Reduced variability of acetabular cup positioning with use of an imageless navigation system

Positioning the acetabular component is one of the most important steps in total hip arthroplasty; malpositioned components can result in dislocations, impingement, limited range of motion, and increased polyethylene wear. Conventional surgery makes use of specialized alignment guides provided by the manufacturers of the implants. The use of mechanical guides has been shown to result in large variations of cup inclination and version. We investigated acetabular cup alignment with the nonimage-based hip navigation system compared with a conventional mechanically guided procedure in 12 human cadavers. Postoperative cup position relative to the pelvic reference plane was assessed in both groups with the use of a three-dimensional digitizing arm. In the navigated group, a median inclination of 45.5 degrees and a median anteversion of 21.9 degrees (goals, 45 degrees and 20 degrees) were reached. In the control group, the median inclination was 41.8 degrees and the median anteversion was 24.6 degrees. The ninetieth percentile showed a much wider range for the control group (36.1 degrees-51.8 degrees inclination, 15 degrees-33.5 degrees anteversion) than for the navigated group (43.9 degrees-48.2 degrees inclination, 18.3 degrees-25.4 degrees anteversion). This cadaver study shows that computer-assisted cup positioning using a nonimage-based hip navigation system allowed for more consistent placement of the acetabular component.     

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.     

Comparison of conventional versus computer-navigated acetabular component insertion

This retrospective study compared the efficacy of computer navigation and conventional freehand techniques to place acetabular component orientation in the target position of acetabular cup inclination of 45 degrees and anteversion of 20 degrees . We selected 69 patients who had undergone total hip arthroplasty with freehand cup insertion who had computed tomography (CT) to plan for acetabular cup placement of the contralateral side. This group was compared with 98 patients who underwent CT-based cup insertion, and all had postoperative CT. After CT-based cup placement, average cup position was 43 degrees inclination (95% confidence interval [CI], 0.97; range, 30 degrees -58 degrees ) and 22.2 degrees anteversion (95% CI, 1.72; range, 5 degrees -38 degrees ). For freehand, average cup position was 45.7 degrees inclination (95% CI, 2.63 degrees ; range, 26 degrees -64 degrees ) and 28.5 degrees anteversion (95% CI, 3.80 degrees ; range, 9 degrees -53 degrees ). F ratio was 5.56 for inclination and 3.67 for anteversion (P < .0001). This study demonstrated substantial statistical improvement in accuracy of cup placement using CT-based navigation compared with freehand methods.     

Computer-assisted navigated cup placement of different cup types in hip arthroplasty--a randomised controlled trial]

AIM:The position of the acetabular implant has a great influence on the outcome of total hip arthroplasty. The acetabular implants differ in their characteristics during insertion. In order to control the influence of the implant in different axes, 80 implantations of acetabular component were performed under computer-assisted control by a surogate navigation system. METHOD: Within a prospective randomised study, 70 total hip replacements were performed by one surgeon using two different uncemented implants (PPF screw cup/Hedrocel pressfit cup). The position of the cup was measured at different times during surgery and was compared to 10 cemented PE cup placements. RESULTS: During the milling procedure the anteversion of the two uncemented cups differed significantly (p = 0.026). The standard deviation of anteversion during implantation different significantly relating to the cup type: press-fit cup 3.86 degrees, screw cup 2.1 degrees and cemented PE cups 0,84 degrees. The inclination was less susceptible. CONCLUSION: The cup design of uncemented acetabular implants influences the outcome of surgery. In our trial a spherical press-fit cup showed a greater variability than the screw-cup. The CAS system can be used as a tool to minimise these effects.     

计算机导航系统辅助下全髋关节表面置换的初步经验

目的 初步评估计算机辅助下行全髋关节表面置换的手术方法及假体植入的精确性.方法 对40例患者分成导航组(20例)和对照组(20例),分别采用计算机导航系统辅助和标准常规技术完成全髋关节表面置换术.比较两组手术时间、术中失血量、Harris髋关节评分(HHS),以及髋臼杯外展角和前倾角的偏移度及股骨假体柄干角和前倾角的偏移度.结果 手术时间导航组较对照组长38.7min(P＜0.05),术中平均出血量导航组较对照组多109.4 ml(P＜0.05).随访平均14.5个月,HHS评分两组比较差异无统计学意义(P＞0.05).对照组髋臼杯外展角偏移度(7.3°±4.3°)大于导航组(1.9°±1.3°)(P＜0.05),对照组髋臼杯前倾角偏移度(3.9°±2.3°)大于导航组(2.4°±1.5°)(P＜0.05).导航组股骨假体柄干角偏移度(1.5°±1.0°)小于对照组(11.3°±1.3°)(P＜0.05),导航组股骨假体前倾角偏移度(1.8°±2.3°)小于对照组(6.5°±5.3°)(P＜0.05).结论 计算机导航系统辅助下行全髋关节表面置换术可以提高假体植入的精确性,避免股骨颈皮质骨切迹,降低股骨颈骨折的风险.     

The effect of the orientation of the acetabular and femoral components on the range of motion of the hip at different head-neck ratios

BACKGROUND: Prosthetic impingement due to poor positioning can limit the range of motion of the hip after total hip arthroplasty. In this study, a computer model was used to determine the effects of the positions of the acetabular and femoral components and of varying head-neck ratios on impingement and range of motion. METHODS: A three-dimensional generic hip prosthesis with a hemispherical cup, a neck diameter of 12.25 millimeters, and a head size ranging from twenty-two to thirty-two millimeters was simulated on a computer. The maximum range of motion of the hip was measured, before the neck impinged on the liner of the cup, for acetabular abduction angles ranging from 35 to 55 degrees and acetabular and femoral anteversion ranging from 0 to 30 degrees. Stability of the hip was estimated as the maximum possible flexion coupled with 10 degrees of adduction and 10 degrees of internal rotation and also as the maximum possible extension coupled with 10 degrees of external rotation. The effects of prosthetic orientation on activities of daily living were analyzed as well. RESULTS: Acetabular abduction angles of less than 45 degrees decreased flexion and abduction of the hip, whereas higher angles decreased adduction and rotation. Femoral and acetabular anteversion increased flexion but decreased extension. Acetabular abduction angles of between 45 and 55 degrees permitted a better overall range of motion and stability when combined with appropriate acetabular and femoral anteversion. Lower head-neck ratios decreased the range of motion that was possible without prosthetic impingement. The addition of a modular sleeve that increased the diameter of the femoral neck by two millimeters decreased the range of motion by 1.5 to 8.5 degrees, depending on the direction of motion that was studied. CONCLUSIONS: There is a complex interplay between the angles of orientation of the femoral and acetabular components. Acetabular abduction angles between 45 and 55 degrees, when combined with appropriate acetabular and femoral anteversion, resulted in a maximum overall range of motion and stability with respect to prosthetic impingement. CLINICAL RELEVANCE: During total hip arthroplasty, acetabular abduction is often constrained by available bone coverage, while femoral anteversion may be dictated by the geometry of the femoral shaft. For each combination of acetabular abduction and femoral anteversion, there is an optimum range of acetabular anteversion that allows the potential for a maximum range of motion without prosthetic impingement after total hip arthroplasty. These data can be used intraoperatively to determine optimum position.     

Navigating the Reaming of the Acetabular Cavity in Total Hip Arthroplasty: Does it Improve Implantation Accuracy?

Computer navigation in total hip arthroplasty is used to improve accuracy of component implantation. Reaming of the acetabular cavity during total hip arthroplasty (THA) can be navigated although this is not done routinely. We hypothesised that navigating the reaming of the acetabular cavity will improve implantation accuracy. A single surgeon series of 100 navigated THAs were analysed retrospectively. In 49 the reaming of the acetabular cavity was done using navigation and in 51 this was done freehand. The verified cup position and the error from the planned position were recorded. The mean error from planned to verified inclination was 2.20 degrees (SD 1.59°) in the navigated group versus 2.33 degrees (SD 1.96°) in the freehand group. The mean anteversion error was 1.92 degrees (SD 1.51°) for the navigated group and 1.45 degrees (SD 1.38°) for the freehand group. This was not statistically significant. This rejects our hypothesis. Navigating the reaming of the acetabular cavity did not improve the accuracy of the implantation against the set inclination and anteversion target during computer navigated THA.     

The acetabulum: The elusive home run of THR

The objective is to describe the current factors for optimizing positioning of the acetabular component in THR. The emphasis in cup placement today is personalizing the component position for each patientʼs anatomy rather than a “one size fits all” (always put the cup in the same position such as 45° inclination and 15° anteversion). To individualize, the arthroplasty requires remembering the operation is on both sides of the joint (combined anteversion) and implanting the cup in the functional plane of that patient, which requires knowledge of the pelvic tilt at surgery and the changes in the spine−pelvic−hip construct between standing and sitting for that patient. To individualize, the cup position will demand higher precision than has been accepted in the past, such as computer navigation, to augment the experience and instinct of the surgeon in performing total hip replacement. In conclusion, acetabular cup placement is an elusive home run because the complexity of its positioning for each patient requires new preoperative planning, and more precise intraoperative positioning.     

Improving acetabular cup orientation in total hip arthroplasty by using smartphone technology

Acetabular cup placement in total hip arthroplasty is often difficult to assess, especially in the lateral position and using the posterior approach. Conventional techniques and computer-assisted surgery are the 2 most popular methods for proper placement of the acetabular cup in Lewinnek's safe zone of orientation (anteversion 15° ± 10° and lateral inclination 40° ± 10°). We developed a system that uses the accelerometer and camera function of the iPhone. A level indicator application and protractor application were downloaded to the iPhone and used to improve acetabular cup placement. This system has proven to be accurate and quick. Our series of 50 prospective cases showed good results with all our acetabular cups being placed within a narrow range in the safe zone and with less than 5% difference between the preoperative, intraoperative, and postoperative acetabular inclinations.     

Validation and usefulness of a computer-assisted cup-positioning system in total hip arthroplasty. A prospective, randomized, controlled study

BACKGROUND: Malpositioning of the acetabular component during total hip arthroplasty increases the risk of dislocation, reduces the range of motion, and can be responsible for early wear and loosening. The purpose of this study was to compare computer-assisted with freehand insertion of the acetabular component. METHODS: A randomized, controlled, matched prospective study of two groups of thirty patients each was performed. In the first group, cup positioning was assisted by an imageless computer-assisted surgical system based on bone morphing. In the control group, the cup was placed freehand. All of the patients were operated on by the same surgeon through an anterolateral approach. Cup anteversion and abduction angles were measured on three-dimensional computed tomography reconstructions postoperatively for each patient by an independent observer using special cup-evaluation software. RESULTS: There were sixteen men and fourteen women in each group, and the mean body-mass index was approximately 25 in each group. The computer-assisted procedure took a mean of twelve minutes longer than the freehand procedure. Fifty-seven percent (seventeen) of the thirty cups placed freehand and 20% (six) of the thirty in the computer-assisted group were outside of the defined safe zone (outliers). This difference was significant (p = 0.002). There were no differences between the computer-assisted group and the freehand-placement group with regard to the mean abduction and anteversion angles, but there was a significant heterogeneity of variances, with the lowest variations in the computer-assisted group. CONCLUSIONS: Use of an imageless navigation system can improve cup positioning in total hip arthroplasty by reducing the percentage of outliers.     

Image-based hip navigation

After experimental and preclinical evaluation (HAP Paul Award 2001) of a CT-free image-guided surgical navigation system for acetabular cup placement, the system was introduced into clinical routine. The computation of the angular orientation of the cup is based on reference coordinates from the anterior pelvic plane (APP) concept. A hybrid strategy for pelvic landmark acquisition has been introduced involving percutaneous pointer-based digitization with the noninvasive biplanar landmark reconstruction using multiple registered fluoroscopy images. From January 2001 to May 2002, 118 consecutive patients (mean age 68 years, 82 male, 36 female, and 62 left and 56 right hip joints) were operated on with the hybrid CT-free navigation system. During each operation, the angular orientation of the inserted implant was recorded. To determine the placement accuracy of the acetabular components, the first 50 consecutive patients underwent a CT scan 7-10 days postoperatively to analyze the cup position relative to the APP. This was done blinded with commercial planning software. There was no significant learning curve observed for the use of the system. Mean values for postoperative inclination read 43 degrees (SD 3.0, range 37-49) and anteversion 19 degrees (SD 3.9, range 10-28). The resulting system accuracy, i.e., the difference between intraoperatively calculated cup orientation and postoperatively measured implant position, shows a maximum error of 5 degrees for the inclination (mean 1.5 degrees, SD 1.1) and 6 degrees for the anteversion (mean 2.4 degrees, SD 1.3). An accuracy of better than 5 degrees inclination and 6 degrees anteversion was achieved under clinical conditions, which implies that there is no significant difference in performance from the established CT-based navigation methods. Image-guided CT-free cup navigation provides a reliable solution for future total hip arthroplasty (THA).     

C-arm based navigation in total hip arthroplasty-background and clinical experience

After experimental and preclinical evaluation of a CT-free image guided surgical navigation system for acetabular cup placement, the system was introduced into clinical routine. The computation of the angular orientation of the cup is based on reference coordinates from the anterior pelvic plane concept. A hybrid strategy for pelvic landmark acquisition has been introduced, involving percutaneous pointer-based digitization with the noninvasive bi-planar landmark reconstruction using multiple registered fluoroscopy images. From January 2001 to October 2003, a total of 236 consecutive patients (mean age 66 years, 144 male, 92 female, 124 left and 112 right hip joints) were operated on with the hybrid CT-free navigation system. During each operation, the angular orientation of the inserted implant was recorded. To determine the placement accuracy of the acetabular components, the first 50 consecutive patients underwent a CT scan 7-10 days postoperatively to analyze the cup position relative to the anterior pelvic plane. This procedure was done blinded and with commercial planning software. There was no significant learning curve observed for the use of the system. Mean values for postoperative inclination read 42 degrees (SD 3.6, range (37-49)) and anteversion 21 degrees (SD 3.9, range (10-28)). The resulting system accuracy, ie, the difference between intraoperatively calculated cup orientation and postoperatively measured implant position shows a maximum error of 5 degrees for the inclination (mean 1.5 degrees, SD 1.1) and 6 degrees for the anteversion (mean 2.4 degrees, SD 1.3). An accuracy of better than 5 degrees inclination and 6 degrees anteversion was achieved under clinical conditions, which implies that there is no significant difference in performance from the established CT-based navigation methods. Image-guided CT-free cup navigation provides a reliable solution for future total hip arthroplasty (THA).     

Measuring anatomical acetabular cup orientation with a new X-ray technique.

Proper alignment of the acetabular cup component is one of the most important requisites for a successful long-term outcome in total hip replacement. However, measurement and indication of cup orientation in an anatomical pelvic reference system is very difficult. We propose a new C-arm-based X-ray technique for determining the values for inclination and anteversion of the acetabular cup component. The proposed method is validated by computer simulation and sources of error are evaluated. The method predicts an accuracy of better then 5 degrees for determination of anteversion of the cup.     

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

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

髋臼横韧带定位髋臼假体前倾校正骨盆旋转

目的通过观察后外侧入路全髋关节置换术中骨盆的旋转变化，探讨术中骨盆旋转对髋臼假体前倾角植入的影响，评估使用髋臼横韧带作髋臼假体前倾定位的准确性以及对骨盆旋转角度变化的校正作用。 方法2015年1月至2016年1月河池市第三人民医院关节外科收治的行初次THA的40例44髋的髋关节疾病患者纳入本研究。纳入标准为：初次THA术的患者，术前、术后CT扫描质量符合标准、能确定髋臼解剖前倾角、髋臼假体前倾角的患者。排除标准：髋臼发育不良、强直性脊柱炎、既往有髋关节严重创伤手术史以及翻修术等，术前、术后双髋关节CT扫描，CT横断面上骨盆明显倾斜、两侧髋关节的中心显示明显不在同一层面、难以确定水平线测量前倾角的患者，予以排除。手术均采用侧卧位后外侧入路，切皮前将1枚施氏针以垂直于地面方向打入髂骨嵴，术中髋臼假体的前倾角，以髋臼横韧带为参照标志，通过直接参照或间接参照髋臼横韧带进行髋臼挫磨及安装臼杯，使髋臼假体开口平行韧带进行安放，在此过程中用摄像机记录施氏针相对于地面的角度变化，确定手术过程中骨盆旋转度数，同时测量并记录挫磨髋臼及安放假体时，相对于身体长轴髋臼手术前倾角的数值。术后通过CT测量髋臼假体前倾角，对术中手术前倾角和术后髋臼假体的前倾角、术前髋臼解剖前倾角数据进行t检验分析。 结果术中骨盆旋转发生在本研究中的平均度数为(18±4)°。44个髋关节中，所有的病例都能对髋臼横韧带进行辨认，术中手术前倾角平均为(33±5)°，有93%(41髋)的病例大于Lewinnek提出的"安全区"的前倾角上限25°，余下的7%(3个髋)也全部大于24°。术后CT测量髋臼假体的解剖前倾角为(21±10)°，与术前髋臼解剖前倾角度(19±7)°比较，差异无统计学意义(t＝1.264，P >0.05)。 结论在后外侧入路THA术中，体位改变骨盆前旋转会影响髋臼假体植入的准确性，使用髋臼横韧带作为解剖标志指导髋臼假体前倾角度的植入，可以排除患者体位改变骨盆旋转对前倾角的影响，提高髋臼假体放置的准确性。     

Cup placement in hip replacement surgery -- A comparison of free-hand and computer assisted cup placement in total hip arthroplasty -- a multi-center study

AIM: The purpose of the current study was to determine the accuracy of free hand and computer-assisted (CAS) cup placement. MATERIAL AND METHODS: Between June 1985 and August 2001, 105 free-hand and from March to November 1999 80 total hip arthroplasties under computer assistance were implanted in the above mentioned centers. To determine the accuracy of the cups, the inclination and anteversion angles were measured. In all cases the cup position was measured with a CT-investigation of the pelvis. Statistical analysis was performed with the F-test. RESULTS: With regard to the inclination and anteversion angles the variability of the cup position was significantly higher in the group of free hand implanted cups. In the CAS group we could not find any "extreme" positions. CONCLUSION: Using computer-assisted surgery a significantly higher reproducible cup position can be obtained. Long time survey may present a lowering of the rate of early and late complications caused by better prostheses alignment in the follow up.     

Accuracy of acetabular cup placement using CT-based navigation in total hip arthroplasty: Comparison between obese and non-obese patients

BackgroundIn obese patients, malpositioning of the acetabular cup increases the risk of dislocation in total hip arthroplasty (THA). The aim of this study was to determine whether obesity affects the accuracy of acetabular cup positioning using a computed tomography (CT)-based navigation system.MethodsWe retrospectively evaluated 226 consecutive patients who underwent cementless primary THAs assisted by the CT-based hip navigation system. We divided the patients into three groups according to body mass index (BMI) and examined the difference between preoperative planning and postoperative implantation angles from CT data.ResultsThere was no significant correlation between BMI and both inclination and anteversion differences (R = 0.028 and R = 0.045, respectively). There were no significant differences among the BMI < 25, 25 ≦ BMI < 30, and BMI ≧ 30 groups (p value: 0.725, 0.934, respectively); between the BMI < 25 and BMI ≧ 25 groups (p value: 0.542, 0.697, respectively); and between the BMI < 30 and BMI ≧ 30 groups with regard to inclination and anteversion (p value: 0.859, 0.456, respectively). Moreover, similar findings were observed with regard to the distance between the preoperative planning and postoperative cup positioning for the transverse, anteroposterior, and craniocaudal axes of the pelvis.ConclusionWe found that the accuracy of acetabular cup placement using CT based-navigation in THA was not affected in obese patients. Therefore, THAs with a CT-based navigation system are considered useful in obese patients.     

Implant position calculation for acetabular cup placement considering pelvic lateral tilt and inclination.

OBJECTIVE: The purpose of this study was to investigate the relationship of cup position versus pelvic lateral tilt and inclination. While computer assisted navigation systems use only the pelvic frontal plane for cup placement, it is important to realize the effect of leg length differences or increased pelvic inclination, i.e., caused by contracted hip flexors. MATERIALS AND METHODS: Using a sawbone model of the pelvis, cup position measurements were performed with different pelvic inclination and lateral tilt angles. The measured values were compared with outcome variables from a mathematical model. A computer program was developed to perform a reverse calculation to verify the mathematical model. RESULTS: The mathematical model proved correct in the sawbone pelvis experiment. The cup position changed from 15 degrees anteversion in 0 degrees pelvic inclination to 0.5 degrees retroversion in 15 degrees pelvic inclination. Regardless of pelvic inclination, the vertical cup projection stayed in a safe range of 37-47 degrees. Leg length differences greater than 3 cm have significant effects on the positioning of acetabular cups in the frontal plane. CONCLUSIONS: Using computer navigation, it is possible to determine pelvic inclination and lateral tilt during an operation by calculating the angular difference between the anatomic frontal plane and the "real world" frontal plane (i.e., the OR table). This method may be helpful in increasing the accuracy of positioning of acetabular cups.     

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° (SD 6.7) of abduction and 16.0° (SD 8.1) of anteversion. Estimation of placement was 46.3° (SD 4.3) of abduction and 14.6° (SD 5.9) of anteversion. Of more interest is the fact that for the orthopaedic surgeons the mean inaccuracy of estimation was 4.1° (SD 3.9) for abduction and 5.2° (SD 4.5) for anteversion and for their residents this was respectively, 6.3° (SD 4.6) and 5.7° (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° anteversion and 30–50° 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° 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°. 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. No benefits or funds were received in support of this study.     

Accuracy of computer-navigated total hip arthroplasty

Proper acetabular cup orientation is essential in total hip arthroplasty. The purpose of this study was to evaluate the accuracy of a particular imageless computer navigation system in determining cup position. Thirty-nine computer-navigated total hip arthroplasty intraoperative measurements of cup abduction and anteversion were compared with those from follow-up radiographs. Sensitivity, specificity, accuracy, prevalence-adjusted positive value (PPV), and negative predictive value were calculated for both navigation and radiographs. Navigation measurements had high specificity and PPV when assessing cup abduction and anteversion (specificity >90%, PPV >94%). In contrast, the system was not very effective in detecting suboptimal cup position (sensitivity abduction, 50%; anteversion, 33%). Intraoperative navigation readings in the safe zone have high probability of indicating correct placement. However, confirmation of suboptimal cup position intraoperatively requires additional diagnostic methods.