Femoroacetabular impingement syndrome is increasingly recognized as a contributor to cartilage injury and early hip osteoarthritis. Both structured conservative care and arthroscopic surgery can improve pain and function, but a major unresolved clinical problem is deciding who should continue conservative care and who should escalate to surgery, and when. Evidence indicates that shorter symptom duration before surgery is associated with better long-term improvement, meaning delays may reduce the chance of achieving meaningful recovery. Current decision-making still depends largely on static imaging and passive clinical examination, which do not capture the dynamic, movement-related nature of the condition, while advanced three-dimensional imaging and laboratory motion analysis are not practical for routine clinical monitoring. This study aims to address this gap by developing and validating feasible, clinic-ready dynamic assessment methods and integrating weight-bearing pelvic and spinal alignment with three-dimensional hip modeling to support more objective, individualized, and timely treatment decisions.

The escalating socioeconomic burden of hip osteoarthritis has driven a strategic shift toward earlier, joint-preserving interventions. Over the past two decades, femoroacetabular impingement syndrome (FAIS) has been increasingly recognized as a key contributor to cartilage injury and the development of early osteoarthritis. This recognition has not only led to the wider adoption of surgical interventions but also fueled significant advances in arthroscopic management. Yet, despite these developments, a central clinical challenge remains: determining which patients will benefit from prolonged conservative care and which require timely surgical care to optimize outcomes. Indeed, several randomized controlled trials have compared arthroscopy with conservative treatment, consistently showing that both approaches improve pain and function. Arthroscopy has demonstrated reliable short- to mid-term gains in patient-reported outcomes, while rehabilitation and conservative management also enable a substantial proportion of patients to achieve meaningful improvement. Although, these findings highlight the value of both strategies, the optimal sequence and timing of non-surgical and surgical care remains unknown.

Emerging literature makes it clear that the timing of surgical intervention is a critical factor in hip preservation. A recent multi-level systematic review showed that shorter symptom duration before arthroscopy is consistently associated with superior outcomes, including greater functional gains, higher rates of achieving clinically meaningful thresholds, and a lower risk of persistent pain. Similarly, a 10-year longitudinal study demonstrated that durable improvements are most likely when surgery is performed earlier in the disease course, whereas delays reduce the likelihood of achieving Minimal Clinically Important Difference (MCID) and Patient Acceptable Symptom State (PASS) thresholds.

Together, these findings establish timing as a critical determinant of long-term outcomes and underscore the pressing need for dynamic, clinic-ready tools that can guide the transition from rehabilitation to surgery in young adults with FAIS.

However, current diagnosis and treatment planning for FAIS relies heavily on static imaging and passive clinical examination, which cannot capture the dynamic, movement-related nature of the condition. Advanced biomechanical studies have shown that patients with FAIS differ from controls not only in femoral and spinopelvic morphology[8] but also in movement strategies during hip-hinging tasks such as deep squats. While these insights have enhanced our understanding of FAIS, their clinical translation remains very limited due to ethical and practical limitations. Firstly, while CT-based 3D imaging can provide detailed morphological assessment, its relatively high radiation dose makes it unsuitable for repeated monitoring in young adults. Similarly, 3D motion analysis has demonstrated important alterations in kinematics, yet the cost, time demands, and need for technical expertise restrict its use to research laboratories. As a result, there are still no feasible, clinic-ready protocols that adequately capture the dynamic nature of FAIS for everyday decision-making. Consequently, treatment planning in young adults, including the above combination of non-surgical and surgical care, often depends on subjective judgment and limited examination factors, which increases the risk of suboptimal care.

The central challenge, therefore, is not whether FAIS should be managed conservatively or surgically, but how to effectively determine which patients are best suited for each pathway and at what point in their care. As hip preservation moves toward preventive strategies and increasingly refined arthroscopic techniques, developing practical, individualized and objective tools to support timely decision-making is essential. This project aims to directly address that need.

Inclusion Criteria:

1. Voluntary written informed consent of the participant or their legally authorized representative has been obtained prior to any screening procedures
2. Male or Female, Age 18-35 years inclusive.
3. First consultation at participating site with clinical diagnosis of FAIS by a hip orthopaedic surgeon, including positive impingement tests and reduced hip ROM in the transverse plane compatible with FAIS.
4. Radiographic cam and/or pincer morphology on standard imaging (e.g. α-angle >60°, lateral center-edge angle >40°, crossover sign) as per local protocol.
5. Willingness and ability to participate rehabilitation and complete the recommended physiotherapy sessions and attend scheduled follow-up visits.
6. Ability to perform basic squat / hip-hinge tasks safely in the motion lab, as judged by the treating clinician.

Exclusion Criteria:

1. Previous surgery on the bilareral hip.
2. History of major hip trauma or pediatric hip disease (e.g. Slipped capital femoral epiphysis, Perthes disease).
3. Developmental dysplasia (lateral center-edge ≤20°) or other severe structural deformity incompatible with the standard FAIS pathway.
4. Radiographic signs of hip degeneration (Tönnis grade >2).
5. Other musculoskeletal conditions that significantly interfere with assessments (e.g. symptomatic lumbar disc disease, severe knee pathology, recent adductor muscle pathology).
6. Contraindications to MRI (e.g. non-MRI compatible pacemaker or implant, severe claustrophobia not manageable with standard care).
7. Contraindications to X-ray or MRI related to pregnancy: known pregnancy at any time, or positive pregnancy test prior to imaging; breastfeeding women will not undergo additional research imaging.
8. Inability to perform basic squat/hip-hinge tasks safely (e.g. due to balance, pain or cardiopulmonary limitations).