Pathomechanics of Femoroacetabular Impingement
FAI – What is it?
Femoroacetabular Impingement syndrome, otherwise referred to as FAI or FAIS, is a clinical diagnosis primarily related to hip motion 8, 6. This disorder affects healthy and proper movement occurring between the femoral head and the acetabulum of the hip and is often a predecessor to labral degeneration and/or hip osteoarthritis 8. According to the Warwick Agreement made in 2016, a proper diagnosis of FAI requires the presence of three factors: symptoms, clinical results from assessment, and relevant imaging findings 6. Symptoms of FAI most often present as pain in the hip and groin region, typically when the hip is undergoing movement, as well as various degrees of restriction, noises such as clicking, locking, and stiffness 6. Pain occurs most commonly in the anterior aspect of the hip, specifically when the hip is brought into the position of FADIR, that being flexion, adduction, and internal rotation, but can also be experienced in the posterior chain of the hip 6, 12, 2.
Pictured here is a short video highlighting what FAI is, in addition to the differences between the classifications of FAI 3.
Anatomical Relevance and Classification
The femoral head, the acetabulum, and numerous structures of soft tissue make up the bulk anatomy of the hip joint 14. The labrum of the acetabulum forms a “U” shape within the socket and functions as a seal between the bone of both the acetabulum and the femoral head, which works to increase depth of the socket and stability to the joint 14. To assess medial and lateral coverage of the acetabulum around the femoral head, the center edge angle can be measured from the center of the femoral head to the edge of the respective side of the acetabulum 14. When this angle is greater than 40 degrees, impingement may be present 14.
There are two major classifications of FAI which differ mainly in the boney morphology of the proximal femur and the acetabulum. The first type of FAI is Cam, which is characterized by the irregular shape of the femoral head, normally rounded, causing a grinding sensation against the labrum 9, 6. In contrast, the Pincer type of FAI is present when the acetabular bone is longer and thicker than normal and excessively surrounds the head of the femur, causing the labrum to be crushed under the pressure of the boney rim 9, 6. While the two exist as distinct classifications, femoroacetabular impingement may occur with both morphologies present, called Mixed FAI 2, 6. As previously mentioned, impingement comes about when the hip moves into positions of flexion, adduction, and internal rotation, which causes the bone-on-bone grinding and possible deterioration of the bone, labrum, ligaments, and tendons in the region 4.
While boney morphologies are highly common in causing the development of FAI, other factors can play a role in its upbringing. Biomechanics and an individual’s muscular strengths and mobility can affect how the hip moves, especially concerning the contact between the femoral head and its socket. A study carried out by Casertelli et al, focused on the muscles of the hip in FAI-diagnosed patients, where each subject was symptomatic and were compared to an asymptomatic control group 2. The results of this study demonstrated that individuals diagnosed with FAI had significantly lower maximal velocity contraction strengths than the control group for hip adduction, flexion, external rotation, and abduction 2. With weaker muscle action in and around the hip, it is possible that this can result in poor dynamic hip instability and can lead to increased femoral head translation against the labrum of the acetabulum 2.
Prevalence of FAI
Between the two classifications of FAI, cam is the more common, shown in various research studies. The Copenhagen Osteoarthritis study took a group of 3202 people and discovered that 17% of the men had presented with cam malformation and had reported hip pain, while that number was only 4% of the women 5. Another study carried out by Hack et al looked at the prevalence of cam-type FAI morphology and demonstrated that cam was present in 25% of the men and only 5% of the women 7. Furthermore, a study carried out by Beck et al confirmed that aforementioned numbers and concluded that cam type FAI was more common in men than women, with an overall general consensus that cam type FAI was more common than pincer type FAI 1. In a systematic review carried out by Mascarenhas et al the existence of cam and pincer type FAI were examined via the results of radiographic imaging. This study illustrated that both the presence of cam type morphology and pincer type morphology were significantly more common among symptomatically affected patients than asymptomatic 11.
Biomechanics Affected by FAI
A handful of studies have recruited various groups of FAI affected individuals and had them perform a range of functional movements to test how these movements were affected by the presence of symptomatic or asymptomatic FAI.
A systematic review carried out by Freke et al examined twelve studies and reported that individuals who presented with FAIS showed significantly lowered ranges of motion for abduction and flexion of the hip 8. Additionally, six studies showed conflicting however moderate muscular weakness in hip adductors and external rotators, as well as limited evidence for reduced strength in the hip flexors, extensors, and abductors for FAI affected individuals 8. The review noted that there were no significant differences in depth of squat performance and pelvic range of motion between those of the FAI affected group and the control group 8. More notably, a handful of the studies within the review suggested the presence of FAI negatively affects the ranges of motion in the sagittal and frontal planes while observing gait mechanics in addition to lower values of peak hip extension, abduction, adduction, and internal rotation 8.
Additionally, a relevant systematic review and meta-analysis was performed by King and colleagues, which involved 14 studies and measured various biomechanical and functional movements such as gait and squatting 9. In examination of gait patterns, a lower peak hip extension angle during the stance phase was found, with no difference in peak hip flexion angle 9. Patients who presented with FAI demonstrated gait with a reduced total ROM in the sagittal plane with no significant differences in either adduction or abduction angles within the frontal plane 9. There was, however, significant reduction in the peak internal rotation angle of the hip in the transverse plane of gait 9. In examination of squatting function, patients with FAI displayed a lower degree of squat depth when compared to the control group, however no significant differences with hip flexion angles 9.
Physical Therapy Route to FAI Management
It is generally understood that hip arthroscopy as a surgical option for FAI is the most common form of management for this diagnosis 13, 10. A systematic review and meta-analysis completed by Schwabe and colleagues demonstrated this notion by reporting that two of three studies involved concluded a significant difference between surgery and conservative treatment, with the former having greater iHOT-33 results 13.
Nevertheless, physical therapy can be viewed as a sequential treatment option alongside surgery and can occur either as a management leading up to surgery or as a follow-up post-surgery 10. With the aim of increasing strength and function of muscles in the hip and further increasing stability of the hip joint and decreasing impingement, Mansell et al designed the Australian FASHIoN protocol 10. The Personalised Hip Therapy protocol includes an assessment of patient’s experiences with pain, ROM, and function, in addition to a progressive exercise program and patient education 10.
1. Beck, M., Kalhor, M., Leunig, M., & Ganz, R. (2005). Hip morphology influences the pattern of damage to the acetabular cartilage: femoroacetabular impingement as a cause of early osteoarthritis of the hip. The Journal of bone and joint surgery. British volume, 87(7), 1012–1018. https://doi.org/10.1302/0301-620X.87B7.15203
2. Casartelli, N. C., Maffiuletti, N. A., Item-Glatthorn, J. F., Staehli, S., Bizzini, M., Impellizzeri, F. M., & Leunig, M. (2011). Hip muscle weakness in patients with symptomatic femoroacetabular impingement. Osteoarthritis and cartilage, 19(7), 816–821. https://doi.org/10.1016/j.joca.2011.04.001
3. Children’s Hospital Colorado. What is FAI? Available from: https://www.youtube.com/watch?v=CNgQpbZPflU .
4. Diamond, L. E., Dobson, F. L., Bennell, K. L., Wrigley, T. V., Hodges, P. W., & Hinman, R. S. (2015). Physical impairments and activity limitations in people with femoroacetabular impingement: a systematic review. British journal of sports medicine, 49(4), 230–242. https://doi.org/10.1136/bjsports-2013-093340
5. Gosvig, K. K., Jacobsen, S., Sonne-Holm, S., & Gebuhr, P. (2008). The prevalence of cam-type deformity of the hip joint: a survey of 4151 subjects of the Copenhagen Osteoarthritis Study. Acta radiologica (Stockholm, Sweden : 1987), 49(4), 436–441. https://doi.org/10.1080/02841850801935567
6. Griffin DR, Dickenson EJ, O'Donnell J, Agricola R, Awan T, Beck M et al. (2016) The Warwick Agreement on femoroacetabular impingement syndrome (FAI syndrome): an international consensus statement. Br J Sports Med. 50(19):1169-76.
7. Hack, K., Di Primio, G., Rakhra, K., & Beaulé, P. E. (2010). Prevalence of cam-type femoroacetabular impingement morphology in asymptomatic volunteers. The Journal of bone and joint surgery. American volume, 92(14), 2436–2444. https://doi.org/10.2106/JBJS.J.01280
8. Freke, M. et al. (2016) Physical Impairments in Symptomatic Femoroacetabular Impingement: A Systematic Review of the Evidence. Br J Sports Med.
9. King, G. M. et al. (2018) Lower Limb Biomechanics in Femoroacetabular Impingement Syndrome: A Systematic Review and Meta-analysis. Br J Sports Med. 52:566-580. doi: 10.1136/bjsports-2017-097839
10. Mansell, N. S., Rhon, D. I., Marchant, B. G., Slevin, J. M., & Meyer, J. L. (2016). Two-year outcomes after arthroscopic surgery compared to physical therapy for femoracetabular impingement: A protocol for a randomized clinical trial. BMC musculoskeletal disorders, 17, 60. https://doi.org/10.1186/s12891-016-0914-1
11. Mascarenhas V, Rego P, Dantas P, Gaspar A, Soldado F, Consciencia J et al. (2015). Imaging prevalence of femoroacetabular impingement in symptomatic patients, athletes, and asymptomatic individuals: A systematic review. Eourpean Jounral of Radiology. 85(1):73-95. Doi: https://doi.rog/101016/j.ejrad.2015.10.016
12. Reiman MP, Thorborg K. (2015) Femoroacetabular impingement surgery: are we moving too fast and too far beyond the evidence? Br J Sports Med. 49(12):782-4. doi: 10.1136/bjsports-2014-093821. Epub PMID: 25677797.
13. Schwabe, M. T., Clohisy, J. C., Cheng, A. L., Pascual-Garrido, C., Harris-Hayes, M., Hunt, D. M., Harris, M. D., Prather, H., & Nepple, J. J. (2020). Short-term Clinical Outcomes of Hip Arthroscopy Versus Physical Therapy in Patients With Femoroacetabular Impingement: A Systematic Review and Meta-analysis of Randomized Controlled Trials. Orthopaedic journal of sports medicine, 8(11), 2325967120968490. https://doi.org/10.1177/2325967120968490
14. Tranovich MJ, Salzler MJ, Enseki KR, Wright VJ. (2014) A review of femoroacetabular impingement and hip arthroscopy in the athlete. Phys Sportsmed. 42(1):75-87. doi: 10.3810/psm.2014.02.2050. PMID: 24565824.