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Purpose: To report the clinical outcomes and reoperation rates of arthroscopic and endoscopic iliopsoas release at short-term follow-up after ipsilateral total hip arthroplasty (THA) at 2 separate medical institutions and to evaluate whether demographic and radiographic parameters are associated with postoperative patient-reported outcomes (PROs). Methods: Patients with iliopsoas tendinitis in the setting of prior THA who underwent arthroscopic iliopsoas fractional lengthening from 1988 to 2023 at 2 academic institutions were reviewed. Patients were included if they had 12 months of follow-up and underwent evaluation of preoperative anterior acetabular component overhang, surgery satisfaction, postoperative subjective hip flexion strength and anterior groin pain improvement, modified Harris Hip Score, Single Assessment Numeric Evaluation score, Tegner activity scale score, visual analog scale (VAS) score, and revision hip arthroplasty. Results: Sixty hips in 58 patients (19 male and 39 female patients) were followed up for a mean of 39.3 months (range, 12.0-105.9 months) postoperatively. Of the patients, 77% reported feeling “much better” or “slightly better,” 75% reported improved anterior groin pain, and 60% reported improved subjective hip flexion strength. The surgery satisfaction rating was 7.2 ± 3.3 (scale of 0 to 10). The mean postoperative modified Harris Hip Score, VAS score for pain at rest, VAS score for pain with use, and Single Assessment Numeric Evaluation score were 73.9 ± 19.4, 1.3 ± 2.4, 3.8 ± 2.9, and 71.9 ± 21.9, respectively. Preoperative anterior acetabular component overhang was 3.3 ± 6.5 mm and did not significantly correlate with postoperative PROs (P ≥ .45). The Tegner score improved from 2.5 ± 1.7 preoperatively to 2.9 ± 1.4 postoperatively (P = .0253). Three patients underwent revision arthroplasty at a mean of 25.3 months (range, 11.6-40.4 months) postoperatively, with an acetabular component revision rate of 3.3%. Conclusions: Satisfactory outcomes and low revision arthroplasty rates were observed in patients undergoing arthroscopic iliopsoas lengthening after THA. There was no statistically significant relation between anterior acetabular component overhang and final PROs. Level of Evidence: Level IV, therapeutic case series.

Purpose: The purpose of this study was to use a smartphone app to collect gait metrics in patients undergoing hip arthroscopy for femoroacetabular impingement syndrome (FAIS) and determine when gait metrics return to preoperative levels and if gait metrics correlate with postoperative hip-specific patient-reported outcomes. Methods: Patients undergoing primary hip arthroscopy for FAIS from August 2021 to December 2022 were prospectively enrolled. Patients downloaded the iPhone app, rHip, at ∼6 months postoperatively, enabling retroactive access to Apple Health data. Gait metrics included step count, step length, and walking speed. Each patient’s preoperative baseline was compared to that individual’s successive postoperative week metrics. All patients underwent a standard 4-stage rehabilitation protocol. We compared metrics from beginning to end of each phase (i.e., week 1 to 6 for phase 1) and evaluated correlations with patient outcomes. Results: Fifty patients (40 women; age: 31.9 ± 14.5 years) were included. Patients had significantly reduced step count from postoperative weeks 1 to 5 and improved upon baseline at weeks 13 and 16 to 26. Patients had significantly reduced step length from postoperative weeks 1 to 4 and improved upon baseline at weeks 16 and 20. Patients had significantly lower walking speed from postoperative weeks 1 to 6 and improved upon baseline at week 20 (P < .05). Step count significantly improved over phase 1 rehabilitation (P < .05). Step length and walking speed significantly improved over each of phases 1 to 3 (P < .05 for all). Preoperatively, weak correlations were noted between step length and Patient-Reported Outcomes Measurement Information System (PROMIS) for Physical Function (PF)/Pain, as well as walking speed and Hip Outcome Score–Activities of Daily Living (HOS-ADL)/PROMIS-Pain. At 3 months, moderate correlations were noted between step count and HOS-ADL/12-item international Hip Outcome Tool (iHOT-12), step length and HOS-ADL/PROMIS-PF, and walking speed and HOS-ADL/iHOT-12/PROMIS-PF/PROMIS-Pain. At 6 months, weak to moderate correlations were noted between all 3 gait metrics and HOS-ADL. Conclusions: A smartphone app was capable of capturing health data gait metrics. Patients undergoing hip arthroscopy for FAIS returned to baseline levels in step count, step length, and walking speed after phase 1 (6 weeks) of rehabilitation. The most consistent correlations between gait metrics and step count were seen at 3 months, although only weak to moderate. Gait metrics similarly had weak to moderate correlations with HOS-ADL at 6 months. Level of Evidence: Level IV, case series.

Purpose: To investigate the type of questions patients undergoing hip arthroscopy for femoroacetabular impingement (FAI) are searching online and determine the type and quality of the online sources from the top results to each query by the “people also ask” Google algorithm. Methods: Three search strings pertaining to FAI were carried out through Google. The webpage information was manually collected from the “People also ask” Google algorithm. Questions were categorized using Rothwell’s classification method. Each website was assessed using Journal of the American Medical Association Benchmark Criteria for source quality. Results: A total of 286 unique questions were collected with their associated webpages. The most common questions included: “How do you treat femoroacetabular impingement and labral tears without surgery?” “What is the recovery process after hip arthroscopy and are there limitations after surgery?” and “How do you diagnose hip impingement and differentiate from other causes of hip pain?” The Rothwell Classification of questions were fact (43.4%), policy (34.3%), and value (20.6%). The most common webpage categories were Medical Practice (30.4%), Academic (25.8%), and Commercial (20.6%). The most common subcategories were Indications/Management (29.7%) and Pain (13.6%). Government websites had the highest average Journal of the American Medical Association score (3.42), whereas Single Surgeon Practice websites had the lowest (1.35). Conclusions: Commonly asked questions on Google regarding FAI and labral tears pertain to the indications and management of pathology as well as pain control and restrictions in activity. The majority of information is provided by medical practice, academic, and commercial sources, which have highly variable academic transparency. Clinical Relevance: By better understanding which questions patients ask online, surgeons can personalize patient education and enhance patient satisfaction and treatment outcomes after hip arthroscopy.

Purpose: To evaluate the influence of spinopelvic parameters on short-term postoperative patient-reported outcomes (PROs) following primary hip arthroscopy for the treatment of femoroacetabular impingement syndrome (FAIS). Methods: Patients undergoing primary hip arthroscopy between January 2012 and December 2015 were retrospectively reviewed. Hip Outcome Score – Activities of Daily Living, Hip Outcome Score – Sports-Specific Subscale, modified Harris Hip Score, International Hip Outcome Tool-12, and visual analog scale pain were recorded preoperatively and at final follow-up. Lumbar lordosis (LL), pelvic tilt (PT), sacral slope, and pelvic incidence (PI) were measured on lateral radiographs in standing position. Patients were split into subgroups for individual analyses based on previous literature cutoffs: |PI-LL|> or or 65°. PROs and rate of achievement of patient acceptable symptom state (PASS) were compared between subgroups at final follow-up. Results: Sixty-one patients who underwent unilateral hip arthroscopy were included in the analysis, and 66% of patients were female. Mean patient age was 37.6 ± 11.3 years, whereas mean body mass index was 25.0 ± 5.7. Mean follow-up time was 27.6 ± 9.0 months. No significant difference in preoperative nor postoperative PROs were appreciated in patients with spinopelvic mismatch (|PI-LL| >10°) versus those without, whereas patients with mismatch achieved PASS according to the modified Harris Hip Score (P = .037) and International Hip Outcome Tool-12 (P = .030) at greater rates. When we compared patients with a PT ≥20° versus PT 65°, no significant differences in 2-year PROs or rates of PASS achievement for any PRO were appreciated (P > .05 for all). Conclusions: In this study, spinopelvic parameters and traditional measures of sagittal imbalance did not influence PROs in patients undergoing primary hip arthroscopy for FAIS. Patients with sagittal imbalance (|PI-LL|> 10° or PT >20°) achieved a greater rate of PASS. Level of Evidence: IV; Prognostic case series.

Purpose: To compare the early postoperative outcomes of patients undergoing inpatient versus outpatient hip arthroscopy and identify any characteristics that may serve as predictors of these complications Methods: The PearlDiver Mariner insurance database was queried for all patients who underwent hip arthroscopy from 2010 to 2019. Patients were matched based on Charlson Comorbidity Index, age, and sex. Outcomes recorded included postoperative complications and return to care within 90 days. Predictors of complications were assessed via multivariable logistic regression controlling for age, sex, Charlson Comorbidity Index, comorbidities, surgical setting, and procedure type Results: The final matched cohort included 832 inpatients and 1,356 matched patients. Fifty-eight patients (7.0%) who underwent inpatient surgery versus 25 patients (1.8%) who underwent outpatient surgery were readmitted (P < .01). Of the readmitted patients, 31 inpatients (3.7%) and 5 outpatients (0.4%) were readmitted for hip-related reasons (P < .01). No significant differences were observed in emergency department visits (67 inpatients [8.1%] vs 84 outpatients [6.2%], P = .11), intensive care unit admissions (3 [0.4%] vs 1 [0.1%], P = .31), or revision hip arthroscopies (43 [5.2%] vs 65 [4.8%], P = .77). A multivariable model of complications correcting for baseline differences in chronic obstructive pulmonary disease, coronary artery disease, diabetes, hypertension, ischemic heart disease, tobacco use, and inpatient status found that age (adjusted odds ratio [OR], 0.92; 95% confidence interval [CI], 0.85-0.99; P = .03), coronary artery disease (adjusted OR, 12.82; 95% CI, 1.18-140.02; P = .03), and inpatient setting (adjusted OR, 20.59; 95% CI, 3.48-401.65; P = .01) were significantly associated with complications. No procedure type was associated with complication rates Conclusions: Compared with the outpatient setting, inpatient hip arthroscopy is associated with higher rates of readmission in a cohort matched for age, sex, and comorbidities. Complications after inpatient hip arthroscopy appear to be related to comorbidities rather than procedure-related factors. The decision to conduct an inpatient hip arthroscopy should prioritize consideration of patient comorbidities over the type of procedure Level of Evidence: Level III, retrospective cohort study.

Purpose: To compare joint distraction measured on ultrasound (US) with joint space width (JSW) measured on fluoroscopy in hip arthroscopy and to determine whether ultrasound guidance is as safe and effective as fluoroscopy, the current gold standard, for establishing arthroscopic portals. Methods: Cadaveric whole-body specimens were positioned supine and subjected to 60 lbs. of unilateral axial traction using a distal femoral Steinman pin. Joint distraction was measured via JSW on fluoroscopic and ultrasound images. A single, fellowship-trained orthopaedic surgeon established anterolateral arthroscopy portals via ultrasound or fluoroscopic guidance in a randomized sequence. Total procedure time, number of times the spinal needle pierced the capsule, and iatrogenic chondral or labral injury were recorded. Results: Twelve full-body specimens (20 hips) underwent distraction, and 17 hips underwent portal placement with fluoroscopic (n = 8) or ultrasound (n = 9) guidance. JSW measured on ultrasound was significantly less laterally (13.0 vs 9.2 mm, P < .001), apically (16.7 vs 9.2 mm, P < .001), and medially (17.9 vs 9.2 mm, P < .001). Successful portal entry was achieved in every specimen. Average procedure time was 133 ± 51 seconds for the fluoroscopy group and 371 ± 260 seconds for the ultrasound group (P = .026). Fluoroscopic guidance required significantly less needle insertion attempts at 1.13 compared with 3.33 attempts for ultrasound (P = .022). Labral damage was greater in the ultrasound group at 66.67% compared with 12.50% for fluoroscopy (P = .0497). Conclusions: Joint distraction measured on ultrasound can be used to subjectively determine if the joint is adequately distracted in hip arthroscopy. Ultrasound-guided portal placement was associated with more needle insertion attempts, iatrogenic injury of the labrum, and overall procedure time in comparison to fluoroscopic guidance. Clinical Relevance: Fluoroscopy is the gold standard to confirm adequate joint distraction, aid in establishing arthroscopy portals, and evaluate resection of the femoral head during hip arthroscopy but exposes the patient to ionizing radiation, requires additional operators in the operating room, and involves the need for a heavy lead shield. Alternatives to fluoroscopy are needed, but ultrasound has not proven superior in our cadaveric model.

Purpose: To evaluate and compare the Patient-Reported Outcomes Measurement Information System (PROMIS) Physical Function (PF) computerized adaptive test and PROMIS Pain Interference (PI) instruments versus legacy patient-reported outcome measures (PROMs) in patients undergoing hip arthroscopy for femoroacetabular impingement syndrome at 1-year follow-up. Methods: Patients undergoing primary hip arthroscopy for the treatment of femoroacetabular impingement syndrome without concomitant procedures performed by a single surgeon between August 2018 and January 2019 were identified. The PROMIS PF score, PROMIS PI score, Hip Outcome Score–Activities of Daily Living Subscale (HOS-ADL), Hip Outcome Score–Sports Subscale (HOS-SS), modified Harris Hip Score (mHHS), International Hip Outcome Tool 12 (iHOT-12), and visual analog scale (VAS) pain score were obtained preoperatively and at 6 months and 1 year postoperatively. Bivariate correlation analyses between PROMIS and legacy PROMs were performed. The floor and ceiling effects, responsiveness, and relative efficiency (RE) of each PROM were calculated. Results: This study included 124 patients with an average age of 32.7 ± 12.3 years. The PROMIS PF score showed very good correlations with the HOS-ADL (r = 0.73, P < .001) and mHHS (r = 0.70, P < .001) and good correlations with the HOS-SS (r = 0.62, P < .001), iHOT-12 score (r = 0.62, P < .001), and VAS pain score (r = –0.64, P < .001). The PROMIS PI score showed very good inverse correlations with the HOS-ADL (r = –0.72, P < .001) and mHHS (r = –0.79, P < .001) and good correlations with the HOS-SS (r = –0.64, P < .001), iHOT-12 score (r = –0.65, P < .001), and VAS pain score (r = 0.65, P < .001). No floor effect was observed for any measure. Ceiling effects were not observed in the PROMIS PROMs but were detected for the HOS-ADL (16.1%) and mHHS (19.3%). The effect size was large for all outcomes: iHOT-12 score, d = 1.77; HOS-ADL, d = 1.37; HOS-SS, d = 1.45; PROMIS PI score, d = 1.05; and PROMIS PF score, d = 1.01. The iHOT-12 score was more responsive than the PROMIS PI score (RE = 1.69), PROMIS PF score (RE = 1.75), HOS-ADL (RE = 1.29), and HOS-SS (RE = 1.22). Conclusions: At 1-year follow-up, PROMIS PROMs displayed very good correlations with legacy PROMs. However, PROMIS PROMs had lower effect sizes and were not as responsive as legacy PROMs. Level of Evidence: Level II, development of diagnostic criteria on the basis of consecutive patients.

Purpose: To determine whether 3-dimensional (3D)–reconstructed proximal femoral bone models can be used to quantify femoral osteochondroplasty and to determine whether the 3D-based metrics are related to clinical alpha angle measures. Methods: Six cadaveric specimens with cam-type morphology underwent open femoral osteochondroplasty. Alpha angles were measured on the oblique axial computed tomography slice before and after femoral osteochondroplasty. Preoperative and postoperative computed tomography–based 3D reconstructed femur models were generated for each cadaveric specimen. A 3D-3D registration technique was used to merge the preoperative and postoperative models to measure the surface-to-surface distance between the model surfaces. Bivariate correlation analyses were used to determine the correlations between the preoperative, and the difference between the preoperative and postoperative alpha angle (Δ alpha angle) measures and each of the femoral osteochondroplasty variables of surface area (mm2), volume (mm3), maximum height (mm), and mean height (mm). The strength of the bivariate correlations was defined as follows: weak 0.1 to 0.3, moderate 0.3 to 0.5, and strong as 0.5 to 1.00. Results: Bivariate correlations revealed a strong positive correlation between preoperative alpha angle with femoral osteochondroplasty volume (r = 0.899, P = .007) and surface area (r = 0.899, P = .007). No significant correlations were found between the change in alpha angle and the osteochondroplasty variables. Conclusions: In this study, pre- and postoperative 3D bone models could be used to quantify femoral osteochondroplasty and to determine if the 3D-based metrics are related to clinical alpha angle measures. Clinical Relevance: 3D-reconstructed image bone models may be helpful to ensure that adequate femoroplasty is performed intraoperatively, in particular with arthroscopic approach in which visualization may be challenging due to capsular management issues and surgeon experience.

Purpose: The purpose of this study is to identify and summarize the current utility of intraoperative ultrasound (US) during hip arthroscopy. Methods: A systematic database query of the Cochrane Database of Systematic Reviews, the Cochrane Central Register of Controlled Trials, EMBASE, PubMed (1980 to 2019), and Ovid MEDLINE (1980 to 2019) was performed. After article identification, descriptions of the surgical procedure, use of intraoperative US, procedural complications, and conclusions from each article were recorded and summarized. Results: Five studies met inclusion criteria, all of which were surgical techniques or technical notes. Four of the 5 studies described US used for placement of arthroscopic portals, and 1 described the use of an intraarticular US catheter for the assessment of an osteochondritis dissecans (OCD) lesion. Of the 4 studies using US for portal placement, 3 were performed supine and 1 was performed in the lateral decubitus position. All studies recognized the need for additional US training or the required assistance of a radiologist to incorporate US into a surgical practice. Conclusion: Descriptions of intraoperative US during hip arthroscopy are limited in the literature. However, existing technique reports demonstrate the feasibility of US for both portal placement with superficial probes and limited evaluation of cartilage using intraarticular US catheters. Level of Evidence: V, systematic review.

Purpose: To determine factors predictive of patients who are at risk for being lost to follow-up after hip arthroscopy for femoroacetabular impingement syndrome (FAIS). Methods: A prospective clinical repository was queried between January 2012 and October 2017 and all patients who underwent hip arthroscopy for primary or revision FAIS with minimum 2-year follow-up were included. A total of 27 potential risk factors for loss to follow-up were available and tested for predictive value. An 80:20 random sample split of all patients was performed to create training and testing sets. Cross-validation, minimum Bayes information criteria, and adaptive machine-learning algorithms were used to develop the predictive model. The model with the best predictive performance was selected based off of the lowest postestimation deviance between the training and testing samples. The c-statistic is a measure of discrimination. It ranges from 0.5 to 1.0, with 1.0 being perfect discrimination and 0.5 indicating the model is no better than chance. A log-likelihood χ2 test was used to evaluate the goodness-of-fit of the logistic regression model. Results: A total of 2113 patients were included. Inference of minimum Bayes information criteria model indicated that male sex (odds ratio [OR] 1.82, P = .028), non-white race (African American OR 2.41, P = .013; other non-white OR 1.42, P = .042), smoking (OR 1.07, P = .021), and failure to provide a phone number (OR 1.78, P = .032) increased the risk for being lost to follow-up. Furthermore, greater preoperative International Hip Outcome Tool 12-item component questionnaire (OR 1.03, P = .004), and modified Harris Hip Score (OR 1.05, P = .014) scores increased the risk of being lost to follow-up. The c-statistic was 0.76 (95% confidence interval 0.701-0.848). The log-likelihood indicated that the regression model as a whole was statistically significant (P = .002). Conclusions: Patients who are male, non-white, smokers, fail to provide a telephone number, and have greater preoperative modified Harris Hip Score and International Hip Outcome Tool 12-item component questionnaire scores are at an increased risk for being lost to follow-up 2 years after hip arthroscopy for FAIS. Level of Evidence: Level III, case control study

Purpose: To (1) investigate trends in kinesiophobia and pain catastrophizing after hip arthroscopy for femoroacetabular impingement syndrome (FAIS), and (2) determine whether kinesiophobia and pain catastrophizing scores are associated with achieving minimal clinically important difference (MCID) for any of the hip-specific patient-reported outcome questionnaires. Methods: Patients undergoing primary hip arthroscopy for treatment of FAIS between December 2016 and March 2017 were prospectively enrolled. Patients received the Tampa Scale of Kinesiophoibia-11 (TSK-11) and Pain Catastrophizing Scale (PCS) questionnaires preoperatively, 6 months, and 1 year postoperatively. They also received the hip-specific patient-reported outcome questionnaires (Hip Outcome Score Activities of Daily Living and Sport-Specific subscales, modified Harris Hip Score, and International Hip Outcome Tool-12), as well as visual analog scale for satisfaction and pain preoperatively and 1-year postoperatively. The threshold for achieving MCID was determined for each hip outcome tool, and patients achieving MCID were compared with those who did not. Results: A total of 85 (80.2%) patients (mean age: 33.7 ± 12.4 years; female: 75.3%) were included in the final analysis. At 1-year follow-up, there was a significant reduction in TSK-11 scores (26.22 ± 5.99 to 18.70 ± 6.49; P < .001) and PCS scores (17.81 ± 10.13 to 4.77 ± 7.57; P < .001) when compared with preoperative scores. 1-year PCS scores were significantly lower in patients achieving MCID compared with patients failing to achieve MCID (3.2 ± 4.4 vs 10.8 ± 15.2; P = .006). There were no significant differences in TSK-11 scores between those achieving and not achieving MCID. Conclusions: Patient kinesiophobia and pain catastrophizing both show significant improvements 1 year after undergoing hip arthroscopy for FAIS. However, pain catastrophizing scores at 1 year are significantly greater in patients not achieving MCID, whereas no association was identified between kinesiophobia and likelihood for MCID achievement. This suggests PCS may be a more useful tool than TSK-11 during postoperative rehabilitation for identifying patients at risk for not achieving MCID. Level of Evidence: Level II, prospective case series

Purpose: To determine the 3-dimensional (3D) in vivo hip translation in patients with symptomatic femoroacetabular impingement syndrome (FAIS) using 3D computed tomography (CT) models with the hip in neutral and FABER (flexion, abduction, and the external rotation) positions and to identify patient predictors associated with the degree of hip translation. Methods: Seventy-eight patients with FAIS and cam lesions underwent CT scans in neutral and FABER positions. Demographics including age, sex, and body mass index (BMI) were recorded for each patient. The cam deformity was characterized both in plain x-ray film and 3D. Translation between both positions was calculated using a validated high-precision 3D-3D registration technique. Univariate and multivariate regression analyses sought factors correlated with translation. Results: The mean age of the patients included in the analysis was 36.3 ± 9.2 years, with 51% of the study group being female. The mean 3D femoral head center translation was 0.84 ± 0.37 mm, decomposed into vectors on standard anatomical directions as 0.13 ± 0.58 mm medial, 0.10 ± 0.54 mm posterior, and 0.08 ± 0.46 mm inferior. Multivariate analysis demonstrated that total translation was associated with larger alpha angles (β = 0.014; 95% confidence interval [CI] 0.003-0.024; P = .013), and greater BMI (β = 0.033; 95% CI 0.001-0.065; P = .042). Furthermore, posterior–inferior translation was associated with BMI (β = 0.032; 95% CI 0.003-0.061; P = .031), whereas medial–lateral translation is associated with the female sex (β = 0.388; 95% CI 0.124-0.634; P = .002), and smaller head radius (β = –0.068; 95% CI –0.128 to –0.007; P = .029). Conclusions: As a provocative maneuver, FABER positioning in patients with FAIS resulted in an average measurable translation of the femoral head center in the posterior, medial, and inferior direction. Factors including sex, BMI, and alpha angle predicted the degree of translation. Clinical Relevance: The current study demonstrates that there is measurable hip translation between the neutral and FABER positions in patients with symptomatic FAIS, which may cause hip microinstability. Furthermore, the study found an association between hip translation and both modifiable and nonmodifiable factors. This may indicate the need for more comprehensive preoperative surgical planning, intraoperative dynamic examination of the hip, and consideration of capsular plication in certain patients.

Purpose: To use mirror imaging to identify the location and magnitude of difference in acetabular rim morphology between the symptomatic and unaffected acetabula in patients with symptomatic unilateral pincer-type or mixed femoroacetabular impingement syndrome (FAIS) using 1-dimensional models created with computed tomography (CT). Methods: CT scans of bilateral hips in 33 patients diagnosed with unilateral pincer-type or mixed FAIS were obtained. Three-dimensional bilateral hip models were constructed, and the unaffected hemipelvis was superimposed onto the symptomatic side to compare acetabular thickness. Protrusion of the symptomatic side was recorded, and rim morphology was divided into clock face quadrants to analyze the location of greatest magnitude of difference between affected and unaffected acetabula. Analysis of the quadrants was performed using analysis of variance with post hoc Bonferroni correction. Results: The study group consisted of more females (51.6%) than males, with an average age of 35.72 ± 7.8 years and an average body mass index of 24.3 ± 4.1 kg/m2. Of the 33 hips included, 14 were isolated pincer-type FAIS and 19 were mixed. The average preoperative symptomatic side lateral center edge angle was 37.5° ± 7.2° compared with 29° ± 5.1° on the asymptomatic side (P = .001). The symptomatic acetabular rim was on average 0.43 ± 0.18 mm thicker than the corresponding location on the unaffected rim. When the acetabulum was divided into clock face quadrants, the 12 to 3 o'clock position showed the greatest difference between symptomatic and unaffected sides (0.55 ± 0.18 mm) compared with the 3 to 6 o'clock position (0.4 ± 0.28 mm; P = .006), 6 to 9 o'clock (0.34 ± 0.07 mm; P < .001), and 9 to 12 o'clock (0.38 ± 0.03; P = .001). Conclusions: Patients with unilateral, symptomatic pincer-type or mixed FAIS show statistical differences in rim thickness between the affected and unaffected acetabula. Small changes in acetabular rim morphology on the order of ≤0.5 mm may be the difference between symptomatic FAIS and the unaffected hip. Level of Evidence: IV, case series.