The 2014 Frank Stinchfield Award: The 'landing zone' for wear and stability in total hip arthroplasty is smaller than we thought: a computational analysis

Accuracy of cup placement compared with preoperative surgeon targets in primary total hip arthroplasty using standard instrumentation and techniques: a global, multicenter study

BACKGROUND

Acetabular cup positioning in total hip arthroplasty (THA) is closely related to outcomes. The literature has suggested cup parameters defined by the Lewinnek safe zone; however, the validity of such measures is in question. Several studies have raised concerns about the benefits of using the Lewinnek safe zone as a predictor of success. In this study we elected to use prospective surgeon targets as the basis for comparison to see how successful surgeons are positioning their cup using standard instruments and techniques.

METHODS

A prospective, global, multicenter study was conducted. Cup positioning success was defined as a composite endpoint. Both cup inclination and version needed to be within 10° of the surgeon target to be considered a success. Radiographic analysis was conducted by a third-party reviewer.

RESULTS

In 170 subjects, inclination, target versus actual, was 44.8° [standard deviation (SD 0.9°)] and 43.1° (SD 7.6°), respectively (p = 0.0029). Inclination was considered successful in 84.1% of cases. Mean version, target versus actual, was 19.4° (SD 3.9°) and 27.2° (SD 5.6°), respectively (p < 0.0001). Version was considered successful in 63.4% of cases, and combined position (inclination and version) was considered successful in 53.1%.

CONCLUSION

This study shows that with traditional methods of placing the cup intraoperatively, surgeons are only accurate 53.1% of the time compared with a predicted preoperative plan. This study suggests that the inconsistency in cup positioning based on the surgeon's planned target is potentially another important variable to consider while using a mechanical guide or in freehand techniques for cup placement in THA.

TRIAL REGISTRATION

This study is registered on ClinicalTrials.gov, NCT03189303.

Variations in pelvic tilt during day-to-day activities after total hip arthroplasty measured with an ultrasound system

INTRODUCTION

The pelvis plays a major role in the orientation of total hip arthroplasty (THA) implants and consequently, on the postoperative stability of the new hip joint. The relationship between the spine and pelvis affects the functional orientation of the acetabular cup, although this relationship differs between individuals and the positions encountered in everyday life. We believe this should be factored in when planning the THA procedure. This led us to conduct an in vivo ultrasound study to document the values of pelvic tilt in standing, sitting and lying positions in patients who have advanced hip osteoarthritis, before and after they undergo THA.

HYPOTHESIS

The pelvic tilt when standing, sitting and lying supine does not change significantly after THA.

MATERIALS AND METHODS

The pelvic tilt, defined as the angle between the anterior pelvic plane and a vertical line was measured using an ultrasound device in 30 patients before the THA procedure and again 6 months later in standing, sitting and lying positions. The measurements were done during an office visit with the patient on an examination table for the lying position, on a chair for the sitting position and with the heels and occiput against the wall for the standing position.

RESULTS

The preoperative and 6-month postoperative pelvic tilt were -5.6̊±10.4̊ [-30.6̊; 11.7̊] and -5.8̊±7.9̊ [-20.6̊; 10.4̊] (p=0.4129) when standing, -44.5̊±6.10 [-53.8̊; -23.9̊] and -43.5̊±6.9̊ [-54.4̊; -17.3̊] (p=0.5760) when sitting, -88.3̊±5.1̊ [-99.3̊; -78.7̊] and -87.9̊±5.9̊ [-97.6̊; -72.4̊] (p=0.6106) when lying supine, respectively. There was no significant difference in the pelvic tilt before and 6 months after THA procedure. Variations of±5̊; ±5-10̊; ±10-15̊ and 15-20̊ were found in 72% (18); 20% (5); 4% (1) and 4% (1) of patients when lying supine, 56% (14); 36% (9); 8% (2); 0% (0) of patients when standing and 76% (19); 24% (6); 0% (0); 0% (0) when sitting. The mean variation for each patient between the preoperative and 6 months postoperative measurement was 4.8±3.4̊ [0.8̊; 10.4̊] (p=0.4129), 3.5±2.3̊ [0.2̊; 9.1̊] (p=0.5760), and 4.6̊±3.2̊ [1.2̊; 15.4̊] (p=0.6106) for the standing, sitting and lying positions respectively.

CONCLUSION

The pelvic tilt does not significantly change 6 months after THA. Taking into account the various positions used in everyday life may help to optimize the implant position. We have described the use of an affordable, accurate and non-irradiating device that provides fast and easy measurements of pelvic tilt in various positions.

LEVEL OF EVIDENCE

IV.

Intraoperative Assessment of Stability in Total Hip Arthroplasty: A Novel Method Utilizing Implant Collinearity

Background

Hip dislocation remains a leading cause of revision following total hip arthroplasty, and intraoperative assessment of acetabular positioning can be difficult to perform. There are multiple clinical tests, as well as computer and robotic techniques, that aim to reduce dislocation rates after hip arthroplasty. These approaches aim to optimize the relationship between acetabular and femoral component positioning. It is hypothesized in this study that implant collinearity assessment intraoperatively can help determine appropriate acetabular component position. More specifically, implant collinearity, when achieved with a specific leg position, can reliably predict appropriate acetabular positioning.

Methods

A single-surgeon, single-centered study with 55 patients was undertaken between August 2017 and March 2020. Using preoperative imaging and computer simulation, data points were taken for 3 different acetabular configurations and comparing using 2 femoral positions. The angle differences from the collinear position (0 degrees) were compared between groups.

Results

A total of 55 patients' imaging was analyzed using 3 acetabular configurations and 2 femoral positions. The test leg position (30-degree flexion/30-degree internal rotation/10-degree adduction) was closer to collinearity than the control position (30-degree flexion/30-degree internal rotation/0-degree adduction) in all 3 acetabular configurations by a mean of 8 degrees (P < .001), 7 degrees (P < .001), and 4 degrees (P < .001), respectively.

Conclusions

The use of a 10-degree adducted position more reliably recreates implant collinearity when determining acetabular positioning during total hip arthroplasty. This intraoperative test adds another data point to assist the surgeon in achieving safe component positioning.

Intraoperative Pelvic Tilt and Axial Rotation During Total Hip Arthroplasty Through the Direct Anterior Approach is Affected by the Acetabular Retractor and Cup Impactor

Background

Intraoperative pelvic motion during total hip arthroplasty (THA) in the supine position affects acetabular cup placement and occurs at each step of THA; however, there are no reports of pelvic motion changes during each stage of THA via the direct anterior approach (DAA). This study aimed to evaluate pelvic motion at each step of THA through the DAA.

Methods

From March to October 2022, 71 hips were prospectively measured for intraoperative pelvic tilt and axial rotation during THA through the DAA at a single center. These parameters were measured during each surgical step using the augmented reality-hip navigation system.

Results

Both pelvic tilt and axial rotation were maximal during acetabular cup placement. The mean intraoperative pelvic tilt and axial rotation during cup placement were 4.8 ± 2.6° (95% confidence interval, 4.19-5.41°) and 4.2 ± 3.3° (95% confidence interval, 3.42-4.98°), respectively. The effects of the acetabular retractor and cup impactor on pelvic tilt and axial rotation were comparable. Spearman's correlation tests showed significant correlation between axial rotation and body mass index (r = -0.444, P = .00011).

Conclusions

The pelvis tilts forward and rotates toward the surgical side during THA through the DAA. The effects of the acetabular retractor and cup impactor on pelvic motion are comparable. Cup implantation must take into account pelvic movement, and it must be recognized that the pelvis is moving at that time, even with only the acetabular retractor inserted, compared to before the skin incision.

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 Functional Mechanics of the Acetabular Component in Total Hip Arthroplasty

BACKGROUND

Total hip arthroplasty functional safe zones match postural hip changes to dynamic positioning of the acetabular component. We studied integrating the Anteinclination (AI) cup angle into the spinopelvic environment, defining normative values for all parameters and calculating adjustments to AI for each degree of altered standing pelvic position and postural mobility from these values. A sagittal geometric model was employed to determine these values using established spinopelvic parameter angles.

METHODS

Theoretical normative Pelvic Incidence (PI) specific values were calculated using a triangular construct employing a linear equation describing the functional relationship between the pelvic parameters at a mobility producing an isosceles solution for normative acetabular angles. Individual optimal AI cup values for altered Sacral Slope (SS)/pelvic tilt (sPT) and mobility (dSS) were calculated using specific ratios of angular change between parameters correcting from these normative values.

RESULTS

A PI:SS:sPT ratio of angular change of 3:2:1 at dSS = 25° mobility creates an isosceles condition solving for PI specific theoretical normative values for all construct parameters. Individualized tilt correction applies to each posture a +0.25° AI alteration for each +1° sPT increase from this architectural value. Mobility correction applies a +0.5° standing AI and -0.5° sitting AI alteration for each -1° dSS < 25°, the opposite for each +1° dSS > 25°. The Sacroacetabular angle/Pelvic acetabular angle (SAA/PAA) index describes the underlying spinopelvic environment the cup functions within.

CONCLUSION

This model quantitatively integrates an implanted acetabular component into the host spinopelvic environment. Theoretical normative and individual optimal cup orientations are passively determined by these conditions of standing pelvic position and mobility.

Orthopedic Surgeons’ Accuracy When Orienting an Acetabular Cup. A Comparison with Untrained Individuals

Background and Objectives: Previous studies demonstrated a huge variability among surgeons when it comes to reproducing the position of an acetabular cup in total hip arthroplasty. Our main objective is to determine if orthopedic surgeons can replicate a given orientation on a pelvic model better than untrained individuals. Our secondary objective is to determine if experience has any influence on their ability for this task. Materials and Methods: A group of specialist orthopedic hip surgeons and a group of volunteers with no medical training were asked to reproduce three given (randomly generated) acetabular cup orientations (inclination and anteversion) on a pelvic model. Error was measured by means of a hip navigation system and comparisons between groups were made using the appropriate statistical methods. Results: The study included 107 individuals, 36 orthopedic surgeons and 71 untrained volunteers. The mean error among surgeons was slightly greater as regards both inclination (7.84 ± 5.53 vs. 6.70 ± 4.03) and anteversion (5.85 ± 4.52 vs. 5.48 ± 3.44), although statistical significance was not reached (p = 0.226 and p = 0.639, respectively). Similarly, although surgeons with more than 100 procedures a year obtained better results than those with less surgical experience (8.01 vs. 7.67 degrees of error in inclination and 5.83 vs. 5.87 in anteversion), this difference was not statistically significant, either (p = 0.852 and p = 0.981). Conclusions: No differences were found in the average error made by orthopedic surgeons and untrained individuals. Furthermore, the surgeons’ cup orientation accuracy was not seen to improve significantly with experience.

Model-Based Acetabular Cup Orientation Optimization Based On Minimizing the Risk of Edge-Loading and Implant Impingement Following Total Hip Arthroplasty

In this paper, a computationally-efficient model-based method for determining patient-specific optimal acetabular cup alignment for total hip arthroplasty (THA) is presented. The proposed algorithm minimizes the risk of implant impingement and edge-loading, which are reported as the major causes of hip dislocation following THA. First, by using motion capture data recorded from the patient performing different daily activities, the hip contact force and the relative orientation of the femur and pelvis are calculated by a musculoskeletal model. Then, by defining two quantitative indices i.e., angular impingement distance and angular edge-loading distance, the risk of impingement and edge-loading are assessed for a wide range of cup alignments. And finally, three optimization criteria are introduced to estimate the optimal cup alignment with a tradeoff between the risk of impingement and edge-loading. The results show that patient-specific characteristics such as pelvic tilt could significantly change the optimal cup alignment, especially the value of cup anteversion. Therefore, in some cases, the well-known Lewinnek safe zone may not be optimal, or even safe. Unlike other dynamic model-based methods, in this work, the need for force plate measurements is eliminated by estimating the ground reaction forces and moments, which makes this method more practical and cost-efficient. Furthermore, the low computational complexity due to analytical formulas makes this method suitable for both preoperative and intra-operative planning.

Abnormal spinopelvic mobility as a risk factor for acetabular placement error in total hip arthroplasty using optical computer-assisted surgical navigation system

Aims

Navigation devices are designed to improve a surgeon’s accuracy in positioning the acetabular and femoral components in total hip arthroplasty (THA). The purpose of this study was to both evaluate the accuracy of an optical computer-assisted surgery (CAS) navigation system and determine whether preoperative spinopelvic mobility (categorized as hypermobile, normal, or stiff) increased the risk of acetabular component placement error.

Methods

A total of 356 patients undergoing primary THA were prospectively enrolled from November 2016 to March 2018. Clinically relevant error using the CAS system was defined as a difference of > 5° between CAS and 3D radiological reconstruction measurements for acetabular component inclination and anteversion. Univariate and multiple logistic regression analyses were conducted to determine whether hypermobile ([Formula: see text]sacral slope(SS)stand-sit > 30°), or stiff ([Formula: see text]SSstand-sit < 10°) spinopelvic mobility contributed to increased error rates.

Results

The paired absolute difference between CAS and postoperative imaging measurements was 2.3° (standard deviation (SD) 2.6°) for inclination and 3.1° (SD 4.2°) for anteversion. Using a target zone of 40° (± 10°) (inclination) and 20° (± 10°) (anteversion), postoperative standing radiographs measured 96% of acetabular components within the target zone for both inclination and anteversion. Multiple logistic regression analysis controlling for BMI and sex revealed that hypermobile spinopelvic mobility significantly increased error rates for anteversion (odds ratio (OR) 2.48, p = 0.009) and inclination (OR 2.44, p = 0.016), whereas stiff spinopelvic mobility increased error rates for anteversion (OR 1.97, p = 0.028). There were no dislocations at a minimum three-year follow-up.

Conclusion

Despite high reliability in acetabular positioning for inclination in a large patient cohort using an optical CAS system, hypermobile and stiff spinopelvic mobility significantly increased the risk of clinically relevant errors. In patients with abnormal spinopelvic mobility, CAS systems should be adjusted for use to avoid acetabular component misalignment and subsequent risk for long-term dislocation. Cite this article: Bone Jt Open 2022;3(6):475–484.

Integrating the Combined Sagittal Index Reduces the Risk of Dislocation Following Total Hip Replacement

BACKGROUND

The aims of this matched cohort study were to (1) assess differences in spinopelvic characteristics between patients who sustained a dislocation after total hip arthroplasty (THA) and a control group without a dislocation, (2) identify spinopelvic characteristics associated with the risk of dislocation, and (3) propose an algorithm including individual spinopelvic characteristics to define an optimized cup orientation target to minimize dislocation risk.

METHODS

Fifty patients with a history of THA dislocation (29 posterior and 21 anterior dislocations) were matched for age, sex, body mass index (BMI), index diagnosis, surgical approach, and femoral head size with 200 controls. All patients underwent detailed quasi-static radiographic evaluations of the coronal (offset, center of rotation, and cup inclination/anteversion) and sagittal (pelvic tilt [PT], sacral slope [SS], pelvic incidence [PI], lumbar lordosis [LL], pelvic-femoral angle [PFA], and cup anteinclination [AI]) reconstructions. The spinopelvic balance (PI - LL), combined sagittal index (CSI = PFA + cup AI), and Hip-User Index were determined. Parameters were compared between the control and dislocation groups (2-group analysis) and between the controls and 2 dislocation groups identified according to the direction of the dislocation (3-group analysis). Important thresholds were determined from receiver operating characteristic (ROC) curve analyses and the mean values of the control group; thresholds were expanded incrementally in conjunction with running-hypothesis tests.

RESULTS

There were no coronal differences, other than cup anteversion, between groups. However, most sagittal parameters (LL, PT, CSI, PI - LL, and Hip-User Index) differed significantly. The 3 strongest predictors of instability were PI - LL >10° (sensitivity of 70% and specificity of 65% for instability regardless of direction), CSIstanding of <216° (posterior instability), and CSIstanding of >244° (anterior instability). A CSI that was not between 205° and 245° on the standing radiograph (CSIstanding) was associated with a significantly increased dislocation risk (odds ratio [OR]: 4.2; 95% confidence interval [CI]: 2.2 to 8.2; p < 0.001). In patients with an unbalanced and/or rigid lumbar spine, a CSIstanding that was not 215° to 235° was associated with a significantly increased dislocation risk (OR: 5.1; 95% CI: 1.8 to 14.9; p = 0.001).

CONCLUSIONS

Spinopelvic imbalance (PI - LL >10°) determined from a preoperative standing lateral spinopelvic radiograph can be a useful screening tool, alerting surgeons that a patient is at increased dislocation risk. Measurement of the PFA preoperatively provides valuable information to determine the optimum cup orientation to aim for a CSIstanding of 205° to 245°, which is associated with a reduced dislocation risk. For patients at increased dislocation risk due to spinopelvic imbalance (PI - LL >10°), the range for the optimum CSI is narrower.

LEVEL OF EVIDENCE

Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

An Online Learning Tool to Obtain, Optimize, and Interpret Radiographs During Total Hip Arthroplasty

Total hip arthroplasty (THA) is a common orthopedic procedure which has been growing in popularity with the elderly population. With more surgeons completing anterior THAs, intraoperative radiographs have become commonplace. Unfortunately, there is a lack of education in regard to obtaining, optimizing, and interpreting these radiographs. The purpose of this study was to develop and test the efficacy of an online learning tool that medical students, residents, and C-arm technicians could use to improve their understanding of THA radiography. The learning tool taught users how to obtain an optimal AP pelvis radiograph and how to interpret radiographs so THA components could be placed in their optimal position. This learning tool was sent to medical students, orthopedic surgery residents, and C-arm technicians along with a pre-test, post-test, and feedback survey. Twenty users (eleven medical students and nine orthopedic surgery residents) completed the learning tool. Post-test scores (M=96.4%, SD=2.9%) were significantly greater than pre-test scores (M=68.3%, SD=23.9%) for all users (t=5.5069, P<.0001). The user's level of training was positively correlated with pre-test scores. Surveys from the users revealed that the learning tool provided significant learning opportunities, was relatively easy to understand, but was slightly too long. Users felt that this learning tool would be best suited for senior medical students, junior orthopedic surgery residents, and C-arm technicians. With the positive results of this study, the authors hope to further develop this learning tool for widespread adoption and to develop similar learning tools in the future.

Smartphone Application Helps Improve the Accuracy of Cup Placement by Young, Less-Experienced Surgeons during Primary Total Hip Arthroplasty

BACKGROUND

Dislocation after total hip arthroplasty (THA) partly under the surgeon's control, by appropriate placement of the components. We aimed in this study to determine the accuracy of using intra-operative smartphone applications (Apps) to place the acetabular cup within the safe abduction angle by less experienced surgeons during THA surgery when compared to the conventional freehand technique for cup placement.

METHODS

Sixty primary THAs were performed, 30 using the conventional freehand technique (control group) and 30 using the smartphone app technique (study group) to determine the acetabular cup abduction angle by the same young surgeon with less than one year of experience. Postoperative mean cup abduction angle, mean cup anteversion angle, and the percentage of cups within the safe abduction zone as measured on radiographs were compared between the two groups.

RESULTS

In the study group, the mean cup abduction angle was significantly lower (P=0.0008), and the acetabular cup was placed within the safe zone in a significantly higher (P<0.001) percentage of patients (93% vs 63%) when compared to the control group. However, there was no significant difference (P=0.40) between the two groups when the mean cup anteversion angle was compared.

CONCLUSION

The smartphone app technique may help achieve an accurate acetabular cup abduction angle and a higher percentage of cups placed within the safe zone of abduction by a less experienced surgeon when compared to the conventional freehand technique. Using tools such as the smartphone app to measure the acetabular cup position can reduce intraoperative errors by young and less experienced surgeons during THA surgery.

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.

Computer simulation based on in vivo kinematics of a replaced hip during chair-rising for elucidating target cup and stem positioning with a safety range of hip rotation

Background After total hip arthroplasty, dislocation can occur when a patient unexpectedly assumes internal/external limb positions, even during chair-rising, which is a frequently activity of daily life. Therefore, determining the target cup position to avoid impingement in unexpected limb positions using in vivo data of daily life activities is critical. Methods A computer simulation was performed on 21 total hip arthroplasty patients using patient-specific component placements and hip kinematics obtained during chair-rising analysis using image-matching techniques. The liner-to-neck distance and impingement were evaluated by simulating the change in internal/external rotation angle at maximum hip flexion/extension from 0 to 90°. The cutoff values of cup anteversion and combined anteversion at 60° of internal/external rotation were determined. Findings The anterior/posterior liner-to-neck distances were negatively correlated with internal/external rotation angles (r = -0.82 and -0.78, respectively) and decreased by 1.7 and 1.8 mm for every 15° increase, respectively. Three cases (14%) of anterior/posterior impingement were observed at 60° of internal/external rotation angle, respectively. The cutoff values for cup anteversion and combined anteversion to avoid impingement at 60° of internal/external rotation angle were 12°-25°/38°-62°, respectively. The stem anteversion, adjustable by cup anteversion to meet both the target cup anteversion and combined anteversion, was 13°-50°. Interpretation Simulated unintentional internal or external hip rotation, even during chair-rising, caused impingement and posed a dislocation risk. If the stem anteversion is excessively small or large in meeting the target combined anteversion, adjustments to stem anteversion could be recommended in addition to adjustments in cup anteversion.

Evaluating Alternate Registration Planes for Imageless, Computer-Assisted Navigation During Total Hip Arthroplasty

BACKGROUND

Imageless computer navigation improves component placement accuracy in total hip arthroplasty (THA), but variations in the registration process are known to impact final accuracy measurements. We sought to evaluate the registration accuracy of an imageless navigation device during THA performed in the lateral decubitus position.

METHODS

A prospective, observational study of 94 patients undergoing a primary THA with imageless navigation assistance was conducted. Patient position was registered using 4 planes of reference: the patient's coronal plane (standard method), the long axis of the surgical table (longitudinal plane), the lumbosacral spine (lumbosacral plane), and the plane intersecting the greater trochanter and glenoid fossa (hip-shoulder plane). Navigation measurements of cup position for each plane were compared to measurements from postoperative radiographs.

RESULTS

Mean inclination from radiographs (41.5° ± 5.6°) did not differ significantly from inclination using the coronal plane (40.9° ± 3.9°, P = .39), the hip-shoulder plane (42.4° ± 4.7°, P = .26), or the longitudinal plane (41.2° ± 4.3°, P = .66). Inclination measured using the lumbosacral plane (45.8° ± 4.3°) differed significantly from radiographic measurements (P < .0001). Anteversion measured from radiographs (mean: 26.1° ± 5.4°) did not differ significantly from the hip-shoulder plane (26.6° ± 5.2°, P = .50). All other planes differed significantly from radiographs: coronal (22.6° ± 6.8°, P = .001), lumbosacral (32.5° ± 6.4°, P < .0001), and longitudinal (23.7° ± 5.2°, P < .0001).

CONCLUSION

Patient registration using any plane approximating the long axis of the body provided a frame of reference that accurately measured intraoperative cup position. Registration using a plane approximating the hip-shoulder axis, however, provided the most accurate and consistent measurement of acetabular component position.

Low dislocation rates with the use of patient specific "Safe zones" in total hip arthroplasty

Introduction

The purpose of this study was to compare patient-specific acetabular cup target orientation using functional simulation to the Lewinnek Safe Zone (LSZ) and determine associated rates of postoperative dislocation.

Methods

A retrospective review of 1500 consecutive primary THAs was performed. Inclination, anteversion, pelvic tilt, pelvic incidence, lumbar flexion, and dislocation rates were recorded.

Results

56% of dynamically planned cups were within LSZ (p < 0.05). 6/1500 (0.4%) of these cups dislocated at two year follow-up, and all were within LSZ.

Conclusion

Optimal acetabular cup positioning using dynamic imaging differs significantly from historical target parameters but results in low rates of dislocation.

Level of evidence

Level III: Retrospective.

Endoscopic Tenotomy for Iliopsoas Tendinopathy Following Total Hip Arthroplasty Can Relieve Pain Regardless of Acetabular Cup Overhang or Anteversion

PURPOSE

To report outcomes of endoscopic iliopsoas tenotomy (EIT) in patients with iliopsoas tendinopathy following total hip arthroplasty (THA) and determine whether improvements in clinical scores are associated with acetabular cup anteversion measured on plain radiographs or overhang measured using established and alternative computed tomography (CT)-based methods.

METHODS

We evaluated patients who underwent EIT for iliopsoas tendinopathy after THA (2014-2017), performed between the lesser trochanter and psoas valley. Indications were groin pain during active hip flexion, exclusion of other complications, and no pain relief after 6 months of conservative treatment. Pretenotomy inclination and anteversion were measured on radiographs; sagittal and axial overhang were measured on CT scans on slices passing through: (Method 1) prosthetic head center and (Method 2) anterior margin of acetabular cup. Modified Harris hip score (mHHS), Oxford Hip Score (OHS), and level of groin pain were recorded at 12 or more months. Wilcoxon signed-rank tests were used to compare pre- and post-tenotomy scores, and intraclass correlation coefficients (ICCs) to assess intermethod agreement.

RESULTS

The cohort comprised 16 men (17 hips) and 32 women (33 hips), aged 60.8 ± 10.5 years at EIT. For the 30 (60%) hips that had pretenotomy CT scans, axial and sagittal overhang were, respectively, 3.4 ± 3.7 mm and 4.6 ± 4.6 mm, using Method 1, compared with 3.9 ± 3.9 mm and 6.9 ± 5.0 mm using Method 2. Intermethod agreement was good for axial (ICC, 0.67; P < .001) and sagittal (ICC, 0.68; P < .001) overhang. At a minimum 1 year after EIT, 3 hips had cup and stem revision and 1 hip had isolated cup revision, leaving 46 hips for clinical assessment. No complications were noted. mHHS improved by 26 ± 19 (P < .001) and OHS improved by 20 ± 11 (P < .001). Twenty-six hips (57%) achieved the patient acceptable symptom state for mHHS, whereas 42 hips (91%) achieved the patient acceptable symptom state for OHS. Post-tenotomy groin pain was slight in 15%, mild in 17%, and moderate in 11%. Regression analyses revealed no associations between clinical scores and overhang/anteversion.

CONCLUSIONS

For patients with iliopsoas tendinopathy following THA, endoscopic iliopsoas tenotomy granted clinically important improvements of mHHS in 76% and OHS in 89%, despite moderate residual groin pain in 11%. Improvements in clinical scores did not seem to be associated with the extent of cup overhang or anteversion in the cases for which adequate preoperative imaging was available.

LEVEL OF EVIDENCE

Level IV, retrospective cohort study.

The Patient-Specific Combined Target Zone for Morpho-Functional Planning of Total Hip Arthroplasty

Background Relevant criteria for total hip arthroplasty (THA) planning have been introduced in the literature which include the hip range of motion, bony coverage, anterior cup overhang, leg length discrepancy, edge loading risk, and wear. The optimal implant design and alignment depends on the patient’s anatomy and patient-specific functional parameters such as the pelvic tilt. The approaches proposed in literature often consider one or more criteria for THA planning. but to the best of our knowledge none of them follow an integrated approach including all criteria for the definition of a patient-specific combined target zone (PSCTZ). Questions/purposes (1) How can we calculate suitable THA implant and implantation parameters for a specific patient considering all relevant criteria? (2) Are the resulting target zones in the range of conventional safe zones? (3) Do patients who fulfil these combined criteria have a better outcome score? Methods A method is presented that calculates individual target zones based on the morphology, range of motion and load acting on the hip joint and merges them into the PSCTZ. In a retrospective analysis of 198 THA patients, it was calculated whether the patients were inside or outside the Lewinnek safe zone, Dorr combined anteversion range and PSCTZ. The postoperative Harris Hip Scores (HHS) between insiders and outsiders were compared. Results 11 patients were inside the PSCTZ. Patients inside and outside the PSCTZ showed no significant difference in the HHS. However, a significant higher HHS was observed for the insiders of two of the three sub-target zones incorporated in the PSCTZ. By combining the sub-target zones in the PSCTZ, all PSCTZ insiders except one had an HHS higher than 90. Conclusions The results might suggest that, for a prosthesis implanted in the PSCTZ a low outcome score of the patient is less likely than using the conventional safe zones by Lewinnek and Dorr. For future studies, a larger cohort of patients inside the PSCTZ is needed which can only be achieved if the cases are planned prospectively with the method introduced in this paper. Clinical Relevance The method presented in this paper could help the surgeon combining multiple different criteria during THA planning and find the suitable implant design and alignment for a specific patient.

The learning curve for the direct anterior total hip arthroplasty: a systematic review

BACKGROUND

The direct anterior approach (DAA) for total hip arthroplasty (THA) is a muscle-sparing approach thought to have less post-operative pain and quicker recovery, with similar functional outcomes to other approaches. However, it is technically challenging and transitioning surgeons may experience increased complication rates. The purpose of this systematic review is to identify reported learning curves associated with the DAA.

METHODS

Three databases (MEDLINE, Embase, and Web of Science) were searched using terms including "total hip arthroplasty," "direct anterior approach," and "learning curve." Study characteristics, patient demographics, learning curve analyses, and complications were abstracted.

RESULTS

Twenty-one studies met inclusion criteria, with a total of 9738 patients (60% female), an average age of 63.7 years (range: 13-94), body mass index of 27.0 kg/m2 (range: 16.8-58.9), and follow-up of 19 months (range: 1.5-100). There were five retrospective cohort studies and 13 case series representing fair methodological quality. Six studies depicted a true learning curve, with mean operative time of 156.59 ± 41.71 minutes for the first case, 93.18 ± 14.68 minutes by case 30, and 80.45 ± 12.28 minutes by case 100. Mean complication rate was 20.8 ± 12.7% in early groups and decreased to 7.6 ± 7.1% in late groups.

CONCLUSION

This review demonstrated a substantial learning curve associated with the DAA to THA. Operative time plateaued after approximately 100 cases. Complication rates decreased substantially from early to late groups.

Spinopelvic mobility affects accuracy of acetabular anteversion measurements on cross-table lateral radiographs

AIMS

Cross-table lateral (CTL) radiographs are commonly used to measure acetabular component anteversion after total hip arthroplasty (THA). The CTL measurements may differ by > 10° from CT scan measurements but the reasons for this discrepancy are poorly understood. Anteversion measurements from CTL radiographs and CT scans are compared to identify spinopelvic parameters predictive of inaccuracy.

METHODS

THA patients (n = 47; 27 males, 20 females; mean age 62.9 years (SD 6.95)) with preoperative spinopelvic mobility, radiological analysis, and postoperative CT scans were retrospectively reviewed. Acetabular component anteversion was measured on postoperative CTL radiographs and CT scans using 3D reconstructions of the pelvis. Two cohorts were identified based on a CTL-CT error of ≥ 10° (n = 11) or < 10° (n = 36). Spinopelvic mobility parameters were compared using independent-samples t-tests. Correlation between error and mobility parameters were assessed with Pearson's coefficient.

RESULTS

Patients with CTL error > 10° (10° to 14°) had stiffer lumbar spines with less mean lumbar flexion (38.9°(SD 11.6°) vs 47.4° (SD 13.1°); p = 0.030), different sagittal balance measured by pelvic incidence-lumbar lordosis mismatch (5.9° (SD 18.8°) vs -1.7° (SD 9.8°); p = 0.042), more pelvic extension when seated (pelvic tilt -9.7° (SD 14.1°) vs -2.2° (SD 13.2°); p = 0.050), and greater change in pelvic tilt between supine and seated positions (12.6° (SD 12.1°) vs 4.7° (SD 12.5°); p = 0.036). The CTL measurement error showed a positive correlation with increased CTL anteversion (r = 0.5; p = 0.001), standing lordosis (r = 0.23; p = 0.050), seated lordosis (r = 0.4; p = 0.009), and pelvic tilt change between supine and step-up positions (r = 0.34; p = 0.010).

CONCLUSION

Differences in spinopelvic mobility may explain the variability of acetabular anteversion measurements made on CTL radiographs. Patients with stiff spines and increased compensatory pelvic movement have less accurate measurements on CTL radiographs. Flexion of the contralateral hip is required to obtain clear CTL radiographs. In patients with lumbar stiffness, this movement may extend the pelvis and increase anteversion of the acetabulum on CTL views. Reliable analysis of acetabular component anteversion in this patient population may require advanced imaging with a CT scan. Cite this article: Bone Joint J 2021;103-B(7 Supple B):59-65.

Comparison of accuracy of cup position using portable navigation versus alignment guide in total hip arthroplasty in supine position

BACKGROUND

Accelerometer-based portable navigation systems in supine total hip arthroplasty (THA) have been developed, but there are no reports on the accuracy of cup placement. We aimed to investigate and compare the accuracy of the accelerometer-based portable navigation system versus the acetabular alignment guide placed on the pelvis in THA using the direct anterior approach (DAA). Both devices tracked changes in the pelvic position.

METHODS

In this single-centre, retrospective study, we reviewed 115 hips in 113 patients who underwent primary THA via the DAA using an accelerometer-based portable navigation system in the supine position (portable navigation group) and 106 hips in 101 patients who underwent THA using an acetabular alignment guide (alignment guide group) as controls. Hips were evaluated postoperatively using computed tomography to measure cup orientation. The accuracy of cup orientation was compared between the 2 groups.

RESULTS

Absolute values of inclination error were 3.1° ± 2.2° and 2.9° ± 2.3° (p = 0.708) in the portable navigation and alignment guide groups and those of anteversion error were 2.8° ± 2.3° and 3.7° ± 2.7°, respectively (p = 0.005). The number of cups placed within 10° of error was 98.3% and 96.2% in the portable navigation and alignment guide groups, respectively (p = 0.304). The portable navigation group had significantly more hips (72.2%) placed within a 5° margin of error than did the alignment guide group (56.6%) (p = 0.016).

CONCLUSION

High accuracy in cup placement was achieved using accelerometer-based portable navigation in supine THA. Using a navigation system may contribute to improved long-term outcomes.

Well-Placed Acetabular Component Oriented Outside the Safe Zone During Weight-Bearing Daily Activities

Background: A comprehensive and thorough understanding of functional acetabular component orientation is essential for optimizing the clinical outcome after total hip arthroplasty (THA). This study aimed to quantify the functional acetabular anteversion and inclination of unilateral THA patients during walking and static standing and to determine whether the functional acetabular orientation falls within the Lewinnek safe zone. Methods: Seventeen patients with unilateral THA received a CT scan and dual fluoroscopic imaging during level walking and static standing to evaluate in vivo hip kinematics. The pelvic functional coordinate system of the 3D CT-based computer model was defined by the line of gravity and anterior pelvic plane (APP) to measure functional acetabular anteversion and inclination in different postures. The Lewinnek safe zone was used to determine the acetabular malposition during functional activities. Results: The THA side demonstrated an average of 10.1° (± 9.6°, range -7.5° to 29.9°) larger functional anteversion and 16.0° (± 9.2°, range -7.2° to 29.9°) smaller inclination than native hips during level walking. Functional acetabular anteversion in the THA side during level walking and static standing was significantly larger than anatomical measurements (p < 0.05). Acetabular orientation of most well-placed THA components anatomically in the Lewinnek safe zone fell outside the safe zone during more than half of the gait cycle and static standing. Conclusion: The current study revealed that an anatomically well-placed acetabular cup does not guarantee a well-functional orientation during daily activities. The in vivo mechanical performance and loading conditions of the THA component during other weight-bearing activities should be investigated in further studies.

Measurement of operative femoral anteversion during cementless total hip arthroplasty and influencing factors for using neck-adjustable femoral stem

BACKGROUND

Placement of femoral stem in excessive anteversion or retroversion can cause reduced range of motion, prosthetic impingement, and dislocation. The aim of this study was to assess the operative femoral anteversion in patients treated with total hip arthroplasty (THA) and analyze the need of adjusting stem anteversion.

METHODS

We retrospectively included 101 patients (126 hips) who underwent cementless THA with a manual goniometer to determine the femoral anteversion between October 2017 and December 2018. The operative femoral anteversion we measured was recorded during THA. We further divided those hips into three subgroups based on the range of operative femoral anteversion: group 1 (<10°), group 2 (10-30°), and group 3 (>30°) and compared the differences of their demographic data. Univariate and multivariate logistic regression were used to identify the influencing factors for the need of neck-adjustable femoral stem. The clinical and radiographic outcomes were also assessed. Perioperative complications were recorded.

RESULTS

After THA, the Harris hip scores improved from 52.87 ± 15.30 preoperatively to 90.04 ± 3.31 at the last follow-up (p < 0.001). No implant loosening, stem subsidence, and radiolucent lines were observed on radiographs. No severe complications occurred and no components needed revision at the latest follow-up. The mean operative femoral anteversion was 14.21° ± 11.80° (range, -9 to 60°). Patients with femoral anteversion more than 30° were about 10 years younger than others. Femoral anteversion >30° was more common in patients with developmental dysplasia of the hip (DDH). There were totally 14 hips treated with the neck-adjustable femoral stem. From the univariate analysis, we can observe that female sex, diagnosis of DDH (compared with osteonecrosis), and higher operative femoral anteversion and its value >30° (compared with <10°) are associated with higher rates of using the neck-adjustable femoral stem. However, all these factors were no longer considered as independent influencing factors when mixed with other factors.

CONCLUSIONS

This study highlighted the significance of operative femoral anteversion. Identification of abnormal femoral anteversion could assist in adjusting stem anteversion and reduce the risk of dislocation after THA.

Effect of the underlying cadaver data and patient-specific adaptation of the femur and pelvis on the prediction of the hip joint force estimated using static models

The prediction of the hip joint force (HJF) is a fundamental factor for the prevention of edge loading in total hip arthroplasty. Naturally, the loading of the liner of the acetabular component depends on the HJF acting on the artificial joint. In contrast to dynamic musculoskeletal models, static models for HJF prediction do not require motion analysis of the patient. However, patient-specific adaptability and validity of static models have to be scrutinized. In this study, a modular framework for HJF prediction using static models is introduced to compare the results of different cadaver templates that are the basis of most static and dynamic models, and different scaling laws for the patient-specific adaptation with in vivo HJF of ten patients for one-leg stance and level walking. The results revealed the significant effect of the underlying cadaver template used for the prediction of the HJF (p < 0.01). A higher degree of patient-specific scaling of the cadaver template often did not significantly reduce the prediction error. Three static models with the lowest prediction errors were compared to results of dynamic models from literature. The prediction error of the peak HJF of the static models (median absolute errors below 15% body weight in magnitude and below 5° in direction) was similar in magnitude and even smaller in direction compared to dynamic models. The necessary reduction of a load-based target zone for the prevention of edge loading due to the uncertainty of the HJF prediction has to be considered in the preoperative planning. The framework for HJF prediction is openly accessible at https://github.com/RWTHmediTEC/HipJointForceModel.

A systematic review and meta-analysis of the hip capsule innervation and its clinical implications

Detailed understanding of the innervation of the hip capsule (HC) helps inform surgeons’ and anaesthetists’ clinical practice. Post-interventional pain following radiofrequency nerve ablation (RFA) and dislocation following total hip arthroplasty (THA) remain poorly understood, highlighting the need for more knowledge on the topic. This systematic review and meta-analysis focuses on gross anatomical studies investigating HC innervation. The main outcomes were defined as the prevalence, course, density and distribution of the nerves innervating the HC and changes according to demographic variables. HC innervation is highly variable; its primary nerve supply seems to be from the nerve to quadratus femoris and obturator nerve. Many articular branches originated from muscular branches of the lumbosacral plexus. It remains unclear whether demographic or anthropometric variables may help predict potential differences in HC innervation. Consequently, primary targets for RFA should be the anterior inferomedial aspect of the HC. For THA performed on non-risk patients, the posterior approach with capsular repair appears to be most appropriate with the lowest risk of articular nerve damage. Care should also be taken to avoid damaging vessels and muscles of the hip joint. Further investigation is required to form a coherent map of HC innervation, utilizing combined gross and histological investigation.

Effects of body mass index and range of motion on intraoperative change in pelvic tilt during total hip arthroplasty using the direct anterior approach

BACKGROUND

Intraoperative pelvic tilt changes that occurs during total hip arthroplasty (THA) in the supine position affects cup placement and sometimes causes malalignment. The relationship between body mass index (BMI) and pelvic movement has been reported for some procedures, but not the direct anterior approach (DAA). The purpose of this study was to investigate intraoperative pelvic tilt changes that occurs during DAA.

METHODS

In this single-center, retrospective study, we reviewed 200 hips that underwent primary THA via DAA in the supine position using an accelerometer-based navigation system. Intraoperative changes in pelvic tilt and axial rotation from the start of surgery to cup placement were assessed using the navigation system. Preoperative clinical factors that increased pelvic tilt and axial rotation toward the surgical side by > 10° were analyzed via univariate and multiple logistic regression analyses.

RESULTS

The mean pelvic tilt value increased by 7.6° ± 3.8° (95% confidence interval [CI], 7.1-8.2; range, - 5.0-19.0) intraoperatively, and the axial rotation increased by 3.2° ± 2.7° (95% CI, 2.7-3.7; range, - 13.0-12.0). Univariate analysis revealed that the group with increased pelvic tilt showed significantly greater range of abduction and internal rotation, and significantly lower BMI than the group with no increased tilt. Pre-incisional pelvic tilt was significantly greater in the group with increased axial rotation than in the group with no increased rotation. On logistic regression analysis, BMI (odds ratio [OR], 0.889; 95% CI, 0.809-0.977; p = 0.014) and the range of internal rotation (OR, 1.310; 95% CI, 1.002-1.061; p = 0.038) were predictors of large increases in pelvic tilt. No predictors of large increases in axial rotation were identified.

CONCLUSION

Significant forward pelvic tilt was observed in patients with a low BMI values and high ranges of internal rotation via THA using the DAA. Findings indicated that surgeons should pay attention to intraoperative pelvic movements, which may help identify patients with significant pelvic tilt changes.

Targeting the safe zones for cup position without fluoroscopic guidance in total hip arthroplasty: does the surgical approach affect the outcomes?

BACKGROUND

Proper acetabular cup positioning is crucial for achieving implant longevity and stability in total hip arthroplasty (THA). One of the elements that may affect the accuracy and precision of the positioning of the cup is the type of surgical approach used. This study compares the accuracy and the precision of conventional free-hand acetabular cup positioning relative to different "safe zones" in two groups of patients operated with THA using two different surgical approach.

METHODS

Retrospective radiological comparative matched-pair controlled study of two groups of patients operated with primary THA: the first group was operated in supine decubitus with a mini-invasive direct anterior approach (DA group), the second group was operated in lateral decubitus with a conventional posterolateral approach (PL group). Cup inclination and anteversion were assessed using the digital planning software TraumaCad. Cup position in the two groups were compared in terms of accuracy and precision.

RESULTS

Forty four patients were enrolled for each group. The DA group showed a less inclined and less anteverted cup compared to the PL group (respectively 38.5 ± 5.2 vs. 49.6 ± 5.6 and 16.2 ± 3.6 vs. 22.9 ± 6.4; p < 0.01). The DA group showed a significantly higher percentage of cups within the "safe zone" in 4 out of 6 reference zones and a significantly lower variance in anteversion (12.96 vs. 40.96, p < .01). There was no difference in the variance for inclination.

CONCLUSIONS

Our study found greater accuracy and precision in the positioning of the cup when surgery was performed through a direct anterior approach compared to the posterolateral approach due to the supine position of the patient which allows greater stability of the pelvis during surgery and makes it easier to target the desired angular references for cup positioning.

Is a portable accelerometer-based navigation system useful in total hip arthroplasty?: A systematic review and meta-analysis

BACKGROUND

Precise implant alignment is a crucial prognostic factor for successful outcomes following total hip arthroplasty (THA). A portable accelerometer-based navigation (PN) device may achieve the same accuracy as that achieved by the computer-assisted navigation surgery technique, with the convenience of a conventional technique. Although the usefulness of PN in THA (PN-THA) has been reported, whether it is more accurate than performing THA with a conventional technique (CON-THA) remains controversial. The difference in surgical time between PN-THA and CON-THA is also unclear. Therefore, we conducted a systematic review and meta-analysis of studies comparing results of PN-THA with those of CON-THA. We focused on the following question: is PN-THA superior to CON-THA in terms of radiological parameters and surgical time?

PATIENTS AND METHODS

A literature search was conducted in PubMed, Web of Science, and Cochrane Library, to identify studies that met the following inclusion criteria: randomised controlled trials (RCT) or non-RCT, studies involving patients who underwent PN-THA and patients who underwent CON-THA, studies including data on radiological parameters and surgical outcomes. Author names, publication year, country, study design, surgical approach, demographic characteristics of the participants (diagnosis, gender, age, and body mass index), and surgical outcomes (the radiological parameters and the surgical time) were extracted. We calculated the mean differences (MDs) for continuous data with 95% confidence intervals (CIs) for each outcome. p<0.05 was considered significant.

RESULTS

Three studies were included in this meta-analysis. The meta-analysis showed that absolute deviation of the postoperative measured angles from the target position for the cup anteversion was significantly smaller in PN-THA than in CON-THA (MD=-1.70, 95% CI=-2.91 to -0.50, [p=0.005]). There was no significant difference in the absolute deviation of the postoperative measured angles from the target position for cup abduction between the groups (MD=-1.82, 95% CI=-4.32-0.67, [p=0.15]). The surgical time was significantly longer in PN-THA than in CON-THA (MD=8.58, 95% CI=4.05-13.10, [p=0.0002]).

DISCUSSION

This systematic review and meta-analysis of studies comparing the results of PN-THA with those of CON-THA showed that the PN-THA is advantageous for precise cup implantation compared to CON-THA, although PN-THA has a longer surgical time compared to CON-THA.

LEVEL OF EVIDENCE

III.

Reliability of Spinopelvic Measurements That May Influence the Cup Position in Total Hip Arthroplasty

BACKGROUND

Spinopelvic pathology has been identified as a potential risk factor for instability after total hip arthroplasty. Spinopelvic radiographic parameters used to diagnose spinopelvic disease may also impact optimal cup placement. The purpose of this study was to assess the arthroplasty surgeon accuracy in making spinopelvic measurements.

METHODS

Five fellowship-trained or in-training arthroplasty surgeons reviewed 44 lateral lumbar radiographs in two sessions. All evaluators were instructed how to perform measurements but had little experience in doing so. Traditional measurements included the pelvic tilt (PT), pelvic incidence (PI), and sacral slope (SS), and novel measurements included the acetabular ante-inclination (AI) and pelvic femoral angle. Surgeon measurements were compared with those made by experienced engineers. intraclass coefficients (ICCs) were calculated and interpreted.

RESULTS

The interobserver reliability for 4 of 5 of the spinopelvic parameters was good to excellent (ICC >0.75) with the highest reliability seen for PI measurement (ICC = 0.939). Only moderate interobserver reliability was observed for AI measurement (ICC = 0.559). Intraobserver reliability was the highest for the PI and SS, ranging from moderate to good (ICC, 0.718 to 0.896). The lowest intraobserver reliability was seen for the AI (ICC range, 0.026 to 0.545) and pelvic femoral angle (ICC range, 0.035 to 0.828). Surgeon measurements of PT and SS were compared with engineer measurements with extremely poor correlation observed (ICC <0 for all).

CONCLUSION

Surgeon intraobserver and interobserver reliability in making novel radiographic measurements was low compared with traditional radiographic measurements. Surgeon reliability in making traditional measurements of the PT and SS was very poor compared with experienced assessors using software-based measurements.

Transitioning the total hip arthroplasty technique from posterior approach in lateral position to direct anterior approach in supine position-risk factors for acetabular malpositioning and the learning curve

PURPOSE

Cup positioning is important for optimum hip stability, avoiding component impingement and decreasing both bearing surface wear and revision rate. Transitioning from posterior approach in a lateral position to direct anterior approach (DAA) in a supine presents unique challenges for surgeons. The aim of this study was to examine the learning curve when using standard instrumentation that was not specific to DAA.

METHODS

A consecutive retrospective series of 537 total hip arthroplasty by DAA from May 2013 to December 2017. Cup positioning was analysed on radiographs and classified whether inside or outside two safe zones (inclination 30-50° and anteversion 10-30°). The demographic data (age, BMI, gender, neck shaft angle (NSA)), surgeon's dominant side and experience were assessed as risk factors.

RESULTS

Eighty per cent of cups (n = 426) were in the combined safe zones. Eighty-eight per cent (n = 470) were in appropriate anteversion and 87% (n = 463) abduction. Two factors that were significant were identified: Cups of left hips operated by right-handed surgeons were more anteverted (OR = 4.06) and more vertical (OR = 2.23); females had a higher anteversion of the cup (OR = 2.42). Obesity, age and NSA were not risk factors for cup malposition. There was a spike of cups too horizontal at the beginning of the experience (OR = 3.86), and no learning curve was observed in the other orientations.

CONCLUSION

With our DAA technique using standard instrumentation, there were no risk factors linked to the patient identified for cup malposition. DAA-specific instrumentation is not required to achieve optimum positioning of the cup. Surgeon has to be aware of an excess of abduction at the beginning of his experience and an excess of anteversion and adduction when performing THA on the opposite side of his dominant hand.

The Impact of Spinopelvic Mobility on Arthroplasty: Implications for Hip and Spine Surgeons

Spinopelvic mobility represents the complex interaction of hip, pelvis, and spine. Understanding this interaction is relevant for both arthroplasty and spine surgeons, as a predicted increasing number of patients will suffer from hip and spinal pathologies simultaneously. We conducted a comprehensive literature review, defined the nomenclature, summarized the various classifications of spinopelvic mobility, and outlined the corresponding treatment algorithms. In addition, we developed a step-by-step workup for spinopelvic mobility and total hip arthroplasty (THA). Normal spinopelvic mobility changes from standing to sitting; the hip flexes, and the posterior pelvic tilt increases with a concomitant increase in acetabular anteversion and decreasing lumbar lordosis. Most classifications are based on a division of spinopelvic mobility based on ΔSS (sacral slope) into stiff, normal, and hypermobile, and a categorization of the sagittal spinal balance regarding pelvic incidence (PI) and lumbar lordosis (LL) mismatch (PI–LL = ± 10° balanced versus PI–LL > 10° unbalanced) and corresponding adjustment of the acetabular component implantation. When performing THA, patients with suspected pathologic spinopelvic mobility should be identified by medical history and examination, and a radiological evaluation (a.p. pelvis standing and lateral femur to L1 or C7 (if EOS (EOS imaging, Paris, France) is available), respectively, for standing and sitting radiographs) of spinopelvic parameters should be conducted in order to classify the patient and determine the appropriate treatment strategy. Spine surgeons, before planned spinal fusion in the presence of osteoarthritis of the hip, should consider a hip flexion contracture and inform the patient of an increased risk of complications with existing or planned THA.

The Impingement-free, Prosthesis-specific, and Anatomy-adjusted Combined Target Zone for Component Positioning in THA Depends on Design and Implantation Parameters of both Components

BACKGROUND

Lewinnek's recommendation for orienting the cup in THA is criticized because it involves a static assessment of the safe zone and because it does not consider stem geometry. A revised concept of the safe zone should consider those factors, but to our knowledge, this has not been assessed.

QUESTIONS/PURPOSES

(1) To determine the shape, size, and location of target zones for combined cup and stem orientation for a straight stem/hemispheric cup THA to maximize the impingement-free ROM and (2) To determine whether and how these implant positions change as stem anteversion, neck-shaft angle, prosthetic head size and target range of movements are varied.

METHODS

A three-dimensional computer-assisted design model, in which design geometry was expressed in terms of parameters, of a straight stem/hemispheric cup hip prosthesis was designed, its design parameters modified systematically, and each prosthesis model was implanted virtually at predefined component orientations. Functional component orientation referencing to body planes was used: cups were abducted from 20° to 70°, and anteverted from -10° to 40°. Stems were rotated from -10° to 40° anteversion, neck-shaft angles varied from 115° to 143°, and head sizes varied from 28 to 40 mm. Hip movements up to the point of prosthetic impingement were tested, including simple flexion/extension, internal/external rotation, ab/adduction, combinations of these, and activities of daily living that were known to trigger dislocation. For each combination of parameters, the impingement-free combined target zone was determined. Maximizing the size of the combined target zone was the optimization criterion.

RESULTS

The combined target zones for impingement-free cup orientation had polygonal boundaries. Their size and position in the diagram changed with stem anteversion, neck-shaft angle, head size, and target ROM. The largest target zones were at neck-shaft angles from 125° to 127°, at stem anteversions from 10° to 20°, and at radiographic cup anteversions between 17° and 25°. Cup anteversion and stem anteversion were inverse-linearly correlated supporting the combined-anteversion concept. The range of impingement-free cup inclinations depended on head size, stem anteversion, and neck-shaft angle. For a 127°-neck-shaft angle, the lowest cup inclinations that fell within the target zone were 42° for the 28-mm and 35° for the 40-mm head. Cup anteversion and combined version depended on neck-shaft angle. For head size 32-mm cup, anteversion was 6° for a 115° neck-shaft angle and 25° for a 135°-neck-shaft angle, and combined version was 15° and 34° respectively.

CONCLUSIONS

The shape, size, and location of the combined target zones were dependent on design and implantation parameters of both components. Changing the prosthesis design or changing implantation parameters also changed the combined target zone. A maximized combined target zone was found. It is mandatory to consider both components to determine the accurate impingement-free prosthetic ROM in THA.

CLINICAL RELEVANCE

This study accurately defines the hypothetical impingement-free, design-specific component orientation in THA. Transforming it into clinical precision may be the case for navigation and/or robotics, but this is speculative, and as of now, unproven.

Increased risk for dislocation after introduction of the Continuum cup system: lessons learnt from a cohort of 1,381 THRs after 1-year follow-up

Background and purpose - The introduction of new total hip replacements (THRs) is known to be associated with an increased risk for complications. On completion of a competitive procurement process, a new uncemented cup system was introduced into general use at our institution in 2016. We launched this study after the introduction to assess (1) the incidence of early dislocations of the old (Pinnacle) and the new (Continuum) cup systems, and (2) whether the cup design would affect the risk for dislocation.Patients and methods - We assessed the incidence of dislocations after 1,381 primary THRs performed at our institution during 2016. Also, the effect of the cup system (Pinnacle, Continuum with neutral liner, Continuum with elevated rim liner) on dislocation rates was analyzed using a multivariable regression model.Results - 47 (3.4%) early dislocations were identified. The incidence of dislocations was 1.3% for the Pinnacle, 5.1% for the Continuum with neutral liner, and 1.2% for the Continuum with elevated rim liner. The Continuum with neutral liner was found to have an increased risk for dislocations compared with the Pinnacle (aOR 5, 95% CI 1.4-17). However, when an elevated rim liner was used with the Continuum, the risk for dislocation between the Continuum and the Pinnacle was similar.Interpretation - Our results emphasize the need for both careful consideration before the introduction of new implants and the systematic monitoring of early outcomes thereafter. The elevated rim liner should be preferred for use with the Continuum cup because of the poor coverage of the neutral liner that may result in dislocations.

Using artificial neural networks to predict impingement and dislocation in total hip arthroplasty

Dislocation after total hip arthroplasty (THA) remains a major issue and an important post-surgical complication. Impingement and subsequent dislocation are influenced by the design (head size) and position (anteversion and abduction angles) of the acetabulum and different movements of the patient, with external extension and internal flexion the most critical movements. The aim of this study is to develop a computational tool based on a three-dimensional (3D) parametric finite element (FE) model and an artificial neural network (ANN) to assist clinicians in identifying the optimal prosthesis design and position of the acetabular cup to reduce the probability of impingement and dislocation. A 3D parametric model of a THA was used. The model parameters were the femoral head size and the acetabulum abduction and anteversion angles. Simulations run with this parametric model were used to train an ANN, which predicts the range of movement (ROM) before impingement and dislocation. This study recreates different configurations and obtains absolute errors lower than 5.5° between the ROM obtained from the FE simulations and the ANN predictions. The ROM is also predicted for patients who had already suffered dislocation after THA, and the computational predictions confirm the patient's dislocations. Summarising, the combination of a 3D parametric FE model of a THA and an ANN is a useful computational tool to predict the ROM allowed for different designs of prosthesis heads.

In vivo kinematics, component alignment and hardware variables influence on the liner-to-neck clearance during chair-rising after total hip arthroplasty

BACKGROUND

There is an interest in quantifying dynamic hip kinematics before and after total hip arthroplasty (THA) during chair-rising: one of daily life activities.

METHODS

The study consisted of 21 patients who underwent unilateral total hip arthroplasty for symptomatic osteoarthritis. We obtained continuous radiographs using a flat-panel X-ray detector while the participants rose from chair. We assessed the pre and postoperative hip joint's movements using three-dimensional-to-two-dimensional model-to-image registration techniques. We also measured minimum liner-to-neck distances at maximum hip flexion and extension as anterior and posterior liner-to-neck distances, respectively. Multivariate analyses were applied to determine which factors were associated with liner-to-neck distances.

RESULTS

The cup inclination, cup anteversion, and stem anteversion averaged 37.4°, 23.1°, and 30.1°, respectively. Significantly larger maximum hip flexion angle (72°) was found during chair-rising after THA compared to that before THA (63°, P < 0.01). The anterior pelvic tilt at the maximum hip flexion after THA (3° of anterior tilt) was significantly (P < 0.05) anterior compared to that before THA (1° of posterior tilt). The anterior and posterior liner-to-neck distances averaged 12.3 mm and 8.1 mm, respectively, with a significant difference (P < 0.01). No liner-to-neck contact was found in any hips. In multivariate analysis, the hip flexion angle, cup inclination, stem anteversion and head diameter were significantly associated with the anterior liner-to-neck distance (P < 0.05), the hip extension angle, cup anteversion, neck length and with or without elevated rim were significantly associated with the posterior liner-to-neck distance (P < 0.05, 0.01, 0.05, 0.01, respectively).

CONCLUSION

This study indicates that well-positioned THA provide increased range of hip flexion with sufficient anterior liner-to-neck clearance during chair-rising. Dynamic hip kinematics, component position, and hardware variables significantly influenced on the liner-to-neck clearance under weight-bearing conditions.

Use of smartphone to improve acetabular component positioning in total hip athroplasty: A comparative clinical study

BACKGROUND

The position of the acetabular cup is important to the outcome of total hip athroplasty (THA). We devised an instrument that uses the level indicator application of smartphone together with a mechanical alignment guide to improve the precision of cup placement. This study aims to determine the percentage of acetabular cups positioned in the Lewinnek safe zone comparing between the conventional technique (using a mechanical alignment guide alone) and the smartphone technique (using a mechanical alignment guide combined with the devised instrument and smartphone).

METHODS

A historical controlled trial was conducted among 82 patients who underwent primary THAs through a posterolateral approach. In the conventional group, 41 cups were placed during January 2013 and December 2014, whereas 41 cups in the smartphone group were placed during January 2015 and March 2016. Inclination and anteversion angles were measured in standardized pelvic radiographs. The cup orientation was compared between groups.

RESULTS

The inclination angle in the smartphone group was significantly lower than in the conventional group (40.9° (SD 3.8) vs. 46.3° (SD 6.7), p < 0.001), but the anteversion angle was higher (19.6° (SD 4.4) vs. 16.5° (SD 6.1), p = 0.010). The smartphone group had more cups positioned in the Lewinnek safe zone (90.2% vs. 56.1%, p = 0.001) and longer operative times (136 (SD 27) vs. 119 (SD 23) min, p = 0.011). No significant difference was found for blood loss ( p== 0.384) or dislocation rate ( p = 0.494).

CONCLUSION

Using the computerized function of smartphone could improve the precision of cup positioning. Most cups were placed within a narrow margin inside the Lewinnek safe zone.

Fully Immersive Virtual Reality for Total Hip Arthroplasty: Objective Measurement of Skills and Transfer of Visuospatial Performance After a Competency-Based Simulation Curriculum

BACKGROUND

Fully immersive virtual reality (VR) uses headsets to situate a surgeon in a virtual operating room to perform open surgical procedures. The aims of this study were to determine (1) if a VR curriculum for training residents to perform anterior approach total hip replacement (AA-THR) was feasible, (2) if VR enabled residents' performance to be measured objectively, and (3) if cognitive and motor skills that were learned with use of VR were transferred to the physical world.

METHODS

The performance of 32 orthopaedic residents (surgical postgraduate years [PGY]-1 through 4) with no prior experience with AA-THR was measured during 5 consecutive VR training and assessment sessions. Outcome measures were related to procedural sequence, efficiency of movement, duration of surgery, and visuospatial precision in acetabular component positioning and femoral neck osteotomy, and were compared with the performance of 4 expert hip surgeons to establish competency-based criteria. Pretraining and post-training assessments on dry bone models were used to assess the transfer of visuospatial skills from VR to the physical world.

RESULTS

Residents progressively developed surgical skills in VR on a learning curve through repeated practice, plateauing, on average, after 4 sessions (4.1 ± 0.6 hours); they reached expert VR levels for 9 of 10 metrics (except femoral osteotomy angle). Procedural errors were reduced by 79%, assistive prompts were reduced by 70%, and procedural duration was reduced by 28%. Dominant and nondominant hand movements were reduced by 35% and 36%, respectively, and head movement was reduced by 44%. Femoral osteotomy was performed more accurately, and acetabular implant orientation improved in VR assessments. In the physical world assessments, experts were more accurate than residents prior to simulation, but were matched by residents after simulation for all of the metrics except femoral osteotomy angle. The residents who performed best in VR were the most accurate in the physical world, while 2 residents were unable to achieve competence despite sustained practice.

CONCLUSIONS

For novice surgeons learning AA-THR skills, fully immersive VR technology can objectively measure progress in the acquisition of surgical skills as measured by procedural sequence, efficiency of movement, and visuospatial accuracy. Skills learned in this environment are transferred to the physical environment.

Does the use of intraoperative fluoroscopy improve postoperative radiographic component positioning and implant size in total hip arthroplasty utilizing a direct anterior approach?

Background

The direct anterior approach (DAA) for total hip arthroplasty (THA) has gained recent popularity, with 1 purported benefit being access to intraoperative fluoroscopy. However, there are limited data demonstrating improved component position with the use of intraoperative fluoroscopy. The aim of this study is to compare radiographic implant positioning on 2 consecutive cohorts of patients undergoing DAA THA performed by 1 surgeon either utilizing intraoperative fluoroscopy or not. We hypothesized that there would be no relevant radiographic differences between the cohorts.

Methods

Forty-two consecutive patients underwent DAA THA utilizing fluoroscopy (IFC), and 42 consecutive patients then underwent DAA THA without fluoroscopy (NFC). Using preoperative pelvis radiographs and 6-week postoperative pelvis radiographs, acetabular anteversion, inclination, femoral offset, and templated component sizes vs final intraoperatively selected sizes were recorded and compared between cohorts.

Results

Acetabular inclination was 45.0° for IFC and 45.6° for NFC (P = .629). Femoral offset difference preoperatively and postoperatively was 0.8 mm for IFC and 1.3 mm for NFC (P = .734). Number of hips within the so-called safe zone was 32 for IFC and 33 for NFC (P = .794). These all demonstrated no significant difference between the cohorts. However, acetabular anteversion was 13.7° for IFC and 11.2° for NFC (P = .02).

Conclusions

In this limited series, the routine use of intraoperative fluoroscopy did not improve implant positioning or sizing. This may be surgeon-specific or due to the result of the use of acetabular landmarks to guide placement of the components without fluoroscopy.

Implications of Spinopelvic Mobility on Total Hip Arthroplasty: Review of Current Literature

Understanding the impact of pathologic spinopelvic mobility on total hip arthroplasty instability requires an appreciation of the dynamic interplay between and the spine, hip and pelvis. This complex interdependent relationship changes with position, pathology and surgical intervention. Spinal pathology may prevent normal dynamic motion leading to spinopelvic stiffness and abnormal pelvic position. Patients at high risk for pathologic spinopelvic motion and subsequent total hip arthroplasty (THA) dislocation should be assessed with a functional imaging series with lateral standing, sitting and AP standing radiographs. Common patterns of stiffness and imbalance as well as proposed surgical treatment algorithms are presented and discussed in this review.

It is safe to use minimal restrictions following posterior approach total hip arthroplasty: results from a large cohort study

BACKGROUND

To prevent early postoperative dislocation following a total hip arthroplasty (THA) procedure, patients must adhere to restrictions. Restrictive protocols are common if THA surgery is performed using the posterior approach, but scientific evidence form larger studies that supports these restrictions are scarce. In this large cohort study we compare the <90-days dislocation rate between patients receiving posterior approach THA managed with minimal versus extensive restrictions.

METHODS

Prospective cohort (n = 1049) of consecutive elective primary hip replacement surgery procedures (September 2014-July 2017) managed with minimal postoperative restrictions. Hospital charts were prospectively reviewed for patient demographics, risk factors and any hip dislocation. Control (n = 1102) consecutive primary elective THAs (January 2011-August 2014) managed with a traditional restrictive protocol. A posterior surgical approach was used in all procedures.

RESULTS

Minimal restrictions group: 17 dislocations <90 days (1.6%); Restricted group: 28 (2.5%), chi-square p = 0.1. Testing the hypothesis of inferiority by a minimum of 1% increase in ⩽90 days dislocation risk: p = 0.14 (test for difference) and p < 0.001 (non-inferiority test), allowing us to discard the null hypothesis (absolute increase in risk of ⩾1% with minimal restrictions). The proportion of surgeries performed with a femoral head size ⩾32 mm was higher in the minimal restrictions group.

CONCLUSIONS

Patients can be managed safely with minimal restrictions following posterior approach THA if combined with frequent use of larger femoral heads.

In vivo kinematic analysis of replaced hip during stationary cycling and computer simulation of optimal cup positioning against prosthetic impingement

BACKGROUND

Dynamic hip kinematics during stationary cycling after total hip arthroplasty (THA) have been unclear. Furthermore, no computer simulation of optimal cup position based on in vivo kinematics has yet been assessed.

METHODS

This study consisted of 7 patients who underwent unilateral primary THA for symptomatic osteoarthritis. Using a flat-panel X-ray detector, continuous radiographs were obtained during stationary cycling. We analyzed the three-dimensional replaced hip kinematics from the top to the bottom position of the crank using image-matching techniques and quantified minimum liner-to-stem neck distance. Simulation analyses with in vivo kinematics were performed to examine patient-specific optimal cup placement against prosthetic impingement.

FINDINGS

During stationary cycling, pelvis showed 27.1° of posterior tilt on average without significant change. Hip flexed by 59.4° and 19.3° on average at the top and bottom positions, respectively. Minimum liner-to-neck distance was 8.8 mm on average at the bottom position. Liner-to-neck, bone-to-bone, or bone-to-component impingement was not observed at any positions in any hips. Simulation analysis of cup placement showed that larger cup anteversion, inclination, and use of elevated liner significantly decreased the minimum distance between posterior liner and stem neck. Cup anteversion of more than 30° with elevated liner could cause posterior liner-to-neck impingement at bottom position.

INTERPRETATION

Stationary cycling after THA provides no excessive hip range of motion or liner-to-neck contact. Cup placement and use of elevated liner significantly influence the minimum liner-to-neck distance, in some cases simulating posterior prosthetic impingement.

Spinopelvic Motion and Impingement in Total Hip Arthroplasty

The stability of a total hip arthroplasty relies on proper positioning of the acetabular cup. Recent research has shown that this cup position is more dynamic than previously thought. The 3-dimensional orientation of the acetabular cup changes when the pelvis tilts anteriorly or posteriorly. These changes in pelvic tilt are directly related to the biomechanics of the lumbosacral junction. In normal physiology, the lumbar spine straightens with sitting and becomes more lordotic with standing. This directly translates to posterior or anterior pelvic tilt due to the rigid sacroiliac attachments. During sitting, increased posterior pelvic tilt opens the acetabulum to accommodate flexion and internal rotation of the hip. This helps prevent anterior impingement and posterior hip dislocation. During standing, anterior pelvic tilt increases superior coverage of the acetabulum. This helps prevent posterior impingement and anterior hip dislocations. When lumbosacral motion becomes pathologic, spinopelvic motion changes and acetabular cup orientation is affected. In cases of decreased lumbosacral motion, patients rely on greater hip motion to reach standing or sitting positions. This can cause pathologic impingement. In addition, traditional safe zones for cup position may not apply in the presence of pathologic spinopelvic motion. This article discusses the normal physiology of spinopelvic motion, the patterns of pathologic change, and the clinical implications therein.

A Geometric Model to Determine Patient-Specific Cup Anteversion Based on Pelvic Motion in Total Hip Arthroplasty

Introduction

Cup position is critical to stability in total hip arthroplasty and is affected by pelvis motion during positions of daily life. The purpose of this study was to explicitly define the relationship between sagittal pelvic motion and resultant cup functional anteversion and create a tool to guide the surgeon to a patient-specific intra-operative anteversion.

Materials and Methods

10,560 combinations of inclination, anteversion, and pelvic tilt were generated using a geometric model. Resultant functional anteversion was calculated for each iteration and variables were correlated. An electronic mobile tool was created that compares inputted patient-specific values to population-based averages to determine pelvic positions and dynamics that may lead to instability.

Results

A third-degree polynomial equation was used to describe the relationship between variables. The freely downloadable mobile tool uses input from pre-operative plain radiographic measurements to provide the surgeon a quantitative correction to intra-operative cup anteversion based on differences in functional anteversion compared to population-based averages.

Conclusion

This study provides a geometric relationship between planned cup position, pelvic position and motion, and the resultant functional anteversion. This mathematical model was applied to an electronic tool that seeks to determine an individualized intra-operative cup anteversion based on measured patient-specific pelvic dynamics.

Functional Safe Zone Is Superior to the Lewinnek Safe Zone for Total Hip Arthroplasty: Why the Lewinnek Safe Zone Is Not Always Predictive of Stability

BACKGROUND

The Lewinnek "safe zone" is not always predictive of stability after total hip arthroplasty (THA). Recent studies have focused on functional hip motion as observed on lateral spine-pelvis-hip x-rays. The purpose of this study was to assess the correlation between the Lewinnek safe zone and the functional safe zone based on hip and pelvic motion in the sagittal plane.

METHODS

Three hundred twenty hips (291 patients) underwent primary THA using computer navigation. Two hundred ninety-six of these hips (92.5%) were within the Lewinnek safe zone as determined by inclination of 40° ± 10° and anteversion of 15° ± 10°. All patients had preoperative and postoperative standing and sitting lateral spinopelvic x-rays. The combined sagittal index (CSI), a combination of sagittal acetabular and femoral position, was measured for each patient and used to assess the functional safe zone. Data analysis was performed to identify hips in the Lewinnek safe zone inside and outside the sagittal functional safe zone. Predictive factors for hips outside the functional safe zone were identified.

RESULTS

Of the 296 hips within the Lewinnek safe zone, 254 (85.8%) were also in the functional safe zone. Forty-two patients were outside the functional safe zone based on CSI; 19 had an increased standing CSI and 23 had a decreased sitting CSI, all were considered at risk for dislocation. Predictive factors for falling outside the functional safe zone were increased femoral mobility (P < .001, r = 0.632), decreased spinopelvic mobility (P < .001, r = 0.455), and pelvic incidence (P < .001, r = 0.400).

CONCLUSION

In this study, 14.2% of hips within the Lewinnek safe zone were outside the functional safe zone, identifying a potential reason hips dislocate despite having "normal" cup angles. The best predictor for falling outside the functional safe zone, both preoperatively and postoperatively, was femoral mobility, not the sagittal cup position (ie, cup anteinclination).

LEVEL OF EVIDENCE

Level III, retrospective review.

In vivo sliding distance on the metal-on-polyethylene total hip arthroplasty articulation using patient-specific gait analysis

Metal-on-polyethylene (MoP) is the most commonly used bearing surface in primary total hip arthroplasty (THA). Polyethylene wear debris remains a major concern. Studies investigating the wear performance based on patient-specific in vivo kinematics and component orientation remains largely lacking. The primary goal of this study was to identify patterns of the distribution of sliding distance and cross-shear ratio among THA patients. A validated approach combining dual fluoroscopic imaging system and computed-tomography was utilized to quantify in vivo gait kinematics and component orientations in 48 total hips. The distribution of accumulated sliding distance and cross-shear ratio over the polyethylene bearing surface was calculated and analyzed using principal component analysis (PCA). Strong patient-specific variation in sliding distance and cross-shear ratio was observed. PCA detected two principal components (PCs) of the sliding distance that together contribute to 94.8% of the total variation. PCA detected four PCs that together contribute to 86% of the total variation of the cross-shear ratio. Regression analysis identified a positive association between cross-shear magnitude and axial and frontal range of motion (RoM). Increased cup inclination, stem anteversion, and reduced cup anteversion may lead to superiorly distributed high cross-shear region, potentially accelerating wear. Our study investigated, in vivo sliding distance and cross-shear pattern using a comprehensive patient-specific dataset and detected several wear indicators under in vivo conditions. These findings provided useful reference values that may help to assess wear in MoP THA patients. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3151-3160, 2018.