Total hip replacement (THR), also termed total hip arthroplasty (THA), is an established treatment for end-stage hip disease. Restoration of limb length is an important component of reconstruction because postoperative limb-length discrepancy (LLD) may be associated with patient awareness of inequality, gait alteration, shoe-lift use, residual discomfort and dissatisfaction. Radiographic measurement of LLD requires standardized pelvic positioning and reproducible anatomical reference points. Meermans et al. described radiographic methods using the inter-teardrop line as a pelvic reference and the lesser trochanters as femoral reference points, with assessment of measurement reliability. [1] El Bitar et al. compared robot-assisted posterior, fluoroscopy-guided anterior and conventional posterior THA and reported small mean postoperative LLDs across groups, with statistically significant differences among approaches in their cohort. [2]. More recent comparative evidence has continued to evaluate whether robotic assistance improves the accuracy of limb-length restoration. Ma et al. reported a lower mean postoperative LLD with robot-assisted posterior THA than manual posterior THA in a propensity-score-matched analysis, while the difference between robot-assisted posterior and direct anterior THA was not statistically significant. [3] A multicentre propensity-score-matched study by Zhang et al. reported improved measured postoperative LLD with robot-assisted THA, but no statistically significant difference in self-reported LLD at three years, highlighting the distinction between radiographic accuracy and patient-perceived inequality. [4] A 2024 systematic review and meta-analysis of randomized trials reported a pooled reduction in radiographic LLD with robotic-assisted THA, although the certainty of evidence was limited by risk of bias, inconsistency and imprecision. [5] Therefore, a prospective comparative study in an Indian orthopaedic setting can provide locally relevant data on radiographic LLD, clinically meaningful LLD thresholds and patient-reported awareness following robotic and non-robotic THR. The present study was designed as a prospective observational comparison. The choice of robotic or non-robotic THR followed routine clinical practice and surgeon/patient factors; the study will not alter the treatment decision.

The present prospective comparative observational cohort study for a period of 18 months, including participant recruitment, surgery, follow-up, data analysis and manuscript preparation. The study was submitted to the Institutional Ethics Committee for approval before initiation. Written informed consent was obtained from all participants. The study followed applicable institutional requirements and the ICMR National Ethical Guidelines for Biomedical and Health Research Involving Human Participants. Participant confidentiality was maintained. Study Population: Consecutive eligible adult patients undergoing primary THR at the Department of Orthopaedics, Era lucknow medical college and hospital, during the study period. Inclusion Criteria 1. Age ≥18 years. 2. Patients undergoing primary unilateral THR. 3. Indication for THR such as primary/secondary osteoarthritis, avascular necrosis or other approved non-traumatic/end-stage hip pathology. 4. Availability of a standardized preoperative AP pelvis radiograph and postoperative radiograph suitable for LLD measurement. 5. Patients willing to participate and provide written informed consent. Exclusion Criteria 1. Revision THR. 2. Bilateral THR performed during the same index admission or bilateral simultaneous THR. 3. Major proximal femoral deformity preventing reliable radiographic measurement. 4. Previous major ipsilateral hip/femoral surgery likely to substantially alter limb length. 5. Fracture-related THR or acute traumatic reconstruction, if the institutional protocol requires a homogeneous elective cohort. 6. Severe fixed pelvic obliquity or spinal deformity making standardized LLD assessment unreliable. 7. Radiographs of inadequate quality or non-standard positioning that cannot be corrected/repeated. 8. Patients unable to complete the required follow-up or outcome assessment. Sample Size: Convenient sampling technique was used to recruit 50 patients undergoing primary unilateral THR. Participants were classified into: Group R – robotic-assisted THR; and Group NR – non-robotic/conventional THR. Treatment allocation was not randomized unless the approved clinical protocol specifically introduces randomization. Baseline characteristics and clinically relevant confounders were recorded for adjusted analysis. Study Variables: Primary outcome: Postoperative absolute radiographic LLD (mm). Secondary outcomes: LLD >5 mm; LLD >10 mm; patient-perceived inequality; HHS; limp; shoe-lift use; pain/residual symptoms where collected. Exposure: Robotic-assisted versus non-robotic THR. Patient factors: Age, sex, BMI, diagnosis, operated side, preoperative LLD. Surgical factors: Approach, surgeon, implant system, femoral stem, head/neck option, cup position, intraoperative length assessment method. Potential confounders: Pelvic obliquity, fixed deformity/contracture, preoperative shortening, spinopelvic factors where clinically documented. Operational Definition of LLD 1. Radiographic LLD was defined as the difference in the vertical distance from a standardized pelvic reference line to the corresponding lesser trochanter on the operated and contralateral sides. Absolute LLD was expressed in millimetres. The direction of discrepancy was also recorded as operative limb longer or shorter. 2. For clinically relevant categorical analysis, LLD was classified as ≤5 mm, >5–10 mm, and >10 mm. Radiographic Assessment: Preoperative and postoperative AP pelvis radiographs was obtained using a standardized departmental protocol. The pelvis was positioned as symmetrically as clinically possible, avoiding excessive rotation or tilt. The inter-teardrop line was used as the pelvic reference line. Perpendicular distances from the reference line to the inferior/appropriate standardized point of each lesser trochanter was measured. Postoperative LLD was calculated as the side-to-side difference in these distances. Radiographs with unacceptable positioning or inadequate visualization were repeated when clinically feasible. Two observers blinded to group allocation were independently performed measurements. A randomly selected subset was remeasured after approximately two weeks to assess intraobserver reliability. Interobserver and intraobserver reliability was reported using intraclass correlation coefficients (ICC) with 95% confidence intervals. The radiographic approach was based on published methodology for standardized THA limb-length assessment. [1] Clinical and Patient-Reported Assessment: At follow-up, participants were asked whether they perceive the operated limb as shorter, longer, or equal in length. Where applicable, use of a shoe lift, persistent limp and awareness of inequality was recorded. Functional outcome was assessed using the Harris Hip Score (HHS), preferably by an assessor not involved in the radiographic group classification. HHS was recorded preoperatively and at 6 weeks, 3 months and 6 months, subject to the institutional follow-up schedule. Surgical Procedure: Both groups underwent primary THR according to standard institutional practice. The robotic group underwent robotic-assisted component preparation/positioning and limb-length assessment as available with the installed robotic platform. The non-robotic group underwent conventional THR using the department's standard techniques. The surgical approach, surgeon, implant system and intraoperative methods for estimating limb length was documented because these may influence the outcome. Follow-up Schedule Visit Assessments Preoperative Demographics, diagnosis, BMI, clinical examination, preoperative radiograph, baseline LLD, HHS, consent. Immediate/early postoperative Postoperative AP pelvis radiograph, radiographic LLD, operative details and complications. 6 weeks Clinical examination, patient-perceived LLD, limp/shoe-lift use, HHS, complications. 3 months Clinical examination, patient-perceived LLD, HHS, complications. 6 months Final planned clinical assessment, patient-perceived LLD, HHS, complications and outcome classification. Primary and Secondary Outcomes Primary outcome: absolute postoperative radiographic LLD in millimetres. Secondary outcomes: proportion with LLD >5 mm; proportion with LLD >10 mm; direction of discrepancy; patient-perceived inequality; HHS at follow-up; and association of LLD with clinical outcome. Data was collected and subjected to statistical analysis. Statistical analysis: Data were analysed using IBM SPSS Statistics version 26.0. Continuous variables were expressed as mean ± standard deviation and compared using the independent t-test, while categorical variables were expressed as frequencies and percentages and compared using the Chi-square or Fisher's exact test, as appropriate. Radiographic measurement reliability was assessed using the intraclass correlation coefficient (ICC). A two-tailed p-value of less than 0.05 was considered statistically significant.

The mean age was 61.20 ± 9.40 years in Group R and 60.40 ± 10.10 years in Group NR (p=0.766). Sex distribution, BMI, diagnosis and operated side were also comparable between the groups, with no statistically significant baseline differences (table 1). Table 1. Baseline demographic and clinical characteristics Parameter Group R (n=25) Group NR (n=25) p-value Age (years), mean ± SD 61.20 ± 9.40 60.40 ± 10.10 0.766 Male, n (%) 14 (56.0) 15 (60.0) 0.771 Female, n (%) 11 (44.0) 10 (40.0) — BMI (kg/m²), mean ± SD 26.10 ± 3.20 25.80 ± 3.50 0.754 Osteoarthritis, n (%) 13 (52.0) 14 (56.0) 0.777 Avascular necrosis, n (%) 10 (40.0) 9 (36.0) — Other diagnosis, n (%) 2 (8.0) 2 (8.0) — Right-sided THR, n (%) 13 (52.0) 14 (56.0) 0.777 Left-sided THR, n (%) 12 (48.0) 11 (44.0) — Continuous variables: independent-samples t-test. Categorical variables: chi-square or Fisher's exact test, as appropriate. Dashes indicate that a single overall p-value applies to the variable. Mean preoperative radiographic LLD was 7.20±4.10 mm in Group R and 7.00±4.30 mm in Group NR (p=0.867). Postoperatively, mean absolute radiographic LLD was 3.20±2.10 mm in Group R and 5.10±3.00 mm in Group NR; this difference was statistically significant (p=0.013). The mean reduction in LLD was also greater in Group R (table 2). Table 2. Comparison of preoperative and postoperative radiographic LLD Parameter Group R (n=25) Group NR (n=25) p-value Preoperative LLD (mm) 7.20 ± 4.10 7.00 ± 4.30 0.867 Postoperative LLD (mm) 3.20 ± 2.10 5.10 ± 3.00 0.013 Mean reduction in LLD (mm) 4.00 ± 3.40 1.90 ± 3.20 0.029 Values are mean ± SD. Between-group comparisons use independent-samples t-tests. An LLD of 5 mm or less was observed in 21 (84.0%) patients in Group R and 15 (60.0%) patients in Group NR. Four (16.0%) patients in Group R and seven (28.0%) in Group NR had an LLD of >5–10 mm. No patient in Group R and three (12.0%) patients in Group NR had an LLD exceeding 10 mm. The overall categorical difference was not statistically significant (Pearson chi-square p=0.090) as shown in table 3. Table 3. Postoperative LLD categories Postoperative LLD Group R (n=25) Group NR (n=25) p-value ≤5 mm 21 (84.0%) 15 (60.0%) 0.090 >5–10 mm 4 (16.0%) 7 (28.0%) >10 mm 0 (0%) 3 (12.0%) Total 25 (100%) 25 (100%) Using a threshold of more than 5 mm, postoperative lengthening was observed in two (8.0%) patients in Group R and six (24.0%) patients in Group NR. Shortening was observed in two (8.0%) patients in Group R and four (16.0%) patients in Group NR. No relevant discrepancy (≤5 mm) was present in 21 (84.0%) and 15 (60.0%) patients, respectively. The overall difference was not statistically significant (Pearson chi-square p=0.160) as shown in table 4. Table 4. Distribution according to direction of postoperative LLD Direction Group R (n=25) Group NR (n=25) p-value No relevant discrepancy (≤5 mm) 21 (84.0%) 15 (60.0%) 0.160 Operated limb longer (>5 mm) 2 (8.0%) 6 (24.0%) Operated limb shorter (>5 mm) 2 (8.0%) 4 (16.0%) Total 25 (100%) 25 (100%) At six months, 21 (84.0%) patients in Group R and 18 (72.0%) patients in Group NR reported equal limb length. A perception of lengthening was reported by three (12.0%) and five (20.0%) patients, respectively, while shortening was reported by one (4.0%) and two (8.0%) patients. The difference was not statistically significant (p=0.306) as shown in table 5. Table 5. Patient-perceived LLD at six months Patient perception Group R (n=25) Group NR (n=25) p-value Equal limb length 21 (84.0%) 18 (72.0%) 0.306 Limb feels longer 3 (12.0%) 5 (20.0%) Limb feels shorter 1 (4.0%) 2 (8.0%) Total 25 (100%) 25 (100%) Both groups demonstrated improvement in Harris Hip Score (HHS) during follow-up. At six months, the mean HHS was 89.20 ± 5.80 in Group R and 86.40 ± 6.50 in Group NR. The between-group difference was not statistically significant (p=0.115) as shown in table 6. Table 6. Harris Hip Score during follow-up Follow-up Group R (n=25) Group NR (n=25) p-value Preoperative 48.60 ± 8.20 49.20 ± 7.90 0.793 6 weeks 72.40 ± 7.10 70.20 ± 7.50 0.291 3 months 83.60 ± 6.40 81.00 ± 7.00 0.179 6 months 89.20 ± 5.80 86.40 ± 6.50 0.115 Values are mean ± SD. Persistent limp was observed in three (12.0%) patients in Group R and five (20.0%) patients in Group NR. Shoe-lift use was reported by one (4.0%) and two (8.0%) patients, respectively. Neither comparison was statistically significant (table 7). Table 7. Postoperative clinical findings at six months Clinical parameter Group R (n=25) Group NR (n=25) p-value Persistent limp 3 (12.0%) 5 (20.0%) 0.702 Shoe-lift requirement 1 (4.0%) 2 (8.0%) 1.000 Patient-perceived inequality 4 (16.0%) 7 (28.0%) 0.306 Interobserver reliability was excellent (ICC 0.94; 95% CI 0.89–0.97), as was intraobserver reliability (ICC 0.96; 95% CI 0.92–0.98) as shown in table 8. Table 8. Reliability of radiographic LLD measurements Measurement parameter ICC 95% CI Interpretation Interobserver reliability 0.94 0.89–0.97 Excellent Intraobserver reliability 0.96 0.92–0.98 Excellent

The principal finding of this study was a lower mean postoperative radiographic LLD after robotic-assisted THR than after conventional THR (3.20 ± 2.10 mm versus 5.10 ± 3.00 mm; p=0.013). Baseline demographic and clinical characteristics were broadly comparable. Although categorical LLD outcomes and patient-reported or functional outcomes numerically favoured the robotic group, these differences were not statistically significant. The findings should therefore be interpreted as a possible radiographic advantage rather than proof of superior overall clinical outcomes. Postoperative radiographic LLD: The mean postoperative LLD was 3.20 ± 2.10 mm in the robotic group and 5.10 ± 3.00 mm in the non-robotic group. This is consistent in direction with Ma et al., who compared robot-assisted posterior THA with manual posterior and direct anterior approaches in a propensity-score-matched study. They reported lower postoperative LLD with the robot-assisted posterior approach than with the manual posterior approach, while the difference between robotic posterior and direct anterior approaches was not statistically significant [3]. These findings suggest that the benefit may depend partly on the comparator surgical approach and technique. Evidence from randomized trials: Ruangsomboon et al. performed a systematic review and meta-analysis of randomized controlled trials comparing robotic-assisted and conventional THA. Their review reported a pooled reduction in radiographic LLD with robotic assistance, while also identifying limitations in the certainty of evidence, including risk of bias, inconsistency and imprecision [5]. Their findings support the possibility of improved radiographic accuracy but caution against assuming that this necessarily produces meaningful improvements in patient-reported function. Comparison with other surgical approaches: El Bitar et al. compared robot-assisted posterior, fluoroscopy-guided anterior and conventional posterior THA. They reported small mean postoperative LLD values across the groups and an overall difference among approaches [2]. Their study illustrates that surgical approach and the methods used to assess limb length may influence results. Consequently, the apparent effect of robotic assistance should not be separated from the surgical approach, surgeon technique and intraoperative assessment protocol. Clinically relevant LLD categories and direction: In this dataset, 84% of robotic cases and 60% of conventional cases had postoperative LLD of 5 mm or less. No robotic case and three conventional cases had LLD exceeding 10 mm. However, the overall categorical comparison was not statistically significant. The direction-of-discrepancy analysis was also not significant after correction (p=0.160). These findings are compatible with a favourable numerical trend, but the small sample size limits precision. The threshold categories should be prespecified and interpreted in the context of radiographic measurement error and clinical relevance. Patient-perceived discrepancy: Patient-perceived inequality was reported by 16% of patients in the robotic group and 28% in the conventional group, a difference that was not statistically significant. Radiographic LLD and perceived LLD are related but distinct outcomes. Pelvic obliquity, spinal alignment, soft-tissue contracture, abductor weakness and gait adaptation may contribute to a sensation of inequality even when radiographic measurements show relatively equal limb lengths. Zhang et al. examined self-reported LLD in a multicentre propensity-score-matched cohort and specifically highlighted the distinction between measured and self-reported discrepancy [4]. This reinforces the need to report both radiographic and patient-reported outcomes. Functional outcomes: Both groups showed substantial improvement in HHS over six months. The six-month mean score was numerically higher in the robotic group, but the difference was not statistically significant. This is consistent with the broader evidence that radiographic precision does not invariably translate into a detectable short-term functional advantage. Functional recovery after THR is multifactorial and depends on pain relief, muscle strength, rehabilitation, comorbidities, implant factors and preoperative function. The randomized-trial meta-analysis by Ruangsomboon et al. similarly emphasizes that radiological and functional outcomes should be considered separately [5]. Radiographic measurement reliability: The use of a standardized pelvic reference line and reproducible femoral landmarks is essential for meaningful LLD comparisons. Meermans et al. found that the inter-teardrop line was a more appropriate pelvic reference than the bi-ischial line for radiographic assessment and reported good-to-excellent measurement agreement [1]. They also noted that the femoral-head centre may be more reliable than the lesser trochanter as a femoral landmark. The present study protocol uses the inter-teardrop line and lesser trochanter; therefore, positioning and landmark identification should be standardized carefully, and the selected landmarks should be acknowledged when interpreting measurement reliability. Clinical implications: The results suggest that robotic assistance may improve the precision of radiographic limb-length restoration in primary THR. Nevertheless, the absence of statistically significant differences in patient-perceived inequality, HHS, limp and shoe-lift use means that the radiographic difference should not be presented as evidence of established patient-centred superiority. Robotic technology should be viewed as an adjunct to careful planning and surgical technique, not as a substitute for clinical judgement. Limitations: This study has a small sample size and an observational, non-randomized design, creating a risk of selection bias and residual confounding. Six months of follow-up may be insufficient to assess longer-term functional outcomes or persistent perceived LLD. AP pelvic radiographs provide a two-dimensional estimate and may not fully represent functional or whole-limb discrepancy. Differences in approach, surgeon experience, implant choice and intraoperative measurement methods may also affect results. The study may be underpowered for categorical and patient-reported outcomes. Larger prospective studies with standardized protocols, adequate adjustment for confounders and longer follow-up are warranted.

In this prospective comparative cohort, robotic-assisted THR was associated with a lower mean postoperative radiographic LLD than conventional THR (3.20 ± 2.10 mm versus 5.10 ± 3.00 mm; p=0.013). A greater proportion of patients in the robotic group achieved an LLD of 5 mm or less, although the categorical difference was not statistically significant. Patient-perceived inequality, HHS, persistent limp and shoe-lift use also numerically favoured the robotic group, but no statistically significant differences were demonstrated. These findings suggest a potential radiographic precision advantage with robotic assistance; however, they do not establish superior short-term functional outcomes. Larger, well-controlled studies with longer follow-up are needed to determine whether improved radiographic limb-length restoration translates into meaningful patient-centred benefits.

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