Femoroacetabular impingement (FAI) – enlarged, aspherical femoral head deformity (cam-type) or retroversion/overcoverage of the acetabulum (pincer-type) – is a leading cause for early hip osteoarthritis. Although anteverting/reverse periacetabular osteotomy (PAO) to address FAI aims to preserve the native hip and restore joint function, it is still unclear how it affects joint mobility and stability. This in vitro cadaveric study examined the effects of surgical anteverting PAO on range of motion and capsular mechanics in hips with acetabular retroversion. Twelve cadaveric hips (n = 12, m:f = 9:3; age = 41 ± 9 years; BMI = 23 ± 4 kg/m2) were included in this study. Each hip was CT imaged and indicated acetabular retroversion (i.e., crossover sign, posterior wall sign, ischial wall sign, retroversion index > 20%, axial plane acetabular version < 15°); and showed no other abnormalities on CT data. Each hip was denuded to the bone-and-capsule and mounted onto a 6-DOF robot tester (TX90, Stäubli), equipped with a universal force-torque sensor (Omega85, ATI). The robot positioned each hip in five sagittal angles: Extension, Neutral 0°, Flexion 30°, Flexion 60°, Flexion 90°; and performed hip internal-external rotations and abduction-adduction motions to 5 Nm in each position. After the intact stage was tested, each hip underwent an anteverting PAO, anteverting the acetabulum and securing the fragment with long bone screws. The capsular ligaments were preserved during the surgery and each hip was retested postoperatively in the robot. Postoperative CT imaging confirmed that the acetabular fragment was properly positioned with adequate version and head coverage. Paired sample t-tests compared the differences in range of motion before and after PAO (CI = 95%; SPSS v.24, IBM). Preoperatively, the intact hips with acetabular retroversion demonstrated constrained internal-external rotations and abduction-adduction motions. The PAO reoriented the acetabular fragment and medialized the hip joint centre, which tightened the iliofemoral ligament and slackenend the pubofemoral ligament. Postoperatively, internal rotation increased in the deep hip flexion positions of Flexion 60° (∆IR = +7°, p = 0.001) and Flexion 90° (∆IR = +8°, p = 0.001); while also demonstrating marginal decreases in external rotation in all positions. In addition, adduction increased in the deep flexion positions of Flexion 60° (∆ADD = +11°, p = 0.002) and Flexion 90° (∆ADD = +12°, p = 0.001); but also showed marginal increases in abduction in all positions. The anteverting PAO restored anterosuperior acetabular clearance and increased internal rotation (28–33%) and adduction motions (29–31%) in deep hip flexion. Restricted movements and positive impingement tests typically experienced in these positions with acetabular retroversion are associated with clinical symptoms of FAI (i.e., FADIR). However, PAO altered capsular tensions by further tightening the anterolateral hip capsule which resulted in a limited external rotation and a stiffer and tighter hip. Capsular tightness may still be secondary to acetabular retroversion, thus capsular management may be warranted for larger corrections or rotational osteotomies. In efforts to optimize surgical management and clinical outcomes, anteverting PAO is a viable option to address FAI due to acetabular retroversion or overcoverage

INTRODUCTION:. Acetabular retroversion has been implicated as a risk factor for the development of early hip osteoarthritis. In clinical practice standard osseous signs such as the cross-over sign (COS) and the posterior wall sign (PWS) are widely used to establish the diagnosis of acetabular retroversion on plain radiographs. Despite standardized radiological evaluation protocols, an increased pelvic tilt can lead to a misdiagnosis of acetabular retroversion in AP radiographs and 2D MR or CT scans. Previous studies have shown that the elimination of observer bias using a standardized methodology based on 3D-CT models and the anterior pelvic plane (APP) for the assessment of COS and PWS results in greater diagnostic accuracy. Using this method a prevalence of 28% for COS and 24% for PWS has been found in a cohort of patients with symptoms indicative of FAI, however the prevalence of both signs in asymptomatic adults remains unknown. This study therefore sought to establish the prevalence of the COS and PWS in relation to the APP in an asymptomatic population using a reliable and accurate 3 D-CT based assessment. METHODS:. A large pool of consecutive CT scans of the pelvis undertaken in our department for conditions unrelated to disorders of the hip was available for analysis. Scans in subjects with a Harris hip score of less than 90 points were excluded leaving a sample of 100 asymptomatic subjects (200 hips) for this study. A previously established 3D analysis method designed to eliminate errors resulting from variations in the position and orientation of the pelvis during CT imaging was applied to determine in order to assess the prevalence of the COS and PWS in relation to the APP. Here, the acetabuli were defined as retroverted if either the COS, PWS or both were positive. RESULTS:. From the total of 200 hips a positive COS was identified in 24% (48/200) and a positive PWS was detected in 6.5% (13/200) relative to the APP using the CT data. A. In male adults a COS was observed in 25.4% (29/114) and a PWS in 10.5% (12/114). In female adults a COS were observed in 22.1% (19/86) and a PWS in 1.2% (1/86). DISCUSSION:. The high incidence of acetabular retroversion observed using an accurate 3D-CT based methodology shows that this anatomic configuration might not differ in frequency between asymptomatic individuals and patients with symptomatic FAI. Patients presenting with hip pain and evidence of FAI should therefore be subjected to strict diagnostic scrutiny, as the presence of a COS and/or PWS shows a poor correlation with the presence of symptomatic disease. In our collective of asymptomatic adults the COS showed a higher incidence than the PWS. Additionally a deficiency of the posterior acetabular wall was rare in asymptomatic adults compared to FAI patients. Therefore, the question whether an abnormal acetabular version does indeed lead to the development of osteoarthritis in all patients warrants further study. Although an association between osteoarthritis and femuro-acetabular impingement is believed to exist, long-term epidemiological studies are needed to establish the natural history of these anatomical configurations

Introduction. Acetabular retroversion is an accepted cause of Pincer-type femoroacetabular impingement. There is increasing evidence that acetabular retroversion is rather a rotational abnormality of the pelvis than an overgrowth of the acetabular wall or even a dysplasia of the posterior wall. Initially, patients with a retroverted acetabulum were treated with an open rim trimming through a surgical hip dislocation (SHD) based on the early understanding of the pathomorphology. Theoretically, the reduction of the anterior wall can decrease the already small joint contact area in retroverted hips to a critical size. Based on the most recent literature, anteverting periacetabular osteotomy (PAO) seems to be the more appropriate surgical treatment. With this technique, the anterior impingement conflict can be treated efficiently without compromising the joint contact area. However, it is unknown whether this theoretical advantage in turn results in better mid term results of treatment. Objectives. We asked if anteverting PAO results in better clinical and radiographical mid term results compared to rim trimming through a surgical hip dislocation. Methods. We performed a retrospective comparative study based on 257 hips (221 patients) with symptomatic femoroacetabular impingement due to acetabular retroversion. Acetabular retroversion was defined by a cross-over sign, a posterior wall sign, and a positive ischial spine sign. We then formed two matched groups consisting of 73 hips undergoing open acetabular rim trimming and 54 hips with an anteverting periacetabular osteotomy. Patients with incomplete radiographic documentation, previous surgery, and hip dysplasia (LCE < 20°) were excluded. Some patients were excluded due to a matching process (Fig. 1). Patients were generally followed clinically and radiographically after two, five and ten years. A Kaplan-Meier survivorship analysis was performed using the following endpoints: a low clinical score, radiographic progression of osteoarthritis, and/or the conversion to total hip arthroplasty. The Log Rank Score was used to compare the two survivorship curves. Results. Patients undergoing anteverting PAO for acetabular retroversion had a significantly increased survivorship (82%, 95% confidence interval, 72–91%) at seven years in comparison to open surgical rim trimming (63%, 95%CI, 49–76%, p<0.0001). The two survivorship curves are comparable for the first four years with a substantial drop for the rim trimming group after year five (Fig. 2). Conclusion. This study proofs for the first time that the theoretical advantages of anteverting periacetabular osteotomy in hips with symptomatic acetabular retroversion results in an increased survivorship at mid term follow-up in comparison to open rim trimming. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Purpose. Diagnosis of acetabular retroversion is essential in femoroacetabular impingement (FAI), but its assessment from radiographs is complicated by pelvic tilt and the two-dimensional nature of plain films. We performed a study to validate the diagnostic accuracy of the crossover sign (COS) and the posterior wall sign (PWS) in identifying acetabular retroversion. Method. COS and PWS were evaluated from radiographs and computed tomography (CT) scans as the standard of reference in 50 hips of subjects with symptoms of FAI. A CT-based method using 3-D models was developed to measure the COS, PWS, true acetabular version and pelvic tilt relative to the anterior pelvic plane. The new CT-based method aimed to eliminate errors resulting from variations in the position and orientation of the pelvis during imaging. Results. A low level of agreement for COS and PWS was found between radiographs and CT scans. A positive COS strongly correlated with pelvic tilt. Conclusion. These results suggest that COS and PWS determined from anteroposterior (AP) radiographs are considerably limited by pelvic tilt and inherent limitations of radiographs. Their use as the sole basis for deciding whether or not surgical intervention is indicated seems questionable. NO DISCLOSURES

Introduction. In the evaluation of patients with pre-arthritic hip disorders, making the correct diagnosis and identifying the underlying bone pathology is of upmost importance to achieve optimal patient outcomes. 3-dimensional imaging adds information for proper preoperative planning. CT scans have become the gold standard for this, but with the associated risk of radiation exposure to this generally younger patient cohort. Purpose. To determine if 3D-MR reconstructions of the hip can be used to accurately demonstrate femoral and acetabular morphology in the setting of femoroacetabular impingement (FAI) and development dysplasia of the hip (DDH) that is comparable to CT imaging. Materials and Methods. We performed a retrospective review of 14 consecutive patients with a diagnosis of FAI or DDH that underwent both CT and MRI scans of the same hip with 3D reconstructions. 2 fellowship trained musculoskeletal radiologists reviewed all scans, and a fellowship trained hip preservation surgeon separately reviewed scans for relevant surgical parameters. All were blinded to the patients' clinical history. The 3D reconstructions were evaluated by radiologists for the presence of a CAM lesion and acetabular retroversion, while the hip preservation surgeon also evaluated CAM extent using a clock face convention of a right hip, location of femoral head blood supply, and morphological anterior inferior iliac spine (AIIS) variant. The findings on the 3D CT reconstructions were considered the reference standard. Results. Of 14 patients, there were 9 females and 5 males with a mean age 32 (range 15–42). There was no difference in the ability of MRI to detect the presence of a CAM lesion (100% agreement between 3D-MR and 3D-CT, p=1), AIIS morphology (p=1, mode=type 1 variant), or acetabular retroversion (85.7%, p=0.5). 3D-MR had a sensitivity and specificity of 100 in detecting a CAM lesion relative to 3D-CT. Four CT studies were inadequate to adequately evaluate for presence of a CAM. Five CT studies were inadequate to evaluate for location of the femoral head vessels, while MRI was able to determine location in those patients. In the 10 remaining patients for presence of CAM, and nine patients for femoral head vessel location, there was no statistically significant difference between 3D-MR and 3D-CT in determining the location of CAM lesion on a clock face (p=0.8, mean MRI = 12:54, mean CT: 12:51, SD = 66 mins MR, 81 mins CT) or in determining vessel location (p=0.4, MR mean 11:23, CT mean 11:36, SD 33 mins for both). Conclusion. 3D MRI reconstructions are as accurate as 3D CT reconstructions in evaluating osseous morphology of the hip, and may be superior to CT in determining other certain clinically relevant hip parameters. 3D-MR was equally useful in determining the presence and extent of a CAM lesion, acetabular retroversion, and AIIS morphologic variant, and more useful than 3D CT in determining location of the femoral head vessels. In evaluating FAI or hip dysplasia, a 3D-MR study is sufficient to evaluate both soft tissue and osseous anatomy, sparing the need for a 3D CT scan and its associated radiation exposure and cost

Introduction:. Most cases of hip osteoarthritis (OA) are believed to be caused by alterations in joint contact mechanics resulting from pathomorphologies such as acetabular dysplasia and acetabular retroversion. Over the past 13 years, our research group has focused on developing approaches for patient-specific modeling of cartilage and labrum in the human hip, and applying these approaches to study hip pathomorphology. The long term objective is to improve the understanding of the etiology of OA related to hip pathomorphology, and to improve diagnosis and treatment. The objectives of this presentation are to provide a summary of our subject-specific modeling approach, and to describe the results of our analysis of hips from three populations of subjects: normal, traditional dysplastic, and retroverted. Methods:. A combined experimental and computational protocol was used to investigate contact mechanics in ten normal subjects (normal center edge angles (CEA), no history of hip pain), ten subjects with hip pain secondary to acetabular dysplasia (CEA less than 25°), and ten patients with a radiographic crossover sign, pain and clinical exams consistent with acetabular retroversion. CT arthrography was used to image cartilage and bone. Volumetric image data were segmented and discretized, and subject-specific finite element models were produced using validated methods [Fig. 1]. Boundary and loading conditions were obtained from instrumented implant and gait data. Contact mechanics were evaluated on the acetabular cartilage and labrum. Labrum contact area and peak contact stress were evaluated. Cartilage contact area, peak and average contact stress were evaluated in six anatomical regions in the acetabulum. Results:. Hip contact patterns were subject-specific, but distinct patterns emerged in the groups. Dysplastic hips had a larger contact area in the lateral region of the acetabulum, while normal hips demonstrated a more distributed contact pattern. The labrum in dysplastic hips supported significantly more load than the labrum in normal hips in all activities [Fig. 2]. Contact in retroverted hips tended to be focused medially and superiorly [Fig. 3]. Retroverted subjects had smaller contact stress and area in most regions. Discussion:. The differences in labrum mechanics between the normal and dysplastic groups provide clear support for the mechanical importance of the acetabular labrum in dysplastic hips. There were only minor differences in cartilage contact stress and area between normal and dysplastic groups, because of a lateral shift in the location of contact and subsequent loading on the acetabular labra in the dysplastic hips. The larger labrum load support and contact area in dysplastic hips indicates that the labrum compensates for the shallow acetabula. Clinically, this may account for the pattern of OA onset in dysplastic hips. The results for the retroverted group do not support the commonly held belief that concentrated posterior loading in retroverted hips leads OA because there were lower contact stresses and areas in the posterior regions of retroverted hips. Further, these results suggest that rim trimming may be appropriate for retroverted hips. The preferred surgery likely depends on subtle patient specific aspects of hip pathoanotomy in both retroverted and dysplastic hips

The majority of patients who develop hip arthritis have a mechanical abnormality of the joint. The structural abnormalities range from instability (DDH) to impingement. Impingement leads to osteoarthritis by chronic damage to the acetabular labrum and adjacent cartilage. In situations of end-stage secondary DJD, hip arthroplasty is the most reliable treatment choice. In young patients with viable articular cartilage, joint salvage is indicated. Treatment should be directed at resolving the structural abnormalities that create the impingement. Femoral abnormalities corrected by osteotomy or increased head-neck offset by chondro-osteoplasty creating a satisfactory head-neck offset. This can safely be done via anterior surgical dislocation. The acetabular-labral lesions can be debrided and/or repaired. Acetabular abnormalities should be corrected by “reverse” PAO in those with acetabular retroversion or anterior acetabular debridement in those with satisfactory posterior coverage and a damaged anterior rim. Often combinations of the above are indicated

Over the past fifteen years hip preservation surgery has rapidly evolved. Improved understanding of the pathomechanics and associated intra-articular degeneration of both hip instability and femoroacetabular impingement have led to improved surgical indications, refined surgical techniques and more effective joint preservation surgical procedures. The periacetabular osteotomy (PAO) was initially introduced by Ganz and colleagues and has become the preferred treatment in North America for pre-arthritic, symptomatic acetabular dysplasia. Both hip arthroscopy and safe surgical dislocation of the hip have been popularised for the treatment of symptomatic femoroacetabular impingement disorders. Hip arthroscopy is effective for focal and\or accessible impingement lesions while the surgical dislocation approach is reserved for nonfocal disease patterns as seen in complex FAI, and residual Perthes and SCFE deformities. Femoroacetabular impingement from major acetabular retroversion can be managed with the PAO if there is coexistent posterosuperior acetabular insufficiency. Short- to mid-term results of these procedures are generally good to excellent for most patients and the complication rates associated with these procedures are very acceptable. Long-term outcomes are best known for the PAO. Several recent studies have documented survivorship rates of 65–90% at 10–20-year follow-up. Certain factors are associated with long-term success including minimal pre-operative radiographic OA, early symptoms, accurate acetabular correction, and younger age. These data strongly suggest that the PAO can defer THA to an older age for most patients while completely avoiding arthroplasty may only be possible in select patients with excellent congruency, no secondary OA and an ideal surgical correction

A study by Harris reported a 40% incidence of femoral and acetabular dysplasia in routine idiopathic osteoarthritic patients. Due to pediatric screening in the United States, today most cases are minimally dysplastic requiring little modification from standard total hip surgical techniques. As the degree of dysplasia increases numerous anatomic distortions are present. These include high hip centers, relative acetabular retroversion, soft bone in the true acetabular area, increased femoral neck anteversion and relative posteriorly positioned greater trochanters, metaphyseal/diaphyseal size mismatch, and small femoral canals. Total hip replacements for these patients have known higher risks for earlier loosening, dislocation, and neurovascular injuries. Use of medialised small uncemented acetabular components placed in the anatomic acetabulum, modular uncemented femoral components, and diaphyseal rotational and shortening osteotomies has become a preferred method of treatment. In 2007, we reported our experience with this technique in 23 cases utilizing a subtrochanteric femoral osteotomy with a 5–14 year follow-up. There were 4 Crowe I, 3 Crowe II, 5 Crowe III, and 11 Crowe IV cases. All osteotomies healed. There were no femoral components revised. In most cases, small (mean 46 mm) hemispherical components were used without bulk allografts in all but 5 early cases. One acetabular component was revised for a recalled component. 3 acetabular liners were revised for wear (2 were very small cups with 4.7 mm poly thickness). Four patients sustained dislocations, with 2 closed and 2 open reductions. There were no neurovascular injuries. The Crowe classification is commonly used to preoperatively classify the degree of dysplasia. However, there are large variations in these anatomic distortions within each class, so it is difficult to preoperatively plan the acetabular component size needed and if one will need to do shortening and/or rotational osteotomy. So the surgeon needs to be prepared for these cases with smaller acetabular components and be prepared to perform a femoral osteotomy

The majority of patients who develop hip arthritis have a mechanical abnormality of the joint. The structural abnormalities range from instability (DDH) to impingement. Impingement leads to osteoarthritis by chronic damage to the acetabular labrum and adjacent cartilage. In situations of endstage secondary DJD, hip arthroplasty is the most reliable treatment choice. In young patients with viable articular cartilage, joint salvage is indicated. Treatment should be directed at resolving the structural abnormalities that create the impingement. Femoral abnormalities corrected by osteotomy or increased head-neck offset by chondro-osteoplasty creating a satisfactory head-neck offset. This can safely be done via anterior surgical dislocation. The acetabular-labral lesions can be debrided and/or repaired. Acetabular abnormalities should be corrected by “reverse” PAO in those with acetabular retroversion or anterior acetabular debridement in those with satisfactory posterior coverage and a damaged anterior rim. Often combinations of the above are indicated. This talk will also update issues related to hip impingement and joint salvage surgery that have arisen over the past year

The majority of patients who develop hip arthritis have a mechanical abnormality of the joint. The structural abnormalities range from instability (DDH) to impingement. Impingement leads to osteoarthritis by chronic damage to the acetabular labrum and adjacent cartilage. In situations of endstage secondary DJD, hip arthroplasty is the most reliable treatment choice. In young patients with viable articular cartilage, joint salvage is indicated. Treatment should be directed at resolving the structural abnormalities that create the impingement. Femoral abnormalities corrected by osteotomy or increased head-neck offset by chondro-osteoplasty creating a satisfactory head-neck offset. This can safely be done via anterior surgical dislocation or arthroscopically. The acetabular-labral lesions can be debrided and/or repaired. Acetabular abnormalities should be corrected by “reverse” PAO in those with acetabular retroversion or anterior acetabular debridement in those with satisfactory posterior coverage and a damaged anterior rim. Often combinations of the above are indicated

A study by Harris reported a 40% incidence of femoral and acetabular dysplasia in routine idiopathic osteoarthritic patients. Fortunately most are minimally dysplastic requiring little modification from standard total hip surgical techniques. However, as the degree of dysplasia increases numerous anatomic distortions are present. These include high hip centres, relative acetabular retroversion, soft bone in the true acetabular area, increased femoral neck anteversion and relative posteriorly positioned greater trochanters, metaphyseal/diaphyseal size mismatch, and small femoral canals. Total hip replacements for these patients have known higher risks for earlier loosening, dislocation, and neurovascular injuries. Use of medialised small uncemented acetabular components placed in the anatomic acetabulum, modular uncemented femoral components, and diaphyseal rotational and shortening osteotomies has become the standard method of treatment. In 2007, we reported our experience with this technique in 23 cases utilising a subtrochanteric femoral osteotomy with a 5–14 year follow-up. There were 4 Crowe I, 3 Crowe II, 5 Crowe III, and 11 Crowe IV cases. All osteotomies healed. There were no femoral components revised. One acetabular component was revised for a recalled component. 3 acetabular liners were revised for wear (2 were very small cups with 4.7 mm poly thickness). 4 patients sustained dislocations, with 2 closed and 2 open reductions. There were no neurovascular injuries. The Crowe classification is commonly used to pre-operatively classify the degree of dysplasia. However, there are large variations in these anatomic distortions within each class, so it is difficult to pre-operatively plan the acetabular component size needed and if one will need to do shortening and/or rotational osteotomy. So the surgeon needs to bring their entire bag of tricks and tools for these surgeries

Pre-existing hip pathology such as femoroacetabular impingement is believed by some, to have a direct causal relationship with osteoarthritis of the hip. The strength of this relationship remains unknown. We investigate the prevalence of abnormal bone morphology in the symptomatic hip on the pre-operative anteroposterior pelvic radiograph of consecutive patients undergoing hip resurfacing. Rotated radiographs were excluded. One hundred patients, of mean age 53.5 years were included (range 33.4–71.4 years, 32% female). We examined the films for evidence of a cam-type impingement lesion (alpha angle >50.5°, a pistol grip, Pitt's pits, a medial hook, an os acetabuli and rim ossification), signs of acetabular retroversion or a pincer-type impingement lesion (crossover sign, posterior wall sign, ischial sign, coxa profunda, protrusio, coxa vara, Tonnis angle < 5°), and hip dysplasia (a Tonnis acetabular angle >14° and a lateral centre-edge angle of Wiberg <20°). Pre-existing radiographic signs of pathology were present in a large proportion of hips with low grade (Tonnis grade 1–2) arthritis. There is a group of patients who presented with more advanced osteoarthritis in which we suspect abnormal bone morphology to be a causative factor but, for example, neck osteophytes obscure the diagnosis of a primary cam lesion. Our findings corroborate those of Harris and Ganz. Impingement is radiographically detectable in a large proportion of patients who present with early arthritis of the hip, and therefore we agree that it is a likely pre-cursor for osteoarthritis. Treatments directed at reducing hip impingement may stifle the progression of osteoarthritis

Background. Rotational acetabular osteotomy (RAO) is an effective treatment option for symptomatic acetabular dysplasia. However, excessive lateral and anterior correction during the periacetabular osteotomy may lead to femoroacetabular impingement. We used preoperative planning software for total hip arthroplasty to perform femoroacetabular impingement simulations before and after rotational acetabular osteotomies. Methods. We evaluated 11 hips in 11 patients with available computed tomography taken before and after RAO. All cases were female and mean age at the time of surgery was 35.9 years. All cases were early stage osteoarthritis without obvious osteophytes or joint space narrowing. Radiographic analysis included the center-edge (CE) angle, Sharp's acetabular angle, the acetabular roof angle, the acetabular head index (AHI), cross-over sign, and posterior wall sign. Acetabular anteversion was measured at every 5 mm slice level in the femoral head using preoperative and postoperative computed tomography. Impingement simulations were performed using the preoperative planning software ZedHip (LEXI, Tokyo, Japan). In brief, we created a three-dimensional model. The range of motion which causes bone-to-bone impingement was evaluated in flexion (flex), abduction (abd), external rotation in flex 0°, and internal rotation in flex 90°. The lesions caused by impingement were evaluated. Results. In the radiographic measurements, the CE angle, Sharp's angle, acetabular roof angle, and AHI all indicated improved postoperative acetabular coverage. The cross-over sign was recognized pre- and postoperatively in each case. Acetabular retroversion appeared in one case before RAO and in three cases after RAO. Preoperatively, there was a tendency to reduce the acetabular anteverison angle in the hips from distal levels to proximal. In contrast, there was no postoperative difference in the acetabular anteversion angle at any level. In our simulation study, bone-to-bone impingement occurred in flex (preoperative/postoperative, 137°/114°), abd (73°/54°), external rotation in flex 0°(34°/43°), and internal rotation in flex 90°(70°/36°). Impingement occurred within internal rotation 45°in flexion 90°in two preoperative and eight postoperative cases. The impingement lesions were anterosuperior of the acetabulum in all cases. Discussion. It is easy to make and assess an impingement simulation using preoperative planning software, and our data suggest the simulation was helpful in a clinical setting, though there were some remaining problems such as approximation of the femoral head center and differences in femur movement between the simulation and reality. In the postoperative simulation there was a tendency to reduce the range of motion in flex, abd, and internal rotation in flex 90°. There was a correlation between acetabular anteversion angle and flex. Since impingement occurred within internal rotation 45°in flexion 90°in eight postoperative simulations, we consider there is a strong potential for an increase in femoroacetabular impingement after RAO