The purpose of this study was to evaluate spinopelvic mechanics from standing and sitting positions in subjects with and without femoroacetabular impingement (FAI). We hypothesize that FAI patients will experience less flexion at the lumbar spine and more flexion at the hip whilst changing from standing to sitting positions than subjects without FAI. This increase in hip flexion may contribute to symptomatology in FAI.
Patients and Methods
Male subjects were prospectively enrolled to the study (n = 20). Mean age was 31 years old (22 to 41). All underwent clinical examination, plain radiographs, and dynamic imaging using EOS. Subjects were categorized into three groups: non-FAI (no radiographic or clinical FAI or pain), asymptomatic FAI (radiographic and clinical FAI but no pain), and symptomatic FAI (patients with both pain and radiographic FAI). FAI was defined as internal rotation less than 15° and alpha angle greater than 60°. Subjects underwent standing and sitting radiographs in order to measure spine and femoroacetabular flexion.
Compared with non-FAI controls, symptomatic patients with FAI had less flexion at the spine (mean 22°, sd 12°, vs mean 35°, sd 8°; p = 0.04) and more at the hip (mean 72°, sd 6°, vs mean 62°, sd 8°; p = 0.047). Subjects with asymptomatic FAI had more spine flexion and similar hip flexion when compared to symptomatic FAI patients. Both FAI groups also sat with more anterior pelvic tilt than control patients. There were no differences in standing alignment among groups.
Symptomatic patients with FAI require more flexion at the hip to achieve sitting position due to their inability to compensate through the lumbar spine. With limited spine flexion, FAI patients sit with more anterior pelvic tilt, which may lead to impingement between the acetabulum and proximal femur. Differences in spinopelvic mechanics between FAI and non-FAI patients may contribute to the progression of FAI symptoms.
Cite this article: Bone Joint J 2018;100-B:1275–9.
Take home message
Spinopelvic mechanics, including compensatory lumbar mobility or spine flexion, has an impact on symptoms in the femoroacetabular impingement (FAI) patient cohort during functional activities.
Lumbar mobility appears to be a reasonable target for asymptomatic and early symptomatic FAI patients.
How patients achieve sitting position may be an important consideration in the FAI population, where compensatory or limited lumbar mobility may influence the hip flexion necessary to achieve functional positions, and thus modulate symptomology in the setting of altered femoroacetabular morphology.
Femoroacetabular impingement (FAI) is a common cause of hip pain and disability. Whilst in many cases the morphological appearances of FAI are associated with debilitating hip pain, a large number of patients, ranging between 14% and 35% of hips in the general population, have radiographic evidence of FAI but have no symptoms.1-3 Likewise, some patients with FAI respond to conservative treatment modalities while others have little or no response.
It is not clear what factors lead to symptoms in FAI. Variations in lumbar lordosis, pelvic incidence, pelvic tilt, and sacral slope exist within the general population and may modify the degree of impingement and consequent symptoms.4 The link between the lumbar spine, pelvic sagittal alignment, and the hip joint has been demonstrated in the arthroplasty literature, where extremes of pelvic alignment, lumbar disease, and stiffness are associated with impingement and early failure of total hip arthroplasty (THA).5,6 Similarly, a cadaveric study has noted significant differences in lumbar spine alignment for cadaveric hips with FAI, suggesting that the lumbar spine may play a role in hip impingement.7
Variability in spinopelvic alignment during different postures, such as standing, sitting, and lying flat, has been shown in patients undergoing THA.8-10 Positions and activities requiring high degrees of flexion of the hip, such as sitting, tend to result in symptoms in patients with FAI because of engagement of the osseous abnormality with the anterosuperior labrum and chondrolabral junction. The sitting position is achieved through a combination of flexion of the spine and the hip. Following THA, patients with degenerative disc disease have to compensate for the stiffness of the lumbar spine by increased flexion of the hip in order to achieve the sitting position.8 A similar effect is likely to be seen in the FAI population, where stiffness of the lumbar spine may require the patient to flex the hip further to achieve the sitting position, resulting in symptoms in the context of abnormal femoroacetabular morphology.
No previous study has formally evaluated the influence of the alignment of the lumbar spine on the symptomatology of FAI. The purpose of this study was to evaluate spinopelvic mechanics in standing and sitting positions in patients without FAI, compared with asymptomatic patients with FAI, and those with symptomatic FAI. The aim of the study was to assess whether the degree of mobility of the lumbar spine contributes to the degree of symptomatology experienced by patients with FAI. We hypothesize the symptomatic FAI cohort will exhibit less functional lumbar mobility leading to greater flexion of the hip than the cohorts without the symptoms or without the morphology of FAI.
Patients and Methods
From November 2015 to April 2017, 30 subjects were prospectively enrolled to participate in this Institutional Review Board (IRB)-approved study. To be included, participants had to be healthy, active males between the ages of 18 and 40 years. Those with osteoarthritis of the hip or spine, or a history of congenital deformity, trauma, or surgery of the hip or spine were excluded, as were those with a history of lumbar pain, pincer morphology (lateral centre edge angle > 40°, retroverted acetabulum), acetabular dysplasia (lateral centre edge angle < 20°), an ongoing claim for worker’s compensation, or a limb length discrepancy > 2 cm.
Subjects were recruited and categorized into one of the following groups: subjects with neither hip pain nor radiographic evidence of FAI (non-FAI group); subjects with no pain in the hip but with radiographic signs of FAI (asymptomatic FAI group); or those with pain in the hip and radiographic signs of FAI (symptomatic FAI group). Symptomatic patients were recruited from the senior author’s clinic (ASR). Subjects in the asymptomatic and non-FAI group were volunteers. All participants gave a detailed history of any symptomatology they may have, were examined to assess the range of movement (ROM) and any signs of impingement, and underwent radiographic imaging. Subjects were assigned to the non-FAI cohort if there was no clinical or radiographic evidence of FAI (defined as those with an alpha angle < 50° and internal rotation (IR) > 15°).11 Subjects demonstrating clinical or radiological evidence of FAI (alpha angle > 60°, IR < 15°), but who had no pain from the hip during normal activities or during provocative testing were assigned to the asymptomatic FAI group. Subjects in the symptomatic FAI group demonstrated both clinic and radiographic evidence of FAI (alpha angle > 60°, IR < 15°) with positive examination findings and pain in activities of daily living.
Ten subjects were recruited for each of the three cohorts. However, after radiographic imaging was acquired, ten subjects did not meet inclusion criteria. Four patients with symptomatic FAI were excluded as their alpha angle was too small; five asymptomatic subjects were excluded as they did not have sufficient internal rotation; and one asymptomatic subject was excluded for reporting pain in the hip during provocative testing. The final study group contained 20 subjects, six in the non-FAI group, seven in the asymptomatic FAI group, and seven in the symptomatic FAI group. All subjects had a morphologically normal, asymptomatic contralateral hip. The mean age for the study cohort was 31 years old (22 to 41), with no difference in age between groups (p = 0.07, analysis of variance, ANOVA).
All clinical examinations were performed by a senior surgeon specializing in sports medicine (ASR). The range of movement of the hip joint was assessed with the subject supine (flexion, extension, internal and external rotation), and provocative testing for FAI was performed in the positions of flexion-adduction internal rotation (FADIR) and abduction-external rotation (ABER).
Plain radiographs (anteroposterior (AP) pelvis and Dunn lateral view) were obtained of each subject. Alpha angle was evaluated independently in a standardized method by two fellowship-trained sports surgeons (RRF, MAT), who were blinded to each other’s measurements. Measurements were taken from digital radiographs using the tools within our institution’s Picture Archiving and Communication System (Sectra PACS; Sectra Imtec AB, Linköping, Sweden). The alpha angle was measured on the Dunn view by measuring the angle between a line drawn from the centre of the femoral head bisecting the femoral neck, and a line from the centre of the femoral neck intersecting the point at the head-neck junction. (Fig. 1). FAI was defined as a combination of an alpha angle > 60° on the Dunn lateral and internal rotation < 15° on examination.12
In addition to plain radiographs, each participant also underwent low-dose, biplanar 2D standing and sitting stereoradiograph from the thoracolumbar junction (T12-L1) to the femur using the EOS Imaging System (EOS Imaging Inc., Paris, France). Standing and sitting position were standardized for all subjects. Subjects were imaged with their femora aligned approximately parallel to the floor to assume 90° of apparent hip flexion and allow measurement of the degree of true flexion of the spine and hip. This relaxed seated position has been previously used in THA studies.6,8 Lumbar lordosis, proximal femoral angle, and sacral slope were measured digitally for both the standing and sitting EOS scans according to a previously described method (Fig. 2).8 Lumbar lordosis was measured as the angle between the superior endplate of L1 and the superior endplate of S1. The proximal femur angle was measured between a vertical line and the line defined by the anterior cortex of the most visible femur. Sacral slope was measured as the angle between a line parallel to the superior endplate of S1 and a line parallel to the horizontal border of the radiograph. Sacral slope angle was used as a surrogate for pelvic alignment in sitting and standing positions.
Measurements of flexion of the spine and hip were derived from the above measurements. Flexion of the spine was calculated as the change in lumbar lordosis between standing and sitting positions. Flexion of the hip joint was calculated by adding the change in position of the proximal femur to the change in position of the pelvis (using the sacral slope angle), always subtracting sitting position values from standing position values. For example, to calculate flexion of the hip joint in the subject without FAI shown in Figure 2, we added the difference in proximal femur angle (89° - 1° = ٨٨°) to the change in sacral slope (8° - 39° = -31°), to find that he had 57°of flexion at the hip.
The measurements of the two examiners were compared using the intraclass correlation coefficient (ICC). In cases of poor correlation, a third examiner was used to decide. Spinopelvic alignment parameters were compared for three groups (non-FAI, asymptomatic FAI and symptomatic FAI) using a one-way ANOVA test or Kruskal–Wallis test. In the case of a significant one-way ANOVA result, post hoc tests were performed, namely Tukey’s test if the data had equal variances or Dunn’s multiple comparison test if not. A linear regression was also calculated. All statistical analyses were performed using SigmaPlot 13.0 (Systat, San Jose, California) with a significance level of 0.05.
Spinopelvic parameters are shown in Table I. Subjects with FAI, whether symptomatic or asymptomatic, had significantly higher sacral slope angles while sitting compared with subjects without FAI (p = 0.01, ANOVA), meaning that those with FAI sat with more anterior pelvic tilt (Fig. 2). Symptomatic subjects with FAI exhibited less flexion of the spine (mean 22° (sd 12°) vs mean 35° (sd 8°); p = 0.04, ANOVA) than those without FAI, with a compensatory increase in flexion at the hip joint (mean 72° (sd 6°) vs mean 62° (sd 8°); p = 0.02, ANOVA). For the group as a whole, there is an inverse linear relationship between flexion at the hip and at the spine (R2 = 0.48, Fig. 3).
|Non-FAI||Asymptomatic FAI||Symptomatic FAI||p-value*||Differences|
|Mean alpha angle, ° (sd)||41 (5)||70 (8)||67 (12)||0.004||Asymptomatic FAI > non-FAI; symptomatic FAI > non-FAI|
|Mean pelvic incidence, ° (sd)||44 (5)||53 (12)||48 (7)||0.26||N/A|
|Mean standing proximal femoral angle, ° (sd)||6 (3)||6 (3)||7 (3)||0.76||N/A|
|Mean standing sacral slope, ° (sd)||37 (5)||45 (6)||42 (9)||0.11||N/A|
|Mean standing lumbar lordosis, ° (sd)||52 (14)||62 (6)||56 (10)||0.25||N/A|
|Mean sitting proximal femoral angle, ° (sd)||90 (5)||92 (8)||90 (3)||0.81||N/A|
|Mean sitting sacral slope, ° (sd)||14 (10)||27 (7)||30 (9)||0.01||Asymptomatic FAI > non-FAI; symptomatic FAI > non-FAI|
|Mean sitting lumbar lordosis, ° (sd)||17 (16)||29 (8)||34 (14)||0.1||N/A|
|Mean spine flexion, ° (sd)||35 (8)||33 (8)||22 (12)||0.04||Symptomatic FAI < non-FAI|
|Mean femoroacetabular flexion, ° (sd)||62 (8)||68 (10)||72 (6)||0.02||Symptomatic FAI > non-FAI|
One-way analysis of variance
N/A, not applicable
In this study, we compared dynamic spinopelvic parameters in symptomatic and asymptomatic individuals with radiographic evidence of FAI as well as normal controls. Subjects with symptomatic FAI had a mean of 13° less spine flexion and 10° more hip flexion versus those without FAI. Subjects with asymptomatic FAI, however, demonstrated no significant difference in flexion of the spine or hip joint compared with controls. These findings suggest that individuals with the morphological features of FAI may remain asymptomatic if the spine is sufficiently mobile to provide compensatory flexion to avoid excess flexion at the hip joint. Without this compensatory lumbar flexion, deep flexion of the hip joint causes cam lesions to engage, precipitating pain seen in FAI. There may be a role for rehabilitation strategies in FAI to focus on improving the mobility of both the hip joint and the lumbar spine.
The current study confirms and extends the findings reported in other studies that proposed an interaction between the kinematics of the spine and the hip.5,12,13 To our knowledge, this is the first study to investigate the role of spinopelvic kinematics in the setting of FAI. Our findings are similar to those described by Esposito et al5 in their study of patients following THA. Those with concomitant degenerative disc disease of the lumbar spine had altered flexion at the hip joint particularly during changes in functional position. We examined similar variables in a younger population of patients with FAI and found that the degree of mobility of the lumbar spine influenced the degree of symptoms in FAI. It is worth noting that subjects included in our cohort did not have radiographic evidence of lumbar spine disease, and therefore the origin of decreased lumbar spine excursion in these symptomatic patients is unclear.
The findings of our study have several important clinical applications. Our results suggest that rehabilitation strategies can be applied to improving flexion of the lumbar spine in patients with symptomatic FAI. We hypothesize that a certain amount of spinopelvic flexion is required to perform functional activities such as sitting, tying of shoelaces, or ascent of stairs. In patients with limited flexion at the lumbar spine, the hip joint is recruited to a greater extent to provide the requisite spinopelvic flexion for functional activities. Other studies have shown that patients with a large anterosuperior cam deformity may impinge throughout the arc of flexion, even during activities such as walking where flexion of the hip only reaches 40°.13 We suggest that mobility of the lumbar spine should be considered and evaluated in the investigation of patients with hip pain and possible FAI. As such, patients with limited mobility of the lumbar spine may be identified and directed toward targeted physiotherapy, in addition to traditional hip- and core-based treatments. Fusion of the lumbar spine may be expected to lead to an increase in the degree of flexion required at the hip joint, and may precipitate or exacerbate symptoms in hitherto asymptomatic patients with radiographic evidence of FAI; likewise, patients who present with new symptoms of FAI on the background of a previous fusion of the lumbar spine may have limited benefit from rehabilitative strategies.
Our study has limitations. We did not perform a formal power calculation and we had a small sample size, limiting our ability to detect significant differences between groups. Our sample size was limited by our ability to recruit control subjects, particularly asymptomatic subjects with radiographic evidence of FAI. In spite of our limited sample size, we did find strongly significant relationships, demonstrating the strength of association among the variables under investigation. Additionally, we confined our data set to male subjects. We did this to remove the potential confounding effect of gender, but it may mean that our findings are not generalizable to females. There is an opportunity in future studies to investigate the role of demographic factors such as age, gender, and ethnicity on parameters relating to spinopelvic flexion. We also focused primarily on cam-type FAI and we did not include subgroups with acetabular pathology. Again, this decision was made to preserve homogeneity of the cohort but patients with pincer-type FAI may be examined in future studies. Finally, the images we acquired were dependent on how subjects were positioned in the imaging system. We attempted to standardize this between subjects and this may not replicate how the subjects sit comfortably during their daily activities.
Spinopelvic mechanics appear to have an impact on symptoms in patients with FAI. When lumbar mobility is limited, this leads to an increase in the amount of flexion required at the hip joint, leading to symptomatic engagement of the cam lesion. Patients with symptomatic FAI demonstrate limited compensatory flexion of the lumbar spine, and therefore increased flexion angles at the hip joint during standing to sitting versus those without FAI or with asymptomatic FAI. Spinopelvic mechanics and lumbar mobility may be a target for physiotherapy in patients without degenerative lumbar disease.
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R. R. Fader: Collecting the data, Statistical analysis, Recruiting the subjects, Writing and editing the manuscript.
M. A. Tao: Collecting the data, Statistical analysis, Recruiting the subjects, Writing and editing the manuscript.
M. A. Gaudiani: Collecting the data, Statistical analysis, Recruiting the subjects, Writing and editing the manuscript.
R. Turk: Collecting the data, Recruiting the subjects, Editing the manuscript.
B. U. Nwachukwu: Designing the study, Statistical analysis, Writing and editing the manuscript.
C. I. Esposito: Designing the study, Statistical analysis, Writing and editing the manuscript.
A. S. Ranawat: Designing the study, Statistical analysis, Recruiting the subjects, Writing and editing the manuscript.
No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
A. S. Ranawat reports a grant from the United States Department of Defense (PRORP Clinical Trial Development Award; Grant Number: W81XWH-15-1-065).
This article was primary edited by A. D. Liddle and first proof edited by G. Scott.