The Effect of a Degenerative Spine and Adverse Pelvic Mobility on Prosthetic Impingement in Patients Undergoing Total Hip Arthroplasty

Variations in pelvic tilt between relaxed-seated and flexed-seated positions affect stability assessment in 3D modelling in total hip replacement

BACKGROUND

The spinopelvic axis is becoming recognized as an essential contributor to impingement and instability leading to dislocation. Computer-assisted hip surgery uses standing and relaxed-seated radiographs as a surrogate marker of pelvic tilt in all seated positions. However, the flexed-seated position is a high-risk position for dislocation, and the standing and relaxed-seated radiographs may not reflect this risk. This study aims to determine whether adding a flexed-seated radiograph affects stability assessment in 3D modelling of THR.

METHODS

Ninety patients with osteoarthritis underwent computer-assisted THR and received standing, relaxed-seated, and flexed-seated radiographs. Sacral slope (SS) was measured and analysed using Pearson correlation. Key measures were degree of tilt between positions, as well as correlations between dynamic hip movements.

RESULTS

Of the examined patients, 96.7% anteriorly tilted their pelvis moving from relaxed-seated to flexed-seated, and 50% of patients anteriorly tilted by >10° SS. There was a moderate correlation between standing SS and flexed-seated SS (r = 0.33, P ≤ 0.1). There was a strong correlation between relaxed-seated SS and flexed-seated SS (r = 0.77, P ≤ 0.001); however, there was a wide variance of flexed-seated SS for any given relaxed-seated or standing SS.

CONCLUSION

The flexed-seated position poses a higher risk of anterior impingement in 96.7% of patients compared to the relaxed seated-position. The flexed-seated position cannot be predicted by existing radiographs, making it a valuable marker in surgical planning to mitigate the risk of hip instability.

Adverse spinopelvic mobility in patients undergoing total hip arthroplasty is associated with high mobility of the hip in a flexed seated position

PURPOSE

Adverse spinopelvic mobility from a standing to a flexed seated position of more than 20° of the spinopelvic tilt (∆SPT) has been shown to have a high risk of dislocation. If hypermobility of the hip analysed with the pelvic femoral angle (∆PFA) has a high risk of impingement, the correlation between the range of motion of the hip from a standing to a flexed seated position and its implication in adverse spinopelvic mobility has not been described.

METHODS

A series of 337 patients treated with primary THA underwent lateral x-ray in standing and flexed seated positions to analyse ∆SPT, ∆PFA and spinopelvic parameters. The objectives were to establish a ∆PFA threshold associated with a ∆SPT ⩾20° and to subsequently investigate its influence in conjunction with spinopelvic risk factors on the occurrence of adverse spinopelvic mobility.

RESULTS

The area under the curve was 0.904 (95%CI, 0.864-0.945) for ∆PFA to predict ∆SPT ⩾ 20°; it was predicted by ∆PFA ⩾ 95° with a sensitivity of 91.7% and a specificity of 74.4% at the Youden optimal threshold. Patients with a ∆SPT < 20° (277 patients) had a mean ∆PFA of 83° compared to 110° if ∆SPT ⩾ 20° (60 patients) (p < 0.001). Patients with a ∆PFA < 95° (203 patients) had a mean ∆SPT of -6° compared to 18° if ∆PFA ⩾ 95° (134 patients) (p < 0.001). ∆PFA ⩾ 95° rates were 95% (57/60) and 27.8% (77/200) in patients with ∆SPT ⩾ 20° and ∆SPT < 20°, respectively (OR 49.35; CI, 15.01-162.28; p < 0.001).

CONCLUSIONS

High mobility of the hip (∆PFA ⩾ 95°) seems to be a necessary condition for adverse spinopelvic mobility. A preoperative analysis of patients with lower hip mobility, associated with spinopelvic risk factors, might identify patients with abnormal spinopelvic mobility after the restoration of femoral flexion.

TRIAL REGISTRATION

IDRCB 2023-A01390, CNIL MR004 2225508 (07/06/2023), retrospectively registered.

Can we identify abnormal pelvic tilt using pre-THA anteroposterior pelvic radiographs?

BACKGROUND

Patients with increased pelvic tilt (PT) are at risk for instability following total hip arthroplasty (THA). Identification of increased PT using anteroposterior (AP) pelvic radiographs could avoid additional spinopelvic radiographs. This study aimed to (1) describe which AP pelvic parameters most accurately estimate sagittal PT, and (2) determine thresholds for these parameters that can identify patients with increased PT.

METHODS

This was a retrospective, consecutive, cohort study in a tertiary referral hospital on 225 patients (age: 66 ± 12 years-old; 52% female) listed for THA. Patients underwent pre-operative standing AP pelvic radiographs to measure distance- and angular- based parameters from several anatomical landmarks. Sagittal PT was measured on a standing lateral spinopelvic radiograph and considered high when ≥ 20°.

RESULTS

No AP pelvic parameters correlated strongly with sagittal PT. Ratio between horizontal and vertical diameter of the pelvic foramen (C/D ratio) (rho - 0.341; p < 0.001); and vertical distance between trans-SIJ and trans-ASIS line (SITA) (rho 0.307; p < 0.001) correlated moderately with sagittal PT. Sacro-femoral-pubic (SFP) angle < 60° had highest sensitivity (85%), but lowest specificity (52%) to differentiate between patients with and without increased PT. If SITA > 62 mm, C/D ratio < 0.5 and SFP < 60°, specificity increased (88%), but sensitivity was low (49%).

CONCLUSION

In the absence of computerized models, AP pelvic parameters cannot accurately predict sagittal PT. However, an SFP < 60° should alert a hip surgeon that a patient may have an increased PT, and would benefit from additional lateral spinopelvic imaging prior to THA.

LEVEL OF EVIDENCE

Level II, diagnostic study.

Archetype analysis of the spine-hip relationship identifies distinct spinopelvic profiles

INTRODUCTION

The position of the pelvis in the sagittal plane can vary considerably between different functional positions. Adapting the position of the acetabular cup in relation to the alignment between the spine and the hip of each individual, prior to prosthesis placement, can prevent the risk of prosthetic impingement. Taken individually, risk factors for unfavorable spinopelvic kinematics can be difficult to interpret when trying to precisely predict which patients are at risk. Furthermore, the use of classifications or algorithms can be complex, most often associated with limited values and often difficult to apply in current practices of risk assessment.

HYPOTHESIS

We hypothesized that the deconstruction of the data matrix including age and spinopelvic parameters (SPT, LL, PI, LF and PI-LL) correlated with the analysis of spinopelvic kinematics could be used to define an individualized hip-spine relationship.

MATERIAL AND METHOD

We applied archetypal analysis, which is a probabilistic, data-driven and unsupervised approach, to a complete phenotype cohort of 330 patients before total hip arthroplasty to define the spinopelvic profile of each individual using the spinopelvic parameters without threshold value. For each archetype, we analyzed the spinopelvic kinematics, not implemented in the creation of the archetypes.

RESULTS

An unsupervised learning method revealed seven archetypes with distinct spinopelvic kinematic profiles ranging from -8.9 ° to 13.15 ° (p = 0.0001) from standing to sitting and -5. 35 ° to -10.81 ° (p = 0.0001) from supine to standing. Archetype 1 represents the "ideal" patient (A1); young patients without spinopelvic anomaly and the least at risk of mobility anomaly. Followed by 3 archetypes without sagittal imbalance according to their lumbar lordosis and pelvic incidence, from the highest to the lowest (archetypes 2-4), archetype 4 exposing a greater risk of spinopelvic kinematic anomaly compared to others. Then 2 archetypes with sagittal imbalance: archetype 5, with an immobile pelvis in the horizontal plane from standing to sitting position in anterior tilt and archetype A6, with significant posterior pelvic tilt standing, likely compensating for the imbalance and associated with the greatest anomaly of spinopelvic kinematics. Finally, archetype 7 with the stiffest lumbar spine without sagittal imbalance and significant unfavorable kinematics from standing to sitting.

CONCLUSION

An archetypal approach to patients before hip replacement can refine diagnostic and prognostic features associated with the hip-spine relationship and reduced heterogeneity, thereby improving spinopelvic characterization. This risk stratification of spinopelvic kinematic abnormalities could make it possible to target patients who require adapted positioning or types of implants before prosthetic surgery.

LEVEL OF EVIDENCE

IV retrospective study.

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

INTRODUCTION

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

HYPOTHESIS

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

LEVEL OF EVIDENCE

IV; retrospective study.

Ceramic-on-Ceramic Total Hip Arthroplasty Using a Double Tapered, Proximally Coated Stem: 15 to 24-year Clinical and Radiologic Follow-Up

BACKGROUND

Total hip arthroplasty implant choice profoundly affects survivorship, complications, and failure modes. This study evaluates the long-term (average 18 year) outcomes of ceramic-on-ceramic hip arthroplasty using uncemented shells and stems. Despite an impressive 20-year cumulative percent revision of 5.9%, the hydroxyapatite proximally coated femoral components evaluated in this study have seen declining use since 2003.

METHODS

A review of 349 consecutive total hip arthroplasties from 1999 to 2007 was matched to 272 cases with registry data. A survivorship analysis included 274 hips (Group A) after excluding patients lost to follow-up and navigated cases. Group B comprised 135 patients who had complete datasets spanning a minimum of 15 years.

RESULTS

Kaplan-Meier analysis identified a 95.6% survivorship plateau at 16 to 24 years, with no significant impact from age, sex, component size, or original pathology. In Group B, EuroQol-5 Dimensions-5 Levels (EQ5D5L) scores indicated favorable outcomes in mobility, self-care, activities, pain/discomfort, and anxiety/depression, with an EQ5D visual analog score mean of 79.24. Functional scores, including the Harris Hip Score, Oxford Hip Score, and Forgotten Joint Score, showed positive outcomes. Radiologic assessments revealed no osteolysis or loose components, with a mean Engh score of 21.69. Dorr classification identified bone quality variations. Better Engh scores corresponded to higher levels of patient satisfaction. Age at surgery was correlated with better functional scores, while sex influenced various outcomes.

CONCLUSIONS

This comprehensive study, spanning an average of 18.23 years, combined multiple patient-reported outcome measures with extensive clinical and radiologic follow-up. It reported a notably high survivorship rate for this implant combination but highlighted the declining use of the hydroxyapatite proximally coated femoral stem used in this study, potentially facing withdrawal risks in Australia.

LEVEL OF EVIDENCE

Therapeutic Level IV.

Hip-spine parameters change with increasing age

Aims

Spinopelvic pathology increases the risk for instability following total hip arthroplasty (THA), yet few studies have evaluated how pathology varies with age or sex. The aims of this study were: 1) to report differences in spinopelvic parameters with advancing age and between the sexes; and 2) to determine variation in the prevalence of THA instability risk factors with advancing age.

Methods

A multicentre database with preoperative imaging for 15,830 THA patients was reviewed. Spinopelvic parameter measurements were made by experienced engineers, including anterior pelvic plane tilt (APPT), spinopelvic tilt (SPT), sacral slope (SS), lumbar lordosis (LL), and pelvic incidence (PI). Lumbar flexion (LF), sagittal spinal deformity, and hip user index (HUI) were calculated using parameter measurements.

Results

With advancing age, patients demonstrate increased posterior APPT, decreased standing LL, decreased LF, higher pelvic incidence minus lumbar lordosis (PI-LL) mismatch, higher prevalence of abnormal spinopelvic mobility, and higher HUI percentage. With each decade, APPT progressed posteriorly 2.1°, LF declined 6.0°, PI-LL mismatch increased 2.9°, and spinopelvic mobility increased 3.8°. Significant differences were found between the sexes for APPT, SPT, SS, LL, and LF, but were not felt to be clinically relevant.

Conclusion

With advancing age, spinopelvic biomechanics demonstrate decreased spinal mobility and increased pelvic/hip mobility. Surgeons should consider the higher prevalence of instability risk factors in elderly patients and anticipate changes evolving in spinopelvic biomechanics for young patients.

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

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

How to Screen for Lumbar Spine Stiffness in Patients Awaiting Total Hip Arthroplasty

BACKGROUND

This study aimed to (1) define the prevalence of spinopelvic abnormalities among patients who have hip osteoarthritis (OA) and controls (asymptomatic volunteers) and (2) identify factors that reliably predict the presence of lumbar spine stiffness.

METHODS

This is a prospective, cross-sectional, case-cohort study of patients who have end-stage primary hip OA, who underwent primary total hip arthroplasty (THA). Patients were compared with a cohort of asymptomatic volunteers, matched for age, sex, and body mass index (BMI), serving as a control group. Spinopelvic pathologies were defined as: lumbar spine flatback deformity (difference of 10 or more degrees for pelvic incidence minus lumbar lordosis angle), a standing sagittal pelvic tilt of 19° or more and lumbar spine stiffness (lumbar flexion of less than 20° between both postures).

RESULTS

The prevalence of spinopelvic pathologies was similar between patients and controls (flatback deformity: 16% versus 10%, P = .209; standing pelvic tilt >19°: 17% versus 24%, P = .218; lumbar spine stiffness: 6% versus 5%, P = .827). Age over 65 years-old and standing lumbar lordosis angle less than 45° were associated with high sensitivity and specificity for identifying lumbar spine stiffness (age >65 years: 82% and 66%; standing lumbar lordosis angle <45°: 85% and 73%).

CONCLUSION

The presence of end-stage hip osteoarthritis was not associated with increased prevalence of adverse spinopelvic characteristics compared to matched, asymptomatic volunteers. Age and LLstanding are the strongest predictors of lumbar spine flexion and can guide clinical practice on when to obtain additional radiographs for patients who have hip OA before arthroplasty to identify at-risk patients.

LEVEL OF EVIDENCE

II (prospective, cohort study).

Acetabular cup positioning in primary routine total hip arthroplasty-a review of current concepts and technologies

INTRODUCTION

Total hip arthroplasty (THA) has revolutionized the treatment of hip joint arthritis. With the increased popularity and success of the procedure, research has focused on improving implant survival and reducing surgical complications. Optimal component orientation has been a constant focus with various philosophies proposed. Regardless of the philosophy, achieving an accurate acetabular position for each clinical scenario is crucial. In this paper, we review the recent developments in improving the accuracy and ideal positioning of the acetabular cup in routine primary THA.

METHODOLOGY

A review of the recent scientific literature for acetabular cup placement in primary THA was performed, with available evidence for safe zones, spinopelvic relationship, preoperative planning, patient-specific instrumentation, navigation THA and robotic THA.

CONCLUSION

Though the applicability of Lewinnek safe zones has been questioned with an improved understanding of spinopelvic relationships, its role remains in positioning the acetabular cup in a patient with normal spinopelvic alignment and mobility. Evaluation of spinopelvic relationships and accordingly adjusting acetabular anteversion and inclination can significantly reduce the incidence of dislocation in patients with a rigid spine. In using preoperative radiography, the acetabular inclination, anteversion and intraoperative pelvic position should be evaluated. With improving technology and the advent of artificial intelligence, superior and more accurate preoperative planning is possible. Patient-specific instrumentation, navigated and robotic THA have been reported to improve accuracy in acetabular cup positioning as decided preoperatively but any significant clinical advantage over conventional THA is yet to be elucidated.

Variation in Functional Pelvic Tilt in Female Patients Undergoing Total Hip Arthroplasty With Acetabular Dysplasia

BACKGROUND

As the pelvis is a dynamic structure, the quantification of pelvic tilt (PT) should be done in different hip positions prior to total hip arthroplasty (THA). We sought to investigate functional PT in young female patients undergoing THA and explore the correlation of PT with the extent of acetabular dysplasia. Additionally, we aimed to define the PS-SI (pubic symphysis-sacroiliac joint) index as a PT quantifier on AP pelvis X-ray.

METHODS

Pre-THA female patients under the age of 50 years (n = 678) were investigated. Functional PT in 3 positions (supine, standing, and sitting) were measured. Hip parameters including lateral center-edge angle (LCEA), Tönnis angle, head extrusion index (HEI), and femoro-epiphyseal acetabular roof (FEAR) index were correlated to PT values. The PS-SI/SI-SH (sacroiliac joint-sacral height) ratio was also correlated to PT.

RESULTS

From the 678 patients, 80% were classified as having acetabular dysplasia. Among these patients, 50.6% were bilaterally dysplastic. The mean functional PT of the entire patient group was 7.4°, 4.1°, and -1.3° in the supine, standing and seated positions. The mean functional PT of the dysplastic group was 7.4°, 4.0°, and -1.2° in the supine, standing and seated positions. The PS-SI/SI-SH ratio was found to be correlated to PT.

CONCLUSION

Most of the pre-THA patients had acetabular dysplasia and exhibited anterior PT in the supine and standing positions, most pronounced in the standing position. PT values were comparable between the dysplastic and non-dysplastic group without change with worsening dysplasia. PS-SI/SI-SH ratio can be used to easily characterize PT.

Low pelvic incidence with low lordosis and distal apex of lumbar lordosis associated with higher rates of abnormal spinopelvic mobility in patients undergoing THA

Aims

The risk factors for abnormal spinopelvic mobility (SPM), defined as an anterior rotation of the spinopelvic tilt (∆SPT) ≥ 20° in a flexed-seated position, have been described. The implication of pelvic incidence (PI) is unclear, and the concept of lumbar lordosis (LL) based on anatomical limits may be erroneous. The distribution of LL, including a unusual shape in patients with a high lordosis, a low pelvic incidence, and an anteverted pelvis seems more relevant.

Methods

The clinical data of 311 consecutive patients who underwent total hip arthroplasty was retrospectively analyzed. We analyzed the different types of lumbar shapes that can present in patients to identify their potential associations with abnormal pelvic mobility, and we analyzed the potential risk factors associated with a ∆SPT ≥ 20° in the overall population.

Results

ΔSPT ≥ 20° rates were 28.3%, 11.8%, and 14.3% for patients whose spine shape was low PI/low lordosis (group 1), low PI anteverted (group 2), and high PI/high lordosis (group 3), respectively (p = 0.034). There was no association between ΔSPT ≥ 20° and PI ≤ 41° (odds ratio (OR) 2.01 (95% confidence interval (CI)0.88 to 4.62), p = 0.136). In the multivariate analysis, the following independent predictors of ΔSPT ≥ 20° were identified: SPT ≤ -10° (OR 3.49 (95% CI 1.59 to 7.66), p = 0.002), IP-LL ≥ 20 (OR 4.38 (95% CI 1.16 to 16.48), p = 0.029), and group 1 (OR 2.47 (95% CI 1.19; to 5.09), p = 0.0148).

Conclusion

If the PI value alone is not indicative of SPM, patients with a low PI, low lordosis and a lumbar apex at L4-L5 or below will have higher rates of abnormal SPM than patients with a low PI anteverted and high lordosis.

Creating Consensus in the Definition of Spinopelvic Mobility

INTRODUCTION

The term "spinopelvic mobility" is most often applied to motion within the spinopelvic segment. It has also been used to describe changes in pelvic tilt between various functional positions, which is influenced by motion at the hip, knee, ankle and spinopelvic segment. In the interest of establishing a consistent language for spinopelvic mobility, we sought to clarify and simplify its definition to create consensus, improve communication, and increase consistency with research into the hip-spine relationship.

METHODS

A literature search was performed using the Medline (PubMed) library to identify all existing articles pertaining to spinopelvic mobility. We reported on the varying definitions of spinopelvic mobility including how different radiographic imaging techniques are used to define mobility.

RESULTS

The search term "spinopelvic mobility" returned a total of 72 articles. The frequency and context for the varying definitions of mobility were reported. 41 papers used standing and upright relaxed-seated radiographs without the use of extreme positioning, and 17 papers discussed the use of extreme positioning to define spinopelvic mobility.

DISCUSSION

Our review suggests that the definitions of spinopelvic mobility is not consistent in the majority of published literature. We suggest descriptions of spinopelvic mobility independently consider spinal motion, hip motion, and pelvic position, while recognizing and describing their interdependence.

Does Individualization of Cup Position Affect Prosthetic or Bone Impingement Following THA?

INTRODUCTION

Spinopelvic (SP) mobility patterns during postural changes affect three-dimensional acetabular component position, the incidence of prosthetic impingement, and total hip arthroplasty (THA) instability. Surgeons have commonly placed the acetabular component in a similar "safe zone" for most patients. Our purpose was to determine the incidence of bone and prosthetic impingement with various cup orientations and determine if a preoperative SP analysis with individualized cup orientation lessens impingement.

METHODS

A preoperative SP evaluation of 78 THA subjects was performed. Data was analyzed using a software program to determine the prevalence of prosthetic and bone impingement with a patient individualized cup orientation versus six commonly selected cup orientations. Impingement was correlated with known SP risk factors for dislocation.

RESULTS

Prosthetic impingement was least with the individualized choice of cup position (9%) vs. preselected cup positions (18 to 61%). The presence of bone impingement (33%) was similar in all groups and not affected by cup position. Factors associated with impingement in flexion were age, lumbar flexion, pelvic tilt (stand to flexed seated), and functional femoral stem anteversion. Risk factors in extension included standing pelvic tilt, standing SP tilt, lumbar flexion, pelvic rotation (supine to stand and stand to flexed seated), and functional femoral stem anteversion.

CONCLUSION

Prosthetic impingement is reduced with individualized cup positioning based on SP mobility patterns. Bone impingement occurred in one-third of patients and is a noteworthy consideration in preoperative THA planning. Known SP risk factors for THA instability correlated with the It dependspresence of prosthetic impingement in both flexion and extension.

Three-Dimensional Functional Impingement in Total Hip Arthroplasty: A Biomechanical Analysis

BACKGROUND

Although component offset can affect impingement after total hip arthroplasty, the exact impact is unclear. Evaluation of offset on an anterior-posterior pelvic radiograph is different than evaluation in functional positions of impingement, namely flexion/internal rotation and extension/external rotation. We quantified the effect of acetabular (cup/liner) vs femoral (head/stem) offsets on changes in range of motion to extra-prosthetic impingement in these 2 impingement-prone functional positions.

METHODS

We retrospectively identified 16 total hip arthroplasty patients (age 61.5 ± 12.1 years, body mass index 28.3 ± 4.9 kg/m²) with preoperative and postoperative computerized tomography scans. To eliminate metal artifact, femoral and pelvic 3-dimensional models were created using preoperative scans aligned with postoperative scans, and 3-dimensional scanned implant models were used to reproduce clinical implantation. We tested ±5 mm acetabular cup, acetabular liner, femoral stem, and femoral head offsets. Maximum range of motion (ROM) to bone-bone impingement was calculated for internal rotation at 90° flexion and external rotation at 10° extension.

RESULTS

In all cases, increased offset increased ROM to impingement, and vice versa. During internal rotation at 90° flexion, ±5 mm liner offset had the greatest impact on ROM (+9°/-10°), followed by cup (+8°/-9°), head (+5°/-7°), and stem (+3°/-5°) offset. During external rotation at 10° extension, ±5 mm cup offset had the greatest impact on ROM (+10°/-10°), followed by liner (+9°/-9°), head (+7°/-8°), and stem (+4°/-4°) offset. However, no statistically significant differences were found in the changes to ROM in flexion obtained through cup and liner offsets, the changes to ROM in extension obtained through liner and head offsets, and the changes to ROM in extension obtained through increasing stem and head offsets.

CONCLUSION

Increasing offset by any method reduces impingement. Center-of-rotation offset changes via acetabular cup or liner have the greatest impact on extra-prosthetic impingement.