1 Introduction

Total hip arthroplasty (THA) is an effective treatment for severe hip disorders, particularly in patients with end-stage hip osteoarthritis (OA). This procedure relieves pain and improves functional status and quality of life (QOL).¹ The evaluation of patient outcomes after THA is essential for optimizing treatment strategies. Patient-reported outcome measures (PROM) are commonly used to assess treatment effectiveness.² Among these, the Hip disability and Osteoarthritis Outcome Score (HOOS) is a highly reliable, valid, and responsive tool that is widely applied in clinical settings to evaluate postoperative outcomes.^3–5 By assessing symptoms, pain, activities of daily living (ADL), sport and recreation (SR), and QOL, HOOS is internationally recommended for THA outcome assessment.⁶

In recent years, the patient acceptable symptom state (PASS) has gained attention as an indicator for evaluating the therapeutic effects of THA from the patient's perspective.⁷ This approach dichotomizes PROM results, allowing clinicians to interpret outcomes not only in terms of average improvement but also in relation to whether patients have reached a personally satisfactory state. PASS refers to the threshold score that distinguishes patients who consider their current state acceptable from those who do not.⁷ Previous studies have reported PASS thresholds for individual HOOS subscales; for example, at 1 year post-THA, the PASS thresholds for pain, physical function short form, and QOL have been reported as 91, 88, and 83, respectively.⁸ Furthermore, a study using the HOOS Joint Replacement (JR) reported a PASS threshold of 76.7 at 2 years post-THA.⁹ Additionally, when examined by preoperative diagnosis, the PASS thresholds for HOOS-pain and HOOS-JR in patients with OA were 80.6 and 76.8, respectively, whereas those in patients without OA were 77.5 and 73.5, respectively.¹⁰ These previous studies suggest that PASS scores after THA vary according to patient characteristics, timing of assessment, and the subscales used.

Most previous studies have focused on mid-to long-term outcomes (≥1 year postoperatively), and the PASS threshold during the early recovery phase (including 3–6 months) remains unclear.^8,9 The early postoperative phase represents the most critical period for clinical decision-making, rehabilitation adjustment, and patient counseling. At this stage, clinicians must judge whether a patient's recovery trajectory is acceptable or delayed, often without concrete patient-centered benchmarks. Importantly, PASS thresholds derived from mid- or long-term outcomes—particularly those based on single composite indices, such as HOOS-JR—may not be directly extrapolated to the early postoperative phase because symptom burden, functional priorities, and patient expectations differ fundamentally between early and late recovery.

No studies have established short-term PASS thresholds for each HOOS subscale after THA. This is clinically important because symptoms, pain, ADL, SR, and QOL recover at different speeds and contribute differently to early postoperative satisfaction. Reliance on a single composite score, such as the HOOS-JR, may obscure domain-specific recovery patterns and limit individualized rehabilitation planning. Therefore, the establishment of subscale-specific short-term PASS thresholds provides novel, domain-sensitive benchmarks that cannot be derived from existing mid- or long-term PASS studies.

Moreover, cultural and regional factors may influence patients’ perceptions of acceptable symptom states. In Japan, THA is predominantly performed in older adults who often have lower preoperative activity levels, place greater emphasis on independence in basic ADL, and typically undergo longer rehabilitation-focused inpatient stays than those in Western countries.¹¹ These sociocultural and healthcare system factors may shape expectations for postoperative function and pain relief, potentially affecting HOOS scoring patterns as well as the symptom levels that patients consider acceptable. Establishing an early postoperative PASS threshold would provide an objective reference, enabling clinicians to identify patients whose recovery is delayed relative to patient-centered expectations.

Clarifying short-term PASS thresholds can support personalized postoperative monitoring by identifying patients who may require extended or modified rehabilitation programs. It can also facilitate timely communication and shared decision-making, for example, by helping clinicians explain recovery progress or prognosis to patients who are anxious about their improvement. Unlike the minimal clinically important difference (MCID), which focuses on the magnitude change in symptoms or function, the PASS represents a specific threshold indicating a state that patients consider satisfactory.⁷ It serves as a clinically intuitive criterion for judging the adequacy of recovery rather than merely the presence or size of improvement.

This study aimed to establish PASS thresholds for each HOOS subscale during the early postoperative period (3 and 6 months) after THA. These thresholds offer objective criteria for assessing patients’ perceived recovery during this phase, thereby supporting patient-centered monitoring, individualized rehabilitation planning, and effective patient education in clinical settings.

2 Materials and methods

3 Results

3.1

3.1 Cohort characteristics

Of the 195 patients scheduled for THA, 45 were excluded because of rheumatoid arthritis (n = 7), systemic lupus erythematosus (n = 2), dementia (n = 4), psychiatric disorders (n = 1), femoral head necrosis (n = 24), bilateral THA (n = 2), or revision THA (n = 5). Consequently, 150 patients were included in the analysis (Fig. 1). At 3 months post-THA, 20 of the 150 eligible patients were excluded because of complications (n = 1), contralateral THA (n = 1), or missing HOOS (n = 18), leaving 130 patients for analysis. At 6 months post-THA, 10 patients were excluded from the 130 eligible patients because of contralateral THA (n = 5) or missing HOOS (n = 5), leaving 120 patients for analysis. The demographic and clinical characteristics of the participants are shown in Table 1.

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Fig. 1 Flow diagram. HOOS, Hip disability and Osteoarthritis Outcome Score; THA, total hip arthroplasty.

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Table 1 Baseline patient characteristics.

	THA study cohort
Variables	(n = 150)
Age	69.00 ± 9.13
Female, n (%)	141 (94.00)
Height (cm)	153.38 ± 6.71
Weight (kg)	60.14 ± 12.31
BMI (kg/m2)	25.52 ± 4.76
WHO classification of obesity, n (%)
Underweight	7 (4.67)
Normal weight	73 (48.67)
Pre-obese	49 (32.67)
Obese class I	14 (9.33)
Obese class II	5 (3.33)
Obese class III	2 (1.33)
ASA, n (%)
1	14 (9.33)
2	113 (75.33)
3	22 (14.67)
4	1 (0.67)
CCI, n (%)
0	131 (87.33)
1–2	19 (12.67)
≥3	0 (0)
Surgical approach, n (%)
Anterolateral approach	128 (85.33)
Direct lateral approach	6 (4.00)
Posterolateral approach	16 (10.67)
Fixation method, n (%)
Cementless	146 (97.33)
Cemented	4 (2.67)
Femoral stem length, n (%)
Short	110 (73.33)
Standard	40 (26.67)
Bearing surface, n (%)
Ceramic-on-polyethylene	146 (97.33)
Metal-on-polyethylene	4 (2.67)

4 Discussion

This study identified the PASS thresholds for each HOOS subscale at 3 and 6 months after THA in patients with hip OA. At 3 months post-THA, the PASS thresholds calculated using the percentile method were 60.00 for symptoms, 70.00 for pain, and 60.29 for ADL, whereas SR and QOL showed lower values of 37.50. At 6 months post-THA, the PASS thresholds for symptoms, pain, ADL, and QOL each increased compared with those at 3 months post-THA, whereas the SR remained at 37.50. Furthermore, in the ROC analysis evaluating discriminatory ability, the AUC for HOOS pain at 3 months post-THA was exceptionally high at 0.96, while symptoms, ADL, and QOL also showed favorable values at 0.84. By contrast, SR yielded a lower value of 0.75. Similarly, at 6 months post-THA, the AUCs for symptoms, pain, ADL, and QOL were all 0.95 or higher, indicating excellent discriminatory ability; however, the AUC for SR was low at 0.72.

A comparison of the PASS thresholds obtained in this study with the mid-to long-term outcomes reported in previous studies revealed several noteworthy differences. In a study by Emara et al., the PASS threshold calculated using ROC analysis at 1 year post-THA was higher than the PASS thresholds at 3 and 6 months post-THA observed in this study.¹⁰ In a study by Paulsen et al., the PASS thresholds identified using the percentile method at 1 year post-THA were calculated as 91 for pain and 83 for QOL.⁸ These values exceeded the corresponding PASS thresholds obtained in this study at 6 months post-THA (pain 82.50, QOL 62.50). Considering the results of these previous studies, it is suggested that the PASS threshold in the early post-THA period (3 or 6 months) should be set lower than that in the 1- or 2-years post-THA period. Indeed, in a study by Galea et al., the PASS threshold for the Oxford Hip Score was 34 points at 3 months post-THA, 40 points at 1 year, and 39 points at 2 years, indicating a lower PASS threshold at the 3-month time point.²³ Collectively, these findings suggest that patients tend to set higher standards for what they consider acceptable as recovery progresses after THA, requiring correspondingly higher HOOS scores to perceive their condition as satisfactory.

In addition to differences in follow-up duration, several other factors may have contributed to the discrepancies between the short-term PASS thresholds observed in this study and the higher thresholds reported in previous long-term studies. Methodological variations, including differences in anchor questions, cut-off determination methods (percentile vs. ROC-based approaches), and outcome instruments, may influence PASS estimation. Furthermore, patient characteristics, such as older age and lower preoperative activity levels in our cohort, could have led to lower expectations of acceptable symptom states in the early postoperative phase. Differences in healthcare systems, including the longer inpatient rehabilitation periods typical of Japan, may also shape postoperative recovery trajectories and patients' perceptions of acceptability.¹¹ These factors likely interact with timing effects to produce the observed differences in PASS thresholds. Important insights were also gained regarding differences in PASS thresholds across HOOS subscales. In particular, for the SR subscale, the PASS thresholds at both 3 and 6 months post-THA were substantially lower than those observed for the other HOOS subscales, and the corresponding AUC values were comparatively reduced. This finding suggests that limitations in sports and recreation during the early postoperative period may not strongly influence patients’ overall subjective acceptance of their condition. However, these observations should be interpreted with caution. Although older age has been associated with lower HOOS-SR scores in previous studies, expectations for sports and recreation were not directly assessed in our cohort.²⁴ Therefore, the relatively low SR PASS thresholds observed in this study cannot be solely attributed to age-related differences in expectations. Other factors, such as lower preoperative activity levels, prioritization of pain relief and basic daily function during early recovery, and potential measurement characteristics of the SR subscale, may also contribute to this finding.²⁵

The clinical implications of this study are multifaceted. A key implication of this study is that identifying PASS thresholds during the early postoperative period provides a practical benchmark for evaluating patient-perceived recovery. By establishing percentile-based PASS thresholds for each HOOS subscale, clinicians can more effectively recognize patients who may perceive their condition as unacceptable even after surgery. Such cases may warrant closer monitoring, reassessment of pain control, or modification of rehabilitation programs. Conversely, certain domains, such as SR, may not strongly influence patient satisfaction in older adults during the early postoperative phase, suggesting that treatment goals should be tailored to individual expectations and activity levels. Using PASS as a reference enables clinicians to assess subjective recovery more objectively, detect potential problems early, and implement timely interventions. Unlike evaluations based solely on mean scores or statistical significance, PASS directly reflects patient satisfaction and offers valuable guidance for patient-centered postoperative care.²⁶ Furthermore, integrating PASS-based assessment into follow-up may enhance shared decision-making and promote individualized rehabilitation strategies after THA.

The primary strength of this study was the first calculation of PASS thresholds using HOOS subscales during the short-term period after THA in patients with hip OA. The ability to clarify treatment effects at an earlier stage would be beneficial to both clinicians and patients, contributing considerably to patient-centered care. Nevertheless, this study has several limitations. First, the sample size was insufficient to ensure broad generalizability. Although the sample size allowed validation of a moderate predictive value, the findings may not represent the entire population of patients undergoing THA worldwide. The study cohort showed a marked female predominance (94 %), which reflects the epidemiology of hip OA and THA in Japan, but may limit the generalizability of the present findings, particularly to male patients.²⁷ In addition, because the present cohort consisted primarily of older adults, the PASS thresholds identified in this study, particularly those for the SR subscale, may not be generalizable to younger or more physically active THA populations, who typically have different postoperative expectations and recovery goals. Moreover, this study focused exclusively on short-term outcomes at 3 and 6 months postoperatively, and long-term PASS comparisons within the same cohort were unavailable. PASS thresholds are known to evolve over time as symptoms improve and patient expectations change; therefore, longitudinal assessment within individuals would provide important insights into the temporal trajectory and stability of acceptable symptom states after THA. Collectively, future studies incorporating larger and more diverse samples with balanced sex distributions and broader age ranges, as well as extended longitudinal follow-up, are needed to enhance generalizability and to compare short-, mid-, and long-term PASS thresholds within the same cohort. Second, important confounders that may influence HOOS outcomes, such as socioeconomic status, preoperative functional status, physical activity level, and postoperative rehabilitation intensity, were not fully adjusted for in this analysis. Lower educational levels have been associated with worse pain and physical function,²⁸ and inactive patients undergoing THA experience greater improvements in physical activity levels than those who were active before surgery.²⁵ In addition, variations in baseline functional severity and the intensity or adherence to rehabilitation programs may substantially affect postoperative recovery trajectories. The lack of comprehensive adjustment for these confounders limits the strength of the causal interpretations drawn from the present findings. Third, potential cultural influences on PROM interpretation should be considered. The present cohort consisted entirely of Japanese patients, whose expectations regarding pain relief, independence in ADL, and SR participation may differ from those of Western populations because of sociocultural norms and healthcare system characteristics. These factors may affect how acceptable symptom states are perceived and limit the direct cross-cultural comparability of PASS thresholds. Fourth, attrition resulting from missing HOOS responses, particularly at 3 months postoperatively, may have reduced the robustness of the findings and introduced the potential for non-response bias. Most missing data were attributed to logistical follow-up issues after hospital discharge and incomplete questionnaire returns rather than clinical deterioration. Future studies should implement structured follow-up strategies, such as electronic PROM systems, automated reminders, and centralized data monitoring, to improve response rates and minimize bias. Fifth, the PASS was anchored solely to the GRC scale, and objective functional recovery markers, such as hip range of motion, muscle strength, and gait performance, were not available for triangulation. Although the GRC scale is widely used and conceptually aligned with the patient-centered definition of PASS, the lack of objective anchors may limit the multidimensional validation of acceptable recovery. Future studies that integrate PROM-based PASS with objective physical performance measures would allow for more robust and comprehensive assessments. Finally, we were unable to adjust for the surgical approach or implant characteristics used in the analysis. Because differences in surgical techniques and implant-related factors may influence postoperative pain relief and the speed of functional recovery, future studies should incorporate appropriate adjustment for these variables.^29,30

5 Conclusions

This study identified short-term PASS thresholds for HOOS after THA in patients with hip OA. These thresholds provide clinicians with practical benchmarks to detect unsatisfactory recovery as early as 3 months after THA and to adjust pain management and rehabilitation strategies accordingly. For patients, PASS offers clear and understandable indicators of acceptable recovery, supporting realistic expectations and shared decision-making in the early THA follow-up period.

Guardian/patient's consent

The study was approved by the institutional ethics committee, and written informed consent was obtained from all participants.

Ethical statement

This study was conducted in accordance with the Declaration of Helsinki. The protocol was reviewed and approved by the Ethics Committee of Saitama Medical Center, Saitama Medical University, which served as the central review board and provided approval covering all participating investigators and sites (approval number: E2022-109). Written informed consent was obtained from all participants.

CRediT authorship contribution statement

Junji Nishimoto: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Data Curation, Writing – original draft, Visualization, Supervision, Project administration. Rikumi Kurahashi: Investigation, Data curation, Writing – review & editing. Kotaro Tamari: Methodology, Writing –review & editing. Ryo Tanaka: Methodology, Writing – review & editing.

Funding

The authors received no financial support for the research, authorship, and publication of this article.