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Femoroacetabular impingement and hip OsteoaRthritis Cohort (FORCe). Protocol for a prospective study

Kay M. Crossley1, Marcus G. Pandy2, Sharmila Majumdar3, Anne J. Smith4, Rintje Agricola5, Adam I. Semciw1,6, Joanne L. Kemp1, Joshua J. Heerey1, Matthew G. King1, Peter R. Lawrenson6, Yi-Chung Lin2, Richard B. Souza3,7, Andrea B. Mosler1, Thomas M. Link3, Ramya Srinivasan3, Anthony G. Schache1,2.

1 La Trobe Sport and Exercise Medicine Research Centre, College of Science, Health and Engineering, La Trobe University, Bundoora, AUSTRALIA2 Department of Mechanical Engineering, University of Melbourne, Melbourne, Victoria, AUSTRALIA3 Department of Radiology and Biomedical Imaging, University of California, San Francisco, USA4 School of Physiotherapy and Exercise Science, Curtin University, Perth, AUSTRALIA 5 Department of Orthopaedics, Erasmus University Medical Centre, Rotterdam, THE NETHERLANDS6 School of Health and Rehabilitation Sciences, University of Queensland, Brisbane, AUSTRALIA 7 Department of Physical Therapy and Rehabilitation Science, University of California, San Francisco, USA

This project was approved by the La Trobe University Human Ethics Committee: HEC No. 15-019; and the University of Queensland Medical Research Ethics Committee: MREC No: 2015000916

Crossley: FAI and OsteoaRthritis Cohort (FORCe): Protocol 1

INTRODUCTIONFemoroacetebular impingement (FAI) is a common cause of hip-related pain in active young adults 1,2 with up to 50% of people presenting with hip-related pain being diagnosed with FAI 3. An international consensus meeting of clinicians and researchers in 2016 determined that a triad of: imaging findings, symptoms, and signs are required for a diagnosis of FAI1. Bony hip morphology is a key feature of FAI and is characterised by extra bone formation at the femoral head-neck junction known as “cam morphology”, and/or overcoverage of the acetabulum known as “pincer morphology”1. Cam morphology is problematic in young adults, evident in 15-25% of men, and 0-15% of women4, and appears to develop during skeletal maturation 4,5. Femoroacetabular impingement is the pathologic, mechanical abutment of the proximal femur and acetabular rim during movements of hip flexion and internal rotation 6,7. Cam morphology is proposed to place stress on hip joint structures with negative consequences for joint integrity 4,8.

The natural history of, and factors associated with, structural disease progression in young adults with hip-related pain and cam morphology is not known. In order to assess this progression, we need measures to identify early joint degeneration that are sensitive to change. Radiography, the gold-standard to assess osteoarthritis (OA), is insensitive to the early stages of the disease 9. Magnetic resonance (MR) imaging can be used to derive semi-quantitative grading of features such as anatomical alterations (lesions) in cartilage, subchondral bone and soft tissues 10.

People with hip-related pain demonstrate early OA changesPatients with hip-related pain demonstrate hip joint intra-articular pathology, both at arthroscopy and on MR imaging. A recent systematic review by Heerey et al 11 found a pooled prevalence of labral tears in people with hip-related pain of 62% (95%CI 47% to 75%) using MR arthrogram (MRA). The pooled prevalence of labral tears using MRI (non-enhanced) imaging was 32% in people with hip-related pain. The same systematic review reported the pooled prevalence of chondropathy as 64% (95%CI 25% to 91%) using MRA in people with hip-related pain 11. This prevalence is similar to that observed (72%) at arthroscopy in people with hip-related pain 12. In a similar fashion to intra-articular knee pathology, such as meniscal tears representing early knee OA, labral tears and cartilage pathology may represent hip OA in its early form 13. To date, no studies have investigated the progression of intra-articular joint pathology consistent with early hip OA in young and middle-aged people.

Hip-related pain is associated with poor quality-of-lifeImportantly, hip-related pain is a condition of young adults, mostly affecting those in their late teens and early adulthood. However, available evidence indicates that hip-related pain is associated with low levels of quality of life (QoL). The International Hip Outcome Tool (iHOT33) is a quality of life questionnaire for young, active people with hip-related pain. It comprises of 4 subscales, each with a maximum score of 100 representing full capacity and no symptoms. On the iHOT33, people with FAI reported scores for symptoms and functional limitations of: 60 ± 23; sports-and recreational activity: 29 ± 23, job-related concerns: 60 ± 30, social concerns: 46 ± 24 14, indicating poor quality of life, as well as concerns about job and social participation in these young adults.

Cam deformity is associated with end-stage hip OACam morphologies have been established as a risk factor for end-stage OA in four large population-based studies of people aged 45-67yrs. Over a 19 year follow-up period in 268 hips of middle-aged and elderly women 15, the risk of total hip arthroplasty (THA) per 1° increase in alpha angle (measure of cam morphology) was 5.8% (95% CI: 2.3–9.3) (p<0.001). A cam morphology (moderate: alpha angle >60°, or


severe alpha angle >83°) predicted end-stage hip OA (adjusted OR: 3.7 (95% CI 1.7-8.0) and 9.7 (95% CI 4.7-19.8), respectively) in 1002 patients at 5 years follow-up 6. A subsequent study identified that a moderate cam morphology (alpha angle >60) predicted incident hip OA (OR: 2.11 (95%CI: 1.55-2.87) in 4438 people at 9 years follow up 16. Furthermore, every 1° increase in alpha angle above 65° was associated with an increased odds of 5% for incident radiographic hip OA over 20 years in 1003 middle-aged and elderly women 17. In contrast, pincer morphology was not a risk factor, and is possibly protective for OA development 18. When data from these two cohorts were combined, an alpha angle of 78˚ (95% CI: 62°–87°) was considered to be pathological, with maximum area under the ROC curve (0.69 (95% CI 0.62–0.75)) for an association with end-stage OA 19. Therefore, a clear relationship exists between both the presence and size of cam morphology, and development of end-stage hip OA in people aged 45-67 years. However, there is limited understanding of this relationship in younger people and thus, in the early stages of hip degeneration.

Identifying risk factors for hip joint deteriorationInterventions with the capacity to change the natural history of FAI are required to guide primary prevention strategies early in the disease process. In order to develop and test such interventions, we need to identify factors that lead to hip joint deterioration. A number of factors, including the size of cam-morphology and/or altered biomechanical characteristics (such as walking patterns, hip contact force (HCF), muscle strength and joint range of motion) may mediate the relationship between hip-related pain and joint deterioration.

Hip contact forces exceed 4 times bodyweight during walking 20,21 and hence, aberrant force distribution or magnitudes exceeding the joint’s capacity may render the joint susceptible to structural deterioration. The hip contact force represents the contact force of the femoral head on the acetabulum, and the net effect of muscle, gravitational, and inertial forces. Direct measurement of hip contact force in vivo is not feasible because current methods are extremely invasive and thus, are limited to studies using instrumented hip prostheses in people with advanced hip OA 21. Three-dimensional gait analysis accurately assesses joint motion, net joint torques, and muscle activation patterns, and there is evidence from cross-sectional studies that walking patterns in people with hip-related pain (FAI) differ from those of healthy controls 22-26. However, joint motions, net joint torques, and muscle activation patterns do not directly quantify joint loading. Detailed musculoskeletal modelling is the only available method to calculate muscle and joint contact forces in a non-invasive way. The role that biomechanical characteristics, such as joint motions, joint torques, or contact forces have in structural and symptomatic decline of hip-related pain is currently unknown.

Muscles are critical to maintain joint mobility, stability and function, as they aid in shock absorption and provide dynamic joint stability. Consequently, muscle weakness is prevalent in OA and is associated with knee OA progression 27. At the hip, reduced strength is evident in people with hip-related pain 28,29, but its relationship with disease progression hasn’t been studied. A number of muscles have potential to provide dynamic hip joint stability. Our narrative review 30 and electromyography (EMG) studies 31 highlighted that the deep hip external rotators, through their morphology, have potential to stabilise the femoral head within the acetabulum, while the capsular attachments of anterior hip joint stabilisers such as anterior gluteus minimus (GMin), rectus femoris (RFem) and iliocapsularis may minimise anterior hip joint shearing 32-34. Our modelling study determined that gluteus maximus (hip extensor) has the capacity to reduce the anterior hip contact force 20. Since the anterosuperior labrum is most vulnerable to joint degeneration,


weakness and dysfunction of the anterior hip stabilisers, deep hip external rotators and hip extensors may affect disease progression.

Restricted hip range of motion is associated with hip-joint pathology. Loss of hip internal rotation range combined with cam morphology, in individuals aged 45-65 years, increased the risk of developing end-stage hip OA, to a greater extent than cam morphology alone, with the positive predictive value increasing to 53% 6. In addition, adolescents (mean age 17±7 years) with cam morphology had reduced flexion and internal rotation range compared to those without cam morphology 35. However, the relationship between movement, morphology and hip joint disease progression in younger adults remains unknown.

We propose to address these knowledge gaps by evaluating whether structural or biomechanical factors can predict people at greater risk of structural disease progression. For example, the biomechanical characteristics associated with cam morphology may play a larger role in joint deterioration than the structural cam morphology itself. Identifying the relative contribution of these factors will indicate the extent to which structural features (such as size of cam morphology, which can be addressed through surgical resection) and biomechanical characteristics (such as hip contact force, muscle strength or joint range of motion, which all could be potentially addressed with targeted physiotherapy) may influence the natural history of hip OA.

The primary aims of this project are to:(i) Evaluate changes in hip joint structure over a 2-year period in people (football players) with hip-

related pain; and (ii) Determine if baseline measures of potentially modifiable factors (cam morphology, hip joint

force, muscle strength, and joint range of motion) predict structural decline over 2 years in people (football players) with hip-related pain.

The secondary aim of this project is to determine if changes in QoL over 2 years are related to changes in joint structure in people (football players) with hip-related pain.

Primary hypotheses: In people with hip-related pain: (H1): Structural deterioration will be evident in at least 40% of people over 2 years. (H2): Deterioration in joint structure over 2 years will be associated with baseline measures: (a) larger cam morphology; (b) higher hip contact force; (c) lower muscle strength and (d) reduced joint range of motion. (H3): Worsening QoL will accompany deterioration in joint structure.

In addition to our primary and secondary aims, we propose to undertake further investigations. These are categorised into four extensions: (a) using additional measures of modifiable factors to explore associations with our primary (changes in hip joint structure) and secondary (changes in hip-related QoL) aims. These include additional gait/movement tasks, additional muscle function (strength and activation) and muscle morphology (size and fatty infiltrates) measures and additional patient-reported outcomes; (b) exploring baseline associations (inter-relationships between physical measures, patient reported outcomes and hip joint structure); (c) exploring relationship between predictor variables and changes in patient-reported outcomes at one year; and (d) comparisons of features with additional cohorts of age and activity matched pain-free controls. These additional investigations are described below.

METHODSExperimental design and overview of procedureCrossley: FAI and OsteoaRthritis Cohort (FORCe): Protocol 4

Our prospective, longitudinal study will be conducted over two sites (Melbourne and Brisbane) to ensure recruitment feasibility, and will use standardised measurement protocols across both sites. Eligible participants will undergo baseline assessments including biomechanical evaluation (La Trobe University, University of Queensland movement laboratories) and X-rays at private radiology clinics. Across both sites, we have a biomechanical team leader (AGS), and at each site we have a site team leader (La Trobe University: JK; University of Queensland: AS). MR imaging will be collected at baseline and 2 year follow-up, and patient reported outcomes (PROs) will be collected at baseline, 6 months, 1 year and 2 year follow-up.

Participants: 200 participants fulfilling clinical findings indicating hip-related pain 2,4,36 will be included. Inclusion Criteria: (i) men or women aged 18-50 yr; (ii) currently playing football (soccer or Australian Rules Football at amateur or semi-professional level) with >6 month history of hip and/or groin pain; (iii) symptoms indicative of impingement: gradual onset of hip pain (may radiate to groin), and aggravated by prolonged sitting or hip movements (such as squatting, twisting, stair climbing, running or sports specific activities) 36; (iv) positive flexion, adduction, internal rotation (FADIR) test; (v) hip-related pain >30 and <80 on a 100mm Visual Analogue Scale.

Exclusion Criteria:(i) planned lower-limb surgery in the following 2 years (e.g. arthroscopy); (ii) previous hip surgery; (iii) self-report of other diagnosed significant hip condition (e.g. trauma, rheumatoid arthritis, congenital dislocation of the hip, Perthes disease, subluxation, slipped upper femoral epiphysis, osteochondritis dissecans, fracture, septic arthritis, bursitis or tendinitis); (iv) contra-indications to MR imaging; (v) Kellgren and Lawrence (K&L) grade ≥2, (vi) physical inability to weight-bear fully or undertake testing procedures; (vii) inability to understand written and spoken English.

Participant recruitment: Potential participants will be recruited via advertisements in print or radio media at sporting venues and health/medical practices and from orthopedic, sports medicine or physiotherapy practices. Social media (e.g. twitter, Facebook, La Trobe Sport and Exercise Medicine Blogs), and the websites of sporting organisations will also be utilised. Eligibility will be confirmed via interview (face-to face or telephone), followed by physical screen. Research personnel will also attend football training, to interview potential participants and conduct physical screening.

Participant characteristics and disease-specific informationBaseline questionnaires: age, gender, occupational and sporting history, symptom duration and severity, type and amount of rehabilitation, medication use. Follow-up questionnaires: hip injuries or surgeries.

Primary outcome measure: Magnetic resonance imaging (baseline and 2 year follow-up)MR images will be obtained on all participants at baseline and at the 2-year follow-up using a 3T MRI scanner and an eight-channel cardiac coil. Patient positioning aids will immobilise and support patients, ensuring consistent, reproducible, and comfortable hip positioning during scanning. Sagittal, coronal, and axial oblique T2-weighted fat-suppressed FSE images will be acquired to evaluate cartilage lesions, bone marrow oedema lesions, potential labral damage, and alpha angle. Sagittal high-resolution gradient echo images will be acquired to quantify cartilage volume and thickness 37.

Joint structure measures: e.g. cartilage, labrum, bone marrow, subchondral cysts.


The Scoring Hip Osteoarthritis with MRI (SHOMRI) classification will be used to score the 8 features (cartilage loss, bone marrow lesions (BML), subchondral cysts, labral abnormalities, loose bodies, joint effusion and ligamentum teres abnormalities). For cartilage lesions, BML and subchondral cysts, the femoral and acetabular segments will be divided into six subregions (four femoral, two acetabular) on the coronal studies and four subregions (two femoral, two acetabular) on the sagittal studies, for a total of 10 subregions (Fig.1) 38,39. Intra- and inter-reader reliability is excellent (ICC: 0.91-0.97; κ: 0.55-0.79).

Classifications will be performed by a single, experienced radiologist, with random checks performed on a subset by a second, senior radiologist. Each structure (e.g. cartilage, labrum, subchondral bone) will be graded using semi-quantitative methods. All radiologists will be blinded to participant demographics, symptoms and physical screening results when performing their image analysis.

The mid portion of the femoral head will be defined on the sagittal images and subdivided into four subregions on the coronal images, from lateral to medial. The landmark for division is the lateral acetabular rim for lateral and superolateral subregions, a vertical line from the centre of the femoral head for superolateral and superomedial subregions, and ligamentum teres for superomedial and inferior subregions. On the sagittal MRI study, the anterior subregion represents the anterior 1 cm of the femoral head and the posterior subregion represents the posterior 1 cm of the femoral head. This division is based on a simplified version of the geographic zone method described for hip arthroscopy, which showed superior inter-observer reproducibility compared to the clock-face method 40.

Cartilage defects are graded as 0 (no defect), 1 (partial thickness) and 2 (full thickness). BMELs are graded as 0 (absent), 1 (< or =0.5 cm), 2 (0.5- 1.5 cm) and 3 (> or =1.5 cm). Subchondral cysts are graded as 0 (absent), 1 (< or =0.5 cm) and 2 (>0.5 cm). The labrum is graded on the sagittal images in the anterosuperior subregion, coronal images in the superolateral subregions and on the axial images in the anterior and posterior subregions. Labral tears are graded as 0 (normal), 1 (fraying or signal abnormality), 2 (simple tear), 3 (labral-cartilage separation), 4 (complex tear) and 5 (maceration).

Primary predictor variablesCam morphology severity (baseline)


Cam morphology severity will be quantified using the alpha angle, as described below.Radiographic alpha angle: Supine antero-posterior (AP) pelvis, with hips internally rotated 45° and Dunn 45° radiographs will be obtained according to standardised protocols. The alpha angle represents the extent of asphericity of the femoral head and therefore quantifies cam morphology. The alpha angle is measured by first drawing a best fitting circle around the femoral head, and then a line through the centre of the neck from the centre of the femoral head. A second line is then drawn from the centre of the head to the point where the superior surface of the head-neck junction first departs from the circle. The angle between these two lines is the alpha angle (Fig 2). A recently validated alpha angle threshold value of 60 degrees will be used to define the presence of cam morphology. The alpha angle will be calculated using a semi-automatic method 4,6 to avoid the bias and inaccuracy that can affect manual measurements. On both the AP and the Dunn 45° radiographs, a set of points will be positioned by the researcher along the contour of the bone of the femoral head and the head-neck junction (Figure 4) using statistical shape modelling (SSM) software (ASM toolkit, Manchester University, UK). From these points, the best fitting circle and the midpoint of the femoral neck will be automatically calculated using Matlab (version R2009; The Mathworks Inc., Natick, MA). This method (Fig 2) is reliable (inter-observer: ICC; 0.73; 95%CI: 0.56-0.86).

Hip contact force (baseline) For our primary aim, we will use a computer-based musculoskeletal model, implemented in OpenSim 41, to calculate HCFs during walking at a self-selected speed.

Experimental gait data collection (La Trobe University and University Queensland)Three dimensional kinematic data will be captured using motion capture systems (VICON Motion Systems Ltd, Oxford, UK), with 10 cameras (La Trobe) and 8 cameras (Queensland), sampling at a frequency of 100Hz. Ground reaction force data will be recorded from two force plates (Kistler, Switzerland and/or AMTI, Massachusetts, USA) embedded in the laboratory floor in series, sampling at 1000Hz.

Participants will wear loose fitting shorts (plus a crop top for women) and Teva Original Universe Sandals (Teva, Deckers Outdoor Corporation, California, USA) to allow exposure of the foot for marker placement. Small reflective markers (14 mm diameter) will be placed at various locations on the trunk, upper-limbs and lower-limbs (Appendix 1) consistent with previously published protocols 42,43. For the upper limbs, markers will be placed over the lateral epicondyle on the humerus and the dorsal aspect of the wrist. For the trunk, markers will be placed on the tips of both shoulders (acromio-clavicular joints) and over the spinous process of the 7th cervical vertebra. For the pelvis, markers will be placed over the left and right anterior


superior iliac spines (ASISs) and a four-marker thermoplastic plate will be positioned over the sacrum. For the lower-limbs, markers will be placed over the medial and lateral femoral epicondyles, the medial and lateral malleoli, the heel, the medial and lateral midfoot, the medial aspect of the 1 st metatarso-phalangeal joint, the lateral aspect of the 5th metatarso-phalangeal joint, and on the dorsal surface of the 1 st toe. Three tracking markers per segment will be placed over the anterolateral aspect of the thigh and the middle third of the shank. Prior to performing walking trials, a static trial will be captured with the participant assuming a neutral upright stance pose with all markers in situ.

TasksFor the primary aim, we will investigate walking only. Participants will be asked to walk at a comfortable, self-selected walking speed along a 10-metre walkway. A successful trial will be defined as one with complete force plate contact of each limb (left then right or vice versa), at a speed within 5% of their average walking speed (established during warm-up). Participants will continue this process until eight successful trials are obtained.

Hip contact forceHip contact forces will be calculated using a three-dimensional computer-based musculoskeletal model implemented in OpenSim 41. The static calibration trial will be used to scale the generic musculoskeletal model to each participant’s anthropometric measurements. An inverse kinematics problem will be solved to determine the model joint angles that best match the marker data obtained from the gait analysis experiment 44. Individual muscle forces will be calculated by solving an optimization problem using a computed muscle control algorithm 45. This algorithm determines the muscle activation patterns that are needed to drive the musculoskeletal model so it accurately tracks the recorded experimental gait data. Muscle redundancy in the optimization problem will be resolved by including a cost function that minimises the sum of all muscle activations squared subject to the physiological bounds on muscle force imposed by each muscle’s force-length-velocity property. The three components of the HCF as well as the resultant HCF will be calculated at each time point using the Joint Reaction analysis in OpenSim 46. This process will involve two steps: first, each muscle’s force together with its contribution to the recorded ground reaction force will be applied to the musculoskeletal model; and second, the equations of motion and subsequently the joint reaction equations will be solved to obtain HCFs 20. HCFs will be normalized to participants’ body weight and expressed with respect to the femoral coordinate system.

Hip muscle strength and range of motion (baseline)Hip muscle strength: We will use hand held dynamometry (HHD) to evaluate hip muscle strength. One tester will perform HHD at each testing location. Isometric (make) and eccentric (break) strength tests will be performed. Each participant will perform hip flexion, extension, abduction, adduction, external rotation and internal rotation isometric strength tests. In addition, the participant will also perform hip abduction and hip adduction eccentric strength tests. The testing positions were based on previously used methods 47-

49 (see Appendix 2 for further details). The participant will be instructed to stabilise themselves by holding onto the examination table. Each participant will perform one sub-maximal familiarisation test for each hip muscle group, followed by 3 maximum voluntary contractions. Each isometric muscle contraction will be performed for a 5 second period, with 5 seconds break provided between each of the 3 trials. Eccentric testing will be performed using a 5 second muscle contraction (3 second isometric and 2 seconds eccentric breaking movement) with 30 seconds break between each muscle contraction. Thirty seconds break will be provided prior to testing the next muscle group. The tester will instruct the participant when to commence muscle contraction using previously advocated standardised verbal encouragement 48 “go ahead and push-


push-push-push-relax”. The highest of the 3 maximal voluntary contractions will be used for analysis. Intra-rater reliability for our hip muscle strength testing protocol has been established with ICCs ranging from 0.76 to 0.9847,50.

Hip range of motion: One tester will perform hip range of motion testing at each testing location. Hip flexion active range of motion will be evaluated using a digital inclinometer and established methods 50. Passive hip external and internal range of motion will be assessed in supine 90° of hip flexion using a manual goniometer 51. Bent knee fall out (BKFO) will be performed bilaterally using previously described methods 52. See Appendix 2 for full details of the testing protocol.

Secondary outcome measure: Hip-related quality of lifeHip-related QoL (baseline, 6 month, 1 year and 2 year follow-up)The iHOT-33 is a quality of life questionnaire for young active people with hip-related pain. It is reliable (ICC for subscales range from 0.86; 95%CIs: 0.70-0.96), with a minimal detectable change (group) ranging from 3.3 to 4.9 in this patient population 14.

Secondary predictor variable: Change in the SHOMRI score Changes in the SHOMRI score, between baseline and the 2-year follow-up time point, will be the predictor variable for our secondary aim.

Other variables to be included as covariatesBody size (measured at baseline, 2 years): Height and weight will be measured, and BMI calculated.Acetabular morphology (measured at baseline): The presence of pincer morphology and acetabular dysplasia will be determined from the AP radiographs. The same SSM software (Manchester University, Manchester, UK), will be used to automatically calculate the lateral centre edge angle (LCEA). The LCEA is the angle between line A (vertical line from the centre of the femoral head) and line B (line to acetabular edge from the centre of the femoral head) (Fig. 3). These measures describe the amount of lateral acetabular coverage with respect to the centre of the femoral head (inter-observer reliability: ICC: 0.97; 95% CI: 0.94-0.99 to 0.99; 95% CI: 0.97-0.99). Acetabular dysplasia will be defined as LCEA <20°, and pincer morphology determined to be present when LCEA is ≥40°.


Crossley: FAI and OsteoaRthritis Cohort (FORCe): Protocol10

Physical activity participation (measured at baseline, 1 and 2 years): Participation in activities of daily living during the two year study will be measured using; (i) the hip sports activity score (HSAS), a score measuring level of sports participation 53 and questions related to pre-injury and current physical activity (intensity, frequency and duration) (HUNT 3: https://www.ntnu.no/hunt/skjema); and (ii) daily steps measured with the Fitbit© in a subset of participants54.

Data management and analysisSample size calculation: A sample size of 170 will enable the proportion of participants with structural deterioration over 2 years to be estimated at the hypothesised value of 0.40 with a precision (half-width of 95% CI) of ±0.07. A sample size of 170 provides 90% power to detect at least an additional 5% of explained variation in the primary outcome of change in SHOMRI score due to each predictor of interest, over an estimated 25% accounted for by a set of 10 covariates, at a significance level of α=0.05. This corresponds to 90% power to detect a partial correlation of 0.24 between the specified predictor and change in SHOMRI (Stata/IC 13.1). Allowing for 15% dropouts and missing data, 200 subjects will be recruited. For our secondary aim, 170 people will enable correlation coefficients of 0.24 (R2 6%) or more to be detected.

Data analysis plan: The independent association between change in SHOMRI score over 2 years and baseline measures of cam morphology, HCF, muscle strength and ROM will be assessed using multivariable linear regression adjusting for potential confounding factors. Data will be transformed as necessary. Nonlinear associations will be explored and piecewise/fractional polynomial modelling used as appropriate. Estimates of association will be presented as standardised and unstandardised regression coefficients with 95% confidence intervals. Linear regression will be used to assess the strength of association between the independent variable change in SHOMRI score and dependent variable change in quality of life measures (iHOT-33) over 2 years.

Additional investigationsAdditional Patient-reported outcome measures Hip-related symptoms and function (baseline, 6 month, 1 year and 2 year follow-up)The Hip Osteoarthritis and disability Outcome Score (HOOS) is reliable (ICC range from 0.84; 95% CI: 0.67-0.93, to 0.96; 95% CI: 0.88-0.98)14, with a minimal detectable change (group) ranging from 6.0 to 11.055.

Psychological measures (baseline, 6 month, 1 year and 2 year follow-up)Self-efficacy will be measured using the Arthritis Self-efficacy Scale (ASES)56. Fear of movement will be measured using the Tampa scale of Kinesiophobia.57

Functional measures (baseline, 6 month, 1 year and 2 year follow-up) The Work Limitations Questionnaire measures work-related functional performance in time management, physical demands, mental-interpersonal demands, and output demands 58. The Patient specific functional scale (PSFS) will be used to capture individual functional performance specific to that patient 59. Patients will report three physical tasks performed in daily life rating difficulty on a numeric rating scale from zero (unable to perform) to 10 (able to perform).


Additional biomechanical measures Hip joint mechanics (baseline and two years follow up: La Trobe University): We will also calculate hip joint angles, torques and power for walking at a self-selected speed as well as for a range of more challenging or complex functional tasks associated with higher loads. Data collection procedures (instrumentation and marker locations) are outlined in the previous section for measuring HCFs.

TasksSix additional tasks (illustrated in Appendix 3) will be evaluated: (a) double leg drop jump; (b) single leg drop jumps; (c) lateral hop; (d) step-up, (e) step-down and pivot; and (f) running.

(a) Double and Single Leg Drop JumpParticipants will stand on a 30 cm high step, which is placed 10 cm behind 2 imbedded force plates (Kistler, New York, USA; AMTI, Massachusetts, USA). They will be instructed to drop off the step, landing with one foot on each plate, and jump directly up to the roof as quickly and as high as possible. Participants will complete 3 practice trials followed by 3 experimental trials.

(b) Single Leg Drop Jump For the single leg drop jump, participants will stand on one limb on the 30 cm step, drop off the step, landing with their contralateral foot on the force plate and hop vertically up, reaching for the roof. Participants will complete three practice trials in a self-selected order and six experimental trials (three each side) in a random order.

(c) Lateral HopMethods for the lateral hop task were adapted from Sayer et al. 60 Participants will stand on one limb, with arms crossed on a step at a height of 30% of their leg length (measured as outermost point of their greater trochanter to the floor). This step is placed 10 cm behind a ground embedded force plate (Kistler, New York, USA). Participants will be asked to drop down from the step onto the centre of the force plate with their contralateral foot, then hop laterally as quickly as possible to a mark on the floor placed a distance of 150% of their leg length. Participants will complete 3 practice trials and 3 experimental trials per limb.

(d) Step-upMethods for the step up task were adapted from Sims, Brauer 61. Participants will be instructed to stand with their arms crossed over their chest, with each foot on separate force plates (Kistler, New York, USA; AMTI, Massachusetts, USA), 15 cm behind a 15 cm high step with a 4.5 cm AMTI Accugait Force plate (AMTI, Massachusetts, USA) sitting on top of it (total height 19.5 cm). Participants receive an instruction of left or right foot, and once they receive the instruction they are required to step up to the centre of the force plate on the step, with the nominated foot first, as quickly as possible. Participants complete 5 practice trials followed by 10 experimental trials in a random order (5 left and 5right).

(e) Step down and pivotParticipants will be instructed to stand on a 19.5 cm high step (15 cm step with a 4.5 cm AMTI force plate located on top) 15 cm behind dual force plates with their feet half a foot length apart and arms folded. Participants will be asked to step down with their left or right foot (depending on step location) to a mark located on the ground embedded force plate in front of them, with toes pointing directly forward. They will then immediately pivot, and place their contralateral foot on a line on the final force plate, perpendicular to their weight bearing limb and then walk off. Participants complete 5 practice trials and 5 experimental trials on one limb prior to switching to the opposite limb.

(f) Running


Timing gates will be set up 10 m apart, either side of the calibrated measurement field for recording experimental data. Participants will be instructed to run through a 25 m runway at a speed of no less than 3.0 m/s (3.33 seconds to cover the 10 m distance between the timing gates). Participants will be provided with feedback after 5 practice runs at a comfortable speed to facilitate the required running speed. Participants will complete 8 successful trials. A successful trial is defined as a speed of at least 3.0 m/s and clear foot contact on a 1.2m x 0.6m Gen5 AMTI force plate (4 left foot contact and 4 right foot contact).

Data processing: Hip joint angles, moments and powerA seven-segment biomechanical model (pelvis, left/right thigh; left/right shank; left/right foot) implemented in Bodybuilder software (VICON Motion Systems Ltd., Oxford, UK) will be used to calculate hip joint joint angles, torques, and power. The structure of the model will be consistent with that described by Schache et al. 62. Details regarding anatomical coordinate system definitions for the pelvis and the femur have been previously outlined 63. The location of the hip joint centre will be defined using the method of Harrington et al 64, whereby the three-dimensional coordinates of the HJC with respect to the origin of the pelvis coordinate system are predicted using linear regression equations involving anthropometric measurements of pelvic depth (distance between the midpoints of the line segments connecting the two ASISs and the two PSISs) and pelvic width (inter-ASIS distance). Three-dimensional joint angles will be calculated using a joint coordinate system convention 65. Specifically, the axis of rotation for flexion-extension will be fixed to the pelvis and coincident with the mediolateral axis of the pelvis coordinate system; the axis of rotation for internal-external (longitudinal) rotation will be fixed to the femur and coincident with the vertical axis of the femoral coordinate system; and the axis of rotation for adduction-abduction will be the ‘floating’ axis, i.e. the common axis perpendicular to the flexion-extension and longitudinal rotation axes 66. A standard inverse dynamics approach will be used to calculate three-dimensional hip joint torques 67. Segmental inertial parameters will be taken from de Leva 68. Joint torques will be reported in the units of Newton-metre per kilogram (Nm/kg). Hip joint torques will expressed in the same non-orthogonal joint coordinate system used to calculate hip joint angles 63. The net joint power at the hip will be calculated by taking the product of the net joint moment and the joint angular velocity. Net joint power will be reported in the units of Watts per kilogram (W/kg).

Additional physical function measures Physical functional measures that will evaluated are single leg hop for distance 50, side bridge test 50 and single leg rise test 69. Each participant will perform these functional measures on each leg. Five minutes rest will be provided to ensure adequate recovery after completing each functional measure (see appendix 4for full testing protocol).

Additional hip morphology measures Hip muscle morphology (muscle size and adiposity): Muscle size: An estimate of hip muscle volume will be obtained for gluteus minimus, medius, maximus and tensor fascia lata from MR imaging scans as described previously 70. In addition, the volume of the deep hip rotators (e.g. quadratus femoris) 71 and the anterior hip joint muscles 72 will also be obtained. De-identified images will be stored on a CD for offline processing, where the fascial border of each muscle will be traced manually in each axial slice using Sante DICOM Editor software (Santesoft, Athens, Greece). The cross-sectional area of each muscle will be summed, and multiplied by the slice thickness to generate a muscle volume estimate. The final estimate will be normalized to bodyweight.


Adiposity We will quantify adiposity from MR imaging using an established 3D multi-echo gradient echo acquisition, which collects the data required for the analysis of phase related to the precessional differences in fat and water 73. The fat/water separation algorithm uses two echoes with non-uniformly spaced echo times (phase shifts between water and fat) to optimize fat/muscle measures. This technique has been validated with the gold standard biopsy 74. A muscle-fat index will be calculated as the proportion of fat to total muscle (fat/fat+muscle) 74. Given the segmental nature of gluteus minimus 33 and gluteus medius 75, this value will be determined for two separate segments of gluteus medius and minimus (anterior and posterior). A customized program has been developed to process these images with MATLAB (The MathWorks, Inc, Natick, MA).

Additional hip muscle function measures Hip muscle activationFor a subset of people (~n=70), hip muscle activation will be recorded bilaterally using surface electromyography for gluteus maximus, gluteus medius, tensor fascia lata, rectus femoris, and adductor longus. Deep hip muscles such as gluteus minimus and iliocapsularis are inaccessible with surface electrodes, so a subset of participants (~n=10-20) will undergo additional testing with fine wire electrodes to record the activation of iliocapsularis 34 and gluteus minimus 33.

DISCUSSION and SIGNIFICANCEOur primary outcome will, for the first time, determine the amount of deterioration in hip joint structure in people with hip-related pain, and whether modifiable factors can predict deterioration in hip structure in this high risk group. Identifying such factors will facilitate transition from treating OA symptoms, to programs designed for primary prevention.

Cam morphology size can potentially be addressed through surgical correction (arthroscopically resecting the asphericity of the femoral head/neck junction). Such surgeries are rapidly increasing for FAI in Australia 76, where the rates of arthroscopy have doubled between 2010 and 2013 77. Despite this increase, the contribution of cam morphology and associated intra-articular pathology, such as chondropathy and labral pathology, to structural deterioration is not known. While international groups are investigating the effects of hip arthroscopic surgical correction of morphology and pathology on symptoms 78,79, it will be many years before we know whether surgical intervention for FAI effectively modifies disease progression in people with this condition. Therefore, this study will provide valuable information regarding the natural history, and relative contribution, of cam morphology to joint deterioration in individuals not undergoing arthroscopic intervention.

Biomechanical characteristics, such as HCF, muscle strength and joint ROM can be addressed with targeted physiotherapy interventions. There are no full-scale randomised clinical trials of conservative therapies for FAI 80,81, and current programs are largely based on clinical experience 2, or an understanding of impairments seen in people with FAI 28,82. Importantly, no conservative therapies with the potential to change structural deterioration over time have been studied or discussed. Thus, if we identify potentially modifiable factors that can predict joint deterioration, evidence-based conservative interventions likely to change the natural course of the disease can be developed. For example, if HCF predicts disease progression, then interventions such as gait retraining 83 could potentially change the timing and/or magnitude of HCFs during walking.


If modifiable factors can be identified early in the disease process, before evidence of radiographic disease and in younger adults (<50 years), the potential to address these factors and reduce the impact of hip OA will be substantial. The results of this project are likely to be of global importance, given the expanding awareness of the problem of FAI in Australia and internationally.


CONFLICT OF INTEREST DECLARATIONThe authors have no conflict of interest to declare

ACKNOWLEDGEMENTSThis work was supported by the Australian National Health and Medical Research Council (NHMRC) (Project GNT 1088683); Early Career Fellowship (JLK): GNT 1119971; Australian Postgraduate Scholarship (JJH, MGK).


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Appendix 1: Marker placement for gait data collection


Appendix 2: Procedures for hip strength and range of motion measuresHip Muscle Strength

Hip abduction isometric (make test) Hip adduction isometric (make test)

The participant will lie on the examination table in supine. The tested leg will be placed in a neutral hip position, with the knee in 0° extension and ankle in plantar grade. The contralateral leg will be in 45° hip flexion and 90° knee flexion. The participant will stabilise themselves by holding onto the examination table. The HHD sensor will be placed on a marked location on the lateral leg (8 cm proximal to the most prominent aspect of the lateral malleoli). The participant will be instructed to push into hip abduction and provided a standardised command of “go ahead and push-push-push-push and relax”

The participant will lie on the examination table in supine. The tested leg will be placed in a neutral hip position, with the knee in 0° extension and ankle in plantar grade. The contralateral leg will be in 45° hip flexion and 90° knee flexion. The participant will stabilise themselves by holding onto the examination table. The HHD sensor will be placed on a marked location on the medial leg (8 cm proximal to the most prominent aspect of the lateral malleoli). The participant will be instructed to push into hip adduction and provided a standardised command of “go ahead and push-push-push-push and relax”


Hip external rotation isometric (make test) Hip internal rotation isometric (make test)

The participant will lie on the examination table in prone with their head rested to one side. The tested leg will be placed in neutral hip position, with the knee in 90° flexion and ankle in plantar grade. The contralateral leg will be in neutral hip position and 0° knee extension. The participant will stabilise themselves by holding onto the examination table. The HHD sensor will be placed on a marked location on the medial leg (8 cm proximal to the most prominent aspect of the lateral malleoli). The participant will be instructed to push into hip external rotation and provided a standardised command of “go ahead and push-push-push-push and relax”

The participant will lie on the examination table in prone with their head rested to one side. The tested leg will be placed in neutral hip position, with the knee in 90° flexion and ankle in plantar grade. The contralateral leg will be in neutral hip position and 0° knee extension. The participant will stabilise themselves by holding onto the examination table. The HHD sensor will be placed on a marked location on the lateral leg (8 cm proximal to the most prominent aspect of the lateral malleoli). The participant will be instructed to push into hip internal rotation and provided a standardised command of “go ahead and push-push-push-push and relax”


Hip flexion isometric (make test) Hip extension isometric (make test)The participant will be sitting in a neutral spinal position on the edge of the examination table. Both legs will be placed in 90° hip flexion, with the knee in 90° flexion and ankle in plantar grade. The participant will stabilise themselves by holding onto the examination table. The HHD sensor will be placed on the tested leg in a marked location 5cm proximal to the superior border of the patella. The HHD sensor will be stabilised using a belt that is fixed to the floor using a glass suction cup. The participant will be instructed to push up into hip flexion and provided a standardised command of “go ahead and push-push-push-push and relax”

The participant will lie on the examination table in prone with their head rested to one side. They will be positioned so the superior aspect of the patella is located 10cm over the end of the examination table. The tested leg will be placed in neutral hip position, with the knee in 90° flexion and ankle in plantar grade. The contralateral leg will be in neutral hip position, 0° knee extension and ankle in a neutral position. The participant will stabilise themselves by holding onto the examination table. The HHD sensor will be placed on the centre of the heel and stabilised using a belt that is fixed to the floor using a glass suction cup. The participant will be instructed to push up into hip extension and provided a standardised command of “go ahead and push-push-push-push and relax”


Hip abduction eccentric (break test) Hip adduction eccentric (break test)

The participant will be in side-lying on the examination table on the non-tested side. The tested leg will be in a neutral hip position, with the knee in 0° extension and ankle in plantar grade. The contralateral leg will be in 90° hip flexion and 90° knee flexion. The participant will stabilise themselves by holding onto the examination table. The HHD sensor will be placed on the marked location on the lateral leg (8 cm proximal to the most prominent aspect of the lateral malleoli). The participant will be instructed to push into hip abduction for 3 seconds then resist the eccentric breaking movement that will be performed by the tester over a 2 second period. The participant will be provided a standardised command of “go ahead and push-push-push-push and relax”

The participant will be positioned in side-lying on the examination table on the tested side. The tested leg will be in a neutral hip position, with the knee in 0° extension and ankle in plantar grade. The contralateral leg will be bent to 90° hip flexion and 90° knee flexion and placed on towels to maintain a neutral pelvis position. The participant will stabilise themselves by holding onto the examination table. The tester will lift the tested leg up into maximal hip adduction range of motion. The HHD sensor will be placed on the marked location on the medial leg (8 cm proximal to the most prominent aspect of the lateral malleoli). The participant will be instructed to push into hip adduction for 3 seconds and then resist the eccentric breaking movement that will be performed by the tester over a 2 second period. The participant will be provided a standardised command of “go ahead and push-push-push-push and relax”


Range of Motion

Passive hip external rotation Passive hip internal rotation

The participant will lie on the examination table in supine. A seatbelt will be placed across the pelvis at the level of the anterior superior iliac spines. The tested leg will be placed in 90° hip flexion and 90° knee flexion. The contralateral leg will be placed in neutral hip position, 0° knee extension and ankle in plantar grade. The stationary arm of the goniometer will be aligned parallel to the line made by the seatbelt as it lies across the pelvis. The goniometer is centred over the patella apex, and the moving arm placed along the tibia. Maximal external rotation will be measured as the deviation from the 90 starting position. The end range of hip motion will be considered as the earliest visible lateral tilting movement of the pelvis, indicating lateral flexion of the lumbar spine.

The participant will lie on the examination table in supine. A seatbelt will be placed across the pelvis at the level of the anterior superior iliac spines. The tested leg will be placed in 90° hip flexion and 90° knee flexion. The contralateral leg will be placed in neutral hip position, 0° knee extension and ankle in plantar grade. The stationary arm of the goniometer will be aligned parallel to the line made by the seatbelt as it lies across the pelvis. The goniometer is centred over the patella apex, and the moving arm placed along the tibia. Maximal internal rotation will be measured as the deviation from the 90 starting position. The end range of hip motion will be considered as the earliest visible lateral tilting movement of the pelvis, indicating lateral flexion of the lumbar spine.


Active hip flexion Bent knee fall out

The participant will lie on the examination table in supine. The tested leg will be placed in a neutral hip position, with the knee in 0° extension and ankle in plantar grade. The contralateral leg will be restrained with a seatbelt (placed over distal thigh superior to patella). The inclinometer (fixed to a 15cm ruler) is placed 5cm superior to the patella. The tester holds both sides of the ruler and the thigh. The participant is asked to “keep your arms folded and bend your knee towards your chest as far as possible”. The end of range is determined by the participant’s subjective feeling, or pain restriction.

The participant will be lying on the examination table in supine with knees bent to 90° knee flexion. A manual goniometer will be used to confirm correct knee position. The participant will be instructed to bring the soles of their feet together and lower their knees out to the side. The tester will provide gentle overpressure to each knee to ensure the participant has achieved full range of motion. A 1 metre ruler will be placed on the bed with distance between the most distal point of the fibula head and the ruler measured with an inflexible measuring tape. The distance to the nearest 0.5cm is recorded.


Functional tests

Single leg hop for distance Single leg rise test

The participant will stand on the tested leg in a sports shoe. The participant will place their hands behind their back. The shoe of the jumping leg will be placed behind the start of the measuring tape. The participant will then be instructed to perform a maximal distance jump along a measuring tape fixed on the ground. The participant will need to land with a flat foot, maintaining the landing position for 3 seconds, to be considered a successful trial. The hands will need to stay behind the back during the entire hop. The distance of the jump will be measured from the back of the heel of the landing leg. One practice trial is performed for familiarisation. Three successful trials will be recorded with the furthest distance used for data analysis.

The participant will sit on the edge of the examination table. The hip of the tested leg will be in 90° hip and knee flexion. The heel of tested leg will be placed 10cm from the end of the examination table. The contralateral leg will be positioned in 90° hip flexion, and 45° knee flexion. The participant will cross arms over their chest. A commercially available metronome app on a mobile phone will be used to ensure the participant performs each single leg rise at a standardised speed. The metronome will be set a rate of 45 counts per minute, with a single leg rise being completed over two counts i.e. one count up/one count down. The tester will issue a warning if the participant places the contralateral foot on the ground during the test, or if the descent is not controlled adequately. If 3 warnings are issued the test will be stopped. The test will be stopped if the participant performs 50 single leg rises.


Side plank endurance

The participant will be positioned in side-lying, with the tested side facing down. A towel will be placed on the ground under the participant’s elbow. Both hips will be in a neutral position and knees in 0° extension. The contralateral arm will be crossed over the chest. The participant will then push up from their elbow and feet into side plank position. They will be instructed by the tester to hold this position for as long as they can. The participant will be timed using a commercially available stop watch. The tester will provide standardised verbal encouragement “keep going” at 30 second time intervals. Completion of the test occurs when the lateral aspect of the tested hip touches the ground.


Appendix 3: Illustration of additional tasks in biomechanical assessment.


Appendix 4: Procedures for hip functional measures

a. Single leg hop for distance b. One leg rise testThe participant will stand on the leg to be tested in a sports shoe. The participant will place their hands behind their back. The shoe of the jumping leg will be placed behind the start of the measuring tape.The participant will then be instructed to perform a maximal distance jump along a fixated measuring tape located on the ground. The individual will need to land with a flat foot, maintaining the landing position for 3 seconds to be considered a successful trial. The hands will need to stay behind the back during the entirety of each hop. The distance of the jump will be measured from the back of the heel of the land leg. One practice trial is performed for familiarisation. Three successful trials will be recorded with the furthest distance used for data analysis.

The participant will be seated on the edge of the examination table. The hip of the leg to be tested will be 90° of flexion, with the knee in 90° flexion. The heel of tested leg will be placed 10cm away from the edge of the examination table. The contralateral hip will be positioned in 90° flexion, with the knee in 45° flexion. The participant will cross arms over their chest. A commercially available metronome app on a mobile phone will be used to ensure the participant performs each single leg rise at a standardised speed. The metronome will be set a rate of 45 counts per minute, with a single leg rise being completed over two counts i.e. one count up/one count down. The tester will issue a warning if the individual places the contralateral foot on the ground during the test or if the descent is not controlled adequately. If 3 warnings are issued the test will be stopped. If the participant performs 50 single leg rises the test will be stopped.


c. Side plank enduranceThe participant will be positioned in sidelying, with the side to be examined facing down most. A towel will be placed on the ground for the participant to place their elbow on. The hips and knees are in neutral and 0° knee extension. The contralateral arm will be crossed over the chest. The participant will then push up from elbow and feet into side plank position. They will be instructed by the tester to hold this position for as long as they can. The participant will be timed using a commercially available stop watch. The tester will provide standardised verbal encouragement “keep going” at 30 second time intervals. Completion of the test occurs when the lateral aspect of the down most hip touches the ground.


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