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Sexual Dimorphic Features Associated with Femoroacetabular Impingement.

Sexual dimorphism describes differences in biologic response between males and females due to inherent chromosomal differences. (1) Though often used to describe anthropomorphic differences between sexes, the term can be extended to include specific anatomic, physiologic, psychological, and behavioral differences. (2)

Thus, sexual dimorphism similarly affects orthopaedic-related injuries and treatment outcomes as seen with shoulder instability, ACL injuries, stress fractures, and femoroacetabular impingement. (3) Dimorphic factors that have been shown to increase the susceptibility of females to musculoskeletal injuries and/or worse outcomes may include joint laxity, hip morphology, and osseous biology. (4)

Femoroacetabular impingement (FAI) is an abnormal hip morphology resulting in an abutment between the proximal femur and acetabular rim. (5) These abnormalities of the acetabulum and/or femur ultimately lead to damage within the joint. (6) Seen most frequently in active adolescents and young adults, FAI is widely recognized in the athletic patient population. (7, 8) The 2 most common types of FAI include cam-type, most prevalent in young males, and pincer-type, most frequently seen in middle-aged females. (5, 8-12) While the clinical presentations are similar for both types of FAI, treatment outcomes have differed between sexes. The purpose of this article is to review the relevant literature to characterize sexual dimorphism as it relates to FAI.


The 2016 Warwick Agreement defines FAI as a syndrome of a motion-related clinical disorder of the hip with a triad of symptoms, clinical signs, and imaging findings. (13) It represents symptomatic premature contact between the proximal femur and the acetabulum. (13) There are 2 main morphologies described for FAI syndrome: cam-type and pincer-type. In cam-type morphology, the proximal femur osseous morphology is characterized by an aspherical cartilage extension at the anterosuperior head-neck junction that abuts against the acetabular rim with flexion and internal rotation of the hip. (14-18) In pincer-type morphology, the abnormal acetabular rim produces relative overcoverage of the femoral head leading to a decrease in motion between the femoral head-neck junction and acetabulum. (5, 6) Over time, these changes can cause significant pain, chondral delamination, and labral dysfunction, ultimately leading to degenerative changes in the hip and premature osteoarthritis. (9, 18-21) As pathologic changes rarely occur independently of one another, a mixed cam and pincer-type morphology is the most common type of FAI syndrome seen in the general population. (5, 22)


The prevalence of FAI syndrome is estimated to be between 10% and 15% of the general population and is predominantly seen in active adolescents and young adults involved in impact sports such as collegiate football and basketball. (19, 23-25) Moreover, there is a high prevalence of asymptomatic abnormal radiographic impingement morphology in the adolescent population. (26) Consequently, due to the wide spectrum of normal variance, using hard-set static radiographic parameters when weighing surgical management should be used cautiously in patients with FAI syndrome. (26)

Typically, FAI syndrome presents as an insidious onset of poorly characterized deep groin pain or discomfort. However, there can be wide variation in the location, nature, radiation, severity, and precipitating factors that characterize this pain. While most patients report pain in the groin or hip, pain is also reported in the lateral hip, anterior thigh, buttock, knee, lower back, lateral or posterior thigh. (27) This pain is aggravated with activity and active hip motion. (28) In females, FAI syndrome can present with significantly more disability, despite generally having less severe deformities and less intra-articular disease in comparison to their male counterparts. (29) Furthermore, although FAI is most commonly associated with developmental morphogenic changes, a high index of suspicion should also be maintained for patients with clinical signs of impingement secondary to trauma or previous surgeries undertaken for correction of developmental hip pathology.

The most commonly tested and the most sensitive clinical examination finding suggestive of impingement of anterior impingement is the flexion, adduction, and internal rotation test (FADIR). (19, 30) In the FADIR test, the hip is flexed to 90[degrees] and forcibly adducted and internally rotated. (31) If anterior impingement is present, the patient will describe reproduction of their pain with a sensitivity of 0.99. (30)

The cam-type morphology is most prevalent in young and athletic males. (8, 18, 32) It is theorized that due to intense sports participation in male adolescents, there is an increased risk of developing cam-type morphology in this population. (33) Schmitz et al compared anteroposterior pelvic radiographs made with EOS imaging in asymptomatic adolescents, demonstrating that 92.8% of participants demonstrated at least one parameter suggesting impingement morphology. (26) These findings demonstrate that the emphasis for surgical management should be tailored towards physical examination findings in conjunction with the radiographic findings, and not radiographic findings alone.

Within the US adult population, the ratio of asymptomatic cam-type morphology is 2:1 when comparing males to females. (22) Furthermore, the prevalence of cam-type morphologies in asymptomatic adults has been reported to be as high as 24% in males and 5.4% of females with alpha angles parameters greater than 50.5[degrees] (34) The Copenhagen Osteoarthritis Study reported a similar prevalence of abnormal alpha angle finding with 17% in males and 4% in female adults on anteroposterior pelvis radiographs. (35)

Diagnostic radiographic findings in cam-type impingements include the alpha angle, pistol grip deformity, and the femoral head-neck offset (Figure 1). (14, 18, 36) Specifically, the pistol grip deformity is more common in males, with a prevalence of 19.6% compared to 5.2% in females. Similar findings were reported by Laborie and colleagues with a prevalence of 21.5% in males and 3.3% in females. (32) Moreover, alpha angles are significantly decreased among females at 47.8[degrees] in comparison to males at 63.6[degrees] (21, 37) The lower symptomatic alpha angles in females tend to be more shallow, have a lower total volume (males 433 [+ or -] 471 [mm.sup.3]; females 89 [+ or -] 124 [mm.sup.3]), and are more symptomatic with alpha angles less than 50[degrees] (37, 38) Additionally, these subtle symptomatic cam-type morphologies in females tend to be in an atypical location and smaller in size, thereby suggesting that radiographic parameters may be modified based on the above sexual dimorphic features to improve potential surgical decision-making. (5, 37)

Pincer-type morphologies are most prevalent in middle aged active females and are more predominantly associated with anterior acetabular wall over coverage. (5, 9-12, 39) Population-based studies have shown males to have a significantly decreased focal and global acetabular version compared with females, where focal acetabular version in the 1-o'clock position was 15.5[degrees] and 18.3[degrees]; 21.5[degrees] and 24.0[degrees] in the 2-o'clock position; and 20.2[degrees] and 24.3[degrees] in the 3-o'clock position for males and females respectively. (39) While Tannebaum et al found no statistical difference between sexes when assessing true acetabular version (males: 19.1[degrees], females 22.2[degrees]), males were shown to have a posterior wall deficiency contributing to a related acetabular retroversion. (39) These findings lead one to consider the relevance of dynamic factors that contribute to the prevalence of pincer-type morphologies in females, such as increased joint laxity that facilitates greater extremes of motion and increased pelvic tilt, or forward tilt from weaker core musculature. (40, 41) Understanding these differences between sexes in the presentation of FAI syndrome is important for establishing accurate diagnostic algorithms for treatment and decision-making. (42)

Diagnostic radiographic findings for the pincer morphologies include the crossover sign, posterior wall sign, ischial spine sign, coxa profunda, protrusio acetabuli, and lateral center-edge (LCE) (Figure 2). Schmitz et al showed a significantly higher prevalence of coxa profunda in females compared with males, 87.8% and 75.6% respectively. (26) Furthermore, their data on asymptomatic adolescents also suggest that the upper limit of normal for LCE angle may be as high as 44[degrees] based on normative values from their study. (26) The high prevalence of coxa profunda illustrated above suggests that this pincer-type morphology may be a variant of normal rather than abnormal morphology, particularly among females. As such, coxa profunda morphology in isolation of other pathology may not represent true impingement pathology in all cases and additional studies should be considered. There was no significant difference detected among other signs of acetabular overcoverage. (26)


Currently, there are no data to suggest a sex difference in conservative treatment modalities. (43) As such, the goal of first-line conservative management for FAI syndrome includes education, activity modification, NSAIDs, and core strengthening exercises. Although the above conservative therapies improve hip function and reduce pain, the overall hip range of motion of the symptomatic extremity does not improve in comparison to the unaffected extremity. (43) Once conservative measures have been exhausted, surgical management is often explored. However, due to the vague insidious symptoms of FAI syndrome and overlap with other musculoskeletal conditions of the hip, pelvis, and lumbar spine, definitive diagnosis and surgical management of symptomatic FAI syndrome is often delayed with average symptom duration of 3.1 years and an average of 4.2 ([+ or -] 2.9) healthcare providers seen before eventual diagnosis. (44) Moreover, surgical management that is excessively delayed can place a patient at risk for an accelerated degenerative disease and end-stage arthritic changes that may ultimately require total hip arthroplasty. This postponed diagnosis is attributed to under-recognition of symptomatic FAI syndrome as a cause of hip pain secondary to the degree of education and training among the various healthcare providers regarding FAI syndrome. (43)

Current surgical interventions to manage FAI syndrome include hip arthroscopy, a combined arthroscopic and open approach, mini-open approach, surgical hip dislocation, and anteverting periacetabular osteotomy (PAO). However, few studies focusing on the surgical management of open and arthroscopic techniques have shown a large influence of sex in determining outcomes in the surgical treatment for FAI syndrome. (7) Clohisy et al (45) examined the rate of surgically treated FAI syndrome and found that females were more likely to undergo surgical treatment for symptomatic FAI syndrome than males. Multiple studies have shown both open and arthroscopic hip approaches were effective in pain relief and improvement in function with short-term (2 years or less) and midterm (2 to 5 years) follow-up for both male and females. (46) Arthroscopy has shown equivalent functional outcomes, using various functional outcome measures, with a lower rate of major complications that include osteonecrosis, deep infection, femoral neck fracture, loss of fixation, and trochanteric nonunion when compared to mini-open approaches and surgical hip dislocation techniques. (47, 48) Furthermore, hip arthroscopy yields meaningful improvements in hip function, as validated with a mean difference before and after surgery exceeding the minimal clinically important difference of the iHOT-33 in majority of patients, regardless of sex. (49-51)

An alternative form of surgical management is the PAO surgical technique often used for pincer-type impingement to reduce acetabular retroversion, and is one of the few surgical approaches to have an association as a sex-specific treatment secondary to the increased prevalence of pincer-type morphology in the female population. (37) The anterosuperior quadrant of the acetabulum is a primary weight-bearing area and removal potentiates a high amount of stress during weight bearing. Thus, performing preoperative hip version analyses with axial CT or MR imaging should be considered to assess for the smaller subtle cam morphologies found in females prior to anterosuperior and superolateral acetabulum rim trimming to avoid increasing contact stresses at weight-bearing areas that may accelerate hip degeneration. (6, 37)

We are unaware of any studies comparing PAO surgical outcomes for acetabular retroversion between sexes; however, Ziebarth et al demonstrated males presented with a high postoperative rate of clinical signs of FAI syndrome after PAO for developmental dysplasia of the hip compared to the female population. (52)


Frank et al showed that older female patients have significantly lower hip function scores than male counterparts before and after surgery. (50) Furthermore, females have a greater overall improvement of quality of life scores after surgery with short-term (2 years or less) follow-up, suggesting that females undergo surgery when they are more highly disabled compared to males and may have more to gain in terms of quality of life after surgery. (50, 53, 54) These sex differences observed in outcome measures are multifactorial and may include a component of perception where females perceive pain as more limiting to function, whereas males may over-estimate pain relief and functional performance. (55)

However a study by Byrd et al showed no meaningful difference in long-term outcomes based on impingement morphology or between male and female patients within a 2- to 10-year period after arthroscopic surgery in a population consisting of young athletes with ages ranging from 12 to 17 years. (56) In comparison, a study by Philippon et al demonstrated contrary findings in a smaller prospective study of 60 active adolescents, with female patients reporting significantly poorer hip function than their male counterparts with midterm (2 to 5 years) follow-up after arthroscopic surgery in adolescent population aged 11 to 16 years. (57) This suggests that females may have worse functional outcomes when compared to males with short to midterm follow-up. Continued research following surgical outcomes with variances of duration of follow-up may narrow the clinical outcome differences amongst sexes.

Outcome measurements are an effective tool for evaluating surgical functional outcomes for FAI syndrome pre-and postoperatively. The most common functional outcomes measurements reported to assess FAI syndrome are the Harris Hip Score (HHS), Nonarthritic Hip Scale, range of motion, pain scores, and patient satisfaction (on visual analogue scales). (58) Other primary clinical outcomes cited in the literature to assess postoperative outcomes include the use of the anterior impingement test (flexion, adduction, internal rotation), modified HHS, Short Form 12, Hip Outcome Score--activities of daily living, and/or Hip Outcome Score--sport-specific. (58) The aforementioned measures are not validated in patients with FAI syndrome but for labral tears, nor are they applicable to the nonadult population (less than 23 years old). Interestingly, Impellizerri et al compared the validity, reproducibility, and responsiveness of the Oxford Hip Score (OHS) and Hip Outcome Score measurements in patients undergoing surgery for FAI syndrome and concluded that the OHS was an appropriate instrument for pain and function assessment in those status post arthroscopic FAI surgery (47% male and 53 % female patients). (59) Despite the promise of these outcome measures, evaluating and validating them in all populations, particularly between males and females, will be important for global use and assessing clinical difference amongst various populations. (58)

Patient satisfaction has been suggested as an important domain for functional outcome assessment with preoperative expectations and the fulfillment of those expectations influencing postoperative satisfaction more than functional outcomes. (60) A review by Kahlenberg et al found that the quality of studies reporting patient satisfaction after FAI surgery are low due to several discrepancies in the reporting, including failure to gauge patients' initial expectations before the operative procedure, and further distinguishing expectations of outcomes versus process of care satisfaction. Failure to define patient expectations before the operative procedure may inaccurately assume that the functional outcome of care is assessed rather than the process of care. (61) These discrepancies may factor into the worse reported functional outcomes in females after undergoing arthroscopy for FAI syndrome. There is currently no standardized means for reporting patient satisfaction. With hip arthroscopy being increasingly used to address FAI syndrome, little is known about the state of patient satisfaction reporting after the treatment of FAI syndrome as well as the relation of patient satisfaction to overall outcome. (60) Delineating satisfaction relating to delivery of care versus outcome of care plays an increasingly larger role in defining the value of emerging orthopaedic interventions, particularly with surgical management of FAI syndrome which has consistently shown worse patient reported functional outcomes when comparing males to females. (62)

Sexual dimorphisms in hip morphology, soft tissue laxity, muscle mass, increased pelvic tilt, and dynamic joint stabilization have been proposed as causes of the reported outcome differences between males and females with nonarthritic hip pain. (41, 49) In a 2014 study by Frank et al, hip capsular volumes of 97 patients were analyzed using magnetic resonance arthrograms. (63) Interestingly, female patients were found to have a larger ratio of capsular volume to femoral head volume compared with male patients, providing a possible explanation for the known prevalence of hip joint hypermobility in female patients compared with male patients. Pontiff et al examined the differences in perceived function and quality of life outcomes in females with and without generalized joint laxity (GJL) prior to and 6 months following hip arthroscopy for FAI syndrome. (64) Differences in self-reported hip function were not identified preoperatively or 6 months postoperatively, suggesting that GJL has no effect on worse self-reported hip function and quality of life outcomes perioperatively for arthroscopic treatment of FAI syndrome in the female population. (64) This is the first prospective study to examine the effect of laxity on patients who undergo surgical treatment for intra-articular hip pathology. (64) While little data exists, the lack of differences in group outcome scores may be attributed to limitations of having a single postoperative follow-up and the use of a laxity score index not specific to the hip joint. (64) Identification of variables such as generalized joint laxity should continue to be explored, along with adjusting the course of postoperative care to modulate improved outcomes in females after hip arthroscopy for FAI syndrome. (64)

Female sex was also shown by Lee et al (49) to be predictive of a longer recovery time after arthroscopic hip surgery for the treatment of common hip disorders. This correlates with a study by Boyd et al, (65) which showed that longer durations of follow-up increased recovery time and ultimately offset differences between sexes in functional outcomes postoperatively from arthroscopic hip surgery. As such, further reducing recovery time may optimize functional outcomes with short-term follow-up and more emphasis directed toward rehabilitation protocols within a specified time interval should be considered. Likewise, Joseph et al demonstrated that functional improvements plateaued 6 months after hip arthroscopic surgery. (49) A greater understanding of physical impairments and activity limitations may aid in the effects of future physical therapy protocols and potentially minimize the discrepancy in hip functional outcomes when comparing males to females after hip arthroscopy for FAI syndrome. (66)


Sexual dimorphic features associated with FAI treatment and outcomes are multifactorial and include factors such as hip morphology, management including conservative and surgical options, and patient-reported outcomes. Future domains of research should include randomized control trials that examine treatment effects on FAI syndrome and consider whether conservative treatments are a viable and/or a preferable alternative to surgical interventions. (66) Current evidence supporting surgical and conservative interventions for FAI syndrome is based on low-level research. Moreover, multicenter randomized controlled trials are needed to help establish the value of these interventions in the management of FAI syndrome. Such trials will also help define appropriate clinical pathways to further differentiate sex-specific treatments and ultimately improve functional outcomes. (67, 68)


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CPT Okpala is a Field Surgeon with the 3rd Cavalry Regiment, Fort Hood, Texas.

MAJ Tennent is an Orthopaedic Surgeon at the Keller Army Community Hospital, US Military Academy, West Point, New York.

When this article was written, COL Johnson was Chairman, Department of Orthopaedic Surgery, San Antonio Military Medical Center, Joint Base San Antonio-Fort Sam Houston, Texas.

LTC Schmitz is with the Department of Orthopaedics, San Antonio Miltary Medical Center, Joint Base San Antonio-Fort Sam Houston, Texas.

CPT Victoria B. Okpala, MC

MAJ David J. Tennent, MC

COL (Ret) Anthony E. Johnson, MC

LTC Matthew R. Schmitz, MC

Caption: Figure 1. Panel A: Lateral view of the right hip showing increased alpha angle (yellow lines with normal being less than 50[degrees]). Also with decrease in femoral head neck offset (the difference between the solid white and dashed white lines (normal 8 mm). Panel B: Anterior posterior view of the right hip showing the pistol grip deformity of the proximal femur with abnormal extension of the epiphysis down the proximal femoral neck.

Caption: Figure 2. Panel A: Crossover sign with the anterior acetabular wall (solid line) crossing over the posterior wall (dashed line) indicating focal anterior over coverage.

Panel B: Ischial spine sign indicated by the arrow with the ischial spine projecting into the true pelvis, indicative of global acetabular retroversion.

Panel C: Coxa profunda on the left with the acetabular depth (dashed line) projecting medial to the ilioischial line (solid line).

Panel D: Coxa protrusio on the right with the femoral head (dashed line) projecting medial to the ilioischial line (solid line).

Panel E: Posterior wall signs with the posterior wall (dashed line) lateral to the center of the femoral head indicated by a circle. This represents posterior overcoverage or relative anteversion of the acetabulum.

Panel F: Decreased lateral center edge (LCE) angle of less than 25[degrees] demonstrating dysplasia.

Panel G: Normal LCE shown with LCE between 25[degrees] and 39[degrees].

Panel H: Increased LCE shown with overcoverage greater than 40[degrees].
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Author:Okpala, Victoria B.; Tennent, David J.; Johnson, Anthony E.; Schmitz, Matthew R.
Publication:U.S. Army Medical Department Journal
Date:Jul 1, 2018
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