid, were recorded on 20 test-rig and 3 phantom setups. A commercially available image-guided surgical navigation system (Vector Vision, BrainLAB), originally designed for instrument navigation, was employed to register and retrieve the C-arm positions. A newly developed software computed the necessary moves to reposition the C-arm on an intuitive visual display.

RESULTS

Retrieving a given C-arm position with the conventional non-navigated technique required an average of 7 scout images (range, 3 to 12 images). In contrast, navigation-assisted repositioning did not necessitate a single scout image. Deviations from the original projection were minimal for both navigated (0.9 degrees, 95% CI 0.8 to 1.1 degrees) and non-navigated repositioning (0.8 degrees, 95% CI 0.7 to 0.9 degrees). Average positioning times were comparable when navigating the C-arm (46 seconds, 95% CI 41 to 51 seconds) and in scout image-based positioning (49 seconds, 95% CI 44 to 53 seconds).

CONCLUSIONS

Navigated C-arm positioning avoids multiple scout images and yields sufficient precision for clinical deployment. Radiation exposure can be reduced considerably by a combination of instrument navigation and navigated C-arm positioning.

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.

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.

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.

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.

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.