Etiology, Evaluation, and Management of Dislocation After Primary Total Hip Arthroplasty

Analgesia and anesthesia for closed reduction of dislocated total hip arthroplasty: Protocol for a scoping review

BACKGROUND

Total hip arthroplasty is a common surgical procedure, but dislocation remains a significant complication often requiring closed reduction. Current anesthetic practices for closed reduction of total hip arthroplasty vary widely, and evidence on the efficacy, safety, and cost-effectiveness of different anesthetic and analgetic regimens is limited.

METHODS

This scoping review follows the Joanna Briggs Institute (JBI) guidelines and PRISMA-ScR framework. A systematic search will be conducted in major databases to identify studies on anesthetic approaches for closed reduction of dislocated total hip arthroplasty. Eligible studies will include adult patients and focus on outcomes such as success rate, complications, patient satisfaction, and procedural efficiency. Data will be extracted and synthesized narratively and descriptively.

RESULTS

The review will map the existing evidence on anesthetic and analgetic regimens, including their success rates, associated complications, and cost-effectiveness. It will also highlight gaps in the literature and variations in practice across different settings.

CONCLUSION

By summarizing current evidence and identifying research gaps, this scoping review aims to inform clinical practice and guide future studies, ultimately improving the safety, effectiveness, and efficiency of anesthetic strategies for closed reduction of dislocated total hip arthroplasties.

Analysis of posterior hip joint impingement about developmental dysplasia of the hip after total hip arthroplasty

BACKGROUND

Total hip arthroplasty is the preferred treatment for advanced hip osteoarthritis, yet complications like hip dislocation (0.2 %-10 %) persist due to factors such as implant design, positioning, surgical technique, and patient-specific conditions. Impingement between prosthetic components or the acetabulum and proximal femur is a primary cause of instability.

METHODS

A retrospective analysis of 120 patients (130 hips) undergoing total hip arthroplasty at XX Hospital (2011-2023) categorized patients by lateral center-edge angle: developmental dysplasia of the hip, borderline developmental dysplasia of the hip, and normal hip. ct imaging facilitated virtual 3D bone models for motion analysis, excluding cases with prior hip surgery or spinal fusion. Statistical analyses utilized the Mann-Whitney U test and one-way ANOVA.

FINDINGS

The developmental dysplasia of the hip group showed the highest posterior impingement frequency, significantly affecting range of motion in flexion, adduction, and external rotation compared to borderline developmental dysplasia of the hip and normal hip groups. Variations in femoral neck anteversion, lesser trochanter version, and ischiofemoral space were notable. Posterior impingement commonly occurred between the lesser trochanter and ischial tuberosity.

INTERPRETATION

Posterior bony impingement emerged as a predominant factor reducing range of motion and increasing dislocation risk, particularly in developmental dysplasia of the hip patients, challenging the conventional "safe zone" for implant positioning. Personalized surgical strategies tailored to individual bone morphology are crucial for enhancing total hip arthroplasty outcomes and minimizing complications, although limitations include the exclusion of soft tissue influences and focusing solely on range of motion until impingement occurs.

Role of the Conjoined Tendon in Hip Stability Post-Total Hip Arthroplasty: Insights From a Direct Anterior Approach Cadaver Study

Background

Hip dislocation represents a significant complication encountered following hip arthroplasty procedures. In this investigation, we conducted a comparative analysis of the biomechanical characteristics exhibited by the piriformis and the conjoined tendon after total hip arthroplasty (THA) via the direct anterior approach (DAA), utilizing cadaveric specimens. The objective is to ascertain the relative significance of the piriformis muscle and the conjoined tendon in mitigating hip dislocation.

Methods

A total of 16 hip joints from 8 freshly frozen cadavers were selected and stratified into 2 groups: the piriformis tendon (PT) group and the conjoined tendon (CT) group. Following THA via the DAA, measurements were taken to record the torque required to induce hip dislocation under various conditions. Torque readings were obtained with the tendon in its intact state (intact group) and after preservation or reconstruction of either the PT or the CT.

Results

The torques of anterior and posterior dislocation in PT group were 22.24 ± 4.53 N.m and 30.7 ± 15.5 N.m following tendon severed, and 20.04 ± 2.67 N.m and 17.5 ± 6.7 N.m following reconstruction. There were no differences compared to the intact group. The torque decreased in the CT group after CT was severed with the torques dropping from 31.2 ± 7.6 N.m to 8.18 ± 2.6 N.m (P < .0001) for anterior dislocation, and from 34.9 ± 8.3 N.m to 9.8 ± 2.8 N.m (P < .0001) for posterior dislocation. Following reconstruction, the torque required for dislocation significantly increased (P < .0001).

Conclusions

This study underscores the preeminent role of the CT in ensuring hip stability following THA via DAA, highlighting the cruciality of its preservation and reconstruction during surgical interventions.

In silico analysis of the patient-specific acetabular cup anteversion safe zone

INTRODUCTION

Various computer-assisted surgical systems claim to improve the accuracy of cup placement in total hip arthroplasties after assessing spinopelvic mobility to prevent prosthetic impingement. However, no study has yet analyzed the extent of the patient-specific cup anteversion safe zones.

HYPOTHESIS

We hypothesized that most patients have a safe zone >10 °, except those with abnormal spinopelvic mobility, who have a much narrower safe zone.

MATERIALS AND METHODS

We simulated the risks of prosthetic impingement using the planned cup anteversion. The consecutive cohort included 341 patients who underwent total hip arthroplasty. Our primary endpoint was the patient-specific impingement-free zone for cup anteversion, which was then divided into four subgroups: 0 °, 1 ° to 5 °, 6 ° to 10 °, and >10 °. This data was then secondarily analyzed for abnormal spinopelvic mobility (the difference in the spinopelvic tilt [ΔSPT] from a standing to a flexed seated position >20 °).

RESULTS

The mean anteversion safe zone was 22.8 ° with 82.4% (281/341) of patients with a zone strictly >10 °. The mean safe zone was 8.9 ° (+/- 9 °) in patients with an ΔSPT ≥20 ° (18.2%), with 37.1% of these patients having a zone of 0 °, 16.13% a zone between 1 ° and 5 °, 8.06% a zone between 6 ° and 10 ° and 38.71% a zone >10 °. The mean safe zone was 25.9 ° (+/- 9 °) in patients with an ΔSPT <20 ° (81.8%), and the proportion of cases in each zone was 2.51%, 1.08%, 4.3%, and 92.11%, respectively (p < 0.001).

CONCLUSION

The safe zone for anteversion appears to be fairly wide in most patients. However, identifying patients at risk of abnormal spinopelvic mobility seems necessary to identify the two-thirds of patients with a narrow safe zone.

LEVEL OF EVIDENCE

IV; retrospective study.

Dislocation of primary total hip arthroplasty: Analysis of risk factors and preventive options

Total hip arthroplasty (THA) is one of the most successful elective operations in orthopedic surgery for improving pain and functional disability in patients with end-stage joint disease. However, dislocation continues to be a troublesome complication after THA, as it is a leading cause of revision and is associated with substantial social, health, and economic costs. It is a relatively rare, usually early occurrence that depends on both the patients' characteristics and the surgical aspects. The most recent and important finding is the special attention to be given preoperatively to spinopelvic mobility, which is closely related to the incidence of dislocation. Consequently, clinical and radiographic assessment of the lumbar spine is mandatory to identify an altered pelvic tilt that could suggest a different positioning of the cup. Lumbar spinal fusion is currently considered a risk factor for dislocation and revision regardless of whether it is performed prior to or after THA. Surgical options for its treatment and prevention include the use of prostheses with large diameter of femoral head size, dual mobility constructs, constrained liners, and modular neck stems.

Cup safe zone and optimal stem anteversion in total hip arthroplasty for patients with highly required range of motion

To avoid dislocation after total hip arthroplasty, it is desirable to avoid implant impingement during activities of daily living. Numerous simulation studies have been performed at 30° of internal rotation (IR) with 90° of flexion. However, these studies may not reflect the impingement that occurs during activities such as floor sitting, which require a larger IR during flexion. The purpose of this study was to assess the impact of varying IR angles during flexion on the impingement-free safe zone and optimal stem anteversion. In this study, implant impingement simulation was evaluated in computer simulation. The prosthesis used a flat liner, and a 32- or 40-mm femoral head and stem. Three patterns of required IR angle (30° IR/40° IR/50° IR with 90° flexion) combined with 13 directions of the required range of motion were simulated. The optimal stem anteversion to maximize the safe zone was analyzed. Increasing the required IR at 90° flexion decreased the safe zone, particularly with small stem anteversion angles. With a 32-mm head, the desirable stem anteversion at 40° of cup inclination was 15°/25°/35° in required 30° IR/40° IR/50° IR with 90° flexion, respectively. The safe zone area of the 32-mm head was smaller than that of the 40-mm head. For patients who require a larger IR with 90° flexion, the stem and cup target anteversion should be adjusted according to the implant design, head diameter, and patient's required IR at flexion in their lifestyle.

Elucidation of target implant orientations with the safety range of hip rotation with adduction or abduction during squatting: Simulation based on in vivo replaced hip kinematics

Objectives: The study aimed to elucidate target cup orientation and stem anteversions to avoid impingement between the liner and stem neck even at hip rotation with adduction during the deeply flexed posture.

Methods: A computer simulation analysis was performed on 32 total hip arthroplasty patients applying patient-specific orientation of the components and in vivo hip kinematics obtained from three-dimensional analysis of the squatting motion. The anterior/posterior liner-to-neck distance and impingement were evaluated based on a virtual change in internal/external rotation (0°–60°) and adduction/abduction (0°–20°) at actual maximum flexion/extension during squatting. Cutoff values of cup orientations, stem anteversion, and combined anteversion to avoid liner-to-neck impingements were determined.

Results: The anterior liner-to-neck distance decreased as internal rotation or adduction increased, and the posterior liner-to-neck distance decreased as external rotation or adduction increased. Negative correlations were found between anterior/posterior liner-to-neck distances at maximum flexion/extension and internal/external rotation. Anterior/posterior liner-to-neck impingements were observed in 6/18 hips (18/56%) at 45° internal/external rotation with 20° adduction. The range of target cup anteversion, stem anteversion, and combined anteversion to avoid both anterior and posterior liner-to-neck impingements during squatting were 15°–18°, 19°–34°, and 41°–56°, respectively.

Conclusion: Simulated hip rotations caused prosthetic impingement during squatting. Surgeons could gain valuable insights into target cup orientations and stem anteversion based on postoperative simulations during the deeply flexed posture.

The incidence of hip dislocation after posterior approach primary total hip arthroplasty: comparison of two different posterior repair techniques

INTRODUCTION

Total hip arthroplasty (THA) remains one of the most successful orthopedic surgical procedures. The posterior approach is associated with a higher incidence of post-operative dislocations than others. Adequate posterior soft tissue repair techniques, including capsulorrhaphy and transosseous bone sutures in the greater trochanter effectively reduce the dislocation rate. Post-operative "posterior hip precautions" were historically believed to reduce dislocation risks, although not clearly proven. The first protocol consists of capsulorrhaphy with the prescription of post-operative posterior hip precautions (TT) and the second, transosseous bone sutures without precautions (TB). This study aims to determine the optimal protocol to decrease the dislocation rate following posterior approach primary THA.

MATERIALS AND METHODS

A 10-year retrospective case-control chart review analyzed demographic, pre-, intra-, and post-surgical parameters. Primary outcomes were the difference in dislocation and revision surgery rates between protocols. Secondary outcomes included the incidence of recurrent dislocations and the identification of predictors of dislocation.

RESULTS

2,242 THAs were reviewed and 26 (1.2%) resulted in dislocation. Increased age (p = 0.04) ASA score (p = 0.03) and larger acetabular cup size (p < 0.001) were associated with heightened risk. Tendon to tendon (TT) repair saw a 1.62% dislocation rate versus 0.98% for tendon-to-bone (TB) repair, although statistically insignificant (p = 0.2). Transosseous repair resulted in recurrent dislocations for 8/16 (50%) patients compared to 6/10 (60%) in the suture group (p ≤ 0.001). No significance was found for prescription of posterior hip precautions.

CONCLUSIONS

To our knowledge, this is the first study to perform a direct comparison of TT repair with posterior precautions to TB repair without posterior precautions. Similarity in dislocation rate, decreased recurrent events and the alleviated patient burden from precautions leads the authors to recommend the TB repair without precautions for a successful THA.

Current Occupational Perspective of Total Hip Joint Surgeons in China: A Survey of Members of the Chinese Orthopedic Association

OBJECTIVES

To explore the basic occupational information and the preferences of surgical techniques in Chinese joint surgeons.

METHODS

A survey study was conducted during the Chinese Hip Society (CHS) conference of 2020. Participants from hospitals all over the country were included as the potential respondents. The questions were asked for respondents' basic information, overviews of total hip arthroplasty (THA), surgical techniques, and prosthesis selection. After data collection and filtration, analysis was conducted using chi-square test or Fisher's exact test.

RESULTS

Only 8.31% of joint surgeons used robotics or navigation systems to assisted their total hip arthroplasty. For the approach preference, posterolateral approaches were generally favored, being used commonly by 75.36% of respondents while the direct anterior approach was considered the preferred choice by 8.31% of Chinese joint surgeons. 24.36% of the respondents choose to use domestic prosthesis in over 80% of their patients.

CONCLUSIONS

The intelligent THA is in the early stage at present but developing rapidly in China. The composition of surgical approach is simple at present and the continuing education should aim at increasing the diversity of approach selection in the following years. Domestic prosthesis is playing significant role in the prosthesis market. Joint registration system is urgently needed for Chinese joint surgery.

Squatting After Total Hip Arthroplasty: Patient-Reported Outcomes and In Vivo Three-Dimensional Kinematic Study

BACKGROUND

Squatting is an important function for many daily activities, but has not been well documented after total hip arthroplasty (THA). This study investigated the participation rate of squatting and in vivo kinematics during squatting.

METHODS

A survey questionnaire about squatting was mailed to patients who underwent primary THA and 328 patients returned acceptable responses. Additionally, 32 hips were evaluated for dynamic 3-dimensional kinematics of squatting using density-based image-matching techniques. Multivariate analyses were applied to determine which factors were associated with anterior liner-to-neck distance at maximum hip flexion.

RESULTS

Patients who could easily squat significantly increased this ability postoperatively (23.5% vs 46%, P < .01). In 29.5% of the patients there was still no ability to squat after THA; the main reason was anxiety of dislocation (34.2%). Kinematic analysis revealed that maximum hip flexion averaged 80.7° ± 12.3° with 12.8° ± 10.7° of posterior pelvic tilt and 9.7 ± 3.0 mm of anterior liner-to-neck distance. Neither liner-to-neck, bone-to-bone, nor bone-to-implant contact was observed in any of the hips. Larger hip flexion and smaller cup anteversion were negatively associated with the anterior liner-to-neck distance at maximum hip flexion (P < .05).

CONCLUSION

Postoperatively, approximately 70% of patients squatted easily or with support. Anxiety of dislocation made patients avoid squatting after THA. In vivo squatting kinematics suggest no danger of impingement or subsequent dislocation, but excessively large hip flexion and small cup anteversion remain as risks.

Pseudotumor and delayed recurrent dislocation after total hip arthroplasty with a modular femoral neck: A case report

RATIONALE

Pseudotumor formation after hip arthroplasty is a rare complication that can occur not only at the head-neck junction but also at the modular neck-stem junction. Dislocation is a challenging and common complication of primary and revision total hip arthroplasty compared with other complications. Similarly, the association between pseudotumors and delayed recurrent dislocation remains unclear.

PATIENT CONCERNS

We report the case of a 73-year-old woman with pseudotumor formation after total hip arthroplasty combined with a modular femoral neck. A delayed recurrent dislocation occurred in this case. Approximately 4weeks after the first revision surgery, redislocation occurred.

DIAGNOSIS

The patient was eventually diagnosed with delayed recurrent artificial hip dislocation combined with a periprosthetic pseudotumor of the right hip.

INTERVENTIONS

During the first revision surgery, a thickened, indurated cyst measuring 8×3×8cm with a red-brown wall containing brown fluid was completely excised. A cemented stem, combined with a BIOLOX Forte ceramic head, was implanted. Approximately 4weeks after surgery, redislocation occurred, and we cemented an elevated rim liner on the acetabular component with a metal head.

OUTCOMES

At the last follow-up, 49 months after revision surgery, the patient was asymptomatic with a Harris hip score of 90. The patient had a satisfactory prognosis after treatment.

LESSONS

The application of the modular-neck stem should be cautiously performed, particularly for modular prostheses containing different alloys. Pseudotumors and insufficient soft-tissue tension both contribute to hip instability, which may eventually lead to delayed repeated dislocation. In addition, femoral offset must be considered. Cement-liner technology may be used for aging patients who are less active. This case report, focusing on pseudotumors and delayed recurrent dislocations, aimed to identify factors that may support this diagnosis, which is easy to miss. Consequently, it can provide further details on the treatment process and alert orthopedic surgeons to this infrequent but important cause of delayed recurrent dislocation.

Evaluation of optimal implant alignment in total hip arthroplasty based on postoperative range of motion simulation

BACKGROUND

Dislocation after total hip arthroplasty is a frequent cause of revision surgery. This study was performed to determine the optimal implant alignment in total hip arthroplasty by simulating the postoperative range of motion.

METHODS

All operations were performed via posterolateral approach using combined anteversion of the stem and cup technique. Maximum range of motion without implant impingement was simulated in 79 replaced hips using postoperative computed tomography and the achievement of the required range of motion defined by previous studies was assessed. Optimal cup and stem alignment for impingement-free range of motion were statistically determined using the receiver operator coefficient curve.

FINDINGS

Cup inclination and anteversion, stem anteversion, and combined anteversion were 37.6°, 20.1°, 26.2°, and 46.3°, respectively. Maximum range of motion in flexion, extension, internal rotation at 90° of flexion, and external rotation were 131.8°, 42.3°, 56.4°, and 64.5°, respectively. Flexion >110°, extension >30°, internal rotation >30° at 90° of flexion, and external rotation >30° were fulfilled by 96%, 86%, 92%, and 96% of all replaced hips, respectively. Optimal implant alignment for impingement-free range of motion was 34°-43° of cup inclination, 18°-26° of cup anteversion, 17°-29° of stem anteversion, and 35°-56° of combined anteversion. Both cup and stem anteversion showed significant relationship with postoperative range of motion.

INTERPRETATION

Surgeons could gain valuable insights into optimal cup and stem alignment to perform postoperative range of motion simulations.

Modular dual mobility total hip arthroplasty is a viable option for young, active patients : a mid-term follow-up study

AIMS

Dual mobility (DM) implants have been shown to reduce the dislocation rate after total hip arthroplasty (THA), but there remain concerns about the use of cobalt chrome liners inserted into titanium shells. The aim of this study was to assess the clinical outcomes, metal ion levels, and periprosthetic femoral bone mineral density (BMD) at mid-term follow-up in young, active patients receiving a modular DM THA.

METHODS

This was a prospective study involving patients aged < 65 years, with a BMI of < 35 kg/m², and University of California, Los Angeles activity score of > 6 who underwent primary THA with a modular cobalt chrome acetabular liner, highly cross-linked polyethylene mobile bearing, and a cementless titanium femoral stem. Patient-reported outcome measures, whole blood metal ion levels (μg/l), and periprosthetic femoral BMD were measured at baseline and at one, two, and five years postoperatively. The results two years postoperatively for this cohort have been previously reported.

RESULTS

A total of 43 patients were enrolled. At minimum follow-up of five years, 23 (53.4%) returned for clinical and radiological review, 25 (58.1%) had metal ion analysis performed, 19 (44.2%) underwent dual energy x-ray absorptiometry scans, and 25 (58%) completed a pain-drawing questionnaire. The mean modified Harris Hip Scores improved significantly from 54.8 (SD 19) preoperatively to 93.08 (SD 10.5) five years postoperatively (p < 0.001). One patient was revised for aseptic acetabular loosening. The mean cobalt levels increased from 0.065 μg/l (SD 0.03) to 0.08 (SD 0.05) and the mean titanium levels increased from 0.35 (SD 0.13) to 0.78 (SD 0.29). The femoral BMD ratio decreased in Gruen Zone 1 (91.9%) at five years postoperatively compared with the baseline scores at six weeks potoperatively. The femoral BMD ratio was maintained in Gruen zones 2 to 7.

CONCLUSION

The use of a modular DM component and a cementless, tapered femoral stem shows excellent mid-term survivorship with minimal concerns for corrosion and metal ion release in a cohort of young, active patients undergoing primary THA. Cite this article: Bone Joint J 2021;103-B(7 Supple B):73-77.

Development of a diagnostic algorithm identifying cases of dislocation after primary total hip arthroplasty-based on 31,762 patients from the Danish Hip Arthroplasty Register

Background and purpose - Dislocation of total hip arthroplasties (THA) is often treated with closed reduction and traditionally not registered in orthopedic registers. This study aimed to create an algorithm designed to identify cases of dislocations of THAs with high sensitivity, specificity, and positive predictive value (PPV) based on codes from the Danish National Patient Register (DNPR).Patients and methods - All patients (n = 31,762) with primary osteoarthritis undergoing THA from January 1, 2010 to December 31, 2014 were included from the Danish Hip Arthroplasty Register (DHR). We extracted available data for every hospital contact in the DNPR during a 2-year follow-up period, then conducted a comprehensive nationwide review of 5,096 patient files to register all dislocations and applied codes.Results - We identified 1,890 hip dislocations among 1,094 of the included 31,762 THAs. More than 70 different diagnoses and 55 procedural codes were coupled to the hospital contacts with dislocation. A combination of the correct codes produced a sensitivity of 63% and a PPV of 98%. Adding alternative and often applied codes increased the sensitivity to 91%, while the PPV was maintained at 93%. Additional steps increased sensitivity to 95% but at the expense of an unacceptable decrease in the PPV to 82%. Specificity was, in all steps, greater than 99%.Interpretation - The developed algorithm achieved high and acceptable values for sensitivity, specificity, and predictive values. We found that surgeons in most cases coded correctly. However, the codes were not always transferred to the discharge summary. In perspective, this kind of algorithm may be used in Danish quality registers.

Simultaneous bilateral dual mobility total hip arthroplasty dislocation in a patient with hepatic encephalopathy: A case report

INTRODUCTION AND IMPORTANCE

Dislocation is a severe complication after total hip arthroplasty (THA). It is one of the most common reasons for failure and revision surgery. This is the first case of a documented simultaneous bilateral dual mobility (DM) THA dislocation.

CASE PRESENTATION

A forty-nine-year-old man presented with bilateral hip pain, immobility and deformity. X-ray images demonstrated simultaneous bilateral posterior THA dislocation. Previously, the patient had presented atraumatic dislocations recurrently. When he was thoroughly re-interrogated, he complained of uncontrolled and generalized muscle contractions, which were compatible with myoclonus due to hepatic encephalopathy (HE). Multidisciplinary treatment was performed satisfactorily to control myoclonus symptomatology and to prevent dislocation.

CLINICAL DISCUSSION

Patient's most important risk factor was a neuromuscular disorder, which we initially gave little notice and undervalued. HE is a serious but reversible syndrome, observed in patients with liver dysfunction. It leads to a wide spectrum of neuropsychiatric abnormalities. Management is based on prevention of episodes, avoiding the underlying triggers. Due to the high risk for dislocation of our patient, we decided to use DM cups bilaterally. This system has demonstrated lower rates of dislocation.

CONCLUSION

This case report reminds us that a careful evaluation through meticulous history and physical examination are mandatory when faced with recurrent instability. Furthermore, prevention of dislocation is vastly preferable to treating this challenging complication. High-risk patients should be identified, and appropriate surgical approach, technique and implants have to be collectively used to reach a strategy that mitigates and ideally prevents dislocation.

Factors affecting impingement and dislocation after total hip arthroplasty - Computer simulation analysis

BACKGROUND

Studies on the causes and factors affecting dislocation after total hip arthroplasty have revealed conflicting results. The purpose of this study was to evaluate the factors affecting impingement and dislocation after total hip arthroplasty, using a 3-dimensional dynamic motion analysis.

METHODS

The CT data of 53 patients (53 hips: anterior dislocation; 11 cases, and posterior dislocation; 42 cases) who experienced hip dislocation after total hip arthroplasty with posterior approach, and 120 control patients (120 hips) without dislocation were analyzed. Parameters related to implant alignment, offset and leg length were evaluated. The impingement type was also analyzed using a software.

FINDINGS

Considering implant settings affecting dislocation, patients at risk for posterior dislocation had decreased stem anteversion, combined anteversion, femoral offset, and leg length. Nevertheless, patients at risk for anterior dislocation had only lower leg length, and these patients may also be at risk for a higher incidence of recurrent dislocation. Bony impingement occurred in almost half of the cases with posterior dislocation, while implant impingement was associated with anterior dislocation. Importantly, anterior dislocation was not as common as posterior dislocation even in cases with occurrence of posterior impingement.

INTERPRETATION

Bony impingement substantially affects dislocation even in the situation where the implant position and alignment are determined by the so-called "safe zone", especially on the anterior side, while implant impingement affects anterior dislocation. The restoration of anterior offset (i.e., prescribed by the stem anteversion and femoral offset) and combined anteversion is critical for avoidance of posterior dislocation after total hip arthroplasty.